Niobate in Silicate and Phosphate Glasses: Effect of Glass Basicity on Crucible Dissolution

Abstract

Using niobium crucibles for melting phosphate and silicate glasses of various modifier oxide contents, and therefore varying optical basicity (Λ), was found to result in varying dissolution rates of niobate during melting. Because of their high electronic polarizability, even small concentrations of niobates are detectable in the Raman spectra of glasses. Even <1 mol% Nb2O5 can be identified, as independently confirmed by SEM-EDX analysis. Silica-rich glasses (~60% SiO2, Λ ~0.6) did not show significant Nb dissolution from the crucible, while higher basicity metasilicate glasses (~50% SiO2, Λ ~0.65) and pyrophosphate glasses (~30% P2O5, Λ ~0.7) did show the typical niobate signature in the Raman spectra at 810–840 cm−1, depending on composition. While niobium is well-dissolved throughout the pyrophosphate glass, metasilicate glasses showed a much more intense Raman signature of niobate units near the outer surface of the glass. Measurements along the cross-section of a fractured metasilicate glass showed a steady decrease of the strength of the niobate signature from the surface toward the bulk of the material. Besides correlation with optical basicity, the tendency of melts to dissolve Nb crucible was discussed in terms of the connectivity or polymerization of the network and the corresponding melt viscosity.