Effects Of A/B-site Substitutions On Structure,piezoelectric And Multiferroic Properties Of BiFeO₃-BaTiO₃-based Lead-free Ceramics

Multiferroics refers to a kind of multifunctional material which exhibit both ferroelectric and ferromagnetic properties. As a rare single-phase multiferroic material at room temperature, Bi Fe O₃ has been attracted much attention because it shows both ferroelectricity（TC = 830 °C） and G-type antiferromagnetism（TN = 370 °C）. However,for pure Bi Fe O₃ ceramics, there are some inherent impediments to its ferroelectricity,piezoelectricity and ferromagnetism. Besides, pure Bi Fe O₃ usually possesses high leakage current because of the volatilization of Bi and reduction of Fe³⁺ to Fe²⁺ during sintering. To prevent the formation of impurities, improve resistivity and multiferroic of the Bi Fe O₃ ceramics, A-site substitution or A, B-site co-substitution with ions(A-site substitution with La³⁺ 、 Sm³⁺ for Bi³⁺ and B-site substitution with(Mg_0.5Zr_0.5)³⁺,(Ni_0.5Mn_0.5)³⁺and(Co_0.5Mn_0.5)³⁺) were carried out. The main results are as follows:（1）（0.725-x）Bi Fe O₃-x Bi(Mg_0.5Zr_0.5)O₃-0.275 Ba Ti O₃ + 1 mol% Mn O2 lead-free ceramics（x = 0-0.08） were synthesized by a conventional solid state reaction method and the effects of Bi(Mg_0.5Zr_0.5)O₃ on phase transition, piezoelectric and ferroelectric properties of the ceramics were investigated. After the addition of Bi(Mg_0.5Zr_0.5)O₃, the crystal structure of the ceramics is transformed from rhombohedral to tetragonal phase and the morphotropic phase boundary（MPB） of rhombohedral and tetragonal phase is formed at x = 0.01. The grain size of the ceramics increases with x increasing from 0 to0.02 and then decreases with x further increasing. The dielectric peak of the ceramics becomes diffusive with x increasing after the addition of Bi(Mg_0.5Zr_0.5)O₃. The ceramics with x = 0-0.08 exhibit much better electric insulation with the resistivity of1.0×109-5.0×109 Ω·cm than pure Bi Fe O₃ ceramic with the resistivity of ⁵×107 Ω·cm.Due to the formation of the MPB, the ceramics with x = 0-0.02 possess gooddensification with the relative densities ρr of 94.9-96.3 %, strong piezoelectricity with the d33 of 129-135 p C/N and very high Curie temperature with the TC of 559-610 °C.（2）（0.725-x）Bi Fe O₃-x Bi(Ni_0.5Mn_0.5)O₃-0.275 Ba Ti O₃ + 1 mol% Mn O2 multiferroic ceramics were synthesized by a conventional solid state reaction method and the effects of Bi(Ni_0.5Mn_0.5)O₃ on phase transition, piezoelectric and multiferroic properties of the ceramics were investigated. All the ceramics sintered at 950 o C for 2 h possess a pure perovskite structure. Rietveld refinement of X-ray diffraction data revealed that a coexistence of rhombohedral and pseudocubic phases are formed at x =0.05-0.07. After the addition of Bi(Ni_0.5Mn_0.5)O₃, the grain growth is suppressed because of the diffusion of Mn4+into the grain boundary. The dielectric peak becomes gradually diffusive with increasing x due to the small difference between Fe³⁺ and(Ni_0.5Mn_0.5)³⁺. After the addition of a small amount of Bi(Ni_0.5Mn_0.5)O₃（x ￡ 0.02）, the ceramics possess strong piezoelectricity with d33 of 124-145 p C/N, strong ferroelectric with Pr of 16.7-19.3 μC/cm2 and ME coefficient performance with α33 of ⁵53.65 m V cm-1 Oe-1.（3）（0.75-x）Bi Fe O₃-0.25 Ba Ti O₃-x La(Co_0.5Mn_0.5)O₃ + 1 mol% Mn O2 lead-free ceramics（x = 0-0.10） were synthesized using conventional solid-state reaction route and the effects of La(Co_0.5Mn_0.5)O₃ on phase transition, piezoelectric, ferroelectric and multiferroic properties of the ceramics were investigated. After the addition of La(Co_0.5Mn_0.5)O₃, the crystal structure of the ceramics is transformed from rhombohedral（space group R3c） to orthorhombic symmetry（space group Amm2）, and the morphotropic phase boundary（MPB） of rhombohedral and orthorhombic phase is formed at x = 0.01-0.03. The dielectric peak of the ceramics was found to become diffusive with increasing x. The ceramics with x = 0-0.03 possess strong piezoelectricity with d33 of 95–108 p C/N, strong ferroelectricity with Pr of 2.2-18.1μC/cm2, considerable magnetoelectric coefficient of 372-672 m V/cm·Oe, and high Curie temperature of 523-659 °C.（4）（0.75-x）Bi Fe O₃-0.25 Ba Ti O₃-x Sm Co O₃ + 1 mol% Mn O2 lead-free multiferroic ceramics were synthesized using a conventional solid-state reaction route. The effect of Sm Co O₃ on phase structure, piezoelectricity and multiferroicity of the ceramics were investigated. The crystalline structure of the ceramics is transformed from rhombohedral to tetragonal symmetry with increasing the amount of Sm Co O₃. A morphotropic phase boundary of rhombohedral and tetragonal phases is formed at x =0.01-0.04. A small doping amount of Sm Co O₃ is shown to improve the ferroelectric,piezoelectric and magnetoelectric properties of the ceramics. For the ceramics with x =0.01-0.03, high resistivity, piezoelectricity and ferroelectricity are obtained. The ferromagnetism of the materials is greatly enhanced by the doping of Sm Co O₃ such that a very high magnetoelectric coefficient of ⁷42 m V/cm·Oe is obtained at x = 0.01.