| The composite materials composing of two or more different compositions have attracted more and more attention because of their developed properties by combining two-phase advantages.Bismuth ferrite,a typical material with coexisting behaviors of ferroelectricity and ferromagnetism at room temperature,has good application prospects in nonvolatile memories,spin electronic devices and magnetoelectric sensors.However,bismuth ferrite ceramics has large leakage current causing by defects such as impurities and oxygen vacancies,so it is difficult to obtain high remnant polarization.PVDF and their copolymer,as common ferroelectric polymers,are widely used in organic electronic devices,piezoelectric sensors and other fields because of their high elasticity,excellent dielectric and piezoelectric ferroelectric properties.However,its high coercivity field makes it difficult to obtain high remnant polarization via low electric field.Therefore,in this thesis,BFO/P(VDF-TrFE)composite thin-film devices are designed and fabricated on an FTO conductive glass substrate by spin-coating method.The ferroelectric and dielectric properties of the optimized structure are analyzed.The main contents are as follows:(1)Prepare bismuth ferrite nano-powder by sol-gel method,and explore the influence of sintering temperature on BFO.As a result,for an one-step sintering,as the sintering temperature increases,the BFO second phases will decrease accompany with an improvment of crystallization and crystal size.Within the sintering temperature of 600?700?,BFO nano powder with less impurity phase can be prepared with the particle size around 300 nm,and weak ferromagnetism as well.As using a two-step sintering method,pure phased BFO nano powder with smaller particle size(?100 nm)can be prepared at even lower temperature.(2)BFO/P(VDF-TrFE)composite films were prepared by spin coating method,and the influences of BFO mass fraction on the structure and morphology of the composite films were studied.Moreover,FTO/x BFO(1-x)P(VDF-TrFE)/Pt(Structure I)device was designed,and its ferroelectric and dielectric properties was studied.As a result,as the mass ratio of BFO powder increases,the surface roughness and thickness of the composite films increase.;With increasing the content of BFO powder,the remnant polarization of the composite films increase at first and decrease then.The leakage current increases as increasing the content of BFO powder,while the dielectric constant of the composite films increases.When the mass ratio of BFO is 20 wt%,the composite film device exhibits the best ferroelectric and dielectric properties:for example,at the electric field of 80 MV/m,the remnant polarization Prreaches 13?C/cm2with the coercive field Ecof 48.1 MV/m,and the leakage current is5×10-7A.While the dielectric constant is 18.27 at 103Hz,with the dielectric loss as low as0.04.(3)Thestructureofthecompositefilmswasoptimized.FTO/x BFO(1-x)P(VDF-TrFE)/P(VDF-TrFE)/Pt device(structure II)was designed.and the structure,morphology,ferroelectric and dielectric properties of the composite film were studied.The results show that after spinning the copolymer layer,the surface of the composite film is smoother and denser with the ferroelectric and dielectric properties being greatly improved.As to the structure II with BFO ratio of 25 wt%,the remnant polarization increases from 10.2?C/cm2of structure I to 24.1?C/cm2,with the leakage current density at50 MV/m reducing about 5%,that is,from 6.92×10-5A/cm2to 3.79×10-6A/cm2.Additionally,the electric constant increases to 43.48,with the dielectric loss of 0.0927 at 103Hz.While for the composite film with BFO content of 50 wt%,the improvement of leakage current behaviors is more obvious.For example,the leakage current density under the electric field of 50 MV/m is reduced about 0.8%,from 8.8×10-4A/cm2of structure I to 6.8×10-6A/cm2of structure II.Besides,the composite films with structure II can bear much higher field of 250MV/m. |
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