DETERMINATION OF ACTIVATION-ENERGIES FOR THE CRYSTALLIZATION OF A CORDIERITE-TYPE GLASS

The determination of apparent activation energies for the crystallizat ion of a P2O5-modified cordierite-type glass was performed utilizing s tatic experiments as well as a dynamic test method. In static experime nts, bulk glass specimens were isothermally annealed and the crystal g rowth velocity was determined by measuring the thickness of the crysta l layer. A modified technique utilizes 'thermal markings' induced by q uick temperature changes. In these experiments, an apparent activation energy of 469 kJ mol-1 is obtained. Dynamic tests were performed on p owdered glass specimens using DTA with different heating rates. The ac tivation energies are calculated using the modified Ozawa and modified Kissinger equation. For the widely used calculation with the peak max imum temperature, both models result in activation energies around 300 kJ mol-1. Introducing the peak start temperature, activation energies in the range of 470-500 kJ mol-1 are obtained, which are in good agre ement with the static experiments. The differences are discussed in te rms of the microstructural processes during crystallization.