| Multiferroic materialshave potential applications in multifunctionaldevices(storage, transducer, sensor, etc.) due to their special physical properties. As a single-phase room temperature multiferroic material, bismuth ferrite(Bi Fe O3) has attracted extensive attention. It has a rhombohedrally distorted perovskite structure at room temperature and possessesvery high Curie temperature(TC) of830?C and Néel temperature(TN) of370?C. However, it is very difficult to prepare pure Bi Fe O3 because of the narrow temperature range of phase stabilization. And it is restricted for the use of Bi Fe O3 due to its large leakage current and weak ferromagnetism. In this thesis, we prepared Sm3+-doped and Bi0.5Na0.5Ti O3 modified Bi Fe O3-based solid solutions. The composition and sintering condition dependences on structure, microstructure, multiferroic and piezoelectric propertiesof the ceramics have been studiedsystematically. The main results are listed as following:(1) 0.725Bi1-xSmxFe O3-0.275 Ba Ti O3+1mol% Mn O2 ceramics were prepared by a conventionalceramic technique and the effects of Smdoping and sintering temperature on structure, microstructure, piezoelectric and multiferroic properties of the ceramics were studied. As x increases from 0 to 0.09, the ceramicsare changed from rhombohedral totetragonal symmetry. The addition of Sm3+ inhibits the grain growth, while the increase in Ts promotes the grain growth. The d33 gradually decreases with x increasing from 0 to 0.09; when x=0.015, the ceramic obtains enhanced ferroelectric property(Pr= 15.4μC/cm2,Ec= 3.41 k V/mm); the strong ferromagnetic property(Mr= 0.21 emu/g,Hc= 3.76 k Oe) is obtained at x =0.09. Moreover, when Ts=960-1020 oC, the ceramic obtainsgood ferroelectricity(Pr=12.6-15.4μC/cm2) and ferromagnetism(Mr = 0.11-0.15 emu/g, Hc = 3.26-3.76 k Oe).(2) Novel(0.725-x)Bi Fe O3-0.275 Ba Ti O3-x Bi0.5Na0.5Ti O3+1mol%Mn O2 multiferroic ceramics were prepared by a conventional sintering method. The structure, microstructure, dielectric, piezoelectric and multiferroic properties of the ceramics were studied. The XRD results show that a morphotropic phase boundary between rhombohedral and tetragonal phases is formed at x=0.02. After the addition of Bi0.5Na0.5Ti O3, two dielectric anomalies at Tm(~510-570oC) and T2(~720oC) are induced. The phase transition around Tm becomes wider gradually with increasing x. When x=0.02, the ceramic obtains high density, high resistivity(~ 1.3×109?·cm), strong ferroelectricity(Pr = 27.4 μC/cm2) and good piezoelectricity(d33=140p C/N, kp=31.4%). Weak magnetic property(Mr = 0.19 emu/g) is observed after the addition of a small amount of Bi0.5Na0.5Ti O3.(3) 0.705 Bi Fe O3-0.275 Ba Ti O3-0.02Bi0.5Na0.5Ti O3+1mol%Mn O2 multiferroic ceramics were prepared. The effects of sintering temperature(Ts) and dwell time(ts) on the structure, microstructure, ferroelectric, piezoelectric and ferromagnetic properties of the ceramics were studied. As Ts increases, the phase structure of the ceramic changes from rhombohedral to tetragonal symmetry and a morphotropic phase boundary between rhombohedral and tetragonal phases is formed at Ts = 1000 oC. The increase in Ts greatly improves the densification, electric insulation, ferroelectricity and piezoelectricity of the ceramics, while relatively weak dependences on ts are observed. As Ts increases from 880 oC to 1000 oC, the magnetism of the ceramics is significantly enhanced and the values of Mr are increased by ~400%. The optimum sintering conditions of the ceramic is 1000oC/2h, in which high density, good electric insulation and enhanced ferroelectricity(Pr= 18.4 μC/cm2), piezoelectricity(d33 = 140 p C/N,kp= 31.4%) and ferromagnetism(Mr = 0.099 emu/g) are simultaneously obtained. |
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