Hydrogen absorption/desorption cycling 

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Synthesis of the alloys and related hydrides

The alloys reported in this manuscript were prepared from pure elemental powders, or bulk pieces, and used as purchased without further treatment. Each element was weighted on microbalances Metler Toledo (±0.0001 g) according to the desired stoichiometry. Table 2.1 lists the optimized composition of the main MPEAs studied in this project. Subsequently, the metals were mixed following the protocol for each synthetic approach: powder elements were used for ball milling, while bulk pieces (sheets, rods, pellets, or chips) were used for high-temperature arc melting. The characteristics of the as purchased elements are described in Table 2.2. For the direct hydride syntheses (reactive ball milling) and the hydrogenation experiments, pure H2 from Alphagaz grade 6 N (99,9999 %) was used with a maximum pressure of 80 bar. For neutron diffraction experiments, deuterium gas (D2) from Alphagaz (99.999 %) was used instead of hydrogen.

High-energy ball milling

High-energy ball milling (HEBM) is a mechanical alloying process in which powdered metals are alloyed by the collision of grinding balls inside a mill, fusing the elements (cold-welding), reducing the particle size to μm range and crystallite size to the nm scale[1]. There are different ball milling instruments, or setups, which are characterized by the motion of the mill, namely: attritor, vibrational (or shaker), drum, horizontal, and planetary mills. Planetary mills are one of the preferred instruments at a laboratory scale due to its high efficiency for alloying, yielding a fine and homogeneous powder material suitable to study the properties of alloys and nanocomposites[2].

High-temperature arc melting

Arc melting is a high-temperature fusion technique (HT-AM) widely used in the production of alloys at the industrial level, e.g. in steel manufacture, but it is also commonly found in laboratories at a smaller scale. Contrary to other high-temperature techniques in which the temperature is increased by heat radiation, HT-AM uses an electric discharge to melt and fuse the elements.
A typical laboratory arc melting instrument consists of a vacuum chamber containing two electrodes set at low potential difference (voltage) and connected to a welder generator to supply an electron current. When the current is high enough, an electric arc is created between the electrodes passing through the material inside the sample chamber, as illustrated in Figure 2.2. The flow of electrons heats the metals above their melting point until the fusion of the elements takes place. The MPEAs were synthesized using a homemade arc melting furnace with a tungsten tip used as the cathode, and a water-cooled copper crucible was used as the anode and sample holder.
Each MPEA was prepared by cutting and weighting bulk pieces of the elements for a total mass of 3 g per alloy. The metals were placed into the copper crucible and evacuated using secondary vacuum (10-5 mbar) for 2 hours. Next, around 400 mbar of Ar were introduced as the inert atmosphere. The welder generator was set to supply a constant current of 90 amperes and the electric discharge was maintained for nearly 60 seconds in each melting. The alloy was re-melted 15-20 times to ensure complete homogeneity of the elements, flipping the ingot over after each melting.

Hydrogenation cycling test

Other important properties for hydrogen storage materials, besides their maximum capacity and kinetics, are the life-cycle and reversibility of the absorption/desorption reaction over several hydrogenation cycles. It is known that alloys can suffer from hydrogen-induced decomposition where the material loses part of its capacity due to irreversible structural deformation, e.g. phase segregation. This evaluation consists of measuring the hydrogen storage capacity of the alloys over several hydrogen absorption and desorption cycles.
Between 300-500 mg of the MPEAs were loaded into a stainless steel cell and submitted to activation before the first hydrogenation cycle, heating the sample under dynamic vacuum (340 °C, 10-5 mbar). All cycling tests were carried out at 25 °C and a final equilibrium pressure of 25 bar. Hydrogen desorption was carried out by heating the sample to 350-400 °C under a continuous secondary vacuum for 4-10 hours (10-5 mbar). All experimental conditions are listed in Table 2.6 for each alloy composition.

Thermo-desorption spectroscopy

Thermo-desorption spectroscopy (TDS) is a characterization technique used to study the desorption behavior of the hydrides by supplying thermal energy at a constant heating rate while under dynamic vacuum. Upon heating, the thermal energy destabilizes the hydride phase, causing hydrogen to desorb from the material in the form of a gas[5]. A mass-spectrometer is coupled to the instrument to analyze the gases during desorption. The partial pressure of the hydrogen released is then plotted as a function of temperature revealing a desorption profile, where certain information can be subtracted such as the onset temperature for desorption (Tonset) and the temperature at maximum desorption rate (Tmax).
TDS measurements were performed in a homemade instrument (Figure 2.6) that consists of a quadrupole mass spectrometer (QMS), Microvision Plus RGA from MKS instruments, connected to a vacuum rig and a turbopump for secondary vacuum pressure (10-6 mbar). A silica tube, which holds the sample, is connected at the instrument and placed inside an electric furnace. The temperature of the sample is monitored by a thermocouple in close contact with the material.

Laboratory X-ray diffraction

X-ray radiation is one of the most common probes used to study diffraction (XRD) due to easy accessibility in laboratory instruments, generating wavelengths with magnitudes close to the unit cell parameters of many crystalline materials. In laboratory XRD, X-rays are produced by bombarding electrons towards a metal target, exciting the core electrons of the atom and emitting characteristic X-rays by the photo-emission effect. Here, a Bruker D8 Advance powder diffractometer was used for the structural characterization of the MPEAs before and after hydrogenation. The characteristic X-rays come from the Kα emission from a copper anticathode with wavelength λ= 1.5406 Å. And the scanning ranged from 20-90° in 2θ domain.

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Synchrotron radiation X-ray diffraction

Synchrotron radiation (SR-XRD) is a more powerful source for X-ray diffraction, using high energy X-rays of 3-40 keV[9]. In SR-XRD, electrons are accelerated near the speed of light and injected into a vacuum ring. The highly energetic electrons are bent using strong magnetic coils to run through the circumference of the ring. The change in the trajectory of the electron beam releases electromagnetic radiation, X-rays, with small and tunable wavelengths. There are many advantages of SR-XRD over laboratory XRD, few worth mentioning are: the radiation wave from SR sources has a higher brightness and brilliance than that from laboratory X-rays, which improves the signal-to-noise ratio and reduces the time of acquisition for measurement; the X-ray beam is highly collimated, and monochromatic, granting a better 2θ angle resolution, and; the energy spectrum for X-rays produced in SR can be tuned on command for different purposes[10].
SR-XRD measurements were performed at the Cristal beamline in SOLEIL synchrotron facilities in France. The hydride samples were prepared by grinding the material below 20 μm particle size using a hand-pestle and mortar and subsequently loaded into capillary tubes of 0.1 and 0.2 mm of diameter. The silica capillary was cut to 5 cm length and sealed using a torch. The energy of the X-rays was set to 17.008 keV, equivalent to a wavelength of λ= 0.72896 Å. The acquisition time was of 10 minutes approximately in a scanning range from 1° – 85° (2θ) and the collection of data was carried out using a Debye-Scherrer (or capillary) geometry.

Neutron diffraction

Neutrons are small particles that show wave-like properties such as interference due to the wave-particle dualism proposed by Louis de Broglie. This allows the use of neutrons as a probe to exploit Bragg’s law of diffraction. There are some notable differences when using X-rays and neutrons as diffraction probes due to their nature and interaction with matter. X-rays are electromagnetic waves that are scattered from the interaction with the electron cloud, and its cross-section typically increases whit the atomic number, Z. Lightweight elements such as hydrogen have none-to-little interaction with X-rays. Elements with low, or none, scattering cross-section are considered transparent to diffraction. On the other hand, neutrons are non-charged particles that will interact mainly with the nuclei of the atom. The size of the cross-section in neutron scattering does not follow a trend in the periodic table of the elements and instead, this seems random. Interestingly, neutrons have a significant large cross-section for smaller elements like hydrogen, and can even discriminate between its isotopes (Figure 2.10). This is particularly attractive for the structural characterization of hydrides because it allows to locate hydrogen within the unit cell. Although, hydrogen is usually replaced with deuterium to have a better diffraction resolution. It is because of these differences that neutron and X-ray diffraction are complementary techniques for structural characterization.

Table of contents :

1.1 Hydrogen: energy carrier
1.2 Hydrogen storage
1.2.1 Physical storage
1.2.2 Chemical storage
1.3 Metal hydride formation
1.4 High-entropy alloys
1.4.1 Hydrogen storage in bcc HEAs
1.5 Project objectives and proposal
2.1 Synthesis of the alloys and related hydrides
2.1.1 High-energy ball milling
2.1.2 High-temperature arc melting
2.1.3 Hydride synthesis: hydrogenation of alloys
2.2 Hydrogen absorption characterization
2.2.1 Sievert’s methodology
2.2.2 Kinetics of absorption
2.2.3 Pressure-composition isotherms
2.2.4 Hydrogenation cycling test
2.3 Thermal analyses
2.3.1 Thermo-desorption spectroscopy
2.3.2 Differential scanning calorimetry
2.4 Structural characterization: diffraction techniques
2.4.1 Principle of diffraction
2.4.2 Laboratory X-ray diffraction
2.4.3 Synchrotron radiation X-ray diffraction
2.4.4 Neutron diffraction
2.4.5 The Rietveld method: structural refinement
2.5 Microstructural characterization
2.5.1 Scanning electron microscopy
2.5.2 Energy dispersive spectroscopy
(I) Chemical Optimization
(I.I) Equimolar compositions
(I.II) Non-equimolar composition
(II) Analysis of three different syntheses methods.
(II.I) Synthesis of Ti-V-Zr-Nb: ArBM and HT-AM
(II.II) Synthesis of the hydride Ti-V-Zr-Nb-H by RBM
(II.III) Hydrogenation of the bcc alloys: ArBM and HT-AM
(II.IV) Hydrogen desorption of the hydrides: ArBM, HT-AM, and RBM
(II.V) Hydrogen absorption/desorption cycling
(II.VI) Summary and conclusions
3.1 Synthesis of Ti-V-Zr-Nb
3.2 Hydrogenation of Ti-V-Zr-Nb
3.3 In-situ neutron diffraction
3.4 Thermo-desorption characterization
3.5 Hydrogen absorption/desorption cycling
3.6 Discussion with reported cases in the literature
4.1 Synthesis of Ti-V-Zr-Nb-Ta
4.2 Hydrogenation of Ti-V-Zr-Nb-Ta
4.3 In-situ neutron diffraction
4.4 Thermo-desorption characterization
4.5 Hydrogen absorption/desorption cycling
4.6 Comparison with the quaternary alloy
5.1 Synthesis of Ti-V-Zr-Nb-Al
5.2 Hydrogenation of Ti-V-Zr-Nb-Al
5.3 In-situ neutron diffraction
5.4 Thermo-desorption characterization
4.5 Hydrogen absorption/desorption cycling
5.6 Comparison with the quaternary alloy
6.1 Synthesis of Ti-V-Zr-Nb-Mg
6.2 Hydrogenation of Ti-V-Zr-Nb-Mg
6.3 In-situ neutron diffraction
6.4 Thermo-desorption characterization
6.5 Hydrogen absorption/desorption cycling
6.6 Comparison with the quaternary alloy
6.5 Bibliography


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