## 3 Structures
### Thin Films
& A discussion of the main challenges of PEHs and an explanation of the main limitations of piezoelectric materials & Nonlinear systems, two-way and three-way systems, geometric optimization, design improvements for piezoelectric cantilevers, hybrid energy harvesting mechanisms, piezoelectric materials and their selection criteria & Finding a balance between the bandwidth and the output power will enable piezo harvesters to perform as expected & Importance of storage circuits and electric interface circuits & [86] \\ \hline [PERSON] and [PERSON] & Report on the power output of PEH, including a mathematical model that matches the resonance frequency to the driving frequency & Manual frequency adjustment, autonomous frequency adjustment, mathematical modeling of PEH & Review, comparison, and classification of several autonomous and manual tuning methods. Addressing the challenges of energy consumption & Importance of storage circuits and electric interface circuits & [87] \\ \hline \multirow{4}{*}{[PERSON] et al.} & Considering the energy conversion efficiency, the EPI output power, the systems, ph-directional and tri-directional coupling factor and the mode of operation. This includes a piezocal electric materials and their selection criteria, improved piezoelectric cantilever cantilever & Describes various important aspects of improving the overall performance of PEHs and their criteria, improved piezoelectric cantilever performance improvement approaches & Increasing endurance and lifetime of PEHs and their manufacturing, size reduction, and packaging piezoelectric basins, hybrid energy harvesting & [64] \\ \hline \multirow{4}{*}{[PERSON] et al.} & Report of PEH output power, including mathematical model to match PEH resonance frequency to driving frequency & Manual frequency tuning, multi-frequency system, autonomous frequency tuning, non-linear system, floating mass system, frequency up-conversion approach, bi-directional and three-way systems & A new taxonomy of performance-enhancing techniques. Comparison of many bandwidth & Increasing the PEHs/ frequency tuning and bandwidth & [88] \\ \hline \multirow{4}{*}{[PERSON] et al.} & PEH output power report with a mathematical model that matches the HF frequency and accounts for the energy conversion efficiency & Manual frequency tuning, multi-frequency systems, autonomous frequency tuning, non-linear systems, frequency up conversion & A proposal for a new general classification using approaches based on aspects of harvester improvement, such as techniques for extending operating frequencies, design of non-resonant systems, and multi-directional bandwidths & [89] \\ \cline{1-1}  
Table 3 presents the properties of some basic piezoelectric materials in terms of their stress and strain constants, charge constants, coupling factor, mechanical quality factor, voltage constant, and curie temperature. PZN-PT and PMN-PT single-crystal structures exhibit great electromechanical properties as compared to PZT ceramics [89]. Due to the enhanced piezoelectric properties and high Curie temperatures, solid solutions of lead titanate and bismuth-based oxides have a wide range of possible applications. Compared to the other materials, ZnO and AIN have a lower piezoelectric effect. Mostly, they are used in thin-film structures at the micro-scale, where their performance metrics differ from that of the bulk materials. In general, we find that d15 >> d33 > d31, as indicated in Table 3. The maximum piezo-electric properties are displayed by the PMN-PT and PZN-PT. However, they are highly sensitive to temperature changes and more challenging to fabricate than the PZT. As a result, PZT is the most widely used piezo-electric material in energy harvesters. Energy harvesters could be divided into two operation modes: d31 and d33, depending on the polarization and stress directions. In d31 mode, the electric field's direction, \"3,\" is perpendicular to applied stress \"1\". In contrast, in d33 mode, the polarization and applied stress are both in the same direction.