Electromechanical energy demands homogenous thick films of piezoceramics with sufficiently large piezoelectric constant and reproducible performance. Single-phase LiTaO 3 films deposited by sol-gel processing have been fabricated as cantilevers to investigate the interdependence of dielectric and piezoelectric properties as a function of film thickness. Phase pure LiTaO 3 films with varying thickness in the range of 2.07-4.37 µm on stainless steel substrates were obtained after calcination of samples at Accepted Article This article is protected by copyright. All rights reserved 650 °C. The relative permittivity of optimized spin-coated films peaked at 479.73 (1 kHz), whereas the piezoelectric coefficient (d 33 mode) determined by piezo force microscopy was in the range of 21-24 pm/V. The effect of poling was studied through the butterfly and phase curves. A figure of merit up to 3.29 (10-18 m 2 /V 2) was determined for cantilever devices, which were able to generate a peak-to-peak voltage of 0.046-0.15 V using a 1 MΩ resistor as an impedance load at a fixed acceleration of 1.5 m/s 2. While the power density was in the range of ~ 4-20x10-9 W/m 3 , increased with the increasing film thickness. The leakage current density decreased in the range of 4x10-5-6x10-7 A/m 2 in the same direction. As both ferroelectric and piezoelectric properties of LiTaO 3 films are dependent on film thickness, an optimal energy conversion efficiency was obtained for a thickness of ~3 µm. Furthermore, these devices were tested up to a temperature of 150 °C for voltage generation. Given the need for lead-free piezoelectric materials for environmental applications, these LiTaO 3 cantilevers are very promising for vibrational energy harvester applications especially due to their cost effectiveness, small size, stability at higher temperatures, and repeatable properties which makes them suitable for MEMS devices for industrial applications.

Wireless sensor nodes (WSNs) are the fundamental part of an Internet of Things (IoT) system for detecting and transmitting data to a master node for processing. Several research studies reveal that one of the disadvantages of conventional, battery-powered WSNs, however, is that they typically require periodic maintenance. This paper aims to contribute to existing research studies on this issue by exploring a new energy-autonomous and battery-free WSN concept for monitor vibrations. The node is self-powered from the conversion of ambient mechanical vibration energy into electrical energy through a piezoelectric transducer implemented with lead-free lithium niobate piezoelectric material to also explore solutions that go towards a greener and more sustainable IoT. Instead of implementing any particular sensors, the vibration measurement system exploits the proportionality between the mechanical power generated by a piezoelectric transducer and the time taken to store it as electrical energy in a capacitor. This helps reduce the component count with respect to conventional WSNs, as well as energy consumption and production costs, while optimizing the overall node size and weight. The readout is therefore a function of the time it takes for the energy storage capacitor to charge between two constant voltage levels. The result of this work is a system that includes a specially designed lead-free piezoelectric vibrational transducer and a battery-less sensor platform with Bluetooth low energy (BLE) connectivity. The system can harvest energy in the acceleration range [0.5 g–1.2 g] and measure vibrations with a limit of detection (LoD) of 0.6 g.

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