Electrocaloric Effect And Energy Storage Performance Of PbZrTiO₃-based Composite Films

The electrocaloric effect?ECE?of ferroelectric thin films is regarded as a new type of solid-state refrigeration,which has an important significance to solve the heat dissipation for microelectronic devices.Meanwhile,the polarization and depolarization behaviors of ferroelectric films correspond to the charging and discharging process of capacitors,so ferroelectric films play an important role in the pulse power capacitors for energy storage.Although the ECE and energy storage performance of ferroelectric films have been widely studied,there are still some problems.The problems existing in the ECE are low refrigeration capacity,low refrigeration efficiency,and it is difficult to refrigerate at room temperature or to work within a wide temperature range.The problems existing in capacitors are low energy storage density and low energy storage efficiency.In this thesis,the PZT based composite films are chosen as research object.The structure characteristics,phase transition behavior,interface effect and electrical properties of each layer film have been investigated,which can be used for regulating the polarization behavior and the distribution of electric field.The aims of this thesis are to improve the ECE and energy storage performance of composite films.The model of field-induced entropy change in bilayer films has been put forward in this thesis.The amplifying effect of electric field can be realized in the bilayer films because of the great difference of dielectric constant of each layer films.The ratio of dielectric constant and thickness of each layer have a deep influence on the amplifying effect of electric field.The micro structure,phase transition,polarization behavior and amplifying effect of electric field of the PZ1-xTx/PZ_0.52T_0.48 ferroelectric bilayer films have investigated systemly.The entropy change ?S is 1 1.9J·K-1·kg-1,and temperature change ?T is 10.7K at 25?.The entropy change ?S and temperature change ?T is 11.9J·K-1·kg-1 and 24.8K,respectively,at 125?.The defect dipoles have been forming in PZ0.8T0.2 thin films when the crystallization process is carried out under the argon atmosphere.Based on the amplying effect of electric field,the PZ_0.52T_0.48/PZ0.8T0.2 bilayer films were prepared.The effect of amplitude of electric field and temperature on the micro structure,polarization behavior have been investigated.In addition,the pinched hysteresis loops induced by the pinning effect of defect dipoles and its response to the amplitudes of applied electric field and temperature have been studied.The excellent negative ECE has been obtained by the pinning and de-pinning effect induced by the defect dipoles,and the related mechanisms have been illuminated.The entropy change ?S and temperature change ? T is-6.4J·K-1·kg-1 and-8.4K,respectively.The polarization behavior of PZO antiferroelectric films has been studied.It can be found that the negative ECE can be induced by applied a moderate electric field.This threshold electric field separate the negative ECE and positive ECE.The mechanisms for negative ECE caused by field-induced phase transition have been illuminated.In addition,an effective way to improve the threshold electric field is provided by doping Li+-La3+ ionic pairs,which is significative to refrigerate by negative ECE.In addition,the results based on Laudau theory indicate that the critical electric field can be enhanced by the uniaxial compressive stress.When the Li+-La3+ doping content is 4mol.%,the energy storage density reaches 16.2J/Cm3,which is 1.8 times than that of pure PZO thin films.The PZ_0.52T_0.48 film is chosen as a buffered layer for the growth of PZO layer film,which is bebeficial to obtain the better crystallized PZO film.The PZO/PZ_0.52T_0.48 bilayer films possesses excellent electric breakdown strength compared to the pure PZ_0.52T_0.48 individual films,which is caused by the PZO layer with smaller dielectric constant and high crystallized quality.The value of electric breakdown strength of PZO/PZ_0.52T_0.48 bilayer films is 2615kV/cm,which is more than twice that of the pure PZ_0.52T_0.48 individual films.The energy storage density is 28.2J/cm3,and the energy storage efficiency is about 50%at room temperature,both of which exhibit good temperature stability.The microstruce and polarization behavior of PZO/AO heterostructures have been investigated.It can be found that the antiferroeletricity of PZO layer films was disappeared,meanwhile,the characteristics of ferroelectricity appears in the heterostructures.This phenomenon was attributed to the build-in electric field in the interface of heterostructures.The electric breakdown strength can be enhanced by the build-in electric field and the high insulated AO layer films.Thus,the PZO/AO/PZO and PZT/AO/PZT composite films were prepared.The crystallization and polarization behavior can be regulated by controlling the annealing temperature.When a electric field of 3780kV/cm was applied for the PZO/AO/PZO annealed at 700?,the energy storage density is 40.6J/cm3 and the efficiency is just 37.5%.For the PZT/AO/PZT composite films annealed at 550?,the energy storage density is 63.7J/cm3 and the efficiency is 81.3%.