Faculty Profiles - TOKUNAGA, Tomoharu

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

Web of Science

Scopus

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

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

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN Vol. 130 ( 11 ) page： 895 - 898 2022.11

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

Web of Science

Scopus

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 Reviewed

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

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN Vol. 130 ( 4 ) page： 327 - 330 2022.4

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

Web of Science

Scopus

Crystalline boron monosulfide nanosheets with tunable bandgaps Reviewed

Kusaka Haruki, Ishibiki Ryota, Toyoda Masayuki, Fujita Takeshi, Tokunaga Tomoharu, Yamamoto Akiyasu, Miyakawa Masashi, Matsushita Kyosuke, Miyazaki Keisuke, Li Linghui, Shinde Satish Laxman, Lima Mariana S. L., Sakurai Takeaki, Nishibori Eiji, Masuda Takuya, Horiba Koji, Watanabe Kenji, Saito Susumu, Miyauchi Masahiro, Taniguchi Takashi, Hosono Hideo, Kondo Takahiro

JOURNAL OF MATERIALS CHEMISTRY A Vol. 9 ( 43 ) page： 24631 - 24640 2021.11

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

Ishino Yuki, Taguchi Kimihiro, Koike Masao, Tokunaga Tomoharu, Yamamoto Takahisa

JOURNAL OF THE AMERICAN CERAMIC SOCIETY Vol. 104 ( 10 ) page： 4960 - 4967 2021.10

Rapid sintering of 3 mol % Y2O3-doped ZrO2 by a combined rapid furnace heating and shrinkage-controlled flash sintering protocol Reviewed

Ishino Yuki, Taguchi Kimihiro, Kodaira Ayu, Tokunaga Tomoharu, Yamamoto Takahisa

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN Vol. 129 ( 8 ) page： 551 - 554 2021.8

Near complete densification of flash sintered 8YSZ: controlled shrinkage rate effects Reviewed

Taguchi Kimihiro, Ishino Yuki, Tokunaga Tomoharu, Yamamoto Takahisa

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY Vol. 41 ( 8 ) page： 4567 - 4571 2021.7

Constant shrinkage rate control during a flash event for 8 mol%Y2O3-doped ZrO2 polycrystals Reviewed

Taguchi Kimihiro, Ishino Yuki, Tokunaga Tomoharu, Yamamoto Takahisa

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN Vol. 129 ( 3 ) page： 204 - 207 2021.3

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

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

Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy Vol. 68 ( 11 ) page： 482 - 486 2021

Publisher：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

Scopus

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Papers - TOKUNAGA, Tomoharu