Fabrication of high thermal-conductive silicon nitride ceramics with low dielectric loss

In order to develop high thermal-conductive silicon nitrides with low dielectric loss, three silicon nitrides with different compositions were pressurelessly sintered with Yb2O3 and SiO2 as sintering additives and the effect of both composition and annealing on the microwave dielectric properties were studied. When the Yb2O3/SiO2 ratio in the starting powder increased from 0.33 to 1.3, the thermal conductivity of the samples increased from 46 W m−1 K−1 to ∼100 W m−1 K−1, while the tan δ of the sample at 2 GHz decreased significantly from 11.5 × 10−4 to 1.4 × 10−4. The crystallization of the intergranular glassy phases in the as-sintered specimens was observed for all the samples after thermal treatments at 1300 °C for 24 h. However, the changes in tan δ were different depending on the compositions; the tan δ of the sample with the lowest Yb2O3/SiO2 ratio reduced to half, that doubled for the sample with the intermediate Yb2O3/SiO2 ratio and that remained almost constant in the case of the highest Yb2O3/SiO2 ratio. These results suggested that the tan δ of the glassy phases as well as the crystalline phases depend greatly on the composition and that the tan δ of the glassy phases of a certain composition could be lower than that of the crystalline phases. It was revealed that selecting the proper composition for silicon nitrides could combine both high thermal conductivity and low dielectric loss even though the intergranular glassy phase was not completely crystallized.