Temperature-dependent free energy of crystallization and viscosity of glassy materials

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 1994-05, Vol.179, p.186-188
• Main Authors: Pratap, Arun, Prasad, Anamika, Saxena, N.S, Saksena, M.P
• Format: Article
• Language: English
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/0921-5093(94)90190-2

Elementary thermodynamic procedures have been used to calculate the Gibbs free energy difference ΔG from a knowledge of the specific heat difference ΔC p between the undercooled melt and solid phase. However, the metastability of the liquid makes the experimental determination of ΔC p very difficult over a wide range of temperatures. In the case of glass-forming materials, ΔG can be best measured between the melting temperature T f and the glass transition temperature T g. Several investigators have attempted to propose an appropriate expression for ΔG in terms of more easily measured parameters. The present work incorporates a new expression for the difference in Gibbs free energy proposed in the power law form ΔG = ΔS m ( ΔT) x . This relation contains a relatively more physical parameter x and is able to predict non-linearity between ΔG and ΔT and hence the deviation from the Turnbull formula. The calculated values using this approach agree fairly well with experimental results in liquids as well as binary and ternary glasses over the temperature range of interest. The temperature dependence of the viscosity can also be arrived at from the C p data up to the glass transition temperature. The variation in viscosity above T g has been very well accounted for by two different proposed expressions in the case of O-terphenyl and Na 0.32K 0.68 glassy systems.