Electromechanical materials properties

In the schematic shown in Figure 4.2.10, the RF path is visible between the two signal sources (RF ports) used for extracting the S parameters, and is composed of a length of microstrip transmission line from each port connected to a model for a series-switch plate . Driven by the 6 mechanical wires at each side, which control its position, the switch plate is internally modeled as an equivalent circuit including transmission line, frequency-dependent resistance, and variable capacitance between the conductor on the plate and the underlap of the ends of the microstrip lines separated by the gap for the switch isolation. As with the beams, this model is defined by a complete set of parameters, such as the dimensions and material properties. Parameters can be adjusted quickly to achieve the desired RF performance for different closing states of the electromechanical structure. 

Berlinite crystals are very much similar to quartz. Stmctural phase change takes place at the temperature 584°C (Blistanov et al. 1982). For material properties see Table 7.6. Similarly to quartz berlinite crystals undergo a- to jS-phase transition. Crystals are grown by hydrothermal method (Detaint et al. 1985 Motchary and Chvanski 2001) Berlinite crystals exhibit higher electromechanical coupling coefficient than quartz. 

Electromechanical coupling coefficient is higher for LGS than for quartz. Some of the LGS non-linear material coefficients are pubhshed in Sorokin et al. (1996). Temperature coefficients published by different authors show values widely scattered. For material properties and their temperature coefficierrts see Tables 7.7 and 7.8. (Adachi et al. 1999 Bohm et al. 1999, 2000 Ilyaev et al. 1986 Kaminskii et al. 1983a,b, 1984 Onozato et al. 2000 Pisarevskii et al. 1998 Silvestrova et al. 1986, 1987, 1993 Sorokin etal. 1996). 

In this chapter, I will conduct a review on some of the fundamental material properties of relaxor ferroelectric PLZT ceramics, which include the dielectric, ferroelectric, electromechanical, electro-optical and thermo-optical behaviours. Further details on each section can be found in the references (Levesque and Sabat 2011 Sabat, Rochon, and Mukherjee 2008 Sabat and Rochon 2009b Sabat and Rochon 2009c Sabat and Rochon 2009a). 

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. 

As mentioned above, the correlation between specified and measured bulk material properties is problematic and further implications will become obvious in the subsequent detailed comparison between experiments and theory. Within the bounds of such variations and of the measurement accuracy, both representatives of the different micro-electromechanical modeling methodologies can be successfully validated. 

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. 

Figure 2.14 M-Test structures. (Top) Fixed-fixed beam (FB). (Bottom) Clamped diaphragm (CD). (Reprinted with permission from J. Micro-electromechanical Systems, M-TEST a test chip for MEMS material property measurement using electrostatically actuated test structures, P.M. Osterberg and S.D. Senturia, 1967 IEEE.)...

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). 

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. 

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... 

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