Investigation On The Dielectric And Tunable Mechanism Of BZT-based Dielectric Materials

With the rapid development of electronic information technology,the demand for voltage-controlled tunable dielectric devices with wide tuning range and low insertion loss is increasingly urgent.The capacity of such devices varies nonlinearly with bias voltage and can be used in microwave circuits to adjust the phase,frequency or amplitude of signals.The voltage-controlled tunable dielectric devices are fast response,low cost and high power capacity,and have broad application prospects in civil and military fields such as mobile communication and satellite system.The devices need high quality voltage-controlled tunable dielectric materials.Tunability and dielectric loss are the key parameters for evaluating performance,but it is difficult to realize the simultaneous optimization between them at present.In this doctoral thesis,the dielectric and tunable properties of BaZrxTi1-xO3(BZT)based dielectric materials were investigated.The interfacial effect mechanism was established and the influence factors of tunability and dielectric loss were analyzed.The thesis has important theoretical and application value.In order to have large tunability and low dielectric loss,Zr4+ ions were used to suppress electron transition.Homogeneous bilayer dielectrics were prepared to reduce dielectric loss and the tunability of homogeneous bilayer dielectrics was improved by heterogenous bilayer composite method.For the purpose of simultaneous optimization of tunability and dielectric loss,dielectric surface was deposited by thin films to improve tunable characteristics,and the activation energy was introduced to reveal the interfacial effect between thin film and substrate.The influences of interface between electrode and substrate on the dielectric properties were systematically analyzed,which provides a theoretical basis for the preparation of next generation voltage-controlled tunable dielectric devices.The innovative works and conclusions are as follows:(1)Homogeneous/heterogenous dielectric layer composite method was used to improve dielectric properties,including the following two parts:(a)Ba0.9988Bi0.0008Zr0.2Ti0.8O3/Ba0.9988Bi0.0008Zr0.3Ti0.7O3(BBZT20/BBZT3 0)homogeneous bilayer dielectrics were prepared.The theoretical model for bilayer dielectrics was established to calculate the theoretical values of dielectric constant and dielectric loss.The effects of space charges at interface on the dielectric properties were revealed.Compared to BBZT20,the dielectric loss of homogeneous bilayer dielectrics was reduced by?60%(0.0026).(b)BBZT20/BBST heterogenous bilayer dielectrics were prepared by Ba0.59928Sr0.39952Bi0.0008TiO3(BBST)with Curie temperature near room temperature.The heterogenous interface effect mechanism was proposed by the analyses of crystal structure,frequency characteristics,temperature characteristics and tunable properties.The tunability of homogeneous bilayer dielectrics was nearly doubled(34.9%).(2)The effects of dielectric surface deposited by NiO thin films on the dielectric properties were studied,including the following two parts:(a)NiO thin films were prepared at different annealing temperatures(100?-400?).The phase,texture coefficient and lattice stress were analyzed.The tunable characteristics of NiO thin films were investigated and the tunable mechanism was revealed using double potential well model.The tunability was 68.1%at relatively low electric field.(b)NiO thin films were deposited on the BaZr0.2Ti0.8O3(BZT20)dielectrics.The intrinsic mechanisms of dielectric constant,tunability and dielectric loss varying with the thickness of NiO thin films were analyzed,which laid a foundation for optimizing the dielectric properties of thin films deposited on dielectrics.(3)Dielectric surface was deposited by BZT20 thin films in order to improve the tunable characteristics,including the following three parts:(a)BZT20 thin films were prepared under different oxygen pressures(0 Pa-60 Pa).The influences of oxygen vacancies on the crystal structure were analyzed.The theoretical mechanism for frequency dispersion of dielectric constant was established by calculating the dispersion factor,and the internal mechanism for electric field dependence of dielectric loss was proposed.(b)BZT20 thin films were deposited on the BZT20 dielectrics.The effect mechanism of interface barrier between thin film and substrate was analyzed according to activation energy.Compared with BZT20 dielectrics,the dielectric constant was increased to 17121 and the dielectric loss was decreased by 30%(0.012).(c)BZT20 thin films were deposited on the BBZT20 dielectrics.The tunability,dielectric loss and quality factor were 68.7%,0.0051 and 134.7,respectively.Compared to substrate material,the dielectric loss was reduced by 22%and the quality factor was increased by 35%.The simultaneous optimization of tunability and dielectric loss was realized.(4)BBZT20 dielectrics with different thicknesses(variation range?15%)were prepared.The microstructures were analyzed by X-ray diffraction patterns and SEM images,the frequency characteristics were studied,the equivalent capacitance model was established to calculate the dielectric constant of BBZT20 substrate layer(?s=9949).The temperature characteristics were analyzed by modified Curie-Weiss law,and the effects of interface between electrode and substrate on the dielectric properties were investigated.These provide a theoretical basis for the preparation of novel voltage-controlled tunable dielectric devices.