Phase Transition Behaviors And Electrocaloric Effect Of BCZT Ceramics

In the early 1940s,researchers found piezoelectric effect in the BaTiO₃ materials.After then,Pb?Zr,Ti?O₃ materials was also proved to own excellent piezoelectric effect,which occupied dominate status in piezoelectric field.It was reported in 2009by Ren et al.that Ba_1-xCa_xZr_yTi_1-yO₃?BCZT?ceramics could hold a competition in piezoelectric performance with that of Pb?Zr,Ti?O₃ materials,which is mainly due to the coexist of tetragonal and rhombohedral phases.However,the studies about BCZT materials have mainly focus on the piezoelectric performance still today,and only few studies reported the electrocaloric effect of BCZT materials.It was noted that not only excellent piezoelectric performance was reported in the BCZT ceramics.On the other hand,BCZT materials are also show outstanding ability as ferroelectric materials.The degree of relaxor,which is helpful to enhance electrocaloric effect,can be easily modified through change the content of Zr⁴⁺ions.Moreover,the coexist of tetragonal and rhombohedral phases is also considered to show exceeding potential to obtain large electrocaloric temperature change.In this regard,this thesis mainly focuses on the electrocaloric effect of lead-free BCZT ceramics,and some basic scientific issues are deeply discussed.The main centents and conclusions are seen in the following:?1?The(Ba_1-xCa_x)(Zr_0.2Ti_0.8)O₃ ceramic samples are synthesized by conventional solid state reaction methods.The microstructure,dielectric and ferroelectric properties of the ceramics have been studied.In the range of x=0.05⁰.11,with Ca²⁺addition,lattice parameters decreased,Curie temperature decreased slightly and permittivity increased.However,when x?0.12,the second phase CaTiO₃ occurred and increased monotonously with the extension of Ca²⁺content.Thus,Curie temperature and ferroelectric properties decreased sharply.All the BCZT samples are relaxor ferroelectrics and its diffuseness increased with Ca²⁺addition until complete diffuseness.?2?The influence of phase evolution on the polarization change and electrocaloric responses are studied in(Ba_0.9Ca_0.1)(Ti_1-xZr_x)O₃ ceramics.It is found that with increasing content of Zr⁴⁺ions,tetragonal and orthorhombic phases become destabilized but stabilizes rhombohedral phase,meanwhile the samples become relaxor ceramics step by step.It is proved that the electrocaloric response could be adjustable through changing Zr/Ti ratio.With increasing Zr/Ti ratios,electrocaloric temperature change decreases gradually but present wider electrocaloric operating temperature range.For x=0.05,the maximal?T=1.64K is obtained at an electric field of 7kVmm^-1 at 130^oC,meanwhile corresponding entropy change?S is 1.78Jkg^-1K^-1.At the same time,appearance of small negative electrocaloric effect?-0.1K?is also found in this composition induced by orthorhombic-tetragonal phase transition.For x=0.2,a wide EC response ranging from room temperature to 130 ^oC with?T over0.58⁰.8K(corresponding?S over 0.61⁰.98Jkg^-1K^-1)is obtained.The field dependent of?T and?T/?E indicate that the electrocaloric response is closely relate to the origin of the polarization response.?3?Effect of doping Ga³⁺and Fe³⁺ions on the electrocaloric effect of BCZT ceramics are studied.By doping Ga³⁺ions,BCZT-xGa samples show an enhanced Curie temperature but decreased ferroelectric performance.As a result,the electrocaloric temperature change and corresponding entropy change are deteriorated.However,the addition of Ga³⁺ions is considered helpful to enhance temperature stability of electrocaloric effect.On the other hand,the BCZT-xFe samples show decreased Curie temperature with increasing content of Fe³⁺ions,accompanying with decreased electrocaloric effect.?4?The relationship between polarization behavior and electrocaloric effect are studied by comparing temperature change of Ba_0.9Ca_0.1Zr_0.2Ti_0.8O₃ ceramics and PbZr_0.95Ti_0.05O₃ ceramics.The Ba_0.9Ca_0.1Zr_0.2Ti_0.8O₃ samples with slim-like hysteresis loops exhibit a typical electrocaloric effect response,while PbZr_0.95Ti_0.05O₃ samples with square-like hysteresis loops show an unusual electrocaloric effect response.Our results present that an inverse electrocaloric effect response can be induced by applying an inverse electric field.This inverse electrocaloric effect response shows its maximal value at 0.5E_C^-.With further increase in the inverse electric field,the inverse EC response has become weak and finally disappeared.A competitive mechanism between the reversible process at the small inverse field and the irreversible process at the large inverse field is sufficient to explain the unusual electrocaloric response.?5?Dielectric and ferroelectric performance of Ba_0.9Ca_0.1Zr_xTi_1-xO₃ ceramics and Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ ceramics under hydrostatic pressure are studied.The hydrostatic pressure is found to depolarize ferroelectric BCZT ceramics.With the increase of hyderstatic pressure,the Curie temperature decrease.For Ba_0.9Ca_0.1Zr_xTi_1-xO₃ ceramics,the dielectric performance go through three status as steady,increased and decreased tendencies.And for Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ ceramics,the different depolarize effects are mainly due to the different phase structure.?6?By doping 1.2mol%Mn element,the Mn-BCZT ceramics are fabricated by conventional solid state reaction method.Not only the observation of ferroelecric-paraelectric phase transition at 333K provides ferroelectricity,but also ferromagnetic properties around room temperature is presented,which confirms multiferroic property.The multiferroic material exhibits a large magnetocapacitance effect?12%?at a moderate field?2000Oe?above room temperature?324K?,indicating a strong coupling between ferroelectricity and magnetism.