Microstructure And Hydrogen Transport Properties Of Nb-Co-Hf Hydrogen Separation Alloys

In recent years,due to the increasing depletion of fossil energy and the increasingly prominent environmental problems,the development of new clean and renewable new energy sources,such as hydrogen energy,has become the focus of energy strategies in all countries around the world.Nevertheless,many by-products,such as CO,CO₂,N₂,H₂S and NH₃.Prior to use,a"hydrogen separation technique"is required to separate(or filter/permeate)H₂from the mixture to obtain pure H₂of 99.999%(mass percentage).For the above separation process,the current commercially applied membrane materials are generally palladium(Pd)based alloys.However,the high price of palladium and the scarcity of resources seriously limit the large-scale application of the above alloy films.Under the above background,VB groups of metals with relatively low price and excellent hydrogen groups of metals(i.e.vanadium(V),niobium(Nb),tantalum((Ta),etc.)have attracted the attention of scholars all over the world,and have successively developed Nb-Ti-Ni,Nb-Ti-Co,Nb-Hf-Ni and Nb-Ti-Fe two-phase hydrogen permeation alloys.However,there are few reports on Nb-Co-Hf biphasic hydrogen-permeable alloys,and the location of the hydrogen-permeable component region in the phase diagram is not clear due to the absence of the complete phase diagram of the alloy.This paper selects Nb-Co-Hf alloy system as the research object,from the experimental analysis and phase diagram(CALPHAD)thermodynamic calculation two aspects,combined with the SEM?XRD?TEM?XPS?PCT and hydrogen permeation experiments,the phase diagram,microstructure,solidification path and hydrogen transport properties are studied in detail,as follows:1.The key information of Nb-Co-Hf alloy phase diagram and equilibrium solidification reaction,as well as the type and number of phase.On this basis,50 alloy compositions with Nb angle-rich phase diagram were studied experimentally.The microscopic characteristics,solidification law and hydrogen permeation properties and their intrinsic relationship were systematically analyzed.For the first time,the position of"hydrogen permeation composition region"in Nb-Co-Hf alloy phase diagram was explored.2.On the basis of the above results,the influence of different Hf/Co ratios on the microstructure and hydrogen transport properties of Nb-Co-Hf biphasic alloys is studied.The results show that the microstructure changes with the increase of Hf/Co ratio,and with the above changes,the hydrogen permeation performance is improved,and the hydrogen diffusion and hydrogen dissolution coefficient are increased.Among them,Nb₄₀Co₂₈Hf₃₂(x=2)alloy has the highest hydrogen permeation performance at 673 K,which is 5.21×10^-8(mol H₂m^-1s^-1Pa^-0.5),and has good resistance to hydrogen embrittlement and hydrogen permeation durability;3.Changes of microstructure and hydrogen transport properties of Nb-Co-Hf biphasic alloys under different heat treatment conditions were obtained,and the essential reasons for these changes were clarified by combining the mixed medium model..Heat treatment will increase the primary BCC-(Nb,Hf)phase in the microstructure of the alloy and thus improve the hydrogen transport performance.In addition,the corrosion resistance of the alloy is also studied,and the mechanism of hydrogen permeation and hydrogen embrittlement resistance before and after corrosion of the alloy film is clarified.4.the hydrogen permeation properties of the alloy at 673 K decreased from 3.3×10^-8mol H₂m^-1s^-1Pa^-0.5to(1.5-0.82)×10^-8mol H₂m^-1s^-1Pa^-0.5after annealing 168 h at 1073 and1173 mol H₂m^-1s^-1Pa^-0.5,indicating that annealing heat treatment could not improve the hydrogen permeation properties of the alloy.