Identifying internal and surface crystallization by differential thermal analysis for the glass-to-crystal transformations

Abstract

A differential thermal analysis (DTA) method has been developed that identifies and distinguishes surface and internal (bulk) crystallization that occurs during the crystallization of a glass. This method is rapid, convenient and requires only a few (about 6–8) DTA experiments to identify the dominant crystallization mechanism (bulk vs. surface) in the glass. In this method, either the maximum height of the DTA crystallization peak, ( δT) p, or the ratio T 2 p (ΔT) p where T p is the temperature at ( δT) p and ( ΔT) p is the peak half-width, is determined as a function of size of the glass particles used for the DTA measurements. When analyzed by this technique, an as-quenched lithium disilicate (LS 2) glass was found to crystallize predominantly by surface crystallization. The tendency for surface crystallization was enhanced when the glass particles were exposed to moisture prior to DTA. Internal or bulk crystallization dominated over surface crystallization when this LS 2 glass was doped with small amounts of platinum. The DTA curves in the literature for several soda-lime-silica glasses as a function of particle size were analyzed by the present method. The analysis showed that Na 2O.CaO.2SiO 2 and Na 2O.2CaO.3SiO 2 glasses crystallized by internal crystallization, but surface crystallization was the dominant crystallization mechanism for an Na 2O.CaO.3SiO 2 glass. These results agree with those obtained from an analysis of the apparent activation energy for crystallization as a function of particle size for these glasses.