Density, refractive index and sorption capacity of solid CO 2 layers

Abstract

The density and refractive index of solid CO 2 layers were measured by an interferometric method, as was the sorption capacity of these layers for hydrogen, as a function of the condensation conditions such as temperature, growth rate and layer thickness. These properties, i.e. the density ϱ, refractive index n and sorption capacity, are strongly influenced by the condensation temperature. Above ≈70 K, ϱ and n of solid CO 2 layers agree well with those given in the literature for the bulk crystal, but below 70 K they decrease monotonically with decreasing condensation temperature. The charges of ϱ and n for a condensation temperature of 10 K are 40% and 16%, respectively. Below about 50 K, the sorption capacity of solid CO 2 layers for H 2 becomes measurable and increases considerably with decreasing condensation temperature indicating a porous structure of these layers. At about 20 K the maximum value of 220 mmol H 2/mol CO 2 has been measured. Below about 10 K drastic changes of the layer properties occur; the sorption capacity reduces considerably and an increase of the density is observed. This behaviour is due to spontaneous transitions of the layer structure during the CO 2 condensation, which can be observed by a sudden increase of the layer temperature as well as by an abrupt change of the interference pattern. The frequency of these spontaneous transitions increases with decreasing condensation temperature and increasing deposition speed. The released energy was measured by 7 K and estimated to be ≈ 1% of the total condensation energy of the CO 2 quantity condensed after a previous transition.