Results 1 - 10 of 93
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[en] Pure VB_2 powders with a particle size less than 300 nm were synthesized by a novel borothermal reduction of NH_4VO_3 in Ar atmosphere at 900 - 1 000 C with or without the presence of NaCl/KCl. Appropriate amounts of NaCl/KCl addition not only accelerated the reaction rate but also inhibited the grain growth of VB_2 particles.
[en] Research highlights: → A comparative study on the preparation of various V-B compounds (including V3B2, VB, V5B6, V3B4, V2B3, and VB2) was firstly conducted by the SHS process. → Sustainability of solid state combustion depended strongly on the starting stoichiometry and preheating temperature of the reactant compact. → Variations of the combustion temperature and flame-front velocity with sample stoichiometry were presented. → The phase composition of the final products was examined and the associated activation energy was determined. - Abstract: An experimental study on the preparation of various vanadium borides (including V3B2, VB, V5B6, V3B4, V2B3, and VB2) in the V-B system was conducted by self-propagating high-temperature synthesis (SHS) from the elemental powder compacts. Close flame-front velocities and comparable combustion temperatures were observed for the reactant compacts with V:B = 1:1, 5:6, and 3:4, and they were much higher than those for the samples of other compositions. The lowest combustion velocity and temperature were detected in the sample of V:B = 1:2. According to the XRD analysis, pure VB was produced not only from the sample of V:B = 1:1, but also from those of V:B = 5:6 and 3:4. For the powder compact with V:B = 1:2, VB2 was dominantly formed along with trivial amounts of VB and V3B4. A multiphase product composed of VB and V3B4 at comparable quantities was yielded from the sample of V:B = 2:3. The only V-rich sample formulated at V:B = 3:2 generated monoboride VB and left the end product a large amount of unreacted vanadium.
[en] Interaction in the systems SiC-Me5B2 (Me-V, Nb, Ta) which ae quasibinary cross sections in the relevent, quaternary systems Si-B-C-Me5, was studied. High-temperature treatment of the samples with subsequent X-ray diffraction and metallographic analysis was carried out
[en] Defect structures of VB2 and NbB2 single crystals grown by the floating zone process were examined by chemical etching and X-ray topography. In both diborides the etched sections of single crystals had numerous etch pits along the <11-20> directions in the (0001) plane, and etch pits and line patterns along the  direction in the [10-10] and [11-20] planes respectively. Etch pit arrays and line patterns along the <1-210> and <-2113> directions were observed in the [10-11] planes of NbB2 single crystals. Lang X-ray topographs of thin sections of both diborides showed pronounced dislocation images along the  direction. It is concluded from contrast experiments that these dislocations have a pure edge character and that their Burgers vectors lie in the (0001) plane. (Auth.)
[en] The chemical stability of boride thin films with aluminum is investigated. Only two dioborides, VB2 and HfB2 have a positive heat of reaction which makes them potential candidates for thermodynamically stable diffusion barriers between Al and Si. Thin films of BV2, and ZrB2 for comparison, prepared by rf sputtering of composite targets were chosen for this study. Multilayer samples of these borides and aluminum were investigated by differential scanning calorimetry to determine if according to calculations, a reaction between Al and the borides takes place, and to measure the heat of reaction. The authors find that an exothermic chemical reaction occurs between ZrB2 and Al and that an exothermic crystallization reaction takes place in the VB2 and Al sample. The reaction products were determined using x-ray diffraction
[en] Results of investigation of interaction in the systems GdB6-M5B2 (M-V,Nb,Ta) are presented. According to metallographic and X-ray diffraction analyses, determination of the melt starting temperature, microhardness of separate phases, parameters of unit cells, the systems GdB6-M5B2 (M5-V,Nb,Ta) are quasibinary and can be described by eutectic type phase diagrams
[en] The interaction of B4C with diborides of group 5 metals is investigated in a wide temperature and concentration range. The paper presents quasi-binary eutectic state diagrams in which the components are practically insoluble in one another. The composition of eutectics and its melting temperature for each system are as follows: 46-48% VB2 and Teut=2170±30 deg C; 35-37% NbB2 and Teut=2250±30 deg C; 31-33% TaB2 and Teut=2370±30 deg C, respectively