Considering the environment and human health, highly transparent lead-free (1 − x)K0.5Na0.5NbO3–xCa(Zn1/3Nb2/3)O3 (abbreviated as KNN–xCZN) piezoceramics were synthesized by a solid state reaction method. The effects of Ca(Zn1/3Nb2/3)O3 on the microstructures, phase structures, characteristic frequency and electrical properties were investigated. Fine grain and relaxor-like behaviors were obtained, and the Ca(Zn1/3Nb2/3)O3 also increased the crystal structure of the ceramics transformation from orthorhombic to pseudo-cubic. It was found that the highest transmittance of 70.42% (with a thickness of 0.5 mm in the visible spectrum) was obtained for the KNN–xCZN ceramics at x = 0.07, which was due to the high relative density, low apparent porosity, uniform and fine-grained microstructures, high symmetry of the pseudo-cubic structure, NbO6 octahedron reduced distortion and more relaxor-like behavior. In addition, the transparent piezoelectric ceramics KNN–xCZN with x = 0.07 possessed high transmittance (∼84% at near-infrared wavelengths) and excellent electrical properties (er = 1162, d33 = 102 pC N−1). All of the above demonstrated that the KNN–xCZN ceramics could be promising lead-free transparent piezoceramics.

Lead-free (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) ceramics with excellent electrical properties were successfully synthesized by a molten salt method (MSS). The submicron BCZT powders with pure perovskite phase were obtained by adjusting the KCl-NaCl content that was used as the eutectic salt. The effects of salt content and reaction temperature on the structure and properties of the BCZT materials were systematically investigated. Comparing with BCZT ceramics prepared by solid state method (SS), the reaction temperature of BCZT ceramics synthesized by MSS decreased approximately 200 °C. Moreover, BCZT ceramics sintered at 1360 °C with 50% eutectic salt showed the most outstanding electrical properties, which are as follows: d33 = 604 pC/N, kp = 57%, Ps = 17.11 µC/cm2, Pr = 9.98 µC/cm2, εm = 15872, εr = 2654 and tan δ = 0.013. In addition, this work revealed a possible reaction course processes and mechanism about MSS. The results provide a new design to optimize the performance of BCZT lead-free piezoelectric ceramics.

Bi2/3Cu3Ti4O12 (BCTO) ceramics were successfully prepared by traditional solid-state reaction method (BCTO-SS) and sol–gel method (BCTO-SG). Pure perovskite phase and dense structure were obtained in BCTO ceramics prepared by both methods. BCTO-SG ceramics showed a large dielectric constant of ~1.1×104 while BCTO-SS ceramics exhibited a low dielectric constant of ~3200. At 100 kHz, the dielectric constant of BCTO-SS ceramics decreased with applied voltage increasing, while the dielectric constant of BCTO-SG ceramics increased with applied voltage increasing. Further study of the relationship between dielectric constant and voltage suggested BCTO-SG ceramics had larger defect concentration than BCTO-SS ceramics. The investigation of complex impedance indicated that the electrical properties of grain boundaries for all BCTO ceramics were evidently affected by applied voltages and the electrical properties of grains were independent of applied voltages. In addition, the non-Ohmic properties of BCTO ceramics were studied in detail. The non-linear coefficients of BCTO-SS and BCTO-SG ceramics were 1.65 and 1.01, respectively. The breakdown electric fields of BCTO-SS and BCTO-SG ceramics were found to be 1.21 and 0.48 kV/cm, respectively. The potential barrier heights of BCTO-SS and BCTO-SG ceramics were calculated to be 0.549 and 0.485 eV, indicating that the potential barriers at the grain boundaries for BCTO-SS and BCTO-SG ceramics are the Schottky-type barrier.

Lead-free (1 − x)wt.% (Ba0.85Ca0.15)(Zr0.10Ti0.90)O3 − x wt.% (Ba(Cu0.5W0.5)O3 (BCZT − x wt.% BCW) piezoelectric ceramics were prepared by solid-state reaction. The phase structure of the ceramics showed that the orthorhombic–tetragonal phase boundary was identified in the range of 1.2 ≤ x ≤ 1.6 wt.%. The dense microstructure and optimal electrical properties of the ceramics were obtained at x = 1.2 wt.%. Fatigue behavior of the ceramics was also investigated as a function of temperature and keeping time, where the BCZT − 1.2 wt.% BCW materials were found to possess improved fatigue characteristic. In addition, the sound pressure level (SPL) of 16.0 mm length × 16.0 mm width × 0.1 mm piezoelectric loudspeakers using lead-free BCZT ceramics with 1.2 wt.% BCW content at drive voltage of 10–20 V was more than 80 dB at frequency of 2.2–20.0 kHz. These results indicated that the 98.8 wt.% BCZT − 1.2 wt.% BCW ceramics was potential for piezoelectric loudspeaker applications.

To improve the temperature stability of (K, Na, Li) (Nb, Ta)O3 (KNLNT) ceramics, the effect of BiFeO3(BFO) additives on the phase structure, microstructure, electrical properties, strain and temperature stability was investigated and evaluated. The value of To–t decreased to near or below room temperature. It was calculated that the average grain size was 2.5, 2.3, 1.9, 1.7 and 1.3 µm when x was increased x from 0.0 to 0.8 mol%. The diffuse phase transition behavior was first reduced and then enhanced by adding BFO additive. The results indicated that the (1−x)KNLNT–xBFO ceramics exhibited favorable electrical properties (d33=261 pC/N, kp=0.58, Pr=23.7 μC/cm2, Ec=22.9 kV/mm and strain=0.3%) at x=0.4 mol%. The KNLNT–0.4 mol%BFO ceramics had good temperature stability from room temperature up to 100 °C. The results represented a new direction for developing electrostrictive material that possesses high piezoelectric properties, large strain and good stability.

(1 − x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3–xBiAlO3((1 − x)BCZT–xBAO) piezoelectric ceramics were synthesized by a solid-state reaction method. Effects of BAO content on the sintering temperature, phase structure, microstructure and electrical properties of BCZT ceramics were investigated. X-ray diffraction revealed a phase transition from orthorhombic to tetragonal phases at x = 0.3–1.0 mol%. The grain size gradually becomes smaller with increasing the BAO content. It was observed that the introduction of BAO content not only decreased the sintering temperature from 1450 to 1300 °C but also enhanced the electrical properties and temperature stability. The favorable electrical properties (d33 = 568 pC/N, kp = 0.54, Pr = 8.17 µC/cm2, Ec = 3.45 kV/cm, εm = 13,298, εr = 3375, tanδ = 0.021, Tc = 72 °C and S = 0.06 %) were demonstrated for the BCZT based ceramics sintered at 1300 °C with x = 0.8 mol%. These results indicated that the ceramics sintered at low temperatures was promising for piezoelectric applications.

•Ta0.05Ti0.95O2 samples with εr = 50000 and tanδ = 3.07% at 10 kHz were obtained.•The samples exhibit a relatively good temperature stability from −170 to 100 °C.•The samples also show a good frequency stability from 40 to 106 Hz.•The 2(Ta5+)Ti·→4(Ti3+)Ti′←Vo·· defect complex is introduced after Ta5+ doping.•Electron-pinned defect-dipoles are responsible for the observed giant permittivity.Colossal permittivity in tantalum-doped TiO2 (TaxTi1−xO2) (x = 1%, 2%, 3%, 4%, 5% and 6%) fabricated by the conventional solid-state reaction method in N2 atmosphere were achieved. Especially, by optimizing components and sintering temperatures, the dielectric loss could significantly decreased. The effects of Ta doping on their microstructure, dielectric properties, and temperature stability were revealed in detail. When the composition with x = 5% and sintered at 1400 °C, the dielectric constant reached to ∼30000 and the dielectric loss decreased to 3%. Interestingly, all the samples also exhibit good temperature stability of dielectric properties in a wide temperature range from 100 to 350 K. Based on XPS analysis, the formation of defect-dipole clusters, e.g. 2(Ta5+)Ti·→4(Ti3+)Ti′←Vo·· should be mainly responsible for the improved dielectric properties in tantalum-doped TiO2.