2023/11/14 更新

写真a

コバヤシ リョウタ
小林 領太
KOBAYASHI Ryouta
所属
理工学域理学系 理工学研究科(理学系) 理学専攻 物理・宇宙プログラム 特任助教
職名
特任助教

経歴

  • 鹿児島大学   大学院理工学研究科 理学専攻物理・宇宙プログラム   特任助教

    2022年7月 - 現在

 

論文

  • Kobayashi R., Mitsui Y., Yoshizaki Y., Takahashi K., Takamine K., Koyama K. .  Controlling the growth of yeast by culturing in high magnetic fields .  Journal of Magnetism and Magnetic Materials586   2023年11月

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    記述言語:日本語   出版者・発行元:Journal of Magnetism and Magnetic Materials  

    The effect of static magnetic fields up to a strength of 15 T on the growth of three types of yeast which were all classified as Saccharomyces cerevisiae was studied. The growth rate of the yeasts was suppressed by applying a magnetic field. The degree of suppression depended on the type of yeast. The reduction in the numbers of yeast cells was due to growth suppression, not sterilization. Among the three types of Saccharomyces cerevisiae, the degree of suppression on the growth of two yeasts was strongest in a magnetic field at 8 T with different suppression degree of 19% and 12% for a culture time of 24 h. Growth of the other yeast most suppressed by 10% at a magnetic field of around 4 T. Therefore, the yeasts have their characteristic magnetic field sensitivity on the growth. The results were discussed using a model based on the competition between yeast osmotic pressure changes by magnetic field and magnetic forces induced by magnetic field gradient. In low magnetic field regions, change of the osmotic pressure shrink the yeast cell and reduce the contact frequency between yeast cells and glucose-based ions. In higher magnetic field regions, effect of magnetic force become apparent, and recover the growth rate of the yeast. It suggested that the selective growth of yeast can be expected by in-magnetic-field culturing.

    DOI: 10.1016/j.jmmm.2023.171193

    Scopus

  • Nakamoto K., Mitsui Y., Kobayashi R., Koyama K. .  In-Magnetic-Field Heat Treatment Effects on Phase Growth of Mn–Bi–Sn Composite .  Materials Transactions64 ( 9 ) 2113 - 2117   2023年

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    記述言語:日本語   出版者・発行元:Materials Transactions  

    To investigate the magnetic field effect on the synthesis reaction and phase growth in a Mn–Bi–Sn composite pellet, in-field heat treatments were performed in 5 T at 573 K for 1 to 48 hours. X-ray diffraction measurements and electron probe micro analysis revealed that MnBi phase did not appear but Mn3Sn, Mn3Sn2 and MnSn2 phases were detected in the pellet. In zero-field heat treatment, the phase fraction of Mn3Sn2 increased and that of MnSn2 decreased as the heat treatment time increased. In contrast, no significant change in the phase fractions of Mn3Sn2 and MnSn2 phase was observed in 5 T even if the heat treatment time was increased to 48 h. The obtained results suggested that the application of magnetic field suppressed the diffusion of Mn atoms between Mn–Sn phases.

    DOI: 10.2320/matertrans.MT-MG2022014

    Scopus

  • Takahashi J., Mitsui Y., Onoue M., Kobayashi R., Koyama K. .  Nitridation kinetics of Sm<inf>2</inf>Fe<inf>17</inf> probed using Mössbauer spectroscopy .  Journal of Magnetism and Magnetic Materials554   2022年7月

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    記述言語:日本語   出版者・発行元:Journal of Magnetism and Magnetic Materials  

    In this study, the Sm2Fe17 powder was subjected to nitridation at 743 K under N2 pressures of 0.05 and 0.1 MPa to investigate the growth kinetics of Sm–Fe–N. Two-phase growth of the fully nitride (FN) Sm2Fe17N3 phase and the nitrogen-poor (NP) Sm2Fe17Nx phase (0 < x < 3; x denotes the nitrogen content) was observed using 57Fe Mössbauer spectroscopy. Although the x value was greater at 0.05 MPa than at 0.1 MPa in the early stage of nitridation, the opposite trend was observed in the later stage. At a nitrogenation time of 24 h, nitridation was nearly complete at 0.1 MPa, while x tended to saturate and did not reach 3 at 0.05 MPa. The nitridation process was discussed based on two simultaneous growth processes: (i) the diffusion of the nitrogen atom in the NP phase and (ii) the nucleation and growth of the FN phase. A process combining N2 pressure and nitrogenation, which facilitated nitrogenation under low N2 pressure and high N2 pressure at the beginning and end of the nitridation process, respectively, achieved faster nitridation than process with a single N2 pressure.

    DOI: 10.1016/j.jmmm.2022.169295

    Scopus

  • Kobayashi R., Mitsui Y., Umetsu R.Y., Takahashi K., Koyama K. .  Acceleration of B2/L2<inf>1</inf> order-disorder transformation in Ni<inf>2</inf>MnAl Heusler alloys by in-magnetic-field annealing .  Journal of Magnetism and Magnetic Materials547   2022年4月

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    記述言語:日本語   出版者・発行元:Journal of Magnetism and Magnetic Materials  

    Ferromagnetic L21 phase of Ni2MnAl with high order degree is difficult to synthesize because of low L21/B2 order-disorder transformation temperature. It is found that magnetic field accelerated the transformation from the antiferromagnetic disordered B2 to ordered L21 phase. Magnetization increased by in-magnetic-field annealing at 673 K, which indicated the acceleration of the ordering to L21 phase. Magnetic field effects on the B2/L21 order-disorder transformation were discussed based on magnetic-field-induced change of transformation temperature, enhancement of degree of order at annealing temperature, and reduction of critical radius of L21 nuclei. Growth kinetics of L21 phase were evaluated by Johnson-Mehl-Avrami equation, which indicated that 15-T annealing accelerated the growth of L21 phase 5.7 times faster than zero-field annealing. Our results showed that magnetic field can contribute to shortening the annealing time for synthesis of ferromagnetic L21-Ni2MnAl alloy.

    DOI: 10.1016/j.jmmm.2021.168908

    Scopus

  • Mitsui Y., Onoue M., Kobayashi R., Sato K., Kuzuhara S., Ito W., Takahashi K., Koyama K. .  High Magnetic Field Effects on Cu-precipitation Behavior of Fe-1mass%Cu at 773 K .  ISIJ International62 ( 3 ) 413 - 417   2022年

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    記述言語:日本語   出版者・発行元:ISIJ International  

    Tramp elements in steel, such as Cu and Sn, cannot be removed by acid treatment. Since these elements condense by repeating recycling process, leading to deterioration of strength. Therefore, the methods for avoiding condensation or removing tramp elements are required. In this study, in-magnetic-field annealing process was focused on because magnetic field is effective for diffusion, phase transformation, phase diagram and precipitation. In-magnetic-field annealing of Fe-1mass%Cu at 773 K was performed in 5 and 10 T for investigating precipitation behavior of supersaturated Cu. From microstructural observation, precipitation of Cu-rich phase in Fe-matrix, and magnetic field effect on itself were not observed clearly. Increase of the hyperfine field was detected for the samples annealed at 5 T by Mössbauer spectroscopy, indicating the enhancement of the Cu-precipitation. On the contrary, hyperfine field for 10T-annealed sample was slightly smaller than that for 0 T. Therefore, in-field annealing effect on Cu-precipitation became unclear at 10 T. These magnetic field effects were discussed in the viewpoints of the change of Cu–Fe phase diagram and the atomic diffusion under magnetic field. Difference of the magnetic field effects on precipitation between 5 T and 10 T is explained by the competition between the enhancement of the driving force of the precipitation and suppression of the atomic diffusion. The obtained results indicated that there is optimized magnetic field intensity for controlling Cu-precipitation.

    DOI: 10.2355/isijinternational.ISIJINT-2021-404

    Scopus

  • Watanabe Y., Kobayashi R., Mitsui Y., Umetsu R.Y., Koyama K. .  Magnetic-field-induced selective retardation of reaction in Mn/Ga diffusion couple .  Journal of Alloys and Compounds887   2021年12月

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    記述言語:日本語   出版者・発行元:Journal of Alloys and Compounds  

    In-field heat treatments of a solid-liquid Mn/Ga diffusion couple were conducted at 573 and 673 K with and without a magnetic field of 5 T to investigate magnetic field effects on the phase formation. Formation of the β-Mn, Mn3Ga, γ-MnGa, λ-MnGa, Mn2Ga5, and MnGa4 phases was observed. The obtained results indicated that the reaction and phase growth of λ-MnGa and Mn2Ga5 proceeded by diffusion-controlled process for both heat treatments at 0 and 5 T. We confirmed that a magnetic field suppressed the phase growth of λ-MnGa and Mn2Ga5. The parabolic coefficients of the λ-MnGa and Mn2Ga5 phases were exponentially suppressed by applying magnetic field of 5 T. It is suggested that gain of Zeeman energy of Mn-rich phase played a key role for the magnetic-field-induced suppression of the formation of the λ-MnGa and Mn2Ga5 phases.

    DOI: 10.1016/j.jallcom.2021.161310

    Scopus

  • Nakagawa S., Mitsui Y., Kobayashi R., Umetsu R.Y., Takahashi K., Koyama K. .  Magnetic-field-induced decomposition in Cu<inf>2</inf>MnAl Heusler alloys .  Journal of Magnetism and Magnetic Materials540   2021年12月

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    記述言語:日本語   出版者・発行元:Journal of Magnetism and Magnetic Materials  

    A magnetic field enhanced the decomposition of a Cu2MnAl Heusler alloy. An aging treatment at 573 K under 10 T promoted decomposition of a L21 phase and hard magnetic properties were exhibited. The crystallite size of the L21 phase decreased on annealing over 12 h and approached 22 nm at zero field. However, the crystallite size of the L21 phase of the 5 T-annealed sample was almost half that of the 0-T-annealed sample and decreased rapidly, indicating enhanced grain refinement by in-field annealing. Decomposition of the L21 phase was more pronounced in the 10-T-aged sample. The hysteresis loop for 10-T-aged sample had a large coercive force of 232 mT and a magnetization of 13 Am2/kg. These properties were markedly different from those of the 0-T-annealed sample. An increase of nucleation owing to a decrease of critical radius caused grain refinement associated with decomposition and magnetic hardening of Cu2MnAl.

    DOI: 10.1016/j.jmmm.2021.168411

    Scopus

  • Kobayashi R., Mitsui Y., Umetsu R.Y., Mizuguchi M., Koyama K. .  Synthesis of ferromagnetic ¸-mnalc by reactive sintering .  Materials Transactions62 ( 1 ) 130 - 134   2021年1月

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    記述言語:日本語   出版者・発行元:Materials Transactions  

    Ferromagnetic MnAlC (¸-phase) can be synthesized by a single-route conventional reactive sintering method. The maximum magnetization and coercivity were 75.8 Am2/kg and 57 mT, respectively. The ¸-phase fraction was evaluated to be 81 mass% for Mn55Al45C2 annealed at 1273 K. The ¸-phase of Mn55Al45C2 can be synthesized at an annealing temperature from 873 to 1273 K, whereas that of Mn55Al45 cannot be synthesized. It was indicated that the phase stability of the hcp-phase (¾-phase) was improved by adding carbon, resulting in an ¾¸ transformation.

    DOI: 10.2320/matertrans.MT-M2020166

    Scopus

  • Kobayashi R., Takaki A., Mitsui Y., Umetsu R.Y., Takahashi K., Mizuguchi M., Koyama K. .  Dual acceleration of ¾-¸ transformation in mnal induced by Zn-addition and in-magnetic-field annealing .  Materials Transactions62 ( 1 ) 124 - 129   2021年1月

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    記述言語:日本語   出版者・発行元:Materials Transactions  

    Combined accelerating effects of ¾¸ phase transformation by in-magnetic-field annealing and Zn-addition were investigated for Mn53Zn2Al45 in magnetic fields up to 15 T. The in-field annealing for Zn-added sample accelerated the ¾¸ transformation 16 times faster than Mn55Al45 at annealing temperature of 623 K. The reduction of magnetization after finishing ¾¸ transformation was also more rapid than that of Mn55Al45. However, magnetic-field-induced acceleration of ¾¸ phase transformation and suppression of precipitation of ¢-phase were observed for Zn-added alloy Mn53Zn2Al45 annealed at 573 K. The obtained results suggested that the ¢-phase stabilized against both ¸- and ¾-phases by Zn-addition. Therefore, although the ¾¸ transformation was efficiently accelerated by in-field annealing of Mn53Zn2Al45, more precisely control of annealing time is required for obtaining high fraction of ferromagnetic ¸-phase.

    DOI: 10.2320/matertrans.MT-M2020235

    Scopus

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