Thermochemical insights into rapid solid-state reaction synthesis of β-sialon

Sialons (Si-Al-O-N) are useful as engineering components because they combine the toughness of silicon nitride with phase stability. Competing reactions occur during conventional processing, making it difficult to form single-phase material without high-temperature/long-time heat treatments. Thermochemical insights into sialon formation are complicated by impure samples, glassy phases, and slow reactions. Here we report the first direct thermochemical measurement of the energetics of a specially synthesized (single-phase, glass-free) β-sialon of composition Si_3.17Al_2.83O_2.83N_5.17 by dissolution calorimetry in molten (Li,Na)BO₂ at 770°C. A remarkable feature of our synthesis by a pulsed electric discharge method is that the unreacted starting powders (aluminum oxide, aluminum nitride, and silicon nitride) were simultaneously reacted and densified to form the single-phase β sialon, all in just 7 min - a hitherto unattained processing record. Thermochemical measurements gave the first direct determination of the standard enthalpy of formation: -67.8 ± 15.5 kJ/mol from the compounds and -2768.3 ± 8.2 kJ/mol from the elements. The thermochemical data suggest new, possibly self-sustaining, reaction paths to refractory nitride materials.