Influence of Crystal Phase on Dielectric Properties of Silicon Nitride Ceramics

The influences of crystal phase on dielectric properties of silicon nitride ceramics produced by pressureless sintering with MgO-Al2O3-SiO2(MAS) as sintering additive were investigated. The samples with different crystal phase were obtained at different sintering temperatures. The microstructure of the materials sintered at 1850 degrees C consists of elongated grains, with almost identical size and aspect ratio, which distribute uniformly throughout the body. The difference in dielectric properties of Si3N ceramics was mainly due to the difference of the relative content of alpha-Si3N4, beta-Si3N4, the intermediate product (Si2N2O) and glass phase in the samples. Compared with alpha-Si3N4 and Si2N2O, beta-Si3N4 is believed to be a major factor influencing the dielectric constant. Al and 0 atoms can exist in beta-Si3N4 and form a solid solution. Because of the different valence between Si, N and Al, 0, there will occur dangling bonds and unpaired electrons. The free charges would move in response to the electric field, and diffusion current results from the field propagation. The high-dielectric constant of beta-Si3N4 could be attributed to the ionic relaxation polarization.