Determination of the appropriate piezoelectric materials for various types of piezoelectric energy harvesters with high output power

For a type-1 piezoelectric energy harvester (PEH), in which stress develops in the supporting system of the piezoelectric materials, the electromechanical coupling factor (k_ij) of the piezoelectric materials is important for the output power at the resonance frequency. Therefore, soft-piezoelectric materials are good candidates for these PEHs. For type-2 PEHs, in which stress develops in the piezoelectric material and supporting system, the figure of merit (FOM) of the output power at the resonance frequency is (k_ij ² × Q_m)/s₁₁ ^E, where Q_m and s₁₁ ^E are the mechanical quality factor and the elastic compliance of piezoelectric materials, respectively. In particular, the effect of Q_m is very large for these PEHs, indicating that hard-piezoelectric materials are good candidates for type-2 PEHs operating at the resonance frequency. For both type-1 and type-2 PEHs operating at off-resonance frequency, the k_ij ² × d_ij × g_ij is the FOM of the output power of the PEHs, where g_ij is a piezoelectric voltage constant. Therefore, soft-piezoelectric materials are also good candidates for both type-1 and type-2 PEHs operating at the off-resonance frequency.