Electrostriction, ceramic piezoelectrics

Piezoelectrics. AH ceramics display a slight change ia dimension, or strain, under the appHcation of an electric field. When the iaduced strain is proportional to the square of the field iatensity, it is known as the electrostrictive effect, and is expressed by ... 

Polycrystalline materials in which the crystal axes of the grains are randomly oriented all behave electrostrictively whatever the structural class of the crystallites comprising them. If the crystals belong to a piezoelectric class and their crystal axes can be suitably aligned, then a piezoelectric polycrystalline ceramic becomes possible. 

Electrostrictive materials offer important advantages over piezoelectric ceramics in actuator applications. They do not contain domains (of the usual ferroelectric type), and so return to their original dimensions immediately a field is reduced to zero, and they do not age. Figure 6.24(a) shows the strain-electric field characteristic for a PLZT (7/62/38) piezoelectric and Fig. 6.24(b) the absence of significant hysteresis in a PMN (0.9Pb(Mg1/3Nb2/303-0.1 PbTi03) electrostrictive ceramic. 

The converse electrostrictive effect—the stress dependence of the permittivity—is also used in stress sensors [19]. A himorph structure provides superior stress sensitivity and temperature stability. A measuring system with a himorph structure, which subtracts the static capacitances of two dielectric ceramic plates, has been proposed [ 19]. The capacitance changes of the top and bottom plates have opposite signs for uniaxial stress and the same sign for temperature deviation. The response speed is limited by the capacitance measuring frequency to about 1 kHz. Unlike piezoelectric sensors, electrostrictive sensors are effective in the low-frequency range, especially DC. 

Piezoelectric and electrostrictive devices have become key components in smart actuator systems such as precision positioners, miniature ultrasonic motors and adaptive mechanical dampers. This section reviews the developments of piezoelectric and related ceramic actuators with particular focus on the improvement of actuator materials, device designs and applications of the actuators. 

The elastic energy density characterize the elastic energy stored in the E-M materials and are defined as Wv = and Wg = where p is the density of the material. Wy is related to the volume of the device, while Wg is related to the mass of the device. A device made of an E-M material with a higher elastic energy density would have a smaller size/mass. As shown in Tables 16.1 and 16.2, the newly developed electrostrictive polymers exhibit a much higher elastic energy density than piezoelectric ceramics and polymers. 

If all the coefficients of equation (2) are known, one can accurately predict the longitudinal strain under a varying electric field for a given piezoelectric or electrostrictive material, and even for a material exhibiting both piezoelectric and electrostrictive effects, such as irreversible electrostrictive materials. For ideal reversible electrostrictive materials, which possess no remnant polarization at zero electric field, the odd power term of the electric field in equation (2) vanishes. However, we will consider the relaxor PLZT ceramics studied in this chapter as irreversible electrostrictives, to account for any ferroelectric behaviour under dc bias fields, and we will therefore include both terms of the electric field in equation (2). 

Figure 7 shows the ac strain amplitude versus dc bias fields, measured up to the fourth harmonic, with a driving 0.37 MV.m-i peak-to-peak ac field at 120 Hz for PLZT (9.5/65/35). In this case, the first harmonic piezoelectric strain is dominant and seems to increase with the dc bias field until a maximum is reached at 1.2 MV.m-i dc. The theoretical curve seen in this figure is the result of fitting the data collected at 120 Hz to the first harmonic term in equation (4), while fixing the ac field value. This general behaviour of relaxor ferroelectrics has been previously observed for PMN electrostrictive ceramics (Masys et al. 2003). 

Microelectromechanical devices, MEMS, usually contain piezoelectric, magnetostrictive or electrostrictive materials. Piezoelectric materials are ceramics or polymers characterized by a swift, linear shape change in response to an electric field. Most piezoelectric transducer formulations are based on PZT, Pb(Zr,Ti)03. The problem of thin monolithic piezoceramic wafers and fibers... 

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