Piezoelectric ceramics polymers

Janas, V.E. and Safari, A. (1995) Overview of fine-scale piezoelectric ceramic/ polymer composite processing, J. Am. Ceram. Soc., 78, 2945-55. 

Fig. 7.25. Connectivity of constituent phases in piezoelectric ceramic-polymer composites [85]...

H.L.W. Chan, J. Unsworth, Simple model for piezoelectric Ceramic/Polymer 1-3 composites used in ultrasonic transducer appHcations. IEEE Trans. Ultra-son., Ferroelectr. Preq. Control 36(4), 434 441 (1989)... 

Normally the filler is an inert inorganic powder or fibre. However, among the modern composites most actively investigated at present are those classified as electronic composites. Multilayer dielectric materials and piezoelectric ceramic-polymer composites are becoming of particular importance as technological and industrial advances demand more versatile and responsive transducer devices. 

GUR87] GURURAJA T.R., SAFARI A., NEWNHAM R.E. and CROSS L.E., Piezoelectric ceramic-polymer composites for transducer applications , in Electronic Ceramics, Levinson L.M. (Ed.), p. 92-128, Marcel Dekker Inc., 1987. 

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). 

Ferroelectric Ceramic—Polymer Composites. The motivation for the development of composite ferroelectric materials arose from the need for a combination of desirable properties that often caimot be obtained in single-phase materials. For example, in an electromechanical transducer, the piezoelectric sensitivity might be maximized and the density minimized to obtain a good acoustic matching with water, and the transducer made mechanically flexible to conform to a curved surface (see COMPOSITE MATERIALS, CERAMiC-MATRix). 

The development of active ceramic-polymer composites was undertaken for underwater hydrophones having hydrostatic piezoelectric coefficients larger than those of the commonly used lead zirconate titanate (PZT) ceramics (60—70). It has been demonstrated that certain composite hydrophone materials are two to three orders of magnitude more sensitive than PZT ceramics while satisfying such other requirements as pressure dependency of sensitivity. The idea of composite ferroelectrics has been extended to other appHcations such as ultrasonic transducers for acoustic imaging, thermistors having both negative and positive temperature coefficients of resistance, and active sound absorbers. 

Ferroelectric—polymer composite devices have been developed for large-area transducers, active noise control, and medical imaging appHcations. North American Philips, Hewlett-Packard, and Toshiba make composite medical imaging probes for in-house use. Krautkramer Branson Co. produces the same purpose composite transducer for the open market. NTK Technical Ceramics and Mitsubishi Petrochemical market ferroelectric—polymer composite materials (108) for various device appHcations, such as a towed array hydrophone and robotic use. Whereas the composite market is growing with the invention of new devices, total unit volume and doUar amounts are small compared to the ferroelectric capacitor and ferroelectric—piezoelectric ceramic markets (see Medical imaging technology). 

Composite piezoelectric transducers made from poled Pb-Ti-Zr (PZT) ceramics and epoxy polymers form an interesting family of materials which highlight the advantages of composite structures in improving coupled properties in soilds for transduction applications A number of different connection patterns have been fabricated with the piezoelectric ceramic in the form of spheres, fibers, layered, or three-dimensional skeletons Adding a polymer phase lowers the density, the dielectric constant, and the mechanical stiffness of the composite, thereby altering electric field and concentrating mechanical stresses on the piezoelectric ceramic phase. 

For low frequency electromechanical applications in which the acoustic wavelength is much larger than the scale of component phases, some of the ceramic-polymer composites have piezoelectric voltage coefficients orders of magnitude larger than solid PZT. Such materials have obvious applications in hydrophones and other listening devices. 

Piezoelectric ceramics and polymers can play the double role of sensors and actuators these materials can either generate an electric field under an applied load or change dimensions when subjected to a voltage difference. When fabrication problems associated with their embedding are solved, they will find wide application for structure vibration control. 

Nunes-Pereira, J., Sencadas, V, Correia, V, Rocha, J.G., Lanceros-M ndez, S., 2013. Energy harvesting performance of piezoelectric electrospun polymer fibers and polymer/ceramic composites. Sensors Actuat. A Phys. 196,55-62. 

Two common piezoelectric materials are polymers (polyvinylidene fluoride, PVDF) and c mics (lead zirconate titanate, PZT). The polymer materials are soft and flexible however have lower dielectric and piezoelectric properties than ceramics. Conventional piezoelectric ceramic materials are rigid, heavy and can only be produced in block form. Ceramic materials add additional mass and stiffiiess to the host structure, especially when working with flexible/lightweight materials. This and their fragile nature limit possibilities for wearable devices. Comparisons of several piezoelectric materials are presented in Table 1. 

Capacitive sensors can be used to detect displacement from the fact that capacitance between two parallel metal plates, C = eoEt A/x, where eo = dielectric constant of free space, Cf = relative dielectric constant of media. Displacement can be measured by changing aU these three parameters. A good example is the capacitance microphone that is responding to displacement by sound pressure. Piezoelectric sensors are used to measure physiological displacement and record heart sounds. These sensors are fabricated from piezoelectric ceramics and piezoelectric polymers. For flexible wearable sensors, fiber or film form of polymer piezoelectric materials such as polyvinyli-dene fluoride (PVDF) are desirable. 

Currently, transducer arrays are formed by ceramic polymer composite elements. A variety of technical solutions have been applied to improve their mechanical characteristics. In particular, piano-concave elements can be used to provide a uniform elevation plane radiation pattern both in the near and in the far field (Jedrzejewicz 1999). Similar results have been obtained producing multi-layered piezoelectric elements (Whittingham 1999b) or shaping the elements in other suitable ways. These refinements led to the use of very short pulses, increased bandwidth and better intrinsic collimation of the ultrasound beam (Fig. 1.1). 

Some ceramic and polymer materials also display piezoelectric properties. For ceramics these include polycrystal hne materials based on solid solutions of Pb(Zr, Ti)03. The ratio of zirconium and titanium constituents help to determine the crystal symmetry structure that results after heating and then cooling of the samples. Modern piezoelectric ceramics are referred to as PZT ceramics. Other materials such as barium, strontium, caldmn, and/or trivalent rare earth elements can be added to the ceramics in small quantities to modify the piezoelectric properties. 

Banno H (1983) Recent developments of piezoelectric ceramic products and composites of synthetic rubber and piezoelectric ceramic particles. Ferroelectrics 50 3-12 Bauer F, Brown LF, Fukada E (eds.) (1995) Special issue on piezo/pyro/ferroelectric polymers. Ferroelectrics 171 1 03... 

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