Investigation On Hydrogen Permeation In Niobium

Dense metal films play an important role in the separation of hydrogen and its isotopes.Palladium and its alloy films are commonly used in industry for hydrogen separation and purification,but their high price limits their wide application of hydrogen isotope separation,especially in the field of hydrogen energy.Refractory metal niobium is paid more attention owing to its advantages of high mechanical strength,lower price and higher intrinsic permeability coefficient,and is considered as a substitute for the hydrogen separation material of metal palladium.However,due to the poor catalytic activity of the metal niobium on the dissociation of hydrogen molecules and its surface easy formation of a dense oxide film with a strong bond with the matrix,the application of the metal niobium in the field of hydrogen per-meation separation has not yet been reported,especially the effect of grain size on the behavior of hydrogen permeation.Therefore,in this paper,nio-bium with different grain size was used as the research object.The relative"clean" niobium surface was obtained by means of mechanical polishing,ion sputtering and ultra high vacuum annealing,and the permeability of hydrogen and deuterium was also investigated.The surface composition and microstructure of the original niobium with two different metallurgical processes and the Nb after mechanical pol-ishing were analyzed.The grain sizes of the two kinds of niobium were found to be 10～200 and 200～500 nm,respectively.According to the grain size,they were named microncrystalline and nanocrystalline niobium re-spectively.The oxide layer on niobium surface is mainly Nb2O5.By me-chanical polishing,most of the oxides on the surface of niobium can be removed,and the remaining oxide layer is very thin,about 20 nm.The com-position of niobium on the Nb surface after mechanical polishing was in-vestigated by high vacuum annealing and ion sputtering.The results showed that ultra-high vacuum annealing and argon ion sputtering could effectively remove the residual oxide film on the niobium surface,and obtain a”clean"niobium surface.After the removal of niobium surface oxide by mechanical polishing,the permeation behavior of deuterium in microcrystalline niobium and nano-crystalline niobium under the temperature range(600～750 0C)and gas pres-sure(50 KPa)was investigated by gas phase driving method.The results show that the relationship between the deuterium permeability,the diffusion coefficient and the solubility constant with temperature T in the microncrys-talline niobium is as follows:Φm=2.45×10-8 exp(-6309.9/T)mol· m-1·s-1·Pa-0.5,Dm=1.60x1 0-6exp(-10511/T)m2·S-1,,Sm=1.53 × 10-2exp(-34927/T)mol· m-3·Pa-0.5.For nanocrystalline niobium,the relationship is as follows:Φn = 3.23×10-6exp(-6656.5/T)mol.m-1·s-1·Pa-0.5,Dn=2.59×10-6exp(-10342/T)m2·s-1,Sn = 7.20×10-1exp(-35712/T)mol·m-3·Pa-0.5.Compared with nanocrystalline niobium and micron crystalline niobium,it is found that the permeability of the former is better.The permeation behavior of hydrogen and deuterium in nanocrystalline niobium at temperature range(600～800 ℃)and gas pressure(50 KPa)was investigated by gas phase driving.The results show that the relationship between hydrogen and deuterium permeation with temperature were:ΦH = 4.98×10-6exp(-6406.9/T)mol·m-1·S-1·Pa-0.5,ΦD = 3.51 ×10-6exp(641$.8/T)mol m-1· s-1· Pa-0.5.Under the same temperature and pressure,the permeability of hydrogen in niobium film is higher than that of deuterium in niobium film,the ratio of is aboutφH/vD= 1.44,which is close to the theoretical value and satisfies the iso-tope effect relationship.Compared with other dense metal films,the deuter-iun permeability of niobium in this article at the same temperature is higher than that of CLAM steel by one order of magnitude and than that of 310 austenitic stainless steel by five orders of magnitude,and slightly lower than that of Pd8.5Y0.19Ru alloy by one order of magnitude.