Electrostrictive actuators applications

Beginning with the work by Ohno and Yonezawa on PFN-PFW systems in the late 1970s [8], many multicomponent dielectric systems have been evaluated and put into manufacture. Some of the patented compositions developed for multilayer capacitor (MLC) application were recently summarized by Shrout and Dougherty [9]. Other compositions were developed for piezoelectric sensors and electrostrictive actuator applications [10]. Most of the compositions used for capacitor dielectrics are based on PFN [8], PMN [11-14], or PZN [15]. 

Although the polycrystalline relaxor-based compositions have useful piezoelectric characteristics, taking their properties overall into account they do not offer significant advantages over the well established PZT system. Their high electrostriction coefficients make them attractive for certain actuator applications (see Section 6.5.2), but the potentially important advance has been the production of single crystals. 

Through additions of PbTi03 to the prototype relaxor Pb(Mg1/3Nb2/3)03 the temperature at which the permittivity peaks can be adjusted and, close to the composition 0.9PMN-0.1 PT, it occurs at around room temperature. This is a composition suited to actuator applications. The dependence of electrostriction on grain size (Fig. 6.15) [14] is believed to be linked to the dependence of the size of the nanosized ordered regions on grain size (T. Mishima et al. [15]). 

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. 

Uchino, K., Electrostrictive actuators Materials and applications, Ceram. Bull., 65, 4 (1986). 

Relaxor-type electrostrictive materials, such as those from the lead magnesium niobate-lead titanate, Pb(Mgp 3Nb2/3)03-PbTi03 (or PMN-PT), solid solution are highly suitable for actuator applications. This relaxor ferroelectric also exhibits an induced piezoelectric effect. That is, the electromechanical coupling factor kt varies with the applied DC bias field. As the DC bias field increases, the coupling increases and saturates. Since this behavior is reproducible, these materials can be applied as ultrasonic transducers which are tunable by the bias field [12]. 

Electrostriction can generate mechanical deformation that is independent of the polarity of the electric field. It exists in almost all materials but is usually too weak for any practical use. However, it can be very large in electrostrictive materials, such as lead magnesium niobate (PMN) systems [11]. The nonlinearity often works to the advantage in such systems since it can produce tunable functional properties. As the effect is nonre-versible, electrostrictive materials are better for actuator applications. Unlike the piezoelectric effect, electrostriction can even exist in systems with center symmetry. Electrostrictive materials become piezoelectric under a dc bias field. 

A specific type of electrostrictive material with large elec-trostrictive coupling coefficients, and important for applications that require high relative permittivities (i. e., supercapacitors) of 20,000-35,000, and actuator applications in particular. Single crystal PMN-PT is a popular composition in this class of material. 

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. 

This book intends to provide a comprehensive and updated insight into both the fundamentals of each class of EAP, and examples of the most significant applications of EAP actuators in the biomedical field, either already demonstrated or currently under development. Eor this purpose, the book comprises five sections devoted to the most technologically mature EAPs, namely polymer gels, ionic polymer-metal composites, conjugated polymers, piezoelectric/electrostrictive polymers and dielectric elastomers. Each section is... 

Vogan, J., Wingert, A., Hafez, M., et al. (2004) Manipulation in MRI Devices Using Electrostrictive Polymer Actuators with an Application to Reconfigurable Imaging Coils, Proceedings of the 2004 IEEE International Conference on Robotics and Automation, New Orleans, LA, 23 April-2 May, 2498-504. 

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