Hot isostatic pressing to increase thermal conductivity of Si3N4 ceramics

Highly anisotropic Si3N4 ceramics were successfully fabricated by tape-cast ing of raw alpha-Si3N4 powders with beta-Si3N4 single-crystal particles as seed particles and Y2O3 as an effective sintering aid, followed by hot isos tatic pressing at a temperature of 2773 K for 2 h under a nitrogen gas pres sure of 200 MPa. The microstructure consists of very large elongated grains (diameter similar to 10 mu m; length of similar to 200 mu m), highly orien ted in the tape-casting direction. The thermal conductivity along this dire ction reaches 155 W m(-1) K-1 at room temperature, but varies significantly between room temperature and 1273 K. This thermal conductivity is closely related to (1) formation of extremely large elongated beta -Si3N4 grains wi th a reduced amount of crystal defects due to the high-temperature firing a nd to (2) orientation of beta-Si3N3 grains due to addition of seed particle s and to tape-casting.