Fabrication Of Multi-components Coatings On Zirconium Hydride And Research On Hydrogen Resistance Mechanism

Zirconium hydride is one of the most ideal neutron moderating materials due to its high hydrogen density,low neutron absorption cross section,high neutron scattering cross section and negative temperature coefficient of reactivity.As part of neutron moderator which can reduce the mass and volume of the reactor effectively.However,the problem of hydrogen loss from zirconium hydride exists in working temperature range,which reduces the neutron moderating efficiency and service life.Therefore,to prevent or slow down the hydrogen loss in moderator from zirconium hydride,in-situ oxidation technique was adopted to prepare hydrogen permeation coatings with low diffusion coefficient and combining densely.The modification technique was the effective way to solve the problem of hydrogen loss in working temperature range.The fabrication of muti-element composite coating on zirconium hydride and research on its hydrogen resistance were investigated in present study.It includes preparation of zirconium hydride,analysis of abnormal occurrence in fabrication process,preparation and properties characterization of coaitngs.The phase structure and morphology of coatings were analyzed by X-ray diffraction?XRD?and scanning electron microscope?SEM?.The composition and depth of elements,bond states were tested by auger electron spectroscopy?AES?and X-ray photoelectron spectroscopy?XPS?technique.The hydrogen resistance was tested in thermal vacuum system.The hydrogen permeation mechanism of coatings was discussion utilizing AES and XPS techniques.The major results and conclusions are as followed:Within the zirconium hydride manufacturing process,as the value of nH:nzr rise,the radial,axial direction and volume expansion rate increasing.There exist certain differences between the expansion rate radial and axial direction.With the height-diameter ratio goes up,the expansion rate radial increased,however the axial direction one decreased.XRD shows the golden yellow coatings were C-N-O-Zr compounds.AES depicts the relative depth of O,C,N elements were 28,6120,5400nm,respectively.In the machine cutting or grinding.The impurity material in the cutting fluid was infiltrated along the cracks.SEM and AES that the impurity material,such as sulfur,calcium,sodium,oxygen et al,was passed into the inner zirconium hydride.The relative depth is about 2250nm.The machine process accelerated the formation of passivation coating.Carbamide TG-DSC indicates the mass loss was 64.17%at 200?,12.69%at 248°C,19.22%at 300?,and decomposed completely at 400?.When the oxidation temperature was 400?,500?,oxidation time for 20h,the oxidation mass gain raise with amounts of urea significantly.When T=400?,500?,carbamide for 0.5g,the oxidation mass gain were 1.62g/m²,2.23g/m².The oxidation mass gain increased twice when oxidation temperature?T?went up 100? under the same amounts of urea.While oxidation temperature rise to 550?,the oxidation mass gain increasd unobviouly.T=600?,oxidation mass was 3.052g/m²,respectively.A parabolic law was observed in oxidation kinetics for zirconium hydride oxidation in oxidizing atmosphere at 400?-600?.The curves of square thickness?D2?of the oxide film with oxidation time?t?follow linear law.The oxidation progress of zirconium hydride is controlled by inward diffusion of carbon,oxygen,nitrogen atoms.The relationship of diffusion coefficient and temperature is represented as Arrhenius Law.The diffusion active energy of the oxidation reaction is 73.28 J·mol^-1,indicating a grain boundary diffusion mechanism.XRD shows the phase structure of coating consists mainly of ZrN and ZrO2,including tetragonal zirconia?t-ZrO2?and monoclinic zirconia?m-ZrO2?.SEM depicts coatings were accumulated and combined densely with substrate,and there no holes or cracks obviously.EDS gives the coatings were made up of carbon?C?,nitrogen?N?,oxygen?O?and zirconium?Zr?elements.AES indicates that the coating is mainly composed of,which show that the atomic concentration of carbon and nitrogen decreased continuously,while,oxygen and zirconium increased with sputter time incresing.And carbon-nitrogen-rich layers were distributed on the surface of coatings,the oxygen layers passed through the whole coatings.XPS investigates the chemical binding states in the coatings were Zr-O?Zr-C?Zr-N-O?Zr-N?O-H bonds.When oxidation temperature was 600?,various intermediate non-stoichiometric oxides were formed first near the substrate boundary.When T=500?,t=20h,the average speed of hydrogen desorption?J?were 1.75,1.75,1.60,1.63,1.43×10^-5 mol·m^-2·s^-1 for m=0.1g,0.2g,0.3g,0.4g,0.5g,respectively.When t=20h,m=0.5g,the value of J were 2.05,1.43,1.22×10^-5 mol·m^-2·s^-1,for T=400?,500?,600?,respectively.When T=500,m=0.5g,the value of J were 1.66,1.43,1.73,1.54×10^-5 mol·m^-2·s^-1,for t=10,20,40,80h,respectively.Utilizing gas chromatograph system,the total hydrogen osmotic quantity was 1.29×10-3mol,the average hydrogen permeation rate was 5.33 × 10^-5mol·m^-2 min^-1.The last 5?7th day,the hydrogen permeation rate was 2.51?2.84× 10^-5 mol·m^-2·min^-1,while the average hydrogen permeation rate was 2.72 × 10^-5 mol·m^-2·min^-1.The intensity has no abvously changed before and after the thermal vacuum tests,which deipicts there little amount hydrogen lost.XPS shows there C-H,C-Zr,O-Zr,O-H,N-H,Zr-O-N bonds in the muti-components coatings.While the amouts of C-H,O-H,N-H bonds increased remarkably after the thermal vacuum tests.The hydrogen permeation mechanism of coatings was that the existence of C,N,O capture the H atoms when it outward diffusion,generating the C-H,O-H,N-H bonds.Moreover,the concentration of hydrogen trap,generated by oxygen vacancy structure,such as Zr₂N₂O,t-ZrO2,could capture the H atoms as well.Both the two ways were collaborated to prevent hydrogen permeation from zirconium hydride,that's the reason why the components coatings have the good resistance effect of hydrogen.