Sythesis And Characterization Multiferroic Composite Nanofibers With Core Shell Structure

Multiferroic material, a type of multifunctional material, possesses two or threecoupling properties of ferroic orders simultaneously. Recent studies have shown thatmultiferroic composite materials can be made from single phase ferroelectric andferromagnetic material. The special multifunctional material is promising for a widerange of applications including information storage, wide band magnetic detection,micro-electromechanical systems （MEMS） and multi-functional electronic devicessuch as sensors, tranformers, gyrators, and drivers. In this dissertation, core-shellmultiferroic nanofibers have been prepared by coaxial electrospinning. Moreover,their microstructure, macroscopic ferroelectric and ferromagnetic properties arecharacteraized. The main research findings are summarized as follows:1. Single-phase Pb（Zr0.52Ti0.48）O3（PZT）, CoFe₂O₄（CFO） and PZT-CFOcomposite nanofibers with different ferroelectric and ferromagnetic molar ratio as1:0.5,1:1,1:1.5are synthesized by electrospinning. The perovskite structure of PZTand spinel structure of CFO are confirmed by X-ray diffraction （XRD）, and themorphology of the fibers is observed by scanning electron microscope （SEM）. Themechanical properties of PZT, CFO and PZT-CFO composite nanofibers withdifferent components are measured by nanoindentation technique. The results showthat both Young’s modulus and hardness of PZT nanofibers are the maximum,corresponding parameters of CFO nanofibers are the minimum, while those ofPZT-CFO composite nanofibers decrease with the increased proportion of CFO.2. Multiferroic Pb（Zr0.52Ti0.48）O3-CoFe₂O₄（PZT-CFO） core-shell nanofibers aresynthesized by coaxial electrospinning. The core-shell configurations of nanofibersare verified by scanning electron microscope. X-ray diffraction analysis show that thecomposite nanofiber are composed of spinel structure of CFO and perovskite structureof PZT, without obvious impurity, and CFO phase content is larger than that of PZTphase. The structure of nanofibers is characterized by transmission electronmicroscope, high resolution transmission electron microscope and selected areaelectron diffraction. It’s observed that the PZT-CFO core-shell fiber diameter is in therange of100-200nm. However, the core shell interface bonding is not compact, andsome gaps exist at interfaces. The macroscopic ferromagnetic properties of core-shellnanofibers are measured by magnetic hysteresis loop, the remanent magnetization Mr is7.5emu/g, corresponding coercive magnetic field Hc is700Oe. The surfacepotential of core-shell nanofiber is measured by scanning Kelvin probe microscope,and the corresponding piezoelectricity is tested by piezoresponse force microscopywith dual frequency resonance tracking technique, which confirms the ferroelectricityof PZT-CFO core-shell nanofibers.3. Multiferroic CoFe₂O₄-BiFeO3（CFO-BFO） core-shell nanofibers are fabricatedwith coaxial electrospinning. The spinel structure of CFO and perovskite structure ofBFO are confirmed by X-ray diffraction. The core-shell configuration of nanofibers isverified by scanning electron microscope and transmission electron microscope. Thediameter of CFO-BFO core-shell fiber is in the range of200to500nm. Themacroscopic ferromagnetic property of core-shell nanofibers is demonstrated bymagnetic hysteresis loop. The butterfly and phase-voltage hysteresis loops ofCFO-BFO core-shell nanofibers are tested by the piezoresponse force microscopy（PFM） in the "OFF" and "ON" state, respectively. The coercive field Ec is measuredto be400kV/cm and the maximum electric displacement is350pm. The polarizationswitching phenomenon occurs when appropriate DC voltage is loaded on the fiber,which proves the ferroelectricity of CFO-BFO core-shell nanofibers. The localmagnetoelectric coupling of the core-shell nanofiber is further confirmed by PFMswitching characteristics before and after the application of magnetic field, with smallintrinsic piezoresponse amplitude and phase contrast reduced owing to the porousinterface. Two group of butterfly and voltage-phase hysteresis loops under0Oe and2000Oe in plane magnetic field are figured out, the corresponoding magnetoelectriccoupling coefficients are218.2V/cm Oe and254.5V/cm Oe, respectively.