Thermal Stability And Magnetoelectric Coupling Of Barium Titanate Based Ferroelectric Ceramics

The morphotropic phase boundary（MPB）can induce high piezoelectric,dielectric and ferroelectric properties in perovskite ferroelectrics,which can be widely used in actuators,sensors and transducers.Barium titanate-based（BT-）ferroelectric ceramics,as one of the alternative materials for toxic lead-based ceramics,have not yet been fully understood about their MPB-induced high performance,and MPB in BT-based is sensitive to relative temperature changes,resulting in materials poor thermal stability.Based on BT-based ferroelectric materials,this thesis mainly studies the response mechanism of MPB to external driving forces to improve the thermal stability of BT-based materials;and uses its characteristics of small strain and large polarization response to study charge-mediated magnetoelectricity coupling,the innovative results obtained are as follows:1.The response mechanism of MPB in BT-base ceramics to external driving forces was obtained.The response behavior of MPB in（1-x）Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)Ti O₃（BZT-x BCT）during application of electric field through in-situ X-ray diffraction.And the scanning electron microscope observation of ferroelectric domains before and after application of an electric field.The results show that the domain conversion coefficient of MPB under an applied electric field is approximately equal to the sum of the domain turnover coefficients of the R-phase and T-phase,and the phase transition near the MPB may be the key factor for the further improvement of the piezoelectric coefficient.2.A composition gradient was prepared to improve the thermal stability of BZT-0.5BCT series ceramics.The pre-calcined BCT and BZT gel were mixed and sintered in the ceramic to obtain the gradient distribution of the composition,and the thermal stability of the sample was controlled by the width of the composition gradient.The results show that the thermal stability of the samples prepared by pre-calcination BCT at 400 ^oC is the best,from-73 ^oC to 23 ^oC,and the specific thermal stability is improved by 33%compared with the standard BZT-0.5BCT.The results show that the composition gradient not only improves the thermal stability of the samples,but also maintains the excellent ferroelectric properties.3.Diffuse MPB was prepared to improve the thermal stability of BZT-0.5BCT series ceramics.The diffuse MPB was obtained by mixing and sintering the R-phase and T-phase powders with a molar ratio of 1:1.The thermal stability of the dispersed MPB was improved by 46%compared with the standard BZT-0.5BCT ceramic.In addition,adding MPB powder to the R phase and T phase increases the dispersion of MPB,and the thermal stability of the sample is improved by 63%compared with the standard BZT-0.5BCT ceramic.4.Magnetoelectric coupling dominated by spin screening effect was observed using ferromagnetic/ferroelectric heterojunctions.Co₉₂Zr₈ thin films were sputtered on0.7Bi Fe O₃-0.3Ba Ti O₃ ferroelectric substrates with small stress strain and large polarization response to exclude the magnetoelectric coupling of stress effects.By measuring the dynamic magnetism under an applied electric field,the magnetoelectric coupling of the spin screening effect is observed,and it is demonstrated that the effect arises from the polarization of the ferroelectric substrate.