Effect of calcium additive on structural changes under heat treatment in sodium borosilicate glasses

The effect of calcium additive on structural change and phase separation behavior in response to heat treatment in 9.4Na2O–25.4B2O3–65.2SiO2–xCaO (mol%) glass, where x was from 0 to 15 mol%, was investigated using 11B NMR, 29Si MAS NMR and SEM. With increasing CaO content, the fraction of four-coordinated boron showed a dramatic increase up to x = 2% but only slight increase at above x = 5%. Interestingly, four-coordinated boron showed no significant increase in response to heat treatment at below x = 2 mol%, but a clear increase at above x = 5 mol%. These changes can be explained by a model in which calcium ions are confined to borate units in samples with low calcium content, but begin to form Si–O−Ca2+–O−–Si or Si–O−Ca2+–O−–B units in samples with high calcium content, which also agrees with the results of 29Si MAS NMR spectra. Here we discuss the relationship between the reduced size of the phase-separation structure in samples with calcium added and the formation of Si–O−Ca2+–O−–B units.