All-Inorganic Flexible (K, Na)NbO₃-Based Lead-Free Piezoelectric Thin Films Spin-Coated on Metallic Foils

Abstract

Flexible piezoelectric thin films are raising interest in energy harvesting and wearable electronics, although their direct fabrication is challenging in the selection of substrates and thermal processing. In this work, we developed direct fabrication of flexible lead-free (K, Na)NbO 3 (KNN)-based piezoelectric films on commercially available metallic foils by sol−gel processing. Stainless steel and platinum foils are selected as flexible substrates because of their good thermal stability, robust flexibility, and cost-efficiency. The sol−gel-processed KNN-based thin films on both of the metallic foils show good flexibility, with the bending radii reaching ±3 mm. The flexible thin films grown on stainless steel and platinum foils present high breakdown electric fields that reach 1760 and 2530 kV/cm, respectively, resulting from the fine-grained dense structure, limited leakage current density, and suppressed mobility of charged carriers. Improved effective piezoelectric coefficient d 33, eff * (75.4 pm/V) with a slight decrease after bending was obtained in the flexible thin films on Pt when compared to their rigid counterparts. The flexible lead-free piezoelectric thin films with combined high breakdown electric fields and piezoelectric and energy storage properties may pave the way for integrating KNN-based multifunctional thin films into flexible electronics.

Amorphous/Crystalline Engineering of BaTiO3-Based Thin Films for Energy-Storage Capacitors

“…Last but certainly not least, the comparison in energy-storage performances for 0.92BT-0.08BMZ thin films with other material systems is shown in Figure . ,,,− Considering that the substrate difference tends to induce the mismatch strain and the phase evolutions, only thin films grown on a Pt/Si substrate are chosen in this figure. It can be seen that the 0.92BT-0.08BMZ thin films depicted in this work possess high values of W D and η at a moderate electric field, being a strong competitor and indicating its potential in future integrated circuits for energy-storage applications.…”

“…Previous reports have proved that the sol–gel technique is a simple and effective method to prepare an amorphous/crystalline phase structure to adjust dielectric and ferroelectric properties by controlling annealing temperature. ,− It is noted that the amorphous phase sustains a high electric field and the crystalline phase provides high polarization, and hence a coexisting heterostructure with the amorphous phase surrounding the crystalline phase would be better to enhance energy-storage performances. Moreover, the crystallinity of thin films is also closely associated with the type of substrates (Si, Nb-SrTiO 3 , flexible mica substrates, and so on) due to the existence of lattice mismatch. − Note that the phase structure evolution driven by different substrates is rarely investigated for BT-based perovskite thin films.…”

Flexible inorganic piezoelectric functional films and their applications

“…In recent years, researchers have proposed various flexible substrates based on suitable fabrication methods, such as metal foils [15,16], 2D mica [158], and polymers [156,159]. Metal foils show good compatibility with most deposition techniques, such as SS [160], nickel (Ni) [161], platinum (Pt) [40], and Al foils [162]. Meanwhile, they could serve as electrodes due to good electrical and thermal conductivity.…”

“…These inorganic piezoelectric thin films have been demonstrated as functional layers in the flexible electronics, which acquire good flexibility and biocompatibility. These devices can not only store energy from various human activities for power supply as energy harvesters, but also serve as sensors or actuators for motion sensing, human-machine interaction, and clinical diagnosis [34,[37][38][39][40][41]. In addition, they can also respond to environmental mechanical vibrations or external stimuli, broadening the application of flexible piezoelectric devices in other fields [8,42].…”

High Energy Storage Performance of All-Inorganic Flexible Antiferroelectric–Insulator Multilayered Thin Films

“…The continuous consumption of fossil energy makes it increasingly important to develop new energy sources . At present, various new energy storage technologies have emerged, so the demand for energy storage devices that can effectively store and convert electrical energy is becoming increasingly urgent. − As a very important type of the energy storage device, dielectric capacitors have higher power density compared with electrochemical capacitors, batteries, and fuel cells, − which have been widely used in advanced pulse power supplies, inverters for new energy vehicles, embedded capacitors, and nuclear magnetic weapons. − …”