Piezoelectric groups

Piezopolymers are a fascinating piezoelectric group of materials, having good piezo performance and much higher flexibility than piezoceramics. Natural polymers such as polysaccharides, proteins and polynucleotides have shown some piezoelectric properties (Fukada, 2000). The polymers have superior benefits over other materials due to their ability to form yams and fabrics. Polyvinylidene fluoride (PVDF) is a piezoelectric polymer with remarkable piezo properties and is a highly suitable material for textile applications. 

A major advance in force measurement was the development by Tabor, Win-terton and Israelachvili of a surface force apparatus (SFA) involving crossed cylinders coated with molecularly smooth cleaved mica sheets [11, 28]. A current version of an apparatus is shown in Fig. VI-4 from Ref. 29. The separation between surfaces is measured interferometrically to a precision of 0.1 nm the surfaces are driven together with piezoelectric transducers. The combination of a stiff double-cantilever spring with one of a number of measuring leaf springs provides force resolution down to 10 dyn (10 N). Since its development, several groups have used the SFA to measure the retarded and unretarded dispersion forces, electrostatic repulsions in a variety of electrolytes, structural and solvation forces (see below), and numerous studies of polymeric and biological systems. 

Some electrical properties are shown in Table 3. Values of other parameters have been pubflshed (146). Polymorphism of the PVDF chains and the orientation of the two distinct dipole groups, —CF2— and —CH2—, rather than trapped space charges (147) contribute to the exceptional dielectric properties and the extraordinarily large piezoelectric and pyroelectric activity of the polymer (146,148,149). 

It should be noted that, whereas ferroelectrics are necessarily piezoelectrics, the converse need not apply. The necessary condition for a crystal to be piezoelectric is that it must lack a centre of inversion symmetry. Of the 32 point groups, 20 qualify for piezoelectricity on this criterion, but for ferroelectric behaviour a further criterion is required (the possession of a single non-equivalent direction) and only 10 space groups meet this additional requirement. An example of a crystal that is piezoelectric but not ferroelectric is quartz, and ind this is a particularly important example since the use of quartz for oscillator stabilization has permitted the development of extremely accurate clocks (I in 10 ) and has also made possible the whole of modern radio and television broadcasting including mobile radio communications with aircraft and ground vehicles. 

Crystals with one of the ten polar point-group symmetries (Ci, C2, Cs, C2V, C4, C4V, C3, C3v, C(, Cgv) are called polar crystals. They display spontaneous polarization and form a family of ferroelectric materials. The main properties of ferroelectric materials include relatively high dielectric permittivity, ferroelectric-paraelectric phase transition that occurs at a certain temperature called the Curie temperature, piezoelectric effect, pyroelectric effect, nonlinear optic property - the ability to multiply frequencies, ferroelectric hysteresis loop, and electrostrictive, electro-optic and other properties [16, 388],... 

An alternative method to position two electrodes at nanometer distances apart is the mechanically-controlled, break junction (MCBJ) technique. An ultra-thin, notched Au wire on a flexible substrate can be broken reliably by pushing on the Au with a piezoelectric piston, cracking the Au (Fig. 4). This produces a gap between the Au shards whose size can be finely varied to 1 A by a piston or control rod [46, 47]. When UE molecules with thiol groups on both ends are present in a surrounding solution, the gap can be adjusted until the molecules can span it. A dilute solution means the number of spanning molecules will be small, and the least-common-multiple of current flow among many junctions indicates those spanned by a single molecule [47]. 

An altemahve approach is the "mass-sensing" multianalyte microarray immunoassay first described by researchers at Beckman Coulter, Fullerton, CA (Silzel et al., 1998). As early as 1991, other groups at Beckman Coulter had adapted commercially available ink-jet printers (e.g., the Hewlett Packard Deskjet) for deposihng oligonucleohdes or proteins such as streptavidin onto substrates to create arrays (Matson, xmpublished data). Piezoelectric... 

University of Alberta. Educational Software for Micromachines and Related Technologies. Available online. URL http //www. cs.ualberta.ca/ database/MEMS/sma mems/index2.html. Accessed May 28,2009. Research groups at the University of Alberta in Canada constructed this Web resource, which discusses a variety of smart materials, including shape-memory alloys, piezoelectric materials, and electrorheological and magnetorheological fluids. 

This crystal, which belongs to the space group monoclinic C2, is associated with the piezoelectric tensor (chain axis = c-axis) ... 

Group (A) includes materials with intrinsic piezoelectricity and dtA is twice the d-eonstant for elongation along the direction at 45° to the draw-axis. The value depends on the degree of orientation and degree of crystallinity. 

Group (Q includes as-cast films which exhibit a weak piezoelectricity due to charges embedded in the film. The charge may be a true charge and/or a polarization charge and the film forms a weak mechano-electret... 

The piezoelectric effect has been shown to exhibit a relaxational nature and the complex piezoeletric constant is a function of frequency and temperature. In Group (A), the relaxation is ascribed to either the nature of the crystallite itself or the viscoelasticity of the amorphous phase in which the crystallites are embedded. In the former case, the piezoelectric relaxation is a cross-coupling phenomenon of dielectric relaxation and mechanical relaxation of the crystallite. In the latter, on the other hand, the relaxation is governed by the mechanical relaxation of the amorphous phase. 

In Group (B), the piezoelectricity is strongly affected by the electro-striction effect in its anisotropy and relaxational behavior. 

The pyroelectricity or depolarization current is closely related to the piezoelectricity of Groups (B) and (C), because the pyroelectric current is caused by the thermal activation of charges in the film. The application of polypeptide film to an acoustic transducer was proposed by Fukada, Tamura, and Yamamoto (1968). 

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