Study Of The Kinetics Of The Initial Oxidation Of Uranium With Pure Oxygen By Spectroscopic Ellipsometry

The kinetics of the surface oxidation of uranium with pure oxygen below 100℃was studied by spectroscopic ellipsometry(SE)systemically.In the study,the focus was on the change of uranium oxide layer thickness with time and the measurement of the optical constants of metallic uranium and its oxide.The relative factors during measurement were studied.Because of the influences of sample roughness,we need treat with the specimen surface to reduce the roughness,which was grinded and polished before experiment.During testing process,light travels through glasses windows,and the glasses' refraction almost doesn't affect testing results,so it can be ignored.Meanwhile,measuring the given thickness of Si/SiO₂ oxide layer,the experimental datas are close to actual values;in situ testing the change plot of Ce-La alloy oxide layer thickness with times,parabolic law agrees with that of literatures.The plot of uranium and its oxide layer's optical constant(refractive index n and extinction coefficient k)with wavelength was measured by SE.we obtained the n and k values of uranium,which were 1.3～2.5 and 1.9～4.5 with wavelength in 350nm～1000nm range,respectively.The thickness of oxide layer being 0.6nm,the value of n was between 2.20～2.34.While the thickness was 129nm,the value of n was between 2～2.12.In visible 520nm wavelength,the n of uranium oxide layer in different thicknesses were 2.54(0.6nm of uranium oxide film)and 2.01(129nm of uranium oxide film),respectively;in 546.1nm wavelength,the n and k of U were 1.5 and 2.6 respectively,and the n of UO₂ film was 2.2.In different temperatures(45℃～95℃)and different oxygen pressures(500Pa～50000Pa), the change of the oxide layer thicknesses along with times was measured in situ,and the results indicated the oxide thickness change obeyed parabolic kinetics rule,which showed that diffusing of oxygen controlled oxidation mostly.The reaction activation energy of uranium in 5000Pa of oxygen pressure and below 100℃was calculated,which was 73.66kJ·mol^-1.The result of in situ testing also showed that the oxidation rate of uranium depended on reaction temperature.The oxidation rate of uranium increased quickly with the temperature enhancement;effect of oxygen pressure on reaction rate was not apparent in high oxygen pressure(more than 500Pa).Between the range of 500Pa～50000Pa oxygen pressure,the oxidation rate change of uranium was little.By repeated experiments,we obtained the primary relationship of uranium oxide layer surface colour and thickness;when the oxide layer thicknesses were approximate 9nm, 50nm～60nm and 90～102nm,the surface colour are yellow,light blue and yellow,respectively. The colour of yellow of uranium surface at 90nm～102nm thickness of oxide layer is the second colour period.