HIGH THERMAL CONDUCTIVITY AND HIGH STRENGTH SINTERED REACTION - BONDED SILICON NITRIDE CERAMICS FABRICATED BY USING LOW GRADE Si POWDER

摘要

Sintered reaction bonded silicon nitrides (SRBSNs) were fabricated from commercially available low grade Si powder containing 1.6 mass % of impurity oxygen and 500 ppm of metallic impurities. Powder compacts of the raw Si powder doped with Y_2O_3 and MgO/MgSiN_2 as sintering additives were nitrided at 1400℃ for 8h under a N2 pressure of 0.1 MPa, followed by post-sintering at 1900C for 6h under a N2 pressure of 0.9 MPa. The SRBSN with Y_2O_3 and MgO as sintering additives had four-point bending strength of about 700 MPa, but lower thermal conductivity of about 89 W/m/K. Thermal conductivity could be improved to over 100 W/m/K without degrading bending strength by replacing MgO with MgSiN*2. It is thought that MgSiNa decreased the amount of oxygen in the liquid phase formed during sintering which resulted in reduced lattice oxygen content in the developed β-Si_3N_1 grains. These good properties were equivalent to those of the SRBSN fabricated by using a reagent grade high purity Si powder containing only 0.3 mass% of impurity oxygen.