Ewald Veleckis
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Thermodynamics of the Lithium-Lithium Deuteride System Ewald Veleckls Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received July 1, 1976; Revised Manuscript Received Janusry 10, 1977) Publication costs assisted by Argonne Nations/ Laboratory
Pressure-composition-temperature data were collected for the Li-LiD system in the ranges 0-750 Torr, 1-99 mol % LiD, and 705-871 "C by measuring equilibrium deuterium pressures over encapsulated Li-LiD mixtures. The data yielded a family of five PD;lz vs. NLiD isotherms whose shapes indicate the existence of two homogeneous terminal solutions that are separated by a wide miscibility gap. Beginning at the consolute point (1000 f 10 "C, 61 f 3 mol % LiD), the gap widens to a range of 21.3-99.0 mol % LiD at the monotectic temperature (689
"C). The mole-fraction solubility at the Li-rich gap boundary and plateau pressures above and below the monotectic temperature may be represented by N'L&atd) = exp(2.604 - 3992T1),PPIJ(Torr)= exp(21.21 - 16940T1),and Ppl,JTorr) = exp(28.04- 23510T1),respectively. The data yielded the equilibrium constant [K(atm-'j2)= exp(-6.630 + 7995T')I for the reaction Li(so1n) + l/zDz(g) LiD(so1n) and expressions for the chemical potentials and activity coefficients of each species as functions of temperature and composition. Activity coefficients, evaluated at N L ~ D 0 and N L ~ 0, were combined with the equilibrium constant to generate temperature-dependenceequations for various constants associated with dilute solutions. For the Li-rich limit, the Sieverts' constant [K' = NLiD/(PD2)'"] is given by K'(mo1 fraction LiD/atm1'2) = exp(-6.138 + 5599T'). Advantages of the Ostwald coefficient in describing the distribution of the dissolved species between liquid and gas phase are discussed.
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Introduction The interaction of gaseous hydrogen isotopes with alkali metals produces ionic monohydrides that, in many respects, are similar to the corresponding ha1ides.l Their solutions in the parent metals are usually characterized by the formation of wide miscibility gaps that separate homogeneous terminal solutions. The hydrides of lithium (LiH, LiD, and LiT) are of particular interest since, owing to their simple electronic configurations, they are amenable to theoretical studies.' They also have great practical significance, especially in D-T-fueled fusion reactor concepts that employ lithium as a tritium-breeding blanket material.2 Comprehensive studies of solution thermodynamics and phase relations in the lithium-hydrogen isotope systems have been under way in this laboratory for several years. Studies of the Li-LiH system have been reported previo u s l ~ Work . ~ on the Li-LiT system is near completion and will be described in a subsequent paper. The results obtained for the Li-LiD system are presented here. Literature information on the Li-LiD system is sparse. Heumann and Salmon4measured deuterium dissociation pressures over Li-LiD alloys at 700,750, and 800 "C and deduced approximate decomposition pressures in the two-phase region (plateau pressures) and miscibility gap limits. More recently, Smith et aL5 reported plateau pressures and Sieverts' law constants at four temperature between 700 and 1000 "C. Goodall and McCracken6used a mass-spectrometric technique to measure deuterium dissociation pressures at 700 "C at very low deuterium < NL~D < 0.05). concentrations All of the above studies were made with lithium sealed in metal capsules whose walls were sufficiently thin to permit rapid permeation by deuterium. Ihle and W U ,on ~ the other hand, made mass-spectrometric measurements on the vapor phase above dilute solutions (5 X < NL~D 24 h at each corresponding value of NOL~D. The corrections are somewhat smaller than those of the Li-LiH s y ~ t e m , ~ temperature. The results are shown in Table I. When the logarithm of the plateau pressure is plotted probably because of the more rigorous initial degassing in the present work. (4) In the LiD-rich region, a similar against 1/T, two distinct linear segments are obtained extrapolation of the data points to 1 / P D , = 0 produced which correspond to the following least-squares equations:
during the capsule-filling step which consisted of immersing the filter end of the capsule (5-pm porosity filter) in a pot of molten lithium at 400 "C and applying gentle suction through the top end. The purpose of the filter was the removal of solid lithium oxide which has a solubility under these conditions of
