Rare Earth Oxides on Properties of Pressureless Sintered Si3N4 Ceramics

High thermal conductivity Si3N4 ceramic is a prospective substrate material for high-power electronic devices. In this paper, pressureless and liquid-phase sintering was proposed using Re2O3 (Re=Sm, Er, Lu) - TiO2 as sintering additives to effectively reduce the cost for applications. Effect of the additive type and content on microstructure, mechanical properties and thermal conductivity of the ceramic were investigated. Results showed that the relative density, thermal conductivity and grain size of Si3N4 decrease gradually with the increase of Re ionic (Re3+) radius. With addition of Sm2O3, the highest density can only reach 3.14 g/cm(3), while the fracture toughness about 5.76 MPa.m(1/2) can be obtained when 8wt% Sm2O3-TiO2 is used. With 12wt% Lu2O3-TiO2 as sintering aid, Si3N4 ceramics show high density of 3.28 g/cm(3) as well as high fracture toughness, while the thermal conductivity is only 42 W/(m.K) due to the presence of large amount of second phase. Thermal conductivity of Si3N4 reaches 51.8 W/(m.K) with the addition of 8wt% Er2O3-TiO2, which can meet the requirement for substrate materials for power electronic device.