Piezo-composites composed of a piezoelectric ceramic and a polymer are promising materials because of their excellent tailorable properties. The geometry for two-phase composites can be classified according to the connectivity of each phase (1, 2 or 3 dimensionally) into 10 structures; 0-0, 0-1, 0-2, 0-3, 1-1, 1-2, 1-3, 2-2, 2-3 and 3-3. In particular, a 1-3 piezo-composite, or PZT-rod/polymer-matrix composite is considered most useful. The advantages of this composite are high coupling factors, low acoustic impedance, good matching to water or human tissue, mechanical flexibility, broad bandwidth in combination with a low mechanical quality factor and the possibility of making undiced arrays by simply patterning the electrodes. The acoustic match to tissue or water of the typical piezoceramics is significantly improved when it is incorporated into such a composite structure, that is, by replacing some of the dense and stiff ceramic with a less dense, more pliant polymer. Piezoelectric composite materials are especially useful for underwater sonar and medical diagnostic ultrasonic transducer applications. Other types of composites based on piezoelectric ceramics are also introduced in this chapter. Piezo-passive-dampers comprise a piezoelectric ceramic particle, polymer, and a carbon black, which suppress the noise vibration more effectively than traditional rubbers. A composite with a magnetostrictive ceramic and a piezoelectric ceramic produces an intriguing product effect, the magnetoelectric effect in which an electric field is produced in the material in response to an applied magnetic field.
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