Fumio Kawamura

Synthesis of rhenium nitride crystals with MoS2 structure

Rhenium nitride (ReN2) crystals were synthesized from a metathesis reaction between ReCl5 and Li3N under high pressure. The reaction was well controlled by the addition of a large amount of NaCl as reaction inhibitor to prevent a violent exothermic reaction. The largest rhenium nitride crystals obtained had a millimeter-order size with a platelet shape. X-ray diffraction analysis revealed that rhenium nitride has MoS2 structure similar to hexagonal rhenium diboride (ReB2) which has recently been investigated as an ultra-hard material. The structure was different from any structures previously predicted for ReN2 by theoretical calculations.

Stability of Amino Acids and Their Oligomerization Under High-Pressure Conditions: Implications for Prebiotic Chemistry

The polymerization of amino acids leading to the formation of peptides and proteins is a significant problem for the origin of life. This problem stems from the instability of amino acids and the difficulty of their oligomerization in aqueous environments, such as seafloor hydrothermal systems. We investigated the stability of amino acids and their oligomerization reactions under high-temperature (180-400°C) and high-pressure (1.0-5.5u2009GPa) conditions, based on the hypothesis that the polymerization of amino acids occurred in marine sediments during diagenesis and metamorphism, at convergent margins on early Earth. Our results show that the amino acids glycine and alanine are stabilized by high pressure. Oligomers up to pentamers were formed, which has never been reported for alanine in the absence of a catalyst. The yields of peptides at a given temperature and reaction time were higher under higher-pressure conditions. Elemental, infrared, and isotopic analyses of the reaction products indicated that deamination is a key degradation process for amino acids and peptides under high-pressure conditions. A possible NH(3)-rich environment in marine sediments on early Earth may have further stabilized amino acids and peptides by inhibiting their deamination.

High-pressure synthesis and compressive behavior of tantalum nitrides

WC- and NaCl-type tantalum mononitrides and hexagonal Ta5N6 were prepared at high pressure and temperature, and their compressive behaviors were examined using in situ high-pressure X-ray diffraction. Comparison of the formula volumes of the tantalum mononitrides indicated that the NaCl type was the densest phase. The P–V data showed that the WC-type structure had the highest bulk modulus value (K0u2009=u2009351(1) GPa). An analysis of the compression properties in terms of the crystallographic characteristics of the structures indicated that a prismatic polyhedral array with face-sharing connectivity was responsible for the incompressible nature of these tantalum nitrides.

Growth of Bulk Nitrides from a Na Flux

The history of research and development of the Na flux method for GaN single crystal growth from its discovery in 1994 until now are summarized. Today this method becomes one of the important growth measures of high quality GaN crystals. The development of the Na flux method starting from clarification of the growth mechanism, then, showing a method of controlling nucleation by adding carbon, reporting a high quality crystallization technique by using solution stirring, and, finally, latest data on the point seeding are described.

Conduction-band effective mass and bandgap of ZnSnN 2 earth-abundant solar absorber

Pseudo III-V nitride ZnSnN2 is an earth-abundant semiconductor with a high optical absorption coefficient in the solar spectrum. Its bandgap can be tuned by controlling the cation sublattice disorder. Thus, it is a potential candidate for photovoltaic absorber materials. However, its important basic properties such as the intrinsic bandgap and effective mass have not yet been quantitatively determined. This paper presents a detailed optical absorption analysis of disordered ZnSnN2 degenerately doped with oxygen (ZnSnN2−xOx) in the ultraviolet to infrared region to determine the conduction-band effective mass (mc*) and intrinsic bandgap (Eg). ZnSnN2−xOx epilayers are n-type degenerate semiconductors, which exhibit clear free-electron absorption in the infrared region. By analysing the free-electron absorption using the Drude model, mc* was determined to be (0.37u2009±u20090.05)m0 (m0 denotes the free electron mass). The fundamental absorption edge in the visible to ultraviolet region shows a blue shift with increasing electron density. The analysis of the blue shift in the framework of the Burstein-Moss effect gives the Eg value of 0.94u2009±u20090.02u2009eV. We believe that the findings of this study will provide important information to establish this material as a photovoltaic absorber.

Synthesis of ammonia using sodium melt

Research into inexpensive ammonia synthesis has increased recently because ammonia can be used as a hydrogen carrier or as a next generation fuel which does not emit CO2. Furthermore, improving the efficiency of ammonia synthesis is necessary, because current synthesis methods emit significant amounts of CO2. To achieve these goals, catalysts that can effectively reduce the synthesis temperature and pressure, relative to those required in the Haber-Bosch process, are required. Although several catalysts and novel ammonia synthesis methods have been developed previously, expensive materials or low conversion efficiency have prevented the displacement of the Haber-Bosch process. Herein, we present novel ammonia synthesis route using a Na-melt as a catalyst. Using this route, ammonia can be synthesized using a simple process in which H2-N2 mixed gas passes through the Na-melt at 500–590u2009°C under atmospheric pressure. Nitrogen molecules dissociated by reaction with sodium then react with hydrogen, resulting in the formation of ammonia. Because of the high catalytic efficiency and low-cost of this molten-Na catalyst, it provides new opportunities for the inexpensive synthesis of ammonia and the utilization of ammonia as an energy carrier and next generation fuel.

Synthesis of cubic-GaN nanoparticles using the Na flux method: A novel use for the ultra-high pressure apparatus

Nano-scale cubic-GaN particles were successfully synthesized using the Na flux method under about 500 atm with a belt-type ultra-high pressure apparatus. High pressure nitrogen gas of about 500 atm was sealed in the ultra-high pressure apparatus, which enabled the dissolution of pressurized nitrogen gas into a Ga-Na melt at 500°C without a compressor. In contrast, the conventional Na flux method is carried out under a pressure of 150 atm, the maximum pressure of a nitrogen gas cylinder. A characteristic feature of the process used herein is that the high-pressure reaction gas is dissolved into a flux within the ultra-high pressure apparatus. The c-GaN nanoparticles obtained by this method show excellent crystallinity and a low mixing ratio of hexagonal-GaN, and thus the method solves two common problems in the synthesis of c-GaN.