Preparation And Sintering Mechanism Of Nd Doped YAG Transparent Laser Ceramics

As the rapid development and wide application of laser devices,Nd:YAG transparent ceramics have played an important role in military,industrial production,medical and health fields as an important laser medium material.In this thesis,Nd:YAG transparent ceramics with high light transmittance and excellent optical properties was prepared by studying the relationship between raw material-process-structure-property.As the rapid development of computational materials science,molecular dynamics（MD）simulation was used effectively to guide and explain experimental phenomena.At present,the theoretical research on the sintering mechanism of YAG transparent ceramics has not been perfected,which is an urgent problem to further explain the sintering mechanism of YAG transparent ceramics through MD simulation.In this thesis,the structure,optical properties and mechanical properties were studied starts from the preparation of Nd:YAG transparent ceramics,and the sintering mechanism of YAG transparent ceramics was studied in detail based on the MD simulation.Using commercial nano powders as raw materials,the mixed powder with uniform particle size was prepared by ball milling.The sintering curve of Nd:YAG transparent ceramic was developed according to the thermal expansion curve of the green body,and the effects of sintering temperature,holding time and TEOS additional amount on the densification process was studied.The results show that the best sintering temperature of Nd:YAG transparent ceramics was 1750℃.The best sintering process of ceramic samples was sintered at 1750℃for 50 h.The optimal amount of TEOS was 0.5 wt%.In the study of the sintering kinetics of Nd:YAG transparent ceramics,the sintering mechanism and the kinetic characteristics of the sintering in the middle and late stages of YAG transparent ceramics were studied,respectively.The action mechanism of Si O₂and diffusion path of O^2-in the sintering process of YAG transparent ceramics were analyzed.The results of sintering kinetics equation indicated that the activation energy of sintering decreases from569.81 k J·mol^-1to 265.69 k J·mol^-1with the increase of TEOS content from 0 wt%to 3.0wt%.Starting from the structure of YAG transparent ceramics,the temperature of crystal phase transition was discussed.The effect of the doping concentration of rare earth ions(Nd³⁺,Lu³⁺)on the crystal structure of YAG transparent ceramics was studied.The structure of the grain boundary of ceramic sample was analyzed,and the distribution pattern of sintering aid Si O₂and Mg O at the end of sintering was discussed.The three-dimensional spherical diameters(D_3S),average specific surface area per unit volume（S_V）,average specific surface area per single grain(S_VP)and three-dimensional mean free distance（λ）of ceramic grains were studied with sintering temperature and holding time by stereology.The relationship between three-dimensional structure parameter and the transmittance and mechanical properties of Nd:YAG transparent ceramics was further studied quantitatively.The change of fractal dimension with holding time was studied by fractal theory,and the relationship between fractal dimension and the transmittance and mechanical properties of Nd:YAG transparent ceramics were quantitatively studied.Besides,the change of the micro-structure of Nd:YAG transparent ceramics with temperature was studied,and the transition temperature of the crystal phase was further determined.The optical loss caused by incident light entering ceramics was discussed,and the transmittance,reflectance,absorption and scattering rate of ceramics samples of different thicknesses at 1064 nm were analyzed.The effects of Nd³⁺doping concentration on transmittance,luminescence intensity and fluorescence lifetime of ceramic samples were discussed.The results indicated that the Nd³⁺is lower than 1.0 at%,the decay trend of the fluorescence lifetime is relatively small,if the Nd³⁺is greater than 1.0 at%,the fluorescence lifetime of ceramics and the luminous intensity begins to drop drastically due to the effect of concentration quenching.The laser performance of Nd:YAG transparent ceramics at1064 nm was studied,and the effect of TEOS addition on the laser performance of Nd:YAG transparent ceramics was discussed.Besides,the compressive strength,flexural strength,Vickers hardness and fracture toughness of Nd:YAG ceramics were tested.The sintering mechanism of YAG ceramics was studied by MD simulation method,the physical properties of YAG transparent ceramics was simulated,the diffusion coefficient and activation energy of YAG transparent ceramics were calculated,and evaluated the applicability of Reax FF reaction force field in YAG transparent ceramics.The results indicated that the melting point and lattice constant of YAG obtained by MD simulation is basically consistent with the reference,and the diffusion coefficients of Y,Al and O satisfy the inequality D_O>D_Al>D_Y.The point defects and oxygen diffusion in YAG transparent ceramics were studied by MD method,which not only makes up for the deficiency in the theoretical research of YAG transparent ceramics,but also explains the experimental phenomenon of YAG transparent ceramics in the sintering process.The results indicated that the diffusion coefficient of the YAG simulation system increased with the increase of yttrium vacancy content;Increasing Si⁴⁺is not the only method to improve the diffusion rate of YAG transparent ceramics;Adding appropriate oxygen vacancy would improve the diffusion rate of YAG simulation system;the diffusion activation energy and the sintering activation energy showed decreasing trend with the increase of SiO₂ addition.This paper provides an important theoretical basis and technical support for the research and development of Nd:YAG transparent ceramics.The application of Nd:YAG transparent ceramics is preliminaries explored through the assembled solid-state laser.