In the present article we consider a novel approach to analyze the processes and kinetics of transformation of magnesium to hydride. Here approach consists to take into account the contributions of mechanics factors such as the work of external forces, the energy of elastic deformation and the energy required to form a unit volume nucleate of new phase. Request for a stable state of either a mechanical system or thermodynamic conditions governing a phase transformation, is to determine a minimum value to the total energy of the system. Analyzing the stability conditions needed when forming a metal hydride nucleus at constant temperature and pressure needs to consider the following: - the volumetric effects at the phase transformation, - the ratio of elastic moduli of metal to hydride phase, - the work spent at the formation of a unit volume of hydride. Under these considerations, it was shown that the most energetically favorable is forming an ellipsoidal hydride nucleus. Moreover, the larger difference in semi-axes, the more stable ellipsoid nucleus. Then, calculations of the stress-strain level states on both sides of the metal/hydride interface have been carried out. It was shown that the near-boundary range during the phase transformation is the place where accumulate inhomogeneous and intense stresses, which can contribute to two parallel processes. Firstly, increase of hydrogen concentration could appear in the distorted area. Secondly, a local accumulation of incoherent boundaries in between the two phases will develop stresses exceeding by more twice the shear yield stress. The presence of these compression zones could give rise new defects such as dislocations, micro-cracks. Consequently this should lead to decrease in magnitude of the powder of material.

Energy-level diagrams for cubic metallic Fe4N and Mn4N were proposed by Goodenough in the late 1960s. Fe4N is ferromagnetic, but Mn4N is ferrimagnetic with a large moment on Mnc at the cube corner site and a much smaller antiparallel contribution from Mnf at the three face-centre sites. Neutron diffraction revealed noncollinear ferrimagnetism with no compensation where the Mnf moments form 120° triangular antiferromagnetic sublattices but are tilted out of the kagome (111) planes to give the small net sublattice moment. A rich variety of magnetic ordering exists in the ternary Mn3−xM′xN metallic perovskites. Partial substitution of nonmagnetic M′ on Mnc sites leads to a tunable ferrimagnetic compensation point. Two possible antiferromagnetic modes in the kagome planes are a topological Γ4g mode, and a nontopological Γ5g mode where the in-plane components of the Mnf spins lie, respectively, perpendicular and parallel to the edges if the triangles in the kagome planes . Interest in the metallic perovskites has revived with the availability of high-quality thin films that facilitate measurements of magneto-transport properties, strain effects and spin wave velocity. The range of magnetic structures, magnetotransport, magnetocaloric and magnetovolume effects is exceptionally large. The topological ferrimagnets exhibit large anomalous Hall effects. The magnetism is compared with materials where N is replaced by C.

The microstructure transformation of ternary Ti-Cr-V alloys after introduction of hydrogen has been analyzed for a long time. Assessment of the impact of vanadium concentration and ratio of Ti and V concentrations on composition stability has been carried out. Investigated alloys system corresponds to relation (TiCr1.8)100–xVx. The atomic ratio Ti/Cr is constant. Vanadium content changes with the step 20 at.%. The hydrogen charging has been carried out in a thermostatic three-electrode electrochemical cell using 1M KOH electrolyte (ic = 10–30 mA/cm2) at 293 K for three hours. It was established that the hydrogen introduction leads to sur-face migration of alloy components. Their distribution oscillates as time passed. This is due to the fact that hydrogen interacts differently with titanium and vanadium. The electrolytic hydrogen introduction initiates deformation of the crystal lattice and self-diffusion of alloy atoms. The statistically nonuniform distribution of electrolytic hydrogen increases the intensity of the process. The relaxation of internal stresses leads to fur-ther redistribution of components. The observed changes depend on the vanadium content in the alloys and have a different character inside the grain and near grain boundaries. A significant change has been estab-lished for (TiCr1.8)60V40.

There are many different systems of an autonomous energy storage including accumulators and storage devices for renewable energy. Systems based on reversible metal hydrogenation have recently been introduced. The selection of metals is based on considerations of temperature and pressure conditions of the hydrogenation/dehydrogenation cycle, as well as the desired storage hydrogen capacity. Magnesium is one of the main challenging metals with respect to these main conditions since having a hydrogen capacity up to 7.6 w.%. For Mg forming MgH2, it was soon established that the size of particles plays a critical role since the kinetics (rate) of hydride formation accelerates when the size of the particles decreases. The present study shows that the overall diameter of the particles is the main characteristic controlling the kinetics of hydride formation because of distinct issues. A distribution of the size entails a strong dispersion transferring the heat of reaction, which characterizes Mg to MgH2 phase transition. Moreover, the formation of MgH2, is accompanied by a great increase of the unit-cell volume, developing noticeable internal stresses within the surface layers of the particles, thus turning to a systematic flaking and a systematic decrease of sizes of the powder particles. The results of the numerical modeling comply with the experimental data. This makes it possible to predict the best size of the initial Mg powder able to achieve fast kinetics during hydrogenation. Furthermore, the present analysis demonstrates the best hydrogenation kinetics, not only when using fine powders, but also when the deviation from the average particle size is minimized.

In order to optimize hydrogen storage properties of bcc Ti-V-Cr alloys it was found that alloying with a few 4 at% of Zr7Ni10 results in acceleration the hydrogen sorption kinetics in the composite material. The novel intergranular phase plays a role of gate for hydrogen, leading parallel to its easy decrepitate, thus enhancing fast formation of Ti-V-Cr hydrides. Nevertheless, the question on how such a composite microstructure affects hydrogen mobility in the material is still open. Here we report on the results of the studies of hydrogen self-diffusion in hydrogenated (TiCr1.8)(1-x)V-x based alloys (x = 0.2, 0.4, 0.6 and 0.8) carried out using proton nuclear magnetic resonance diffusiometry in a static field gradient For all compounds the method has proved itself as a powerful tool to probe the microstructure of the multicomponent alloys with inhomogeneous element distribution in a few micrometer scale. It has been found that addition of Zr7Ni10 lowers the activation energy of hydrogen motion in (TiCr1.8)(1-x)V-x alloys and leads appear two different diffusion areas. They can be associated with a redistribution of elements within the intra-granular phase due to opposite substitution of Ti and Ni atoms during synthesis and blurring the boundaries between the intra-granular and inter-granular phases. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

A series of (TiCr1.8)(1-x)V-x alloys with x = 0.2, 0.4, 0.6 and 0.8 was synthetized by induction melting of a mixture of TiCr1.8 alloy and pure V. It has been found that after hydrogenation all the compounds turn into bct structure. The present bct metal hydrides could be considered forming a pre-martensite state during the transformation (Ti-V-Cr)H-xmin (bcc) <-> (Ti-V-Cr)H-xmax (fcc). Proton NMR studies reveal that distribution of hydrogen over all samples is not homogeneous. The modified Bloembergen-Purcell-Pound model, which supposes existence of two hydrogen fractions in different metal environment, provides the lowest averaged activation energy of hydrogen motion corresponding to (TiCr1.8)(0.6)V-0.4, the composition that exhibits the highest reversible hydrogen capacity. Following a regularity in changes of both structural parameters of the studied compounds before and after hydrogenation and those parameters that answer for hydrogen mobility we anticipate the existence of a percolation threshold in properties of the alloys (close to the compositions x = 0.4 divided by 0.5). (C) 2018 Published by Elsevier B.V.

In-situ hydrogenated Mg-Ti-H films with different Ti contents (0 <= at.% Ti <= 19.5) were deposited in a single-step process using the microwave reactive plasma-assisted co-sputtering technique. Multi-phase films containing tetragonal beta-MgH2, metastable orthorhombic gamma-MgH2 and metastable hexagonal Mg-Ti were obtained depending on the Ti content. Subsequent to their deposition, the films underwent various thermal treatments aimed at studying their stability and sorption behavior. The structural and morphological properties were investigated by X-ray diffraction and scanning electron microscopy, before and after treatments. The thermal stability was studied by thermogravimetric and differential thermal analysis coupled with mass spectrometry. The results showed that the Mg-Ti phase remains structurally stable up to at least 500 degrees C after dehydrogenation, while the orthorhombic gamma-MgH2 phase no longer re-forms after its decomposition. A significant decrease of the Mg-Ti-H films desorption temperature compared to single phase MgH2 and TiH2 films was observed for the films with a Ti content in the 2.7-6.6 at.% Ti range. (C) 2018 Elsevier B.V. All rights reserved.

Received: 20.06.2014 Accepted: 24.06.2014 Published: 30.06.2018 Metal hydrides are among the optimum solutions for hydrogen storage in terms of effectiveness and safety. Magnesium and its alloys can reversibly absorb hydrogen in large amounts, so according to the DOE's requirements and making those materials attractive for applications. At first, determining a fast hydrogen saturation of Mg-based alloys consisted in grinding the materials up to micrometric grain size. A significant increase of the specific surface of the treated powders by plastic strain processing leads to delivering very reactive samples. Also, huge improvement of H-sorption characteristics of bulk Mg-alloys was shown to be efficient under Equal Channel Angular Pressing (ECAP) treatments. During ECAP treatments, the achievement of a fine grained microstructure in bulk samples is accompanied by the formation of a clear texture. The main achievements expected from the application of ECAP treatments to Mg-rich alloys are the formation of ultra-fine microstructures with high angle boundaries, which drastically changes the characteristics of the alloy; volume homogenization of the microstructure for the best final stability of the hydrogenation properties of the refined material. Since, in most cases, a two or even more ECAP passes should be applied to deliver highly reacting materials, the operating temperature must be adjusted in terms of ductile to fragile characteristics in order to avoid irreversible cracking of the bulk sample. After the application of the ECAP process, the resulting strain was characterized using different methods, such as mechanical engineering, numerical simulations and experimental methods. The present article reports on the sample strain process by using the grid evaluation method.

Mg-Ti-H films with a Ti content in the range 0 ≤ at.% Ti < 20 were obtained in a single-step process using the microwave reactive plasma-assisted co-sputtering technique. The morphology, crystal structure, chemical composition and spatial distribution of the elements were investigated by X-ray diffraction, scanning and transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy and precession electron diffraction. Our results show that the Ti-poor films (0 ≤ at.% Ti ≤ 0.45) exhibit mainly the tetragonal β–MgH2 phase and have a dense microstructure with discontinuous columnar grains. For the films with a medium Ti content (2.7 ≤ at.% Ti ≤ 6.6), the β–MgH2, metastable orthorhombic γ–MgH2 and Mg-Ti phases were found in different proportions, with a typical columnar growth. The abrupt microstructural changes of the films with a Ti content >10 at.% correlate with the change in crystallinity. These films exhibit a mainly amorphous/nanocrystalline structure with a granular morphology. We showed that the Ti content plays a significant role in the formation of structural and microstructural features of the Mg-Ti-H films, which might be a useful parameter in tuning the functional properties of magnesium hydride.

The paper reviews the state of the art of hydrogen storage systems based on magnesium hydride, emphasizing the role of thermal management, whose effectiveness depends on the effective thermal conductivity of the hydride, but also depends of other limiting factors such as wall contact resistance and convective exchanges with the heat transfer fluid. For daily cycles, the use of phase change material to store the heat of reaction appears to be the most effective solution. The integration with fuel cells (1 kWe proton exchange membrane fuel cell and solid oxide fuel cell) highlights the dynamic behaviour of these systems, which is related to the thermodynamic properties of MgH2. This allows for "self-adaptive" systems that do not require control of the hydrogen flow rate at the inlet of the fuel cell.

A thermal Energy Storage Unit (ESU) using liquid hydrogen has been developed as a solution for absorbing the heat peaks released by the recycling phase of a 300 mK cooler that is a part of the cryogenic chain of one of ESA's new satellites for science missions. This device is capable of storing 400 J of thermal energy between 15 and 16 K by taking advantage of the liquid-to-vapor latent heat of hydrogen in a closed system. This paper describes some results obtained with the development model of the ESU under different configurations and using two types of hydrogen storage: a large expansion volume for ground testing and a much more compact unit, suitable for space applications and that can comply with ESA's mass budget.

The critical exponents in Fe80−xVxB12Si8 (x=8, 10 and 13.7) amorphous alloys were investigated near ferromagnetic to paramagnetic phase transition temperature. All amorphous alloys exhibit a second order ferromagnetic to paramagnetic phase transition (SOMT). The critical exponents (β, γ and δ) were estimated using the modified Arrott plot technique (MAP), the Widom scaling relation (WSR), and the critical isotherm analysis (CIA). In addition, an independent analysis of the critical behavior is presented in terms of the magnetocaloric effect (MCE). It shows in accordance with conclusion from magnetization data analysis. The estimated critical exponent values are found to be consistent and comparable to those predicted by the mean field model. This result points out to the ferromagnetic exchange interaction of long-range type.

This study analyzes the mechanical behavior of compacted disks made of MgH2 powders co-milled with vanadium or Ti-V-Cr alloy as an additive, through the evolution of the microstructure and mechanical properties upon hydrogen cycling. The recrystallization of MgH2 particles results from a dynamic recrystallization phenomenon associated with the reaction of hydrogenation itself. The coalescence of the nanometric particles tends to create large agglomerates, which induces an increase in porosity, and explains the progressive swelling of the composites. A relaxation of the maximum strain is observed after 10 cycles for vanadium whereas for Ti-V-Cr the expansion increases until 200 cycles. This difference of behavior is correlated to the ability of vanadium particles to prevent the recrystallization mechanisms, then to limit the agglomeration of the MgH2 particles. From the Vickers hardness measured on compacted powders, a hardness of 0.58 GPa was estimated for highly densified magnesium hydride. Nano-indentation tests performed on compacted pellets show an enhancement of about 20% of the Young modulus as the amount of additives raises from 4 to 8 wt. %. The Young modulus markedly improves as the number of hydrogen cycles increases up to 10. The H/E ratio calculated from these data is characteristic of an intermediate state in between elastic and plastic behavior. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

One-dimensional Co-Nd ferrite nanostructures were prepared by controlled co-precipitation method. Co-Nd ferrite nanorods exhibits different magnetic phases: blocking phase with the corresponding blocking temperature (T-B) of about 350 K, metamagnetic phase with the corresponding Morin transition temperature (T-M) of about 550 K and canted ferrimagntic phase with the corresponding Neel temperature (T-N) of about 800 K. These novel magnetic one-dimensional structures can potentially be used in nanoelectronic devices, magnetic sensors, and flexible magnets. (C) 2015 Elsevier Ltd. All rights reserved.

Solubility of several transition metal chlorides (NiCl 2 , CrCl 2 , MoCl 3 , FeCl 2 ) was measured in KCl-AlCl 3 based melts. It was found that the solubility of studied metal chlorides depends on K : Al mole ratio. MoCl 3 solubility decreases with increasing AlCl 3 content. Solubility of CrCl 2 and FeCl 2 reaches maximum at K : Al ratio of 1 and decreases when this ratio either de-creases or increases. The dependence of NiCl 2 solubility on K : Al mole ratio is V-shaped with the maximum near 0.9–0.95. The effect of temperature on solubility of transition metal chlorides in KCl-AlCl 3 melts was also investigated. Increasing temperature does not alter the character of «solubility – K : Al mole ratio» dependences.

Solubility of several transition metal chlorides (NiCl2, CrCl2, MoCl3, FeCl2) was measured in KCl-AlCl3 based melts. It was found that the solubility of studied metal chlorides depends on K : Al mole ratio. MoCl3 solubility decreases with increasing AlCl3 content. Solubility of CrCl2 and FeCl2 reaches maximum at K : Al ratio of 1 and decreases when this ratio either de-creases or increases. The dependence of NiCl2 solubility on K : Al mole ratio is V-shaped with the maximum near 0.9–0.95. The effect of temperature on solubility of transition metal chlorides in KCl-AlCl3 melts was also investigated. Increasing temperature does not alter the character of «solubility – K : Al mole ratio» dependences.

Amorphous soft magnetic Fe 80−x V x B 12 Si 8 ribbons (0 ≤ x ≤ 14) have been fabricated by melt spinning technique, and their magnetic and magnetocaloric properties have been studied. The value of magnetocaloric effect has been determined from the measurements of magnetization as a function of temperature and an external magnetic field. The addition of vanadium to the ternary Fe 80 B 12 Si 8 alloy results in a decrease of the Curie temperature of amorphous alloys, T C , from 473.5 to 335 K. With an increasing V content, the average magnetic moment of Fe atom and the magnetic entropy change also decrease. Fe 66.3 V 13.7 B 12 Si 8 alloy exhibits the highest refrigeration capacity of 93.7 J kg −1 and moderate peak magnetic entropy of 1.034 J kg −1 K −1 ( T C = 335 K) under the maximum applied field of 2 T. The results from this work showed that V containing amorphous alloy 13.7 at. % is an interesting material and potential candidate for magnetic refrigerants working near room temperature. The observed −ΔS M max values compare favorably with other amorphous Fe-based alloys.

The thermal expansion and magnetostriction of the HoFe11-xCoxTi (0 <= x <= 11) alloys have been investigated, using the strain gauge technique in the temperature range 77-600 K. Both thermal expansion and thermal expansion coefficient exhibit an anomalous behaviour and Invar effect below Curie temperature (T-C =516 K) in sample with x=0. The increase of x in HoFe11-xCoxTi system leads to disappearing of the Invar effect and increasing of the average thermal expansion coefficient from 6.370 x 10(-6) K-1 in x=0 to 10.735 x 10(-6) K-1 in x=11 below room temperature. In addition, the spontaneous volumemagnetostriction decreases with Co content. The magnetostriction compensation point is observed in the anisotropic magnetostriction curve of all samples. The maximum value of anisotropic magnetostriction (Delta lambda approximate to 50 x 10(-6)) at room temperature is observed in sample with x=9. The saturation behaviour only appears in samples with x=5, 7 and 9. In samples with x=3, 5 and 7, a noticeable volume magnetostriction (Delta V/V) is observed in low temperature due to first-order magnetisation process. Moreover, Delta V/V exhibits a large anomaly about 45 x 10(-6) and 20 x 10(-6) around room temperature related to spin reorientation transition in samples with x=9 and 11, respectively. The results are discussed based on the local magnetic moment model and irreducible magnetoelastic coupling modes. (C) 2014 Elsevier B.V. All rights reserved.

A reliable process for compressing hydrogen and for removing all contaminants is that of the metal hydride thermal compression. The use of metal hydride technology in hydrogen compression applications, though, requires thorough structural characterization of the alloys and investigation of their sorption properties. The samples have been synthesized by induction – levitation melting and characterized by Rietveld analysis of the X‐ray diffraction patterns. Volumetric pressure–composition isotherm measurements have been conducted at 20, 60 and 90 °C, in order to investigate the maximum pressure that can be reached from the selected alloys using water of 90 °C. Experimental evidence shows that the maximum hydrogen uptake is low since all the alloys are consisted of Laves phases, but it is of minor importance if they have fast kinetics, given a constant volumetric hydrogen flow. Hysteresis is almost absent while all the alloys release nearly all the absorbed hydrogen during desorption. Due to hardware restrictions, the maximum hydrogen pressure for the measurements was limited at 100 bars. Practically, the maximum pressure that can be reached from the last alloy is more than 150 bars. Copyright © 2014 John Wiley & Sons, Ltd.

Amorphous soft magnetic Fe80-xVxB12Si8 ribbons (0 <= x <= 14) have been fabricated by melt spinning technique, and their magnetic and magnetocaloric properties have been studied. The value of magnetocaloric effect has been determined from the measurements of magnetization as a function of temperature and an external magnetic field. The addition of vanadium to the ternary Fe80B12Si8 alloy results in a decrease of the Curie temperature of amorphous alloys, T-C, from 473.5 to 335 K. With an increasing V content, the average magnetic moment of Fe atom and the magnetic entropy change also decrease. Fe66.3V13.7B12Si8 alloy exhibits the highest refrigeration capacity of 93.7 J kg(-1) and moderate peak magnetic entropy of 1.034 J kg(-1) K-1 (TC = 335 K) under the maximum applied field of 2 T. The results from this work showed that V containing amorphous alloy 13.7 at. % is an interesting material and potential candidate for magnetic refrigerants working near room temperature. The observed -Delta S-M(max) values compare favorably with other amorphous Fe-based alloys.