Thermochemistry of Si_(6-z)Al_zN_(8-z) (Z=0 to 3.6) materials

摘要

Enthalpies of formation were determined for#beta#-sialon phases (Si_(6-z)Al_zO_zN_(8-z), z = 0.46 to 3.6)by high-temperature oxidative drop solution calorimetry using analkali-metal borate (52 wtpercent LiBo_2; 48 wtpercent NaBO_2)solvent. Oxygen gas was bubbled through the melt to accelerateoxidation of the oxynitride samples during dissolution. Sialonsnear z = 2 appear less stable energetically than ones with higher orlower nitrogen content. A large configurational entropycontribution for sialons with z> 2 may further stabilize thesematerials. This larger free energy driving form may be the reasonfor success in pulse-activated processing of these materials. Theenthalpies of formation further suggest that a greater driving formfor oxynitride formation exists in batch synthesis using SiO_2rather than Al_2O_3.