Thermal stability of Na2O-FeO-Fe2O3-P2O5 glasses

The crystallization tendencies of five series of Na2O-FeO-Fe2O3-P2O5 (NFP) glasses with different O/P (3.0-3.5) and Fe/P (0.13-0.67) ratios were studied. Characteristic temperatures, including the glass transition temperature (Tg) and crystallization temperature (Txh), were obtained using differential thermal analysis (DTA), and liquidus temperatures (TL) were determined by microscopic evaluation of heat-treated samples. The compositional dependence of glass structure and the characteristic temperatures are discussed. The glass stability (GS) against crystallization is described using parameters based on the characteristic temperatures. For the glass series with O/P ratios near the pyrophosphate composition (O/P ~ 3.5), the glass stability goes through a minimum value around Fe/P ~ 0.3 to 0.4, which corresponds to the crystalline phase NaFeP2O7 and Na7Fe3(P2O7)4.