Differential thermal analysis of nucleation and crystal growth rates in glasses

A generalized experimental method that uses differential thermal analysis (DTA) has been developed for determining the nucleation rate (I), the crystal growth rate (U), and the concentration of quenched-in nuclei (N_q) in glasses. The method is applicable even for glasses, whose I and U curves (as a function of temperature) overlap to a considerable degree. The present DTA method yields values for I, U, and N_q that are in excellent agreement with the same values determined by conventional methods.; The effect of the concentration of platinum particles on nucleation was investigated for lithium disilicate glass using the DTA method. The heterogeneous nucleation rate for 440°C, 450°C, and 470°C was measured for different concentrations of platinum particles.; The DTA method was numerically simulated for a model lithium disilicate glass. Exothermic DTA-type curves were observed. Time and temperature dependent nucleation rates for the modeled glass were predicted using the proposed DTA method and compared with the simulated nucleation rates. The quantitative agreement demonstrates the validity of the numerical model.; It was observed that the inclusion of impurities, e.g., platinum particles, to the melt, enhances the nucleation and crystal growth in a glass forming melt. When glass is processed in a container, e.g., a platinum crucible, avoiding the addition of container-induced impurities at high temperatures is difficult. Containerless processing has the advantage of eliminating those container-induced impurities into the melt. To investigate the effect of a high purity of a glassy melt, containerless processing of LS₂ glass was carried out. The results suggest that the glass formation for a containerless melt is four to five times larger than a similar melt in a container due to the suppression of heterogeneous nucleation/crystallization in the melt compared with the melt processed in a containerless melt.