Cr2O3 Composites Prepared by Internal Oxidation

Feb 10, 2009 - H2 Solubility in Pd/Cr2O3 Composites Prepared by Internal Oxidation of ... determined H2 isotherms for the IOed, internally oxidized, a...
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J. Phys. Chem. C 2009, 113, 4078–4087

H2 Solubility in Pd/Cr2O3 Composites Prepared by Internal Oxidation of Pd-Cr Alloys: Equilibrium Pressure-Composition-Temperature Data D. Wang,† Ted B. Flanagan,*,† Gouthama,‡ and R. Balasubramaniam‡ Chemistry Department, UniVersity of Vermont, Burlington, Vermont 05405, and Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, Kanpur, 208 016 India ReceiVed: October 21, 2008; ReVised Manuscript ReceiVed: January 5, 2009

The solution of hydrogen and hydride formation in Pd/Cr2O3 composites has been investigated. Internal oxidation of Pd-Cr alloys is employed to prepare the composites consisting of nanosized Cr2O3 precipitates within Pd matrices. The dissolved H segregates to the internal Pd/Cr2O3 interfaces. Plots of pH1/22 versus H content for internally oxidized Pd-Cr alloys exhibit small positive intercepts along the H/Pd axis which can be attributed to trapping of H by Pd/Cr2O3 internal interfaces. The amount of trapping is found to be directly proportional to the atom fraction Cr in the alloys after internal oxidation at the same temperature. In addition to the enhanced solubility noted from these intercepts, H2 solubilities in the dilute phase of these internally oxidized alloys are larger than in pure Pd or in other internally oxidized Pd-rich alloys and these solubilities also increase with atom fraction Cr, XCr. In contrast with internally oxidized Pd-Al alloys studied earlier, internally oxidized Pd-Cr alloys with XCr g 0.02 have an initial plateau pH2 lower than those of Pd-H. The H2 isotherms for internally oxidized alloys with XCr g 0.05 exhibit gradual phase transitions to the two phase coexistence region rather than an abrupt one as for annealed Pd. Introduction The work described here is part of an effort to characterize the effects of nanosized oxide precipitates on H2 absorption in Pd. Metal/oxide interfaces are important for several technological applications.1 An advantage of employing H for investigation of the segregation to internal metal/oxide interfaces is that it diffuses rapidly at moderate temperatures whereas other solutes do not. It can be safely assumed that only H segregates to the interfaces at moderate temperatures, and therefore, these internal interfaces will be clean with respect to impurities. The oxide precipitates resulting from IO, internal oxidation, are generally nanometer-sized.2 In internal oxidization (IO), a solute metal is oxidized within a less oxidizable metallic matrix to form small oxide precipitates.2 In order to relieve internal stress caused by the expansion of the oxide precipitates during IO, matrix metal atoms must be transported from the interfacial region to the surface and simultaneously there must be a counterflow of vacancies from the surface to the interface.3 Evidence for this transport is the appearance of nodules of the matrix metal on the surface with a total volume closely equal to that caused by the expansion due to the growing oxide precipitate.3,4 Dissolved H was first employed by Kirchheim et al.5-8 as a probe of metal/oxide internal interfaces prepared by internal oxidation of several Pd-M alloys, e.g., M ) Al, Zr, and Zn. Kirchheim et al. found by comparison of electrochemically determined H2 isotherms for the IOed, internally oxidized, alloys with those of pure Pd that H was both strongly and weakly trapped by Pd/oxide interfaces. The existence of strong trapping of H by metal/oxide interfaces was indicated by regions of the isotherms where pH2, or its equivalent in electrode potential, was ∼0 from the origin along the (H/Pd) () r) axis.8 Further support * To whom correspondence should be addressed. † University of Vermont. ‡ Indian Institute of Technology.

for strong trapping at the interfaces is that the electrical resistivity and H diffusion constants become characteristic of pure Pd only after the strong traps are filled, i.e., at H contents beyond where pH2 ≈ 0 region.8 The strongly trapped H was believed to be attached to outer oxygen atoms at the metal/ oxide interface.8,9 The present workers found that the strongly trapped H in Pd/Al2O3 composites could not be removed by evacuation at 473 K but could by evacuation at 573 K.10 The weak traps were attributed by Huang et al.8 to both the stress fields adjacent to the precipitates and a chemical interaction between the H and the precipitates; later they proposed that there may be an interaction with stress fields about misfit dislocations and/or thermal residual stress (TRS).11 TRS arises from the difference in the coefficients of thermal expansion of the Pd matrix and the oxide when the alloy is cooled from the IO temperature to ambient. It was shown to be the probable cause of the weak trapping in Pd/alumina composites.12 TEM photomicrographs show that there are negligible dislocation densities after IO of Pd-Al alloys with XAl e 3 at %,12,13 but by contrast, dislocation loops are found in IOed Pd-Cr alloys with more loops than oxide precipitates.14 Hydride formation/decomposition, cycling, causes an abrupt volume expansion/contraction introducing large dislocation densities into Pd15 which subsequently increases its dilute phase solubility relative to annealed Pd at a given pH2 because of the attraction of H to the dislocation stress fields.16,17 Experimental Section Pd-Cr alloys were prepared by arc-melting the pure component metals and then annealing the buttons for 72 h at 1173 K. The buttons were then rolled into thin foil (120 µ) and reannealed. The times needed for complete IO in the atmosphere ranged from 12 h (1273 K) to 144 h (998 K). After IO, alloys were quenched in water (at 298 K) from the IO temperatures. The extent of IO was determined from the weight changes and

10.1021/jp8093148 CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

H2 Solubility in Pd/Cr2O3 Composites

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verified from the H2 isotherms.18 Hydrogen isotherms were measured in an all-metal Sieverts’ apparatus; several diaphragm gauges were available permitting pressure determinations over various ranges, e.g., 0-1.4 MPa and 0-14 MPa. A series of anneals in vacuo were carried out on some of the IOed alloys which were done cumulatively; that is, the same sample was subjected to annealing at progressively higher temperatures. TEM were taken with a JEOL 2000FX electron microscope. The alloy strips were thinned to about 0.5 mm, and then 3 mm discs were punched out from the strips. Thin foils suitable for TEM were prepared from the discs by jet polishing in a solution of 77% acetic and 23% perchloric acid. Results and Discussion Stoichiometry of the Cr Oxide Formed from IO. The stoichiometry of the Cr oxide formed from IO was determined by weighing the Pd-Cr foils before and after IO. Cr2O3 (chromia) is the most likely oxide to form because it is the most stable and the weight increases after IO were closest to it although they were generally on the high side of 100% Cr2O3, e.g., for six determinations the average was 107% after IO at 1098 K for a period of about 72 h. A Pd0.97Cr0.03 alloy was IOed at 1273 K for 24 h, and it gave 102% Cr2O3. In any case, the identification of the oxide precipitate as Cr2O3 was confirmed directly from electron diffraction patterns of the precipitates using TEM. The observation of greater weights than predicted from the oxide stoichiometry was also observed for IOed Pd-Co alloys19 but was not found for other alloys such as Pd-Al10 and Pd-Ni.19 For Pd-H, isotherms at H contents greater than the twophase plateau in the single hydride phase region, p1/2 H2 rises steeply with r and, at about pH2 ) 0.1 MPa, r ) 0.67 (298 K). The H capacities of IOed Pd-Cr alloys, which should be essentially Pd, are ∼2% greater than 0.67 at the same conditions and this increased slightly with % Cr. The second and third cycles for a given IOed alloy had the same r at 0.1 MPa and 298 K as the initial value. XRD, SEM, and TEM Observations of IOed Pd-Cr Alloys. An XRD (X-ray diffraction) pattern of an IOed (1073 K) Pd0.93Cr0.07 alloy gave reflections at the same angles as bulk Pd but they were weaker and somewhat broader; this supports the view that the matrix of the composite is pure Pd. SEM of an IOed Pd0.98Cr0.02 alloy showed several relatively large pores (≈10 µm) not observed for IOed Pd-Al alloys10 and there were extruded Pd nodules on the surface; however, these were not as plentiful as for IOed Pd-Al alloys.12 The surfaces of the IOed Pd-Cr alloys had thermal grooves, and the IOed alloys cracked intergranularly. A network of pores appeared on the grain boundary walls which were extensively connected; pores were also observed for Pd-Al alloys.20 The presence of pores indicates transport of vacancies to the surface during IO,3,4 but the connectivity has not been observed for other IOed alloys. When the IOed Pd-Cr alloy was cycled, no obvious new features were introduced into the SEM images. After IO at 1098 K, the resulting Pd/chromia composites were examined by TEM, and some of the results were shown earlier.14 For the IOed Pd0.98Cr0.02 alloy dislocations are seen to be pinned by the precipitates and there are also a few dislocation loops. Faceted chromia precipitates were about 20 nm in size, and the Pd matrix showed some strain contrast. Larger chromia precipitates are seen in grain boundaries with adjacent zones nearly precipitate-free. The dislocation density does not appear to be very great.

Figure 1. Dilute phase solubilities (323 K) for IOed (1098 K, 72 h) Pd-Cr alloys and Pd(b). Continuous line without points, Pd(b); dashed line, cycled Pd; O, Pd0.98Cr0.02; ∇, Pd0.97Cr0.03; 0, Pd0.95Cr0.05 and ∆, Pd0.93Cr0.07 alloy. The empty and corresponding filled symbols refer to the initial and second cycle, respectively. The (a) inset shows the intercepts as a function of XCr and the (b) inset shows (r′ - rPd) against the intercept where r′ is for the internally oxidized alloy and r is Pd(b)-H.

An IOed Pd0.96Cr0.04 alloy had a much greater density of dislocation loops than the IOed Pd0.98Cr0.02 alloy.14 There were many more loops than precipitates for the Pd0.96Cr0.04 alloy, and a series of punched-out dislocation loops could be seen around some precipitates. After hydriding/dehydriing (cycling), TEM revealed dislocation forests arranged in cells and the formation of some subgrain boundaries having very high dislocation densities. TEM for the IOed Pd0.94Cr0.06 alloy is not shown because its surface is quite irregular due to profound changes occurring during IO and consequently it was difficult to image but, nonetheless, it clearly has a very high density of loop and other dislocations, i.e, significantly greater than the IOed Pd0.96Cr0.04 alloy. As noted above such large dislocation densities were not observed in the IOed Pd-Al alloys.20 Strongly Trapped H (323) K in IOed Pd-Cr Alloys. In the absence of perturbations caused by the precipitates, e.g., stress, interfaces, etc., H2 solubilities in the Pd matrix of IOed alloys should be the same as in Pd(bulk), e.g., the XRD patterns are the same. For purposes of identification, Pd in the composites will be referred to as Pd matrix and bulk Pd as Pd(b) with the understanding that both are pure and bulk phases rather than thin film or nanocrystalline, etc. The dilute phase H2 solubilities in the Pd/chromia composites resulting from IO of Pd-Cr alloys at 1098 K are shown in Figure 1 (323 K). For meaningful comparisons, the alloys were all IOed at the same temperature because the temperature is an important factor determining the precipitate size2 which influences the stresses and interfacial areas and, consequently, the subsequent H2 solubilities. The pH1/2 2-r plots can be seen to have positive intercepts along the r axis indicating strong trapping.8,12 The values of the intercepts increase linearly with XCr, atom fraction Cr, as seen in the inset of Figure 1. Positive intercepts have been observed for the other IOed Pd-M alloys which have been investigated,8,10 and although the details differ among the various alloys, the magnitudes of the intercepts are not unreasonable for attribution to H segregation at the internal Pd/oxide interfaces.8 After the strong trapping, the solubilities are seen to be greater than for Pd(b) in the dilute phase and these solubilities increase with Cr content.

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Figure 2. Dilute phase solubilities (323 K) for IOed (72 h) Pd0.97Cr0.03. Dashed line, Pd(b); O, Pd0.97Cr0.03 IOed 1098 K where the arrow shows data after cycling; ∆, IOed 1273 K where the arrow shows data after cycling. b, repeat solubility of Pd0.97Cr0.03 IOed at 1098 K after evacuating at 323 K (2 h); 2, repeat solubility of Pd0.97Cr0.03 IOed at 1273 K after evacuating at 323 K (2 h).

A “control” experiment was carried out in which Pd(b) was heated in the atmosphere for 96 h (1123 K) and then quenched, i.e., typical conditions for IO.8 This experiment indicates the role of any dissolved O introduced at the elevated temperature of IO (1123 K).21 An isotherm was measured (323 K) after this atmospheric exposure and a very small intercept was found, r ≈ 0.0002, which arises from H trapping by dissolved O.21 This will be neglected as compared to the much larger intercepts for the IOed Pd-Cr alloys (Figure 2). Solutes are attracted to or repelled from internal interfaces due to a difference of surface free energies of the two phases according to the Gibbs equation.22 The specific type of binding at the interface may be elastic or chemical. Dilute phase hydrogen solubilities are shown in Figure 2 for two Pd0.97Cr0.03 alloys one of which had been IOed at 1098 K and the other at 1273 K. The alloy with the smaller oxide precipitates, i.e., the one IOed at 1098 K, has a slightly greater intercept (Figure 2) than the alloy IOed at 1273 K; however, the dilute phase solubilities differ dramatically. The alloy IOed at 1273 K has a solubility similar to Pd(b), whereas that IOed at 1098 K has a significantly greater solubility. This suggests that the trapping of H at the Pd/chromia interfaces and the enhanced dilute phase solubilities may be independent of each other, however, a linear correlation of the strong trapping and solubility enhancement was shown in Figure 1. The reason for this may be that the alloys in Figure 1 were all IOed at 1098 K, whereas in Figure 2, the alloys are IOed at different temperatures. Before the two phase plateau region was reached, these two IOed alloys were evacuated (323 K, ∼2 h) and their remeasured solubilities were found to nearly pass through the origin (Figure 2) which means that there is no strong trapping after this evacuation. It follows from this that the trapped H is not removed by evacuation at 323 K. Repeat solubility data for IOed Pd-Al alloys10 also pass through the origin indicating that the H is strongly held in both systems. If the initial solubility relations are translated along the r axis in order to pass through the origin, they nearly coincide with the repeat determinations. Cycled IOed Alloys. It is seen in Figure 1 that the intercepts of the solubility relations of the IOed alloys, after their cycling and evacuation at 323 K, are the same as the original ones.

Wang et al.

Figure 3. Dilute phase solubilities (323 K) for the Pd0.93Cr0.07 alloy after IO at 1098 K, 72 h. Dashed line, Pd(b); O, initial solubility after IO; b, repeat solubility after evacuation at 323 K; ∆, solubility after cycling; 2, solubility for the seventh cycle after evacuation at 323 K (2 h).

This demonstrates that the cycling affects the strongly trapped H allowing it to be removed by evacuation at 323 K, whereas before cycling, it is not removed. It has been suggested to explain a similar observation for an IOed Pd0.97Al0.03 alloy that dislocations generated during cycling interact with the H trapped at the Pd/oxide interfaces thereby weakening the traps and allowing removal of H at 323 K.10 Dilute Phase H2 Solubilities before Cycling. In this section the unusually large H2 dilute phase solubilities in IOed Pd-Cr alloys (Figures 1-3) will be examined and discussed. Shown in Figure 3 are the initial and repeat solubility relations for an IOed (1098 K) Pd0.93Cr0.07 alloy which exhibits a remarkably large solubility enhancement in the dilute phase. Its repeat solubility passes through the origin after evacuation at 323 K as found for the IOed Pd0.97Cr0.03 alloy (Figure 2). The TEM results discussed above clearly show that there are large dislocation densities in the IOed alloys resulting from IO, and they increase with XCr and many of these are dislocation loops. Since such high dislocation densities have not been found after IO of other Pd--alloys and such large solubility enhancements have also not been found, the large dislocation densities appear to be the cause of the solubility enhancements. These dislocation densities must be very large indeed to explain these large dilute phase solubilities (Figures 1-3). The presence of the small precipitates after IO must play an important role in the formation of the large dislocation densities, e.g., by anchoring the dislocations23 and also by punching-out loops. The solubility enhancement of the initially IOed Pd0.93Cr0.07 alloy is especially large, (r′IOed/rPd(b)) ≈ 4.3 where r′IOed refers to the H content of the IOed alloy and rPd(b) to Pd(b) at the same pH2 and temperature. These solubility enhancements are greater than that for Pd after its cycling and/or cold-working.15,16 Some solubility enhancements for IOed Pd-Cr alloys are shown in Table 1. In column 2, (r′IOed/rPd(b))pH2, appears to depend directly on XCr, and it also depends on the IO temperature; there is very little enhancement after IO at 1273 K (Figure 2). Dilute Phase H2 Solubilities after Cycling. Solubility enhancements due to cycling Pd-Cr alloys, which were all IOed at 1098 K for 72 h, (Figure 2) are shown in Table 1. It is of interest that the solubility enhancements due to cycling the IOed alloys, r′cycl,IOed/rIOed, decrease with increase of XCr (Figure 1, Table 1 (column 4)) and are quite small for XCr ) 0.07. Although

H2 Solubility in Pd/Cr2O3 Composites

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TABLE 1: Free Energy Changes (323 K) for Hydride Formation of IOed (1098 K) Pd-Cr Alloys and Solubility Enhancementsa XCr

(r′IOed/ rPd(b))

(r′cycl.IOed/ rPd(b))

(r′cycl,IOed/ rIOed,alloy)

f RT ln pf1/2 ) ∆Gplat in kJ mol H

0 0.02 0.03 0.05 0.07

1.0 1.47 1.97 3.0 4.3

1.35 2.03 2.28 3.29 4.62

1.35 1.38 1.16 1.09 1.02

-3.65 -3.94 -3.97 -4.16 -4.29

a The solubility enhancements have been evaluated at pH2 )1.2 kPa and 323 K. The strongly trapped H has been removed in the calculation of the enhancements.

TABLE 2: Hydrogen Solubility Increments at 323 K, and 1.2 kPa, ∆r, in IOed (1098 K, 72 h) Pd-Cr Alloys

XCr

∆r (IO)

∆r (Pd(b),cycle)

0.02 0.03 0.05 0.07

0.0029 0.0073 0.0215 0.0337

0.003 0.003 0.003 0.003

sum of columns 2 and 3

∆r (experimental)

0.0059 0.0103 0.0245 0.0367

0.0074 0.0093 0.0238 0.0349

the solubility enhancement due to cycling is nil for the IOed Pd0.93Cr0.07 alloy, dislocations must still form and move during the cycling in order to accommodate the large, abrupt volume changes which occur in the Pd matrix. It is suggested that dislocations form during the cycling for this alloy but annihilate after formation such that their density, and the subsequent solubility enhancement, remain nearly unchanged. The efforts to confirm this directly using TEM was unsuccessful for the high Cr alloys because their dislocation densities were already so large that it could not be ascertained whether or not additional dislocations had been introduced by cycling. If the solubility enhancement increment from cycling IOed alloys is the same as that for cycling Pd(b) plus that due to IO, the solubility enhancement increments, ∆r ) rIOed,cycl - rPd(b), would be equal to ∆r(IO) + ∆r(cycle Pd(b)). For the Pd0.98Cr0.02 and Pd0.97Cr0.03 alloys, the experimental values are greater than the sum indicating that more dislocations are generated than for Pd(b) due to the presence of small precipitates during cycling (Table 2). The small oxide precipitates interact with dislocations generated by cycling. It is known that precipitates can act as barriers for dislocation movement and the resulting hardening is referred to as dispersion- or precipitation-hardening.24 For the Pd0.95Cr0.05 and Pd0.93Cr0.07 alloys the experimental values are slightly lower than the sum. This can be due either to the cycling annihilating some dislocations created by IO or that the dislocations needed to accommodate the volume changes during cycling are simultaneously annihilated along with some of those from the IO. In an attempt to introduce more dislocations and greater dilute phase solubilities in the Pd0.95Cr0.05 and Pd0.93Cr0.07 alloys, they were IOed at a lower temperature, 998 K, than usually employed. This should result in smaller precipitates2 which interact more strongly with dislocations; the alloys were also cycled several times. The results show that the intercepts are about the same as for the alloy IOed at 1098 K (Figure 4) but the dilute phase solubilities are somewhat greater especially as reflected by the repeat solubility for the Pd0.95Cr0.05 alloy shown in Figure 4. During measurement of the solubility for the IOed (998 K), cycled alloy, absorption was reversed to desorb H2 along the isotherm at several different H contents (shown by the letters

Figure 4. Dilute phase solubilities (323 K) for a Pd0.95Cr0.05 alloy IOed at 1098 K (72 h) and an Pd0.95Cr0.05 alloy IOed at 998 K (120 h). Solid line without symbols, Pd(b); O, initial solubility after IO at 1098 K; b, repeat solubility after evacuation 323 K; ∆, initial solubility after IO at 998 K; 2, repeat solubility after evacuation at 323 K; 0, solubility after IO at 998 K and many cycles; A, B, C, and D are the initiation points for desorption as shown by 9.

Figure 5. Dilute phase hydrogen isotherms at several temperatures for an IOed, cycled Pd0.93Cr0.07 alloy. The temperatures are indicated. The open and filled symbols indicate separate determinations at a given temperature.

in Figure 4), and if they are nearly the same, the process is reversible. This is the case up to r ) 0.045, i.e., absorption and desorption isotherms coincide. The content of r ) 0.045 is much greater than the terminal hydrogen solubility of Pd(b) which is 0.019 at 323 K.25 This reversible behavior indicates that the solubility in this region corresponds to solution in the dilute phase rather than to some irreversible hydride formation which is accompanied by hysteresis. Partial Thermodynamic Parameters from Dilute Phase Isotherm Data. Dilute phase isotherms have been measured for IOed (998 K, 336 h) Pd0.95Cr0.05 and Pd0.93Cr0.07 alloys, which had been cycled after IO, at three and four different temperatures, respectively. Between each isotherm measurement, H was removed by evacuation at 323 K (2 h), and it should be noted that H can be removed by such an evacuation of cycled but not of uncycled alloys. Figure 5 shows results for the Pd0.93Cr0.07 alloy where the filled symbols represent a repeat set of measurements after evacuation. Since these dilute phase isotherms are linear, their slopes can be employed to determine |∆H°H| ) 16.1 kJ/mol H and |∆S°H| ) 59.8 J/K mol H. Both of these values are greater than for Pd(b), where the former is 10.3 kJ/mol H and the latter 55 J/K mol H.25 The standard designation refers to infinite dilution of H. Similar experiments were carried out at three different temperatures with an IOed (998 K, 624 h) Pd0.95Cr0.05 alloy cycled (10 times) with an intercept of r ) 0.005. This alloy

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Figure 6. Complete H2 isotherms (323 K) for Pd-Cr alloys after IO at 1098 K (72 h) and for Pd(b). O, Pd(b); ∆, IOed Pd0.98Cr0.02 alloy; 0, IOed Pd0.97Cr0.03 alloy; ∇, IOed Pd0.95Cr0.05 alloy; ], IOed Pd0.93Cr0.07 alloy. The filled symbols are the corresponding desorption isotherms. The inset shows a correlation of log(pf/bar) with XCr where pf is the plateau pressure for hydride formation.

had decrepitated from its multiple cycling into a finely divided material. |∆H°H| ) 17.5 kJ/mol H and |∆S°H| ) 63 J/K mol H. The values were determined in the same way but these results are not as accurate because only three temperatures were employed whereas four were employed for the Pd0.93Cr0.07 alloy. Complete Isotherms for IOed Pd–Cr Alloys. Figure 6 shows complete initial H2 isotherms (323 K) for IOed (1098 K) Pd-Cr alloys where it can be seen that both plateau pressures decrease with XCr and hysteresis decreases slightly. The sloping of the absorption plateau, pf, increases with XCr. The inset of Figure 6 shows a linear correlation between log pf, which is proportional to the free energy change for hydride formation, and XCr for these IOed alloys (303 K). The lowering of both pf and the decomposition plateau pressure, pd, (Figure 6) may arise from long-range macroscopic stresses from dislocation pile-ups at the precipitates formed during hydriding. This is the first observation of IO decreasing the plateau pH2 for the initial isotherms. For instance, the pf values for IOed Pd-Rh alloys increase with XRh27 and for Pd-Al alloys they are basically unchanged from that of Pd(b) at least at 323 K.10 In addition to lowering plateaux pressures, the dilute f hydride transitions for the Pd0.95Cr0.05 and Pd0.93Cr0.07 alloys are more gradual after IO (1098 K) (Figure 6). The dilute f hydride transition is significantly more gradual for the IOed Pd0.95Cr0.05 than the Pd0.97Cr0.03 (Figure 6). Supersaturation of the dilute phase with respect to the hydride phase is found in well annealed Pd(b) but not in cycled Pd(b)28 indicating that dislocations serve as nucleation sites. Supersaturation occurs for the IOed (1098 K) Pd0.98Cr0.02 alloy indicating that few dislocations form during its IO; however, there is no supersaturation for XCr > 0.02 alloys IOed at e1098 K because of their large dislocation densities. For IOed Pd-Al alloys, there is supersaturation only for the Pd0.985Al0.015 alloy and not for

Wang et al.

Figure 7. Complete H2 isotherms (323 K) for a Pd0.97Cr0.03 alloy after IO at 1098 K (72 h). O, Pd(b). The following isotherms were carried out on the IOed Pd0.97Cr0.03 alloy: ∆, first cycle; ∇, second cycle; 0, fifth cycle. ], after heating the alloy in the atmosphere at 1273 K; ×, after annealing in vacuo, 1173 K. Open symbols are for absorption and filled ones for desorption.

XAl > 0.015.10 Gradual transitions are found for IOed Pd-Al alloys with XAl > 0.015; however, the transition to the hydride phase is more gradual for the IOed Pd-Cr alloys. Plateau pressures after the second cycle do not change with further cycling (323 K) for the IOed (1098 K) Pd0.97Cr0.03 alloy, but there is a greater degree of sloping (Figure 7). (Larger changes can be seen for isotherms measured at higher temperatures, e.g., g373 K.29) Initially there is a sharp dilute f hydride transition for the IOed (1098 K) Pd0.97Cr0.03 alloy which becomes more gradual after one cycle (323 K). Although no further changes are found after the second cycle (Figure 7), changes continue for a Pd0.97Cr0.03 alloy IOed at 998 K. A Pd0.97Cr0.03 alloy was IOed at 1273 K and its initial pf is slightly lower than that for Pd(b) but not nearly as low as for the same alloy IOed at 1098 K, and its pd is essentially unchanged from Pd(b). A small supersaturation can be seen for the initial cycle (323 K) (Figure 8) which is not present in this alloy IOed at 1098 K. The supersaturation observed indicates an absence of dislocations and also indicates that the relatively large chromia precipitates are unsuitable as nucleation sites. The second cycle has a slightly higher pf which is very close to that for Pd(b). The absorption plateaux for both cycles are quite horizontal (Figure 8). The effect of sequential cycles of hydriding/dehydriding on both pf and pd will now be discussed. For the IOed (1098 K) Pd0.95Cr0.05 alloy sequential isotherms (323 K) are shown in Figure 9 where the changes with respect to Pd(b)-H are more dramatic than for alloys with XCr < 0.05. In the initial cycle, pf decreases compared to Pd(b), and for the second cycle, there is a further decrease with the significant sloping. The pd values increase with cycling. There are continual changes in pf and pd from the second to the fourth cycle, but there are no corre-

H2 Solubility in Pd/Cr2O3 Composites

Figure 8. Complete H2 isotherms for a Pd0.97Cr0.03 alloy after IO at 1273 K (12 h). 0, Pd(b). The following isotherms (323 K) were carried in sequence: O, first cycle for the IOed alloy; ∇, second cycle for IOed alloy. Open symbols are for absorption and filled ones for desorption.

Figure 9. Complete H2 isotherms (323 K) for the Pd0.95Cr0.05 alloy after IO at 1098 K (72 h). 0, Pd(b). O, first cycle IOed Pd0.95Cr0.05 alloy; ∆, second cycle; ∇, fourth cycle. Another Pd0.95Cr0.05 alloy IOed at 1098 K was annealed at 1173 K for 12 h prior to cycling. ×, first cycle; ], second cycle. Open symbols are for absorption and filled ones for desorption.

sponding changes in the dilute phase solubility; therefore, the source of the plateau changes does not affect the dilute phase behavior. For the Pd0.93Cr0.07 alloy IOed at 1098 K, the changes with cycling are qualitatively similar to those for the Pd0.95Cr0.05 alloy, but they are somewhat greater (Figure 10). Sloping becomes quite pronounced and, e.g., pf of the seventh cycle of the IOed (1098 K) Pd0.93Cr0.07 alloy is lower at r < 0.35 but similar to that of the initial cycle at greater r values (Figure 10). The value of pd for the first cycle is significantly lower than that for Pd(b)-H, but it increases with cycling thus reducing hysteresis. After the initial cycle, pf increases slightly for Pd(b), but there are no further changes.26

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Figure 10. Complete H2 isotherms (323 K) for a Pd0.93Cr0.07 alloy after IO at 1098 K (72 h). 0, Pd(b); O, initial cycle; ∆, second cycle; ∇, third cycle; ×, seventh cycle. The filled symbols are the corresponding desorption isotherms.

Macroscopic stresses introduced by cycling are not the cause of the plateau changes because they would affect the subsequent dilute solubilities. Sloping of these plateaux cannot be attributed to compositional inhomogeneities since the Pd matrix should be the same as Pd(b), i.e., essentially all of the Cr is oxidized to chromia. It is clear from these results that the effect of cycling on the plateau behavior increases with XCr and with decreasing IO temperatures. The former leads to a greater number of precipitates and the latter to smaller ones. The sloping must be caused by stresses which develop during hydriding from the juxtaposition of the two phases in the presence of the precipitates. Hydride phase may form initially not only at a penetrating interface but also within the bulk. These stresses introduced by the juxtaposition of the phases and precipitates disappear after complete conversion to the hydride phase since pd is reasonably horizontal. Other stresses remain which affect the dilute phase solubilities. Hysteresis, 1/2RT ln(pf/pd), for the IOed Pd0.93Cr0.07 alloy decreases as a result of this cycling from 1070 J/mol H (Pd) to 945 J/mol H (first) to 470 (third) J/mol H each evaluated at r ) 0.30 (Figure 10). The almost complete elimination of hysteresis for an IOed Pd-Cr alloy has been described elsewhere.29 The effect of cycling on the plateau pH2 seems to level-off after about 4 cycles and plateau pressures have been evaluated after four and more cycles at r ) 0.3. Van’t Hoff plots for cycled IOed Pd0.93Cr0.07 (Figure 11) show that both plateau pH2 are smaller than those for Pd(b) and that they are very close, merging at higher temperatures. Enthalpy and entropy changes for the plateau reaction were determined from these van’t Hoff plots (Figure 11) and the average magnitudes for the enthalpies and entropies of hydride formation and decomposition are 20.6 kJ/ mol H and 48.5 J/K mol H, respectively. The |∆Hplat| values are larger for the IOed alloy than for Pd(b), which was redetermined in this research with agreement with earlier values.30 |∆Splat| is greater for the IOed alloys than for Pd(b)-H which would contribute to an increase in the plateau

4084 J. Phys. Chem. C, Vol. 113, No. 10, 2009

Figure 11. Van’t Hoff plots for Pd(b) and a multicycled IOed Pd0.93Cr0.07 alloy. O, Pd(b) and ∆, IOed Pd0.93Cr0.07. The data for Pd(b) were determined in this work. The open and filled symbols are for hydride formation and decomposition, respectively.

pressures, however, the increased enthalpy dominates, causing the plateau pH2 to decrease. Passing around the critical point of Pd(b)-H (563 K)31 and returning to 323 K, while maintaining an H content greater than that of the upper phase boundary, avoids the abrupt lattice expansion and dislocation generation/annihilation normally accompanying hydriding.32 This was done here with an uncycled IOed Pd0.93Cr0.07 alloy. After loading with H as described, H was desorbed until pd was reached. The H contents in the single hydride phase were obtained from the equilibrium pH2 values using a previous isotherm for the Pd0.93Cr0.07 alloy. The value of pd for this alloy is similar to the first cycle measured in the conventional way for another IOed Pd0.93Cr0.07 alloy. The only difference is that the onset of the hydride f dilute transition to the decomposition plateau was slightly sharper. This alloy which has passed around Tc already contained a large dislocation density from the IO and that is perhaps why it did not differ significantly from the usual ones which passed through the abrupt dilute f hydride transition. The IOed Pd0.93Cr0.07 alloy, which had passed around Tc, was then dehydrided by evacuation (323 K); that is, it was subjected to a half-cycle of hydriding/dehydriding, and its remeasured dilute phase solubility has an intercept r ) 0.0046. The solubility enhancement in the dilute phase was, however, similar to that for the initial cycle demonstrating again that most of the dislocation density arises from IO. After this alloy had undergone a complete cycle and evacuation (323 K), its subsequent intercept was r ) 0.007 or the same as shown in Figure 4 for a complete cycle. Thus only about 65% of H is lost from the strong traps following the hydride f dilute phase transition (half-cycle) and evacuation (323 K) showing that there is only about 65% of the dislocation interaction with precipitates of a full cycle where all of the strongly trapped H is removed by evacuation at 323 K. Effects of Annealing on IOed Alloys Containing Strongly Trapped H. RemoWal of the Strongly Trapped H. The dilute H2 solubility was determined for an IOed (1098 K) Pd0.93Cr0.07 alloy which had not been cycled and its intercept was similar to that in Figure 1. It was then evacuated at 323 K (2 h) and, as before, its repeat solubility intersects the origin. This IOed alloy, containing strongly trapped H, was then simultaneously heated and evacuated at various temperatures and, after each treatment, its repeat solubilities were redetermined (323 K) before any hydride formation had occurred. The amounts of strongly trapped H can be determined after each of these

Wang et al.

Figure 12. Dilute phase solubilities (323 K) for the Pd0.95Cr0.05 alloy after IO at 1098 K (72 h). Solid line without symbols, Pd(b); O, initial solubility of Pd0.95Cr0.05 after IO at 1098 K; b, repeat solubility after evacuation 323 K; ∇, after cycling. ×, H2 solubility for another IOed Pd0.95Cr0.05 alloy after cycling showing the good reproducibility. Solubility data for a third IOed Pd0.95Cr0.05 alloy which was annealed at 1173 K prior to cycling: ∆, first solubility measurement; 0, solubility after cycling; ], fifth cycle.

progressively higher temperature treatments from the redetermined intercepts. If all of the H is removed, the intercept is the same as that shown following IO in Figure 2 but, if none is removed, the intercept is zero and for intermediate amounts removed % H ) [(intercept)/(initial intercept)] × 100%. At higher temperatures the strong traps themselves are removed, but this will not be a factor at the temperatures of these heat treatments. H was not removed from the strong traps after heating/evacuating (2 h) at 423 K. After heating/evacuating (2 h) at 473 K, 14% of the strongly trapped H is removed. No further H is evolved at 523 K but, at 573 K (2 h), further H is removed for a total of 65%. For the Pd/Al2O3 composite strongly trapped H is not removed at 473 K but it was all lost at 573 K (2 h).12 After cycling (323 K) the strongly trapped H can be removed at 323 K but without cycling it cannot be removed by evacuation at e473 K demonstrating the profound effect of cycling on the nature of the H traps. None of these annealing treatments had any significant effect on the dilute phase solubility enhancements. Annihilation of Strong Traps Themselves by Annealing. Traps from the Initial IO. The initial and repeat H2 solubilities of a freshly IOed (1098 K) Pd0.95Cr0.05 alloy were redetermined with the expected results of a appreciable intercept and solubility enhancement (Figure 12). Another Pd0.95Cr0.05 alloy was IOed (1098 K) then heated in vacuo at 1173 K, and its subsequent solubility intercept was about half of that of the IOed alloy not subjected to the heating in vacuo (1173 K) (Figure 12) and its solubility enhancement much smaller, although there is still a dilute phase solubility enhancement compared to Pd(b). Since 1173 K is a sufficiently high temperature to remove all of the strongly trapped H, the heating/evacuation must annihilate some of the strong traps, otherwise, the intercept would not be reduced. This IOed Pd0.95Cr0.05 alloy was then cycled and its intercept was restored to that of the initially IOed alloy. This is noteworthy because it shows that dislocation production, movement, and interaction with the precipitates during cycling restores the traps (Figure 12). After a further cycling and evacuation (323 K), the intercept was unchanged again indicating that H was removed from these traps after cycling. The removal of the intercept after annealing cannot be due to growth

H2 Solubility in Pd/Cr2O3 Composites

Figure 13. Dilute phase solubilities (323 K) for a partially (71%) IOed (953 K) Pd0.95Cr0.05 alloy (11 d). Solid line without symbols, Pd(b); dashed line without symbols, alloy after IO; O, solubility of alloy after cycling; ∆, solubility after annealing at 923 K (12 h); ∇, solubility after annealing at 1023 K (12 h); 0, solubility after annealing at 1098 K (12 h); •, after annealing at 1173 K (12 h); 2, after annealing at 1223 K (8 d).

of the precipitates because, if this were the case, they could not be restored by cycling. Kirchheim et al.8 suggest that the strong traps are due to bonding of H to O at the internal interfaces and, in support of this, have shown that the traps can be removed by vacuum annealing (1273 K) and can be restored by subsequent exposure to oxygen. In the present work about half of the traps were removed by evacuating at 1173 K which supports this model, however, the restoration of the traps by cycling indicates that O is not needed to restore the traps so that the nature of the traps remains uncertain. RemoWal of Traps Introduced by Cycling. The following has to do with the removal of the traps which have been altered or produced by cycling. In order to obtain a large solubility enhancement a Pd0.95Cr0.05 alloy was IOed at 953 K for 12 d; however, even after this long time, it was only 71% IOed. In any case, this alloy was employed to investigate removal of the traps resulting from cycling. The effect of cycling is somewhat greater for this partially IOed Pd0.95Cr0.05 alloy, i.e., r′/rIOed,alloy ) 1.15, than for a completely IOed alloy, r′/rIOed,alloy ) 1.09 at 1.2 kPa (323 K). There is a greater increase of dislocation density by cycling in the partially IOed (71%) alloy because its unoxidized portion is affected more by cycling than the IOed portions. This partially IOed alloy was then annealed cumulatively (in vacuo) at a series of consecutively higher temperatures and, after each, a dilute phase isotherm was remeasured (323 K) (Figure 13). After annealing at 923 K (12 h), the dilute solubility decreases (Figure 13) partly because dislocations are annealed out in the unoxidized fraction but not in the IOed fraction. It is more difficult to anneal dislocations in the presence of the very small oxide precipitates.33 The intercept is unchanged after this annealing because the temperature is apparently too low for annihilation of these traps. After an anneal at 1023 K, the solubility enhancement decreases further and the intercept decreases from r ) 0.005 to 0.0018 and the pH1/22 -r plot levelsoff at a lower pH2 (Figure 13). A subsequent annealing at 1098 K (12 h) leads to the same intercept as the 1023 K anneal. It is interesting that nearly the same temperatures are needed to the annihilate traps reintroduced by cycling and those formed from the initial IO although their trapping depths for H differ markedly. The next anneal, 1173 K, leads to an even smaller

J. Phys. Chem. C, Vol. 113, No. 10, 2009 4085 intercept, and after an anneal at 1223 K for 72 h, the intercept is nearly zero indicating that the strong traps have all been removed. Annealing out of the Dilute Phase Solubility Enhancement. During the annealing/evacuation to remove the strong traps, the dilute phase solubility enhancements are also removed. After annealing at 923 K, only a little of the solubility enhancement is removed, but annealing at 1098 K removed about half of the solubility enhancement. It can be seen from Figure 12 that, following the annealing at 1173 K, the dilute phase solubility enhancement at 323 K, r′ann,IOed/rPd(b) ) 1.67 whereas after IO, the solubility enhancement is 3.08. The effect of cycling is much greater for the annealed than for the unannealed alloy, i.e., r′cycl.,IOed/runcyc ) 1.50 and 1.05 (at 1.2 kPa) indicating that many dislocations are introduced into the former but not the latter. Annealing at 1223 K for 8 days removes nearly all of the solubility enhancement (Figure 13). Since most dislocations should be removed by prolonged annealing at 1223 K, even in the presence of precipitates, there should be no contribution of dislocations to the solubility enhancement and this appears to be the case (Figure 13). Annealing Effects on the Complete Isotherms of IOed Pd-Cr Alloys. An IOed (1098 K) Pd0.97Cr0.03 alloy, which had already been cycled, was heated in the atmosphere at 1273 K for 12 h, and after this, its remeasured isotherm was compared with that after its original cycling (Figure 6, 323 K). The isotherms before and after this heating are very similar. The transition to the hydride phase is gradual for the second cycle, i.e., the same as before annealing (Figure 7). Further cycles did not lead to any changes regarding the gradual transition to the hydride phase. The same cycled Pd0.97Cr0.03 sample was then annealed in vacuo at 1173 K for 12 h, and its isotherm now exhibits a relatively sharp transition to the hydride phase similar to Pd(b) and pf is the same as Pd(b) (Figure 9). It seems that heating for similar times and temperatures g1173 K either in the atmosphere or in vacuo would both annihilate dislocations to a similar extent because this is a bulk phenomenon. Annealing in vacuo, however, leads to a sharp rather than gradual dilute f hydride phase transition and to higher pf and pd values. The explanation for this is not obvious. Complete isotherms are shown in Figure 14 for the partially (71%) IOed Pd0.95Cr0.05 alloy discussed above. There is a welldefined plateau and an indication of another one at pH1/22 ≈ 7.6 kPa which correspond to the IOed and unoxidized fractions, respectively, where the latter plateaux pH2 are known from an Pd0.95Cr0.05 alloy isotherm measured earlier.34 Hysteresis for the unoxidized portion is appreciable, i.e., see the dashed lines in Figure 14. After cycling, the plateau-like region for the unoxidized phase is only slightly affected; however, as expected, pf decreases for the IOed fraction. This partially IOed alloy was then subjected to a series of in vacuo anneals. After annealing at 923 K (12 h), there was almost no effect on the subsequent isotherm. The alloy was then annealed at 1023 K (12 h) causing pf of the IOed fraction to increase slightly and to shift the pH2 data at the higher plateau to greater H contents indicating that some limited metal atom diffusion occurs (Figure 14). If Cr diffusion from the unoxidized portion occurs into the oxidized portion, it will form an alloy with a Cr content smaller than Pd0.95Cr0.05 because the Cr in the chromia is not free to diffuse. Pd must diffuse into the unoxidized portion as the Cr diffuses out. An advantage in employing this partially IOed alloy is that an indication of the progress of Cr metal atom diffusion into the IOed portion can be followed by the isotherms after each annealing treatment.

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Wang et al.

Figure 15. log pH2 against t for H2 absorption (323 K) by IOed Pd0.97Cr0.03 alloys. Empty and solid symbols are for the initial absorption and that after cycling, respectively. ∆ after IO at 1273 K; O, after IO at 1098 K.

were also found to increase with an increase of XCr; however, the rates are slightly slower than those for the IOed Pd-Al alloys under comparable conditions. Conclusions

Figure 14. Complete isotherms (323 K) for a partially IOed (953 K) Pd0.95Cr0.05 alloy (11 d). Dashed lines, first cycle for alloy after IO; O, second cycle; ∇, after annealing at 1023 K (12 h); 0, after annealing at 1098 K (12 h); ×, after annealing at 1173 K (12 h); ∆, after annealing at 1223 K for 8 d. The filled symbols are the corresponding desorption isotherms.

After a further annealing, at 1098 K, additional shifts of the higher plateau region to greater H contents are observed and pf for the IOed fraction increases. After annealing at 1173 K for 12 h, these changes are accentuated and the higher plateau has vanished and pf of the IOed fraction increases markedly. The alloy was then annealed for an additional 120 h (1223 K) which leads to only one sloping plateau with an increased pf indicative of a more homogeneous distribution of Cr because Pd-Cr alloys have greater pf values than Pd(b). Comparison of the plateau pressures with the literature values34 indicate that the alloy is approximately Pd≈0.985Cr≈0.015 in agreement with that expected from diffusion of the Cr into the unoxidized portion throughout the alloy. The appreciable sloping and the large hysteresis, e.g., as compared to an annealed Pd0.98Cr0.02 alloy,34 may be caused by the small precipitates. Kinetics of H2 Absorption for Some IOed Pd-Cr Alloys. It has been shown that internally oxidized (1073 K) Pd-Al alloys exhibit faster rates of H2 absorption compared to Pd or to the corresponding unoxidized alloys.10 It is of interest to learn if the IOed Pd-Cr alloys also have relatively fast kinetics. Plots are shown in Figure 15 of data for the IOed Pd-Cr alloys plotted as log pH2 against time (323 K); pH2 is not constant but decreases during absorption. These rate studies are for comparison purposes and are not for fundamental characterization of the kinetic steps. As found for the IOed Pd-Al alloys,10 the rates are faster after lower, rather than higher, temperatures of IO, e.g., 1098 vs 1273 K, because the precipitates are smaller after the former and they presumably cause more favorable changes in the surface and near surface layers for the kinetics. The rates

The dilute phase solubilities of IOed Pd-Cr alloys have positive intercepts along the r axis and they increase in direct proportion to XCr. These intercepts are larger than those for IOed Pd-Al alloys.10 The most interesting features of the IOed Pd-Cr alloys are their large dilute phase solubilities compared to Pd(b) which increase in proportion to XCr. TEM showed that after IO of XCr g 0.05 alloys, large dislocation densities are found with many dislocation loops. These must be the reason for the large dilute phase solubilities. The extent of the solubility increase in the dilute phase region depends on the IO temperature, e.g., it is negligible after IO at 1273 K but large after IO at 1098 K. Cycling increases the dilute phase solubility in Pd(b) by creating dislocations which lead to H segregation into the stress fields about the dislocations, although the solubility enhancement, i.e., the increase in solubility relative to the IOed alloy, from cycling the IOed (1098 K) Pd0.98Cr0.02 alloy is similar to Pd(b). For XCr > 0.02, cycling does not increase the solubility as much, and for the IOed Pd0.93Cr0.07 alloy, there is only a minimal additional solubility enhancement. This is an interesting result which is consistent with the large dislocation densities, including many loops, introduced by IO especially for the higher Cr content alloys. Apparently the dislocation densities of these high Cr content alloys are saturated and although dislocations must still form during cycling in order to accommodate the large abrupt volume changes, they must be simultaneously annihilated. The dislocation densities are quite large because the solubility enhancements of the IOed alloys with XCr g 0.05 are significantly greater than for (cold-worked + cycled) Pd15 where the densities are known to be large. Strongly trapped H is not removed by evacuation (323 K); however, after cycling the IOed alloy, evacuation at 323 K does remove the strongly trapped H. This shows that the strong traps are altered by their interaction with dislocations generated by the phase transition such that the H can be removed by evacuation at 323 K. After the traps are removed by annealing at high temperature in vacuo, traps are reintroduced by cycling. Alloys with XCr g 0.05 are the first IOed (e1098 K) alloys found where the dilute f hydride transition for the initial isotherms (323 K) is very gradual. This is indicative of large dislocation densities because Pd(b) exhibits a sharp phase

H2 Solubility in Pd/Cr2O3 Composites transition whereas cycled Pd, containing a large dislocation density, does not.28 The pf of the IOed Pd-Cr alloys are lower than for Pd(b) and log pf decreases linearly with XCr (Figure 2). The pd are also lower than for Pd(b) and the desorption plateaux are rather horizontal (323 K) despite sloping of the absorption plateaux. Cycling decreases pf and increases pd thus decreasing hysteresis. Compared to Pd(b) hysteresis is quite small for the IOed, cycled Pd0.93Cr0.07 alloy. Acknowledgment. T.B.F. acknowledges financial support from Westinghouse Savannah River Co. References and Notes (1) Sinnott, S.; Dickey, E. Mat. Sci. Eng. 2003, R43, 1. (2) Meijering, J. in AdVances in Materials Research; Herman, H., Ed.; Wiley: New York, 1971; Vol. 5 p 1. (3) Mackert, J.; Ringle, R.; Fairhurst, C. J. Dent. Res. 1983, 62, 1229. (4) Guruswamy, S.; Park, S.; Hirth, J.; Rapp, R. Oxid. Mets. 1986, 26, 77. (5) Huang, X. Y.; Mader, W.; Eastman, J.; Kirchheim, R. Scripta Met. 1988, 22, 1114. (6) Huang, X. Ph.D. Thesis; University of Stuttgart: Stuttgart, 1989. (7) Kirchheim, R. Huang, X. Y. Mu¨tschele, T. In Hydrogen Effects on Material BehaVior; Moody, N., Thompson, A., Eds.; The Minerals, Metals and Materials Society: 1990; p 85. (8) Huang, X. Y.; Mader, W.; Kirchheim, R. Acta Metall. Mater. 1991, 39, 893. (9) Gegner, J.; Ho¨rz, G.; Kirchheim, R. Interfacial Sci. 1997, 5, 231. (10) Wang, D.; Noh, H.; Luo, S.; Flanagan, T.; Clewley, J.; Balasubramaniam, R. J. Alloys Compd. 2002, 339, 76. (11) Kirchheim, R. Encl. Materials: Sience and Technology; Elsevier: The Netherlands, 2005; p 3882. (12) Balasubramaniam, R.; Noh, H.; Flanagan, T.; Sakamoto, Y. Acta Metall. Mater. 1997, 39, 893.

J. Phys. Chem. C, Vol. 113, No. 10, 2009 4087 (13) Eastman, J.; Ru¨hle, M. Ceram. Eng. Sci. Proc. 1989, 10, 1515. (14) Gouthama, G.; Balasubramaniam, R.; Wang, D.; Flanagan, T. J. Alloys Compd. . in press. (15) Lynch, J.; Clewley, J.; Curran, T.; Flanagan, T. J. Less-Common Mets. 1977, 55, 153. (16) Flanagan, T.; Lynch, J.; Clewley, J.; von Turkovich, B. J. LessCommon Mets. 1976, 49, 13. (17) Kirchheim, R. Prog. Mater. Sci. 1988, 32, 261. (18) Noh, H.; Flanagan, T.; Balasubramaniam, R.; Eastman, J. Scripta Mat. 1996, 34, 863. (19) Wang, D. Flanagan, T. to be published. (20) Wang, D.; Clewley, J.; Flanagan, T.; Balasubramaniam, R.; Shanahan, K. J. Alloys Compd. 2000, 298, 261. (21) Wang, D.; Flanagan, T. Scripta Mat. 2003, 49, 77. (22) Sutton, A. Balluffi, R. Interfaces in Crystalline Materials; Clarenden Press: Oxford, 1995. (23) Lewis, M.; Martin, J. Acta Met. 1963, 11, 1207. (24) Haasen, P. Physical Metallurgy; Cambridge University Press: Cambridge, 1986. (25) Flanagan, T.; Oates, W. Annu. ReV. Mater. Sci. 1991, 21, 269. (26) Luo, S.; Flanagan, T. J. Alloys Compd. 2002, 330-332, 29. (27) Wang, D.; Clewley, J.; Flanagan, T.; Balasubramaniam, R.; Shanahan, K. Acta Mat. 2002, 50, 259. (28) Luo, S.; Flanagan, T. Scripta Mat. 2005, 53, 1269. (29) Wang, D.; Flanagan, T.; Balasubramaniam, R. J. Alloys Compd. 2005, 404-406, 38. (30) Flanagan, T.; Luo, W.; Clewley, J. J. Alloys Compd. 1991, 172174, 42. (31) Wicke, E.; Blaurock, J. J. Less-Common Mets. 1987, 130, 351. (32) Jamieson, H.; Weatherly, G.; Manchester, F. J. Less-Common Mets. 1976, 50, 85. (33) Wang, D.; Flanagan, T.; Balasubramaniam, R. Scripta Mat. 1999, 41, 517. (34) Wang, D.; Clewley, J.; Luo, S.; Flanagan, T. J. Alloys Compd. 2001, 325, 151.

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