Thermodynamics of Ca-Ga alloys
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I. INTRODUCTION
IN the pyrochemical processes involving actinide metals, it is necessary to recover the actinides left in the spent molten salt after the completion of the process.[1] The recovered actinides will be recycled when the amounts are large and discarded as waste when the amounts are very small. For recovering the actinides, generally, Ca-Ga alloys are equilibrated with the spent molten salt. Hence, the thermodynamic properties of the CaGa alloys are important. There are nine intermetallic compounds—Ca28Ga11, Ca5Ga3, Ca11Ga7, Ca3Ga5, CaGa, CaGa2, CaGa21x, Ca3Ga8, and CaGa4—in the Ca-Ga system.[2] There are no experimental data on the enthalpy of formation of these intermetallic compounds. In this study, we have measured the enthalpy of formation of CaGa2, CaGa4, and Ca3Ga8 at 298.15 K by high-temperature gallium solution calorimetry. The enthalpies of formation of CaGa4 at 988 K and that of Ca3Ga8 at 1070 K were derived from the integral enthalpies of formation of Ca-Ga alloys at these temperatures, measured over the biphasic regions {Ca-Ga} 1 ^CaGa4& and {Ca-Ga} 1 ^Ca3Ga8&, respectively. In this system, there are two compounds (CaGa21x) having a homogeneity range from 67.7 to 70.6 at. pct Ga and CaGa2. Because of this, the two-phase regions of {Ca-Ga} 1 ^CaGa2& and of {Ca-Ga} 1 ^CaGa21x& have only a spread of ,2 at. pct each and, hence, precipitation calorimetric measurements on CaGa2 and/or CaGa21x could not be carried out. The integral enthalpies of mixing of Ca-Ga liquid alloys were measured at 1309 K. In this article, these calorimetric results are discussed. II. EXPERIMENTAL A. Sample Preparation High-purity gallium (99.99 pct) and calcium (99.9 pct) were used for the preparation of the samples. The intermetallic compounds CaGa4, Ca3Ga8, and CaGa2 were prepared B. PRABHAKARA REDDY and R. BABU, Scientific Officers, K. NAGARAJAN, Head, Pyrochemical Processing Section, and P.R. VASUDEVA RAO, Head, are with the Fuel-Chemistry Division, Department of Atomic Energy, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India. Manuscript submitted July 31, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
by arc melting together calcium and gallium under an argon atmosphere in an arc-melting set up. The buttons thus prepared were wrapped in tantalum foils and sealed in quartz tubes under reduced argon pressure and annealed for 170 hours. The annealing temperatures were 773 K for CaGa4 and 923 K for Ca3Ga8 and CaGa2. The X-ray diffraction patterns of the annealed samples confirmed the presence of single-phase intermetallic compounds. Since standard X-ray diffraction patterns were not available for CaGa2 and Ca3Ga8 compounds in the JCPDS files, the lattice parameters reported by Bruzzone et al.[3] and the crystal structures were used to generate them. The samples were stored in an argon-atmosphere glove box until experiments were carried out. B. Equipment The calorimetric measurements were carried out by using a high-temperature differential calorimeter (Model HT-1500 of M/s. SETARAM, Lyon
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