Preparation And Electrical Properties Of Bismuth Ferrate Multilayer Composite Films

The great potential of magnetoelectric effect in the fields of information storage,sensors,adjustable transformers and so on has attracted great interest of researchers both at home and abroad.Finding the best composite method to obtain the high magnetoelectric coupling coefficient has been the focus of research in this field.Multilayer polymer-based composite film based on interface strain is one of the common ways to study magnetoelectric effect.In this paper,BiFeO₃（BFO）nanoparticles were prepared by sol-gel method,and the effects of different processing conditions on the crystal structure and micro-morphology of BFO nanoparticles were investigated.The optimal spin coating of BFO nanoparticles and polymer P（VDF-TrFE）was used to fabricate the composite films.The preparation process of the composite films was preliminarily explored and the crystal structure and surface morphology of the composite films were characterized.The ferroelectric properties and conductive mechanism of the composite films were studied,and the preparation parameters of the composite films were adjusted to obtain the best properties.Here,the main conclusions are as follow:（1）Preparation of BFO nanometer powder by sol-gel method.Adjust the sol-gel process to prepare BFO nanoparticles.First of all,the effectiveness of dispersing agent polyethylene glycol was studied.Secondly,under the same technological conditions after adding dispersing agent,the effects of different annealing temperatures on the crystal phase structure,miscellaneous phase content and microstructure of BFO nanoparticles were investigated.The results show that the addition of polyethylene glycol increases the diffraction intensity of BFO nanoparticles,reduces the particle size and homogenizes the size,and effectively weakens the agglomeration effect of BFO nanoparticles after annealing.The higher the annealing temperature,the higher the crystallization degree of BFO nanoparticles,the weaker the peak strength and the decrease of the number of impurity phases,but the grain size also increases.After adding dispersing agent,compared with BFO nanoparticles at different annealing temperatures,the particle size became larger with the increase of annealing temperature.Similar to the previous reports,the magnetic properties of BFO nanoparticles prepared by the experiment are very weak,but the magnetic properties of BFO nanoparticles are improved obviously when the dispersion agent is added to reduce the agglomeration phenomenon.（2）Preparation of BFO/P（VDF-TrFE）composite thin films.The effects of different composite ratios on the crystal phase structure and surface morphology of the composite thin films were studied.The diffraction intensity corresponding to theβphase of the matrix P（VDF-TrFE）decreased with the increase of BFO content.However,the number and intensity of BFO diffraction peaks in the composite films increased.The results of surface scanning electron microscope（SEM）show that BFO nanoparticles in the composite films are uniformly dispersed on the surface of the film,but there is still particle aggregation in some areas.（3）The ferroelectric properties of the composite films of different proportions are investigated.The results show that the ferroelectric properties of BFO nanoparticles can be better than that of pure P（VDF-TrFE）.There are two factors in the improvement of the ferroelectric properties of the composite thin film:（i）With the increase of BFO content,the remnant polarization intensity and maximum polarization intensity of the4/1BFO/P（VDF-TrFE）composite films increased gradually,which reached 0.53μC/cm²and0.85μC/cm²,respectively.Therefore,the increase of BFO content optimizes the ferroelectric properties of the composite film.（ii）An increase in the content of the matrix P（VDF-TrFE）.The residual polarization strength of 1/2BFO/P（VDF-TrFE）increased by about twice as compared to 1/1BFO/P（VDF-TrFE）,but the coercivity increased by about twice,that is,the remnant polarization intensity,the maximum polarization intensity and the remnant polarization rate increased from 0.1μC/cm²,0.28μC/cm²,0.35 to 0.8μC/cm²,1.13μC/cm²,0.7.The coercive field of the composite film is doubled,which is not conducive to the application of the composite film under the condition of small voltage.（4）Study on the conductive mechanism of BFO/P（VDF-TrFE）composite film.The motion pattern of the charge in the composite film is closely related to the performance.The conductive mechanism of BFO/P（VDF-TrFE）composite film was discussed in detail,and four common thin-film conductive mechanism models were selected for data analysis.The composite film is subjected to a current-to-voltage test under a periodic varying scan voltage.The positive and negative bias of the current-voltage（I-V）is in accordance with the SCLC conductive mechanism,but due to the difference in the work function of the upper and lower electrodes,the I-V curve is different.The results show that the larger the content of BFO nanoparticles in the composite film,the larger the leakage current,but it is of the order of micro-ampere.（5）The dielectric properties of composite films with different composite ratios were investigated.According to the previous reports,the dielectric constant of BFO is larger than that of P（VDF-TrFE）.Under the same test conditions,the dielectric constant of the thin films is increased with the increase of BFO content（1/1BFO/P（VDF-TrFE）、2/1BFO/P（VDF-TrFE）、3/1BFO/P（VDF-TrFE）、4/1BFO/P（VDF-TrFE））,such as the dielectric constant of 10.8,16,10.9,18.5,when the frequency is 1kHz,the dielectric constant of4/1BFO/P（VDF-TrFE）is largest.However,there is no significant difference in dielectric loss among different proportion composite films,such as 0.035,0.046,0.041 and 0.09 at 1kHz.Therefore,the dielectric properties of the composite films were optimized with the increase of BFO nanoparticles content.