Published Papers - NAKAGAWA Tessui

X-ray Absorption Spectroscopic Study on Valence State and Local Atomic Structure of Transition Metal Oxides Doped in MgH2

Nobuko Hanada , Takayuki Ichikawa , Shigehito Isobe , Tessui Nakagawa , Kazuhiko Tokoyoda , Tetsuo Honma , Hironobu Fujii , Yosbitsugu Kojima

JOURNAL OF PHYSICAL CHEMISTRY C ( American Chemical Society )  113 ( 30 ) 13450 - 13455   2009.07 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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A valence state and a local structure of transition metals (Nb, V, and Ti) in MgH2 doped with metal oxides (Nb2O5, V2O5, and TiO2nano) by ball milling were examined by X-ray absorption spectroscopy (XAS). The main edge regions of the Nb, V, and Ti K-edges in the X-ray absorption near edge structure (XANES) profiles are located between 0 and +5 in the oxidation states. Since these spectra coincide with those of NbO, VO, and Ti2O3, respectively, the additives are reduced by MgH2 to the metal oxides, which have lower oxidation states than those of the starting materials. Furthermore, in order to examine the local structures around the transition metal atoms, the extended X-ray absorption fine structure (EXAFS) spectra were analyzed. In the Fourier transformation curves of the EXAFS spectra, all samples doped with the metal oxides show two peaks corresponding to metal−oxygen and metal−metal bonds, being the same as the references of NbO, VO, and Ti2O3. The local structure formed after ball milling or dehydrogenation is close to that of each of the reduced metal oxides (NbO, VO, and Ti2O3) but in a more disarrangement state.

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Simultaneous structure and carrier tuning of dimorphic clathrate Ba8Ga16Sn30

Koichiro Suekuni, Marcos A. Avila, Kazunori Umeo, Hiroshi Fukuoka , Shoji Yamanaka, Tessui Nakagawa, Toshiro Takabatake

PHYSICAL REVIEW B ( American Physical Society )  77 ( 23 ) 1 - 8   2008.06 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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We report structural, transport, and thermal properties of carrier-tuned Ba8Ga16Sn30 single crystals with the type-1 clathrate structure (beta phase), demonstrating that Ba8Ga16Sn30 is a unique thermoelectric clathrate material wherein both the structure type and the carrier type are tunable. The results are compared with the properties of the better known type-8 structure (alpha phase) and of A(8)Ga(16)Ge(30) (A=Sr,Eu). Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of 520 +/- 3 degrees C. Refinements of single-crystal x-ray diffraction data indicate that the Ba(2) guest ion in the tetrakaidecahedron occupies the off-center 24k sites which are 0.43-0.44 angstrom away from the centered 6d site. The temperature-linear coefficient of the specific heat is 29 mJ/mol K-2 for both n- and p-type carriers in the beta phase, four times larger than that for the alpha phase, suggesting contributions from tunneling of the Ba(2) guest ions between off-center minima. Analysis of specific heat with a soft-potential model (SPM) gives a characteristic energy of 20 K for the Ba(2) vibration, significantly lower than 50 K for the alpha phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities kappa(L) for the beta phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in kappa(L)(T) is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the beta phase is significantly larger than that for Sr8Ga16Ge30.

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Thermal analysis on the Li-Mg-B-H systems

Tessui Nakagawa , Takayuki Ichikawa , Nobuko Hanada , Yoshitsugu Kojima , Hironobu Fujii

JOURNAL OF ALLOYS AND COMPOUNDS ( PERGAMON-ELSEVIER SCIENCE LTD )  446 ( SI ) 306 - 309   2007.10 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Thermal analyses of the mixture of MgH2 and LiBH4 doped with TiCl3, and each element MgH2 or LiBH4 doped with TiCl3 were performed under an inert gas flow and 0.5 MPa H2-gas conditions. It was indicated that the hydrogen desorption reaction of MgH2 + 2LiBH4 to MgB2 + 2LiH + 4H2 phases proceeded at temperature above 400 °C under 0.5 MPa hydrogen, whereas, under an inert gas atmosphere, the reaction of the same mixture was transformed into Mg + 2B + 2LiH + 4H2 phases in a temperature range from 350 to 430 °C. Before these reactions, the dehydrogenation of MgH2 and the melting of LiBH4 took place under both hydrogen and the inert gas atmospheres with increasing temperature. In addition, the molten LiBH4 did not decompose into LiH, B and H2 below 450 °C under 1 MPa H2, while the LiBH4 decomposed even below 450 °C under an inert gas atmosphere. From these results, it is deduced that the reaction producing MgB2 is a solid–liquid reaction between solid Mg and liquid LiBH4 above 400 °C without decomposition of molten LiBH4 under a hydrogen atmosphere, while MgH2 dehydrogenate. A characteristic solid–liquid reaction is realized under a hydrogen atmosphere for proceeding of the MgB2 producing reaction in this system.

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Gas emission properties of the MgHx-Zn(BH4)(2) systems

Tessui Nakagawa, Takayuki Ichikawa, Yoshitsugu Kojima, Hironobu Fujii

MATERIALS TRANSACTIONS ( 日本金属学会 )  48 ( 3 ) 556 - 559   2007.03 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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The gas emission properties of MgH2-Zn(BH4)2 and Mg-Zn(BH4)2 systems have been investigated to design a new Metal-B-H system which has superior hydrogen storage properties. Thermal decomposition properties of two-layered samples with MgH2 on Zn(BH4)2, Mg on Zn(BH4)2, the mixture of Mg and Zn(BH4)2 and the mixture of MgH2 and Zn(BH4)2 were investigated by DTA-MS. The experimental results indicated that the B2H6 emission decreases in the case of the two-layered samples and the mixtures when compared with that of Zn(BH4)2, as a result of reaction of the solid MgH2 or Mg with B2H6. Then, the reactions between Mg and Zn(BH4)2 as well as MgH2 and Zn(BH4)2 were exothermic. In the case of the Mg-Zn(BH4)2 mixture, the reaction between Mg and Zn(BH4)2 surprisingly proceeded below 100°C before the decomposition of Zn(BH4)2 occurred. This indicates that the reaction between Mg and Zn(BH4)2 proceeds by a solid-solid reaction.

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Observation of hydrogen absorption/desorption reaction processes in Li-Mg-N-H system by in-situ X-ray diffractmetry

Tessui Nakagawa , Takayuki Ichikawa , Ryo Iida , Haiyan Leng , Nobuhiko Takeichi , Tetsu Kiyobayashi , Hiroyuki Takeshita , Hironobu Fujii

JOURNAL OF ALLOYS AND COMPOUNDS ( PERGAMON-ELSEVIER SCIENCE LTD )  430 ( 1-2 ) 217 - 221   2006.06 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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The in-situ XRD measurements on dehydrogenation/rehydrogenation of the Li–Mg–N–H system were performed in this work. The ballmilled mixture of 8LiH and 3Mg(NH2)2 as a hydrogenated phase gradually changed into Li2NH as a dehydrogenated phase during heat-treatment at 200 °C in vacuum for 50 h. Neither Mg-related phases nor other intermediate phases were recognized in the dehydrogenated phase. With respect to the hydrogenation process, the dehydrogenated state gradually returned to the mixed phase of the LiH and Mg(NH2)2 without appearance of any intermediate phases during heat treatment at 200 °C under 5 MPa H2 for 37 h and during slow cooling down to room temperature through 24 h. In the hydrogenation process at 200 °C under 1 MPa H2, however, the growing up of the LiNH2 and LiH phase was observed in the XRD profiles before the 3Mg(NH2)2 and 8LiH phases were formed as the final hydrogenated state. This indicates that the LiNH2 and LiH phase essentially appears as an intermediate state in the Li–Mg–N–H system composed of 3Mg(NH2)2 and 8LiH.

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Mechanism of hydrogenation reaction in the Li-Mg-N-H system

Haiyan Leng, Takayuki Ichikawa, Satoshi Hino, Tessui Nakagawa, Hironobu Fujii

JOURNAL OF PHYSICAL CHEMISTRY B ( American Chemical Society )  109 ( 21 ) 10744 - 10748   2005.05 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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The Li−Mg−N−H system composed of 3Mg(NH2)2 and 8LiH reversibly desorbs/absorbs ∼7 wt % of H2 at 120−200 °C and transforms into 4Li2NH and Mg3N2 after dehydrogenation. In this work, the mechanism of the hydrogenation reaction from 4Li2NH and Mg3N2 to 8LiH and 3Mg(NH2)2 was investigated in detail. Experimental results indicate that 4Li2NH is first hydrogenated into 4LiH and 4LiNH2. At the next step, 4LiNH2 decomposes into 2Li2NH and 2NH3, and the emitted 2NH3 reacts with (1/2)Mg3N2 and produces the (3/2)Mg(NH2)2 phase, while the produced 2Li2NH is hydrogenated into 2LiH and 2LiNH2 again. Such successive steps continue until all 4Li2NH and Mg3N2 completely transform into 8LiH and 3Mg(NH2)2 by hydrogenation.

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Divalent state in YbGaGe: magnetic, thermal, transport and structural studies

Yuji Muro, Tessui Nakagawa, Kazunori Umeo, Masakazu Itoh, Takashi Suzuki, and Toshiro Takabatake

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN ( 日本物理学会 )  73 ( 6 ) 1450 - 1452   2004.06 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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YbGaGe was reported as the first example of a material with zero thermal expansion that is accomplished by the temperature-variant Yb 2+ /Yb 3+ configuration [Salvador et al. : Nature 425 (2003) 702]. Here we report magnetic, transport, X-ray diffraction and thermal expansion measurements of single-crystal samples. The results reveal a stable divalent state of Yb ions in YbGaGe and a distinct volume-thermal contraction with decreasing temperature.

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