Piezoelectric polymers displays

A typical example for the frequency- and temperature-dependent dielectric properties of a piezoelectric polymer is given in Fig. 7 that displays the a-relaxation, related the dynamic glass transition, of a polyvinylidene fluoride (PVDF) film along with an upswing of the dielectric loss at low frequencies due to electrical conduction. 

The ink-jet process relies on using a piezoelectric printhead that can create deformation on a closed cavity through the application of an electric field. This causes the fluid in the cavity to be ejected through the nozzle whose volume is determined by the applied voltage, nozzle diameter, and ink viscosity. The final width of the drop of the substrate is a result of the volume of fluid expelled and the thickness of the droplet on the surface. In addition, the drop placement is critical to the ultimate resolution of the display. Typical volumes expelled from a printhead are 10 to 40 pi, resulting in a subpixel width between 65 and 100 pm. Drop accuracies of +15 pm have been reported such that resolutions better than 130 ppi are achievable however, because the solvent to polymer ratio is so high, the drops must be contained during the evaporation process to obtain the desired resolution and film thickness. This containment can be a patterned photoresist layer that has been chemically modified so that the EL polymer ink does not stick to it. 

Ferroelectric crystals (especially oxides in the form of ceramics) are important basic materials for technological applications in capacitors and in piezoelectric, pyroelectric, and optical devices. In many cases their nonlinear characteristics turn out to be very useful, for example in optical second-harmonic generators and other nonlinear optical devices. In recent decades, ceramic thin-film ferroelectrics have been utilized intensively as parts of memory devices. Liquid crystal and polymer ferroelectrics are utilized in the broad field of fast displays in electronic equipment. 

Some ceramic and polymer materials also display piezoelectric properties. For ceramics these include polycrystal hne materials based on solid solutions of Pb(Zr, Ti)03. The ratio of zirconium and titanium constituents help to determine the crystal symmetry structure that results after heating and then cooling of the samples. Modern piezoelectric ceramics are referred to as PZT ceramics. Other materials such as barium, strontium, caldmn, and/or trivalent rare earth elements can be added to the ceramics in small quantities to modify the piezoelectric properties. 

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]. 

Materials or surfaces are said to be responsive if they display a pronounced response to an environmental stimulus, particularly a response that may be suitable for application. Some responses in the form of physical or phase changes can be switchable or reversible. With the development of materials science, especially with the development of synthetic polymers and surface chemistry, these materials and surfaces have been designed for broad applications. Smart or intelligent has also been used to describe these materials since the 1980s. Shape-memory alloys and polymers, piezoelectric materials, and switchable glass are all good examples. 

This chapter presents a brief overview on sensor and transducer applications of piezoelectric and electrostrictive polymers. Piezoelectric and electrostrictive polymers are smart electromechanical materials which have already found commercial applications in various transducer configurations. Novel applications may arise in the emerging fields of autonomous robots, electronic skin, and flexible energy generators. This chapter focuses on recent device demonstrations of piezoelectric and electrostrictive polymers in these novel fields of research to stimulate and to facilitate the exchange of ideas between disciplines. The applications considered include piezoelectric sensors for electronic skin, piezoelectric loudspeakers and transducers for mechanically flexible energy harvesters, as well as electrostrictive transducers for haptic feedback in displays. 

Ihble 2 shows a comparison of physical quantities for three optically active polymers described above. With an increase of polarity in chemical structure, the magnitude of the piezoelectric constant increases remarkably, although the degree of crystallinity and the degree of orientation are not exactly the same for the three polymers. The chemical structure of poly-lactic acid is the simplest form to oou de an asymmetric carbon atom and a polar group CO-O and is most suitable for displaying the piezoelectric effect in this aeries. 

The display of piezoelectricity of Sc FLC polymers can be explained by the inherent C symmetry. For this symmetry the piezo tensor has the following form, with eight nonzero coefficients [ISO] ... 

An nnusual phenomenon exhibited by a few ceramic materials (as well as some polymers) is piezoelectricity—WiexaWy, pressure electricity. Electric polarization (i.e., an electric field or voltage) is induced in the piezoelectric crystal as a result of a mechanical strain (dimensional change) produced from the application of an external force (Figure 18.36). Reversing the sign of the force (e.g., from tension to compression) reverses the direction of the field. The inverse piezoelectric effect is also displayed by this group of materials—that is, a mechanical strain results from the imposition of an electrical field. 

---