Dielectric properties, ferroelectric polymers

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. 

Considerable interest also has been directed at the use of multicomponent composites where, in theory, the most useful properties from each phase can be realized in the whole. This includes metallodielectric structures where a metallic phase imparts, for example, a high index or more exotic effect (e.g., plasmon resonance) and a low-loss or property-tunable dielectric phase. The dielectric phase can be ceramic or polymeric and also has included ferroelectric polymers, embedded nanoparticles, and organic/inorganic hybrids. ... 

CONCEPTS More about relaxation process within solids Typical loss peaks are broader and asymmetric in solids, and frequency is often too low compared with Debye peaks. A model using hypotheses based on nearest-neighbor interactions predicts a loss peak with broader width, asymmetric shape, and lower frequency [27]. This behavior is well suited to polymeric, glassy materials and ferroelectrics. Low temperature loss peaks typically observed for polymers need many-body interactions to be obtained. Although current understanding of these processes is not yet sufficient to enable quantitative forecasting the dielectric properties of solids may offer insight into the mechanisms of many-body interactions. 

Conduction and dielectric properties are not the only electrical properties that polymers can exhibit. Some polymers, in common with certain other types of materials, can exhibit ferroelectric properties, i.e. they can acquire a permanent electric dipole, or photoconductive properties, i.e. exposure to light can cause them to become conductors. Ferroelectric materials also have piezoelectric properties, i.e. there is an interaction between their states of stress or strain and the electric field across them. All of these properties have potential applications but they are not considered further in this book. 

Ferroelectric composites are alternatives to standard piezoelectric and pyroelectric ceramics such as lead zirconate titanate (PZT) and BaHOs (BT). They combine the strong ferroelectric and dielectric properties of ceramics with the easy processing and good mechanical properties of polymers. Dispersion of micrometer-sized ferroelectric particles in an electrically passive epoxy matrix was first published by Furukawa et al. [1976] and later extended to ferroelectric matrices such as poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-3-fluoroethylene) (PVDF-TrFE) [Hsiang et al., 2001 Hilczer et al., 2002 Gimenes et al., 2004 Lam et al., 2005 Beloti et al., 2006]. However, the necessity of miniaturization of electronic components and... 

PVDF was discovered by Dr. Heiji Kawai in 1969 [32]. Although PVDF is a piezoelectric system, it is a ferroelectric cum piezoelectric material as explained earlier, with a Curie point of 103 °C. PVDF possesses various phases such as a, p, y, and 5, among which p-phase has the most responsive piezoelectric properties. Compared with all ferroelectric polymers, PVDF has a dielectric constant with a reasonable chemical and mechanical durability [4,32]. In general, the physical properties of PVDF make it the most valuable material for application in sensors. 

Nalwa H, Fukada E (eds) (1995) Ferroelectric polymers. Marcel dekker. New York Newman B et al (1980) The piezoelectricity of poly(vinyhdene fluoride). J Appl Phys 51 5161 Omote et al (1997) Temperature dependence of elastic, dielectric, and piezoelectric properties of single crystalline films of vinylidene fluoride trifluoroethylene copolymer. J Appl Phys 81 2760... 

The ferroelectric effect was discovered in 1920 by Valasek, who obtained hysteresis curves for Rochelle salt analogous to the B-H curves of ferromagnetism [5.5], and studied the electric hysteresis and piezoelectric response of the crystal in some detail [5.6]. For about 15 years thereafter, ferroelectricity was considered as a very specific property of Rochelle salt, until Busch and Scherrer discovered ferroelectricity in KH2PO4 and its sister crystals in 1935. During World War II, the anomalous dielectric properties of BaTiOs were discovered in ceramic specimens independently by Wainer and Solomon in the USA in 1942, by Ogawa in Japan in 1944, and by Wul and Goldman in Russia in 1946. Since then, many ferroelectrics have been discovered and research activity has rapidly increased. In recent decades, active studies have been made on ferroelectric liquid crystals and high polymers, after ferroelectricity had been considered as a characteristic property of solids for more than 50 years. 

There are only a few publications which treat the dielectric and ferroelectric properties and the related features of blentb of PVDF with polymers other than PMMA [53,97]. 

The most interesting properties of polymers are their high mechanical and electrical strength and low electrical conductivity and acoustic impedance, whereas the ferroelectric ceramics exhibit good dielectric, pyroelectric, and piezoelectric properties (4,8,67]. 

Polymer composites are multiphase materials containing, usually, inorganic fillers or reinforcing materials embedded in an anurphous or polycrystalline matrix. The dielectric properties of the inclusions arc, usually, very different from those of the nutrix. For ferroelectric appUcatioos, inorganic ferroelectric materials (e.g.. ceramics) are often used u fillers. 

Levstik A, Carlsson T, Filipic C, Levstik I, Zeks B (1987) Goldstone and soft mode at the smectic-A-smectic-C phase transition studied by dielectric relaxation. Phys Rev A 35 3527-3534 Li J, Wang Z, Cai Y, Huang X (1998) Study of EO properties of polymer network stabilized of ferroelectric hquid crystal in smectic C phase. Ferroelectrics 213 91-98 Li J, Zhu X, Xuan L, Huang X (2002) V-shaped electro-optic characteristics in ELC gels. Ferroelectrics 277 85-105... 

Conducting polymers also can be utilized to form core-shell structures with high dielectric constant particles. Fang et al. used PANl to encapsulate barium titanate via in situ oxidative polymerization. They examined the influence of the fraction of BaTiOs particles on the ER behavior, and found that the PANl/ BaTiOs compo-sites-based ERFs exhibit a better ER effect than does pure PANl, which result might be due to the unique ferroelectric properties as well as the high dielectric constant of BaTiOs nanoparticles. 

Like solid ferroelectrics, the ferroelectric liquid crystals, particularly the FLCPs, show a pyroelectric effect and a piezoelectric effect and are capable of switching polarization direction (dielectric hysteresis). Moreover, they can switch propagating or reflected polarized light. Finally, the polar symmetry of the phase leads to nonlinear optical properties of the FLCPs such as second-harmonic generation, the Pockels effect, and the Kerr effect. These physical properties of the ferroelectric LC polymers are discussed in the following sections. 

Pfeiffer, M., Beresnev, L. A., Haase, W., Scherowsky, G., Kuehnpast, K., and Jung-bauer, D., Dielectric and electrooptic properties of a switchable ferroelectric liquid crystalline side chain polymer. Mol. Cryst. Liq. Cryst., 214, 125-141 (1992). 

Li Z, Grimsditch M, Xu X, Chan SK (1993) The elastic, piezoelectric and dielectric constants of tetragonal PbTiOs single crystals. Ferroelectrics 141 313-325 MarraSP, Ramesh KT, Douglas AS (1999) The Mechanical properties of lead-titanate/polymer 0-3 composites. Compos Sci Technol 59 2163-2173 Materials Data Sheets of APC International, Tokin, Ferroperm, Morgan Matroc, Siemens Mattiat OE (1971) Ultrasonic transducer materials. Plenum Press, Tokyo McLachlan DS, Blaszkiewicz M, Newnham RE (1990) Electrical resistivity of composites. J Am Ceram Soc 73 2187-2203... 

Poly(vinylidene fluoride), PVDF or PVF2, is usually manufactured from radical initiated batch polymerization process in aqueous emulsion or suspension of CH2=CF2 monomer. PVDF is a thermoplastic that exhibits interesting properties, such as piezoelectric, pyroelectrical, and ferroelectric behaviors. PVDF has even superior dielectric permittivity arising from the strong polymerization originating from C—F bonds, and the spontaneous orientation of dipoles in the crystalline phases makes it a polar polymer with good compatibility with polar chemicals. 

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