Study On Diffusion-controlled Growth And Diffusion Behavior Of ZrO₂ And Y₂O₃-stabilized ZrO₂

ZrO₂ materials have both excellent mechanical and functional properties,but temperature-induced allotropic transformations limit their applications.Oxide doping is an important means to stabilize the high-temperature phase of ZrO₂ and broaden its application range.Among them,Y₂O₃-stabilized ZrO₂（Y₂O₃-ZrO₂:YSZ）is the most mature and widely used in thermal barrier coatings,fuel cells and oxygen sensors.To clarify the problems of Zr oxidation corrosion,thermal barrier coating failure,and fuel cell life,it is necessary to quantitatively describe the diffusion growth and diffusion behavior of ZrO₂ and YSZ.In recent years,diffusion kinetics simulation based on the CALculation of Phase Diagram（CALPHAD）has become an effective method for material diffusion research.However,the quantitative description of the entire oxidation process of metals with large oxygen solid solubility（e.g.Zr,etc.）and the diffusion behavior of linear oxides is an urgent problem to be solved in the field of materials.There is also a lack of systematic research on the complex diffusion behavior of doped oxides such as YSZ in the world,and there are no relevant diffusion kinetics database reports.The thesis takes ZrO₂ and YSZ as the research objects.Firstly,the diffusion kinetic database of HCP Zr（O）solid solution,α-ZrO₂ and YSZ are constructed by the CALPHAD method,and then the pure Zr oxidation process and diffusion behavior of fuel cells YSZ electrolyte were quantitatively simulated by coupled thermodynamic description.The main research contents and innovative achievements of this thesis are as follows:（1）A database of diffusion kinetics of single-crystal HCP Zr（O）solid solution phase along the a-b plane and c axis was constructed,and the anisotropic diffusion behavior in the HCP Zr（O）phase in the initial stage of pure Zr oxidation was systematically studied by combining with thermodynamic description.It is found that the diffusion of HCP Zr（O）solid solution tends to be isotropic with the temperature increase.The interdiffusion coefficient varies significantly with temperature,crystal orientation and oxygen concentration.It is isotropic when x（O）=0.19;when x（O）<0.19,D_a-b>D_c;when x（O）>0.19,the result is the opposite.In the initial oxidation stage of pure Zr,oxygen diffusion along the a-b plane predominates the oxidation process,and the contribution of short-range diffusion is weak.（2）A two-step numerical simulation method that can quantitatively describe the oxidation process of metals with large oxygen solubility is developed,and its programmed and high-throughput calculation/simulation is realized.At the same time,the bulk,grain boundary,dislocation and effective diffusion coefficient ofα-ZrO₂ were systematically evaluated,and the kinetic database ofα-ZrO₂ was constructed.Based on a reliable thermodynamic and kinetic database,the entire oxidation process of HCP Zr was quantitatively simulated by the two-step method.The simulation results under different temperatures,oxygen partial pressures and times conditions show that the HCP Zr oxidation process is jointly controlled by the solid solution stage and the diffusion growth ofα-ZrO₂,the contribution of the solid solution stage cannot be ignored,and the contribution of short-range diffusion ofα-ZrO₂is larger at low temperature.The increase in temperature and oxygen partial pressure will accelerate the diffusion of oxygen ions,increasing the oxidation rate.In addition,the growth ofα-ZrO₂ grains will reduce the oxidation rate of Zr,and the smaller the initial grain size,the higher the oxidation rate.（3）Based on rigorous evaluation of experimental data,the atomic mobility parameter database of cubic YSZ was constructed.Coupled with thermodynamic information,the diffusion coefficients of Y³⁺,Zr⁴⁺and O^2-at different temperatures were calculated.,and the tracer diffusion behavior of single-crystal 9.5 mol%YSZ in the range of 423～1370 K was accurately reproduced.The diffusion rate of oxygen ions in cubic YSZ is much greater than that of cations（by about ten orders of magnitude at 1000°C）.In addition,the relationship of the conductivity of YSZ with temperature is also accurately predicted.The conductivity of 9.5 mol%single-crystal YSZ at 1000°C is 0.092 S·cm^-1,and the conductivity is controlled by the diffusion rate of O^2-in YSZ.This thesis contains 37 figures,6 tables,175 references.