Preparation And Dielectric Tunable Performance Of Barium Calcium Zirconate Titanate Based Dielectric Materials

With the continued advancement of mobile/satellite communications,Internet of Things,aerospace and other fields,electronic circuits and systems are facing problems such as crowded electromagnetic spectrum and complex application scenarios.Improving the efficiency of spectrum utilization and work stability is urgently needed,which provide high-performance and high-reliability dielectric tunable devices a huge momentum for development.The voltage-controlled tunable dielectric materials,as the core of dielectric tunable devices,its dielectric tunable performance（tunability,figure of merit（FOM））and temperature stability（temperature-varied rate of FOM value）are important indicators for evaluating comprehensive dielectric tunable performance.However,the difficulty in synergistic optimization of these performance evaluations is still in trouble,which limits the further development of those fields.In this thesis,Barium Calcium Zirconate Titanate((Ba_1-xCa_x)(Zr_yTi_1-y)O₃,BCZT)lead-free ceramics are focused on.Some research methodologies,such as preparation process optimization,composition adjustment,and system designing,are employed to comprehensively improve the dielectric tunable performance and its temperature-stability,and the related characteristics and physical mechanisms are explored.The principle works and critical conclusions of this thesis are as follows:Firstly,(Ba_0.95Ca_0.05)(Zr_0.18Ti_0.82)O₃ ceramics were prepared by a high-temperature solid-state method.The influence of process parameters（calcining temperature and sintering temperature）on the surface morphology and crystal structure and other microstructure is studied.The mechanism of microstructure on the dielectric tunable performance is established.The results show that calcining temperature～1250°C and sintering temperatures～1450°C are the optimum process parameter of BCZT ceramics in this thesis.Under this condition,larger grains and dense morphology are obtained,and optimum dielectric tunable performance is achieved:unvder a DC bias electric field of 6.95 k V/cm,the tunability is 86.7%@6.95 k V/cm and the maximum FOM reaches322.Then,a series of(Ba_1-xCa_x)(Zr_0.18Ti_0.82)O₃ ceramics were prepared by adjusting the proportion of A site.The results show that Ca²⁺has an inhibitory effect on the growth of grains,and excessive Ca²⁺causes the formation of the second phase Ca Ti O₃.At this time,the diffuse phase transition（DPT）behavior of the sample is gradually increased.In addition,the composition dependence of the dielectric tunable performance is studied.When x=0.09,the tunability of the sample at room temperature is 86.1%@7.42k V/cm,and the maximum FOM value is 448.On this foundation,partial Schottky defects are introduced,and a novel non-stoichiometric(Ba_0.91Ca_0.09)_x(Zr_0.18Ti_0.82)O₃ceramics is designed.The mechanism of the defects on the microstructure is discussed.The results show that the insufficient A-site cations will promote the growth of grains,and the surface morphology of the sample is improved.The influence of the defect structure on the dielectric tunable performance is analyzed.One can see that a further improved dielectric tunable performance at room temperature is achieved:when x=0.99,the tunability of the samples is 87.8%@7.28 k V/cm and the maximum FOM reaches 623.The further improved dielectric tunable performance indicates that the introduction of local Schottky defects can strengthen the nonlinear dielectric response of polarized dipoles and domain walls under a certain electric field,while a low dielectric loss is maintained.Finally,a binary system of（1-x）Ba(Zr_0.36Ti_0.64)O₃–x(Ba_0.82Ca_0.18)Ti O₃ is designed based on the obtained(Ba_0.91Ca_0.09)(Zr_0.18Ti_0.82)O₃ composition.The microstructure,dielectric properties and dielectric tunable performance of the system were adjusted by compositional customization.The effect of internal structural evolution on the temperature characteristics of dielectric tunability is analyzed by describing the relaxation characteristics and diffuse phase transition behavior using the Curie-Weiss law and empirical Lorentz formula.The results show that lower dielectric loss in the paraelectric phase can have a compensating effect on the dielectric tunable performance,so that the FOM value of BZ_0.36T–0.3BC_0.18T ceramics is maintained at 154～269 in the range of 0℃～100℃and varied more smoothly with temperature,which realizes a comprehensive improvement of the dielectric tunable performance of the dielectric material.Furthermore,for(Ba_0.91Ca_0.09)(Zr_0.18Ti_0.82)O₃ ceramics,the law of dielectric tunable characteristics influenced by temperature was analyzed and its relationship with ferroelectric characteristics was established.A modified multipolarization model is employed to analyze the source mechanism of dielectric tunable characteristics at different temperatures.The contribution of the intrinsic and non-intrinsic polarization mechanisms to the dielectric tunable characteristics in ferroelectrics,ferroelectrics–paraelectrics coexistence,and paraelectrics phase is investigated.In order to discuss the evolution process of the internal domain structure with temperature,the influence law of the dielectric polarization characteristics by the applied electric field measurement history is investigated.