Relaxation and glass transition in an isostatically compressed diopside glass

Relaxation of isostatically compressed CaMgSi2O6 (diopside) glass is explored by ex situ differential scanning calorimetric (DSC) experiments. A diopside melt was compressed above its glass transition temperature under isostatic conditions at a pressure of 500 MPa. DSC analysis was performed at ambient pressure after slowly cooling the compressed melt under pressure (i.e., after freezing-in the densified state). Compression-induced enhancement of the overshoot in heat capacity was observed in the glass transition region. This indicates that a densified, quenched glass possesses a lower apparent fictive temperature, TfA, than a glass that was cooled under ambient pressure at the same cooling rate. However, a thermodynamic analysis indicates that the fictive temperature produced under pressure, Tf0, is actually much higher than that determined from DSC experiments at ambient pressure (Tf0>TfA).