Thermodynamics of liquid Al-Na alloys determined by using CaF₂ solid electrolyte

Abstract

The purpose of this work has been to establish activity data on sodium in liquid aluminum-sodium alloys at temperatures applied by the industry in liquid metal refining processes. A coulometric titration technique using a galvanic cell employing CaF₂ as a solid electrolyte has enabled measurements to be done under very clean and well-defined conditions over the entire range of compositions from highly diluted up to nearly sodium-saturated solutions. Sodium in liquid aluminum of 99.9999 pct purity is found to exhibit strong negative deviation from Henry’s law, corresponding to a large negative self-interaction coefficient ɛ ^Na_Na as expressed by the equation ɛ ^Na_Na =16,318−(191.1·10⁵ K)·T ⁻¹. This behavior is normal for elements, which exhibit strong positive deviation from Raoult’s law and is explained by formation of Na clusters. The activity coefficient at infinite dilution, γ ^o_Na , is expressed by the equation: RT ln γ ^o_Na =86,729−26.237T. The magnitude of γ ^o_Na from this equation agrees with the value predicted from the Miedema’s semiempirical model. Sodium in liquid Al-Si5 pct alloy of 99.9999 pct purity exhibits strong positive deviation from Henry’s law, which is in agreement with earlier investigations of the activity of sodium in liquid Al-Si alloys. The activity coefficient of sodium in pure liquid aluminum at saturation, γ ^sat_Na , is expressed by RT ln γ ^sat_Na =−67,476+102.33T, which gives for the sodium concentration at saturation x ^sat_Na =exp(8115.5/T−12.307). This implies that the solubility of sodium in liquid aluminum at temperatures around the melting point of aluminum is about 10 times higher than previously reported and decreases rapidly with increasing temperature, possibly due to a decreasing stability of Na clusters. Analysis of the experimental conditions used by previous investigators supports these findings.