Kiyokata Iizumi

Mechanochemical synthesis of MoB2 and Mo2B5

Abstract Ball-milling of elemental powder mixtures without external heat application has been utilized as a process for the mechanochemical synthesis of high melting compounds. Molybdenum and amorphous boron elemental powder mixtures of atomic ratios of 1:2 and 2:5 were comminuted for 10–40 h in a planetary ball-mill. MoB2 was partially formed after 10 h of milling using tungsten carbide balls in both cases. The single phase of MoB2 (hexagonal) was found by XRD after 40 and 30 h of milling of Mo–B (1:2) and Mo–B (2:5), respectively. By annealing the as-milled Mo–B (1:2) mixed powder at temperatures above 1000°C, a mixed phase of MoB2 and Mo2B5−x type (rhombohedral) was formed, whereas by annealing the as-milled Mo–B (2:5) mixed powder at temperatures between 1000 and 1500°C, a single phase of the Mo2B5−x type was obtained. The lattice parameters of the molybdenum borides obtained agreed with previously reported data. Increased contamination of the Mo–B (2:5) mixture compared with the Mo–B (1:2) mixture was found. The mechanochemical reactions were the diffusion-controlled reaction type.

Single-crystal growth and properties of CrB, Cr3B4, Cr2B3 and CrB2 from high-temperature aluminum solutions

Single crystals of CrB, Cr3B4, Cr2B3, and CrB2 were grown from high-temperature Al solutions. The crystals were examined by X-ray diffraction and chemical analysis. The CrB, Cr3B4, and Cr2B3 single crystals were obtained as columnar and thick-platelet crystals having well-developed (010) and (100) faces with maximum dimensions of 3 to 8 mm. CrB2 single crystals 4–8 mm in size were obtained as thick platelets having well-developed (0001) planes or as needles elongated in the 〈0001〉 direction. The Vickers microhardness and electrical resistivity of the crystals were measured, and oxidation at high temperature in air was studied.

Crystal Growth and Properties of AlLiB14

Single crystals of AlLiB14 were grown from a high temperature aluminum solution using Li2B4O7 and amorphous boron as raw materials under an argon atmosphere. The AlLiB14 crystals obtained had well developed (010) or (001) faces, and were reddish black with a metallic luster. The maximum dimensions of AlLiB14 crystals obtained were about 4.6×0.3×0.3 mm3. Magnetic susceptibility at low temperature and Vickers microhardness of the as-grown crystals were measured, and the oxidation resistance in air at high temperatures was studied.

Boron-carbon atomic ratio dependence on the hardness and oxidation resistance of solid solutions of perovskite-type borocarbide YRh3BxC1-x (0 ≤ x ≤ 1)

Solid solutions of the quaternary borocarbide system YRh3BxC1-x have been synthesized and studied from the viewpoints of their microhardness and oxidation resistance. Perovskite-type YRh3B and YRh3C form a continuous solid solution in the range of 0?x?1 with a cubic structure (space group: Pm3m, Z=1). The microhardness of YRh3BxC1-x increases with increasing boron content. The oxidation onset temperature also increases with boron content. Thus, it is indicated that the chemical stability of solid-solution YRh3BxC1-x is strongly dependent on the boron content.

TG-DTA-MS of chromium(III) formate

Abstract The thermal decomposition of chromium(III) formate pentahydrate, Cr3(OH)2(HCO2)7·5H2O, in helium atmosphere and 20% O2 in helium atmosphere has been successfully studied by means of TG-DTA-MS, i.e., thermogravimetry-differential thermal analysis (TG-DTA) coupled with evolved gas analysis (EGA) using mass spectrometry (MS). The TG-DTA-MS is useful to interpret the complicated successive reactions and to determine the mechanism of the thermal decomposition. The thermal process in He atmosphere proceeded by four steps, while in 20% O2–He atmosphere it proceeded by three steps. The decomposition scheme of Cr3(OH)2(HCO2)7·5H2O in He atmosphere was proposed by the following decomposition mechanism via formation of three intermediates: Cr 3 ( OH ) 2 ( HCO 2 ) 7 ·5 H 3 O → Cr 3 ( OH ) 2 ( HCO 2 ) 7 ·2 H 2 O +3 H 2 O Cr 3 ( OH ) 2 ( HCO 2 ) 7 ·2 H 2 O → Cr 3 ( OH ) 2 ( HCO 2 ) 7 +2 H 2 O Cr 3 ( OH ) 2 ( HCO 2 ) 7 → Cr 2 O 3 + Cr ( HCO 2 ) 3 + CO +3 CO 2 +3 H 2 2 Cr ( HCO 2 ) 3 → Cr 2 O 3 +3 CO +3 CO 2 +3 H 2 . After the decomposition, a slight mass loss was observed with a sharp exothermic crystallization of Cr2O3 due to evolution of H2. In 20% O2–He atmosphere, a drastic exothermic mass loss accompanying evolution of CO2, H2O and H2 was explicable by one step combustion of the dehydrated formate, and a small amount of mass loss observed with a crystallization of Cr2O3 was attributed to evolution of O2. The decomposition process without an intermediate was indicated by the temperature increase of the specimen produced by combustion heat.

Crystal growth and some properties of REMn2Si2 (RE=Y, Tb, Dy, Ho)

Single crystals of REMn2Si2 (RE=Y, Tb, Dy, Ho) were grown from a high-temperature lead metal solution in an argon atmosphere. Single crystals of REMn2Si2 were obtained in the form of thin plates with well-developed (001) faces. The as-grown REMn2Si2 crystals were used for measurements of micro-Vickers hardness at room temperature, oxidation resistance in air and magnetic susceptibility at low temperatures. The values of the micro-Vickers hardness for the (001) faces of REMn2Si2 crystals are about 5.5 GPa. The oxidation process of REMn2Si2 crystals was studied at a temperature below 1473 K by TG-DTA analyses. The TG curves show that the oxidation of YMn2Si2, TbMn2Si2, DyMn2Si2 and HoMn2Si2 crystals starts at about 811, 807, 844, and 849 K, respectively. Weight gains of the compounds after TG determination were measured to be in the range of 14.7–19.7 mass%. The results of magnetic susceptibility measurements at low temperatures of the compounds are discussed.

REMn2Si2 (RE=Y, Tb, Dy, Ho) Single Crystals Grown from Lead Flux and Magnetic Properties

Single crystals of REMn2Si2 (RE=Y, Tb, Dy, Ho) were grown from a high-temperature lead metal flux by slowly cooling the melt in an argon atmosphere. The REMn2Si2 crystals were obtained as thin plates with well-developed (001) faces, and the largest crystals have maximum dimensions of about 2.6×2.6×0.02 mm3. The as-grown REMn2Si2 crystals were used for chemical analyses, measurements of unit cell parameters and magnetic susceptibility at low temperatures. The results of magnetic susceptibility measurements of the crystals are discussed.

Sintering and Mechanical Properties of Chromium Boride - Chromium Carbide Composites

Several boride sintered bodies such as TiB2, ZrB2, and SiB6 were previously reported. In the present study, the sinterability and physical properties of chromium boride (CrB2) containing chromium carbide (Cr3C2) sintered bodies were investigated in order to determine its new advanced material. The samples were sintered at desired temperature for 1 hour in vacuum under a pressure by hot pressing. The relative density of sintered bodies was measured by Archimedes’ method. The relative densities of CrB2 addition of 0, 5, 10, 15 and 20 mass % Cr3C2 composites were 92 to 95 %. The Vickers hardness of the CrB2 with 10 and 15 mass % Cr3C2 composites were about 14 and 15 GPa at room temperature, respectively. The Vickers hardness at high temperature of the CrB2 addition of 10 mass % Cr3C2 composite decreased with increasing measurement temperature. The Vickers hardness at 1273 K of the sample was 6 GPa. The Vickers hardness of CrB2 addition of Cr3C2 composites was higher than monolithic CrB2 sintered body. The powder X-ray diffraction analysis detected CrB and B4C phases in CrB2 containing Cr3C2 composites.

Hardness and oxidation resistance of perovskite-type borocarbide system YRh3BxC1−x (0≦x≦1)

Abstract Quaternary borocarbide system YRh 3 B x C 1− x phase have been synthesized by arc melting method and studied for their microhardness and oxidation resistance. Perovskite-type YRh 3 B and YRh 3 C form a continuous solid–solution in the range of 0≦ x ≦l with a cubic structure (space group: Pm 3 m , Z =1). The microhardness of YRh 3 B x C 1− x increases with increasing boron content. Thermogravimetric analysis indicates that the oxidation onset temperature also increases with boron content. Thus, it is indicated that the chemical stability of the perovskite-type borocarbide solid–solution YRh 3 B x C 1− x is strongly dependent on the boron content.

Hardness and oxidation resistance of perovskite-type solid solution of the ScRh3B-ScRh3C system

Perovskite-type ScRh3B and ScRh3C form a continuous solid solution, ScRh3BxC1-x, in the range of 0x1 with cubic structure (space group: Pm3m, Z=1). The microhardness of the ScRh3BxC1-x phase increases with increasing boron content. Thermogravimetric analysis of this phase indicates that the oxidation onset temperature also increases with boron content. Thus, it appears that the mechanical strength and chemical stability of the ScRh3BxC1-x phase strongly depend on its boron content.