Microstructure And Hydrogen Transport Of As-Cast And Cold-rolled Cu-Nb-Ti(Zr) Alloys

With increasing demands for pure hydrogen in industrial fields,such as aerospace,petroleum processing and electronic industry,much attention has been paid to the purification of hydrogen.High purity hydrogen can be obtained by metal membranes.Metal membranes are well known for a high mechanical strength,strong pressure-bearing ability,good thermal conductivity and high chemical stability.Therefore,using metal membranes for the separation of hydrogen gains increasing industrial appeal,and has led to the development of related scientific research.In the present study,Cu-30Nb-5Ti and Cu-30Nb-5Zr(at%)ternary alloys are used to fabricate metal membranes for hydrogen separation.By means of scanning electron microscope(SEM)and X-ray diffraction(XRD),the microstructures are characterized and phase compositions are analyzed for the ascast and cold-rolled states.For both states,pressure-concentration-temperature(PCT)curves are measured and hydrogen-permeation tests are performed using self-developed Sieverts equipment and high-precision hydrogen permeation equipment.A relationship is revealed between alloy type,microstructure,hydrogen absorption and hydrogen permeation.The experimental results show that the as-cast and cold-rolled Cu-30Nb-5Ti alloy consist of the same phases.They are primary phase bcc-(Nb,Ti)and solid solution matrix phase fcc-Cu.The primary phase presents dendritic morphology and is embedded into the matrix phase in the as-cast state.After cold rolling,the morphology of primary phase is modified to slim lath.The as-cast and coldrolled Cu-30Nb-5Zr alloy also consist of the same phases,primary phase bcc-(Nb,Zr)and solid solution matrix phase fcc-Cu.For the as-cast alloy,the dendrite morphology for primary phase is observed.This morphology is changed to lath shape after cold rolling.For Cu-30Nb-5Ti and Cu-30Nb-5Zr alloy membranes at as-cast and cold-rolled states,the hydrogen absorption and hydrogen permeation tests are performed.Based on the experimental results,the general trend is the hydrogen solubility decreases with the temperature and increases with the pressure.The hydrogen solubility of Cu-30Nb-5Zr alloy membrane is higher than that of Cu-30Nb-5Ti alloy membrane.The hydrogen permeability increases with the temperature raising.The hydrogen permeability coefficient is in the linear relation with temperature.The as-cast Cu-30Nb-5Zr ternary alloy shows a higher permeability.At 673 K,this permeability value is 1.83×10-8 molH2 m-1 s-1 Pa-0.5 that is about1.1 times higher than pure Pd(1.6×10-8 mol H2 m-1 s-1 P-0.5).Although the thickness of as-cast metal membrane is reduced from 0.6 mm to 0.2 mm after cold rolling,at the same temperature and pressure,alloy membranes at as-cast state presents higher hydrogen solubility and hydrogen permeation in comparison with the counterpart at cold-rolled state.The hydrogen diffusion coefficient increases with temperature.Compared to Cu-30Nb-5Ti alloy membranes at ascast and cold-rolled states,the as-cast Cu-30Nb-5Zr alloy membrane presents the largest hydrogen diffusion coefficient 1.98×10-9 m2 s-1 which is lower than pure Pd(7.69×10-9 m2 s-1).