Hot Isostatic Pressing to Increase Thermal Conductivity of Si₃N₄ Ceramics

Highly anisotropic Si₃N₄ ceramics were successfully fabricated by tape-casting of raw α–Si₃N₄ powders with β–Si₃N₄ single-crystal particles as seed particles and Y₂O₃ as an effective sintering aid, followed by hot isostatic pressing at a temperature of 2773 K for 2 h under a nitrogen gas pressure of 200 MPa. The microstructure consists of very large elongated grains (diameter ~10 μm; length of ~200 μm), highly oriented in the tape-casting direction. The thermal conductivity along this direction reaches 155 W m^-1K^-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 β–Si₃N₄ grains with a reduced amount of crystal defects due to the high-temperature firing and to (2) orientation of β–Si₃N₄ grains due to addition of seed particles and to tape-casting.