Preparation And Properties Of Percolative Ferroelectric/Ferromagnetic Composites By Liquid-Coating Method

With the rapid development of electronic industry and technology,trends toward device miniaturization have led to increased interest in multifunctional composites,so that a single device component can perform more than one task.Among them, ferroelectric/ferromagnetic composites are particularly appealing not only because they possess the properties of both parent compound,but also because interactions between the magnetic and electric polarizations may lead to novel functionalities, such as magneto-electric properties.However,it's difficult to obtain composite material with both giant dielectric constant and high permeability simultaneously due to the limitation of compound law,which is undesirable to truly realize miniaturization.Percolation theory can be used to develop ferroelectric/ferromagnetic composites with high dielectric constant.According to percolation theory,the dielectric constant of composite can be enhanced dramatically near percolation threshold by introducing semi-conducting ferrite particles into insulating ferroelectric matrix.However,the magnetic properties decrease distinctly since the percolation threshold is very low in conventional percolative system.Furthermore,in all the conventional percolative composites reported so far,a high dielectric constant can only be achieved when the fraction of conductive phase is very close to the percolation threshold,i.e.,the dielectric constant suffers from an abrupt variation near the percolation threshold, which imposes much challenge and risk in controlling the percolative composites with the threshold composition in order to obtain reproducible products for practical applications.In the present work,semi-conductive Ni_0.5Zn_0.5Fe₂O₄ particles were coated with barium titanate layers by liquid coating method.The existence of insulating dielectric layer can effectively block off electron transport between ferrite particles,thus leading to high percolation threshold.Therefore,ferroelectric/ferromagnetic composite with both high dielectric constant and excellent magnetic properties were obtained.In this article,the dielectric and magnetic properties of composite ceramics were studied systematically.Furthermore,the physic mechanism of high dielectric constant in percoaltive composite was also analyzed.Ni_0.5Zn_0.5Fe₂O₄ was synthesized by self-combustion method,using metal nitrates and glycine as the starting materials.The results reveal that Ni_0.5Zn_0.5Fe₂O₄ nanoparticles with excellent soft magnetic properties can be obtained directly without calcination at high temperature.Ni_0.5Zn_0.5Fe₂O₄/BaTiO₃ composite powders were obtained through sol-gel coating.The resultant powders were pressed into green pellets and rings and then sintered at different temperatures.The influence of sintering condition on dielectric properties and magnetic properties was studied.It's found that when the content of ferrite is larger than 50%,the dielectric constant of the composite remain stable at a high constant value,which is completely different from the conventional percolative system.A Debye-like relaxation was observed,and relaxation peak shifts to higher frequency with increasing temperature,the relaxation process follows well the Arrhenius law.The high dielectric response of the composite ceramics is mainly enhanced by Maxwell-Wagner polarization.The experimental results reveal that for composites with 80%ferrite content,the permeability is about 20,the value of saturation magnetization is about 50 emu/g,and the dielectric constant at 1kHz reaches a maximal value of about 10000,while the dielectric loss is lower than 0.5. Furthermore,the dielectric constant changes slightly with applied frequency and temperature at low frequency areas.Such multifunctional composits possessing both high dielectric constant and excellent magnetic properties has a great potential application in electromagnetic-wave absorption.The experimental results also reveal that the dielectric loss can be reduced by introducing glass dopant.The melt amorphous dopant can serve as electron barrier to restrict the electron transportation between ferrite particles,thus leading to a low dielectric loss.