ENTHALPIES OF FORMATION OF UA14 AND UA13 BY CALORIMETRY

Enthalpies of formation of the intermetallic compounds UAl4 and UAl3 a t 298.15 K were determined by high temperature solution calorimetry in which liquid aluminium was used as the solvent. The thermal effects o f dissolution of UAl4, UAl3 and U in liquid aluminium were measured in separate experiments by dropping the samples held at the ambient temp erature into liquid aluminium maintained at 980 K in the calorimeter. The thermal effects of dissolution of these samples at infinite diluti on 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 DELTAH(f,298.15)-degrees(UAl4) = - 126.5 +/- 13.3 kJ Mol-L and DELTAH(f,298.15)-degrees(UAl3) = - 118 .1 +/- 8.2 kJ mol - 1. The integral enthalpies of formation of U-Al al loys at 978 K, 1078 K and 1094 K were measured by dropping U samples m aintained at the ambient temperature into liquid aluminium in the calo rimeter at the experimental temperature. From the integral enthalpies of formation of U-Al alloys in the two phase regions, {U-Al}+[UAl4] an d {U-Al}+[UAl3], the enthalpies of formation of UAl4 and UAl3, respect ively, at the temperatures of measurement were derived. These results are discussed in comparison with the literature data.