X-ray sensing materials stability: influence of ambient storage temperature on essential thermal properties of undoped vitreous selenium

[摘要] Amorphous selenium (a-Se) is currently used in x-ray image detectors as an x-ray photoconductor. Normally a-Se films used in device applications are fabricated by the evaporation of vitreous bulk material loaded into boats in a typical vacuum deposition system. The resistance against crystallization is an important factor in both film and bulk forms of a-Se. Previous work has indicted that the resistance to crystallization is surprisingly more pronounced around 35 °C [1]. In this work we have therefore examined the essential thermal properties of vitreous selenium (99.999%) samples that have been stored at different temperatures. The thermal characterization experiments involved a series of DSC (Differential Scanning Calorimetry) measurements in which have monitored the glass transition and melting endotherms, and the crystallization exotherm in heating-cooling-heating scans. In DSC experiments, a sample would be heated to a temperature above the melting temperature, equilibrated, then cooled at a fixed rate down to 20 °C, then equilibrated and finally scanned again under a heating schedule. The samples were isothermally stored at temperatures corresponding to 18, 35 and 55 °C. The thermal analysis results show that there are distinct differences in the thermal properties. We have examined the stability in terms of the difference in the crystallization onset temperature (Tc) and the onset of glass transition temperature (Tg). We also examined the Hruby coefficient (Kgl) of these samples, that is Kgl (Tc-Tg)/(Tm-Tc) where Tcis the crystallization onset temperature and Tmis the melting onset temperature. We have found Kgl to depend on the storage temperature. Surprisingly, we observed that the Hruby coefficient is actually larger at 35 °C compared to the values at 18 and 55 °C.

[发布机构] Department of Electrical and Computer Engineering, University of Saskatchewan, Saskatoon; SK; S7N 5A9, Canada^1;Analogic Canada Corporation, 4950 Levy Street, Saint-Laurent; QC; H4R 2P1, Canada^2;Canadian Light Source (CLS), University of Saskatchewan, Saskatoon; SK; S7N0X4, Canada^3;Department of Chemical Technology, Plovdiv University Paisii Hilendarski, Plovdiv; 4000, Bulgaria^4

[关键词] Crystallization exotherm;Crystallization onset temperature;Device application;Dsc(differential scanning calorimetry);Storage temperatures;Thermal characterization;X-ray image detectors;X-ray photoconductor