Preparation And Investigation Of TiO₂ And α-Al₂O₃/TiO₂ Composite Tritium Permeation Barrier

The raw materials of the magnetically constrained nuclear fusion reactor are hydrogen isotopes:deuterium and tritium.Low-activated ferritic/martensitic steel（RAFM steel）is one of the candidate structural materials for the fusion reactor cladding,but the tritium permeability of RAFM steel is high.Reducing tritium penetration is one of the key scientific and technical issues in fusion reactor research.A thin ceramic coating as a tritium permeation barrier（TPB）on metal structural materials is an internationally recognized solution,which not only ensures the structural properties,but also greatly improves its barrier properties against tritium penetration.The oxide composite coating has excellent barrier properties against tritium penetration and good mechanical properties.Therefore,in this study,the preparation and research of TiO₂ andα-Al₂O₃/TiO₂ composite coatings were carried out,and the mechanism of TiO₂ promoting the formation ofα-Al₂O₃ at low temperature was explored.（1）Firstly,the preparation of TiO₂ composite coating was studied to solve the current problem of coating preparation technology.A new combined process including pack cementation and thermochemical treatment was utilized to prepare coatings.The three-layer“TiO₂/nitrogen layer/transition layer”composite coating with a thickness of about 11μm was successfully prepared.The outer layer was rutile TiO₂ layer with a thickness of about2.5μm,the middle layer was Ti-N layer with a thickness of about 5.5μm,the transition layer was Ti-C layer mainly consisted of Ti and C,and it also had a gradually increasing content of Fe.Nitrogen pretreatment before oxidation can reduce the oxidation rate of the coating,change the nucleation and growth process of the final oxide,refine the coating grains,and improve the compactness of the coating.The coating exhibited good barrier properties against hydrogen penetration,its steady-state permeation current density was only 0.13μA/cm²,and the steady-state permeation current density of the substrate was 150times of that of the coating.The bonding strength of the coating was 49.8 MPa,which was well bonded to the substrate.The surface of the coating began to fail after 200 cycles,showing excellent thermal shock resistance.（2）Subsequently,in order to solve the problems ofα-Al₂O₃ coating preparation,such as too high preparation temperature,easy to peel off and difficult inner surface forming,etc Based on the above research,theα-Al₂O₃/TiO₂ composite barrier coating was produced by the combined process including pack cementation and thermochemical treatment.The coating obtained consisted of three layers,and the multilayer structure of the coating was“α-Al₂O₃/TiO₂/transition layer”.The surface of the coating was very fine and compact.The nitriding treatment reduced the oxidation rate and changed the oxidation behavior of the coating,and the surface of the resulting coating was very fine and compact.The total thickness of the coating was approximately 21μm.The outer oxide layer had a thickness of about 4μm and exhibited a two-layer structure:an outerα-Al₂O₃ layer and an inner TiO₂layer.The inner transition layer consisted of Al,Ti and Fe.A continuous nitride layer of about 2.5μm was present beneath the oxide scale,the presence of which significantly reduced the oxidation rate of the coating and the formation of TiO₂ on the surface of the coating.The coating exhibited good barrier properties against hydrogen penetration,its steady-state permeation current density was less than 0.05μA/cm².The bonding strength of the coating was 54.1 MPa,which was well bonded to the substrate.The surface of the coating began to fail after 220 cycles,showing excellent thermal shock resistance.（3）Finally,a series of sol-gel experiments were designed to explore the mechanism of TiO₂ promoting the formation ofα-Al₂O₃.The doping effect of TiO₂ was verified by comparison with the templating effect of Cr₂O₃.Cr₂O₃ and TiO₂ both can accelerate the formation ofα-Al₂O₃.TiO₂ promoted the formation ofα-Al₂O₃ through its doping effect,which was different with the templating effect of Cr₂O₃.The difference of ionic radius between Ti⁴⁺and Al³⁺is small,and the relatively slight difference between the sizes of these cations suggests that Ti can substitute partially for Al in the octahedral sites of alumina.Ti⁴⁺doping into the lattice structure of transition state Al₂O₃ can replace Al³⁺,which will lead to an increase in the concentration of aluminium ion vacancy V_Al′′′.These vacancies can decrease the nucleation barrier ofα-Al₂O₃ and promoteθ→α-Al₂O₃ phase transformation,thus promoting the formation ofα-Al₂O₃.