Permanent Densification of Calcium Aluminophosphate Glasses

Saurabh Kapoor,Morten M Smedskjaer,Lars R Jensen,Lina Ma,Michal Bockowski,Randall E Youngman,Mathieu Bauchy,Sylwester J Rzoska,Nadja Lönnroth

doi:10.3389/fmats.2019.00063

Abstract

High temperature densification of oxide glasses influences their interatomic distances and bonding patterns, resulting in changes in the mechanical and chemical properties. Most of the 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 pressure-induced changes in structure and properties of phosphate based glasses, although P2O5 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 (Vickers’s hardness), and chemical durability (weight loss in water) of binary CaO-P2O5 and ternary CaO-Al2O3-P2O5 glasses. The densification of bulk glasses is obtained through isostatic compression (1-2 GPa) at the glass transition temperature. The binary CaO-P2O5 series is prepared with varying [CaO]/[P2O5] ratio to obtain glasses with different O/P ratios, while the ternary series CaO-Al2O3-P2O5 is prepared with constant O/P ratio of 3 (metaphosphate) but with varying [CaO]/([CaO]+[Al2O3]) ratio from 0~1. Using Raman and 31P NMR spectroscopy, we observed minor, yet systematic and composition-dependent changes in the phosphate network connectivity upon compression. On the other hand, 27Al NMR analysis of the compressed CaO-Al2O3-P2O5 glasses highlights an increase in the Al coordination number. We discuss these structural changes in relation to the pressure-induced increase in density, Vickers’s hardness, and chemical durability.

Highlights

The application of pressure enables tuning of the interatomic distances and bonding patterns in glassy solids (Kapoor et al, 2017c), providing an additional degree of freedom for altering glass properties compared to varying composition or temperature alone
We investigated the influence of high-temperature densification on the structure, mechanical properties (Vicker’s hardness), and chemical durability of binary CaO-P2O5 and ternary CaO-Al2O3-P2O5 glasses
We found that the changes in P speciation in P2O5-based glasses upon hot compression are generally small and highly composition dependent

Summary

Introduction

The application of pressure enables tuning of the interatomic distances and bonding patterns in glassy solids (Kapoor et al, 2017c), providing an additional degree of freedom for altering glass properties compared to varying composition or temperature alone. Previous studies have reported similar shift in the positions of 31P NMR peaks in alumino-metaphosphate glasses upon Al2O3 addition and have been correlated to general strengthening, or shortening, of the average P–O bonds related with the different metal polyhedra (Smith et al, 2014). Hot compression of the glasses results in insignificant changes in the 31P chemical shifts and relative areas of the peaks corresponding to different Qn phosphorous units (Table 2).

Results

Conclusion