purpose of the study was to obtain the correct thermodynamic description of binary and ternary liquid-metal systems as a basis for coolants of the new kind with regulated properties for power generation and technology in the range of compositions and temperatures 0 < Xj < 1 and 200 < T < 1500 K. This means building the set of characteristic functions (potentials) which in coordinates P,T,Xj are Gibbs energies of formation of the systems (partial and integral functions, absolute and excessive values) and thus the activities and coefficients of activity of components AG;, AG, AGi, AG , aj, Yj. Then all the derivatives i.e. the rest thermodynamic functions which are enthalpy and entropy of formation, excessive heat capacity, &H-, AH, AS, , AS, AS/, AS and ACp can be obtained. Direct measurement of the change in the partial Gibbs energy of the components at formation of the liquid-metal alloy in this broad temperature range, using the EMF method [1-4], is extraordinarily difficult because of the absence of steady high- temperature solid electrolytes with the necessary ionic composition for use as membranes in concentration cells. Determination of partial pressure and activity of the components via an experimental study of absorption spectra by the atomic absorption method [5,6] is limited to a small range of temperatures because of the influence of the radiation contribution to corresponding lines. Effusion methods at high temperatures, with either optical or mass spectroscopic or chemical methods of determining of flow composition, are not suitable for alkali- metal systems because of high saturation pressures and technical difficulties of operating with high intensity atomic flows of alkali metals coming out of the apparatus. Method Instead of direct measurement of the Gibbs energy of alloy formation, AG;, or activity of components, aj, at high temperature the following methods are described: • experimental determination of the temperature and concentration dependencies of enthalpy of formation (heat of mixing) AH, A/7Z in the required range of parameters: AH, AH, = f(xj, T), that is easier and can be fulfilled with hermetic working cells • experimental determination of the activity of components aj or the partial Gibbs energy AGZ at a certain (not high) reference temperature T j= 400K: aj = f(xj ,T const) or AG, = f(xj, Tj= const) Solving the differential equation of chemical thermodynamics of the type ,
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