Electromechanical materials

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 ... 

Table 1 Comparison of electromechanical materials and their fundamental modes of operation (Modified and amended fi-om the Whitehead Memorial Lecture 2014 (Gerhard 2014))...

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. 

More recent development of complex perovskite structures has resulted in a new class of electromechanical materials, electmstrictors. Purely electrostrictive materials are para-electric and centrosymmetric that is, they do not possess a polar axis and are typically cubic. The electrostrictive materials of most interest are ferroelectrics that are operated above or near their transition temperatures. The electrostrictive effect is a second-order phenomenon whereby an applied electric field results in a lattice distortion and mechanical distortion in the material. 

In general, most electromechanical materials have contributions from both effects. However, typically one effect is significantly more dominant than the other. All materials... 

In most electromechanical materials, dopants are used to tailor the properties for specific applications. Isovalent substitutions are often used to modify the dielectric properties of these materials—for instance, Ba or Sr " substitution for Pb in perovskite and tungsten-bronze structures or Sn for Zr in PZT. The perovskite and tungsten-bionze structures will allow significant substitution with isovalent ions of similar size. 

By definition, electromechanical materials provide a coupling between electrical and mechanical energy. While the mechanical displacements are relatively small, these materials can generate significant electrical and mechanical forces. 

RCT are designed to successfully solve a whole number of tasks in nuclear power when testing fuel elements, in aviation and space industry when testing construction materials, nozzles and engine units, turbine blades and parts, in electromechanical industry-cables switching elements, electric motors in defense sphere- charges, equipment in prospecting for research of rock distribution and detection of precious stones in samples. 

Polarization which can be induced in nonconducting materials by means of an externally appHed electric field is one of the most important parameters in the theory of insulators, which are called dielectrics when their polarizabiUty is under consideration (1). Experimental investigations have shown that these materials can be divided into linear and nonlinear dielectrics in accordance with their behavior in a realizable range of the electric field. The electric polarization PI of linear dielectrics depends linearly on the electric field E, whereas that of nonlinear dielectrics is a nonlinear function of the electric field (2). The polarization values which can be measured in linear (normal) dielectrics upon appHcation of experimentally attainable electric fields are usually small. However, a certain group of nonlinear dielectrics exhibit polarization values which are several orders of magnitude larger than those observed in normal dielectrics (3). Consequentiy, a number of useful physical properties related to the polarization of the materials, such as elastic, thermal, optical, electromechanical, etc, are observed in these groups of nonlinear dielectrics (4). 

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). 

Magnetic and Electromechanical Separation Magnetic separation of ferrous materials is a weh-established technique. More recently, a variety of electromechanical techniques have been developed for the removal of several nonferrous materials (see Table 25-62). 

With the advent of these compounds in the 1960s, the hitherto more conventional insulating materials, such as phenol formaldehyde (popularly known as Bakelite) and wood (veneered impregnated) have been almost replaced by them. These compounds offer better electromechanical properties than conventional materials. Below we describe the basic mix and properties of these two basic compounds, for a brief reference. 

This is also known as Bulk Moulding Compound (BMC). It is blended through a mix of unsaturated polyester resin, crosslinking monomer, catalyst, mineral fillers and short-length fibrous reinforcement materials such as chopped glass fibre, usually in lengths of 6-25 mm. They are all mixed in different proportions to obtain the required electromechanical properties. The mix is processed and cured for a specific time, under a prescribed pressure and temperature, to obtain the DMC. 

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 contribution to the stress from electromechanical coupling is readily estimated from the constitutive relation [Eq. (4.2)]. Under conditions of uniaxial strain and field, and for an open circuit, we find that the elastic stiffness is increased by the multiplying factor (1 -i- K ) where the square of the electromechanical coupling factor for uniaxial strain, is a measure of the stiffening effect of the electric field. Values of for various materials are for x-cut quartz, 0.0008, for z-cut lithium niobate, 0.055 for y-cut lithium niobate, 0.074 for barium titanate ceramic, 0.5 and for PZT-5H ceramic, 0.75. These examples show that electromechanical coupling effects can be expected to vary from barely detectable to quite substantial. 

The Failure and Inventory Reporting System (FIRS) program was developed by the Geological Survey Division of the U.S. Department of the Interior for safety and pollution prevention devices on offshore structures that produce or process hydrocarbons. The program collected data on mechanical and some electromechanical systems on offshore oil platforms. About 8,000 failure events were documented. Access has been limited to internal materials management system use. No real-time access or periodic output products have been available. 

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... 

Carpi, F., Chiarelli, P., Mazzoldi, A., and De Rossi, D., Electromechanical characterisation of dielectric elastomer planar actuators Comparative evaluation of different electrode materials and different counterloads, Sensors Actuators, A107, 85, 2003. 

There are more issues and complexity to be considered if various micro-electromechanical (MEMS)-type devices are included in the macroelectronics tool kit. As described previously, the materials and devices required for TFTs and circuits can provide adequate electromagnetic (visible and RF) sensitivity for many image-type applications. These materials may also provide satisfactory performance in pressure and strain sensors. Nanotube/nanowire-based devices look promising for various chem-bio sensors.85 However, there is little that is known about the ability to integrate printed microfluidic devices (and other such devices with moving parts) into a roll-to-roll-type process. 

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