Pyroelectrics large piezoelectricity

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). 

Theoretical estimations and experimental investigations tirmly established (J ) that large electron delocalization is a perequisite for large values of the nonlinear optical coefficients and this can be met with the ir-electrons in conjugated molecules and polymers where also charge asymmetry can be adequately introduced in order to obtain non-centrosymmetric structures. Since the electronic density distribution of these systems seems to be easily modified by their interaction with the molecular vibrations we anticipate that these materials may possess large piezoelectric, pyroelectric and photoacoustic coefficients. 

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... 

Spun-cast polymers, such as polyvinylidenefluoride and its copolymers, show good values of pyroelectric coefficient, and favourable values of the figure of merit p/er. The major disadvantages of such materials are their large piezoelectric activity, and the necessity for electrical poling. In practical devices, piezoelectricity is a problem,because it can cause problems associated with microphony. 

Poly(vinylidene fluoride)(PVDF) is one of the most polar polymers among synthetic polymers and shows the most unique phenomena in many fields of pol3nner science. Especially, the electrical properties of PVDF have been the subject of intensive investigations in recent years since it was reported that it could exhibit a large dielectric constant and internal polarization for its B-form films, a very large piezoelectric and pyroelectric effects for polarized monoaxially and biaxially stretched films.[1,2,3]... 

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. 

All pyroelectric materials are piezoelectric and therefore develop electric charges in response to external stresses that may interfere with the response to radiation. This can largely be compensated for by the provision of a duplicate of the detecting element that is protected from the radiation by reflecting electrodes or masking, but which is equally exposed to air and mounting vibrations. The principle is illustrated in Fig. 7.7. The duplicate is connected in series with the detector and with its polarity opposed so that the piezoelectric outputs cancel. This results in a small reduction in sensitivity (< 3 dB) but compensation is an... 

Electret materials are meanwhile used in a large number of modern high-tech applications including microphones, acoustic sensors, transducers, radiation and pollution dosimeters, power generators, filters, and many more. Additionally, electret technology is of great interest in the field of biomaterials, for instance in callus formation and wound healing [10, 11], When used in cellular or in multilayer sandwich structures, polymer electrets can exhibit piezoelectricity. Such materials are ferroelectrets, as they show typical features of ferroelectric materials such as piezo-and pyroelectricity [12-17],... 

Chemical and physical processing techniques for ferroelectric thin films have undergone explosive advancement in the past few years (see Ref. 1, for example). The use of PZT (PbZri- cTi c03) family ferroelectrics in the nonvolatile and dynamic random access memory applications present potentially large markets [2]. Other thin-film devices based on a wide variety of ferroelectrics have also been explored. These include multilayer thin-film capacitors [3], piezoelectric or electroacoustic transducer and piezoelectric actuators [4-6], piezoelectric ultrasonic micromotors [7], high-frequency surface acoustic devices [8,9], pyroelectric intrared (IR) detectors [10-12], ferroelectric/photoconduc-tive displays [13], electrooptic waveguide devices or optical modulators [14], and ferroelectric gate and metal/insulator/semiconductor transistor (MIST) devices [15,16]. 

A large number of apphcations have been proposed for piezoelectric polymers. The types of applications can be grouped into live major categories sonar hydrophones, ultrasonic transducers, audio-frequency transducers, pyroelectric sensors, and electromechanical devices. The principal polymers of interest in these applications are PVDF and copolymers of vinylidene fluoride and trifluoroethylene. 

Since pyroelectric ceramics are also piezoelectric, a temperature change also induces a change in the polarization due to the secondary pyroelectric effect, which is described by the product of the thermal expansion strain times the piezoelectric coupling coefficient. While this secondary effect can be large in polymers due to their large thermal expansion coefficients, in ceramics, it is typically small compared with the (first-order) pyroelectric effect. 

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