Phase Transformation Behavior And Energy Storage Performance And Electrocaloric Effect Of Doped PbZrO₃-based Thin Films

Antiferroelectric materials have potential applications in micro-energy systemsand refrigeration technology of microelectromechanical System （MEMS） due to thepresence of a unique field-induced antiferroelectric to ferroelectric （AFE-FE） phasetransition behavior. In recent years, PbZrO₃（PZO） with typical ABO3perovskitephase structure is one of the most widely studied antiferroelectric materials.Therefore, in this paper, Nb-modified, Eu-modified, and Eu-doped compositionallygraded multilayer PbZrO₃thin films have been deposited on Pt（111）/Ti/SiO₂/Sisubstrates by using sol-gel method and rapid thermal annealing process, respectively.Firstly, the microstructure, phase transformation behavior and electrical propertiesof modified PbZrO₃thin films have been investigated comprehensively by usingX-ray diffractometer （XRD）, scanning electron microcopy （SEM）, ferroelectrictester, impedance analyzer and other means. The modification mechanism of thePbZrO₃films has been also proposed. And then the energy storage density, energyefficiency, electrocaloric entropy change, and electrocaloric temperature change ofmodified PbZrO₃thin films have been evaluated.After the optimization of the process,20mol%of excess of lead in theprecursor is determined. All films specimen which are pyrolyzed at450oC, andannealed at650oC for3min display pure perovskite structure. XRD analysis showsthat the （100）-orientation degree of the thin films increases at the low content of Eu,and decreases at high content of Eu. Nb doping content has an obvious influence onthe orientation of the films. With increasing the content of Nb, the （100） preferredorientation gradually changes to （111） orientation. It was found from the XRD andSEM results that the gradient sequence of the composition had an obvious influenceon the microstructure of Eu-doped compositionally graded multilayer PbZrO₃thinfilms. The down-graded films shows a big grain and clear grain boundaries whichalmost is not observed in the up-graded films.The electrical properties and phase transition characteristics of modifiedPbZrO₃thin film have been investigated. All Eu-modified PbZrO₃films areantiferroelectric at room temperature. With the increase of Eu content, the FE-AFEand AFE-FE phase transition switch are shifted to the higher electric field. Itindicated that the temperature range for the AFE phase is enlarged and the stabilityof the AFE is stabilized by the modification of Eu. The gradient sequence of thecomposition has a great influence on the phase transition characteristics of Eu-dopedcompositionally graded multilayer PbZrO₃thin films. At room temperature, the down-graded films show an AFE phase, while the up-graded films show coexistenceof FE and AFE phases. The degree of diffusion for up-graded films is enhanced,compared with that of down-graded films. The AFE phase is destabilized, and theFE state is stabilized. The diffusion phase transition characteristic is observed inmodified PbZrO₃thin films by doping Nb. With increasing content of Nb, theferroelectric-paraelectric （PE） phase transition temperature decreases, and thedegree of diffuse phase transition is enhanced.The energy storage performance of modified PbZrO₃films has been evaluated.The reversible energy density （Jrev）, energy loss （Jloss）, energy efficiency （η） andstability were found to be strongly dependent on Eu modifying content and gradientsequence. With the increase of Eu content, energy storage density has been enhancedfollowed by their subsequent reduction. A maximum energy density （18.8J/cm3）,minimum energy loss （7.3J/cm3） and energy efficiency of72%have been achievedon3mol%Eu-modified PbZrO₃thin films. Meanwhile,3mol%Eu-modified PbZrO₃thin films show a stabler storage performance in the measurement frequency andtemperature range, and better fatigue resistance. The down-graded films show ahigher energy density due to double P-E hysteresis loops, small hysteresis switchand high polarization, compared with that of the up-graded films.The electrocaloric effect of modified PbZrO₃films has been evaluated. Theelectrocaloric entropy change （ΔS） and electrocaloric temperature change （ΔT） arecalculated by measured hysteresis loops of7mol%Nb-modified PbZrO₃thin filmsat various temperatures. With the increase of temperature, the ΔT and ΔS had beenenhanced followed by their subsequent reduction. The maximum ΔT=9.3K, and ΔS=7.4J/K/kg are obtained near the FE-PE phase transition temperature point （140°C）.The point of maximum electrocaloric temperature change decreases about90°C,compared with pure PbZrO₃, which is closer to room temperature.Based on above results and analysis, the energy storage performance andelectrocaloric effect of PbZrO₃thin films can be optimized by a rationalmodification, which laid solid foundations for the research and development ofmicro-energy systems and refrigeration technology in MEMS.