Research On Thermal Sprayed Tritium Permeation Barriers And Theoretical Calculation Of Hydrogen Permeation

It is one of the most important and difficult problems to reduce the permeation and leakage of tritium in a magnetic confinement reactor with deuterium and tritium.Preparing ceramic tritium permeation barriers?TPBs?on metal structure materials is a commonly accepted solution at present,which can obtain good process property,high mechanical properties and high tritium permeation resistance at the same time.Most TPBs have shortcomings at present,such as complex preparation process,easy to produce cracks,poor radiation resistance or poor chemical stability.At the same time,there are very few studies on the diffusion mechanism of hydrogen and its isotopes.High porosity is the largest problem of the thermal spryayed coatings.The self-healing coating of TiC+Mixture?TiC/Al₂O₃?+Al₂O₃?TLC for short?using TiC as the core was designed by our group.Volume increased by the oxidation of TiC can reduce the high porosity of the coating prepared by air plasma spraying?APS?.The design of TLC,oxidation process,nitridation process,properties,binding mechanism,hydrogen permeation test method were studied.The different permeation rates of hydrogen in TiN coatings with different preferred orientations were tested.The diffusion process of hydrogen in TiN with different crystal direction was researched based on the first principles to explain the results mentioned above.The main conclusions are as follows:?1?According to the existing theoretical calculation and experimental results,it is the control step of the whole diffusion process that the H atom diffuse from the most surface to the subsurface of the coating.The structure and composition design based on the interface theory mentioned above were innovatively proposed,and several more efficient multi interface composite TPBs,such as TLC,were designed.?2?The radiation test of the TLC coating with and without AlNi was conducted.The AlNi had obvious segregation of components after irradiation.Holes appeared and the coating fell off seriously.The interface and the inner of the TLC without NiAl was intact.?3?Based on experimental results and the stress simulation,an accelerating test with a thick TiC+mixture?TiC/Al₂O₃??DLC?was designed to get a more accurate crack-filling heat-treatment process.The thick DCL samples warped from the edge when the heat treatment temperature was lower than 600°C,while crashed to pieces when the temperature was higher than 650°C.These two different failures were well explained by ANSYS stress simulation.The optimum heat treatment temperature was 650°C,12 hours?called standard heat treatment?.The TLC sample whose diameter was 20mm after standard heat treatment remains intact after 400 thermal shocks at 600°C.The200mm*500mm large sample also remained intact after 10 thermal shocks.?4?The bonding strength between TLC and substrate after self-healing was much higher than that of spraying one,exceeding 50MPa.The microscopic of the failure zone after thermal shock was studied.The Mixture?TiC/Al₂O₃?and Al₂O₃ layers fell off,while the TiC layer was still intact.The TiC monolayer coating samples after heat treatment?TiC has changed to TiO₂?was researched by XRD,TEM.There was a very thin layer of Cr₂O₃ between TiO₂ and the substrate.Between the TiO₂ and Cr₂O₃ there was a typical semi-coherent interface.That was why the TiC layer after hear treatment had unusually strong bonding strength with the substrate.The surface should be very smooth to seal well in the high temperature gas driven deuterium permeation test.To meet the requirement the sand blasting process and the bottom layer in the traditional thermal spraying were both removed and the coating was prepared directly on the smooth substrate.The results showed that the coating sample after heat treatment also had rather high bonding strength,thermal shock resistance and good hydrogen resistance.After polishing the samples were intact,the surface was very smooth and the Rz decreased by94%.?5?In this paper,the liquid phase electrochemical hydrogen permeation test was innovatively used to test the hydrogen permeability of the TPBs.Since the coatings involved in this paper contained TiC,TiO₂,TiN,it has certain conductivity and can be tested by this method.Compared with the conventional gas driven deuterium permeation method,this method has the advantages of low cost,simple sealing,no damage to the coating,and high speed.The test results showed that the sample after the heat treatment of 40h at 650°C had the best hydrogen permeation resistance,so it was the new best heat treatment process.?6?The nitridation process was introduced in this paper to improve the hydrogen permeation resistance of the TLC.The results showed that the hydrogen permeation resistance coefficient of the samples after an additional nitridation at 760°C for 1h doubled,comparing with the sample after the oxidation treatment at 650°C for 40h.A thin layer of AlN appears between the coating and the substrate after nitridation,while it disappeared after additional oxidation.Ti migrated to the surface of the coating during the nitridation process,and the coating surface became much smoother.This article explained the phenomenon innovatively with the principle of catalyst and the reversible reaction.?7?The TiN coating with different preferred orientation was prepared by magnetron sputtering.The test results showed that,the?111?preferred orientation coating had better hydrogen permeation resistance than that of the?200?preferred orientation coating.In this paper,Materials Studio software was the first time to calculate the diffusion process of hydrogen in TiN coatings with different orientations.There is only one termination of?200?surface,while two of?111?surface.The most surface was Ti?code name 111Ti?or N?code name 111N?,respectively.The results showed that,the controlling step of the diffusion in these three TiN coatings were the same.It was the step that H diffused from the most surface to the subsurface.The maximum diffusion energy barrier and the maximum diffusion reaction heat were both 111Ti>111N>TiN?200?.That was to say,the?111?oriented TiN had better hydrogen permeation resistance than the?200?oriented TiN in both dynamics and thermodynamics.