Thermochemistry of Si_6–zAl_zO_zN_8–z(z = 0 to 3.6) materials

Enthalpies of formation were determined for β-sialon phases (Si_6–zAl_zO_zN_8–z, z = 0.46 to 3.6) by high-temperature oxidative drop solution calorimetry using an alkali-metal borate (52 wt% LiBo₂; 48 wt% NaBO₂) solvent. Oxygen gas was bubbled through the melt to accelerate oxidation of the oxynitride samples during dissolution. Sialons near z = 2 appear less stable energetically than ones with higher or lower nitrogen content. A large configurational entropy contribution for sialons with z > 2 may further stabilize these materials. This larger free energy driving form may be the reason for success in pulse-activated processing of these materials. The enthalpies of formation further suggest that a greater driving form for oxynitride formation exists in batch synthesis using SiO₂ rather than Al₂O₃.