| In recent years, Bai-xCaxTiO3based lead-free ceramics have been considered as one of the most promising alternatives for lead-based ceramics due to their excellent electric field-induced strain, ferroelectric and piezoelectric properties, and have potential applications in electromechanical devices such as multilayer ceramic capacitors, transducers, sensors and ferroelectric random access memories, etc. However, their high sintering temperatures (Ts), low Curie temperatures (TC) and poor temperature stabilities hamper their practical applications. So, it is significant to investigate modified Ba1-xCaxTiO3based lead-free ceramics. BiFeO3is one of the very few single phase multiferroics above room temperature with a high ferroelectric TC of830℃, an antiferromagnetic Neel temperature (TN) of370℃and a low melting point of930℃. Generally, forming solid solutions of BiFeO3and other ABO3perovskite ferroelectics is suggested to be an effective way to decrease Ts, modify the phase transition temperature, electrical and magnetic properties of the solid solutions. In this paper, the optimization of Ts and poling conditions, temperature stabilities, aging behaviors of Ba0.70Ca0.30TiO3ceramics have been investigated. Modification of Ba0.70Ca0.30TiO3ceramics such as Fe doping, Bi and Fe codoping, solid solution of BiFeO3and Ba0.70Ca0.30TiO3, and substitution of B-site by Mn in0.33Ba0.70Ca0.30TiO3-0.67BiFeO3solid solutions have been intensively studied. The room temperature electrical properties and polymorphic phase transition (PPT) of the Ba1-xCaxTi0.9sSn0.02O3ceramics have been studied. Phase transition, dielectric, ferroelectric, piezoelectric, magnetic properties, electric field-induced strain, magentocapacitance and the electrical properties-structure relationship of the above modified Ba1-xCaxTiO3-based ceramics have been systematically investigated.Firstly, the Ba0.70Ca0.30TiO3ceramics were prepared by a conventional solid-state reaction method. Ts and poling conditions were optimized to be1340℃and100℃,30kV/cm for20min respectively. The temperature stabilites and aging behaviors of Ba0.70Ca0.30TiO3ceramics with Ts=1340℃were studied. It is found that with Ts increases, all ceramics have coexisted tetragonal and orthorhombic phases, the densification increases,TC increases firstly, reaches the maximum of125℃for the ceramics with Ts=1340℃, and then decreases slightly. The grain size of tetragonal phase, the remanent polarization (Pr), the piezoelectric constant (d33), the planar electromechanical coupling factors (kp) and the mechanical quality factor (Qm) reach the maximum for Ts=1340℃ceramics, indicating the enhanced ferroelectricity and piezoelectricity. The mechanism of the enhanced electrical properties were analyzed. Temperature dependent ferroelectric properties and kv curves indicate that the ceramics with Ts=1340℃have good temperature stability at30-80℃. It is suggested that high inner stress induced by90°domain wall motion and rotation of ferroelectric domains under electric field tends to force part of oriented domains to switch back dramatically and relax the stress, which are responsible for the significant decrease of piezoelectric properties within the initial1000s after the removal of electric field.Fe2O3was added into Ba0.70Ca0.30TiO3ceramics to improve sintering ability and modify electrical properties. Phase transition, dielectric, ferroelectric and piezoelectric properties of Ba0.70Ca0.30Ti1-xFexO3(x=0-0.03) ceramics were intensively studied. B-sites Fe doping improves densification. The ceramics are diphasic composites of tetragonal and orthorhombic phases when x<0.02, diphasic composites of pseudocubic and orthorhombic phases when x=0.03. With x increases, the average grain size of tetragonal phase inreases, reaches the maximum of5.45μm at x=0.005, and then decreases, while that of orthorhombic phase inreases monotonically. Temperature dependent dielectric properties, polarization electric field (P-E) hysteresis loops, bipolar strain-electric field (S-E) curves and piezoelectric properties were measured and analyzed. The variation’s mechanism of dielectric properties,TC, ferroelectricity and piezoelectricity with x increasing were investigated.With x increases, the relative dielectric permittivity (εr) increases, dielectric loss (tanδ) and TC decreases monotonically, the ferroelectricity enhances, reaches the maximum near x=0.005, and then weakens, d33and kp decrease simultaneously, whereas Qm increases sharply. The structure-electrical properties relationship was discussed intensively. The enhanced ferroelectricity and decreased piezoelectricity are described to combined action of the four aspects:the phase transition, the variation of grain size, the pinning effect of oxygen vacancies for domains, the coupling of the large ionic polarization in orthorhombic phases with the non-1800domains in tetragonal phases.The Ca content was adjusted to modify the PPT and room temperature electrical peoperties of Ba1-xCaxTi0.98Sn0.02O3ceramics. Structure, dielectric, ferroelectric, electric field induced strain, piezoelectric properties, PPT and the related mechanism of the Ba1-xCaxTi0.98Sn0.02O3ceramics with x=0-0.015were inestigated. It is found that the substitution of A-site by Ca in Ba1-xCaxTi0.98Sn0.02O3ceramics induces a negligible change of TC, but a decrease of orthorhombic tetragonal phase transition temperature (TO-T) and rhombohedral-orthorhombic phase transition temperature (TR-O), which improves the dielectric temperature stabilities. The Ba0.995Ca0.005Ti0.98Sn0.02O3ceramics have the maximum average grain size, the optimal piezoelectric properties (d33=417pC/N) with TC=117℃, the enhanced piezoelectric properties were described to combined action of PPT near room temperature, diphasic tetragonal-orthorhombic structures and grain size effect.Bi and Fe codoping was performed to modify TC and electrical properties of Ba0.70Ca0.30TiO3ceramics. Sintering characteristics, structure, relaxor behaviors, ferroelectricity, piezoelectricity and temperature stability of Bi and Fe co-doped Ba0.70Ca0.30TiO3ceramics were investigated. The structure-electrical property relationship was discussed. It is found that Bi and Fe codoping improves the densification of Ba0.70Ca0.30TiO3ceramics. The ceramics have diphasic tetragonal and orthorhombic phases when x≤0.06, diphasic pseudocubic and orthorhombic phases when x=0.065. With x increases, the average grain size of the tetragonal phase increases, while that of the orthorhombic phase keeps almost constant. The P-E loops and S-E curves indicate that the ferroelectricity enhances slightly, reaches the maximum near x=0.05, and then weakens with increasing x. Bi and Fe codoping decreases the ferroelectric temperature stability and piezoelectricity. Temperature dependent dielectric properties were measured and analyzed. Frequency dependent the temperature corresponding to the dielectric constant maximum (Tm) were analyzed and fitted according to the Vogel-Fulcher formula. With increasing x from0to0.065, Tm decreases monotonically from125℃to50℃, accompanied by enhanced ferroelectric relaxor behavior. The mechanism of the enhanced relaxor degree, the variation of ferroelectricity, electic field induced strain and piezoelectricity were discussed.Phase transition, relaxor behavior, ferroelectricity, piezoelectricity, ferromagnetism and magnetocapacitive effect of the (1-x)Ba0.70Ca0.30TiO3-xBiFeO3(x=0.07-0.90) solid solutions were studied. The variation’s mechanism of electrical and multiferroic properties were discussed. It is found that Ba0.70Ca0.30TiO3and BiFeO3form continuous solid solutions. The ceramics have coexisted tetragonal and orthorhombic phases when x<0.06, coexisted pseudocubic and orthorhombic phases when x=0.065, coexisted cubic and orthorhombic phases when0.07<x<0.12, pseudocubic phase when0.21<x<0.42, coexisted tetragonal and rhombohedral phases when0.52<x<0.70, and rhombohedral phase when x‰0.75. Significantly composition dependent microstructures and Tm are observed, the average grain size increases from1.9μm for x=0, reaches the maximum of12.0μm for x=0.67, and then decreases to1.3μm for x=0.90. With x increases, the relaxor degree enhances, Tm decreases, reaches the minimum of3℃measured at10kHz at x=0.07, then increases, reaches the maximum of361℃measured at10kHz at x=0.67. The room temperature P-E loops, polarization current intensity-electric field (j-E) curves and magnetization-magnetic field curves indicate that the x=0.42-0.70ceramics have piezoelectric and multiferroic properties, ferromagnetism increases monotonically with increasing x, the variation’s mechanism of multiferroic properties and magnetocapcitance were discussed. The x=0.67ceramic has the optimal mutiferroic and piezoelectric properties with Pr, coercive electric field (Ec), remnant magnetization (Mr), coercive magnetic field (HC) and d33of9.06μC/cm2,32.77kV/cm,0.14emu/g,1135Oe and20pC/N respectively. Magnetocapacitance is evidenced by the increased εr with increasing H. With△H=8000Oe, the x=0.67ceramic has the largest magnetocapacitive coefficient (MD) of2.96%at room temperature, indicating that strong coupling exists between the ferroelectric and ferromagnetic orders, the x=0.67ceramic has potential applications in multiple state memory devices, spintronics, transducers and electric field controlled ferromagnetic resonance devices, etc. Structure, relaxor behaviors, multiferroic and magnetocapacitive properties of the0.33Ba0.70Ca0.30TiO3-0.67BiFeO3+x wt%MnO2(x=0-0.6) ceramics were investigated. Multiferroic and magentocapacitive mechanism were discussed. All ceramics have diphasic tetragonal-rhombohedral phases, whereas Mn doping decreases Ts and grain size, increases the densification and magnetism, weakens the ferroelectricity, piezoelectricity and magentocapacitance, the MD decreases with increasing x, from2.96%for x=0to0.086%for x=0.6. The enhanced magnetism is described to suppressing or destroying of the spiral spin structure resulted from the structural aberration by Mn doping and the enhanced interactions between magnetic ions. |
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