Multilayer ceramic actuator (MLCA) has been widely employed in actuators due to the large cumulative displacement under the low driving voltage. In this work, the MLCA devices consisting of a lead-free MnCO₃- and CuO-doped 0.96(K_0.48Na_0.52)(Nb_0.96Ta_0.04)O₃–0.04CaZrO₃ piezoelectric ceramics and a base nickel (Ni) metal inner electrode were well co-fired by the two-step sintering process in a reducing atmosphere. The ceramic layer/electrode interface is well-integrated and clearly continuous without distinct interdiffusion and chemical reaction, which is beneficial to the electrical reliability of the MLCA. As a result, the MLCA laminated with nine active ceramic layers obtains an ultrahigh piezoelectric coefficient d₃₃ of 3157 pC/N, about 9 times than bulk ceramics. The 0.5 mm-thick MLCA composed of a series of ~50 μm-thick ceramic layers and ~3 μm-thick Ni electrodes reaches a high 1.8 μm displacement under the low applied voltage of 200 V (the same displacement requires a voltage as high as 3700 V for ~1 mm-thick bulk ceramics). The excellent electrical performance and low-cost base electrode reveal that the (K,Na)NbO₃ (KNN)-based MLCAs are promising lead-free candidate for actuator application.

Piezoelectric energy harvesters (PEHs) fabricated using piezoceramics could convert directly the mechanical vibration energy in the environment into electrical energy. The high piezoelectric charge coefficient (d₃₃) and large piezoelectric voltage coefficient (g₃₃) are key factors for the high-performance PEHs. However, high d₃₃ and large g₃₃ are difficult to simultaneously achieve with respect to $g_{33}=d_{33}/({\epsilon }_0{\epsilon }_{\text{r}})$ and $d_{33}=2Q{\epsilon }_0{\epsilon }_{\text{r}}{P}_{\text{r}}$. Herein, the energy harvesting performance is optimized by tailoring the CaZrO₃ content in (0.964-x)(K_0.52Na_0.48)(Nb_0.96Sb_0.04)O₃ -0.036(Bi_0.5Na_0.5)ZrO₃-xCaZrO₃ ceramics. First, the doping CaZrO₃ could enhance the dielectric relaxation due to the compositional fluctuation and structural disordering, and thus reduce the domain size to ~30 nm for x = 0.006 sample. The nanodomains switch easily to external electric field, resulting in large polarization. Second, the rhombohedral-orthorhombic-tetragonal phases coexist in x = 0.006 sample, which reduces the polarization anisotropy and thus improves the piezoelectric properties. The multiphase coexistence structures and miniaturized domains contribute to the excellent piezoelectric properties of d₃₃ (354 pC/N). Furthermore, the dielectric relative permittivity (ε_r) reduces monotonously as the CaZrO₃ content increases due to the relatively low ion polarizability of Ca²⁺ and Zr⁴⁺. As a result, the optimized energy conversion coefficient (d₃₃ × g₃₃, 5508 × 10^-15 m²/N) is achieved for x = 0.006 sample. Most importantly, the assembled PEH with the optimal specimen shows the excellent output power (~48 μW) and lights up 45 red commercial light-emitting diodes (LEDs). This work demonstrates that tailoring ferroelectric/relaxor behavior in (K,Na)NbO₃-based piezoelectric ceramics could effectively enhance the electrical output of PEHs.

The intrinsic conduction mechanism and optimal sintering atmosphere of (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ (BCZT) ceramics were regulated by Mn-doping element in this work. By Hall and impedance analysis, the undoped BCZT ceramics exhibit a typical n-type conduction mechanism, and the electron concentration decreases with the increasing oxygen partial pressure. Therefore, the undoped ceramics exhibit best electrical properties (piezoelectrical constant d₃₃ = 585 pC·N^-1, electro-mechanical coupling factor k_p = 56%) in O₂. A handful of Mn-doping element would transfer the conduction mechanism from n-type into p-type. And the hole concentration reduces with the decreasing oxygen partial pressure for Mn-doped BCZT ceramics. Therefore, the Mn-doped ceramics sintered in N₂ have the highest insulation resistance and best piezoelectric properties (d₃₃ = 505 pC·N^-1, k_p = 50%). The experimental results demonstrate that the Mn-doping element can effectively adjust the intrinsic conduction mechanism and then predict the optimal atmosphere.

The thermal stability and fatigue resistance of piezoelectric ceramics are of great importance for industrialized application. In this study, the electrical properties of (0.99-x)(K_0.48Na_0.52)(Nb_0.975Sb_0.025)O₃- 0.01CaZrO₃-x(Bi_0.5Na_0.5)HfO₃ ceramics are investigated. When x = 0.03, the ceramics exhibit the optimal electrical properties at room temperature and high Curie temperature (T_C = 253 ℃). In addition, the ceramic has outstanding thermal stability (d₃^*₃ ≈ 301 pm/V at 160 ℃) and fatigue resistance (variation of P_r and d₃^*₃ ~10% after 10⁴ electrical cycles). Subsequently, the defect configuration and crystal structure of the ceramics are studied by X-ray diffraction, temperature- dielectric property curves and impedance analysis. On one hand, the doping (Bi_0.5Na_0.5)HfO₃ makes the dielectric constant peaks flatten. On the other hand, the defect concentration and migration are obviously depressed in the doped ceramics. Both of them can enhance the piezoelectrical properties and improve the temperature and cycling reliabilities. The present study reveals that the good piezoelectric properties can be obtained in 0.96(K_0.48Na_0.52)(Nb_0.975Sb_0.025)O₃-0.01CaZrO₃-0.03(Bi_0.5Na_0.5) HfO₃ ceramics.