The Research On The Structures, Properties And Applications Of Doped BiFeO₃Multiferrioc Materials

The research on multiferroic materials is a rising area in material science and condensed matterphysics, containing abundant physics and materials subjects. Multiferroic materials have simultaneousferroelectric and magnetic ordering, which enables a coupling interaction between them. Thiscoupling interaction, called the magnetoelectric （ME） effect, produces various possibilities for therealization of the mutual control between ferroelectric and magnetic ordering which opens the newopportunities in magnetic recording, spintronics, etc. Among the rare multiferroic materials, BiFeO₃has received much attention due to that both the ferroelectric and antiferromagnetic ordering are quiterobust at room temperature （Tc¹103K and TN⁶43K）. However, the ferroelectricity and magnetismin BiFeO₃are attributed to different ions. Furthermore, the magnetism in pure BiFeO₃is rather weakand the saturate ferroelectric hysteresis is difficult to be observed in BiFeO₃ceramic due to theleakage current which is caused by impurity and defects. All this restrictions inhibit the observationof the linear ME effect in BiFeO₃ceramic. In our studies, we observe the structure and propertieschanges through A site doping, thus to explore the internal magnetic, electric structure andmultiferroic material characteristic of BiFeO₃. In additon, we provides a preliminary exploration forthe application of the multiferroic materials in the electrowetting display and photocatalysisadsorption. This work develop new application fields of BiFeO₃.Ba doped Bi_1.04-xBa_xFeO₃ceramics with x up to0.30have been prepared by the tartaric acidmodified sol-gel method. The X ray diffraction patterns show that the structure transforms fromrhombohedral to tetragonal with increasing the Ba substitution concentration from10%to30%andthe coexistence of distorted rhombohedral and tetragonal phases in20%Ba substituted BiFeO₃, whichwas further confirmed by the Raman spectra. Bi0.84Ba0.20FeO3exhibits the highest magnetization （1.6emu/g under magnetic field of12kOe） compared with the other samples of different Ba substitutionconcentration. Significant enhancement of the ferroelectricity has been observed in20%and30%Basubstituted BiFeO₃with saturate polarization close to6.6μC/cm2for Bi0.74Ba0.30FeO3. Themagnetoelectric coupling of Bi0.84Ba0.20FeO3has been measured and the maximum decrease ofmagnetization under magnetic field of9.8kOe was about0.06emu/g with increasing applied electricfield to11kV/cm, and the magnetoelectric coefficient is1.5×1012s/m.In order to explore the new application of BiFeO₃in electrowetting display technology, we havedesigned and successfully fabricated the electrowetting display unit with Pr and Mn cosubstituted BiFeO₃as dielectric layer. With such kind of typical multiferroic materials as its dielectric layer, theelectrowetting display unit shows excellent eletrowetting effect. We successfully control thecontraction and expansion of the oil droplet with much smaller leakage current. The contraction of theoil droplet occurs at low voltage of11V and contraction area gradually increases along with theenhancement of the applied voltage. The unit shows favorable repeatability since the shrinked oildroplets restitute as soon as the applied voltage was removed. This experiment provides a preliminaryexploration for the application of the multiferroic materials in the electrowetting display.In order to explore the new application of BiFeO₃in dye adsorption, pure phase non-porousBiFeO₃and porous BiFeO₃were prepared through the tartaric acid sol-gel method and cottontemplate method. Firstly, we systematic studied the adsorption performance of non-porous BiFeO₃.The results show that non-porous BiFeO₃performs better in low temperature and pH=4conditions.And we further contrast the adsorption effect when they adsorb dye rhodamine B （RHB） in the pH=4conditions. It was found that the porous BiFeO₃performs better than the ordinary non-porous BiFeO₃.Comparing the morphology and nitrogen adsorption desorption results of two kinds of samples, webelieve that the excellent adsorption efficiency of porous BiFeO₃is attributed to the high surface areawhich is caused by pipe and cavity structure existed in the sample. On the other hand, we comparedthe structure and magnetic properties of the two kinds of BiFeO₃samples before and after adsorption.We found that the structure of the two kind samples is not changed and the magnetic propertieschanges a little. Overall, compared with ordinary non-porous BiFeO₃, porous BiFeO₃prepared bycotton template method shows higher adsorption efficiency, strong ferromagnetic and stable chemicalproperties. Those excellent properties of adsorbent is of great significance for recycling. Our workprovides BiFeO₃a new application prospect in the flied of environmental protection.