Piezoceramic-polymer composites

The following discussion should be supplemented by reference to the article by T.R. Gururaja el al. [4], one of the contributing authors being R.E. Newnham, a pioneer in the field of piezoelectric composites. 

It is instructive to compare the basic properties of the piezoelectric polymer, polyvinylidene fluoride (PYDF) with those of PZT . The flexibility and low density of the polymer contrasts with the stiffness, brittleness and high density of PZT . On the other hand the piezoelectric d coefficient for PYDF is relatively small ( — 30pCN the mechanisms by which the polarisation in PVDF 

Composite technology in general sets out to combine materials in such a way that the properties of the composite are the optimum for a particular application. The property, whether mechanical, thermal, electrical etc., is determined by the choice of component and their relative amounts and, most importantly, the connectivity , that is the manner in which the components are interconnected. 

It follows from a consideration of Fig. 6.2, assuming cylindrical symmetry about the poling direction, that 

The other important consideration concerns the transmission of ultrasound (and other forms of energy) from one medium to another and the importance of impedance matching . When wave energy is transferred from one medium to another then a part is transmitted and the rest reflected. The ratio of reflected to transmitted energies depends on the characteristic impedances of the two media and the transmission is total if these are matched. In the case of acoustic waves the specific impedance (Z) of a medium is given by the product of the density p and the velocity of sound v. that is 

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Recently injection-moulding has been exploited by L.J. Bowen [10] to fabricate a range of piezoceramic-polymer composites (see Section 6.4.6) for the active elements of sonar devices. 

Fig. 6.19 (a) 1-3 piezoceramic-polymer composite, (b) Fabricating the rod array for a 1-3 piezoceramic polymer composite by injection-moulding. (Courtesy of Materials Systems Inc. Littleton MA the MSI website www.matsjrinc.com is an excellent source of information.)... 

The high values of the cdh gh figure of merit offered by piezoceramic-polymer composites can be appreciated from Table 6.4. 

An experimental 1-3 piezoceramic/polymer composite disc transducer operating at the fundamental thickness mode resonance frequency is intended to detect objects in water having dimensions of approximately 10 mm. It is firmly fixed to a backing. 

Piezoceramic/polymer composite materials are a means of overcoming the limitations of both ceramic and polymer materials to provide complementary properties that produce a superior piezoelectric device. The polymer phase lowers the density of the material, providing better acoustic coupling to water as well as more easily adjusted buoyancy than that obtained for a high-density homogeneous ceramic. The low dielectric constant of the polymer phase effectively increases the coefficients and... 

Piezocomposite transducers are an advancement of piezoelectric ceramics. Instead of the classic piezoceramic material, a compound of polymer and piezoceramic is used for the composite element to improve specific properties. The 1-3 structure, which is nowadays mostly used as transducer material, refers to parallel ceramic rods incorporated in an epoxy-resin matrix (see Fig. 1). 

A 0-3 composite consists of discrete piezoceramic particles dispersed in a polymer the isolated particles have zero connectivity and the polymer matrix 3-dimensional connectivity. A 1-3 composite consists of piezoceramic rods extending from electrode to electrode and embedded in a polymer, as shown in Fig. 6.19. The rods have one-dimensional connectivity and the polymer again 3-dimensional connectivity. Some important considerations concerning the two types are summarized below. 

Fine scaled composites can be prepared starting with molded and sintered ceramic arrays made by the soft mold process. Once the piezoceramic pillars have been formed, the remaining spaces are filled with a polymer matrix material. Next the base is removed by grinding. Metal electrodes are then bonded to the ends of the fibers and first used to polarize the piezoceramic at an elevated temperamre and then to apply an electric field or collect developed charges from the material. At this point, the active elements are ready to be used as piezoelectric transducer element. Experiments have shown that high-performance composites can be prepared . 

Active fiber composites (AFC), macro fiber composites (MFC) [88,97] and piezo fiber composites (PFC) [87,98] are advanced variants of 1-3 composites that are particularly designed for adaptronic applications. Those variants are based on a basic design developed at the Massachusetts Institute of Technology (MIT) [89,93-96]. For that purpose uniaxial arranged piezoceramic fibers from PZT are embedded in a polymer matrix and contacted via interdigital electrodes at the surface. Such composites have a high flexibility and robustness and can therefore be easily applied to components or structures... 

Multiple layers of piezoelectric composites consisting of polymer matrix material and piezoceramic fibers form the walls of the considered beams. The properties of typical materials are given in Tables A.2 and A.3, respectively. For the calculations which are presented later in this chapter, the data of Epon... 

The requirements for medical ultrasonic imaging transducers are met with 1-3 PZT-rod/polymer-matrix piezoelectric composites. The composites provide high electromechanical coupling and low impedance properties superior to those of the conventional piezoceramics and piezopolymers. Also composite plates can be formed into complex shapes for beam focusing and steering. 

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