In the 4^th industrial age, electronic devices are becoming smaller and lighter with a low power consumption to overcome spatial limitation. The piezoelectric energy harvesters can convert mechanical kinetic energy into electric energy; thus, enabling the operation of small electronic devices. Recently, various piezoelectric harvesters have been reported and the electric output from these harvesters could be anticipated by theoretical analysis methods. For example, COMSOL Multiphysics software provides a theoretical simulation of piezoelectric effect with a combination of mechanical and electrical phenomena in the piezoelectric materials. This article introduces a brief modeling of piezoelectric harvester to investigate mechanical stress and electrical output of harvesting devices by the COMSOL Multiphysics software.

The piezoelectric properties of 0.65Pb(Zr_1-xTi_x)O₃-0.35Pb(Zn_1/6Ni_1/6Nb_2/3)O₃ (PZT_x-PZNN) ceramics with 0.530≤ x≤0.555 were investigated for application to piezoelectric energy harvesters. Although a morphotropic phase boundary (MPB) was found at approximately x＝0.545, the ceramic with the highest figure of merit (FOM) (d₃₃×g₃₃) was observed at a composition of x＝0.540. Values of this figure of merit, d₃₃×g₃₃, of 19.6 pm²/N and 20.2 pm²/N were obtained from PZT_0.540-PZNN ceramics sintered at 920℃ and 950℃, respectively. A high output power of 937 μW and a high power density of 3.3 mW/cm³ were obtained from unimorph-type piezoelectric energy harvesters fabricated using PZT_0.540-PZNN ceramic sintered at 920℃ for 4h.

Recently, energy harvesting technology is increasing due to the fossil fuel shortages. To compensate problem of low generating power than other energy harvesters, many researchers have studied about piezoelectric harvester for obtaining high output. In this paper, four kinds of unimorph based piezoelectric harvesters were proposed and its generating characteristics were studied. Each of the piezoelectric harvesters has three, four, and six unimorph arms, respectively, and the arms are symmetrically arranged from one central point. The centrosymmetric structure of the harvesters guarantees more stable and multiplied generation than a cantilever-type harvester since the arms of the harvester resonate at same frequency. Resonance frequency, output voltage, displacement, and stress characteristics of the generator were analyzed by using a FEM (finite element method) program. Harvesters were fabricated on the basis of analysis results. Experimental results were compared with simulated results.

Spring supported piezoelectric cantilever structures (SPCS) were fabricated for vibration-based energy harvester application. We selected four elastic springs (A, B, C, and D type) as cantilever`s supporter, each elastic spring has a different spring constant (S). The C type of SPCS (SC: 4,649 N/m) showed a extremely low resonance frequency of 81 Hz along with the highest power output of 38.5 mW while the A type of SPCS (SA: 40,629 N/m) didn`t show a resonance frequency while. Therefore, it is considered that the lower spring constant lead to a lower resonance frequency of the SPCS. In addition, a tip mass (18 g) at one end of the SPCS could further reduce the resonance frequency without heavy degradation of power output.

Owing to the rapid growth of mobile and electronic equipment miniaturization technology, the supply of micro mobile computing machine has been fast raised. Accordingly they have performed many researches on energy harvesting technology to provide promising power supply equipment to substitute existing batteries. In this paper, in order to have low resonance frequency for piezoelectric energy harvester, we have tried to make it larger than before by adopting nickel that has much higher density than silicon. We have applied it for our energy harvesting actuator instead of the existing silicon based actuator. Through such new concept and approach, we have designed energy harvesting device and made it personally by making with micromachining process. The energy harvester structure has a cantilever type and has a dimension of 10×2.5×0.1 mm3 for length, width and thickness respectively. Its electrode type is formed by using Au/Ti of interdigitate d33 mode. The pattern size and gap size is 50 μm. Based on the measurement of the nickel-based piezoelectric energy harvester, it is found to have 778 Hz for a resonant frequency with no proof mass. In that resonance frequency we could get a maximum output power of 76 μW at 4.8 MΩ being applied with 1 g acceleration.