Katsuya Kudaka

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.

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.

Some Observation on the Crystal Surfaces of Cubic Boron Nitride

Crystal surfaces of cubic boron nitride synthesized at 55 to 60 k bars an d 1600 to 1800°C and identified by X ray diffraction analysis, has been observed optically. Although cubic boron nitride has similar structure to diamond, it shows somewhat different aspect of crystal growth from diamond. The most dominant crystal face is (111). However it must be emphasized that (110) face prefers to appear more frequently than (100) face in cubic boron nitride in this experimental condition. The crystal growth of cubic boron nitride is by means of plane sheet growth on the (111) face as in diamond and two types of plane sheet growth are observed; one is the growth of various shapes of layers on (111) face and another the growth of triangular pyramids on (111) face.

Preparation of Chromium Monoboride by Reaction Between Chromium and Amorphous Boron in the Presence of Halides.

NaC1,KC1およびKIのハロゲン化物を添加して,アルゴン雰囲気中で金属クロムと非晶質ホウ素粉末間の反応により一ホウ化クロムCrBの合成を行った。得られた結果は次の通りであった。ハロゲン化物を添加することによってCrBの生成開始温度は300℃ 低下した,600～1300℃ の各反応温度におけるCrBの生成割合はハロゲン化物を添加した場合の方が無添加の場合より高い値を示していた。1300℃,2時間の反応で生成した粉末は,無添加の場禽はよく発達した2μm程度の粒子が焼結した,粗大な二次粒子であった。NaC1,KC1添加の場合は微粒子の焼結体として得られた。KI添加の場合は焼結がさらに進み個々の粒子は認められなかった。NaC1を添加して生成したCrBの格子定数値はa=2.964(1)A,b=7.878(1)A,c=2.932(1)A,V=68.5(1)A3,KClを添加して生成したCrBの格子定数はa=2.970(1)A,b=7.871(1)A,c=2.928(1)A,V=68.4(1)A8,KIを添加して生成したCrBの格子定数はα=2.970(1)A,b=7.865(1)A,c=2.929(1)A,V=68.4(1)A8であった。また,ハロゲン化物の添加効果についてはKI>KC1z6gt;NaClの順で,CrBの坐成温度が低温側に移ることがわかった。