Electromechanical materials piezoelectrics

The phase structure of the phase is at the origin of the piezoelectric effects. While low molar mass Sq liquid crystals flow under the influence of an external mechanical held, the network structure of the Sq elastomers prevents macro-Brownian motions of the mesogens and deformations with large amplitudes are feasible. On the other hand, compared to solid-state crystals, the modulus of the elastomers is smaller by orders of magnitude and, moreover, can be modified by the cross-linking density of the network. With these exceptional properties, S() elastomers offer a new class of electromechanical materials that stimulate theoretical and experimental activities. 

Affine deformation Bend Dielectrostriction Direct piezoelectricity Electric polarization Electromechanical materials Electrostriction Flexoelectricity Gibbs free energy Inverse piezoelectrieity Non-affine deformation ... 

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. 

Poly(3HB) and poly(3HB-co-3HV) are piezoelectric materials, whereas the piezoelectric properties of other PHAs have not been investigated (Steinbuchel 1996). The piezoelectric materials produce electric charges on parts of their surface when mechanical pressure is applied to the crystalline material, and an electric current will result from the charges if the crystal is short circuited. Conversely, application of a voltage between certain faces of the material produces a mechanical distortion (a deformation) of the material. Piezoelectric materials have important applications in electromechanical transducers, such as microphones. In medicine, chemically synthesized piezoelectric polymers such as polyvinylidene fluoride stimulated bone growth. The piezoelectric property of poly(3HB) may be important for some medical applications (Steinbuchel 1996). 

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). 

Uncoupled solutions for current and electric field give simple and explicit descriptions of the response of piezoelectric solids to shock compression, but the neglect of the influence of the electric field on mechanical behavior (i.e., the electromechanical coupling effects) is a troublesome inconsistency. A first step toward an improved solution is a weak-coupling approximation in which it is recognized that the effects of coupling may be relatively small in certain materials and it is assumed that electromechanical effects can be treated as a perturbation on the uncoupled solution. 

The major piezoelectric applications are sensors (pickups, keyboards, microphones, etc.), electromechanical transducers (actuators, vibrators, etc ), signal devices, and surface acoustic wave devices (resonators, traps, filters, etc ). Typical materials are ZnO, AIN, PbTiOg, LiTaOg, and Pb(Zr.Ti)03 (PZT). 

The semiconducting properties of the compounds of the SbSI type (see Table XXVIII) were predicted by Mooser and Pearson in 1958 228). They were first confirmed for SbSI, for which photoconductivity was found in 1960 243). The breakthrough was the observation of fer-roelectricity in this material 117) and other SbSI type compounds 244 see Table XXIX), in addition to phase transitions 184), nonlinear optical behavior 156), piezoelectric behavior 44), and electromechanical 183) and other properties. These photoconductors exhibit abnormally large temperature-coefficients for their band gaps they are strongly piezoelectric. Some are ferroelectric (see Table XXIX). They have anomalous electrooptic and optomechanical properties, namely, elongation or contraction under illumination. As already mentioned, these fields cannot be treated in any detail in this review for those interested in ferroelectricity, review articles 224, 352) are mentioned. The heat capacity of SbSI has been measured from - 180 to -l- 40°C and, from these data, the excess entropy of the ferro-paraelectric transition... 

Probably the best measure of the effectiveness of a piezoelectric material is its electromechanical coupling constant, k, defined as... 

The piezoelectric effect is an electromechanical effect in which mechanical evoke and reverse an electric reaction in a ferroelectric material and vice versa. The word piezo has been derived from the Greek piezein which means press . Compounds are composed of positive and negative ions and are electrically neutral as a whole. The fact that electrically charged particles are still present in the crystal can for example be demonstrated by means of the electric... 

The class of ferroelectric materials have a lot of useful properties. High dielectric coefficients over a wide temperature and frequency range are used as dielectrics in integrated or in smd (surface mounted device) capacitors. The large piezoelectric effect is applied in a variety of electromechanical sensors, actuators and transducers. Infrared sensors need a high pyroelectric coefficient which is available with this class of materials. Tunable thermistor properties in semiconducting ferroelectrics are used in ptcr (positive temperature coefficient... 

The electromechanical coupling coefficient (k) is a measure of the ability of a piezoelectric material to transform mechanical energy into electrical energy, and vice versa. It is defined [5] by... 

Relaxor ferroelectrics47-49 (RFEs) have attracted considerable attention in recent years due to their unusual physical behaviour. Relaxors are technologically important as transducer/actuator materials. Relaxors are intermediate between dipolar glasses and classical FEs and exhibit both substitutional and charge disorder. They exhibit very large dielectric, piezoelectric, and electromechanical... 

For low frequency electromechanical applications in which the acoustic wavelength is much larger than the scale of component phases, some of the ceramic-polymer composites have piezoelectric voltage coefficients orders of magnitude larger than solid PZT. Such materials have obvious applications in hydrophones and other listening devices. 

Selected classes of asymmetric crystal structures exhibit the property of piezoelectricity. With the application of a mechanical strain, piezoelectric materials develop an electrical potential difference across them conversely, when a potential difference is applied to these materials, a displacement occurs. The efficiency of the conversion between mechanical energy and electrical energy is described by the electromechanical coupling constant, which practically ranges to values as high as 0.7 a value of 1 would imply complete conversion between mechanical and electrical energy. 

A review of micro-electromechanical systems (MEMS)-based delivery systems provides more detailed information of present and future possibilities (52). This covers both micropumps [electrostatic, piezoelectric, thermopneumatic, shape memory alloy bimetallic, and ionic conductive polymer films (ICPF)] and nonmechanical micropumps [magnetohydrodynamic (MHD), electrohydrodynamic (EHD), electroosmotic (EO), chemical, osmotic-type, capillary-type, and bubble-type systems]. The biocompatibility of materials for MEMS fabrication is also covered. The range of technologies available is very large and bodes well for the future. 

The main types of SiOj used in indu.siry are high-purity a-quanz, vimeous silica, silica gel. fumed silica and diaiomaceous eanh. The most important application of quartz is as a piezoelectric material (p. 58) it is used in crystal oscillators and filters for frequency control and modulation, and in electromechanical devices such as transducers and pickups tens of millions of such devices are made each year. There is insufficient natural quartz of adequate purity so it must be synthesized by hydrothermal growth of a s crystal using dilute aqueous NaOH and vitreous SiOy at 4(I0°C and... 

The performance of oscillators depends essentially on the stabihty of the acoustic device [7-9] no matter if working as electromechanical resonator or delay line. Because of its extraordinary importance we will concentrate further on resonators, namely quartz crystal resonators. However, the analysis is descriptive also for other piezoelectric materials and partly for delay line elements as well. 

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]. 

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