Kinetic Process of Crystal Formation

The phase sequence and solid-state reactions involving the formation of different phases within a glass-ceramic can be determined by thermal analysis. Differential thermal analysis (DTA) and DSC are particularly useful methods for studying these processes. Sestak (1996) determined phenomenological kinetics based on DTA and Donald (1998) analyzed the crystallization process of iron-containing glasses using the same technique. 

Ray and Day (1990) used DTA investigations of the crystallization peak as a rapid method to determine the temperature for maximum nucleation of glasses to develop glass-ceramics. The height of the exothermic signal of the crystallization of lithium disilicate, Li2Si205, was measured at different temperatures for a constant time of 3 h. The results clearly showed a maximum of the crystallization peak with a sample heat treated at 453 3 C for nucleation of the base glass. Therefore, the temperature of 453 C was determined as the temperature for maximum nucleation. 

The DTA method is also very usefiil to distinguish between surface and volume crystallization. Ray et al. (1996) demonstrated in different types of glasses that the height of the exothermic peak decreased with increasing particle size when surface crystallization is the dominant mechanism. But on the other hand, the peak intensity will increase with increasing particle size when volume crystallization of glasses is dominant. 

Furthermore, Ray and Day (1997) determined the influence of heterogeneous nucleation on the crystallization rate of a Na20 2Ca0 3Si02 glass in comparison to an undoped glass. Platinum (0.1 wt%) was used as a 

In comparison to the undoped base glass, the Pt-doped glass showed a shifting of the DTA crystallization peak to lower temperatures (Fig. 3-24) than the undoped glass. This result is a very good indication that heterogeneous nucleation (in this case Pt) increases the crystal growth rate 

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