Effects Of B-site Substitution On Structure,piezoelectric And Multiferroic Properties Of BiFeO₃-BaTiO₃-based High Temperature Piezoelectric/single-phase Multiferroic Ceramics

Single-phase multiferroic materials possess the coexistence of magnetism and ferroelectricity in the same material. Bi Fe O3 is a rare single-phase multiferroic material and has been extensively investigated in recent years due to the coupling of ferroelectricity and ferromagnetism. It shows high Curie temperature TC of830 oC and Néel temperature TN of 370 oC and thus may be a promising candidate for lead-free high temperature, piezoelectric and room-temperature multiferroic materials.In this thesis, the structure, multiferroic and piezoelectric properties of Bi Fe1-x Mx O3-BaTiO₃（M:Al, Yb and Co）ceramics have been systematically investigated.The main results are listed as follows:（1） 0.725BiFe_1-xAl_xO₃-0.275 BaTiO₃ + 1mol% Mn O2 ceramics were prepared by a conventional ceramic technique. The effects of Al doping and sintering temperature on microstructure, piezoelectricity and magnetism of the ceramics were studied. The crystalline structures of BFO-BT-BA-x ceramics sintered at 970 o C for 2 h evolve from rhombohedral structure at x = 0 to the coexistence of rhombohedral structure and orthorhombic phases at x = 0.01-0.03 and finally to orthorhombic phases at x > 0.03.The critical sintering temperature is 970 o C. For the ceramics with x = 0.01 sintered at/above 970 oC, large grains, good densification, high resistivity, and enhanced ferromag-netic properties are obtained. The ceramic with x = 0.01 sintered at 970 o C possesses high resistivity（¹09 Ωcm） and good piezoelectric properties（d33 = 138 p CN-1,kp = 32.7 %）. Al doping decreases the ferromagnetic properties of the ceramics, whilethe increase in the sintering temperature enhances the ferromagnetic properties of the ceramics.（2） 0.725Bi Fe0.98Yb0.02O3-0.275 BaTiO₃ + 1 mol% Mn O2 multiferroic ceramics were prepared by the conventional ceramic processing and the effects of sintering temperature on structure, ferroelectric, piezoelectric and ferromagnetic properties of the ceramics were studied. The crystalline structures of the ceramics evolve from rhombohedral structure at 850 o C-940 o C to the coexistence of rhombohedral and tetragonal structures at 970 o C-1000 o C and finally to the tetragonal structure at 1030 o C.The grain size of the ceramics sintered at 850oC-1000 oC for 2 h increases from 1.02μm to 4.69 μm with sintering temperature increasing from 850 o C to 1000 o C. As sintering temperature increases, the ferroelectric and ferromagnetic properties of the ceramics are largely enhanced. The ceramics sintered at/above 970 oC possess high density of 94.7-95.9 %, large d33 of 125-138 p C/N, strong ferroelectricity with Pr of¹3.2 μC/cm2 and enhanced magnetism with Mr of 0.118 emu/g.（3） 0.75BiFe_1-x Co_xO₃-0.25 BaTiO₃ + 1 mol% Mn O2 multiferroic ceramics were synthesized by a conventional solid-state reaction route and the effects of Co doping and sintering temperature on structure, ferroelectric, piezoelectric and ferromagnetic properties of the ceramics were investigated. After the addition of Co, the crystal structure of the ceramics is transformed from rhombohedral to tetragonal symmetry. The ferroelectric and ferromagnetic of the ceramics was found to increase with the increasing of x. The ceramic with x = 0.10 sintered at 970 oC possesses highest Pr and Mr values of 12.8 μC/cm2 and 0.97 emu/g, respectively. The crystalline structures of the ceramics evolve from rhombohedral structure at 850 o C-940 o C to the coexistence of rhombohedral and tetragonal structures at 970oC-1000oC。The ceramics sintered at 970oC-1000 o C possess strong ferroelectricity（Pr = 14.5-21.6 μC/cm2）and enhanced magnetism（Mr = 0.09-0.13 emu/g）.