Raman Study of Isothermal Devitrificatn Kinetics of NaPO3 Glass

A laser Raman spectroscopic method was developed by which the increase in crystallinity of a devitrifying glass can be monitored continuously and isothermally at the devitrification temperature, without sample quenching, in a molecularly specific manner that yields high-quality kinetic data for the glass-crystal transformation. Applied to the study of vitreous (NaPO₃)_n( gl ), the method yielded continuous data which were fitted, by a nonlinear least-squares method, to an equation of the form α( t ) = 1- exp (- k_Nt^N ) with sufficient precision to permit optimization with respect to both N and k_N. The values of N and k_N vary with glass thermal history, i.e. with variations in time of heating at T_g for otherwise identical samples, although the material does not phase separate. The activation energy from any plot of In ( k_N )^{1/ N} vs (1/ T ) does not vary, but the offset of the curves indicates different preexponential terms in the Arrhenius expression. The results are analyzed in terms of current theories.