Top-down approach using supercritical carbon dioxide ball milling for producing sub-10 nm Bi2Te3 grains

We compare Bi2Te3 powders prepared by conventional ball milling to powders milled in supercritical carbon dioxide (scCO2). We demonstrate that scCO2 milling overcomes size-reduction limitations reported for conventional milling. XRD and TEM reveal nanograins with smaller average sizes (< 10 nm) and narrower grain size distributions in the scCO2 milled case. scCO2 milling also preserves the crystallinity and shows less oxidation than conventional milling. This is the first report of Bi2Te3 with a sub-10 nm grain size whilst conserving high quality crystallinity, made using a top-down approach. Our study offers a route for developing unprecedentedly fine bulk nanostructured Bi2Te3-based thermoelectric materials

金属ホウ化物を用いた金属-ホウ素-水素系複合物質における水素貯蔵特性

広島大学(Hiroshima University)博士(学術)Sciencedoctora

Synthesis, structural characterization, and hydrogen desorption properties of Na[Al(NH2BH3)4]

Na[Al(NH2BH3)4], a mixed-metal amidoborane, was synthesized by ball-milling (solid method) and the chemical reaction in THF (solution method). Solid method has a tendency to remain unreacted NaAlH4 and AB. In the solution method, the partial decomposition of Na[Al(NH2BH3)4] would proceed during mixing in THF. The local structural characterization of as-synthesized material was performed by MAS NMR and FT-IR. While Na[Al(NH2BH3)4] desorbed hydrogen in two steps as reported, the results of structural characterization suggested that the hydrogen desorption in the 2nd step would originate from the AlNBH phase. Effect of hydrogen pressure during ball-milling was also investigated for nNH3BH3NaAlH4 (n = 1, 4) composites. In the case of n = 4, Na[Al(NH2BH3)4] was formed under both Ar and H2 atmosphere. However, in the case of n = 1, Na[Al(NH2BH3)4] was only formed under H2 atmosphere, whereas most of H2 was desorbed during ball-milling under Ar atmosphere. Thus, the hydrogen pressure is necessary for the synthesis in the case of n = 1. Potential energy diagram of AB-NaAlH4 system was described

Surface-Controlled Conversion of Ammonia Borane from Boron Nitride

“One-pot regeneration”, which is simple regneneration method of ammonia borane (AB) using hydrazine and liquid ammonia, enables conversion of AB from hexagonal boron nitride (h-BN) after milling hydrogenation. Solution ^B-NMR revealed the presence of AB after NH_3/N_2H_4 treatment of milled h-BN (BNH_x) although the yield of AB was less than 5%. The conversion mechanism was clarified as B-H bonds on the h-BN surface created by ball-milling under hydrogen pressure have an ability to form AB, which was confirmed by Thermogravimetry-Residual Gas Analysis (TG-RGA) and Infrared (IR) analysis. The reaction routes are also the same as regeneration route of polyborazylene because intermediates of AB such as (B(NH_2)_3 and hydrazine borane were found by solution ^B-NMR after soaking BNH_x in liquid NH_3 and hydrazine, respectively. Because of the fact that all reactions proceed on the h-BN surface and no reaction proceeds when neat h-BN is treated, breaking of B_3N_3 ring structure and then creation of B-H bond is the key issue to increase conversion yield of AB

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

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