WATER SELF-DIFFUSION ALONG GRAIN BOUNDARIES OF PURE AND DOPED POLYCRYSTALLINE ICE AT TEMPERATURES NEAR ITS MELTING POINT

Ice is a mineral ubiquitously present on Earth, and is involved in a vast array of geological and environmental phenomena. Nevertheless, in spite of centuries of scientific enquire many of its physical and chemical properties remain incompletely understood. A particular interest to environmental science is the transport of impurities along grain boundaries and related defects in polycrystalline ice. At temperatures near 0 deg. C, grain boundary diffusion is further complicated by the premelting phenomena, i.e. formation of a network of quasi-liquid in the polycrystalline ice at temperatures significantly below its normal melting point.

With the objective of gaining insights into transport and premelting phenomena in polycrystalline ice, we have used Fast Thermal Desorption Spectroscopy (FTDS), a novel technique developed in our laboratory, to study H/D isotopic exchange in 2-4 micrometer polycrystalline ice films undergoing rapid vaporization from a ten micrometer in diameter tungsten filament. According to our results, H/D isotopic exchange reaction, at temperatures from -2 deg. C to -20 deg. C, occurred in 50-500 nm wide reaction zones between polycrystalline H₂O and D₂O layers. Because the extent of the H/D isotope exchange reaction depends on the rates of inter-diffusion, and inter-diffusion is facilitated by grain boundary premelting, we were able to obtain a variety of information on both of these phenomena. For example, we show that grain boundary premelting in pure polycrystalline ice is unlikely at temperatures below -2 deg. C. However this is not the case, when even a small amount of ionic impurities is present in the ice sample bulk.