V-based Alloys Research Articles

A modified embedded-atom method interatomic potential for the V–H system

An interatomic potential for the vanadium–hydrogen binary system has been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) potential formalism, in combination with the previously developed potentials for V and H. Also, first-principles calculation has been carried out to provide data on the physical properties of this system, which are necessary for the optimization of the potential parameters. The developed potential reasonably reproduces the fundamental physical properties (thermodynamic, diffusion, elastic and volumetric properties) of V-rich bcc solid solution and some of the vanadium hydride phases. The applicability of this potential to the development of V-based alloys for hydrogen applications is discussed.

Mechanical properties and defective effects of bcc V–4Cr–4Ti and V–5Cr–5Ti alloys by first-principles simulations

V–(4–5) wt.% Cr–(4–5) wt.% Ti alloys are important candidate structural materials for the first-wall and blanket in future fusion reactor. Thus it is necessary to study the fundamental mechanical properties and the irradiation effects of the V-based alloys. Within a random solid solution model, the elastic constants and ideal strength of the V–4Cr–4Ti and the V–5Cr–5Ti alloys were calculated and compared with those of pure V solid. According to the theoretical Cauchy pressure and the ratio of bulk modulus and shear modulus, both alloys exhibit good ductility. Within the 250-atom supercell, inclusion of one vacancy defect or one interstitial H (He) atom will further enhance the ductility of these alloys.

Hydrogen transport through V 85Ni 10M 5 alloy membranes

Despite their inherent high permeability, unalloyed body-centred cubic (BCC) metals are prone to brittle failure due to their excessive hydrogen solubility. The primary challenge for BCC metal membrane development is therefore to control the solubility to a point where embrittlement is inhibited, while increasing the rate of hydrogen diffusion through the alloy. This can be potentially achieved through alloying, with several V-based BCC alloys exhibiting much improved resistance to embrittlement compared to pure vanadium, while maintaining higher hydrogen permeabilities than palladium alloys. While several binary and ternary V-based alloys have been investigated, a systematic approach is needed to properly evaluate potential alloying elements. In response, alloys of the general formula V 85Ni 10M 5 (atom%), where M is Si, Mn, Fe, Co, Ni, Cu, Pd, Ag, or Al have been fabricated, coated with 500 nm of Pd, and their microstructure, hydrogen permeability and hydrogen solubility evaluated. The results obtained showed small compositional variations can lead to large changes in permeability, whereas diffusivity is less-dependent on composition. Formation of multi-phase microstructures can enhance the permeability by increasing the vanadium content and hydrogen solubility of the BCC primary phase. The multiphase V 85Ni 10Ti 5 alloy exhibited a hydrogen permeability of 9.3 × 10 −8 mol m −1 s −1 Pa −0.5 at 400 °C.

Advanced hydrogen storage alloys for Ni/MH rechargeable batteries

Hydrogen storage alloys are of particular interest as a novel group in functional materials owing to their potential and practical applications in Ni/MH rechargeable batteries. This review is devoted to the specific alloy families developed for high-energy and high-power Ni/MH batteries in the last decades, especially for EV, HEV and PHEV applications. The scope of the work encompasses principles of Ni/MH batteries, electrochemical hydrogen storage thermodynamics and kinetics, prerequisites for hydrogen storage electrode alloys and recent advances in hydrogen storage electrode alloys. Rare earth AB5-type alloys, Ti- and Zr-based AB2-type alloys, Mg-based amorphous/nanocrystalline alloys, rare earth-Mg–Ni-based alloys and Ti–V-based alloys are highlighted. Additionally, the challenges met in developing advanced hydrogen storage alloys for Ni/MH rechargeable batteries are pointed out and some research directions are suggested.

First-principles study of hydrogen behavior in V–Cr–Ti alloys

V–Cr–Ti alloys are considered as one of the most promising candidate structural materials for fusion reactor. Hydrogen impurities originated from bombarding with reaction plasma as well as various nuclear transmutation reactions have a considerable influence on the performance and stability of V–Cr–Ti alloys. To understand the microscopic mechanism, we have simulated the behaviors of H atoms inside the pure vanadium solid as well as V-based alloys (V–4Cr–4Ti and V–5Cr–5Ti) using first-principles approach. The binding energies or absorption energies for H atom inside different interstitial sites were compared to determine the optimal trapping sites. Interaction between two interstitial H atoms was calculated and fit into a screened Coulomb potential. H atom prefers to diffuse along path between two nearest interstitial sites with low energy barriers of 0.20–0.34eV.

V–W alloy membranes for hydrogen purification

The alloying effects of tungsten on the hydrogen solubility and the hydrogen permeability are investigated for V-based hydrogen permeable membranes. The hydrogen solubility is found to decrease by the addition of tungsten into vanadium or by increasing the temperature. It is shown that the ductile fracture occurs for V–5 mol%W alloy even in the hydrogen pressures of 0.3 MPa at 773 K. It is also found that the mechanical properties (i.e., strength and ductility) of V-based alloy are better than that of Nb-based alloy in hydrogen atmosphere at high temperature. It is demonstrated that the V–5 mol%W alloy possess excellent hydrogen permeability without showing any hydrogen embrittlement when used under appropriate permeation conditions, i.e., temperature and hydrogen pressures.

Influence of processing conditions on the microstructure and permeability of BCC V–Ni membranes

V-based alloy membranes with the body-centred-cubic structure are of great interest for hydrogen separation applications due to their low cost and high permeability. As microstructure can greatly influence membrane performance, internal microstructures resulting from different processing conditions, and their effects on hydrogen permeability, have been investigated for the V–15Ni (wt%) BCC alloy. The initial coarse-grained, as-cast microstructure evolved into a fibrous/lamellar microstructure with a small grain size during cold-rolling deformation, and a significant reduction in hydrogen permeability accompanied this deformation. Subsequent annealing decreased the defect density and increased the grain size and hydrogen permeability. These results show that, aside from compositional optimization of these BCC alloys to minimize the effects of hydrogen embrittlement, the control of microstructural defects is central to the development of high-permeability alloy membranes.

Post-Hydrogen Permeation Characterization of V-Based Crystalline Alloy Membranes

The surface preparation and hydrogen embrittlement in particular are research challenges facing the practical application of vanadium alloy membranes. These two issues are addressed by surface characterization and fracture analysis in order to find the reasons why delamination and crack failures occur during hydrogen permeation. Post-failure analysis of the hydrogen-induced cracking membrane specimen suggests a new failure mechanism for hydrogen embrittlement.

Electrocatalysis induced by surface-modification with Pd through sol–gel method for Ti 33V 20Cr 47 alloy

A body-centered-cubic (bcc) phase Ti 33V 20Cr 47 alloy surface-modified with Pd particles was prepared through sol–gel method. The composite showed significantly improved electrochemical hydrogen release capacities, reaching 225 mAh g −1 at a discharge current of 60 mA g −1 at 293 K in the second cycle. Such a large capacity could be attributed to the contribution of catalysis of Pd particles, which induced the Ti 33V 20Cr 47 alloy to release hydrogen more easily. These results provide a new approach to wide applications of Ti–V-based bcc phase alloys in high-energy batteries.

Pulverization mechanism of the multiphase Ti–V-based hydrogen storage electrode alloy during charge/discharge cycling

Pulverization is an important key factor for the electrochemical cycle stability of many hydrogen storage alloys. In this paper, the pulverization mechanism of the multiphase Ti–V-based hydrogen storage alloy which mainly consists of a V-based solid solution phase with the BCC structure and a C14 Laves phase is studied based on a sample material of the Ti 0 .8 Zr 0 .2 V 2.7Mn 0.5Cr 0.6Ni 1.25Fe 0.2 alloy. The microstructure of the alloy and the morphology change of the alloy electrode during the charge/discharge process were observed by transmission electron microscope, scanning electron microscope and atomic force microscope, etc. The effect of mechanical properties of the V-based phase and the C14 Laves phase on the pulverization behavior of the Ti–V-based alloy is discussed. The results show that microcracks initially occur at the phase boundary of the V-based phase and the C14 Laves phase and then extend to the C14 Laves phase in the charge/discharge process. The phase boundary is composed of a Ti segregated amorphous layer with a thickness of about 90 nm, mismatching with the crystallized V-base phase and C14 Laves phase. The toughness of the C14 Laves phase is much lower and the hardness is higher than that of the V-based phase. The weak bonding strength of the phase boundary, the lower toughness of the C14 Laves phase and the large volume expansion/contraction of the C14 Laves phase during charge/discharge cycling are the main factors that cause the pulverization of the Ti–V-based alloy.

A study on crystal structure and chemical state of TiCrVMn hydrogen storage alloys during hydrogen absorption-desorption cycling

The evolution of crystal structure and chemical state of the V-based hydrogen storage alloy (Ti 0.32Cr 0.46V 0.22) 96Mn 4 during hydrogen absorption/desorption cycling was examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Reasons for the degradation in cycling capacity of the alloy are presented and discussed. One reason is the continuous reduction of the V-based cell volume during cycling, which cannot hold further hydrogen atoms. The decrease in cycling capacity can also be attributed to the oxidation of Ti, V, and Cr elements during cycling.

Combustion-type hydrogenation of nanostructured Mg-based composites for hydrogen storage

In this study Reactive Ball Milling in hydrogen gas was used to synthesize nanostructured hydrogenated composites of Mg and V-based alloy. After hydrogen desorption, the nanocomposites exhibited a dramatic facilitation of the rate of H absorption by Mg and reduction of the temperature of onset of hydrogenation. These favourable changes were caused by a synergy of catalytic effect of the V-based alloy on hydrogen absorption by Mg and heat release caused by exothermic hydrogen absorption by the V-based alloy. When the initial interaction temperature exceeded a threshold, rather low, value of 20–125°C, depending on the H2 pressure, composition of the sample and its total amount, a combustion-type hydrogenation took place. With optimal interaction parameters applied, H absorption was completed in just 5–70 s and was accompanied by a significant heat release. The observed features can be utilized to reach fast recharge of the Mg-based H stores and to develop efficient heat management systems. Copyright © 2009 John Wiley & Sons, Ltd.

Effect of La partial substitution for Zr on the Structural and electrochemical properties of Ti0.17Zr0.08-xLaxV0.35Cr0.1Ni0.3 (x=0–0.04) electrode alloys

In order to elucidate the effects of metallic La addition on the performance of Ti–V-based hydrogen storage alloys as negative electrodes for nickel/metal-hydrides batteries, Ti 0.17 Zr 0.08- x La x V 0.35 Cr 0.1 Ni 0.3 ( x = 0, 0.01, 0.02, 0.03, 0.04) alloys were prepared and their structural and electrochemical properties were systematically investigated. X-ray powder diffraction (XRD) results showed that these alloys were mainly consisted of C14 Laves phase with a hexagonal structure, V-based solid solution phase with BCC structure and C15 Laves phase with a cubic structure. The electrochemical measurements indicated that the maximum discharge capacities of the alloy electrodes decreased from 337.3 mAh/g ( x = 0) to 262.5 mAh/g ( x = 0.04) and that the substitution of Zr with metallic La in the alloys had no obvious effect on the capacity retention rate ( C 100 / C max , C 200 / C max ). The high-rate dischargeability ( HRD ) of the alloy electrodes at the discharge current density of 800 mA/g first increased from 69.01% ( x = 0) to 71.13% ( x = 0.01) and then decreased to 65.35% ( x = 0.04). In brief, the HRD was improved with an optimum La content in the alloy ( x = 0.01). The electrochemical hydrogen kinetics of the alloy electrodes was further studied by means of electrochemical impedance spectroscopy, linear polarization, anodic polarization and potential-step measurements. The charge-transfer reaction resistance R ct decreased for x = 0.01 with respect to x = 0 and then increased with the increase of x , while exchange current density I 0 , limiting current density I L and hydrogen diffusion coefficient D were all increased for x = 0.01 with respect to x = 0 and then decreased with the increase of x . The optimal content of La in Ti 0.17 Zr 0.08- x La x V 0.35 Cr 0.1 Ni 0.3 alloys for negative electrodes in alkaline rechargeable secondary batteries is x = 0.01 in this study.

Effects of rare earth elements substitution for Ti on the structure and electrochemical properties of a Fe-doped Ti–V-based hydrogen storage alloy

Effects of a partial substitution of rare-earth elements of Y, La, Ce, Pr, Nd in an atom fraction of 1/8 for Ti on the structure and electrochemical property of a Fe-doped Ti–V-based hydrogen storage alloy, Ti 0.8Zr 0.2V 2.7Mn 0.5Cr 0.6Ni 1.25Fe 0.2, have been investigated systematically using X-ray diffraction, scanning electron microscope, energy dispersive spectroscope and electrochemical tests including charge/discharge, high rate dischargeability, polarization, etc. The results indicate that all the alloys mainly consist of a C14 Laves phase in a three-dimensional (3D) network and a dendritic V-based BCC phase. The lattice parameters and the unit cell volumes of the two main phases are almost unchanged with the substitution of the rare earth elements for Ti except for Y, which increase slightly . Additionally, the La, Ce, Pr and Nd element added mostly formed a new phase with an approximate composition of R 2Ni 3 (R = La, Ce, Pr, Nd). The formation of the R 2Ni 3 phase favors the activation property of the alloy. As a result, with the substitution of the rare earth elements for Ti, the maximum discharge capacities ( C max) of the alloy electrodes increase, and a maximum value of 360 mAh/g is obtained for the substitution of Y. The HRD 600 of the alloy electrodes increases from 63% to 64–70% depending on the different substituent elements, but the cycle stability slightly decreases. In addition, the exchange current density, the limiting current density and the hydrogen diffusion coefficient of the alloy electrodes further indicate that the electrochemical kinetics of the alloy electrodes is improved by the increase in both the proximity effect between C14/BCC and the catalytic effect from R 2Ni 3 phase.

The first-principles study on the elasticity of V-based solid solution hydrogen storage materials

The lattice parameters， elastic properties and electronic density of state of the （59Cr-41Ti）100－xVx（x=5， 15， 30， 60， 80， 100） V-based solid solution hydrogen storage alloys were calculated using the first-principles plane-wave pseudo-potential method based on the density functional theory， and the calculated results were in agreement with the experimental results. It was found that the V-based alloy x=60 showing better elastic properties， with the Young modulus Yzz=14930 GPa， shear modulus Ct=5442 GPa and bulk elastic modulus B=19396 GPa. By combining the experimental cyclic durability performance， we found plastic deformations occurred in the alloys. Thus， we concluded that the elastic properties were not the primary factor that determines the cyclic durability of the V-based hydrogen storage alloys. The micromechanism of the elastic properties of the impure alloys were also explained with the electronic density of state.

The electrochemical performances of Ti–V-based hydrogen storage composite electrodes prepared by ball milling method

In order to overcome the inherent disadvantages of Ti–V-based hydrogen storage alloys, such as poor activation behavior and low high-rate dischargeability, the novel composites Ti 0.17Zr 0.08V 0.35Cr 0.1Ni 0.3– x wt.% La 0.7Mg 0.3Ni 2.75Co 0.75 ( x = 0, 5, 10 and 20) were successfully synthesized by ball milling method in the present study. And the structure and overall electrochemical properties of as-prepared composites are investigated systemically. The electrochemical studies show that the maximum discharge capacity of the composite electrodes displays no variation with the increase of La 0.7Mg 0.3Ni 2.75Co 0.75 content, whereas the high-rate dischargeability (HRD) and the activation behavior are distinctly improved with increasing x. The electrochemical hydrogen kinetics of composite electrodes is also studied by means of electrochemical impedance spectroscopy (EIS), linear polarization (LP), anodic polarization (AP) and potential-step measurements. It is found that the charge-transfer reaction resistance R ct is decreased with increasing the amount of La 0.7Mg 0.3Ni 2.75Co 0.75 while exchange current density I 0, limiting current density I L and hydrogen diffusion coefficient D are all increased with increasing the amount of La 0.7Mg 0.3Ni 2.75Co 0.75. These results suggest that the formation of composite with La 0.7Mg 0.3Ni 2.75Co 0.75 alloy is a promising strategy for improving the HRD, activation behavior and electrochemical kinetics of Ti–V-based alloy electrodes.

Intrinsic/Extrinsic Degradation of Ti−V-Based Hydrogen Storage Electrode Alloys upon Cycling

The capacity degradation mechanism of the Ti−V-based hydrogen storage electrode alloys was systematically investigated from the viewpoint of intrinsic/extrinsic degradation for the first time. The oxidation/corrosion of the active components and the appearance of the irreversible hydrogen were found to be the two dominating intrinsic factors for the cycling capacity degradation of the Ti−V-based electrode alloys, rather than the segregation and dissolution of the main hydrogen absorbing element V, the loss of which had some effect on the degradation of its discharge capacity but not dominating. The extrinsic degradation of the electrode alloys was mainly caused by the pulverization of the alloy particles, the increase of the contact resistance between particles, and the reaction resistance on surfaces during charge/discharge cycling. In the meantime the pulverization of the alloy particles accelerates the oxidation/corrosion of the active components, which further promotes the formation of the oxide layer and decreases the reaction speed. The decay of the apparent discharge capacity of the Ti−V-based electrode alloys is jointly affected by these factors.

Effect of rare earth (RE) elements on V-based hydrogen storage alloys

In this work, the effect of RE additives on the properties of V 55 Ti 22.5 Cr 16.1 Fe 6.4 alloy ( RE = La , Pr, Ce and Nd, separately) was discussed. It was demonstrated that RE additives improve the activation property rather than kinetics during cycling, absorption capacity and the plateau pressure. Two phases, including BCC main phase and Ce second one, were found in Ce-containing alloy. It is inferred that RE element offers a route for hydrogen to enter the alloys more easily, which leads to the improvement of activation property of the alloys.

Effect of rapid solidification on the structural and electrochemical properties of the Ti–V-based hydrogen storage electrode alloy

Abstract The structural and electrochemical properties of the as-cast and rapidly solidified Ti0.8Zr0.2V2.4Mn0.48Cr0.72Ni0.9 alloys were studied. Both the as-cast and the rapidly solidified alloys were mainly composed of a C14 Laves phase matrix with hexagonal structure and a V-based solid solution phase with body centered cubic (BCC) structure. The V-based solid solution phase showed very fine dendrites in the rapidly solidified alloy instead of the large dendrites as observed in the as-cast alloy. In addition, the content of the C14 Laves phase in the alloy decreased greatly after rapid solidification. Electrochemical measurements indicated that the rapidly solidified alloy had a lower discharge capacity, a slower activation rate, a worse high rate dischargeability, a smaller exchange current density and limiting current density, but an improved cycle life compared with that of the as-cast alloy.

Structure and electrochemical properties of the Fe substituted Ti–V-based hydrogen storage alloys

Abstract To elucidate the effects of Fe on the Ti–V-based hydrogen storage electrode alloys, the Ti 0.8 Zr 0.2 V 2.7− x Mn 0.5 Cr 0.8 Ni 1.0 Fe x ( x = 0.0–0.5) alloys were prepared and their structures and electrochemical properties were systematically investigated. XRD results show that all the alloys consist of a C14 Laves phase with hexagonal structure and a V-based solid solution phase with bcc structure. With increasing Fe content, the abundance of the C14 Laves phase gradually decreases from 43.4 wt% ( x = 0.0) to 28.5 wt% ( x = 0.5), on the contrary, that of the V-based solid solution phase monotonously increases from 56.6 wt% to 71.5 wt%. In addition, SEM observation finds that the grain size of the V-based solid solution phase is first gradually reduced and then enlarged with increasing x . Electrochemical investigations indicate that the substitution of Fe for V markedly improves the cycling stability and the high rate dischargeability of the alloy electrodes, but decreases the maximum discharge capacity and the activation performance. Further electrochemical impedance spectra, the linear polarization curve and the potentiostatic step discharge measurements reveal that the electrochemical kinetics of the alloy electrodes should be jointly controlled by the charge-transfer reaction rate on the alloy surface and the hydrogen diffusion rate in the bulk of the alloys. For the alloy electrodes with the lower Fe content ( x = 0.0–0.2), the hydrogen diffusion in the bulk of the alloys should be the rate-determining step of its discharge process, and while x increases from 0.3 to 0.5, the charge-transfer reaction on the alloy surface becomes to the rate-determining step, which induces that the electrochemical kinetics of the alloy electrodes is firstly improved and then decreased with increasing Fe content.