Influence of Cr on the hydrogen storage properties of Ti-rich Ti–V–Cr alloys

In the present work, we studied the effects of Cr on the crystal structures and hydrogen storage properties of ternary alloys, Ti_0.7V_0.3−xCr_x and Ti_0.8V_0.2−xCr_x. Metal–hydrogen interactions were characterised by Thermal Desorption Spectroscopy (TDS) and in situ Synchrotron X-ray diffraction (SR-XRD). All initial alloys crystallise with body-centred cubic (BCC) crystal structures formed as solid solutions of V and Cr in Ti. Upon hydrogenation, the dihydrides (Ti,V,Cr)H₂ with face-centred cubic (FCC) structures are formed. An increase in the Cr content leads to systematic changes in the structure and hydrogenation behaviours. The changes include (a) contraction of the unit cells for the initial alloys and for the corresponding dihydrides; (b) slower hydrogen absorption kinetics and an increase in the incubation period for hydrogenation; (c) a decrease in the thermal stability of the saturated hydrides; and (d) a reduction in the apparent activation energy of hydrogen desorption. In situ SR-XRD and TDS studies of the FCC Ti–V–Cr hydrides indicated that their decomposition consists of five individual desorption events.     

Strengthening of Ti–6Al–4V alloys by thermohydrogen processing

The relationships among the hydrogen-induced phase transformation, grain refinement and improvement of mechanical properties of Ti–6Al–4V alloys are investigated. For this purpose, the decomposition of the α″ martensite and metastable β phase of hydrogenated Ti–6Al–4V alloys was investigated first. As a result, it is found that the key role leading to grain refinement is the decomposition of metastable phase. On the other hand, the precipitation of the TiH₂ hydride under lower temperature aging shows no significant refining effect after dehydrogenation. Mechanical properties of the materials after dehydrogenation show that the decomposition of the metastable phase strengthens the materials. In particular, specimens containing 0.45 wt% H in their fully martensites structures show a marked increase in strength. However, a fine grain and comprehensive mechanical properties are obtained only in specimens containing 0.8 wt% H.     

Hydrogen absorption characteristics of mechanically alloyed Ti–Zr–Ni and Ti–V–Ni powders

A first investigation into the production of amorphous and nanostructured Ti-based alloys with nominal compositions Ti_41.5Zr_41.5Ni₁₇, Ti₆₁Zr₂₂Ni₁₇, Ti_41.5V_41.5Ni₁₇ and Ti₆₁V₂₂Ni₁₇ by mechanical alloying (MA) technique is presented. This technique was adopted to produce alloys' powders with high fresh surface area that were active for hydrogen storage. Hydrogen absorption characteristics and structure changes in the alloys after hydrogenation were investigated. Gas phase hydrogenation of the Ti–Zr–Ni alloys, at 573 K and an initial hydrogen pressure of 2 MPa, exhibited good hydriding properties and started at a maximal rate without induction period with a hydrogenation capacity up to 1.2 wt%. However, hydriding of Ti–V–Ni alloys at the same conditions exhibited slower rates. The Ti₆₁V₂₂Ni₁₇ composition showed high hydrogen absorption capacity of 1.8 wt% and exceeded 4 wt% at 345 K. In addition, the Ti–V–Ni alloys showed structure stability after hydrogenation and retained the amorphous structure.     

Structure and hydrogen permeability of V–15Ni alloy

The structure of the V–15Ni at.% alloy before and after hydrogen permeability tests was investigated by means of XRD and SEM with EDS analysis. We have found that decomposition of supersaturated V-based solid solution with variable Ni content occurred during testing. The volume fraction of the solid solution decreased and the fraction of V₃Ni phase increased during permeability testing, thus bringing the alloy to nearly equilibrium. The membrane without Pd coating showed satisfactory hydrogen fluxes with a significant impact of the surface dissociation rate of hydrogen. The shape of hydrogen permeation curves at the downstream side of the membrane at various temperatures was unusual. We attribute it to the high concentration of dissolved hydrogen in the metal lattice and its effect on the hydrogen diffusivity and solubility. In addition, the multiphase structure with non-uniform distribution of nickel both between the phases and within the BCC solid solution (and, consequently, different hydrogen concentrations) may cause dilatation or compressing effect on neighbouring micro-volumes of the alloy.     

Synchrotron EXAFS and XRD studies of Ti–V–Cr hydrogen absorbing alloy

We reported extended X-ray absorption fine structure (EXAFS) measurements at the Ti, V and Cr K-absorption edges in a Ti–V–Cr alloy before and after hydrogen charging. The results indicated that all the bond lengths increased significantly after hydrogenation, and Ti atoms interacted strongly with V, while V and Cr atoms interacted weakly. The Ti and V atom's coordination numbers in hydrogenated alloy decreased obviously in comparison with the pre-hydrogenation situation. X-ray diffraction (XRD) patterns demonstrated that the structure of Ti–V–Cr alloy had transformed from BCC to FCC during hydrogenation. In addition, we investigated the nanoparticle size distribution in the as-cast and hydrogenated Ti–V–Cr alloy by small-angle X-ray scattering (SAXS) technique.     

Measurement of fracture toughness transition behaviour of Cr–Ni–Mo–V pressure vessel steel using pre-cracked Charpy specimens

The potential application of small pre-cracked Charpy specimens for the prediction of the fracture toughness of the 1T-thickness specimens and the reference temperature T₀ has been examined. Transition fracture behaviour of plane sided, side-grooved and 1T SENB specimens, respectively, was investigated over a wide temperature range. The fracture toughness regions with various fracture initiation mechanisms were defined and ductile to brittle transition temperatures denoted. The fracture toughness transition region of small pre-cracked specimens was shifted to lower temperatures as compared with that of 1T SENB specimens. The fracture toughness data of small pre-cracked specimens have been size corrected (weakest link) to 1T thickness and used to establish the reference temperature T₀ and K_Jc(mean) fracture toughness vs. temperature curve. The calculated temperature T₀ has been in consistence with that of the 1T SENB specimen. However, some corrected fracture toughness data lay outside the scatter band of 1T thickness specimens and the shape of the K_Jc(mean) curve has been quite different from the K_Jc(med)(1T) curve. It was found out that the original measured fracture toughness results of corrected data points lying outside the scatter band violated the validity condition b₀R_p0.2/J_c≥30. Bearing in mind the work of Koppenhoefer and Dodds Jr. (Engng Fract Mechanics (1997);58:249–270), and the most recent analysis of Ruggieri et al. (Engng Fract Mechanics (1998);60:19–36), the fracture data of small pre-cracked specimens having the validity parameter lower than 50 have been first constraint adjusted using the cleavage fracture toughness scaling model of Dodds and coworkers (J Testing Evaluation (1991);19:123–134; Int J Fracture (1995); 74:131–161; Engng Fract Mechanics (1997);58:249–270), and only then size corrected. The K_Jc(mean) curve of such treated data was identical with K_Jc(med)(1T).     

Hydrogen-induced softening of Ti–44Al–6Nb–1Cr–2V alloy during hot deformation

The effect of hydrogen on the hot deformation behavior and microstructural evolution of Ti–44Al–6Nb–1Cr–2V (at.%) alloys were investigated at temperature range of 1373–1523 K under strain rate of 0.01 s^?1. The true stress–strain curves show that the peak stress decreases from 323 MPa to 97 MPa when deformation temperature increases from 1373 K to 1523 K. The peak stress is decreased by 30% after hydrogenation with 2% H, which corresponds to the decrease of deformation temperature by about 50 K, it denotes that hydrogen can promote a solution softening effect in TiAl alloys. This is attributed to hydrogen-promoted the dynamic recrystallization, hydrogen-induced dislocation movement and hydrogen-stabilized the B2 phase. For dynamic recrystallization, the calculated results show that hydrogen accelerates the onset of dynamic recrystallization, which means that hydrogen promotes the dynamic recrystallization kinetics. For dislocation movement, EBSD results show that the fraction of low-density dislocation region increases from 59.6% to 79.7% after hydrogenation with 2% H, which indicates that hydrogen reduces the dislocation tangles and dislocation density. For B2 phase, more softening B2 phases are observed in hydrogenated alloy compared with that in unhydrogenated alloy, which results from hydrogen-promoted the transition of L (α₂/γ) → γ + B2. The positive effect of hydrogen on TiAl alloys provides an effective method to improve the hot workability of TiAl alloys.     

Microstructure and hydrogen storage properties of non-stoichiometric Zr–Ti–V Laves phase alloys

The non-stoichiometric C15 Laves phase alloys namely Zr_0.9Ti_0.1V_x (x = 1.7, 1.8, 1.9, 2.1, 2.2, 2.3) are designed and expected to investigate the role of defect and microstructure on hydrogenation kinetics of AB₂ type Zr-based alloys. The alloys are prepared by non-consumable arc melting in argon atmosphere and annealed at 1273 K for 168 h to ensure the homogeneity. The microstructure and phase constitute of these alloys are examined by SEM, TEM and XRD. The results indicate the homogenizing can reduce the minor phases α-Zr and abundant V solid solution originating from the non-equilibrium solidification of as-cast alloys. Twin defects with {111}<011 > orientation relationship are observed, and the role of defects on hydrogenation kinetics is discussed. Hydrogen absorption PCT characteristics and hydrogenation kinetics of Zr_0.9Ti_0.1V_x at 673–823 K are investigated by the pressure reduction method using a Sievert apparatus. The results show the hypo-stoichiometric alloys preserve faster hydrogenation kinetics than the hyper-stoichiometric ones due to the decrease of dendritic V. The excess content of Zr₃V₃O phase decreases the hydrogenation kinetics and the stability of hydrides. In addition, the different rate controlled mechanisms during hydrogen absorption are analyzed. The effects of non-stoichiometry on the crystal structure and hydrogen storage properties of Zr_0.9Ti_0.1V_x Laves alloys are discussed.     

Effect of hydrogen on fracture behavior of Ti–6Al–4V alloy by in-situ tensile test

The fracture behaviors of non-hydrogenated, hydrogenated and dehydrogenated Ti–6Al–4V alloys were investigated by in-situ tensile test at room temperature. The distributions of stress and strain near the notch of in-situ tensile specimen were calculated using finite element method. Results indicate that hydrogen has an important effect on the fracture behavior of Ti–6Al–4V alloy. Crack initiation sites differ in the specimens treated by various procedures, and they are related to the stress intensity in specimens under different loads. The fracture mode of non-hydrogenated specimen is a ductile fracture by the initiation and coalescence of microvoids. The hydrogenated specimen shows a mixture of intergranular and transgranular brittle fractures. The dehydrogenated specimen is characterized by a mixture of intergranular and transgranular ductile fractures. The transition of fracture mode is attributed to the hydrogen atoms in solid solution and hydrides.     

Microstructure and electrochemical properties of Zr–Ti–V–Ni–Mn–LM alloys for application in Ni–MH secondary battery

A series of multi-component Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3, 0.4; y=0.0,0.02,0.05,0.1,0.2,0.3, LM; lantanum-rich-mischmetal) alloys are prepared and their crystal structure and P–C–T curves are analyzed. The alloys have been modified by adding LM and their gaseous and electrochemical hydrogenation properties are studied to find out the effect of LM elements. Also, the second phase and initial activation performance are investigated. The Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3,0.4; y=0.0,0.02,0.05,0.1,0.2,0.3) alloys have C14 Laves phase hexagonal structure, so the volume expansion ratio of lattice parameters with LM has increased. As the amount of LM in alloy has increased, correspondingly the second phase is also increased. The second phase is LM, Ti and V-rich. The second phase improve the activation of La-rich misch-metal, and also the concentration of elements Ti, V〉LM〉 matrix in alloys.The addition of LM in Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3, 0.4) alloys have increased the activation rate and hydrogen storage capacity significantly, but the plateau pressure and the discharge capacity have been decreased due to the formation of second phase. For more Zr in electrode alloys, the activation of rate becomes slow.     

Synthesis and characterization of metastable Li–Mn–O spinels from Mn(V)

Li₃MnO₄ Mn(V) was synthesized from LiOH and LiMnO₄, and characterized by gravimetric, X-ray, and XPS analysis. This precursor was used to improve the calcination time required for the low temperature solid state synthesis of highly oxidized spinels, Li₂Mn₄O_8+x. TG calculations were used to show the effect of calcination conditions on phase purity. The effect of a second phase on cycling performance was evaluated.     

First-principle modeling of hydrogen site solubility and diffusion in disordered Ti–V–Cr alloys

Hydrogen diffusion and solubility in disordered alloys are of paramount importance to a variety of practical applications from hydrogen storage materials to separation membranes and protection against hydrogen embrittlement. By employing density functional theory calculations we unveil the atomic-level understanding of hydrogen diffusion in disordered Ti–V–Cr alloys used for hydrogen storage. Hydrogen distribution over interstitial sites of the bcc and fcc lattices of TiV_0.8Cr_1.2 has been simulated using a supercell approach. Taking into account both structural and energy factors we identify tetrahedral sites coordinated by three different metal atoms as the most favorable for hydrogen. The calculations carried out within the nudged elastic band method show that hydrogen diffusion between two tetrahedral site in fcc TiV_0.8.Cr_1.2H_5.25 occurs nearby an intermediate octahedral site with the activation barrier of 0.158 eV for the most probable diffusion pathway. An estimation of the hydrogen diffusion coefficient in fcc TiV_0.8.Cr_1.2H_5.25 at 294 K provides the value of 2.6 × 10^?11 m²/s that is in fair agreement with experiment data. Despite the modeling was done for a hydride of a definite composition we anticipate that the present results could be extended to Ti–V–Cr hydrides with various compositions.     

Safety comparisons of various gas source deposition technologies for large-scale production of III–V solar cells

III–V solar cells offer potential advantages in efficiency and cost for making photovoltaic power systems economic in large-scale utility applications. Relatively safe manufacturing technologies have been developed to fabricate III–V electronic devices but the scale involved in utility power has introduced new safety challengs. Gas source fabrication offers large-scale manufacturing advantages including the ease of gas source materials handling and control. However, the present gas source technologies such as metal-organic chemical vapor deposition (MOCVD) and metal-organic molecular beam epitaxy (MOMBE) involve the potential for significant injury from the toxic materials used. This risk can be greatly reduced by advanced technology vacuum chemical epitaxy (VCE). MOCVD has the problems of low materials utilization with large quantities of toxic effluent, fragile quartz components, cold chamber walls that rapidly build-up residues, no load lock for personnel protection and the potential for hydrogen gas explosions. However MOCVD has been scaled up to medium production levels of III–V solar cells. In contrast MOMBE has the safety advantages of rugged metal chamber walls, load locks and higher utilization of some of the source gases. But MOMBE rapidly builds up toxic residues on cold surfaces and it is not readily scaleable to utility level production volumes. VCE combines many of the best characteristics of MOCVD and MBE. These include higher utilization of all the source materials, hot chamber walls with low residue build-up rates, rugged metal outer chamber walls, load locks and an inherent scalability to large production levels.     

Analytical derivation of explicit J–V model of a solar cell from physics based implicit model

Recently a simple explicit model was introduced to represent the J–V characteristics of an illuminated solar cell with parasitic resistances and bias dependent photocurrent as v^m + jⁿ = 1. Here the normalized voltage, v and normalized current density j can be represented as v = V/V_oc and j = J/J_sc respectively, where V_oc is the open circuit voltage and J_sc is the short circuit current density. This model is useful for design, characterization and simple fill factor calculation and its applicability was demonstrated with the measured data of a wide variety of solar cells. This explicit form is intuitive and hence the model lacks the analytical support. In this paper an analytical derivation of this closed form explicit model is presented, which is derived from the physics based implicit J–V equation. The derivation expands the scope of model applicability and provides a new insight of analytical modeling of the solar cell.     

Photocatalytic water splitting into hydrogen and research on synergistic of Bi/Sm with solid solution of Bi–Sm–V photocatalyst

Photocatalyst Bi_1−xSm_xVO₄ were prepared by solid phase reaction and characterized by XRD, UV–visible DRS, BET, and SEM. Bi_1−xSm_xVO₄ showed two structures with the component content. When the composition was above x = 0.3, Bi_1−xSm_xVO₄ were of single phase with tetragonal type and can be regarded as solid solutions of was BiVO₄ and SmVO₄. When Bi_1−xSm_xVO₄ was loaded with 0.3 wt% Pt, the samples showed photocatalytic activities for water decomposition to hydrogen under UV light. Among these catalysts, Bi_0.5Sm_0.5VO₄ showed the best photocatalytic activity for water splitting, which indicated synergistic of Bi/Sm enhanced the photocatalysis efficiency. What's more, Bi_0.5Sm_0.5VO₄ loaded with other co-catalysts was found to act as a photocatalyst for water decomposition to hydrogen and oxygen under UV light irradiation, and the photocatalyst loaded with Pt/Cr₂O₃ had the best photocatalytic property. The amounts of the produced hydrogen and oxygen, respectively, were about 188.25 μmol h⁻¹ g⁻¹ and 95.90 μmol h⁻¹ g⁻¹. This study indicated that the formation of solid solution was the feasible method to adjust energy band and synergistic of Bi/Sm can enhance the photocatalytic activities of water decomposition.