Electret

Nordtest. Electret Filters Determination of the Electrostatic Enhancement Factor of Filter Media. Method NT WS 117. December 1997. 

Electret A type of filter that does not require a power supply, and depends on the use of a filter medium with a permanent charge. Best performance is achieved with dry air. 

Electret Another application for plastics which uses the intrinsic properties is in elec-trets (a dielectric body in which a permanent state of electric polarization has been set up). Some materials such as highly polar plastics can be cooled from the melt under an intense electrical field and develop a permanent electrical field that is constantly on or constantly renewable. 

E. Fukada, Depolarization Current and Molecular Motion in Polyethylene and Polytetrafluoro-ethylene Electrets , Rfept No AST-18401-120-75, FSTC-1901-75, Army For Sci Tech Center, Charlottesville (1976)... 

An electret is a crystal which has dipoles oriented permanently in one direction. The crystal therefore is a macroscopic dipole. 

Structure of ferroelectric BaTi03 and Pb(Ti,Zr)03 and the analogous situation in the electret WOCl4... 

There have been a number of investigations of the electret behavior of anodic oxides formed on various valve metals in different electrolytes and anodization regimes. The purpose of these... 

Various mechanisms for electret effect formation in anodic oxides have been proposed. Lobushkin and co-workers241,242 assumed that it is caused by electrons captured at deep trap levels in oxides. This point of view was supported by Zudov and Zudova.244,250 Mikho and Koleboshin272 postulated that the surface charge of anodic oxides is caused by dissociation of water molecules at the oxide-electrolyte interface and absorption of OH groups. This mechanism was put forward to explain the restoration of the electret effect by UV irradiation of depolarized samples. Parkhutik and Shershulskii62 assumed that the electret effect is caused by the accumulation of incorporated anions into the growing oxide. They based their conclusions on measurements of the kinetics of Us accumulation in anodic oxides and comparative analyses of the kinetics of chemical composition variation of growing oxides. 

The negative resistance effect is observed when anodic oxides are subjected to so-called electroforming (i.e., annealing in vacuum).93 Such a treatment removes the special features of the anodic oxides (asymmetry of conduction and electric strength, electret effect, etc.), and the negative resistance effect may be explained using the general approach developed for amorphous dielectrics.5... 

A. M. Bhagwat, and S. D. Soman, Passive Measurement of Radon and Thoron Using TLD of SSNTD on Electrets, Health Physics, 43 399-404... 

Khan, A., and C.R. Philips, Electrets for Passive Radon Daughter Dosimetry, Health Phys., 46, 141-149, (1984). 

They produced high performance electrets from thin polymer films metallized so as to yield high capacitance. Both electrical and mechanical properties of these transducers have been remarkable examples of how applications of science of solids, including knowledge of electron traps, conduction processes in insulators and the viscoelastic phenomena of semicrystalline polymers, can be combined.(6) Incidentally, similar ideas have been applied to optimization of the properties of particle microphones, through assemblies of perfectly microspherical polymer carbon systems. These have shown what limits of performance... 

Consideration of the structure of polyvinylidene fluoride (65) assuming a barrier of 3 kilo cal per mole for rotational minima of conformation of the chain by A. E. Tonelli (66) led to detailed conformation and its implications for dipole structure (Fig. 22). Indeed, the material can approximate a ferro electric. It is thus of interest in our expectations of the environments that polymers can provide for the creation of new phenomena. The total array of dipoles in polyvinylidene fluoride will switch in about 3 microseconds at 20°C with 200 megavolts per meter field. The system becomes much slower at lower temperatures and fields. But we do have a case of macroscopic polarization intrinsic to the polymer molecules, which thus supplements the extensive trapping and other charge of distribution phenomena that we have discussed in connection with electrets. 

Figure 3 shows one of our photoacoustic cell for X-ray spectroscopy of solid samples The cylindrical cell has a sample chamber at the center with volume of 0.16 cm which has two windows of beryllium (18 mm x 0.5 mm thickness). A microphone cartridge is commercially available electret type (10 mm ) and the electronics of preamplifier for this microphone is detailed elsewhere Figure 4 shows the typical experimental setup for spectroscopic study X-ray was monochromated by channel cut silicon double crystal (111) and ion chamber was set to monitor the beam intensity. Photoacoustic signal intensity was always divided by the ion chamber current for the normalization against the photon flux. X-ray was modulated by a rotating lead plate (1 mm thick) chopper with two blades. 

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