Permanent Densification of Calcium Aluminophosphate Glasses

Abstract

High-temperature densification of oxide glasses influences their interatomic distances and bonding patterns, resulting in changes in the mechanical and chemical properties. Most high-pressure investigations have focused on aluminosilicate and aluminoborosilicate based glasses, due to their relevance for the glass industry as well as the geological sciences. Relatively few studies have explored the pressureinduced changes in the structure and properties of phosphate-based glasses, although P 2 O 5 is an important component in various multicomponent oxide glasses of industrial interest. In this work, we investigate the influence of permanent densification on the structure, mechanical properties (Vicker's hardness), and chemical durability (weight loss in water) of binary CaO-P 2 O 5 and ternary CaO-Al 2 O 3-P 2 O 5 glasses. The densification of bulk glasses is obtained through isostatic compression (1-2 GPa) at the glass transition temperature. The binary CaO-P 2 O 5 series is prepared with varying [CaO]/[P 2 O 5 ] ratios to obtain glasses with different O/P ratios, while the ternary series CaO-Al 2 O 3-P 2 O 5 is prepared with a constant O/P ratio of 3 (metaphosphate) but with varying [CaO]/([CaO]+[Al 2 O 3 ]) ratio. Using Raman and 31 P NMR spectroscopy, we observe minor, yet systematic and composition-dependent changes in the phosphate network connectivity upon compression. On the other hand, 27 Al NMR analysis of the compressed CaO-Al 2 O 3-P 2 O 5 glasses highlights an increase in the Al coordination number. We discuss these structural changes in relation to the pressure-induced increase in density, Vicker's hardness, and chemical durability.

Statistical mechanical model for the formation of octahedral silicon in phosphosilicate glasses

“… Note : The values have been obtained when fitting the statistical mechanical model to structural data from literature 25–28,33,39–46 for binary phosphate and silicate glasses. Root‐mean‐square error (RMSE) is provided to compare the quality of the fits.…”

“…Fraction of structural units in binary (A) sodium silicate and (B) sodium phosphate glasses as a function of modifier content. Solid lines model predictions using Equation () and the experimental data from literature are shown as symbols 25–28,41,42 …”

“…We note that parameters and glass compositions for sodium silicate and sodium phosphate glasses have already been established in previous work. 21,24 The model is established based on experimental 29 Si and 31 P magic angle spinning (MAS) NMR spectroscopy data from literature, [25][26][27][28] and does not consider formation of Si 6 units in the binary silicate glasses in agreement with the experimental data (at ambient pressure). We find that the model captures the evolution of structural Q n units in the binary glasses with the modifier concentration very well with the F I G U R E 2 Fraction of structural units in binary (A) sodium silicate and (B) sodium phosphate glasses as a function of modifier content.…”

“…The H i values obtained by fitting the statistical mechanical model to binary oxide glasses have previously been used directly to calculate the structures of ternary borosilicate and aluminoborate glasses. 18,19 As the H i values in Table 1 for silicate glasses are relative to that of the modifier-to-Si 4 interaction and the H i values for the phosphate glasses Note: The values have been obtained when fitting the statistical mechanical model to structural data from literature [25][26][27][28]33,[39][40][41][42][43][44][45][46] for binary phosphate and silicate glasses. Root-mean-square error (RMSE) is provided to compare the quality of the fits.…”

“…We find that the model captures the evolution of structural Q n units in the binary glasses with the modifier concentration very well with the F I G U R E 2 Fraction of structural units in binary (A) sodium silicate and (B) sodium phosphate glasses as a function of modifier content. Solid lines model predictions using Equation (1) and the experimental data from literature are shown as symbols [25][26][27][28]41,42 obtained H i values as reported in Table 1. To obtain relative H i values for the binary phosphate and silicate systems, T g values are required as T g is assumed to be the temperature where the structure stops rearranging.…”