研究者詳細 - 徳永　智春

Rapid consolidation of cemented tungsten carbide via ultrafast high-temperature sintering 査読有り Open Access

Akutagawa, D; Ihara, K; Sato, T; Tokunaga, T; Yamamoto, T

INTERNATIONAL JOURNAL OF REFRACTORY METALS & HARD MATERIALS 136 巻 2026年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：International Journal of Refractory Metals and Hard Materials

Ultrafast high-temperature sintering (UHS) has garnered attention for facilitating rapid sintering by employing carbon felt as a heat source. In particular, the low oxygen partial pressure achieved by the carbon heater is effective for sintering carbides in a short duration. In this study, WC-11mass%Co was used as a model cemented carbide material to investigate the densification conditions and the resulting alloy microstructure obtained during UHS. A dense alloy composed of only WC and γ-phases was obtained using a dwell time of only 60 s by setting the sintering temperature (carbon felt current) above the eutectic temperature between WC and the γ-phase. Scanning transmission electron microscopy revealed a characteristic microstructure at the WC/γ-phase interface, resulting from the rapid cooling rate achieved by turning off the carbon felt current at the end of UHS. The presence of steps was observed on the WC (0001) surface, but not on the {101¯0} surfaces. Additionally, the WC<inf>1-X</inf> phase, a structurally relaxed phase, was not observed on the WC (0001) surface. However, extending the dwell time to 90 s led to the formation of numerous voids, substantially decreasing the density. The formation of these voids is attributed to localized current concentration within the alloy during UHS. To prevent current inflow during UHS, a thin alumina single-crystal plate was inserted between the carbon felt and the green compact. This effectively suppressed the current inflow, resulting in a dense alloy without voids. To ensure electrical insulation from the carbon felt is essentially necessary for UHS of cemented carbides.

DOI： 10.1016/j.ijrmhm.2025.107582

Enhanced Hydrogen Release from Hydrogen Boride Nanosheets via Carbon Doping 査読有り Open Access

Kawamura, R; Kawamura, R; Varadwaj, A; Kisu, K; Tanaka, R; Yamamoto, A; Ishibiki, R; Ito, S; Fujita, T; Tokunaga, T; Oki, O; Fujimoto, Y; Toyoda, M; Saito, S; Sato, T; Orimo, S; Hosono, H; Matsuda, I; Kotsugi, M; Miyauchi, M; Kondo, T

SMALL STRUCTURES 6 巻 ( 12 ) 2025年12月

記述言語：英語出版者・発行元：Small Structures

Hydrogen boride (HB) nanosheets, which exhibit unique B.H.B bonding and intrinsic hydrogen release properties, show improved performance upon carbon doping. Herein, the synthesis of carbon-doped HB via ion exchange is demonstrated by using carbon-doped MgB<inf>2</inf> as a precursor. Comprehensive spectroscopic analyses inferred carbon substitution at boron sites, leading to structural distortions and electronic modifications. These changes weaken B.H.B bonds, thereby facilitating improved hydrogen release under both thermal and ultraviolet (UV) stimuli. Temperature-programed desorption measurements reveal a reduction in the activation energy for intralayer hydrogen desorption with increasing carbon content, while UV-induced hydrogen evolution shows a markedly lower onset threshold. Density functional theory calculations support these experimental findings, further demonstrating elongation of B.B distances and shifts in vibrational modes. Overall, these results establish carbon doping as an effective strategy for modulating the chemical and electronic environment of HB, offering a promising pathway toward high-efficiency, tunable hydrogen storage, and release materials.

DOI： 10.1002/sstr.202500578

The variation of oxygen and cation vacancies in strontium titanate polycrystals flash-sintered under direct and alternating electric fields 査読有り

Inagaki, S; Tokunaga, T; Kobayashi, K; Yamamoto, T

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 45 巻 ( 15 ) 2025年12月

記述言語：英語出版者・発行元：Journal of the European Ceramic Society

Point defect formation was investigated based on the electron energy-loss near-edge structure observed within the grains and 10 nm from the grain boundaries of strontium titanate flash-sintered under direct (DC) and alternating (AC) electric fields with strengths of 100 and 200 V/cm. More oxygen vacancies were generated in both regions under the DC field than under no field, but the same behavior was only observed under the AC field of 200 V/cm. Similarly, strontium vacancies were generated in both regions under the DC field but only within the grains under the AC field. The discrepancies in vacancy formation may be related to their generation and annihilation rates at grain boundaries under AC electric fields. The intricacies of the electric field-induced vacancy formation mechanism require further elucidation, but this work underscores the importance of considering the roles of both oxygen and cation vacancies in the accelerated mass diffusion during flash sintering.

DOI： 10.1016/j.jeurceramsoc.2025.117611

Rapid titanium carbide synthesis from titanium and graphite powders via ultrafast high-temperature sintering 査読有り Open Access

Akutagawa, D; Sato, T; Tokunaga, T; Kobayashi, K; Yamamoto, T

MATERIALS LETTERS 398 巻 2025年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Letters

Self-propagating high-temperature synthesis (SHS) is a combustion-based method that involves a self-heating chain reaction, which is effective for rapidly producing materials with high melting points. Herein, ultrafast high-temperature sintering (UHS) was leveraged to ignite the SHS of titanium carbide (TiC) from titanium and graphite powders, and the heat generation and microstructural evolution during the process were investigated. The exothermic heat generated during TiC synthesis was confirmed, demonstrating that SHS was initiated in the initial stage of UHS. The incubation time before SHS ignition decreased with an increase in the UHS temperature above the melting point of titanium. Scanning electron microscopy revealed that fine TiC grains formed during SHS, which became denser under continued UHS. Additionally, molten Ti spread over the graphite particles during SHS, which transformed into plate-like TiC grains with elongated voids between them under subsequent heating by UHS. This structural feature was found to hinder further densification.

DOI： 10.1016/j.matlet.2025.138890

Surface step motion on the (001) surface of strontium titanate single crystals during DC electric field-assisted flash treatment 査読有り Open Access

Komori, S; Katsuyama, Y; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 133 巻 ( 11 ) 頁： 635 - 641 2025年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

A DC electric field-assisted flash treatment was performed on the (001) surface of a SrTiO<inf>3</inf> single crystal on which a step-terrace structure had been formed in advance (by solution etching). The DC electric field was applied along the direction that the surface height increases (up-step) or decreases (down-step) for a very short treatment duration of 1 min. The step-terrace morphology on the (001) surface was observed by atomic force microscopy, and the results were primarily discussed in terms of step stabilization or destabilization. Applying the DC electric field in the up-step direction appeared to stabilize the steps overall, although slight wandering was observed. Conversely, the steps were destabilized when the DC electric field was applied in the down-step direction, and the steps wandered significantly, with terraces protruding in the direction that the surface height decreases. The dependence of the step-terrace morphology on the direction of the DC electric field was consistent with previous studies that employed static electric fields (without a flash event). However, the step-terrace morphology may eventually become destabilized when the DC flash treatment time is extended, regardless of the applied field direction, considering the treatment duration of 10 min used in a previous study. This is inferred to be attributed to the amount of material diffusion relative to that of surface diffusion, which is increased by athermal effects during flash treatment.

DOI： 10.2109/jcersj2.25102

Surface mound formation and growth via step destabilization under a direct current electric field on (001) strontium titanate single crystals 査読有り

Katsuyama, Y; Inagaki, S; Tokunaga, T; Yamamoto, T

JOURNAL OF CRYSTAL GROWTH 667 巻 2025年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Crystal Growth

The specific heat-treatment conditions are crucial for facilitating surface-diffusion processes that lead to the reconstruction of surface structures in oxide single crystals. This reconstruction gives rise to a step-terrace structure, which is characterized by steps with heights equivalent to the unit cell and atomically flat terraces. A method using a power spike (i.e., a flash event) generated under an applied electric field above the threshold value has emerged as a promising approach for promoting such surface reconstruction. In this study, we performed Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) to examine the (001) surface structure of a strontium titanate single crystal treated with a flash event induced by a Direct Current (DC) electric field. We observed the formation of a step-terrace structure on the (001) surface even with a short holding time of 1 min under the flash event (initial voltage of 300 V/cm at a limiting current of 35 mA). However, a surface mound with a height of approximately 5.5 nm also formed on the surface, the height of which increased with holding time. SEM and AFM observations of the same surface location revealed that the formation of the surface mound is due to the formation of anti-bands as well as the bunching of surface steps, which is caused by their destabilization under the DC electric field. To avoid the formation of surface mounds, we suggest using an alternating electric field to trigger the flash event, which does not destabilize the steps.

DOI： 10.1016/j.jcrysgro.2025.128281

Electron-Beam-Induced Crystallization of Amorphous Gallium Oxide Thin Films Using Scanning Transmission Electron Microscopy 査読有り

Higuchi, K; Tokunaga, T; Sato, T; Yamamoto, T

CRYSTAL GROWTH & DESIGN 25 巻 ( 11 ) 頁： 3749 - 3755 2025年5月

記述言語：英語出版者・発行元：Crystal Growth and Design

Crystallization of amorphous materials using an electron beam is a promising technique for crystallizing minute regions. The crystallization behavior of amorphous gallium oxide (Ga<inf>2</inf>O<inf>3</inf>) thin films under electron-beam irradiation was investigated with a scanning transmission electron microscope, and patterned line drawing was performed using its line-scan mode. The Ga<inf>2</inf>O<inf>3</inf> thin films exhibited crystallization at threshold electron-beam doses in the range of 53–144 C/cm2. The γ- and β-Ga<inf>2</inf>O<inf>3</inf> forms crystallized as grains with an average size of 7 nm. The crystallization volume increased with doses. Crystallized Ga<inf>2</inf>O<inf>3</inf> polymorphs varied predominantly from γ- to β-Ga<inf>2</inf>O<inf>3</inf> with increased doses. The dose dependence of Ga<inf>2</inf>O<inf>3</inf> polymorphs was attributed to the variation in the coordination number of oxygen ions to gallium ions and the formation of oxygen vacancies caused by the electron-beam irradiation. The letter N predominantly composed of β-Ga<inf>2</inf>O<inf>3</inf> single crystals could be drawn with an approximately 10 nm line width in an amorphous Ga<inf>2</inf>O<inf>3</inf> thin film using the line-scan mode. Crystallized β-Ga<inf>2</inf>O<inf>3</inf> at the onset of the line-scan was inferred to grow epitaxially with subsequent scanning, realizing a letter N predominantly composed of single-crystal β-Ga<inf>2</inf>O<inf>3</inf>.

DOI： 10.1021/acs.cgd.5c00148

Healing behavior of microcracks on the surface of yttria-stabilized polycrystals using an ultrafast high-temperature sintering method 査読有り

Sato, T; Kodaira, A; Tokunaga, T; Morita, K; Yamamoto, T

CERAMICS INTERNATIONAL 51 巻 ( 4 ) 頁： 4178 - 4184 2025年2月

記述言語：英語出版者・発行元：Ceramics International

A technique to heal microcracks on ceramic surfaces in a relatively short time is an attractive proposition for extending the lifetime of ceramics. To this end, we used the ultrafast high-temperature sintering (UHS) method, which permits a rapid temperature increase, to heal Vickers indentation microcracks generated in a 4 mol% Y2O3-doped ZrO2 polycrystal. We examined the microstructure following microcrack healing using transmission electron microscopy and scanning electron microscopy with continuous focused ion beam slicing along the microcrack plane. This revealed that the microcrack healing process involves crack pinching and neck formation, followed by the growth of the neck. The microcracks were fully healed at the surface by UHS treatment at 2000 °C for 20 s. However, residual pores corresponding to the unhealed state were observed inside the material. We hypothesized that these pore residues were caused by significant grain coarsening during UHS. It is essential to optimize the UHS temperature and time to account for grain coarsening and facilitate the application of this method to microcrack healing.

DOI： 10.1016/j.ceramint.2024.11.394

Microstructural analysis of flash-healed Vickers-indented microcracks near positive/negative electrodes on the (001) surface of cubic zirconia single crystals under direct current electric fields 査読有り

Kayukawa, S; Katsuyama, Y; Kodaira, A; Tokunaga, T; Morita, K; Yamamoto, T

CERAMICS INTERNATIONAL 50 巻 ( 19 ) 頁： 37348 - 37355 2024年10月

記述言語：英語出版者・発行元：Ceramics International

Flash healing is a technique that uses power spikes to rapidly repair microcracks on ceramic surfaces. In this study, the microstructures of flash-healed Vickers-indented microcracks near the positive/negative electrodes under a DC electric field were examined by scanning transmission electron microscopy. The microstructural morphology near the positive electrode was almost identical to that of the microcracks located between both electrodes, as previously reported. However, a decrease in the degree of healing was observed near the negative electrode. This was attributed to the insufficient recovery of dislocations generated at the indentation state, which should have occurred in the initial stage of healing. The delay in dislocation recovery prevented contact between the inner surfaces of the microcracks and is possibly related to the increase in oxygen vacancies near the negative electrode.

DOI： 10.1016/j.ceramint.2024.04.251

Variation of surface morphology of material uplift near flash-healed Vickers indents under direct-current electric field in cubic zirconia single crystals 査読有り

Kayukawa, S; Katsuyama, Y; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 132 巻 ( 8 ) 頁： 528 - 531 2024年8月

記述言語：英語出版者・発行元：Journal of the Ceramic Society of Japan

Flash events can cause rapid mass diffusion through instantaneous power spikes that occur in electric fields and at temperatures above certain thresholds. Recent research suggests that such mass diffusion caused by the flash event may include athermal effects in addition to Joule heating due to the power spikes. However, this perspective has not been applied to surface diffusion, one of the mass diffusion pathways. This study examines the athermal effect on surface diffusion during the flash event by analyzing the surface morphology variation of material uplift formed after the flash-/thermal-healing of Vickers-indentation-induced microcracks. The results show that the formation of surface facet structures, one of the material uplift surface morphologies, varies depending on the direction of the direct current (DC) electric field. The direction dependence of the facet structure formed on the material up-lifted surface due to the DC electric field is a result of the electric field drift effect. The athermal effects associated with flash events can be considered to exist also in mass diffusion involving surface diffusion.

DOI： 10.2109/jcersj2.24036

Pulsed laser deposition and structural analysis of Ba2Ti6O13 epitaxial thin film on the (210) surface of SrTiO3 single crystal 査読有り

Kono, R; Hieda, K; Tokunaga, T; Tamura, R; Masamura, R; Hikita, Y; Kano, K; Yamamoto, T

APPLIED PHYSICS EXPRESS 17 巻 ( 7 ) 2024年7月

記述言語：英語出版者・発行元：Applied Physics Express

A single crystalline thin film of Ba2Ti6O13 was grown on the (210) surface of a SrTiO3 single-crystal substrate by pulsed laser deposition. Scanning transmission electron microscopy shows that the Ba2Ti6O13 thin film grew epitaxially with the a-plane, and the b-/c-axes of Ba2Ti6O13 parallel to the (210) plane and the [00 1 ¯ ]/[ 1 ¯ 20] directions of SrTiO3, respectively. The electrical conductance of the Ba2Ti6O13 film was approximately one order larger than that of the reduced SrTiO3−δ single crystal with oxygen deficiency of roughly δ = 0.05. Electronic structure calculations predict that Ba2Ti6O13 is an n-type degenerate semiconductor with spin-split states primarily of Ti 3d orbital.

DOI： 10.35848/1882-0786/ad605d

Phase textures of metal-oxide nanocomposites self-orchestrated by atomic diffusions through precursor alloys 査読有り

Shudin, NH; Eguchi, R; Fujita, T; Tokunaga, T; Hashimoto, A; Abe, H

PHYSICAL CHEMISTRY CHEMICAL PHYSICS 26 巻 ( 19 ) 頁： 14103 - 14107 2024年5月

記述言語：英語出版者・発行元：Physical Chemistry Chemical Physics

Metal-oxide nanocomposites (MONs) are of pivotal importance as electrode materials, yet lack a guiding principle to tune their phase texture. Here we report that the phase texture of MONs can be tuned at the nanoscale by controlling the nanophase separation of precursor alloys. In situ transmission electron microscopy (in situ TEM) has demonstrated that a MON material of platinum (Pt) and cerium oxide (CeO2) is obtained through promoted nanophase separation of a Pt5Ce precursor alloy in an atmosphere containing oxygen (O2) and carbon monoxide (CO). The Pt-CeO2 MON material comprised an alternating stack of nanometre-thick layers of Pt and CeO2 in different phase textures ranging from lamellae to mazes, depending on the O2 fraction in the atmosphere. Mathematical simulations have demonstrated that the phase texture of MONs originates from a balance in the atomic diffusions across the alloy precursor, which is controllable by the O2 fraction, temperature, and composition of the precursor alloys.

DOI： 10.1039/d3cp05157a

Non-uniformity of shrinkage rate as a function of current acceleration during flash event under voltage-to-current protocol in 8 mol %Y2O3-ZrO2 査読有り

Seko, M; Inagaki, S; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 132 巻 ( 3 ) 頁： 93 - 98 2024年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

This study verified the behavior of sample current, volumetric power dissipation, and shrinkage during flash sintering with the voltage-to-current protocol in the time frame of 90 s before and after the flash event. The volumetric power dissipation exhibited a peak value, followed by a decrease to the minimum value, and then increased asymptotically and approached a steady-state value. The discrepancy between the peak and minimum values, ¦P, increased with the limiting current value pre-set at the power supply during the heat-up stage. As ¦P increased, the degree of shrinkage progress decreased in stage III after the flash event, which is caused by the heterogeneity of shrinkage rate inside a compact. To consider the complete shrinkage amount, including stage III, in the current-to-voltage protocol, it is crucial to ensure that ¦P does not reach an excessive level when setting the maximum current.

DOI： 10.2109/jcersj2.23179

Variation of grain boundary dihedral angles caused by voltage-to-current-based flash treatment in fully stabilized yttria-doped zirconia polycrystals 査読有り

Seko, M; Kayukawa, S; Inagaki, S; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 132 巻 ( 2 ) 頁： 63 - 67 2024年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

The flash sintering method has garnered attention for its ability to significantly accelerate the densification rate of ceramic green compact through the generation of an electric power spike under an electric field exceeding a defined threshold. In order to amass foundational knowledge about the phenomenon that results from an electric field during flash sintering via a voltage-to-current protocol, the dihedral angles of grain boundaries were surveyed in flash-treated pre-sintered fully stabilized zirconia polycrystal. The flash treatment resulted in a reduction of the grain boundary dihedral angle for grain boundaries that are approximately perpendicular to the direction of the electric field. The reduction in the dihedral angle is possibly attributed to a reduction in surface energy which can be observed through the formation of sinuous pores during flash sintering.

DOI： 10.2109/jcersj2.23170

Effect of Initial Grain Size on Crack Healing Behavior under DC Electric Field of Zirconia (8Y-CSZ) Ceramic 査読有り

Takahashi, S; Morita, K; Nambu, K; Terada, D; Kobayashi, K; Tokunaga, T; Yamamoto, T

ADVANCED ENGINEERING MATERIALS 25 巻 ( 18 ) 2023年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Advanced Engineering Materials

The effect of initial grain size on the flash event was examined in microcrack healing behavior under DC electric field in fine- and coarse-grained 8 mol% Y2O3-stabilized cubic ZrO2 (8Y-CSZ) polycrystals. The healing behavior depends strongly on the initial grain size and is apparently accelerated in fine-grained 8Y-CSZ than in coarse-grained one. Since the crack healing under the flash event is higher than that of static annealing treatment without the electric field, the enhanced healing phenomena cannot be explained only by the thermal effect caused by Joule heating, but nonthermal effects caused by the flash event. The grain size-dependent healing behavior indicates that the grain boundaries play an important role in the flash event and the flash healing would be accelerated through the field/current-enhanced diffusional processes, especially through the grain boundary diffusivity of the cations.

DOI： 10.1002/adem.202201807

Layer transfer of ultrathin Ge crystal segregated on Al/Ge(111) structure 査読有り Open Access

Matsushita, K; Ohta, A; Shibayama, S; Tokunaga, T; Taoka, N; Makihara, K; Miyazaki, S

JAPANESE JOURNAL OF APPLIED PHYSICS 62 巻 ( SG ) 2023年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Japanese Journal of Applied Physics

Effects of the surface modification by O2 plasma exposure on the Al/Ge(111) structure have been investigated in order to get an insight into a layer transfer technique of the ultrathin Ge layer segregated on the Al/Ge(111) structure toward the device fabrication, and then the wafer bonding of the Al/Ge(111) structure to the thermally-grown SiO2/Si structure has been demonstrated. The O2 plasma treatment and the subsequent pure water rinse were found to be effective to form the hydrophilic surface of the Al/Ge(111) structure with a suppression of the segregated Ge layer oxidation. The Al/Ge(111) structure with the hydrophilic surface was then bonded to the SiO2/Si substrate, and its bonding strength was enough to perform Ge thinning by the chemical mechanical polishing and the wet-chemical etching using H2O2-based solutions. Ohmic contact of the ring-type device pattern with the segregated Ge/Al stack was achieved by using the remaining p-type Ge substrate as the contact pads.

DOI： 10.35848/1347-4065/acb65c

Scanning electron microscopy observation in wide field of view for SrO/TiO2-terminated step-terrace structure formed on SrTiO3(001) single crystal surface 査読有り Open Access

Katsuyama, Y; Kodaira, A; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 131 巻 ( 5 ) 頁： 135 - 139 2023年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

SrO-terminated terraces were formed along TiO2-terminated step edges by annealing (001) surfaces of SrTiO3 single crystal originally with step-terrace structure fully terminated at TiO2 atomic layer. The prepared step-terrace structure with the two different terminations were visualized in a wide field of view using scanning electron microscopy (SEM) under a low acceleration voltage of 1 kV and a short working distance of 2 mm. The different termination was clearly identified by an in-column secondary electron detector even in the wide-field region with dimensions of 30 © 30 ¯m2. As results, the uniformity of the step-terrace structure was found to be limited only at a localized area. The wide-field observation revealed that there exist many “step-defects” such as pinning of the step lines and wandering so large that it goes around to an opposite side. Step-defects were present in at least several for 30 © 30 ¯m2 sized observation areas with a probability of about 60 %.

DOI： 10.2109/jcersj2.23011

Local Crystallization of Amorphous Aluminum Oxide Thin Film Using Electron Beam Irradiation by SEM Open Access

Tanahashi, S; Okawara, S; Tokunaga, T; Yamamoto, T

MATERIALS TRANSACTIONS 64 巻 ( 1 ) 頁： 184 - 190 2023年

Shrinkage-rate controlled flash sintering for 3-10 mol %Y2O3-doped ZrO2 polycrystals 査読有り Open Access

Koike, M; Kodaira, A; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 130 巻 ( 12 ) 頁： 906 - 912 2022年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

Shrinkage-rate controlled flash (SCF) sintering using alternative current electric field was systematically performed for 310 mol% Y2O3-doped ZrO2. As results, the uniformly densified compacts with about 99% relative densities could be produced at furnace temperatures of about 754781 °C for about 30 min as SCF-regime irrespective of Y concentration, while the achieved densities were limited by conventional flash sintering. The shrinkage-rates during SCF-sintering were found to be enhanced in lower temperature range comparing those during thermal-sintering. The enhancement of shrinkage-rates in lower temperature range observed in SCFsintering is possibly related to the reduction of apparent activation energy of sintering under electric fields.

DOI： 10.2109/jcersj2.22114

Shell-like structure to limit further densification formed during flash sintering under alternative current electric field for 8 mol %Y2O3-doped ZrO2 査読有り Open Access

Seko, M; Hattori, K; Ishino, Y; Kodaira, A; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 130 巻 ( 11 ) 頁： 895 - 898 2022年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

A phenomenon often appears that the final achieved density is limited during conventional flash sintering even under alternative current electric field. By observing the cross-sectional microstructure of the flash sintered 8 mol %Y2O3-doped ZrO2 at this state, we found that a thin surface portion with near full density surrounds a porous inside with lower density. The densified thin surface portion is shell-like structure covering a whole of the sintered compact. The formation of this characteristic shell-like structure in the early stage of flash sintering greatly restricts subsequent densification of porous inside. In order to suppress the formation of the shell that inhibit densification, a technique of controlling a steep power spike at a flash event must be necessary to be appropriately transitioned.

DOI： 10.2109/jcersj2.22109

A first attempt of automated shrinkage-rate control flash sintering using a current profile without feedback of shrinkage behavior for 8 mol % Y2O3 -doped ZrO2 査読有り Open Access

Koike, M; Ishino, Y; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 130 巻 ( 4 ) 頁： 327 - 330 2022年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

Shrinkage-rate control flash (SCF) sintering is an attractive flash sintering that is modified to densify ceramic green compacts up to high density, which controls electric current to keep shrinkage-rate constant by feedback from shrinkage behavior during a flash state. To build up easier sintering protocol and to realize faster shrinkage-rate, we attempted automated SCF-sintering using the predicted current profile without feedback of the shrinkage behavior. As results, 8YSZ polycrystal with a density of 5.83 g/cm3 and a grain size of 1.84.5 μm could be produced within 10 min during SCF and soaking regimes at the furnace temperature of 870 °C, although the linearity of shrinkage-rate during automated SCF-sintering is degraded from the assumed shrinkage-rate. In order to improve automated SCF-sintering further, a more accurate method of approximating the current profile would be necessary to realize the higher linearity of shrinkage-rate.

DOI： 10.2109/jcersj2.22009

Crystalline boron monosulfide nanosheets with tunable bandgaps 査読有り Open Access

Kusaka, H; Ishibiki, R; Toyoda, M; Fujita, T; Tokunaga, T; Yamamoto, A; Miyakawa, M; Matsushita, K; Miyazaki, K; Li, LH; Shinde, SL; Lima, MSL; Sakurai, T; Nishibori, E; Masuda, T; Horiba, K; Watanabe, K; Saito, S; Miyauchi, M; Taniguchi, T; Hosono, H; Kondo, T

JOURNAL OF MATERIALS CHEMISTRY A 9 巻 ( 43 ) 頁： 24631 - 24640 2021年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Materials Chemistry A

Two-dimensional (2D) boron monosulfide (BS) nanosheets are predicted to have several stable phases and unique electronic structures, endowing them with interesting attributes, including superconducting, thermoelectric, and hydrogen storage properties. In this paper, we report the experimental realization of 2D BS nanosheets by the physical exfoliation of rhombohedral boron monosulfide (r-BS). Moreover, we demonstrate the facile separation of a mixture of 2D BS nanosheets and the r-BS powder in acetonitrile; the former were selectively separated as a dispersion in the supernatant, whereas the latter remained in the precipitate. In addition, density functional theory calculations reveal a clear dependence of the bandgap energy (Eg) on the number of layers of stacked BS nanosheets, whereEgfor BS nanosheets is approximately 1.0 eV higher than that for r-BS. Atomic force microscopy, cathode luminescence, ultraviolet-visible absorption spectroscopy, and excitation emission matrix experiments revealed a consistent bandgap difference of approximately 1.0 eV between the BS nanosheets and r-BS. We also demonstrate the applications based on the properties that originated from the difference in the bandgap between r-BS and BS nanosheets using photoelectrochemical current switching. These results indicate that the nanosheet bandgap can be tuned to a desired value by controlling the number of stacked 2D BS nanosheets. Therefore, BS nanosheets are promising non-metal 2D materials for applications requiring bandgap control, such as electronics and photocatalysis.

DOI： 10.1039/d1ta03307g

Shrinkage rate control during a flash state by current-ramping for 3 mol% Y2O3-doped ZrO2 polycrystals 査読有り

Ishino, Y; Taguchi, K; Koike, M; Tokunaga, T; Yamamoto, T

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 104 巻 ( 10 ) 頁： 4960 - 4967 2021年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the American Ceramic Society

3 mol% Y2O3-doped ZrO2 green compacts with rectangular shapes were sintered by maintaining the shrinkage rates at constant values under alternating electric fields by ramping the electric current during flash states. Green compacts were furnace-heated under 40-100 Vrms/cm until current hits an initial current limit of 100 mArms. After then, current-ramping was started to keep the shrinkage rates constant by increasing the limit current value using a programmable power supply operating in a current control mode. Highly densified 3 mol% Y2O3-doped ZrO2 polycrystals with a density of 6.05 g/cm3 as a bulk density and a grain size of about 0.4 μm were obtained at a furnace temperature of about 930℃, 50 Vrms/cm with 1000 Hz and shrinkage rate of about 120 μm/min (0.8%/min against initial lengths of green compacts). The Vickers hardness and indentation fracture toughness of the compact exhibit similar values to those obtained from thermally sintered compacts.

DOI： 10.1111/jace.17912

Rapid sintering of 3 mol % Y2O3-doped ZrO2 by a combined rapid furnace heating and shrinkage-controlled flash sintering protocol 査読有り Open Access

Ishino, Y; Taguchi, K; Kodaira, A; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 129 巻 ( 8 ) 頁： 551 - 554 2021年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

Flash sintering has attracted much attention as a useful sintering technique that is able to lower the sintering furnace temperature and shorten the sintering time. To further improve this original flash sintering technique, we have developed a technique to facilitate shrinkage at a constant shrinkage rate by controlling the power dissipation during a flash state, which is termed as shrinkage-controlled flash (SCF) sintering. Here, we combine this SCF sintering method with a rapid furnace heating process to shorten the entire sintering process time. Accordingly, we succeed in sintering 3 mol % Y2O3-doped ZrO2 (3YSZ) polycrystals with a high density of 6.05 g/cm3 and a uniform grain size of approximately 0.47 μm with a short sintering time of approximately 40 min. The obtained 3YSZ polycrystals are confirmed to exhibit a Vickers hardness and fracture toughness [by an indentation fracture method] of approximately 1295 HV and 5.3 MPa m1/2, respectively.

DOI： 10.2109/jcersj2.21066

Near complete densification of flash sintered 8YSZ: controlled shrinkage rate effects 査読有り Open Access

Taguchi, K; Ishino, Y; Tokunaga, T; Yamamoto, T

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 41 巻 ( 8 ) 頁： 4567 - 4571 2021年7月

記述言語：英語出版者・発行元：Journal of the European Ceramic Society

Rate-controlled sintering of 8 mol%Y2O3-doped ZrO2 (8YSZ) polycrystals was performed by rating the current limit of alternating current (AC) electric field during a flash event; this method promotes constant linear shrinkage of the green compact with respect to the sintering time. A power dissipation peak associated with the flash event was suppressed by setting the initial current limit at 100 mA; after AC current hits 100 mA, the current limit was rated to a maximum value of 1100−2000 mA to keep shrinkage rate constant. The power dissipation behavior during the current-rating process exhibited a characteristic inverted S-shaped curve as a function of time. 8YSZ polycrystal with a near-theoretical density of 5.97 g/cm3 and a grain size of approximately 3.4 μm was produced in AC electric field (150 V/cm at 1000 Hz) with a final maximum current limit of 1150 mA as a rectangular bulk style.

DOI： 10.1016/j.jeurceramsoc.2021.03.005

Constant shrinkage rate control during a flash event for 8 mol%Y2O3-doped ZrO2 polycrystals 査読有り Open Access

Taguchi, K; Ishino, Y; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 129 巻 ( 3 ) 頁： 204 - 207 2021年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

The shrinkage rate during a flash event is controlled at a constant value by current ramping for 8 mol %Y2O3ZrO2 green compact with a rectangular shape. The green compact is heated at a constant heating rate of 5 °C/min until the occurrence of a flash event under a constant alternative current (AC) field of 50 Vrms/cm and 1000 Hz at a limit current of 100 mArms. After the flash event, current ramping is started to keep the shrinkage rate at a constant value of 120 ¯m/min. Current ramping is conducted by controlling the limit current using a programmable power supply operating in a current control mode. When the limit current reaches 1200 mArms, the sintering condition is kept as it is for 5 min, and the sintering process is completed. 8 mol %Y2O3ZrO2 sintered polycrystal with a relative density of approximately 99 % and a grain size of about 2.5 ¯m can be obtained at a furnace temperature of 870 °C for a current ramping and soaking regime of approximately 30 min.

DOI： 10.2109/jcersj2.20192

Power Dissipation Behaviors during SCF-sintering for 8 mol% Y<inf>2</inf>O<inf>3</inf>-doped ZrO<inf>2</inf> Open Access

Ishino Y., Taguchi K., Kodaira A., Tokunaga T., Yamamoto T.

Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy 68 巻 ( 11 ) 頁： 482 - 486 2021年

出版者・発行元：Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy

Shrinkage behaviors during shrinkage rate-controlled flash (SCF) sintering were investigated for 8 mol% Y2O3–ZrO2 (8YSZ) when shrinkage rates were varied between 60 and 300 μm/min under alternating current fields of 50 Vrms/cm, 1200 mArms, and 1000 Hz at a furnace temperature of approximately 870°C. Volumetric power dissipation and electrical conductivity were shown to exhibit similar dependencies against linear shrinkages even if the shrinkage rates were changed. The 8YSZ polycrystals, with similar microstructure and density, could be produced under all SCF conditions used in this study, which was confirmed as a characteristic feature of SCF sintering. Finally, the 8YSZ polycrystals with a density of 5.91–5.93 g/cm3 and a grain size of 2.2–2.6 μm could be produced with a shrinkage process period of 15–65 min at a furnace temperature of approximately 870°C.

DOI： 10.2497/jjspm.68.482

Cross-sectional Area Dependency of Shrinkages and Grain Sizes of Flash-sintered 3 mol%Y<inf>2</inf>O<inf>3</inf>–ZrO<inf>2</inf> Polycrystals with a Circular Truncated Cone-shape at High Frequency Alternating Electric Current Fields Open Access

Kurachi T., Kobayashi K., Tokunaga T., Yamamoto T.

Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy 68 巻 ( 11 ) 頁： 487 - 493 2021年

出版者・発行元：Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy

The uniformity of shrinkages and the distribution of grain sizes were investigated for flash-sintered 3 mol%Y2O3-ZrO2 compacts in direct current and alternating current (AC) electric fields using rectangular-shaped and circular truncated cone-shaped green compacts. The use of a high-frequency AC electric field enough to suppress electrode overvoltage was confirmed to be advantageous for uniform shrinkages, regardless of the shapes of the green compacts. However, the grain size distribution became larger in cone-shaped green compacts, the cross-sectional areas of which varied along the compact. The grain sizes in flashed cone-shaped green compacts were found to exhibit a near linear relation to current density, which provides a means for determining the grain size distribution upon the flash sintering of green compacts with different cross-sectional areas along an electric field.

DOI： 10.2497/jjspm.68.487

EBV status has prognostic implication among young patients with angioimmunoblastic T-cell lymphoma Open Access

Eladl Ahmed E., Shimada Kazuyuki, Suzuki Yuka, Takahara Taishi, Kato Seiichi, Kohno Kei, Elsayed Ahmed Ali, Wu Chun-Chieh, Tokunaga Takashi, Kinoshita Tomohiro, Sakata-Yanagimoto Mamiko, Nakamura Shigeo, Satou Akira

CANCER MEDICINE 9 巻 ( 2 ) 頁： 678 - 688 2020年1月

記述言語：日本語出版者・発行元：Cancer Medicine

Epstein-Barr virus (EBV)-positive B cells have been detected in 66%-86% of patients with angioimmunoblastic T-cell lymphoma (AITL). However, it remains controversial whether EBV status has an impact on the survival of patients with AITL. In this study, we aimed to reevaluate the impact of EBV on the clinicopathological characteristics of AITL. In particular, we focused on the impact of EBV in younger patients with AITL. In total, 270 cases of AITL were studied. Epstein-Barr virus-positive B cells were detected in 191 (71%) cases (EBER+ group). Among the patients who received anthracycline-based therapy, the EBER status did not affect the overall survival (OS) or progression-free survival (PFS). In the younger group of AITL (≤60 years), PFS was significantly worse in the EBER− group compared to the EBER+ group (P =.0013). Furthermore, the multivariate analysis identified EBER-negative status, thrombocytopenia, and elevated serum IgA level as significant adverse prognostic factors for PFS (P <.001, P <.001, and P =.002). Based on these findings, we constructed new prognostic model for the younger group, based on three adverse factors. We classified the patients into two risk groups: low risk (no or 1 adverse factor) and high risk (2 or 3 adverse factors). This new model for younger patients with AITL showed that both OS and PFS were significantly related to the level of risk (P <.0001). In summary, this study showed that, among younger patients with AITL, an EBER+ status significantly improved prognosis compared to an EBER− status. Our new prognostic model should be applicable to younger patients with AITL.

DOI： 10.1002/cam4.2742

Characterisation of bovine embryos following prolonged culture in embryonic stem cell medium containing leukaemia inhibitory factor Open Access

Hosoe Misa, Furusawa Tadashi, Hayashi Ken-Go, Takahashi Toru, Hashiyada Yutaka, Kizaki Keiichiro, Hashizume Kazuyoshi, Tokunaga Tomoyuki, Matsuyama Shuichi, Sakumoto Ryosuke

REPRODUCTION FERTILITY AND DEVELOPMENT 31 巻 ( 6 ) 頁： 1157 - 1165 2019年5月

記述言語：日本語出版者・発行元：Reproduction, Fertility and Development

In order to help elucidate the process of epiblast and trophoblast cell differentiation in bovine embryos in vitro, we attempted to develop a suitable culture medium to allow extended embryo culture. Day 7 bovine blastocysts developed in conventional medium were cultured further in embryonic stem cell medium with or without leukaemia inhibitory factor (LIF) until Day 23. At Day 14, the expression of octamer-binding transcription factor 3/4 (OCT3/4) and VIMENTIN was significantly higher in embryos cultured with than without LIF, but embryonic disc formation was not observed. Although expression of SRY (sex determining region Y)-box 17 (SOX17) mRNA was significantly lower in Day 14 embryos cultured with and without LIF than in in vivo embryos, hypoblast cells formed just inside the trophoblast cells of the in vitro-cultured embryos. On Day 23, expression of placental lactogen (PL) and prolactin-related protein 1 (PRP1) was not affected by LIF in in vitro-cultured embryos, levels of both genes were significantly lower in the in vitro than in vivo embryos. Similar to in vivo embryos, binucleate cell clusters seen in Day 23 in vitro-cultured embryos were composed of PL-negative and -positive cells. These results suggest that our culture system partially reproduced the differentiation process of trophoblast cells in vivo.

DOI： 10.1071/RD18343

Removal of carbon contamination in ETEM by introducing Ar during electron beam irradiation Open Access

Tokunaga, T; Saito, K; Kuno, K; Higuchi, K; Yamamoto, Y; Yamamoto, T

JOURNAL OF MICROSCOPY 273 巻 ( 1 ) 頁： 46 - 52 2019年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Microscopy

Organic materials, including carbon, exist inside the transmission electron microscope (TEM) chamber and are adsorbed onto samples under observation during TEM. When these adsorbed organic materials are irradiated by an electron beam, the adsorbed gas is decomposed. Carbon atoms remain on the sample and bond with each other forming a material with an amorphous structure. Due to the carbon deposition on the observation area of the sample, it is contaminated and the TEM image quality is decreased. Ar was introduced into environmental TEM (ETEM) to purge organic material from the sample chamber to reduce contamination growth. After Ar gas was introduced, the contamination was gradually removed. The contamination removal rate was dependent on the Ar pressure. Moreover, it was clear that Ar was ionised by electron beam irradiation and the Ar ions were produced in the ETEM during electron beam irradiation. It is proposed that the Ar ions removed the carbon contamination. Lay Description: Organic materials, including carbon, exist inside the transmission electron microscope (TEM) chamber and are adsorbed onto samples under observation during TEM. When these adsorbed organic materials are irradiated by an electron beam, the adsorbed gas is decomposed. Carbon atoms remain on the sample and bond with each other forming a material with an amorphous structure. Due to the carbon deposition on the observation area of the sample, it is contaminated and the TEM image quality is decreased. Ar was introduced into environmental TEM (ETEM) to purge organic material from the sample chamber to reduce contamination growth. After Ar gas was introduced, the contamination was gradually removed. The contamination removal rate was dependent on the Ar pressure. Moreover, it was clear that Ar was ionised by electron beam irradiation and the Ar ions were produced in the ETEM during electron beam irradiation. It is proposed that the Ar ions removed the carbon contamination.

DOI： 10.1111/jmi.12759

Intentionally encapsulated metal alloys within vertically aligned multi-walled carbon nanotube array via chemical vapor deposition technique

Inoue H., Nishikawa T., Hayashi Y., Hada M., Tokunaga T., Amaratunga G., Amaratunga G.

Conference Program Digest - 7th International Conference on Manipulation, Manufacturing and Measurement on the Nanoscale, IEEE 3M-NANO 2017 2018-January 巻頁： 357 - 361 2018年2月

記述言語：日本語出版者・発行元：Conference Program Digest - 7th International Conference on Manipulation, Manufacturing and Measurement on the Nanoscale, IEEE 3M-NANO 2017

This paper presents a growth and characterization of vertically aligned PdxCoi-x alloy encapsulated inside Multi-Walled Carbon Nanotube (MWCNT) arrays on Pd/Co thin layers coated on Si substrate by a dc bias-enhanced plasma chemical vapor deposition (CVD) method. The Metal Alloy Encapsulated within MWCNTs (MAE-MWCNTs) were characterized by a scanning electron microscope (SEM) and a transmission electron microscopy (TEM). The SEM images show the teardrop-shape particles encapsulated within the tube top of MWCNTs. A vibrating sample magnetometer was used to study the magnetism of a large number of MAE-MWCNTs on Si substrate at room temperature. The hysteresis loop of the ME-MWCNTs shows clear ferromagnetic behavior and the easy axis of magnetization is parallel to the MEA-MWCNT tube axis, as can be elucidated from the large coercive fields and remanence values. Moreover, TEM off-axis electron holograms were used to study the magnetism of the individual MAE-MWCNT or the two pair of MAE-MWCNTs. Based on electron holography, we have successfully obtained the saturation magnetization of 0.7 T and 1.12 T for the individual MAE-MWCNT with diameters of 41 nm and 83 nm, respectively.

DOI： 10.1109/3M-NANO.2017.8286262

Dendritic WS2 Nanoribbon Networks Grown in Interfacial Confinement Space: Edge-Rich Architectures for Enhanced Hydrogen Evolution Open Access

Suzuki, H; Hirata, K; Takahashi, Y; Fujii, S; Misawa, M; Tokunaga, T; Nakaya, I; Senda, Y; Hayashi, Y; Takahashi, Y

SMALL STRUCTURES 7 巻 ( 1 ) 2026年1月

掲載種別：研究論文（学術雑誌）出版者・発行元：Small Structures

Fractally grown dendritic nanostructures exhibit exceptional potential across various applications, particularly in electrochemical catalysis, owing to their high surface area and abundant reactive sites. Such structures can be synthesized via diffusion-limited aggregation (DLA), which requires tightly controlled growth conditions, such as a low precursor supply rate. Two-dimensional materials, especially transition-metal dichalcogenides (TMDCs), provide an ideal platform for realizing atomically precise dendritic architectures. In this study, we present a strategy for synthesizing fractal dendritic networks of monolayer WS<inf>2</inf> nanoribbons by employing an interfacial nanoreactor to create a highly confined growth environment. The dendritic WS<inf>2</inf> nanoribbons were formed at the interface between the monolayer WS<inf>2</inf> and the growth substrate, facilitating a DLA-dominated growth regime. Atomic-scale structural analysis revealed atomically sharp edges of dendritic WS<inf>2</inf> nanoribbon networks. High-resolution scanning electrochemical cell microscopy demonstrated enhanced hydrogen evolution reaction activity, with the catalytic current localized at the dense edges. Furthermore, a combination of experimental analysis and theoretical modeling provided insights into the surface-diffusion-governed growth mechanism. These findings offer a new approach for designing edge-rich TMDC nanostructures and pave the way for their integration into high-performance catalytic and electrochemical devices.

DOI： 10.1002/sstr.202500542

Suppression of carbon contamination in tungsten oxide by oxygen-plasma-assisted electron beam-induced deposition

Kato, S; Tsuji, A; Ogasawara, H; Tokunaga, T; Yamamoto, T; Kobayashi, S

JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 43 巻 ( 6 ) 2025年12月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Vacuum Science and Technology B

Electron beam-induced deposition (EBID) is a direct-writing technique that utilizes a focused electron beam and a precursor gas to fabricate materials with arbitrary shapes. One of the challenges of EBID is the incorporation of residual carbon, primarily originating from organometallic precursors, which hampers the formation of high-purity deposits. In this study, we present a plasma-assisted EBID method in which externally generated oxygen plasma is introduced into the deposition chamber during growth to mitigate carbon contamination. Using tungsten oxide as a model system, thin films were deposited from tungsten hexacarbonyl in different plasma environments. The tungsten oxide fabricated under oxygen plasma showed a significant reduction in carbon content, as confirmed by scanning transmission electron microscopy and electron energy loss spectroscopy. Furthermore, the tungsten oxide deposited under the oxygen plasma condition was found to have an oxygen-rich composition, and an increase in conductivity was also observed. These findings highlight the critical role of the oxygen plasma in enhancing the chemical and electrical properties of oxide films fabricated by EBID.

DOI： 10.1116/6.0004857

Conversion of the (001) Strontium Titanate Surface to Titanium Monoxide via Ultrafast High-Temperature Sintering

Ihara, K; Sato, T; Tokunaga, T; Kobayashi, S; Kobayashi, K; Yamamoto, T

CRYSTAL GROWTH & DESIGN 25 巻 ( 20 ) 頁： 8393 - 8398 2025年10月

掲載種別：研究論文（学術雑誌）出版者・発行元：Crystal Growth and Design

The reconstruction of crystal surfaces (the alteration of the surface structure from that of the bulk) is imperative for the development of catalysts and energy-related materials. In this study, ultrafast high-temperature sintering (UHS) was employed to rapidly heat and reconstruct a model surface, namely the (001) surface of a SrTiO<inf>3</inf>single crystal. The structural changes were investigated by X-ray diffraction, scanning transmission electron microscopy, and electron energy-loss spectroscopy. Following treatment at 1650 °C for 180 s using UHS, a gold-colored transformation was observed on the surface; however, this was not the case after treatment at 1550 °C. Microscopic and spectroscopic analysis have revealed that the gold-colored surface layer possesses a thickness of approximately 100 nm, predominantly comprising rock salt-type cubic TiO. Furthermore, the electrical conductivity exhibited by the surface layer was very close to that reported for cubic TiO. The reconstruction mechanism of the (001) surface during UHS treatment was inferred to be similar to the incongruent sublimation of the SrTiO<inf>3</inf>surface that has previously been observed under vacuum at extremely low oxygen partial pressures. In conclusion, UHS has been demonstrated to be a highly efficacious technique for expeditious reconstruction of crystal surfaces, encompassing other metal oxides.

DOI： 10.1021/acs.cgd.5c00622

Fracture strength recovery in zirconia (8Y-CSZ) polycrystal through crack healing under strong DC electric field

Morita, K; Takahashi, S; Nambu, K; Terada, D; Tokunaga, T; Yamamoto, T

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 45 巻 ( 12 ) 頁： 117406 - 117406 2025年9月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the European Ceramic Society

The effect of the flash event on crack healing and the recovery of mechanical strength was examined under direct electric field/current (DC) in 8 mol% Y<inf>2</inf>O<inf>3</inf> stabilized cubic ZrO<inf>2</inf> (8Y-CSZ) polycrystal. Under the DC flash event, the micro-cracks with a length of 20 μm can be healed at furnace/sample temperatures for 10 min. As compared to the crack healing under heating treatment (HT) without the electric field/current, the crack healing was accelerated by more than 180 times under the DC flash even at the same specimen temperature of T<inf>s</inf> = 1223 °C. The rapid flash healing succeeded to recover the bending strength from ∼180 MPa with microcracks to ∼400 MPa, which is almost the same to that of the as-sintered state. This indicates that the flash treatment is an effective method not only to heal the micro-crack microstructure, but also to recover the mechanical strength through the crack healing.

DOI： 10.1016/j.jeurceramsoc.2025.117406

Microstructure analysis for zirconium oxynitrides formed in 8 mol% yttria-doped zirconia by a flash event under a direct current electric field in an ambient atmosphere 査読有り

45 巻 ( 7 ) 2025年7月

Ultrasonically Activated Liquid Metal Catalysts in Water for Enhanced Hydrogenation Efficiency

Manyuan, N; Tanimoto, N; Ueda, K; Yamamoto, K; Tokunaga, T; Nishio, M; Yonezawa, T; Kawasaki, H

ACS APPLIED MATERIALS & INTERFACES 17 巻 ( 4 ) 頁： 6414 - 6427 2025年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Applied Materials and Interfaces

Hydride (H-) species on oxides have been extensively studied over the past few decades because of their critical role in various catalytic processes. Their syntheses require high temperatures and the presence of hydrogen, which involves complex equipment, high energy costs, and strict safety protocols. Hydride species tend to decompose in the presence of atmospheric oxygen and water, which reduces their catalytic activities. These challenges highlight the need for further research to improve the stability and efficiency of catalytic processes and develop safer and cost-effective synthesis methods. This paper introduces an ultrasonic fabrication method for gallium hydride species on liquid metal (LM) nanoparticles (Ga-H@LM NPs) in water and describes the evaluation of their catalytic properties. The Ga-H@LM NPs were synthesized by dispersing liquid metals of eutectic gallium-indium in water using a two-step ultrasonication process in an ice bath. The presence of Ga-H species was confirmed by Fourier-transform infrared spectroscopy. The Ga-H@LM NPs demonstrated the rapid catalytic hydrogenation of 4-nitrophenol and reductive degradation of azo dyes within minutes without the need for external reducing agents like NaBH4. The proposed mechanism involves high-energy ultrasonic cavitation at the interface between LM NPs and water, which promotes the formation of H2 from water and its activation to form Ga-H on particles surface during ultrasonication. This study has significant implications for advancing the field of catalysis because it provides a novel and efficient catalytic method for the synthesis of stable hydride species on gallium oxides.

DOI： 10.1021/acsami.4c19936

Variation of (001) surface structures of strontium titanate single crystals caused by flash event under direct/alternative current electric fields

Katsuyama, Y; Kayukawa, S; Tokunaga, T; Kobayashi, S; Hidaka, S; Nakamura, A; Yamamoto, T

CERAMICS INTERNATIONAL 50 巻 ( 19 ) 頁： 37366 - 37372 2024年10月

掲載種別：研究論文（学術雑誌）出版者・発行元：Ceramics International

Flash events, which are electric power spikes occurring under an electric field and temperature above the threshold, can significantly accelerate mass diffusion over short periods of time. This study investigated the surface structure of the (001) surface of strontium titanate single-crystal treated by flash events caused by direct current (DC) and alternating current (AC) electric fields from a viewpoint of surface-related mass diffusion. The surface structuring caused by flash events was significantly different among DC and AC electric fields. The DC-flash event destabilized surface steps, causing them to wander and form surface mounds with a height of several tens of nanometers. In addition, numerous two-dimensional (2D) nuclei with a size of several nanometers are formed on the terraces. In contrast, the AC-flash event stabilized surface steps without the formation of surface mounds, causing them nearly straight with uniform terrace width at approximately 500 nm. 2D nuclei with a size of a few 10 nm were formed at the step edges with round shapes. Both methods were confirmed as evidenced by the formation of two-dimensional nuclei on the surface after the flash events, to enhance surface-related mass diffusion.

DOI： 10.1016/j.ceramint.2024.05.055

Topographical and morphological changes of the single-crystal (La0.3Sr0.7)(Al0.65Ta0.35)O3 (001) surface treated with high-purity deionized water

Tokuda, Y; Nishikawa, N; Irimoto, T; Katsuyama, Y; Kobayashi, S; Tokunaga, T; Yamamoto, T

SURFACE AND INTERFACE ANALYSIS 56 巻 ( 8 ) 頁： 564 - 572 2024年8月

掲載種別：研究論文（学術雑誌）出版者・発行元：Surface and Interface Analysis

The topological and morphological changes of the (001) surface of single-crystal (La0.3Sr0.7)(Al0.65Ta0.35)O3 during treatment with high-purity deionized water (DI) have been investigated by atomic force microscopy and scanning transmission electron microscopy (STEM). Etching by DI treatment at room temperature proceeded mainly in anisotropic etching mode, resulting in rectangular etch pits with {001} inner surfaces forming in the surface. The dominant etching mode changed from anisotropic to isotropic with increasing DI-treatment temperature, and the lowest surface roughness occurred for DI treatment at 50°C for 60 min. Direct observation of the atomic structure of the surface by STEM showed that the etched surface was the B-site-terminated (001) surface after anisotropic etching at 50°C for 60 min, in which the B-site-terminated atomic layer was found to maintain the B-site ordered structure existing in the crystal. This is because DI treatment, which does not require high-temperature heat treatment of the substrate, suppresses element diffusion, sublimation, and formation of point defects on and in the vicinity of the surface.

DOI： 10.1002/sia.7313

Optical and mechanical properties of rutile TiO2 subjected to AC flash processing

Masuda, H; Tokunaga, T; Yamamoto, T; Yoshida, H

SCRIPTA MATERIALIA 246 巻頁： 116088 - 116088 2024年6月

掲載種別：研究論文（学術雑誌）出版者・発行元：Scripta Materialia

Unusual physical properties associated with lattice defects induced by flash sintering and processing of oxide ceramics have attracted attentions. In this study, optical and mechanical properties of undoped and polycrystalline rutile TiO2 subjected to alternating current (AC) flash processing were characterized through diffuse reflectance spectroscopy and nanoindentation experiments at room temperature. The flashed sample exhibited substantial optical blackening and elastic softening but negligible changes in the hardness and crystal structure. The elastic softening demonstrated a significant rate dependence (i.e., elastic moduli decreased with testing rate), which was possibly caused by stress-induced reordering of point defects induced by AC flash processing. This elastic softening suggests that pronounced ionic mobility is obtained by flash processing even in undoped systems, which can potentially be extended to a general understanding of mass transport phenomena in transient metal oxides in strong electric fields.

DOI： 10.1016/j.scriptamat.2024.116088

Photoinduced dynamics during electronic transfer from narrow to wide bandgap layers in one-dimensional heterostructured materials 査読有り

Saida, Y; Gauthier, T; Suzuki, H; Ohmura, S; Shikata, R; Iwasaki, Y; Noyama, G; Kishibuchi, M; Tanaka, Y; Yajima, W; Godin, N; Privault, G; Tokunaga, T; Ono, S; Koshihara, SY; Tsuruta, K; Hayashi, Y; Bertoni, R; Hada, M

NATURE COMMUNICATIONS 15 巻 ( 1 ) 頁： 4600 2024年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Nature Communications

Electron transfer is a fundamental energy conversion process widely present in synthetic, industrial, and natural systems. Understanding the electron transfer process is important to exploit the uniqueness of the low-dimensional van der Waals (vdW) heterostructures because interlayer electron transfer produces the function of this class of material. Here, we show the occurrence of an electron transfer process in one-dimensional layer-stacking of carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs). This observation makes use of femtosecond broadband optical spectroscopy, ultrafast time-resolved electron diffraction, and first-principles theoretical calculations. These results reveal that near-ultraviolet photoexcitation induces an electron transfer from the conduction bands of CNT to BNNT layers via electronic decay channels. This physical process subsequently generates radial phonons in the one-dimensional vdW heterostructure material. The gathered insights unveil the fundamentals physics of interfacial interactions in low dimensional vdW heterostructures and their photoinduced dynamics, pushing their limits for photoactive multifunctional applications.

DOI： 10.1038/s41467-024-48880-3

The use of He buffer gas for moderating the plume kinetic energy during Nd:YAG-PLD growth of EuxY2-xO3 phosphor films

Suzuki, S; Dazai, T; Tokunaga, T; Yamamoto, T; Katoh, R; Lippmaa, M; Takahashi, R

JOURNAL OF APPLIED PHYSICS 135 巻 ( 19 ) 2024年5月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Applied Physics

We have investigated the He buffer gas process of moderating the kinetic energy of the pulsed laser deposition (PLD) plume during EuxY2−xO3 phosphor film growth. When using a neodymium yttrium aluminum garnet laser for PLD thin film growth, the kinetic energy of the ablation plumes can be high enough to cause the formation of point defects in the film. The buffer gas pressure is an important process parameter in PLD film growth. We find that the presence of the He buffer gas reduces the kinetic energy of the laser deposition plume through many low-angle collisions in the gas phase by a factor of 7 without reducing the deposition rate. This is because He is much lighter than any of the elements in the plume and it does not affect the composition of the oxide films. Consequently, the resputtering of the Y2O3 film surface by the plume was significantly suppressed in the presence of the He gas moderator, leading to a decrease of the defect density in the Y2O3 films. The improvement of the film quality was verified by a systematic analysis of time-resolved photoluminescence (PL) data for EuxY2−xO3 composition-gradient films. The PL lifetime and intensity of Eu0.2Y1.8O3, which shows the highest PL intensity, increased by 13.3% and 36.4%, respectively, when the He gas moderation process was used. The He buffer gas process is applicable to the PLD growth of the other oxide materials as well, where the reduction of the kinetic energy of the plume would bring the PLD process closer to the molecular beam epitaxy growth condition.

DOI： 10.1063/5.0196987

Applying an alternating current to strontium titanate minimizes the change in oxidation state caused by flash sintering

Seko, M; Inagaki, S; Tokunaga, T; Kobayashi, S; Nakamura, A; Yamamoto, T

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 44 巻 ( 5 ) 頁： 2965 - 2971 2024年5月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the European Ceramic Society

This study estimated the amount of oxygen deficiency in SrTiO3 polycrystals that were flash sintered under direct current (DC) and alternating current (AC) electric fields, mainly by using the fine structural change of the Ti L23-edges collected by electron energy-loss spectroscopy. The DC electric field corresponded to formation of the maximum amount of oxygen deficiency at approximately δ = 0.07 as SrTiO3−δ, which corresponds to a lattice expansion of roughly 0.1% as a lattice constant of SrTiO3. Furthermore, the oxygen deficiency was distributed unevenly. The formation of dislocation networks in DC-flash-sintered SrTiO3 polycrystals might be related to the lattice strain field caused by this nonuniform oxygen deficiency. In contrast, an AC electric field did not correspond to the oxygen deficiency of SrTiO3, which is similar to the results in thermally sintered SrTiO3 polycrystals.

DOI： 10.1016/j.jeurceramsoc.2023.12.046

Flash healing of Vickers microcracks formed on the (001) surface of cubic zirconia single crystals

Kayukawa, S; Katsuyama, Y; Kodaira, A; Tokunaga, T; Morita, K; Nakamura, A; Yamamoto, T

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 43 巻 ( 14 ) 頁： 6272 - 6278 2023年11月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the European Ceramic Society

Flash healing, a technique for microcrack healing using a flash event, has recently attracted attention for effective microcrack healing of ceramics. To investigate the diffusion-accelerated phenomena that occur during flash healing, the flash-healing behavior of Vickers indentation microcracks on (001) zirconia single crystals was investigated, mainly from the viewpoint of microstructure analysis of the healed microcracks. Scanning electron microscopy and atomic force microscopy showed that the surface microcracks were rapidly healed. However, scanning transmission electron microscopy showed that a row of pores remained along the original microcrack inside the crystal, and the crystallographic orientation relationship on both sides of the microcrack was restored to that of the original single crystal. Compared with the findings for healing without an electric field, the distance from the pore closest to the surface was larger and the total pore volume was smaller for flash healing. These results suggest that flash treatment increases lattice diffusion.

DOI： 10.1016/j.jeurceramsoc.2023.06.001

Near-full compaction within only 30 min of total sintering time by shrinkage-rate-controlled flash sintering in 3 mol % Y2O3-doped ZrO2 polycrystal Open Access

Ishino, Y; Koike, M; Kodaira, A; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 131 巻 ( 10 ) 頁： 600 - 607 2023年10月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

The development of a high-speed sintering method that can shorten the entire sintering time while guaranteeing the mechanical properties is very attractive as a sintering process for structural ceramic polycrystals. For this purpose, a sintering protocol that combines shrinkage-rate-controlled flash sintering, which controls the shrinkage rate to be constant with the use of an alternative current electric field, and a rapid furnace ramp were applied to 3-mol % Y2O3-doped ZrO2 (3YSZ). As a result, a 3YSZ sintered polycrystal with about the relative density of 99.5 % (6.06 g/cm3) could be successfully fabricated within 30 min as the entire sintering time, which includes the furnace-heating regime. The sintered 3YSZ polycrystal is confirmed to exhibit a Vickers hardness of approximately 1290 HV and a fracture toughness (KIC) of approximately 5.2 MPa m1/2, which are similar values to those obtained from a typical thermally sintered 3YSZ polycrystal.

DOI： 10.2109/jcersj2.23041

Structural Defect Effect on the Concentration Quenching of TbXY2-XO3 Phosphor Thin Films

Suzuki, S; Dazai, T; Tokunaga, T; Yamamoto, T; Katoh, R; Takahashi, R

JOURNAL OF PHYSICAL CHEMISTRY C 127 巻 ( 19 ) 頁： 9380 - 9387 2023年5月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Physical Chemistry C

This study investigates the structural defect effect on the luminescence properties of TbxY2-xO3 (x = 0-0.5) composition-gradient films using combinatorial pulsed laser deposition (PLD). TbxY2-xO3 films were grown on a SrTiO3 (001) substrate in an O2/He mixed atmosphere at PO2 = 1 × 10-4 Torr. A He buffer gas moderated the kinetic energy of the PLD plume and suppressed the resputtering of the film surface, resulting in the fabrication of defect-rich and poor TbxY2-xO3 films. The TbxY2-xO3 composition-gradient films exhibited varying green emissions from the Tb-doped Y2O3 films. The luminescence intensity of the defect-rich film was significantly weaker than that of the defect-poor film. Furthermore, concentration quenching of the luminescence occurred at high Tb concentrations. The analysis of the decay profiles of the luminescence as a function of the Tb concentration suggested two possible reasons for luminescence quenching: dynamic quenching induced by structural defects and static quenching induced by the shorter average distance of adjacent Tb ions. The investigation of the structural defect effect on the luminescence properties provides insights into the primary material parameters of phosphor materials and offers a pathway for enhancing their luminescence properties.

DOI： 10.1021/acs.jpcc.3c01679

Mechanistic consideration of ZnTe microspheres formation in a PVP-contained polyol system via hot injection method

Chau, YTR; Nguyen, MT; Tokunaga, T; Yonezawa, T

ADVANCED POWDER TECHNOLOGY 34 巻 ( 4 ) 頁： 103970 - 103970 2023年4月

掲載種別：研究論文（学術雑誌）出版者・発行元：Advanced Powder Technology

Zinc telluride (ZnTe) microspheres have been synthesized in a polyol system with adding various amount of poly(N-vinyl-2-pyrrolidone) (PVP, K-90) at 200 °C via a hot injection method. The smallest sized ZnTe microspheres obtained in this study had an average size of 634 nm through reaction for 48 h using 518 mg of PVP. A retrospective study was conducted to evaluate the formation process of ZnTe microspheres by conducting experiments from 0.2 h to 48 h. Nucleation and growth of ZnTe at the tips of Te rods in a PVP-contained polyol system were observed in sample reacted less than 1 h. More ZnTe microspheres and less Te rods were observed when the reaction was proceeded for longer time, suggesting that Te rods were the sacrificial template for the growth of ZnTe. Amount of PVP is the key factor of controlling dimensions of Te rods formed at the initial stage as well as the sizes of ZnTe microspheres by influencing the nucleation and growth rate of ZnTe. The obtained ZnTe microspheres at 48 h consisted of voids, which were originated by the detachment of ZnTe microspheres from Te rods after their growth.

DOI： 10.1016/j.apt.2023.103970

Microcrack healing in single-crystal cubic zirconia by thermal annealing 査読有り

Kayukawa, S; Katsuyama, Y; Kodaira, A; Tokunaga, T; Morita, K; Nakamura, A; Higuchi, K; Yamamoto, T

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 43 巻 ( 3 ) 頁： 1078 - 1086 2023年3月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the European Ceramic Society

Thermally healed Vickers microcracks perpendicular to (001) surface of cubic zirconia single crystals were investigated mainly with scanning transmission electron microscopy. The orientational relationship of areas containing healed microcracks was completely restored to that of the original single crystal, but a row of pores corresponding to the unhealed state remained where the original microcrack was located. Isolated pore formation proceeded from dislocation recovery via heat treatment, which brought the inner surfaces of the microcrack into contact, followed by diffusion bonding via surface diffusion. Once isolated pores were formed, the total pore volume did not significantly change by increasing the healing temperature, although pore sizes increased and the number of pores decreased. Ostwald ripening became pronounced in pore microstructure evolution when pore elimination was dominated by lattice diffusion after complete crystallographic restoration to a single crystal.

DOI： 10.1016/j.jeurceramsoc.2022.10.065

Flash self-joining of Y-TZP ceramics assisted with an AC electric field 査読有り

Nambu, K; Kitaoka, T; Morita, K; Soga, K; Tokunaga, T; Yamamoto, T; Masuda, H; Yoshida, H

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 106 巻 ( 3 ) 頁： 2073 - 2082 2023年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the American Ceramic Society

In this study, flash joining experiments were conducted using an AC field on 3 mol% Y2O3‐stabilized tetragonal ZrO2 polycrystal (Y‐TZP) bodies. Furthermore, the necessary conditions to obtain an almost complete self‐joining of Y‐TZP bodies were clarified. The specimens were successfully joined by applying an AC field at 60 mA·mm−2 for 80 s at a furnace temperature of 1000°C, thus resulting in a successfully joined specimen with 92% of the flexural strength of the as‐sintered Y‐TZP body. Almost complete self‐joining of Y‐TZP was achieved at current densities above 30 mA·mm−2, and input energy densities of >24 J·mm−3. Both the input energy density and electric current were critical factors for producing the reliable joining of ceramics.

DOI： 10.1111/jace.18889

A technique to modify the photoluminescence intensity of β-Ga2O3 polycrystals using an electric field during sintering 査読有り

Ono, R; Kodaira, A; Tokunaga, T; Yoshida, H; Yamamoto, T

JOURNAL OF LUMINESCENCE 254 巻頁： 119508 - 119508 2023年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Luminescence

Beta gallium trioxide (β-Ga2O3) is an attractive wide bandgap oxide that exhibits photoluminescent emission in the ultraviolet and blue/green regions. In the present study, we describe the tuning and enhancement of photoluminescent emission in the blue/green region by applying an instantaneous electric field during the sintering of a β-Ga2O3 green compact. Blue/green emission at around 480 nm increases by a factor of approximately 16 in β-Ga2O3 polycrystal prepared with the sintering protocol that a green compact was heated up to 1300 °C and an alternating current electric field (50 V/cm at 1000 Hz) was isothermally loaded. Considering that the enhancement occurs at the cathode side under a direct current electric field, the enhanced blue/green photoluminescence is likely due to an increase in the number of oxygen vacancies.

DOI： 10.1016/j.jlumin.2022.119508

Local Crystallization of Amorphous Aluminum Oxide Thin Film Using Electron Beam Irradiation by SEM Open Access

Tanahashi S., Okawara S., Tokunaga T., Yamamoto T.

Materials Transactions 64 巻 ( 1 ) 頁： 184 - 190 2023年1月

掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Transactions

Amorphous thin film thinned for transmission electron microscopy observation is reported to crystallize by electron beam (EB) irradiation of TEM. If thin film deposited on substrates can be crystallized by EB irradiation of scanning electron microscopy (SEM), new nanoscale devices can be fabricated such as electronic nanoscale circuits in amorphous thin films. Therefore, an amorphous AlOx (am-AlOx) thin film is deposited onto a Si substrate by atomic layer deposition (ALD) and point and area scan irradiation of EBs were performed by SEM. Then, structure and elemental analyses are carried out by TEM. Only the point EB irradiated area crystallized from the surface to the interface between am-AlOx and the Si substrate, and £-Al2O3 appeared. Moreover, the oxygen content of the not irradiated area close to the irradiated area was lower than that of the not irradiated area far from the irradiated area. These results show that the EB irradiated area was positively charged by the emission of secondary and Auger electrons under point irradiation conditions, and that oxygen was supplied by attracting its negative charge to the EB irradiated area. Also, the electrostatic repulsion between positively charged atoms and the structure relaxation due to electron excitation were additional driving forces for the crystallization.

DOI： 10.2320/matertrans.mt-n2022004

Apparent Sintering Activation Energy Estimated by Master Sintering Curves for 8 mol% Y<inf>2</inf>O<inf>3</inf> Doped ZrO<inf>2</inf> Polycrystals during Shrinkage-Rate Controlled Flash Sintering Open Access

Koike M., Kodaira A., Tokunaga T., Yamamoto T.

Materials Transactions 64 巻 ( 10 ) 頁： 2315 - 2320 2023年

掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Transactions

The apparent activation energy of sintering for 8-mol%-Y2O3-doped ZrO2 during shrinkage-rate-controlled flash sintering was determined using a master sintering curve. The activation energy during SCF sintering was found to decrease from approximately 700 to 430 kJ/mol with density. The activation energy in a high-density region is approximately similar value of lattice diffusion, 460 kJ/mol. The relative density range over which the decrease in activation energy occurred during SCF sintering was smaller than during thermal sintering and further moved to a lower temperature range. The increase in shrinkage rate at lower temperatures that occurs during SCF sintering could be considered to be related to this decrease in activation energy.

DOI： 10.2320/matertrans.MT-T2023002

Apparent Sintering Activation Energy Estimated by Master Sintering Curves for 8mol% Y2O3 Doped ZrO2 Polycrystals during Shrinkage-Rate Controlled Flash Sintering Open Access

Koike, M; Kodaira, A; Tokunaga, T; Yamamoto, T

MATERIALS TRANSACTIONS 64 巻 ( 9 ) 頁： 2315 - 2320 2023年

Variation of photoluminescence intensity depending on the timing of electric field application during isothermal flash sintering for 3mol% Y2O3-ZrO2 polycrystal 査読有り

Itoh, A; Tokunaga, T; Kodaira, A; Yoshida, H; Yamamoto, T

CERAMICS INTERNATIONAL 48 巻 ( 19 ) 頁： 28712 - 28717 2022年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Ceramics International

The shrinkage process and the change in photoluminescence (PL) intensity during isothermal flash sintering were investigated for 3 mol% Y2O3–ZrO2 (3YSZ) polycrystals doped with 300 ppm Bi2O3 as a PL impurity centre. The PL intensity of 3YSZ is enhanced by isothermal flash sintering. However, the enhancement of the PL intensity was revealed to become smaller when flash-treatment was applied for thermally pre-sintered 3YSZ polycrystal. To maximize the efficiency of electric field on the enhancement of the PL intensity, the electric field needs to be applied throughout the sintering from the initial stage to the final densification. The necessity for continuing to apply the electric field until the densification is completed is due to the fact that the recovery of oxygen vacancies, the origin of the F+ band, must be suppressed.

DOI： 10.1016/j.ceramint.2022.06.185

Effect of He Buffer Gas on Pulsed Nd:YAG Laser Deposition of EuxY2-xO3 Phosphor Thin Films 査読有り

Suzuki, S; Dazai, T; Kawashima, K; Tokunaga, T; Yamamoto, T; Takahashi, R

ACS APPLIED ELECTRONIC MATERIALS 4 巻 ( 9 ) 頁： 4419 - 4426 2022年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Applied Electronic Materials

We investigated the feasibility of using He buffer gas to moderate the kinetic energy of pulsed laser deposition (PLD) plumes during the growth of EuxY2-xO3phosphor films. The PLD growth of Y2O3films using a Nd yttrium aluminum garnet (Nd:YAG) laser results in expanded lattice parameters compared to those of the bulk crystals, indicating the presence of crystal defects in the Y2O3films. Ablation plumes with a high kinetic energy are generated by the Nd:YAG laser that operates at high energy densities, leading to defect formation. To moderate the kinetic energy without reducing the film growth rate, He buffer gas was used for the Nd:YAG-PLD growth of Y2O3films, resulting in the suppression of crystal defect generation. This process was demonstrated to improve the phosphor properties of EuxY2-xO3thin films grown via combinatorial PLD as the He buffer gas-induced reduction in crystal defect density enhances the phosphor properties of the EuxY2-xO3films. Using He buffer gas to moderate the kinetic energy of PLD plumes is expected to be useful in reducing the crystal defect density and improving the film characteristics.

DOI： 10.1021/acsaelm.2c00691

A liquid metal catalyst for the conversion of ethanol into graphitic carbon layers under an ultrasonic cavitation field Open Access

Kawasaki, H; Otsuki, T; Sugino, F; Yamamoto, K; Tokunaga, T; Tokura, R; Yonezawa, T

CHEMICAL COMMUNICATIONS 58 巻 ( 56 ) 頁： 7741 - 7744 2022年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Chemical Communications

Eutectic gallium indium (EGaIn) has drawn considerable research interest in potential liquid catalysis. Herein, we report that EGaIn liquid metal acts as a catalyst for the growth of a graphitic carbon layer from ethanol under ultrasonication. High-speed imaging demonstrated the formation of ultrasonic cavitation bubbles at the liquid metal/ethanol interface, which facilitated the pyrolysis of ethanol into graphitic carbon on the liquid metal surface.

DOI： 10.1039/d2cc02510h

Effect of electron beam irradiation in gas atmosphere during ETEM 査読有り Open Access

Tokunaga, T; Kawakami, T; Higuchi, K; Yamamoto, Y; Yamamoto, T

MICRON 158 巻頁： 103289 - 103289 2022年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Micron

Transmission electron microscopy (TEM) is used to observe the atomic structures of materials. Environmental TEM (ETEM) is a method wherein a gas can be evaluated and it has been used to observe the dynamic reaction between materials and gases at the atomic level. An electron beam (EB), which has a sufficiently high energy (exceeding a few tens of kilovolts), can be used to ionize gas molecules. Subsequently, the ionized molecules might react with the materials during ETEM. Therefore, the current generated by the ions and electrons were measured to verify the presence of ions generated due to the ionization of the N2 gas atmosphere during EB irradiation in ETEM. The electron energy loss spectra (EELS) were acquired from the N2 gas atmosphere to estimate the types of ions generated. The results demonstrated that ions and electrons were generated in the N2 atmosphere during ETEM and EB irradiation. Moreover, the EELS analysis indicated that the generated ion was N2+. The material observation conducted using gas ETEM can detect the reaction between gases, ions, and materials.

DOI： 10.1016/j.micron.2022.103289

The role of surface oxides and stabilising carboxylic acids of copper nanoparticles during low-temperature sintering Open Access

Tokura, R; Tsukamoto, H; Tokunaga, T; Nguyen, MT; Yonezawa, T

MATERIALS ADVANCES 3 巻 ( 12 ) 頁： 4802 - 4812 2022年6月

掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Advances

This article provides a detailed discussion of 1-hexanoic-acid-stabilised copper nanoparticles with an average diameter of ∼80 nm prepared from cupric oxide micro-powders for low-temperature sintering applications. The obtained fine particles were dispersed in dipropylene glycol using a bead mill to obtain a stable paste containing 50 wt% copper. Sintering experiments at temperatures in the range of 120-250 °C were performed under a nitrogen or 3%H2-N2 gas flow. The lowest resistivity, approximately three times that of bulk copper, was obtained at 250 °C. These particles exhibited good conductivity upon sintering under nitrogen only. 1-Hexanoic acid contributed to the acceleration of sintering by removing the Cu64O oxide layer of the particles and activating the surface. The dispersed copper paste and the copper layer after sintering were observed using SEM.

DOI： 10.1039/d1ma01242h

Excess oxygen-vacancy formed by FAST regime of direct-current electric field during flash sintering for 3 mol%-10 mol% Y2O3-doped ZrO2 査読有り Open Access

Morisaki, N; Tokunaga, T; Kobayashi, K; Kodaira, A; Yamamoto, T

CERAMICS INTERNATIONAL 48 巻 ( 9 ) 頁： 12091 - 12097 2022年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Ceramics International

The amount of excess oxygen vacancies that are generated during FAST regime of direct-current flash sintering was estimated semi-quantitatively for 3 mol%–10 mol% Y2O3-doped ZrO2 (3–10YSZ) using the temperature dependence of the electrical conductivity. For the estimation, the electrical conductivity in the temperature range, where shrinkages are below a few percent, was used to avoid the effect of large microstructural changes on the electric conductivity. For all Y2O3 contents except 3YSZ, the amount of excess oxygen vacancies increased above a compact temperature of approximately 840 °C, which depended on Y2O3 content, and was highest in 8YSZ. Approximately 0.13% excess oxygen vacancy was generated by direct-current flashing in 8YSZ in the present electric field condition. In contrast, the increase in the excess oxygen vacancies is negligible in 3YSZ. The dependence of the amount of excess oxygen vacancies on the Y2O3 content was related possibly to the dependence of the electrode overvoltage on the Y2O3 content.

DOI： 10.1016/j.ceramint.2022.01.069

Anelasticityinduced by AC flash processing of cubic zirconia 査読有り Open Access

Masuda, H; Morita, K; Tokunaga, T; Yamamoto, T; Yoshida, H

ACTA MATERIALIA 227 巻頁： 117704 - 117704 2022年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Acta Materialia

The defect physics is important for understanding the athermal effect of electric fields and/or currents on the enhanced ionic diffusion during flash sintering of ceramics. In this study, conventionally sintered 8 mol% yttria-stabilized zirconia with a fluorite cubic structure was processed by AC flash and subsequently quenched to partially freeze the flash-induced defect structures. The dynamic and quasi-static mechanical properties of flashed and non-flashed samples were evaluated by a sound velocity measurement and a nanoindentation technique to characterize the mechanical responses attributed to the flash-induced defects. A negligible change was confirmed in the dynamic elastic constants even after flash, while the quasi-static elastic properties of the flashed sample exhibited a considerable rate dependence (i.e., the slower the loading rate, the lower the measured contact modulus). The contact modulus of the flashed sample tested at the slowest loading rate in this study was reduced to approximately 72% of the non-flashed one, where the contact modulus was rate-independent. The time-dependent mechanical behavior was also confirmed during nanoindentation creep. Such rate- and time-dependent mechanical properties are characterized as anelasticity (i.e., viscoelasticity or pseudoelasticity in other terms), which can be attributed to stress-induced and thermally activated recoverable motions of point defects. The flash process of cubic zirconia possibly induced point defects associated with short-range structural change, which could be the origin of anelasticity.

DOI： 10.1016/j.actamat.2022.117704

Direct determination of a topmost atomic layer of (100) surfaces on (La0.3Sr0.7)(Al0.65Ta0.35)O3 single crystals annealed in air by high-resolution scanning transmission electron microscopy 査読有り

Anisotropic Growth of Copper Nanorods Mediated by Cl- Ions 査読有り国際共著 Open Access

Saw, MJ; Nguyen, MT; Kunisada, Y; Tokunaga, T; Yonezawa, T

ACS OMEGA 7 巻 ( 8 ) 頁： 7414 - 7420 2022年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Omega

Anisotropic growth to form Cu particles of rod and wire shapes has been obtained typically in a complex system that involves both organic capping agents and Cl- ions. However, the sole effect of Cl- ions on the formation of Cu wires has yet to be fully understood, especially in an organic system. This present work determines the effect of Cl- ions on the morphologies of Cu particles in an organic phase without any capping agents. The results revealed that anisotropic Cu rods could be grown with the sole presence of Cl- ions. The rods have the (011) facets as the long axis, the (111) facets as the tip, and the (100) facets as the side surface. By increasing the Cl- ion concentration, more Cu atoms contributed to the formation of Cu rods and the kinetic growth of the length and the diameter of the rods varied. This suggests that Cl- ions have preferential adsorption on the (100) Cu surfaces to promote the anisotropic growth of Cu. Meanwhile, the adsorption of Cl- to the (111) and (100) surfaces at high Cl- concentrations regulates the relative growth of the particle length and diameter.

DOI： 10.1021/acsomega.2c00359

Tokuda, Y; Irimoto, T; Nishikawa, N; Katsuyama, Y; Kobayashi, S; Tokunaga, T; Yamamoto, T

SURFACE AND INTERFACE ANALYSIS 54 巻 ( 3 ) 頁： 212 - 217 2022年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Surface and Interface Analysis

Herein, we investigated the surface of an (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) single crystal via scanning transmission electron microscopy (STEM) after annealing in air. We found amorphous mounds and atomically flat surfaces formed on the surface of the (100) LSAT single crystal subsequent to heat treatment. These surface mounds mainly constituted SrO, including ~50 at.% Al. The atomically flat surface showed a B-site termination determined by the direct surface observation at the atomic scale through HAADF-STEM. The atomic arrangements on the surface were accommodated by surface-mound formation and La evaporation. Consequently, the ordered structure disappeared because of the increase in the Ta content at the terminated B-site layer.

DOI： 10.1002/sia.7043

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/sia.7043

Alginate-Stabilized Gold Nanoparticles Prepared Using the Microwave-Induced Plasma-in-Liquid Process with Long-Term Storage Stability for Potential Biomedical Applications 査読有り Open Access

Liu, HR; Ikeda, K; Nguyen, MT; Sato, S; Matsuda, N; Tsukamoto, H; Tokunaga, T; Yonezawa, T

ACS OMEGA 7 巻 ( 7 ) 頁： 6238 - 6247 2022年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Omega

A one-step preparation of alginate-stabilized gold nanoparticles (Au NPs) using the microwave-induced plasma-in-liquid process (MWPLP) was reported. Effects of alginate with various concentrations on the preparation and properties of the synthesized Au NPs, including reaction rate, morphology, size, and optical absorption property, were studied. The introduction of alginate (1) accelerated the reaction rate, (2) prevented aggregation and precipitation due to long time discharge in MWPLP, and (3) provided long-term colloidal stability. An abnormal size change (from large to small) of Au NPs during particle growth, which was opposite to the typical change in bottom-up chemical reduction, was observed and a possible mechanism was proposed based on the dynamical and thermodynamical instability of particles during growth. The strategy of drying and redispersion of Au NPs in alginate solution was also studied. The drying and redispersion process had an imperceptible effect on the Au NPs. As a consequence, this strategy might be an effective technique for the long-term storage of Au NPs and other metal NPs. The alginate-stabilized Au NPs without the addition of toxic reducing or stabilizing agents can be appropriate to biomedical applications.

DOI： 10.1021/acsomega.1c06769

Nanoporous ultra-high-entropy alloys containing fourteen elements for water splitting electrocatalysis Open Access

Cai, ZX; Goou, H; Ito, Y; Tokunaga, T; Miyauchi, M; Abe, H; Fujita, T

CHEMICAL SCIENCE 12 巻 ( 34 ) 頁： 11306 - 11315 2021年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Chemical Science

High-entropy alloys (HEAs) are near-equimolar alloys comprising five or more elements. In recent years, catalysis using HEAs has attracted considerable attention across various fields. Herein, we demonstrate the facile synthesis of nanoporous ultra-high-entropy alloys (np-UHEAs) with hierarchical porosityviadealloying. These np-UHEAs contain up to 14 elements, namely, Al, Ag, Au, Co, Cu, Fe, Ir, Mo, Ni, Pd, Pt, Rh, Ru, and Ti. Furthermore, they exhibit high catalytic activities and electrochemical stabilities in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic media, superior to that of commercial Pt/graphene and IrO2catalysts. Our results offer valuable insights for the selection of elements as catalysts for various applications.

DOI： 10.1039/d1sc01981c

Flash sintering for BaTiO3 with square alternating current field including zero-field duration 招待有り査読有り Open Access

Umemura, R; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 128 巻 ( 12 ) 頁： 1018 - 1023 2020年12月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

The shrinkage behavior of barium titanate (BaTiO3) green compacts during flash sintering with a square alternating current (AC) field including a zero-field duration (AC-pulse wave) was investigated. It was found that an AC-pulse wave is effective for the densification of BaTiO3 green compacts without reduction and discharge, which is confirmed by transmission electron microscopy and electron energy loss spectroscopy. BaTiO3 sintered polycrystal was obtained at a furnace temperature of 1000 °C by the optimal AC-pulse conditions of 285 V/cm at 250 Hz and a duty ratio of 30 %. A waveform with alternating polarity and the inclusion of a zero-field duration is shown to be possibly useful for flash sintering of oxide materials including altervalent cations.

DOI： 10.2109/jcersj2.20181

H2 production from methane decomposition by fullerene at low temperature 査読有り

Tokunaga, T; Kuno, K; Kawakami, T; Yamamoto, T; Yoshigoe, A

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 45 巻 ( 28 ) 頁： 14347 - 14353 2020年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：International Journal of Hydrogen Energy

Carbon materials have previously been reported to work as catalysts for hydrogen (H2) production from hydrocarbons. Mechanisms of the catalytic behavior of graphite and carbon black (CB) have often been discussed in literature. Graphite and CB is constructed from mainly 6-membered rings with sp2 bonds. To understand the catalytic behavior of carbon materials for H2 production by methane (CH4) decomposition, the catalytic behavior of fullerenes with 6-membered rings and also those comprising 5- and 7-membered rings with sp2 bonds and their associated mechanisms should be investigated. In this study, the fullerene catalyst activity has been investigated using gas chromatography and the electronic states and nanoscale structures have been analyzed. H2 production started at 400 °C and the H2 production rate gradually increased with time, and the activation energy of the fullerene for H2 production by CH4 decomposition was found to be 166 kJ/mol. Moreover, in situ heating X-ray photon spectroscopy (XPS) measurements showed that the π-π∗ transition signal becomes stronger with increasing temperature above the threshold of 300 °C. The transition of the π electrons to π∗ orbitals upon heating is expected to decompose CH4 absorbed on fullerene. Moreover, transmission electron microscopy (TEM) analysis revealed that the generated carbon atoms from the CH4 decomposition were deposited onto the surfaces of the fullerenes, forming amorphous and layered concentric sphere carbon. Amorphous carbon is reported to not work as a catalyst for CH4 decomposition at around 400 °C. From XPS analysis and TEM observations of these two structures, it is anticipated that the ring structures without 6-membered rings in carbon materials with sp2 bonding contribute to this catalytic behavior for CH4 decomposition at a low temperature of 400 °C.

DOI： 10.1016/j.ijhydene.2020.03.178

Blue photo luminescence from 3 mol%Y2O3-doped ZrO2 polycrystals sintered by flash sintering under an alternating current electric field 査読有り Open Access

Yamashita, Y; Kurachi, T; Tokunaga, T; Yoshida, H; Yamamoto, T

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 40 巻 ( 5 ) 頁： 2072 - 2076 2020年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the European Ceramic Society

We investigated the photoluminescence (PL) properties of 3 mol%Y2O3-ZrO2 (3YSZ) polycrystals. The 3YSZ polycrystals were prepared by an isothermal flash sintering technique in which an alternating electric field was applied at a predetermined furnace temperature. After flash sintering, clear, visibly blue PL at 2.82 eV (440 nm) was confirmed under an ultraviolet light (3.4 eV (365 nm)); however, no visible PL was observed from 3YSZ polycrystals conventionally sintered using the same raw powders. Thus, the emission of blue PL was induced by the electric field applied during sintering. The obtained PL band was related to excitation at 3.87 eV (320 nm), which is possibly assigned to the singly ionized associated oxygen vacancy defects (AOD + centers). Furthermore, the blue emission was stable after high temperature annealing at 1400 °C for 1 h in air. We possibly attribute the emission of this PL to the stabilization of AOD + centers by the electric field loading during flash sintering.

DOI： 10.1016/j.jeurceramsoc.2019.12.060

Suppression of nitridation of yttria-doped zirconia during flash sintering 査読有り国際共著

Kurachi, T; Yamashita, Y; Tokunaga, T; Yoshida, H; Yamamoto, T

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 103 巻 ( 5 ) 頁： 3002 - 3007 2020年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the American Ceramic Society

The effect of direct current (DC) and alternating current (AC) on nitridation of 3 mol% Y2O3-doped ZrO2 (3YSZ) after keeping in a flash state for 1 hour was investigated. The inside of the DC-flashed compact was confirmed to exhibit blacking. Scanning transmission electron microscopy, electron energy loss spectroscopy, and X-ray diffraction analysis revealed that zirconium nitrides formed in the blackened area. In contrast, a uniformly densified compact without blackening was obtained by AC fields. No zirconium nitrides formed in the compacts exposed to AC fields even when the flash state was maintained for 1 hour. Therefore, AC fields are effective to suppress nitridation of 3YSZ during flash sintering.

DOI： 10.1111/jace.16990

In situ TEM observation of liquid-state Sn nanoparticles vanishing in a SiO2 structure: a potential synthetic tool for controllable morphology evolution from core-shell to yolk-shell and hollow structures 査読有り国際共著 Open Access

Zhu, S; Nguyen, MT; Tokunaga, T; Wen, CY; Yonezawa, T

NANOSCALE ADVANCES 2 巻 ( 4 ) 頁： 1456 - 1464 2020年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Nanoscale Advances

Precise design of hollow nanostructures can be realized via various approaches developed in the last two decades, endowing nanomaterials with unique structures and outstanding performances, showing their usefulness in a broad range of fields. Herein, we demonstrate the formation of SnO2@SiO2 hollow nanostructures, for the first time, by interaction between liquid state Sn cores and SiO2 shell structures inside Sn@SiO2 core-shell nanoparticles with real-time observation via in situ transmission electron microscopy (TEM). Based on the in situ results, designed transformation of the nanoparticle structure from core-shell Sn@SiO2 to yolk-shell Sn@SiO2 and hollow SnO2@SiO2 is demonstrated, showing the controllable structure of core-shell Sn@SiO2 nanoparticles via fixing liquid-state Sn inside a SiO2 shell which has a certain Sn containing capacity. The present approach expands the toolbox for the design and preparation of yolk-shell and hollow nanostructures, thus providing us with a new strategy for fabrication of more complicated nanostructures.

DOI： 10.1039/c9na00782b

Deng, LL; Nguyen, MT; Shi, JM; Chau, YTR; Tokunaga, T; Kudo, M; Matsumura, S; Hashimoto, N; Yonezawa, T

LANGMUIR 36 巻 ( 12 ) 頁： 3004 - 3015 2020年3月

記述言語：英語出版者・発行元：Langmuir

Pt/Au alloy nanoparticles (NPs) in a wide composition range have been synthesized by room-temperature simultaneous sputter deposition from two independent magnetron sources onto liquid PEG (MW = 600). The prepared NPs were alloyed with the face-centered cubic (fcc) structure. In addition, the particle sizes, composition, and shape are strongly correlated but can be tailored by an appropriate variation of the sputtering parameters. No individual particle but large agglomerates with partial alloy structure formed at Pt content of less than 16 atom %. Highly dispersed NPs with no agglomeration were observed in PEG when the quantity of Pt is more than 26 atom %. On the other hand, a small amount of Pt could terminate the agglomeration of Au when sputtering on the grids for transmission electron microscope observation. Our experiment and computer simulation carried out by two different methods indicate that the composition-dependent particle size of Pt/Au can be explained by the atomic concentration, formation energy of the cluster, and interaction between different metal atoms and the PEG molecule.

DOI： 10.1021/acs.langmuir.0c00152

Synthesis of composition-tunable Pd-Cu alloy nanoparticles by double target sputtering

Chau, YTR; Nguyen, MT; Zhu, MB; Romier, A; Tokunaga, T; Yonezawa, T

NEW JOURNAL OF CHEMISTRY 44 巻 ( 12 ) 頁： 4704 - 4712 2020年3月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：New Journal of Chemistry

In this work, we introduce a green synthesis technique, double-target sputtering into a liquid polymer-polyethylene glycol (PEG, M. W. = 600), to synthesize palladium-copper (Pd-Cu) alloy nanoparticles (NPs) dispersed in PEG. The composition of Pd and Cu in the alloy NPs can be controlled and varied by applying different sputtering currents for Pd and Cu targets. The particle sizes of the Pd-Cu NPs are maintained in the range of 2.4-2.5 nm for various Pd-Cu compositions. Oxidation issues and crystallinity enhancement are discussed.

DOI： 10.1039/d0nj00288g

Blue photoluminescence at room temperature from Y2O3-doped ZrO2 polycrystals sintered by flash sintering

Yamashita, Y; Itoh, A; Tokunaga, T; Yoshida, H; Yamamoto, T

APPLIED PHYSICS EXPRESS 13 巻 ( 3 ) 2020年3月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Applied Physics Express

Photoluminescence band variation of Y2O3-doped ZrO2 (YSZ) polycrystals was investigated for flash-sintered YSZ polycrystals. Nearly full density could be obtained by flash sintering under an alternating current field of 100 V cm-1, 100 Hz, limiting current of 1000 mA and at 1200 °C for 5 min. The flash-sintered YSZ polycrystals emitted blue photoluminescence even at room temperature under excitation of a handheld ultraviolet light, which could be observed with the naked eye. By contrast, conventionally sintered YSZ polycrystals emitted no photoluminescence despite using the same raw powders. In addition, the photoluminescence bands were tunable by varying the Y2O3 content.

DOI： 10.35848/1882-0786/ab7710

Green Synthesis of Size-Tunable Iron Oxides and Iron Nanoparticles in a Salt Matrix 査読有り

Nguyen, MT; Yu, K; Tokunaga, T; Boonyaperm, K; Kheawhom, S; Arita, M; Yonezawa, T

ACS SUSTAINABLE CHEMISTRY & ENGINEERING 7 巻 ( 21 ) 頁： 17697 - 17705 2019年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Sustainable Chemistry and Engineering

This research is devoted for the synthesis of Fe oxide and Fe nanoparticles (NPs) via, respectively, thermal decomposition of iron(III) oleate and H2 reduction of either iron(III) oleate or iron oxide NPs in a NaCl matrix. Lowering the weight ratios of metal precursor to NaCl and synthesis temperature results in smaller particle sizes. A higher uniformity of Fe NPs can be achieved with a two-step synthesis, that is, thermal decomposition of iron oleate followed by H2 reduction of iron oxides, with the milling of iron oxides and NaCl prior to the reduction step. Fe NPs obtained in this study are stable in the air for long-time storage. It is found that thin oxide and C layers on the particle surface are responsible for their stability.

DOI： 10.1021/acssuschemeng.9b03950

Technique to control specimen electric current during a flash state with alternating current electric fields 査読有り Open Access

Yamashita, Y; Kurachi, T; Tokunaga, T; Yoshida, H; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 127 巻 ( 11 ) 頁： 849 - 851 2019年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

A simple technique to control the specimen electric current during flash sintering was developed using AC electric fields in 3 mol% Y2O3-doped ZrO2. Flash sintering is characterized by the occurrence of a flash event, i.e., a spike in the specimen electric current, which makes it difficult to control the specimen electric current at/ after the flash event for AC electric fields. However, AC electric fields are very attractive from the viewpoint of a suppression of the severe reduction arising from the unidirectional ionic flow during the flash event, which is a fatal phenomenon with DC electric fields. In this study, we develop a simple technique to control the AC electric fields at/after the flash event by limiting the AC electric fields under certain values. As a result, the flash state after the flash event is successfully maintained. Further, it is found that 3 mol% Y2O3-doped ZrO2 compacts prepared by DC- and AC-flash sintering emit blue fluorescence (FL) under 254 nm ultraviolet light. The blue FL is not obtained in the compacts prepared by conventionally-sintering. The application of electric fields is confirmed to result in the blue florescent properties of 3 mol% Y2O3-doped ZrO2 compacts.

DOI： 10.2109/jcersj2.19144

One-Minute Joule Annealing Enhances the Thermoelectric Properties of Carbon Nanotube Yarns via the Formation of Graphene at the Interface 査読有り

Hada, M; Hasegawa, T; Inoue, H; Takagi, M; Omoto, K; Chujo, D; Iemoto, S; Kuroda, T; Morimoto, T; Hayashi, T; Iijima, T; Tokunaga, T; Ikeda, N; Fujimori, K; Itoh, C; Nishikawa, T; Yamashita, Y; Kiwa, T; Koshihara, S; Maeda, S; Hayashi, Y

ACS APPLIED ENERGY MATERIALS 2 巻 ( 10 ) 頁： 7700 - 7708 2019年10月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Applied Energy Materials

Interfaces in nanocarbon materials are highly important, as they determine the properties of carbon-based devices. In terms of carrier and thermal transport properties, the interfacial features are often more important than the intrinsic characteristics. Herein, we describe how 1 min Joule annealing of carbon nanotube (CNT) yarns can convert the interfacial amorphous carbon into graphene fragments. After 1 min Joule annealing, we have obtained multiwalled CNT yarns with extremely high Seebeck coefficients (±100 μV/K) and high thermoelectric power factor (400 and 1000 μW/mK2) at room temperature, both with or without polyethylenimine doping. Theoretical simulations and experimental measurements helped to determine the optimal annealing conditions in terms of a rapid transformation of the interfacial amorphous carbon between the bundled CNTs in the yarn into graphene fragments at â¼2000 K. The present approach represents significant progress in energy materials science, as it provides a guiding principle for the design of interfaces in nanocarbon materials with potential applications in energy-harvesting systems.

DOI： 10.1021/acsaem.9b01736

Selective Reduction Mechanism of Graphene Oxide Driven by the Photon Mode versus the Thermal Mode 査読有り国際誌

Hada, M; Miyata, K; Ohmura, S; Arashida, Y; Ichiyanagi, K; Katayama, I; Suzuki, T; Chen, W; Mizote, S; Sawa, T; Yokoya, T; Seki, T; Matsuo, J; Tokunaga, T; Itoh, C; Tsuruta, K; Fukaya, R; Nozawa, S; Adachi, S; Takeda, J; Onda, K; Koshihara, S; Hayashi, Y; Nishina, Y

ACS NANO 13 巻 ( 9 ) 頁： 10103 - 10112 2019年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Nano

A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems.

DOI： 10.1021/acsnano.9b03060

Porous ZnV2O4 Nanowire for Stable and High-Rate Lithium-Ion Battery Anodes 査読有り

De Juan-Corpuz, LMZ; Nguyen, MT; Corpuz, RD; Yonezawa, T; Rosero-Navarro, NC; Tadanaga, K; Tokunaga, T; Kheawhom, S

ACS APPLIED NANO MATERIALS 2 巻 ( 7 ) 頁： 4247 - 4256 2019年7月

記述言語：日本語出版者・発行元：American Chemical Society ({ACS})

Porous ZnV2O4 nanowires (NWs) were successfully prepared by hydrothermal reaction followed by calcination. Despite the porous structure, these porous ZnV2O4 NWs are single crystal with {220} facets and a wire direction along the c-axis. On the basis of an electrochemical test, these porous ZnV2O4 NWs have better cycling stability and higher specific capacity (i.e., 460 mA h g-1 after 100 cycles and 149 mA h g-1 after 1000 cycles using 1 and 5 A g-1 current densities, respectively) compared to other morphologies (i.e., spherical and coral-like morphologies). As a ternary transition metal oxide, the produced porous ZnV2O4 NWs undergo phase transformation without compromising the resulting capacity. On the other hand, the CV curves at different scan rates indicate a pseudocapacitive electrochemical behavior of the porous ZnV2O4.

DOI： 10.1021/acsanm.9b00703

Preparation and Growth Mechanism of Pt/Cu Alloy Nanoparticles by Sputter Deposition onto a Liquid Polymer 査読有り国際誌

Deng, LL; Nguyen, MT; Mei, S; Tokunaga, T; Kudo, M; Matsumura, S; Yonezawa, T

LANGMUIR 35 巻 ( 25 ) 頁： 8418 - 8427 2019年6月

記述言語：英語出版者・発行元：Langmuir

We use a green sputtering technique to deposit a Pt/Cu alloy target on liquid polyethylene glycol (PEG) to obtain well-dispersed and stable Pt29Cu71 alloy nanoparticles (NPs). The effects of sputtering current, rotation speed of the stirrer, sputtering time, sputtering period, and temperature of PEG on the particle size are studied systematically. Our key results demonstrate that the aggregation and growth of Pt/Cu alloy NPs occurred at the surface as well as inside the liquid polymer after the particles landed on the liquid surface. According to particle size analysis, a low sputtering current, high rotation speed for the stirrer, short sputtering period, and short sputtering time are found to be favorable for producing small-sized single crystalline alloy NPs. On the other hand, varying the temperature of the liquid PEG does not have any significant impact on the particle size. Thus, our findings shed light on controlling NP growth using the newly developed green sputtering deposition technique.

DOI： 10.1021/acs.langmuir.9b01112

Size-Tunable Alumina-Encapsulated Sn-Based Phase Change Materials for Thermal Energy Storage 査読有り

Zhu, SL; Nguyen, MT; Tokunaga, T; Yonezawa, T

ACS APPLIED NANO MATERIALS 2 巻 ( 6 ) 頁： 3752 - 3760 2019年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Applied Nano Materials

The most commonly used phase change materials (PCMs), like organic compounds and inorganic salts, were limited in application by their low thermal conductivity. Herein, for the first time, alumina-encapsulated metallic Sn-based PCMs, named Sn@Al2O3, were successfully fabricated with tunable size (60-2000 nm) by a facile process from low-cost chemicals. The robust fabrication process consists of a surfactant-free solvothermal synthesis of SnO2 spheres, boehmite treatment on SnO2 spheres, calcination in the air, and the final hydrogen reduction to transform SnO2 to metallic Sn, endowing the PCMs with high potential for mass production. The as-obtained Sn@Al2O3 showed a core-shell structure, in which a main metallic Sn core located in the center covered with an Al2O3 shell with small Sn nanoparticles distributed inside. The boehmite treatment, in which the penetration of aluminum species into SnO2 spheres played an important role, was found to be responsible for the unique structure formation of final Sn@Al2O3. The understanding of structure formation mechanism gives the possibilities of a new facile way for the synthesis of metal nanoparticles and particle-distributed nanostructures. The obtained Sn@Al2O3 particles not only have high PCM content (92.37 wt %) but also show a stable thermal behavior and morphology during 100 melt-freeze cycles in the air atmosphere. Furthermore, the low melting temperature of the PCM core, combined with high thermal conductivity of both core material (Sn, 66.8 W m-1 K-1) and shell material (Al2O3, 35 W m-1 K-1), makes Sn@Al2O3 potentially suitable for rapid thermal energy storage in the range 100-300 °C.

DOI： 10.1021/acsanm.9b00649

Topologically immobilized catalysis centre for long-term stable carbon dioxide reforming of methane 査読有り国際誌 Open Access

Shoji, S; Peng, XB; Imai, T; Kumar, PSM; Higuchi, K; Yamamoto, Y; Tokunaga, T; Arai, S; Ueda, S; Hashimoto, A; Tsubaki, N; Miyauchi, M; Fujita, T; Abe, H

CHEMICAL SCIENCE 10 巻 ( 13 ) 頁： 3701 - 3705 2019年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Chemical Science

Methane reforming at low temperatures is of growing importance to mitigate the environmental impact of the production of synthesis gas, but it suffers from short catalyst lifetimes due to the severe deposition of carbon byproducts. Herein, we introduce a new class of topology-tailored catalyst in which tens-of-nanometer-thick fibrous networks of Ni metal and oxygen-deficient Y2O3 are entangled with each other to form a rooted structure, i.e., Ni#Y2O3. We demonstrate that the rooted Ni#Y2O3 catalyst stably promotes the carbon-dioxide reforming of methane at 723 K for over 1000 h, where the performance of traditional supported catalysts such as Ni/Y2O3 diminishes within 100 h due to the precluded mass transport by accumulated carbon byproducts. In situ TEM demonstrates that the supported Ni nanoparticles are readily detached from the support surface in the reaction atmosphere, and migrate around to result in widespread accumulation of the carbon byproducts. The long-term stable methane reforming over the rooted catalyst is ultimately attributed to the topologically immobilized Ni catalysis centre and the synergistic function of the oxygen-deficient Y2O3 matrix, which successfully inhibits the accumulation of byproducts.

DOI： 10.1039/c8sc04965c

Nanoporous Nickel Composite Catalyst for the Dry Reforming of Methane 査読有り国際誌 Open Access

Fujita, T; Peng, XB; Yamaguchi, A; Cho, Y; Zhang, YZ; Higuchi, K; Yamamoto, Y; Tokunaga, T; Arai, S; Miyauchi, M; Abe, H

ACS OMEGA 3 巻 ( 12 ) 頁： 16651 - 16657 2018年12月

記述言語：英語出版者・発行元：ACS Omega

The development of efficient catalysts with high activities and durabilities for use in the dry reforming of methane (DRM) is desirable but challenging. We report the development of a nanoporous nickel composite (nanoporous Ni/Y2O3) via a facile one-step dealloying technique, for use in the DRM. Focusing on the low-temperature DRM, our composite possessed remarkable activity and durability against coking compared with conventional particle-based Ni catalysts. This was attributed to the aluminum oxides present on the Ni surface, which suppress pore coarsening. In addition, the inert bundled Y2O3 nanowires are suitable for use as substrates for nanoporous Ni.

DOI： 10.1021/acsomega.8b02023

Intergranular amorphous films formed by DC electric field in pure zirconia 査読有り

Morisaki, N; Yoshida, H; Kobayashi, T; Tokunaga, T; Yamamoto, T

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 101 巻 ( 8 ) 頁： 3282 - 3287 2018年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the American Ceramic Society

It is difficult to obtain pure ZrO2 sintered compacts with a bulk style at room temperature because a large volumetric expansion from tetragonal to monoclinic phase (t/m) transformation occurs at around 1000°C, which is lower than the sintering temperature. In contrast, pure monoclinic ZrO2 can be consolidated without shattering using flash-sintering at 1350°C for 5 minutes under an applied DC electric field of 175 V/cm. High-resolution transmission electron microscopy and electron energy loss spectroscopy have revealed that amorphous films are formed along grain boundaries after flash-sintering at 1350°C for 5 minutes. Monoclinic ZrO2 flash-sintered compact including the amorphous films are able to survive without shattering through the t/m transformation, as the amorphous films partially absorb the large volumetric expansion arising from the t/m transformation. The formation of the amorphous films results from the severe reducing condition due to the applied DC electric fields during flash-sintering.

DOI： 10.1111/jace.15485

Ligand free green plasma-in-liquid synthesis of Au/Ag alloy nanoparticles 査読有り

Cempel, D; Nguyen, MT; Ishida, Y; Tokunaga, T; Yonezawa, T

NEW JOURNAL OF CHEMISTRY 42 巻 ( 8 ) 頁： 5681 - U9 2018年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：New Journal of Chemistry

Au/Ag alloy nanoparticles (NPs) were successfully prepared by a one step synthesis using HAuCl4 and [Ag(NH3)2]+ as the corresponding precursors in a microwave-induced plasma in liquid process (MWPLP) without the addition of any organic protecting or reducing agents. The time dependent reduction and particle formation during the 140 min plasma discharge were studied with TEM, UV-vis extinction spectra, and ICP-OES. After 140 min, the HAADF-STEM and EDX results indicated that the obtained Au/Ag NPs were alloy NPs. Varying the input molar ratios of the metal precursors allowed for control of the composition of the Au/Ag alloy NPs.

DOI： 10.1039/c7nj05154a

Sputter Deposition toward Short Cationic Thiolated Fluorescent Gold Nanoclusters: Investigation of Their Unique Structural and Photophysical Characteristics Using High-Performance Liquid Chromatography 査読有り国際誌

Ishida, Y; Morita, A; Tokunaga, T; Yonezawa, T

LANGMUIR 34 巻 ( 13 ) 頁： 4024 - 4030 2018年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Langmuir

We herein present the preparation of short, bulky cationic thiolate (thiocholine)-protected fluorescent Au nanoclusters via sputter deposition over a liquid polymer matrix. The obtained Au nanoclusters showed near-infrared fluorescence and had an average core diameter of 1.7 ± 0.6 nm, which is too large compared to that of the reported fluorescent Au nanoclusters prepared via chemical means. We revealed the mechanism of formation of this unique material using single-particle electron microscopy, optical measurements, X-ray photoelectron spectroscopy (XPS), and high-performance liquid chromatography fractionations. The noncrystallized image was observed via single-particle high-angle annular dark-field scanning transmission electron microscopy observations and compared with chemically synthesized crystalline Au nanoparticle with the same diameter, which demonstrated the unique structural characteristic speculated via XPS. The size fractionation and size-dependent fluorescence measurement, together with other observations, indicated that the nanoclusters most probably contained a mixture of very small fluorescent species in their aggregated form and were derived from the sputtering process itself and not from the interaction between thiol ligands.

DOI： 10.1021/acs.langmuir.8b00067

Au/Cu Bimetallic Nanoparticles via Double-Target Sputtering onto a Liquid Polymer 査読有り国際誌

Nguyen, MT; Zhang, H; Deng, LL; Tokunaga, T; Yonezawa, T

LANGMUIR 33 巻 ( 43 ) 頁： 12389 - 12397 2017年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Langmuir

Alloy nanoparticles (NPs) of a bimetal system, Au/Cu, that form intermetallic compounds in a bulk state have been successfully produced using a double-target sputtering technique onto a low-cost and biocompatible liquid polymer (polyethylene glycol, PEG). The formation of an Au/Cu solid solution alloy in individual NPs was revealed by scanning transmission electron microscopy-energy-dispersive X-ray elemental mapping analysis. Altering the sputter currents for Au and Cu targets resulted in a tailored NP composition, but the particle sizes did not significantly vary. We found similar structures, sizes, and optical properties of Au/Cu NPs obtained by double-head sputtering on carbon-coated transmission electron microscopy grids or PEG and by Au/Cu alloy target sputtering. Random alloy formation occurred in matrix sputtering using double-target heads. This method is advantageous for manipulating the alloy composition through highly independent control of sputter parameters for each metal target.

DOI： 10.1021/acs.langmuir.7b03194

In-Situ TEM Study of a Nanoporous Ni-Co Catalyst Used for the Dry Reforming of Methane 査読有り Open Access

Fujita, T; Higuchi, K; Yamamoto, Y; Tokunaga, T; Arai, S; Abe, H

METALS 7 巻 ( 10 ) 頁： 406 2017年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Metals

We performed in-situ transmission electron microscopy (TEM) on a dealloyed nanoporous NiCo catalyst used for the dry reforming of methane (DRM) to investigate the origin of the catalytic activity and structural durability. The in-situ observations and local chemical analysis indicated that the DRM induced chemical demixing of Ni and Co accompanied by grain refinement, implying possible “synergic effects” in a general bimetallic NiCo catalyst when used for the DRM.

DOI： 10.3390/met7100406

Electric current-controlled synthesis of BaTiO3 査読有り

Nakagawa, Y; Yoshida, H; Uehashi, A; Tokunaga, T; Sasaki, K; Yamamoto, T

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 100 巻 ( 9 ) 頁： 3843 - 3850 2017年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the American Ceramic Society

In BaTiO3, flash-sintering associated with a surge of the specimen electric current sometimes results in an inhomogeneous microstructure including Ti-excess secondary phases because of discharging. We applied field-assisted sintering technique (FAST) under precisely controlled specimen current that was set just below the threshold value for the occurrence of flash event for BaTiO3, to avoid the occurrence of the discharging. As a result, uniform and fine-grained compacts were obtained without any secondary phases. A relative density of approximately 92% was achieved under FAST condition of 100 V/cm with a limiting current of 72 mA and soaking time of 3 hours at 1070°C. The voltages during sintering under a constant current of 72 mA were found to decrease during the soaking process. Electron energy loss spectroscopy revealed the generation of excess oxygen vacancies at/near grain boundaries. The excess oxygen vacancies induced by application of DC electric fields were confirmed to reduce the voltages and to retard the shrinkage rate in a final sintering stage.

DOI： 10.1111/jace.14938

Consolidation of undoped, monoclinic zirconia polycrystals by flash sintering 査読有り

Morisaki, N; Yoshida, H; Tokunaga, T; Sasaki, K; Yamamoto, T

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 100 巻 ( 9 ) 頁： 3851 - 3857 2017年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the American Ceramic Society

Consolidated, monoclinic ZrO2 polycrystal was produced from undoped ZrO2 powders in air by flash sintering at the sintering temperature of 1350°C for 5 minutes or 3 hours under an applied DC electric field of 175 V/cm. When the ZrO2 was heated under the applied DC field, the electric current of the specimen steeply increased at the furnace temperature of 1335°C below the sintering temperature of 1350°C. When the furnace temperature was decreased from the sintering temperature of 1350°C to room temperature, volumetric expansion associated with tetragonal-to-monoclinic phase transformation gradually took place at the furnace temperature from 1000°C to 750°C, and monoclinic ZrO2 body was remained consolidated even at room temperature in both specimens. In contrast, conventionally sintered ZrO2 without applying DC field exhibited the abrupt volumetric expansion at about 1000°C, and shattered. SEM observation revealed the presence of grain-boundary second phase in the flash-sintered specimen for 3 hours, which is a possible origin of keeping a bulk body at room temperature. The thinner second phase is considered to be formed also in the flash-sintered specimen for 5 minutes, although the formation of the phase could not be observed clearly by SEM observation. On the other hand, XRD measurements showed that <001> directions of the monoclinic ZrO2 grains were oriented along the applied DC field after the isothermal flash sintering for 3 hours while the grain alignment could not be observed in flash-sintered specimen for 5 minutes. The alignment of ZrO2 grains observed in the isothermal flash sintering is considered to be closely related to the preferential direction of oxygen ionic conduction and the second phase formed along grain boundaries.

DOI： 10.1111/jace.14954

Structural Control Parameters for Formation of Single-Crystalline ss-Sn Nanorods in Organic Phase 査読有り

Lyn Marie Z. De Juan, Mai Thanh Nguyen, Tetsu Yonezawa, Tomoharu Tokunaga, Hiroki Tsukamoto, Yohei Ishida

CRYSTAL GROWTH & DESIGN 17 巻 ( 9 ) 頁： 4554 - 4562 2017年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER CHEMICAL SOC

For the first time, investigation on the effect of lowering the synthesis temperature below room temperature (10 to -10 degrees C) and increasing the viscosity of the reaction solution at certain temperatures has been used to trigger the formation of single-crystalline ss-Sn nanorods (NRs) in a chemical synthesis using an organic solvent. These two parameters govern the energy of the particles in the solution, thereby mediating the selective attachment to the nuclei and the preferential growth of Sn along the c-axis. The length and aspect ratio of the NRs were inversely proportional to the synthesis temperature and directly proportional to the viscosity of the reaction solution. In addition, we found that poly(vinylpyrrolidone) with a high molecular weight aided the formation of longer and higher aspect ratio NRs via the bridge flocculation effect.

DOI： 10.1021/acs.cgd.7b00227

Water transport phenomena through membranes consisting of vertically-aligned double-walled carbon nanotube array

Small Nanosized Oxygen-Deficient Tungsten Oxide Particles: Mechanistic Investigation with Controlled Plasma Generation in Water for Their Preparation 査読有り国際誌 Open Access

Ishida, Y; Motono, S; Doshin, W; Tokunaga, T; Tsukamoto, H; Yonezawa, T

ACS OMEGA 2 巻 ( 8 ) 頁： 5104 - 5110 2017年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Omega

Production of oxygen-deficient tungsten oxide nanoparticles with a diameter of around 10 nm have been successfully developed using a microwave-induced plasma in liquid technique. The prepared blue-green nanoparticles exhibit strong absorption in the visible region; thus, these could be efficient visible-light photocatalysts. The high-angle annular dark-field images revealed the dislocation of tungsten, which causes oxygen deficiencies.

DOI： 10.1021/acsomega.7b00986

Matsumoto, H; Tsuruoka, S; Hayashi, Y; Abe, K; Hata, K; Zhang, SL; Saito, Y; Aiba, M; Tokunaga, T; Iijima, T; Hayashi, T; Inoue, H; Amaratunga, GAJ

CARBON 120 巻頁： 358 - 365 2017年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Carbon

Nanofluidics in CNTs is argumentative though it is theoretically calculated by various reports. It is because only a few of experimental reports are available, and the measured permeability is not so large as that suggested from the theoretical calculations. Also, water motion suppression in the confined space has not been exhibited by flux measurement. The present work explores these yet-unsolved discrepancies using the measurable size membrane of vertically aligned double-walled carbon nanotube array, which is borne with durability and flexibility, and a conventional measurement method is applied to the membranes. Water motion suppression occurs in the CNT confined space significantly, depending on temperature. Additionally, it is confirmed that the obtained permeability correlates to the reported experimental results with regard to the relationship between CNT length and permeability, and the correlation does not agree with permeability calculated from the Hagen-Poiseuille law. These results pose an insight into the inherent water transport characteristics in the CNT confined space.

DOI： 10.1016/j.carbon.2017.05.034

Methane decomposition for hydrogen production by catalytic activity of carbon black under low flow rate conditions Open Access

Nanophase-separated Ni3Nb as an automobile exhaust catalyst 国際誌 Open Access

Tanabe, T; Imai, T; Tokunaga, T; Arai, S; Yamamoto, Y; Ueda, S; Ramesh, GV; Nagao, S; Hirata, H; Matsumoto, S; Fujita, T; Abe, H

CHEMICAL SCIENCE 8 巻 ( 5 ) 頁： 3374 - 3378 2017年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Chemical Science

Catalytic remediation of automobile exhaust has relied on precious metals (PMs) including platinum (Pt). Herein, we report that an intermetallic phase of Ni and niobium (Nb) (i.e., Ni3Nb) exhibits a significantly higher activity than that of Pt for the remediation of the most toxic gas in exhaust (i.e., nitrogen monoxide (NO)) in the presence of carbon monoxide (CO). When subjected to the exhaust-remediation atmosphere, Ni3Nb spontaneously evolves into a catalytically active nanophase-separated structure consisting of filamentous Ni networks (thickness < 10 nm) that are incorporated in a niobium oxide matrix (i.e., NbOx (x < 5/2)). The exposure of the filamentous Ni promotes NO dissociation, CO oxidation and N2 generation, and the NbOx matrix absorbs excessive nitrogen adatoms to retain the active Ni sites at the metal/oxide interface. Furthermore, the NbOx matrix immobilizes the filamentous Ni at elevated temperatures to produce long-term and stable catalytic performance over hundreds of hours.

DOI： 10.1039/c6sc05473k

Tokunaga, T; Kishi, N; Yamakawa, K; Sasaki, K; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 125 巻 ( 4 ) 頁： 185 - 189 2017年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

Recently, carbon materials were reported to work as catalysts for the production of H3 from hydrocarbons. However, the hydrogen production rate is extremely low. To broaden the scope of carbon materials as catalysts for the decomposition of hydrocarbons, a higher H3 production rate is required. Thus, the hydrogen production rate under conditions employing a low flow rate of hydrocarbon per unit area and a large quantity of carbon materials as the catalyst should be evaluated, and the influence of these conditions on the rate of H3 production by carbon materials should be discussed. Herein, the H3 production rate was measured under conditions employing a low flow rate of CH4 and a large quantity of carbon black (CB) as a catalyst. H3 production started at 600°C and the production rate was less than 0.1%. The transmission electron microscope image of CB heated in CH4 atmosphere demonstrated that a low flow rate of CH4 does not affect the reaction site (i.e., graphite edges) of CB. However, the H3 production rate was extremely low compared to that achieved with a high flow rate. It is proposed that there is an optimal flow rate for maintaining the catalytic activity at the graphite edge of CB for decomposition of CH4.

DOI： 10.2109/jcersj2.16246

Preparation of Au/Pd Bimetallic Nanoparticles by a Microwave-Induced Plasma in Liquid Process

Shirai, H; Nguyen, MT; Cempel, D; Tsukamoto, H; Tokunaga, T; Liao, YC; Yonezawa, T

BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 90 巻 ( 3 ) 頁： 279 - 285 2017年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Bulletin of the Chemical Society of Japan

Au/Pd alloy nanoparticles (NPs) were successfully synthesized using HAuCl4, H2PdCl4, and α-thioglycerol as precursors and a stabilizer, respectively, by microwave-induced plasma in liquid process (MWPLP). After only several minutes of microwave irradiation, metallic NPs were obtained in aqueous environment without any reducing agent. The reduction mechanism of metal ions was confirmed by UV-vis spectral changes. The results from HAADF-STEM and EDS indicated that synthesized NPs were bimetallic Au/Pd. Moreover, XRD patterns of synthesized NPs implied that the composition of bimetallic NPs could be controlled by changing the initial ratio of metal precursors.

DOI： 10.1246/bcsj.20160333

Directly determining the polarity of WC grains in WC-12 wt%Co alloy by ABF-STEM 査読有り

Hayashi, Y; Tokunaga, T; Sasaki, K; Yamamoto, T

INTERNATIONAL JOURNAL OF REFRACTORY METALS & HARD MATERIALS 62 巻頁： 9 - 13 2017年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：International Journal of Refractory Metals and Hard Materials

WC grains in WC–Co alloys are shaped as truncated triangular prisms, each with a basal plane of a (0001) truncated triangle and two {11̅00} prismatic planes, one narrower and one wider. This anisotropic shape is related to the polarity of the WC lattice, which comes from the difference in the spacing of the W and C layers in the (0001) projection. We directly determined the relationship between the two {11̅00} prismatic planes and the polarity of the WC lattice by using high-resolution scanning transmission electron microscopy (STEM). High-angle annular dark-field STEM and annular bright-field STEM revealed that the wider prismatic plane had W and C atomic layers with a shorter spacing, while the narrower plane had a longer spacing. The WC/Co interface on the wider prismatic side was atomically flat, while many steps formed on the surface of the narrower prismatic side. This difference in step formation at these interfaces could have come from the difference in how their W[sbnd]C bonds formed.

DOI： 10.1016/j.ijrmhm.2016.10.013

Water transport phenomena through membranes consisting of vertically-aligned double-walled carbon nanotube array

Hidetoshi Matsumoto, Shuji Tsuruoka, Yasuhiko Hayashi, Koji Abe, Kenjiro Hata, Shaoling Zhang, Yoshitaka Saito, Motohiro Aiba, Tomoharu Tokunaga, Toru Iijima, Takuma Hayashi

Carbon 120 巻頁： 358–365 2017年

Nanophase-separated Ni3Nb as Automobile Exhaust Catalysts 査読有り

Toyokazu Tanabe, Tsubasa Imai, Tomoharu Tokunaga, Shigeo Arai, Yuta Yamamoto, Shigenori Ueda, Gubbala V. Ramesh, Satoshi Nagao, Hirohito Hirata, Shin-ichi, Matsumoto, Takeshi Fujita, Hideki Abe

Chemical Science 8 巻頁： 3374-3378 2017年

Preparation of Au/Pd bimetallic nanoparticles by a microwave-induced plasma in liquid process 査読有り

Hiroaki Shirai, Mai Thanh Nguyen, David Čempel, Hiroki Tsukamoto, Tomoharu Tokunaga, Ying-Chih Liao, and Tetsu Yonezawa

Bulletin of the Chemical Society of Japan 90 巻 ( 3 ) 頁： 279-285 2017年

Methane Decomposition for Hydrogen Production by Catalytic Activity of Carbon Black under Low Flow Rate Conditions Open Access

Tomoharu Tokunaga, Naoyuki Kishi, Kenya Yamakawa, Katsuhiro Sasaki, Takahisa Yamamoto

ACS-Japan 125 巻 ( 4 ) 頁： 185-189 2017年

Intentionally Encapsulated Metal Alloys within Vertically Aligned Multi-Walled Carbon Nanotube Array via Chemical Vapor Deposition Technique

Hayashi Yasuhiko, Inoue Hirotaka, Hayashi Takuma, Hada Masaki, Nishikawa Takeshi, Tokunaga Tomoharu, Amaratunga G. A. J.

2017 IEEE INTERNATIONAL CONFERENCE ON MANIPULATION, MANUFACTURING AND MEASUREMENT ON THE NANOSCALE (3M-NANO) 頁： 357-361 2017年

Intentionally Encapsulated Metal Alloys within Vertically Aligned Multi-Walled Carbon Nanotube Array via Chemical Vapor Deposition Technique 査読有り

Yasuhiko Hayashi, Hirotaka Inoue, Takuma Hayashi, Masaki Hada, Takeshi Nishikawa, Tomoharu Tokunaga, G. A. J. Amaratunga

2017 IEEE INTERNATIONAL CONFERENCE ON MANIPULATION, MANUFACTURING AND MEASUREMENT ON THE NANOSCALE (3M-NANO) 頁： 357 - 361 2017年

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）出版者・発行元：IEEE

This paper presents a growth and characterization of vertically aligned PdxCo1-x alloy encapsulated inside Multi-Walled Carbon Nanotube (MWCNT) arrays on Pd/Co thin layers coated on Si substrate by a dc bias-enhanced plasma chemical vapor deposition (CVD) method. The Metal Alloy Encapsulated within MWCNTs (MAE-MWCNTs) were characterized by a scanning electron microscope (SEM) and a transmission electron microscopy (TEM). The SEM images show the teardrop-shape particles encapsulated within the tube top of MWCNTs. A vibrating sample magnetometer was used to study the magnetism of a large number of MAE-MWCNTs on Si substrate at room temperature. The hysteresis loop of the ME-MWCNTs shows clear ferromagnetic behavior and the easy axis of magnetization is parallel to the MEA-MWCNT tube axis, as can be elucidated from the large coercive fields and remanence values. Moreover, TEM off-axis electron holograms were used to study the magnetism of the individual MAE-MWCNT or the two pair of MAE-MWCNTs. Based on electron holography, we have successfully obtained the saturation magnetization of 0.7 T and 1.12 T for the individual MAE-MWCNT with diameters of 41 nm and 83 nm, respectively.