Effect of Grain Size on the Thermal Conductivity of Si ₃ N ₄

Polycrystalline Si 3 N 4 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°C and subsequent HIP sintering at 2400°C. The HIP treatment of hot‐pressed Si 3 N 4 resulted in the formation of a large amount of ß‐Si 3 N 4 grains ∼10 µm in diameter and ∼50 µm long, and the elimination of smaller matrix grains. The room‐temperature thermal conductivities of the HIPed Si 3 N 4 materials were 80 and 102 Wm −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 Wm −1 K −1 ). The calculated phonon mean free path of sintered Si 3 N 4 was ∼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 Si 3 N 4 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.