Reduction processes of amorphous GeO2 by hydrogen

A study of the reactivity of amorphous materials compared with that of crystalline materials would appear worthwhile, but little attention has been paid to this problem. In an earlier study, Baba [1] found a difference in the kinetic behaviour between amorphous and hexagonal GeO2 in the hydrogen reduction process. He suggested that the difference is mainly due to differences in surface conditions, i.e. the number of active sites which play a part in the formation of nuclei of the product and their growth. We have observed the same reaction with special reference to the initial process at temperatures of 450 to 530°C and at hydrogen pressures below 300 torr. The e (extent of reaction) against t (time) curves for crystalline GeO2 were sigmoidal, which is commonly encountered in solid-state reactions, whereas the curves for amorphous materials showed a complicated behaviour. This behaviour appeared to be explicable by glass transition and crystallization of the reactant during the reaction. As far as we know, such anomalous behaviour of amorphous material has not yet been reported. Kinetic measurements were carried out isothermally in a static system using a vacuum electrobalance (Cahn RG-2000). The amorphous sample was prepared as described previously [2]. The glass transition temperature (Tg) was found to be in the range 530 to 550 ° C, according to thermal analysis. It was also confirmed that crystallization occurs after prolonged heating above 700 ° C. Prior to use, about 100rag powdered sample was treated for 1 h with atmospheric dry air at a temperature below Tg and then evacuated at reaction temperature for 30min under a vacuum of 10 -5 torr. In some cases, after the reaction was stopped at known extent of reaction, changes in the surface area and structure of the partially reduced sample were observed by the BET adsorption of nitrogen and X-ray diffractometry, respectively.