Preparation of nano silicon carbide, silicon nitride and nitrogen doped silicon carbide powders and their microwave permittivities

In order to prepare the high-temperature radar absorbing materials, nano SiC, Si₃N₄ and SiC(N) powders were synthesized from SiH₄-C₂H₂, SiH₄-N₂H₃ and hexamethyldisilazane ((Me₃Si)₂NH) (Me:CH₃), by a laser-induced gas-phase reaction. The particles are spherical, loosely agglomerated, with sizes in a range of 20-30 nm. The laser synthesis reactor consists of two reaction zones, which efficiently increase laser efficiency and production yield. The microwave permittivities of nano SiC, Si₃N₄ and SiC(N) powders suspended in paraffin wax matrixes have been studied at the frequency range of 8.2-12.4 GHz. The dissipation factors of the nano SiC(N) powders were high at the microwave frequencies. The microwave permittivity of the mixture of nano SiC(N) powder and paraffin wax (or other dielectric materials) could be tailored by the content of these nano powders. The real part (ε′) and imaginary part (ε″) of the complex permittivity of the nano SiC(N) powders decrease with frequency increase at the frequency range of 8.2-12.4 GHz. But the dissipation factors (tgδ=ε′/ε′) of the nano SiC and Si₃N₄ powders were almost zero at the frequency range of 8.2-12.4 GHz. It was found that the microwave permittivities of the nano composites could not be predicted using classical effective medium functions. The nano SiC(N) powders could be a good candidate for radar absorbing material. The promising features of nano SiC(N) powders could be attributed the more complex Si, C and N atomic chemical environment than that in a mixture of pure SiC and Si₃N₄ phase, so charged defects and quasi-free electrons move in response to the electric field, diffusion or polarization current resulting from the field propagation. The high ε″ and tgδ of nano SiC(N) powders were attributed to the dielectric relaxation.