Microstructure Modulation And Hydrogen Transport Property Of V_SS-Cu_SS Dual Phase Hydrogen Separation Alloys

High-purity hydrogen is widely used in the electronics industry,aerospace,nuclear fusion and fuel cells.Membrane separation is an important technical way to purify hydrogen.The membrane materials mainly used in industry are the precious metal palladium（Pd）and its alloys（e.g.,Pd-Ag,etc.）.However,the extremely high cost of Pd severely limits its large-scale application,which stimulates the development of new hydrogen separation membranes with lower cost and similarly high performance.Vanadium（V）,a metal of Group 5B,has received much attention due to its lower price and significantly higher hydrogen permeability than Pd.The high hydrogen permeability of V originates from its high hydrogen dissolution,which leads to a high tendency to hydrogen embrittlement.Much of current research focused on reducing hydrogen solubility for enhancing resistance to hydrogen embrittlement by selective alloying.Nevertheless,alloying also leads to reductions in hydrogen diffusion coefficient and hydrogen permeability.Therefore,it is crucial to develop V-based hydrogen separation alloys with low hydrogen dissolution and high hydrogen diffusion properties to achieve good matching of hydrogen permeation performance and resistance to hydrogen embrittlement.In this paper,V-based hydrogen separation alloys were designed based on the lattice matching principle,i.e.,reducing hydrogen solubility by alloying while maintaining high hydrogen diffusion properties by matching the lattice parameters to that of pure V.As a result,three types of V-M-Al（M:Cr,Ni,Co）hydrogen separation alloys were designed to achieve low hydrogen solubility and high hydrogen diffusion properties.On this basis,a dual phase alloy of V solid solution(V_SS)and Cu solid solution(Cu _SS)was formed by adding Cu elements.The former(V_SS)maintains high hydrogen permeability and the latter(Cu _SS)improves rolling deformability to achieve the preparation of thin membranes.Finally,the hydrogen permeation behavior of V_SS-Cu _SS biphasic hydrogen separation membranes in typical industrial gas mixtures was investigated to examine the effects of impurity gases on the hydrogen permeation performance of the membranes.The details are as follows.Based on the“lattice matching”principle,three types of V-M-Al（Cr,Ni,Co）hydrogen separation alloys were designed and their as-cast microstructure and hydrogen transport properties were systematically investigated.The results showed that the addition of Cr,Ni,Co and Al in V at less than 10 at.%resulted in the formation of single phase solid solution alloys with reduced hydrogen solubility properties.Among them,V₉₀Cr₅Al₅,V₉₀Ni₅Al₅ and V₉₀Co₅Al₅ alloys were best matches to the pure V lattice.Under the same conditions,V₉₀Ni₅Al₅alloy has a higher tough-brittle transition temperature and poorer resistance to hydrogen embrittlement;V₉₀Co₅Al₅ has insufficient hydrogen permeation performance;V₉₀Cr₅Al₅ has the lowest hydrogen dissolution and highest hydrogen diffusion properties,showing the best match of hydrogen permeability and resistance to hydrogen embrittlement.In particular,V₉₀Cr₅Al₅ exhibits excellent hydrogen permeation stability during the long-term hydrogen permeation at 623 K for 500 h.The hydrogen permeation coefficient in pure hydrogen atmosphere is～2.5×10^-7 mol H₂ m^-1s^-1Pa^-0.5,which is 16 times higher than that of Pd under the same conditions.When the temperature increased to673 K,V₉₀Cr₅Al₅ showed degradation of hydrogen flux due to the formation of an interdiffusion layer between V-based substrate and Pd catalytic film during long-term hydrogen permeation.Based on the idea of“multi-phase composition and function sharing”,dual phase solid solution alloys with V_SS and Cu _SS were developed by adding（10,20,30）at.%Cu to V₉₀Cr₅Al₅ alloy.Cu _SS phase significantly improves the plastic deformation ability of the alloys.Hydrogen separation membranes with a thickness of～0.1 mm can be prepared by cold rolling processing.The microstructure and hydrogen solubility,permeation and diffusion properties of(V₉₀Cr₅Al₅)_100-xCu_x（x=10,20,30 at.%）alloys in the as-cast,rolled and rolled-annealed states were systematically investigated.The results showed that with the increase of Cu content,the volume fraction of Cu _SS phase in the as-cast V_SS-Cu _SS alloys increases obviously and the hydrogen solubility of alloys decreases slightly.While the hydrogen diffusion and permeation properties decrease significantly due to the spatial hindrance of Cu _SS phases on the hydrogen transport performance in V_SS phase.After rolling deformation,the V_SS and Cu _SSphases extend along the rolling direction and are distributed in strips.The hydrogen diffusion and hydrogen permeation properties are further reduced due to the introduction of a large number of dislocations after rolling.The heat treatment of the rolled membranes reduced the dislocation density.Some of the Cu _SS phases with stripe-like distribution undergo dynamic recrystallization into dispersed particles,resulting in continuous channels of V_SS in the hydrogen permeation direction.Compared with the rolled state,the hydrogen diffusion and hydrogen permeation properties of rolled-annealed V_SS-Cu _SS alloys are remarkably improved.Among them,the rolled-annealed(V₉₀Cr₅Al₅)₉₀Cu₁₀membrane shows the optimal hydrogen permeation properties and resistance to hydrogen embrittlement,and the hydrogen permeation flux reaches 0.38 mol H₂m^-2s^-1at 623 K in pure hydrogen atmosphere,which is 5.8 times higher than that of Pd under the same conditions.The hydrogen permeation properties and mechanism of rolled-annealed((V₉₀Cr₅Al₅)₉₀Cu₁₀ membrane in gas mixtures were systematically investigated.Firstly,the effect of concentration polarization on hydrogen transport caused by different conditions on the upstream side was simulated by Open FOAM software,and the test conditions to reduce the mass transfer limitation were determined.Then,the hydrogen permeation performance of the membrane was tested at temperatures ranging from 523 to 673 K with 0～30 mol%of N₂,CO₂,CO and steam on the upstream side,respectively.The results show that when the upstream hydrogen partial pressure was kept as constant,the variation of hydrogen permeation flux was negligible for N₂concentrations from 0 to 30mol%,but gradually decreased with increasing concentrations of CO₂,CO and steam.Based on the“dissolution-diffusion”transport mechanism,a hydrogen transport model including competitive adsorption of different gases on the membrane surface was constructed to predict the quantitative relationship between the effects of impurity gases and the hydrogen permeation flux of the membrane,which agreed with the experimental results.The decay coefficients reflecting the effects of impurity gases on hydrogen permeation flux were obtained,which increased with decreasing temperature or increasing concentration of impurity gases.Under the same conditions,CO brought the largest decay coefficient,followed by CO₂ and H₂O brought the smallest.