Hafnium in peralkaline and peraluminous boro-aluminosilicate glass and glass sub-components: a solubility study

A relationship between the solubility of hafnia (HfO2) and host glass composition was investigated by determining the solubility limits of HfO2 in peralkaline and peraluminous glasses in the system SiO2–Al2O3–B2O3–Na2O in air at 1450 °C. The only Hf-bearing phase to crystallize in the boro-aluminosilicate melts is hafnia. Some peraluminous boro-aluminosilicate glasses also crystallize a B-containing mullite. In glasses with molar Na2O > Al2O3, the solubility of hafnia is linearly and positively correlated with Na2O/(Na2O + Al2O3) or Na2O − Al2O3 (excess sodium), despite the presence of 5–16 mol% B2O3. The results of this solubility study are comparable to other studies of high-valence cation solubility in B-free silicate melts. For peralkaline B-bearing glasses, it is shown that, although the solubility limits are higher, the solution behavior of hafnia is the same as in B-free silicate melts previously studied by other workers. By comparison, it is shown that in peraluminous melts there must be a different solution mechanism for hafnia, different than that for peralkaline sodium-boro-aluminosilicate glasses in this study and different than that for B-free silicate melts studied by others. The formation of moieties in the peralkaline glasses/melts, with approximate stoichiometries of Na4HfO4, Na6Hf2O7 or Na2HfO3 (most likely with associated Si and B), appears to be responsible for the incorporation of large amounts of Hf in the glass matrix, and therefore high HfO2 solubility.