Energy harvesting using the piezoelectric material in the development of compact vibration energy harvesters can be used as a backup power source for wireless sensors or to fully replace the use of fossil-resource-wasting batteries and accumulators to power a device or sensor. Generally, the coefficient is used as the metric for evaluating the property in materials. Recent research reports that accurate measurement and calculation of the coefficient in materials, especially in polymers, can be challenging for various reasons. From the reviewed references, different methods, including the quasi-static, dynamic, interferometric, and acoustic methods, are discussed and compared based on the direct and indirect effect, accuracy, repeatability, frequency range, and so on. A development of an ultrasound piezoelectric transducer is conducted to estimate $d_{33}$ coefficient with a reference value. The purpose of the method was mainly to measure the values of piezoelectric material in order to measure the efficiency of the poling process in piezoelectric materials. The test setup described in this study allowed for the effective measurement of the $d_{33}$ factor of piezoelectric materials using a 1.4 MHz PZT ultrasonic piezoelectric transducer. The arrangement of the components, including the use of organic glass, copper, and aluminum electrodes, ensured accurate and reliable measurements. This setup can be valuable for various applications requiring the characterization of piezoelectric materials and for understanding their behavior under specific conditions. The advantages and challenges in this method are discussed and compared with existing works.

Piezoelectric energy harvesters operate by converting mechanical vibrations or strains into electrical energy. From recent research, it is understood that the choice of coating material for piezoelectric energy harvesters is a critical consideration that impacts the device’s performance, durability, and compatibility with the intended application. Selecting the right coating material involves balancing the electrical, mechanical, and environmental requirements to optimize energy conversion and reliability. There are methods like the thermocycling process that can provide an accelerated ageing of the energy harvester in order to conduct a reliability assessment. The thermocycling process was carried out for 450 h on six samples of piezoelectric cantilever-type energy harvesters made of copper–nickel- and aluminum-coated PVDF (Polyvinylidene fluoride) piezoelectric material. The effect of aluminum and copper–nickel coating on PVDF piezoelectric material before and after the aging process was studied. The numerical results of the generated output voltage, surface resistance, and capacitance values measured before and after the accelerated aging process are presented in this study. This work also discusses the structure of the developed energy harvester, thermocycling experiment setup, and methodology of conducting the ageing process. It aims to provide a conclusion on the suitability of the PVDF metal coating material, the type of conductive adhesive to be used in order to seal the PVDF material to the harvester core, improvements in the structural design and selection of materials to reduce mechanical fatigue and ensure even stress distribution, and minimizing points of stress concentration, to help mitigate piezoelectric material delamination risks.

Energy harvesting technologies and material science has made it possible to tap into the abundant amount of surrounding vibrational energy to efficiently convert it into useable energy providing power to portable electronics and IoT devices. Recent investigations show that the piezoelectric effect is created in cellular polymers called ferroelectrets. These cellular-compliant polymers with polarized pores have a piezoelectric response to generate electrical energy when subjected to mechanical strain or surrounding vibration. It is found that there is a significant difference between ferroelectret polarized cellular polypropylene foam and traditional piezoelectric polymers such as polyvinylidene fluoride (PVDF). The former has approximately ten times higher piezoelectric coefficient than the latter. This means that with an acceleration of 9.81 $\mathrm{m}/{\mathrm{s}}^2$ force on this material, ferroelectrets generate up to 39 (µW/g/${\mathrm{m}\mathrm{m}}^3$) power output. Designing a polypropylene-based piezoelectric energy harvester based on the d33 mode of vibration can be challenging due to several factors, as it requires balancing multiple factors such as mechanical stability, piezoelectric response, circuit topology, electrode size, spacing, placement relative to the piezoelectric material, and so on. This paper proposes the preliminary experimental investigation of ferroelectret cellular polypropylene foam in harvesting performance. Suggestions of different approaches for the structural design of energy harvesters are provided. The vibration-dependent response and generated output are examined concerning pulse or sinusoidal input excitation. The voltage generated for both excitations is compared and suggestions are provided regarding the suitable kind of excitation for the chosen ferroelectret material. Finally, conclusions and prospects for ferroelectret materials used in energy-harvesting applications are given.