Electret Analysis
Electret: Theoretical Foundations
What is an Electret?
Professor, what is an electret?
A dielectric that permanently retains electric charge. The electrical equivalent of a magnet. Used in condenser microphones, air filters, and energy harvesting.
Primarily manufactured by injecting charge via corona discharge into fluoropolymer materials (PTFE, FEP). Capable of retaining charge for decades.
Summary
- A dielectric that retains permanent charge — the electrical version of a permanent magnet
- Condenser microphones — the largest application of electrets
- Calculate electric field distribution with FEM — apply surface charge density as a Neumann BC
The Discovery of Electrets—Mototaro Eguchi Created an Electrical "Fossil" in the 1920s
An electret is a material with permanent electrical polarization, first created in 1919 by Japan's Mototaro Eguchi by applying a strong electric field to a mixture of beeswax and carnauba wax and then cooling it. This material, which could be called an "electrical fossil," revolutionized microphones in the 1960s; nearly all microphones in modern smartphones and hearing aids are of the electret type. In FEM analysis, a special formulation is required that incorporates the permanent polarization P (polarization vector) of the electret as a material constant.
Computational Methods for Electret
Electrets in FEM
Set the surface charge density $\sigma_s$ on the electret surface as a Neumann boundary condition. Treat internal polarization as a volume charge density $\rho_p = -\nabla \cdot \mathbf{P}$.
Summary
- Set surface charge $\sigma_s$ as a Neumann BC
- Treat internal polarization as a volume charge density
FEM Formulation for Electrets—How to Incorporate Polarization Vector P
In FEM formulation for electrets, a common method is to convert the permanent polarization P into equivalent charge densities (volume charge density ρ_pol=-∇·P, surface charge density σ_pol=P·n) and add them to the Poisson equation. Commercial tools often implement material models that perform this conversion automatically, but when using custom materials, it is necessary to manually calculate ρ_pol and σ_pol and provide them as boundary conditions. If the permittivity ratio between the electret layer and the air layer is large, the mesh near the interface must be refined, otherwise the accuracy of the electric field concentration will degrade.
Electret in Practice
Practical Applications
Design of MEMS microphones, vibration power generation devices (electret-type energy harvesters).
Checklist
- [ ] Are the charge density values based on actual measurements? (PTFE: approx. 1 to 10 mC/m²)
- [ ] Is the mesh for the air gap sufficient?
- [ ] Is the calculation of electrostatic force (Maxwell stress tensor) correct?
Electret Fibers in N95 Masks—CAE of the Mechanism that Captures Viruses via Static Electricity
The filter layer of an N95 mask is made of electret non-woven fabric, where the fibers are permanently charged and capture PM2.5 and virus droplets via electrostatic attraction. During the COVID-19 pandemic, information spread that "sterilizing masks in a microwave oven removes the static electricity, making them no longer N95." This is based on the fact that the polarization of electret fibers disappears with heat. By analyzing the relationship between the electric field distribution between fibers and particle capture efficiency using FEM, the design optimization of next-generation filters is advancing.
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