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Dehydriding properties of ternary Mg2Ni1−xZrx hydrides synthesized by ball milling and annealing
Journal of Alloys and Compounds 269 (1998) 278–283
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Dehydriding properties of ternary Mg 2 Ni 12x Zr x hydrides synthesized by ball milling and annealing Yunshi Zhang, Huabin Yang*, Huatang Yuan, Endong Yang, Zuoxiang Zhou, Deying Song Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China Received 12 December 1997; received in revised form 20 January 1998
Abstract The ternary hydrogen-storage alloys Mg 2 Ni 12x Zrx (0,x#0.3) have been successfully synthesized by ball milling followed by annealing. Studies of the synthesis conditions of the ternary alloys and their dehydriding properties have been carried out. The results show that the ternary Mg 2 Ni 12x Zr x (0,x#0.3) alloys have the same hexagonal crystal structure as that of Mg 2 Ni. Compared with a Mg 2 Ni alloy, they have a larger specific surface (|1.20 m 2 g 21 ), more promising dehydriding kinetics, lower enthalpy of formation of hydrides than that of Mg 2 NiH 4 , and lower decomposition temperatures in an open system. An optimum desorption storage capacity of about 3.3 wt.% is observed. 1998 Elsevier Science S.A. Keywords: Ball milling; Diffusion; Mg alloys; Hydrogen storage alloy; Dehydriding kinetics
1. Introduction Among the metal–hydrogen systems, the magnesium– hydrogen system is recognized to be the most promising one. The interest in magnesium as a hydrogen-storage material originates from the lighter weight, the higher hydrogen weight percent (7.6%) of the metal hydride, and the lower price compared with other types of metal– hydrogen systems. However, it is inadequate for practical use for hydrogen storage applications because of its poor hydriding / dehydriding kinetics. In recent years, there has been much effort made to improve the hydriding / dehydriding properties of the Mg– hydrogen system [1–18]. The hydriding / dehydriding properties of the Mg–hydrogen system have been substantially improved in several ways: (i) element partial substitution, e.g. Mg 2 Ni [1], Mg 2 Cu [2], Mg–La [3], etc.; (ii) composite formation with other hydrogen storage materials, such as Mg–Mg 2 Ni [4], Mg–LaNi 5 [5], Mg–ZrFe 1.4 Cr 0.6 [6] and Mg–40wt.%FeTi (Mn) [7,8] to form new phases in order to increase the surface activity; (iii) modification with polynuclear aromatic compounds, e.g. Mg–THF [9] and Mg 2 Ni–tetracyanoethylene and phthalonitrile [10]; (iv) surface treatment to improve the surface activity by coating the surface with Ni, Cu or Pd [11,12] and etching *Corresponding author. Tel.: 186 22 23502604; fax: 186 22 23502604; e-mail: [email protected] 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 98 )00222-9
Mg-based alloys with HCl solution [13], or / and F-solution [14,15]. Another approach to improving the hydriding / dehydriding kinetics is to find new synthesis methods. It is well known that the properties of Mg-based alloys and their hydrides are quite different according to the various preparative methods used. Mg-based alloys are usually prepared according to a normal metallurgical procedure [2]. Ball milling has recently emerged as a novel technique for alloy formation [19–21]. In the metal–hydrogen systems, specific surface and particle sizes of the material are the important parameters for enhancement in the hydrogen uptake, reduction in the activation time, temperature and pressure, and improvement in the kinetics of the material. Since ball milling leads to materials in fine-particle form, it may be typically suited for the synthesis and formation of the Mg-based hydrogen storage alloys. In the present paper, we have used a powerful method to synthesize Mg-based alloys [22]. We have called this method the ball milling-diffusion method (BDM) in the following. One of the striking aspects of this method is that the formation of the Mg-based alloys is proceeded by a two-step process, i.e. ball milling, which is carried out only for 30–120 min, coupled with the interdiffusion of the components in the solid state. However, the temperature during the interdiffusion must not be very high. It must be lower than the melting point of any component of the
Y. Zhang et al. / Journal of Alloys and Compounds 269 (1998) 278 – 283
initial materials which are used for the synthesis of the alloys. If the temperature is higher than the lowest melting point of any initial material, this component must be sintered so as not to be able to interdiffuse in the solid state. For the Mg–Ni–Zr system, the temperature must be lower than the melting point (6508C in an open system) of the metal Mg. In addition, this method is especially suitable for the substitution of metals with poor activity and high melting points for other components. Previous studies carried out in our laboratory on magnesium-based alloys have shown that the hydriding / dehydriding rates and other properties of Mg-based alloys can be strongly improved by the replacement-diffusion method (RDM) [23–25]. However, RDM is only suitable for metals with high activity. As we know, Zr does not form an alloy with Mg, which may be important in maintaining the high hydrogen capacity associated with Mg. Zr can absorb and desorb hydrogen reversibly itself. Moreover, Zr can very easily form alloys with Ni, which can also absorb and desorb hydrogen reversibly. In addition, Zr has a good ability to protect the alloy bulk from being oxidized further. However, the synthesis of the Mg 2 Ni 12x Zr x (0,x#0.3) alloys is very difficult using metallurgical methods (MM) due to the difference between the higher melting point of Zr and the lower melting point of Mg. After fusion, the materials need to be powdered and reheated at higher temperature for several days to ensure homogeneity. Because of the poor activity of Zr, RDM is also not suitable for the synthesis of Zr-containing Mg-based alloys [25]. In the present paper, we successfully synthesized the ternary Mg 2 Ni 12x Zr x (0,x#0.3) alloys by BDM. Studies on the synthesis conditions of the ternary alloys and their dehydriding properties have been carried out.
2. Experimental details
2.1. Materials Magnesium was obtained in powder form (purity of 99.8% and particle size of 60–100 mesh). Nickel and zirconium were also obtained in powder form with a particle size of 60–100 mesh (purity of 99.9%).
2.2. Sample preparation The ternary Mg 2 Ni 12x Zr x samples were produced by BDM. In order to avoid the formation of the non-reacting MgNi 2 phase during the reaction, these samples were slightly Mg-richer than the stoichiometric composition of Mg 2 Ni 12x Zr x . The mixtures of the initial materials (magnesium, nickel and zirconium) and 20 agate balls of 7 mm diameter (weight ratio, 1:20) were placed into a ball milling mortar which was sealed under an argon atmosphere. The mixtures were milled for 2 h with a rotational
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velocity of 300 rev min 21 . After milling, the mixtures were removed from the mortar. The mixtures were compressed under a pressure of 30 MPa to form pellets 20 mm in diameter and 5 mm thick. Then, the pellets were heated employing a resistor furnace under an argon atmosphere (5 atm) in a sealed stainless steel vessel. The vessel was heated to 400, 500, 550, 580, 600 and 6508C, respectively, and held at each temperature for 1, 2, 3, 4, 5 and 6 h, respectively. During this treatment, the components of the mixtures interdiffused in the solid state. Then, the vessel was cooled to room temperature. The samples produced in this way were removed from the vessel, crushed and subjected to the component analysis, which indicated that the samples were Mg 2 Ni 12x Zr x . The X-ray diffraction (XRD) spectra were recorded using a Rigaku / max-rB X-ray diffractometer with Cu Ka radiation.
2.3. Activation procedure of the samples Since the synthesized samples did not absorb hydrogen at room temperature, they had to be suitably activated. It was found that the favorable activation process corresponded to the following conditions: Each synthesized sample was placed in another stainless steel reactor which was evacuated to about 10 22 Torr. Then, 3 MPa of hydrogen was introduced from a highpressure gas cylinder. The bottom portion of the reactor was heated at 3508C for 3 h. Then it was slowly cooled to 3008C and the hydrogen desorbed. Again the reactor was evacuated and hydrogen gas was introduced into it. In the fifth cycle, the quantity of absorbed hydrogen was still small. However, upon continuation of the activation process, each sample absorbed more and more hydrogen. After the tenth cycle, the capacity of the absorbed hydrogen of each sample had become constant.
2.4. Properties of the samples The specific surface of each sample was obtained by the Brunaner–Emmett–Teller (BET) method using nitrogen adsorption at 77 K. The rates and the composition during the desorption were measured using a conventional volumetric method. The pressure–composition desorption isotherms (PCT) measurements were performed at various temperatures on the synthesized samples.
3. Results and discussion
3.1. Moderate synthesis conditions 3.1.1. Effect of reaction temperature on formation of alloys Because the samples were synthesized by BDM rather than by the metallurgical method (MM), the reaction
Y. Zhang et al. / Journal of Alloys and Compounds 269 (1998) 278 – 283
280
Table 1 The temperature effect on the formation of the ternary alloys Temp. (8C)
x50.01
x50.1
x50.2
x50.3
x50.4
400 500 550 580 600 650
Metals Metals Metals1alloy Alloy Alloy Alloy1metals
Metals Metals Metals1alloy Alloy Alloy Alloy1metals
Metals Metals Metals1alloy Alloy Alloy Alloy1metals
Metals Metals Metals1alloy Alloy Alloy Alloy1metals
Metals Metals Metals1alloy Metals1Alloy Metals1Alloy Alloy1metals
Size, 60–100 mesh; ball milling time, 4 h; heating time, 4 h.
temperature must be lower than the melting point of the metal Mg (6508C). A temperature of 6508C was chosen as the maximum temperature. Table 1 shows the effect of the reaction temperature on the formation of the ternary alloys. These results indicate that the alloys cannot be formed unless the reaction temperature is higher than 5508C, but lower than 6508C. When the reaction temperature was set to 5008C or lower, e.g. 4008C, the samples were still mixtures of the original metals. When the temperature was set to 5508C, some amounts of alloys began to be formed. In the temperature range from 580 to 6008C, the samples consisted of only the ternary alloys. However, when the temperature was higher than 6008C, maybe 6508C, the samples could be partly converted into metals again due to the sintering and loss of Mg, as can be seen from Table 1. From Table 1, we can also see that, no matter how high the temperature is, the samples (x50.4) all consisted of both alloys and metals, which is probably due to the excessive addition of zirconium.
3.1.2. Effect of reaction time on formation of alloys Because the metal zirconium does not very easily diffuse into the bulk of Mg, the reaction time should be monitored carefully. According to Section 3.1.1, we chose 5808C as the reaction temperature. Table 2 shows the effect of the reaction time on the formation of the ternary alloys. The results indicate that, after the samples were heated for 4 h, the alloys have been completely formed. Even when heating for longer than 4 h, the samples consisted of only the alloys. However, after 6 h, the samples, with x50.4, still consisted of both metals and alloys, which is also due to the excessive addition of zirconium. From Tables 1 and 2, we chose a temperature of 5808C
and a reaction time of 4 h as the moderate synthesis conditions for the ternary Mg 2 Ni 12x Zr x (0,x#0.3) alloys. In addition, the component of zirconium (x) should not be larger than 0.3. Otherwise, no matter how high the temperature or how long the reaction time, the samples will be mixtures of both alloys and metals.
3.2. Phase structure of ternary Mg2 Ni12 x Zrx alloys In order to unravel the curious hydrogenation behavior of the ternary Mg 2 Ni 12x Zr x alloys, structural characteristics were monitored. X-ray diffraction spectra of these samples were taken at seven separate zirconium concentrations: x50, 0.01, 0.1, 0.2, 0.25, 0.3 and 0.4. Fig. 1 shows the X-ray diffractograms of the ternary Mg 2 Ni 12x Zrx alloys synthesized for 4 h under 5808C and an argon atmosphere (5 atm). They reveal that the main phase of each ternary alloy is similar to that of the Mg 2 Ni alloy, which has a hexagonal crystal structure. There are substantial changes in lattice parameters of the hexagonal structure (Table 3). The results from Table 3 show that the value of the lattice parameter a increases with increasing value of x. However, the value of the lattice parameter c, slightly increases with increasing amount of zirconium. This suggests that the zirconium, which has a larger atom radius than that of Ni, mainly occupies the a-axis, which leads to the increasing of the lattice volume. Table 3 also shows the lattice parameters of the Mg 2 Ni alloy made by MM. It clearly shows that the lattice parameters a and c of the Mg 2 Ni alloy made by BDM are greater than those obtained by MM. The difference is only caused by the different synthesis method. According to the lattice expansion theory [26], the larger the unit cell volume, the better
Table 2 The reaction time effect on the formation of the ternary alloys Time (h)
x50.01
x50.1
x50.2
x50.3
x50.4
1 2 3 4 5 6
Metals1alloy Metals1alloy Alloy Alloy Alloy Alloy
Metals1alloy Metals1alloy Metals1alloy Alloy Alloy Alloy
Metals1alloy Metals1alloy Metals1alloy Alloy Alloy Alloy
Metals1alloy Metals1alloy Metals1alloy Alloy Alloy Alloy
Metals1alloy Metals1alloy Metals1alloy Metals1alloy Metals1alloy Metals1alloy
Size, 60–100 mesh; heating temperature, 5808C; heating time, 4 h.
Y. Zhang et al. / Journal of Alloys and Compounds 269 (1998) 278 – 283
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Fig. 2. Kinetic curves of the desorption of the activated ternary Mg 2 Ni 12x Zr x alloys for three values of x (0, 0.1 and 0.3) at 3408C and under a pressure of 1 atm. Fig. 1. XRD spectra of the ternary Mg 2 Ni 12x Zr x (0,x#0.4) samples.
the characteristics of the alloy. Therefore, the alloys made by BDM may have some advantages. In addition, according to the binary alloy phase diagram of Mg–Zr [27], only a dilute solid solution of Zr in Mg (solubility of 3.8 wt.% at 923 K and 0.3 wt.% at 573 K) is formed. This is verified by the XRD results (Fig. 1). Only diffraction peaks originating from Mg 2 Ni and Zr–Ni phases are observed. However, the peaks of the Zr–Ni phases are so small that they are not very easily observed.
3.3. Dehydriding characteristics of ternary Mg2 Ni12 x Zrx (0, x#0.3) alloys In the metal hydride systems, specific surface is one of the most important parameters for enhancement of hydrogen uptake, reduction in the activation time, temperature and pressure, and improvement in the kinetics of the material. The alloys made by BDM have promising surface characteristics. The specific surfaces of all the alloys obtained by the BET method are listed in Table 4. These results show that the alloys synthesized by BDM
have a very large specific surface. The specific surface of the unsubstituted Mg 2 Ni alloy made by BDM is only 0.5 m 2 g 21 . However, the specific surface of the ternary substituted alloys can approach 1.2 m 2 g 21 . The substituted alloys have a larger specific surface mainly due to the substitution of zirconium for nickel. Using scanning electron microscopy, the surfaces of the ternary alloys are seen to have many more defects and cracks along the interfaces between the particles than those of the Mg 2 Ni alloy. These defects and cracks will lead to the dehydriding characteristics of the ternary substituted alloys being more promising than those of the Mg 2 Ni alloy made by BDM.
3.3.1. Dehydriding kinetics of ternary alloys The hydrogen desorption characteristics of the ternary Mg 2 Ni 12x Zrx (0,x#0.3) alloys for several values of x were measured at 3408C. Fig. 2 shows the kinetic curves of the desorption of the activated ternary alloys for three values of x (0, 0.1 and 0.3) at 3408C and under a pressure of 1 atm. Fig. 2 clearly shows that the desorption kinetic characteristics of the ternary alloys are enhanced by the addition
Table 3 Lattice parameters of the Mg 2 Ni 12x Zr x alloys (0#x#0.4) Mg 2 Ni a ˚ a (A) ˚ c (A) a
a
5.190 a 13.24
x50
x50.01
x50.1
x50.2
x50.3
x50.4
5.201 13.25
5.203 13.19
5.210 13.26
5.212 13.29
5.218 13.30
— —
x50
x50.01
x50.1
x50.2
x50.3
x50.4
0.5
1.20
1.20
1.19
1.21
—
Made by MM.
Table 4 The specific surface of Mg 2 Ni 12x Zr x (0#x#0.4) alloys 2
21
Specific surface (m g ) Size, 100–200 mesh.
282
Y. Zhang et al. / Journal of Alloys and Compounds 269 (1998) 278 – 283
of zirconium. These results indicate that the desorption kinetics of the ternary alloys is faster than that of the Mg 2 Ni alloy. From Fig. 2, it can be observed that the desorption of hydrogen to about 80% of the saturation value of the ternary alloys is accomplished within 10 min, which is due to the larger specific surface (Table 4). However, the total time required for complete desorption has been found to be about 40 min, which is the same as that of the Mg 2 Ni alloy made by BDM. Compared with the Mg 2 Ni alloy made by MM [28], even the Mg 2 Ni alloy made by BDM also shows better desorption kinetics. According to a previous report [28], the Mg 2 Ni alloy made by MM desorbed hydrogen to only about 20% of the saturation value within 60 min. It can also be observed that the maximum desorbed hydrogen capacity of the alloys is about 3.3 wt.%, which is a little higher than that of the Mg 2 Ni alloy in this experiment. The desorption kinetic characteristics of the zirconium substituted alloys are enhanced by the addition of zirconium. This may be attributed to the following factors. Firstly, the substituted alloys have a larger specific surface than that of the Mg 2 Ni alloy made by BDM, which leads to an enhancement of the desorption kinetics (Table 4). The redox reaction of hydrogen proceeds much more easily along the interfaces between the particles, where there are many defects and cracks. Secondly, as discussed in Section 3.2, the unit cell volumes of the alloys increase with increasing zirconium content, which is beneficial for hydrogen diffusion in the alloys. Finally, because only a dilute solid solution of Zr in Mg is formed (Section 3.2), some of the Zr has alloyed with Ni to form Zr–Ni phases. It is known that Zr–Ni alloys can act as hydrogen storage materials themselves, providing a passway for hydrogen atoms to diffuse. The Zr–Ni phases may act as active sites for hydrogen desorption. To find and evaluate the storage capacity, the pressure– composition desorption isotherms of activated Mg 2 Ni 12x Zr x (0,x#0.3) alloys have been monitored for various x values.
3.3.2. Pressure–composition desorption isotherms of ternary Mg2 Ni12 x Zr x (0, x#0.3) alloys The P–C desorption isotherms of the ternary alloys of several compositions (x50.01, 0.1 and 0.3) were determined at 3408C. The maximum storage capacity, obtained for the three separate x values, all corresponds to 3.3 wt.% at 3408C. A clear-cut plateau is observed for all cases. The abrupt termination of the isotherms at a composition corresponding to Mg 2 Ni 12x Zr x H 4 indicates that all the hydrides have well-defined stoichiometry. The desorption isotherms at 3408C for the Mg 2 Ni 0.7 Zn 0.3 alloy is shown in Fig. 3. Because few Zr–Ni phases can be observed in Fig. 1, only one plateau is clearly observed on each isotherm. The desorption reaction can be described as follows:
Fig. 3. P–C desorption isotherms of the Mg 2 Ni 0.7 Zr 0.3 alloy (BDM) and the Mg 2 Ni alloy (MM) at 3408C.
Mg 2 Ni 12x Zr x H 42y → Mg 2 Ni 12x Zr x 1 (2 2 y / 2)H 2
(1)
where the desorption enthalpy is indicated by DH. The desorption process obeyed the linear relationship expected on the basis of the Van’t Hoff equation very well. Based on the relationship between ln PeqH 2 and the absolute temperature 1 /T, the linear equation is given by: ln PeqH 2 5 DH /RT 1 DS /R
(2)
where PeqH 2 is the equilibrium pressure of the plateau, R is the gas constant, T is the absolute temperature, DS is the entropy. The heat of desorption DH is given in Table 5. The results from Table 5 show that the heat of the desorption of the ternary alloys have lower absolute values for increasing the zirconium concentrations. Fig. 3 and Table 6 show that the desorption equilibrium pressures of the ternary alloys (BDM) are slightly higher than that of the Mg 2 Ni alloy (MM). As we know, there is a good correlation between the unit cell volume of compounds and the equilibrium plateau pressure of hydrogen [29,30]. In other words, the larger the unit cell volume, the more stable the hydride. However, there are still some exceptional cases against this rule. The Mg 2 Ni 12x Be x [31] system is a typical example of this Table 5 Dissociation temperatures and enthalpies of Mg 2 Ni 12x Zr x (0,x#0.3) hydrides (PH 2 50.1 MPa)
T (8C) DH (kJ mol 21 )
Mg 2 Ni
x50.1
x50.2
x50.3
253 [1] 264.0 [1]
251 262.7
250 262.3
248 259.8
Table 6 The desorption equilibrium pressures of Mg 2 Ni 12x Zr x (0,x#0.3) hydrides at 3408C Mg 2 Ni Peq (atm) a
8.0
a
x50.01
x50.1
x50.3
8.2
9.0
10.1
Calculated based on the data from Ref. [1].
Y. Zhang et al. / Journal of Alloys and Compounds 269 (1998) 278 – 283
case. In this paper, also, there is another opposite correlation, in a Mg 2 Ni 12x Zr x system, since the hydrides become unstable with increasing unit cell volume. The results from Table 3 show that the unit cell volumes increase with increasing amount of Zr. However, the hydrides become unstable with increasing the unit cell volumes (Table 5). Unfortunately, the reason is not yet clear. Hence, more fundamental approaches are needed for the further understanding of the hdride stability in future work.
3.3.3. Dissociation temperatures of hydrides in an open system Table 5 also shows the dissociation temperatures of the ternary alloy hydrides in an open system (the pressure of H 2 is 1 atm). The results from Table 5 also show that the dissociation temperatures slightly decrease with increasing zirconium concentrations.
4. Conclusions The ternary alloys Mg 2 Ni 12x Zr x (0,x#0.3) have been successfully prepared by the ball milling-diffusion method (BDM) under moderate synthesis conditions (5808C and 4 h). The main phase of each ternary alloy is similar to that of the Mg 2 Ni alloy, which has a hexagonal crystal structure. Partial replacement of nickel by zirconium in Mg 2 Ni leads to a remarkable improvements of the surface properties. The ternary substituted alloys Mg 2 Ni 12x Zr x (0,x# 2 21 0.3) have a larger specific surface (|1.20 m g ). The desorption kinetics of the ternary alloys have been enhanced by the addition of zirconium. The heat of desorption and the decomposition temperature of the ternary alloys Mg 2 Ni 12x Zr x (0,x#0.3) are also lowered by the addition of zirconium. An optimum desorption storage capacity of approximately 3.3 wt.% is observed.
Acknowledgements This research is Project 59781001 supported by the National Natural Science Foundation of China and Project 973605611 supported by the Natural Science Foundation of Tianjin, China.
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Dehydriding properties of ternary Mg 2 Ni 12x Zr x hydrides synthesized by ball milling and annealing Yunshi Zhang, Huabin Yang*, Huatang Yuan, Endong Yang, Zuoxiang Zhou, Deying Song Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China Received 12 December 1997; received in revised form 20 January 1998
Abstract The ternary hydrogen-storage alloys Mg 2 Ni 12x Zrx (0,x#0.3) have been successfully synthesized by ball milling followed by annealing. Studies of the synthesis conditions of the ternary alloys and their dehydriding properties have been carried out. The results show that the ternary Mg 2 Ni 12x Zr x (0,x#0.3) alloys have the same hexagonal crystal structure as that of Mg 2 Ni. Compared with a Mg 2 Ni alloy, they have a larger specific surface (|1.20 m 2 g 21 ), more promising dehydriding kinetics, lower enthalpy of formation of hydrides than that of Mg 2 NiH 4 , and lower decomposition temperatures in an open system. An optimum desorption storage capacity of about 3.3 wt.% is observed. 1998 Elsevier Science S.A. Keywords: Ball milling; Diffusion; Mg alloys; Hydrogen storage alloy; Dehydriding kinetics
1. Introduction Among the metal–hydrogen systems, the magnesium– hydrogen system is recognized to be the most promising one. The interest in magnesium as a hydrogen-storage material originates from the lighter weight, the higher hydrogen weight percent (7.6%) of the metal hydride, and the lower price compared with other types of metal– hydrogen systems. However, it is inadequate for practical use for hydrogen storage applications because of its poor hydriding / dehydriding kinetics. In recent years, there has been much effort made to improve the hydriding / dehydriding properties of the Mg– hydrogen system [1–18]. The hydriding / dehydriding properties of the Mg–hydrogen system have been substantially improved in several ways: (i) element partial substitution, e.g. Mg 2 Ni [1], Mg 2 Cu [2], Mg–La [3], etc.; (ii) composite formation with other hydrogen storage materials, such as Mg–Mg 2 Ni [4], Mg–LaNi 5 [5], Mg–ZrFe 1.4 Cr 0.6 [6] and Mg–40wt.%FeTi (Mn) [7,8] to form new phases in order to increase the surface activity; (iii) modification with polynuclear aromatic compounds, e.g. Mg–THF [9] and Mg 2 Ni–tetracyanoethylene and phthalonitrile [10]; (iv) surface treatment to improve the surface activity by coating the surface with Ni, Cu or Pd [11,12] and etching *Corresponding author. Tel.: 186 22 23502604; fax: 186 22 23502604; e-mail: [email protected] 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 98 )00222-9
Mg-based alloys with HCl solution [13], or / and F-solution [14,15]. Another approach to improving the hydriding / dehydriding kinetics is to find new synthesis methods. It is well known that the properties of Mg-based alloys and their hydrides are quite different according to the various preparative methods used. Mg-based alloys are usually prepared according to a normal metallurgical procedure [2]. Ball milling has recently emerged as a novel technique for alloy formation [19–21]. In the metal–hydrogen systems, specific surface and particle sizes of the material are the important parameters for enhancement in the hydrogen uptake, reduction in the activation time, temperature and pressure, and improvement in the kinetics of the material. Since ball milling leads to materials in fine-particle form, it may be typically suited for the synthesis and formation of the Mg-based hydrogen storage alloys. In the present paper, we have used a powerful method to synthesize Mg-based alloys [22]. We have called this method the ball milling-diffusion method (BDM) in the following. One of the striking aspects of this method is that the formation of the Mg-based alloys is proceeded by a two-step process, i.e. ball milling, which is carried out only for 30–120 min, coupled with the interdiffusion of the components in the solid state. However, the temperature during the interdiffusion must not be very high. It must be lower than the melting point of any component of the
Y. Zhang et al. / Journal of Alloys and Compounds 269 (1998) 278 – 283
initial materials which are used for the synthesis of the alloys. If the temperature is higher than the lowest melting point of any initial material, this component must be sintered so as not to be able to interdiffuse in the solid state. For the Mg–Ni–Zr system, the temperature must be lower than the melting point (6508C in an open system) of the metal Mg. In addition, this method is especially suitable for the substitution of metals with poor activity and high melting points for other components. Previous studies carried out in our laboratory on magnesium-based alloys have shown that the hydriding / dehydriding rates and other properties of Mg-based alloys can be strongly improved by the replacement-diffusion method (RDM) [23–25]. However, RDM is only suitable for metals with high activity. As we know, Zr does not form an alloy with Mg, which may be important in maintaining the high hydrogen capacity associated with Mg. Zr can absorb and desorb hydrogen reversibly itself. Moreover, Zr can very easily form alloys with Ni, which can also absorb and desorb hydrogen reversibly. In addition, Zr has a good ability to protect the alloy bulk from being oxidized further. However, the synthesis of the Mg 2 Ni 12x Zr x (0,x#0.3) alloys is very difficult using metallurgical methods (MM) due to the difference between the higher melting point of Zr and the lower melting point of Mg. After fusion, the materials need to be powdered and reheated at higher temperature for several days to ensure homogeneity. Because of the poor activity of Zr, RDM is also not suitable for the synthesis of Zr-containing Mg-based alloys [25]. In the present paper, we successfully synthesized the ternary Mg 2 Ni 12x Zr x (0,x#0.3) alloys by BDM. Studies on the synthesis conditions of the ternary alloys and their dehydriding properties have been carried out.
2. Experimental details
2.1. Materials Magnesium was obtained in powder form (purity of 99.8% and particle size of 60–100 mesh). Nickel and zirconium were also obtained in powder form with a particle size of 60–100 mesh (purity of 99.9%).
2.2. Sample preparation The ternary Mg 2 Ni 12x Zr x samples were produced by BDM. In order to avoid the formation of the non-reacting MgNi 2 phase during the reaction, these samples were slightly Mg-richer than the stoichiometric composition of Mg 2 Ni 12x Zr x . The mixtures of the initial materials (magnesium, nickel and zirconium) and 20 agate balls of 7 mm diameter (weight ratio, 1:20) were placed into a ball milling mortar which was sealed under an argon atmosphere. The mixtures were milled for 2 h with a rotational
279
velocity of 300 rev min 21 . After milling, the mixtures were removed from the mortar. The mixtures were compressed under a pressure of 30 MPa to form pellets 20 mm in diameter and 5 mm thick. Then, the pellets were heated employing a resistor furnace under an argon atmosphere (5 atm) in a sealed stainless steel vessel. The vessel was heated to 400, 500, 550, 580, 600 and 6508C, respectively, and held at each temperature for 1, 2, 3, 4, 5 and 6 h, respectively. During this treatment, the components of the mixtures interdiffused in the solid state. Then, the vessel was cooled to room temperature. The samples produced in this way were removed from the vessel, crushed and subjected to the component analysis, which indicated that the samples were Mg 2 Ni 12x Zr x . The X-ray diffraction (XRD) spectra were recorded using a Rigaku / max-rB X-ray diffractometer with Cu Ka radiation.
2.3. Activation procedure of the samples Since the synthesized samples did not absorb hydrogen at room temperature, they had to be suitably activated. It was found that the favorable activation process corresponded to the following conditions: Each synthesized sample was placed in another stainless steel reactor which was evacuated to about 10 22 Torr. Then, 3 MPa of hydrogen was introduced from a highpressure gas cylinder. The bottom portion of the reactor was heated at 3508C for 3 h. Then it was slowly cooled to 3008C and the hydrogen desorbed. Again the reactor was evacuated and hydrogen gas was introduced into it. In the fifth cycle, the quantity of absorbed hydrogen was still small. However, upon continuation of the activation process, each sample absorbed more and more hydrogen. After the tenth cycle, the capacity of the absorbed hydrogen of each sample had become constant.
2.4. Properties of the samples The specific surface of each sample was obtained by the Brunaner–Emmett–Teller (BET) method using nitrogen adsorption at 77 K. The rates and the composition during the desorption were measured using a conventional volumetric method. The pressure–composition desorption isotherms (PCT) measurements were performed at various temperatures on the synthesized samples.
3. Results and discussion
3.1. Moderate synthesis conditions 3.1.1. Effect of reaction temperature on formation of alloys Because the samples were synthesized by BDM rather than by the metallurgical method (MM), the reaction
Y. Zhang et al. / Journal of Alloys and Compounds 269 (1998) 278 – 283
280
Table 1 The temperature effect on the formation of the ternary alloys Temp. (8C)
x50.01
x50.1
x50.2
x50.3
x50.4
400 500 550 580 600 650
Metals Metals Metals1alloy Alloy Alloy Alloy1metals
Metals Metals Metals1alloy Alloy Alloy Alloy1metals
Metals Metals Metals1alloy Alloy Alloy Alloy1metals
Metals Metals Metals1alloy Alloy Alloy Alloy1metals
Metals Metals Metals1alloy Metals1Alloy Metals1Alloy Alloy1metals
Size, 60–100 mesh; ball milling time, 4 h; heating time, 4 h.
temperature must be lower than the melting point of the metal Mg (6508C). A temperature of 6508C was chosen as the maximum temperature. Table 1 shows the effect of the reaction temperature on the formation of the ternary alloys. These results indicate that the alloys cannot be formed unless the reaction temperature is higher than 5508C, but lower than 6508C. When the reaction temperature was set to 5008C or lower, e.g. 4008C, the samples were still mixtures of the original metals. When the temperature was set to 5508C, some amounts of alloys began to be formed. In the temperature range from 580 to 6008C, the samples consisted of only the ternary alloys. However, when the temperature was higher than 6008C, maybe 6508C, the samples could be partly converted into metals again due to the sintering and loss of Mg, as can be seen from Table 1. From Table 1, we can also see that, no matter how high the temperature is, the samples (x50.4) all consisted of both alloys and metals, which is probably due to the excessive addition of zirconium.
3.1.2. Effect of reaction time on formation of alloys Because the metal zirconium does not very easily diffuse into the bulk of Mg, the reaction time should be monitored carefully. According to Section 3.1.1, we chose 5808C as the reaction temperature. Table 2 shows the effect of the reaction time on the formation of the ternary alloys. The results indicate that, after the samples were heated for 4 h, the alloys have been completely formed. Even when heating for longer than 4 h, the samples consisted of only the alloys. However, after 6 h, the samples, with x50.4, still consisted of both metals and alloys, which is also due to the excessive addition of zirconium. From Tables 1 and 2, we chose a temperature of 5808C
and a reaction time of 4 h as the moderate synthesis conditions for the ternary Mg 2 Ni 12x Zr x (0,x#0.3) alloys. In addition, the component of zirconium (x) should not be larger than 0.3. Otherwise, no matter how high the temperature or how long the reaction time, the samples will be mixtures of both alloys and metals.
3.2. Phase structure of ternary Mg2 Ni12 x Zrx alloys In order to unravel the curious hydrogenation behavior of the ternary Mg 2 Ni 12x Zr x alloys, structural characteristics were monitored. X-ray diffraction spectra of these samples were taken at seven separate zirconium concentrations: x50, 0.01, 0.1, 0.2, 0.25, 0.3 and 0.4. Fig. 1 shows the X-ray diffractograms of the ternary Mg 2 Ni 12x Zrx alloys synthesized for 4 h under 5808C and an argon atmosphere (5 atm). They reveal that the main phase of each ternary alloy is similar to that of the Mg 2 Ni alloy, which has a hexagonal crystal structure. There are substantial changes in lattice parameters of the hexagonal structure (Table 3). The results from Table 3 show that the value of the lattice parameter a increases with increasing value of x. However, the value of the lattice parameter c, slightly increases with increasing amount of zirconium. This suggests that the zirconium, which has a larger atom radius than that of Ni, mainly occupies the a-axis, which leads to the increasing of the lattice volume. Table 3 also shows the lattice parameters of the Mg 2 Ni alloy made by MM. It clearly shows that the lattice parameters a and c of the Mg 2 Ni alloy made by BDM are greater than those obtained by MM. The difference is only caused by the different synthesis method. According to the lattice expansion theory [26], the larger the unit cell volume, the better
Table 2 The reaction time effect on the formation of the ternary alloys Time (h)
x50.01
x50.1
x50.2
x50.3
x50.4
1 2 3 4 5 6
Metals1alloy Metals1alloy Alloy Alloy Alloy Alloy
Metals1alloy Metals1alloy Metals1alloy Alloy Alloy Alloy
Metals1alloy Metals1alloy Metals1alloy Alloy Alloy Alloy
Metals1alloy Metals1alloy Metals1alloy Alloy Alloy Alloy
Metals1alloy Metals1alloy Metals1alloy Metals1alloy Metals1alloy Metals1alloy
Size, 60–100 mesh; heating temperature, 5808C; heating time, 4 h.
Y. Zhang et al. / Journal of Alloys and Compounds 269 (1998) 278 – 283
281
Fig. 2. Kinetic curves of the desorption of the activated ternary Mg 2 Ni 12x Zr x alloys for three values of x (0, 0.1 and 0.3) at 3408C and under a pressure of 1 atm. Fig. 1. XRD spectra of the ternary Mg 2 Ni 12x Zr x (0,x#0.4) samples.
the characteristics of the alloy. Therefore, the alloys made by BDM may have some advantages. In addition, according to the binary alloy phase diagram of Mg–Zr [27], only a dilute solid solution of Zr in Mg (solubility of 3.8 wt.% at 923 K and 0.3 wt.% at 573 K) is formed. This is verified by the XRD results (Fig. 1). Only diffraction peaks originating from Mg 2 Ni and Zr–Ni phases are observed. However, the peaks of the Zr–Ni phases are so small that they are not very easily observed.
3.3. Dehydriding characteristics of ternary Mg2 Ni12 x Zrx (0, x#0.3) alloys In the metal hydride systems, specific surface is one of the most important parameters for enhancement of hydrogen uptake, reduction in the activation time, temperature and pressure, and improvement in the kinetics of the material. The alloys made by BDM have promising surface characteristics. The specific surfaces of all the alloys obtained by the BET method are listed in Table 4. These results show that the alloys synthesized by BDM
have a very large specific surface. The specific surface of the unsubstituted Mg 2 Ni alloy made by BDM is only 0.5 m 2 g 21 . However, the specific surface of the ternary substituted alloys can approach 1.2 m 2 g 21 . The substituted alloys have a larger specific surface mainly due to the substitution of zirconium for nickel. Using scanning electron microscopy, the surfaces of the ternary alloys are seen to have many more defects and cracks along the interfaces between the particles than those of the Mg 2 Ni alloy. These defects and cracks will lead to the dehydriding characteristics of the ternary substituted alloys being more promising than those of the Mg 2 Ni alloy made by BDM.
3.3.1. Dehydriding kinetics of ternary alloys The hydrogen desorption characteristics of the ternary Mg 2 Ni 12x Zrx (0,x#0.3) alloys for several values of x were measured at 3408C. Fig. 2 shows the kinetic curves of the desorption of the activated ternary alloys for three values of x (0, 0.1 and 0.3) at 3408C and under a pressure of 1 atm. Fig. 2 clearly shows that the desorption kinetic characteristics of the ternary alloys are enhanced by the addition
Table 3 Lattice parameters of the Mg 2 Ni 12x Zr x alloys (0#x#0.4) Mg 2 Ni a ˚ a (A) ˚ c (A) a
a
5.190 a 13.24
x50
x50.01
x50.1
x50.2
x50.3
x50.4
5.201 13.25
5.203 13.19
5.210 13.26
5.212 13.29
5.218 13.30
— —
x50
x50.01
x50.1
x50.2
x50.3
x50.4
0.5
1.20
1.20
1.19
1.21
—
Made by MM.
Table 4 The specific surface of Mg 2 Ni 12x Zr x (0#x#0.4) alloys 2
21
Specific surface (m g ) Size, 100–200 mesh.
282
Y. Zhang et al. / Journal of Alloys and Compounds 269 (1998) 278 – 283
of zirconium. These results indicate that the desorption kinetics of the ternary alloys is faster than that of the Mg 2 Ni alloy. From Fig. 2, it can be observed that the desorption of hydrogen to about 80% of the saturation value of the ternary alloys is accomplished within 10 min, which is due to the larger specific surface (Table 4). However, the total time required for complete desorption has been found to be about 40 min, which is the same as that of the Mg 2 Ni alloy made by BDM. Compared with the Mg 2 Ni alloy made by MM [28], even the Mg 2 Ni alloy made by BDM also shows better desorption kinetics. According to a previous report [28], the Mg 2 Ni alloy made by MM desorbed hydrogen to only about 20% of the saturation value within 60 min. It can also be observed that the maximum desorbed hydrogen capacity of the alloys is about 3.3 wt.%, which is a little higher than that of the Mg 2 Ni alloy in this experiment. The desorption kinetic characteristics of the zirconium substituted alloys are enhanced by the addition of zirconium. This may be attributed to the following factors. Firstly, the substituted alloys have a larger specific surface than that of the Mg 2 Ni alloy made by BDM, which leads to an enhancement of the desorption kinetics (Table 4). The redox reaction of hydrogen proceeds much more easily along the interfaces between the particles, where there are many defects and cracks. Secondly, as discussed in Section 3.2, the unit cell volumes of the alloys increase with increasing zirconium content, which is beneficial for hydrogen diffusion in the alloys. Finally, because only a dilute solid solution of Zr in Mg is formed (Section 3.2), some of the Zr has alloyed with Ni to form Zr–Ni phases. It is known that Zr–Ni alloys can act as hydrogen storage materials themselves, providing a passway for hydrogen atoms to diffuse. The Zr–Ni phases may act as active sites for hydrogen desorption. To find and evaluate the storage capacity, the pressure– composition desorption isotherms of activated Mg 2 Ni 12x Zr x (0,x#0.3) alloys have been monitored for various x values.
3.3.2. Pressure–composition desorption isotherms of ternary Mg2 Ni12 x Zr x (0, x#0.3) alloys The P–C desorption isotherms of the ternary alloys of several compositions (x50.01, 0.1 and 0.3) were determined at 3408C. The maximum storage capacity, obtained for the three separate x values, all corresponds to 3.3 wt.% at 3408C. A clear-cut plateau is observed for all cases. The abrupt termination of the isotherms at a composition corresponding to Mg 2 Ni 12x Zr x H 4 indicates that all the hydrides have well-defined stoichiometry. The desorption isotherms at 3408C for the Mg 2 Ni 0.7 Zn 0.3 alloy is shown in Fig. 3. Because few Zr–Ni phases can be observed in Fig. 1, only one plateau is clearly observed on each isotherm. The desorption reaction can be described as follows:
Fig. 3. P–C desorption isotherms of the Mg 2 Ni 0.7 Zr 0.3 alloy (BDM) and the Mg 2 Ni alloy (MM) at 3408C.
Mg 2 Ni 12x Zr x H 42y → Mg 2 Ni 12x Zr x 1 (2 2 y / 2)H 2
(1)
where the desorption enthalpy is indicated by DH. The desorption process obeyed the linear relationship expected on the basis of the Van’t Hoff equation very well. Based on the relationship between ln PeqH 2 and the absolute temperature 1 /T, the linear equation is given by: ln PeqH 2 5 DH /RT 1 DS /R
(2)
where PeqH 2 is the equilibrium pressure of the plateau, R is the gas constant, T is the absolute temperature, DS is the entropy. The heat of desorption DH is given in Table 5. The results from Table 5 show that the heat of the desorption of the ternary alloys have lower absolute values for increasing the zirconium concentrations. Fig. 3 and Table 6 show that the desorption equilibrium pressures of the ternary alloys (BDM) are slightly higher than that of the Mg 2 Ni alloy (MM). As we know, there is a good correlation between the unit cell volume of compounds and the equilibrium plateau pressure of hydrogen [29,30]. In other words, the larger the unit cell volume, the more stable the hydride. However, there are still some exceptional cases against this rule. The Mg 2 Ni 12x Be x [31] system is a typical example of this Table 5 Dissociation temperatures and enthalpies of Mg 2 Ni 12x Zr x (0,x#0.3) hydrides (PH 2 50.1 MPa)
T (8C) DH (kJ mol 21 )
Mg 2 Ni
x50.1
x50.2
x50.3
253 [1] 264.0 [1]
251 262.7
250 262.3
248 259.8
Table 6 The desorption equilibrium pressures of Mg 2 Ni 12x Zr x (0,x#0.3) hydrides at 3408C Mg 2 Ni Peq (atm) a
8.0
a
x50.01
x50.1
x50.3
8.2
9.0
10.1
Calculated based on the data from Ref. [1].
Y. Zhang et al. / Journal of Alloys and Compounds 269 (1998) 278 – 283
case. In this paper, also, there is another opposite correlation, in a Mg 2 Ni 12x Zr x system, since the hydrides become unstable with increasing unit cell volume. The results from Table 3 show that the unit cell volumes increase with increasing amount of Zr. However, the hydrides become unstable with increasing the unit cell volumes (Table 5). Unfortunately, the reason is not yet clear. Hence, more fundamental approaches are needed for the further understanding of the hdride stability in future work.
3.3.3. Dissociation temperatures of hydrides in an open system Table 5 also shows the dissociation temperatures of the ternary alloy hydrides in an open system (the pressure of H 2 is 1 atm). The results from Table 5 also show that the dissociation temperatures slightly decrease with increasing zirconium concentrations.
4. Conclusions The ternary alloys Mg 2 Ni 12x Zr x (0,x#0.3) have been successfully prepared by the ball milling-diffusion method (BDM) under moderate synthesis conditions (5808C and 4 h). The main phase of each ternary alloy is similar to that of the Mg 2 Ni alloy, which has a hexagonal crystal structure. Partial replacement of nickel by zirconium in Mg 2 Ni leads to a remarkable improvements of the surface properties. The ternary substituted alloys Mg 2 Ni 12x Zr x (0,x# 2 21 0.3) have a larger specific surface (|1.20 m g ). The desorption kinetics of the ternary alloys have been enhanced by the addition of zirconium. The heat of desorption and the decomposition temperature of the ternary alloys Mg 2 Ni 12x Zr x (0,x#0.3) are also lowered by the addition of zirconium. An optimum desorption storage capacity of approximately 3.3 wt.% is observed.
Acknowledgements This research is Project 59781001 supported by the National Natural Science Foundation of China and Project 973605611 supported by the Natural Science Foundation of Tianjin, China.
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