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Hydrogen sorption properties of an Mg + WO3 mixture made by reactive mechanical alloying
Journal of Alloys and Compounds 366 (2004) 303–308
Hydrogen sorption properties of an Mg + WO3 mixture made by reactive mechanical alloying Facundo J. Castro a , Jean-Louis Bobet b,∗ a
Centro Atómico Bariloche, Comisión Nacional de Energ´ıa Atómica (CNEA) and Instituto Balseiro (UNC–CNEA), R8402AGP S.C. de Bariloche, Argentina b Institut de Chimie de la Matière Condenseé de Bordeaux (ICMCB), CNRS (UPR 9048), Université de Bordeaux I, 87 Avenue du Dr. A. Schweitzer, 33608 Pessac Cedex, France Received 18 July 2003; accepted 29 July 2003
Abstract The hydrogen sorption properties of a mixture of Mg + 10 wt.% WO3 made by reactive mechanical grinding at room temperature under 1100 kPa of H2 were studied and compared with those of elemental Mg subjected to a similar preparation procedure. The milling products were also characterized by XRD, EPMA, and granulometric measurements. It was observed that WO3 has an important catalytic effect on hydrogen absorption and desorption, more than doubling the rates observed in the samples milled without the additive. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage material; Reactive milling; Magnesium hydride; Kinetics
1. Introduction Magnesium is one of the most promising metals for hydrogen storage due mainly to its low weight and high H-storage capacity, its abundance on the earth’s crust, and its low cost. However, its main drawback lies in its kinetics. To achieve a convenient absorption and desorption kinetics, Mg and Mg hydride have to be heated up to more than 300 ◦ C. Several options have been explored recently to enhance hydrogen sorption of Mg. Many of them use mechanical alloying to intimately incorporate different additives. Liang et al. [1] have shown that the addition of different transition metal elements have a positive effect on hydrogen sorption properties. More recently, several authors have reported that the addition of metal oxides also produces a catalytic effect on hydrogen absorption and desorption from Mg [2–8]. Wang et al. [2] have shown that TiO2 incorporated by ball milling improves the hydrogenation of Mg, and also that this process improves the resistance of the material towards oxidation. Oelerich et al. [3,4] and Dehouche et al. [5] have explored the effects of the addition to MgH2 of Sc2 O3 , TiO2 , V2 O5 , Cr2 O3 , Mn2 O3 , Fe3 O4 , CuO, Al2 O3 , and SiO2 . They ob∗
Corresponding author. E-mail address: [email protected] (J.-L. Bobet).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00747-3
served an important catalytic effect on hydriding and dehydriding, in particular for the oxides of the transition metals that present several valences. For the specific case of Cr2 O3 , they have also studied the long-term cycling stability of the catalytic effect of the oxide [5] and found that after 1000 cycles, absorption kinetics do not deteriorate, but desorption kinetics show a systematic slow-down of the rate upon cycling. They have also reported that after cycling, the XRD patterns show traces of MgO and Cr instead of Cr2 O3 . Bobet et al. [6,8] and Song et al. [7] have studied the kinetics of Mg mechanically milled with Cr2 O3 , Al2 O3 , and CeO2 . They have observed that multivalence metals induce an important catalytic effect in hydrogen sorption, and also reported that upon cyclically absorbing and desorbing hydrogen, Cr2 O3 reduces to Cr. In this work we present an analysis of the effect of the addition of WO3 to Mg by mechanical milling under H2 atmosphere. The interest for selecting this particular oxide lies, on one hand, in the fact that W is a multivalence metal, and as such it is expected that the addition of the oxide WO3 might produce an improvement in hydrogen sorption. On the other hand, the possibility of the formation of a hydrogen tungsten bronze during milling could provide special paths to hydrogen mobility, resulting in an additional benefit for the hydrogen sorption properties of Mg.
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2. Experimental Mg powder (99.8, Strem Chemicals) and a mixture composed of Mg powder and 10 wt.% hexagonal WO3 (99.9994, Strem Chemicals) were mechanically milled at room temperature under 1100 kPa of H2 pressure up to 16 h. The milling was performed in a Fritsch-Pulverisette P5 apparatus at 200 rpm, with a ball-to-powder ratio equal to 9.8. The milling schedule was 15 min of milling followed by 5 min of rest. Every 30 min the milling chamber was refilled with H2 to ensure a constant pressure. At regular intervals a small amount of powder was removed from the container to follow the evolution of the mixture by XRD (Philips PW 1050 diffractometer using Bragg–Brentano geometry and Cu K␣ radiation), EPMA (CAMECA SX-100 Instrument) and granulometric measurements in liquid media (Malvern Mastersizer Hydro 2000S). For selected samples we also studied the kinetics of absorption and desorption of H2 by performing several hydriding and dehydriding cycles at temperatures ranging from 100 to 330 ◦ C in a Hera volumetric apparatus.
3. Results and discussion In Fig. 1, we present the XRD characterization of the Mg+ WO3 mixture for different milling times (mt). After 1 h mt, only the reflections of Mg (JCPDS Powder Diffraction Card
Fig. 1. XRD patterns of the Mg + WO3 mixture as a function of milling time.
No. 35-0821), hexagonal WO3 (JCPDS Powder Diffraction Card No. 85-2460), and Al (JCPDS Powder Diffraction Card No. 04-0787) from the sample holder can be seen. After 2 h mt, small peaks of tetragonal MgH2 (JCPDS Powder Diffraction Card No. 12-0697) start to appear. After 4 h mt, the reflections of MgH2 grow, and the peaks of Mg start to decrease in intensity, partly due to its consumption to form Mg hydride, and partly due to its amorphization and the introduction of micro-strains, a consequence of the milling process. At 8 h mt, a significant broadening of the reflection peaks can be seen, particularly for MgH2 and Mg. Also, at this milling time, reflections of the orthorhombic phase of MgH2 (i.e. ␥-MgH2 ; JCPDS Powder Diffraction Card No. 35-1184) can be seen [9]. With further milling up to 16 h, the diffraction pattern shows mainly the peaks associated with tetragonal MgH2 , together with the reflections of WO3 and a small signal corresponding to orthorhombic MgH2 . At this milling time, Mg peaks cannot be seen. The observation of the reflections of WO3 up to 16 h seems to indicate that Mg does not reduce this oxide during milling. An estimation of the amount of hydrided Mg as a function of mt was made by means of a whole pattern fitting procedure using the program Fullprof [10] (Fig. 2). It can be seen that the fraction of MgH2 obtained from Rietveld refinement grows almost linearly from 2 to 8 h of milling. After that, it reaches an approximately stationary value around 76%. This value can be compared with the amount of hydride obtained by Huot et al. [11] by milling Mg + 5 at.% V + graphite at 573 K, which goes from 64 wt.% after 30 min of milling to almost the completion after 1 h.
Fig. 2. Mg hydrided fraction determined from Rietveld refinement of XRD data.
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The crystallite size of the MgH2 phase was estimated from XRD data using the Scherrer equation. A decrease in the crystallite size as milling proceeds is observed, in particular the most important decrease takes place between 4 and 8 h mt, when the crystallite size changes from 19 to 12 nm. After this last milling time, the crystallite size changes only slightly (e.g. after 16 h mt, the crystallite size is approximately 10 nm). The milled powders were also characterized by EPMA. Before the analysis, the samples were made into pellets, included in resin and polished. Fig. 3 shows the evolution of the intermixing of the components as a function of milling time. Once again, the main changes occur during the first 8 h of milling. For shorter milling times (1 h), WO3 locates mostly outside the Mg agglomerates (characteristic size around 200 m). After 2 h mt, the average size of the Mg agglomerates decreases significantly with the consequent improvement in intermixing. This process goes on up to 8 h mt, when a good intermixing of Mg and WO3 can be appreciated. After this mt, the distribution of Mg and WO3 does not change significantly. The size distribution of the milled powders as a function of mt, obtained by means of granulometric measurements, is in agreement with the information provided by the other characterization techniques (Fig. 4). The size of the agglomerates diminishes appreciably up to 8 h mt and then reaches a stationary value. The median of the distribution changes from 120 m after 1 h mt to 20 m after 2 h, and to 4 m after 4 h mt. The samples milled for 8, 12, and 16 h present similar median values, approximately equal to 0.5 m. Taking into account the information provided by XRD, EPMA and granulometry, the samples milled for 4, 8, and 16 h were selected to study their hydrogen sorption properties. Hydriding was performed at temperatures in the range 100–300 ◦ C, and under an H2 pressure of 1100 kPa; dehydriding was done in the range 280–330 ◦ C, and under 40 kPa of H2 . Typical absorption and desorption curves for the Mg + WO3 mixture milled for 8 h are presented in Fig. 5. As general remarks we observed that no activation of the samples was required, a good capacity was reached (on average 85% of the theoretical value of 6.9 H2 wt.%), and no significant degradation of the sorption characteristics were observed up to 15 cycles. In the case of absorption at 300 ◦ C and desorption at 330 ◦ C only 10 min are needed to reach full capacity or to completely desorb the sample. A quantitative comparison of the kinetics of absorption and desorption as a function of milling time is presented in Fig. 6. There the absorption and desorption rates calculated as the ratio between the amount absorbed (desorbed) and the time taken to go from 20 to 80% of full capacity are presented (similar results were obtained for the initial rate, i.e. between 0 and 20% of full capacity). The absorption rate improves as milling proceeds, the difference being more important between the samples milled for 4 and 8 h than between the samples milled for 8 and
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Fig. 3. EPMA images showing the evolution of Mg and WO3 intermixing with milling time (Mg characteristic radiation at the left, W at the right).
16 h. This is in agreement with the evolution of the samples already discussed. However, it is interesting to note that in the desorption case an inversion in the rates of the samples milled for 8 and 16 h is observed, though the difference between the rates is not very important (it is of the order of accuracy). Finally, for the sample milled for 8 h, the rates
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Fig. 4. Size distributions of the Mg + WO3 powders for different milling times.
calculated for the first cycle and after 15 cycles at 300 ◦ C for absorption and 330 ◦ C for desorption were the same. Taking all this into account, and to determine the effect of WO3 on the sorption properties of Mg, elemental Mg was milled under similar conditions and after 8 h mt, its hydrogen absorption and desorption properties were studied. The absorption and desorption curves are compared with those of the Mg + WO3 mixture in Fig. 7. It can be seen that WO3 has an important effect on the hydrogen sorption properties of Mg, substantially accelerating the absorption and desorption processes. This is not a surprise, because as it was mentioned before, several oxides of multivalence transition metals also produce a catalytic effect on hydrogen sorption [2–8]. The comparison of the absorption and desorption rates is presented in Fig. 8. It can be seen that the effect of the addition of WO3 is at least to double the absorption and desorption rates (see Fig. 8, insets). Moreover, the effect is more pronounced at lower temperatures. For
absorption at 200 ◦ C the ratio between the rates is almost 8, and for desorption at 300 ◦ C the ratio is almost 9. These results can be compared with the results obtained by Oelerich et al. [3]. When desorbing at 300 ◦ C we obtain a desorption rate of 0.4 × 10−2 wt.%/s, similar to the result obtained by them for nanocrystalline Mg after 120 h of milling. With the addition of different oxides, and after milling for 120 h, also in a P5 apparatus, they reach desorption rates ranging from (1.5 to 3) × 10−2 wt.%/s. This fact suggests that although it seemed that the main changes induced by milling take place during the first 8 milling hours, the evolution of the microstructure could go on by further milling and this could result in even better sorption properties. After 10 hydrogen absorption and desorption cycles on a sample milled for 16 h, XRD measurements on a hydrided sample showed only the reflections of tetragonal MgH2 , some reflections of amorphized MgO (JCPDS Powder Diffraction Card No. 45-0946) and peaks of Al from
Fig. 5. Kinetics of hydrogen absorption and desorption at different temperatures for the Mg + WO3 sample milled for 8 h.
Fig. 6. Absorption and desorption rates for the Mg + WO3 mixture as a function of temperature for different milling times.
F.J. Castro, J.-L. Bobet / Journal of Alloys and Compounds 366 (2004) 303–308
Fig. 7. Kinetics of hydrogen absorption and desorption at different temperatures for Mg (open symbols) and Mg+WO3 mixture (filled symbols), both milled for 8 h.
the sample holder. No reflections corresponding to W or W compounds can be seen, although the presence of MgO could indicate that some WO3 has been reduced by Mg. However, it should be taken into account that no apprecia-
Fig. 8. Absorption and desorption rates as a function of temperature for Mg (open symbols) and Mg + WO3 (filled symbols), both samples milled for 8 h. The insets show the ratio of the rates as a function of temperature.
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ble deterioration of the catalytic effect induced by WO3 has been observed after this cycling. Up to now it is unclear what is the origin of the catalytic effect induced by the oxides on the hydrogen sorption properties of Mg. In this particular case, a possible explanation for the improvement of kinetics could be the formation of a tungsten bronze during milling. If this were the case, the structural characteristics of this compound could provide favorable paths for enhancing hydrogen mobility within the matrix. During the milling of the Mg + WO3 mixture, no evidence of the formation of a tungsten bronze has been observed by XRD. However, to further clarify this point we have milled pure WO3 in the same conditions used when milling the Mg + WO3 mixture but using both H2 and Ar atmospheres. During these milling processes the analysis by XRD showed no reflections corresponding to a tungsten bronze, but an interesting phenomena was observed. After approximately 1 h mt, part of the originally hexagonal WO3 transformed to the monoclinic phase of WO3 (JCPDS Powder Diffraction Card No. 43-1035) (Fig. 9). With further milling up to 4 h, the proportion of monoclinic WO3 increases, and after that, part of the monoclinic phase retransforms to the hexagonal phase. This evolution can be followed observing (Fig. 9) the decrease and posterior increase with milling time of the (1 0 0), (2 0 0), and (2 0 2) reflections of the hexagonal phase; the growth of the (1 1 2) and (2 2 2) reflections of the monoclinic phase; and the per-
Fig. 9. XRD patterns of WO3 milled at different times under H2 atmosphere. The evolution of the hexagonal phase can be followed by the characteristic (1 0 0), (2 0 0) and (2 0 2) reflections indicated in the figure.
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manence of the peaks where the reflections of both phases cannot be resolved, (1 1 2) and (2 0 2), and (2 2 2) and (4 2 0) of the hexagonal and monoclinic phases, respectively. In the literature it is reported that the hexagonal phase of WO3 transforms exothermally and irreversibly to the monoclinic phase of WO3 upon heating [12], but to our knowledge there is no information concerning the phase evolution of WO3 during mechanical milling. Up to now it is not clear to us why this transformation and retransformation occurs, but this is the subject of current research. Returning to the discussion on the origin of the catalytic effect induced by WO3 , we should take into account the steps that limit absorption and desorption kinetics [13]. In the hydriding case, two processes seem to control absorption: the dissociation of the H2 molecule on the Mg surface, and hydrogen diffusion through a growing hydride layer. In the dehydriding case, nucleation seems to be the rate-controlling process. To improve absorption and desorption kinetics, the addition of WO3 should have an effect in these processes. On the one hand, the mixed Mg–WO3 surface contains an important density of defects introduced by the milling process that could improve the dissociation of the H2 molecules [14]. On the other hand, another possible favorable milling effect is the production of an intimate mixture of Mg and WO3 . The presence of dispersed WO3 agglomerates within the matrix can provide special paths for hydrogen diffusion through the hydride layer and also supply nucleation centers to enhance dehydriding.
4. Conclusions By reactive mechanical alloying a mixture of Mg + 10 wt.% WO3 powders at room temperature under 1100 kPa of H2 up to 16 h the hydrogen sorption properties of Mg were greatly enhanced. The samples were hydrided and dehydrided without activation, presented good capacity (on an average 85% of the theoretical value) and showed no deterioration after 15 cycles. Also, an important improvement in absorption and desorption rates was observed. For example, the absorption and desorption rates for the mixture milled
for 8 h increased in a factor between 2 and 8 compared with the rates of elemental Mg subjected to a similar treatment. Although the improvement in the hydrogen sorption properties of Mg induced by WO3 agrees with previously reported effects produced by other oxides, the cause of the improvement remains unclear. The presence of a tungsten bronze in the mixture was discarded as the origin of the effect because no evidence was found of the formation of a tungsten bronze during the milling of the Mg + WO3 mixture under H2 , or during the milling of pure WO3 under H2 or Ar. Other possible explanations for the catalytic effect could be an increase in the dissociation probability of the H2 molecule on the Mg–WO3 surface, the existence of favorable paths for hydrogen mobility within the bulk, and the presence of additional nucleation sites—these last two effects being a consequence of the good intermixing between Mg and WO3 and the high density of defects produced by the milling process. References [1] G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloys Comp. 292 (1999) 247. [2] P. Wang, A.M. Wang, H.F. Zhang, B.Z. Ding, Z.Q. Hu, J. Alloys Comp. 313 (2000) 218. [3] W. Oelerich, T. Klassen, R. Bormann, J. Alloys Comp. 315 (2001) 237. [4] W. Oelerich, T. Klassen, R. Bormann, Adv. Eng. Mater. 3 (2001) 487. [5] Z. Dehouche, T. Klassen, W. Oelerich, J. Goyette, T.K. Bose, R. Schulz, J. Alloys Comp. 347 (2002) 319. [6] J.-L. Bobet, B. Chevalier, M.Y. Song, B. Darriet, J. Etourneau, J. Alloys Comp. 336 (2002) 292. [7] M.Y. Song, J.-L. Bobet, B. Darriet, J. Alloys Comp. 340 (2002) 256. [8] J.-L. Bobet, S. Desmoulins-Krawiec, E. Grigorova, F. Cansell, B. Chevalier, J. Alloys Comp. 351 (2003) 217. [9] F.C. Gennari, F.J. Castro, G. Urretavizcaya, J. Alloys Comp. 321 (2001) 46. [10] J. Rodriguez-Carrajal, in: Proceedings of the Satellite Meeting of the 15th Congress of IUCr, Toulouse, France, 1990, p. 127. [11] J. Huot, M.-L. Tremblay, R. Schulz, J. Alloys Comp., in press. [12] M.V. Šušic, Y.M. Solonin, J. Mater. Sci. 23 (1998) 267. [13] M.Y. Song, J. Mater. Sci. 30 (1995) 1343. [14] V. Henrich, Rep. Prog. Phys. 48 (1985) 1481.
Hydrogen sorption properties of an Mg + WO3 mixture made by reactive mechanical alloying Facundo J. Castro a , Jean-Louis Bobet b,∗ a
Centro Atómico Bariloche, Comisión Nacional de Energ´ıa Atómica (CNEA) and Instituto Balseiro (UNC–CNEA), R8402AGP S.C. de Bariloche, Argentina b Institut de Chimie de la Matière Condenseé de Bordeaux (ICMCB), CNRS (UPR 9048), Université de Bordeaux I, 87 Avenue du Dr. A. Schweitzer, 33608 Pessac Cedex, France Received 18 July 2003; accepted 29 July 2003
Abstract The hydrogen sorption properties of a mixture of Mg + 10 wt.% WO3 made by reactive mechanical grinding at room temperature under 1100 kPa of H2 were studied and compared with those of elemental Mg subjected to a similar preparation procedure. The milling products were also characterized by XRD, EPMA, and granulometric measurements. It was observed that WO3 has an important catalytic effect on hydrogen absorption and desorption, more than doubling the rates observed in the samples milled without the additive. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage material; Reactive milling; Magnesium hydride; Kinetics
1. Introduction Magnesium is one of the most promising metals for hydrogen storage due mainly to its low weight and high H-storage capacity, its abundance on the earth’s crust, and its low cost. However, its main drawback lies in its kinetics. To achieve a convenient absorption and desorption kinetics, Mg and Mg hydride have to be heated up to more than 300 ◦ C. Several options have been explored recently to enhance hydrogen sorption of Mg. Many of them use mechanical alloying to intimately incorporate different additives. Liang et al. [1] have shown that the addition of different transition metal elements have a positive effect on hydrogen sorption properties. More recently, several authors have reported that the addition of metal oxides also produces a catalytic effect on hydrogen absorption and desorption from Mg [2–8]. Wang et al. [2] have shown that TiO2 incorporated by ball milling improves the hydrogenation of Mg, and also that this process improves the resistance of the material towards oxidation. Oelerich et al. [3,4] and Dehouche et al. [5] have explored the effects of the addition to MgH2 of Sc2 O3 , TiO2 , V2 O5 , Cr2 O3 , Mn2 O3 , Fe3 O4 , CuO, Al2 O3 , and SiO2 . They ob∗
Corresponding author. E-mail address: [email protected] (J.-L. Bobet).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00747-3
served an important catalytic effect on hydriding and dehydriding, in particular for the oxides of the transition metals that present several valences. For the specific case of Cr2 O3 , they have also studied the long-term cycling stability of the catalytic effect of the oxide [5] and found that after 1000 cycles, absorption kinetics do not deteriorate, but desorption kinetics show a systematic slow-down of the rate upon cycling. They have also reported that after cycling, the XRD patterns show traces of MgO and Cr instead of Cr2 O3 . Bobet et al. [6,8] and Song et al. [7] have studied the kinetics of Mg mechanically milled with Cr2 O3 , Al2 O3 , and CeO2 . They have observed that multivalence metals induce an important catalytic effect in hydrogen sorption, and also reported that upon cyclically absorbing and desorbing hydrogen, Cr2 O3 reduces to Cr. In this work we present an analysis of the effect of the addition of WO3 to Mg by mechanical milling under H2 atmosphere. The interest for selecting this particular oxide lies, on one hand, in the fact that W is a multivalence metal, and as such it is expected that the addition of the oxide WO3 might produce an improvement in hydrogen sorption. On the other hand, the possibility of the formation of a hydrogen tungsten bronze during milling could provide special paths to hydrogen mobility, resulting in an additional benefit for the hydrogen sorption properties of Mg.
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2. Experimental Mg powder (99.8, Strem Chemicals) and a mixture composed of Mg powder and 10 wt.% hexagonal WO3 (99.9994, Strem Chemicals) were mechanically milled at room temperature under 1100 kPa of H2 pressure up to 16 h. The milling was performed in a Fritsch-Pulverisette P5 apparatus at 200 rpm, with a ball-to-powder ratio equal to 9.8. The milling schedule was 15 min of milling followed by 5 min of rest. Every 30 min the milling chamber was refilled with H2 to ensure a constant pressure. At regular intervals a small amount of powder was removed from the container to follow the evolution of the mixture by XRD (Philips PW 1050 diffractometer using Bragg–Brentano geometry and Cu K␣ radiation), EPMA (CAMECA SX-100 Instrument) and granulometric measurements in liquid media (Malvern Mastersizer Hydro 2000S). For selected samples we also studied the kinetics of absorption and desorption of H2 by performing several hydriding and dehydriding cycles at temperatures ranging from 100 to 330 ◦ C in a Hera volumetric apparatus.
3. Results and discussion In Fig. 1, we present the XRD characterization of the Mg+ WO3 mixture for different milling times (mt). After 1 h mt, only the reflections of Mg (JCPDS Powder Diffraction Card
Fig. 1. XRD patterns of the Mg + WO3 mixture as a function of milling time.
No. 35-0821), hexagonal WO3 (JCPDS Powder Diffraction Card No. 85-2460), and Al (JCPDS Powder Diffraction Card No. 04-0787) from the sample holder can be seen. After 2 h mt, small peaks of tetragonal MgH2 (JCPDS Powder Diffraction Card No. 12-0697) start to appear. After 4 h mt, the reflections of MgH2 grow, and the peaks of Mg start to decrease in intensity, partly due to its consumption to form Mg hydride, and partly due to its amorphization and the introduction of micro-strains, a consequence of the milling process. At 8 h mt, a significant broadening of the reflection peaks can be seen, particularly for MgH2 and Mg. Also, at this milling time, reflections of the orthorhombic phase of MgH2 (i.e. ␥-MgH2 ; JCPDS Powder Diffraction Card No. 35-1184) can be seen [9]. With further milling up to 16 h, the diffraction pattern shows mainly the peaks associated with tetragonal MgH2 , together with the reflections of WO3 and a small signal corresponding to orthorhombic MgH2 . At this milling time, Mg peaks cannot be seen. The observation of the reflections of WO3 up to 16 h seems to indicate that Mg does not reduce this oxide during milling. An estimation of the amount of hydrided Mg as a function of mt was made by means of a whole pattern fitting procedure using the program Fullprof [10] (Fig. 2). It can be seen that the fraction of MgH2 obtained from Rietveld refinement grows almost linearly from 2 to 8 h of milling. After that, it reaches an approximately stationary value around 76%. This value can be compared with the amount of hydride obtained by Huot et al. [11] by milling Mg + 5 at.% V + graphite at 573 K, which goes from 64 wt.% after 30 min of milling to almost the completion after 1 h.
Fig. 2. Mg hydrided fraction determined from Rietveld refinement of XRD data.
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The crystallite size of the MgH2 phase was estimated from XRD data using the Scherrer equation. A decrease in the crystallite size as milling proceeds is observed, in particular the most important decrease takes place between 4 and 8 h mt, when the crystallite size changes from 19 to 12 nm. After this last milling time, the crystallite size changes only slightly (e.g. after 16 h mt, the crystallite size is approximately 10 nm). The milled powders were also characterized by EPMA. Before the analysis, the samples were made into pellets, included in resin and polished. Fig. 3 shows the evolution of the intermixing of the components as a function of milling time. Once again, the main changes occur during the first 8 h of milling. For shorter milling times (1 h), WO3 locates mostly outside the Mg agglomerates (characteristic size around 200 m). After 2 h mt, the average size of the Mg agglomerates decreases significantly with the consequent improvement in intermixing. This process goes on up to 8 h mt, when a good intermixing of Mg and WO3 can be appreciated. After this mt, the distribution of Mg and WO3 does not change significantly. The size distribution of the milled powders as a function of mt, obtained by means of granulometric measurements, is in agreement with the information provided by the other characterization techniques (Fig. 4). The size of the agglomerates diminishes appreciably up to 8 h mt and then reaches a stationary value. The median of the distribution changes from 120 m after 1 h mt to 20 m after 2 h, and to 4 m after 4 h mt. The samples milled for 8, 12, and 16 h present similar median values, approximately equal to 0.5 m. Taking into account the information provided by XRD, EPMA and granulometry, the samples milled for 4, 8, and 16 h were selected to study their hydrogen sorption properties. Hydriding was performed at temperatures in the range 100–300 ◦ C, and under an H2 pressure of 1100 kPa; dehydriding was done in the range 280–330 ◦ C, and under 40 kPa of H2 . Typical absorption and desorption curves for the Mg + WO3 mixture milled for 8 h are presented in Fig. 5. As general remarks we observed that no activation of the samples was required, a good capacity was reached (on average 85% of the theoretical value of 6.9 H2 wt.%), and no significant degradation of the sorption characteristics were observed up to 15 cycles. In the case of absorption at 300 ◦ C and desorption at 330 ◦ C only 10 min are needed to reach full capacity or to completely desorb the sample. A quantitative comparison of the kinetics of absorption and desorption as a function of milling time is presented in Fig. 6. There the absorption and desorption rates calculated as the ratio between the amount absorbed (desorbed) and the time taken to go from 20 to 80% of full capacity are presented (similar results were obtained for the initial rate, i.e. between 0 and 20% of full capacity). The absorption rate improves as milling proceeds, the difference being more important between the samples milled for 4 and 8 h than between the samples milled for 8 and
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Fig. 3. EPMA images showing the evolution of Mg and WO3 intermixing with milling time (Mg characteristic radiation at the left, W at the right).
16 h. This is in agreement with the evolution of the samples already discussed. However, it is interesting to note that in the desorption case an inversion in the rates of the samples milled for 8 and 16 h is observed, though the difference between the rates is not very important (it is of the order of accuracy). Finally, for the sample milled for 8 h, the rates
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Fig. 4. Size distributions of the Mg + WO3 powders for different milling times.
calculated for the first cycle and after 15 cycles at 300 ◦ C for absorption and 330 ◦ C for desorption were the same. Taking all this into account, and to determine the effect of WO3 on the sorption properties of Mg, elemental Mg was milled under similar conditions and after 8 h mt, its hydrogen absorption and desorption properties were studied. The absorption and desorption curves are compared with those of the Mg + WO3 mixture in Fig. 7. It can be seen that WO3 has an important effect on the hydrogen sorption properties of Mg, substantially accelerating the absorption and desorption processes. This is not a surprise, because as it was mentioned before, several oxides of multivalence transition metals also produce a catalytic effect on hydrogen sorption [2–8]. The comparison of the absorption and desorption rates is presented in Fig. 8. It can be seen that the effect of the addition of WO3 is at least to double the absorption and desorption rates (see Fig. 8, insets). Moreover, the effect is more pronounced at lower temperatures. For
absorption at 200 ◦ C the ratio between the rates is almost 8, and for desorption at 300 ◦ C the ratio is almost 9. These results can be compared with the results obtained by Oelerich et al. [3]. When desorbing at 300 ◦ C we obtain a desorption rate of 0.4 × 10−2 wt.%/s, similar to the result obtained by them for nanocrystalline Mg after 120 h of milling. With the addition of different oxides, and after milling for 120 h, also in a P5 apparatus, they reach desorption rates ranging from (1.5 to 3) × 10−2 wt.%/s. This fact suggests that although it seemed that the main changes induced by milling take place during the first 8 milling hours, the evolution of the microstructure could go on by further milling and this could result in even better sorption properties. After 10 hydrogen absorption and desorption cycles on a sample milled for 16 h, XRD measurements on a hydrided sample showed only the reflections of tetragonal MgH2 , some reflections of amorphized MgO (JCPDS Powder Diffraction Card No. 45-0946) and peaks of Al from
Fig. 5. Kinetics of hydrogen absorption and desorption at different temperatures for the Mg + WO3 sample milled for 8 h.
Fig. 6. Absorption and desorption rates for the Mg + WO3 mixture as a function of temperature for different milling times.
F.J. Castro, J.-L. Bobet / Journal of Alloys and Compounds 366 (2004) 303–308
Fig. 7. Kinetics of hydrogen absorption and desorption at different temperatures for Mg (open symbols) and Mg+WO3 mixture (filled symbols), both milled for 8 h.
the sample holder. No reflections corresponding to W or W compounds can be seen, although the presence of MgO could indicate that some WO3 has been reduced by Mg. However, it should be taken into account that no apprecia-
Fig. 8. Absorption and desorption rates as a function of temperature for Mg (open symbols) and Mg + WO3 (filled symbols), both samples milled for 8 h. The insets show the ratio of the rates as a function of temperature.
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ble deterioration of the catalytic effect induced by WO3 has been observed after this cycling. Up to now it is unclear what is the origin of the catalytic effect induced by the oxides on the hydrogen sorption properties of Mg. In this particular case, a possible explanation for the improvement of kinetics could be the formation of a tungsten bronze during milling. If this were the case, the structural characteristics of this compound could provide favorable paths for enhancing hydrogen mobility within the matrix. During the milling of the Mg + WO3 mixture, no evidence of the formation of a tungsten bronze has been observed by XRD. However, to further clarify this point we have milled pure WO3 in the same conditions used when milling the Mg + WO3 mixture but using both H2 and Ar atmospheres. During these milling processes the analysis by XRD showed no reflections corresponding to a tungsten bronze, but an interesting phenomena was observed. After approximately 1 h mt, part of the originally hexagonal WO3 transformed to the monoclinic phase of WO3 (JCPDS Powder Diffraction Card No. 43-1035) (Fig. 9). With further milling up to 4 h, the proportion of monoclinic WO3 increases, and after that, part of the monoclinic phase retransforms to the hexagonal phase. This evolution can be followed observing (Fig. 9) the decrease and posterior increase with milling time of the (1 0 0), (2 0 0), and (2 0 2) reflections of the hexagonal phase; the growth of the (1 1 2) and (2 2 2) reflections of the monoclinic phase; and the per-
Fig. 9. XRD patterns of WO3 milled at different times under H2 atmosphere. The evolution of the hexagonal phase can be followed by the characteristic (1 0 0), (2 0 0) and (2 0 2) reflections indicated in the figure.
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manence of the peaks where the reflections of both phases cannot be resolved, (1 1 2) and (2 0 2), and (2 2 2) and (4 2 0) of the hexagonal and monoclinic phases, respectively. In the literature it is reported that the hexagonal phase of WO3 transforms exothermally and irreversibly to the monoclinic phase of WO3 upon heating [12], but to our knowledge there is no information concerning the phase evolution of WO3 during mechanical milling. Up to now it is not clear to us why this transformation and retransformation occurs, but this is the subject of current research. Returning to the discussion on the origin of the catalytic effect induced by WO3 , we should take into account the steps that limit absorption and desorption kinetics [13]. In the hydriding case, two processes seem to control absorption: the dissociation of the H2 molecule on the Mg surface, and hydrogen diffusion through a growing hydride layer. In the dehydriding case, nucleation seems to be the rate-controlling process. To improve absorption and desorption kinetics, the addition of WO3 should have an effect in these processes. On the one hand, the mixed Mg–WO3 surface contains an important density of defects introduced by the milling process that could improve the dissociation of the H2 molecules [14]. On the other hand, another possible favorable milling effect is the production of an intimate mixture of Mg and WO3 . The presence of dispersed WO3 agglomerates within the matrix can provide special paths for hydrogen diffusion through the hydride layer and also supply nucleation centers to enhance dehydriding.
4. Conclusions By reactive mechanical alloying a mixture of Mg + 10 wt.% WO3 powders at room temperature under 1100 kPa of H2 up to 16 h the hydrogen sorption properties of Mg were greatly enhanced. The samples were hydrided and dehydrided without activation, presented good capacity (on an average 85% of the theoretical value) and showed no deterioration after 15 cycles. Also, an important improvement in absorption and desorption rates was observed. For example, the absorption and desorption rates for the mixture milled
for 8 h increased in a factor between 2 and 8 compared with the rates of elemental Mg subjected to a similar treatment. Although the improvement in the hydrogen sorption properties of Mg induced by WO3 agrees with previously reported effects produced by other oxides, the cause of the improvement remains unclear. The presence of a tungsten bronze in the mixture was discarded as the origin of the effect because no evidence was found of the formation of a tungsten bronze during the milling of the Mg + WO3 mixture under H2 , or during the milling of pure WO3 under H2 or Ar. Other possible explanations for the catalytic effect could be an increase in the dissociation probability of the H2 molecule on the Mg–WO3 surface, the existence of favorable paths for hydrogen mobility within the bulk, and the presence of additional nucleation sites—these last two effects being a consequence of the good intermixing between Mg and WO3 and the high density of defects produced by the milling process. References [1] G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloys Comp. 292 (1999) 247. [2] P. Wang, A.M. Wang, H.F. Zhang, B.Z. Ding, Z.Q. Hu, J. Alloys Comp. 313 (2000) 218. [3] W. Oelerich, T. Klassen, R. Bormann, J. Alloys Comp. 315 (2001) 237. [4] W. Oelerich, T. Klassen, R. Bormann, Adv. Eng. Mater. 3 (2001) 487. [5] Z. Dehouche, T. Klassen, W. Oelerich, J. Goyette, T.K. Bose, R. Schulz, J. Alloys Comp. 347 (2002) 319. [6] J.-L. Bobet, B. Chevalier, M.Y. Song, B. Darriet, J. Etourneau, J. Alloys Comp. 336 (2002) 292. [7] M.Y. Song, J.-L. Bobet, B. Darriet, J. Alloys Comp. 340 (2002) 256. [8] J.-L. Bobet, S. Desmoulins-Krawiec, E. Grigorova, F. Cansell, B. Chevalier, J. Alloys Comp. 351 (2003) 217. [9] F.C. Gennari, F.J. Castro, G. Urretavizcaya, J. Alloys Comp. 321 (2001) 46. [10] J. Rodriguez-Carrajal, in: Proceedings of the Satellite Meeting of the 15th Congress of IUCr, Toulouse, France, 1990, p. 127. [11] J. Huot, M.-L. Tremblay, R. Schulz, J. Alloys Comp., in press. [12] M.V. Šušic, Y.M. Solonin, J. Mater. Sci. 23 (1998) 267. [13] M.Y. Song, J. Mater. Sci. 30 (1995) 1343. [14] V. Henrich, Rep. Prog. Phys. 48 (1985) 1481.