375
Hydrogen and Deuterium Stability in U-Mo (10) (1 1) (12) (13) (14) (15) (16) (17) (18)
B. B. Damaskin, Electrochim. Acta, 9, 231 (1964). R. Lerkkh and B. B. Damaskin, Zh. Fiz. Khim., 38, 1154 (1964). R. Lerkkh and B. B. Damaskin, Zh. Fiz. Khim., 39, 21 1 (1965). B. B. Damaskin and R. Lerkkh, Zh. fiz. Khim., 39, 495 (1965). V. K. Venkatesan, B. B. Damaskin, and N. V. Nikoiaeva-Fedorovich,Zh. Fiz. Khim., 39, 129 (1965). B. Darnaskin, A. Frumkin, and A. Chizhov, J. Electroanai. Chem., 28, 93 (1970). B. B. Damaskin, I. P. Mishutushkina, V. M. Gerovich, and R. i. Kaganovich, Zh. Fiz. Khim., 38, 1797 (1964). B. 8. Damaskin. V. M. Gerovich, I. P. Giadkikh, and R. I. Kaganovich, Zb. Fir. Khim., 38, 2495 (1964). B. 8. Darnaskin, A. N. Frumkin, and S. L. Dyatkina, izv. Akad. Nauk
SSSR, Ser. Khim., No. 10, 2171 (1967). (19) B. B. Damaskin, Eiektrokhimiya, 4, 675 (1968). (20) B. B. Damaskin, Eiektrokhimiya, 5, 346 (1969). (21) B. B. Damaskin, S.L. Dyatkina, and S.L. Petrochenko, Eiektrokhimiya. 5, 935 (1969). (22) D. E. Broadhead, R. S. Hansen, and G. W. Potter, J. Colloid interface Sci., 31, 61 (1969). (23) K. G. Baikerikar and R . S.Hansen, J. Colloid interface Scl., accepted for publication. (24) 0. C. Grahame, J. Am. Chem. Soc., 88,301 (1946). (25) B. B. Damaskin, Zh. Fir. Khim., 32, 2199 (1958). (26) Equation 7 is the equation of state corresponding to the Frumkin Is& therm and is derivable from eq 1 and the Gibbs adsorptiontheorem.
Solubility of Hydrogen and Deuterium in a Uranium-Molybdenum Alloy G. Lovis Powell Nuclear Division, Union Carbide Corporation, Oak Ridge, Tennessee 37830 (ReceivedMay 27, 1975) Publication costs assisted by Nuclear Division, Union Carbide Corporation for The UnitedStates Energy Research and Development Administration
The solubility of hydrogen and of deuterium in uranium-0.222 mole fraction molybdenum alloy has been measured over the temperature range from 700 to 1350 K with a precision of fl%. The solubility displays a minimum value near 900 K. A model, which is generally applicable to hydrogen dissolved in other metals, is presented that describes the observed phenomena, that can be extrapolated to lower temperatures, that yields the enthalpy of formation of hydrogen dissolved in the alloy at 0 K, and that suggests a quantum mechanical treatment for interstitial diffusion.
I. Introduction
Single-phase metal-hydrogen alloy systems have been studied extensively and the literature in this field has been reviewed several t i m e ~ . l -Experimentally, ~ the equilibrium constant ( K H )for the reaction between hydrogen gas a t pressure P H and ~ hydrogen dissolved in a metal at concentration [HM] is determined using pressure-volume measurements yielding the relationship:
Here F ’ H ~ , Tis the standard Gibbs free energy per gram atom hydrogen dissolved in the metal. Activity coefficients are usually added as [HM] becomes large. F’H~,Tis the standard Gibbs free energy per mole for hydrogen gas. For most metals K H either increases (Y, Ti, Nb) or decreases (Ni, Cu, W) markedly with temperature and appears to converge to approximately the same value at elevated temperatures (-1500 K). Equation 1is known as Sievert’s law. Ebisuzaki et al.234have developed a quantitative description of the contribution of the vibrational states of a hydrogen atom in an interstitial site of a metal to K H based on the ratio of K H for hydrogen to that for deuterium (KD). Qualitative correlations have been drawn between K H and the electronic properties of some metal^.^.^ A quantitative theory generally applicable to all metals and derivable from K H , that allow (1) the extrapolation of K H into temperature regions inaccessible to direct measurements and (2)
the direct comparison of K H for various metals have not been reported. The value for K H for the high-temperature, body-centered-cubic (bccub) phase of uranium has a magnitude that is intermediate between that for niobium and that for nickel.* Additions of molybdenum as an alloying element broadens the temperature range over which the bccub phase is thermodynamically and kinetically stable with the maximum temperature range near 0.2 mole fraction molybdenum.&‘ This report describes the very precise determination of K H and K D for the uranium-0.222 mole fraction molybdenum alloy (U-0.222 Mo) over the temperature range from 700 to 1400 K. A minimum value was observed for K H and K D near 900 K. A simple model, consistent with the “isotope effect theory” of Ebisuzaki and O’Keefe,2*4is presented that describes the observed values of K H and K D and yields parameters by which K H and K D may be extrapolated to lower temperatures. 11. Materials
The argon (99.999%pure) was used without further purification. The hydrogen gas (99.99% pure) was filtered through UH3 powder to remove oxygen (water).8 The deuterium gas (99.8% Dz, 99.99% hydrogen isotopes) was filtered through UD3 powder. The gas manifold had ultrahigh-vacuum integrity and was evacuated to less than 1 x atm before a gas was introduced. The U-0.222 Mo alloy composition was determined from The Journal of physicel Chemistry, Voi. 80, No. 4, 1976
376
x-ray adsorption measurements of a solution containing a dissolved aliquote of the U-0.222 Mo alloy. Major impurities were the following: 300 pg of C/g, 125 pg of Si/g, 125 pg of Felg, 40 pg of Ni/g, 30 pg of Culg, 23 pg of N/g, 23 pg of O/g. The U-0.222 Mo specimen used for this experiment weighed 102.082 g (0.4943 mol of metal atoms). T o prevent contamination of the alloy by reaction with the vacuum chamber wall a t temperatures near 1400 K, a yttria-lined alumina boat (1.2 cm i.d. X 7.6 cm long X 0.1 cm wall thickness) was prepared to support the sample by plasma spraying the A1203 and Y2O3 over a tungsten mandrel followed by sintering at 1500 K.9 The U-0.222 Mo alloy exists as an equilibrium bccub phase above 860 K. The alloy can retain this bccub phase indefinitely if rapidly cooled below 700 K.596This bccub phase is retained for 25 ks or longer a t temperatures slightly below 860 K before transforming into a two-phase alloy consisting of a uranium-rich phase similar to a-phase uranium and a molybdenum-rich phasea7 I
111. Instrumentation
The experimental arrangement was the Sievert's apparatus shown in Figure 1. All-metal, ultra-high-vacuum equipment was used exclusively. The pressure could be reproducibly measured within 1 X atm over the pressure range from 0.13 to 1.3 atm. At pressures below 0.13 atm, the pressure measurements were reproducible within f l X 10-5 atm over a 90-ks period. The reference volume (VO= 207.2 cm3) was calibrated by the Oak Ridge Y-12 Plant Standards Laboratory and this calibration is traceable to the National Bureau of Standards. The working standard volume (VI = 52.76 f 0.21 cm3) was determined by argon expansions from VO. A proportional temperature controller maintained the sample temperature within 1 K over the temperature range from 700 to 1350 K as measured by a platinum-platinum10% rhodium thermocouple with a precision of 0.01 mV. The sample temperature could be changed incrementally at a preset time interval by a temperature programmer that repeated itself every 26-time intervals. Another proportional temperature controller maintained Vi and the portion of V2 outside the high-temperature furnace a t 300.3 f 0.1 K. The sample temperature and the pressure gauge output were alternately recorded on a digital voltmeter-printer system a t 60-s intervals. Thermocouples were checked against National Bureau of Standards Reference thermocouples and found to agree within 1 K. IV. Procedure The sample is positioned in the Mullite tube exposing only Vz to air, evacuated, and heated to 1100 K under vacuum, Once a high vacuum is achieved, V3 = V1 t V2 (Figure 1) is backfilled with H2 to 0.5 atm and left undisturbed for 173 ks to reduce oxides of copper and iron on the walls of VB. The system is then evacuated to
