TABLE 50.2 Selection Critera for Relaxor Ferroelectrics for Electrostrictive devices Material Behavior ·La induced piezoelectricity Large operating temperature range Tma-Ta is large Broad dielectric transition Low-loss, low-joule heating, Operation in paraelectric regime inimal hysteresis, no remanent In selecting electrostrictive relaxor ferroelectrics for actuator and sensor applications, the following criteria are commonly used. A large dielectric constant and field stability in the K vs. e relations are useful in achieving large electrostrictive strains. These criteria also lead to large induced polarizations and large induced piezoelec tric coefficients through the second converse effect. Broad dielectric transitions allow for a large operating temperature range. In the case of relaxors, this implies a large difference between Tmax and T. Minimal E-P hysteresis and no remanent polarization are useful in achieving a low-loss material that is not susceptible to joule heating effects. These factors are listed in Table 50.2. 50.5 Summary lectrostriction is a fundamental electromechanical coupling effect In ceramics with large dielectric constants and in some polymers, large electrostrictive strains may be induced that are comparable in magnitude with piezoelectric strains in actuator materials such as PZT. The converse electrostrictive effect, which is the change in dielectric susceptibility with applied stress, facilitates the use of the electrostrictor as a stress gauge. The second converse effect may be used to tune the piezoelectric coefficients of the material as a function of the applied field. Electrostrictive materials offer tunable nonlinear properties that are suitable for application in very smart syste Defining Terms Elastic compliance: A fourth-rank tensor(Sit)relating the stress(X) applied on a material and the strain (x)developed in it, x;=Sil Xy. Its inverse is the elastic stiffness tensor(o Electrostriction: The quadratic coupling between strain and applied field or induced polarization. Conversely, it is the linear coupling between dielectric susceptibility and applied stress. It is present in all insulating Ferroelectricity: The phenomenon by which a material exhibits a permanent spontaneous polarization that can be reoriented(switched) between two or more equilibrium positions by the application of a realistic electric field (i.e, less than the breakdown field of the material) Perovskite: A crystal structure with the formula ABO,, with A atoms at the corners of a cubic unit cell, B atoms at the body-center position, and O atoms at the centers of the faces. Many oxide perovskites are used as transducers, capacitors, and thermistors. Piezoelectricity: The linear coupling between applied electric field and induced strain in acentric materials The converse effect is the induction of polarization when stress is applied Relaxor ferroelectric: Relaxor ferroelectric materials exhibit a diffuse phase transition between paraelectric and ferroelectric phases, and a frequency dependence of the dielectric propert Smart and very smart systems: A system that can sense a change in its environment, and tune its response suitably to the stimulus. a system that is only smart can sense a change in its environment and react to it Related Topic 58.5 State-of-the-Art Smart Materials c 2000 by CRC Press LLC© 2000 by CRC Press LLC In selecting electrostrictive relaxor ferroelectrics for actuator and sensor applications, the following criteria are commonly used. A large dielectric constant and field stability in the K vs. E relations are useful in achieving large electrostrictive strains. These criteria also lead to large induced polarizations and large induced piezoelec￾tric coefficients through the second converse effect. Broad dielectric transitions allow for a large operating temperature range. In the case of relaxors, this implies a large difference between Tmax and Td . Minimal E–P hysteresis and no remanent polarization are useful in achieving a low-loss material that is not susceptible to joule heating effects. These factors are listed in Table 50.2. 50.5 Summary Electrostriction is a fundamental electromechanical coupling effect. In ceramics with large dielectric constants and in some polymers, large electrostrictive strains may be induced that are comparable in magnitude with piezoelectric strains in actuator materials such as PZT. The converse electrostrictive effect, which is the change in dielectric susceptibility with applied stress, facilitates the use of the electrostrictor as a stress gauge. The second converse effect may be used to tune the piezoelectric coefficients of the material as a function of the applied field. Electrostrictive materials offer tunable nonlinear properties that are suitable for application in very smart systems. Defining Terms Elastic compliance: A fourth-rank tensor (sijkl) relating the stress (X) applied on a material and the strain (x) developed in it, xij = sijklXkl . Its inverse is the elastic stiffness tensor (cijkl). Electrostriction: The quadratic coupling between strain and applied field or induced polarization. Conversely, it is the linear coupling between dielectric susceptibility and applied stress. It is present in all insulating materials. Ferroelectricity: The phenomenon by which a material exhibits a permanent spontaneous polarization that can be reoriented (switched) between two or more equilibrium positions by the application of a realistic electric field (i.e., less than the breakdown field of the material). Perovskite: A crystal structure with the formula ABO3, with A atoms at the corners of a cubic unit cell, B atoms at the body-center position, and O atoms at the centers of the faces. Many oxide perovskites are used as transducers, capacitors, and thermistors. Piezoelectricity: The linear coupling between applied electric field and induced strain in acentric materials. The converse effect is the induction of polarization when stress is applied. Relaxor ferroelectric: Relaxor ferroelectric materials exhibit a diffuse phase transition between paraelectric and ferroelectric phases, and a frequency dependence of the dielectric properties. Smart and very smart systems: A system that can sense a change in its environment, and tune its response suitably to the stimulus. A system that is only smart can sense a change in its environment and react to it. Related Topic 58.5 State-of-the-Art Smart Materials TABLE 50.2 Selection Critera for Relaxor Ferroelectrics for Electrostrictive Devices Desirable Properties Material Behavior • Large strain, induced polarization, and induced piezoelectricity • Large dielectric constants • Large operating temperature range • Tmax – Td is large • Broad dielectric transition • Low-loss, low-joule heating, minimal hysteresis, no remanent polarization • Operation in paraelectric regime (T > Tmax)