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Characteristics of hydrogen-absorbing Zr-Mn alloys for heat utilization
Journal of the Less-Common
201
Metals, 168( 1991) 201-209
CHARACTERISTICS OF HYDROGEN-ABSORBING HEAT UTILIZATION
Zr-Mn ALLOYS FOR
I. YONEZU, S. FUJITANI, A. FURUKAWA, K. NASAKO, T. YONESAKI, T. SAITO and N. FURUKAWA Sanyo Electric Co. Ltd, Functional Materials Research Center, l-18-13, Hashiridani, Hirakata, Osaka, 5 73 (Japan)
(Received April 23, 1990; in revised form July 9, 1990)
Summary The substitution of various metals into ZrMn, to produce possible materials for heat utilization systems was studied in the temperature range 200-250 “C. As a result, vanadium was found to be the most effective element for decreasing the equilibrium hydrogen pressure of the ZrMn, alloy, cobalt was found to be an effective element for reducing the plateau slope of the ZrMn, alloy and the ternary Zr-Mn-V alloy, and vanadium was found to be an effective element for reducing the hysteresis of the ZrMn, alloy. Results show that quaternary Zr-Mn-Co-V alloy has excellent equilibrium characteristics in the temperature range 200-250 “C for heat utilization systems.
1. Introduction Because of growing awareness of the earth’s environmental problems, hydrogen energy is expected to become one of the clean energy sources to replace fossil fuel. In order to be able to make use of hydrogen energy, hydrogen-absorbing alloys are being given attention for energy conversion, hydrogen purification and so on. We have developed the technology necessary for the effective utilization of heat, i.e. heat storage systems and heat transportation systems, using a hydrogenabsorbing alloy [ l-41. When heat from a source, such as solar heat, is excessive in a system, excess heat is transferred to a long-term heat storage system. Because the storage system uses a chemical heat storage method, there is no heat loss during long-term heat storage. When the heat is needed at a distant heat load, it is transported through a long-distance heat transportation system. The heat transportation system uses the energy conversion function of the hydrogen-absorbing alloy, and is therefore theoretically characterized by the absence of heat loss even over a long distance. 0022-5088/91/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
202
In these heat utilization systems, heat at temperatures in the range loo-250 “C is in great demand for industrial use. For this purpose, it is necessary to develop hydrogen-absorbing alloys suitable for this temperature range. We have already developed Zr-Mn-Co-Al quaternary alloys suitable for use in the temperature range 100-200 “C [S]. It is therefore necessary to develop a hydrogenabsorbing alloy suitable for use in the range 200-250 “C. In this study, a hydrogen-absorbing alloy was developed which has equilibrium characteristics suitable for use at temperatures of 200-250 “C, as is required for industrial heat utilization to expand the high temperature range (200-250 “C). A heat utilization system requires a hydrogen-absorbing alloy with an equilibrium pressure of 0.5-l .O MPa, a small hysteresis and a small plateau slope, for easy and effective operation. The original ZrMn, alloy exhibits difficulty in the following areas: first, the equilibrium hydrogen pressure at 200-250°C is very high, second, the hysteresis is very large, and third, the slope in the plateau is also very large. Though several substituted Zr-Mn alloys have been previously investigated [6-91, they were not suitable for heat utilization at 200-250 “C. We then attempted to improve the equilibrium characteristics of ZrMn, by substituting various elements. As a result, a Zr-Mn-Co-V alloy was developed, which has an optimum equilibrium hydrogen pressure for system operations and superior reversibility in the temperature range 200-250 “C.
2. Experimental details Various multicomponent Zr-Mn alloys were prepared using an argon arc furnace (Daia Vacuum Engineering Co., Ltd.) without annealing. The buttonshaped alloy ingots obtained were crushed and put through a 100 mesh as test samples. Equilibrium characteristics and crystal structures were examined with respect to the various multicomponent Zr-Mn alloys previously obtained. Equilibrium characteristics were determined using the ordinary Sieverts unit [lo] to measure pressure-composition (hydrogen content) isotherms, and from these data, van? Hoff plots were obtained. Activation treatment was conducted on each sample by repeated hydrogen gas pressurization at 1 MPa and evacuation at 200-250 “C. Analysis of the crystal structure was conducted with an X-ray powder diffractometer (Shimadzu Corporation, XD-3), and crystal lattice parameters and cell volumes were calculated using the data obtained.
3. Results and discussion Figure 1 shows pressure-composition isotherms for the ZrMn, alloy at 150 “C. Figure 2 shows the van? Hoff plots. As shown in Fig. 1, there are great differences (hysteresis) between the absorption pressure and the desorption pressure in the plateau, with a large slope in the desorption plateau. When this alloy is used
203
for heat storage, for example at a temperature of 200 “C, it is not possible to obtain an outlet temperature of more than about 160 “C during the heat generation process because the pressure difference in the pressure-composition isotherms induces a drop in temperature. The decrease in temperature is extremely disadvantageous from the standpoint of thermal efficiency. In addition, the equilibrium hydrogen pressure for this alloy at 200-250 “C is high: higher than 1 MPa during absorption, as shown in Fig. 2. To use the original ZrMn, alloy in a heat utilization system at 200-250 “C, it was therefore necessary to (a) decrease the equilibrium hydrogen pressure, (b) reduce plateau slopes, and (c) reduce hysteresis. Improvements in these characteristics were attempted through multicomposing of the original ZrMn, alloy. H/M
P z
‘Or---=-+
P
200
150
(‘C) 100
1
E
r : : L 0.1 f
F
I t t 0
2
0.0
Temperature 250
11 0
I
0.6 1.0 Hacontent (WI%)
Fig. 1. Pressure-composition
1.5
0.0 1
isotherms for ZrMn? at 150 “C.
Fig. 2. Van7 Hoff plots for ZrMnz (Hz content, 0.65 wt.%).
M
content (x1
Fig. 3. Equilibrium hydrogen pressures of ternary ZrMn,_,M,
alloys ( 150 “C; Hz content, 0.65 wt.%).
204
First, a series of ternary alloys was prepared by substituting some first transition metals and rare earth metals for a portion of the manganese in the ZrMn, alloy, and their equilibrium characteristics were studied. Figure 3 shows the relationship between equilibrium hydrogen pressure and the amounts of vanadium, iron, cobalt, nickel and cerium substituted. As shown in this figure, vanadium, nickel and cerium effected a decrease in equilibrium hydrogen pressure. In contrast, iron and cobalt effected an increase in equilibrium hydrogen pressure. Hysteresis of the ternary alloys was also investigated. Figure 4 shows the pressure-composition isotherms for the ZrMn, alloy and the Zr-Mn-V alloys. Vanadium was found to be effective in reducing hysteresis, as shown in this figure. The substitution of only 0.2 vanadium in atomic ratio with zirconium effectively reduced hysteresis. Plateau slopes of the ternary alloys were then investigated. Figure 5 shows pressure-composition isotherms for the ZrMn, and Zr-Mn-Co alloys. As shown in Fig. 5, cobalt was found to be effective in reducing plateau slopes. We summarized the effects of the various elements in improving the equilibrium characteristics of Zr-Mn alloys, as obtained through investigation of the ternary alloys mentioned above. Using Table 1 we selected alloy systems with a H/M
H/M 0
0 10
1.0
0.5
0.5
0.5
1.0
lo -
P e
0
1.0
,
1
s t 2 0.1 P
p
O.OlL-----
0
0.5 Hz content(wt%)
0.5 1.0 1.5 HI content (vd %I
(a)
W
ZrMnn
Fig. 4. Pressure-composition
Zr Mn 1.9 Vo.1
o.oi0
0.5 1.0 1.5 H2 content (wt %)
(d
ZrMnr.8 Vo.2
isotherms for Zr-Mn and Zr-Mn-V
alloys at 200 “C.
H/M 0
0.5
1.0
10
1
Zr(Mno..s Coo.2) 2
I ,I 0.0
1 0
I 0.5
1.0
1.5
H2content (wt%)
Fig. 5. Pressure-composition
isotherms for Zr-Mn and Zr-Mn-Co
alloys at 150 “C.
205
TABLE 1 Effects of additional elements on improvement
of equilibrium characteristics
of Zr-Mn alloys
Characteristics
Element
Reversibility
Plateau pressure Increase
Hysteresis
Decrease
Plateau flatness
co
0
Fe
0
0
Ni Ce
0
V
0
0 0
0 denotes an effective element.
10
Equation for alloy design lnPd=-1.02+2.8x-6.8y (200°C) PdDesorption pressure (spa) x:Co content y:V content
ao.ol i 0
0.5 1.0 Co,V content (x,y)
Fig. 6. Equilibrium hydrogen pressure of ZrMn2_x_jCo,VY (200 “C; Hz content, 0.65 wt.%).
suitable hydrogen pressure, small hysteresis and a flat plateau at 200-250 “C. Consequently, the quarternary Zr-Mn-Co-V alloy system, which had a lower hydrogen pressure than the ZrMn, alloy, was selected as a material for use at 200-250 “C. The quarternary alloys selected were then investigated in order to determine the alloy composition which gave the optimum hydrogen pressure of about 1 MPa necessary for the efficient operation of heat utilization systems. The equilibrium hydrogen pressure of various Zr-Mn-Co-V alloys was investigated. As shown in Fig. 6, the desorption pressure of the Zr-Mn-Co-V alloy system with a vanadium content of 0.2 in atomic ratio with zirconium, increased linearly with increasing cobalt content. This increase paralleled that of the Zr-Mn-Co alloy system without vanadium. The effects of cobalt and vanadium on hydrogen pressure were considered to be independent. Thus, the
206
equation for the alloy design of the Zr-Mn-Co-V for 200 “C,
system is shown in Fig. 6, i.e.,
In Pd = - 1.03 + 2.8x + 6.8~ where Pd is the desorption pressure in megapascals, x is the cobalt content and y is the vanadium content. Furthermore, a study was done on the relationship between hydrogen pressure and cell volume in multicomponent Zr-Mn alloys analyzed by X-ray diffraction, as shown in Fig. 7. The larger the cell volume became, owing to a decrease in cobalt substituted or an increase in vanadium substituted, the lower the hydrogen pressure became. This relationship has also been recognized in part for the rareearth-nickel alloy system [ 111, and it is thought that it will prove to be an important index for the improvement of equilibrium characteristics of hydrogen-absorbing alloys. Next, Fig. 8 shows the effect of cobalt on the reduction of plateau slopes observed in our investigations. The substitution of effectively cobalt created flat plateau areas not only in ternary alloys but also in quarternary alloys. This was thought to be due to homogenizing of the alloys, because of the change in the X-ray powder diffraction profiles, as shown in Fig. 9, which is an example of the results of X-ray analysis. This figure shows X-ray powder diffraction profiles of the (220) plane for the Zr-Mn-V and Zr-Mn-Co-V alloys. The profiles became sharp with the substitution of cobalt as shown in this figure. This was believed to explain why the substitution of cobalt produced flat plateau areas.
5u\ 0
2i! 5 0 5
l-
0.5 -
c .e z
0 :Zr-h-Co-V
0
0
\
ii ii 0.2 -
k O.lf D 0.05 ff 0.02 0.01 w 175
:Zr-M-Co
A :Zf-Mn-V
0 \
Ti
0 0
0
A \
Cell Zme
(A31
4 v z 2
0
10
0.5
1.0
10
2 B al 5
1
0
0.6
1.0
’
:
h 0.1 =
go.1
i! 0.0 1
P
0
0.5 1.0 1.5 Hz content (wt%)
0.01’ 0
Fig. 7. Relation between cell volume and H, desorption pressure for various multicomponent alloys (150 Y; H, content, 0.65 wt.%). Fig. 8. The effect of cobalt on the reduction of plateau inclinations for Zr-Mn-V alloys at 200 “C.
’
0.5 1.0 i.5 H2 content (wt%)
Zr-Mn
and Zr-Mn-Co-V
207
15
16
(a)
15
(“I
28 ZrMn
1.3 V 0.2
Half values
16 20
(b)
(“I
ZrMnl.6
cOo.2VO.2
for (110),(112) and (220)
Fig. 9. Results of X-ray analysis for ZrMn,,XV,,,z and ZrMn,,,Co,,,zV,,z.
25bT1
HZ
content
(a)ZrMn,.,&o
(wt%)
04 V00s
Fig. 10. Pressure-composition
Hz
content (wt%)
(b)ZrMnl.7,Coo.4V,.o,
isotherms for Zr-Mn-Co-V
alloys.
An alloy with an optimum hydrogen pressure of 1 MPa at temperatures between 200 and 250 “C was then designed, made and tested. Consequently, we successfully developed quaternary alloys which exhibited an equilibrium hydrogen pressure of about 1 MPa at temperatures of 200-250 “C by designing an alloy with cobalt and vanadium contents, as shown in Fig. 10. At the same time, these alloys
208 2.4
2.2 7 Y 7 ;;‘ z 2.0
/ 1.8
,-
/
/
////
0.2
0.3
V.4
co content(x) ,n ZrMn,
,,-J&V,
oe
Fig. 11. Relationship between alloy composition and operating temperature for ZrMn,,,z _ xCo,V,, ,,#.
showed excellent reversibility, i.e. small hysteresis and a flat plateau. Figure 11 shows the relationship between cobalt content in ZrMn,,,,_XCo,V,,, alloys and operation temperature with an equilibrium hydrogen pressure of 1 MPa. As shown in this figure, it is possible to obtain optimum alloy composition by adjusting the cobalt content.
4. Conclusion We have successfully developed Zr-Mn-Co-V quarternary alloys with equilibrium characteristics superior to any previously available alloys. In this study, the following was found. (1) Vanadium was the most effective element for decreasing the equilibrium hydrogen pressure of a ZrMn, alloy. (2) Cobalt was effective in reducing the plateau slope of the ZrMn, and Zr-Mn-V alloys. (3) Vanadium was effective in reducing the hysteresis of the ZrMn, alloy. (4) Zr-Mn-Co-V alloys have suitable equilibrium characteristics for heat utilization in the range 200-250 “C. Furthermore, these alloys were able to be designed with an optimum equilibrium hydrogen pressure of 1 MPa. These alloys can therefore be used as hydrogen-absorbing alloys in heat utilization systems with an operating range of 200 to 250 “C, as required for industrial heat utilization.
209
Acknowledgments
A part of this work was supported by NED0 (New Energy and Industrial Technology Development Organization) as part of the Sunshine Project under the Ministry of International Trade and Industry in Japan.
References 1 I. Yonezu, K. Nasako, N. Honda and T. Sakai, J. Less-Common Met., 89 (1983) 35 1. 2 N. Honda, N. Furukawa and T. Sakai, Sunshine J., 9 (1) (1988) 20. 3 K. Nasako, I. Yonezu, S. Fujitani, A. Furukawa, T. Yonesaki and T. Sakai, Proc. Mat. Res. So;., Int. Meet. on Advanced Materials, Materials Research Society, Pittsburgh, PA, 1989, Vol. 2, pp. 113. 4 K. Nasako, T. Yonesaki, I. Yonezu, S. Fujitani, T. Saito, M. Moroto, M. Osumi and N. Furukawa, Proc. ISES Solar World Congress, Kobe, 1989, to be published. 5 I. Yonezu, S. Fujitani, A. Furukawa, K. Nasako, T. Yonesaki and N. Furukawa, Proc. Materials Research Society, Int. Meet. on Advanced Materials, Materials Research Society, Pittsburgh, PA, 1989, Vol. 2, p. 149. 6 D. Shaltiel, L. Jacob and D. Davidov, J. Less-Common Met., 53 (1977) 117. 7 Y. Ishido, N. Nishimiya and A. Suzuki, DENKI KAGAKU, 45 (1) (1977) 52. 8 A. Suzuki, N. Nishimiya and S. Ono, J. Less-Common Met., 89( 1983) 263. 9 D. G. Ivey and D. 0. Northwood, J. Mater. EnergySystems, 4 (1983) 222. IO K. Nomura and S. Ono, Kagaku Kogaku, 49(5) (1985) 630. 11 M. H. Mendelshon and D. M. Gruen, Nature, 269 (1977) 45.
201
Metals, 168( 1991) 201-209
CHARACTERISTICS OF HYDROGEN-ABSORBING HEAT UTILIZATION
Zr-Mn ALLOYS FOR
I. YONEZU, S. FUJITANI, A. FURUKAWA, K. NASAKO, T. YONESAKI, T. SAITO and N. FURUKAWA Sanyo Electric Co. Ltd, Functional Materials Research Center, l-18-13, Hashiridani, Hirakata, Osaka, 5 73 (Japan)
(Received April 23, 1990; in revised form July 9, 1990)
Summary The substitution of various metals into ZrMn, to produce possible materials for heat utilization systems was studied in the temperature range 200-250 “C. As a result, vanadium was found to be the most effective element for decreasing the equilibrium hydrogen pressure of the ZrMn, alloy, cobalt was found to be an effective element for reducing the plateau slope of the ZrMn, alloy and the ternary Zr-Mn-V alloy, and vanadium was found to be an effective element for reducing the hysteresis of the ZrMn, alloy. Results show that quaternary Zr-Mn-Co-V alloy has excellent equilibrium characteristics in the temperature range 200-250 “C for heat utilization systems.
1. Introduction Because of growing awareness of the earth’s environmental problems, hydrogen energy is expected to become one of the clean energy sources to replace fossil fuel. In order to be able to make use of hydrogen energy, hydrogen-absorbing alloys are being given attention for energy conversion, hydrogen purification and so on. We have developed the technology necessary for the effective utilization of heat, i.e. heat storage systems and heat transportation systems, using a hydrogenabsorbing alloy [ l-41. When heat from a source, such as solar heat, is excessive in a system, excess heat is transferred to a long-term heat storage system. Because the storage system uses a chemical heat storage method, there is no heat loss during long-term heat storage. When the heat is needed at a distant heat load, it is transported through a long-distance heat transportation system. The heat transportation system uses the energy conversion function of the hydrogen-absorbing alloy, and is therefore theoretically characterized by the absence of heat loss even over a long distance. 0022-5088/91/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
202
In these heat utilization systems, heat at temperatures in the range loo-250 “C is in great demand for industrial use. For this purpose, it is necessary to develop hydrogen-absorbing alloys suitable for this temperature range. We have already developed Zr-Mn-Co-Al quaternary alloys suitable for use in the temperature range 100-200 “C [S]. It is therefore necessary to develop a hydrogenabsorbing alloy suitable for use in the range 200-250 “C. In this study, a hydrogen-absorbing alloy was developed which has equilibrium characteristics suitable for use at temperatures of 200-250 “C, as is required for industrial heat utilization to expand the high temperature range (200-250 “C). A heat utilization system requires a hydrogen-absorbing alloy with an equilibrium pressure of 0.5-l .O MPa, a small hysteresis and a small plateau slope, for easy and effective operation. The original ZrMn, alloy exhibits difficulty in the following areas: first, the equilibrium hydrogen pressure at 200-250°C is very high, second, the hysteresis is very large, and third, the slope in the plateau is also very large. Though several substituted Zr-Mn alloys have been previously investigated [6-91, they were not suitable for heat utilization at 200-250 “C. We then attempted to improve the equilibrium characteristics of ZrMn, by substituting various elements. As a result, a Zr-Mn-Co-V alloy was developed, which has an optimum equilibrium hydrogen pressure for system operations and superior reversibility in the temperature range 200-250 “C.
2. Experimental details Various multicomponent Zr-Mn alloys were prepared using an argon arc furnace (Daia Vacuum Engineering Co., Ltd.) without annealing. The buttonshaped alloy ingots obtained were crushed and put through a 100 mesh as test samples. Equilibrium characteristics and crystal structures were examined with respect to the various multicomponent Zr-Mn alloys previously obtained. Equilibrium characteristics were determined using the ordinary Sieverts unit [lo] to measure pressure-composition (hydrogen content) isotherms, and from these data, van? Hoff plots were obtained. Activation treatment was conducted on each sample by repeated hydrogen gas pressurization at 1 MPa and evacuation at 200-250 “C. Analysis of the crystal structure was conducted with an X-ray powder diffractometer (Shimadzu Corporation, XD-3), and crystal lattice parameters and cell volumes were calculated using the data obtained.
3. Results and discussion Figure 1 shows pressure-composition isotherms for the ZrMn, alloy at 150 “C. Figure 2 shows the van? Hoff plots. As shown in Fig. 1, there are great differences (hysteresis) between the absorption pressure and the desorption pressure in the plateau, with a large slope in the desorption plateau. When this alloy is used
203
for heat storage, for example at a temperature of 200 “C, it is not possible to obtain an outlet temperature of more than about 160 “C during the heat generation process because the pressure difference in the pressure-composition isotherms induces a drop in temperature. The decrease in temperature is extremely disadvantageous from the standpoint of thermal efficiency. In addition, the equilibrium hydrogen pressure for this alloy at 200-250 “C is high: higher than 1 MPa during absorption, as shown in Fig. 2. To use the original ZrMn, alloy in a heat utilization system at 200-250 “C, it was therefore necessary to (a) decrease the equilibrium hydrogen pressure, (b) reduce plateau slopes, and (c) reduce hysteresis. Improvements in these characteristics were attempted through multicomposing of the original ZrMn, alloy. H/M
P z
‘Or---=-+
P
200
150
(‘C) 100
1
E
r : : L 0.1 f
F
I t t 0
2
0.0
Temperature 250
11 0
I
0.6 1.0 Hacontent (WI%)
Fig. 1. Pressure-composition
1.5
0.0 1
isotherms for ZrMn? at 150 “C.
Fig. 2. Van7 Hoff plots for ZrMnz (Hz content, 0.65 wt.%).
M
content (x1
Fig. 3. Equilibrium hydrogen pressures of ternary ZrMn,_,M,
alloys ( 150 “C; Hz content, 0.65 wt.%).
204
First, a series of ternary alloys was prepared by substituting some first transition metals and rare earth metals for a portion of the manganese in the ZrMn, alloy, and their equilibrium characteristics were studied. Figure 3 shows the relationship between equilibrium hydrogen pressure and the amounts of vanadium, iron, cobalt, nickel and cerium substituted. As shown in this figure, vanadium, nickel and cerium effected a decrease in equilibrium hydrogen pressure. In contrast, iron and cobalt effected an increase in equilibrium hydrogen pressure. Hysteresis of the ternary alloys was also investigated. Figure 4 shows the pressure-composition isotherms for the ZrMn, alloy and the Zr-Mn-V alloys. Vanadium was found to be effective in reducing hysteresis, as shown in this figure. The substitution of only 0.2 vanadium in atomic ratio with zirconium effectively reduced hysteresis. Plateau slopes of the ternary alloys were then investigated. Figure 5 shows pressure-composition isotherms for the ZrMn, and Zr-Mn-Co alloys. As shown in Fig. 5, cobalt was found to be effective in reducing plateau slopes. We summarized the effects of the various elements in improving the equilibrium characteristics of Zr-Mn alloys, as obtained through investigation of the ternary alloys mentioned above. Using Table 1 we selected alloy systems with a H/M
H/M 0
0 10
1.0
0.5
0.5
0.5
1.0
lo -
P e
0
1.0
,
1
s t 2 0.1 P
p
O.OlL-----
0
0.5 Hz content(wt%)
0.5 1.0 1.5 HI content (vd %I
(a)
W
ZrMnn
Fig. 4. Pressure-composition
Zr Mn 1.9 Vo.1
o.oi0
0.5 1.0 1.5 H2 content (wt %)
(d
ZrMnr.8 Vo.2
isotherms for Zr-Mn and Zr-Mn-V
alloys at 200 “C.
H/M 0
0.5
1.0
10
1
Zr(Mno..s Coo.2) 2
I ,I 0.0
1 0
I 0.5
1.0
1.5
H2content (wt%)
Fig. 5. Pressure-composition
isotherms for Zr-Mn and Zr-Mn-Co
alloys at 150 “C.
205
TABLE 1 Effects of additional elements on improvement
of equilibrium characteristics
of Zr-Mn alloys
Characteristics
Element
Reversibility
Plateau pressure Increase
Hysteresis
Decrease
Plateau flatness
co
0
Fe
0
0
Ni Ce
0
V
0
0 0
0 denotes an effective element.
10
Equation for alloy design lnPd=-1.02+2.8x-6.8y (200°C) PdDesorption pressure (spa) x:Co content y:V content
ao.ol i 0
0.5 1.0 Co,V content (x,y)
Fig. 6. Equilibrium hydrogen pressure of ZrMn2_x_jCo,VY (200 “C; Hz content, 0.65 wt.%).
suitable hydrogen pressure, small hysteresis and a flat plateau at 200-250 “C. Consequently, the quarternary Zr-Mn-Co-V alloy system, which had a lower hydrogen pressure than the ZrMn, alloy, was selected as a material for use at 200-250 “C. The quarternary alloys selected were then investigated in order to determine the alloy composition which gave the optimum hydrogen pressure of about 1 MPa necessary for the efficient operation of heat utilization systems. The equilibrium hydrogen pressure of various Zr-Mn-Co-V alloys was investigated. As shown in Fig. 6, the desorption pressure of the Zr-Mn-Co-V alloy system with a vanadium content of 0.2 in atomic ratio with zirconium, increased linearly with increasing cobalt content. This increase paralleled that of the Zr-Mn-Co alloy system without vanadium. The effects of cobalt and vanadium on hydrogen pressure were considered to be independent. Thus, the
206
equation for the alloy design of the Zr-Mn-Co-V for 200 “C,
system is shown in Fig. 6, i.e.,
In Pd = - 1.03 + 2.8x + 6.8~ where Pd is the desorption pressure in megapascals, x is the cobalt content and y is the vanadium content. Furthermore, a study was done on the relationship between hydrogen pressure and cell volume in multicomponent Zr-Mn alloys analyzed by X-ray diffraction, as shown in Fig. 7. The larger the cell volume became, owing to a decrease in cobalt substituted or an increase in vanadium substituted, the lower the hydrogen pressure became. This relationship has also been recognized in part for the rareearth-nickel alloy system [ 111, and it is thought that it will prove to be an important index for the improvement of equilibrium characteristics of hydrogen-absorbing alloys. Next, Fig. 8 shows the effect of cobalt on the reduction of plateau slopes observed in our investigations. The substitution of effectively cobalt created flat plateau areas not only in ternary alloys but also in quarternary alloys. This was thought to be due to homogenizing of the alloys, because of the change in the X-ray powder diffraction profiles, as shown in Fig. 9, which is an example of the results of X-ray analysis. This figure shows X-ray powder diffraction profiles of the (220) plane for the Zr-Mn-V and Zr-Mn-Co-V alloys. The profiles became sharp with the substitution of cobalt as shown in this figure. This was believed to explain why the substitution of cobalt produced flat plateau areas.
5u\ 0
2i! 5 0 5
l-
0.5 -
c .e z
0 :Zr-h-Co-V
0
0
\
ii ii 0.2 -
k O.lf D 0.05 ff 0.02 0.01 w 175
:Zr-M-Co
A :Zf-Mn-V
0 \
Ti
0 0
0
A \
Cell Zme
(A31
4 v z 2
0
10
0.5
1.0
10
2 B al 5
1
0
0.6
1.0
’
:
h 0.1 =
go.1
i! 0.0 1
P
0
0.5 1.0 1.5 Hz content (wt%)
0.01’ 0
Fig. 7. Relation between cell volume and H, desorption pressure for various multicomponent alloys (150 Y; H, content, 0.65 wt.%). Fig. 8. The effect of cobalt on the reduction of plateau inclinations for Zr-Mn-V alloys at 200 “C.
’
0.5 1.0 i.5 H2 content (wt%)
Zr-Mn
and Zr-Mn-Co-V
207
15
16
(a)
15
(“I
28 ZrMn
1.3 V 0.2
Half values
16 20
(b)
(“I
ZrMnl.6
cOo.2VO.2
for (110),(112) and (220)
Fig. 9. Results of X-ray analysis for ZrMn,,XV,,,z and ZrMn,,,Co,,,zV,,z.
25bT1
HZ
content
(a)ZrMn,.,&o
(wt%)
04 V00s
Fig. 10. Pressure-composition
Hz
content (wt%)
(b)ZrMnl.7,Coo.4V,.o,
isotherms for Zr-Mn-Co-V
alloys.
An alloy with an optimum hydrogen pressure of 1 MPa at temperatures between 200 and 250 “C was then designed, made and tested. Consequently, we successfully developed quaternary alloys which exhibited an equilibrium hydrogen pressure of about 1 MPa at temperatures of 200-250 “C by designing an alloy with cobalt and vanadium contents, as shown in Fig. 10. At the same time, these alloys
208 2.4
2.2 7 Y 7 ;;‘ z 2.0
/ 1.8
,-
/
/
////
0.2
0.3
V.4
co content(x) ,n ZrMn,
,,-J&V,
oe
Fig. 11. Relationship between alloy composition and operating temperature for ZrMn,,,z _ xCo,V,, ,,#.
showed excellent reversibility, i.e. small hysteresis and a flat plateau. Figure 11 shows the relationship between cobalt content in ZrMn,,,,_XCo,V,,, alloys and operation temperature with an equilibrium hydrogen pressure of 1 MPa. As shown in this figure, it is possible to obtain optimum alloy composition by adjusting the cobalt content.
4. Conclusion We have successfully developed Zr-Mn-Co-V quarternary alloys with equilibrium characteristics superior to any previously available alloys. In this study, the following was found. (1) Vanadium was the most effective element for decreasing the equilibrium hydrogen pressure of a ZrMn, alloy. (2) Cobalt was effective in reducing the plateau slope of the ZrMn, and Zr-Mn-V alloys. (3) Vanadium was effective in reducing the hysteresis of the ZrMn, alloy. (4) Zr-Mn-Co-V alloys have suitable equilibrium characteristics for heat utilization in the range 200-250 “C. Furthermore, these alloys were able to be designed with an optimum equilibrium hydrogen pressure of 1 MPa. These alloys can therefore be used as hydrogen-absorbing alloys in heat utilization systems with an operating range of 200 to 250 “C, as required for industrial heat utilization.
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Acknowledgments
A part of this work was supported by NED0 (New Energy and Industrial Technology Development Organization) as part of the Sunshine Project under the Ministry of International Trade and Industry in Japan.
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