Types of electrets

In order to conclude the above discussion of the spring model in which electric charges of both polarities are coupled to materials with nonuniform elastic behavior, a comparative overview of electroactive dielectric materials that exhibit piezoelectricity and/or electrostriction is presented in Table 1. The table was developed in order to smnmarize the possible roles of electric charges, their coupling to materials, and their interplay with elastic heterogeneities at different length scales (Gerhard 2014). Most of the fundamental material concepts listed in Table 1 are known as various types of electrets as indicated, but often also under other technical terms. 

Of the various types of electrets, thermoekctreti ate most important bom the scientific and application points of view. Therefore, most studies have been carried out on ther moelcctrets prepared bom polymeric nutcrials. 

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. 

The electret behavior of a polymer material is not only the result of the chemical structure of the polymer itself. Especially when employing commercial resins and films, end groups, catalyst residues, concentration, the type of processing aids and additives, processing conditions, and the thermal history have a great impact on the electret behavior of the particular material. All these parameters are commonly not... 

A comparative study on the influence of chemical structure and solubility of a series of low-molecular-weight 1,4-phenylene-bisamides in i-PP was conducted to explore their performance as electret additives [30]. Bisamides are also capable of forming well-defined nano-aggregates via self-assembly from the i-PP melt upon cooling. Here, a comparison of a dicyclohexyl-substituted 1,4-phenylene-bisamide 5 and the asymmetrically substituted cyclohexyl/ -alkyl-l,4-phenylenebisamide 6 will be presented the chemical structures are included in Fig. 24. The symmetry of the molecule, the type of substitution, and the length of the alkyl chain have an influence on the solubility in the i-PP melt while the nucleation efficiency and the ratio of the a- to [3-crystal modification will have an effect on the charge storage properties. 

EAPs can be broadly divided into two categories based on their method of actuation ionic and field-activated. Further subdivision based on their actuation mechanism and the type of material involved is also possible. Ionic polymer-metal composites, ionic gels, carbon nanotubes, and conductive polymers fall under the ionic classification. Ferroelectric polymers, polymer electrets, electrostrictive polymers, and dielectric elastomers fall under the electronic classification. 

In principle, different types of passive detectors can be used such as thermoluminescent materials, electret devices, and solid-state nuclear track detectors. 

Electrets are another type of electrically active polsrmer. These are materials that contain embedded electrical charges (155). Electret films are extensively used as membranes in microphones. The embedded electrical charges in the electret film eliminate the need to supply an external d-c bias voltage to the microphone. 

Another type of condenser microphone is the electret (more accurately called an electret-biased condenser microphone). In this case, the condenser s DC bias is supplied by an electret material, rather than a battery or power supply. The electret material, generally a fluorocarbon polymer, can be a part of the diaphragm or the backplate. Its electrostatic charge lasts almost indefinitely. A well-designed electret capsule retains its charge and sensitivity for 100 years or so. After more than 30 years of use, the sensitivity may drop by no more than 3 dB. 

The short excursion into the history of electret research already shows that there are numerous different material types and materials with the ability to quasi-permanently store charges. Depending on the charge carrier type, electrets are divided into charge electrets and dipole electrets. The following section introduces both concepts and the most important polymers belonging to each category. 

Besides, high-performance polymer electrets, sueh as Parylene HT and certain types of CYTOP, were introduced to the family of electret materials. They exhibit not only excellent electret properties (high surfaee charge density with exceptional long-term and thermal stability) but also good compatibility with MEMS fabrication process. 

The particular choice of the authors was rather to put emphasis on experimental techniques that are either specifically relevant or powerfiil with respect to ferroelectric polymers and fenoelectrets or represent recent experimental developments and trends. In this sense, room was given to nonlinear dielectric properties that can be probed by nonlinear dielectric spectroscopy and various types of hysteresis experiments. Besides a systematic description of piezoelectric and inverse piezoelectric techniques, we have added dielectric resonance spectroscopy as an all-round approach yielding elastic, piezoelectric, and dielectric properties of polymer electrets in a single dielectric measurement. 

Polymer electret and ferroelectrets have unique characteristics such as electrostatic transduction wifliout external voltage, light weight, flexibility, and so on. Their most successful apphcations are microphones and air Alters, but various other types of devices have also been proposed. In this chapter, after giving overview of their apphcations, developments of acoustic devices and power generators/energy harvesters arc discussed. 

The physical properties of low-melting-point organic materials are inadequate for certain types of applications. Hans Jaffe 27,28 showed that ceramic materials can be poled in the desired manner alter they are fired and cooled if they are properly compounded and processed. This important work opened up a new field for electret researdi and applications. 

Polymer blends have attracted a great deal of attention in recent years. Most of the studies repotted on these class of materials are based on tbeir mechanical properties. Although persistent polarization and relaxation properties of various types of mdividual polymers have been studied by several researchers, not much work hvs been repotted on the electrical and electret-forming properties of polymer blends. 

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. 

Fig. 8. Cross section of a miniature electret microphone with a separate membrane. This type allows an independent optimization of the membrane and the electret material. Holes to a back chamber decrease the stiffness of the air gap...

A photoacoustic cell of novel design was described by Patel and Kerl This cell consists of a rectangular waveguide with six electret miniature microphones. For excitation of NO a spin-flip Raman laser was used with 0,1 W power at 5,3 pm. With this apparatus a NO concentration of 1,0 10 molecules/cm could be detected. This corresponds to a measurable absorption coefficient of 10 ° cm" . A similar sensitivity may also be obtainable with Nodov s H-type cell (see Fig. 7 b)... 

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