Introduction of electret
After removing the external electric field or applying mechanical action, the dielectric that can maintain the polarization state for a long time is called electret. The electret has both piezoelectric effect and pyroelectric effect. In technology, polar polymer is mostly used as electret material. The electret can generate a strong electric field of 30 kV / cm. The electret's ability to store charge has been used in electrophotography and gas filters that adsorb fine particles in gases.