Control of hydrogen equilibrium pressure for C14-type laves phase alloys

1028

Journal of the Less-Common Metals, 172 174 (1991) 1028 1035

Control of hydrogen equilibrium pressure for C14-type Laves phase alloys Y. M o r i w a k i , T. G a m o and T. I w a k i Living Systems Research Centre, Matsushita Electric Industrial Co. Ltd., 3-15 Yagumonakamachi, Moriguchi, Osaka 570 (Japan)

Abstract It is very important to control continuously the hydrogen equilibrium pressure P - t e m p e r a t u r e T characteristics of hydrogen storage alloys to have them applicable in a wide range of temperatures. The relation between the alloy phases and the P - C - T characteristics was determined for C14 (MgZn2) type Laves phase alloys based on Ti(Zr)Mn 2 by changing the alloy composition. The P - C - T characteristics, hysteresis and plateau pressure were changed substantially by substituting zirconium for titanium in the A site and other elements such as chromium, copper or nickel in the B site. In this case, the crystal lattice constants and hydrogen equilibrium pressure were changed more effectively by substituting zirconium for titanium in the A site. The range of equilibrium pressures covered was from 10 to l0 s Pa at 20 ~ (this corresponds to the range from - 5 0 ~ to +250 "C at 1.013 x 105 Pa). The equilibrium pressure changes particularly markedly for lattice constants a = (4.84-5.05) x 10-'~ m and c = (7.94-8.28) x 10 -'~ m. These results show that the C14-type Laves phase alloys with excellent flatness of the plateau pressure and small hysteresis, are applicable in these ranges of temperatures and pressures for hydrogen storage. We have confirmed these results by fabricating a prototype heat pump with a hydrogen storage alloy that utilizes high temperature waste heat.

1. I n t r o d u c t i o n H y d r o g e n s t o r a g e alloys (or m e t a l h y d r i d e s ) h a v e b e e n c o n s i d e r e d for a p p l i c a t i o n n o t o n l y to h y d r o g e n s t o r a g e and t r a n s p o r t a t i o n b u t also to m a n y o t h e r fields, s u c h as h y d r o g e n p u r i f i c a t i o n , s e c o n d a r y b a t t e r i e s , h e a t s t o r a g e , h e a t p u m p and r o b o t a c t u a t i o n . T h e alloys n e e d to h a v e h y d r o g e n e q u i l i b r i u m p r e s s u r e s a t d i f f e r e n t t e m p e r a t u r e s for s u c h a p p l i c a t i o n s . F o r example, t h e a l l o y h a s to be operat i o n a l at n o r m a l t e m p e r a t u r e for h y d r o g e n s t o r a g e , for h e a t p u m p s for low t e m p e r a t u r e g e n e r a t i o n , - 2 0 ~ to - 6 0 ~ is r e q u i r e d and, for h i g h t e m p e r a t u r e w a s t e h e a t u t i l i z a t i o n , 200 ~ ~C is r e q u i r e d . T h u s the c o n t r o l o f t h e e q u i l i b r i u m p r e s s u r e P - C - T r e l a t i o n is a v e r y i m p o r t a n t f a c t o r in h y d r o g e n s t o r a g e alloy a p p l i a n c e s . A l t h o u g h v a r i o u s alloys h a v e b e e n d e v e l o p e d a n d a n n o u n c e d , c o n t r o l l i n g e q u i l i b r i u m p r e s s u r e o v e r a wide r a n g e of c o n d i t i o n s was t e c h n i c a l l y difficult. T h e p r e s e n t a u t h o r s studied, in detail, t h e C14 (MgZn2) t y p e L a v e s p h a s e [1] alloys b a s e d on T i ( Z r ) - M n 2 , c o n s i d e r i n g p r e v i o u s s t u d i e s on alloys

0022-5088/91/$3.50

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1029

of two metallic elements and/or several metallic elements such as TiMnl. 5 and Tio.sZro.2Mn~.~Cr0.2Vo. 2 [2-4]. Various alloy phases were analysed, and their P - C - T characteristics were measured. With regard to the results, a continuous variation in equilibrium pressure in a wide temperature range has been obtained for the basic composition AB2, where A = Ti or Zr and B = Mn or Cr. In other words, their stoichiometric compositions are basically AB 2. In addition to the above alloys, other alloys such as Zrl _xTi~(Cr~_yFey) 2 a r e popular [5, 6]. We studied a wide range of control of the equilibrium pressure by examining the relationshi p b e t w e e n crystalline properties and hydriding properties of C14 (MgZn2) type Laves phase alloys based on Ti (Zr)Mn2. We fabricated a prototype high temperature waste-heat-utilizing heat pump system as a demonstration of control of the equilibrium pressure characteristics of these alloys.

2. Experimental details 2.1. M a t e r i a l s

Til_xZrxMn2 alloys, based on TiMn2 and ZrMn2, were examined to control the equilibrium pressure of C14 (MgZn2) type Laves phase alloys. Substitution of zirconium for titanium will promote an increase in crystal lattice constants and a decrease in equilibrium pressure. Furthermore, partial substitution of chromium for manganese that is Til_ x ZrxMn2_y Cry and Til _xZrxMn2_y_ z CryMz (M = Cu, Ni, etc.), will be effective in improving the plateau and hysteresis of Ti~_xZrxMn2 [7]. Also, TiMn~Cry with a high equilibrium pressure and ZrMn~ Cry and ZrMn~ Cry Niz for the low pressures are examined to increase the control range of the equilibrium pressure. Alloys were produced by compounding market-affordable metals (more than 99.5% pure) and melting them four times with an argon arc. After this process, annealing was carried out at 1000-1200 ~ in a vacuum. 2.2. E v a l u a t i o n of the characteristics

The alloys were analysed by inductively coupled plasma emission spectrometry. The crystalline characteristics and the lattice constants were determined by X-ray diffraction patterns and the formation of alloys was observed. The P - C - T characteristics were measured with a general Sieverts system. The hydrogen absorption and desorption quantities, the p l a t e a u and the hysteresis characteristics were obtained as the hydriding properties. The plateau factor Sf and hysteresis factor H~ were as shown in Fig. 1 and were evaluated as follows: St = ln(Ps:M =0.75/PH:M =o.25) H~ = ln(Pa/Pd)

where Pa and Pd are the equilibrium pressure and the release equilibrium

1030

(at constant.T)

A ~ o z ~

i/,,.,.,,,,,o,, ~0 O

~

"Sf

In ( P H / M = 0.? S / P H / M = OJ S) ( P ' , P a or Pd, T-con=tant)

9 Hf :

In ( P a / P d )

(C, T: constant) |

0.2 5

0.5

0.7 5

,m

1.0

Hydrogen/Metal atom ratio, ( H / M ) Fig. 1. Definitions of the slope factor S e and hysteresis factor H e on P - C - T

curves.

pressure at H:M = 0.5. The reaction heat can be measured also with a Van't Hoff plot using plateau pressures.

3. R e s u l t s a n d d i s c u s s i o n 3.1. H y d r i d e characteristics

The T i l _ x Z r x M n e alloy exhibits a C14-type Laves phase of hexagonal structure with lattice constants a and c increasing almost linearly in proportion to the amount of zirconium substitution (Fig. 2). The alloys cannot store hydrogen in the range 0 < x <0.1 b u t can store with H : M = I . 0 for 0.1
1031

Ti~.x ZrxMnz

0.3

/

8.Z

'X=q.s

X=O.6 =e X.~-O.S

8,'1

0.4L

o !

ot,--4

ILO

X=O,Z

~_/*

./~=o I

7.a

:~ ,..4.o4-5.o~

"

7.S

CI4 tavms-type

,:a

4:, all0

,:o

-1~ m

Fig. 2. L a t t i c e p a r a m e t e r s of Ti 1 _ x Zrx Mn2 systems.

1.0

Ti~.x Zrx Mno.s C rl z (annea ed) =(103)

g.

9,(I 12) ,,(201}

\ -

o.5

O_

0

9

0'2

i

0A

i

0.6

Is

0.

1.0

X value Fig. 3. R e l a t i o n b e t w e e n x a n d h a l f - w i d t h of t h e d i f f r a c t i o n p e a k of Ti 1 _xZrx Mno.sCrl,2 alloys.

The P - C - T characteristics of Til _ x Zr~ Mny Crz under the constraint Mn:Cr = 1:1.5 and with y + z being changed gradually, the p l a t e a u became flattest when y + z = 2 . 0 (Fig. 6). It is clear t h a t a d e p a r t u r e from the theoretical composition ratio A:B = 1:2 causes a decrease in the flatness of the plateau pressures. The relations b e t w e e n x and the lattice p a r a m e t e r and between x and equilibrium pressure of Til_xZrxMn~_y Cry are shown in Fig. 7 and Fig. 8 respectively.

1032

il

Tl~s Z r o zMno,s C rtz ( aunea led ) I : ;ill2) ! ! ! (201) 103i

==

,i

/

CI4 ype La~'es-phase .

,

:

..... !t

ooJ

9

i

i

i

!

I ;

; I

'.

'.

I

t

, 'i~i

45

50

,

flier

......

i t~176

"-

i

i !

Angle

i ,

, ;

ii , i

35

40

Scattering

i

2e/degree

F i g . 4. X - r a y d i f f r a c t i o n p a t t e r n o f Ti0.sZr0.2Mn0.sCrt.2 L a v e s p h a s e a l l o y s .

r

y -II

T i l - X Zrx M n z - l C r

30 *C

5

A,

c,st

_i/'7

1/"

t-i

"1 ='

1

-7

f

....

~

~

. . . . ~/_.

.~-.:.~~

;/~ ,

...........

~

E 0.5

//i-

I r O" aJ

t=

il

~!

. . . .

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~_-.

0 .1

i-

._ ----"" ...~-"~

~ar-"

.-,.---

1 .J

.--- "I t o.~Z re.3 M n i.z C r o.a

.......

F

ia0

O

.~ o.o5 0

!

!

I

I

0.25

0.5

0.75

1.0

Hydrogen/Metal atom ratio, (H/M) F i g . 5. P - C - T

c u r v e s for t h e Ti 1 _ x Z r = M n 2 _ y C r y

s y s t e m s a t 30 ~

Copper and chromium are also as effective in flattening the plateau pressure (St changes from 2.04 to 1.0 on substituting copper for manganese), which can be improved to 0.24 by an annealing process (Fig. 9). In the annealing process, copper improved the crystalline characteristics of the C14 Laves phase by completely dissolving into the main phase and producing a sharp peak in the diffraction pattern and decreasing the halfwidth. St values are improved in the range 1.9 < ~ < 2.1 (where ~ gives the deviation from the AB2 stoichiometry) by the addition of chromium and copper and the annealing process. Substitution of zirconium for titanium and chromium, molybdenum and cobalt for manganese was effective in the improvement of H~.

1033

TiJ-xZrxMnyCrz--}l

(M~E, = VI.= }

30 'C

5 OJ

As Castz=Zr

~M

(/I r L.

E

0.5

9E

8.*

r,~ O"

eJ

=

---- ~'...~:L .......

0.1

r o

r

1 ..

o

....

t/

0.05

9

:.~'~-_Y]'ir Z rr M n0., Crr

I

Y

1

f

l

2

3

I[.rlmoi Fig. 6. P - C - T curves for the Ti, _=Zr=MnyCr= systems with the Mn:Cr ratio fixed at 1:1.5.

4.0

55

Ti,-,Zr,MnuCr~=

(annealed)

3.5

oa

x\ x

T 3.0 8.0 'o

~ 5.0 0 LI

x ,%

7 b-

2.5

-..I

1.51

X

o:8

1.5

,.o

value for Tin-x ZrxMno.sCrl.z(:lnne~led)

xx x.

2.(1

0:6

x %x

~

'Z

4.5

x

0

x.q

0.2

0.4

0.6

0.8

1.0

X value

Fig. 7. The relation between x and the crystal lattice parameters of Ti, _ = Zr x Mno.sCr!.2 systems. Fig. 8. Temperature dependence on x at 0.5 MPa (the equilibrium pressure) of Ti 1 _xZr=Mno.s Cr1.2 systems.

3.2. Equilibrium pressure control From Section 3.1, the equilibrium pressures are effectively controlled by the adjustment of ~ for AB~ and the selection of elements for A and B substitutions. The equilibrium pressures of the alloys at various temperatures are shown in Fig. 10. TiMno.6Cro.94, for example, is more suitable for higher equilibrium pressure and ZrMnl.sCro.5 for lower pressures. These results show that we can range from - 5 0 to + 2 5 0 ~ at an equilibrium pressure of 1.013 • 105 Pa with these T i ( Z r ) - M n alloys.

1034

~

5

T;o.sZrosMn,TCuo~

IlV

Q., = o .M

I A,.,,I',

0.5

o

t~

I.

~

|

~,L7 ~"

~

/

..'/

..-..."~" Tio.s~r~MntlCuo.z

,...,"- / , z

IA..-Cllt I

r /

0.1

/ #~'~o.sZr

I

e.,

0

d. 110"0'C 9 Z'41,

Si=o.14

"5 o m ~

I ~

1O0 'C

0.05

>., I

O

.

0.2 5

I

I

f

0.5

0.75

1.0

Hydrogen/Metal

atom

ratio,

(I-[/M)

Fig. 9. Sr improvement resulting from copper substitution and annealing in the Tio.sZro.sMn2 systems.

300 200

9 , ............

r~

9

o

~q

(~ 50 25 0 -25 -50

100

9

o

o , 9 9 r- . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . .

i

1

E .N

o.5

q~ r~

oA

o 0.0s 1.5

!

!

!

!

1

f

2.0

2.5

3.0

3.5

4.0

/,.5

"/'-lll0-~ K-t Fig. 10. Temperature dependence of the hydrogen equilibrium pressure for different T i ( Z r ) - M n alloy compositions: curve 1, TiMno.eCro.~; curve 2, Tio.sZro.2Mno.sCrl.oCuo.2; curve 3, Tio.~ZroAMno.sCr~.oCuo.2; curve 4, Tio.4Zro.sMno.sCr~.oCuo.2; curve 5, Tio.2Zro.sMno.sCrl.oCuo.2; curve 6, ZrMn~.sCro.5.

1035 3,3. E x a m p l e of a heat p u m p system a p p l y i n g e q u i l i b r i u m p r e s s u r e control

We confirmed the c on t rol of equilibrium pressure c h a r a c t e r i s t i c s with an a c t ua l h e a t pump system [8]. T h r e e kinds of alloys with different equilibrium pressures were selected for low, mid- and high t e m p e r a t u r e s in a h e a t pump system which is o p e r a t i o n a l at low pressures and, doubly effective, designed for high t e m p e r a t u r e waste heat. Details of this h e a t pump system are published elsewhere [8].

4. C o n c l u s i o n s The equilibrium pressures of C14-type Laves phase alloys based on Ti(Zr)-Mn2 have been c o n t r o l l e d mainly by c h a n g i n g the rat i o of t i t a n i u m and zirconium. Also, the hysteresis and p l a t e a u were improved so t h a t Hf and Sf values were a p p r o x i m a t e l y 0.5. We applied this alloy design t e c h n i q u e to a low pressure o p e r a t i o n a l and doubly effective h e a t pump system. It was found t h a t the alloys have suitable equilibrium pressures, small hysteresis and good plateaux, and an excellent coefficiency of p e r f o r m a n c e (0.65 in cooling, 1.78 in heating) was a c h i e v e d in this system.

References 1 T. Nakamichi, Nonstoichiometric Metal Compounds, Nippon Kinzoku Gakkai, 1975, p. 277. 2 T. Yamashita, T. Gamo, Y. Moriwaki and M. Fukuda, Nippon Kinzoku Gakkaishi, 41 (1977) 148. 3 T. Gamo, Y. Moriwaki, N. Yanagihara and T. Iwaki, J. Less-Common Met., 89 (1983) 495. 4 T. Gamo, Y. Moriwaki, N. Yanagihara, T. Yamashita and T. Iwaki, Int. J. Hydrogen Energy, 10 (1985) 39. 5 F. Pourarian, W. E. Wallace, J. Less.Common Met., 107 (1985) 69. 6 D. Shaltiel, J. Less-Common Met., 73 (1980) 329. 7 Y. Moriwaki, T. Gamo, N. Yanagihara and T. Iwaki, Proc. 6th Hydrogen Energy Systems Meet. of Japan, Tokyo, September 1983, p. 69. 8 Y. Moriwaki, T. Gamo, I. Takeshita and T. Iwaki, Nippon Kagaku Kaishi, 8 (1988) 1282.