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Hydride formation and fracture of vanadium alloys

What is vanadium hydride?

A vanadium aryl hydride gel was prepared by thermal decomposition and subsequent hydrogenation of tetraphenyl vanadium and evaluated for electrochemical and hydrogen storage performance. Characterization by IR, XRD, XPS, nitrogen adsorption, and TGA suggests that the material consists predominantly of a mixture of vanadium centers in oxidation states of IV bound together by bridging hydride and phenyl groups. Electrochemical properties were explored to probe the reversible oxidation state behavior and possible applications to Li batteries, with the hypothesis that the low mass of the hydride ligand may lead to superior gravimetric performance relative to heavier vanadium oxides and phosphates. The material shows reversible redox activity and has a promising peak capacity of 131 mAhg -1 at a discharge rate of 1 mAcm-2, comparable to bulk VO2 samples also tested in this study. After repeated charge-discharge cycling for 50 cycles, the material retained 36% of its capacity. The material also shows improved hydrogen storage performance relative to previously synthesized VH3-based gels, reaching a reversible gravimetric storage capacity of 5.8 wt% at 130 bar and 25 °C. Based on the measured density, this corresponds to a volumetric capacity of 79.77 kgH2 m-3, demonstrating that the 2017 US DOE system goals of 5.5 wt% and 40 kg H2 m-3 may be achievable upon containment in a Type 1 tank and coupling to a fuel cell.


Hydride formation and fracture of vanadium alloys

The effects of hydrogen in vanadium, V–5Cr, and V–5Ti have been evaluated by the change in surface and microstructure and by the aspect of fractured surfaces. Hydrogen accumulation promotes the local concentration of hydrides and assists in generating stacking faults. Under increased stress, crack propagation occurs by the successive formation of crack fronts. The perspective of using vanadium alloys as a component of the blanket of fusion reactors has motivated significant research in the last decades. The detrimental effects of hydrogen remain a main concern and have been widely documented. Common sources of hydrogen include neutron-induced transmutation of vanadium alloys, intake from primary fuel components, coolants, and byproducts of corrosion reactions. In evaluating the effects of hydrogen, particular attention has been given to the characteristics and formation of hydrides and their influence on the embrittlement process. Theoretical models, however, tend to imply assumptions and restrictions, which sometimes limit their reach and often lead to overemphasized abstractions. Back to the roots of research on embrittlement lies the direct observation of fractured surfaces, which is worthy of revisiting and reconsidering in the search for new ideas and clues and as a pivotal point in applying theoretical models. The present study uses SEM and TEM techniques to approach the embrittlement process by directly observing damaged surfaces. It considers the effects of sample preparation as an intrinsic part of the results.


Experimental procedure of vanadium hydride

Vanadium, V–5Cr, and V–5Ti alloys were prepared from 99.9% pure vanadium by arc melting in an argon atmosphere. The nuggets were cold-rolled and punched into tensile specimens for in situ straining experiment with a gauge size of 2×7 mm2 and 0.1 mm in thickness. The samples were evacuated in a quartz tube and annealed in a vacuum at 1000 °C for one h. Hydrogen was doped into the samples in a high-pressure oven at 450 °C with 0.5 and 0.8 MPa hydrogen pressures. The first visible evidence of the effect of hydrogen in the samples comes from early examination during sample preparation. In the electro-polishing process, an acid solution is projected from both sides of the sample, making the material in the center progressively thinner until a hole is formed. Fig. 1 compares the final shape of the electro-polished hole of non-hydrogenated V–5Ti with the same alloy charged with a metal/hydrogen ratio of 0.24. The hydrogen distribution in the samples is not uniform. Regions rich in hydrogen are prone to form the hydride, which makes some regions more brittle than others altering stress distributions and promoting the failure of susceptible regions. Stacking faults bounded by dislocations are promoted in zones where hydrogen is accumulated. The main characteristic of fracture in hydrogenated samples is the formation of barriers that block the free movement of dislocations. The reaction of hydrogen gas with magnesium metal, which is important for hydrogen storage purposes, is enhanced significantly by adding catalysts such as Nb and V and using nanostructured powders. In situ, neutron diffraction on MgNb0.05 and MgV0.05 powders give a detailed insight into the magnesium and catalyst phases that exist during the various stages of hydrogen cycling. During the early stage of hydriding (and deuteride), an MgH1<x<2 phase is observed, which does not occur in bulk MgH2 and, thus, appears characteristic for the small particles. The abundant H vacancies will cause this phase to have a much larger hydrogen diffusion coefficient, partly explaining the enhanced kinetics of nanostructured magnesium. It is shown that under relevant experimental conditions, the niobium catalyst is present as NbH1. Second, a hitherto unknown Mg−Nb perovskite phase could be identified that has to result from the mechanical alloying of Nb and the MgO layer of the particles. Vanadium is not visible in the diffraction patterns, but electron micrographs show that the V particle size becomes very small, 2−20 nm. Nanostructuring and catalyzing the Mg enhance the adsorption speed so much that now temperature variations effectively limit the absorption speed and not, as for bulk, the slow kinetics through bulk MgH2 layers.


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