Structure Control And Microwave Dielectric Properties Research Of Complex Perovskite Ba（Co,Zn）_1/3Nb_2/3O₃ Based Microwave Ceramics

Ba(B′_1/3B″_2/3)O₃microwave dielectric ceramics have been widely concerned because of their unique B-site ordering structure,which can improve the microwave dielectric properties.However,the effect of point defects of Ba(B′_1/3B″_2/3)O₃microwave dielectric ceramics on the microwave dielectric properties is often ignored.At present,the influence of defects on the microstructure and crystal structure of microwave dielectric ceramics and the dielectric loss mechanism related to defects are not fully understood.In addition,with the development of microwave components toward miniaturization and integration in the future,it is urgent to prepare high-quality dielectric ceramic films.Due to the lattice mismatch of thin films and substrate materials,different degrees of strain will occur in thin films.The method of regulating the physical and chemical properties of thin films by adjusting the crystal structure changes through epitaxial strain is called strain engineering.In this paper,the effects of microstructure,defects and crystal structure changes on microwave dielectric properties of Ba(B′_1/3B″_2/3)O₃ceramics with complex perovskite structure are clarified.The main research contents and conclusions are as follows:（1）The annealed BCZN have high 1:2 ordering degree and similar microstructures,but exhibit different microwave dielectric properties.With the increase of annealing temperature,the 1:2 ordering degree of BCZN ceramics increases continuously,but the Q×f values decreases from 84,700GHz to 53,900GHz.This work found that crystal defects induced by the evaporation of Zn and Co during the annealing processing significantly affect the dielectric loss of BCZN ceramics.When the annealing temperature increases,the deficiency of zinc and cobalt in BCZN ceramic increase.Both the resistivity and carrier transport activation energy E_aof as-annealed BCZN ceramics decrease continuously,indicating higher crystal defects density.The increase of defect concentration results in conductive dielectric loss and leads to the decrease of Q×f values.In addition,the volatilization of Zn and Co results in the lattice distortion of BCZN ceramics.With the increase of defect concentration,the lattice constant c/a value and cell volume of BCZN ceramics decrease.The variation of activation energy E_apresents a similar trend with that of the Q×f values of BCZN ceramics.The crystal defects rather than B-site 1:2 ordering play a dominant role in determining the dielectric loss of BCZN ceramic.（2）CuO-doped BCZN ceramics exhibit lower maximum densification rate temperature(T_max)and a much higher densification rate than that of undoped ones.BCZN ceramics with dense microstructures was obtained by sintering at 1300℃.With increasing the concentration of CuO,the ordering parameter of BCZN ceramics increase from 38.6%to 70.6%.The brightness of supperlattice diffraction spots related with 1:2 ordering structure became stronger and the average size of ordering domain increase form 10-20 nm to 50-60nm.The addition of CuO can accelerate mass transport process by forming grain boundary quasi-liquid Cu₂O-CuO films.As a result,the improvement of relative density and the 1:2 ordering degree optimize the dielectric properties of BCZN ceramics.Compared with undoped BCZN ceramics,0.2wt%CuO doped BCZN ceramics sintered at1300℃exhibits excellent dielectric properties:Q×f=69,300GHz,ε_r=34,andτ_f=1.1ppm/℃.CuO doping has an important effect on the densification behavior,order-disorder transition and dielectric properties of BCZN microwave ceramics.（3）The optimization of microwave dielectric properties of（1-x）BaZn_1/3Nb_2/3O₃-xBaGa_1/2Ta_1/2O₃（x=0-0.3）ceramics does not depend on the 1:2 ordering structure of B-site cations,but on the inhibition of the formation of second phase and the reduction of carrier defects.It provides a new way for the design and improvement of complex perovskite ceramic system.With the increase of BaGa_1/2Ta_1/2O₃content,the crystal structure of BZN-BGT ceramics changes from 1:2 ordering structure to disordering structure,but the microwave dielectric properties of BZN-BGT ceramics are optimized.The addition of BaGa_1/2Ta_1/2O₃inhibits the formation of the Nb-rich second phase on the surface of BZN-BGT ceramics induced by volatilization of Zn,and reduce the defect concentration.With the increase of x,the activation energy E_aof BZN-BGT ceramics increases from 0.40e V to 0.55e V.The thermal activation relaxation process of BZN-BGT ceramics weakens and the electrical conductivity decreases,which reduces the conductive dielectric loss.0.7BZN-0.3BGT ceramic shows excellent microwave dielectric properties:Q×f=70,300GHz,ε_r=40.4,τ_f=2.7ppm/℃.（4）High quality epitaxial growth of complex perovskite BCZN thin films was grown on SrTiO₃and LaAlO₃substrates by pulsed laser deposition.Thin films grows along c-axis on the substrates and exhibits the epitaxial relationship of BCZN（001）//SrTiO₃（001）and BCZN（001）//LaAlO₃（001）.As the substrate temperature increases,epitaxial strain increase continuously and the crystal structure of BCZN/SrTiO₃and BCZN/LaAlO₃films change from untilted to in-phase tilting.The dispersion parameters and dielectric loss derived from fitted far-infrared reflectance spectra reveal that the intrinsic dielectric loss of BCZN thin films are mainly controlled by the crystallization quality of the films.However,the in-phase tilt of oxygen octahedron will increase the dielectric constant but have little effect on the dielectric loss.Both the substrate temperature and epitaxial strain have significant influence on the crystal structure and dielectric properties of thin films.BCZN/SrTiO₃epitaxial films exhibit better intrinsic dielectric properties due to better crystallinity and smaller lattice distortion.These findings can be helpful to understand the influence of crystal structure on microwave dielectric properties of complex perovskite Ba(B′_1/3B″_2/3)O₃based dielectric ceramics.