# A Review of MEMS Scale Piezoelectric Energy Harvester
## 2 Research Progress of PEH
### Research and Improvement of Piezoelectric Modes
#### 2.3.1 Piezoelectric Energy Harvesters in \(d_{31}\) and \(d_{33}\) Mode  
The PEHs possess parallel plate electrodes in \(d_{31}\) mode with the direction of polarization perpendicular to the direction of applied stress. By contrast, a \(d_{33}\) mode PEH is covered with the interdigital electrodes. In addition, the polarization direction is parallel to the direction of applied stress. Furthermore, the device in \(d_{33}\) mode has relatively complex structure compared with \(d_{31}\). The electrode configurations and poling direction of the \(d_{31}\) devices and the \(d_{33}\) devices are, respectively, shown in Figures 3 and 4. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Materials**} & \(\mathbf{d_{31}}\) & \(\mathbf{d_{33}}\) & \(\mathbf{K_{p}}\) (\%) & \(\varepsilon_{r}\) & \(\mathbf{T_{C}rie}\) & \multirow{2}{*}{**Reference**} \\  & (\(\mathbf{\mathrm{p}}\)/\(\mathbf{\mathrm{C}}\)) & (\(\mathbf{\mathrm{p}}\)/\(\mathbf{\mathrm{C}}\)) & & & & (\(\mathbf{\mathrm{{}^{\circ}}}\)/\(\mathbf{\mathrm{C}}\)) & \\ \hline PZT-2 & \(-\)60.2 & 152 & 47 & - & 370 & \\ \hline PZT-4 & \(-\)123 & 289 & 58 & - & 328 & \\ \hline PZT-5A & \(-\)171 & 374 & 60 & - & 365 & \\ \hline PZT-5H & \(-\)274 & 593 & 65 & - & 193 & \\ \hline PZT-8 & \(-\)37 & 225 & 51 & - & 300 & \\ \hline (Na\({}_{0.5}\)K\({}_{0.5}\))NbO\({}_{3}\)-LiTaO\({}_{3}\) & - & 200 & 36 & 570 & - & [34] \\ \hline (Na\({}_{0.5}\)K\({}_{0.5}\))NbO\({}_{3}\) & - & 148 & 38.9 & 606 & 395 & [35] \\ \hline \([\mathrm{Bi}_{0.5}\)(Na\({}_{1-\mathrm{x-y}}\)K\({}_{\mathrm{x}}\)Li\({}_{y}\))\({}_{0.5}\)]TiO\({}_{3}\) & - & 231 & 41.0 & 1190 & - & [36] \\ \hline \(0.69\)BiFeO-\(0.04\)Bi(ZnTi)O-\(0.27\)BaTiO (MoOdoped) & - & 145 & 41.0 & - & 510 & [37] \\ \hline \(\mathrm{Li}_{0.058}\)(K\({}_{0.480}\)Na\({}_{0.535}\))\({}_{0.966}\)(Nb\({}_{0.9}\)Ta\({}_{0.11}\))O\({}_{3}\)(ZnO doped) & - & 272 & 44 & 5081 & 395 & [38] \\ \hline \(\mathrm{Bi}_{0.5}\)(Na\({}_{0.68}\))\({}_{0.22}\)Li\({}_{0.1,0.5}\)TiO\({}_{3}\) & - & \multirow{2}{*}{295} & \multirow{2}{*}{-} & \multirow{2}{*}{2720} & \multirow{2}{*}{-} & [42] \\ ((Ba\({}_{0.90}\)Ca\({}_{0.10}\))(T\({}_{0.85}\)Zr\({}_{0.15}\))O\({}_{3}\) doped) & & & & & & \\ \hline (Bi\({}_{0.5}\)Na\({}_{0.5}\))TiO\({}_{3}\)(BaTiO\({}_{3}\)doped) & 37 & 180 & - & 1557 & - & [43] \\ \hline ((Na\({}_{0.535}\)K\({}_{0.48}\))NbO\({}_{3}\)(LiNbO\({}_{3}\) doped) & & 150 & 50 & 5500 & 470 & [44] \\ \hline
0.995(K\({}_{0.5}\)Na\({}_{0.5}\))\({}_{1-\mathrm{x}}\)Li\({}_{x}\)NbO\({}_{3}\)-\(0.005\)BiAlO\({}_{3}\) & & 182 & 37.5 & 5000 & 450 & [45] \\ \hline PVDF & 8–22 & \(-\)24\(\cdots\)34 & - & - & - & \\ \hline P(VDF-TrFE) & 12 & \(-\)38 & - & - & - & \\ \hline P(VDF-HFP) & 30 & \(-\)24 & - & - & - & \\ \hline P(VDF-CTFE) & - & \(-\)140 & - & - & - & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of some parameters of piezoelectric material. The coefficients for piezoelectric polymers are given by thesis [41]. Reports on \(d_{31}\) mode PEHs are relatively more compared to the other two modes PEHs owing to its simplicity of fabrication. By contrast, \(d_{33}\) is normally twice \(d_{31}\) according to the study of piezoelectric material listed in [PERSON] and [PERSON]'s research [49]. Typically, the piezoelectric material in the PEH is thin, which leads to smaller distance between electrodes of the \(d_{31}\) mode device than that of the \(d_{33}\) mode device. Therefore, the \(d_{33}\) mode PEHs have advantage in generating higher output voltage while the \(d_{31}\) mode PEHs have potential in generating larger electric current. Since most of the rectifier circuit designs contain diodes, the generated voltage after flowing through the rectifier circuit will be reduced. This is also the reason the high output voltage can reduce the power loss in the rectifier circuit. In 2013, [PERSON] and others [50] studied the performance difference between the \(d_{31}\) mode and the \(d_{33}\) mode of PEHs.