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Designing with Materials and Devices and Fabrication Processes
Published in Kenji Uchino, Ferroelectric Devices, 2018
Unimorph and bimorph devices are defined by the number of piezoelectric ceramic plates: only one ceramic plate is bonded onto an elastic shim, or two ceramic plates are bonded together, respectively. The bimorphs are mainly considered here. The bimorph causes bending deformation because two piezoelectric plates are bonded together and each plate produces extension or contraction under an electric field. This effect is employed for various speakers. The bimorph induces voltage for a bending deformation, which can be used for accelerometers. Since the fabrication process is simple (two ceramic plates are just bonded using a resin) and large magnification of the displacement is easily obtained, this structure has been widely used. However, the drawbacks include a low response speed (1 kHz) and low generative force due to the bending mode (tensile stress concentrates on the top of the piezoplate). A metallic sheet (called the shim) is occasionally sandwiched between the two piezoceramic plates to increase the reliability, that is, the structure is maintained even if the ceramic fractures (Figure 3.24). Also notice the tapering of the bimorph, which increases the response frequency by maintaining the tip displacement magnitude.
Bimorph piezoelectric vibration energy harvester with flexible 3D meshed-core structure for low frequency vibration
Published in Science and Technology of Advanced Materials, 2018
Takuya Tsukamoto, Yohei Umino, Sachie Shiomi, Kou Yamada, Takaaki Suzuki
Vibration energy harvesting is categorized into three types: electrostatic, electromagnetic, and piezoelectric. Piezoelectric vibration energy harvesters (PVEHs) exhibit (1) simple configuration, (2) high output voltage, and (3) high compatibility with microfabrication technology [10,11]. Many unimorph cantilever-type harvesters (i.e. consisting of a piezoelectric film and an elastic layer to support the piezoelectric film and to control vibration characteristics) have been proposed for PVEHs [12–16]. Because the configuration of a unimorph cantilever is simple and the fabrication process is relatively easy, many unimorph harvesters are fabricated using piezoelectric materials such as lead zirconate titanate (PZT) and polyvinylidene fluoride (PVDF). However, their output power is low because there is only one piezoelectric layer. Moreover, to harvest electric energy from low-frequency band vibration, it is necessary to select a flexible material for the elastic layer so as to lower the resonance frequency, because the types of the piezoelectric materials are limited. However, if the elastic layer becomes flexible, the neutral axis is shifted from the elastic layer side toward the piezoelectric layer side. This shift reduces both the output power and the strain generated in the piezoelectric layer. Therefore, it is difficult to achieve both low resonance frequency and high output power in a unimorph cantilever-type vibration energy harvester.