Abstract: The nano Si/C/N composite powders were synthesized from hexamethyldisilazane ((Me₃Si)₂NH) (Me:CH₃) by a laser-induced gas-phase reaction. The powders
are spherical, loosely agglomerate with sizes in range of 20～30nm. The laser synthesis reactor consists of two reaction zones, which can efficiently increase
laser efficiency and production yield. The microwave permittivities of nano Si/C/N composite powders suspended in different matrixes were studied at the frequency range of
8.2～12.4GHz. The ε’ and ε’’ of the nano Si/C/N composite powder decrease with frequency at the frequency range of 8.2～12.4GHz.
The difference being the microwave resonance is not sharply peaked but rather smeared out over a large frequency range. The dissipation factors tgδ(ε"/ε’)
of the nano Si/C/N composite powders are high at the microwave frequencies. The SiC microcrystallines in the nano composite powders can dissolve a great deal of nitrogen,
so charged defects and quasi-free electrons move in response to the electric field, diffusion or polarization current resulted from the field propagation.
The high ε’’ and tgδ of nano Si/C/N composite powders are due to the dielectric relaxation. The unusual ε’, ε’’ and tgδ of the nano Si/C/N composite powders suspended
in different matrixes are attributed to the interface effects between nano Si/C/N composite powders and matrixes.

Key words: microwave permittivity, nano Si/C/N composite powder, microstructure, interface effect