The effect of Ti on the microstructure, hydrogen absorption and diffusivity of V–Ni alloy membranes

Abstract

Vanadium is highly permeable to hydrogen which makes it one of the leading alternatives to Pd alloys for hydrogen-selective alloy membrane applications. The tendency of vanadium to embrittle can be reduced through alloying, thereby enabling V-based membranes to be developed for specific operating regimes. Foremost amongst these alloying additives are Ni and Ti. Ni is known to form V–Ni solid solutions with decreased hydrogen absorption and diffusivity compared to V. The influence of Ti is more complex due to phase segregation, and this work focuses the influence of Ti on the microstructural and chemical properties of hydrogenated vanadium–nickel solid solution alloys. Alloy membranes of the compositions V90−xTixNi10 (where 2≤x≤10) were prepared by the sectioning arc-melted ingots and the deposition of a Pd dissociation/recombination catalyst at each surface. Hydrogen permeation measurements were undertaken using the constant pressure method, with feed pressures ranging from 1 to 10 bar, permeate pressures <0.05 bar via use of a sweep gas, and temperatures of 350 and 400 °C. Hydrogen absorption of these same alloys was measured under corresponding conditions using the Sieverts' method. The microstructures featured a dendritic (branched) body-centred cubic vanadium solid solution matrix with interdendritic Ni–Ti compounds. The Ti content in the matrix increased, while the Ni content decreased, with increasing bulk Ti content. Hydrogen diffusivity increased with increasing bulk Ti content and increasing dissolved hydrogen concentration. The Ni–Ti compounds, known for being weakly hydrogen absorbing, also scavenge oxygen from the vanadium matrix, both of which are important in enhancing the embrittlement resistance of these alloys. This work has shown that Ti is a favourable additive for vanadium–nickel alloy membranes when added in the amount of ∼10 at%, with a hydrogen diffusivity (DH) equal to 4.2×10−8 m2 s−1 and hydrogen permeability of 1.2×10−7 mol−1 s−1 Pa−0.5 corresponding to a feed pressure of 10 bar and temperature of 400 °C.