Effect of sintering additive composition on the thermal conductivity of silicon nitride

The thermal conductivity of silicon nitride prepared with varying sintering additive compositions was studied. Samples of Si₃N₄ + 0.5 mol% Y₂O₃ + 0.5 mol% Nd₂O₃ and a further additional agent were gas pressure sintered at 2173 K. MgO or Al₂O₃ was employed as the additional agent. While both agents improved sinterability, the former promoted grain growth and the latter suppressed it. Thermal conductivity increased with increasing MgO content, and a maximum value of 128 Wm^-1 K^-1 was attained when 2 mol% MgO was added. In contrast, addition of Al₂O₃ degrades thermal conductivity. This is probably due to the suppression of grain growth and the dissolution of Al₂O₃ into Si₃N₄ grains.