Poling ferroelectric polymers

These materials have shown piezoelectric responses after appropriate poling [18]. Their piezoelectric actuation properties are typically worse than ceramic piezoelectric crystals however, they have the advantages of being lightweight, flexible, easily formed, and not brittle. Additionally, while ceramics are limited to strains on the order of 0.1%, ferroelectric polymers are capable of strains of 10% [91] and very high electromechanical coupling efficiencies [93]. 

P. Gilthner and K. Dransfeld, Local poling of ferroelectric polymers by scanning force microscopy, Appl Phys. Lett. 61, 1137 1139 (1992). 

In this context, the possibility to tune tire piezo- and pyroelectricity of specific composites (Floss et al. 2000) by means of separate poling of the inorganic particles and of the polymer crystallites should also be mentioned. In addition, piezo-, pyro-, and ferroelectric polymers such as PVDF and its relevant copolymers may be optimized by controlling fire poling of the amorphous and of the crystalline phase, as well as of the interface between fiiem (Maxwell-Wagner interface polarization) separately (Rollik et al. 1999). Furthermore, it is possible to follow the examples of the classical electret transducers (witti polymeric space-charge electrets) or of the dielectric-elastomer transducers (sometimes also called electro-electrets) and to... 

Relaxor ferroelectric polymers produce large strains up to 5 % under large electric fields of 150 V/pm without the necessity of poling. Such polymers are very interesting for the development of tactile-feedback devices on various substrates a potential mass application are flexible touch screens in future mobile appliances (Ju et al. 2014). CXurently, such applications are hindered by the need of high voltages to drive the relaxor ferroelectric transducers. To overcome this limitation, it is necessary to develop multilayer ferroeleclric relaxor film structures (Fig. 8a). 

After the discovery of significant piezoelectricity in polymers in the late 60s, they were immediately considered for applications in the early 70s. Interest faded in the early 80s due to problems in producing reproducible devices. After solving these initial problems by developing efficient poling procedures and suitable electrical contacting methods, piezoelectric polymers became a commercial success in many niche applications. Science progresses in waves, and currently, we face a very active research phase where piezoelectric and relaxor ferroelectric polymers are employed... 

Qin X (2010) Patterned piezo-, pyro- and ferroelectricity in poled polymer electrets. J Appl Phys 108 011101-1 11101-19... 

Norwood RA, Findakly T et al (1992) Optical polymers and multifunctional materials. In Homak LA (ed) Polymers for lightwave and integrated optics. Marcel Dekker, New York, pp 287-320 Qiu X (2010) Patterned piezo-, pyro-, and ferroelectricity of poled polymer electrets. J Appl Phys 108 011101... 

The application of high fields to charge or polarize polymer electrets inherently involves the risk of sample damage due to electrical breakdown. While local air breakdown in voids is part of the poling process, energy-rich discharges over large areas of ferroelectric polymers will lead to sample failure. 

An array of backing electrodes, realized by photolithography and chemical etching techniques, can be bonded on a polymeric subs te and can lodge a ferroelectric polymer film onto which a front electrode has been previously coaled. PVDF or copolymer films can be poled locally through such electrodes, but also a film poled ova its whole surface may be used without affecting the distribution of the acoustic field or the cross-talk between adjacent elements. 

It should be noted that the correct choice of initiator, and any other additives necessary for efficient polymerization, is crucial to the preparation of material for use as a ferroelectric polymer. This is because of the essential requirement to subject the polymer to very high electric fields to render it electrically active (a process known as poling ). Any conductive residues in the polymer can make it polable only with great difficulty, or in extreme cases impossible, owing to the high probability of dielectric breakdown. Because of this, details of polymerization methods used to prepare commercially available electrically active material are not generally accessible. 

Anisotropy is introduced into ferroelectric polymer films by the poling process, which aligns the dipoles in the direction of the poling field. By convention, this axis is known as the 3 direction, as illustrated in Fig. 5.6. For PVDF, an additional anisotropy is caused by the mechanical stretching procedure (section 5.2.5). The T axis is parallel to the stretch direction, while the 2 axis is perpendicular to this in the plane of the film. 

Organic piezo-polymers such as polyvinylidene fluoride along with trifluoroethylene and their copolymers can be made ferroelectric by poling the crystalline regions of these polymers. Because the density of these materials is close to that of human tissue, they have foimd use as transducers for ultrasound imaging with no acoustic mismatch. 

It is assumed that the composite matrix is made of polyvinylidene fluoride (PVDF). This ferroelectric polymer can be poled so that its remanent polarisation veetor 14- OX, and... 

Ferroelectric side chain liquid crystalline polymers have been synthesized recently. Because of their non-central symmetry they don t need to be poled. This has led to the recent active research efforts in the field of ferroelectric side chain liquid crystalline polymers (Guglielminetti et al., 1990 Spassky et al., 1989 Kapitza et al., 1986). 

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