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Hydride formation in metal-hydrogen systems at equilibrium pressures below the plateau values
Journal of the Less-Common
Metals, 99 (1984)
L5 - L8
L5
Letter
Hydride fo~ation in metal-hydrogen below the plateau values TED B. FLANAGAN Department (Received
systems at equilibrium pressures
and T. KUJI
of Chemistry,
University
of Vermont,
Burlington,
VT 05405
(U.S.A.)
January 30,1984)
1. Introduction Evidence has been offered in several instances that a hydride phase can form in metal-hydrogen systems at pressures of hydrogen below the plateau pressure Pf for hydride formation. This hydride formation can occur in the dilute o phase solubility range [l] and during hysteresis scans [l, 21. A satisfactory explanation has not been given for this phenomenon. cx phase solubility data must reflect only CYphase solubility and must not contain any contributions from hydride phase formation; if such contributions are present, the solubility data will, of course, give erroneous 01 phase thermodynamic parameters. The solubility in the cx phase is important because the non-ideality of the interstitial hydrogen soIution can be evaluated from these data and this is important for an understanding of hydride formation [3]. In this letter a simple explanation is provided for this effect in the cr phase solubility range and experimental evidence is given supporting the mechanism and the fact that a hydride phase can form below pf in this region. 2. An explanation for hydride formation below pr in the a phase region Let us consider a metal-hydrogen system in the dilute CY phase and assume that it absorbs hydrogen very rapidly in this region, i.e. inhibition of hydrogen uptake due to surface barriers is minimal. Such a rapid uptake can occur in practice with pure palladium or with materials which have been coated with palladium and also with some intermetallic compounds, e.g. LaNi,. It will also be assumed that the diffusion constant of hydrogen in the cwphase is sufficiently small that significant concentration gradients can develop in the initial stages of the hydrogen uptake. If a dose of hydrogen is added to such a system at a dosing pressure pl, which is greater than Pf, the plateau pressure for hydride formation, a concentration gradient will develop which will cause the chemical potential ~.r, of hydrogen in regions adjacent to the surface to be greater than the hydrogen chemical potential corresponding to pf, i.e. pHf. It seems clear that if these circumstances occur a hydride phase will form in localized Elsevier Sequoia/Printed
in The Netherlands
L6
regions near the surface where the condition & > pHf obtains as a result of hydrogen atom pileup. Now during this same addition of hydrogen, as it continues to be absorbed by the sample, the pressure will fall and the concentration gradient in the region where the hydride phase has formed will also fall, leading to PH < I_tHf . However, because of hysteresis, once formed, the hydride phase cannot decompose until PH > pHd where pHd is the chemical potential of hydrogen corresponding to the decomposition plateau pressure pd_ The hydride phase which initially forms when fiH > pHf has the hydrogen composition of the hydride phase which coexists at the plateau pressure pi; however, when PH falls below PH f in the regions where hydride has formed, the hydrogen concentration in the hydride also falls to become equal to that of the coexisting 01 phase. This is analogous to the behavior which occurs during hysteresis scans when hydrogen is desorbed starting from the formation plateau [ 23. ft is suggested that a hydride phase can even exist when PH < juHd because the decomposition isotherms of Pd-H do not follow a horizontal pathway from the plateau (decomposition) to the Q phase solubility curve; the decomposition data curve downwards to meet the CYphase solubility curve at values of PH less than PHd. It will be shown below that the conditions necessary for hydride formation below pIltf are fulfilled for the Pd-H system in the cy phase at low temperatures. 3. Experimen tul de tails Thin sheets of palladium of thickness 0.001 m were employed. The apparatus is a grease- and mercury-free system and the rates of hydrogen uptake were extremely rapid, e.g. eciuilibrium was obtained in about 10 min at -40 “CLPressures were measured with a series uf diaphragm gauges (MKS). Variable dosing volumes were available for the determination of the isotherms. For example, for the isotherms which were determined under conditions where p1 > pf a dosing volume of 189 cm3 was employed and for isotherms with p1 < pf a dosing volume of 3760 cm3 was generally used. All the data were corrected for thermal transpiration [4]. 4. Results and discussiun Figure 1 shows low hydrogen content at phase solubility data for Pd-H. In one set of isotherms dosing pressures in excess of pf were employed and in the other the dosing pressures were kept less than pf a It can be seen that the measured isotherms differ markedly at the two temperatures. The isotherms determined with p1 > pf exhibit a considerable amount of curvature as they approach the plateau pressures. The plateau pressures pf and the initial slopes ([HI /[Pd] -+ 0) are the same for the two types of isotherms but the intermediate regions are quite different. These data were determined before the effect described above was realized and it should be remarked that at these low temperatures the natural experimental procedure is to employ values of p1 > pf because the values of pf are quite small, e.g. at -40 “C pf = 2.9 X 10d4 atm. It is suggested that the isotherms which were determined with p1 > pf contain contributions due to hydride formation, i,e, the system fulfils the
L7
0.025-
Pf(-30% ) --___----~--+__~__---. 0 0
Pf(-40-c ) __ ~___~~~~~~~__~___~_ 0 “I___ +_---___;-______‘ pd(-30 c ) & (-4o.c ) --_*_---------___.
Fig. 1. The dilute phase solubility of hydrogen in well-annealed palladium determined after absorption of hydrogen with various dosing pressures (all the data points correspond to the attainment of apparent equilibrium, i.e. no further pressure changes occurred): 0, data determined under conditions where the dosing pressure (initial) was always less than pr; 0, data determined under conditions where the dosing pressure was greater than pf ; A, data determined by cooling the sample to the temperatures shown at constant hydrogen contents; *, data determined by desorption of hydrogen from the sample starting from the full circle (-40 “C) with the largest hydrogen content. The broken lines from the full circles to the open circles represent a transition from dosing pressures below Pf to a dose with an initial pressure greater than nf. The vertical broken line at -30 “C represents the behavior observed after heating the sample to 0 “C and retooling to -30 “C.
conditions described above which lead to hydride formation before the plateau is reached. The isotherms which were determined using p1 < pf are the true a phase isotherms. These data are quite compatible with higher temperature data for the CYphase [ 51. Evidence to support these conclusions will now be cited. Data were also obtained with the same palladium sample by isochoric methods, i.e. by variation of the temperature at a constant hydrogen content. In this method the sample is loaded to a small hydrogen content in the a phase at 0 “C and then cooled while equilibrium pressure data are recorded as a function of temperature. Conditions of constant hydrogen content are closely approached by employing a large sample and a small dead volume and by working in a low temperature range where the pressures are small. It is impossible for a hydride phase to form under these conditions, provided that the sample is loaded at 0 “C! with p1 < pf, because the hydrogen is uniformly distributed before the cooling commences. Several of these data points are shown in Fig. 1 where it can be seen that they agree with the isotherms determined withpi < pi. At -30 “C!, while on the true (Yphase isotherm, a dose of hydrogen was added with p1 > pi (Fig. 1) and it can be seen that this datum point is shifted
LS
to a higher hydrogen content relative to the true solubility curve. After “equilibrium” was obtained at -30 “C, the sample was briefly heated to 0 “C and then retooled to -30 “C; after this, the equilibrium hydrogen pressure had increased relative to the first reading at -30 “C and now fell onto the true solubility curve. This experiment shows that the hydride phase which forms at -30 ‘C! because p1 > pf subsequently decomposes on heating to 0 “C. The fully homogenized sample when retooled to -30 “C then reflects the true CYphase solubility behavior. The isothermal determinations at -40 “C with p1 < pi are shown to be reversible (Fig l), again demonstrating the presence of only the (Yphase. The data obtained with pi > pf have been shown to be irreversible (not shown in Fig. 1) which ...ldicates the presence of a hydride phase. The difference between the two types of isotherms commences at lower hydrogen contents at -40 “C compared with -30 “C (Fig. 1). This is because the diffusion constant of hydrogen is smaller at -40 “C and because the dosing pressures, which were similar at the two temperatures, are greater relative to pi at -40 “C than at -30 “C. It is also seen in Fig. 1 that the hydride phase persists to well below pd especially at -40 “C. 5. Conclusions It has been shown that dilute phase solubility data can be erroneous if dosing pressures in excess of pm are employed and if certain circumstances obtain. This is caused by hydride formation because of a combination of kinetic effects and hysteresis. The problem can be circumvented if dosing pressures less than pf are employed, Although the effect has been demonstrated experimentally with Pd-H, it is expected to be general if the conditions outlined above hold. For example, it may be the origin of the very gradual transitions to the plateau pressures for hydride formation frequently observed in (intermetallic compound)-H systems [6]. Finally it should be pointed out that, while the explanation given above is believed to be correct for the formation of a hydride phase below pi in the dilute phase of wellannealed samples as described above, a hydride phase may form below pf for different reasons under different circumstances, e.g. during hysteresis scans from the decomposition to the formation plateau. The authors wish to acknowledge Science Foundation.
the financial
support
of the National
I ‘I’. B. Flanagan, S. Kishimoto and G. E. Biehl, in N. A. Gokcen (ed.), Chemical h&tab lurgy -A Tribute to Carl Wagner, Metallurgical Society of AIME, 1981, p. 471. 2 B. S. Bowerman, G. E. Biehl, C. A. Wulff and T. B. Flanagan, Ber. Bunsenges. Phys. Chem., 84 (1980) 536. 3 W. A. Oates and T. B. Flanagan, Prog. Solid State Chem., 13 (1981) 193. 4 M. J. Bennett and F. C. Tompkins, Trans. Faraday Sot., 53 (1957) 185. 5 E. Wicke and G. N. Nernst, Ber. Bunsenges. Phys. Chem., 68 (1964) 224. 6 T. B. Flanagan,C. A. Wulff and B. S. Bowerman, J. Solid State Chem., 34 (1980) 215.
Metals, 99 (1984)
L5 - L8
L5
Letter
Hydride fo~ation in metal-hydrogen below the plateau values TED B. FLANAGAN Department (Received
systems at equilibrium pressures
and T. KUJI
of Chemistry,
University
of Vermont,
Burlington,
VT 05405
(U.S.A.)
January 30,1984)
1. Introduction Evidence has been offered in several instances that a hydride phase can form in metal-hydrogen systems at pressures of hydrogen below the plateau pressure Pf for hydride formation. This hydride formation can occur in the dilute o phase solubility range [l] and during hysteresis scans [l, 21. A satisfactory explanation has not been given for this phenomenon. cx phase solubility data must reflect only CYphase solubility and must not contain any contributions from hydride phase formation; if such contributions are present, the solubility data will, of course, give erroneous 01 phase thermodynamic parameters. The solubility in the cx phase is important because the non-ideality of the interstitial hydrogen soIution can be evaluated from these data and this is important for an understanding of hydride formation [3]. In this letter a simple explanation is provided for this effect in the cr phase solubility range and experimental evidence is given supporting the mechanism and the fact that a hydride phase can form below pf in this region. 2. An explanation for hydride formation below pr in the a phase region Let us consider a metal-hydrogen system in the dilute CY phase and assume that it absorbs hydrogen very rapidly in this region, i.e. inhibition of hydrogen uptake due to surface barriers is minimal. Such a rapid uptake can occur in practice with pure palladium or with materials which have been coated with palladium and also with some intermetallic compounds, e.g. LaNi,. It will also be assumed that the diffusion constant of hydrogen in the cwphase is sufficiently small that significant concentration gradients can develop in the initial stages of the hydrogen uptake. If a dose of hydrogen is added to such a system at a dosing pressure pl, which is greater than Pf, the plateau pressure for hydride formation, a concentration gradient will develop which will cause the chemical potential ~.r, of hydrogen in regions adjacent to the surface to be greater than the hydrogen chemical potential corresponding to pf, i.e. pHf. It seems clear that if these circumstances occur a hydride phase will form in localized Elsevier Sequoia/Printed
in The Netherlands
L6
regions near the surface where the condition & > pHf obtains as a result of hydrogen atom pileup. Now during this same addition of hydrogen, as it continues to be absorbed by the sample, the pressure will fall and the concentration gradient in the region where the hydride phase has formed will also fall, leading to PH < I_tHf . However, because of hysteresis, once formed, the hydride phase cannot decompose until PH > pHd where pHd is the chemical potential of hydrogen corresponding to the decomposition plateau pressure pd_ The hydride phase which initially forms when fiH > pHf has the hydrogen composition of the hydride phase which coexists at the plateau pressure pi; however, when PH falls below PH f in the regions where hydride has formed, the hydrogen concentration in the hydride also falls to become equal to that of the coexisting 01 phase. This is analogous to the behavior which occurs during hysteresis scans when hydrogen is desorbed starting from the formation plateau [ 23. ft is suggested that a hydride phase can even exist when PH < juHd because the decomposition isotherms of Pd-H do not follow a horizontal pathway from the plateau (decomposition) to the Q phase solubility curve; the decomposition data curve downwards to meet the CYphase solubility curve at values of PH less than PHd. It will be shown below that the conditions necessary for hydride formation below pIltf are fulfilled for the Pd-H system in the cy phase at low temperatures. 3. Experimen tul de tails Thin sheets of palladium of thickness 0.001 m were employed. The apparatus is a grease- and mercury-free system and the rates of hydrogen uptake were extremely rapid, e.g. eciuilibrium was obtained in about 10 min at -40 “CLPressures were measured with a series uf diaphragm gauges (MKS). Variable dosing volumes were available for the determination of the isotherms. For example, for the isotherms which were determined under conditions where p1 > pf a dosing volume of 189 cm3 was employed and for isotherms with p1 < pf a dosing volume of 3760 cm3 was generally used. All the data were corrected for thermal transpiration [4]. 4. Results and discussiun Figure 1 shows low hydrogen content at phase solubility data for Pd-H. In one set of isotherms dosing pressures in excess of pf were employed and in the other the dosing pressures were kept less than pf a It can be seen that the measured isotherms differ markedly at the two temperatures. The isotherms determined with p1 > pf exhibit a considerable amount of curvature as they approach the plateau pressures. The plateau pressures pf and the initial slopes ([HI /[Pd] -+ 0) are the same for the two types of isotherms but the intermediate regions are quite different. These data were determined before the effect described above was realized and it should be remarked that at these low temperatures the natural experimental procedure is to employ values of p1 > pf because the values of pf are quite small, e.g. at -40 “C pf = 2.9 X 10d4 atm. It is suggested that the isotherms which were determined with p1 > pf contain contributions due to hydride formation, i,e, the system fulfils the
L7
0.025-
Pf(-30% ) --___----~--+__~__---. 0 0
Pf(-40-c ) __ ~___~~~~~~~__~___~_ 0 “I___ +_---___;-______‘ pd(-30 c ) & (-4o.c ) --_*_---------___.
Fig. 1. The dilute phase solubility of hydrogen in well-annealed palladium determined after absorption of hydrogen with various dosing pressures (all the data points correspond to the attainment of apparent equilibrium, i.e. no further pressure changes occurred): 0, data determined under conditions where the dosing pressure (initial) was always less than pr; 0, data determined under conditions where the dosing pressure was greater than pf ; A, data determined by cooling the sample to the temperatures shown at constant hydrogen contents; *, data determined by desorption of hydrogen from the sample starting from the full circle (-40 “C) with the largest hydrogen content. The broken lines from the full circles to the open circles represent a transition from dosing pressures below Pf to a dose with an initial pressure greater than nf. The vertical broken line at -30 “C represents the behavior observed after heating the sample to 0 “C and retooling to -30 “C.
conditions described above which lead to hydride formation before the plateau is reached. The isotherms which were determined using p1 < pf are the true a phase isotherms. These data are quite compatible with higher temperature data for the CYphase [ 51. Evidence to support these conclusions will now be cited. Data were also obtained with the same palladium sample by isochoric methods, i.e. by variation of the temperature at a constant hydrogen content. In this method the sample is loaded to a small hydrogen content in the a phase at 0 “C and then cooled while equilibrium pressure data are recorded as a function of temperature. Conditions of constant hydrogen content are closely approached by employing a large sample and a small dead volume and by working in a low temperature range where the pressures are small. It is impossible for a hydride phase to form under these conditions, provided that the sample is loaded at 0 “C! with p1 < pf, because the hydrogen is uniformly distributed before the cooling commences. Several of these data points are shown in Fig. 1 where it can be seen that they agree with the isotherms determined withpi < pi. At -30 “C!, while on the true (Yphase isotherm, a dose of hydrogen was added with p1 > pi (Fig. 1) and it can be seen that this datum point is shifted
LS
to a higher hydrogen content relative to the true solubility curve. After “equilibrium” was obtained at -30 “C, the sample was briefly heated to 0 “C and then retooled to -30 “C; after this, the equilibrium hydrogen pressure had increased relative to the first reading at -30 “C and now fell onto the true solubility curve. This experiment shows that the hydride phase which forms at -30 ‘C! because p1 > pf subsequently decomposes on heating to 0 “C. The fully homogenized sample when retooled to -30 “C then reflects the true CYphase solubility behavior. The isothermal determinations at -40 “C with p1 < pi are shown to be reversible (Fig l), again demonstrating the presence of only the (Yphase. The data obtained with pi > pf have been shown to be irreversible (not shown in Fig. 1) which ...ldicates the presence of a hydride phase. The difference between the two types of isotherms commences at lower hydrogen contents at -40 “C compared with -30 “C (Fig. 1). This is because the diffusion constant of hydrogen is smaller at -40 “C and because the dosing pressures, which were similar at the two temperatures, are greater relative to pi at -40 “C than at -30 “C. It is also seen in Fig. 1 that the hydride phase persists to well below pd especially at -40 “C. 5. Conclusions It has been shown that dilute phase solubility data can be erroneous if dosing pressures in excess of pm are employed and if certain circumstances obtain. This is caused by hydride formation because of a combination of kinetic effects and hysteresis. The problem can be circumvented if dosing pressures less than pf are employed, Although the effect has been demonstrated experimentally with Pd-H, it is expected to be general if the conditions outlined above hold. For example, it may be the origin of the very gradual transitions to the plateau pressures for hydride formation frequently observed in (intermetallic compound)-H systems [6]. Finally it should be pointed out that, while the explanation given above is believed to be correct for the formation of a hydride phase below pi in the dilute phase of wellannealed samples as described above, a hydride phase may form below pf for different reasons under different circumstances, e.g. during hysteresis scans from the decomposition to the formation plateau. The authors wish to acknowledge Science Foundation.
the financial
support
of the National
I ‘I’. B. Flanagan, S. Kishimoto and G. E. Biehl, in N. A. Gokcen (ed.), Chemical h&tab lurgy -A Tribute to Carl Wagner, Metallurgical Society of AIME, 1981, p. 471. 2 B. S. Bowerman, G. E. Biehl, C. A. Wulff and T. B. Flanagan, Ber. Bunsenges. Phys. Chem., 84 (1980) 536. 3 W. A. Oates and T. B. Flanagan, Prog. Solid State Chem., 13 (1981) 193. 4 M. J. Bennett and F. C. Tompkins, Trans. Faraday Sot., 53 (1957) 185. 5 E. Wicke and G. N. Nernst, Ber. Bunsenges. Phys. Chem., 68 (1964) 224. 6 T. B. Flanagan,C. A. Wulff and B. S. Bowerman, J. Solid State Chem., 34 (1980) 215.