Development of metal hydride with high dissociation pressure

Journal of Alloys and Compounds 419 (2006) 256–261

Development of metal hydride with high dissociation pressure Yoshitsugu Kojima a,∗ , Yasuaki Kawai a , Shin-ichi Towata a , Tomoya Matsunaga b , Tamio Shinozawa b , Masahiko Kimbara c b

a Toyota Central R&D Labs. Inc., Nagakute, Aichi, Japan Material Engineering Div. 3, Toyota Motor Corporation, Susono, Shizuoka, Japan c Corporate Technical Center, Toyota Industries Corporation, Obu, Aichi, Japan

Received 5 July 2005; accepted 29 August 2005 Available online 15 November 2005

Abstract The effective hydrogen capacity of Ti1.1 CrMn exhibited the maximum value of 1.8 wt% in the pressure range of 33 and 0.1 MPa at 296 K (dissociation pressure: 11 MPa), and the alloy provided over 10% more capacity than conventional Ti Cr Mn (Ti1.2 CrMn: 1.6 wt%, Ti1.2 Cr1.9 Mn0.1 : 1.3 wt%). At the low temperature of 233 K, the alloy absorbed 2.0 wt% of hydrogen and the hydrogen desorption capacity at 0.1 MPa was 1.6 wt%. The desorption capacity of conventional Ti Cr V was 0 at 233 K due to the low dissociation pressure. The dissociation pressure decreased with the Ti and the Mn contents and was explained by the function of the bulk modulus and the cell volume. According to the Van’t Hoff plots, the standard enthalpy difference (heat of formation) of the Ti1.1 CrMn hydride was −22 kJ/mol H2 . The absolute value was about 10 kJ/mol H2 smaller than those of LaNi5 and Ti Cr V. The alloy had sufficient hydriding and dehydriding kinetics. In the pressure range of 33 and 0.1 MPa at 296 K, the alloy absorbed and desorbed 1.8 wt% of hydrogen in 60 and 300 s, respectively. The hydrogen capacity changed gradually over many cycles and that after 1000 cycles was 94% of the initial capacity. Thus, Ti1.1 CrMn can be utilized for a high-pressure MH tank, which contains a hydrogen-absorbing alloy with high dissociation pressure and compressed hydrogen. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage materials; Gas–solid reaction; X-ray diffraction; Thermodynamic properties; High-pressure

1. Introduction A fuel cell is a device that continuously converts the chemical energy of hydrogen (H2 ) and oxygen (O2 ) into electrical energy. Since the fuel cell has efficiency much higher than that of conventional combustion engines, a fuel cell vehicle (FCV) is expected to have high efficiency [1]. A polymer electrolyte fuel cell (PEFC, PEM fuel cell) is the prime power source for FCV. One of the most widely envisioned sources of fuel for FCV is H2 . Therefore, it is necessary to have a storage tank of H2 to start the system on demand. The first FCVs were delivered on December 2, 2002. FCVs feature a 35-MPa H2 storage tank and can travel 300–355 km on a full tank. The driving ranges of the vehicles are small compared to those of gasoline vehicles. It is the biggest hurdle to FCVs and the improvement of the range is required for a new H2 storage system.

∗

Corresponding author. Tel.: +81 561 63 5325; fax: +81 561 63 6137. E-mail address: [email protected] (Y. Kojima).

0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.08.078

Hydrogen (H2 ) can be stored in tanks as compressed [1] or liquefied H2 [1] or by adsorption on carbon materials [1–4]. It can also be stored in hydrogen-absorbing alloys [5,6] or as a chemical hydride, such as NaBH4 [7–12], LiBH4 [13,14], NaAlH4 [15–18], metal nitrides [19–23] or MgH2 [24–27] as well as in an organic hydride, such as methylcyclohexane or decalin [28,29]. Among these methods, the hydrogen-absorbing alloy was considered to play an important role in a fuel cell vehicle. For the hydrogen-absorbing alloy, however, there are also many problems to overcome, such as weight (small amount of storage per unit weight due to its nature as an alloy), large heat of absorption, decrease of hydrogen desorption at low temperature, deterioration (the alloy turning into finer particles or changing its structure) upon repeated storage and release, securing of its resources when it includes rare metals. A high-pressure MH tank containing a hydrogen-absorbing alloy with high dissociation pressure (metal hydride with high dissociation pressure) and compressed H2 can improve various issues of hydrogen-absorbing alloys and volumetric hydrogen density of a high-pressure hydrogen tank. Ti Cr Mn hydride is a useful metal hydride with high

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dissociation pressure. However, very few studies have been reported on the hydrogen absorption and desorption properties of Ti Cr Mn at high-pressure above 10 MPa [30–32]. The purpose of this study is to investigate the H2 absorption–desorption properties of Ti Cr Mn alloys at highpressure of 33 MPa by varying the contents of Ti, Cr and Mn to improve the desorption capacity. We also report a method to describe the dissociation pressure from the bulk modulus and the cell volume. 2. Experimental The materials used were commercially available high purity materials (titanium, >99% pure; chromium, >99% pure; manganese, >99% pure). Ti Cr Mn alloys Tix Cr2−y Mny (0.96 ≤ x ≤ 1.41, 0.09 ≤ y ≤ 1.66) and Ti1.34 Mn2.00 used as specimens were prepared by arc melting the materials in a water-cooled copper crucible in a high purity argon atmosphere. They were melted three times to ensure homogeneity and then pulverized. Ti Cr V (Ti0.22 Cr0.39 V0.39 ) was also prepared by the method of arc melting. Commercially available compressed hydrogen (99.999%) was used without further purification. X-ray diffraction intensity curves were recorded with Cu K␣ radiation (50 kV, 300 mA) filtered by a monochrometer using Rigaku Rint-TTR. The specimens were characterized for elemental composition by a inductively coupled plasma spectroscopy (ICP). This examination indicated Ti:Cr:Mn atomic ratios (x, y). We measured H2 absorption and desorption properties with a specially designed high-pressure ‘pressure–composition–temperature’ (PCT) automatic measuring system (Sieverts type apparatus, maximum pressure: 33 MPa) provided by Suzuki Shokan Co. Ltd., Japan. The specimen (about 10 g) was packed into a stainless steel tube (internal volume of 20 cm3 ). The tube was first evacuated down to below 0.5 Pa at room temperature and pressure–composition (P–C) isotherm measurements were then started. The P–C isotherms were obtained by calculation of the H2 capacity from the pressure change in the tube. The H2 desorption capacity at 0.1 MPa was also measured using a flow meter. In this experiment, from the H2 desorption isotherm, the dissociation pressure is defined to be the pressure when the H2 desorption capacity (33–0.01 MPa) is half.

3. Results and discussion The X-ray diffraction intensity curve of Ti1.16 Cr0.92 Mn1.08 (Fig. 1) has the hexagonal MgZn2 (C14 Laves phase) structure. The crystal structure of those alloys Tix Cr2−y Mny (0.96 ≤ x ≤ 1.41, 0.09 ≤ y ≤ 1.66), Ti1.34 Mn2.00 did not change. It was clear that an increase of the Ti content (x) and decrease of the Mn content (y) resulted in increase of the cell volume (cell volume V0 = 0.163–0.174 nm3 ). Tix Cr2−y Mny (1.02 ≤ x ≤ 1.41,

Fig. 1. X-ray diffraction intensity curve for Ti1.16 Cr0.92 Mn1.08 .

Fig. 2. H2 absorption and desorption isotherms for Ti Cr Mn H system.

0.09 ≤ y ≤ 1.66) was easily activated at H2 pressure of 33 MPa and room temperature. The experimental typical absorption and desorption P–C isotherms for a Ti Cr Mn H system at 296 and 233 K are presented in Fig. 2. When H2 is added into the tube containing Ti Cr Mn at 296 K, H2 is absorbed. The H2 absorption capacity reaches a value of 2.0 wt% in the H2 pressure of 33 MPa. The H2 desorbed at the pressure of 0.1 MPa (effective H2 capacity) was 1.8 wt% and a little small compared with conventional Ti Cr V (2.2 wt%). The effective H2 capacity of Ti Cr Mn increased to 1.9 wt% at 353 K. At the temperature of 233 K, the H2 absorption capacity at 10 MPa and the desorption capacity at 0.1 MPa is 2.0 and 1.6 wt%, respectively. One can see that the 296 K dissociation pressure is larger than the 233 K value. The desorption capacity of Ti Cr V was 0 at 233 K due to the low dissociation pressure. Ti Cr Mn displays a hysteresis effect. The hysteresis in the absorption–desorption isotherms for the Ti Cr Mn H system increases with decreasing temperature. The hysteresis in the absorption–desorption isotherms for the Tix Cr2−y Mny –H system decreased with increasing the Ti content and decreasing the Mn content. The hysteresis has its origin in the large strain associated with the metal ↔ hydride transformation [5]. The H2 absorption and desorption properties of Ti Cr Mn were measured by varying the contents of Ti, Cr and Mn to obtain the optimum composition. Figs. 3 and 4 show the Ti and the Mn content dependencies of the H2 absorption capacities and the effective H2 capacities, respectively. We found that Tix Cr2−y Mny (1.08 ≤ x ≤ 1.16, 0.96 ≤ y ≤ 1.08) has the highest effective H2 capacity at 296 K. Conventional Ti Cr Mn alloys such as Ti1.2 CrMn, Ti1.2 Cr0.9 Mn0.1 , Ti1.1 Cr1.2 Mn0.8 , Ti1.2 Cr r1.2 Mn0.8 and Ti1.3 Cr1.2 Mn0.8 [31] were prepared as reference specimens and the H2 absorption–desorption experiments were carried out. The effective H2 capacities of those Ti Cr Mn alloys prepared as reference specimens were 1.3–1.6 wt% (Ti1.2 CrMn: 1.6 wt%, Ti1.2 Cr0.9 Mn0.1 : 1.3 wt%, Ti1.1 Cr1.2 Mn0.8 : 1.5 wt%, Ti1.2 Cr1.2 Mn0.8 : 1.4 wt%, Ti1.3 Cr1.2 Mn0.8 : 1.4 wt%). Thus, Tix Cr2−y Mny (x ∼ = 1.1, 1.08 < x <1.16, y ∼ = 1.0, 0.96 < y < 1.08) provides over 10% more capacity than conventional Ti Cr Mn.

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Fig. 6. Plots of dissociation pressure and cell volume against Mn content. Fig. 3. H2 absorption capacity and H2 desorption capacity (effective H2 capacity) in Ti Cr Mn vs. Ti content.

Fig. 4. H2 absorption capacity and H2 desorption capacity (effective H2 capacity) in Ti Cr Mn vs. Mn content.

The dissociation pressure decreases with the Ti and the Mn contents as shown in Figs. 5 and 6. Below Ti content of 1.0, it is considered that the activation of the specimen is insufficient at 33 MPa based on high dissociation pressure. Above Ti content of 1.16, irreversible stored H2 is increased because the dissociation pressure is decreased. Figs. 5 and 6 also show the plots of the

cell volume against Ti and Mn contents for Ti Cr Mn. The cell volume increases with the Ti content. This is due to the fact that Ti partially substituted for Cr or Mn has larger metallic radius (Ti: 0.132 nm, Cr: 0.119 nm, Mn: 0.118 nm) [33]. It is well known that the order of the decrease of the dissociation pressure agreed with the increase of the unit cell volume [34,35]. The cell volume decreases with increasing Mn content. Mn with smaller metallic radius has been substituted in part for Cr. The cell volume decreases with the Mn content. The decrease of the unit cell volume from 0.1743 to 0.1636 nm3 does not result in an increase of the dissociation pressure, which is different from the case of Ti. Gibbs free energy is a property that provides a convenient measure of the driving force of hydrogen absorption and desorption. Recently, it has been reported that a bulk modulus of host metal is a key to describe the heat of formation [36] of the hydride. Thus, we estimate the change in the dissociation pressure using the free energy with excess energy (elastic energy). The stability of Ti Cr Mn H system depends on its formation energy of the reaction metal (Ti–Cr–Mn) + H2 ↔ metal hydride (Ti–Cr–Mn–H)

(1)

It is useful to normalize all coefficients in the formation reactions per molecule of H2 . The dissociation process starts when GMH0 = GM+H2 , where GMH0 and GM+H2 are the thermodynamic Gibbs free energy for the metal hydride without elastic energy, and H2 gas and the alloy (Fig. 7). When a certain elastic energy is applied to the metal hydride, it increases the free energy. The change in the free energy follows the plot GMH in Fig. 7. As the free energy for the H2 gas and the alloy phase does not change, the dissociation pressure will increase from PMH0 to PMH . Then, the free energy difference
G of the system with elastic energy could be expressed by
G =
Gab +
Uel + T
Sel + P
Vel =
Gab +
Uel (2)

Fig. 5. Plots of dissociation pressure and cell volume against Ti content.

where
Gab is the free energy difference for the H2 absorption,
Uel the internal energy difference for the energy elasticity,
Sel the entropy difference and
Vel is the volume difference. As T
Sel and P
Vel is much less than
Uel ,
Gab is expressed

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Fig. 8. Bulk modulus dependence of dissociation pressure. Fig. 7. The dependence of Gibbs free energy on pressure: GMH , Gibbs free energy of the hydride with elastic energy; GMH0 , Gibbs free energy of the hydride without elastic energy; GM+H2 , Gibbs free energy of H2 gas and the alloy.

as
Gab =
G0 + RT lnK =
G0 − RT lnP

(3)

where
G0 is the standard free energy difference, R the gas constant, T the absolute temperature, K the equilibrium constant and P is the pressure of H2 gas. The activities of metal (Ti Cr Mn) and metal hydride (Ti Cr Mn H) is approximately equal to 1.
Uel is expressed as
Uel =

nB(V − V0 )2 nA2 B = 2V0 2V0

(4)

where B is the bulk modulus, and V0 and V are the cell volume before and after H2 absorption. nV0 and nV are the cell volume per one H2 molecule before and after H2 absorption. We assumed that the volume difference (V − V0 ) is proportional to the hydrogen molecule absorbed in the metal, A is constant (V − V0 = A). Substituting Eqs. (4) and (3) into (2)
G =
G0 − RT lnP +

CB V0

where C = nA2 /2. At the dissociation
G = GMH − GM+H2 = 0, Eq. (5) becomes lnP =

(5) pressure,

(
H0 + CB/V0 ) RT −


S0 R

as

V =

X1 M1 X 2 M2 X 3 M3 + + d1 d2 d3

Here, suffixes 1, 2 and 3 indicate Ti, Cr and Mn, respectively. X, M, d and V are the atomic ratio, the atomic weight, the density and the volume, respectively. The bulk moduli of Ti, Cr and Mn are 105, 190 and 59.6 GPa, respectively [37]. The values of d1 , d2 and d3 are 4.51, 7.19 and 7.43 g/cm3 , respectively. M1 , M2 and M3 are 47.9, 52.0 and 54.9, respectively [38]. The experimental bulk moduli of LaAlNi4 , LaSn0.4 Ni4.6 , LaNi5 , ZrCo, ZrCr2 , TiCr1.8 , TaV2 are 127, 128, 137–139, 140, 154, 164, 195 GPa [39–43] and correspond to the calculated values (LaAlNi4 : 105 GPa, LaSn0.4 Ni4.6 : 116 GPa, LaNi5 : 120 GPa, ZrCo: 118 GPa, ZrCr2 : 138 GPa, TiCr1.8 : 152 GPa, TaV2 : 177 GPa) by Eqs. (8) and (9) [37,38]. Thus, it is reasonable to use the calculated modulus. The dissociation pressure P is plotted against the bulk modulus B calculated by Eq. (8) with V0 = 0.164 nm3 as shown in Fig. 8. ln P linearly increases with B. Fig. 9 shows the relation between ln P and V0 for the alloy with B = 112–117 GPa. ln P is inversely proportional to V0 . Thus, the dissociation pressure is described by Eq. (7), which is a function of the bulk modulus and the cell volume. The properties of Ti1.1 CrMn (Ti1.08 Cr1.04 Mn0.96 ) that we have investigated so far and of Ti Cr V (Ti0.22 Cr0.39 V0.39 ) are summarized in Table 1. The Ti1.1 CrMn alloy has sufficient

(6)

where
H0 is the standard enthalpy difference by mixture and
S0 is the standard entropy difference by mixture. We thus have lnP ∝

B V0

(7)

Eq. (7) shows that logarithm of the dissociation pressure increases with the bulk modulus and decrease with the cell volume. The alloy bulk modulus was calculated by addition relationship of Ti, Cr and Mn bulk moduli. The modulus is given by B=

B1 (X1 M1 /d1 ) B2 (X2 M2 /d2 ) B3 (X3 M3 /d3 ) + + V V V

(8)

(9)

Fig. 9. Cell volume dependence of dissociation pressure.

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Table 1 Properties of Ti1.1 CrMn and Ti Cr V (Ti0.22 Cr0.39 V0.39 ) Properties Effective H2 capacity (wt%; 33–0.1 MPa, 296 K) H2 absorption capacity (wt%; 33–0.1 MPa, 296 K, 5 min) H2 desorption capacity (wt%; 33–0.1 MPa, 296 K, 5 min) Effective H2 capacity (wt%; 9–0.1 MPa, 233 K) Dissociation pressure (MPa; 296 K) Heat of formation (kJ/mol H2 ) Capacity/initial capacity (%; 20 cycles) a

Ti1.1 CrMn

Ti Cr V

1.8

2.2

1.8

1.6

1.8

0.4

1.6

0

11 −22 100 (94)a

0.3 −34 82

Thousand cycles.

hydriding and dehydriding kinetics. Absorption of hydrogen to 100% of saturated value (1.8 wt% in the pressure range of 33 and 0.1 MPa at 296 K) is accomplished with in 60 s. The hydrogen desorbed increases with time and the time to desorb 1.8 wt% of hydrogen is within 300 s, being comparable to compressed hydrogen. The effective H2 capacity of Ti1.1 CrMn is 1.6 wt% at 233 K. The capacity of Ti Cr V decreased at low temperature, which is 0 at 233 K due to the low dissociation pressure. The Van’t Hoff plot for the Ti Cr Mn H (Ti1.08 Cr1.04 Mn0.96 ) system indicates that the standard enthalpy difference (heat of formation) is −22 kJ/mol H2 (Fig. 10). The absolute value is 3–4 kJ/mol H2 smaller than those of Ti1.2 CrMn, Ti1.1 Cr1.2 Mn0.8 , Ti1.2 Cr1.2 Mn0.8 and Ti1.3 Cr1.2 Mn0.8 [31]. The values are also about 10 kJ/mol H2 smaller than those of LaNi5 [5] and Ti Cr V (Fig. 10; Table 1). In cycling test, the capacity showed a little decrease. After 1000 cycles, the capacity is 94% of the initial capacity. We conclude from the forgoing results that Ti1.1 CrMn has properties that make it suitable for consideration as a hydrogen-absorbing alloy with high dissociation pressure. We have calculated the gravimetric and the volumetric H2 densities of the H2 storage system combining the Ti Cr Mn alloy and compressed hydrogen. At an operating pressure of 35 MPa at 298 K, the volume of a tank (100 L) for gaseous hydrogen allows storage density of 2.3 kg H2 [44]. We assumed that the density of Ti Cr Mn was 6 g/cm3 [5]. When we filled the tank (100 L) with Ti Cr Mn alloy of 100 kg, the gravimetric and the volumetric H2 densities were calculated as 3.7 wt% and

Fig. 10. Van’t Hoff plots for Ti1.1 CrMn H and Ti Cr V H systems.

3.7 kg/100 L, respectively. The volumetric H2 density provides 60% more capacity than compressed hydrogen at 35 MPa. The gravimetric H2 density of the system containing Ti1.1 CrMn and compressed hydrogen is similar to the absorption capacity of NaAlH4 with Ti clusters (3.4 wt%) at 10 MPa and 373 K for 420 s [18]. The packing density of 0.64 g/cm3 (void fraction: 50%) is used to calculate the volumetric H2 density. The volumetric H2 density of NaAlH4 is 2.2 kg H2 /100 L, provides similar capacity of compressed hydrogen. Therefore, a high-pressure MH tank containing Ti1.1 CrMn and compressed hydrogen can improve various issues of hydrogen-absorbing alloys, a complex hydride and a high-pressure hydrogen tank. 4. Conclusions Ti1.1 CrMn prepared in this experiment provides over 10% more capacity than conventional Ti Cr Mn (1.3–1.6 wt%). The dissociation pressure was explained by the function of the bulk modulus and the cell volume. The kinetics of the alloy were similar to compressed hydrogen. Thus, the developed Ti1.1 CrMn alloy can be utilized for a high-pressure MH tank. Acknowledgements The authors are greatly indebted to T. Noritake and Dr. G.J. Shafer of the Toyota Central R&D Labs. Inc., for their help and discussions. References [1] L. Schlapbach, A. Z¨uttel, Nature 414 (2001) 353. [2] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [3] R. Chahine, T.K. Bose, Int. J. Hydrogen Energy 19 (1994) 161. [4] Y. Kojima, N. Suzuki, Appl. Phys. Lett. 84 (2004) 4113. [5] G. Sandrock, Final Report, Contract N00014-97-M-0001, SunaTech Inc., Ringwood, NJ, July 24, 1997. [6] T. Tamura, Y. Tominaga, K. Matumoto, T. Fuda, T. Kuriiwa, A. Kamegawa, H. Takamura, M. Okada, J. Alloys Compd. 330–332 (2002) 522. [7] S.C. Amendola, S.L. Sharp-Goldman, M.S. Janjua, M.T. Kelly, P.J. Petillo, M. Binder, J. Power Sources 85 (2000) 186. [8] Y. Kojima, K. Suzuki, K. Fukumoto, M. Sasaki, T. Yamamoto, Y. Kawai, H. Hayashi, Int. J. Hydrogen Energy 27 (2002) 1029. [9] Y. Kojima, K. Suzuki, K. Fukumoto, Y. Kawai, M. Kimbara, H. Nakanishi, S. Matsumoto, J. Power Sources 125 (2004) 22. [10] Y. Kojima, Y. Kawai, H. Nakanishi, S. Matsumoto, J. Power Sources 135 (2004) 36. [11] Y. Kojima, T. Haga, Int. J. Hydrogen Energy 28 (2003) 989. [12] Z.P. Li, B.H. Liu, K. Arai, N. Morigazaki, S. Suda, Int. J. Hydrogen Energy 356–357 (2003) 469. [13] Y. Kojima, Y. Kawai, M. Kimbara, H. Nakanishi, S. Matsumoto, Int. J. Hydrogen Energy 29 (2004) 1213. [14] A. Z¨uttel, P. Wenger, S. Rentsch, P. Sudan, Ph. Mauron, Ch. Emmenegger, J. Power Sources 118 (2003) 1. [15] B. Bogdanovi´c, M. Schwickardi, J. Alloys Compd. 253–254 (1997) 1. [16] C.M. Jensen, R. Zidan, N. Mariels, A. Hee, C. Hagen, Int. J. Hydrogen Energy 24 (1999) 461. [17] K.J. Gross, G.J. Thomas, C.M. Jensen, J. Alloys Compd. 330–332 (2002) 683. [18] M. Fichtner, J. Engel, O. Fuhr, O. Kircher, O. Rubner, Mater. Sci. Eng. B 108 (2004) 42.

Y. Kojima et al. / Journal of Alloys and Compounds 419 (2006) 256–261 [19] P. Chen, Z. Xiong, J. Luo, J. Lin, K.L. Tan, Nature 420 (2002) 302. [20] Y. Kojima, Y. Kawai, Chem. Commun. 19 (2004) 2210. [21] H.Y. Leng, T. Ichikawa, S. Hino, N. Hanada, S. Isobe, H. Fujii, J. Phys. Chem. B 108 (2004) 8763. [22] W. Luo, J. Alloys Compd. 381 (2004) 284. [23] Y. Nakamori, G. Kitahara, S. Orimo, J. Power Sources 138 (2004) 309. [24] G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloys Compd. 292 (1999) 247. [25] G. Barkhordarian, T. Klassen, R. Bormann, J. Alloys Compd. 364 (2004) 242. [26] Y. Kojima, K. Suzuki, Y. Kawai, J. Mater. Sci. Lett. 39 (2004) 2227. [27] Y. Kojima, Y. Kawai, T Haga, in: T. Vogt, R. Stumpf, M. Heben, I. Robertson (Eds.), Proceedings of Symposium N Materials for Hydrogen Storage, Mat. Res. Soc. Symp. Proc., vol. 837, 2004. [28] E. Newson, Th. Haueter, P. Hottinger, F. Von Roth, G.W.H. Scherer, Th. H. Schucan, Int. J. Hydrogen Energy 23 (1998) 905. [29] N. Kariya, A. Fukuoka, M. Ichikawa, Appl. Catal. A: Gen. 233 (2002) 91. [30] J.J. Reilly, in: A.F. Anderson, M.J. Maeland (Eds.), Hydrides for Energy Storage, Pergamon, Oxford, 1978, pp. 301–322. [31] Y. Osumi, H. Suzuki, A. Kato, K. Oguro, T. Sugioka, T. Fujita, J. LessCommon Met. 89 (1983) 257. [32] O. Beeri, D. Cohen, Z. Gavra, J.R. Johnson, M.H. Mintz, J. Alloys Compd. 299 (2000) 217.

261

[33] L. Pauling, The Chemical Bond, Cornell University Press, Ithaca, NY, 1967. [34] M.H. Mendelsohn, D.M. Gruen, A.E. Dwight, Nature 269 (1977) 45. [35] S. Fujitani, I. Yonezu, T. Saito, N. Furukawa, E. Akiba, H. Hayakawa, S. Ono, J. Less-Common Met. 172–174 (1991) 220. [36] N. Nagasako, A. Fukumoto, K. Miwa, Phys. Rev., B 66 (2002) 155106. [37] C. Kittel, Introduction to Solid State Physics, seventh ed., John Wiley & Sons, New York, 1996. [38] H.E. Boyer, T.L. Gall (Eds.), Metals Handbook Desk Edition, American Society of Metals, 1985. [39] J.E. Atteberry, D.S. Agosta, C.E. Nattrass, R.G. Leisure, I. Jacob, R.C. Bowman Jr., O. Yeheskel, J. Alloys Compd. 376 (2004) 139. [40] J.E. Atteberry, D.S. Agosta, R.G. Leisure, O. Beeri, M.H. Mintz, J. Alloys Compd. 365 (2004) 68. [41] D.S. Agosta, J.E. Hightower, K. Foster, R.G. Leisure, Z. Gava, J. Alloys Compd. 346 (2002) 1. [42] L.G. Hector Jr., J.F. Herbst, T.W. Capehart, J. Alloys Compd. 353 (2003) 74. [43] K. Foster, J.E. Hightower, R.G. Leisure, A.V. Skripov, Philos. Mag. B80 (2000) 1667. [44] E.W. Comings, High Pressure Technology, McGraw-Hill, New York, 1956, pp. 486–487.