Preparation And Ferroelectric Properties Of BiFeO₃-Bi_1-xNd_xFeO₃ Multiferroic Composite Ceramic

Bismuth ferrite(Bi Fe O3) has both ferroelectricity and weak magnetism, which is an only room-temperature multiferroic material. Because of its high Curie temperature(Tc~830oC), high antiferromagnetic Neil temperature(TN~370 oC) and high spontaneous polarization values(>90μC?cm-2) in single crystal, it has attracted a lot of research interests that focused on magnetoelectric coupling for potential applications. However, its G-type magnetic structure determines that it only has weak magnetism, which cannot meet the practice applications. Therefore, we have successfully used the doping Rare earth element to change bismuth ferrite’s crystal structure and then construct the two-phase boundaries of composites, controlling the multiferroic behavior of Bi Fe O3. The effects of phase constitution and sintering technology on the multiferroic properties of composites are also studied.Bi1-xNdxFe O3(x=0,0.1,0.15,0.2) powers were prepared by homogeneous coprecipitation method, and multiferroic Bi Fe O3-Bi1-xNdxFe O3 composite ceramic were prepared in different atmosphere, different temperature and holding time by the method of pressureless sintering. Protective atmosphere, calcination temperature and holding time were studied on the change of phase content in the composite ceramic block, density and crystalline. The differences between phase content and the sintering prosess were analyzed, through the ferroelectric,ferromagnetic and dielectric properties.The results show that neither the low temperature sintering for a long time nor the high temperature sintering for a short time can make complete crystallization and dense composite ceramic; High temperature sintering(800 oC for 2h) for a long time is the optimum temperature to make Bi Fe O3-Bi1-xNdxFe O3 composite ceramic crystal and achieve high densification.The density can reach 93%; Too high temperature can cause a slight burnt and ceramic coarsening resulting in the drops of density.The results of XRD show that composite ceramic block were consisted of Bi Fe O3 of R phase and Bi1-xNdxFe O3 of T phase. The results of phase contents show that the content of R phase and T phase were between 45%~55%. It was in the phase transition interval. Controlling of sintering process and rare earth elements doped could inhibit the decomposition of Bi Fe O3,reduce the noise generation and improve the density of pottery and porcelain.The results of scanning electron microscope(SEM) and transmission electron microscope(TEM) show that high temperature sintering(800 oC insulation 2h) for a long time is the best optimum temperature to make Bi Fe O3-Bi1-xNdxFe O3 composite ceramic crystal. R phase was regular and T phase was ellipsoid.Under this condition, when x=0.15, the Pr was 0.38μC/cm-2 and the Ec was 6kv/cm. The highest stable dielectric constant is about 25 and the smallest dielectric loss is 0.25 for the composite ceramic block prepared in oxygen insulation. Along with the increase in amount of Nd doped rare earth elements, the dielectric constant and dielectric loss all showed a trend of increase, but they are lower than the pure phase(R) Bi Fe O3.Argon atmosphere can improve the remanent magnetization of Bi Fe O3 and the dopof Nd element can enhance remanent magnetization. When x= 0.2, the remanent magnetization is about 0.06 emu/g belonging to the biggest and the saturated magnetization is 0.28 emu/g with coercive field 250 oe.