Effect of Rare-Earth Additives on Thermal Conductivity, Mechanical and Electrical Properties of Silicon Nitride Ceramics

Silicon nitride ceramics of high thermal conductivity is a promising candidate for application as ceramic IC substrates. The thermal conductivity of Si3N4 can be changed greatly by the use of different sintering additives. In this work, different systems of additives were used to sinter the silicon nitride ceramics, including Y2O3-MgO, Y2O3-CaO, CeO2-MgO, CeO2-CaO, La2O3-MgO and La2O3-CaO, and their effects on the thermal conductivity and the mechanical and electrical properties of the sintered Si3N4 ceramics were studied. The ceramics were sintered by the spark plasma sintering (SPS) method at 1500 degrees C for 5 min which was then heat treated at 1900 degrees C for 3 h under N-2 atmosphere. Results showed that the conductivities of the ceramics sintered from the rare-earth additives tended to decrease with the increase of the cation radius in the rare-earth oxides. Different from the use of MgO, the additives of CaO did not generate the desired rod-like grains of silicon nitride which thus led to a lower thermal conductivity and a lower strength of the ceramics, although it gave a higher ceramic hardness. In contrast, the Y2O3-MgO additive system, under the optimized sintering conditions, provided a higher thermal conductivity of 80 W//m.K, with the bend strength larger than 1000 MPa. Meanwhile, the bulk resistivity was higher than 10(13)Omega.m, the dielectric constant was lower than 10 and the dielectric dissipation fraction below 3x10(-3).