XRD Investigation of SHS-Produced Boron Carbide

Boron carbide with compositions corresponding to the homogeneity region in its phase diagram was obtained by self-propagating high temperature synthesis (SHS). The received samples were characterized by X-ray diffraction (XRD) analysis, lattice parameters and half-width of reflections have been investigated. The lattice parameters of boron carbide reported in the literature exhibit a too wide spread in their values for equal carbon content. It is unusually for covalent compounds such as boron carbide which has strong chemical bonds. The largest spread in lattice parameters was observed at 13.2 % (atomic) of carbon in boron carbide. In other hand, for SHS-produced samples the spread was minimal. Using XRD analysis cell prameters of boron carbide in our experiments was found to depend on carbon concentration in non-linear way. Lattice parameter was found to reach the unusually high value of 12.31 Å. The half-widths of boron carbide diffraction lines were found to depend on carbon concentration and reach their maximum values at 13.2 atomic % of carbon when the lattice is most disordered. The structure analysis allows to associate it with the process of crystal structure ordering caused by replacement of boron atoms by carbon ones during formation of boron carbide structure. The carbon atoms can be incorporated into different positions in both the linear groups and the icosahedra. In other words, some certain composition can correspond to different structure. Therefore, both experimental data and crystal-chemical considerations allow to conclude the possibility of different kinds of ordering in boron carbide structure, resulting in instability of lattice parameters and consequently in properties

Гетерофазная керамика в системе Hf–Si–Mo–B, полученная сочетанием методов СВС и горячего прессования

The paper focuses on obtaining heterophase powder ceramics and consolidated ceramics based on borides and silicides of hafnium and molybdenum by combining the methods of self-propagating high-temperature synthesis (SHS) and hot pressing (HP). Composite ceramic SHS powders HfB2–HfSi2–MoSi2 were obtained according to the scheme of magnesium-thermal reduction from oxide raw materials where the combustion wave is characterized by temperatures of 1750–2119 K and high mass combustion rates of 8,4– 9,3 g/s. The structure of synthesized SHS powders consists of relatively large MoSi2 grains up to 10 μm in size and submicron elongated HfB2 grains located mainly inside the MoSi2 grains and rounded Si precipitates. The composition with a lower concentration of boron contains a large number of polyhedral HfSi2 grains with a size of less than 10 μm. The resulting powders are characterized by an average particle size of ~6 μm with a maximum size up to 26 μm. Phase compositions of ceramics consolidated by the HP method and SHS synthesized powders are identical. The microstructure of compact samples consists of faceted HfB2 elongated grains 0,5– 10,0 μm in size, polyhedral HfSi2 and MoSi2 grains up to 8–10 μm in size and silicon interlayers. Consolidated ceramics has a high structural and chemical homogeneity, low residual porosity of 1,1–1,7 %, high hardness of 11,7–12,6 GPa and thermal conductivity of 62–87 W/(m·K).Работа посвящена получению гетерофазной порошковой и консолидированной керамики на основе боридов и силицидов гафния и молибдена путем комбинирования методов самораспространяющегося высокотемпературного синтеза (СВС) и горячего прессования (ГП). Композиционные керамические СВС-порошки HfB2–HfSi2–MoSi2 получали по схеме магнийтермического восстановления из оксидного сырья, при котором волна горения характеризуется температурами 1750–2119 К и довольно высокими значениями массовых скоростей горения 8,4–9,3 г/с. Структура синтезированных СВС-порошков состоит из относительно крупных зерен MoSi2 размером до 10 мкм и субмикронных вытянутых зерен HfB2, преимущественно расположенных внутри зерен MoSi2, а также округлых выделений Si. Состав с меньшей концентрацией бора содержит большое количество полиэдрических зерен HfSi2 размером менее 10 мкм. Полученные порошки характеризуются средним размером частиц ~6 мкм при максимальном размере до 26 мкм. Фазовые составы консолидированной методом ГП керамики и синтезированных СВС-порошков идентичны. Микроструктура компактных образцов состоит из ограненных вытянутых зерен HfB2 размером 0,5–10,0 мкм, полиэдрических зерен HfSi2 и MoSi2 размером до 8–10 мкм и прослоек кремния. Консолидированная керамика обладает высокой структурной и химической однородностью, низкой остаточной пористостью 1,1–1,7 %, высокой твердостью 11,7–12,6 ГПа и теплопроводностью 62–87 Вт/(м·К)