Effect of grain size on the thermal conductivity of Si3N4

Polycrystalline Si3N4 samples with different grain-size distributions and a nearly constant volume content of grain-boundary phase (6.3 vol%) were fabricated by hot-pressing at 1800 degrees C and subsequent HIP sintering at 2400 degrees C. The HIP treatment of hot-pressed Si3N4 resulted in the formation of a large amount of beta-Si3N4 grains similar to 10 mu m in diameter and similar to 50 mu m long, and the elimination of smaller matrix grains. The room-temperature thermal conductivities of the HIPed Si3N4 materials were 80 and 102 W m(-1) K-1, respectively, in the directions parallel and perpendicular to the hot-pressing axis. These values are slightly higher than those obtained for hot-pressed samples (78 and 93 W m(-1) K-1). The calculated phonon mean free path of sintered Si3N4 was similar to 20 nm at room temperature, which is very small as compared to the grain size. Experimental observations and theoretical calculations showed that the thermal conductivity of Si3N4 at room temperature is independent of grain size, but is controlled by the internal defect structure of the grains such as point defects and dislocations.