Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt.......140s&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF VIRGINIA, 1992.Source: Dissertation Abstracts International, Volume: 55-04, Section: B, page: 150
Astronomy and Astrophysics
Astrophysics
2
Thin Films
Scientific paper
The structural and thermal properties of thin vapor-deposited water ice films were investigated experimentally at temperatures below 170 K. Generally, the sublimation rate of water ice was found to follow the well-known exponential dependence on the sublimation temperature above 140 K. The sublimation energy was determined to be 0.45 +/- 0.03 eV. At around 135 K, however, deviations of the sublimation rate from the expected value were detected, which were dependent on the growth conditions. The observation of a decrease of the sublimation rate with time at 135 K led to the conclusion that fresh vapor-deposited films, which were concluded to be partially amorphous, have a higher sublimation rate than crystalline films; at 135 K, the films transform slowly into crystalline ice and hence the sublimation rate decreases with time. The activation energy for the phase transformation was 0.44 +/- 0.04 eV. By exposing the films to 300 K heat radiation and measuring the sublimation rate at a substrate temperature of 135 K, the thermal conductivity of both fresh and annealed films was determined to be less than 10^ {-3} WK^{-1} m^{-1} for films of thicknesses less than 6 mum. This low value was explained by a granular structure of the water ice films with the grains barely touching. Since the thermal conductivity was found to increase more than linearly with film thickness, we concluded that the film's growth is inhomogeneous in thickness. Vapor deposited water ice films only 100 μm thick were not transparent in the UV and visible. Since the absorption coefficient of water ice in that wavelength region is very low, it was concluded that the sample must scatter light strongly. Scattering occurs at grain boundaries and internal defects, to which we also attributed the observed low thermal conductivity of the ice films. These 100 μm water ice films were irradiated with keV He, Ar, and S ions. It was found that the reflectance spectra of the films changed upon irradiation. The effects observed in reflectance were explained either by absorption by new species created in radiation chemical reactions, or by changes in the scattering behavior caused by changes in the surface morphology and the grain structure of the ice films induced by the ion bombardment. Application of the results to various icy surfaces in the solar system are pointed out. The physical properties of astrophysical ice depend strongly on the growth conditions of the ice and that care should be used in applying laboratory data to astrophysical problems for which growth conditions are not well known. (Abstract shortened by UMI.).
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