ИСТИНА

Grain boundary wetting in polycrystals may produce a special kind of heterogeneous matter consisting of a solid framework containing a continuous intergranular liquid phase. It occurs when the fractions of wetted and dry boundaries exceed the percolation thresholds for liquid and solid structure elements, respectively. To understand whether or not an interconnected network of wet boundaries is likely to arise in a given system, in the simplest cases a knowledge is needed of ranges covering the solid-liquid and solid-solid energies, and of the grains geometry. On the contrary, to get an idea of viscous flow or diffusion along intergranular films, a detailed information about their thickness is necessary, which is by far less available due to the scatter in this parameter and the increasingly reported complexity of films structure. In this work, water-containing polycrystalline sodium chloride was used to measure the mean effective thickness of intergranular aqueous interlayers by study of the diffusion permeability, electric conductivity, filtration coefficient and label concentration. Experiments were conducted on coarse-grained pore-free NaCl samples prepared by hot pressing and recrystallization of single crystals. Helium diffusivity under a weak pressure gradient through brine-presaturated salt disks was determined by means of a mass spectrometer. Transport of Fe2+ ions was measured analytically in a two-chambers diffusion cell containing saturated brine in both parts, with Mohr’s salt additive in one of them. Electric conductivity was measured at 50 Hz or 1 kHz AC from the linear volt-ampere plots. For estimating the Darcy permeability coefficient, a high sensitive displacement device was used, and several days long filtration experiments under a constant pressure were performed. Dense fine-grained extruded NaCl polycrystals were taken to study grain boundary wetting in brine containing a foreign ion (Ni+2 or Fe+2) as a label. After soaking in brine, samples were dissolved in water, and the label concentration was determined analytically. All the experiments have shown a good agreement between the values calculated for the effective thickness of liquid intergranular water layers. The mean thickness of transport rate determining pathway elements was found to be 120 ± 30 nm, the scatter within each group of results being comparable with the averaged scatter. The value obtained seems to be close to the upper limit of liquid films thickness in the NaCl – H2O system.