Swansiger, Swisher, Darginis, and Schoenfelder
308
Hydrogen Permeation in Palladium-Chromium Alloys W. A. Swanslger, * J. H. Swisher, J. P. Darginis, and C. W. Schoenfelder Sandia Laboratories, Livermore, California 94550
(Received July 2, 1975)
Publication costs assisted by the U S . Energy Research and Development Administration
As part of a search for new palladium alloys with attractive permeation characteristics for hydrogen isotopes, alloys containing 9.3, 16.8, and 24.2 atom % chromium were evaluated in the temperature range from 322 to 449 K. The permeation coefficient, diffusion coefficient, and hydrogen solubility were all found to decrease with increasing chromium content. The solubility was found to obey Sievert’s law in all alloys. The permeation coefficients deviated slightly from classical diffusion-controlled behavior. These deviations are attributed to slow molecular dissociation and adsorption of hydrogen on the surface a t low pressures. In some preliminary experiments at high pressure, the Pd-16.8 atom % Cr alloy retained its structural integrity while a commercially available Pd-25 atom % Ag alloy was susceptible to cracking and blistering. Introduction
inverted and remelted several times to produce homogeneous material. The alloy compositions were obtained by Palladium has the scientifically interesting and practichemical analysis. Hot and cold rolling were used to break cally important characteristic of having a very high permeup the cast structure and obtain the desired sheet thickation rate for hydrogen isotopes. At low temperatures and ness of approximately 0.06 cm (23 mils). Specimens in the moderate hydrogen pressures, however, a phase change ocform of disks approximately 2 cm in diameter were cut curs on hydrogen absorption which adversely affects the from the sheet, polished metallographically, and washed mechanical properties of palladium. A number of P d alloy with distilled water and ethyl alcohol. The cold-rolled disks systems have been studied for improved permeation and were sputter-cleaned before mounting in the permeation mechanical properties (see as examples ref 1-12). Alloying apparatus. One sample of each composition was used. of palladium with silver has led to greatly improved propPermeation and diffusion coefficients were determined erties,l and a Pd-25 atom % Ag alloy is now used commerin an all-metal ion-pumped vacuum system using a gas percially for hydrogen isotope purification and separation promeation technique. The specimens were placed between cesses. two 0.10-cm thick copper gaskets, which in turn were Because of the highly catalytic nature of P d and Pd alplaced between two Varian mini-conflat fittings. The asloys, surface poisoning by impurities is a concern, particusembly was bolted into the permeation apparatus and enlarly at low temperatures. For this reason, prior to 1965 closed in a 7.6-cm diameter copper heat sink. The hydrogen there had been rather poor agreement between various inpressure on the upstream side of the sample was held convestigators who measured permeation constants for hydrostant from 0.13 to 13 kPa (1to 100 mmHg) by a servo-congen in P d and Pd-Ag alloys. More recently, Holleck,8 for trolled leak valve receiving feedback from a capacitance example, has succeeded in obtaining permeation, diffusion, manometer. Before admission to the upstream side of the and solubility data which me internally consistent and free system, the hydrogen was purified by passage through a from anomalies attributable to surface defects. heated Pd-25 atom % Ag tube. Hydrogen permeating into Nearly all the published data are based on measurethe downstream volume, which was continuously pumped, ments made at pressure differentials less than 1 atm. A notable exception is the work of Ackerman and K o ~ k i n a s , ~ was detected by either a quadrupole mass spectrometer or an ionization gage. Permeation rates were determined by who measured permeation rates through Pd-25 atom % Ag comparison with calibrated orifice leaks, while diffusion alloy tubes at high pressure and a temperature of 673 K. coefficients were determined by least-squares fits to the Our calculations show that if Holleck’s resultss are extrapotime-dependent hydrogen flux. lated by Sievert’s law to a pressure of 68 atm, the agreeSolubility measurements were made by sealing speciment with Ackerman’s results is within a few percent. mens in glass capsules with known quantities of hydrogen The only related work in the past on Pd-Cr alloys was gas. After holding for sufficient time to reach the equilibridone by L ~ e b i c h He . ~ found that foils containing 5.0-26.5 um solubility a t 449 K, the capsules were quenched in liqatom % chromium were warp and fissure resistant :vhen uid nitrogen, The amount of hydrogen remaining in the gas used for hydrogen purifteation a t high temperatures. No phase was then measured volumetrically, and the amount permeation data were published by Loebich. of hydrogen absorbed by the specimens was measured after In the work described here, permeation and diffusion heating under vacuum to release the gas. coefficient d a t d were obtained for alloys containing 9.3, 16.8, and 24.2 atom % chromium. Some direct measureResults ments of the hydrogen solubility and the pressure dependence of the permeation rate were obtained at one temperPermeation. The temperature dependence of the steadyature, 449 K. state permeation coefficient of hydrogen in the experimental alloys is shown in Figure 1 in the form of Arrhenius Experimental Section plots. Re-ults obtained by Hollecks for unalloyed P d and Pd-25 atom % Ag are shown for comparison. The experiThe experimental alloys were prepared by arc melting ments were all performed with an upstream pressure of buttons from mixtures of the two metals. The buttons were The Journal of Physlcal Chemistry, Vol. 80, No. 3, 7976
Hydrogen Permeation in Palladium-Chromium Alloys
309
TABLE I. Diffusion and Permeation Constants for Hydrogen in Pd and Pd Alloys _______ Alloy compositions @Oa E@ DO __
TEMPERATURE I’C)
c , - /3- 200
-
100
150
50
25
9
Pd-25 atom % Agd Pdd Pd-9.3 atom % Cr Pd-16.g atom % Cr Pd-24.2 atom % Cr
(u
8
7 x lov4 1 x 10-3
7.96 13.65
2.2 x 2.9 x 10-3
23.03 22.02
9x
23.49
1.0 x
23.87
2 x loF3
34.71
1.0 x
28.72
1x 1 O - j 42.12 1 . 3 x 32.66 ______ ____ cm3 H,(STP)/sec cm (kPa)%.b E Q and E D in KJ/ g atom. =Doin cm2/sec.d Reference 8 .
-9
a @ , , in
0
9
%
.in.-
2.0
2.5
3.0
3.5
,
TEMPERATURE (‘C)
lOOO/T (K-l)
-4(
200
150
100
50
Figure 1. Temperature dependence of the permeation rate of hydrogen In Pd-Cr alloys. 13.33 kPa (100 Torr). As will be discussed later, there was a slight dependence of the permeation coefficient on pressure at 449 K. Figure 1 shows that the permeation coefficient decreases and the temperature coefficient of permeation increases with increasing chromium content. Least-squares calculations were used to obtain equations for the results in the form 4 = 40 exp(-Em/RT) (1)
Values for 40 and E , for each alloy are given in Table I. The $0 values are within approximately a factor of 2 of each other, and E, increases approximately linearly with chromium content. Diffusion. Corresponding data (determined a t 13.33 kPa upstream pressure) for the temperature dependence of the diffusion coefficients are plotted in Figure 2. Again the diffusion coefficient decreases and the temperature coefficient for diffusion increases with increasing chromium content. The values of D Oand EDfrom equations of the form
D = DOexp(-EDIRT)
Pd - 9.3% Cr
D
-
Pd 16.8%Cr Pd .24.2% Cr
4 2.0 2.5 3.0
-9
1 O O O R ( K-’ )
Figure 2. Temperature dependence of the diffusion coefficient of hydrogen in Pd-Cr alloys. PRESSURE (mm Hg)
50 150
400
760
(2)
are given in Table I. The DOvalues are nearly the same for all alloys, and the E D values increase linearly with chromium content. At 100 Torr upstream pressure the time-dependent hydrogen flux compared very well with the theoretical curve obtained by solution of Fick’s second law. Solubility. Experimental measurements of hydrogen solubility were made only a t a single temperature (449 K). The data are plotted in Figure 3 as hydrogen concentration vs. p1/2 to show conformance to Sievert’s law. Only one value was obtained for the 24.2 atom % Cr alloy because the solubility was so low. A line corresponding to Holleck’s results8 for unalloyed P d is given for comparison. His results were checked with our technique and the agreement was good; our value of 3.19 cm3 H2/[cm3 metal kPa1/2] was 5% less than a value of 3.36 obtained from Holleck’s data. A summary of the data obtained a t 449 K for the variation of permeation coefficient, 4, diffusion coefficient, D, and solubility, s, with chromium contentis given in ~i~~~~ 4. Equations fitted to the results are log 4 = -4.68 - 0,120(% Cr)
(3)
log D = -5.15 - 0.069(% Cr)
(4)
log S = +0.47 - 0.051(% Cr)
(5)
p%
(kPa)%
Figure 3. Hydrogen solubility in Pd-Cr alloys at 449 K.
Pressure Dependence of Permeation. As alluded to earlier, variation of the upstream pressure during permeation measurenlents a t 449 K showed a relatively small but systematic effect on the permeation coefficient, 4. In principle, q5 contains a p 1 / 2term to make it a pressure-independent parameter. The observed effect of pressure on the permeation flux, J , is shown in Figure 5 in the form log J vs. log p . The linear behavior corresponds to a pressure depenThe Journal of Physical Chemistry, Vol. 80, No. 3, 1976
310
-i
10.6
x P
m.2
0.2
"2
0.6
E -
.1.0
v)
0
9
*L 0
aO
8
4
9
12
16
20
24
ATOMIC PERCENT Cr
Figwe 4. Permeation coefficient.diffusion coefficient. and solubility of hy&ogan as a function of chromium content in Pd-Cr alloys at 449 K. PRESSURE
3
h m Hpl
10
-5,
33
103
1
-
Pd .93% Cr
SLOPE 0.53
- ..
(a) Pd-25 atom % A g specimen (75X) and (b) Pd-16.8 atom YO Cr specimen (75X) after exposure to 50 MPa H2 at 373 K for 2 days. Flgura 0.
SLOPE = 0.61
SLOPE
0
I 4 a.4
-
0.65
-MEASUREDFLUX FLUX COMPUTED FR0MO.S
___ 0
~
m.4 LOG
1,
ta.8
~~~
I
+1.2
IkPal
EIgwe 5. Permeation flux of hydrogen as a function of pressure in
W-Cr alloys at 449 K.
dence of the type J = Ap", where n = 0.53 for 9.3 atom % Cr. 0.61 for 16.8 atom % Cr, and 0.65 for 24.2 atom % Cr. The uncertainty in n is believed to be i0.03. Values for n would probably vary with temperature. Additional lines are shown in Figure 5 for fluxes computed from the relation 6 = J P " ~= DS, with D and S values taken from Figures 2 and 3. The differences between the computed and experimentally determined results are believed to be deviations from strictly diffusion-controlled permeation. Extrapolation of the results for the 16.8 and 24.2 atom % Cr alloys to higher pressures indicates that the experimental and computed lines should meet at pressures of a few atmospheres. As will be discussed, the deviations from classical behavior may be attributed to slow adsorption andlor dissociation of hydrogen molecules on the surface at low pressures. High Pressure Experiments. A few experiments were performed at high pressure to investigate structural integmaJmmiormwiUwMMw. vol. so, NO. 3. 1976
rity of the alloys. These tests consisted of exposing specimens of the commercial Pd-25 atom % Ag alloy and the experimental Pd-16.8 atom % Cr alloy a t 373 K to a hydrogen pressure of 50 MPa. After holding under these conditions for 2 days, the specimens were examined metallographically for structural damage. As can be seen in Figure fia and 6b, there was no evidence for damage in the Pd-16.8 atom % Cr specimen, but the Pd-25 atom % Ag specimen was badly cracked, due presumably to excessive swelling.
Discussion In many permeation studies, the permeation and diffusion coefficients are determined independently, and the solubility is computed from the S = h/D relationship. In our results, if the small pressure dependence effect is neglected, equations for S for the various alloys determined from @ID for a pressure of 13.33kPa (100Torr) are 0 atom % C$
S = 0.3 exp(+8.37/RT)
(6)
9.3 atom % Cr
S = 0.9 exp(+0.38/RT)
(7)
16.8 atom % Cr
S = 2.0 exp(-5.99/RT) S = 0.8 exp(-9.46/RT)
(8)
24.2 atom % Cr
S
(9)
i s given in units of cm? H&TP)/[cm" alloy kPa'/'l, and the constant in the exponential term is in kJ/g atom. The solubility decreases with increasing chromium content, and the temperature coefficient changes sign. becoming more negative with increasing chromium content. A change in sign of the temperature coefficient of solubility was also observed in the Pd-Ni system."' Table I1 shows a comparison of the solubility values
Hydrogen Permeation in Palladium-Chromium Alloys
311
TABLE 11: Comparison of Hydrogen Solubility Values at 449 K Measured Directly and Computed from .__I
_ _ _ I -
Atom % Cr I _ _ _
-
9.3 16.8 24.2 -_
a S i s given
S (measd)
---____-. S (compd) 0.81
0.95 0.54
0.12
0.40 0.06
in units of cm3 H,(STP)/[cm3alloy kPa%].
computed from eq 7-9 with values measured experimentally at 449 K. The agreement is reasonably good for the first two alloys. For the third alloy, the agreement is not as good; the measured value may be in error because of the difficulty in measuring the lower hydrogen concentration. Rapid advances are being made in applying band theory to explain hydrogen solubility in metals.I3 Hydrogen solution in palladium can be considered basically to follow the proton model, in which the hydrogen electrons go into existing holes in the palladium d bands. In fact, 0.4 electrons per Pd atom can be accommodated in these holes,14but another 0.3 electrons per P d atom go into new hydrogenmetal hybrid states created by the presence of hydrogenI6 (a feature of the anion model). Now, when the palladium is alloyed with another metal, the Fermi energy will change, causing either a decrease or increase in the number of holes in the d bands, depending on the metal added. Using a rigid band approach, one would predict that the Fermi energy would increase with silver additions, Le., the density of states at the Fermi energy would decrease.16 If this occurred, the hydrogen solubility should decrease when silver is added to palladium. Experimental data show that the opposite occurs, so it is probable that silver additions create hybridized S-like bands below the Fermi energy (as hydrogen does in yttrium17), thereby providing effectively for more holes in the palladium d bands. In the Pd-Cr system, a rigid band model could produce either an increase or a decrease in electron holes in the d band, depending on whether the Cr bands were Pd-like or the Cr electrons had to be put into existing holes in the palladium d band. Our experimental results suggest that some Cr electrons go into Pd d-band holes, since a decrease in hydrogen solubility is observed. The solubility effects observed here can also be discussed in terms of a thermodynamic “interaction parameter” formalism developed for liquid iron alloys.ls A large number of interaction parameters have been compiled for computing the effect of one alloying element on the solubility of another in liquid iron. While the model applies also to solid solutions, a compilation has not been made of available data. In the interaction parameter model, the activity coefficient, y, of a constituent in a ternary or multicomponent alloy is expressed as a series of terms. For the Pd-Cr-H system, the expression for y~ is (10) where is the activity coefficient of hydrogen a t infinite dilution e# = a In yHlaNII, = a In yH/aNcr, and the N’s are concentrations in atom fraction. This model is strictly applicable only to very dilute solutions, but in practice it has been used successfully for moderately concentrated solutions. If YH is defined as P H J N H it , can be shown from the data for 449 K in Figures 3 and 4 that eq 10 reduces to In YH = 3.57
+ 11.9Ncr
(11)
The second term in eq 10 is zero, which follows from Sievert’s law, and the Cr-H interaction coefficient e ~ is, found to be +11.9. This value represents a relatively strong thermodynamic interaction of the repulsive type between Cr and H. Equation 11 can be used to compute hydrogen solubility values for various hydrogen pressures and chromium concentrations at 449 K. There is some risk, however, in using the equation outside the range of the experimental measurements, especially for higher hydrogen pressures and higher chromium concentrations. With respect to the diffusion coefficient data, the observed effects can be explained in terms of contraction of the crystal lattice of palladium by chromium. Hydrogen is known to dissolve interstitially in metals, so one of the parameters which affects the ease of migration of hydrogen in the material is the size of the interstitial holes. Grube and Knabel9 have shown that the lattice parameter of palladium decreases continuously with additions up to 60 atom 96 chromium. Thus there is a correlation between contraction of the crystal lattice and lowering of the diffusion coefficient of hydrogen in Pd-Cr alloys. The size of the effect in Pd-Cr alloys is approximately the same as reported for Pd-Rh alloys.ll It is not felt that the size of interstitial holes is the only parameter of importance in affecting hydrogen diffusion rates. Band theory might shed more light on diffusion effects, but any conclusions drawn at the present time would be more speculative than for solubility effects and will not be attempted here. Another topic that merits discussion is the pressure dependence of the permeation rate. The observation that the permeation rate a t low pressures is lower than one would predict from the solubility and diffusion coefficient could be due to any one of several possible effects: (i) insufficient flux of hydrogen molecules impinging on the surface, (ii) low sticking coefficient of hydrogen,20 (iii) deviations from Sievert’s law behavior, (iv) slow dissociation of molecules to form chemisorbed hydrogen atoms. When calculations were made and consistency with the experimental results examined, explanations based on (i), (ii), and (iii) were dismissed. The formation of chemisorbed hydrogen on the surface appears to be the only process which is affecting the permeation rate. Smithells and Ransley21 observed similar effects a t low pressures and interpreted their results with the aid of the Langmuir adsorption equation. Our results in Figure 5 and those of others follow a socalled parabolic adsorption isotherm in which the volume of gas adsorbed varies logarithmically with p r e ~ s u r e . ~ ~ ~ ~ ~ It should be mentioned that the presence of chromium in palladium could play a significant role in adsorption behavior, particularly if the concentration of chromium atoms at the surface is higher than in the bulk due to surface energy considerations. Another potentially important effect is the presence of impurities on the surface. Auger spectroscopy showed the presence of sulfur, oxygen, and carbon on the specimen surfaces, all of which could reduce the sites available for hydrogen adsorption. It is of interest to examine the pressure dependence effect using a chemical reaction model. The reaction under study is
Hz (gas)
2H (adsorbed)
The overall rate of this reaction is the difference between the rates of the forward and reverse reactions: The Journal of Physical Chemistry, Vol,80, No. 3, 1976
Swansiger, Swisher, Darginis, and Schoenfelder
312
/
7
where hl and k2 are rate constants and [HI is the concentration of hydrogen at the surface. At steady state, the overall rate of the chemical reaction must be equal to the permeation flux through the specimen. [HI will in general be lower than the value obtained from Sievert's law for the same pressure, and it can be evaluated from the measured permeation flux, specimen dimensions, and diffusion coefficient. This model was tested by using two experimental data points to evaluate k l and k 2 from simultaneous equations derived from eq 12. In Figure 7, the curve obtained from the model is compared to the experimental curve. Even though the model is probably oversimplified, the agreement is rather good. The agreement was nearly the same when the model was applied to the data for the Pd-16.8 atom % Cr alloy. It should be mentioned that at higher pressures, the [HI2 term dominates, the computed curve passes through a maximum and the model can no longer apply. The last topic to be discussed is the behavior of palladium alloys under conditions of high hydrogen pressure and relatively low temperature. In unalloyed palladium a t temperatures below approximately 673 K, a hydride phase forms at moderate to high hydrogen pressures. Addition of nonhydride-forming elements such as silver and chromium to palladium tends to suppress hydride formation. No phase equilibrium data are available on either system, but it is known that silver has a beneficial effect on the structural integrity of palladium when it is thermally cycled in a hydrogen environment. In the few experiments completed to date, it has been demonstrated that the Pd-25 atom % alloy is seriously degraded in 50 MPa H2 a t 373 K but the Pd-16.8 atom % Cr alloy is not (see Figure 6). For the Pd16.8 atom % Cr alloy, no defects were present in the microstructure after the hydrogen was removed. Thus, while the permeation rate in the Cr alloy is not as high, it may be more suitable for high-pressure, low-temperature applications than the commercial Pd-Ag alloy because of better structural integrity. In addition to gas purification systems, Pd-Cr alloys might also have attractive properties as catalysts used in hydrogen at high pressures and low temperatures.
Summary The hydrogen permeation behavior of three Pd-Cr alloys has been characterized at temperatures to 449 K and at low pressures. The permeation coefficient, diffusion coefficient, and hydrogen solubility all decrease substantially with increasing chromium content. The hydrogen solubility in the alloys was found to obey Sievert's law. Measurements of the pressure dependence of the permeation rate showed that the rates of surface adsorption and dissociation of hy-
The Journal of Physical Chemistry, Vol. 80, No. 3, 1976
I '
p% (kPa1' Figure 7. Comparison of pressure dependence of permeation flux with model for slow surface reaction.
drogen molecules had a slight effect on the overall rate. In a few hydrogen absorption experiments at 373 K and 50 MPa HP,it was found that Pd-25 atom % Ag is susceptible to severe cracking while Pd-16.8 atom % Cr is not.
Acknowledgments. The authors wish to acknowledge D. R. Folk for making the permeation measurements, L. A. West for Auger spectroscopy, T. L. Bryant for metallographic assistance, M. C. Nichols for x-ray diffraction measurements, R. B. Anderson for high pressure testing, and S. C. Keeton for comments on the application of band theory. References and Notes (1)J. B. Hunter, U.S. Patent No. 2 773 561,Dec 1956. (2)D. L. McKinley, US. Patent No. 3 247 648,April 1966. (3)0.Loebich, German Patent No. 1 533 234,June 1970. (4)J. P. G. Farr and I. R. Harris, U S . Patent No. 3 713 270,Jan 1973. (5) J. W. Simons and T. B. Flanagan, J. Phys. Chem., 69,3581 (1965). (6)G. Bohmholdt and E. Wicke, 2. Phys. Chem. (Frankfurt am Main), 56, 332 (1967). (7)F. J. Ackerman and G. J. Koskinas, ind. Eng. Chem., Fundam., 11, 332 (1972). (8)G. L. Holleck, J. Phys. Chem., 74,503 (1970). (9)R. A. Karpova and I. P. Tverdosvskii, Zh. Fiz. Khim., 33, 1393 (1959). (IO) V. A. Gol' Tsov, V. B. Demin, P. V. Gel'd, and G. E. Kagan, Fiz. Khim. Mekh. Mater., 7,56 (1971). (11) D. Artman and T. E. Fianagan, J. Phys. Chem., 77; 2804 (1973). (12)K . D. Allard and T. B. Flanagan, J. Phys. Chem., 74,298 (1970). (13)R. B. McLellan and C. G. Harkins, Mater. Sci. Eng., 18,5 (1975). (14)F. M. Mueller, A. J. Freeman, J. 0 . Dimrnoils. and A. M. Furdyna, Phys. Rev. B., 1, 4617 (1970). (15)A. C. Swltendick, Ber. Bunsenges. Phys. Chem., 7 6 , 5 3 5 (1972). (16)0.K. Anderson, J. Appl. Phys., 41, 1225 (1970). (17)A. C.Switendick, Solid State Commun.,8, 1463 (1970). (18)J. F. Elliott, M. Gleiser, and V. Ramakrishna, "Thermochemistry for Steelmaking", Vol. 2,Addison-Wesley, Reading,.Mass., 1963,p 562. (19) G. Grube and R. Knabe, 2.Elektrochem.,42,739 (1936). (20) H. Conrad, G. Ertl, and E. E. Latta, Surface Sci., 41,435 (1974). (21)C. J. Smithells and C. E. Ransley, Proc. R. SOC.London, Ser. A, 150, 172 (1935):152,706 (1963). (22)H.Freundlich, "Kapillarchemie", I, 153 (1930). (23)D. H. Baughm and F. P. Burt, Proc. R. SOC. London, Ser. A, 105, 461 (1924).
