Enthalpies of formation of UAl1 and UAl3 by calorimetry

Enthalpies of formation of the intermetallic compounds UAl4 and UAl3 at 298.15 K were determined by high temperature solution calorimetry in which liquid aluminium was used as the solvent. The thermal effects of dissolution of UAl4, UAl3 and U in liquid aluminium were measured in separate experiments by dropping the samples held at the ambient temperature into liquid aluminium maintained at 980 K in the calorimeter. The thermal effects of dissolution of these samples at infinite dilution in liquid aluminium were derived from these measurements and based on this data the enthalpies of formation of UAl4 and UAl3 at 298.15 K were computed. The values obtained are ΔH f,298.15(UAl4)=-126.5±13.3 kJ mol-1 and ΔH f,298.15(UAl3)=-118.1±8.2 kJ mol-1. The integral enthalpies of formation of U-Al alloys at 978 K, 1078 K and 1094 K were measured by dropping U samples maintained at the ambient temperature into liquid aluminium in the calorimeter at the experimental temperature. From the integral enthalpies of formation of U-Al alloys in the two phase regions, {U-Al}+ and {U-Al}+, the enthalpies of formation of UAl4 and UAl3, respectively, at the temperatures of measurement were derived. These results are discussed in comparison with the literature data. (orig.)