Pressure induced nucleation in a Li2O•2SiO2 glass

The concentration of nuclei (N-v) in a Li2O . 2SiO(2) (LS2) glass was measured using a newly developed differential thermal analysis technique, after densifying the glass at 1, 3, or 6 GPa pressure at 400degreesC for 20 min. N-v increased from 38 x 10(12) m(-1) for an as-melted glass to 82 x 10(12) m(-3) for a glass subjected to a pressure of I GPa, and remained nearly constant with further increase of pressure of up to 6 GPa. This increase in N-v is equivalent to nucleating this LS2 glass at the temperature (455degreesC)of its maximum nucleation rate for 2 h. The average effective activation energy for crystallization for the densified glasses, 292 +/- 10 kJ/mol, was also the same as that of the (undensified) glass nucleated at 455degreesC for 2 It, but was significantly smaller than that (358 +/- 10 kJ/mol) for an as-melted (undensified and unnucleated) glass. The present results show that applying an external pressure on this LS2 glass shifts its nucleation curve to lower temperatures causing nuclei to form even at 400degreesC. A decrease in viscosity at all temperatures with the application of pressure is suspected to be a reason for the enhanced nucleation observed for this LS2 glass. (C) 2002 Published by Elsevier Science B.V.