Faculty Profiles - TOKUNAGA, Tomoharu

Papers - TOKUNAGA, Tomoharu

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

Shunsuke Tanahashi, Sae Okawara, Tomoharu Tokunaga, Takahisa Yamamoto

MATERIALS TRANSACTIONS Vol. 64 ( 1 ) page： 184 - 190 2023.1

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

Masao Koike, Ayu Kodaira, Tomoharu Tokunaga, Takahisa Yamamoto

Materials Transactions Vol. 64 ( 10 ) page： 2315 - 2320 2023

Publishing type：Research paper (scientific journal)

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

Scopus

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

Masao Koike, Ayu Kodaira, Tomoharu Tokunaga, Takahisa Yamamoto

MATERIALS TRANSACTIONS Vol. 64 ( 9 ) page： 2315 - 2320 2023

Variation of photoluminescence intensity depending on the timing of electric field application during isothermal flash sintering for 3mol%Y2O3–ZrO2 polycrystal Reviewed

Asa Itoh, Tomoharu Tokunaga, Ayu Kodaira, Hidehiro Yoshida, Takahisa Yamamoto

Ceramics International Vol. 48 ( 19 ) page： 28712 - 28717 2022.10

Effect of He Buffer Gas on Pulsed Nd:YAG Laser Deposition of EuxY2–xO3 Phosphor Thin Films Reviewed

Shizuka Suzuki, Takuro Dazai, Kazuhiro Kawashima, Tomoharu Tokunaga, Takahisa Yamamoto, Ryota Takahashi

ACS Applied Electronic Materials Vol. 4 ( 9 ) page： 4419 - 4426 2022.9

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

Hideya Kawasaki, Tomoko Otsuki, Fumiya Sugino, Ken Yamamoto, Tomoharu Tokunaga, Rintaro Tokura, Tetsu Yonezawa

Chemical Communications Vol. 58 ( 56 ) page： 7741 - 7744 2022.7

Effect of electron beam irradiation in gas atmosphere during ETEM Reviewed

Tomoharu Tokunaga, Takumi Kawakami, Kimitaka Higuchi, Yuta Yamamoto, Takahisa Yamamoto

Micron Vol. 158 page： 103289 - 103289 2022.7

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

Rintaro Tokura, Hiroki Tsukamoto, Tomoharu Tokunaga, Mai Thanh Nguyen, Tetsu Yonezawa

Materials Advances Vol. 3 ( 12 ) page： 4802 - 4812 2022.6

Publishing type：Research paper (scientific journal) Publisher：Royal Society of Chemistry (RSC)

1-Hexanoic acid stabilised copper nanoparticles were obtained. Cu64O phase could be found on the particle surface. During sintering, the stabilising molecule, 1-hexanoic acid, reduced Cu64O to Cu and it introduced neckings between the particles.

DOI： 10.1039/d1ma01242h

Web of Science

Scopus

Excess oxygen-vacancy formed by FAST regime of direct-current electric field during flash sintering for 3 mol%–10 mol% Y2O3-doped ZrO2 Reviewed

Nobuhiro Morisaki, Tomoharu Tokunaga, Kiyoshi Kobayashi, Ayu Kodaira, Takahisa Yamamoto

Ceramics International Vol. 48 ( 9 ) page： 12091 - 12097 2022.5

Anelasticity induced by AC flash processing of cubic zirconia Reviewed

Hiroshi Masuda, Koji Morita, Tomoharu Tokunaga, Takahisa Yamamoto, Hidehiro Yoshida

Acta Materialia Vol. 227 page： 117704 - 117704 2022.4

Anisotropic Growth of Copper Nanorods Mediated by Cl– Ions Reviewed International coauthorship

Min Jia Saw, Mai Thanh Nguyen, Yuji Kunisada, Tomoharu Tokunaga, Tetsu Yonezawa

ACS Omega Vol. 7 ( 8 ) page： 7414 - 7420 2022.3

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 Reviewed

Yoshinori Tokuda, Takeshi Irimoto, Naoki Nishikawa, Yutaro Katsuyama, Shunsuke Kobayashi, Tomoharu Tokunaga, Takahisa Yamamoto

Surface and Interface Analysis Vol. 54 ( 3 ) page： 212 - 217 2022.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Wiley

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

Web of Science

Scopus

Other Link： 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 Reviewed

Haoran Liu, Kai Ikeda, Mai Thanh Nguyen, Susumu Sato, Naoki Matsuda, Hiroki Tsukamoto, Tomoharu Tokunaga, Tetsu Yonezawa

ACS Omega Vol. 7 ( 7 ) page： 6238 - 6247 2022.2

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

Ze-Xing Cai, Hiromi Goou, Yoshikazu Ito, Tomoharu Tokunaga, Masahiro Miyauchi, Hideki Abe, Takeshi Fujita

Chemical Science Vol. 12 ( 34 ) page： 11306 - 11315 2021.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Royal Society of Chemistry (RSC)

Nanoporous ultra-high-entropy alloys containing 14 elements (Al, Ag, Au, Co, Cu, Fe, Ir, Mo, Ni, Pd, Pt, Rh, Ru, and Ti) were obtained by dealloying. The products showed excellent electrocatalytic performance for water splitting in acidic media.

DOI： 10.1039/d1sc01981c

Web of Science

Scopus

PubMed

Flash sintering for BaTiO<inf>3</inf> with square alternating current field including zero-field duration Invited Reviewed

Ryosuke Umemura, Tomoharu Tokunaga, Takahisa Yamamoto

Journal of the Ceramic Society of Japan Vol. 128 ( 12 ) page： 1018 - 1023 2020.12

Language：Japanese Publishing type：Research paper (scientific journal)

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

Web of Science

Scopus

H-2 production from methane decomposition by fullerene at low temperature Reviewed

Tomoharu Tokunaga, Kohei Kuno, Takumi Kawakami, Takahisa Yamamoto, Akitaka Yoshigoe

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY Vol. 45 ( 28 ) page： 14347 - 14353 2020.5

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Carbon materials have previously been reported to work as catalysts for hydrogen (H-2) 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 sp(2) bonds. To understand the catalytic behavior of carbon materials for H-2 production by methane (CH4) decomposition, the catalytic behavior of fullerenes with 6-membered rings and also those comprising 5- and 7-membered rings with sp(2) 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.H-2 production started at 400 degrees C and the H-2 production rate gradually increased with time, and the activation energy of the fullerene for H-2 production by CH4 decomposition was found to be 166 kJ/mol. Moreover, in situ heating X-ray photon spectroscopy (XPS) measurements showed that the pi-pi* transition signal becomes stronger with increasing temperature above the threshold of 300 degrees C. The transition of the pi. electrons to pi* orbitals upon heating is expected to decompose CH 4 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 degrees 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 sp(2) bonding contribute to this catalytic behavior for CH4 decomposition at a low temperature of 400 degrees C. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2020.03.178

Web of Science

Scopus

Blue photo luminescence from 3 mol%Y2O3-doped ZrO2 polycrystals sintered by flash sintering under an alternating current electric field Reviewed

Yudai Yamashita, Tsuyoshi Kurachi, Tomoharu Tokunaga, Hidehiro Yoshida, Takahisa Yamamoto

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY Vol. 40 ( 5 ) page： 2072 - 2076 2020.5

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

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 degrees 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

Web of Science

Scopus

Suppression of nitridation of yttria-doped zirconia during flash sintering Reviewed International coauthorship

Tsuyoshi Kurachi, Yudai Yamashita, Tomoharu Tokunaga, Hidehiro Yoshida, Takahisa Yamamoto

JOURNAL OF THE AMERICAN CERAMIC SOCIETY Vol. 103 ( 5 ) page： 3002 - 3007 2020.5

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY

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

Web of Science

Scopus

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 Reviewed International coauthorship

Shilei Zhu, Mai Thanh Nguyen, Tomoharu Tokunaga, Cheng-Yen Wen, Tetsu Yonezawa

NANOSCALE ADVANCES Vol. 2 ( 4 ) page： 1456 - 1464 2020.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

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

Web of Science

Scopus

PubMed

Lianlian Deng, Mai Thanh Nguyen, Jingming Shi, Yuen-Ting Rachel Chau, Tomoharu Tokunaga, Masaki Kudo, Syo Matsumura, Naoyuki Hashimoto, Tetsu Yonezawa

Langmuir : the ACS journal of surfaces and colloids Vol. 36 ( 12 ) page： 3004 - 3015 2020.3

Language：English

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

Web of Science

Scopus

PubMed

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Papers - TOKUNAGA, Tomoharu