SURFACE AND INTERNAL CRYSTALLIZATION IN CLASSES AS DETERMINED BY DIFFERENTIAL THERMAL-ANALYSIS

A differential thermal analysis (DTA) method has been developed that i dentifies and distinguishes surface and internal (volume) crystallizat ion in glasses, This method is fast, convenient, and requires only a s mall quantity of sample, similar to 500 mg, to identify the dominant c rystallization, surface versus internal, in the glass, In this method, either the maximum height of the DTA crystallization 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) the peak half width, is plotted as a func tion of particle size, The composition of the glasses that have been i nvestigated in the present work include (in mol%) 33.3BaO . 66.7SiO(2) (BS2), 20Na(2)O . 80TeO(2) (NT4), 22.2PbO . 11.1Nb(2)O(5) . 66.7TeO(2 ) (PNT), 66.7PbO . 10.0Bi(2)O(3) . 23.3Ga(2)O(3) (PBG), and xLi(2)O .( 100 - x)SiO2 (lithium silicate, x = 33.3, 35, 37, and 40 mol%), Both ( delta T)(p) and T-p(2)/(Delta T)(p) decrease with increasing particle size when surface crystallization is the dominant mechanism and increa se when internal crystallization becomes predominant, The surface and internal crystallization have been identified by crystallizing the gla sses at temperatures, as determined by DTA, and then examining the mic rostructure by scanning electron microscopy. The temperature at the cr ystallization peak maximum, T-p, shows no dependence on the crystalliz ation mechanism (surface versus internal) and increases with increasin g particle size for all the preceding glasses.