An electric-field-induced large strain and strong photoluminescence was achieved by introducing trivalent Pr3+ as the activator into 0.92(Bi0.5Na0.5)TiO3 − 0.08(Ba0.90Ca0.10)(Ti0.92Sn0.08)O3 (BNT−8BCST) ceramics. Around a critical composition of 0.4 mol% Pr3+, a large strain of ∼0.39% with a relatively small hysteresis compared with existing lead-free Bi-perovskite ceramics was obtained. In particular, the strain is very resistant to field cycling and thermal shock, giving the materials attractive for its exceptionally good fatigue resistance and high temperature stability. Besides the excellent electrical properties, Pr3+-modified BNT−8BCST host exhibits a strong photoluminescence with a bright red emission at 610 nm assigned to 1D2 → 3H4 transitions of the Pr3+ ions upon a blue light excitation of 400–500 nm. The photoluminescence can be enhanced through poling treatment of the samples. Moreover, samples have a superior water resistance property which almost maintaining the same photoluminescence intensity after 40 h water immersion time. These results suggest the material may have potential application as a multifunctional device such as “on-off” actuator and electric field-controlled photoluminescence devices by integrating its excellent luminescence and electrical properties.

The effects of M2O5 (M = Nb, Ta, Sb) addition on the phase transition and electrical properties of 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 (BNBT6) lead-free ceramics were studied. Results showed that M substitution into BNBT6 induced a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic with a significant disruption of the long-range ferroelectric order. Accordingly, large strain response of 0.42–0.43% with fatigue-free behavior, which is due to a reversible field-induced ergodic relaxor-to-ferroelectric phase transformation, was obtained at 0.5 mol% M content. Furthermore, as the compositions (x ≥ 0.06) lied deep in the pseudocubic region of the phase diagram, a high, purely electrostrictive effect with large electrostrictive coefficient Q33 of 0.020–0.024 m4/C2 was achieved. More interestingly, the obtained BNBT6-based electrostrictors showed good resistance to both long-term electric cycling (up to 106 cycles) and thermal influence (20–160 °C), rendering them suitable for high-precision positioning devices and other actuators.

Lead-free piezoelectric ceramics, (Bi1/2Na1/2)0.935Ba0.065Ti1−x(Fe1/2Nb1/2)xO3, were prepared by the conventional solid-state reaction method. The room temperature ferroelectric P–E loops illustrated the transition of ferroelectric domains. The composition and electric field dependent strain behavior of this system were investigated. A highest unipolar strain of ∼0.422% and corresponding normalized strain, d*33 ( = Smax/Emax) of 844 pm/V under an applied field of 50 kV/cm were observed at x = 0.020, due to the destabilization of the ferroelectric order. It is observed that the unipolar strain of BNBT-0.02FN is temperature insensitive, and the d*33 maintains a high value of ∼600 pm/V at 90 °C. The observed large strain behavior can be attributed to the transformation from the ferroelectric phase at zero electric field into a relaxor ferroelectric phase under an applied electric field. The large strain response with good temperature stability would be quite suitable for environmental-friendly solid-state actuator applications.

Lead-free (Bi1/2Na1/2)0.935+xBa0.065Ti1−x(Pr1/2Nb1/2)xO3 (BNBT–x PN) piezoelectric ceramics were prepared by the conventional solid-state reaction method and the crystal structure and electrical properties were systematically investigated. X-ray diffraction patterns revealed that the (Pr1/2Nb1/2)4+ completely diffused in the BNBT lattice in the studied composition range. An appropriate amount of (Pr1/2Nb1/2)4+ additive enhanced the dielectric, ferroelectric, and piezoelectric properties of BNBT ceramics. The Pr and d33 increased from 29.1 μC/cm2 and 121 pC/N of pure BNT to 32.8 μC /cm2 and 221 pC/N of modified ceramics with x=0.0050, respectively. Furthermore, electric field-induced strain was enhanced to its maximum value (Smax=0.34%) with normalized strain (d33*=Smax/Emax=567 pm/V) at an applied electric field of 60 kV/cm for x=0.0075. These findings have great potential for actuator and multifunctional device applications, which may also open up a range of new applications.

•An enhancement of the field-induced strain with a peak at a value of 0.40% is obtained in this work.•The high strain response of the composition stems from the composition proximity to the ferroelectric-relaxor phase boundary.•This high-strain material is also attractive for its exceptionally good fatigue resistance (up to 106 cycles).High-strain lead-free (Bi0.5Na0.5)1−xBaxTi0.98(Fe0.5Ta0.5)0.02O3 (BNBT100x–2FT) piezoelectric ceramics were designed and fabricated through conventional techniques. Results showed that changes in Ba content of BNBT100x–2FT induced transition from the ferroelectric phase to the ergodic relaxor phase. These changes also significantly disrupted long-range ferroelectric order, thereby correspondingly adjusting the ferroelectric-relaxor transition point TF-R to room temperature. A giant strain of 0.40% (corresponding to a large signal d33∗ of 500 pm/V) was obtained at x = 0.06, approaching to that of lead-based materials. High-strain responses of the ceramic composition originated from the composition proximity to the ferroelectric-relaxor phase boundary. This phenomenon led to reversible transformation between the ergodic relaxor phase and the polar ferroelectric phase under cyclic field. Moreover, the high-strain material exhibited exceptional fatigue resistance (up to 106 cycles) as a result of the reversible field-induced phase transition. The proposed material exhibits potential for novel ultra-large stroke and nonlinear actuators that require enhanced cycling reliability.

•The BCZT ceramics with Pr doped in A-site exhibit optimum electrical properties.•BCZT-A ceramics show large piezoelectric constant d33 of 344 pC/N.•BCZT-A ceramics exhibit strong red emission centered at 649 nm.•The Pr doped in BCZT ceramics are promising lead-free piezoelectric candidates.(Ba0.99Ca0.01)(Ti0.98Zr0.02)O3 (BCZT) and Pr-doped samples (Pr0.002Ba0.988Ca0.01)(Ti0.98Zr0.02)O3 (BCZT-A) as well as 0.998(Ba0.99Ca0.01)(Ti0.98Zr0.02)O3-0.002PrO1.5 (BCZT-W) lead-free piezoelectric ceramics were prepared by conventional solid state reaction method. The structural, dielectric, piezoelectric, ferroelectric and luminescence properties of the Pr-doped BCZT ceramics were systematically studied. The orthorhombic-tetragonal (O–T) phase transition temperature decreased obviously, while Curie temperature (TC) maintained at 118 °C for both of the Pr-doped BCZT ceramics. The BCZT-A ceramics exhibited the best piezoelectric properties (d33 = 344 pC/N) and ferroelectric properties (Pr = 13.042 μC/cm2). Meanwhile, desired luminescence properties (bright reddish-orange light, emission peak centered at 649 nm) were obtained under an excitation of 448 nm at room temperature for BCZT-A and BCZT-W ceramics. The luminescence properties of the BCZT-A ceramics were better than those of the BCZT-W ceramics. Asymmetric structure, coexistence of orthorhombic and tetragonal phases at room temperature and less oxygen vacancies exhibited in BCZT-A ceramics could result in the improved piezoelectric properties and luminescence properties simultaneously. These results suggest that the BCZT-A ceramics have potential application as a multifunctional device by integrating its excellent electrical and luminescence properties.

•The BNBT-x SZMO with appropriated SZMO exhibited optimum electrical properties.•It has a relatively high strain S of 0.388%.•It exhibits high Smax/Emax=597 pm/V and large d33=141 pC/N at x=0.005.•Modified sample has great potential for actuator and multifunctional device applications.The obvious conflicts between large piezoelectricity and high strain could be solved by adding new components in BNT. Moreover, this urgent problem has been solved and induced a large strain as well as a high piezoelectricity in BNT. Large normalized strain (d33*=526–597 pm/V) and high strain (0.342%–0.388%) were achieved at the composition range of 0.0025≤x≤0.0050, suggesting that such a system is a promising lead-free candidate for electromechanical actuator applications. Furthermore, high d33 values of 141–204 pC/N have also been attained at the above composition range due to adding new components.

The (100) oriented and random oriented 0.755Bi0.5Na0.5TiO3–0.065BaTiO3–0.18SrTiO3 (BNT–BT–ST) thin films were deposited on LaNiO3 (LNO) buffered Pt(111)/Ti/SiO2/Si substrates by the sol–gel processing technique. The orientation is controlled by the concentration of solution. The structure, dielectric and piezoelectric properties of the thin films are significantly affected by the crystallographic orientation. The (100) oriented BNT–BT–ST thin film has improved dielectric and piezoelectric properties. For the (100) oriented and random oriented BNT–BT–ST thin films, the dielectric constants are 660 and 550, the dielectric losses are 0.045 and 0.076 and the effective piezoelectric coefficients are 140 and 110 pm/V, respectively. The large piezoelectric response is attributed to the uniform microstructure and increased lattice distortion along (100) direction.

Lead-free ceramics (1−x−y) Bi1/2Na1/2TiO3–xBaTiO3–yY2NiMnO6 (BNT–BT–YNMO–x/y) were prepared by conventional solid-state reaction method and their structure and electrical properties were systematically investigated. The X-ray diffraction patterns showed that all the samples exhibit a pure perovskite phase with rhombohedral structure. All the modified ceramics with a clear grain boundary and uniformly distributed grain size were observed in SEM images. Either increasing x or y content can enhance ferroelectric and piezoelectric properties. The optimum electrical properties can be obtained at x=0.065 and y=0.002, which are as follows: Pr=38.05 μC/cm2, Ec=21.42 kV/cm, d33=219 pC/N and kp=29.5%. The results indicate that the BNT–BT–YNMO–x/y ceramics are promising lead-free piezoelectric candidates for practical applications.

(1 − x)Bi0.5(Na0.80K0.20)0.5TiO3–x(K0.5Na0.5)MO3 (M = Sb, Ta) (BNKT20–KNM100x) lead-free piezoelectric ceramics were designed and fabricated using a conventional fabrication process to achieve large strain response in BKNT20-based ceramics. The KNM substitution was found to induce a transition from ferroelectric to relaxor pseudocubic phase, and such transition is accompanied with the significant disruption of ferroelectric order and the shift of the ferroelectric–relaxor transition temperature TF−R down to room temperature. Accordingly, large electric-field-induced strains of 0.39–0.41% (at 80 kV cm−1, equivalently 488–513 pm−1 V), which are derived from a reversible field-induced ergodic relaxor to ferroelectric phase transformation, were obtained in 1.25 mol% KNM-modified compositions near the phase boundary. Moreover, an attractive property for application as actuators was obtained in the present system, compositions near the phase boundary with an ergodic relaxor state exhibited fatigue-free behavior after 106 cycles. Furthermore, unexpected almost fatigue-free behavior was also observed in 0.5 mol% KNM-modified samples with a typical ferroelectric long-range order. Results of the enhanced activation energy (Ea) for electrical conduction suggest the well-observed fatigue-resistant behavior in the present system should be mainly attributed to the lower defect density. These findings give the current material great opportunity for actuator applications demanding improved cycling reliabilities.

Lead-free piezoelectric ceramics (1 − x)(0.935Bi1/2Na1/2TiO3–0.065BaTiO3)–xAl6Bi2O12 (BNT–BT6.5–xAB, with x = 0–0.020) were prepared using a conventional solid-state reaction method and the crystal structure and electrical properties were systematically investigated. For all BNT–BT6.5–xAB ceramics form the pure perovskite phase structure, SEM-analysis revealed an increase first and then a decrease in the grain size with increasing AB content with no obvious change in grain morphology. Appropriate AB doping into BNT–BT6.5 ceramics induces the enhancement of piezoelectric and ferroelectric properties. Improved Pr = 32.8 μC cm−2, low Ec of 18.2 kV cm−1, and high d33 = 234 pC N−1 were observed at x = 0.005. Furthermore, electric field-induced strain was enhanced to its maximum value (Smax = 0.33%) with normalized strain (d33* = Smax/Emax = 413 pm V−1) at an applied electric field of 80 kV cm−1 for x = 0.010. The enhanced strain can be attributed to the coexistence of ferroelectric and relaxor ferroelectric phases. It is obvious that this piezoceramic is promising candidate for lead-free piezoceramics and can be used in practical applications.

The structure, electric field-induced strain (EFIS), polarization and piezoelectric response of lead-free Sr3FeNb2O9-modified (Bi1/2Na1/2)0.935Ba0.065TiO3 (BNBT–xSFN, with x = 0–0.012) ceramics were investigated. XRD patterns show that all compositions have a pure perovskite structure and SFN effectively diffused into the BNBT lattice during sintering to form a solid solution. A large EFIS of 0.35% was obtained at the critical composition of x = 0.009 which corresponds to a normalized strain (Smax/Emax) of 583 pm V−1. A maximum value of piezoelectric constant (254 pC N−1) was obtained for x = 0.006. These results show our research can benefit the developments of Bi1/2Na1/2TiO3 ceramics and widen their range of applications.

Multifunctional Ba1−xTi0.96Sn0.04O3 + x mol% Er (BST-Er) ceramics were prepared using a solid state reaction method. The large piezoelectric coefficient (d33 = 400 pC N−1) and field induced strain (S = 0.175%) were obtained for the BST-Er ceramics at x = 0.5. The polymorphic phase transitions (PPT) from the orthorhombic phase to the tetragonal phase, which contributes to the high piezoelectricities, was identified at room temperature (RT). There are bright green emission bands centered at 520 and 550 nm in up-conversion (UC) luminescence spectra measured under 980 nm laser excitation, which correspond to the radiative transitions from 2H11/2 and 4S3/2 to 4I15/2, respectively. The fluorescence intensity ratio of green UC emissions at 520 and 550 nm was investigated in the temperature range of 233–413 K. The maximum sensing sensitivity was found to be 0.0033 K−1. The results reveal that the BST-Er ceramics with simple composition are promising multifunctional sensing materials.

•The BNT–BT–xBNMO with appropriated BNMO exhibited optimum electrical properties.•It has a relatively high remnant polarization Pr of 31 μC/cm2.•It exhibits high planar electromechanical coefficient kp of 31%.•It shows large piezoelectric constant d33 of 229 pC/N.•The modified BNT–BT ceramics are promising lead-free piezoelectric candidates.The 0.935Bi1/2Na1/2TiO3–0.065BaTiO3–xmol⋅Bi2NiMnO6(x = 0–0.008) (abbreviated as BNT–BT6.5–xBNMO) lead-free piezoelectric ceramics were fabricated by conventional solid-state reaction method and the effects of BNMO addition on microstructure and electrical properties of the ceramics were investigated. Results show that all samples have formed dense structures with a large relative density > 95%. X-ray diffraction (XRD) patterns show all compositions had a pure perovskite structure, suggesting BNMO effectively diffused into the BNT–BT6.5 lattice to form a solid solution. SEM images indicate that BNMO modified ceramics have a clear grain boundary and a uniformly distributed grain size. The measurements of electrical properties reveal that the electrical properties of BNT–BT–xBNMO ceramics have been greatly improved by certain amount of BNMO substitutions. At room temperature, the BNT–BT–xBNMO ceramics with appropriated BNMO exhibited optimum ferroelectric and piezoelectric properties with a relatively high remnant polarization Pr of 31 μC/cm2, high planar electromechanical coefficient kp of 31% and large piezoelectric constant d33 of 229 pC/N, respectively. These results indicate that the modified BNT–BT6.5 ceramics are promising lead-free piezoelectric candidates for practical applications.

•KNN substitution into BNKT100y tends to enhance the energy storage density of the ceramics.•A large energy storage density value of 1.20 J/cm3 at 100 kV/cm is achieved in BNKT100y–xKNN (y=0, x=0.16) samples.•BNKT100y–xKNN (y=0, x=0.16) samples are attractive for their fatigue-free behavior and good temperature stability.In this work, we designed a series of compositions within peudocubic region based on ternary (1−x)[(1−y)(Bi0.5Na0.5)TiO3–y(Bi0.5K0.5)TiO3]–x(K0.5Na0.5)NbO3 (BNKT100y–xKNN) system for energy storage applications. Results showed that the KNN substitution into BNKT100y induced a significant disruption of the ferroelectric order, and tended to enhance the energy storage density of the ceramics. With the external electric field, the energy storage density increased drastically, and a maximum value of 1.20 J/cm3 at 100 kV/cm was achieved in BNKT100y–xKNN (y=0, x=0.16) samples. Furthermore, BNKT100y–xKNN (y=0, x=0.16) ceramics not only exhibited high energy density but also possessed fatigue-free behavior and temperature-independent characteristic. Temperature-dependent structural analysis suggested that the good energy-storage properties insensitive to temperature can be ascribed to the stable relaxor pseudocubic (antiferroelectric-like) phase over a wide temperature range. These results indicate that BNKT100y–xKNN system should be a promising lead-free material for energy-storage capacitor application.

•Enhanced temperature stability was obtained in the range from 20 to 100 °C.•TO–T shifted to low temperature while Tc shifted conversely with the increase of Er.•Bright upconversion luminescence was obtained under excitation (980 nm).Lead-free (Ba0.99Ca0.01)(Ti0.98Zr0.02)O3 (BCZT) + x mol% (x = 0–0.8) Erbium (Er) ceramics were prepared using the solid-state reaction technique. Dielectric, ferroelectric, piezoelectric and photoluminescence properties were systematically studied. Greatly enhanced temperature stabilities of the piezoelectric properties were obtained in the temperature range from 20 to 100 °C. Broad ferroelectric phase (orthorhombic–tetragonal) transition (TO–T) tuned by Er doping was deduced from the dielectric properties. With the increase of Er content, the TO–T shifted towards low temperature, while Curie temperature (TC) shifted towards high temperature. Furthermore, bright green (525 and 550 nm) and weak red (670 nm) emission bands were obtained under excitation (980 nm) at room temperature, corresponding to the transitions of 2H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively. As multifunctional material, Er-doped BCZT ceramics show great potential in mechano-electro-optic integration and coupling device applications.

Lead-free (Ba0.9Ca0.1)(Ti1−xSnx)O3 (BCTS) (x=0.02–0.1) ceramics were fabricated using conventional solid state reaction method. The effects of Sn substitution on structure and electrical properties of the BCTS ceramics were researched. At room temperature, a polymorphic phase transition (PPT) from tetragonal phase to rhombohedral phase is identified from XRD patterns with increasing Sn content. High piezoelectric coefficient of d33=405 pC/N and planar mode electromechanical coupling factor of kp=43.2% are obtained for the sample at x=0.06. The ceramic for x=0.06 also shows uniform microstructure that contributes to the excellent electrical properties. These results indicate that this BCTS system with optimal composition is a promising lead-free piezoelectric material.Highlights► We achieve enhanced piezoelectricity in (Ba0.9Ca0.1)(Ti1−xSnx)O3 ceramics. ► We used the different functions of Sn4+ and Ca2+ to enhance properties. ► Our work could promote the research of (Ba,Ca)(Ti,Sn)O3 systems.

In present study, pyrochlore-free 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 (0.67PMN–0.33PT) powders and ceramics have been successfully prepared. Using oxides as raw materials, pyrochlore-free 0.67PMN–0.33PT powders were obtained by two-step particle-coating method. The XRD and EDS results confirmed that the Mg–citric acid polymeric complex coatings effectively prevent the direct contact between PbO and Nb2O5 and thus avoid the formation of pyrochlore phase. The obtained pyrochlore-free 0.67PMN–0.33PT powders at 800 °C showed uniform and even grain size. The 0.67PMN–0.33PT ceramics sintered at 1150 °C for 2 h exhibited 99% of relative density and a piezoelectric coefficient (d33) of 576pC/N, a remnant polarization (Pr) of 28.4 μC/cm2, a planar electromechanical coupling factor (kp) of 0.55 and a mechanical quality factor (Qm) of 90.Highlights► Pyrochlore-free PMN–PT powders were obtained by two-step particle-coating method. ► Mg–citric acid polymeric complex coatings avoid the formation of pyrochlore phase. ► Pyrochlore-free PMN–PT powders have been successfully prepared at 800 °C. ► The PMN–PT ceramics sintered at 1150 °C exhibited excellent electrical properties.

Lead-free (Ba0.93Ca0.07)(Ti0.95Zr0.05)O3 (BCZT) ceramics were prepared using a solid-state reaction technique. The structure and electrical properties were investigated with a special emphasis on the influence of sintering temperature. Crystalline structures and microstructures were analyzed by X-ray diffraction and scanning electron microscope (SEM) at room temperature. The BCZT ceramics sintered at 1450 °C show the highest densification and exhibit excellent piezoelectric properties of high piezoelectric coefficient d33 = 387 pC/N, planar mode electromechanical coupling coefficient kp = 44.2%, mechanical quality factor Qm = 140 and Curie temperature Tc = 108 °C.

La2O3 (0–0.8 wt.%)-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 (abbreviated as BNBT6) lead-free piezoelectric ceramics were synthesized by conventional solid-state reaction. The influences of La2O3 on the microstructure, the dielectric, ferroelectric and piezoelectric properties of the composites were investigated. X-ray diffraction (XRD) patterns indicate that 0.2-0.8 wt.% of La2O3 has diffused into the lattice of BNBT6 ceramics. Consequently, a pure perovskite phase is formed. SEM images show that the microstructure of the ceramics is changed with the addition of a small amount of La2O3. The temperature dependence of the relative dielectric constant shows that Curie point decreases with the increase of La2O3. At room temperature, the ceramics doped with 0.6 wt.% La2O3 show superior performance with high piezoelectric constant (d33 = 167 pC/N), high planar electromechanical coupling factor (kp = 0.30), high mechanical quality factor (Qm = 118), high relative dielectric constant (εr = 1470) and lower dissipation factor (tanδ = 0.056) at a frequency of 10 kHz.

(K0.5Na0.5)NbO3 powders and ceramics were prepared by a novel hybrid method of sol–gel and ultrasonic atomization, in which Nb2O5 was used as the niobium source to replace those expensive soluble niobium salts. X-ray diffraction and thermal analysis were performed to investigate the synthesis process and phase transformation behavior of (K0.5Na0.5)NbO3 powders. The results showed that (K0.5Na0.5)NbO3 powders with a reasonably fine particle size and single-phase perovskite structure were formed at a temperature as low as 650 °C. Dense (K0.5Na0.5)NbO3 ceramics with a relative density of 93% were obtained using the refined powders. The (K0.5Na0.5)NbO3 ceramics prepared by the novel hybrid method exhibited relatively good properties (d33 = 90 pC/N, kp = 0.32, Pr = 20.6 μC/cm2, Tc = 405 °C, εr = 712), suggesting that this novel hybrid method might be a promising method for the powders and ceramics preparation.

Sm2O3 (0–0.7 wt.%)-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 (abbreviated as BNBT6) lead-free piezoelectric ceramics were synthesized by conventional solid-state processes. The effects of Sm2O3 on the microstructure, dielectric properties and piezoelectric properties were investigated. X-ray diffraction (XRD) patterns show that 0.1–0.7 wt.% of Sm2O3 can diffuse into the lattice of BNBT6 ceramics forming pure perovskite phase. The SEM images indicate that the microstructure can be affected obviously by a small amount of added Sm2O3. The temperature dependence of the relative dielectric constant shows that Curie point decreases with the increase of Sm2O3. At room temperature, the ceramics doped with 0.3 wt.% Sm2O3 show quite good performance with excellent piezoelectric properties (piezoelectric constant d33 of 202 pC N−1, planar coupling factor kp of 0.30 and mechanical quality factor Qm of 101) and improved dielectric properties (relative dielectric constant ɛr of 2018 and dissipation factor tan δ of 0.056). Moreover, all BNBT6 − x (wt.%) Sm2O3 ceramics exhibit a typical relaxor behavior with diffuse phase transition characteristics and the degree of ferroelectric relaxation behavior is enhanced with the increase of Sm2O3 content.

Lead-free (Ba1 − xCax)(Ti0.98Zr0.02)O3 (x = 0–0.04) ceramics were prepared successfully using a solid-state reaction technique. The polymorphic phase transitions (PPT) from orthorhombic to tetragonal phase around room temperature were identified in the composition range of 0 < x < 0.03. High piezoelectric coefficient of d33 = 375 pC/N and planar electromechanical coupling factor of kp = 44.1% were obtained for the samples at x = 0.01. With the increase of Ca content, the orthorhombic–tetragonal phase transitions shifted towards room temperature, while relative high Curie temperature (TC) was still maintained about 115 °C.

(1 − x)(K0.5Na0.5)0.94Li0.06NbO3-x(Bi0.5Na0.5)TiO3 (KNLN6-xBNT) lead-free piezoelectric ceramics with dense microstructures and good temperature stability were prepared by conventional sintering. The effects of the BNT on the structure, electrical properties, and temperature stability of the ceramics were studied. The results showed that BNT doping shifted the polymorphic phase transition below room temperature and significantly improved the temperature stability of KNLN6 ceramics. It was found that the KNLN6 ceramics doped with 3 mol.% BNT exhibited good temperature stability in the temperature range of 25°C to 180°C, as well as good electrical properties (d33 = 152 pC/N, kp = 37.1%, Qm = 108, εr = 634, tanδ = 0.018 and Tc = 415°C), suggesting that this material should be an attractive lead-free material for piezoelectric applications.

Nd2O3 (0–0.8 wt.%)-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 (abbreviated as BNBT6) lead-free piezoelectric ceramics were synthesized by conventional solid-state processes. The effects of Nd2O3 on the microstructure, the dielectric, ferroelectric and piezoelectric properties were investigated. X-ray diffraction (XRD) data shows that Nd2O3 in an amount of 0.2–0.8 wt.% can diffuse into the lattice of BNBT6 ceramics and form a pure perovskite phase. SEM images indicate that all the modified ceramics have a clear grain boundary and a uniformly distributed grain size, and the BNBT6 ceramics doped with Nd2O3 become denser. The temperature dependence of the relative dielectric constant shows that Curie point does not change obviously with the addition of Nd2O3. At room temperature, the BNBT6 ceramics doped with 0.4 wt.% Nd2O3 show quite good performance with excellent piezoelectric properties (piezoelectric constant d33 of 175 pC/N, planar coupling factor kp of 0.31 and mechanical quality factor Qm of 118) and enhanced dielectric properties (relative dielectric constant ɛr of 1947 and dissipation factor tan δ of 0.057). Moreover, all BNBT6 − x (wt.%) Nd2O3 ceramics exhibit a typical relaxor behavior with diffuse phase transition characteristics.

Lead-free relaxor ceramics with composition (Na0.5K0.5)1−3xLaxNb0.95Ta0.05O3 (KNTN-Lax) were prepared by the conventional solid-state route. The ferroelectric relaxor behavior and dielectric properties of KNTN-Lax ceramics were systemically investigated. X-ray diffraction analysis showed that the crystal structure of the ceramics changed from orthorhombic to pseudo-cubic with increasing La-doping level. Dielectric study revealed that KNTN-Lax ceramics underwent a transition from a normal ferroelectric to relaxor-like ferroelectric. Furthermore, P–E hysteresis loops, which were of slim-loop type with small remnant polarization values in the composition range of 0.015 ≤ x ≤ 0.03 confirmed the ferroelectric relaxor behavior of KNTN-Lax ceramics. The samples of KNTN-Lax ceramics in the composition range from x = 0 to 0.015 exhibited good electrical properties, piezoelectric constant d33 of 98–119 pC/N and electromechanical coupling coefficients kp of 33.2–37.0%, respectively. The results show that the series of KNTN-Lax ceramics is one of the promising lead-free materials for electromechanical transducer applications.

The effects of a small amount MnO2 (0–1.20 mol%) doping on the phase transition behavior, microstructure and electrical properties of (K0.5Na0.5)0.94Li0.06NbO3 (KNLN6) ceramics have been investigated. X-ray diffraction analysis showed that the ceramics underwent a perovskite structure transition from tetragonal to orthorhombic symmetry with increasing the MnO2 content, which is similar to the morphtropic phase boundary (MPB). SEM and density studies revealed that a small amount of MnO2 improved effectively the densification of the KNLN6 ceramics. Because of the high densification, the mechanical quality factor Qm of the KNLN6 ceramics were significantly enhanced by MnO2 addition which meant MnO2-doping changed the KNLN6 to “hard” ceramics. For 1.20 mol% MnO2-doped KNLN6 ceramics, the mechanical quality factor Qm reached up to 301, which increased by four times as compared with KNLN6. The piezoelectric constant d33 and the planar electromechanical coefficient kp still maintained relatively high levels, which were about 141 pC/N and 32.0%, respectively.