Effect of Boron on Polytype Phase Transformation Occurring During Sintering of Si-C-B Powders Obtained by Combustion (SH
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ABSTRACT Silicon carbide powders were synthesized by SHS method with boron added in various quantities as either amorphous elemental or boric acid additive. The powder samples were then heat treated at a selected temperature (up to 2200QC). Crystallographic structure of the powders were analyzed by means of X-ray powder diffraction technique. It was found that boron added in the form of H3BO 3 tends to precipitate in the form of small clusters and does not dissolve evenly in the SiC lattice. Upon annealing the boron clusters dissociate. The boron atoms diffusing across the SiC lattice generate a large number of stacking faults with simultaneous transformation of hexagonal polytype 6H (a-phase) present in as-synthesized powders into the cubic 3C (frphase) modification. INTRODUCTION Silicon carbide ceramic materials have a number of commercial applications. However, not all potential properties of such ceramics have been explored so far. Also, the exact mechanism of SiC sintering process is not well understood yet. As a part of our continuous studies on silicon carbide ceramics we focused our attention on some crystallographic aspects of pressureless hightemperature densification of boron-doped SiC powders. Silicon carbide is a prominent polytypic material. Its hcp lattice is built of Si-C double layers that may be stacked in different sequences. As a result, a number of crystallographic modifications, polytypes having different lattice period along hexagonal axis exist. In addition, structures with no periodicity along the hexagonal axis abound. A 3-layer period structure of silicon carbide has a cubic symmetry, while all other polytypes can be described as having trigonal lattice with either hexagonal or rhombohedral symmetry. All polytypes are known to be stable under ambient conditions but may transform into other polytypes when subjected to a pressure and/or thermal treatment. From thermodynamic point of view the cubic modification of SiC, 3C, is metastable [1] and is expected to transform upon annealing into one of the trigonal polytypes. On the other hand trigonal polytypes, although thermodynamically stable under ambient conditions, may transform back into cubic modification under some combination of pressure, temperature, and doping conditions [2]. Silicon carbide polycrystals are usually classified as either a- or fr-type, P3being a purely cubic phase and a being a mixture of trigonal lattice modifications. Pressureless sintering of SiC always produces a modification independent on the structure of the starting powder material. Since the process of the synthesis of silicon carbide produces a mixture of different crystallographic modifications of the crystalline grains, sintering has to lead to a considerable reconstruction of the crystal lattice. In order to investigate correlation between polytypic transformations and the process of sintering, we conducted a detailed study of the structural transformations of SiC powders annealed in the presence of boron, a commonly used additive in SiC sintering. EXPERIME
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