Preparation And Doping Modification Of Single-phase Multiferroic BiFeO₃

Materials which possess two or more than two of ferroic order?such as ferromagnetic?ferroelectric,ferroelasticity and so on?are called multiferroics.It means that not only more than one kind of physical appearance exists in the materials,but also shows an interaction between these physical appearances.The interplays offer the ability for designing the next generation function devices.The BiFeO₃ is the only single-phase multiferroics possessing not only large ferroelectric polarization?⁹0?C/cm²?but also G-type antiferromagnetic order above room temperature,which has attracted intense attention and comprehensive study in past ten years.However,it is diffcult to synthesis the high quality BiFeO₃-based materials.It is the fact that the defects and the impurity phases would form during the inaccurately controlled process that leads to the problem of the leakage current,which seriously hinder its application.In this paper,the BiFeO₃ which is successfully synthesized by a sol-gel method,is used to study its structure?thermal stability and physical properties.The research focuses on how the processes and the substituted element affect BiFeO₃,The results can be summaried as follows:?1?The influences of processes on the BiFeO₃ powders have been studied by changing the content of solution and the calcinating temperature.The results show that single phase appears in BiFeO₃ powders which was synthesized under the condition that the content of solution is 0.4mol/L and that the calcinating temperature is 550?.In terms of ceramics,the samples are synthesized by different processes.The results indicated that many point defects would form during the conventional sintering.These point defects with charge can move when the external electrical field exists,which seriously weaken the property of the Bi FeO₃.Finally,the results of the BiFeO₃ sintered by SPS demonstrated that the point defects can be remarkly reduced and therefore the quality of the BiFeO₃ ceramic has a significant improvement;?2?Synthesizing the BiFe_1-xAl_xO₃ and BiFe_1-xGa_xO₃ ceramics to study the effect of Al³⁺or Ga³⁺cation on the BiFeO₃.The results show that a decomposition occurred during sintering.It is because the substitution of Al³⁺or Ga³⁺cation will increase the total energy of the system.So,while the external environment is above a specifical temperature,the total energy could promote the decomposition,leading to the formation of Bi₂Fe₄O₉ and Bi₂₅FeO₃₉.The results of theoretical calculation and the curves of TG-DSC can explain this phenomenon.According to the plots of the TG-DSC,the decomposition occurred while the temperature located in the range of 650?-700?.Besides,the substitution of Al³⁺or Ga³⁺may lead to the formation of the BiAlO₃ and the BiGaO₃ in system.The results of theoretical calculation show that the formation enthalpy of the BiGaO₃ is higher than that of Bi AlO₃,but the TG-DSC curves show that decomposition of BiFe_1-xGa_xO₃ ceramics occurred at a little higher temperature.This phenomenon could be described to the fact that the radius of Ga³⁺is closer to Fe³⁺,which cause the less distortion in the system;?3?Synthesizing the BiFe_1-2xZn_xTi_x O₃ ceramics to study the effect of Zn²⁺and Ti⁴⁺ation codoping at Fe site.The XRD patterns show that the Bi Fe_1-2XZn_xTi_xO₃ceramics belong to rhombohedral structure with space group R3c.While the substitution increased,a tendency of phase transition from rhombohedral?R3c?to a tetragonal-like phase can be observed.This tendency can be ascribed to the distinction of the different ions radius.Based on the measure of physical properties,it is founded that the codopant of Zn²⁺and Ti⁴⁺plays an important role in improving the electrical properties of BiFeO₃.This enhancement contributed to the formation of defects complexes in the system.The innovation of this paper is to study the influence of different elements on BiFe O₃.The influences of Al³⁺or Ga³⁺doped into BiFeO₃ on the thermal stablity have been studied in the article,and the deeply mechanism of decomposition was discussed.Besides,the effect on the structure of the Zn²⁺and Ti⁴⁺co-replacing partly the Fe-site in BiFe O₃ and its reason have been studied.At the same time,the effect of the Zn²⁺and Ti⁴⁺codoped system on physical properties and its micromechanism have been studied.