In this paper, the progress of micro-energy harvesters by using piezoelectret-based transducers as a core element is reviewed, including basic physical principle and properties of piezoelectrets, and their applications in micro-energy harvesting. Piezoelectret is electret-based piezoelectric polymer with a foamed structure. The piezoelectric effect of such material is a synergistic effect of the electret property of the matrix polymer and the foam mechanical structure in the material. Piezoelectret, featuring strong piezoelectric effect, flexibility, low density, very small acoustic impedance and film form, is an ideal electromechanical material for lightweight flexible sensors and mechanical energy harvesters. The piezoelectret prepared by means of grid, template patterning, supercritical CO
2assisted low-temperature assembly, lithography mold combined with rotary coating and hot pressing has regular voids and good piezoelectric properties. Piezoelectret has been used to harvest vibrational energy, human motion energy and sound energy.
According to the stress direction applied to the piezoelectrets, operating modes of energy harvesters can be divided into 33 and 31 modes. The vibrational energy harvesters based on piezoelectret are utilized to harvest medium frequency vibrational energy generated by factory machines, aircrafts, automobiles, etc. Such energy harvesters can generate considerable power even in a small size. Human motion energy harvesters are generally used to power wearable sensors. The high sensitivity, lightweight, and flexibility of the piezoelectret make such a material a promising candidate for harvesting human motion energy. Owing to very small acoustic impedance, high figure-of-merit, flat response in audio and low-frequency ultrasonic range, the piezoelectrets are more appropriate for acoustic energy harvesting in air medium than conventional PZT and ferroelectric polymer PVDF.
In the future, specific micro-energy harvesters using piezoelectrets as transduction material can be designed and fabricated according to the practical application environment, and their performance can be enhanced by using flexible connections of transduction elements.
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