Glass-forming region in the Ca–Si–O–N system using CaH2 as Ca source

The present work explores the glass-forming region in the Ca–Si–O–N system, particularly in the N-rich part of the system. Bulk oxynitride glasses in the Ca–Si–O–N system were prepared by melting mixtures of CaH2, SiO2 and Si3N4 powders in a nitrogen atmosphere at 1500–1650 °C. The glasses were characterized by X-ray powder diffraction, differential thermal analysis, scanning electron microscopy and transmission electron microscopy. Glass compositions were calculated from analyses obtained by energy dispersive X-ray (EDX) spectroscopy and combustion analysis, for cation and anion compositions, respectively. The glasses were found to be X-ray amorphous and gray to black in color. The glasses retain up to 58 e/o of nitrogen and 42 e/o of calcium. The glass formation depends on reaction kinetics and the precursor used. A strong exothermic reaction is observed at 850–1000 °C, leading to formation of amorphous and crystalline oxynitride phases that melt at high temperatures upon further heating. Glass transition temperatures (Tg) were observed to vary between 798 °C and 1050 °C, and crystallization occurs typically 130 °C above the glass transition temperature. The glass densities vary between 2.79 g/cm3 and 3.25 g/cm3.