Study On The Influence Of Surface Properties In U-H₂ Reaction And Characterization Of α-UH₃

Metal uranium and its alloys have attracted wide attention as one of the most important materials for the energy industry and defense industry.Uranium and uranium alloys are reactive and prone to corrosion in the ambient atmosphere in their application conditions,which leads to degradation or even failure of the materials,and causes some pollution and damage to the environment.Hydrogen corrosion of uranium is one of the most serious corrosion of uranium due to its strong permeability to hydrogen,large hydride expansion,serious destruction of the matrix and spontaneous combustion.The related scientific research has been focused for over 70 years in the field of nuclear materials science.The major laboratories in this field,such as LANL,LLNL,AWE,NRC-Negev and CAEP have conducted long-term and systematic studies on the thermodynamic and kinetic characteristics of U-H₂ reaction,the nucleation and growth mechanisms of uranium hydrides,and the impact of hydriding corrosion on material properties.However,at present,the microscopic mechanism of nucleation of hydride during the initial stage of U-H₂ reaction is still ambiguous.The surface oxide layer or the metal matrix,which is the key factor affecting the nucleation of hydride is still in dispute.In addition,most of current literature is only P-UH₃ involved,while the study of the metastable a-UH₃ for its physical and chemical properties and formation conditions is rarely discussed.It is generally believed that a-UH₃ can only be formed under the conditions of low temperature,low hydrogen pressure or dilution for a slow reaction.However,at normal temperature and hydrogen partial pressure,the product of U-H₂ reaction is almost pure P-UH₃.There is no credible literature reporting on the selectivity of the products during reaction.In our work,we have selected several representative surface layers:Ar+ion sputtering surface with weak enhancing on hydriding,oxidized surface with weak inhibition on hydriding,Pd deposited surface with strong catalytic effect on hydriding and Dimond like carbon deposited surface with strong resistance against hydriding.The effects of these surface layers on the hydriding kinetics,nucleation mechanism and hydride microscopic morphologies were analyzed in terms of inhibiting/enhancing hydrogen diffusion or suppressing the growth of hydride nuclei.By analyzing the reaction conditions for the preparation of a-UH₃,we innovatively adopted the method of covering the uranium surface with Pd,and prepared the hydridng products of high a-UH₃ content under conventional experimental conditions（70℃,70 kPa）.The morphologies and growth characteristics were analyzed to obtain the growing environments and conditions for a-UH₃ and P-UH₃.Theoretically,we used the first-principles principle to calculate the energies of formation of two hydrides,and established a doping model for H in the four media α-UH₃,β-UH₃,UO₂,and β-PdH_x to calculate the strain energies and diffusion barriers.The main conclusions of this paper are as follows:1.The influence of the surface layer on the hydride formation at the interface is mainly manifested in（1）retarding or enhancing the diffusion of hydrogen in the surface layer and hydride nucleation at the interface,（2）the membrane-substrate stress which suppresses hydride geometry and growth at the interface.Specifically:（1）Surface Ar+ion sputtering can remove the residual oxide layer,stress-damaged layer and most of the adsorbed impurities on the surface after machining,as well as introduce a high concentration superficial defect layer with a thickness of about 15 nm.The introduction of the defect layer facilitates the adsorption,dissociation,diffusion and accomulation of hydrogen,and therefore the induction period of U-H₂ reaction of the sputtered surface is shortened or disappeared.When the depth of oxidation of the sputtered sample is greater than the thickness of the defect layer,the induction period of the reaction reappears.The surface deposition of Pd layer as a high concentration surface hydrogen source can make the interface easier to reach the critical concentration of nucleation.so that the U-H₂ reaction induction period disappears and has some homogeneous reaction characteristics.The diamond-like film,an inert surface layer that blocks diffusion of hydrogen,can effectively prevent the penetration of hydrogen so that the interface hardly reacts;the surface oxide layer can also suppress the diffusion of hydrogen,weaker than the diamond-like film,and dense defects in the oxide layer provides short-circuit diffusion channels for hydrogen.（2）The thickness of the surface layer,the permeability of hydrogen,and the stress of the surface layer on the substrate affect the nucleation and growth behavior of the hydride.Without the constraint of the oxide layer,the hydriding corrosion pit on sputtering surface is more open;as the oxide thickness increases,U-H₂ is more dependent on the defects in the surface oxide layer,and the density of the nucleation sites is increased but the individual hydride is reduced.The top of the blister-like hydrides crackes at where the stress is most concentrated.The intense compressive stress of the diamond-like film on the metal substrate has a significant inhibitory effect on the growth of the hydride at the interface,and no hydride is observed during the reaction.In addition to the properties of the surface layer,the thickness of the surface layer also has an influence on the growth characteristics of the hydride.The thin surface layer,less compression and larger hydrogen flux allow the hydride to grow in the superficial lateral direction,with a high surface area ratio;The larger stress and the smaller hydrogen flux due to the thicker surface layer make the hydride grows in the depth direction and occupies a low proportion;Hydrogen can be transported through the cracks produced by stress release during U-H₂ reaction,thus forming corrosion inside the matrix.2.In the case of surface-covered Pd film,we obtained a remarkable(X-UH₃ concentration under common reaction conditions（70℃,70 kPa）.Hydrogen accumulation and hydride nucleation are preferred at grain boundaries and inclusion-matrix interfaces,and we furtherly interpretated it as a definite correlation between β-UH₃ and grain boundaries and inclusion-matrix interfaces.On single crystal samples,where grain boundaries and inclusion-matrix interfaces are absent,the U-H₂ reaction has an induction period of one order of magnitude higher than that of polycrystalline samples with rich grain boundaries and inclusion-matrix interfaces.The comparison of the hydrogenation kinetics of single-crystal/polycrystalline samples with Pd deposition shows that at the initial stage of the U-H₂ reaction,the hydrogen enrichment at the interface leads to nucleation.causing uneven expansion and the formation of a volcano shaped β-UH₃ corrosion sites;while a-UH₃ is growing homogeneously beneath the uniform Pd layer.A comparison of the reaction kinetics of single crystal and polycrystalline samples deposited with Pd films shows that the at the initial stage there is a relatively lower and constant reaction rate and the main product is a-UH₃,which indicates that covering the surface with Pd to enhance the formation of a-UH₃,is essentially utilizes the limiting effect of the intact Pd film on the diffusion of hydrogen from the gas phase to the reaction interface.3.The first-principles calculation of the formation energy of two hydrides,α-UH₃ and β-UH₃,was performed.The results showed that the difference in formation energy between the two hydrides was only 0.029 eV,thus the product selectivity could not be explained from the perspective of thermodynamics.The doping models of H in a-UH₃ and P-UH₃ are also developed and the energy change of the system under different doping concentrations was calculated.It was found that the β-UH₃,with larger hexahedral interstitials comparing to a-UH₃,has a higher hydrogen solubility.The diffusion paths and diffusion barriers of H in the four media a-UH₃,P-UH₃,UO₂,and β-PdH_x were calculated using CI-NEB method.The results show that the diffusion barrier of H in UO₂ lattice is as high as 2.2 eV,which indicates that H diffusion in compact UO₂ lattice is almost impossible.The diffusion barriers of H in α-UH₃,β-UH₃ and β-PdHoO5 are 0.16 eV,0.09 eV,and 0.19 eV,respectively.This indicates that the diffusion rate of hydrogen in β-PdH0.5 is similar to the diffusion rate of hydrogen in α-UH₃ and is lower than the hydrogen diffusion rate in P-UH₃.Given the autocatalytic characterisitc of β-UH₃.Diffusion flux may be responsible for the selectivity of hydride generation.The work of this doctoral thesis verifies the correlation between grain boundary,inclusion substrate interface and nucleation and growth of β-UH₃.On condition that the hydrogen flux diffused from the gas phase to the reaction interface is limited,the product of the U-H₂ reaction is mainly α-UH₃.This not only promote us for deeper understandings of the nucleation properties of uranium hydride,but also construct a method for the preparation of α-UH₃ and further research.