Correlation between hydrogen storage properties and textures induced in magnesium through ECAP and cold rolling

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 3 8 1 0 e3 8 2 1

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Correlation between hydrogen storage properties and textures induced in magnesium through ECAP and cold rolling Alberto Moreira Jorge Jr.a,b,c,*, Gisele Ferreira de Lima d, Maria Regina Martins Triques a, Walter Jose´ Botta a, Claudio Shyinti Kiminami a, Ricardo Pereira Nogueira c, Alain Reza Yavari b, Terence G. Langdon e,f a Departamento de Engenharia de Materiais, Universidade Federal de Sa˜o Carlos, Via Washington Luiz, km 235, Sa˜o Carlos 13565-905, SP, Brazil b SiMap Laboratory CNRS, UMR 5266 INPG e UJF, Grenoble, BP 75, 38402 St-Martin d’He`res, France c LEPMI CNRS, UMR 5279 INPG e UJF, Grenoble, BP 75, 38402 Saint Martin d’He`res, France d Engenharia de Materiais Meta´licos, Universidade Federal de Sa˜o Paulo, R. Talim 330, 12231-280 Sa˜o Jose´ dos Campos, SP, Brazil e Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA f Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK

article info

abstract

Article history:

It is feasible to obtain a significant enhancement of the hydrogen storage capability in

Received 17 September 2013

magnesium by selecting an appropriate sequence of mechanical processing. The Mg metal

Received in revised form

may be produced with different textures which will then give significant differences in the

16 December 2013

absorption/desorption kinetics and in the incubation times for hydrogenation. Using pro-

Accepted 24 December 2013

cessing by equal-channel angular pressing (ECAP), different textures may be produced by

Available online 28 January 2014

changing both the numbers of passes through the ECAP die and the ram speed. Significant grain refinement is easily avoided by using commercial coarse-grained magnesium as the

Keywords:

starting material. The use of cold rolling after ECAP further increases the preferential

Hydrogen storage

texture for hydrogenation. The results show that the hydriding properties are enhanced

Mg hydrides

with a (002) texture where the improved kinetics lie mainly in the initial stages of hydro-

Hydrogen-absorbing materials

genation. An incubation time is associated with the presence of a (101) texture and this is

ECAP

probably due to the magnesium oxide stability in this direction.

Cold rolling

Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Departamento de Engenharia de Materiais, Universidade Federal de Sa˜o Carlos, Via Washington Luiz, km 235, Sa˜o Carlos 13565-905, SP, Brazil. Tel.: þ55 16 33519478; fax: þ55 16 33615404. E-mail addresses: [email protected], [email protected], [email protected] (A.M. Jorge). 0360-3199/$ e see front matter Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.12.154

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1.

Introduction

The safe and convenient storage of hydrogen has been a challenge during the past decade. In practice, magnesium alloys are attractive for use in hydrogen storage applications both because of the light weight of these alloys and because their hydrogen storage capacity is greater than all known reversible hydrides [1,2]. Thus, magnesium forms a hydride (MgH2) with a hydrogen capacity of about 7.6 wt.% [1,3]. This feature, combined, with an abundance and low cost, makes magnesium and its alloys ideal candidates for use in commercial applications. Furthermore, hydrogen storage is most effective for use in the solid state rather than in a pressurized or liquefied condition. It is important to note that the practical use of magnesium hydride for storing hydrogen continues to be a challenge because the reactions of absorption and desorption of hydrogen occur only at very high temperatures and are sluggish. Thus, both hydrogenation and dehydrogenation require very high temperatures, at least 350e400  C, but over a very slow time scale of several hours. The kinetics are reduced primarily because the diffusion rate of hydrogen is low within the magnesium hydride [4,5] and there are generally oxide layers on the surfaces which delay or even prevent the penetration of hydrogen [6,7]. Specifically, the formation of the metal hydride can be split, at least, into the following “basic” reactions [4,5,8e10]: (i) Dissociation/adsorption; (ii) Surface penetration; (iii) Bulk diffusion; (iv) Hydride formation. This picture becomes more complicated since the diffusion of hydrogen can also occur through the hydride. It is reasonable to anticipate that a reduction in grain size into nanocrystalline dimensions may lead to a considerable improvement in the diffusion and thermo-dynamical properties in magnesium hydride. In order to overcome these problems, several different processes have been investigated with the objective of producing micro or nanostructured MgH2 [5,7,11e13]. For example, nanostructured Mg or MgH2 may be prepared by means of mechanical milling to improve the kinetic of reactions but without simultaneously reducing the high hydrogen capacity [5,7,13]. In addition, nanocomposites of Mg-based hydrides and catalysts have been prepared by mechanical milling especially in the production of nanocrystalline magnesium [14e17]. High-energy ball milling (HEBM) techniques have been utilized successfully in order to prepare Mg-based nanocomposites and this provides fast Hsorption kinetics at 300  C or even at lower temperatures [1,18e22]. The aforementioned Mg-based systems have revealed important improvements in the properties of hydrogen storing. Nevertheless, the occurrence of surface contamination, the expended time, the potential fire and health risk are all concerns that hinder the further development of processing techniques using powder metallurgy. To overcome these various shortcomings, it is possible to use techniques based on the application of severe plastic deformation (SPD) [23e27]. It is now well established that SPD processing provides a capability for converting conventional coarse-grained metals into materials having ultrafine-grains or even a nanocrystalline structure by using a high hydrostatic pressure and a relatively low deformation temperature

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[28e30]. Furthermore, SPD processing creates defects such as vacancies and dislocations which produce a positive effect on the diffusion kinetics. In practice, the presence of porosity is essentially absent after using SPD techniques although porosity generally improves the diffusion kinetics by creating an easy path for hydrogen penetration in relatively large bulk samples [31,32]. In fact, investigations have suggested that there is an improvement in the diffusion and H2 storage capacity in Mg alloys after processing by SPD due to the presence of excess vacancies. This allows the entrapment of up to six hydrogen atoms per vacancy and, thus, dramatically accelerates the diffusion process [33e37]. Equal-channel angular pressing (ECAP) [29] is an SPD processing technique which has been used extensively for achieving exceptional grain refinement in light metal alloys and, in practice, improvement of the H-kinetics sorption properties and also the structural stability during cycles of absorption/desorption have been reported due to the defect structure [23] and the refined microstructure [23,24]. It also appears that processing by ECAP may produce textures which serve to improve the H-sorption properties [26]. An investigation of the structural and hydrogen storage properties in thin films of nanostructured Mg deposited on Si (001) substrates showed that the conversion of Mg to MgH2 generally follows a martensitic-like orientation relationship with Mg (002)//MgH2 (110) and Mg [120]//MgH2 [111] [38]. Other orientation relationships can be found in other reports ([39] and citations therein). In an early study of the processing of Mg by ECAP, it was established that the basal planes become aligned with the theoretical shearing plane and thus it follows that different directions within the processed samples may exhibit different properties which relate directly to the texture [40]. In earlier work [26] the (002) texture was confirmed as being favorable for the absorption of hydrogen in bulk materials processed by ECAP. Cold rolling (CR), as one SPD processing technique, has been used also [41e43] to produce a reduced grain size and the good hydrogenation properties were also attributed to the presence of a (002) texture. However, the effect of a (002) texture was not clearly confirmed in any of these investigations and there was also no analysis of the effects of other remaining textures. Based on the limitations inherent in the information available to date, the objective of the present investigation was to use ECAP processing followed by CR with commercial coarse-grained magnesium in order to produce different textures in samples with reasonably large grain sizes, to systematically investigate the hydrogen storage properties as a function of such textures and to clarify the correlation between the crystallographic orientation and the hydrogen absorption properties.

2.

Experimental materials and procedures

The experiments were conducted using a commercial coarsegrained magnesium which was supplied in the form of an ingot by Baofull Trading Co. (Liuzhou, China). The measured initial grain size in the ingot was w940 mm. This large grain size was chosen specifically because, based on available evidence [44e46], it is necessary in order to avoid grain

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refinement to the submicrometer level and thereby to maintain a grain size of around tens of microns. The chemical composition of the commercial magnesium (in wt.%) was reported as 0.005 Fe, 0.03 Si, 0.002 Ni, 0.02 Cu, 0.05 Al, 0.005 Cl, 0.06 Mn and 0.2 impurities with the balance as Mg. Billets were machined from the ingot with a square crosssection of 10 mm and lengths of w60 mm for processing by ECAP using a mechanical press having a 60 tons capacity. The samples were pressed at 573 K where this temperature was selected in order to permit the imposition of a large number of passes, to avoid increasing the numbers of defects and to maintain the processed grain sizes within the micron scale. The processing temperature was maintained stable within a range of 2 and the processing was conducted after the die reached the selected temperature within 2 . All billets were held at room temperature until the die attained a stable processing temperature. The processing by ECAP was performed using a solid die having an internal channel angle of F ¼ 110 and an angle at the outer arc of curvature of the two parts of the channel of J ¼ 30 . It is now well established that these angles lead to a strain of w0.8 on each separate passage through the die [47] and in these experiments repetitive pressings were performed using route Bc where the billet is pressed through the die and the sample is rotated by 90 in the same sense between passes [48]. In the present investigation, the billets were processed by ECAP in terms of the numbers of passes and processing speeds and in this way it was possible to achieve a range of possible textures. Specifically, Mg samples were pressed from 1 to 4 passes at pressing speeds of either 3 or 25 mm/min giving a total strain of w3.2 in the fourth pass. Following ECAP, the processed samples were cut in directions perpendicular to the pressing direction to give the cross-sectional plane and then inserted between two AISI 304 stainless steel plates and subjected to cold rolling using a duoreversible FENN conventional rolling facility with a 50% reduction in each pass. Different numbers of passes were used in order to equalize the amount of deformation in the various samples processed by ECAP and to produce the maximum number of different textures. Accordingly, the number of passes in CR was varied from 20 to 30 as shown in Table 1. This procedure gave a final thickness of w150 mm for all conditions even when using different numbers of CR passes. Because the samples processed with 1 and 2 ECAP passes in the condition of 3 mm/min produced similar textures to those

for samples processed at 25 mm/min, detailed analyses were undertaken only on samples processed at 25 mm/min and those processed at 3 mm/min for 3 and 4 passes. For subsequent analysis, pieces of these samples were prepared having mean thicknesses of about 500 mm. Details on the hydrogenation and kinetic measurements of hydrogen absorption were obtained using a Sieverts apparatus with the samples hydrogenated at a temperature of 623 K (350  C) under a hydrogen pressure of 1.5 MPa (15 bar). All samples were ground as in metallographic preparation and then stored in air to analyze the resistance to oxide formation. The desorption analysis of the commercial Mg was performed using a Netzsch Simultaneous Thermal Analyzer (STA) 449 Jupiter calorimeter coupled to quadrupole mass spectrometer (QMS) Aeolos equipment. This instrument takes simultaneous differential scanning calorimetric (DSC) and thermogravimetric (TG) measurements and on-line evolved gas analysis. The hydrogen desorption temperatures were recorded during continuous heating in DSC using purified and dried argon gas in an overflow regime. The buoyancy effect was taken into consideration due to the use of argon as a carrier gas and therefore the necessary background treatment was also undertaken. Phases and orientations were identified using X-ray diffraction (XRD) with monochromatic Cu-Ka radiation having an angular pass of 0.032 in a Rigaku DMAX diffractometer equipped with a C-monochromator. The microstructure was characterized by optical microscopy (OM) and scanning electron microscopy (SEM). The amount of oxygen present on the surface of the samples was measured by Microanalysis (EDAX) in an FEI INSPECT S50, using an acceleration voltage of 5 KV to minimize the effect of the interaction volume and improve the spatial resolution of the analysis. This was performed in several areas to obtain an average value by using magnifications of 1000 times. Since the samples were subjected to the same procedure and put together within the microscope chamber, this procedure will provide a good indication of the oxygen level. As mentioned earlier, the formation of the metal hydride can be split into at least four “basic” reactions [4,5,8e10]. However, this problem is difficult to solve analytically and it is preferable to assume a rate-limiting step. This may be performed by fitting the experimental kinetic data to a rate equation where the limitations due to either hydride nucleation and growth or diffusion are normally favored.

Table 1 e Summary of relative amounts of textures for the three main peaks of a-Mg observed in Figs. 1 and 2 and number of CR passes. ECAP þ CR

ECAP ECAP condition. Passes 3 4 1 2 3 4

CR Passes

Relative amount of textures (%)

Relative amount of textures (%)

Speed (mm/min)

(1 0 0)

(0 0 2)

(1 0 1)

(1 0 0)

(0 0 2)

(1 0 1)

3 3 25 25 25 25

30.51 15.12 11.39 24.80 9.52 16.94

14.12 15.59 10.42 8.40 22.32 15.02

55.37 69.30 78.19 66.80 68.16 68.04

0.98 0.85 0.84 0.47 0.27 4.02

98.29 95.65 93.10 97.05 96.18 87.47

0.73 3.50 6.06 2.48 3.55 8.51

25 25 30 30 20 20

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Generally, the nucleation and growth kinetics is usually explained by the Johnson-Mehl-Avrami (JMA) equation [49e52]: n


xðtÞ ¼ 1  exp  ðktÞ

where x(t) is the transformed fraction, k is the kinetic rate constant (nucleation and growth rates) and t is the reaction time. The value of the Avrami exponent n, which depends on the type of nucleation, the dimensionality of growth and the rate-limiting step of growth can be obtained from the slope of the straight line when plotting ln[ln(1/(1  x(t)))] vs. ln(t).

3.

Experimental results

Figs. 1 and 2 show representative XRD patterns after processing by ECAP (left) and CR (right) for the Mg alloy taken on the cross-sectional plane. Fig. 1 shows results for samples processed at 3 mm/min and Fig. 2 shows results for samples processed at 25 mm/min. These patterns reveal the presence of a-Mg but, when comparing the theoretical and measured relative intensities, it is readily apparent that the a-Mg phase contains preferred orientations after different numbers of ECAP passes and after the subsequent CR. Table 1 summarizes all relative intensities for the three main peaks of a-Mg. From Fig. 1 and Table 1 it is observed that the a-Mg phase has changed preferred orientations between ECAP passes along the pyramidal (101) plane and prismatic (100) which are activated at high temperatures and the basal (002) plane and it is apparent that the (101) plane becomes more pronounced. Also, as observed earlier [48], it is noticeable that the orientation trends are restored every 4 passes using route Bc. For CR samples, the basal (002) orientation is strong and this orientation grows at the expense of all other orientations. In practice, the (002) orientation is the main slip plane for the a-Mg and, as observed earlier [26,53], this is the preferred orientation for hydrogen absorption. Analysis by OM reveals clear evidence for a grain size distribution which is bimodal in character for all conditions. Separate measurements gave average grain sizes of w70 and w42 mm, respectively, for conditions of 3 and 4 ECAP passes with 3 mm/min and w115, w85, w60 and w37 mm,

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respectively, for conditions of 1, 2, 3 and 4 ECAP passes with 25 mm/min Fig. 3 shows OM images taken at the crosssections of the processed Mg alloy for samples processed only by ECAP for conditions of 4 passes at 3 mm/min and 4 passes at 25 mm/min and SEM images for samples processed by ECAP þ CR in the same conditions. Despite the obvious capacity of ECAP to produce significant grain refinement, there is no marked influence of processing speed on the final grain size. After cold rolling, all samples had average grain sizes of w20 mm. Kinetic measurements are presented in Fig. 4 where the hydrogen absorption is plotted against time for (a) samples processed by ECAP and (b) samples processed by ECAP þ CR. These plots compare the first absorption curves between all the processing conditions. In general, by linking Fig. 4a and b with Table 1, examination shows that much higher hydrogen capacities and kinetics and much lower incubation times are attained for samples having larger amounts of (002) preferential texture. These graphs are plotted on the same scales to provide a direct comparison of the capacities between the ECAP and ECAP þ CR samples. As can be observed, the capacities for samples processed by ECAP are much lower than for ECAP þ CR and, as previously reported [26,31,32], this can be promptly correlated with the different sample thicknesses in both conditions which were respectively 500 and 150 mm since the excess of vacancies produced by ECAP processing, when combined with an absence of any residual porosity, is insufficient to create an easy path for any hydrogen penetration [26]. The amount of oxygen measured on the surface of the samples gave average values of w35.0 and w35.9 at. %, respectively, for conditions of 3 and 4 ECAP passes with 3 mm/ min and w34.7, w34.8, w34.9 and w34 at. %, respectively, for conditions of 1, 2, 3 and 4 ECAP passes with 25 mm/min. After cold rolling, the average values were w39.6 and w38.5 at. %, respectively, for conditions of 3 and 4 ECAP passes with 3 mm/ min and w38.8, w39.0, w38.5 and w39.4 at. % %, respectively, for conditions of 1, 2, 3 and 4 ECAP passes with 25 mm/min. These results essentially show that samples with higher amounts of (002) texture present higher levels of oxygen than samples with lower amounts, with average amounts of 34.7 and 38.9 at.%, respectively, for conditions processed by ECAP and by ECAP þ CR. As the incubation times are associated with

Fig. 1 e XRD patterns of cross-section of Mg alloy comparing samples processed by ECAP (left) and ECAP D CR (right) in the condition of 3 mm/min for (a) 3 ECAP passes and 25 CR passes and (b) 4 ECAP passes and 25 CR passes.

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Fig. 2 e XRD patterns of cross-section of Mg alloy comparing samples processed by ECAP (left) and ECAP D CR (right) in the condition of 25 mm/min for (a) 1 ECAP pass and 30 CR passes, (b) 2 ECAP passes and 30 CR passes, (c) 3 ECAP passes and 20 CR passes and (d) 4 ECAP passes and 20 CR passes.

Fig. 3 e OM images of cross-sections of Mg alloy processed by ECAP for the conditions (a) 4 passes and 3 mm/min, (b) 4 passes and 25 mm/min and SEM images of cross-sections of Mg alloy processed by ECAP D CR for the conditions (c) 4 passes and 3 mm/min plus 25 CR passes and (d) 4 passes and 25 mm/min plus 20 CR passes.

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Fig. 4 e Hydrogen absorption at 623 K under 1.5 MPa of H2 for the Mg alloy for (a) samples processed by ECAP and (b) samples processed by ECAP D CR.

surface oxidation, these results are significant and it is noted that the contrary may be expected because the lowest incubation times were attained for samples having larger amounts of (002) preferential texture. Fig. 5 shows the thermogravimetric desorption properties of the hydrogenated samples processed by ECAP þ CR compared with commercial MgH2 powders. From these plots, it is evident that the amount of desorbed hydrogen also increases and, although it is not so evident, there is a slight decrease in desorption temperature with the amount of acquired (002) texture when compared with the MgH2 powders. At this point it is of interest to note that the desorbed amount of hydrogen for the bulk samples tends to reach the same amount for the MgH2 powders and the temperatures are lower for the bulk by comparison with the powder.

4.

Discussion

In order to simplify the discussion on the influence of texture on capacities, kinetics and incubation times, it is instructive to plot the graphs shown in Fig. 6 which summarize the main features observed in Fig. 4. Fig. 6a and b present the behavior of the maximum capacity against the percentage of the (002) direction for samples processed by ECAP and ECAP þ CR,

Fig. 5 e The TG desorption analysis for samples processed by ECAP D CR compared with commercial MgH2 powders.

respectively. As already noted, because of different thicknesses a direct comparison between the capacities is not feasible but from these graphs it is apparent that the maximum capacity increases nearly linearly with the amount of (002) texture for both processing conditions. It is important to note that this behavior is due only to the amount of texture since the observed large grain sizes will have no influence on the hydrogenation behavior. Also, it can be seen that the grain sizes do not follow any relationship with the maximum observed capacities. However, the slopes are different for both graphs and the slope for samples processed by ECAP þ CR is higher when compared with samples processed by ECAP and this is attributed to the more deformed state that is introduced by the additional CR. The above described behavior for different capacities can be related directly with the different acquired kinetics. Thus, Fig. 6c shows the influence of the acquired textures on the kinetics where the first and second stages of hydrogenation (as shown by the inset) have the same exponential growth behavior but it is more pronounced in the first stage of hydrogenation. This means the process is controlled by the first stage of hydrogenation and this stage tends to disappear with the amount of (002) direction. Another important aspect related to textures is presented in Fig. 6d where the incubation times and amounts of surface oxygen are plotted as a function of the amount of (101) direction. In general, the incubation times are associated with surface oxidation and they can be related also with the difficulty of bulk hydrogenation. Fig. 6d shows a clear logarithmic relationship between the (101) direction and the incubation time, where the mean fitted curve reaches a steady state with higher amounts of such direction. Together with the increasing difficulty of hydrogen penetration due the reduced amount of (002) direction, oxidation can also play a role and the presence of the (101) direction influences the amount of oxidation on the sample surface which is discussed later. Apparently, the data can be fitted with a logarithmic curve (shown by the dashed line) which means that in this position, or lower than this, the bulk should present incubation times due to limited hydrogen diffusion even without oxidation. In order to explain the behavior on hydrogenation and the incubation times, JEMS software [54] was used to simulate high resolution transmission electron microscopy (HRTEM) Weak Phase-Object Approximation Images (WPOA) for the

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Fig. 6 e Graphs of the main features in Fig. 4 plotted as a function of the amount of texture. Against the (002) direction: (a) the maximum H content for ECAP samples, (b) the maximum H content for ECAP D CR samples and (c) kinetics for ECAP D CR samples. Against the (101) direction: (d) the incubation time and amounts of surface oxygen for samples processed by ECAP and ECAP D CR. The meanings of the letters are defined in (a) and (b).

three main directions of Mg ((001), (002) and (101)) and these simulations are given in Fig. 7 for the three directions in (a), (b) and (c), respectively. The WPOA are essentially showing that the amplitude of a transmitted wave function will be linearly related to the projected potential of the specimen. From these images it is possible to observe that all possible good positions for hydrogen entrance, the hcp (tetrahedral) and fcc (octahedral) sites, are free in the (002) direction. However, this is not true for the (001) and (101) directions and it is worst for (001). Magnesium is kinetically slow when reacting with hydrogen and this may be due to its relatively poor ability to dissociate H2 or to the creation of a stable surface hydride structure which limits the diffusion of atomic hydrogen into the magnesium matrix. By using WPOA images, it is possible to simulate the density distribution of the projected potential taking account of variations in the z-direction on the frozen (001), (002) and (101) surfaces which are presented in Fig. 7 (d), (e) and (f), respectively. From these images it is observed that the highest extent of minimal energy positions are found for the (002) direction and the lowest extent is found for the (001) direction. Similar quantitative results were already found by Jacobson et al. [55] for (002) surface. They stated that the minimum energy values are for fcc sites at 0.88 eV and for hcp sites at 0.852 eV (blue colors in Fig. 7e) and the maximum value is for the Mg atoms sites at 0.09 eV (red color in Fig. 7e) (in the web version). This means the local minima in Fig. 7 (d), (e) and (f), which correspond to the blue color, are more pronounced for the hcp and fcc sites in Fig. 7e and these positions can be a stable surface for the hydride structure. This

enhances the rate-limiting dissociation of the hydrogen molecules at the sample surface and reduces the limit for hydrogen atomic diffusion into the Mg-matrix so that a greater extent of texture means that less time is required for the second stage. Ideally, this required time will tend to zero and the process will have only one stage for hydrogenation if 100% of the (002) texture is reached. In practice, this will also depend on the oxidation level which is dependent on the amount of (002) texture as discussed later. After overcoming this obstacle in the first stage of hydrogenation, the kinetics are increased for all processing conditions according to the amount of (002) texture but, even with the observed exponential growth in the second stage in Fig. 6c, this will be fixed at an almost constant level which will depend on the amount of the (002) direction. As the transformation upon hydrogenation of Mg into MgH2 has a fixed martensiticlike orientation relationship according to Mg(002)//MgH2(110) [38], the presence of higher amounts of (002) directions can then lower significantly the hydrogen loading times because this transformation is already favored by the presence of aligned Mg on the correct orientation. This will increase the kinetics by reducing the time to place the hydrogen atoms in the correct positions for this transformation. However, the samples are bulk structures having large grain sizes with reduced superficial areas and only small amounts of grain boundaries which may facilitate hydrogen entrance in the bulk as in the case for powders or for bulk solids having very small grain sizes. Comparing the hydrogenation kinetics for powders and the bulks used in this study, the kinetics in the

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Fig. 7 e Simulation of high resolution electron transmission microscopy Weak Phase-Object Approximation Images (WPOA) for the three main directions of Mg: (a) (001), (b) (002) and (c) (101). Simulation of density distributions of the projected potential on the WPOA images taking account of the variations in the z-direction on the frozen (001), (002) and (101) surfaces which are presented in (d), (e) and (f), respectively.

bulks will be sluggish. Thus, even with very large amounts of (002) texture this conclusion remains the same but in the present case the reaction was facilitated to some extent by the presence of such texture. Although the above discussion can explain the ability for hydrogenation, it cannot explain directly the observed incubation times. As was already reported [56], the (002) direction is also the best for oxidation. A good explanation can be found in the activation energies for oxidation in both directions. Thus, the activation energy for (101) is higher than for (002) and this means that oxide layers are more stable in the (101) direction than in (002) so that it is easier to remove such oxide layers in the (002) direction than in the (101) direction. In this way, the incubation times should be lower in the presence of a larger amount of (002) direction. It should be noted that it was not possible to use XRD to measure the amounts of oxide on the sample surfaces because they are probably below the XRD resolution. For example, they are assumed to have a thickness of no more than w20 nm when formed at room temperature [56]. Because of this, EDS analysis was performed to assess the amount of oxygen on the sample surface. The results agree with the above comment so that the (002) direction is also the best for oxidation [56] and, as can be observed in Fig. 6d, there was an increase in the amounts of oxygen with the increase of the amount of (002) direction on average from 34.7 to 38.9 at.%. Thus, the oxidation was more intense in samples with higher amounts of (002) direction. Also, this is in agreement with the

above supposition that the (002) direction can facilitate the process of oxide removing and, as the activation energy for oxide formation for (101) is higher than for (002), the oxide layers are more difficult to form and are more stable in the (101) direction than in (002). Therefore, it is easier to form and to remove such oxide layers in the (002) direction than in the (101) direction. Despite this discussion on the influence of preferential texture, it can be argued that the hydriding and dehydriding reactions are often nucleation-limited because of the huge volumetric and chemical interfacial energy discrepancy between the metal and the hydride. The hydrogenation curves presented in Fig. 4 represent essentially a “macroscopic” answer of this complex question but it is recognized that a deep analysis is required. In practice, the shape of the transformation curves can be divided into two major classes [57]; one with a monotonically growing (A) where the rate of transformation decreases continuously with time and the one with a sigmoidal shape (B) as in the present experiments. In terms of the first principles of the original theory of transformation [57], curves of type A are assigned as relating to a homogeneous reaction. For nucleation and growth reactions, where the situation is more complex, curves of type B are normally observed and a region is formed at a time equal to s (the induction period) after the reaction or transformation is initiated but before the monotonically growing curve (such as type A). However, the

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Johnson-Mehl-Avrami (JMA) equation can be fitted to both kinds of curves and it is also worth noting that a sigmoidal shape curve is also considered as a result of a hydride with an oxide layer [53]. In the present experiments, all the curves have a sigmoidal shape and the analysis was based on first principles by considering the incubation time as being the time before the reaction started and thus the so-called “first stage” in Fig. 6 (aec) was considered to be equal to the induction period (s). Of course, there is also the influence of oxidation and this will be considered. The JMA model [49e52], when applied to these curves, resulted in the graphs presented in Fig. 8a which show the plotting of ln[ln(1/(1  x(t)))] vs. ln(t) for the condition of 4 ECAP passes with 25 mm/min (curve D) and, after cold rolling, for conditions of 3 ECAP passes with 3 mm/min (curve K) and 4 ECAP passes with 25 mm/min (curve J). As can be observed, there are three linear regions and, under this procedure, the appropriate three JMA functions are those which provide a best fit to all absorption experimental data. Two linear regions were covered by Christian [57] and normally three stages are not observed but this may be due to the presence of preferential textures. The last linear region, for all conditions, can be assigned to site saturation [57]. This will lead to a strong influence on the growing of the hydride nuclei and also to a considerable reduction in the transformation rate. Fig. 8b shows a plot of all values obtained for the Avrami exponent (n) and the rate constant (k) against the amount of (002) texture for all processing conditions (where n1 and k1 refer to the first region and n2 and k2 to the second one in Fig. 8a). From Fig. 8b is possible to observe that, when the hydrogenation starts, n1 first increases from values of about 0.57 and 0.58 to about 1.34, respectively, for 8.4%, 15% and 87.5% of (002) texture and then decreases to a value of about 1 (1.1 and 0.99, respectively, for 97.05% and 98.29% of (002) texture). The values of n1 indicate that the rate-limiting step is diffusion for any amount of texture. The obtained n values correspond to one-dimensional grain growth with decreasing nucleation rate. However, one-dimensional for 0.57 and 0.58 may mean that the nucleation is mainly on the sample surface as this value also denotes a thickening of very large plates [57]

indicating a difficulty in hydrogen penetration. In fact, the smallest n1 values suggest that the nucleation and growth during this step of hydrogenation is sluggish, even from the initial reaction stage, due to the fast increase of hydride fraction on the sample surface. This agree with the earlier discussion which addressed the poor ability to dissociate H2 or to create a stable surface hydride structure with very low amounts of (002) direction. In the case of n1 ¼ 1, there is evidence of growing inside the magnesium grain, as this value can mean either needles and plates of finite long dimensions (but small by comparison with their separation) or the thickening of long needles [57]. Nevertheless, the reaction was faster and probably the hydrogen dissociation was easier and the surface was more stable than in the previous case thereby allowing hydrogen penetration in an easier way. As mentioned earlier, MgO is anticipated to act as an obstacle between the gas phase and magnesium and thus limit the diffusion of hydrogen atoms. However, it was not possible to observe any change during the induction time period and in fact the reaction started and continued in the same way during hours after starting, being 0.5 and 1.3 h, respectively, for 8.4% and 98.29% of (002) texture. Also, it is significant that, before this period, the incubation times were 3.3 and 0.73 h, respectively, for 8.4% and 98.29% of (002) texture where probably the oxidation layer was totally removed. For all the samples investigated, the diffusion of hydrogen through MgO contributed significantly to the incubation time as can be observed in Fig. 6d. However, even if the oxide layer could contribute during the induction time, it was very fast and impossible to determine using the JMA methodology. In all the cases, the nucleation and growth rates were very slow. The rate constant k1 is almost constant, about 0.064 s1, for all low amounts of (002) direction and then decreases to 0.027 s1 for 87.51% of (002) direction and increases again to values of around 0.033 s1 for 97.05% and 98.29% of (002) texture. Due to their highest value of k1, the first linear region finishes earlier in the conditions of small amounts of (002) texture than with highest amounts. After the first linear region, the slopes of the JMA curves start to change to the second linear region, very slowly and drastically in the case of smaller amounts of (002) texture and fast and smoothly in the

Fig. 8 e (a) Plotting of ln[ln(1/(1 L x(t)))] vs. ln(t) for the condition of 4 ECAP passes with 25 mm/min (curve D) and, after cold rolling, for conditions of 3 ECAP passes with 3 mm/min (curve K) and 4 ECAP passes with 25 mm/min (curve J). (b) Plotting of all values obtained for the Avrami exponent (n) and rate constant (k) against the amount of (002) texture for all processing conditions (n1 and k1 refer to the first linear region and n2 and k2 to the second one in Fig. 8a).

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Fig. 9 e Phases and orientations identified using XRD for the highest amount of (002) direction after hydrogenation.

case of higher amounts of (002) texture. From Fig. 8b, it is possible to observe that the values of n2 and k2 increased drastically and also there was a change in the behavior. The n2 values decreased from 1.8 to 1.44 and to almost 1.38 and 1.36, respectively, for 8.4%, 87.51%, 97.05% and 98.29% of (002) texture. The values of k2 had an almost parabolic behavior, and the k2 values decreased from 0.23 to 0.10 s1 (8.4% and 87.5% of (002) texture) and then increased to values of about 0.11 and 0.15 (97.05% and 98.29% of (002) texture). The range of n2 values indicates that the rate-limiting step continues to be diffusion for any amount of texture. However, values between 1.6 and 1.8, as for the smallest amounts of (002) texture, means that they changed to a more two-dimensional grain growth character with higher constant rates than in the first linear region and they are growing from small dimensions with decreasing nucleation rate [57]. This is the reason for attaining a very fast saturation in these cases, leading to a smaller amount of hydrogen; even considering the thickness differences, this would tend to happen by comparing these conditions with the conditions for 87.51% of (002). For the highest amounts of (002) texture conditions, they continued to be one-dimensional in all cases with decreasing nucleation rate but in these cases, even with higher constant rate, the saturation did not occur so rapidly leading to the highest amounts of hydrogen in smaller times when

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compared with the other conditions. It is important not to forget that the samples are bulk with very large grain sizes, with reduced superficial area and with very small amounts of grain boundaries which could facilitate the uptake of hydrogen within the bulk. The values of k1 and k2 are sluggish when compared with powders or even bulks of very small grain sizes. Thus, even with very large amounts of (002) texture, these facts continue to be the same but in this case the reaction was facilitated to some extent by the presence of this texture. The JMA analysis is in agreement with the above discussion after modeling because the hydrogen absorption was favored and the one-dimensionality was retained in the same way which was previewed by the modeling. However, within the context of growing facilitated by (002) direction and continued growing favored by the presence of aligned Mg on the correct orientation, it is reasonable to suppose that MgH2 could acquire some specific orientation or again the relationship according to Mg(002)//MgH2(110) [38]. To observe this, phases and orientations were identified using XRD for the highest amount of (002) condition. Fig. 9 presents this result and, as can be observed, the (110) direction is still prevailing in a high amount which is in agreement with the above orientation relationship. However, it is possible to observe that the (200) direction for MgH2 has grown. As observed previously [39], there is the possibility that during nucleation Mg and MgH2 take a given orientation relationship, but as the transformation continues the interface between the phase can rotate to several orientations with lower energies and finally reach the optimum condition. As can be also observed, the peak of the (002) direction for Mg is still present as the maximum capacity reached was 6 wt. % and this is another indication of difficulties of bulk penetration by the hydrogen. It is believed that, allied to the preferential texture, a reduced grain size to the nanoscale can improve both the capacity and the kinetics. In order to simplify the discussion on the influence of textures on the desorption properties, Fig. 10 plots data which summarizes the main features observed in Fig. 5. From these plots, it is evident that texture can be retained after hydrogen absorption and desorption because the amount of desorbed hydrogen increases with the amount of (002) texture. Moreover, it is interesting to note that the desorption temperatures are also influenced by the amount of (002) texture and this

Fig. 10 e Graphs of the main features observed in Fig. 5 plotted as a function of the amount of (002) texture compared with commercial MgH2 powders: (a) maximum desorbed H and (b) desorption temperatures.

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may mean that there is not only a kinetic effect but also a thermodynamic effect. Thus, the preferential textures can reduce the levels of the energy barriers, at least during the dehydrogenation process. When compared with commercial MgH2 powders, these properties become even more evident. It is also possible to examine the effect of grain size in ECAP processing by comparing the acquired capacity in the earlier report [26] with the present data. Thus, the amount of (002) texture was earlier estimated as 73%, the sample thickness was about 300 mm (twice the thickness of samples processed by ECAP þ CR) and the grain size was about 1 mm (almost 20 times smaller than in the present results). In these conditions the sample absorbed almost 4 wt. %. For comparison, extrapolating the straight line in Fig. 5b to a sample thickness of 150 mm and 73% of the (002) direction, it is concluded that a sample with a grain size of 20 mm would absorbed approximately 3.4 wt. % of hydrogen. Thus, considering the difference in thickness of the samples, there may be a large effect of grain size on the capacity. From these results, it is apparent that different hydrogen absorption results may be achieved primarily due to differences in the preferential texture. Minor amounts of (002) or major amounts of (101) direction play an important role in the incubation times which are directly correlated with the ease of oxygen removal in the (002) direction.

5.

Summary and conclusions

1. Experiments on commercial purity magnesium show that the (002) texture influences the capacity, kinetics and desorption temperatures. Higher capacities, faster kinetics and lower desorption temperatures correlate directly with the amount of such texture. 2. The most important influence of (002) is in the first stage of hydrogenation which tends to disappear by increasing the amount of texture. This is correlated with the presence of higher amounts of lower energy sites which produce stable surfaces for hydride structure and thus enhance the ratelimiting dissociation of hydrogen molecules at the sample surface and reduce the limit for hydrogen atomic diffusion into the Mg-matrix 3. After overcoming the first stage of hydrogenation, the kinetics are improved and this is attributed to the presence of a favorable (002) texture which reduces the time for the transformation of Mg in MgH2 according to a martensiticlike orientation relationship. 4. The incubation times are correlated with the minor amounts of (002) or major amounts of (101) direction which is associated with the ease of oxygen removal in the (002) direction.

Acknowledgments This work was supported in part by award FAPESP# 2011/ 51245-8 under a cooperation agreement between the Federal University of Sao Carlos and the University of Southampton, in part by the Conselho Nacional de Desenvolvimento

Cientifico e Tecnolo´gico CNPq# 237345/2012-9 PDE (CsF) and in part by the European Research Council under Grant Agreement No. 267464-SPDMETALS.

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