Research on the electrical conductivity of fluoride electrolytes NaF - AlF3 - CaF2 in liquid and solid state

Abstract

The main aim of the present work was to perform conductivity measurement of fluoride electrolytes in binary system Na3AlF6-CaF 2 and ternary system Na3AlF6-AlF3 - CaF2. The experiments were done in a wide range of temperature (580-1300 K), including the liquid and solid phase. Temperature dependencies of the conductivity of the liquid and solid cryolite mixtures with CaF2 (2, 4 and 6 wt %) and ternary solution (88 wt % Na3AlF6 - 8 wt % AlF3-4 wt % CaF2) were determined by an A.C. two-electrodes technique. The simultaneous measurements of conductivity and temperature were made in 1min steps during heating and cooling of the sample with rate 0,5 K·min-1. It was confirmed, that the addition of CaF2 to liquid electrolyte decreases its electrical conductivity and liquidus temperature, whereas addition of aluminium fluoride up to 4 wt % in solid state has the inverse influence on conductivity. The studied ternary system showed the highest conductivity in the solid state at temperatures up to phase transition α - β in solid cryolite (ca. 780 K). This may be due to the formation of a solid solution of NaCaAlF6 in β-Na 3AlF6 and the formation of cationic vacancies, which facilitate the transport of electrical charge.