Giant Electrocaloric Effect Of Lead-Free Barium Titanate Based Ceramics And Multilayers Ceramic Structure

Electrocaloric effect(ECE)is referred to the change of polarization in polar materials induced by the variation of electric field applied,and consequently,resulting in the variation of adiabatic temperature(?T)/isothermal entropy change(?S).With the remarkably growing demand for refrigeration technology in modern cooling scenarios such as specialized medical equipment,frozen storage and transportation,smart-home design(Home automation).specialized technical equipment,microelectronic circuit heat dissipation and others,in order to overcome the disadvantages,e.g.,refrigeration efficiency,greenhouse effect and limited applications of traditional vapor-compression refrigeration approach,the development of new high-efficiency solid-state refrigeration technology to meet the needs of modern society has become one of the urgent challenges to researchers all over the world.Electrocaloric cooling has been considered as a promising alternative to replace the traditional vapor-compression refrigeration technology widely used currently,and as the state-of-the-art one,due to its high refrigeration cycle efficiency of up to 60%,higher cooling capacity compared with other novel refrigeration technologies such as magnetocaloric effect(MCE),elastic caloric effect(eCE)and thermoelectric cooling technologies,not to mention the solid-state cooling approach which completely avoids the utilization of refrigerants and prevents the environment form damaging by house warming gas emission.For solid-state cooling technology based on the electrocaloric effect(ECE),recently,innumerable researches are still in the primary stage of developing and optimizing ECE materials.How to locate a class of suitable materials which is appropriate for practical solid-state cooling devices design with high electrocaloric response,considerable thermal performance and excellent mechanical properties is still the crucial issue currently in this filed.In this work,based on the environment-friendly lead-free barium titanate(BTO)ceramic material system,we aim to achieve a considerable electrocaloric response to meet the requirement of high-performance practical novel ECE solid-state cooling devices by doping with Zr4+,Sr2+,Mn2+ions and multilayer ceramic structure design.1.Barium zirconate titanate(Ba(ZrxTi1-x)O3)ceramics with Zr4+content x of 5,10,15,20,25 and 30 mol%,were prepared using a solid-state-reaction approach.The microstructures,morphologies,and electric properties were characterized using XRD,SEM and impedance analysis methods,respectively.The dielectric analyses indicate that the BZT bulk ceramics show characteristics of phase transition from a normal ferroelectric to a relaxor ferroelectric with the increasing Zr4+ ionic content.The electrocaloric effect(ECE)adiabatic temperature change decreases with the increasing Zr4+ content.The highest adiabatic temperature change obtained is 2.4 K for BZT ceramics with a 5 mol%of Zr4+ionic content.Moreover,an ECE strength(|?T/?E|)of 0.8 K·MVm-1 was procured for 5%BZT,which is among the largest values reported thus far for ceramics.2.The barium strontium zirconate titanate ceramics with Zr4+ ionic contents of 0.15,0.20 mol%,and Sr2+ ionic content of 0.15,0.2,0.25,0.3 mol%were prepared using an oxide reaction approach.The XRD,SEM were used to characterize the lattice structure and the morphology of the ceramics.Permittivity and polarization versus temperature were characterized using an impedance analyzer and a Tower-Sawyer circuit.The electrocaloric effect(ECE)was measured directly and indirectly.Results indicated that the characteristics of permittivity as a function of temperature and frequency change from a normal ferroelectric to a relaxor ferroelectric with the increasing Zr4+ionic content,which can be modified by the Sr2+ ionic doping.The optimized adiabatic temperature change obtained is 2.4 K in(Ba0.85Sr0.i5)(Zr0.15Ti0.75)O3 ceramics,and T>1.6 K over a wide temperature range over 120? was obtained.Moreover,ECE strength(|?T/?E|)increases,and a value of 0.48 K·MVm-1 was procured.Results based on direct and indirect measurements indicated that indirectly calculated values based on the Maxwell relations are smaller.The reason is that the indirect method uses the polarization extracted from the P-E hysteresis loops,which is usually obtained via a parallel plate capacitor configuration,in which only the projected polarization along the electric field can be measured,therefore the measured polarization is always less than the real polarization of the material.3.The manganese doped barium zirconate titanate ceramics(Ba(ZrxMnyTi1-x-y)O3-BZMT ceramics)and manganese doped barium strontium titanate ceramics((Ba1-xSrx)(MnyTi1-y)O3-BSMT ceramics)were prepared using an oxide reaction approach.The XPS analysis indicated that the Mn4+ ions were converted into Mn2+ions.EDS analyses revealed that the Mn2+ ions were distributed on the surfaces of the grains when the Mn2+ionic content is less than 5 mol.%,and entering into the lattice when larger than 5 mol%.The electrocaloric effect(ECE)was measured directly using a thermocouple.Results revealed that by adjusting the nonequivalent Mn2+ionic concentration within 0.25%to 0.75%,desired dielectric performances with high permittivity's and lower losses were obtained.The polarization was increased with the doping content of Mn2+ions.The largest ECE of 2.75 K under 50 kV/cm of external electric field in(Ba0.6Sr0.4)(Mn0.001Ti0.995)O3 ceramic sample,and a broad temperature range of?40? was accompanied.4.Tape-casting process was used to fabricate the Ba0.7Sr0.3TiO3 multilayers ceramics.An atmosphere sintering furnace was designed and constructed.Two thermocouples were used to instruct the temperatures inside and outside the tube.A sintering temperature can be reached as high as 1400? with a resolution of 1?.The multilayer ceramics shows a dense structure of inner electrode and ceramic layers.Furthermore,a 10-layer Ba0.7Sr0.3TiO3 ceramic structure was procured with a breakdown electric field>30 MV/m,and an electrocaloric temperature change of ?T=2.49K.An energy storage density of 0.71 J/cm3 and an efficacy of 93%were obtained.