Structural and hydriding–dehydriding properties of Ho1−xTixCo2 alloys

Structural and hydriding–dehydriding properties of Ho1−xTixCo2 alloys

Materials Science and Engineering A 472 (2008) 293–298 Structural and hydriding–dehydriding properties of Ho1−xTixCo2 alloys G. Srinivas a,b , V. San...

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Materials Science and Engineering A 472 (2008) 293–298

Structural and hydriding–dehydriding properties of Ho1−xTixCo2 alloys G. Srinivas a,b , V. Sankaranarayanan a , S. Ramaprabhu b,∗ a

Low Temperature Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India b Alternative Energy Technology Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India Received 29 December 2006; received in revised form 8 March 2007; accepted 12 March 2007

Abstract The influence of Ti on the structural and hydrogen absorption–desorption properties of Laves phase Ho1−x Tix Co2 (x = 0–0.6) alloys have been studied using X-ray diffraction (XRD), pressure–composition (PC) isotherms, differential scanning calorimetry (DSC) and thermogravimetry (TG). The crystalline nature and lattice expansion of hydrogenated alloys at different hydrogen concentrations have been reported. In addition, the coexistence of two hydride phases at intermediate hydrogen concentrations has been identified from the XRD. The changes in plateau pressure and hydrogen absorption–desorption capacity upon increasing Ti content have been discussed. The preferential occupation of different types of interstitial sites has been identified from the DSC studies of hydrides. The complete desorption temperature of hydrogen has been obtained from the TG studies of hydrides. © 2007 Elsevier B.V. All rights reserved. Keywords: Laves phase; Hydrogen absorption–desorption; XRD; DSC; TG

1. Introduction The Laves phase AB2 -type alloys are well-known class of hydrogen storage materials. A large number of Laves alloys, especially with rare earth element as one of the components, easily absorb hydrogen and form stable hydrides. The desorption of hydrogen is not possible at room temperature [1–10]. There are a few investigations on the hydrogen absorption and desorption properties of Laves phase RTM2 -hydrides (R = rare earth elements, TM = Mn, Fe, Co and Ni), because of their very lower plateau pressures and complex behavior of hydrogen absorption and desorption properties [5–7]. For example, ErFe2 –H [7] exhibits very low-pressure and multi-plateau behavior with five different hydride phases, whereas GdB2 –H (B = Mn, Fe, Ni, Rh, Ru) [6] exhibit a single plateau region and complete desorption of hydrogen cannot happen even by evacuating the hydride down to 10−5 mbar near room temperatures. However, the Ti- or Zr-based Laves phase alloys show



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very good hydriding–dehydriding properties at ambient conditions [11–18]. Further, the hydrogen absorption–desorption properties and storage capacity of these Laves phases greatly get altered by partial replacement or alloying with different kinds of elements. Most of the hydrogen absorbing materials are of multi-element alloys and lot of effort has been spent to optimize the materials for the best practical performance [19–21]. Significant improvements have been achieved in hydrogen absorption–desorption and structural properties of AB2 and AB5 alloys by substitution or addition of other elements [19,20]. Kleperis et al. [21] have given an excellent explanation on the role of various additives in modification of the properties of hydride-forming alloys. The HoCo2 belongs to the RCo2 compounds family, crystallize in the MgCu2 -type (C15) structure ¯ (space group Fd 3m), where the R atoms form a diamond lattice and the remaining space inside the unit cell is occupied by regular tetrahedra consisting of the Co atoms. In this structure, the R and Co atoms each occupy one crystallographic site, namely the 8a and 16b sites, respectively [22]. The interstitial sites occupied by hydrogen in this structure are tetrahedral sites with four metalatom nearest neighbors. There are 17 tetrahedral interstitial sites per formula unit; contains twelve interstitials (g sites) with two

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R and two Co nearest neighbors (2R2Co), four interstitials (e sites) with one A and three B nearest neighbors (1R3Co) and one interstitial with four Co nearest neighbors (4Co). R or Co atoms within each type of sites are locally equivalent. 2R2Co site has the largest interstitial hole size and the 4Co site has the smallest [23,24]. However, the maximum hydrogen content for these RCo2 based compounds [25–28] under normal temperature and pressure conditions corresponds to x = 3–4, much less than 17. The observed H site occupancy satisfies the empirical geometric criteria proposed by Westlake of minimum hole size of radius ˚ and minimum H–H distance of about 2.1 A. ˚ Therefore, 0.4 A, occupation of these sites is limited through the nearest neighbor distances between two sites available for hydrogen, so that tetrahedral sites with shared faces cannot be occupied by hydrogen atoms at the same time [29]. In addition, pressure–composition (PC) isotherms of HoCo2 –H system [27,28] indicate these compounds exhibit very lower plateau pressures and the hydrides are very stable nature at room temperature and 1 atm pressure. The complete desorption cannot possible under ambient conditions. Further, the X-ray diffraction (XRD) patterns of HoCo2 –H showed that the hydrogen absorption leads considerable disorder in the cubic structure and poor crystallinity. The hydrogen absorption results in an expansion of cell volume to the extent of ∼20–25% [30]. Recently, we have studied the hydriding and dehydriding properties of Ho1−x Mmx Co2 alloys, where Mm = mischmetal, a natural mixture of the light rare earth metals, 50% Ce, 35% La, 8% Pr, 5% Nd and 1.5% other rare earth elements and 0.5% Fe [30,31]. It is found that with increasing Mm content, the plateau pressure further decreased and the hydrides stability increased. The complete hydrogen desorption temperature too increased. Whereas Ramesh and Rama Rao [28] reported that the considerable improvement in the plateau pressure and crystallinity of Zr substituted HoCo2 –hydrogen system. This increase in the plateau pressure with increasing Zr concentration in Zr1−x Hox Co2 alloys is attributed to the decrease in the unit cell volume thereby decreasing interstitial hole size, because of the smaller radius of the Zr than that of Ho. In this present study, the influence of Ti on the structural and hydrogen absorption–desorption properties of Ho1−x Tix Co2 (x = 0–0.6) alloys have been investigated. The crystal structure of different hydride phases, plateau pressure and hydrogen storage capacity have been discussed. In addition, the differential scanning calorimetry (DSC) and thermogravimetry (TG) of Ho1−x Tix Co2 -hydrides have been studied in order to understand the preferential occupation of interstitial sites and complete desorption temperature of hydrogen from interstitials. 2. Experimental details The Ho1−x Tix Co2 alloys were prepared by arc melting of stoichiometric amounts of constituting elements Ho, Ti and Co with purity better than 99.9% under a protective argon atmosphere. Six weight percent excess of Ho and Ti were taken in order to prevent the formation of Co-rich phases. The ingots were melted several times to ensure homogeneity. Samples thus obtained were sealed in an evacuated quartz tube and homogenized at 850 ◦ C for 4 days. The structural

characterization of the Ho1−x Mmx Co2 were carried out by X-ray diffraction (X’pert PRO, PANalytical diffractometer) using the Cu K␣ radiation. Hydrogen absorption–desorption pressure–composition isotherms of Ho1−x Tix Co2 alloys were performed using Sieverts-type apparatus in the pressure range 0.001–1 bar and temperature range 50–200 ◦ C. The samples were activated by repeating hydrogen absorption and desorption cycles: (1) the absorption measurements have been carried out at a constant initial hydrogen pressure of 1 bar and temperature, at 100 ◦ C, (2) desorption carried out in a vacuum and increasing the temperature up to 800 ◦ C. A controlled amount of hydrogen gas has been admitted into the reaction chamber that holds the specimen and the pressure change has been monitored while maintaining constant temperature of the reaction chamber. The amount of hydrogen absorbed by the specimen is determined by calculating the amount of pressure change after the reaction. The DSC (NETZSCH DSC 200 PC) measurements have been carried out in the temperature range 30–450 ◦ C with a constant heating rate of 10 ◦ C/min. A 20–50 mg sample, sealed in an aluminum pan with an aluminum lid of similar diameter using a standard press, was scanned under a constant flow of argon in the calorimeter against a reference pan sealed with the lid. The data were collected by simultaneously deducing a predetermined base line from the scan programmed through a computer. TG measurements (Perkin-Elmer, TGA 6) were csarried out under a protective argon atmosphere from room temperature to 500 ◦ C with a constant heating rate of 10 ◦ C/min. 3. Results and discussion 3.1. Crystal structure and lattice constant XRD patterns of homogenized Ho1−x Tix Co2 (x = 0–0.6) alloys are shown in Fig. 1. The alloys with x = 0 [30], 0.1 and 0.2 ¯ and the exhibit a single C15 Laves structure (space group Fd 3m) alloys with x ≥ 0.3 show the formation of secondary Laves structure. The appearance of new reflections have been indexed based on the C36 hexagonal Laves (space group P63 /mmc) structure [32,33]. The amount of C36 hexagonal phase increases with the increasing Ti content. The calculated lattice parameters for each phase are given in Table 1. 3.2. PC isotherms The variations of the hydrogen absorption–desorption PC isotherms of Ho1−x Tix Co2 alloys for x = 0, 0.2, 0.4 and 0.6 are shown in Fig. 2(a–d). The closed and open symbols represent the absorption and desorption process, respectively. Each experimental data point in the plots represents a balance between hydrogen pressure and hydrogen content in the alloy. On increasing hydrogen pressure, the interaction between the alloy and the hydrogen results in the system exhibit three different phase regions, in which a low concentration phase of hydrogensaturated alloy called the ␣-phase (the first sloping region, extends up to hydrogen concentration, y = 0.2–0.6), a high concentration hydride phase called the ␤-phase and a coexistence of these two ((␣ + ␤)-phase) phase region, called plateau region. In

0.166 – 0.316 – 0.245 – 0.169

wt.%

1.238 1.199 1.126 1.034 0.907 0.816 0.663

0.47 – 0.824 – 0.58 – 0.36

H/f.u.

4.350(7) 4.350(1) 4.349(8) 4.337(1) 7.1781(2) [30] 7.155(8) 7.151(1) 7.151(5) 7.148(4) 7.146(5) 7.146(6) HoCo2 Ho0.9 Ti0.1 Co2 Ho0.8 Ti0.2 Co2 Ho0.7 Ti0.3 Co2 Ho0.6 Ti0.4 Co2 Ho0.5 Ti0.5 Co2 Ho0.4 Ti0.6 Co2

a

17.066(6) 17.097(4) 17.040(3) 17.070(3)

7.70(2) 7.64(1) 7.66(3) 7.62(5) 7.59(6) 7.57(8) 7.51(6)

4.348(3) 4.350(2) 4.347(4) 4.355(3)

17.08(6) 17.06(5) 17.07(2) 17.03(2)

3.50 3.25 2.92 2.56 2.14 1.83 1.41 23.4 (y = 3.6) 21.7 (y = 3.3) 22.9 (y = 3.0) 21.0 (y = 2.6) 19.7 (y = 2.2) 18.9 (y = 1.9) 16.1 (y = 1.5)

wt.% H/f.u. Volume expansion (V/V%) c a

MgNi2 MgCu2 MgNi2

a

MgCu2

c

a

Hydrides Alloys

295

1.222 1.183 1.126 1.040 0.913 0.824 0.657

Desorbed capacity (wt.%) Absorbed capacity (50 ◦ C)

P–C isotherms ˚ Lattice parameters (A) Composition

Table 1 The lattice parameters, volume expansion at maximum hydrogen concentrations and the absorption and desorption capacities of Ho1−x Tix Co2 –hydrogen system

Desorbed capacity (50◦ C)

TG

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Fig. 1. XRD patterns of Ho1−x Tix Co2 (x = 0–0.6) alloys.

the ␤-phase, the isotherms display a continuous (steep) increase of pressure, indicating solid solution behavior in hydride phase. The width of these phase regions become shortens with increasing Ti substitution. The variations in plateau pressure and great reduction in hydrogen storage capacity of Ho1−x Tix Co2 with increasing Ti concentration can be interpreted on the basis of change in lattice parameters [15,34] and the formation of secondary hexagonal Laves phase in the alloys [35]. Further, it is assumed due to the affinity of Ti with H is lesser when compared with that of rare earths [36]. The desorption isotherms of hydrides reveal that the amount of desorbed hydrogen is very less when compared to that of absorbed amount. For example, in the case of HoCo2 –H, the amount of desorbed hydrogen is about 1.55 H/f.u. even the pressure decreases from 1 to 0.001 bar at 200 ◦ C, but the absorption capacity is 2.3 and 3.5 H/f.u. at 200 and 50 ◦ C, respectively, with maximum of 1 bar. This indicates that the hydride phase is rather stable, so that absorbed hydrogen cannot be fully released at present experimental conditions. The maximum hydrogen absorption (at 50 ◦ C, 1 bar) and desorption capacity (at 50 ◦ C, 0.001 bar) is given in Table 1. According to Fig. 2, the maximum of hydrogen absorption content (H/f.u.) increases with the decrease in temperature and it appears in the reverse trend for desorpton capacity. The complete desorption temperature can be estimated from the thermal analyses of the hydrides. 3.3. Structural behavior of hydrides In order to understand the formation of different hydride phases and the contribution of each hydride phase to struc-

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Fig. 2. Hydrogen absorption–desorption PC isotherms of: (a) HoCo2 , (b) Ho0.8 Ti0.2 Co2 , (c) Ho0.6 Ti0.4 Co2 and (d) Ho0.4 Ti0.6 Co2 ; carried out in the pressure range 0.001–1 bar and temperature range 50–200 ◦ C. The closed and open symbols represent absorption and desorption process, respectively.

tural property of Ho1−x Tix Co2 alloys, the XRD of hydrogenated Ho1−x Tix Co2 alloys with various hydrogen concentrations have been investigated. The hydrides of Ho1−x Tix Co2 , with different concentrations have been prepared by controlling the sample temperature and pressure in the range 0.01–1 bar. For example, Fig. 3 shows XRD patterns of the parent [30], the dehydrogenated (hydrogenated and then dehydrogenated) and hydrogenated with concentrations in the ␣-phase (y = 0.6 H/(f.u.)), (␣ + ␤)-phase (y = 1.7 H/(f.u.)) and ␤-phase (y = 3.1 H/(f.u.)) regions of HoCo2 alloys at room temperature. The dehydrogenated sample resembles the parent alloy structure without a shift in the peak positions. A single-phase C15 MgCu2 -type diffraction pattern is observed for y = 0.6 and 3.1. For y = 1.7, the diffraction lines split into two sets of identical lines, representing the coexistence of the two hydride phases. One phase has a lattice parameter close to that of unhydrogenated HoCo2 alloys (i.e., ␣-phase) and the other phase corresponds to hydride phase of HoCo2 , (␤-phase). The appearance of new set of diffraction lines with large shift towards the lower angle side represents large lattice expansion, due to the growth of ␤phase at the expense of ␣-phase. It is clear that in the ␣-phase region, e.g., y ≈ 0.6, the absorbed hydrogen causes a relatively small lattice expansion, whereas in the ␤-phase (y = 3.1), the

large shift of diffraction lines towards the smaller angle side, indicates the expansion of the lattice by about 21 vol.%. Fig. 4 shows the XRD patterns of Ho1−x Tix Co2 –H for x = 0–0.6, with maximum hydrogen concentrations at room temperature. The

Fig. 3. XRD patterns of HoCo2 and dehydrogenated and with different hydrogen concentrations of HoCo2 –Hy .

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297

Fig. 4. XRD patterns of Ho1−x Tix Co2 –H for x = 0–0.6, hydrogenated at 1 bar and 100 ◦ C.

lattice parameters and the corresponding volume expansion of Ho1−x Tix Co2 hydrides with respect to their hydrogen concentration are given in Table 1. It is clear that the considerable volume expansion occurs in the C15 MgCu2 -type Laves structure of hydrides and almost there are no changes in the lattice parameters belonging to C36 MgNi2 -type Laves structure of hydrides.

Fig. 5. DSC plots for hydrogen desorption of Ho1−x Tix Co2 –H (x = 0–0.6); on heating from room temperature to 450 ◦ C at the rate of 10 ◦ C/min (x = 0 [31]).

3.4. DSC and TG analyses As mentioned above, the desorption PC isotherms (Fig. 2) reveal that a little amount of hydrogen can be released from the Ho1−x Tix Co2 –H even when the pressure is reduced to 0.001 bar at 200 ◦ C. In order to further understand the hydrogen desorption process, the conditions under which hydrogen could be fully released, the DSC and TG measurements were performed. Figs. 5 and 6 represent DSC and TG results for Ho1−x Tix Co2 alloy hydrides, which had been hydrided at 100 ◦ C and 1 bar. Upon heating, the DSC curves show two endothermic peaks 1 and 2 at different temperatures about 175–220 and 300 ◦ C, respectively. The separate endothermic peaks suggest that hydrogen desorption is taking place from two different types of interstitial sites, which are having different binding energies. The AB2 -type Laves phase alloy hydrides are generally formed with hydrogen occupying tetrahedral interstitial sites. It is found that the most of the hydrogen atom occupation takes place with (2A2B)-sites at lower hydrogen concentrations and (1A3B)-sites starts filling at higher hydrogen concentrations [37]. Since A (i.e., rare earths) atoms have a large negative heat of mixing with hydrogen, the hydrogen atoms can stay

Fig. 6. TG plots for hydrogen desorption of Ho1−x Tix Co2 –H (x = 0–0.6); on heating from room temperature to 500 ◦ C at the rate of 10 ◦ C/min (x = 0 [31]).

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more stable in the (2A2B)-sites than that of (1A3B)-sites [38–40]. Therefore, it is suggested that the first and the second endothermic peaks are due to hydrogen desorption from the tetrahedral sites surrounded by [1(Ho,Ti)3Co]-sites and those by [2(Ho,Ti)2Co]-sites, respectively. In general, the peak position corresponds to the intensity of hydrogen released from the interstitial sites and the peak area presents the amount of desorbed hydrogen. In the DSC plots, even though the area of the endothermic peak represents the amount of desorbed hydrogen, in this experiment it is difficult to estimate the desorbed amount of hydrogen from the area of the peak, because the DSC tests have been carried out in the sealed aluminum pan. Therefore, the desorbed hydrogen at lower temperatures prevent the amount of hydrogen at higher temperatures, i.e., second endothermic peak. The variations in the endothermic peaks are attributed to the small deformation of the sealed aluminum pans due to the evaluation of hydrogen during DSC run. The shift in the first endothermic peak position towards lower temperature side with increasing Ti substitution represents the more unstable nature of (1A3B)-sites than that of (2A2B)-sites. The TG analysis results of Ho1−x Tix Co2 –H (x = 0–0.6) show the most of the hydrogen desorption taking place at about 150–300 ◦ C (Fig. 6). It is in good agreement with the results of DSC experiments. When the temperature is over 500 ◦ C, no decrease in mass of the samples is observed and the sample shows almost a stable weight. The desorption capacity obtained from the weight loss of hydride at the terminal reaction temperature is in good agreement with that of the capacity calculated from the absorption PC isotherms at 50 ◦ C under the pressure of 1 bar (Table 1). TG results indicate that the substitution of Ti does not change the desorption temperature; but it drastically reduces the storage capacity and the total absorbed hydrogen could be released thoroughly when the Ho1−x Tix Co2 –H is heated to about 500 ◦ C. 4. Conclusion Ho1−x Tix Co2 alloys crystallize in single C15 cubic Laves phase structure for x < 0.3 and secondary C36 hexagonal Laves phase structure exists for x ≥ 0.3. Three different hydride phases of Ho1−x Tix Co2 –H have been distinguished from the difference in lattice constants of hydrides and changes in the slopes of PC isotherms. The hydrogen absorption capacity greatly decreased with increasing Ti content and there is no appreciable increase in hydrogen absorption–desorption plateau. The DSC results reveal that the hydrogen atoms occupy the two [2(Ho,Ti)2Co] and [1(Ho,Ti)3Co] tetrahedral interstitial sites and indicate that [2(Ho,Ti)2Co]-sites are more stable than the [(Ho,Ti)3Co]sites. The TG analysis confirms the large decrease in hydrogen capacity with increasing Ti content and also shows that a temperature more than 500 ◦ C is necessary for complete desorption of hydrogen. Acknowledgements The authors gratefully acknowledge the DRDO and DST for supporting this work. One of the authors (G. Srinivas) is grateful to IIT Madras for the financial support.

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