Abstract:
The influence of strongly reducing conditions of glass melting on phase transformations in titanium-containing glasses of the zinc aluminosilicate system has been investigated. The glass was melted with the addition of aluminium powder as a reducing agent. The glass-ceramics were obtained by heat treatments at temperatures ranging from 720$^\circ$C to 1350$^\circ$C. The processes of liquid phase separation and crystallization occurring in this glass during its secondary heat treatment were studied using structurally sensitive methods. The results are compared with the data for materials obtained under oxidizing conditions. It is shown for the first time that the sequence of phase transformations in glasses melted under strong reducing and oxidizing conditions is similar, but reducing conditions accelerate the gahnite crystallization and slow down the rutile and cristobalite crystallization, which indicates the entry of Ti$^{3+}$ ions into the liquid phase separated zinc alumotitanate and zinc aluminate amorphous regions, as well as into the residual glass at the initial stages of phase decomposition. Broadband absorption of glass-ceramics in the visible and near-IR regions of the spectrum is caused by Ti$^{3+}$ ions and Ti$^{3+}$–Ti$^{4+}$ pairs in gahnite, rutile crystals and in the residual glass. Broadband luminescence of glass-ceramics was found to be due to Ti$^{3+}$ ions and Ti$^{3+}$–Ti$^{4+}$ pairs in gahnite nanocrystals rather than impurity Cr$^{3+}$ ions, which was observed in glass melted under oxidizing conditions.
Keywords:glass-ceramics, red-ox conditions of glass melting, gahnite, rutile, absorption, luminescence.
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