Electret materials

Some of the insulating polymers such as poly (1, 1-difluoroethylene) meet the requirements of electret materials (the electric analogue of a permanent magnet) they respond to applied electric fields by limited movement of charge, and the charge does not leak back when the field is removed. 

Keywords Charge storage properties Commodity polymers Corona charging Electret materials High performance polymers Physical aging Polymer films... 

Electret materials are meanwhile used in a large number of modern high-tech applications including microphones, acoustic sensors, transducers, radiation and pollution dosimeters, power generators, filters, and many more. Additionally, electret technology is of great interest in the field of biomaterials, for instance in callus formation and wound healing [10, 11], When used in cellular or in multilayer sandwich structures, polymer electrets can exhibit piezoelectricity. Such materials are ferroelectrets, as they show typical features of ferroelectric materials such as piezo-and pyroelectricity [12-17],... 

Of special interest for automotive and filter applications are temperature-stable electret materials which maintain their shape and electret properties at high temperatures, typically around 120°C. With commodity polymers (Fig. 4) this requirement cannot be fulfilled due to their low CST of max. 100 °C for instance of i-PP. 

Figure 5 displays the ITPD curves determined at 90 °C of PEEK, PES, and PPS films [27]. Both sulfur-containing polymer films exhibit a pronounced charge drain-off within the first 180 min and hence cannot be considered as electret materials. PEEK, on the other hand, shows a moderate surface potential decay and maintains around 50% charge after 24 h at 90°C. The charge storage characteristics of commercial polyimides and polyetherimide (Ultem 1000) films at 90 °C are displayed in Fig. 6. Kapton HN (PI) films lose almost the complete surface potential within 24 h, exhibiting a steep decay in the first minutes. Upilex R renders better results, since its potential decay is moderate and after 24 h around 45% of the applied charge...

Based on these results, our recent research on polymer electret materials focuses on the understanding of (1) the role of the chemical structure of the polymer, (2) the influence of additives, and (3) the variation of processing parameters on the electret performance. As schematically illustrated in Fig. 7, the objective of our research is to provide concepts and methods to improve substantially the electret performance of polymer materials. 

This review summarizes our work at the University of Bayreuth over the last few years on improving the electret performance of the commodity polymer isotactic polypropylene (Sect. 3) and the commodity polymer blend system polystyrene/polyphenylene ether (Sect. 4) to provide electret materials based on inexpensive and easily processable polymers. To open up polymer materials for electret applications at elevated temperatures we concentrated our research on commercially available high performance thermoplastic polyetherimide resins and synthesized several fluorinaled polyetherimides to identify structure-property relations and to improve further the performance at elevated temperatures (Sect. 5). 

In summary, at comparable concentrations, films containing Irgafos 126 are superior electret materials compared to films containing Irgafos P-EPQ. This result is in agreement with the fact that Irgafos 126 is a more efficient antioxidant than Irgafos P-EPQ [70],... 

In this chapter we have presented concepts and methods to improve polymer electret materials. Corona charging was employed for film charging since this method possesses the advantages of being cost-effective, easy to perform, and applicable in short periods of time. 

A very simple but yet time-consuming treatment to improve amorphous electret materials is physical aging prior to corona charging. For instance, the charge retention ofUltem 1000 films after 24h at 120 °C can be raised from 18% to 95% when the films were physically aged at 200 °C for 4 days. 

Special thanks go to Prof. Gerhard M. Sessler and to Dr. Joachim Hillenbrand (Technical University of Darmstadt) for their continuous support and many fruitful discussions on the physics and characterization of our polymer electret materials. Additional thanks for performing the TSD measurements. 

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...

Electret is an important cross-scientific subject of dielectric physics, material science, sensor engineering, medical and bio-engineering (Sessler G., Sessler et al., Nalwa H., Goel M.)-Over the years, considerable interest has been shown in the surface potential decay of corona-charged polymeric materials. Besides the electrets material and conditions of producing electrets the surface potential decay depends on number of factors under which the electrets have been stored or used, for example temperature, humidity, pressure etc. (Zhongfu et al, Gang-Jin et al., Ribeiro et al.). 

At times, particularly with amorphous materials, the term electret is used instead of piezoelectric to describe a material the term was introduced as an electric counterpart to the magnet, defined as all materials that are able to retain at least a quasi-permanent electric polarization within and perhaps trap charge on the surface (or within in a porous material). Polytetrafluor-ethylene (PTFE) is a powerful electret material. The critical difference among the terms is that electrets retain polarization in thermod3mamic wowequilibrium, pyroelectric materials retain polarization in equilibrium, ferroelectrics permit... 

CaoY, Xia ZF, Li Q, Shen J, Chen LY, Zhou B (1998) Study of porous dielectrics as electret materials. IEEE Trans Dielectr Electr Insul 5 58-62... 

The relative dielectric constant of silica aerogels can be as low as 1.1 [165]. Hence, thin film silica aerogels could be and are being considered as super-low dielectric constant material for integrated circuits in computers. It is also possible to modify the surface of silica aerogel to obtain good electret materials (i.e., materials that produce a permanent external electric field) [166]. 

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. 

Cardioid pattern A microphone pickup pattern whose polar plot is graphically depicted as heart shaped. Electret (or electret-biased) microphone A variation on the basic condenser microphone in which the DC bias is suppHed by an electret material, rather than a battery or power supply. 

Electrets are dielectric materials capable of quasi-permanently storing electric charges at their surface or in their bulk. This chapter presents a brief history of electret research, followed by a classification and introduction of the most important electret materials. The chapter also discusses ferroelectrets and recent developments in charge stability. 

Polytetrafluoroethylene (PTFE) and its copolymers fluoroethylenepropylene (FEP), perfluoroalkoxy (PFA), and amorphous PTFE, a perfluorinated dioxole (Teflon AF), are the best electret materials for storing negative charges due to the large electronegativity of the F atom and the highly symmetrical CF2 group with its... 

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