Thermodynamic properties for solution of hydrogen in Pd-Ag-Ni ternary alloys

Journal of

AILLOYS AND COMPOUNDS Journal of Alloys and Compounds 236 (1996) 42-49

ELSEVIER

Thermodynamic properties for solution of hydrogen in Pd-Ag-Ni ternary alloys K. Ohira a, Y. Sakamoto a'*, T.B. Flanagan b aDepartment of Materials Science and Engineering, Nagasaki University, Nagasaki 852, Japan bDepartment of Chemistry, University of Vermont, Burlington, VT 05405-0125, USA

Received 15 September 1995; in final form 4 October 1995

Abstract

The hydrogen absorption characteristics of well-annealed Pdloo_2xAgxNix ternary solid solution alloys with x (at.%) = 1.5, 3.0, 4.5, 6.0 and 7.5 have been investigated at temperatures between 273 and 433 K at hydrogen pressures up to 1000 Torr (133.32 kPa) by means of pressure-composition isotherm measurements and X-ray measurements of the lattice parameters of hydrogen-free and hydrogenated alloys. The room temperature lattice parameters of hydrogen-free ternary alloys decrease slightly with increasing solute metal content. The relative partial molar enthalpy AH H at infinite dilution for the Pd-Ag-Ni alloys becomes more exothermic with solute metal content, despite the lattice contraction of the alloys. The behaviour is near the mid-point between those of Pd-Ag and Pd-Ni binary alloys. The partial molar entropy AS~(/3 = 1) at infinite dilution decreases with solute metal content. The exceptional behaviour of the AH~ values in the ternary alloys for the hydrogen absorption of Pd alloys on the basis of the lattice 'expanded' or 'contracted' categories may be attributed to the preferential occupation of Pd-rich interstices near Ag atoms rather than near Ni atoms in the alloy solid solutions. The B-hydride in Pd-Ag-Ni ternary alloys becomes more unstable with increasing solute metal content, and the standard free energy change AGp~a, at 298 K for the hydride formation is nearly between that for each binary alloy. Keywords: Pd-Ag-Ni alloys; Hydrogen sorption; Pressure-composition isotherms; Partial molar hydrogenation enthalpy

1. Introduction

It has been generally observed, with some exceptions [1-5], that for substantially disordered palladium-based binary alloys whose unit cells are larger than Pd, more hydrogen dissolves at a given low hydrogen pressure than in Pd, and the plateau pressures corresponding to the a/fl phase transition are lower than that for the P d - H system because the strain energy required for the expansion of the Pd lattice by hydrogen occupation is smaller than for Pd. For alloys with a smaller unit cell than Pd the opposite holds, i.e. the dilute phase solubility is smaller at low pressures than for Pd and the plateau pressures are higher than that of Pd. With respect to the classification of behaviour based on lattice 'expansion'-'contraction' of Pd binary alloys relative to pure Pd, it is of interest to investigate the behaviour of hydrogen solution in Pd ternary solid * Corresponding author. 0925-8388/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0925-8388(95)02103-5

solution alloys. This may shed some light on the interactions between the first and secorid solute metal atoms in the hydrogen-free alloys, and on the interactions between dissolved hydrogen and the individual solute metal atoms. The present authors and co-workers [6-8] have previously studied the solubility of hydrogen in the ternary solid solution alloys P d - C u Au [6], P d - Y - A g [7] and P d - N i - R h [8]. The results o have shown that the relative chemical potential A/z u of dissolved hydrogen at infinite dilution and the o standard free energy change AGplat for r - h y d r i d e formation in these alloys appears to fall near the mid-points of the values for each binary alloy. It seems therefore, that the solubility of hydrogen in these ternary alloys can be attributed mainly to the magnitude of the strain energy for hydrogen occupation in the alloys, because this can explain the relative values o o of A/zH and AGplat for the binary alloys. In the present study, in order to obtain more detailed information about the absorption of hydrogen by Pd ternary solid solution alloys, the alloy system of

K. Ohira et al. / Journal of Alloys and Compounds 236 (1996) 42-49

Pdl00_2xAg~Nix with up to x =7.5 was chosen for investigation. The substituent Ag atom expands the Pd lattice, while the Ni atom contracts the host lattice. Hydrogen absorption isotherms for P d - A g and Pd-Ni binary alloys have been determined [9-12]. The trends in the relative chemical potential A/z~ for hydrogen solution at infinite dilution and the standard free energy change A G p l a t for/3-hydride formation in both P d - A g [7,9] and Pd-Ni [10-12] alloys conform to the lattice 'expanded'-'contracted' classification. Recently, Kishimoto and co-workers [13,14] have studied the dilute phase hydrogen solubilities in Pd~_~Ni~:/2Agx/2 alloys with up to x =0.2, together with that of each binary alloy from a viewpoint of dislocation-enhanced hydrogen solubility due to coldworking. The results have shown that well-annealed ternary alloys behave like an 'expanded' alloy with respect to the partial molar enthalpy AH H at infinite dilution, where the exothermicities are smaller than that for Pd-Ag binary alloys but larger than for Pd-Ni binary alloys. The solubility enhancements for the cold-worked ternary alloys do not change significantly with the solute metal content x. This was explained [13] by ~t model involving changes of the fraction of available Pd-rich interstices in the tensile stressed regions resulting from segregation during cold-working; the segregation of Ag to, and depletion of Ni from, th,e tensile stressed region about dislocations should approximately cancel in the ternary alloys. O

43

3. Results and discussion

3.1. X-ray diffraction study The P d - A g - N i ternary alloys used in this study were confirmed to be a single a-f.c.c. Pd phase by X-ray diffraction. The room temperature lattice parameters a~s of hydrogen-free alloys are plotted in Fig. 1 as a function of the total solute metal atom fraction, x u =XAg +XNi, in comparison with those of Pd-Ag [7,15,16] and Pd-Ni [8,15] binary alloys. Table 1 summarizes the lattice parameters for the ternary alloys. The lattice parameters of hydrogen-free ternary alloys decrease slightly with increasing x~, and they are placed near the mid-points between those of Pd-Ag and Pd-Ni binary alloys. The lattice parameters, a~,,~ and aflmi, at the O/max and flmin phase boundary composition in hydrogenated ternary alloys are plotted against the solute metal content in Fig. 2 together with

0.394

I

0.392

0.390

0.388

2. Experimental details

0.386"

The P:I-Ag-Ni ternary alloys used in this study were as follows: Pd~00_2xAg~Ni~ with x(at.%)= 1.5, 3.0, 4.5, 6.0 and 7.5; they were prepared by arc-melting the pure components under argon atmosphere. The alloy buttons were annealed and rolled into foils about 75/zm thick. Finally, the foil samples were annealed at 1123 K fi)r 2 h in vacuo and then furnace-cooled to room temperature. The lattice parameters as~ of hydrogen-free alloy were me~tsured at room temperature by X-ray diffraction, together with the lattice parameters a~max and at3m~" at ':he phase boundary compositions with the O/+/3 miscibility gaps in hydrogenated alloys, where the hydrc,genation was carried out electrolytically [15]. Pressme (p)-composition (c) isotherms were measured at temperatures between 273 and 433 K and hydrogen pressures up to 1000 Torr (133.32 kPa) using a Sieverts-type apparatus. The procedure for measuring the data has been previously described [6-8]. The hydrogen concentration is expressed as r = H/M, the ratio of the number of hydrogen atoms to the total number ef metal atoms.

0.384

0.382

0

~ 5

' 10 Xu / 10 .2

' 15

20

Fig. 1. T h e room temperature lattice parameters of hydrogen-free Pda0o_2~AgxNix ternary alloys as a function of the total solute atom fraction x , =XAg +XN~, together with those of P d - A g and P d - N i binary alloys: ©, Pdlo0_2~Ag/Ni ; A, P d - A g [7,15,16]; ~ , P d - N i [8,15]. Table 1 T h e r o o m temperature lattice parameters ass of the hydrogen-free f.c.c. Pd-rich Pd~00_2xAg~Nix alloys, together with those for the areax and fl~.~o phase boundaries in the Pdloo_2~Ag~Nix-H systems Alloys

a~s -+ 0.0001 (rim)

a m~x (nm)

aBmin (nm)

Pd Pdl0o 2xAgxNix x = 1.5 x = 3.0 x = 4.5 x = 6.0 x = 7.5

0.3890

0.3895 _+0.0001

0.4025 _+0.0001

0.3890 0.3887 0.3885 0.3884 0.3883

0.3892 0.3888 0.3884 0.3881 0.3875

0.4015 0.4008 0.4001 0.3989 0.3978

_+0.0001 _ 0.0001 _+0.0002 _+0.0002 _+ 0.0002

_+0.0001 _+ 0.0002 _ 0.0002 _+0.0002 -+ 0.0002

44

K. Ohira et al. / Journal of Alloys and Compounds 236 (1996) 42-49

0.405

0.4oo

r

0.395

0.390

0.385

0.380

i

i

i

5

10 Xu / 10 "2

15

20

Fig. 2. The room temperature lattice parameters a max and at3min at the a,,~x and fl=~. phase boundary compositions in hydrogenated Pdlo0 2~Ag,Ni~ alloys as a function of x., together with those of hydrogenated Pd-Ag and Pd-Ni binary alloys: O, Pdmo 2~Ag~Ni~; A Pd-Ag [7,15,16]; O, Pd-Ni [8,15].

those of P d - A g [7,15,16] and Pd-Ni [8,15] binary alloys. These values are also given in Table 1. Both values of %rain and aamax for the ternary alloys decrease gradually with increasing Xu, lying between those of the P d - A g [7,15,16] and Pd-Ni [8,15] binary alloys respectively, although the variation of a,max in the ternary alloys with the alloying content is close to that for Pd-Ni [8,15] binary alloys.

3.2. p-c isotherms The absorption p - c isotherms for a series of Pd~00_2xAgxNi, alloys with x -- 1.5, 3.0, 4.5, 6.0 and 7.5 are shown in Figs. 3(a) to 3(e). The low pressure solubilities at low temperature have a tendency to increase slightly with an increase of the solute metal content, but the solubilities are smaller than found in P d - A g binary alloys [7,9]. The a/fl plateau pressures Pplat for the alloys increase with the solute metal content; however, the Pptat values are smaller than those for Pd-Ni binary alloys [832] under the same conditions. The higher pressure solubilities in the ternary alloys decrease with x u and the solubilities, e.g. at 133.32 kPa, are smaller than those in P d - A g [7,9] and Pd-Ni [8,12] binary alloys.

3.3. Thermodynamic parameters of hydrogen solution in the dilute phase region Fig. 4 shows plots of m~l~oH / T vs. T-1 for Pd~00_2xAgxNix alloys, together with that of pure Pd [6]. The relative chemical potential Atz~ of dissolved hydrogen at infinite dilution was obtained from the

1/2,4 intercepts of plots of RT In Pn2 ~1 - r)/r vS. r and the apparent H - H pair interaction free energies between dissolved hydrogen ga were determined from the slopes of the plots. It can be seen from Fig. 4 that, although the lattice parameters of the ternary alloys decrease slightly with the solute metal content, the dissolved hydrogen becomes more stable as the solute metal content increases; however, hydrogen in the alloy with x = 1.5.is less stable compared with that for pure Pd, and hydrogen in high solute metal content alloy with x = 7.5 becomes less stable at high temperature. The relative partial molar enthalpy AH~ and entropy AS~(fl = 1) of solution of hydrogen at infinite dilution in the ternary alloys are plotted in Figs. 5 and 6 respectively against the atom fraction of the solute atoms x u together with that of the previously reported Pdl00_xAgx/2Nix/2 ternary alloys [13] and also of P d Ag [7,9] and Pd-Ni [8,12] binary alloys. It can be seen that the two sets of data for the binary alloys agree quite well. Table 2 summarizes the thermodynamic parameters for the ternary alloys determined in this study together with the apparent H - H interaction free energy gl at 348 K. The AH~ values for P d - A g - N i alloys becomes gradually more exothermic with increasing solute metal content, despite the lattice contraction of the alloys; this trend is in good agreement with that reported previously [13]. The rate of increase in exothermicity for the ternary alloys with the solute content is small compared with that for P d - A g alloys [7,9], and the alloy composition dependence of AH~ is near the mid-points between those of P d - A g and Pd-Ni binary alloys, except for low solute metal contents. As can be seen from Fig. 6, the AS~(/3 = 1) values in the ternary alloys decrease with increasing solute metal content as for P d - A g binary alloys [7,9], while the values for the Pd-Ni binary alloys are insensitive to the Ni content of the alloys. The relative insensitivity of AS~(/3 = 1) values to Ni content in Pd-Ni alloys, where all the octahedral sites are equally accessible to hydrogen, is consistent with the formation of nickel hydride and is similar to the behaviour of P d - C u and P d - R h alloys [6,8]. However, it has recently been found [5] that for Pd-Li solid solution alloys there is a regular and significant decrease in the relative partial excess entropy of solution of hydrogen with increasing Li content. LiH is, however, an ionic solid and the others are metallic. The apparent H - H attractive pair interaction free energy gl for P d - A g - N i ternary alloys at 348 K (Table 2) decreases with increasing xu; this trend is similar to that for P d - A g binary alloys [7,9], whereas the gl values in Pd-Ni binary alloys become rather more attractive with Ni content [12]. As mentioned above, the exceptional behaviour of

45

K. Ohira et al. I Journal of Alloys and Compounds 236 (t996) 42-49

strain energy necessary for hydrogen occupation of the interstices.

the A H ~ values for the Pdl0o_2xAgxNix ternary alloys with regard to hydrogen absorption by Pd alloys on the basis of the lattice " e x p a n d e d " or " c o n t r a c t e d " alloys is of interest. T h e dilute phase solubility m a y be attributed to the selective occupation of Pd-rich interstices near Ag atoms rather near than Ni atoms dissolved in the solid solutions because of a smaller 400

,

,

432.9 K

- .....

402.9 K

3.4. Plateau and solvus thermodynamic parameters The standard free energy change for /3-hydride o formation is expressed by the relation: AGpla, = ,"

,. . . .

•. . . . .

////:

.o /:

..

S

fgo~-O-

-..o

(//

¢h

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o-" °

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?

....

0

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~

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/

273 K -

fi

______ojO-//

0

.o.~

~0 O 0 ~ O

J

--0---

I

I

I

l

!

l

0.1

0.2

0.3

0.4

0.5

0.6

0.7

r Oq/M)

a)

400

t . . . .

432.8 K

I

"

I. . . . .

I

f

Pdloo-2xAgxNix(x=3.0)

300

!j/ o~°~'

250 t~

ca,

"-- 200 ca,

/

150

o ..-.-~-

-o

-o_

o

°

"

323.1 K . 0

o

0

0

~

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'

0.2

o ~

/!::/ jd

~

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" ~

I

/!/: /

,,/

/

o

/ °

_ o.__.,_...,..o---0--

/

o

/

/ / 6 . /

/

273 K

50

o°

o/:

302.9 K 0

~0-0 ~

['

////i

348.2 K

o

0

/o..~---o

100

. . . . .

402.9 K

350

b)

/

322"8 K__.__..~ ------ - ~ ° ~ ' ° 0

?

]

/o ]

o~'-

//

) O o

~

/s

/1//

348.1 K . _ _ . . _ _ ~ o ~

~:

/

I r/

/: 50

-

Pdloo-2xAgxNix(x=l,5)

,oo :

100

,

"O

0.3

0.4

0.5

0.6

r (H/M)

Fig. 3. Pressure-composition isotherms for hydrogen absorption by Pdl0o z, Ag~Ni~ alloys: (a) x ~- 1.5; (b) x = 3.0; (c) x = 4.5; (d) x = 6.0; (e) x = 7.5.

46

K. Ohira et al. / Journal of Alloys and Compounds 236 (1996) 42-49

400

i

....

'

i

i

i

i

'

'

'

J'

Pdloo-zxAgxNix(x=4.5)

402.5 K

350 300

If.

- - 0 ~ 0

']

372"6°K'K-~ 0~/

/

z

/

250 --- 200

,

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150

I

100

oi I

347.7 K

.

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O.

,

o/

D-

0

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/

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304.1 K 273 K

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b

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--

0

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1

i

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0.2

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0.4

0.5

0.6

e)

r (H/M)

400 433.5 K 402.8 K

300

(o/

250

O

°/°

!

350

e

Pdl00-2xAgxNix(x=6.0)

o ~ o ~ o ~ ---~'°~

~

J

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-'- 200 150

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o

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..o~

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1

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0.2

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0.4

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.

r 0-I/M)

d)

Fig. 3. (contd).

R T "mPpla 1/2 t = AHplat -- TASp],t, where AHpl~t a n d / ~ S p l a t are respectively the standard enthalpy and entropy changes for the hydride formation. o Fig. 7 shows plots of AGp],t/T vs. T - ' for the ternary alloys, where it can be seen that the/3-hydride becomes more unstable at a given temperature with increasing (Ag + Ni) content. This trend is similar to that for Pd-Ni binary alloys [8,12[, whereas the /3o

o

o

o

hydride of Pd-Ag binary alloys becomes more stable with Ag content. Fig. 8 shows the standard free energy o change AGplat,298 K at T = 2 9 8 K against the solute metal atom fraction, where it can be seen that the free energy change for the ternary alloys is near the midpoints between those of P d - A g [7,9] and Pd-Ni [8,12] o binary alloys. The derived AH~t~t and/~kSplat values are o given in Table 2. T h e Anptat values for the ternary

47

K. Ohira et al. / Journal of Alloys and Compounds 236 (1996) 42-49

400 432.6 K

' 402.7 K

350

i °//

0

_

'

/ °/ 3 7 2 9 K"

IT/

300

' ' Pd lOO-2xAgxNix(x=7.5)

ol [ /o

0

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/

;

~

0

o/

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/

rr/ o' o/

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:

/

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o

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°/°/

150 ~/

I~/o /

°"°"~°~

303.8 K

/

100b'~/°/°to,,Of°'--'--° ° ~ ° ~ ° / ~ / odd

50

o/ °/°/

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o/°

o

o-

0 Pr

,

,

0

0.1

0.2

,

0.3

e)

,

x

0.4

0.5

r (H/M) Fig. 3. (contd).

40

0

,

i

,

35 -5

r ~ ~

B

30 '7"

I"1~

"B~

~

-

25

<1

15

102.i

'

I

2.5

~0

I

3.5

4.0

T -1 / 10 "5K'I Fig. 4. Plc.ts of AI-*~/T against 1/T for Pdloo_2~Ag~Ni~ alloys; O, pure Pd [6]; ©, x = 1.5; A, x = 3.0; [2],x = 4.5; ~, x = 6.0; +, x = 7.5.

alloys become less exothermic with increasing solute metal content up to about x~ = 0.12 similar to the P d - N i binary alloys, where the values for the latter alloys are less exothermic. The enthalpy values for P d - A g binary alloys, however, become more exothermic with Ag content [7,9]. The AS~ta, values in the ternary alloys increase with the solute metal content up to about x , = 0.12. As can be seen from Fig. 3, the hydrogen solvus composition of the ternary alloys, i.e. areax composition

-20

' ' ' ~÷ i0 15 20 Xu / I0 "2 Fig. 5. Plots of AH oH vs. atom fraction x. for Pdlo 0 2~AgxNix alloys, compared with those of the previously reported Pd-Ag-Ni alloys [13], and also with those of Pd-Ag and Pd-Ni binary alloys: (3, Pd,0 o 2~Ag~Ni~; O, Pd-Ag-Ni [13]; A, Pd-Ag [7,9]; D, Pd-Ni

0

5'

[8,12]. at a given temperature, increases with increasing solute metal content. Fig. 8 shows plots of AGZo~v/T z vs. T -1 for the ternary alloys, where AG~otv is the standard free energy change for the solvus formation, i.e. it refers to the formation of one mole of dissolved hydrogen in the infinitely dilute solution from the hydride phase. The energy change is expressed [17] Z z z by AG~o,v= - R T ln[a/(1 - a)] = AH~o,v - T ASso,v,

K.

48

.52

•

i

•

Ohira et

i

•

al. /

|

Journal

of Alloys and Compounds 236 (1996) 42-49

•

-54

A

'7

-5

"7 -56 "7 o

~.E.10

~-58

?. o=

<1-60

~-15 ,0

-62

-20

','64

i

,,

0

I

i

5

•

10 X u 1 10 .2

I

I

15

20

Fig. 6. Plots of AS~ (fl = 1) vs. atom fraction x for Pd~o0_2~AgxNi+ alloys, compared with those of the previously reported P d - A g - N i alloys [13], and also with those of P d - A g and P d - N i binary alloys: ©, Pd~0o_2~Ag~Ni~; O, P d - A g - N i [13]; ~ , P d - A g [7,91; rn, P d - N i [8,121.

where a is the solvus composition, and AH~o~v and AS~o], are the standard enthalpy and entropy changes respectively. The derived values are given in Table 2. As has been discussed [1], the AG~o~, values in Pd alloys decrease with increasing solute metal content, independent of their lattice-expanding or -contracting properties, i.e. the extent of the tr phase solid solubility limit of Pd alloys increases.

I

"252.6

218

3~0

I

3~2 3.4 T / 10"3K .t

3.6

3.8

Fig. 7. Plots of AG°maJT against 1/T for Pd~00 2~AgxNi~ alloys: 0 , pure Pd [6]; (3, x = 1.5; A, x = 3.0; ~ , x = 4.5; A, x = 6.0; +, x = 7.5.

'7

~'-2 o

"~-4 -¢1 Ii o.in.

~-6 <1

4. Conclusions (1) The room temperature lattice parameters of hydrogen-free ternary solid solution alloys of Pd~00_2xAg~NL decrease slightly with increasing Xu = XAg + XNi, and they are near the mid-points between those of the Pd-Ag and Pd-Ni binary alloys. The lattice parameters, a%~x and a Stain, at the areax and flrnin

~ z

-8

-10

I

0

I

i

5

I

I

10 Xu / 10"2

I

i

15

20

Fig. 8. Plots of Aapl,t at 298K vs. atom fraction x, for Pdt00_2+Ag+N L alloys, together with those of P d - A g and P d - N i binary alloys: ©, Pdto0_2+Ag~Nix; A, P d - A g [7,9]; [2, P d - N i [8,12].

Table 2 Thermodynamic parameters of hydrogen absorption by Pdt00_2xAgxNi x alloys

Specimens

AH~ kJ(mol H) -1

AS~ J(mol H ) - I K 1

g~ kJ(mol H) -1

AH~ta, kJ(mol H) -1

ASp,at J(mol H) -1 K -~

AH~o,v kJ(mol H) -1

Pd Pdt00-2xAg~Nix x = 1.5 x = 3.0 x = 4.5 x = 6.0 x = 7.5

- 10.1

- 53.7

-47.3

- 18.4

-45.5

10.7

2.3

-10.2 - 10.8 - 11.2 -11.6 - 12.5

-54.4 - 55.3 -56,2 -57.0 -'59.9

-45.8 -41.6 - 38.3 -34.3 -29.8

-17.9 - 17.0 - 16.2 -15.6 - 16.9

-45.3 -43.5 -41.9 -41.3 -46.8

9.6 8.6 7.6 6.8 6.2

1.0 -0.7 - 1,4 -2.3 - 2.4

a At T = 3 4 8 K a n d f l = l .

AS~o,v J(mol H) - t K t

K. Ohira et al. / Journal of Alloys and Compounds 236 (1996) 42-49

49 o

(3) The standard free energy change AGplat f o r fl-hydride formation in P d - A g - N i ternary alloys increases with the solute metal content, and the free energy values are nearly halfway between those of P d - A g and Pd-Ni binary alloys. The hydrogen solvus composition at a given temperature in the ternary alloys, i.e. a . . . . which is related to the standard free energy for solvus formation, increases with increasing solute metal content, independent of their lattice-expanding or -contracting properties.

40

35 "7. '7,

~30

jo

-;25 <1 20

Acknowledgements 15

--

2.6

I

I

2.8

3.0

I

I

3.2 3.4 T / 10"3K'a

I

3.6

3.8

Fig. 9. Plots of AG~ojv/T against 1/T for Pd~00_zxAgxNix alloys: O, pure Pd [61; ©, x - 1.5; •, x = 3.0; D, x = 4.5; A, x = 6.0; +, x = 7.5.

phase beundary compositions in hydrogenated ternary alloys decrease gradually with increasing x u, lying between those of P d - A g and Pd-Ni binary alloys; the variation of ac%ax in the ternary alloys with the alloying content is near that for Pd-Ni binary alloys. (2) T~e dissolved hydrogen as reflected in the o relative chemical potential A#H at infinite dilution in Pdl00_2xAgxNix ternary alloys becomes more negative with solute metal content; however, hydrogen in the ternary ~lloy with x = 1.5 is less stable than in pure Pd, and hydrogen in the high solute metal content alloys with x = 7.5 becomes less stable at high temperature. The relative partial molar enthalpy AH H at infinite dilution in the ternary alloys becomes gradually more exothermic with increasing solute metal content, despite the small lattice contraction of the alloys (Fig. 5). The AH H values in the ternary alloys are near the mid-points between those for the P d - A g and Pd-Ni binary atloys. This indicates that the most important factor for the behaviour of the ternary alloys is that their thermodynamic behaviour is an average of the two binary alloys rather than being governed by the 'expanded'-'contracted' classification. The A S ~ ( f l = 1) values in the ternary alloys decrease with increasing solute metal content. The exceptional behaviour of the AH H in the ternary alloys with respect to hydrogen absorption on the basis of the lattice 'expanded' or 'contracted' categories of Pd alloys may be attributed to the l:referential occupation of Pd-rich interstices near Ag atoms rather than Ni atoms dissolved in the solid solutions. However, the high pressure solubilities in the ternary alloys decrease with increasing solute metal content.

Y. Sakamoto wishes to thank the Grant-in-Aid for Scientific Research from the Ministry of Education (Japan). TBF wishes to acknowledge the NSF for financial support.

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