Piezoelectric properties, combined polymers

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... 

Ferroelectric Ceramic—Polymer Composites. The motivation for the development of composite ferroelectric materials arose from the need for a combination of desirable properties that often caimot be obtained in single-phase materials. For example, in an electromechanical transducer, the piezoelectric sensitivity might be maximized and the density minimized to obtain a good acoustic matching with water, and the transducer made mechanically flexible to conform to a curved surface (see COMPOSITE MATERIALS, CERAMiC-MATRix). 

When the experimentalist set an ambitious objective to evaluate micromechanical properties quantitatively, he will predictably encounter a few fundamental problems. At first, the continuum description which is usually used in contact mechanics might be not applicable for contact areas as small as 1 -10 nm [116,117]. Secondly, since most of the polymers demonstrate a combination of elastic and viscous behaviour, an appropriate model is required to derive the contact area and the stress field upon indentation a viscoelastic and adhesive sample [116,120]. In this case, the duration of the contact and the scanning rate are not unimportant parameters. Moreover, bending of the cantilever results in a complicated motion of the tip including compression, shear and friction effects [131,132]. Third, plastic or inelastic deformation has to be taken into account in data interpretation. Concerning experimental conditions, the most important is to perform a set of calibrations procedures which includes the (x,y,z) calibration of the piezoelectric transducers, the determination of the spring constants of the cantilever, and the evaluation of the tip shape. The experimentalist has to eliminate surface contamination s and be certain about the chemical composition of the tip and the sample. 

The application of combined electrochemical and nonelectrochemical techniques, such as piezoelectric microgravimetry atEQCM [10,40,73,74,132,134,139-144], radiotracing [27,145], various spectroscopies [16,44,72,100,116,117,146] and microscopies [19,29,46,79,97,114,127,147,148], ellipsometry [15,21,26,86], conductivity [80], and probe beam deflection [149], has allowed irs to gain very detailed insights into the nature of electropolymerization and deposition processes, and so the production of conducting polymers, polymeric films, and composites with desired properties is now a well-established area of the electrochemical and material sciences. 

As pointed out already in Section 2.5.5, low-molecular weight ferroelectric liquid crystals (FLCs) and FLCPs are attracting a lot of interest because of their potential for electro-optical applications. The polymers offer new possibilities, e.g., as elastomers for piezoelectric elements or by copolymerization [77, 78, 105] due to the formation of intrinsic mixtures between SmC mesogenic units and other comonomers. This leads to FLCPs combining several material properties which might be utilized for colored displays in the case of comonomers containing chromophores. For the differentiated evaluation of such copolymers with reference to the possible exploitation of nonlinear optical (NLO) properties, the interplay of the different orientation tendencies of the side-chain functionalities is of crucial importance [36,106]. 

The final determining factor for a material s degree of piezoelectric response is the ability of the polymer to strain with applied stress. Since the remanent polarization in amorphous polymers is lost in the vicinity of Tg, the use of these piezoelectric polymers is limited to temperatures well below Tg. This means that the polymers are in their glassy state, and the further away from Tg the use temperature is, the stiffer the polymer. This also means that measurement of the bulk physical properties is crucial both for identifying practical applications and for comparing polymers. The electromechanical coupling coefficient, kai, is a measure of the combination of piezoelectric and mechanical properties of a material (refer to Table III). It can be calculated using the equation below ... 

As mentioned above, a composite in general is a heterostructural materuJ whose properties are determined by the contents, the number of different phases of which the material is composed, their properties, and the ways in which different phases are interconnected (73.67]. The latter is the most important feature of composites, since the mixiiig rules of a given property are controlled by the self-connectiveneas of individual phases. Piezoelectric polymer-ceramic composites, for example, have a number of appUcatioas. since their properties can be tailored to the requirements of various devices by combining the superior properties of a polymer and those of ceramics [67]. 

This development has been initiated by the in )Oftant work of Udiida el al. (lOS] Development of materials which lake advantage of the polymer spectfic properties (glassy pbm) in combination with a high spontaneous polarization for NLO or piezoelectric applicatiofls. 

Elastomeric LCP networks with the potential to show piezoelectric behavior were described by Zentel and co-workers [109, 110, 125, 773]. More recently, Meier and Finkelmann [774, 775] created piezoelectric cholesteric elastomers and described some of their properties. Polymers [776] and networks [126,777] with nonlinear optical responses were recently synthesized. Some of these polymers are liquid crystalline [776], We see no reason why mesogenic monomers and NLO responsive monomers may not soon be combined to form permanently crosslinked NLO LCP networks. 

The coupling between the phases is no longer electrical but mechanical via the elastic constants and 2- Thus, combining a phase 1, which is non-piezoelectric and flexible and a phase 2, which is piezoelectric and rigid, the composite is still piezoelectric (d PP = d2), but the coefficient g PP can be specifically increased by minimizing the denominator of g PP. These special properties of piezoelectric composites, prepared from PZT and a polymer, have been taken advantage of in various apphcations (medical and sonar imaging, for example). 

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