Textured Lead-Free Piezoelectric Ceramics for Flexible Energy Harvesters.

A lead-free (K,Na)NbO3-based piezoelectric ceramic is textured along the (001) direction using the NaNbO3 (NN) seeds. The composition 0.96(K0.5Na0.5)(Nb0.965Sb0.035)O3-0.01CaZrO3-0.03(Bi0.5K0.5)HfO3 (KNN) is found to provide an excellent combination of electromechanical coefficients at room temperature. The textured composition with 5 wt % NN template (KNN-5NN) exhibits considerably improved electromechanical coefficients, d33 ∼ 590 pC/N, k31 ∼ 0.46, and d31 ∼ 215 ×10-12 C/N, at room temperature. A flexible piezoelectric energy harvester (F-PEH) is fabricated using the textured KNN-5NN ceramic and tested under cyclic force. F-PEH exhibits enhanced output voltage (Voc ∼ 25 V), current (I ∼ 0.4 µA), and power density (PD ∼ 5.5 mW/m2) (RL of 10 MΩ) in the off-resonance frequency regime. In comparison to the random ceramic KNN-0NN-based F-PEH (Voc ∼ 8 V and I ∼ 0.1 µA), the textured F-PEH significantly outperformed energy harvesting capability due to the large figure-of-merit value (d31 × g31) ∼ 3354 ×10-15 m3/J. This work provides a methodology for texturing lead-free materials and further implementing them in flexible energy harvesting devices and sensors.

Water Quenched and Acceptor-Doped Textured Piezoelectric Ceramics for Off-Resonance and On-Resonance Devices.

Piezoelectric materials should simultaneously possess the soft properties (high piezoelectric coefficient, d33 ; high voltage coefficient, g33 ; high electromechanical coupling factor, k) and hard properties (high mechanical quality factor, Qm ; low dielectric loss, tan δ) along with wide operation temperature (e.g., high rhombohedral-tetragonal phase transition temperature Tr-t ) for covering off-resonance (figure of merit (FOM), d33  × g33 ) and on-resonance (FOM, Qm  × k2 ) applications. However, achieving hard and soft piezoelectric properties simultaneously along with high transition temperature is quite challenging since these properties are inversely related to each other. Here, through a synergistic design strategy of combining composition/phase selection, crystallographic texturing, defect engineering, and water quenching technique, <001> textured 2 mol% MnO2 doped 0.19PIN-0.445PSN-0.365PT ceramics exhibiting giant FOM values of Qm  × k 31 2 $k_{31}^2$ (227-261) along with high d33  × g33 (28-35 × 10-12 m2 N-1 ), low tan δ (0.3-0.39%) and high Tr-t of 140-190 °C, which is far beyond the performance of the state-of-the-art piezoelectric materials, are fabricated. Further, a novel water quenching (WQ) room temperature poling technique, which results in enhanced piezoelectricity of textured MnO2 doped PIN-PSN-PT ceramics, is reported. Based upon the experiments and phase-field modeling, the enhanced piezoelectricity is explained in terms of the quenching-induced rhombohedral phase formation. These findings will have tremendous impact on development of high performance off-resonance and on-resonance piezoelectric devices with high stability.

Near-ideal electromechanical coupling in textured piezoelectric ceramics.

Electromechanical coupling factor, k, of piezoelectric materials determines the conversion efficiency of mechanical to electrical energy or electrical to mechanical energy. Here, we provide an fundamental approach to design piezoelectric materials that provide near-ideal magnitude of k, via exploiting the electrocrystalline anisotropy through fabrication of grain-oriented or textured ceramics. Coupled phase field simulation and experimental investigation on <001> textured Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3 ceramics illustrate that k can reach same magnitude as that for a single crystal, far beyond the average value of traditional ceramics. To provide atomistic-scale understanding of our approach, we employ a theoretical model to determine the physical origin of k in perovskite ferroelectrics and find that strong covalent bonding between B-site cation and oxygen via d-p hybridization contributes most towards the magnitude of k. This demonstration of near-ideal k value in textured ceramics will have tremendous impact on design of ultra-wide bandwidth, high efficiency, high power density, and high stability piezoelectric devices.

Ultrahigh Piezoelectric Performance through Synergistic Compositional and Microstructural Engineering.

Piezoelectric materials enable the conversion of mechanical energy into electrical energy and vice-versa. Ultrahigh piezoelectricity has been only observed in single crystals. Realization of piezoelectric ceramics with longitudinal piezoelectric constant (d33 ) close to 2000 pC N-1 , which combines single crystal-like high properties and ceramic-like cost effectiveness, large-scale manufacturing, and machinability will be a milestone in advancement of piezoelectric ceramic materials. Here, guided by phenomenological models and phase-field simulations that provide conditions for flattening the energy landscape of polarization, a synergistic design strategy is demonstrated that exploits compositionally driven local structural heterogeneity and microstructural grain orientation/texturing to provide record piezoelectricity in ceramics. This strategy is demonstrated on [001]PC -textured and Eu3+ -doped Pb(Mg1/3 Nb2/3 )O3 -PbTiO3 (PMN-PT) ceramics that exhibit the highest piezoelectric coefficient (small-signal d33 of up to 1950 pC N-1 and large-signal d33 * of ≈2100 pm V-1 ) among all the reported piezoelectric ceramics. Extensive characterization conducted using high-resolution microscopy and diffraction techniques in conjunction with the computational models reveals the underlying mechanisms governing the piezoelectric performance. Further, the impact of losses on the electromechanical coupling is identified, which plays major role in suppressing the percentage of piezoelectricity enhancement, and the fundamental understanding of loss in this study sheds light on further enhancement of piezoelectricity. These results on cost-effective and record performance piezoelectric ceramics will launch a new generation of piezoelectric applications.