Experimental tests of the classical nucleation theory for glasses

Abstract

New data for crystal nucleation rates and viscosities were obtained for Li 2O·2SiO 2 and BaO·2SiO 2 glasses. Special efforts were made to minimise impurities in the glasses. Good general agreement with previously published nucleation results was found. The new and previous results were used to test the applicability of classical nucleation theory. For both lithium and barium disilicates the temperature dependence of nucleation rates was satisfactorily described by theory, ln ( Iη/ T) vs. 1/ ΔG 2 T plots yielding straight lines over a wide range of temperatures ( ΔG is the bulk free energy difference per mole between crystal and liquid at temperature T). However, for lithium disilicate, where experimental ΔG data were available, the experimentally determined preexponential factor A in the nucleation equation was much higher than the theoretical value, in agreement with previous studies. This was the case even allowing for reasonable variations in ΔG. A similar result was obtained for barium disilicate using calculated ΔG values. Possible reasons for the discrepancy in A between theory and experiment are discussed including transient nucleation, experimental errors in the nucleation rates and heterogeneous nucleation. None of these effects can account for the observed discrepancy. Other possible explanations are considered.