Study On Structure, Ferromagnetic, Ferroelectric And Piezoelectric Properties Of Single Phase BiFeO₃-based Multiferroic Ceramics

Multiferroics are defined as the materials with coexistence of ferroelectric andferromagnetic prperties. As a single phase room temperature multiferroic material,BiFeO3has been widely investigated because of its high Curie temperature (TC=830oC), Néel temperature (TN=370oC) and the coupling of ferroelectric and ferromagneticorders. It has been paid considerable attention in recent years in condensed matterphysics and material science. In this thesis, the microstructure, multiferroic,piezoelectric and dielectric properties of (1–x)BiFe1–yMyO3–xATiO3(M=Sc and Mnions, A=compound ions) materials were systematically investigated, and the physicalmechanism of these electric and magnetic properties was discussed. The main resultsare listed as following:(1)0.725BiFe1–xScxO3–0.275BaTiO3+1mol%MnO2multiferroic ceramics werefabricated by a conventional ceramic technique and the effects of Sc doping andsintering temperature on microstructure, multiferroic and piezoelectric properties of theceramics were studied. The ceramics can be well sintered at the wide low sinteringtemperature range of930–990oC and possess a pure perovskite structure. The ceramicswith x=0.01–0.02sintered at960oC possess high resistivity (~2×109·cm), strongferroelectricity (Pr=19.1–20.4μm/cm2), good piezoelectric properties (d33=127–128pC/N, kp=36.6–36.9%) and very high Curie temperature (618–636oC). Sc3+ions withempty d-orbit lie in the oxygen octahedron to form Sc3d O2p hybridization, whichenhanced stabilization of the ferroelectric distortion, and thus leads to improvedferroelectric properties. In addition, Sc3+ions occupy the B site and decrease thetolerance factor t of the ceramics, resulting in reduced TC. The increase in sintering temperature improves the densification, electric insulation, ferroelectric andpiezoelectric properties of the ceramics. A small amount of Sc doping (main reason) andthe increase in the sintering temperature reduce crystal symmetry, resulting in thebreaking of the antiferromagnetic spin structure and the increased magnetization in theBiFeO3ceramics. Improved ferromagnetism with remnant magnetization Mrof0.059and0.10emu/g and coercive field Hcof2.51and2.76kOe are obtained in the ceramicswith x=0.04(sintered at960oC) and0.02(sintered at1050oC), respectively. However,excess Sc3+doping (x>0.04) substituted not only for Fe3+cations but also Bi3+cationssimultaneously. This leads to the emergence of other antiferromagnetic ordering in therhombohedral structure and the observed reduction in magnetization.(2) In order to develop the ceramics which possess more excellent multiferroicperformance and higher Curie temperature,(1x)BiFeO3xBa0.6(Bi0.5K0.5)0.4TiO3+1mol%MnO2lead free multiferroic ceramics were fabricated by a conventionalceramic technique and the effects of Ba0.6(Bi0.5K0.5)0.4TiO3doping and sinteringtemperature on the microstructure, ferroelectric, piezoelectric and ferromagneticproperties of the ceramics were studied. All the ceramics show good electric insulationwith the resistivity values of1.97×1091.20×1010·cm. After the addition ofBa0.6(Bi0.5K0.5)0.4TiO3, two dielectric anomalies are observed at high temperatures (T1=~453710oC and T2=~716755oC, respectively). The ceramic with x=0.275exhibitsthe optimum piezoelectricity (d33=48pC/N and kp=13.6%, respectively). TheBa0.6(Bi0.5K0.5)0.4TiO3doping and the increasing in sintering temperature improvesignificantly the ferromagnetic properties of the ceramics. The ceramic with x=0.25sintered at1040oC gives the optimum remnant magnetization Mrof0.13emu/g.(3) In general, MnO2is frequently used as a modifier for improving theferroelectric and piezoelectric properties in ferroelectric and piezoelectric ceramics.Based on the above mentioned results, the effect of Mn doping on structure, magneticand electrical properties of0.725BiFe0.96Sc0.04O3–0.275BaTiO3+x Mn (x=0–8mol%)was studied. XRD patterns show that the ceramics possess single phase perovskitestructure after the addition of the proper level Mn. A small amount of Mn doping (x=0.005) can results in deep electron trap levels and thus reduces leakage current. Whenthe MnO2doping level further increases, although the concentration of Fe2+and oxygenvacancy is decreased, excess Mn leads to valence fluctuations of Mn ions and thus formshallow states and decreases electron trap, which causes the increase in the leakagecurrent of the materials. In addition, in the ceramics with high MnO2level, charge transfer can be carried out from the Mn3d state to the empty Ti3d state. This leads tounstable ferroelectric structure in the ceramics. For the ceramics with x <0.07, spiralstructure in ceramics is gradually destroyed with Mn doping level increasing and thuslocked magnetic moment in the materials is released. As x increases to0.07, the spiralmodulated spin structure of the ceramics is completely destroyed and thus the largestremanent magnetization value of~0.495emu/g is obtained. As x further increases to0.08, excess Mn doping results in a transformation of the long–range spiral spinmodulation of the ceramics to another antiferromagnetic ordering, leading to theobserved reduction in magnetization.