Effect of dynamic nanostructuring near KCl melting point

Abstract

The existence of transient phenomena near the melting point T m is usually accompanied by an anomalous behavior of various macroscopic parameters (heat capacity, thermal conductivity, electrical conductivity, thermal expansion coefficient, self-diffusion coefficient, surface melting) with an increase in the temperature. In addition, an exponential increase in the concentration of point and linear defects with temperature, structural transitions of some types of crystals to a disordered state indicate that, as the melting point is approached, the structure and properties of the solid undergo changes that are nonlinearly dependent on the heating rate, and in the very the transition point T m change abruptly. Some transient processes manifest themselves only under the conditions when the substance is brought into a strongly nonequilibrium state, for example, at significant heating rates. In this work, we analyze the dynamics of structuring of the transitional phases of premelting of KCl in various kinetic regimes. It is revealed that the premelting process is a nonequilibrium fluctuation process accompanied by the appearance of dissipative states near the melting point, characterized by a set of thermodynamic and spectral parameters that nonlinearly depend on the heating rate. Based on the data of X-ray diffraction analysis, the average size of the coherent scattering regions of the premelting phase of KCl was calculated, which vary in the range of 65-90 nm. The size of the coherent scattering regions is in a satisfactory agreement with model calculations of the parameters of nanocluster structures in the transition region (~10-14 nm). In the premelting phase of KCl, a nonmonotonic change in the peak intensity was found, which is associated with thermal fluctuations in the temperature range T ≥ 0.85 T m. The nonlinear change in the thermodynamic and structural parameters of KCl, as well as the fluctuation character of heat release in the transitional premelting region, indicate the dynamic nature of the formation of the nanophase near the melting point of KCl.