X-Ray Sensing Materials Stability: Influence Of Ambient Storage Temperature On Essential Thermal Properties Of Undoped Vitreous Selenium

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 degrees 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 degrees C, then equilibrated and finally scanned again under a heating schedule. The samples were isothermally stored at temperatures corresponding to 18, 35 and 55 degrees 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 (T-c) and the onset of glass transition temperature (T-g). We also examined the Hruby coefficient (K-gl) of these samples, that is K-gl = (T-c - T-g)/(T-m - T-c) where T-c is the crystallization onset temperature and Tm is the melting onset temperature. We have found K-gl to depend on the storage temperature. Surprisingly, we observed that the Hruby coefficient is actually larger at 35 degrees C compared to the values at 18 and 55 degrees C.