Information AboutElectret |
| CATEGORIES ABOUT ELECTRET | |
| condensed matter physics | |
| electrical phenomena | |
| dielectrics | |
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SIMILARITY TO CAPACITORS There is a similarity between electrets and dielectrics in Capacitors ; the difference is that dielectrics in capacitors possess an induced polarization that is only transient, dependent on the potential applied on the dielectric, while dielectrics with electret properties exhibit quasi-permanent charge storage or dipole polarization in addition. Some materials also display Ferroelectricity , i.e. they react to the external fields with a Hysteresis of the polarization; ferroelectrics can retain the polarization permanently because they are in thermodynamic equilibrium, and are used in Ferroelectric Capacitor s. Electrets are only in a steady state, i.e. metastable, but may still store charge or polarization for extremely long periods of time. MATERIALS Electret materials are quite common in nature. Quartz and other forms of silicon dioxide, for example, are naturally occurring electrets. Today, most electrets are made from highly insulating polymers, e.g. Fluoropolymer s, Polypropylene , Polyethyleneterephthalate , etc. Real-charge electrets contain either positive or negative excess charges or both, while oriented-dipole electrets contain oriented dipoles. Electrets exhibit quasi-permanent internal or external electric fields that can be exploited in applications. ELECTRET TYPES There are two types of electrets:
Some dielectrics are capable of acting both ways. MANUFACTURE A Polarised material consists of Atom s or Molecule s with Electric Dipole Moment . It can be produced by cooling down material composed of long Molecule Chains with Electric Dipole Moment so that so-called Domain s are formed. Electrets are prepared by cooling a suitable Dielectric material in a strong electric field, after heating it to a high temperature. This process repositions the charge carriers or orients the dipoles in the material, then fixes them in position. The effect is not permanent, the charge decays exponentially, but the decay process is slow, in order of decades to centuries at normal temperature.The materials used for preparation of electrets are usually Wax es, Polymers or Resin s. One of the recipes consists of 45% Carnauba Wax , 45% white Rosin , and 10% white Beeswax , melted, mixed together, and left to cool in a static electric field of several kilovolts. Most of commercially produced electrets are based on Fluoropolymer s (eg. Amorphous Teflon ) machined to thin films. APPLICATIONS Electret materials have recently found commercial and technical interest. For example, they are used in one form of Microphone and in Copy Machine s. They are also used in some types of Air Filter s, for electrostatic collection of dust particles, and in electret ion chambers for measuring Ionizing Radiation . See for "Manufacturing Method and Device for Electret Processed Product" FURTHER READING
SEE ALSO PATENTS
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