Microstructure And Hydrogen Permeability In Mixed Atmosphere Of Nb-Hf-Co Alloys

In recent years,the rapid development of global hydrogen energy,it is imperative to mass preparation/production of high purity hydrogen is imperative.As a new hydrogen separation process,membrane separation is considered as one of the most promising hydrogen purification methods because of its low cost and simple operation.Bcc 5B group metals（Nb V and Ta）have the advantages of high permeability and low cost,and have a good application prospect in the field of membrane separation.At present,they have become one of the main alternative materials for hydrogen separation applications of Pd-based alloys.Among the alloys mentioned above,Nb-Hf-Co alloy has excellent hydrogen permeability,which has attracted the attention of domestic and foreign scholars.However,The toughness and brittleness transition concentration（DBTC）of Nb-Hf-Co alloys is not clear,and the brittle fracture problem closely related to DBTC has not been solved;In addition,the catalytic Pd film on the outer layer of the alloy is easily poisoned in the mixed atmosphere（toxic gas H₂S）,resulting in the failure of hydrogen separation and purification.In order to solve these requirements,a series of studies on the microstructure regulation and hydrogen transport performance of Nb-Hf-Co hydrogen separation alloy were carried out in this paper,and the details are as follows:1,Based on the research our group’s previous research on Nb-Hf-Co alloy,in order to further meet the needs of practical application,the range of toughness/brittleness transition concentration of the alloy was studied in depth.Firstly,three kinds of alloys with different compositions were successfully prepared by vacuum melting furnace,namely1#(Nb₄₀Hf₃₀Co₃₀),2#(Nb₄₅Hf_27.5Co_27.5)and 3#(Nb₅₀Hf₂₅Co₂₅)alloy.Then,the microstructure variation was studied by SEM,TEM and XRD.Finally,the hydrogen permeability and hydrogen absorption properties of the alloys at different temperatures were measured,and the DBTC boundary of Nb based multiphase alloys was successfully explored.On this basis,a new design concept of Nb-based multi-phase hydrogen permeation alloy film was established,based on the above ideas,a new Nb-Hf-Co-Ti dual-phase hydrogenation alloy was successfully developed.The highest hydrogenation performance of the alloy at 673 K is 2.11×10^-8mol H₂m^-1s^-1Pa^-0.5.2,In order to further improve the toxicity of the alloy membranes in H₂S atmosphere.A series of studies on alloy membranes at different temperatures and H₂S concentrations were carried out by controlling the composition of catalytic layer.Firstly,Pd-,Pd₆₅Ag₃₅-,Pd₉₀Cu₁₀-,and Pd₈₀Au₂₀-coated Nb₃₀Hf₃₅Co₃₅membranes were synthesized by magnetron sputtering.Then,the hydrogen osmosis behavior in pure H₂and different H₂S/H₂mixtures at 673 K was studied.Finally,the microstructure changes of the alloy film after poisoning were investigated.The permeability of Pd/Nb₃₀Hf₃₅Co₃₅/Pd membranes decreases rapidly with the increase of H₂S concentration,surface Pd-Ag significantly increased the permeability of pure H₂but was prone to membrane poisoning,on the contrary,the Pd-Au alloy catalytic layer on the surface can effectively improve the H₂S corrosion resistance.Among them,the hydrogen permeability of Pd₈₀Au₂₀-Nb₃₀Hf₃₅Co₃₅alloy is about 1.48×10^-8mol H₂m^-1s^-1Pa^-0.5at 120 ppm H₂S,which is 3 times that of traditional Pd film coated alloy.3,Based on the above,Nb-Hf-Co hypereutectic alloy was prepared,the relationship between mechanical properties of microstructure and hydrogenation properties of alloys under 10 different annealing conditions was studied,the precipitation and ablation models of Bcc-（Nb,Hf）phase were established,the effects of eutectic structure,lattice orientation and defects on hydrogen permeability of alloy films were discussed.Finally,the hydrogen permeability mechanism was established based on the relationship between the hydrogen permeability properties and confirm that annealing cannot improve the hydrogen permeability of as-cast Nb-Hf-Co dual-phase alloys.