The properties and structure of SnCaPOF glasses

Low melting modified calcium stannous fluorophosphate glasses based on a basic composition 40P2O5–25SnO–30SnF2–5CaF2 (in mol%) with glass transition temperature Tg ranging 220–240 °C have been prepared. The effect of Al(OH)3 or SiO2 addition on the properties of these glasses has also been determined. Aluminum and silicon compounds decrease the dissolution rate in water and thermal expansion coefficient while increase the density, Tg and softening temperature. The effect of Al(OH)3 addition on the increase of chemical durability is better for these glasses, due to the partial crystallization effect in stannous calcium silicofluorophosphate glasses. With an addition of 4 wt.% Al(OH)3, the dissolution rate of the glass in 30 °C water decreases from 1.0×10−5 to 1.3×10−7 g cm−2 min. The structure of Al(OH)3 added glasses was studied by 27Al and 31P MAS NMR and Fourier transform IR (FTIR) spectroscopies to explain the relationship between glass properties and composition. The 27Al spectra show three different aluminum environments [Al(OP)6, Al(OP)5 and Al(OP)4], and Al(OP)6 species seem to be predominant in these glasses. The formation of POAl covalent bond and more strongly ionic bonds increase the strength of the glass network in stannous calcium aluminofluorophosphate glasses. FTIR spectra indicate the presence of PF bond in the short range glass structure.