Influences of pre-forming on preparation of SiC by microwave heating

Abstract

Silicon carbide (SiC) crystals were synthesized by microwave sintering using coal and tetraethoxysilane (TEOS) as raw materials. A sol-gel method was carried out to coat coal mineral particles with silicon dioxide (SiO2). The mixed raw powders were pre-formed by uniaxial pressing into cylindrical pellets in dimension of ~ 30 × 3 mm2. The pre-forming pressure was selected at 0 MPa, 1 MPa, 2 MPa, 3 MPa, 4 MPa and 5 MPa respectively, which led to different apparent density of the green pellets. The influence of apparent density of green pellets on microwave heating behavior was investigated. Different microwave thermal effects were analyzed. Techniques of XRD、SEM were carried out to characterize samples. It was found that pre-forming pressure showed crucial influences on microwave thermal effects and electric field (E-field) intensification. No SiC crystal could be formed without pre-forming pressure. Pre-forming pressure might be the prerequisite for synthesis of SiC by microwave heating. Five consecutive and indispensable heating stages including accumulation of residual air, microwave plasma generation, complex chemical reactions, nucleation and grain growth of SiC crystallites could be distinguished for samples under pre-forming pressure. Different pre-forming pressure leads to changes in heating behavior as well as morphologies of SiC crystals. ~ 4 MPa might be the optimized pre-forming pressure for both microwave plasma effects and E-field intensification.