- Email: [email protected]
Effect of Al on microstructures and hydrogen storage properties of Ti26.5Cr20(V0.45Fe0.085)100−xAlxCe0.5 alloy
Journal of Alloys and Compounds 485 (2009) 324–327
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of Al on microstructures and hydrogen storage properties of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloy Jing Mi, Xiumei Guo, Xiaopeng Liu ∗ , Lijun Jiang, Zhinian Li, Lei Hao, Shumao Wang General Research Institute for Non-ferrous Metals, Beijing 100088, China
a r t i c l e
i n f o
Article history: Received 26 November 2008 Received in revised form 18 May 2009 Accepted 19 May 2009 Available online 27 May 2009 Keywords: Hydrogen storage materials Ti–V based alloy Ferrovanadium Aluminum
a b s t r a c t The effect of Al additive on microstructures and hydrogen storage characteristics of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 (x = 0, 0.5, 1.0 and 1.5 at.%) alloys have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and P–C-isotherm measurements. All of Al-added alloys consist of major BCC and secondary Al–Ce–O phases. The EDS analysis results show that aluminum is poor in the BCC phase but enriches in the secondary Al–Ce–O phase. As Al additive content increased, both the lattice parameters of the BCC phase and the hydrogen desorption equilibrium pressures increase, but the hydrogen storage capacities decrease. It is revealed that the substitution solid solution with BCC structure is formed by the substitution of Al for V atoms. It decreases the hydride stability of the BCC phase and degrades the hydrogen storage performance of the Al-added Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys. © 2009 Elsevier B.V. All rights reserved.
1. Introduction As a promising future energy source, hydrogen has attracted more and more attention because of its clean and unexhausted characteristics. Among the candidates for hydrogen storage application, Ti–V based solid solution alloys with BCC structure have been investigated extensively due to their high hydrogen storage capacities and good absorption/desorption kinetics [1–5]. Moreover, they also have been selected as anode materials in Ni/MH secondary batteries [6–8]. However, the high cost of pure vanadium limits the practical application of the Ti–V based hydrogen storage alloy. As a cheap vanadium source, commercial ferrovanadium alloy is a good vanadium source to decrease the cost of Ti–V based BCC alloy, moreover iron element is beneficial for improving the hydrogen storage properties of alloys [9–11]. Many investigations have been done on the hydrogen storage properties of Ti–V alloys prepared by using ferrovanadium alloy as raw material [12–15]. Aluminum is a major impurity in the ferrovanadium raw material. The previous reports had showed that the hydrogen absorption/desorption equilibrium pressures of Ti–V based alloy were increased by the Al additive, but the hydrogen storage capacities were considerably decreased [16,17]. The reason for the degradation mechanism of Al additive on the hydrogen storage performance of Ti–V based alloy is not clear. In this paper, the effect mechanism of Al additive on the hydrogen storage performance of Ti26.5 Cr20 V45 Fe8.5 Ce0.5 alloy has been investigated. Moreover, a
∗ Corresponding author. Tel.: +86 10 8224 1240; fax: +86 10 8224 1294. E-mail address: [email protected] (X. Liu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.096
possible way for suppressing the side-effect of Al also has been studied. 2. Experimental Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 (x = 0, 0.5, 1.0 and 1.5 at.%) alloys were prepared by magnetic levitation melting in a pure argon atmosphere. The purities of raw materials of V, Ti, Cr, Fe, Ce and Al are better than 99.5%. The samples were melted four times to ensure homogeneity. The ingots were subsequently annealed at 1673 K for 5 min under argon atmosphere, then immediately quenched into water to room temperature. P–C-isotherms of the alloys have been measured with a Sievert’s type apparatus. Each sample with a mass of 2 g was introduced in a stainless steel container. For the first activation, samples were evacuated at 298 K for 20 min, and then hydrogen with a pressure of 6 MPa was introduced into the reactor. After activation, the samples were evacuated at 673 K for 1 h to ensure that the alloys were fully dehydrogenized, then cooled down to room temperature and brought into hydrogen with a pressure of 6 MPa for absorption. After the 3rd absorption/desorption cycle, P–C-isotherms measurements were carried out at 298 K. X-ray diffraction experiments were performed to analyze crystal structure and lattice parameters on a X-Pert PRO MPD X-ray diffraction measurement using Cu K␣ radiation. The microstructure and chemical composition were studied by field emission scanning electron micrographs using Hitachi model S4800 with an energy dispersive X-ray spectrometer (EDS).
3. Results and discussion Fig. 1 shows XRD patterns of the Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys. The main peaks indicate that all of samples consist of a major BCC phase. The decrease of the 2 values of main peaks reveals that the lattice parameters of the BCC phase increase with increasing the Al addition content. Fig. 2 shows back scattering electron images of the studied alloys. All of alloys consist of BCC (grey) and secondary phases
J. Mi et al. / Journal of Alloys and Compounds 485 (2009) 324–327
325
Table 1 Chemical compositions of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys. Alloys
Phase
Composition (at%) Al
Ti
Fe
Ce
O
Ti26.5 Cr20 V45 Fe8.5 Ce0.5
BCC Secondary phase
– –
27.03 –
45.95 –
18.02 –
9.00 –
– 34.10
– 65.90
Ti26.5 Cr20 (V0.45 Fe0.085 )99.5 Al0.5 Ce0.5
BCC Secondary phase
0.29 2.49
25.39 –
45.25 –
20.46 –
8.61 –
– 45.59
– 51.29
Ti26.5 Cr20 (V0.45 Fe0.085 )99 Al1.0 Ce0.5
BCC Secondary phase
0.79 2.46
25.47 –
46.42 –
19.80 –
7.52 –
– 50.37
– 47.17
Ti26.5 Cr20 (V0.45 Fe0.085 )98.5 Al1.5 Ce0.5
BCC Secondary phase
1.04 5.12
26.61 –
43.84 –
19.95 –
8.56 –
– 31.44
– 63.44
V
Cr
Fig. 1. XRD patterns of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys.
(white). The chemical compositions analyzed by EDS are summarized in Table 1. For Al-free alloy, the chemical compositions of the BCC phase are very close to the nominal value of the devised alloy and no trace of cerium was detected in the BCC phase. The white area corresponds to a CeO2 phase. It is indicated that cerium only reacts with dissolved oxygen atoms to form stable CeO2 phase due to the stronger affinity of cerium for oxygen atoms during alloy melting and annealing processes [18,19]. In the case of the Al-added alloy, the chemical compositions of the BCC phase also are close to the nominal value except for Al. It is seen that the aluminum is poor in the BCC phase but enriches in the secondary phase. Compared with the Al-free alloy, the secondary phase in the Al-added alloy is attributed to an Al–Ce–O phase. It is worth noting that the difference of electronic negativity between Al (1.61) and Ce (1.12) is the largest among element constituents of the Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloy. The segregation of Ce induces the precipitation of Al in the secondary phase. Fig. 3 shows P–C-isotherms of the studied alloys measured at 298 K during the hydrogen desorption process. The hydrogen storage capacities, plateau slope factors (it is used to describe the sloping of plateau and the definition is the same with that in Ref. [20]) and hydrogen desorption equilibrium pressures of the alloys are summarized in Table 2. At 298 K, the maximum hydrogen storage capacity of the Ti26.5 Cr20 (V0.45 Fe0.085 )98.5 Al1.5 Ce0.5 alloy decreases to 3.20 wt.% compared with 3.58 wt.% for the Al-free alloy, while the equilibrium hydrogen desorption pressure and the plateau slope factor of the former alloy increases to 0.307 MPa and 0.79 compared with 0.095 MPa and 0.48 for the latter alloy, respectively. Obviously, although the content of Al dissolved in the BCC
Fig. 2. Back scattering electron images of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys: (a) x = 0 at.%, (b) x = 0.5 at.%, (c) x = 1.0 at.% and (d) x = 1.5 at.%, respectively.
326
J. Mi et al. / Journal of Alloys and Compounds 485 (2009) 324–327
Table 2 Hydrogen desorption properties of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys. Alloys
Hydrogen storage capacity (wt.%)
Equilibrium pressure (MPa)
Slope factor
Ti26.5 Cr20 V45 Fe8.5 Ce0.5 Ti26.5 Cr20 (V0.45 Fe0.085 )99.5 Al0.5 Ce0.5 Ti26.5 Cr20 (V0.45 Fe0.085 )99 Al1.0 Ce0.5 Ti26.5 Cr20 (V0.45 Fe0.085 )98.5 Al1.5 Ce0.5
3.56 3.55 3.45 3.20
0.095 0.104 0.162 0.307
0.48 0.56 0.68 0.79
phase is in the low level, the hydrogen storage properties are seriously degraded. It is supposed that the Al atoms can occupy the interstitial sites of BCC phase to form interstitial solid solution or substitute other atoms to form substitution solid solution. The Ti26.5 Cr20 V45 Fe8.5 Ce0.5 alloy has a homogeneous BCC structure with a lattice parameter a = 3.034 Å. According to the metallographic principle, the sizes of the interstitial site of tetrahedron and octahedron of the BCC phase in √ the Ti26.5 Cr20 V45 Fe8.5 Ce0.5 √ alloy are equal to 0.382 Å ((( 5 − 3)/4)a) and 0.203 Å (((2 − √ 3)/4)a), respectively. The sizes are considerably smaller than the atomic radius of Al (1.43 Å). So very huge potential energy must be overcome if the Al atoms occupy the interstitial sites to form interstitial solid solution. Obviously, it is reasonable to suggest that the substitution solid solution is formed in the Al-added Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys. Moreover, the formation of substitution solid solution is accomplished by the substitution of Al for V atoms because that the atomic radius and electron negativities between Al and V atom are quite similar (the atomic radius of Al and V are equal to 1.43 Å and 1.35 Å, and the electron negativities are 1.61 and 1.60, respectively). The substitution of Al for V atoms increases the lattice parameter of the BCC phase. According to the local environment model proposed by Ivey and Northwood [21], the increase of amount of the atoms with low hydrogenation enthalpy will decrease the hydride stability and impair the plateau of hydrogen storage alloy. In the case of the Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys, the hydrogenation enthalpy (H) of vanadium (−40.1 kJ/mol) is considerably higher than that of Al (−6.95 kJ/mol) [22,23], and the substitution of Al for V in the BCC phase increase the amount of atom sites with low hydrogenation enthalpy, so the hydrogen storage properties of the alloy were worsened. The same phenomenon had been reported by Liu et al. [24].
In our previous work, we found that Ce additive is useful for suppressing the formation of a Ti-rich secondary phase, favoring the chemical homogeneity and improving the hydrogen storage properties of Ti–V based alloys [18,19]. In the case of the Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys, one can see from Fig. 2 and Table 1 that Al is poor in the BCC phase but enriches in the secondary Ce–Al–O phase. That is, Ce additive is useful for decreasing the content of Al dissolved in the BCC phase, so the side-effect of Al on the hydrogen storage performance of the BCC phase can be considerably suppressed. Of course, the appropriate mass of Ce additive for entirely suppressing the side-effect of Al should be further studied. 4. Conclusions (1) All of the Al-added Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys consist of major BCC and secondary Al–Ce–O phases. The chemical compositions of Ti, Cr, V and Fe in the BCC phase are very close to the nominal value. Al is poor in the BCC phase but enriches in the secondary Ce–Al–O phase. (2) In the case of the Al-added alloys, the lattice parameters of the BCC phase and the hydrogen desorption equilibrium pressures increase with increasing Al addition content, but the hydrogen storage capacities decrease. (3) Cerium is considerable useful for suppressing the side-effect of Al on the hydrogen storage performance of alloys. Acknowledgments This research was supported by Hi-Tech Research and Development Program of China (Grant Nos. 2006AA05Z144 and 2007AA05Z106). References
Fig. 3. P–C-isotherms for the studied alloys measured at 298 K during the hydrogen desorption process.
[1] H. Iba, E. Akiba, J. Alloys Compd. 21 (1997) 253–254. [2] X.B. Yu, Z. Wu, F. Li, B.J. Xia, N.X. Xu, Appl. Phys. Lett. 84 (16) (2004) 3199–3201. [3] T. Tamura, T. Kazumi, A. Kamegawa, H. Takamura, M. Okada, J. Alloys Compd. 356–357 (2003) 505–509. [4] M.V. Lototsky, V.A. Yartys, I.Yu. Zavaliy, J. Alloys Compd. 404–406 (2005) 421–426. [5] S.W. Cho, C.N. Park, J.H. Yoo, J. Choi, J.S. Park, C.Y. Suh, G.C. Shim, J. Alloys Compd. 403 (2005) 262–266. [6] H.G. Pan, Y.F. Zhu, M.X. Gao, Q.D. Wang, J. Electrochem. Soc. 149 (2002) A829–A833. [7] Y.F. Zhu, H.G. Pan, M.X. Gao, J.X. Ma, S.Q. Li, Q.D. Wang, Int. J. Hydrogen Energy 27 (2002) 287–293. [8] H.G. Pan, Y.F. Zhu, M.X. Gao, Y.F. Liu, R. Li, Y.Q. Lei, Q.D. Wang, J. Alloys Compd. 370 (2004) 254–260. [9] H. Miyamura, T. Sakai, N. Kuriyama, H. Tanaka, I. Uehara, H. Ishikawa, J. Alloys Compd. 253–254 (1997) 232–234. [10] S. Basak, K. Shashikala, P. Sengupta, S.K. Kulshreshtha, Int. J. Hydrogen Energy 32 (2007) 4973–4977. [11] K. Nomura, E. Akiba, J. Alloys Compd. 231 (1995) 513–517. [12] J.F. Lynch, A.J. Maeland, G.G. Libowitz, Z. Phys. Chem. NF 145 (1985) 51–59. [13] J.G. Park, D.M. Kim, K.J. Jang, J.S. Han, K. Cho, J.Y. Lee, J. Alloys Compd. 293–295 (1999) 150–155. [14] H. Miao, M.X. Gao, Y.F. Liu, Y. Lin, J.H. Wang, H.G. Pan, Int. J. Hydrogen Energy 32 (2007) 3947–3953. [15] H.G. Pan, R. Li, Y.F. Liu, M.X. Gao, H. Miao, Y.Q. Lei, Q.D. Wang, J. Alloys Compd. 463 (2008) 189–195. [16] A. Kagawa, E. Ono, T. Kusakabe, Y. Sakamoto, J. Less Common Met. 172–174 (1991) 64–70.
J. Mi et al. / Journal of Alloys and Compounds 485 (2009) 324–327 [17] Y.G. Yan, Y.G. Chen, H. Liang, C.L. Wu, M.D. Tao, J. Alloys Compd. 426 (2006) 253–255. [18] X.P. Liu, L.J. Jiang, Z.N. Li, Z. Huang, S.M. Wang, J. Alloys Compd. 471 (2008) L36–L38. [19] X.P. Liu, F. Cuevas, L.J. Jiang, M. Latroche, Z.N. Li, S.M. Wang, J. Alloys Compd. 476 (2008) 403–407.
[20] [21] [22] [23] [24]
T. Dou, Z. Wu, J.F. Mao, N.X. Xu, Mater. Sci. Eng. A 476 (2007) 34–38. D.G. Ivey, D.O. Northwood, J. Mater. Sci 18 (1981) 321. J.J. Reilly, R.H. Wiswall, Inorg. Chem. 9 (1970) 1678. C. Wolverton, V. Ozolins, M. Asta, Phys. Rev. B 69 (2004) 109–144. B.H. Liu, D.M. Kim, K.Y. Lee, J.Y. Lee, J. Alloys Compd. 240 (1996) 214–218.
327
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of Al on microstructures and hydrogen storage properties of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloy Jing Mi, Xiumei Guo, Xiaopeng Liu ∗ , Lijun Jiang, Zhinian Li, Lei Hao, Shumao Wang General Research Institute for Non-ferrous Metals, Beijing 100088, China
a r t i c l e
i n f o
Article history: Received 26 November 2008 Received in revised form 18 May 2009 Accepted 19 May 2009 Available online 27 May 2009 Keywords: Hydrogen storage materials Ti–V based alloy Ferrovanadium Aluminum
a b s t r a c t The effect of Al additive on microstructures and hydrogen storage characteristics of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 (x = 0, 0.5, 1.0 and 1.5 at.%) alloys have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and P–C-isotherm measurements. All of Al-added alloys consist of major BCC and secondary Al–Ce–O phases. The EDS analysis results show that aluminum is poor in the BCC phase but enriches in the secondary Al–Ce–O phase. As Al additive content increased, both the lattice parameters of the BCC phase and the hydrogen desorption equilibrium pressures increase, but the hydrogen storage capacities decrease. It is revealed that the substitution solid solution with BCC structure is formed by the substitution of Al for V atoms. It decreases the hydride stability of the BCC phase and degrades the hydrogen storage performance of the Al-added Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys. © 2009 Elsevier B.V. All rights reserved.
1. Introduction As a promising future energy source, hydrogen has attracted more and more attention because of its clean and unexhausted characteristics. Among the candidates for hydrogen storage application, Ti–V based solid solution alloys with BCC structure have been investigated extensively due to their high hydrogen storage capacities and good absorption/desorption kinetics [1–5]. Moreover, they also have been selected as anode materials in Ni/MH secondary batteries [6–8]. However, the high cost of pure vanadium limits the practical application of the Ti–V based hydrogen storage alloy. As a cheap vanadium source, commercial ferrovanadium alloy is a good vanadium source to decrease the cost of Ti–V based BCC alloy, moreover iron element is beneficial for improving the hydrogen storage properties of alloys [9–11]. Many investigations have been done on the hydrogen storage properties of Ti–V alloys prepared by using ferrovanadium alloy as raw material [12–15]. Aluminum is a major impurity in the ferrovanadium raw material. The previous reports had showed that the hydrogen absorption/desorption equilibrium pressures of Ti–V based alloy were increased by the Al additive, but the hydrogen storage capacities were considerably decreased [16,17]. The reason for the degradation mechanism of Al additive on the hydrogen storage performance of Ti–V based alloy is not clear. In this paper, the effect mechanism of Al additive on the hydrogen storage performance of Ti26.5 Cr20 V45 Fe8.5 Ce0.5 alloy has been investigated. Moreover, a
∗ Corresponding author. Tel.: +86 10 8224 1240; fax: +86 10 8224 1294. E-mail address: [email protected] (X. Liu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.096
possible way for suppressing the side-effect of Al also has been studied. 2. Experimental Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 (x = 0, 0.5, 1.0 and 1.5 at.%) alloys were prepared by magnetic levitation melting in a pure argon atmosphere. The purities of raw materials of V, Ti, Cr, Fe, Ce and Al are better than 99.5%. The samples were melted four times to ensure homogeneity. The ingots were subsequently annealed at 1673 K for 5 min under argon atmosphere, then immediately quenched into water to room temperature. P–C-isotherms of the alloys have been measured with a Sievert’s type apparatus. Each sample with a mass of 2 g was introduced in a stainless steel container. For the first activation, samples were evacuated at 298 K for 20 min, and then hydrogen with a pressure of 6 MPa was introduced into the reactor. After activation, the samples were evacuated at 673 K for 1 h to ensure that the alloys were fully dehydrogenized, then cooled down to room temperature and brought into hydrogen with a pressure of 6 MPa for absorption. After the 3rd absorption/desorption cycle, P–C-isotherms measurements were carried out at 298 K. X-ray diffraction experiments were performed to analyze crystal structure and lattice parameters on a X-Pert PRO MPD X-ray diffraction measurement using Cu K␣ radiation. The microstructure and chemical composition were studied by field emission scanning electron micrographs using Hitachi model S4800 with an energy dispersive X-ray spectrometer (EDS).
3. Results and discussion Fig. 1 shows XRD patterns of the Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys. The main peaks indicate that all of samples consist of a major BCC phase. The decrease of the 2 values of main peaks reveals that the lattice parameters of the BCC phase increase with increasing the Al addition content. Fig. 2 shows back scattering electron images of the studied alloys. All of alloys consist of BCC (grey) and secondary phases
J. Mi et al. / Journal of Alloys and Compounds 485 (2009) 324–327
325
Table 1 Chemical compositions of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys. Alloys
Phase
Composition (at%) Al
Ti
Fe
Ce
O
Ti26.5 Cr20 V45 Fe8.5 Ce0.5
BCC Secondary phase
– –
27.03 –
45.95 –
18.02 –
9.00 –
– 34.10
– 65.90
Ti26.5 Cr20 (V0.45 Fe0.085 )99.5 Al0.5 Ce0.5
BCC Secondary phase
0.29 2.49
25.39 –
45.25 –
20.46 –
8.61 –
– 45.59
– 51.29
Ti26.5 Cr20 (V0.45 Fe0.085 )99 Al1.0 Ce0.5
BCC Secondary phase
0.79 2.46
25.47 –
46.42 –
19.80 –
7.52 –
– 50.37
– 47.17
Ti26.5 Cr20 (V0.45 Fe0.085 )98.5 Al1.5 Ce0.5
BCC Secondary phase
1.04 5.12
26.61 –
43.84 –
19.95 –
8.56 –
– 31.44
– 63.44
V
Cr
Fig. 1. XRD patterns of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys.
(white). The chemical compositions analyzed by EDS are summarized in Table 1. For Al-free alloy, the chemical compositions of the BCC phase are very close to the nominal value of the devised alloy and no trace of cerium was detected in the BCC phase. The white area corresponds to a CeO2 phase. It is indicated that cerium only reacts with dissolved oxygen atoms to form stable CeO2 phase due to the stronger affinity of cerium for oxygen atoms during alloy melting and annealing processes [18,19]. In the case of the Al-added alloy, the chemical compositions of the BCC phase also are close to the nominal value except for Al. It is seen that the aluminum is poor in the BCC phase but enriches in the secondary phase. Compared with the Al-free alloy, the secondary phase in the Al-added alloy is attributed to an Al–Ce–O phase. It is worth noting that the difference of electronic negativity between Al (1.61) and Ce (1.12) is the largest among element constituents of the Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloy. The segregation of Ce induces the precipitation of Al in the secondary phase. Fig. 3 shows P–C-isotherms of the studied alloys measured at 298 K during the hydrogen desorption process. The hydrogen storage capacities, plateau slope factors (it is used to describe the sloping of plateau and the definition is the same with that in Ref. [20]) and hydrogen desorption equilibrium pressures of the alloys are summarized in Table 2. At 298 K, the maximum hydrogen storage capacity of the Ti26.5 Cr20 (V0.45 Fe0.085 )98.5 Al1.5 Ce0.5 alloy decreases to 3.20 wt.% compared with 3.58 wt.% for the Al-free alloy, while the equilibrium hydrogen desorption pressure and the plateau slope factor of the former alloy increases to 0.307 MPa and 0.79 compared with 0.095 MPa and 0.48 for the latter alloy, respectively. Obviously, although the content of Al dissolved in the BCC
Fig. 2. Back scattering electron images of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys: (a) x = 0 at.%, (b) x = 0.5 at.%, (c) x = 1.0 at.% and (d) x = 1.5 at.%, respectively.
326
J. Mi et al. / Journal of Alloys and Compounds 485 (2009) 324–327
Table 2 Hydrogen desorption properties of Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys. Alloys
Hydrogen storage capacity (wt.%)
Equilibrium pressure (MPa)
Slope factor
Ti26.5 Cr20 V45 Fe8.5 Ce0.5 Ti26.5 Cr20 (V0.45 Fe0.085 )99.5 Al0.5 Ce0.5 Ti26.5 Cr20 (V0.45 Fe0.085 )99 Al1.0 Ce0.5 Ti26.5 Cr20 (V0.45 Fe0.085 )98.5 Al1.5 Ce0.5
3.56 3.55 3.45 3.20
0.095 0.104 0.162 0.307
0.48 0.56 0.68 0.79
phase is in the low level, the hydrogen storage properties are seriously degraded. It is supposed that the Al atoms can occupy the interstitial sites of BCC phase to form interstitial solid solution or substitute other atoms to form substitution solid solution. The Ti26.5 Cr20 V45 Fe8.5 Ce0.5 alloy has a homogeneous BCC structure with a lattice parameter a = 3.034 Å. According to the metallographic principle, the sizes of the interstitial site of tetrahedron and octahedron of the BCC phase in √ the Ti26.5 Cr20 V45 Fe8.5 Ce0.5 √ alloy are equal to 0.382 Å ((( 5 − 3)/4)a) and 0.203 Å (((2 − √ 3)/4)a), respectively. The sizes are considerably smaller than the atomic radius of Al (1.43 Å). So very huge potential energy must be overcome if the Al atoms occupy the interstitial sites to form interstitial solid solution. Obviously, it is reasonable to suggest that the substitution solid solution is formed in the Al-added Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys. Moreover, the formation of substitution solid solution is accomplished by the substitution of Al for V atoms because that the atomic radius and electron negativities between Al and V atom are quite similar (the atomic radius of Al and V are equal to 1.43 Å and 1.35 Å, and the electron negativities are 1.61 and 1.60, respectively). The substitution of Al for V atoms increases the lattice parameter of the BCC phase. According to the local environment model proposed by Ivey and Northwood [21], the increase of amount of the atoms with low hydrogenation enthalpy will decrease the hydride stability and impair the plateau of hydrogen storage alloy. In the case of the Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys, the hydrogenation enthalpy (H) of vanadium (−40.1 kJ/mol) is considerably higher than that of Al (−6.95 kJ/mol) [22,23], and the substitution of Al for V in the BCC phase increase the amount of atom sites with low hydrogenation enthalpy, so the hydrogen storage properties of the alloy were worsened. The same phenomenon had been reported by Liu et al. [24].
In our previous work, we found that Ce additive is useful for suppressing the formation of a Ti-rich secondary phase, favoring the chemical homogeneity and improving the hydrogen storage properties of Ti–V based alloys [18,19]. In the case of the Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys, one can see from Fig. 2 and Table 1 that Al is poor in the BCC phase but enriches in the secondary Ce–Al–O phase. That is, Ce additive is useful for decreasing the content of Al dissolved in the BCC phase, so the side-effect of Al on the hydrogen storage performance of the BCC phase can be considerably suppressed. Of course, the appropriate mass of Ce additive for entirely suppressing the side-effect of Al should be further studied. 4. Conclusions (1) All of the Al-added Ti26.5 Cr20 (V0.45 Fe0.085 )100−x Alx Ce0.5 alloys consist of major BCC and secondary Al–Ce–O phases. The chemical compositions of Ti, Cr, V and Fe in the BCC phase are very close to the nominal value. Al is poor in the BCC phase but enriches in the secondary Ce–Al–O phase. (2) In the case of the Al-added alloys, the lattice parameters of the BCC phase and the hydrogen desorption equilibrium pressures increase with increasing Al addition content, but the hydrogen storage capacities decrease. (3) Cerium is considerable useful for suppressing the side-effect of Al on the hydrogen storage performance of alloys. Acknowledgments This research was supported by Hi-Tech Research and Development Program of China (Grant Nos. 2006AA05Z144 and 2007AA05Z106). References
Fig. 3. P–C-isotherms for the studied alloys measured at 298 K during the hydrogen desorption process.
[1] H. Iba, E. Akiba, J. Alloys Compd. 21 (1997) 253–254. [2] X.B. Yu, Z. Wu, F. Li, B.J. Xia, N.X. Xu, Appl. Phys. Lett. 84 (16) (2004) 3199–3201. [3] T. Tamura, T. Kazumi, A. Kamegawa, H. Takamura, M. Okada, J. Alloys Compd. 356–357 (2003) 505–509. [4] M.V. Lototsky, V.A. Yartys, I.Yu. Zavaliy, J. Alloys Compd. 404–406 (2005) 421–426. [5] S.W. Cho, C.N. Park, J.H. Yoo, J. Choi, J.S. Park, C.Y. Suh, G.C. Shim, J. Alloys Compd. 403 (2005) 262–266. [6] H.G. Pan, Y.F. Zhu, M.X. Gao, Q.D. Wang, J. Electrochem. Soc. 149 (2002) A829–A833. [7] Y.F. Zhu, H.G. Pan, M.X. Gao, J.X. Ma, S.Q. Li, Q.D. Wang, Int. J. Hydrogen Energy 27 (2002) 287–293. [8] H.G. Pan, Y.F. Zhu, M.X. Gao, Y.F. Liu, R. Li, Y.Q. Lei, Q.D. Wang, J. Alloys Compd. 370 (2004) 254–260. [9] H. Miyamura, T. Sakai, N. Kuriyama, H. Tanaka, I. Uehara, H. Ishikawa, J. Alloys Compd. 253–254 (1997) 232–234. [10] S. Basak, K. Shashikala, P. Sengupta, S.K. Kulshreshtha, Int. J. Hydrogen Energy 32 (2007) 4973–4977. [11] K. Nomura, E. Akiba, J. Alloys Compd. 231 (1995) 513–517. [12] J.F. Lynch, A.J. Maeland, G.G. Libowitz, Z. Phys. Chem. NF 145 (1985) 51–59. [13] J.G. Park, D.M. Kim, K.J. Jang, J.S. Han, K. Cho, J.Y. Lee, J. Alloys Compd. 293–295 (1999) 150–155. [14] H. Miao, M.X. Gao, Y.F. Liu, Y. Lin, J.H. Wang, H.G. Pan, Int. J. Hydrogen Energy 32 (2007) 3947–3953. [15] H.G. Pan, R. Li, Y.F. Liu, M.X. Gao, H. Miao, Y.Q. Lei, Q.D. Wang, J. Alloys Compd. 463 (2008) 189–195. [16] A. Kagawa, E. Ono, T. Kusakabe, Y. Sakamoto, J. Less Common Met. 172–174 (1991) 64–70.
J. Mi et al. / Journal of Alloys and Compounds 485 (2009) 324–327 [17] Y.G. Yan, Y.G. Chen, H. Liang, C.L. Wu, M.D. Tao, J. Alloys Compd. 426 (2006) 253–255. [18] X.P. Liu, L.J. Jiang, Z.N. Li, Z. Huang, S.M. Wang, J. Alloys Compd. 471 (2008) L36–L38. [19] X.P. Liu, F. Cuevas, L.J. Jiang, M. Latroche, Z.N. Li, S.M. Wang, J. Alloys Compd. 476 (2008) 403–407.
[20] [21] [22] [23] [24]
T. Dou, Z. Wu, J.F. Mao, N.X. Xu, Mater. Sci. Eng. A 476 (2007) 34–38. D.G. Ivey, D.O. Northwood, J. Mater. Sci 18 (1981) 321. J.J. Reilly, R.H. Wiswall, Inorg. Chem. 9 (1970) 1678. C. Wolverton, V. Ozolins, M. Asta, Phys. Rev. B 69 (2004) 109–144. B.H. Liu, D.M. Kim, K.Y. Lee, J.Y. Lee, J. Alloys Compd. 240 (1996) 214–218.
327