Piezoelectric behaviour

As mentioned earlier, only certain crystal exhibits piezoelectric behaviour. Only pressure on certain electrically neutral crystals - those not having a centre of structural symmetry - polarizes them by slightly separating the centre of the positive charge from that of the negative charge. Some examples of such crystals are ... 

Capsal, J. -F., Dantras, E., Laffont, L., Dandurand, J., and Lacabanne, C., Nanotexture influence of BaTiOs particles on piezoelectric behaviour of PAll/BaTiOs nanocomposites, J. Non-Cryst. Solids, 356, 623-634 (2010). 

Lead Tantalate. PbTa205 a ferroelectric compound of interest as an electroceramic. The Curie temperature is 260°C. Lead Titanate. PbTi03 added in small amounts to barium titanate ceramics to improve their piezoelectric behaviour a complex lead titanate-zirconate body (P.Z.T.) finds use as a ceramic component in piezoelectric transducers. The Curie temperature is 490 C. 

Other commercial fluoropolymers (all made by free-radical mechanisms) include polychlorotrifluoroethylene (Kel-F), which is similar to PTFE but which can be moulded at temperatures 300 °C. Polyvinylidene difluoride (PVDF) is a thermoplastic (T 160°C) and films of this material show piezoelectric behaviour. When copolymerized with hexafluropropene a very chemically resistant elastomer is obtained (Viton). Polyvinylfluoride (PVF) is another highly crystalline polymer (T 197°C) which is used for high-performance protective coatings. 

R. P. lluidon, R. Singh, R. D. P. Sinha. and S. Chandra. Dideciric and piezoelectric behaviour of lead titanale/polymer composite, Fenvekctrics 720 293 (1991). 

The occurrence of piezoelectric behaviour in LB films has been known for some time [57,58], and a 30 X-type layer LB film of (37) was found to give opposite signs of the piezoelectric strain coefficients d i and d [59], the latter having a value of 1.5 pC which is approximately an order of magnitude lower than that of the well-documented polymer poly(vinylidene fluoride) (PVDF). Values for 31 of 0.023 and 0.170 pC N have also been obtained for alternate-layer structures of 22-tricosenoic acid with docosylamine, and a ruthenium complex with docosanoic acid respectively [60]. As the use of pyroelectric materials in detector applications requires that the materials possess only low levels of piezoelectricity (high levels introduce problems of microphony), this suggests that the former materials would be better suited for pyroelectric detector applications, while the latter system would be more appropriate for piezoelectric-based applications. 

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. 

Sco et al (1987) employed the piezoelectric surface stress technique to study the behaviour of gold on electrochemical cycling in aqueous sulphate solution. They obtained plots of u and vs. E at a range of pH from 0.3 to 14 and a typical example of these is shown in Figure 2.12 which shows the plots obtained at pH 3. It should be emphasised that the data in Figure 2.12 were obtained dynamically by simultaneously scanning the potential in the... 

Bhuiyan, M.T.R., Hirai, N. and Sobue, N. (2002). Behaviour of piezoelectric, dielectric, and elastic constants of wood during about 40 repeated measurements between 100 °C and 220 °C. Journal of Wood Science, 4S(l ), 1-7. 

When the experimentalist set an ambitious objective to evaluate micromechanical properties quantitatively, he will predictably encounter a few fundamental problems. At first, the continuum description which is usually used in contact mechanics might be not applicable for contact areas as small as 1 -10 nm [116,117]. Secondly, since most of the polymers demonstrate a combination of elastic and viscous behaviour, an appropriate model is required to derive the contact area and the stress field upon indentation a viscoelastic and adhesive sample [116,120]. In this case, the duration of the contact and the scanning rate are not unimportant parameters. Moreover, bending of the cantilever results in a complicated motion of the tip including compression, shear and friction effects [131,132]. Third, plastic or inelastic deformation has to be taken into account in data interpretation. Concerning experimental conditions, the most important is to perform a set of calibrations procedures which includes the (x,y,z) calibration of the piezoelectric transducers, the determination of the spring constants of the cantilever, and the evaluation of the tip shape. The experimentalist has to eliminate surface contamination s and be certain about the chemical composition of the tip and the sample. 

Because a ceramic is composed of a large number of randomly oriented crystallites it would normally be expected to be isotropic in its properties. The possibility of altering the direction of the polarization in the crystallites of a ferroelectric ceramic (a process called poling ) makes it capable of piezoelectric, pyroelectric and electro-optic behaviour. The poling process - the application of a static electric field under appropriate conditions of temperature and time -aligns the polar axis as near to the field direction as the local environment and the crystal structure allow. 

The values of the piezoelectric properties of a material can be derived from the resonance behaviour of suitably shaped specimens subjected to a sinusoidally varying electric field. To a good approximation the behaviour of the piezoelectric... 

The optical properties of GalnN/GaN quantum wells differ somewhat from the well-known behaviour of other III-V-based strained quantum well structures, partly due to the rather strong composition and well width fluctuations, possibly induced by a partial phase separation of InN and GaN. The even more dominant effect seems to be the piezoelectric field characteristic for strained wurtzite quantum wells, which strongly modifies the transition energies and the oscillator strengths. However, the relative influence of localisation and piezoelectric field effect is still subject to considerable controversy. 

More recently the treatment was extended to piezoelectric devices in contact with viscoelastic media (i.e., liquids and polymers). It was then realised that if the deposited mass was not rigidly coupled to the oscillating quartz crystal, separation of inertial mass and energy losses was not possible with the measurement of the resonant frequency alone. Quartz crystal impedance in the acoustic frequencies was introduced in order to study mass and viscoelastic changes and a full electrical characterization of the crystal behaviour near resonance was employed. 

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... 

Pseudohalide crystals like the fulminates with asymmetric ions can be piezoelectric, while crystals which are sensitive to electric fields probably have ferroelectric phases. Electric fields generated on the application of thermal shock and pressure on piezoelectric crystals can cause electrical breakdown which readily leads to the formation of localized hot spots and the initiation of an explosive process. These ideas would need detailed study and can provide insights into the behaviour of energy rich crystals. 

Nomura, T., Watanabe, M., and West, T. M. (1985). Behaviour of piezoelectric quartz crystals in solutions with application to the determination of iodide. Anal. Chim. Acta 175,107-112... 

In a piezoelectric solid a surface electric charge develops when the solid is subjected to a mechanical stress such as pressure, even in the absence of an external electric field. This is called the piezoelectric effect, and crystals that exhibit this behaviour are called piezoelectrics. Piezoelectric solids are a subset of dielectrics. All piezoelectrics are dielectrics, but only some dielectrics are piezoelectrics. 

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