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The lead-free x(Bi0.5K0.5)TiO3-0.23BaTiO3-0.02Bi(Mg0.5Ti0.5)O3-(0.75-x)BiFeO3 piezoelectric ceramics (x = 0.05, 0.07, 0.09, and 0.11) were prepared by solid-state reaction method. The increase of (Bi0.5K0.5)TiO3 concentration revealed a decrease of dielectric maximum temperature (Tm), however, the values were higher than 420 °C. The Rietveld structure refinement for each composition showed pseudo-cubic symmetry and the rhombohedral distortion (90 - αF) was gradually decreased from 0.27° to 0.09° for x = 0.05 and 0.11, respectively. The saturation and remanent polarization were increased and a coercive field was decreased, while the critical exponent (γ) was increased from 1.407 (x = 0.07) to 1.821 (x = 0.11) revealing more relaxor-like behavior with higher (Bi0.5K0.5)TiO3 content. The remanent polarization, coercive field, and piezoelectric strain constant of 31.9 μC/cm2, 23.6 kV/cm, and 315 pm/V, respectively, obtained in the composition with x = 0.11 are significantly higher than those reported previously for other Bi-based piezoelectric ceramics with Tm>250 °C.

A large piezoelectric response in ferroelectric ceramics is typically associated with extrinsic contributions from ferroelectric domain structures. However, such domain structures cannot be expected in systems with pseudo-cubic symmetry. In this study, we determine the mechanism of significant piezoelectricity and ferroelectricity in 0.3BaTiO3–0.1Bi(Mg1/2Ti1/2)O3–0.6BiFeO3 ceramic with a perovskite-type pseudo-cubic symmetry. Synchrotron radiation X-ray diffraction reveals that the Bi ions in this ceramic essentially prefer to be off-centered at six sites by approximately 0.4 A, in the cubic directions. A phase transition occurs at TC ~725 K. However, the crystal seems to present a cubic symmetry even at room temperature. The large piezoelectric response is caused by the combinational partial ordering of the off-centered Bi ions, adapted to any direction of the applied electric field to the ceramic grains. The proposed mechanism for the emergence of a high polarization in the above system will enable designing novel Pb-free ceramics by controlling the fluctuated and off-centered ions under an applied electric field. Strong piezoelectricity is typically due to reorientation of ferroelectric domains and is not expected in cubic systems. Here, the strong piezoelectricity of pseudo-cubic 0.3BaTiO3–0.1Bi(Mg1/2Ti1/2)O3–0.6BiFeO3 is explained in terms of partial ordering of Bi ions adapted to any applied field direction.

0.3BaTiO3-0.1Bi(Mg1/2Ti1/2)O3-0.6BiFeO3 ceramics were fabricated by conventional solid-state reaction. Piezoelectric and ferroelectric properties were measured for as-sintered ceramic and ceramics quenched from temperatures of 600, 700, and 800 °C. The crystal system of the ceramics was pseudo-cubic in the as-sintered state and all the quenched states. Pinched P-E hysteresis loops were observed in the as-sintered sample. Ferroelectric P-E hysteresis loops and S-E curves were observed for the samples quenched from 700 and 800 °C. The electrical properties were enhanced with increasing quenching temperature, which could be attributed to an improved homogeneity of defect dipoles.

The origin of the significantly large piezoelectricity and ferroelectricity was investigated in the 0.67BiFeO3–0.33BaTiO3 ceramics. The crystal structure of 0.67BiFeO3–0.33BaTiO3, identified using the Rietveld method, suggested a cubic structure at 950 K and a rhombohedral structure at 300 K with small lattice distortion 89.94(4)°, which is close to 90° of cubic structure. Despite the small lattice distortion at 300 K, significantly large piezoelectricity and ferroelectricity was observed in the 0.67BiFeO3–0.33BaTiO3, which is associated Bi ion off-centering. The Bi3+ ions were off-centered from the corner of the Ba2+ position along the 〈100〉 directions at 300 K, occupying three equivalent neighboring sites in the [100], [010], and [001] directions. The occupancies of Bi ions could be changed, and partially ordered along the applied electric field direction. The off-centered Bi ions are attributed to the ferroelectricity and piezoelectricity in non-centrosymmetry structure with a small lattice distortion, i.e., pseudo-cubic structure, due to partial ordering under the electric field.

Electrophoretic deposition (EPD) process has certain advantages such as it can be applied for a mass production and also can be combined with magnetic crystal alignment technique. In this work, we prepared lead-free 85(Bi0.5Na0.5)TiO3–15BaTiO3 (85BNT–15BT) piezoelectric ceramics by conventional uniaxial pressing and EPD process. Various conditions were optimized such as suspension media, applied electrical field, and deposition time in order to yield dense green ceramics of 85BNT–15BT composition using EPD process. 85BNT–15BT ceramics prepared using EPD process revealed the Curie temperature of about 250 °C, coercive field of about 30 kV/cm, and piezoelectric constant (d 33) of 75 pC/N. The EPD-processed samples exhibited structural and electrical properties similar to that of the conventionally processed one suggesting the successful fabrication of 85BNT–15BT piezoelectric ceramics by EPD method without composition deviation. This study lays a foundation on the fabrication of Bi-based lead-free piezoelectric ceramics by an alternative route other than the conventionally practiced solid-state reaction method maintaining the similar chemical composition, moreover, leaving a large space to explore more in the future.

The structure of lead-free (1-x)(Bi0.5Na0.5)TiO3-xSrTiO3 (BNT-STx) ceramics was analyzed by the Rietveld method, using X-ray diffraction and neutron scattering data. The structural refinement results suggest that the crystal structure successively changes with SrTiO3 concentration, x, from the rhombohedral phase (x = 0.00) to rhombohedral and tetragonal (x = 0.10–0.30), tetragonal and cubic (x = 0.40–0.60), and finally cubic (x = 0.80–1.00) phases. Correlation between the charge sensor constant (d33) and the weighted off-center value (dw) was observed, which may be attributed to the increased dipole motion in the unit cell due to an increased tendency to respond to external stimulation. Furthermore, an improved charge sensor constant (d33) of 140 pC/N was observed for BNT-ST0.20, and a large strain of 0.25% and a d33* value of 443 pm/V were observed from x = 0.30.

The leakage current mechanism in the as-sintered and quenched 0.75BiFeO3–0.25BaTiO3 (0.75BF–0.25BT) ceramics is evaluated by the space-charge-limited current (SCLC), Poole–Frenkel (P–F) emission, Schottky (ST) emission, and Fowler–Nordheim (F–N) tunneling mechanism. The discrepancy observed in the optical dielectric constant of BF and BT between the reported value and the values calculated from the P–F and ST emission plots suggests that the P–F emission and ST emission mechanisms do not contribute to the leakage current behavior of the 0.75BF–0.25BT ceramics. Subsequently, F–N tunneling is observed under a high electric field in the as-sintered 0.75BF–0.25BT ceramics, whereas the direct tunneling effect is exhibited throughout the measured electric field in the quenched ceramics. The SCLC mechanism is dominant in both the as-sintered and quenched 0.75BF–0.25BT ceramics. A change from Ohmic conduction to trap-filled-limit conduction is observed with an increase in the applied electric field in the as-sintered ceramics, whereas the quenched ceramics only revealed Ohmic conduction over the entire range of the measured electric field. The different behaviors exhibited in the SCLC mechanism between the as-sintered and the quenched ceramics imply that the conduction mechanism can be controlled by heat treatment.

A bismuth ferrite and barium titanate solid solution compound can achieve good piezoelectric properties with a high Curie temperature when fabricated with low-temperature sintering followed by a water-quenching process, with no complicated grain alignment processes performed. By adding the super-tetragonal bismuth gallium oxide to the compound, the piezoelectric properties are as good as those of lead zirconate titanate ceramics.

Lead-free ceramics with compositions of (1−x)[0.67Bi1.05FeO3-0.33BaTiO3]-xBaZrO3 (x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05) [BF-BT-BZx] have been prepared through a conventional solid-state reaction method. The effects of BZ substitution on the crystal structural, microstructural, piezoelectric, and electrical properties of the ceramics were investigated. The X-ray diffraction patterns revealed that all ceramics were formed with a mixed structure of rhombohedral and tetragonal perovskite phases. For x = 0.03, good ferroelectric and piezoelectric properties were observed: 2P r = 43 μC/cm2 and 2E c = 61 kV/cm. The static- and dynamic-piezoelectric constants were observed to be 52 pC/N and 330 pm/V, respectively.

Lead-free polycrystalline (K0.48Na0.52)1+x(Nb0.55Ta0.45)O3 (x = 0, 0.01, 0.02 and 0.03) thin films have been grown on Pt(111)/Ti/SiO2/Si substrates using KrF excimer laser ablation to investigate the effects of A-site ion excess. The film at x = 0.01 exhibited good ferroelectric hysteresis loop with a remnant polarization 2Pr of 30 μC/cm2 and coercive field 2Ec of 61 kV/cm. The piezoelectric coefficient d33,f of the KNNT film with x = 0.01 was found to amount to 79 pm/V. These improved ferroelectric and piezoelectric properties were attributed to the excess of A-site Na and K ions.

Mn-modified 0.67BiFeO3-0.33BaTiO3 (BF-BT) bulk ceramics were prepared via a solid-state reaction process using quenching method. The X-ray diffraction peaks shifted toward higher angles as increasing Mn content. Ferroelectric properties were improved by a small amount of Mn content. Leakage current density of 1 mol% Mn-modified ceramic showed the lowest leakage current density, 9.74 × 10−7 A/cm2 at 30 kV/cm. The value of small- and large-field piezoelectric coefficients (d 33 and d 33 *) were 135 pC/N and 250 pm/V, respectively, in 1 mol% Mn-modified BF-BT ceramic. A small amount of Mn content was effective to improve electrical properties and sintering behavior in Mn-modified BF-BT system.

The role of thermal annealing treatment in the enhancement of the piezoelectric and ferroelectric properties of 0.3BaTiO3–0.1Bi(Mg1/2Ti1/2)O3–0.6BiFeO3 (0.3BT–0.1BMT–0.6BF) lead-free piezoelectric ceramics was investigated. Mechanical stress induced by ceramic processing such as polishing and cutting could severely affect the electrical properties of 0.3BT–0.1BMT–0.6BF lead-free piezoelectric ceramics. In order to explore the impact of mechanical processing on their electrical properties, polished and cut samples were thermally annealed to remove damaged surface layers. It was found that the remanent polarization (P r; 27.27 μC cm−2) and large-field piezoelectric coefficient (d 33*; 334 pm V−1) for the thermally annealed sample were remarkably enhanced in comparison to those of the polished sample (P r of 17.57 μC cm−2 and d 33* of 271 pm V−1). This significant enhancement of piezoelectric and ferroelectric properties in thermally annealed ceramics, without any considerable change in grain size, suggests a major contribution of thermal annealing to the removal of damaged surface layers.

0.935(Bi0.5Na0.5)TiO3-0.065BaTiO3 (BNT-BT) thin films have been grown on Pt(111)/Ti/SiO2/Si substrates by using a pulsed laser deposition method. Deposition conditions of oxygen pressure and deposition temperature were changed. The BNT-BT thin film deposited at 750 °C and 300 mTorr exhibited a slim hysteresis loop with a remnant polarization (2P r ) of 11 µC/cm2 and a low leakage current density of 7.3 × 10−5 A/cm2 at 98 kV/cm. The piezoelectric responses of the thin films were investigated using piezoelectric force microscopy with a lock-in amplifier. The thin films exhibited piezoelectric properties with a d 33,f value of 168 pm/V.

Effects of heat-treat procedure and sintering temperatures were investigated in 0.67Bi1.05FeO3-0.33BaTiO3ceramics. The structure, dielectric, ferroelectric, and piezoelectric properties of ceramics made by normal cooling or quenched cooling process were investigated. All ceramics show single pseudo-cubic perovskite crystal structure. Although the structure was not changed but other properties were much changed by heat-treatment. The normally cooled ceramic showed ‘pinched’ ferroelectric hysteresis loops with remanent polarization value of about 10 μC/cm2. On the contrary, the quenched ceramic showed ‘normal’ ferroelectric hysteresis loops with remanent polarization value of about 17 μC/cm2. The piezoelectric constant d 33value of 13 pC/N in normally cooled ceramic was improved to 39 pC/N in quenched ceramic. These improved results were attributed to the reduced populations of defects made during the heat-treatments.

The effects of Ta substitution for B-site Nb in (Na 0.53 K 0.47 )(Nb 1− x Ta x )O 3 (NKNT) ceramics were investigated in the range of x = 0–0.6. It was found that polymorphic phase transitions (PPT) were significantly influenced by Ta substitution. Transitions among orthorhombic, tetragonal, and cubic phases in sequence with temperature, T O-T and T C , respectively, decreased linearly with x . At x = 0.45, T O-T was reduced to room temperature from 182 °C at x = 0, and subsequently piezoelectric coefficient (d 33 ) at room temperature was enhanced up to 284 pC/N from 120 pC/N at x = 0 due to the coexistence of ferroelectric orthorhombic and tetragonal NKNT phases. With x further increasing beyond x = 0.45, d 33 decreased due to there being no orthorhombic but only a tetragonal NKNT phase at room temperature with T O-T below room temperature.

Mechanical damage and stress caused by polishing and cutting of ferroelectric materials could deteriorate their dielectric, ferroelectric, and piezoelectric properties. BaTiO3 piezoelectric ceramics were polished and cut, and thermally annealed at 600, 800, 1000, and 1200 °C for recovery. The electrical measurements revealed a reduction in the coercive field, elimination of imprint behavior, and an increase in saturation polarization in the thermally annealed ceramics. Furthermore, the electromechanical properties measured by the resonance method, prior to thermal annealing were d33 = 184 pC/N, k33 = 0.36, and s33E = 10.9 × 10−12 m2/N, while the values after 1200 °C-4 h thermal annealing were d33 = 287 pC/N, k33 = 0.54, and s33E = 12.1 × 10−12 m2/N. The in situ crystal structure evaluation and Williamson-Hall analysis suggested the decrement of microstrain from 6.38 × 10−4 to 5.20 × 10−4, indicating the retrieval of the residual stress imposed. The enhancement of piezoelectricity by 56% was ascribed to the effective annealing out of the surface mechanical damaged layers.Mechanical damage and stress caused by polishing and cutting of ferroelectric materials could deteriorate their dielectric, ferroelectric, and piezoelectric properties. BaTiO3 piezoelectric ceramics were polished and cut, and thermally annealed at 600, 800, 1000, and 1200 °C for recovery. The electrical measurements revealed a reduction in the coercive field, elimination of imprint behavior, and an increase in saturation polarization in the thermally annealed ceramics. Furthermore, the electromechanical properties measured by the resonance method, prior to thermal annealing were d33 = 184 pC/N, k33 = 0.36, and s33E = 10.9 × 10−12 m2/N, while the values after 1200 °C-4 h thermal annealing were d33 = 287 pC/N, k33 = 0.54, and s33E = 12.1 × 10−12 m2/N. The in situ crystal structure evaluation and Williamson-Hall analysis suggested the decrement of microstrain from 6.38 × 10−4 to 5.20 × 10−4, indicating the retrieval of the residual stress imposed. The enhancement of piezoelectricity by 56% was ascribed to th...

The crystal structures and electrical properties of lead-free BiFeO3-BaTiO3 [(1-x)BFxBT] piezoelectric system are investigated as a function of BaTiO3 concentration. The well-saturated P-E hysteresis loop was observed in the 0.80BiFeO3–0.20BaTiO3 composition system, while a less hysteretic strain-electric field curve was exhibited by the 0.70BiFeO3–0.30BaTiO3 with a strain hysteresis of 16%, the value comparable to PZT-based piezoelectric ceramics. The crystal structures investigated under the synchrotron radiation X-ray diffraction exhibited a rhombohedral structure for BFBT system with x = 0.10–0.25 and a pseudo-cubic structure for BFBT system with x = 0.30–0.40. The structural phase diagram for the BiFeO3-BaTiO3 system is suggested based on the results of temperature-dependent synchrotron radiation X-ray diffraction measurement and investigated electrical properties.

The structural and electrical properties of the virgin, annealed, and quenched samples of 0.80BiFeO3–0.20BaTiO3 lead-free ceramics were investigated. The crystal structures investigated with the structure refinement method exhibited a rhombohedral structure for all the samples. The significant enhancement of ferroelectric and piezoelectric properties is ascribed to the relaxation of the lattice strain induced in the samples after the heat treatment. The bond-length calculated from the final structure refinement results revealed the increment of the A-O bond-length after the heat treatment, while the B-O bond-length was maintained in all the virgin, annealed, and quenched samples suggesting the specific influence of the lattice strain at the A-site of the material system.