Preparation,Structure Control And Multiferroic Properties Of BCZT/LCMO Magnetoelectric Composites

Magnetoelectric(ME)multiferroics are those materials possessing simultaneous ferroelectric ordering and ferromagnetic ordering along with the coupling between them,which can realize the mutual conversion and regulation between the electric field energy and the magnetic field energy,and thus provides a new design freedom for the development and utilization of the next generation multifunctional ME devices.Compared with the single-phase ME multiferroics,the multi-phase ME multiferroic composites show stronger designability and exhibit more significant ME coupling effects at room-temperature,which already become one of the current research hotspots.In this dissertation,Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃/La_0.67Ca_0.33Mn O₃(BCZT/LCMO)ME composites were built using environmentally friendly BCZT as the ferroelectric phase and LCMO as the ferromagnetic phase.The preparation,structure control and multiferroic properties as well as the ME coupling mechanism of the BCZT/LCMO ME composites were mainly studied in detail,for the sake of providing theoretical foundation and technical support for the miniaturization and integration of high-performance ME devices.Highly dense BCZT ceramics were prepared by Plasma Activated Sintering(PAS)due to its technical advantages of field-assisted activation and rapid densification at low temperatures,and the effects of sintering temperature and subsequent annealing treatment on the phase structure,microstructure and electrical properties were investigated.As the sintering temperature increases,the phase structure of BCZT ceramics first changed from rhombohedral phase to the coexistence of rhombohedral/tetragonal phases and then to tetragonal phase,while the density and grain size gradually increased.BCZT ceramic sintered at the optimum sintering temperature(1250°C)displayed a dense structure and uniform grains as well as the morphotropic phase boundary(MPB)structure.Furthermore,the microstructure and electrical properties of the BCZT ceramics were further optimized by annealing treatment,BCZT ceramic annealed at 1400°C exhibited good electrical properties with the piezoelectric coefficient of 501 p C/N,remanent polarization of 18.1?C/cm² and dielectric constant of 14000.LCMO ceramics were prepared by using PAS,and the sintering temperature effect on the phase structure,microstructure and electrical properties was investigated.As the sintering temperature increases,the crystal structure of the LCMO ceramics remained unchanged,all of which were single orthogonal phase structures,but the density and grain size gradually increased,and therefore greatly affected its magnetic and electrical transport properties.The magnetization of the LCMO ceramics gradually decreased with increasing sintering temperature,while the resistivity gradually increased,and the metal-insulation transition temperature shifted to the low temperature.In addition,LCMO ceramic sintered at the optimum temperature of 1000°C showed that the Curie temperature is 148.6 K and the maximum magnetoresistance is 41.1%.On the base of that,BCZT/LCMO ME layered composite ceramics were prepared by silver paste method and PAS method,and the effects of different complex structures and interface characteristics on the interface structure and multiferroic properties were studied.The layered composite ceramics prepared by the two methods exhibited good multiferroic performances,in particular,as compared to the BCZT ceramics,the dielectric constant was significantly increased with the maximum value of 25097.As the thickness of the ferromagnetic phase LCMO increases,the piezoelectric coefficient of the layered composite ceramics gradually reduced,the remanent polarization first increased and then decreased,while the dielectric constant,saturation magnetization,and ME coupling coefficient gradually increased.Furthermore,multiferroic properties of the layered composite ceramics were closely related to theirs interface structure.Compared with the layered composite ceramics prepared by silver paste method,the layered composite ceramics prepared by PAS method showed denser bonding interface which is conducive to improving the multiferroic properties,and in turn displayed a significant ME coupling coefficient with the maximum values of 6.57 m V/cm·Oe.BCZT and LCMO ceramics were utilized as the ceramic targets,and BCZT and LCMO films were separately deposited by using Pulsed Laser Deposition to construct the BCZT/LCMO ME heterostructures with different complex structure and interface characteristics.The effects of the film thickness,crystal orientation,and substrate material on the crystal structure,morphology and multiferroic properties of the ME heterostructures were systematically studied.The relevant factors affecting the ME coupling effect and their correlation with the microstructure and interface strain were discussed,and in turn the internal relationship between the composition,structure,and performance of the ME heterostructures was established.The interfacial structure and strain states of the ME heterostructures were effectively improved by altering the stress relaxation,structural transformation and lattice mismatch,and thus the multiferroic properties were optimized.In the BCZT/LCMO ME heterostructures,the increase of the ferroelectric BCZT layer thickness could result in the reduction of its residual strain,which therefore leaded to the larger electrical properties and ME coupling effect,otherwise,the ME coupling coefficient of the ME heterostructures was more significant than the ME layered composite ceramics;the(111)-oriented ME heterostructures displayed the largest ME coupling coefficient of 207 m V/cm·Oe due to the highest interface transfer efficiency;the ME heterostructure grown on the Sr Ti O₃ substrate exhibited the largest piezoelectric coefficient and stronger ME coupling effect because of the smallest residual strain in the BCZT layer.Results suggested that the stress/strain-mediated mechanism was the main ME coupling mechanism of the BCZT/LCMO ME heterostructures,and thus the reduction of residual strain,the improvement of piezoelectric effect as well as the increase of interface transfer efficiency are benefit to the enhancement in the ME coupling effect of the BCZT/LCMO ME heterostructures.