Study On Defect Behavior And Dielectric Properties Of Ho₂O₃ Doped Barium Strontium Titanate?BST? Based Ceramics

As an important component of electronic products,ceramic capacitors require superior and more stable performance in smaller sizes.This dissertation aims at obtaining BST based ceramic capacitors with high dielectric constant,low dielectric loss and low capacitance change rate.0¹0wt%Ho₂O₃ doped(Ba_1-x-x Sr_x)Ti_1±?O_3±2???=0⁰.03,x=0.05⁰.25?ceramics were prepared by solid state method.The effect of dopant content,primary phase composition and sintering process on the microstructure and macroscopic properties of BST based ceramics were studied systematically.The X-ray diffraction?XRD?,scanning electron microscopy?SEM?and energy dispersion spectroscopy?EDS?were used to analyze the phases and surface morphologies of BST based ceramics.It was revealed that with increasing Ho2O3 doping content,the BST based ceramics were turned into multi-phase polycrystalline materials from perovskite single-phase solid solutions.However,BST based ceramics with different?value and Sr content were still perovskite single phase solid solutions.In addition,proper content of Ho₂O₃ can refine grains of BST based ceramics.Dielectric properties of BST based ceramics at multi-frequencies from low to high temperature were obtained using dielectric property test system and the dielectric properties at room temperature were associated with defect behaviors.It was found that with increasing Ho₂O₃ doping content,Ho³⁺ions first substituted for A sites of perovskite lattice,generating cation vacancies or electrons.The electrons were further weakly bound by Ti⁴⁺ions to form Ti³⁺ions,which increased the dielectric loss.After that,Ho³⁺ions began to occupy B sites of perovskite lattice,producing oxygen vacancies which decreased the dielectric loss.The?value can control defect types and defect behaviors of BST based ceramics.With increasing?value for Ho₂O₃ doped Ti-rich BST ceramics,the concentration of A-site ion vacancies and oxygen vacancies increased,and more Ho³⁺ions filled A-site ion vacancies,leading to the shrinkage of unit cell volume and the increase of internal stress,which increased the dielectric constant.With increasing?value for the Ho₂O₃ doped Ti-deficient BST ceramics,the concentration of B-site ion vacancies and oxygen vacancies increased,and more Ho³⁺ions occupied B-site ion vacancies,resulting in the expansion of unit cell volume and the decrease of internal stress,which decreased the dielectric constant.Increasing sintering temperature or holding time increased the dielectric constant of BST based ceramics at room temperature,but it was not conducive to the improvement of temperature stability.Under suitable sintering process,increasing Ho₂O₃ doping content and non-stoichiometric level or decreasing Sr content will help to obtain BST based ceramic capacitors with high temperature stability of the dielectric constant.The diffuse phase transition and frequency dispersion occurred in the BST ceramics with high Ho₂O₃ content,so Ho₂O₃ doped BST ceramics gradually transformed into relaxor-like ferroelectrics from ferroelectrics with diffused phase transition with increasing Ho₂O₃doping content.The different?value and Sr content did not cause the ferroelectric type transformation of Ti-deficient BST ceramics which were relaxor ferroelectrics.5wt%Ho₂O₃ doped(Ba_0.95Sr_0.05)Ti_0.97O_2.94 ceramics with higher dielectric constant??_r=3435?,lower dielectric loss?tan?=0.00983?at room temperature and lower capacitance change rate??C/C=-36.38⁺1.18%,-30⁸5??are promising for applications in capacitor industry as environment-friendly and temperature stable ceramic materials.