Effect of compression temperature on the structure and properties of hot compressed sodium borosilicate glasses from molecular dynamics simulations

Applying pressure during glass formation or annealing is an effective method to modify the properties of oxide glasses. The effect of compression temperature on the structure and properties of a sodium borosilicate glass has been investigated using molecular dynamics simulations. The connection between macroscopic properties (density and elastic moduli) and microscopic structures (short- and medium-range order) was investigated. Young's modulus increased with the compression temperature, and then reached a plateau at temperatures close to the glass transition temperature. The modulus increase was found to correlate linearly with glass density change. Correlations with structural changes, such as fraction of four-fold coordinated boron, oxygen triclusters, sodium ion coordination number, and the Na-O and Na-Na distances were studied. The results suggest that the pressure-induced transformation from three-fold to four-fold coordinated boron was achieved through the formation of oxygen triclusters, which is different from the commonly observed mechanism involving nonbridging oxygen during the boron coordination change. In addition, the linear correlation between sodium coordination number and glass density suggests that change of sodium ion local environment and redistribution of sodium ions also play an important role in the property behaviour during pressure quenching of the glasses.