Study On The Electrocaloric Effect In Ferroelectric Materials

With the rapid development of 5G,new energy automobiles,biomedical,aerospace engineering,and other fields,the industry has an urgent need for new solidstate refrigeration technology with high efficiency,flexibility,and environmentally friendly.At present,the new solid-state refrigeration technology which has been studied extensively mainly includes the thermoelectric cooler,magnetocaloric cooler,elastocaloric cooler,electrocaloric cooler,and so on.Among them,electrocaloric cooler mainly uses the electrocaloric effect(ECE)of polar materials,that is,changing the polarization of materials by the external electric field,which causes temperature changes of materials.Electrocaloric refrigeration technology has the advantages of high efficiency,environmentally friendly,easy miniaturization,and high reliability.It has broad application prospects in chip-level refrigeration,limited space refrigeration or other fields.The ECE of ferroelectrics has become one of the frontier hot spots in the study of condensed matter physics.There are still some problems to be solved urgently in the research of the ECE of ferroelectrics and the application of devices,such as accurate measurement,material selection and optimization,device reliability,and so on.In this thesis,focusing on the domain switching behavior of ferroelectrics,the key issues mentioned above were systematically investigated by using homemade thermal-mechanical-electrical coupled loading equipment and the setup designed for testing the ECE directly under different preloading stresses.The influences of electric field frequency and uniaxial compression stress on the ECE were studied,and the mechanisms of domain switching on the electrical fatigue behavior of ferroelectric materials were discussed in detail.In addition,researches on ions doping in the novel lead-free ferroelectric relaxor ceramics were also carried out.The main results of this study are listed as follows:(1)The influence of applied electric field frequency for the polarization-electric field(P-E)loops on indirect electrocaloric evaluation.Soft lead zirconate titanate(PZT)ceramic is used as a model material.The P-E loops under 1 and 100 Hz are presented at various temperatures with an electric field amplitude of 2.5 MV/m.The indirect method based on the Maxwell relations is used to calculate the electrocaloric response.The result demonstrates that the electrocaloric responses based on the P-E loops measured at 1 Hz agree well with direct measurement results in the literature,but at 100 Hz,the coexistence of positive and negative electrocaloric effects is obtained,which has not been reported before.Domain switching is a dynamic process and ratedependent,which has a significant influence on the polarization behavior and the subsequent electrocaloric performance of ferroelectric materials.The result also shows that during the indirect ECE evaluation,high attention needs to be paid to the applied electric field frequency to obtain accurate and comparable electrocaloric evaluations in ferroelectric materials.(2)The effects of uniaxial compressive stress on the electrocaloric effect of ferroelectric ceramics are studied by Landau–Ginzburg–Devonshire(LGD)thermodynamic approach,direct method,and indirect techniques.Soft lead zirconate titanate ceramics is used as a model material.The direct measurement results are given by an infrared camera combined with a set of specially designed testing setup.It is demonstrated that ECE can be significantly tuned by uniaxial compressive stress.The direct measurement results are essentially in agreement with the LGD theory calculated results,while significant discrepancies between direct and indirect methods are observed.Thus,the indirect method should be treated with reasonable caution for evaluating the ECE under mechanical stress in ferroelectrics.These results are explained by the complex domain switching and possible phase transition behavior under the coupled thermo-electro-mechanical field.In addition,with compressive stress of 50 MPa,direct measurement shows that an improvement of ? 66.7% in cooling capacity can be achieved at 375 K,which demonstrates that the application of compressive stress is an effective approach for enhancing ECE in ferroelectric ceramics.Our results not only provide insights into the effects of uniaxial compressive stress on ECE,but also offer more opportunities for the design of electrocaloric materials and devices.(3)The fatigue behavior and stability of the electrocaloric materials under electric field cycling is an important consideration in the development of solid-state cooling devices.Here,uniaxial compressive stress(2 MPa?100 MPa)and thermal loading(20 °C?150 °C)were used to tune the domain switching process.Under the same loading condition,the electrical fatigue behavior of soft lead zirconate titanate ceramics was systematically characterized.The amplitude and frequency of the applied electric field are 2 k V/mm and 10 Hz,respectively.By analyzing the evolution of the domain switching process,combined with the measured polarization and strain response,as well as the cracks observed on the surface of the specimen,it is found that the fatigue of ferroelectric ceramics was mainly related to the domain switching process near the coercive electric field.The underlying mechanism was further discussed by considering the interplay between the domain switching process with the main factors affecting the electrical fatigue of ferroelectrics,namely defect redistribution,charge carrier injection,and crack initiation.(4)(Bi_0.5Na_0.5)TiO₃-based relaxor ferroelectrics has great potential in the application of electrocaloric refrigeration and piezoelectric device.Here,Fe-doped0.94(Bi_0.5Na_0.5)TiO₃-0.06 Ba Ti O3 lead-free ceramics were fabricated by conventional solid-phase reaction method,and the phase structure,dielectric,ferroelectric and piezoelectric properties were investigated.The results show that the appearance of double P-E hysteresis loops is deferred with Fe doping.With the increase of Fe doping concentration,the determined ferroelectric-to-relaxor transition temperature TF–R gradually moves to a higher temperature,form 105 ? for the undoped sample to 125 ?for the sample with Fe amount of 1.0%.In addition,the piezoelectric property and thermal stability can be simultaneously improved by Fe doping at an appropriate level(?1.0%),due to the stabilization of long-range ferroelectric order.