Stability Considerations of AB5 Hydrides in Chemical Heat Pump

23 Stability Considerations of AB Hydrides in Chemical Heat Pump Applications with Reference to the New LaNi5-x Alx Ternary System Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch023

5

DIETER M. GRUEN and MARSHALL H. MENDELSOHN Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 AUSTIN E. DWIGHT

1

Materials Science Division, Argonne National Laboratory, Argonne, IL 60439

Alloys of the general composition A B , that react rapidly and reversibly with large quantities of hydrogen gas, can be used in chemical heat pump systems. The theoretical operating temperatures of these heat pumps are determined by the thermodynamic values of the enthalpy (∆H) and the entropy (∆S) of the reactions of hydrogen with metal alloys. A configurational entropy model was developed to account for experimentally observed differences in ∆S. Because the exact theory of dissociation pressures is not fully understood, an empirical correlation relating the alloy cell volumes to the plateau pressures is useful. A new ternary alloy system (composition LaNi Al ), that spans a wide range of pressures without greatly impairing the desirable properties of the alloy LaNi , was developed. 5

5-x

x

5

0

0 T h e discovery of the ability of A B compounds (A = rare earth, Β = transition metal) to f o r m hydrides w i t h u n i q u e properties stimulated research a n d development i n several areas ( J , 2, 3, 4). A l t h o u g h the physicoc h e m i c a l properties of some A B hydrides were measured, the factors that d e termine the hydrogen dissociation pressures of these materials are not completely understood. 5

5

T h e r a p i d kinetics of Reaction 1, the h i g h v o l u m e t r i c h y d r o g e n storage densities, a n d the w i d e range of h y d r o g e n decomposition pressures of the A B hydrides initiated proposals to use t h e m as c h e m i c a l compressors, cryogenic 5

1

Present address: Department of Physics, Northern Illinois University, Dekalb, IL 60115

0-8412-0390-3/78/33-167-327/$05.00/0 © American Chemical Society Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

328

TRANSITION M E T A L HYDRIDES

devices, a n d as m e d i a for thermal energy storage, space heating and cooling, a n d conversion of l o w grade heat to w o r k (5). AB

5

+ nH — AB H 2

5

(1)

2 n

T h e use of A B 5 hydrides as c h e m i c a l l y or t h e r m a l l y d r i v e n heat p u m p s is i n t r i g u i n g (6). Since t w o different A B alloys are i n v o l v e d , the relationship be tween their respective hydrogen decomposition pressures, as a f u n c t i o n of t e m perature, is the key parameter that determines the thermodynamics of heat p u m p action.

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5

It is c r u c i a l to discover the relationship between c h e m i c a l compositions and h y d r o g e n decomposition pressures of the A B 5 compounds. Intermetallic c o m pounds of lanthanide and transition metals f o r m an interesting class of structures. T h e A B series crystallize i n the hexagonal C a C u s ( P 6 / m m m ) structure (see F i g u r e 1). Generally, radius ratios (t\/r ) greater than 1.30 f o r m the C a C u s - t y p e 5

B

Figure

1. CaCu structure

5

type of

Ο Ca ο Cu

configuration a n d compounds w i t h ratios less than 1.30 f o r m the c u b i c UN15 structure. C o m p o u n d s f o r m e d b y rare earths to the right of l a n t h a n u m i n the periodic table display the lanthanide contraction that decreases the AB5 unit cell volume. T h e A B phase is generally stable over the composition range ( A B 3-5 2) (5, 7). 5

4

H e x a g o n a l A B compounds f o r m orthorhombic hydrides. Basal plane ex pansion, caused b y hydrogen occupation of interstitial sites (4), change the c o m p o u n d s structure a n d can be ordered or disordered. F o r example, Kuijpers and Loopstra (4) f o u n d , b y neutron diffraction, that the d e u t e r i u m atoms i n P r C o D were ordered on certain interstitial octahedral a n d tetrahedral sites. T h e sites occupied b y hydrogen i n various A B hydrides are not fully understood because of insufficient neutron diffraction data. T h e available i n f o r m a t i o n is considered i n the section o n configurational entropies. 5

5

4

5

Thermodynamic Properties of AJB5 Hydrides T h e equation RT In Ρ = AG = AH - TAS

Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

(2)


23.

Stability of A B

GRUEN ET AL.

1

0

329

Hydrides

5

2

3

4

5

6

^Hydrogen concentration (at. M /mole L a N i ) 9

Figure

2.

Pressure vs. composition

for

LaNis

expresses the relationships a m o n g the dissociation pressure (P), the free energy of f o r m a t i o n ( A G ) , the enthalpy change (AH), a n d the entropy change (AS) of Reaction 1. Phase diagrams consisting of pressure-composition curves are available for m a n y intermetallic hydrides (5). In some cases ( O C 0 5 , SmCos, and YC05), single plateaus approach the composition AB5H3. In other cases ( P r C o s a n d N d C o s ) , two plateaus exist w i t h a m a x i m u m composition a p p r o a c h i n g AB5H4. C e r t a i n A B intermetallics possessing one ( L a N i ) , two ( N d N i ) , or three (LaCos) plateaus absorb more than four atoms of hydrogen. L a C o s absorbs nine atoms at pressures a p p r o a c h i n g 1250 a t m (8). 5

5

5

D a t a are available on quaternary hydrides, where another element is p a r tially substituted at the A or Β sites (9). Desorption isotherms were measured for L a i _ Y N i , where χ = 0.3, 0.4, 0.5 (JO), a n d for M m ^ x C a ^ N i s , where χ = 0 - 1 at room temperature (11). Several isotherms were reported for L a N i 4 C u a n d one for L a N i 3 C u (12), w h i l e v a n M a l et al. have shown 4 0 ° C isotherms i n the series L a C o 5 _ N i for χ = 0 - 5 (13). x

x

5

2

x

x

T y p i c a l pressure-composition isotherms for the L a N i - H 2 system are shown i n F i g u r e 2 ( 14). F r o m plots of In P p i a u vs. 1 / T , several workers d e t e r m i n e d the experimental heats a n d entropies of Reaction 1. T h e enthalpies are the heats of formation for the β-phase h y d r i d e f r o m the α-phase hydrogen-saturated alloy (see T a b l e I). 5

ate

Ternary Metal Hydrides as Chemical Heat Pumps T h e rapid kinetics of Reaction 1, the ability to vary the hydrogen dissociation pressures b y controlling the c h e m i c a l composition of the A or Β component, and


330


T a b l e I.

Compound (a-phase) CeCosHo.6 YCo H .3 PrCo H .3 LaCoôHo.n S1m1C05H0.3 NdNi H .5 LaNi CuH . LaNi H . 5

5

5


5

a

b

c

5

0

0

0

5

-ΔΗ (kcal/ mol of H) 2

3

3

4

a

0

4

CeCo H YC05H3 PrCo H .5 LaCo5H S111C05H3 NdNi H .5 LaNi CuH LaNi H . 5

a

0

Representative T h e r m o d y n a m i c D a t a

Compound (β-phase) (limiting composi-) tion)

5

5

a

a

4

4

5

6

5

6

9.3 7.7 9.2 10.8 7.8 6.7 8.1 7.2

-AS (cal/deg mol of H) 2

32.6 31.8 30.1 30.0 29.0 27.7 27.1 26.1

6

(ASceCo5H - AS )/ R

3

C

Ref. 16 17 16 16 16 26 26 18

0 -0.4 -1.2 -1.3 -1.8 -2.5 -2.8 -3.3

Estimated from P-C diagrams. Calculated from equation: log Ρ = 6.96-1679/T. See Ref. 4.

Figure

3.

Three examples thermodynamic

of

chemical heat relationships

pump


Struc ture β-phase

0

ιη

β

β" β 1

23.

Stability of AB Hydrides

G R U E N E T AL.

331

5

the h i g h hydrogen-rarrying capacity of the A B alloys suggest that these materials 5

are useful i n c h e m i c a l heat p u m p applications (6). T h e t h e r m a l energy of h y d r o g e n absorption a n d desorption processes suggests the possibility of p u m p i n g heat f r o m a low temperature to an i n t e r m e diate temperature, b y using a higher temperature source a n d appropriate pairs of m e t a l - h y d r o g e n systems.

In such t h e r m a l l y d r i v e n heat p u m p s used for

c o o l i n g or heating purposes, t h e r m a l energy does not have to be converted first to m e c h a n i c a l energy to d r i v e , for example, a compression refrigerator. avoids m o v i n g parts a n d vibration.

This

M o r e i m p o r t a n t l y , l o w grade heat sources,

such as p r o v i d e d b y solar collectors, c o u l d power these c h e m i c a l heat pumps. Suppose a t h e r m a l l y d r i v e n heat p u m p operates at temperatures T h , T , m


a n d T\. A t the h i g h temperature T h , heat Q temperature T , heat Q

M

m

n

is s u p p l i e d to the heat p u m p ; at

is generated by the heat p u m p ; a n d at the temperature

T\ heat Q\ is extracted f r o m a l o w temperature source. T

F o r a C a r n o t c y c l e (re

versible process) the f o l l o w i n g relations hold:

Substitution gives the expression for the efficiency of a C a r n o t process:

Consider a pair of different metal hydrides, Μ χ a n d M , w i t h hydrogen gas 2

f l o w i n g freely between t h e m . peratures T\ a n d T

Suppose h y d r i d e M operates between the t e m 2

w i t h corresponding pressures P\ a n d F

m

Straight lines i n F i g u r e 3 are plots of In Ρ as 1 / T .

2

(see F i g u r e 3).

E q u i l i b r i u m dissociation

pressures i n the two-phase region are g i v e n by enthalpies Δ Η

Μ Ι

and Δ Η

Μ

of

2

R e a c t i o n 1, a n d the intercepts are g i v e n by entropies Δ 5 ι a n d Δ 5 Μ · Μ

T o operate hydrides M i a n d M saturated w i t h h y d r o g e n at T P T

2

m

m

a n d to desorb the hydrogen.

2

i n the heat p u m p m o d e , begin w i t h M i

and P i .

2

m

and M to T\. 2

H e a t M i to T h to raise the pressure to

T h e released h y d r o g e n is absorbed b y M

and P , and the heat of absorption, Δ Η

of M i is lowered to T

2

M 2

> is rejected at T . m

2

at

T h e temperature

H y d r o g e n desorbing f r o m M at T\ absorbs 2

heat f r o m the e n v i r o n m e n t w h i l e the heat of reaction, Δ Η Μ

ρ

is rejected at T . m

H y d r o g e n gas is the w o r k i n g f l u i d i n this closed cycle w h i c h can be repeated indefinitely. T o choose m e t a l h y d r i d e pairs efficiently for heat p u m p operation, the re lationships a m o n g the t h e r m o d y n a m i c quantities that govern Reaction 1 and the values for T h , T , a n d T\ should be established. m

T h r e e examples for the rela

tionships between t h e r m o d y n a m i c quantities of M i and M are shown i n F i g u r e 2

3.

These are the only possibilities that simultaneously satisfy the c o n d i t i o n :

P(M ) 2

> P ( M i ) for any g i v e n 1 / T > 0.


332


2.

Δ Η

3.

AHM

Μ

2

A S

M

I

^ ASM,:

AG , M

T

=

2

2

2

=

2

A G

—R In P

=

2

; A S M

I

^ AH ,

2

M

_

(6)

h

AG

,

M

(7)

T

r.

m

Straightforward substitution and rearrangement shows A H


ΔΗ

M

2

_

T

( A S

M

,

Μ

Atf

A H

T|(AS

M 2

AHM,

M

, - A S

M

M

, ) _ T

M

|

)_

T

(9) m

ASM,:

=

2

(8) H

T|

AHM,

For the special case, A S M

M

T

L

-AS

2

M

(10) * m

from which it follows that T For the special case, A H A S

M

= T T,

2

(H)

h

= AHM,= AH:

2

M

m

, - A S

M

|

M

,

_

1

1

_

1

1

Γι

Γ„

(12) (from (8))

AH A S

M

2

- A S

AH

(13) (from (9))

from which it follows that Τ

Τ

—

+ —

T

Τ,

h

=

2

(14)

Equations 8 and 9 are used to calculate Th and T\ for a range of T values if AH and AS values for a pair of metal hydrides are given. Such calculations were made for the CaNi -hydride pair (AH = - 7 . 5 5 kcal, A S = - 2 3 . 9 cal/deg) and the L a N i - h y d r i d e pair (AH = - 7 . 2 0 kcal, A S = - 2 6 . 1 cal/deg). Results are listed in Table II. Calculations of T\ (using the T and Th values) for the special cases A S M = A S M I and A H = A H , are listed in Table II. As shown in Figure 3 for fixed T and Th, the lowest refrigeration temperature (T\) is reached when A S M = A S M , while the least effective heat pump action occurs when Δ Η Μ = Δ Η Μ , · The differences in achievable refrigeration temperatures are ~ 1 0 % of ( T — T\) and therefore help to determine overall cycle efficien cy. m

5

5

m

2

M

2

M

m

2

2

m


23.

Stability of AB

GRUEN ET AL.

333

Hydrides

5

T a b l e II.

Examples of Chemical Heat P u m p Operating Temperatures T and Th Values of Columns 1 Hydride Pair Involving and 2 m

LaNi


5

and CaNi

ASM

5

ASMι

=

2

ΔΗΛ/

2

= ΔΗΜ ι

m(°C)

T (°C)

T/(°C)

T/( C)

T,(°C)

20 30 40 50 60

64.5 77.2 90.0 102.8 115.8

-15.6 -7.5 -1-0.5 +8.5 +16.4

-18.6 -10.8 -3.1 +4.6 +12.2

-14.1 -6.0 +2.1 + 10.2 +18.2

h

e

Rule of Reversed Stability A scheme for predicting ternary hydride stability was proposed b y workers at the P h i l l i p s Research Laboratories ( 9 , 1 5 , 1 6 , 1 7 ) .

A c c o r d i n g to this scheme,

the heat of f o r m a t i o n of a ternary h y d r i d e A B H

is g i v e n b y :

n

ΔΗ(ΑΒ Η η

2 τ η

) = AH(AH )

2

m

+ AH(H H )

m

n

m

- ΔΗ(ΑΒ )

(15)

η

Most heats of formation of binary hydrides were determined experimentally, a n d they are used i n E q u a t i o n 15 w i t h calculated Δ Η ( Α Β ) values. η

Miedema's

(18) approach for calculating Δ Η ( Α Β ) is summarized b y Stewart, L a k n e r , and η

U r i b e (8): "Assumes that the d r i v i n g force for reactions between metals is a function of two factors: a negative one, arising f r o m the difference i n c h e m i c a l potential, Αφ*, of electrons associated w i t h each m e t a l atom, a n d a positive one that is the d i f ference i n the electron density, An at the boundaries of W i g n e r - S e i t z type cells s u r r o u n d i n g each atom. Values of φ* for the metals are a p p r o x i m a t e d b y the electronic work functions; n is estimated f r o m compressibility data. T h e atomic concentrations i n the alloy must be i n c l u d e d i n the c a l c u l a t i o n . ' ' WSi

w s

T h e equation for Δ Η ( Α Β ) calculations (17) is η

Δ Η ( Α Β ) = N o f ( C , C ) g P [ - * ( Δ 0 * ) 2 + & (An )W η

s

A

s

B

(16)

- Jj

ws

Ο

where C\ a n d C% = surface concentrations of each component; P , Q a n d R = constants that vary between systems of alloys, i.e., transition metal-transition metal alloys, transition-metal-p-metal alloys, etc.; e = electronic charge; N = A v o gadro's n u m b e r ; a n d g is a f u n c t i o n of the m e t a l parameters n , V ^ , a n d the atomic concentrations (17). Values of φ* and n (18) a n d the constants P , C o , a n d R are listed for each of the systems studied. OJ

Q

w s

3

w s

T h e rule of reversed stability is illustrated b y E q u a t i o n 15. W h e n the sta b i l i t y of the intermetallic c o m p o u n d A B is great, the stability of the ternary h y d r i d e is low; therefore, the h y d r o g e n dissociation pressure of the h y d r i d e is h i g h (8). T h e rule of reversed stability aids i n c a l c u l a t i n g approximate values n


334


of AH for h y d r i d e formation. H o w e v e r , recent attempts to p e r f o r m such c a l culations (8) b y using estimated entropy values of b i n a r y hydrides have shown that reliable estimates of ternary h y d r i d e e q u i l i b r i u m pressures cannot be made. T h e s e m i - e m p i r i c a l l y calculated A H ( A B H ) values represent the enthalpies for the phase changes i n the plateau regions a n d are related to the e q u i l i b r i u m h y d r o g e n pressures. T h e calculations of Steward, L a k n e r , a n d U r i b e (8) show that plateau pressures are lower than the experimental values by factors ranging f r o m 1 0 to 1 0 . T o obtain reliable estimates of ternary h y d r i d e e q u i l i b r i u m pressures, the rule of reversed stability should be modified or a new method should be devised. n

6

14

Configurational Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch023

2 m

Entropies and Stabilities of Intermetallic

Hydrides

T h e rule of reversed stability regards the enthalpy as a direct measure of the relative stability of m e t a l - h y d r o g e n phases since it was assumed that the entropy changes of Reaction 1 are relatively constant (9). T o calculate the free energies of Reaction 1, Steward et al. estimate entropy changes and assume that they depend only on the M : H ratio (8). Entropy changes i n Reaction 1 are almost certainly d o m i n a t e d b y the entropy of gaseous hydrogen (31 c a l / d e g m o l H ) since the contribution of hydrogen atoms to the lattice vibrational entropy of the solid A B 5 h y d r i d e is probably quite small near room temperature. T h e m a i n effect is that a n optical phonon branch is added at fairly h i g h frequencies. T h e change i n lattice vibrational entropy because of the lanthanide contraction would be s m a l l since the v o l u m e change i n the two extreme cases of L a C o a n d Y C o is o n l y 6%, w h i c h is about one-half of that for the elements L a and Y . 2

5

5

H o w e v e r , the experimental data (see T a b l e I) show that entropy changes (AS) i n a series of reactions of hydrogen w i t h A B compounds differ by u p to 6.5 e u / m o l H at 300° K . T h i s gives differences of ~ 2 k c a l / m o l H i n the free energies of Reaction 1 a n d more than one order of m a g n i t u d e change i n the h y d r o g e n dissociation pressure (20). Differences i n entropy changes for a ho mologous series of Reaction 1 are based on changes i n the configurational e n tropies of the A B s H ternary hydrides; therefore, the configurational entropies are important i n d e t e r m i n i n g H dissociation pressures, a n d future attempts to explain the theoretical predictions of ternary hydride dissociation pressures should account for the relationship between the h y d r i d e structures a n d their thermo d y n a m i c stabilities. 5

2

2

n

2

H y d r o g e n atoms i n the R E C o H (n < 4) hydrides o c c u p y sixfold tetra hedral sites w i t h two R E and two C o , and threefold octahedral sites w i t h two R E a n d four C o (4). F o r higher stoichiometry hydrides ( A B H , η > 4), an a d d i tional two sets of twelvefold tetrahedral sites can be occupied (21). Configu rational entropies (S ^ = k In w) were calculated on the assumption that the tetrahedral a n d octahedral sites are distinguishable and are equally available to h y d r o g e n at temperatures near 300° Κ a n d that occupation of a particular site does not block other available sites. T h e n u m b e r of possible hydrogen ar5

n

5

n

00


23.

335

Stability of AB Hydrides

G R U E N E T AL.

5

rangements i n a g-atom of metal was obtained b y m u l t i p l y i n g two combinatorial formulas

where X = 3 for the octahedral sites a n d X = 6 for the tetrahedral sites i n the RECo H 5

n

(n < 4) compounds; Ν is Avogadro's n u m b e r a n d θ is the o c c u p i e d

fraction of the X N available sites (22). Several models fit the data for the c o m pounds A B H 5

(n > 4). F o r one m o d e l , we used X = 3 for the octahedral sites

n

a n d X = 30 for the tetrahedral sites.

E q u a t i o n 17 simplifies v i a Stirling's a p


p r o x i m a t i o n to In w = - Χ Ν [ 0 In θ + (1 - θ) In (1 - θ)] Differences i n A S - S

for different A B H 5

n

(18)

compounds c o m p a r e d w i t h A S l %

for CeCo5H3 are listed i n Table III. T h e values of these numbers (see Table III), calculated using the fractional site occupations for the β phase, can be compared w i t h the experimentally d e t e r m i n e d entropy differences listed i n T a b l e I. T h e calculated configurational entropy differences (see Table III) agree satisfactorily w i t h the experimental data (see T a b l e I) currently available for seven A B H 5

compounds.

Structures of some A B H 5

n

n

compounds d e d u c e d f r o m neutron

diffraction data (4) are listed i n Table I. F o r compounds whose structures have not been d e t e r m i n e d , the occupation numbers listed i n T a b l e III are i n best agreement w i t h the t h e r m o d y n a m i c data. A l t h o u g h the m a x i m u m enthalpy difference for the A B H 5

n

group is 4.1

k c a l / m o l H , w h i c h is larger than the m a x i m u m entropy difference of 2.0 2

kcal/mol H

2

(at 300° K ) , the latter quantity helps determine the dissociation

pressure ratio of two A B H 5

T a b l e III.

n

compounds at a given temperature.

F o r example,

Calculated Configurational Entropies ο con/ _ gcon/ _ AS c°e éo5H2 ~~ C

gcon/

C con/

Alloy

(cal/deg)

(cal/deg)

CeCo YCo PrCo LaCo5 SmCo5

3.9 2.4 2.4 1.5 2.4

7.6 7.6 11.7 11.4 11.4

3.1 3.9 5.8

NdNi LaNi Cu LaNi5

3.4 3.4 3.4

19.4

8.0

27.2

8.7 9.7

(mol of Hz)

n

AS (R) c onf c

β-phase

occupancy 3 oct—6

5

5

5

5.2 6.7

0.0 -0.4 -1.3 -1.1 -1.8

tet.

l/3ord_ / l/3ord_!/3 1

1/2—1/3 2/3—1/3 1/3—1/3 3 oct—30

5

4

32.5

-2.5 -2.8 -3.3

3

tet.

l/2ord_ / 1

1 0

2/3—2/15 1/6—1/5


336


based on the enthalpy difference alone, the H dissociation pressure at 300° Κ (~20 atm) should be comparable w i t h that of L a N i H ( - 2 H dissociation pressures differ b y about one order of magnitude. configurational entropy of the former c o m p o u n d is responsible for ior. 2

5

6

2

of Y C o H atm). T h e T h e lower this behav 5

3

A l t h o u g h the model of three octahedral a n d six tetrahedral sites seems sat isfactory for hydrides ( A B H ) where η < 4, several models y i e l d comparable results for the three hydrides where η > 4. T o select the best model, a d d i t i o n a l neutron diffraction data for hydrides w i t h η > 4 w o u l d be useful. 5


Hydrogen Dissociation

n

Pressure—Cell

Volume

Correlation

Considerations of enthalpy, as illustrated by the rule of reversed stability, a n d of configurational entropy p r o v i d e d insight into the factors governing the stabilities of the A B hydrides. Theoretical understanding to predict dissociation pressures should be developed on heat p u m p application, for example. Although 5

5 4

2h

I σ

oh

"2

h

-4

h

o28 -6 83

84

85

86

CELL

Figure

87 VOLUME

89

88

90

91

3

(A )

4. Alloy cell volume vs. In Ρ for A B 5 type hy drides (reference numbers given on figure)


23.

Stability of AB

G R U E N E T AL. Table IV. Compound

5

P l a t e a u Pressure a n d A l l o y C e l l V o l u m e D a t a Cell Volume (A ) Reference P (at 20°C)(atm) 0.04 1.55 0.51 0.68 3.30 24 17.1° 1.5° 8 30 120 0.055 0.12 0.22 12.7° 0.57° 0.22° 0.005

5

YC05

LaNi PrNi SmNi GdNi LaCo Ni LaCo Ni LaCo2Ni NdNi LaNi .8Al .2 LaNi . Al . LaNi Al 5

6

5

6

5


4

c

3

0

6

0

4

4

a 6

c

c

c

2

5

4

19, 29 29 29 29 29 29 30 31 32 32 32 13 13 13 12 28 28 28

b

5

4

3

eq

LaCc-5 CeCc-s PrCc-5 NdCc.5 SmCo5 GdCo

3

337

Hydrides

89.74 84.30 87.13 86.79 85.67 85.19 83.96 86.54 84.73 83.44 82.58 89.01 88.44 87.80 84.32 87.28 88.24 90.51

Calculated from given data or equation. Given at 23° C only. Estimated from data at 40° C

an adequate theory does not exist, attempts were m a d e to correlate dissociation pressures e m p i r i c a l l y w i t h other observations. Geometrical factors are important i n determining transition metal h y d r i d e stability (24).

Recently, other authors cited the importance of crystal structure

and geometrical factors to the a f f i n i t y for hydrogen a n d the stability of metal a l l o y - h y d r o g e n systems (23,24,25,26).

L u n d i n et al. have shown a correlation

of interstitial hole v o l u m e vs. the l o g a r i t h m of h y d r o g e n dissociation pressure (26).

P r e d i c t i o n of the proper alloy composition a p r i o r i w o u l d be h e l p f u l i n

searching for alloys w i t h a particular plateau pressure at a g i v e n temperature. These considerations led to a direct correlation between crystal cell volumes and e q u i l i b r i u m plateau pressures (12).

F i g u r e 4 shows a linear plot of the In P i t e a u p

a

vs. c e l l v o l u m e for r o o m temperature data (see T a b l e I V ) . T h e theoretical explanation of this remarkable correlation is unclear.

Note

however, that Δ η £ { i n E q u a t i o n 16 is proportional to the molar volumes ( V ~ 3

4 / 3

)

of the alloys' elemental constituents. The LaNis- Al x

System:

x

Versatile New Ternary Alloys for Metal Hydride

Applications M a n y A B hydrides have Δ Η ~ 8 k c a l so that at 0 - 1 0 0 ° C (of interest for 5

c h e m i c a l heat p u m p action), the hydrogen dissociation pressures change roughly by one order of magnitude.

This implies that a metal hydride pair could function

i n the heat p u m p mode even if their enthalpies were exactly the same, provided


338


that their entropies of formation differed by 5 eu. Discussion of configurational entropies showed that entropy differences of this m a g n i t u d e occur a m o n g the A B 5 hydrides. Therefore, entropy considerations are important i n selection of candidates for heat p u m p pairs, p a r t i c u l a r l y since cycle efficiency c r i t e r i a also d e p e n d on entropy. F o r successful use of metal hydrides as chemical heat pumps, alloy systems should allow for changes i n the free energies of formation continuously over a w i d e range. G i v e n a particular hydride, it w o u l d be possible to specify AH and A S of a second hydride to reach a given Th and T\. Recent work i n our laboratory has shown that L a N i _ A l alloys provide a close approach to such a system (27). 5

x

x

X - r a y d i f f r a c t i o n measurements were made on some L a N i 5 A l alloys. Results show the a a n d c lattice parameters a n d the crystal cell volumes as a f u n c t i o n of the a l u m i n u m concentration (see F i g u r e 5). Substitution of one A l atom into the five c and g sites results i n a sharp increase i n c a n d a smaller i n crease i n a . Intensity calculations show that a l u m i n u m prefers to enter the 3 g sites rather than the 2c sites. T h i s f i n d i n g is consistent w i t h an earlier investi gation of the Y C 0 5 - Y N 1 5 system (28). F u r t h e r a l u m i n u m substitution causes c to level off but a increases at a faster rate. T h e initial increase i n c must result f r o m a lengthening of the b o n d between layers, w h i l e the subsequent leveling off i n c results f r o m the tendency for the large L a atom to collapse into the hexagonal r i n g of N i ( A l ) atoms above a n d below it.


x

Q

x

G

Q

0

Q

Q

Q

0

U s i n g the cell volume-decomposition pressure correlation, one predicts that a l u m i n u m substitutions i n L a N i should lower the h y d r o g e n decomposition pressures b y one order of m a g n i t u d e for every t w o - A increase i n c e l l v o l u m e (12). 5

3

A l u m i n u m substitution for n i c k e l i n L a N i lowers decomposition pressures without i m p a i r i n g the kinetics or the h y d r o g e n - c a r r y i n g capacity. T h e y allow for continuous spanning of a w i d e range of decomposition pressures. In 0-20% A l , the plateau pressures of the L a N i - L a N i A l h y d r i d e system are reduced by - 3 0 0 (27). 5

5

4

Alloys were prepared f r o m metals of 99.9% p u r i t y b y arc m e l t i n g on a water-cooled copper hearth under an argon atmosphere. T h e alloys were ho mogenized at 800° C. Diffraction patterns of cast material were equally as sharp as those of homogenized alloys. X - r a y diffraction patterns were taken w i t h f i l tered F e K radiation. C o m p u t e r programs verified x-ray pattern indexes. a

A l l o y samples, weighed to ±0.0001 g, were placed i n a stainless steel type 316 reactor fitted w i t h 1μ porous stainless steel filter disc. T h e reactor was connected to a stainless steel (type 316), h i g h pressure m a n i f o l d connected to a 0 - 2 0 0 0 psia Heise pressure gage (accuracy ±0.1%), a v a c u u m p u m p , a n d a high pressure h y d r o g e n gas c y l i n d e r . H y d r o g e n was Matheson's p r e p u r i f ied grade (99.95% m i n ) . H o k e valves rated to 3000 psi were used. T h e reactor was i m mersed i n a water bath whose temperature was m a i n t a i n e d to ± 0 . 2 ° C by a T h e r m i s t e m p temperature controller (for temperatures < 8 0 ° C ) a n d b y a n i -


Stability of AB

G R U E N E T AL.

5

339

Hydrides


23.

X Figure

5.

Parameters

a , c , and v / M vs. χ in LaNi - Al 0

5 x

0

alloys

x

c h r o m e - w o u n d c y l i n d r i c a l furnace (for temperatures > 8 0 ° C ) .

Samples were

activated b y evacuating the reactor containing the alloy and exposing the sample to h i g h pressure h y d r o g e n (300-800 psia) for 1-2 hr. Pressure-composition data were obtained for L a N i 8 A l o . , LaNi 6AI0.4, 4

2

4

L a N i A l , a n d L a N i Αΐχ 5. P r e l i m i n a r y data at several temperatures allow us 4

3 5

to plot In Ρ vs. 1 / T for the hydrides of these four compounds.

F r o m the slope

and intercept of the v a n t H o f f plots (see F i g u r e 6), Δ Η a n d A S values for the reaction L a N i _ A l + n H —* L a N i 5 _ A l H 5

x

x

2

x

x

2 n

are estimated and given i n Table

V.



340


2.0

Figure

Table V . Compound LaNi LaNi LaNi LaNi LaNi

5

4 4 4 3

. Al .2 . Alo. Al . Ali. 8

0

6

4

5

5

6.

2.5 1000/T

3.0

1/T vs. In Ρ for LaNi - Al 5 x

3.5

hydrides

x

T h e r m o d y n a m i c D a t a for the L a N i 5 _ A l - H 2 System Τ (°C)for AH(kcal/mol H ) AS(cal/deg mol H ) Ρ = 2.0 atm x

2

-7.2 -8.3 -9.1 -12.7 -14.5

0

x

2

± 0.1 ± 0.1 ± 0.2 ±0.3 ± 0.6

-26.1 -27.3 -28.1 -29.2 -29.6

± ± ± ± ±

0.4 0.4 0.7 0.7 1.4

-25 -50 -70 -180 -240

T h e increasing hydride stability can be illustrated by isobaric data as in the last column of Table V . T h e decomposition pressures of the L a N i _ A l hydrides at 2 5 ° C are included in Figure 4 and follow the correlation curve in a regular manner.

a

5

x

x

Conclusion M e t a l hydrides that have a linear dependence of In Ρ vs l/T c a n be used i n c h e m i c a l heat p u m p systems. G e n e r a l equations were developed to relate the t h e r m o d y n a m i c variables of the hydrides to the c h e m i c a l heat p u m p oper ating temperatures. A series of hydrides with variable thermodynamic properties were p r e p a r e d f r o m alloys of composition L a N i - A l (x = 0-1.5). 5

x

x


23.

GRUEN ET AL.

Stability of AB Hydrides 5

341


Literature Cited 1. van Vucht, J. H. N., Kuijpers, F. Α., Bruning, H. C. A. M., Philips Res. Rep. (1970), 25, 133. 2. Kuijpers, F. Α., van Mal, Η. H., J. Less-Common Met. (1971), 23, 395. 3. Zijlstra, H., Chem. Technol. (1972), 2, 280. 4. Kuijpers, F. Α., Loopstra, B. O., J. Phys. Chem. Solids (1974), 35, 301. 5. Newkirk, H. W., "Hydrogen Storage by Binary and Ternary Intermetallics for Energy Applications," UCRL-52110, 1976. 6. Gruen, D. M , McBeth, R. L., Mendelsohn, M., Nixon, J. M., Schreiner, F., Sheft, I., Proc. lntersoc. Energy Convers. Eng. Conf., 11th, 1976, 681. 7. Buschow, Κ. H. J., van Mal, Η. H., J. Less-Common Met. (1972), 29, 203. 8. Steward, S. Α., Lakner, J. F., Uribe, F., UCRL-52039 and 77455 (1976). 9. van Mal, Η. H., Buschow, Κ. H. J., Miedema, A. R., J. Less-Common Met. (1974), 35, 65. 10. Gruen, D. M., Mendelsohn, M., Dwight, A. E., Proc. Rare Earth Res. Conf., 13th, 1977. 11. Sandrock, G. D., Proc. lntersoc. Energy Convers. Eng. Conf., 12th, 1977. 12. Gruen, D. M., Mendelsohn, M., Sheft, I., Proc. Symp. Electrode Mater. Processes Energy Conversion Storage, The Electrochemical Society, 1977. 13. van Mal, Η. H., Buschow, Κ. H. J., Kuijpers, F. Α., J. Less-Common Met. (1973), 32, 289. 14. van Mal, Η. H., Thesis, Technische Hogeschool, Delft (1976). 15. Miedema, A. R., J. Less-Common Met. (1973), 32, 117. 16. Buschow, Κ. H. J., van Mal, Η. H., Miedema, A. R., J. Less-Common Met. (1975), 42, 163. 17. Miedema, A. R., J. Less-Common Met. (1976), 46, 67. 18. Miedema, A. R., Boom, R., de Boer, F. R., J. Less-Common Met. (1975), 41, 283. 19. Kuijpers, F. Α., J. Less-Common Met. (1971), 27, 27. 20. Gruen, D. M., Mendelsohn, M. H., J. Less-Common Met. (1977) 55, 149. 21. Bowman, A. L., Anderson, J. L., Nereson, N. G., Proc. Rare Earth Res. Conf., 10th, Arizona, 1973. 22. Gibb, T. R. P., Jr., J. Phys. Chem. (1964), 68, 1096. 23. Buschow, Κ. H. J., van Mal, Η. H., J. Less-Common Met. (1972), 29, 203. 24. Beck, R. L., "Investigation of Hydriding Characteristics of Intermetallic Compounds," Denver Research Institute, DRI-2059 (1962). 25. Bechman, C. Α., Goady, Α., Takeshita, T., Wallace, W. E., Craig, R. S., Inorg. Chem. (1976), 15, 2184. 26. Lundin, C. E., Lynch, F. E., Magee, C. B., Proc. lntersoc. Energy Convers. Eng. Conf., 11th, 1976, p. 961. 27. Mendelsohn, M., Gruen, D. M., Dwight, A. E., Nature (1977) 269, 45. 28. Dwight, A. E., J. Less-Common Met. (1975), 43, 117. 29. Kuijpers, F. Α., Thesis, Technische Hogeschool, Delft (1973). 30. Takeshita, T., Wallace, W. E., Craig, R. S., Inorg. Chem. (1974), 13, 2282. 31. Lundin, C. E., Lynch, F. E., Denver Research Institute, First Annual Report, No. AFOSR, F44620-74-C0020, (DDC #ADA006423) (1975). 32. Anderson, J. L. et al., Los Alamos Report, LA-5320-MS (1973). RECEIVED July 18, 1977. Work performed under the auspices of U.S. Energy Research Development Administration.


Stability Considerations of AB5 Hydrides in Chemical Heat Pump

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