328 Electrets

Electrets are dielectric bodies that can retain an electric field for a certain time after it has been applied. They are only formed by polymers with poor electrical conductivity, for example, poly(styrene), poly(methyl methacrylate), poly(propylene), polyamides, or carnauba wax. 

As yet, the principles of electret formation are not fully understood. It is likely that both volume and surface polarizations can occur. A volume polarization is obtained with fields below 10 kV/cm. That is, if an electret is parted parallel to the charged surface, two new capacitors result. With fields of more than 10 kV/cm, an ionization, electronic failure, or break- 

The origin of the homocharges was explained in the following way. As the volume polarisation increases at the beginning of poling (stage A) the field in 

It took a long time for this somewhat complicated behaviour of electrets to be sorted out, and in the meantime high-grade insulating polymers had been developed. The question then to be settled was what system would give the most stable electret, and this prompted a study of the relevant decay processes in polymers. 

The conclusion from these results is that homocharges are much longer lasting than heterocharges, and this has led to the recent production of electrets 

Another way of investigating the depth in energy to which charges are trapped (a measure of the stability towards thermal decay) may be obtained by thermally stimulated currents (TSC). In this technique, electrodes on the two sides of the electret are connected via a sensitive current meter and the specimen is then heated at a constant, slow rate (1 Cmin-1, say). Discrete current peaks are observed as a function of temperature as successively more deeply trapped charges are released (Fig. 7.22). Dipolar relaxation may also give peaks in the TSC spectrum (van Turnhout, 1975). 

Charge densities as high as 1 mCm 2 can be obtained with a time constant for decay in excess of 20 years. The principal application for such electrets is 

As nonconductors of electricity, dielectrics interact with electric fields through their ability to form dipoles, which is called polarization. There are three sources of polarization electric, ionic, and dipolar. 

Electronic polarization results from the orbits of the electrons being displaced from the nuclear charge by the electric field. The dielectric constant from the electronic polarization is denoted as s(oo). The resonance frequency occurs in the ultraviolet. 

Ionic polarization occurs in ionically and covalently bonded materials on which there is some charge separation. The ionic dielectric constant plus the electronic dielectric constant is denoted by s(0). The resonance frequency for ionic polarization is in the infrared. 

Polar liquids and gases possess permanent dipoles which can be aligned by an applied electric field. The aligning energy provided by the field is in competition with thermal motion which tends to randomize the orientation of the dipoles. Such materials are called paraelectrics. When the field is removed, the dipoles relax with a time constant t, which is on the order of 10 s for liquids such as water where the dipoles are free to rotate. These dipoles can couple with microwave radiation to cause heating of the material, which is the principle on which microwave ovens operate. 

In dipolar solids, frozen polar liquids or solids with built-in dipole moments, the dipoles can also be aligned by an external field and are not free to rotate. In this case, the relaxation time is proportional to exp(— / /fcT), where / is the potential barrier that must be overcome for the dipole to rotate. If the potential barrier is on the order of the chemical bond, the relaxation time becomes virtually infinite. Such solids are said to have spontaneous or permanent dipole moments and are called pyroelectrics. If the dipole moment can be reversed by an electric field, the material is also a ferroelectric. 

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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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