Zdrojový dokument:Scientific papers of the University of Pardubice. Series A, Faculty of Chemical Technology. 17/2011
ISSN:1211-5541
Abstrakt:
Structural relaxation kinetics and crystallization kinetics of chosen compositions
from the Se-Te glassy system were studied under non-isothermal conditions by
using differential scanning calorimetry in dependence on particle size. The purpose
of this contribution is to demonstrate the extent of information accessible by the
present-day kinetic analysis provided by the differential scanning calorimetry and
to suggest its importance and merit for the development of new high-tech PCM
materials. Enthalpic relaxation was described on the basis of
Tool–Narayanaswamy–Moynihan model. Single set of TNM parameters was
obtained from the curve-fitting procedure for each studied glass. A comparison
with our previous measurements on a-Se was made and the development of
particular TNM parameters with increasing Te content was discussed in terms of
changes in molecular structure of the material. The curve-fitting results were
further verified by several independent non-fitting methods. The crystallization
kinetics was described in terms of the nucleation-growth Johnson–Mehl–Avrami
model. Complexity of the crystallization process was in this case represented by
very closely overlapping consecutive competing surface and bulk nucleation-growth mechanisms. Mutual interactions of both mechanisms as well as all other
observed effects were explained in terms of thermal gradients, surface
crystallization centres arising from the sample preparation treatments and
changing amount of volume nuclei originating from the combination of prenucleation
period and the very glass preparation phase. Advanced error analysis
was performed for each step of the kinetic study. A new criterion for quick
determination of the dominating crystallization mechanism — surface or bulk —
was introduced.