Trapping of deuterium during permeation through gold

Scripta

METALLURGICA

Vol. 1O, p p . 3 7 7 - 3 8 0 , P r i n t e d in the U n i t e d

1976 States

Pergamon

Press,

Inc,

TRAPPING OF DEUTERIUM DURING PERMEATION THROUGHGOLD* G. R. Gaskey, Jr. and R. G. Derrick Savannah River Laboratory E. I. du Pont de Nemours and Company Aiken, South Carolina 29801 (Received (Revised

SeDtember M a r c h 12,

30, 1975) 1976)

Trapping of hydrogen during diffusion in ferrous alloys has been studied (i, 2) for a number of years. More recently, trapping has been proposed (3) to explain anomalies in transient permeation curves in several FCC metals. Interpretation of experimental data is complicated, however, by the presence of surface oxide films that may also cause deviations from the ideal behavior predicted (4-6) for a simple diffusion-controlled process. Differentiation between the effects of trapping and surface oxide film is extremely difficult for many metals, such as molybdenum, stainless steel, or titanium. The surface of gold, however, .is apparently free of oxide films that would restrict hydrogen permeation or require prolonged reaction for their removal (7). Trapping, therefore, may be studied in gold foils without the necessity of special attention to obviate surface effects. The deuterium permeation experiments reported here demonstrate the presence of trapping in gold and suggest that vacancies or vacancy clusters are active trapping sites. Deuterium permeation rate measurements were made on gold foils 2.5 x 10 .-2 cm thick over the temperature range 520 to 820°K at pressures ranging between 1 and S atmospheres by techniques described previously (8). Four specimens were cut from cold-rolled foil of 99.99% (VP grade) gold obtained from Materials Research Corporation, Orangeburg, New York. Specimen heat treatments and permeation test conditions were as follows: Specimen 1 - As-received, cold-rolled foil.

Test started at 523°K and ended at 723°K.

Specimen 2 - As-received, cold-rolled foil. Test started at 873°K and ended at 623°K. This specimen was heated to 903°K in the permeation cell during temperature adjustment before start of the test. Specimen 3 - Cold-rolled foil was annealed in air at 1088°K for 30 minutes and furnace cooled. Test began at 623°K and ended at 873OK. Specimen 4 - Cold-rolled foil was annealed in air at 1223°K for 30 minutes and water quenched. The test began at 623°K and ended at 843°K. The permeability (~) to deuterium for the specimens was calculated from the steady-state permeation rate (P~) according to the equation:

¢ = (P~/a)/(h'~),

(1)

where (a) i s t h e specimen t h i c k n e s s i n cm, (A) i s t h e a r e a i n cm, and (p) i s t h e p r e s s u r e i n atmospheres. The combined d a t a f o r a l l f o u r specimens ( F i g u r e 1) and a l e a s t s q u a r e s f i t t o Equation 1 yields: = 0.22 exp (-29,400/RT)

(2)

The c a l c u l a t e d p e r m e a b i l i t y i s i n r e a s o n a b l e a g r e e m e n t w i t h t h e one r e p o r t e d measurement a t 1033°K (9). In a d d i t i o n , p e r m e a b i l i t y may be c a l c u l a t e d from t h e d i f f u s i v i t y r e p o r t e d by E i c h e n a u e r and L i e b s c h e r (10) and t h e s o l u b i l i t y d a t a o f Thomas (11) o r McLellan (12) by t h e relation ~ = D-S; where S is the solubility, and D is the diffusivity~ Even after correcting diffusivity values for the isotope effect, the measured permeabilities lie a factor of I0 or

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more below these calculated values. Furthermore, the activation energy (E~) measured in these experiments is greater (29,400 cal/mol versus 12,200 to 22,900 cal/mol) th~n that calculated from the above data through the relationship E~ = E D + E S. Apparent diffusivities (D*) were determined from measured permeation rates Pt at time t, using solutions (13) for the relation:

P~/P~ = 1 + 2 ~ (-l)nexp(-D*n2~2t/a a) n=l

(3)

Derivation of this relation assumes diffusivity is independent of concentration, the initial concentration within the specimen is zero, and the concentrations on the sheet faces are kept at C and 0 throughout the experiment. Following steady-state permeation, the evolution rate from the sheet (C = 0 at both faces) is given (4) by the ratio:

P t / P = -2 n__~(-l)nexp(-D*n2~2t/a2)_

(4)

Therefore, the permeation and evolution rate curves should coincide after suitable transposition. Implicit in these derivations is the further assumption that diffusion is the only significant rate process operative; thus, surface effects and trapping must be insignificant. When these assumptions are satisfied, the apparent diffusivity calculated from measured permeation rates P. and P is the lattice diffusivity of hydrogen in the specimen. Furthermore, calculated values ~f D* w~ll agree within experimental error for all points on both the permeation and evolution rate curves. The apparent diffusivities for deuterium in gold depend on (a) specimen treatment (Figure 2), and (b) time elapsed during the permeation or evolution transient which is chosen for the calculation, as seen from typical plots of D* as a function of P t / P (Figure 3). Generally, the apparent diffusivities are lower by a factor of i00 than the values of Eichenauer and Liebscher (i0); however, in some instances, calculated diffusivities (Figure 3) equal or exceed these values. These high D* values were obtained from calculations based on the evolution curves near steady-state. Furthermore, temperature dependence of the diffusivity (apparent activation energy for diffusion) depended on the maximum temperature attained in the test. Specimens 1 and 2 were both in the as-rolled condition, but have markedly different activation energies: El z 19,000 cal/mol, and E2 z 13,000 cal/mol. Specimen 2 was heated to %900°K, whereas Specimen 1 only reached 700°K during testing. The large uncertainty in diffusivity among the specimens and within a given isothermal permeation test of an individual specimen would cause a similar spread in solubility calculations; therefore, deuterium solubility was not evaluated. The following features in these data suggest that the diffusing deuterium is interacting in a reversible manner with impurities or imperfections in the gold: •

Diffusivities calculated from the permeation rate measurements are markedly dependent on specimen treatment prior to testing (Specimens 1 and 2 u8 Specimens 3 or 4) and on the maximum temperature attained during the measurements (see Figure 2, Specimen 1 v8 Specimen 2).

•

Diffusivities calculated during rise to steady-state are less than those obtained from the evolution-rate data immediatelp following steady-state.

•

Diffusivities depend on the point on the rise or evolution curve where the calculation is made.

Possible trapping sites for the hydrogen are oxygen atoms present as a substitutional impurity, dislocations, vacancies, or complex vacancy combinations. The effects are sensitive to specimen treatment which should not alter the oxygen content significantly. The dislocation density and vacancy concentrations, however, would be expected to change with the treatments used. Vacancies are believed to be the more likely trapping sites; a supposition supported by

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PER~EATION

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the observation that hydrogen atoms stabilize divacancies introduced into gold by quenching (14). Furthermore, recent calculations for FCC metals (15) indicate that the proton is more stable as a substitutional impurity bound to a vacant lattice site than as an interstitial impurity. Impurities (iron, copper, and zinc) have been observed to diffuse to a gold surface during prolonged heating (60 to 150 hours) in low pressure oxygen at elevated temperature, T >I]00°K (16). In the case of iron, isolated islands of a spinel oxide formed. With the 'heat treating (30 minutes at 1088 or 1223°K) and permeation conditions (~150 hours at T <900°K) of these tests, impurity diffusion and oxide formation would be minimal; therefore, the observed differences among the specimens are not believed to be associated with impurity oxides at the surface. Further investigation of the possible role of impurities, either at the surface or internally, is needed, however, to establish the conditions under which the impurities may affect the permeability and diffusivity of hydrogen in gold.

* The information contained in this article was developed during the course of work under Contract No. AT(07-2)-1 with the U. S. Energy Research and Development Administration.

REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. I0. II. 12. 13. 14. 15. 16.

A. McNabb and P.K. Foster, Trans~ AIME 227, 618 (1963). R.A. Oriani, Acta Met. 18, 147 (1960). ---G.R. Caskey,Jr. and R.G~errick, "TranDing Durina Hydrooen Permeation in FCC Metals". Spring Meeting, Metallurgical Society i f AI~E, Pittsburgh, Pennsylvania, Mav 19-23, 197~. R.A. Strehlow and H.C. Savage, J. Nucl. Mater. 53, 323 (1974). T.S. Elleman and K. Verghese, J. Nucl. Mater. 53~--299 (1974). W.A. Swanzioer, R.G. Musket, L.J. Weirick, and-W. Bauer, J. Nucl. Mater. 53, 307 (1974). J. P. Coughlin, "Contributions to Data on Theoretical Metallurgy. XII. Hea-ts and Free Energies of Formation of Inorganic Oxides." B u l l e t i n 542, Bureau of Mines, Washington, D.C. (1954). G.R. Caskey, R.G. Derrick, and M.R. Louthan, J r . , Scripta Met. 8, 481 (1974). Y . I . Zvezdin and Y . I . Belyakov, Diffusion Data 2, 117 (1968). W. Eichenauer and D. Liebscher, Z. Naturforsch. 17A, 355 (1962). C.L. Thomas, Trans. Met. Soc. AIME, 239, 485 (1967--~. E.B.McLellan, J. Phys. Chem. Solids, 3~, 1137 (1973). J. Crank,"The Mathematics of Diffusion-~,' Clarendon Press, Oxford (1956). I.A. Johnston, P.S. Dobson, and R.E. Smallman, Proc. Roy. Soc., Ser. A 315, 231 (1970). Z.D. PoDovic and M.J. S t o t t , Phys. Rev. Lett. 33, 1164 (1974). D. Clark, T. Dickinson, and W.N. Mair, Trans Far-. Soc., 5_55, 1937 (1959).

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