Structure and mechanical properties of aluminosilicate glasses

Grammes, Thilo GND

The effect of phosphate on the mechanical properties of aluminosilicate glasses so far has barely been studied. Yet, phosphate incorporation bears potential for changing the polymerisation of aluminosilicate glasses and thus their properties. This thesis presents the first detailed mechanical analysis of phospho-aluminosilicate glasses that includes elastic properties. The studied compositions comprised metaluminous glasses from the system SiO2 - Al2O3 - Na2O - P2O5 with 0 to 7.5 mol% P2O5 and 50 to 70 mol% SiO2. The glass hardness and elastic properties were assessed by several techniques of indentation and sound speed measurement and were found to decrease with increasing P2O5 content. Changes of these properties with increasing SiO2 content were less expressed and could be explained by either density changes or polymerisation. Additionally, the densification upon indentation was studied, as well as crack resistance and strain rate sensitivity. Furthermore, phosphate was found to decrease the glass transition temperature and to impede crystallisation. To tailor glass properties, the structure-property relationships need to be understood. This thesis includes a structural analysis by combined infrared and Raman spectroscopy. A far-infrared analysis of the sodium signal indicated a competition between aluminate and phosphate groups for charge-balancing sodium. Also, a correlation was found between shifts in infrared and Raman spectra and the degree of ionic bonding, represented by the theoretical optical basicity. In summary, the mechanical properties and the structure-property relationships of metaluminous phospho-aluminosilicate glasses were characterised and the analysis of the degree of ionic bonding and of the role of sodium provided new structural insights.

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