Some lead-free piezoelectrics cannot meet the requirement for application in extremely harsh high-temperature surroundings. In this work, an enhanced piezoelectric performance with high piezoelectric coefficient (d(33)) and high Curie temperature (T-c) has been achieved by replacing the Ti4+ with the Co3+ in Na0.5Bi4.46Ce0.04Ti4-xCoxOy ceramics. The introduction of Co3+ decreases the B-O bond lengths, leading to the shrinkage of BO6 octahedra and lattice volume. The electron localization degree of B-O bond and the polarity of the BO6 octahedra were improved by diminution of lattice volume concomitantly, which were indicated by the first principle calculations. Therefore, an enhanced piezoelectric coefficient of 22 pC/N and high T-c of 690.2 C have been realized in Na0.5Bi4.46Ce0.04Ti4-xCoxOy ceramics with x = 0.02 mol. Our work gives a good paradigm to design superhigh temperature piezoelectric devices for practical application in extremely harsh circumstances.

The electric and dielectric properties of Na0.5Bi4.50+xTi4Oy (x = -0.02, 0, 0.02) prepared by conventional mixed oxide route have been investigated by impedance spectroscopy (IS) over a wide temperature range. Single-phase bismuth layer-structured perovskite patterns were observed through X-ray diffraction of the three samples Na0.5Bi4.5Ti4O15, Na0.5Bi4.48Ti4Oy, and Na0.5Bi4.52Ti4Oy. The results show that the relative permittivity (epsilon r) increases with the increase in temperature and reaches its maximum at about 675celcius. With the continuous increase in temperature, the permittivity decreases gradually. Both relative permittivity and dielectric loss show great stability at the low-temperature zone. The ceramic of x = 0.02 with Ea of 1.09 eV has the maximum oxygen ionic transport number between 600 and 800celcius for all samples. And at this time, it has the maximum electrical conductivity. All the results indicated that Na0.5Bi4.50+xTi4Oy (x = -0.02, 0, 0.02) ceramics were promising base materials for high-temperature capacitor because of their high dielectric properties.

KxNa0.5-xBi4.46Ce0.04Ti4O15+y (x = 0, 0.04, 0.08, 0.12, 0.16, 0.20) ceramics were synthesized by solid state reaction. The microstructure and electrical properties of the solid solutions were characterized. It is found that the doping of appropriate amount of K improved the degree of lattice distortion of the material, and the characteristic parameter b/a of lattice distortion obtained the maximum value through structural refinement at x = 0.12. A defect dipole pair (K-Bi '' - V-o(..)) is formed by adjusting the defects of the material. The polarizability of defects is 4.88 x 10(-37)F.m(2). The formation of defect dipole pairs that deflect towards the direction of spontaneous polarization reduces the difficulty of polarization and improves the piezoelectric properties. Finally, the optimal piezoelectric coefficient (d(33)) of NBCT-xK ceramics is 23 pC/N, and the residual polarizability (2P(I)) is 4.14 mu C/cm(2). The values of residual polarization rate (2P(I)), piezoelectric coefficient (d(33)) and b/a show the same trend, which will gradually decrease when their value increases to a certain extent. The dielectric loss of all samples below 200 degrees C are lower than 0.01, and the dielectric constant is relatively flat below 400 degrees C. Moreover, the Curie temperature (T-c) of K(x)Na(0.5-x)Bi(4.46)Ce(0.04)Ti(4)O(15+y)based ceramics is increased to 686.0 degrees C, which will enable the ceramics to be used in oilfield logging, aerospace and military fields in high temperature environment.

Electrocaloric effects due to entropy change and dipole coupling upon electric field in second-order phase transition (tetragonal-cubic) ferroelectrics are investigated by a three-dimensional Devonshire's theory and statistic method. In diabatic condition, increase in vibration entropy to heat the ferroelectric is due to decreases in polarization entropy and configuration entropy of dipole reorientation in the specific directions. Coupling effect originated from the reorientation of dipoles accompany with electric hysteresis loop happens in the nearest neighbor dipoles parallel to electric field direction. Numerical simulations exhibit that polarization effect causes electrocaloric peak at the Curie's temperature independent of electric field, reorientation effect of dipole causes a shift of electrocaloric peak to high temperature with electric field, and coupling effect between dipoles gives rise to increase in electrocaloric effect with decreasing temperature. A method to predetermine the excellent electrocaloric effect of ferroelectrics from dielectric and/or polarization experimental results is proposed.

An electric hysteresis loop is derived according to dipole turning upon an applied electric field in 3D ferroelectrics by the Boltzmann statistic method via Gibbs free energy in the Devonshire's theory. The loop shape varies with temperature, dipole coupling, and applied maximum electric field, which provides a corresponding theoretical method to derive temperature dependent energy storage density. By numerical simulation the result demonstrates that energy storage density peak appears and shifts towards high temperature with increasing electric field, which is in good agreement with experimental results. A mechanism revealed that the high energy storage density in paraelectric phase, a state of zero spontaneous polarization, is ascribed to a huge increase in polarization induced by electric field around the Curie's temperature. Since ferroelectric dielectric constant is related to the induced polarization in principle, dielectric constant peak can be a direct indicator for the energy storage density peak.

Na0.5Bi4.5-xCexTi4O15(x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) lead-free piezoelectric ceramics with high Curie temperatures are fabricated using the conventional solid-phase method. The effects of the Ce content on the phase structures, morphologies, and electrical properties of the Na(0.5)Bi(4.5-)(x)Ce(x)Ti(4)O(15)ceramics are systematically investigated. The appropriate content of Ce increasesb/aandc/aand induces the distortion of the crystal structure. The increasedb/aleads to a transverse asymmetry of the Na(0.5)Bi(4.5-)(x)Ce(x)Ti(4)O(15)ceramics, which facilitates the dipole flipping, thus enhancing the piezoelectric properties (d(33) = 20 pC/N). Although the improvedc/aincreases the degree of tetragonality of the Na(0.5)Bi(4.5-)(x)Ce(x)Ti(4)O(15)ceramic, which decreases the Curie temperature (T-C), theT(C)values of all samples are higher than 600 degrees C, considerably higher than the practical application temperature. The Ce doping significantly reduces the dielectric loss of the sample and increases its dielectric performance. The improvements in electric properties by the cerium doping can expand its use in high-temperature environments for oilfield logging, aerospace, and military applications.

The influence of Mg doping on a type of Na0.5Bi0.5TiO3 (NBT)-based ceramic, viz. NaBiTi6O14, has been investigated. NaBi(Ti1−xMgx)6Oy ceramics (x = 0.03, 0.06, 0.09, 0.12) were prepared by a traditional solid-phase method, and the influence of Mg doping on their microstructure, dielectric properties, and ferroelectric performance studied. Scanning electron microscopy (SEM) showed that the doped ceramics had clearer grain boundaries and more uniform grain size than the undoped ceramic. Mg doping enhanced the dielectric properties of the NaBiTi6O14 ceramic, with lower dielectric loss and higher dielectric constant. Z* plots showed that the NaBi(Ti1−xMgx)6Oy ceramics were a kind of dielectric. The activation energy of the ceramics was found to be 1.083 eV, 1.087 eV, 1.086 eV, and 0.861 eV, respectively, confirming their excellent dielectric performance. Ferroelectric hysteresis measurements showed that the NaBi(Ti1−xMgx)6O14−6x ceramics exhibited weak ferroelectric performance with 2Pr values of 0.079 μC cm−2 to 0.195 μC cm−2. The stable behavior of the doped ceramics may enable their application in high-temperature and high-frequency devices.

The fascinating dielectric property of relaxor ferroelectrics is investigated on the basis of phase-transition dynamics within the mean-field theory. With a power-law distribution of the Curie temperature for the second order phase transition ferroelectrics, the complex dielectric permittivity is derived. The result shows that the modified Curie–Weiss relation in the paraelectric phase follows the power-law distribution directly. The significant conclusion is that the “rediscovered” distributed Curie temperature can describe the universal dielectric dispersion for both real part and imaginary part of dielectric permittivity with temperature and frequency in relaxor ferroelectrics, and it conforms well to experimental results.

Lead-free NaBixTi6O12.5+1.5x ceramics (x = 0.98, 0.99, 1.01, 1.02) with a pseudo-perovskite structure were prepared via the traditional solid-phase reaction. The structure, dielectric properties and impedance spectroscopy of the NaBixTi6O12.5+1.5x ceramics were investigated in detail. X-ray diffraction patterns reveal that all of the prepared ceramics have a single phase and the scanning electron microscope confirms the well-growth performance of the ceramics. The dielectric constant and dielectric loss show great stability with a wide temperature range. It can be learned that the resistance, the maximum Debye peak, and the corresponding capacitance simultaneously decrease with increasing temperature by analysing Z* plots. The active energy of the NaBixTi6O12.5+1.5x ceramics (x = 0.98, 0.99, 1.01, 1.02) are respectively 0.67, 0.56, 1.34 and 1.37 eV, meanwhile, there is no phase transition point in 25–750 °C. The results demonstrate that the electrical properties of the NaBixTi6O12.5+1.5x lead-free ceramics can be well tuned by varying the Bi quantity.

As one of the representatives of lead-free NBT ceramics, Na[Formula: see text]Bi[Formula: see text]Ti4O[Formula: see text] has still attracted much attention due to its excellent dielectric properties and has become the focus of research. However, its piezoelectric properties are far from satisfactory. In order to improve the piezoelectric properties of Na[Formula: see text]Bi[Formula: see text]Ti4O[Formula: see text], Na[Formula: see text]Bi[Formula: see text]Ti[Formula: see text]MgxOy–BaTiO3 (NBTM–BT) composite ceramics were synthesized by a conventional mixed oxide route and sintered at 1040∘C through two-step method. We optimized the electrical properties of NBTM–BT by changing the stoichiometric ratio of Mg content and studied its microscopic mechanism. The piezoelectric coefficient ([Formula: see text]) is stable at about 20 pC/N. Moreover, the maximum remanent polarization (2[Formula: see text]) of the ceramic is 3.08[Formula: see text][Formula: see text]C/cm2 with the coercive field of 18.01[Formula: see text]kV/cm. The dielectric constant and dielectric loss for Na[Formula: see text]Bi[Formula: see text]Ti[Formula: see text]Mg[Formula: see text]–BT composite ceramic were found to be 486 and 0.17 at 10[Formula: see text]kHz, respectively. The characteristic peaks of BT and Na[Formula: see text]Bi[Formula: see text]Ti4O[Formula: see text] can be observed clearly from the X-ray diffraction analysis. SEM analysis showed that all samples were well crystallized. Consequently, the piezoelectric and ferroelectric properties of Na[Formula: see text]Bi[Formula: see text]Ti4O[Formula: see text]–BT composite ceramic will be enhanced much by Mg doping, which means it has a wider range of applications in electronic devices such as piezoelectric devices.

The bismuth layer-structured Na0.5Bi4.5-xPrxTi4O15 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) (NBT-xPr(3+)) ceramics were fabricated using the traditional solid reaction process. The effect of different Pr3+ contents on dielectric, ferroelectric and piezoelectric properties of Na0.5Bi4.5Ti4O15 ceramics were investigated. The grain size of Pr3+-doping ceramics was found to be smaller than that of pure one, the maximum dielectric constant and Curie temperature T-c gradually decreased with increasing Pr3+ contents, and the dielectric loss decreased at high temperature by Pr3+-doping. Moreover, thktivation energy (E-a) resistivity (Z'),remanent polarization (2P(r)) and piezoelectric constant (d(33)) increased by Pr3+-doping. The NBT-xPr(3+) ceramics with x = 0.3 achieved the optimal properties with the maximum dielectric constant of 1109.18, minimum loss of 0.00822 (250 kHz), E-a of 1.122 eV, Z' of 7.9 k Omega cm (725 degrees C), d(33) of 18 pC/N, 2P(r) of 12.04 mu C/cm(2). The enhancement was due to the addition of Pr3+ which suppressed the decreasing of resistivity at high temperature and made it possible for NBT-xPr(3+) ceramics to be poled in perpendicular direction, implying that it is a great improvement for Na0.5Bi4.5Ti4O15 ceramics in electrical properties.