NaPO3-AlF3 GLASSES: FLUORINE EVAPORATION DURING MELTING AND THE RESULTING VARIATIONS IN STRUCTURE AND PROPERTIES

Four glass series with identical nominal composition of (100-x) NaPO3-xAlF3 (NAPF), with x= 0 to 40 mol%, were prepared by melting under different conditions and are compared here for their compositional and structural variations. Melting these glasses at different temperatures, with or without a crucible lid, with or without an additional fluorinating agent (NH4. HF2 or NH4F) and with different quality of the raw materials (OH content) was found to determine fluoride retention. In addition, melting in alumina crucibles can change the glass composition through Al2O3 uptake from the crucible, which does not happen to melts prepared in Pt crucibles. Glasses of the differently prepared series were investigated for their actual composition and in regard to structural variations using Raman and also solid-state NMR spectroscopy. The glass density increases for all series in a similar manner with increasing fraction of AlF3. The glass transition temperatures Tg vary by up to 100 C between nominally similar glasses prepared under different conditions. A higher Tg is generally observed for higher aluminum content and lower fluoride levels. Thus, glasses melted in Al2O3 crucibles have the highest F-loss and showed the highest Tg values. The effect of the melting temperature is reflected in the number of PF bonds which break up in NAPF glasses above 850 C, while Al-F bonds are more stable.

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Recommended citation: D. Möncke, M.C.B. Neto, H. Bradtmüller, G.B. de Souza, A.M. Rodrigues, H.S. Elkholy, H.A. Othman, B.J.A. Moulton, E.I. Kamitsos, A.C.M. Rodrigues, D. Ehrt, “NaPO3-AlF3 GLASSES: FLUORINE EVAPORATION DURING MELTING AND THE RESULTING VARIATIONS IN STRUCTURE AND PROPERTIES”, Journal of Chemical Technology and Metallurgy, 2018, 53(6), 1047−1060.