Crystallization and dielectric property of Si-B-C-N ceramics at different annealing temperatures

Polymer derived Si-B-C-N ceramics were prepared from the liquid polyborosilazane. The TG-DSC analysis was carried out to determine the conditions of the crossing, curing and pyrolysis. The as-received Si-B-C-N ceramics were respectively annealed at 1200-1800 oC for 2 hours in a high-purity nitrogen atmosphere. The effects of annealing temperature on the surface morphology and phase composition of Si-B-C-N ceramics were discussed. With the increasing of temperature, the crystallization degree of ceramics increases gradually, the crystallization is more and more adequate, and the grains grow gradually. When temperature is up to 1800 oC, the crystallization of SiC has completed. Owing to the formation of SiC crystals and the increase of grain boundary, the permittivity and dielectric loss of Si-B-C-N ceramics gradually increase as the temperature increases. The reflection coefficient of Si-B-C-N ceramics was calculated in terms of the measured permittivity and permeability. It shows that the reflection coefficient gradually decreases as the annealing temperature increases, which demonstrates that the as-annealed ceramics possess the superior electromagnetic wave absorbing ability compared to as-received ceramics, and the higher annealing temperature is more conducive to the increase of waves-absorbing potential. KEY WORDS Si-B-C-N ceramics; heat treatment; crystallization; dielectric property; electromagnetic wave absorbing ability.