Piezoelectrics, pyroelectrics and ferroelectrics

The loss tangent is a measure of the energy loss in a capacitor. For good dielectrics, tan 8 is about 10 and is relatively insensitive to the frequency of the applied field. 

The relative permittivity (as well as the refractive index) for such crystals is quoted as three values corresponding to the polarisations projected onto the axes. Some representative values are given in Table 11.1. 

It may sometimes be necessary to estimate the polarisability of a solid in the absence of experimental data. Polarisability is not particularly easy to measure, but the relative permittivity is. The Clau-sius-Mossotti equation. Equation (11.6), is generally used to obtain polarisability from relative permittivity. The equation gives reasonable values for isotropic solids showing only ionic and electronic polarisation. If the refractive index is known, the 

Lorentz-Lorentz equation. Equation (11.11), will yield the electronic polarisability of the material. Hence, by difference, the ionic polarisability can be estimated. 

In the absence of relative permittivity data for the solid under consideration it is possible to make use of the additivity rule. In its simplest form, we can write  

---

In this review the basic concepts of piezoelectricity, pyroelectricity and ferroelectricity will be first considered in the light of molecular structure. The... 

As concerns the piezoelectric layer, the first choice often goes to lead zirconate titanate (PZT) because of its outstanding piezoelectric, pyroelectric and ferroelectric properties. Nickel ferrite (NF) is not widely employed for the synthesis of the multilayered composites owing to a strong reduction of its magnetization in the lower grain size limit. However, a very thin NF layer can help to attain entirely different properties and, hence, this material has been chosen as a sandwiched layer in the present work. 

The solids discussed in the remainder of this chapter have one thing in common They exhibit various polar effects, such as piezoelectricity, pyroelectricity, and ferroelectricity. Piezoelectric crystals are those that become electrically polarized or undergo a change in polarization when subjected to a stress, as shown in Fig. 15.12c to /. The application of a compressive stress results in the flow of charge in one direction in the measuring circuit and in the opposite direction for tensile stresses. Conversely, the application of an electric field will stretch or compress the crystal depending on the orientation of the applied field to the polarization in the crystal. 

A number of other polymeric solids have also been the subject of much interest because of their special properties, such as polymers with high photoconductive efficiencies, polymers having nonlinear optical properties, and polymers with piezoelectric, pyroelectric and ferroelectric properties. Many of these polymeric materials offer significant potential advantages over the traditional materials used for the same application, and in some cases applications not possible by other means have been achieved. 

R. G. Kepler, Piezoelectricity, pyroelectricity, and ferroelectricity in organic materials, Ann. Rev. Phys. Chem. 29, 497 (1979). 

An important point now emerges the requirement that the piezoelectric, pyroelectric and ferroelectric effects are restricted to non-centrosymmetric crystals implies that these physical phenomena should not be observed in a polycrystalline solid. This is because the individual grains of a polycrystalline body will polarise in random directions that will cancel overall. This was changed by the discovery, in 1945, of a way to endow polycrystalline ceramic articles with ferroelectric properties. 

Magnetic, Piezoelectric, Pyroelectric, and Ferroelectric Properties of Synthetic and Biological... 

Piezoelectricity, pyroelectricity, and ferroelectricity is hardly confined to synthetic polymers. Some biopolymers also possess these properties, and scientists study them to understand how nature exploits these properties. The earliest studies of biopolymer piezoelectricity, for example, go back to 1960s when Morris Shamos and Leroy Lavine (with Michael Morris) studied bone piezoelectricity [39] and later postulated piezoelectricity as a fundamental property of tissues of biological origins [40], In 1968, RNA ferroelectricity was demonstrated by Stanford and Lorey [41]. However, the scientific interest in these properties of biological molecules was dwarfed by the interest in other materials. In 1999 Sidney Lang [42] indicated that compared to thousands of publications on piezoelectric, pyroelectric, and ferroelectric materials, only less than 100 of them were biologically related. 

There is a growing interest in studying the piezoelectricity, pyroelectricity, and ferroelectricity in biological systems to understand what roles they play in cellular functions, and this interest is expected to increase in the future. 

Poly(vinylidene fluoride), PVDF or PVF2, is usually manufactured from radical initiated batch polymerization process in aqueous emulsion or suspension of CH2=CF2 monomer. PVDF is a thermoplastic that exhibits interesting properties, such as piezoelectric, pyroelectrical, and ferroelectric behaviors. PVDF has even superior dielectric permittivity arising from the strong polymerization originating from C—F bonds, and the spontaneous orientation of dipoles in the crystalline phases makes it a polar polymer with good compatibility with polar chemicals. 

Piezoelectric, pyroelectric and ferroelectric materials are often discussed simultaneously, owing to their interrelationship with each other at the crystalline structure level. For a crystalline structure to exhibit piezoelectricity, there should be no symmetry at the inversion centre for point group(s) (Tilley, 2006). A piezoelectric material can show both pyroelectricity (generation of electric charge on a crystal by change of temperature) and ferroelectricity (a property of certain materials that have a spontaneous electric polarisation). The relationship between different types of materials is shown in Figure 9.1. Ferroelectric materials are known to have superior piezoelectric properties over their non-feiroelectric counterparts. 

---