High-pressure Densification of Glass and the Effects of Shear

Abstract

WHEN certain oxide glasses are temporarily compressed at high pressures, a large ‘permanent’ increase of density may occur when the pressure is released. For example, Bridgman and Simon¹ reported that the density of vitreous silica was increased 7 per cent after room temperature compression at 200 kilobars; below an apparent threshold pressure of about 100 kilobars, however, no densification was possible at room temperature. Roy and Cohen² afterwards reported that a density increase of 7 per cent was achieved at only 55 kilobars, and that from 20 to 160 kilobars densification of silica glass at room temperature was a linear function of pressure. Despite the unexplained large disparity between these results, it was proposed² that silica glass be used as a pressure calibrant for the range 10–200 kilobars. More recently, Christiansen, Kistler and Gogarty³ furnished yet another different set of results. These and some results of the present work at room temperature are shown in Fig. 1. The results of Roy and Cohen which were presented as changes of refractive index have been converted to changes of density from a calibration obtained by us. The results of curves A, B, and C were all obtained on the opposed-anvil apparatus. Whereas Roy and Cohen used powdered glass, the samples of the other workers were in the form of thin glass disks. The results reported here were obtained on solid cylinders about 2 mm in diameter and 2 mm in height with a pyro-phyllite-alumina cell in a ‘belt’ apparatus⁴.