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Effect of mechanical activation on compactibility of metal hydride materials
Accepted Manuscript Effect of mechanical activation on compactibility of metal hydride materials V. Yu Zadorozhnyy, S.N. Klyamkin, M. Yu Zadorozhnyy, D.V. Strugova, G.S. Milovzorov, D.V. Louzguine-Luzgin, S.D. Kaloshkin PII:
S0925-8388(16)33801-4
DOI:
10.1016/j.jallcom.2016.11.320
Reference:
JALCOM 39817
To appear in:
Journal of Alloys and Compounds
Received Date: 21 June 2016 Revised Date:
7 November 2016
Accepted Date: 22 November 2016
Please cite this article as: V.Y. Zadorozhnyy, S.N. Klyamkin, M.Y. Zadorozhnyy, D.V. Strugova, G.S. Milovzorov, D.V. Louzguine-Luzgin, S.D. Kaloshkin, Effect of mechanical activation on compactibility of metal hydride materials, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.320. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of mechanical activation on compactibility of metal hydride materials
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V. Yu. Zadorozhnyy1,*, S. N. Klyamkin2, M. Yu. Zadorozhnyy1, D.V. Strugova1, G.S. Milovzorov1, D. V. Louzguine-Luzgin3 and S. D. Kaloshkin1
National University of Science and Technology «MISIS», Moscow, 119049, Russia
2
Department of Chemistry, M.V. Lomonosov Moscow State University, 119991, Moscow, Russia
3
WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
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1
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* e-mail: [email protected]; tel./fax: +7(495)638-44-13/+7(495)638-45-95
Abstract
The most common hydrogen storage alloys (TiFe, Mg2Ni and LaNi5) were chosen for studying the effect of high energy ball milling (HEBM) on the ability of intermetallic powders to
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form bulk compact samples (pellets) by solid-state bonding without binder. The role of internal energy accumulated in the material during activation treatment on formation of contact surfaces between the particles was considered. It was shown that the consolidation of the intermetallic
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hydrogen storage alloys prepared by HEBM allows obtaining porous bulk nanostructured samples which demonstrate simplified hydrogen activation procedure, higher thermal diffusivity and good
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durability after multiple absorption-desorption cycles.
Keywords: High energy ball milling; Intermetallic compounds; Hydrogen storage
1. Introduction
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Metal hydrides represent a wide class of chemical compounds formed by hydrogen with most individual metals and many intermetallics. Due to a set of unique properties they are considered as advanced functional materials for various applications including hydrogen purification, storage and compression, separation of hydrogen isotopes, liquid H2 control and boil-
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off capture, diamond synthesis, permanent magnet production, heterogeneous catalysis and others [1-5]. The large-scale production of Ni-MH rechargeable batteries is the most mature technology employing metal hydrides [6].
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An intrinsic feature of metal hydride consists in significant changes of the crystal lattice volume during hydrogen absorption and desorption. Those volume effects lead to a spontaneous
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pulverization of the bulk metals with formation of fine powders. Such a behavior causes serious problems in the implementation of metal hydrides, since the use of powdered materials requires additional infiltration systems. Moreover, the thermal conductivity of powder beds is much lower compared to compact alloys that reduce dramatically the rate of hydrogenation/dehydrogenation
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reactions. To eliminate the above drawbacks polymers binding agent or metallic matrices are commonly used [7-11]. Here, we are considering an alternative approach based on preliminary high energy ball milling (HEBM) of hydride forming alloys to produce compact materials
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sustainable in course of hydrogen absorption/desorption cycling. Mechanical activation through ball milling is one of the simplest and the most powerful
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technique to obtain nanostructured materials with peculiar phase compositions (including the nonequilibrium ones) and high reactivity [13, 14]. The high energy ball milling creates new uncontaminated surface but also alters drastically the microstructure of materials. Energy accumulated inside the crystal phases in the form of various lattice defects promotes interaction processes between the solid particles, and reduces the activation barrier for further chemical transformations of the material [15, 16]. We demonstrated recently [17-20] possibility of direct solid-phase synthesis by HEBM of nanostructured single-phase TiFe intermetallic compound from individual components Fe and Ti.
HEBM process was also applied as an effective method to dope TiFe by third components [21-23],
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encapsulate it into protective coatings [24] and produce bulk samples from intermetallic powders [25, 26]. In the present work, applicability of this technique to the most common metal hydride materials including LaNi5 and Mg2Ni in order to evaluate the mechanism of the powder consolidation and to obtain compact alloys resistant with regard to decrepitation during interaction
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with hydrogen was provided.
2. Materials and methods
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The ingots of the TiFe, Mg2Ni and LaNi5 alloys were prepared by induction melting from the stoichiometric mixtures of pure metals (99.9 %) in boron nitride crucible under an argon
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atmosphere. As prepared ingots were crushed into powder and exposed to HEBM. The HEBM process was performed in an AGO-2C water-cooled high energy planetary ball mill. The powder mixture was treated in an argon atmosphere for 120 min at a rotation speed of 840 rpm. The vials and reducing bodies (4 mm in diameter) of stainless steel were used. The ratio
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of the mass of balls and powder was 10/1.
Phase and structure transformations were examined by X-ray diffractometry with monochromatic CuKα radiation. An accuracy of determination of the lattice parameters and the
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phase composition was of 0.0001 nm and 5%, respectively. The crystallite size and the mean square deformation were estimated by the method of approximation of XRD peak widening with
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the accuracy of ±3-5 nm and ±0.05% [27]. Compacting of mechanically treated powders was performed in a ТЕСАР APVM-904
laboratory pressing machine. The procedure included compressing of the powder in a steel mould (8 and 12 mm in diameter) under a pressure of 600 MPa at room temperature, and subsequent heating in vacuum of 10-3 Pa for 10 min to different temperatures: 873 K (TiFe); 573 K (Mg2Ni) and 873 K (LaNi5). Dilatometric analysis was performed on Netzsch DIL402 dilatometer. Analysis of the content of oxygen and nitrogen was performed on the analyzer TC 436.
Testing of physical properties of the prepared materials was carried out using a differential
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scanning calorimeter Exstar DSC 6300 SII and Netzsch LFA 447 NanoFlash (thermal diffusivity analysis). The structure of the prepared samples was examined by scanning electron microscopy (SEM) Hitachi S-4800 at 15 kV. The study of the material interaction with hydrogen was carried out in a steel reactor using
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an original experimental Sievert's type unit for precise P–C–T measurements under high pressure hydrogen atmosphere [28].
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3. Results and Discussion
According to the XRD data, the HEBM powder samples and the related bulk materials have
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similar phase compositions with a certain increase of the crystallite size caused by heating treatment after pressing. The fine structure parameters of the obtained alloys calculated from the XRD peak widening evidence the nanocrystalline state the HEBM samples (Fig. SI-1 - SI-3 and Table SI-1). The high density of the crystal defect generated during HEBM resulted in an
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extension of the lattice, but there is a slight decrease in the unit cell parameters after subsequent compacting. Such an effect can be explained by redistribution of the defects (recovery), formation and migration of small-angle boundaries (polygonization process). This correlates with observed
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changes of microstrain parameters. We should note that the nanostructured state of the alloys remains practically unaffected by thermal treatment during consolidation.
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DSC analysis of the TiFe intermetallic powder subjected to HEBM clearly shows two
main exothermic peaks during the heating process at temperatures of 643 K and 823 K (Fig. 1a). The low-temperature exothermic process (approx. 10 kJ/mol) should be related to matrix relaxation, and the higher one - to crystallization of amorphous phases present in the HEBM samples [19]. The DSC curve obtained for the as-cast macrocrystalline TiFe is presented at the same figure as a reference. It can be clearly seen that there is no pronounced heat effects within the same temperature range for the reference sample. This confirms once more that the exothermic
effects found in the HEBM alloy is caused by some relaxation processes and corresponds to an
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excessive internal energy accumulated in form of defects during synthesis. DSC analysis of the HEBM Mg2Ni demonstrates two similar exothermic peaks shifted to lower temperatures: 443 K and between 493 and 593 K (Fig. 1b). According to Refs [29-31], the first one may be attributed to matrix relaxation and crystallization of the inter-particle amorphous
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phase, while the second exothermic reaction is likely related to growth of the nanoparticles. Fig. 1b also shows the DSC curve of the macrocrystalline as-cast Mg2Ni which does not have any exothermic peaks in the whole temperature range.
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The nanocrystalline HEBM LaNi5 possesses two exothermic effects during the heating process: one at about 550 K and another, small one, at about 670 K (Fig. 2, c). According to Ref
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[32], the first peak corresponds to the matrix relaxation and grain growth. At 670 K crystallization of the amorphous components occurs. As in the previous cases, no heat effects are observed for the macrocrystalline alloy produced by conventional melting.
Excessive internal energy accumulated in all HEBM intermetallic powders is a key factor
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defining the physical properties of the materials. It improves adhesion ability of the powder particles, and thus ensures stronger interaction between them [33, 34]. The general views and the surface images of the obtained bulk samples after compaction
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and annealing process are shown in Figure 2. The samples look rather dense, but a sponge-like microstructure is evident at higher magnification.
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The compacted bulk samples change markedly their physical properties. Thermal
diffusivity increased from 0.3 to 1.2 mm2/s for TiFe, from 0.45 to 0.7 mm2/s for Mg2Ni and from 1.1 to 2.2 mm2/s for LaNi5 (Fig. 3). That confirms pronounced improvement of adhesion between the powder particles and agrees with the above mentioned DSC data. Nanocrystalline structure of the HEBM alloys facilitates significantly the process of the first hydrogenation. This effect is of particular importance for TiFe which commonly requires a prolonged high-temperature degassing and several subsequent hydrogen absorption cycles to complete activation [1, 2]. As it was noted previously, the HEBM ensures the same activated state
already after an exposure to hydrogen atmosphere at pressure of 1 MPa and temperature of 673 K
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for 30 minutes [19]. The pressure-composition isotherms measured for the intermetallic-hydrogen systems under consideration demonstrated that there are no visible changes in hydrogenation behaviour of the powdered HEBM alloys [35, 36] and the derived bulk samples (Fig. SI-4). The total hydrogen
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storage capacity, shape and position of the isotherms, reaction kinetics remains unaffected by compacting and annealing.
The most valuable result of the present work consists in evidence of the durability of the
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compacted metal hydride material pre-treated through HEBM. There was no visible destruction or powdering of the studied samples after 20 hydrogen absorption-desorption cycles in spite of
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significant volume effects accompanying the reactions of hydride formation and decomposition. Some previous observations of similar phenomena on TiFe and Mg2Ni reported in [25, 26], are confirmed here for LaNi5 which exhibits much higher lattice expansion at hydrogenation (∆V/V0 ≈25%). Such pronounced volume effects commonly lead to the spontaneous transformation of the
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alloy ingots into fine powders. Moreover, compacting of those powders in the above described mode (without HEBM) does not ensure the integrity of the pellets, and they completely destroy after 1-2 hydrogenation cycles.
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The "breathing" nature of the materials compacted after HEBM was revealed by a dilatometric analysis of hydrogenated samples in course of heating. The data obtained certify that
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hydrogen release under those conditions results in a shrinking of the cylindrical pellet with its length reduction of 2.5% (Fig. SI-5). Taking into account that the density of the compacted samples is about 65-70% of the crystallographic one, we assume these materials have a peculiar microstructure which allows them to act like a sponge during hydrogen absorption and desorption. To clarify the origin of such a peculiar behaviour of the compacted HEBM powders a detailed microstructure investigation using scanning electron microscopy was performed. It was found that a specific feature of these materials consists in formation of interparticle bridges or necks (Fig. 4). The necking process occurring in the mechanically activated powders during their
pressing and subsequent low-temperature annealing was observed in all of the studied
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intermetallic compounds, namely TiFe, Mg2Ni, and LaNi5. It is important to note that there exist no similar binding between the particles in materials of the same compositions but produced by conventional melting without ball milling treatment and then subjected to the same procedure of compacting (Fig. 4 g,h).
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Superplasticity of the metallic systems in course of HEBM process was previously reported in [37-39]. The planetary activator provides a huge energy impact on the material ensuring high intensity of the solid state processes [40, 41]. Besides, the background temperature
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of the HEBM vials, as it was shown in our previous work [42], may reach 473-673 K that is in a good agreement with the experimental data in [43]. Those treatment conditions provide an
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opportunity to decrease the activation energy for the formation of extra surfaces, grain boundaries and sub-grain boundaries that is equivalent to increasing the temperature for diffusion initialization [44]. The first step of the MA process is the formation of the layered structure from the initial components. The powder particles are rolled in flat sheets and are mixed one with the
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other. The second step is the interdiffusion of the alloy components leading to the formation of solid solutions and intermetallic compounds [45]. During further consolidation process the accumulated excessive internal energy (which is responsible for improved adhesion) can be
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released in local contact regions of the powder particles, and thus create the above mentioned interparticle necks.
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Qualitative analysis performed for the Fe-Ti system allowed us to estimate the partial
diffusion coefficient of Fe in Ti under HEBM conditions as 1.1·10-11 cm2/s [46]. This value agrees with the corresponding parameters calculated in [47-51], while the high temperature (1373 K) diffusion coefficient for the same system DFe = 1.56·10-12 cm2/s [52]. Taking into account the known reduction of diffusion mobility with decreasing temperature (1 order per 473 K) [51], the extrapolation to 873 K results in 10-15-10-16 cm2/s for non-activated alloy.
The knowledge of the diffusion coefficients is essential to evaluate the main characteristics
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of the powder sintering process [34], namely “viscosity coefficient of the defect crystal” and “flow velocity” caused by a diffusion-dislocation-controlled mechanism.
−2 kT 1 ⋅ LD ⋅ ≈ DΩ N
(1)
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η≅
Where η – viscosity coefficient of the defect crystal; k - Boltzmann's constant; T – absolute −2
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temperature; D – diffusion coefficient; Ω – atom volume; L D – average linear distances between
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the dislocations; N - dislocation densities.
ε&D ≅
N V DΩ P kT
(2)
Where ε& D - flow velocity; Nv – density of “free” (movable) dislocation; D – diffusion coefficient;
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Ω – atom volume; P - capillary force; k - Boltzmann's constant; T – absolute temperature.
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An important parameter in the above equations is dislocation density (NV) or its equivalent - linear distances between the dislocations (LD). According to available data, in the alloys prepared
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by SPD (Severe plastic deformation) this value is close to 1014–1015 m-2 [54, 55] and even reaches 1015–1016 m-2 in the HEBM processing [56-59]. This is several orders of magnitude higher than in as-cast and annealed metals (105-106 m-2) [60]. Applying of the coefficients NV (or LD) and D to the equations (1) and (2) evidences
significant reduction of the parameter η and an increase of ε& D compared to conventional metallic powders produced by alternative methods. This obviously simplifies the formation of the interparticle necks between the powder particles during compacting processes, even at the low sintering temperature: 873 K for TiFe and LaNi5 and 573 K for Mg2Ni.
A peculiar combination of the steady bonding between the particles and sufficient porosity
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of the consolidated HEBM materials is the main reason for the observed durability in course of absorption-desorption cycling. The specific sponge-type microstructure makes these bulk samples stable for the dimensional changes and "breathing" at hydrogen uptake and release. Another important factor relates to the nanocrystalline nature of the HEBM alloys. The nanostructure of the
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metallic matrix (including the interparticle necks) provides the nanoanisotropy in the bulk samples that is why the linear scaling during hydrogenation and the resulting strain become much smaller.
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4. Conclusions
The results obtained allow us to conclude that the spectacular compactibility of
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intermetallic materials prepared through ball milling is based on a peculiar combination of their features. The excessive internal energy accumulated during HEBM enhances drastically the interdiffusion, and thus ensures high performance of the sintering process at relatively low temperatures via formation of specific necks between the particles. Besides, the nanostructured
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state of the HEBM materials facilitates the accommodation of the volume distortion of the crystallites at hydrogenation and finally reduces the resulting strain. The breathing sponge-like composites are of particular interest for a wide range of hydride forming alloys in order to
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improve their hydrogen storage performance as well as for a variety of other applications.
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Acknowledgments
The authors appreciate the help and discussions from Prof. F. Wakai from “Secure
Materials Center, Materials and Structures Laboratory, Tokyo Institute of Technology”. The work was support in part by the Ministry of Education and Science of the Russian Federation within the framework of the Increase Competitiveness Program of MISiS. Financial support from Russian Foundation for Basic Research (13-03-12424) is gratefully acknowledged.
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List of Figure Captions ACCEPTED MANUSCRIPT Fig. 1. DSC curves of the prepared powder samples: TiFe (a), Mg2Ni (b), LaNi5 (c) Fig. 2. General view and the surfaces images of the obtained bulk samples: TiFe (a, b); Mg2Ni (c, d); LaNi5 (e, f) Fig. 3. Thermal diffusivity of the bulk samples: TiFe (a); Mg2Ni (b); LaNi5 (c) before and
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after annealing
Fig. 4. Necking between the particles inside the samples consolidated after HEBM: TiFe bulk sample (a) and its nanocrystalline structure (b), Mg2Ni (c) and (d), LaNi5 (e) and (f); and
same procedure of compacting: LaNi5 (g) and TiFe (h)
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structure of the bulk samples produced from as-cast alloys without HEBM, but subjected to the
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Supporting Information
Fig. SI1 - XRD patterns of the TiFe intermetallic alloy samples: as-cast condition (a); after HEBM (b); bulk sample, after HEBM and subjected to subsequent compacting with annealing at 873 K (c)
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Fig. SI2 - XRD patterns of the Mg2Ni intermetallic alloy samples: as-cast condition (a); after HEBM (b); bulk sample, after HEBM and subjected to subsequent compacting with annealing at 573 K (c)
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Fig. SI3 - XRD patterns of the LaNi5 intermetallic alloy samples: as-cast condition (a); after HEBM (b); bulk sample, after HEBM and subjected to subsequent compacting with
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annealing at 873 K (c)
Fig. SI-4. Pressure composition isotherms for hydrogen absorption (closed symbols) and
desorption (open symbols) in nanocrystalline bulk samples: TiFe (a), Mg2Ni (b), LaNi5 (c) Fig. SI-5. Dilatometric analysis of the TiFe bulk hydrogenated samples: length change at heating (a); general view of the sample after 20 hydrogenation cycles (b); powder obtained after 1 hydrogenation cycle from the TiFe bulk sample produced by induction melting (c)
Table SI1. Effect of milling and compacting conditions on the structure parameters of the alloys
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- The bulk nanocrystalline intermetallic compounds have been obtained via high energy ball milling. - The bulk samples have the improved durability to the hydrogen absorptiondesorption process. - The improved durability is related to the formation of a peculiar sponge-type microstructure. - The sponge-type microstructure formation is possible due to the excessive internal energy.
S0925-8388(16)33801-4
DOI:
10.1016/j.jallcom.2016.11.320
Reference:
JALCOM 39817
To appear in:
Journal of Alloys and Compounds
Received Date: 21 June 2016 Revised Date:
7 November 2016
Accepted Date: 22 November 2016
Please cite this article as: V.Y. Zadorozhnyy, S.N. Klyamkin, M.Y. Zadorozhnyy, D.V. Strugova, G.S. Milovzorov, D.V. Louzguine-Luzgin, S.D. Kaloshkin, Effect of mechanical activation on compactibility of metal hydride materials, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.320. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of mechanical activation on compactibility of metal hydride materials
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V. Yu. Zadorozhnyy1,*, S. N. Klyamkin2, M. Yu. Zadorozhnyy1, D.V. Strugova1, G.S. Milovzorov1, D. V. Louzguine-Luzgin3 and S. D. Kaloshkin1
National University of Science and Technology «MISIS», Moscow, 119049, Russia
2
Department of Chemistry, M.V. Lomonosov Moscow State University, 119991, Moscow, Russia
3
WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
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1
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* e-mail: [email protected]; tel./fax: +7(495)638-44-13/+7(495)638-45-95
Abstract
The most common hydrogen storage alloys (TiFe, Mg2Ni and LaNi5) were chosen for studying the effect of high energy ball milling (HEBM) on the ability of intermetallic powders to
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form bulk compact samples (pellets) by solid-state bonding without binder. The role of internal energy accumulated in the material during activation treatment on formation of contact surfaces between the particles was considered. It was shown that the consolidation of the intermetallic
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hydrogen storage alloys prepared by HEBM allows obtaining porous bulk nanostructured samples which demonstrate simplified hydrogen activation procedure, higher thermal diffusivity and good
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durability after multiple absorption-desorption cycles.
Keywords: High energy ball milling; Intermetallic compounds; Hydrogen storage
1. Introduction
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Metal hydrides represent a wide class of chemical compounds formed by hydrogen with most individual metals and many intermetallics. Due to a set of unique properties they are considered as advanced functional materials for various applications including hydrogen purification, storage and compression, separation of hydrogen isotopes, liquid H2 control and boil-
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off capture, diamond synthesis, permanent magnet production, heterogeneous catalysis and others [1-5]. The large-scale production of Ni-MH rechargeable batteries is the most mature technology employing metal hydrides [6].
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An intrinsic feature of metal hydride consists in significant changes of the crystal lattice volume during hydrogen absorption and desorption. Those volume effects lead to a spontaneous
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pulverization of the bulk metals with formation of fine powders. Such a behavior causes serious problems in the implementation of metal hydrides, since the use of powdered materials requires additional infiltration systems. Moreover, the thermal conductivity of powder beds is much lower compared to compact alloys that reduce dramatically the rate of hydrogenation/dehydrogenation
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reactions. To eliminate the above drawbacks polymers binding agent or metallic matrices are commonly used [7-11]. Here, we are considering an alternative approach based on preliminary high energy ball milling (HEBM) of hydride forming alloys to produce compact materials
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sustainable in course of hydrogen absorption/desorption cycling. Mechanical activation through ball milling is one of the simplest and the most powerful
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technique to obtain nanostructured materials with peculiar phase compositions (including the nonequilibrium ones) and high reactivity [13, 14]. The high energy ball milling creates new uncontaminated surface but also alters drastically the microstructure of materials. Energy accumulated inside the crystal phases in the form of various lattice defects promotes interaction processes between the solid particles, and reduces the activation barrier for further chemical transformations of the material [15, 16]. We demonstrated recently [17-20] possibility of direct solid-phase synthesis by HEBM of nanostructured single-phase TiFe intermetallic compound from individual components Fe and Ti.
HEBM process was also applied as an effective method to dope TiFe by third components [21-23],
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encapsulate it into protective coatings [24] and produce bulk samples from intermetallic powders [25, 26]. In the present work, applicability of this technique to the most common metal hydride materials including LaNi5 and Mg2Ni in order to evaluate the mechanism of the powder consolidation and to obtain compact alloys resistant with regard to decrepitation during interaction
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with hydrogen was provided.
2. Materials and methods
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The ingots of the TiFe, Mg2Ni and LaNi5 alloys were prepared by induction melting from the stoichiometric mixtures of pure metals (99.9 %) in boron nitride crucible under an argon
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atmosphere. As prepared ingots were crushed into powder and exposed to HEBM. The HEBM process was performed in an AGO-2C water-cooled high energy planetary ball mill. The powder mixture was treated in an argon atmosphere for 120 min at a rotation speed of 840 rpm. The vials and reducing bodies (4 mm in diameter) of stainless steel were used. The ratio
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of the mass of balls and powder was 10/1.
Phase and structure transformations were examined by X-ray diffractometry with monochromatic CuKα radiation. An accuracy of determination of the lattice parameters and the
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phase composition was of 0.0001 nm and 5%, respectively. The crystallite size and the mean square deformation were estimated by the method of approximation of XRD peak widening with
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the accuracy of ±3-5 nm and ±0.05% [27]. Compacting of mechanically treated powders was performed in a ТЕСАР APVM-904
laboratory pressing machine. The procedure included compressing of the powder in a steel mould (8 and 12 mm in diameter) under a pressure of 600 MPa at room temperature, and subsequent heating in vacuum of 10-3 Pa for 10 min to different temperatures: 873 K (TiFe); 573 K (Mg2Ni) and 873 K (LaNi5). Dilatometric analysis was performed on Netzsch DIL402 dilatometer. Analysis of the content of oxygen and nitrogen was performed on the analyzer TC 436.
Testing of physical properties of the prepared materials was carried out using a differential
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scanning calorimeter Exstar DSC 6300 SII and Netzsch LFA 447 NanoFlash (thermal diffusivity analysis). The structure of the prepared samples was examined by scanning electron microscopy (SEM) Hitachi S-4800 at 15 kV. The study of the material interaction with hydrogen was carried out in a steel reactor using
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an original experimental Sievert's type unit for precise P–C–T measurements under high pressure hydrogen atmosphere [28].
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3. Results and Discussion
According to the XRD data, the HEBM powder samples and the related bulk materials have
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similar phase compositions with a certain increase of the crystallite size caused by heating treatment after pressing. The fine structure parameters of the obtained alloys calculated from the XRD peak widening evidence the nanocrystalline state the HEBM samples (Fig. SI-1 - SI-3 and Table SI-1). The high density of the crystal defect generated during HEBM resulted in an
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extension of the lattice, but there is a slight decrease in the unit cell parameters after subsequent compacting. Such an effect can be explained by redistribution of the defects (recovery), formation and migration of small-angle boundaries (polygonization process). This correlates with observed
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changes of microstrain parameters. We should note that the nanostructured state of the alloys remains practically unaffected by thermal treatment during consolidation.
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DSC analysis of the TiFe intermetallic powder subjected to HEBM clearly shows two
main exothermic peaks during the heating process at temperatures of 643 K and 823 K (Fig. 1a). The low-temperature exothermic process (approx. 10 kJ/mol) should be related to matrix relaxation, and the higher one - to crystallization of amorphous phases present in the HEBM samples [19]. The DSC curve obtained for the as-cast macrocrystalline TiFe is presented at the same figure as a reference. It can be clearly seen that there is no pronounced heat effects within the same temperature range for the reference sample. This confirms once more that the exothermic
effects found in the HEBM alloy is caused by some relaxation processes and corresponds to an
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excessive internal energy accumulated in form of defects during synthesis. DSC analysis of the HEBM Mg2Ni demonstrates two similar exothermic peaks shifted to lower temperatures: 443 K and between 493 and 593 K (Fig. 1b). According to Refs [29-31], the first one may be attributed to matrix relaxation and crystallization of the inter-particle amorphous
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phase, while the second exothermic reaction is likely related to growth of the nanoparticles. Fig. 1b also shows the DSC curve of the macrocrystalline as-cast Mg2Ni which does not have any exothermic peaks in the whole temperature range.
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The nanocrystalline HEBM LaNi5 possesses two exothermic effects during the heating process: one at about 550 K and another, small one, at about 670 K (Fig. 2, c). According to Ref
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[32], the first peak corresponds to the matrix relaxation and grain growth. At 670 K crystallization of the amorphous components occurs. As in the previous cases, no heat effects are observed for the macrocrystalline alloy produced by conventional melting.
Excessive internal energy accumulated in all HEBM intermetallic powders is a key factor
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defining the physical properties of the materials. It improves adhesion ability of the powder particles, and thus ensures stronger interaction between them [33, 34]. The general views and the surface images of the obtained bulk samples after compaction
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and annealing process are shown in Figure 2. The samples look rather dense, but a sponge-like microstructure is evident at higher magnification.
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The compacted bulk samples change markedly their physical properties. Thermal
diffusivity increased from 0.3 to 1.2 mm2/s for TiFe, from 0.45 to 0.7 mm2/s for Mg2Ni and from 1.1 to 2.2 mm2/s for LaNi5 (Fig. 3). That confirms pronounced improvement of adhesion between the powder particles and agrees with the above mentioned DSC data. Nanocrystalline structure of the HEBM alloys facilitates significantly the process of the first hydrogenation. This effect is of particular importance for TiFe which commonly requires a prolonged high-temperature degassing and several subsequent hydrogen absorption cycles to complete activation [1, 2]. As it was noted previously, the HEBM ensures the same activated state
already after an exposure to hydrogen atmosphere at pressure of 1 MPa and temperature of 673 K
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for 30 minutes [19]. The pressure-composition isotherms measured for the intermetallic-hydrogen systems under consideration demonstrated that there are no visible changes in hydrogenation behaviour of the powdered HEBM alloys [35, 36] and the derived bulk samples (Fig. SI-4). The total hydrogen
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storage capacity, shape and position of the isotherms, reaction kinetics remains unaffected by compacting and annealing.
The most valuable result of the present work consists in evidence of the durability of the
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compacted metal hydride material pre-treated through HEBM. There was no visible destruction or powdering of the studied samples after 20 hydrogen absorption-desorption cycles in spite of
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significant volume effects accompanying the reactions of hydride formation and decomposition. Some previous observations of similar phenomena on TiFe and Mg2Ni reported in [25, 26], are confirmed here for LaNi5 which exhibits much higher lattice expansion at hydrogenation (∆V/V0 ≈25%). Such pronounced volume effects commonly lead to the spontaneous transformation of the
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alloy ingots into fine powders. Moreover, compacting of those powders in the above described mode (without HEBM) does not ensure the integrity of the pellets, and they completely destroy after 1-2 hydrogenation cycles.
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The "breathing" nature of the materials compacted after HEBM was revealed by a dilatometric analysis of hydrogenated samples in course of heating. The data obtained certify that
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hydrogen release under those conditions results in a shrinking of the cylindrical pellet with its length reduction of 2.5% (Fig. SI-5). Taking into account that the density of the compacted samples is about 65-70% of the crystallographic one, we assume these materials have a peculiar microstructure which allows them to act like a sponge during hydrogen absorption and desorption. To clarify the origin of such a peculiar behaviour of the compacted HEBM powders a detailed microstructure investigation using scanning electron microscopy was performed. It was found that a specific feature of these materials consists in formation of interparticle bridges or necks (Fig. 4). The necking process occurring in the mechanically activated powders during their
pressing and subsequent low-temperature annealing was observed in all of the studied
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intermetallic compounds, namely TiFe, Mg2Ni, and LaNi5. It is important to note that there exist no similar binding between the particles in materials of the same compositions but produced by conventional melting without ball milling treatment and then subjected to the same procedure of compacting (Fig. 4 g,h).
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Superplasticity of the metallic systems in course of HEBM process was previously reported in [37-39]. The planetary activator provides a huge energy impact on the material ensuring high intensity of the solid state processes [40, 41]. Besides, the background temperature
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of the HEBM vials, as it was shown in our previous work [42], may reach 473-673 K that is in a good agreement with the experimental data in [43]. Those treatment conditions provide an
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opportunity to decrease the activation energy for the formation of extra surfaces, grain boundaries and sub-grain boundaries that is equivalent to increasing the temperature for diffusion initialization [44]. The first step of the MA process is the formation of the layered structure from the initial components. The powder particles are rolled in flat sheets and are mixed one with the
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other. The second step is the interdiffusion of the alloy components leading to the formation of solid solutions and intermetallic compounds [45]. During further consolidation process the accumulated excessive internal energy (which is responsible for improved adhesion) can be
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released in local contact regions of the powder particles, and thus create the above mentioned interparticle necks.
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Qualitative analysis performed for the Fe-Ti system allowed us to estimate the partial
diffusion coefficient of Fe in Ti under HEBM conditions as 1.1·10-11 cm2/s [46]. This value agrees with the corresponding parameters calculated in [47-51], while the high temperature (1373 K) diffusion coefficient for the same system DFe = 1.56·10-12 cm2/s [52]. Taking into account the known reduction of diffusion mobility with decreasing temperature (1 order per 473 K) [51], the extrapolation to 873 K results in 10-15-10-16 cm2/s for non-activated alloy.
The knowledge of the diffusion coefficients is essential to evaluate the main characteristics
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of the powder sintering process [34], namely “viscosity coefficient of the defect crystal” and “flow velocity” caused by a diffusion-dislocation-controlled mechanism.
−2 kT 1 ⋅ LD ⋅ ≈ DΩ N
(1)
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η≅
Where η – viscosity coefficient of the defect crystal; k - Boltzmann's constant; T – absolute −2
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temperature; D – diffusion coefficient; Ω – atom volume; L D – average linear distances between
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the dislocations; N - dislocation densities.
ε&D ≅
N V DΩ P kT
(2)
Where ε& D - flow velocity; Nv – density of “free” (movable) dislocation; D – diffusion coefficient;
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Ω – atom volume; P - capillary force; k - Boltzmann's constant; T – absolute temperature.
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An important parameter in the above equations is dislocation density (NV) or its equivalent - linear distances between the dislocations (LD). According to available data, in the alloys prepared
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by SPD (Severe plastic deformation) this value is close to 1014–1015 m-2 [54, 55] and even reaches 1015–1016 m-2 in the HEBM processing [56-59]. This is several orders of magnitude higher than in as-cast and annealed metals (105-106 m-2) [60]. Applying of the coefficients NV (or LD) and D to the equations (1) and (2) evidences
significant reduction of the parameter η and an increase of ε& D compared to conventional metallic powders produced by alternative methods. This obviously simplifies the formation of the interparticle necks between the powder particles during compacting processes, even at the low sintering temperature: 873 K for TiFe and LaNi5 and 573 K for Mg2Ni.
A peculiar combination of the steady bonding between the particles and sufficient porosity
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of the consolidated HEBM materials is the main reason for the observed durability in course of absorption-desorption cycling. The specific sponge-type microstructure makes these bulk samples stable for the dimensional changes and "breathing" at hydrogen uptake and release. Another important factor relates to the nanocrystalline nature of the HEBM alloys. The nanostructure of the
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metallic matrix (including the interparticle necks) provides the nanoanisotropy in the bulk samples that is why the linear scaling during hydrogenation and the resulting strain become much smaller.
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4. Conclusions
The results obtained allow us to conclude that the spectacular compactibility of
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intermetallic materials prepared through ball milling is based on a peculiar combination of their features. The excessive internal energy accumulated during HEBM enhances drastically the interdiffusion, and thus ensures high performance of the sintering process at relatively low temperatures via formation of specific necks between the particles. Besides, the nanostructured
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state of the HEBM materials facilitates the accommodation of the volume distortion of the crystallites at hydrogenation and finally reduces the resulting strain. The breathing sponge-like composites are of particular interest for a wide range of hydride forming alloys in order to
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improve their hydrogen storage performance as well as for a variety of other applications.
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Acknowledgments
The authors appreciate the help and discussions from Prof. F. Wakai from “Secure
Materials Center, Materials and Structures Laboratory, Tokyo Institute of Technology”. The work was support in part by the Ministry of Education and Science of the Russian Federation within the framework of the Increase Competitiveness Program of MISiS. Financial support from Russian Foundation for Basic Research (13-03-12424) is gratefully acknowledged.
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List of Figure Captions ACCEPTED MANUSCRIPT Fig. 1. DSC curves of the prepared powder samples: TiFe (a), Mg2Ni (b), LaNi5 (c) Fig. 2. General view and the surfaces images of the obtained bulk samples: TiFe (a, b); Mg2Ni (c, d); LaNi5 (e, f) Fig. 3. Thermal diffusivity of the bulk samples: TiFe (a); Mg2Ni (b); LaNi5 (c) before and
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after annealing
Fig. 4. Necking between the particles inside the samples consolidated after HEBM: TiFe bulk sample (a) and its nanocrystalline structure (b), Mg2Ni (c) and (d), LaNi5 (e) and (f); and
same procedure of compacting: LaNi5 (g) and TiFe (h)
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structure of the bulk samples produced from as-cast alloys without HEBM, but subjected to the
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Supporting Information
Fig. SI1 - XRD patterns of the TiFe intermetallic alloy samples: as-cast condition (a); after HEBM (b); bulk sample, after HEBM and subjected to subsequent compacting with annealing at 873 K (c)
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Fig. SI2 - XRD patterns of the Mg2Ni intermetallic alloy samples: as-cast condition (a); after HEBM (b); bulk sample, after HEBM and subjected to subsequent compacting with annealing at 573 K (c)
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Fig. SI3 - XRD patterns of the LaNi5 intermetallic alloy samples: as-cast condition (a); after HEBM (b); bulk sample, after HEBM and subjected to subsequent compacting with
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annealing at 873 K (c)
Fig. SI-4. Pressure composition isotherms for hydrogen absorption (closed symbols) and
desorption (open symbols) in nanocrystalline bulk samples: TiFe (a), Mg2Ni (b), LaNi5 (c) Fig. SI-5. Dilatometric analysis of the TiFe bulk hydrogenated samples: length change at heating (a); general view of the sample after 20 hydrogenation cycles (b); powder obtained after 1 hydrogenation cycle from the TiFe bulk sample produced by induction melting (c)
Table SI1. Effect of milling and compacting conditions on the structure parameters of the alloys
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- The bulk nanocrystalline intermetallic compounds have been obtained via high energy ball milling. - The bulk samples have the improved durability to the hydrogen absorptiondesorption process. - The improved durability is related to the formation of a peculiar sponge-type microstructure. - The sponge-type microstructure formation is possible due to the excessive internal energy.