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Piezoelectricity The piezoelectric effect is exhibited in a certain group of crystalline solid materials whose unit cells do not possess a center of symrnetry. These materials, when mechanically stressed, will produce an electrical charge, and conversely, when an electric field is applied, a mechanical strain will occur, changing the dimensional shape of the material. Currently the most widely-used piezoelectric transducer materials are poly-crystalline ceramics based on lead zirconate titanate and barium titanate compositions. Specific additives are included to give each composition unique dielectric, piezoelectric and physical properties. In their original state, ceramic materials are composed of a multitude of crystallites with random domain orientation. Consequently, they are isotropic and possess no piezoelectric properties. By the application of a temporary high-static electric field, the ceramic material will become anisotropic, retain a remnant polarization and become piezoelectric. This is referred to as the poling operation, and the poling direction, by convention, defines the Z axis of a three-dimensional orthogonal axis system. The X, Y, and Z axes are represented in the piezoelectric constant subscripts as 1, 2 and 3. The subscripts 4, 5 and 6 refer to shear distortions about the 1, 2 and 3 axes respectively. Polar symmetry exists, and even though the 1 and 2 axes are designated, they may be arbitrarily located but must be at right angles to each other. The 1 and 2 axes are identical, so for simplicity, reference is usually made only to the 3 and 1 directions. The piezoelectric constants are generally written with two subscripts, The first subscript is the electrical direction, and the second is the mechanical direction. Piezoelectric Ceramic Materials Channel Industries manufactures lead zirconate titanate and barium titanrate compositions as listed on page14, as well as other specialized compositions. Lead Zirconate Titanate Characterized by high coupling factors and high piezoelectric and dielectric constants over extended temperature ranges and stress amplitudes, lead zirconate titanates are the most extensively used material for elctro-mechanical and electro-acoustic transducers. Barium Titanate Modified barium titanates are widely used in transducers requiring moderate power level and receiving sensitivity. Special characteristics have made barium titanate a proven material for use in devices such as depth sounders and expendable hydrophones. Polarity of Piezoelectric Effect Subsequent to polarization, an electric field applied in the 3 direction and of the same polarity as the poling field will cause elongation along the 3 axis and contraction in all perpendicular directions. A reverse field will cause contraction along the 3 axis and expansion in all transverse directions. These strains remain as long as the field is maintained. A piezoelectrically-generated voltage of the same polarity as the poling field occurs, due to a compressive force applied parallel to, or a tensile force applied perpendicular to, the polar axis. Reversing the direction of the applied forces reverses the polarity of the generated voltage. The positive electrode on the finished ceramic shape is usually identified by a polarity mark. This is the electrode to which the positive voltage is applied during the poling operation. Projector & Sensor Materials Piezoelectric ceramic materials are compounded to have advantageous properties depending upon their end use. Two broad categories include: 1) High coercivity power ceramics that are capable of accepting high continuous input power and operating at high mechanical stress levels. Important properties of this group of materials are low dielectric and mechanical losses, even at large displacement amplitudes or acoustic intensities. 2) Sensor materials characterized by high charge and voltage sensitivity with high dielectric constants and resistivity, as well as low aging of their properties with time. Most of these materials are capable of large displacements with positive applied static electric fields, but are limited in their use as dynamic transducers to very low power projectors, due to dielectric dissipation at high amplitude.
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