IDENTIFYING INTERNAL AND SURFACE CRYSTALLIZATION BY DIFFERENTIAL THERMAL-ANALYSIS FOR THE GLASS-TO-CRYSTAL TRANSFORMATIONS

A differential thermal analysis (DTA) method has been developed that i dentifies and distinguishes surface and internal (bulk) crystallizatio n that occurs during the crystallization of a glass. This method is ra pid, 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 cryst allization peak, (delta T)(p), or the ratio T-p(2)/(Delta T)(p) where T-p is the temperature at (delta T)(p) and (Delta T)(p) is the peak ha lf-width, is determined as a function of size of the glass particles u sed for the DTA measurements. When analyzed by this technique, an as-q uenched lithium disilicate (LS(2)) glass was found to crystallize pred ominantly by surface crystallization. The tendency for surface crystal lization was enhanced when the glass particles were exposed to moistur e prior to DTA. Internal or bulk crystallization dominated over surfac e crystallization when this LS(2) glass was doped with small amounts o f platinum. The DTA curves in the literature for several soda-lime-sil ica glasses as a function of particle size were analyzed by the presen t method. The analysis showed that Na2O.CaO.2SiO(2) and Na2O.2CaO.3SiO (2) glasses crystallized by internal crystallization, but surface crys tallization was the dominant crystallization mechanism for an Na2O.CaO .3SiO(2) glass. These results agree with those obtained from an analys is of the apparent activation energy for crystallization as a function of particle size for these glasses.