Piezoelectric actuators ceramic actuator materials

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. 

High flexibility, low drive voltage, and large bending deflection are definite advantages of IPMCs over other rigid piezoelectric ceramic materials. These characteristics make IPMC actuators and sensors very popular in various biomedical applications. 

Piezoelectric ceramics and polymers can play the double role of sensors and actuators these materials can either generate an electric field under an applied load or change dimensions when subjected to a voltage difference. When fabrication problems associated with their embedding are solved, they will find wide application for structure vibration control. 

An example of a smart material is one developed by Toyota. The multilayer piezoelectric ceramic inside an automobile shock absorber has about five layers for sensing road vibrations. The multilayer stacks positioned near each wheel of the auto also have about 100 layers that act as the actuator, all part of the same ceramic. After analyzing the vibrational signals, a voltage is fed back to the actuator stack, and a response occurs by pushing on the hydraulic system of the auto to cancel the vibration. 

Thin sheets of piezoelectric materials are used in sensors, buzzers, and actuators. In addition to the conventional vibrators, pressure and acceleration sensors are now also being manufactured from these materials. Lead zirconate titanate (PZT) is one of the most common materials used for these applications. The trend is to produce thinner and thinner and smaller and smaller parts. Therefore tape casting has become the manufacturing route of choice. One of the basic applications of piezoelectric ceramics is as a gas igniter where a spark is generated by the piezoelectric under an applied mechanical stress. Microphone discs are also prepared from thin... 

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. 

Actuator materials are classified into three categories piezoelectric, elec-trostrictive and phase-change materials. Modified lead zirconate titanate [PZT, Pb(Zr,Ti)03l ceramics are currently the leading materials for piezoelectric applications. The PLZT [(Pb,La)(Zr,Ti)03l 7/62/38 compound is one such composition [31], The strain curve is shown in Figure 4.1.19a (left). When the applied field is small, the induced strain x is nearly proportional to the field E (x = dE, where d is called the piezoelectric constant). As the field becomes larger (i.e. greater than about IkV/cm), however, the strain curve deviates from this linear trend and significant hysteresis is exhibited due to polarization reorientation. This sometimes limits the use of such materials for actuator applications that require non-hysteretic response. 

Uchino K (1997) Piezoelectric actuators and ultrasonic motors. Kluwer series in Electronic materials Science and technology, Norwell, MA Uchino K (2000) Eerroelectric devices. Marcel Dekker, New York, NY Valasek J (1921) Piezo-electric and allied phenomena in Rochelle Salt. Phys Rev 17 475 81 Waanders JW (1991) Piezoelectric ceramics, properties and applications. Philips Components, Academic, New York... 

Ferroelectric and piezoelectric ceramics, in particular, play an ever-increasing role as materials for electrical and electronic applications that include multilayer capacitors (MLCs), bypass capacitors, dielectric resonators for frequency stabilization of microwave circuits, low-noise oscillators and low-insertion loss bandpass filters for microwave communication components, dielectric waveguide resonators, piezoelectric transducers and sensors, piezomechanical actuators and motors,... 

The Xm of an E-M material is directly related to the displacement generated in an actuator. For most piezoelectric ceramics and polymers, Xm is about 0.1-0.2 %, while the newly developed E-M polymers exhibit a strain response of more than 5%, in some cases achieving as much as 100 %. This makes it possible to create actuators that exhibit a giant displacement. measures the maximum force needed to maintain the zero displacement when the material is under electric field. For an E-M polymer with a linear elastic response, Fb can be expressed as Fb=Fxj . Due to their low Young s modulus, E-M polymers usually exhibit a small block force compared to E-M ceramics. 

These materials have shown piezoelectric responses after appropriate poling [18]. Their piezoelectric actuation properties are typically worse than ceramic piezoelectric crystals however, they have the advantages of being lightweight, flexible, easily formed, and not brittle. Additionally, while ceramics are limited to strains on the order of 0.1%, ferroelectric polymers are capable of strains of 10% [91] and very high electromechanical coupling efficiencies [93]. 

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