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103 results on '"Yamashita, Takako"'

27. Formation of Dehydrohexahydroxydiphenoyl Esters by Oxidative Coupling of Galloyl Esters in an Aqueous Medium Involved in Ellagitannin Biosynthesis.

Author

Yamashita, Takako, Matsuo, Yosuke, Saito, Yoshinori, and Tanaka, Takashi

Subjects
*OXIDATIVE coupling, *BIOSYNTHESIS, *ELLAGITANNINS, *ESTERS, *BIOMIMETIC synthesis, *OXIDATION, *BUTANEDIOL
Abstract

Hexahydroxydiphenoyl (HHDP) and dehydrohexahydroxydiphenoyl (DHHDP) groups are the major acyl components of ellagitannins, which are polyphenols whose biosynthesis have attracted considerable attention; however, the mechanisms of the production of HHDP and DHHDP in the ellagitannin biosynthesis have not been clarified. With the aim of elucidating such a mechanism, this study investigates the CuCl2‐mediated oxidation of simple galloyl derivatives in an aqueous medium. It is shown that the oxidation of methyl gallate affords a DHHDP‐type dimer, whose reduction with Na2S2O4 yields an HHDP‐type dimer. However, the oxidation of the HHDP‐type product over CuCl2 does not afford the parent DHHDP ester. The oxidation of 1,4‐butanediol digallate under the same conditions produces a DHHDP‐type product via the intramolecular coupling of galloyl groups. These results strongly suggest that the DHHDP group is the initial product of the oxidative coupling of two galloyl groups in the ellagitannin biosynthesis, and subsequent reductive metabolism affords HHDP esters. [ABSTRACT FROM AUTHOR]

Published
2021
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28. Excellent plasticity revealed in high purity Si steels by the addition of Cr

Author

Takajo, Shigeaki, Yamashita, Takako, Matsuzaki, Akihiro, and Kondo, Osamu

Abstract

The plasticity of high Si steels is improved remarkably by adding Cr in high purity steels. The factors which contribute to this phenomenon were investigated. At the normal level of purity, the plasticity of Fe –Si alloys is deteriorated by Cr addition. However, in high purity steels, the plasticity drastically improves when Cr is added. When the influence of microstructure was removed, the improvement of plasticity by adding Cr to high purity Fe –Si alloys was attributed to the suppression of deformation twinning formation. The microstructural improvement was explained by grain refinement and by formation of subgrains during hot rolling. The properties of these steels are expected to be applied to new functional steels.

Published
2021
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32. 陽電子消滅法を用いたFe-Cu合金の低温時効硬化の評価

Author

Yamashita, Takako, Katsuyama, Jinya, Sato, Kei, Mizuno, Masataka, Araki, Hideki, and Shirai, Yasuharu

Subjects
Fe–Cu alloy, hardening, aging, positron lifetime, coincidence Doppler broadening, Cu cluster
Abstract

The hardness of Fe–Cu alloys is known to increase by the precipitation of Cu clusters during thermal aging. We reported previously that the precipitation of Cu clusters accelerate due to the introduction of lattice defects by pre-strain obtained through tensile test or cold rolling, thus reducing the starting temperature of the hardness increase. However, the mechanism has not yet been clarified. In this study, the positron annihilation lifetime and coincidence Doppler broadening techniques have been used to investigate the recovery behavior of lattice defects such as vacancies, vacancy clusters and dislocations as well as the diffusion behavior of Cu atoms of cold rolled and thermally-aged Fe–Cu alloys. Lattice defects were densely introduced when Fe–1.5%Cu alloys were cold rolled at a reduction rate of more than 30%, and vacancies were approximately 20% of the defects. Both vacancies and dislocations were annealed out through the aging process. On the other hand, some dislocations remained even after aging at 550℃, although the vacancies almost disappeared after aging at 300℃. Cu clusters started to precipitate mainly on the dislocations above 200℃ in the cold rolled Fe–1.5%Cu alloys. The hardening due to the Cu clusters occurs with the recovery of vacancies. Thus, both vacancies and dislocations accelerate the diffusion of Cu atoms and the precipitation of Cu clusters.

Published
2011

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Author

Yamashita, Takako, Katsuyama, Jinya, Sato, Kei, Mizuno, Masataka, Araki, Hideki, Shirai, Yasuharu, Yamashita, Takako, Katsuyama, Jinya, Sato, Kei, Mizuno, Masataka, Araki, Hideki, and Shirai, Yasuharu

Abstract

The hardness of Fe–Cu alloys is known to increase by the precipitation of Cu clusters during thermal aging. We reported previously that the precipitation of Cu clusters accelerate due to the introduction of lattice defects by pre-strain obtained through tensile test or cold rolling, thus reducing the starting temperature of the hardness increase. However, the mechanism has not yet been clarified. In this study, the positron annihilation lifetime and coincidence Doppler broadening techniques have been used to investigate the recovery behavior of lattice defects such as vacancies, vacancy clusters and dislocations as well as the diffusion behavior of Cu atoms of cold rolled and thermally-aged Fe–Cu alloys. Lattice defects were densely introduced when Fe–1.5%Cu alloys were cold rolled at a reduction rate of more than 30%, and vacancies were approximately 20% of the defects. Both vacancies and dislocations were annealed out through the aging process. On the other hand, some dislocations remained even after aging at 550℃, although the vacancies almost disappeared after aging at 300℃. Cu clusters started to precipitate mainly on the dislocations above 200℃ in the cold rolled Fe–1.5%Cu alloys. The hardening due to the Cu clusters occurs with the recovery of vacancies. Thus, both vacancies and dislocations accelerate the diffusion of Cu atoms and the precipitation of Cu clusters.

Published
2011

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