What Are Electrets?

Electrets are dielectric materials that maintain a quasi-permanent electrostatic charge or polarization without an external power source. In this respect they are analogous to permanent magnets, but they store electric rather than magnetic energy. First described systematically by Japanese physicist Mototaro Eguchi in 1919, who created early specimens by cooling carnauba wax in a strong electric field, electrets have since been realized in a wide variety of polymers, ceramics, and composite materials. The field draws on condensed matter physics, materials science, and electrical engineering.

An electret can hold surface charge densities on the order of 100 microcoulombs per square meter and can sustain voltage gradients of several hundred volts between its surfaces for periods ranging from days to years, depending on the material and environmental conditions. Electrets are classified by their method of formation: thermoelectrets are produced by cooling a dielectric in a strong electric field, coronaelectrets are charged by corona discharge, and piezoelectrets are cellular polymer films whose air-filled voids develop electromechanical coupling when charged. As covered in the ScienceDirect overview of electret materials, the charge in an electret can reside as trapped real charges on or within the material, as frozen-in aligned dipoles, or as both simultaneously.

Charge Storage and Polarization

The durability of charge storage in an electret depends on the bandgap and trap density of the dielectric material. Fluoropolymers such as polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (FEP) are among the most stable electret materials because their deep charge traps resist thermal de-trapping at room temperature. Polyvinylidene fluoride (PVDF) and its copolymers behave differently: they achieve high charge densities through dipolar polarization of crystalline domains rather than through trapped real charges, which gives them strong piezoelectric properties. The IEEE Xplore paper on piezoelectric polymer electrets reviews PVDF-family materials, odd nylons, and vinylidene cyanide copolymers, noting that PVDF remains the most widely studied polymer electret for sensor and actuator applications. Charge stability is further affected by humidity, radiation exposure, and mechanical deformation, all of which can accelerate surface charge decay.

Piezoelectric and Acoustic Applications

Piezoelectrets, sometimes called ferroelectrets, are a class of electret in which mechanical stress produces a measurable change in surface charge, mimicking the behavior of crystalline piezoelectrics at a fraction of the material cost. These materials consist of a cellular polymer film, typically polypropylene, whose gas-filled voids are pre-charged by corona discharge. When the film is compressed, the voids deform and the displaced charge produces a voltage signal proportional to the applied force. This principle underlies the electret condenser microphone, one of the most widely produced transducers in the world, used in consumer electronics, hearing aids, and measurement instruments. The sensitivity, low noise floor, and broad frequency response of electret microphones made them the dominant condenser microphone type through the latter half of the twentieth century and they remain common today. Research on cellular polypropylene electrets has demonstrated piezoelectric coefficients comparable to those of lead zirconate titanate (PZT) ceramics in certain frequency ranges, as documented in technical surveys published through Wiley's Journal of Applied Polymer Science on polymer electrets and their applications.

Applications

Electrets have applications in a range of fields, including:

• Condenser microphones and acoustic sensors in consumer electronics and hearing aids
• Air filtration media, where electrostatic attraction captures fine particles
• Wearable and implantable energy harvesters that convert body motion to electricity
• Radiation dosimetry, where charge decay is used to measure accumulated dose
• Electrostatic voltmeters and non-contact charge measurement instruments