Surface and Internal Crystallization in Glasses as Determined by Differential Thermal Analysis

A differential thermal analysis (DTA) method has been developed that identifies and distinguishes surface and internal (volume) crystallization in glasses. This method is fast, convenient, and requires only a small quantity of sample, ∼500 mg, to identify the dominant crystallization, surface versus internal, 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 the peak half width, is plotted as a function of particle size. The composition of the glasses that have been investigated in the present work include (in mol%) 33.3BaO 66.7SiO 2 (BS 2 ), 20Na 2 O‐80TeO 2 (NT 4 ), 22.2PbOll.lNb 2 O 5 66.7TeO 2 (PNT), 66.7PbO10.0Bi 2 O 3 ‐23.3Ga 2 O 3 (PBG), and xLi 2 O(100 ‐ x)SiO 2 (lithium silicate, x = 33.3, 35, 37, and 40 mol%). Both (δT)p and T 2 p /(ΔT) p decrease with increasing particle size when surface crystallization is the dominant mechanism and increase when internal crystallization becomes predominant. The surface and internal crystallization have been identified by crystallizing the glasses at temperatures, as determined by DTA, and then examining the microstructure by scanning electron microscopy. The temperature at the crystallization peak maximum, Tp, shows no dependence on the crystallization mechanism (surface versus internal) and increases with increasing particle size for all the preceding glasses.