Dimensional Effects of Polymer Piezoelectric Films for Wind Energy Harvesting

Arrays of flexible polymer piezoelectric film cantilevers that mimic grass or leaves is a prospective idea for harvesting wind energy in urban areas, where the use of traditional technologies is problematic due to low wind velocities. Conversion of this idea into an economically attractive technology depends on various factors including the shape and dimensions of individual films to maximize generated power and to minimize associated costs of production, operation, and maintenance. The latter requirement can be satisfied with rectangular films undergoing flutter in ambient air. Flexible piezoelectric films that displace due to low forces and can convert mechanical energy into electrical energy are ideal for this application. The goal of the presented study is to determine the key dimensions of the piezoelectric film to enhance generated power within the wind range characteristic for urban areas from 1.3 to 7.6 m/s. For this purpose, experiments were conducted in a wind tunnel using piezoelectric polymer films of polyvinylidine fluoride with the length, width, and thickness varying in the ranges of 32 - 150 mm, 16 - 22 mm, and 40 - 64 µm, respectively. Voltage and power outputs for individual samples were measured at wind speeds ranging from 0.5 to 16.5 m/s. Results demonstrated that a single film could produce up to 0.74 nW and that the optimal film dimensions are 63 mm × 22 mm × 40 µm (from considered samples) for the wind energy harvesting in urban areas. Further improvement in power production can be expected when using films with reduced thickness, low elastic modulus, and increased length, and by assembling films in arrays.

Enhanced Piezoelectric Coefficient of PVDF-TrFE Films via In Situ Polarization

The d33 coefficient = 28 pC/N of PVDF-TrFE piezoelectric films was achieved by the in situ polarization. Compared with traditional poling methods, the in situ polarization is performed with low poling voltage and short poling time, and it can ensure the PVDF-TrFE film with enhanced piezoelectric performances and uniform distribution among a large area of 200 mm2 × 200 mm2. The processing influence of drying, annealing, and poling on the crystalline properties and piezoelectric performances were investigated. Besides, the obtained PVDF-TrFE films present a good piezoelectric response to different extents of mechanical stimulations, which have great potential in energy harvesting applications.

Piezoelectric Ultrasonic Micromotor

Ultrasonic motors are characterized by low speed and high-torque operation, without the need for gear trains. They can be compact and lightweight, and they can also work in the absence of applied loads, due to the frictional coupling between the rotor and the stator induced by the traveling wave. In this work, we discuss a concept design based on thin piezoelectric films, sol-gel directly deposited onto a silicon substrate to provide high-torque motors compatible with wafer integration technologies. Due to the large dielectric constants and the enhanced breakdown strengths of thin piezoelectric films, such ultrasonic micromotors can lead to meaningful improvements over electrostatic ones in terms of energy density. As far as the fabrication of the micromotor at the mm-scale is concerned, an integrated approach is proposed with significant improvements regarding: the comb-tooth structure, to maximize/optimize the motor torque; a back and front etch lithographic process; and the design of the electrodes, which provide the electric signal at the central anchor of the stator, taking advantage of low-temperature soldering. The proposed design has been assessed through multiphysics simulations, carried out to evaluate the resonant behavior of the stator and the motor performance in terms of angular velocity, torque, and output power, and it is shown to lead to promising results.