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α-SiC particles were used as heating seeds to prepare SiC from coal minerals with microwave heating method. Coating technique was carried out to prepare composite raw powders in three different methods. Heating temperatures were at 1000 °C, 1050 °C, 1100 °C, 1150 °C, 1200 °C for 10min, respectively. XRD, SEM techniques were carried out to characterize samples. It was found that different distribution between C and α-SiC particles from different mixing method leads to different microwave heating behavior and growth mechanisms. V-V reaction leads to in situ nucleation and grain growth on the surface of α-SiC seeds which contact with C particles. Well-grown β-SiC particulates appear. Hybrid V-V reaction V-L reaction lead to local microwave plasma and diffusion-precipitation with a very thin layer of SiO2 between raw C particles and α-SiC seeds. Local fine β-SiC whiskers and particulates on the surface of α-SiC seeds co-exist. Primary V-V reaction leads to nucleation and grain growth along reacted C-SiO2 interface with very thick layer of SiO2 between raw C and α-SiC seeds. Substantial β-SiC whiskers appear. Transformation from β-SiC to α-SiC on the surface of as-formed whiskers will be enhanced by microwave plasma at high temperature.

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.

Chain-like zircona (ZrO2) nanofibers were prepared by microwave sintering without any surfactants or solid templates. Microwave sintering was conducted in a multimode microwave cavity with TE666 resonant mode at 2.45 GHz. Carbon particles were used to activate unique thermal processes when mixed with ZrO2 precursor. The sintering condition was at 1300°C for 10 min. Samples were characterized by XRD, SEM, TEM techniques. It was found that both monolithic and tetragonal ZrO2 co-existed in samples prepared fromthe mixture of ZrO2 precursors and carbon by either microwave or conventional sintering. Only m-ZrO2 exists in samples prepared by ZrO2 precursors without carbon. ZrO2 appeared as chain-like nanofibers, which might be attributed to a so-called carbon-induced self-assembly growth mechanism.

β-SiC powders were synthesized by microwave sintering at 1400-1850°C for 60 minutes with coal minerals. Raw materials were prepared with a sol-gel method by coating SiO2 on coal mineral particles, and microwave sintering was conducted in a microwave chamber with TE666 resonant mode. Heating rates were controlled by input microwave power and heating behavior during the microwave sintering was also investigated. XRD, SEM, Raman, TEM techniques were carried out to characterize samples. It was found that different particle size of coal mineral particles gave rise to different heating effect, leading to different growth mechanisms of SiC crystals. This might be the unique features for microwave sintering. Particular SiC crystals dominated for small coal mineral particles, which attributed to microwave plasma effect. While SiC whiskers or fibers dominated for large coal mineral particles, which corresponded to microwave coupling effect. Microwave coupling effect prevailed for low heating rate which was apt to form particular SiC crystals. Whereas, higher heating rate enhanced microwave plasma and primarily resulted in SiC whiskers or fibers.

Silicon carbide (SiC) whiskers with different morphologies are fabricated by microwave heating of SiO2-coated coal mineral particles at different temperatures and different holding times in an oxygen-containing environment. The atomic diffusion processes and growth mechanism of the SiC whiskers are simulated. It is found that a closed capsule of SiO2 appears during microwave heating, within which the SiC whiskers are formed. SiC crystals can be prepared at 1100 °C for 10 min. The optimized synthesis condition is approximately 1100 °C for 20 min. Higher temperatures or/and holding times lead to the re-oxidation of the SiC crystals. A layer of amorphous SiO2 wraps around the SiC whisker surface and generates coated composites at all temperatures. Crystallite knots are observed embedding on the SiC whiskers at 1300 °C due to the surface cleaning and activating effects of microwave plasma. The knots are smoothed at 1500 °C due to local atomic diffusion and grain growth motivated by the microwave coupling effect. The variations in the microwave plasma and the coupling effect at different heating stages also give rise to unique growth phenomena. For the sample synthesized at 1100 °C for 20 min, the high permittivity values present in the SiC whiskers lead to the excellent EM absorption properties at high frequency.