Understanding of high-pressure torsion (HPT) processing in case of bulk as well as powder materials for relatively thick-samples

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Lightweight metal hydrides

Lightweight metal (Li, Be, Na, Mg, B and Al) based hydrides have a much higher storage capacity by weight, and are therefore preferred for automotive applications. They are especially interesting due to their light weight and the number of hydrogen atoms per metal atom, which is in many cases at the order of H/M = 2. Sodium, lithium and beryllium are the only elements lighter than magnesium that can form solid-state compounds with hydrogen known as complex hydrides. The hydrogen content reaches the value of 18 wt% for LiBH4 [1.14,1.15]. Use of complex hydrides for hydrogen storage is challenging because of both kinetic and thermodynamic limitations. Although the storage capacities of complex hydrides are theoretically high, there is a big difference between the theoretical and the practical attainable hydrogen capacities. The complex hydrides released hydrogen by the step reactions unlike the metallic hydrides. Repeated hydrogenation cycles need to be applied for the potential applications to ensure the reversibility of these materials. Moreover, the slow kinetics problem is a significant obstacle for practical on-board applications [I.7].
Among the lightweight metal hydrides, Mg-based materials for on-board hydrogen storage applications appeal significant research interests mainly due to its high gravimetric and volumetric hydrogen storage capacity, low cost and abundant on the earth’s crust, and its hydrides are reversible and recyclable [1.16,1.17]. However, the main obstacles of Mg-based materials for practical applications are the high temperature of hydrogen desorption, slow absorption/desorption kinetics and a high reactivity towards air and oxygen. High thermodynamic stability of MgH2 results in a relatively high desorption enthalpy, which corresponds to an unfavorable desorption temperature of above 300 C at 1 bar [1.18].
Nevertheless, the Mg-based materials are potential candidates for the following applications:
1) On-board hydrogen storage.
2) Heat-storage and.
3) Stationary and portable energy storage.
Most of the research in Mg based hydrogen storage materials so far aims at on-board storage. According to the targets from DOE, for this kind of applications, it requires a gravimetric storage capacity of at least 4.5 wt% (i.e. 1.5 kWh/kg system) for the year 2020 [1.5]. Although MgH2 can store 7.6 wt% H, when considering the additional weight for necessary components including tank body and heat transfer additives besides the hydrogen storage materials in the system (some amount of catalysts and other elements are necessary to enhance the kinetics, and also to reduce the thermodynamic stability by alloy formation), important issues (rather slow kinetics and high operating temperatures) remain for Mg based materials to be applied for on-board applications based on current technology and the capacity requirement.

Modifications of hydrogen sorption properties

The solid-state hydrogen storage materials possess the highest potential to be used together with fuel cell technology for the generation of electric power in a clean, inexpensive, safe and efficient manner. However, one of the drawbacks for this system is its rather sluggish kinetics in the charging/discharging processes, and, at least for practical applications, strategies have to be developed to solve this problem. The hydrogen sorption kinetics of the lightweight metal hydrides is mainly influenced by:
– particle size/surface area.
– crystallite size/large fraction of grain boundaries.
– additives/catalysts.

Severe plastic deformation (SPD) techniques

Severe plastic deformation (SPD) is a general term used to describe a group of techniques that produce large strains in the material which in turn result in a high defect density and nanocrystalline structure. The SPD techniques could replace and/or complement HEBM for the synthesis and preparation of metal hydrides. The advantages of SPD over the ball milling are that usually SPD can be easier to scale up to industrial level, the impurity level is lower, and it has fewer safety concerns [1.24]. Moreover, due to the comparably short processing time and relatively simple setup, including those for continuous SPD processing, the production costs can be dramatically reduced.
The use of SPD techniques in the processing of metal hydrides is relatively new, and in this field of research is rapidly increasing. Processing by SPD produces multiple defects in the crystalline lattice such as vacancies and dislocations and this has a positive effect on the diffusion kinetics. For example, defects could act as nucleation site for a chemical reaction (such as hydrogenation) while grain boundaries could act as fast diffusion pathways. Therefore, these techniques are particularly attractive in the synthesis and preparation of metal hydrides. Recently, the most popular SPD techniques for the processing of Mg-based metal hydrides include equal channel angular pressing (ECAP), cold rolling (CR) and high-pressure torsion (HPT). These techniques and their related results are shortly discussed in the following paragraphs.

Equal channel angular pressing (ECAP)

In equal channel angular pressing (ECAP) technique, severe plastic deformations are introduced into a material by forcing a sample (billet) with a piston through a die consisting of two channels of equal cross-section, which intersect at an angle (Φ) between 90° and 120° [1.25], see Figure 1.3. The outer arc of curvature where the two channels intersect is labeled Ψ. Since the billet assumes the form of the cross-section of the die, it can be repeatedly processed to increase the micro-strains and reduce the crystallite size in the material. ECAP is quite efficient in processing metals and alloys producing porosity-free materials with average crystallite sizes between 2 μm and 100 nm in substantial quantities with lower concentration of impurities and at a lower cost than ball-milling [1.26]. Through the grain refinement process, the proportion of high-angle grain boundary increases due to dislocations recovery. While ECAP has emerged as well-known procedure for the fabrication of ultrafine-grained metals and alloys with enhanced mechanical and functional properties, it is the recent years that have been investigating the effects of ECAP processing on H-sorption properties of Mg-based hydrogen storage materials.

High-pressure torsion (HPT)

Compared to other SPD techniques, high-pressure torsion (HPT) is a relatively simple and quick processing technique, and also easy to apply on powder materials. It is also very efficient to produce small grain size and large fraction of high-angle grain boundaries. Among the HPT processing routes available, the more versatile one is under processing through quasi-constraint conditions [1.37]. The principle of a quasi-constraint HPT facility is schematically illustrated in Figure 1.6. The sample, generally in the form of a thin disk, is located between two anvils within a cavity. A hydrostatic pressure is applied and plastic torsional straining is achieved by rotation of the lower anvil or both. The diameter of the cylindrical cavities and the initial diameter of the HPT sample are identical. However, the sum of both depths of the cavities is somewhat smaller than the initial height of the HPT sample. This implies that during loading a small amount of the material will flow laterally in the ring shaped region between the two anvils. The friction in this region confines the free flow of the material out of the HPT tool and leads to a back pressure and induces a defined hydrostatic pressure within the processing zone. More details of this technique can be found in Ref. [1.37].

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High-pressure torsion (HPT)

The bulk samples were processed by a severe plastic deformation (SPD) processing route using high-pressure torsion (HPT) to interpret the deformation characteristics of relatively thick-samples. Among the three types of HPT processing conditions, a more versatile one – quasi-constrained HPT facility [1.37] where the materials lateral flow is partially restricted – was employed in the present study. A photograph of the HPT facility and its associated experimental setup available at the laboratory LEM3 are given in Figure 2.5. The left-side of figure shows a hydraulic press along with the experimental set up at the higher magnification. A sketch of the upper and lower anvils is provided at the right-side of the figure. The geometry of both anvil dies used in the present study is given as: depth of the depression – 1.2 mm, the angle of the lateral wall – 15° and lower diameter of the depression – 20 mm.
The bulk thick-samples with 20 mm in diameter and 3 mm in thickness were processed by the single-step HPT facility. The disk-shape samples were first put in the depression of the lower anvil followed by bringing down the upper anvil to compress the sample up to the intended hydrostatic pressures. Immediately after reaching to the applied pressure (~ 1.2 GPa), the motor was run to rotate the lower anvil at a constant angular speed of 0.125 rpm in the direction as shown in Figure 2.5. During the HPT processing, a significant amount of strains was imposed to the materials by rotating the lower anvil up to 180° and 270°. The above experiments were conducted at room temperature and under quasi-constrained conditions.

Two-step HPT consolidation

The powder samples were consolidated into bulk products by using a two-step HPT processing route as illustrated in Figure 2.6. In step – 1, the powder was first compacted by uniaxial compression under a hydrostatic pressure of 1.5 GPa holding for 10 min into an intermediate body in a form of disk with 20 mm diameter and 3 mm thickness. In step – 2, the intermediate disks were subsequently deformed by torsional straining using the quasi-constrained HPT facility (see in Figure 2.5) under a hydrostatic pressure of 1.2 GPa. Different amounts of strains were introduced into the disks by varying the torsional straining up to different levels of revolutions.
Both HPT treatments were carried out at room temperature. After the HPT processing, all the HPT-processed samples (obtained from the bulk or the powder materials) were preserved at -80 °C in order to avoid any kind of restoration processes associated with the low homologous temperature materials (i.e. Al, Mg etc.) [2.5].

Evolutions of microhardness

Vickers microhardness measurements were conducted across the through-thickness as well as along the radial direction of the HPT-disk where the positions selected for measurements can be found from Figure 3.7(e). The average values of microhardness after HPT for the top, middle and bottom planes of the disks are displayed at four radial positions in Figure 3.7(a) and 3.7(c) for the Al alloy and Mg, respectively. For comparison, the hardness values of the initial sample (dashed lines) and after the compression stage (see Figures 3.7(a, c)) are also shown. It is clearly visible that, even after the initial compression stage, the hardness values increases from the center to the periphery of the compressed disk. This is consistent with other results from the literature which showed both from modeling and experimental approaches that the central regions are less deformed than the outer ones in the compressed state which results in different dislocation densities [3.15].

Table of contents :

Chapter 1: Bibliographic review and scope of thesis work
1.1 Hydrogen: a clean energy vector for future
1.2 Options for hydrogen storage
1.2.1 Solid-state storage of hydrogen
1.2.2 Lightweight metal hydrides
1.3 Modifications of hydrogen sorption properties
1.3.1 Mechanical effects of ball milling
1.3.2 Severe plastic deformation (SPD) techniques
1.3.3 Influence of additive/catalysts
1.4 Scope of thesis work
Chapter 2: Experimental materials, methods and characterization techniques ..
2.1 Selected materials
2.2 Processing routes
2.2.1 Arc-plasma method
2.2.2 High-pressure torsion (HPT)
2.2.3 Two-step HPT consolidation
2.3 Characterization techniques
2.3.1 X-ray tomography (XRT)
2.3.2 X-ray diffraction (XRD) .
2.3.3 Hardness testing
2.3.4 Scanning electron microscope (SEM)
2.3.5 Electron backscatter diffraction (EBSD)
2.3.6 SEM based transmission Kikuchi diffraction (SEM-TKD) ..
2.3.7 Transmission electron microscope (TEM)
2.3.8 Sievert-type apparatus
2.3.9 Differential scanning calorimetry-thermogravimetry (DSC-TG)
2.3.10 Raman Spectroscopy
Chapter 3: Understanding of high-pressure torsion (HPT) processing in case of bulk as well as powder materials for relatively thick-samples
Part I: Effects of processing conditions on heterogeneities in strain, microstructure and hardness in two bulk materials: aluminum alloy and commercial purity magnesium
3I.1 X-ray tomography
3I.1.1 Gradients in shear deformation
3I.1.2 Strain analysis
3I.2 EBSD characterizations
3I.2.1 Aluminum alloy
3I.2.2 Commercial purity Mg
3I.3 Evolutions of microhardness
3I.4 Discussion
Part II: HPT consolidation of two distinct Mg powders: Influences of nature of initial powder precursors on evolutions of microstructure, texture and strength
3II.1 Structural characterizations
3II.1.1 X-ray diffraction (XRD)
3II.1.2 SEM observations
3II.2 EBSD and TKD characterizations
3II.2.1 Micro-HPT product obtained from atomized Mg
3II.2.2 Nano-HPT product obtained from condensed Mg
3II.3 Microhardness evolution
3II.4 Discussion
3II.4.1 Effects of severe plastic deformation through HPT
3II.4.2 Significance of MgO oxides
Chapter’s conclusions
Chapter 4: Study of improvements in hydrogen storage properties of high-pressure torsion (HPT) consolidated magnesium products
4.1 First hydrogenation kinetics
4.2 Hydrogen sorption properties
4.2.1 Thermodynamics of absorption/desorption
4.2.2 Kinetics of hydrogen absorption
4.2.3 XRD of the hydrided products
4.3 Desorption performances
4.4 Microstructural modifications upon cycling
4.5 Discussion
4.5.1 Consequences of the processing route
4.5.2 Influences of the nature of initial powder precursors
4.5.3 Effects of the absorption/desorption cycling
Chapter’s conclusions
Chapter 5: Hydrogen storage properties of Mg-Fe based composites fabricated by high-pressure torsion consolidation of magnesium and iron powder precursors
5.1 Structural characterizations of the HPT composites
5.1.1 Consolidated microstructures
5.1.2 XRD analysis of the HPT composites
5.1.3 XRD analysis of the hydrided products
5.2 First hydrogenation kinetics
5.3 Hydrogen sorption properties
5.3.1 Thermodynamics of absorption/desorption
5.3.2 Kinetics of hydrogen absorption
5.3.3 Hydrogen desorption by thermal decomposition
5.4 Discussion
5.4.1 Effects of the processing route
5.4.2 Nature of the initial powder precursors
5.4.3 Catalytic effects
Chapter’s conclusions
Chapter 6: Hydrogen storage properties of as-synthesized and high-pressure torsion (HPT) consolidated magnesium-graphene based composites
6.1 Structural characterizations
6.1.1 Morphology and microstructure
6.1.2 X-ray diffraction (XRD) analyses
6.1.3 Raman spectroscopic analysis
6.2 First hydrogenation characteristics
6.3 Hydrogen sorption properties
6.3.1 Thermodynamics of absorption/desorption
6.3.2 Kinetics of hydrogen absorption
6.3.3 Hydrogen desorption behaviors
6.4 Discussion
6.4.1 Peculiarity in powder formation and its consequences
6.4.2 Effects of severe plastic deformation through HPT
Chapter’s conclusions


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