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Crystal structure, hydrogen absorption and thermodynamics of Zr1−xCoxFe2 alloys
Journal of Alloys and Compounds 438 (2007) 106–109
Crystal structure, hydrogen absorption and thermodynamics of Zr1−xCoxFe2 alloys Ankur Jain ∗ , R.K. Jain, Garima Agarwal, I.P. Jain ∗ Centre for Non-Conventional Energy Resources, University of Rajasthan, Jaipur 302004, India Received 20 June 2006; received in revised form 3 August 2006; accepted 4 August 2006 Available online 8 September 2006
Abstract In this paper, the structural and hydrogen storage properties of Zr1−x Cox Fe2 (x = 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys have been systematically investigated. XRD analysis of the alloys have revealed that all the alloys were formed as a single phase alloys having C14 laves phase hexagonal structure. With increasing Co addition, the unit cell volume decreases. P–C–T curves were measured in the pressure and temperature range of 0.5 ≤ P ≤ 60 bar and 303 ≤ T ≤ 373 K using a Sievert type apparatus. The results indicate that with increasing Co content in the samples, the plateau pressure increases whereas hydrogen storage capacity and stability decreases. © 2006 Elsevier B.V. All rights reserved. Keywords: Intermetallics; Hydrogen storage material; P–C–T isotherm; Thermodynamics
1. Introduction Hydrogen storage alloys are usually intermetallic compounds consisting of a hydride forming metal component (A) and a nonforming metal component (B) and having specific crystal structures such as C14, B2, D2d, and so on. They absorb and desorb hydrogen reversibly keeping the crystalline state at the moderate temperature. Among them AB2 type Zr based laves phase metal hydrides have been attracting much attention recently because of their larger hydrogen storage capacity and relatively longer electrochemically charging discharging cycle life than the commercialized AB5 type alloys [1–3]. A number of studies have been taken place on Zr based alloys [4–10]. Naik et al. [4] has been reported the suitability of the intermetallic compound of zirconium with cobalt for hydrogen storage and recovery applications for hydrogen isotopes, including tritium. Boncour et al. [6] studied Zr M2 Dx system by means of neutron diffraction measurements. Berznitsky et al. [8] replace Fe by Al partially and studied the thermodynamical and structural aspects of the alloys. But the problem associated with ZrFe2 based alloys is that they make very stable hydride, which is not suitable for practi-
∗
Corresponding authors. Tel.: +91 141 2701602; fax: +91 141 2711049. E-mail addresses: [email protected] (A. Jain), [email protected] (I.P. Jain). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.007
cal application [11]. In the present work, we could achieve the reduction in stability of ZrFe2 based ternary alloys by substituting Zr by Co. We are presenting the structural, thermodynamical, and hydrogenation properties of Zr1−x Cox Fe2 alloys. 2. Experimental The Zr1−x Cox Fe2 alloys were prepared by Arc melting under argon atmosphere and remelted four times to ensure homogeneity. All starting materials have purity higher than 99.9%. The weight loss of the samples prepared was found to be less than 1%. Part of the alloys were mechanically crushed and grounded into powder with a dimension of about 300 mesh for XRD measurements. Crystal structures and lattice parameters of the alloys were determined by X-ray powder diffraction (XRD) using Cu K␣ radiation. Microstructures of the ingots were examined by scanning electron microscope (SEM) technique along with an energy dispersive X-ray spectroscope. Pressure–composition isotherms were measured using Sievert’s type apparatus [12] at different temperatures. The purity of used gas is higher than 99.99%. The powder samples of about 1 g were loaded into reaction chamber. Before experiments, the samples were subjected to several hydriding and dehydriding cycles to ensure activation and reproduction of the data.
3. Results and discussion Fig. 1 shows XRD patterns of the as cast Zr1−x Cox Fe2 (x = 0.2, 0.3, 0.4, 0.5) alloys. The cell parameters and cell volume of all the alloys are shown in Table 1. It has been found that all the samples prepared, have been crystallized in the C14 hexagonal
A. Jain et al. / Journal of Alloys and Compounds 438 (2007) 106–109
107
Fig. 2. Variation of lattice constants with cobalt content.
Fig. 1. XRD pattern of Zr1−x Cox Fe2 : (a) x = 0.2; (b) x = 0.3; (c) x = 0.4; (d) x = 0.5.
structure (space group P63 /mmc) and have almost same pattern. The dependence of the cell constants and volume of the unit cell on the Co content in Zr1−x Cox Fe2 is shown in Fig. 2. The lattice constants and the unit cell volume of Zr1−x Cox Fe2 decreases with increasing Co content due to the smaller atomic radius of Co compared to Zr. The SEM images (Fig. 3) of alloys show that all the alloys are homogeneous and have good crystallinity. This is in good agreement with XRD results. The elemental composition of the alloys has also been checked by EDX technique at different sites of the ingots and the elemental composition was Table 1 Lattice parameters and unit cell volume of Zr1−x Cox Fe2 (x = 0.2, 0.3, 0.4, 0.5) System
˚ a (A)
˚ c (A)
Volume, V
Zr0.8 Co0.2 Fe2 Zr0.7 Co0.3 Fe2 Zr0.6 Co0.4 Fe2 Zr0.5 Co0.5 Fe2
4.854 4.846 4.839 4.831
15.901 15.891 15.883 15.874
324.44 323.17 322.08 320.83
˚ 3) (A
found almost same at all the sites. The elemental percentage is shown in Table 2. Fig. 4 presents the P–C–T curves for the alloys at different temperatures. These absorption isotherms clearly indicate the presence of ␣, ␣ + , and  phase regions for all the temperatures. It can also be seen that with increasing Co content, hydrogen storage capacity of the alloy decreases, which is due to the contraction of the lattice. The decrease in unit cell volume led to the contraction of the interstitial hole size. This contraction of interstitial holes disabled many interstitial sites to become occupied by hydrogen, leading to decrease in hydrogen storage capacity. The changes in plateau pressure during hydrogen absorption with increasing Co content at room temperature are shown in Fig. 5. It can be noted that the increase of the amount of Co addition induces a lattice shrinkage that is linear with Co composition. The increase in plateau pressure with decreasing the unit cell volume seen here is consistent with the previous reported [13]. Table 3 summarized all the characteristic parameters of hydrogen absorption of the alloys. The relative partial molar thermodynamics properties of these hydrides were determined from a thermodynamic equation: 1 RT lnPH2 = H − TS 2 where H is relatively partial molar enthalpy and S is the partial molar entropy. The thermodynamic properties are summarized in Table 3. The logarithmic plots of absorption plateaux against reciprocal temperature are shown in Fig. 6. The practical significance of H is that it is an index of thermochemical stabilTable 2 Elemental composition (atomic percentage) according to EDX analysis Sample
V/V% – 0.39 0.73 1.11
Zr0.8 Co0.2 Fe2 Zr0.7 Co0.3 Fe2 Zr0.6 Co0.4 Fe2 Zr0.5 Co0.5 Fe2
Atomic percentage Zr
Co
Fe
26.61 23.29 19.95 16.60
6.69 10.03 13.38 16.73
66.70 66.68 66.67 66.67
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A. Jain et al. / Journal of Alloys and Compounds 438 (2007) 106–109
Fig. 3. SEM images of Zr1−x Cox Fe2 : (a) x = 0.2; (b) x = 0.3; (c) x = 0.4; (d) x = 0.5.
Fig. 4. P–C–T isotherm of Zr1−x Cox Fe2 : (a) x = 0.2; (b) x = 0.3; (c) x = 0.4; (d) x = 0.5.
A. Jain et al. / Journal of Alloys and Compounds 438 (2007) 106–109
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Table 3 The equilibrium pressure, H, S, and μ for these alloy hydrides Alloy hydrides
Plateau pressurea 303 K, 323 K (bar)
−H (kJ mol−1 )
−S (J K−1 mol−1 )
Zr0.8 Co0.2 Fe2 –H Zr0.7 Co0.3 Fe2 –H Zr0.6 Co0.4 Fe2 –H Zr0.5 Co0.5 Fe2 –H
5.198, 12.075 6.658, 16.278 8.698, 18.243 9.483, 19.890
13.556 13.507 12.838 12.302
54.266 51.931 51.661 49.293
a
Plateau pressure at H/M = 1.5.
4. Conclusion Hydrogen absorption isotherms of Zr1−x Cox Fe2 (x = 0.2, 0.3, 0.4, 0.5) alloys have been investigated in the ranges 303 ≤ T ≤ 373 K and 0.5 ≤ P ≤ 60 bar. The present study revealed that substitution of Co at Zr site decreases the unit cell volume thereby raising the plateau pressure. The enthalpy of the alloys was found decreasing with increase in Co content, so Co substitution makes these alloys to form less stable hydrides without much affecting the hydrogen storage capacity, which goes from 3.1 to 2.8 H/f.u. Acknowledgment
Fig. 5. Variation in plateau pressure with Co content.
Authors are highly grateful to University Grants Commission, New Delhi, India, for providing financial support for this work. We would like to thank to Dr. Gautam Ghosh and Mr. Mendole, UGC - DAE CSR, Mumbai, India for their assistance in preparing alloys and their characterization. References
Fig. 6. Van’t Hoff plots for Zr1−x Cox Fe2 .
ity of metal hydride, low dissociation pressure and requirement of moderately higher temperature to decompose it to liberate the hydrogen than hydride formation temperature [11]. It is clear from Table 3 that the value of H decreases with Co addition, which is good indication for practical application.
[1] J.J.G. Willems, K.H.J. Buschow, J. Less Common Met. 129 (1987) 13. [2] M.A. Fet Cenko, S. Venkatesan, K.C. Hong, B. Reichman, J. Power Sources 12 (1989) 411. [3] H.H. Lee, K.Y. Lee, J.Y. Lee, J. Alloys Comp. 239 (1996) 63. [4] Y. Naik, G.A. Rama Rao, V. Venugopal, Intermetallics 9 (2001) 309. [5] J.A.H. Coaquira, H.R. Rechenberg, J. Mestnik-Filho, A.W. Carbonari, J. Alloys Comp. 356–357 (2003) 200. [6] V.P. Boncour, F.B. Vigneron, S.M. Filipek, I. Marchuk, I. Jacob, A.P. Guegan, J. Alloys Comp. 356–357 (2003) 69. [7] M. Kandavel, S. Ramaprabhu, J. Phys. Condens. Matter 15 (2003) 7501. [8] M. Berznitsky, I. Jacob, J. Bloch, M.H. Mintz, J. Alloys Comp. 351 (2003) 180. [9] Y.L. Du, X.G. Yang, Y.Q. Li, M.S. Zhang, Int. J. Hydrogen Energy 27 (2002) 695. [10] S.M. Philipose, N. Mani, T.R. Kesavan, S. Ramaprabhu, Int. J. Hydrogen Energy 27 (2002) 419. [11] B. Viswanathan, M.V.C. Sastry, S. Murthy (Eds.), Metal Hydride, Narosa Publishers, New Delhi, New Delhi, 1998. [12] A. Jain, R.K. Jain, C. Lal, I.P. Jain, Proceedings of DAE Solid State Symposium-2005, 2005, pp. 313. [13] N. Mani, R. Sivakumar, S. Ramaprabhu, J. Alloys Comp. 337 (2002) 148.
Crystal structure, hydrogen absorption and thermodynamics of Zr1−xCoxFe2 alloys Ankur Jain ∗ , R.K. Jain, Garima Agarwal, I.P. Jain ∗ Centre for Non-Conventional Energy Resources, University of Rajasthan, Jaipur 302004, India Received 20 June 2006; received in revised form 3 August 2006; accepted 4 August 2006 Available online 8 September 2006
Abstract In this paper, the structural and hydrogen storage properties of Zr1−x Cox Fe2 (x = 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys have been systematically investigated. XRD analysis of the alloys have revealed that all the alloys were formed as a single phase alloys having C14 laves phase hexagonal structure. With increasing Co addition, the unit cell volume decreases. P–C–T curves were measured in the pressure and temperature range of 0.5 ≤ P ≤ 60 bar and 303 ≤ T ≤ 373 K using a Sievert type apparatus. The results indicate that with increasing Co content in the samples, the plateau pressure increases whereas hydrogen storage capacity and stability decreases. © 2006 Elsevier B.V. All rights reserved. Keywords: Intermetallics; Hydrogen storage material; P–C–T isotherm; Thermodynamics
1. Introduction Hydrogen storage alloys are usually intermetallic compounds consisting of a hydride forming metal component (A) and a nonforming metal component (B) and having specific crystal structures such as C14, B2, D2d, and so on. They absorb and desorb hydrogen reversibly keeping the crystalline state at the moderate temperature. Among them AB2 type Zr based laves phase metal hydrides have been attracting much attention recently because of their larger hydrogen storage capacity and relatively longer electrochemically charging discharging cycle life than the commercialized AB5 type alloys [1–3]. A number of studies have been taken place on Zr based alloys [4–10]. Naik et al. [4] has been reported the suitability of the intermetallic compound of zirconium with cobalt for hydrogen storage and recovery applications for hydrogen isotopes, including tritium. Boncour et al. [6] studied Zr M2 Dx system by means of neutron diffraction measurements. Berznitsky et al. [8] replace Fe by Al partially and studied the thermodynamical and structural aspects of the alloys. But the problem associated with ZrFe2 based alloys is that they make very stable hydride, which is not suitable for practi-
∗
Corresponding authors. Tel.: +91 141 2701602; fax: +91 141 2711049. E-mail addresses: [email protected] (A. Jain), [email protected] (I.P. Jain). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.007
cal application [11]. In the present work, we could achieve the reduction in stability of ZrFe2 based ternary alloys by substituting Zr by Co. We are presenting the structural, thermodynamical, and hydrogenation properties of Zr1−x Cox Fe2 alloys. 2. Experimental The Zr1−x Cox Fe2 alloys were prepared by Arc melting under argon atmosphere and remelted four times to ensure homogeneity. All starting materials have purity higher than 99.9%. The weight loss of the samples prepared was found to be less than 1%. Part of the alloys were mechanically crushed and grounded into powder with a dimension of about 300 mesh for XRD measurements. Crystal structures and lattice parameters of the alloys were determined by X-ray powder diffraction (XRD) using Cu K␣ radiation. Microstructures of the ingots were examined by scanning electron microscope (SEM) technique along with an energy dispersive X-ray spectroscope. Pressure–composition isotherms were measured using Sievert’s type apparatus [12] at different temperatures. The purity of used gas is higher than 99.99%. The powder samples of about 1 g were loaded into reaction chamber. Before experiments, the samples were subjected to several hydriding and dehydriding cycles to ensure activation and reproduction of the data.
3. Results and discussion Fig. 1 shows XRD patterns of the as cast Zr1−x Cox Fe2 (x = 0.2, 0.3, 0.4, 0.5) alloys. The cell parameters and cell volume of all the alloys are shown in Table 1. It has been found that all the samples prepared, have been crystallized in the C14 hexagonal
A. Jain et al. / Journal of Alloys and Compounds 438 (2007) 106–109
107
Fig. 2. Variation of lattice constants with cobalt content.
Fig. 1. XRD pattern of Zr1−x Cox Fe2 : (a) x = 0.2; (b) x = 0.3; (c) x = 0.4; (d) x = 0.5.
structure (space group P63 /mmc) and have almost same pattern. The dependence of the cell constants and volume of the unit cell on the Co content in Zr1−x Cox Fe2 is shown in Fig. 2. The lattice constants and the unit cell volume of Zr1−x Cox Fe2 decreases with increasing Co content due to the smaller atomic radius of Co compared to Zr. The SEM images (Fig. 3) of alloys show that all the alloys are homogeneous and have good crystallinity. This is in good agreement with XRD results. The elemental composition of the alloys has also been checked by EDX technique at different sites of the ingots and the elemental composition was Table 1 Lattice parameters and unit cell volume of Zr1−x Cox Fe2 (x = 0.2, 0.3, 0.4, 0.5) System
˚ a (A)
˚ c (A)
Volume, V
Zr0.8 Co0.2 Fe2 Zr0.7 Co0.3 Fe2 Zr0.6 Co0.4 Fe2 Zr0.5 Co0.5 Fe2
4.854 4.846 4.839 4.831
15.901 15.891 15.883 15.874
324.44 323.17 322.08 320.83
˚ 3) (A
found almost same at all the sites. The elemental percentage is shown in Table 2. Fig. 4 presents the P–C–T curves for the alloys at different temperatures. These absorption isotherms clearly indicate the presence of ␣, ␣ + , and  phase regions for all the temperatures. It can also be seen that with increasing Co content, hydrogen storage capacity of the alloy decreases, which is due to the contraction of the lattice. The decrease in unit cell volume led to the contraction of the interstitial hole size. This contraction of interstitial holes disabled many interstitial sites to become occupied by hydrogen, leading to decrease in hydrogen storage capacity. The changes in plateau pressure during hydrogen absorption with increasing Co content at room temperature are shown in Fig. 5. It can be noted that the increase of the amount of Co addition induces a lattice shrinkage that is linear with Co composition. The increase in plateau pressure with decreasing the unit cell volume seen here is consistent with the previous reported [13]. Table 3 summarized all the characteristic parameters of hydrogen absorption of the alloys. The relative partial molar thermodynamics properties of these hydrides were determined from a thermodynamic equation: 1 RT lnPH2 = H − TS 2 where H is relatively partial molar enthalpy and S is the partial molar entropy. The thermodynamic properties are summarized in Table 3. The logarithmic plots of absorption plateaux against reciprocal temperature are shown in Fig. 6. The practical significance of H is that it is an index of thermochemical stabilTable 2 Elemental composition (atomic percentage) according to EDX analysis Sample
V/V% – 0.39 0.73 1.11
Zr0.8 Co0.2 Fe2 Zr0.7 Co0.3 Fe2 Zr0.6 Co0.4 Fe2 Zr0.5 Co0.5 Fe2
Atomic percentage Zr
Co
Fe
26.61 23.29 19.95 16.60
6.69 10.03 13.38 16.73
66.70 66.68 66.67 66.67
108
A. Jain et al. / Journal of Alloys and Compounds 438 (2007) 106–109
Fig. 3. SEM images of Zr1−x Cox Fe2 : (a) x = 0.2; (b) x = 0.3; (c) x = 0.4; (d) x = 0.5.
Fig. 4. P–C–T isotherm of Zr1−x Cox Fe2 : (a) x = 0.2; (b) x = 0.3; (c) x = 0.4; (d) x = 0.5.
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Table 3 The equilibrium pressure, H, S, and μ for these alloy hydrides Alloy hydrides
Plateau pressurea 303 K, 323 K (bar)
−H (kJ mol−1 )
−S (J K−1 mol−1 )
Zr0.8 Co0.2 Fe2 –H Zr0.7 Co0.3 Fe2 –H Zr0.6 Co0.4 Fe2 –H Zr0.5 Co0.5 Fe2 –H
5.198, 12.075 6.658, 16.278 8.698, 18.243 9.483, 19.890
13.556 13.507 12.838 12.302
54.266 51.931 51.661 49.293
a
Plateau pressure at H/M = 1.5.
4. Conclusion Hydrogen absorption isotherms of Zr1−x Cox Fe2 (x = 0.2, 0.3, 0.4, 0.5) alloys have been investigated in the ranges 303 ≤ T ≤ 373 K and 0.5 ≤ P ≤ 60 bar. The present study revealed that substitution of Co at Zr site decreases the unit cell volume thereby raising the plateau pressure. The enthalpy of the alloys was found decreasing with increase in Co content, so Co substitution makes these alloys to form less stable hydrides without much affecting the hydrogen storage capacity, which goes from 3.1 to 2.8 H/f.u. Acknowledgment
Fig. 5. Variation in plateau pressure with Co content.
Authors are highly grateful to University Grants Commission, New Delhi, India, for providing financial support for this work. We would like to thank to Dr. Gautam Ghosh and Mr. Mendole, UGC - DAE CSR, Mumbai, India for their assistance in preparing alloys and their characterization. References
Fig. 6. Van’t Hoff plots for Zr1−x Cox Fe2 .
ity of metal hydride, low dissociation pressure and requirement of moderately higher temperature to decompose it to liberate the hydrogen than hydride formation temperature [11]. It is clear from Table 3 that the value of H decreases with Co addition, which is good indication for practical application.
[1] J.J.G. Willems, K.H.J. Buschow, J. Less Common Met. 129 (1987) 13. [2] M.A. Fet Cenko, S. Venkatesan, K.C. Hong, B. Reichman, J. Power Sources 12 (1989) 411. [3] H.H. Lee, K.Y. Lee, J.Y. Lee, J. Alloys Comp. 239 (1996) 63. [4] Y. Naik, G.A. Rama Rao, V. Venugopal, Intermetallics 9 (2001) 309. [5] J.A.H. Coaquira, H.R. Rechenberg, J. Mestnik-Filho, A.W. Carbonari, J. Alloys Comp. 356–357 (2003) 200. [6] V.P. Boncour, F.B. Vigneron, S.M. Filipek, I. Marchuk, I. Jacob, A.P. Guegan, J. Alloys Comp. 356–357 (2003) 69. [7] M. Kandavel, S. Ramaprabhu, J. Phys. Condens. Matter 15 (2003) 7501. [8] M. Berznitsky, I. Jacob, J. Bloch, M.H. Mintz, J. Alloys Comp. 351 (2003) 180. [9] Y.L. Du, X.G. Yang, Y.Q. Li, M.S. Zhang, Int. J. Hydrogen Energy 27 (2002) 695. [10] S.M. Philipose, N. Mani, T.R. Kesavan, S. Ramaprabhu, Int. J. Hydrogen Energy 27 (2002) 419. [11] B. Viswanathan, M.V.C. Sastry, S. Murthy (Eds.), Metal Hydride, Narosa Publishers, New Delhi, New Delhi, 1998. [12] A. Jain, R.K. Jain, C. Lal, I.P. Jain, Proceedings of DAE Solid State Symposium-2005, 2005, pp. 313. [13] N. Mani, R. Sivakumar, S. Ramaprabhu, J. Alloys Comp. 337 (2002) 148.