Synthesis And Properties Of Zirconium Carbide Ceramic Coatings For Advanced Nuclear Power Systems

The development of nuclear reactors,the structural materials for advanced nuclear power systems are operated under more severe conditions.The structural materials used for both in-core and out-of-core applications,especially in GenerationⅣ nuclear reactor,are faced with extreme conditions,such as higher temperature,higher irradiation dose and extremely corrosive environment.It is urgent to develop materials with advanced irradiation resistance.Vacancy,as a new approach of material design,can act as the effective "trap" to absorb radiation defects and improve the radiation resistance of materials.Zirconium carbide ceramic can remain stable with a wide range of carbon to zirconium ratio and regard as non-stoichiometric material,which is the one of the promising candidate materials for advanced nuclear reactor systems.Zirconium carbide demonstrates the high melting point,high thermal conductivity and low neutron scattering cross section.In this dissertation,the synthesis,microstructure and properties of ZrC_x coatings were systematically investigated.The main objective is to clarify the damage mechanism and performance degradation of ZrC_x ceramic coatings with different stoichiometry induced by high irradiation dose.The dissertation is developed as follows:Firstly,ZrC_x coatings with different stoichiometry（x=0.49～1.00）were deposited by magnetron sputtering,which were obtained by carefully controlling the power ratio of zirconium target and carbon target.The effects of C/Zr ratio on the lattice constant,microstructure,growth mode and mechanical properties were systematically studied.It is found that the phase constitution remains the NaCl structure with the C/Zr ratio varying from 0.49 to 1.00.With the increase of carbon content,the lattice constant,hardness and elastic modulus first increase and approach a maximum at x～0.86,and then decrease upon the further increase of carbon content.With the increase of carbon content,the surface grain morphology of ZrC_x coating changes from triangular-conical grain to polygonal grain,and the growth mode changes from feather-like grain growth to typical V-shape columnar grain.Based on the investigation of sub-stoichiometric zirconium carbide,three kinds of ZrC_x（x=0.42,0.67,0.73）powder samples were obtained.The vacancy configuration of non-stoichiometric zirconium carbide was studied by using synchrotron radiation technology.It is found that the 3rd nearest-neighbor coordination in ZrC_x decreases evidently with the decreasing carbon content.It is proved that carbon vacancies prefer to occupy the 3rd nearest-neighbor（3NN）sites in ZrC_x coatings with high carbon vacancy concentration.ZrC_x（x=0.86,0.78,0.55,0.49）coatings were irradiated with 3 MeV Au²⁺ions at different fluence（1 × 1014 ions/cm²,1 × 1015 ions/cm²,1 × 1016 ions/cn2,2×1016 ions/cm²）.The carbon content and radiation fluence had influence on the swelling and amorphous degree of zirconium carbide.It is found that low irradiation fluence do not destroy the crystallinity of ZrC_x.Under the 2×1016 ions/cm² irradiation fluence,the localized amorphization apper in ZrC0.86 coating with the 3.3%swelling.While,the ZrC_0.55 coating with low carbon content still maintains its intact lattice structure without amorphization,and the maximum lattice swelling is only 0.9%.It shows that the high concentration of carbon vacancy can act as the trap of point defects effectively,inhibit the irradiation damage,and improve the irradiation resistance of zirconium carbide.After irradiation,two ordered phases with space groups of Fd3m and P3121 and twin structures were observed in the ZrC_x coatings which increased with the increased irradiation fluence and carbon vacancy concentration.The two ordered phases and twins formed by the irradiation in ZrC_x can improve the resistance of irradiation damage.Based on the acknowledgement of ZrC_x coatings with good irradiation resistance and the matched thermal expansion coefficient with Zry-4,ZrC_x was selected as the diffusion barrier in integrated coatings.The thermal stability of Cr₂AlC/ZrC_x integrated coating was investigated during a hypothetical LOCA scenario.The observation shows that ZrCo.55 is the thermochemical stable phase in contact with Zr-4 substrates in the temperature range from 800 to 1200℃.Furthermore,under the condition of rapid annealing at 1000℃,the diffusion of C and Al elements from of Cr₂AlC were effectively inhibited with the introduction of ZrC_0.55 buffer layer.The structural integrity of Cr₂AlC/ZrC_0.55 coating was well retained,demonstrating the promising application of integrated ceramic coatings for accident tolerant fuel（ATF）system.