Guest Editorial: Polymer electrets and ferroelectrets

Electrets are functional dielectrics capable of quasi-permanently storing electric charges at their surface and/or in their bulk. The electret charges are either real charges (space charges) or oriented dipoles (polarisation). Traditionally, electrets are divided into space-charge (non-polar) electrets and dipole (polar) electrets. Ferroelectrets (also called piezoelectrets) are a relatively young member added to the electret family around the end of the last century. These are non-polar polymer foams or cavity-containing polymer-film systems. The air-filled cavities carry positive and negative charges on their top and bottom internal surfaces, respectively, and thus can be considered as macroscopic dipoles, the direction of which can be switched by reversing the polarity of the charging voltage. Therefore, ferroelectrets are non-polar space-charge electrets with ferroic behaviour phenomenologically the same with that of traditional ferroelectrics.

Polymer electrets and ferroelectrets may show peculiar functionalities such as electrostatic effect, piezo-, pyro- and ferroelectricity, biological effects, non-linear optical effects, and therefore attract extensive attention from academia and industry. This special issue collects some of the latest advancements in the field of polymer electrets and ferroelectrets. In total, nine papers are accepted, which cover a wide scope of topics. One paper (of Yan et al.) presents the fundamental open-circuit thermally stimulated discharge technique for electrets. Two papers (of Yang et al. and Feng et al.) study electrets employed in energy harvesters. The papers of Chen et al. and of Jiang et al. propose an electret-based electrostatic motor that can generate a power up to 5.4 mW and electrospun PVDF microfiber sensors capable of capturing weak mechanical signals, respectively. Two papers (of Sun et al. and Wang et al.) report biological effects in electrets. The paper of Ul Hag and Wang investigates the surface potential of epoxy electrets in relation to their insulation properties, while the paper of Wang et al. brings forth compound-structured ferroelectrets that can be used as wearable devices for health monitoring. In the following a brief presentation of each paper in this special issue is given.

Yan B. et al. propose a glass-assisted open-circuit thermally stimulated discharge (GA-OCTSD) technique. The newly developed technique is applied to study fluorinated ethylene-propylene copolymer (FEP) electret films. The influences of the glass thickness, glass dielectric properties, and glass metallisation on the GA-OCTSD spectra are investigated. It turns out that the GA-OCTSD can clearly distinguish contributions from surface charge and bulk/volume charge, which is not feasible with traditional air-gap OCTSD.

Yang X. et al. report a resilient electret film-based vibrational energy harvester with a V-shaped counter electrode. A negatively charged wavy-shaped FEP electret film generates simultaneously a stable embedded bias voltage and a large tensile deformation during vibration. Simulation and experiments are carried out to tune the resonance frequency and to optimise the output power of the device. Influences of such factors as the initial stretching state of the resilient electret film, seismic mass and depth of the V-shaped counter electrode on the performance of the device are investigated. A wide resonant frequency from 28 to 68 Hz is possible by adjusting the initial stretching state of the V-shaped FEP film. An optimised energy harvester, with a volume of only 15 × 5 × 1.7 mm³, and a tiny seismic mass of 25 mg, generates a normalised output power up to 547 μW at its resonant frequency of 28 Hz (referring to 1 × g, where g is the gravity of the earth). The miniaturised vibrational energy harvester is a promising electrical energy supplier for low-power-consumption electronic devices.

Feng Y. et al. introduce a frequency-tunable resonant hybrid vibration energy harvester (HVEH) using a piezoelectric cantilever with electret-based electrostatic coupling. An electret film is placed below the cantilever, such that the electrostatic force acting on the cantilever leads to a tunable resonant frequency and additional electrical damping boosts the output power. The resonant frequency of the HVEH, which depends on both the electret surface potential and the external resistance, can be adjusted in range of 194.6 rad/s. The maximum output power of HVEH reaches 5.2 μW, 27.4 times higher than that of the individual piezoelectric generator. The proposed energy harvester has promising potential for powering microelectronic devices and wireless sensor network node.

Chen G. et al. present a novel electret-based electrostatic micromotor (EEM) with an electret film as the stator and a metal electrode as the rotor. A maximum output power and rotation speed of 5.4 mW and 2864 rpm are realised for an EEM with a dimension of 42 × 44 × 15 mm³, respectively. By using two nickel-metal hydride batteries with a capacity of 1700 mAh, the EEM can continuously drive a fan with a diameter of 40 mm to rotate for 18 h. The easy-to-fabricate EEM, free of mechanical friction dissipation between the stator and the rotor and highly reliable, has promising application potential in microelectromechanical systems.

Jiang H. et al. explore piezoelectric poly(vinylidene fluoride) (PVDF) microfibers electrospun on polyethylene terephthalate substrate as sensor for detection of weak mechanical signals. Bending measurements show that the open-circuit voltage response of the sensor is strain-dependent but independent of the bending frequency. The sensor can detect acoustic signals within a sound pressure level of 70–120 dB and light wind from a low-power hand fan with a sound pressure between 1.31 and 3.09 Pa. Therefore, the flexible and simple-structured microfiber sensor represent a promising solution for detection, recognition and collection of weak mechanical signals.

Sun Z. et al. make use of the stable electric field of electret films to inhibit the formation of bacterial biofilms and weaken the adhesion of bacterial biofilms. It turns out that both the activity and the total amount of the biofilm noticeably decrease with the treatment of electrets of either positive or negative polarity. Employment of electrets is an environmentally friendly method that helps to decrease the resistivity of bacteria, improve the effect of antibiotics, and reduce their dosage and side effects.

Wang H. et al. study the mechanism of the influence of electret films on biofilms. The investigation carried out on staphylococcus aureus suggests that the electric field of electrets likely inhibits the expression of key genes related to bacterial biofilms. This, instead of the direct bactericidal effect, prevents the aggregation of bacteria. It is believed that the conclusion applies to other Gram-positive bacteria, indicating the application of electrostatic materials in the field of biomedicine.

Ul Hag I. and Wang F. propose various surface treatment methods, including ion-beam irradiation, sandpaper polishing, and a combination of both, as means to enhance the flashover threshold of epoxy insulations commonly utilised in high voltage direct current (HVDC) systems. In the case of electret-based electrical insulations, it is advantageous to have shallow trap energy and low trap density, as excessive trapped charges can disrupt the local electric field and trigger flashovers or breakdowns. The researchers investigate the surface potential of the epoxy electrets and find a strong correlation between the results and the flashover properties of the samples. Through surface treatments, the epoxy insulations exhibit shallow trap energy and an improved flashover threshold.

Wang S. et al. propose a compound-structured piezoelectret system consisting of a layer of polypropylene (PP) foam sandwiched between two layers of solid polytetrafluoroethylene (PTFE). The internally charged cavities in the PP foam and the charged PP/PTFE interfaces form macroscopic dipoles respectively. It is found that the foam and the layered-structure contribute individually to the piezoelectric sensitivity of the compound system. Sensors made of the compound piezoelectret films are used for sleep monitoring, carotid pulse and radial pulse monitoring. From the captured signals, such useful physiological information as breath, heartbeat, and pulse details can be extracted. The compound piezoelectret is highly suitable for developing flexible sensors in portable and wearable devices for tactile sensing, micro-energy harvesting, health monitoring, etc.

All of the papers selected in this Special Issue show that the research of polymer electrets and ferroelectrets is actively moving forward along many different avenues. It is foreseeable that polymer electrets and ferroelectrets, particularly their applications-related research and development, will continue to be active and challenging fields.