Preparation And Properties Of BiFeO₃ Based Thin Film And Ceramics

Ferroelectric is a kind of material with spontaneous polarization and the direction of its dipole moment can change with the variation of the external electric field.Due to its ferroelectric properties,it can be widely used in ferroelectric memory and resistance memory,and other devices.Flexible ferroelectric device possesses excellent mechanical flexibility,which can adapt to different working environment and meet the deformation requirements in curved surface environment.Meanwhile,ferroelectric material also has piezoelectric performance,and plays an important role in electronic devices such as transducers,sensors and actuators by virtue of the ability of converting the mechanical energy into electric energy and vice versa.Bismuth ferrite（BiFeO₃）is a kind of ferroelectric with distorted perovskite structure at room temperature,possessing an ultrahigh Curie temperature（T_C～830℃）,a great theoretical polarization intensity(remanent polarization P_r>100μC cm^-2)and weak intrinsic piezoelectric properties(piezoelectric constant d₃₃～40 p C N^-1).In addition,pure BiFeO₃materials usually suffer from impurity phase,and low resistivity and due to the existences of oxygen vacancies caused by the volatilization of Bi ions and the valence fluctuation of Fe ions.While the leakage current of BiFeO₃at high temperature is more deteriorated,which cannot guarantee its normal operation in a wide temperature range.To tackle these problems,This work presents a flexible（Mn,Ti）-codoped multiferroic BiFeO₃（BFOMn Ti）film with excellent performances on mica substrate,and prepared a BiFeO₃-BiScO₃-PbTiO₃ceramic by solid-phase sintering in combination with high Curie temperature and strong piezoelectric properties.The main contents of the research work are as follows:1.This work presents the direct fabrication of a flexible（Mn,Ti）-codoped multiferroic BFOMn Ti film capacitor with excellent performances on mica substrate.The fabricated polycrystalline BFOMn Ti film on mica exhibits superior ferroelectric switching behavior with robust saturated polarization(P_s～93μC cm^-2)and remanent polarization(P_r～66μC cm^-2),as well as excellent piezoelectric property with a self-polarization induced large converse piezoelectric constant d*₃₃～387 pm V^-1.At 400 k V cm^-1,the leakage current density is only3×10⁴A cm^-2.The film possesses excellent frequency stability（1-50 k Hz）and temperature resistance（25-200℃）,as well as reliable long-lifetime operation（1×10⁹cycles）.The BFOMn Ti film memory also exhibits excellent bending resistance.It can be safely bent to a small radius of curvature as low as 2 mm.Compared with the flat state,the converse piezoelectric constant hardly changes under the small radius of r=4 mm.The polarization intensity-electric field（P-E）and leakage current density-electric field（J-E）curves of the film measured in various bending states show no visible difference.The film safely goes through repeated compressive/tensile mechanical flexing for 1×10³bending times at 4 mm radius without any obvious deterioration in polarization,or fatigue resistance after 10⁹switching cycles.2.The BFO-BS-PT ceramics with the co-existence of rhombohedral and tetragonal phases were prepared by solid-phase reaction.The Curie temperature,positive piezoelectric constant of the ceramic are 500℃,287 p C N^-1,respectively.Taking into consideration the piezoelectric constant and Curie temperature comprehensively,the ceramics displays a balance between piezoelectric constant and Curie temperature.The resonant frequency of the ceramics decreases by～14%from room temperature to 300℃,showing good temperature stability.Based on the ceramic and the finite element method（software ANSYS）,a high temperature piezoelectric ceramic based quadrupole transducer was designed and fabricated.In the selected frequency range（3-30 k Hz）,the transmission voltage response is～100 d B,and the receiving sensitivity is～-228 d B.