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Incoming links: Electret
Electret Created 2026-10-20 Updated 2026-11-05
An electret is a dielectric retaining long-lived electric polarization or trapped electric charge after its preparation field is removed. The retained charge distribution can supply a persistent electric field without continuous external polarization power.
An electret condenser microphone uses this property to bias a variable capacitor. Sound moves a diaphragm, changing capacitance and producing an electrical signal in the readout circuit. The electret supplies the polarization condition; it does not provide unlimited electrical energy, and the readout can still require power. Temperature, charge leakage and material history affect long-term stability. [ref]
https://www.grasacoustics.com/faq. GRAS manufacturer FAQ: electret layer and internally supplied microphone polarization
Three standard approaches are piezoelectric, electromagnetic and electrostatic energy harvesting.
- A piezoelectric microcantilever or nanowire develops electric polarization when strained. An inertial mass, bending substrate or vibration bends the element; electrodes and a rectifier collect charge. PZT, ZnO and PVDF illustrate different material choices. This approach needs no external bias for transduction and can give appreciable voltage in a compact element. Its capacitance gives high source impedance and relatively small electric current; brittle materials, fatigue, depolarization and a narrow mechanical resonance can limit performance.
- Electromagnetic harvesting uses relative motion of a magnet and coil to change magnetic flux, producing . A micromachined coil and movable magnet can implement it at the microscale. It can offer low source impedance and useful electric current, but the small flux and coil area make voltage and power increasingly difficult to maintain on miniaturization; coil resistance and fabrication of magnets also matter. A practical microscale device does not imply efficient scaling all the way to individual nanowires.
- Electrostatic harvesting uses a variable capacitor, for example MEMS comb fingers or moving plates. Mechanical motion changes capacitance; an initial charge, electrical bias or electret allows this change to do electrical work. Small gaps favor strong coupling and compatibility with microfabrication. Limitations include the need for charge or bias, parasitic capacitance, leakage, pull-in, stiction and dielectric breakdown.
In all three cases the output is constrained by the available mechanical input and damping. Nanoscale dimensions alone do not supply an energy advantage.