Improved thermal conductivity of sintered reaction-bonded silicon nitride using a BN/graphite powder bed

Sintered reaction-bonded silicon nitride (SRBSN) with improved thermal conductivity was achieved after the green compact of submicron Si powder containing 4.22 wt% impurity oxygen and Y2O3-MgO additives was nitrided at 1400 °C for 6 h and then post-sintered at 1900 °C for 12 h using a BN/graphite powder bed. During nitridation, the BN/10 wt% C powder bed altered the chemistry of secondary phase by promoting the removal of SiO2, which led to the formation of larger, purer and more elongated Si3N4 grains in RBSN sample. Moreover, it also enhanced the elimination of SiO2 and residual Y2Si3O3N4 secondary phase during post-sintering, and thus induced larger elongated grains, decreased lattice oxygen content and increased Si3N4-Si3N4 contiguity in final SRBSN product. These characteristics enabled SRBSN to obtain significant increase (∼40.7%) in thermal conductivity from 86 to 121 W ∙ m−1 ∙ K−1 without obvious decrease in electrical resistivity after the use of BN/graphite instead of BN as powder bed.