Electrets characteristics

All electrical measurements for the electret films were carried out in the group of Hilleringmann in Paderbom. A dark shielded metal box using a HP 4156A parameter semiconductor analyser identified the transistor characteristics. The influence of the interface between the Au top contacts and the Pc film was measured with a Keithley picoamperemeter in darkness as well as under illumination. 

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...

The sorption characteristics of electret coatings are described in Fig. 3.14 [15]. Sorption of liquids by electret coatings is seen to be noticeably lower than by reference ones (charge density 10 -10 C/cm ). This difference increases... 

The principal characteristics of the triboelectret state in polymers recorded experimentally are i) the efficient surface charge density (ESCD) value and ii) the thermally stimulated depolarization (TSD) current spectrum, i.e. the discharge current dependence of the electret on its heating temperature. The analysis of TSD spectra helped to estimate the parameters of the triboelectret state, including the homo- to heterocharge relation in a dielectric, activation energy of the charge relaxation processes, relaxation time and others. 

The polymeric materials are utilized in industry and an ordinary household with the characteristic that a natural organic material does not have. Research and development are performed actively now because the polymers are materials with a variety of functionality (Imai et al., 2002 Ishii et al., 2009 Ishimoto et al., 2009 Varlow Li, 2002). By such a background, we aimed at the polymer system piezoelectric material which let it give piezoelectricity as the sensor function. For typical piezoelectric material(Koga Ohigashi,1985 Lindner etal.,2002), PZT and BaTiCb are well known until now. In contract, the PVDF of the polymer system piezoelectric material immobilized CF2 dipolar orientation. Piezoelectric modulus d.33 of the pwrous polymer electrets is higher than PVDF, and in a polymer system, polaristation reversal happens. 

Optical Absorption and Photoconductivity Experiments, Biochim. Biophys. Acta. 538(1), 164-170 (1978) cf. also P. Baraldi, R. Capelletti, P. R. Crippa, and N. Romeo, Electrical Characteristics and Electret Behavior of Melanin, J. Electrochem. Soc. 126, 1207-1212 (1979). 

We define the electret state of a dielectric material, in agreement with the definition of Refs. 9 and 10, as a metastable state characterized by a stored polarization. Its decay time must be assumed long with respect to the characteristic time of experiments performed on the material. 

The dependence of built-in field on film thickness is reported in Fig. 3.10. It is seen, that this field increases essentially with the film thinning as l/h. The estimations had shown that for the films of several tens nm thickness and reasonable material characteristics, can achieve several hundreds kV/cm as it is shown in the inset to Fig. 3.10. For the sake of comparison the thermodynamic coercive field is shown by dashed line in the inset. The emergence of electret phase due to built-in field will be discussed later. 

In this eontext, ferroelectrets actually represent a third elass of piezoelectric polymers in the sense that these cellular materials share features of charge electrets (real eharges) with those of ferroelectric polymers (hysteresis-type ED characteristics) (Bauer et al. 2004 Lekkala et al. 1996). Moreover, ferroelectrets are characterized by extremely high piezoelectric 33 coefficients as well as an extreme anisotropy in their mechanical and electromechanical properties, which partially require speeifie characterization techniques (Dansachmiiller et al. 2005). 

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. 

ECMs are a form of electrostatic transducer. An electric field generated by the electret charge is used as an alternative to the external bias system of conventional electrostatic transducers. The element technologies of an omnidirectional ECM are compatible with other ECMs such as directive ECMs in the common basic technology. This section describes the basics of omnidirectional ECM and derives a general formula for its sensitivity and fi quency characteristics. 

Recently, electrets made with cellular stmctured polymers have received attention as new piezoelectric materials with a high piezoelectric 33 coefficient exceeding 100 pC/N. In particular, research on cellular polypropylene (PP) film has been conducted to improve both its physical characteristics and its applications (Neugschwandtner et al. 2000 Hillenbrand and Sessler 2000 Paajanen et al. 1998, 2001 Zhang et al. 2007). 

Eguchi (1 prepared the first electrets and studied their various characteristics. He melted equal parts of camauba wax and rosin with the addition of some beeswax, and permitted the mixture to solidify in a strong DC field between two parallel electrodes. He exfriained his success in the foUowing way ... 

Fabel arid Henisch (492) studied the characteristics of the electret by measuriitg the surfree potentiaL The measured surfoce voltage is given by ... 

In addition to the above methods, other techniques based on the chaige oompensatioo method [3S23S3], the thermal pulse method (3S435S], polarization and depolarization current methods, etc. [256,257], have also been used by some workers in the field to study the characteristics of electrets. 

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