Effect of crystallization of intergranular glassy phases on the dielectric properties of silicon nitride ceramics

Effect of annealing on dielectric properties of Si3N4 ceramics at 2 GHz was studied using two kinds of Si3N4 samples which were sintered with different amounts of Yb2O3 and SiO2 as sintering additives. The dielectric loss (tan δ) of the sample containing glassy phase was ∼20 × 10−4, which was significantly higher than that of the sample with crystalline secondary phase. The tan δ of the former sample was reduced significantly by the annealing at 1300 °C for 24 h. The decrease in tan δ due to the annealing was associated with the crystallization of glassy phase, indicating that the contribution of glassy phase to the tan δ was substantially larger than those of crystalline phases. By contrast, dielectric constant was independent on the composition and was not affected by the annealing. It was revealed that tan δ of sintered Si3N4 could be lowered by eliminating the glassy phase.