ABSTRACT
Since the discovery of the piezoelectric effect of crystals by Pierre and Jacques in 1880, a signifi cant progress has been made both in terms of the material itself and its applications. Piezoelectricity is an electromechanical phenomenon that involves interaction between the mechanical (elastic) and the electrical behavior of a material. A typical piezoelectric material produces an electric charge or voltage in response to a mechanical stress, and vice versa. The former is known as the direct piezoelectric phenomenon, while the latter is known as the converse piezoelectric phenomenon. In the application of piezoelectric materials, the direct effect is normally used for sensing technology, while the converse effect is used for actuating technology. The direct and converse effects of commercial piezoelectric materials are achieved by a so-called poling process, which involves exposing the material to high temperatures while imposing high electric fi eld intensity in a desired direction. Before the poling process, the piezoelectric material exhibits no piezoelectric properties, and it is isotropic because of the random orientation of the dipoles, as shown in Figure 1.1a. However, upon developing a poling voltage in the direction of the poling axis, the dipoles reorientate to form a certain class of anisotropic structures as shown in Figure 1.1b. Then, a driving voltage with a certain direction of polarity causes that the cylinder deforms. For example, a driving voltage with an opposite polarity to the poling axis causes that the cylinder elongates.
