Glass transformation phenomena in bulk and film amorphous selenium via DSC heating and cooling scans
- PDF / 581,887 Bytes
- 6 Pages / 593.28 x 841.68 pts Page_size
- 59 Downloads / 165 Views
S. O. Kasap Materials and Devices Laboratories, Department of Electrical Engineering, College of Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N 0W0 (Received 2 June 1989; accepted 8 December 1989) Recently there has been an emphasis on the importance of using cooling scans in DSC experiments in studying the glass transformation kinetics of glasses. The physical interpretation of the apparent activation energy from DSC heating scans has been questioned as not being meaningful. The present paper reports glass transition temperature (Tg) measurements derived from Differential Scanning Calorimetry (DSC) experiments on bulk and film amorphous selenium samples subjected to heating, at constant rate r, and cooling, at constant rate q, scans. Film samples were prepared by thermal evaporation techniques in vacuum. It is shown that for both bulk and film forms of fl-Se, within experimental errors, log(r/Tgm) vs 1/Tgm plot where Tgm is the peak glass transformation temperature, and log(r) vs 1/Tgh plot, where Tgh is the glass transition onset temperature from DSC heating scans, are parallel to the log(g) vs 1/Tgc plot where Tgc is the glass transition temperature from cooling scans. Within the Hutchinson and Kovacs formulation of the glass transformation phenomenon, the results imply that the structural contribution to the mean retardation time, T, is negligible in comparison with the temperature dependent part. The mean structural relaxation time for both bulk and film forms was found to exhibit a typical Vogel-Tammann-Fulcher type of temperature dependence. Furthermore, the structural relaxation rate was observed to be inversely proportional to the viscosity, rj(T), implying that the mean structural retardation time is proportional to the viscosity, T ~ 17. The results also confirm that the earlier studies of glass transformation kinetics in a-Se utilizing only DSC heating scans remain meaningful.
I. INTRODUCTION
Recently Crichton and Moynihan1 emphasized the importance of using cooling rate schedules in studying the kinetics of glass transformation in glasses, and questioned the interpretation obtained from heating schedules, particularly the results of Gutzow and coworkers.2 The doubts of Moynihan and co-workers concentrated on the use of the Ritland equation3 in the analysis of the heating rate (r) dependence of the glass transition temperature, Tgfl. For the simple case of an Arrhenius relaxation time, Macedo and Moynihan4 argued that the apparent activation energy from the semilogarithmic plot of the heating rate versus the reciprocal glass transition temperature can vary as much as 25%, depending on the cooling thermal history. Based on the latter argument, Macedo and Moynihan criticized the thermal analysis of Rasmussen and MacKenzie,5 who employed a Kissinger analysis6 of the heating rate-glass transition temperature to derive the kinetic parameters. J. Mater. Res., Vol. 5, No. 4, Apr 1990
http://journals.cambridge.org
Downloaded: 01 Jan 2015
It is surprising that even in the case
Data Loading...
