Thermal conductivity of sintered silicon nitride has been improved by the alignment of large β-Si3N4 grains. Beta-Si3N4 containing 0.5mol% Y2O3 and 0.5mol% Nd2O3 was seeded with rodlike β-Si3N4 grains and tape-cast to align the seed grains. In-situ composite microstructure of the aligned large grains in smaller matrix grains was developed by the grain growth of seed grains during gas-pressure sintering at 2200°C for 4h. Room-temperature thermal conductivity was anisotropic: 137W·m-1·K-1 in the direction of tape-casting, 97W·m-1·K-1 of seet width, and 72W·m-1·K-1 in the direction of seet stack. The thermal conductivity in the direction parallel to the tape-casting direction was high because of the aligned large grains.
