Smart, Very Smart, and Intelligent Materials
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forms of self-protection. In a crude way, these materials mimic the brain, metabolism, and protective reflex reactions of the human body that augment muscle motion and the five senses. Materials with a built-in learning function are smarter than those without. A very smart material senses a change in the environment and responds by altering one or more of its property coefficients. In this way, it can tune its sensing and actuating capabilities in time or space to optimize behavior. With the help of a feedback system, a very smart material becomes smarter with age. The distinction between smart and very smart materials is essentially one between linear and nonlinear properties. The physical properties of nonlinear materials can be adjusted by bias fields or forces to control response. The tunable transducer developed in our laboratory illustrates the concept of a very smart material. Two of the important properties of the ultrasonic transducers used in biomedical scanners and in nondestructive testing are the resonant frequency and the electromechanical coupling coefficient. The resonant frequency is controlled by the elastic constants of the transducer, and the electromechanical coupling coefficient by its piezoelectric constants. Tunable transducers are made by sandwiching together two highly nonlinear materials: rubber and relaxor ferroelectric ceramics. Rubber, like most elastomers, is elastically nonlinear. In its unstressed compliant state, the molecules are coiled and tangled, but under stress, the molecules align and the material stiffens noticeably. Relaxor ferroelectrics, such as lead magnesium niobate, are electrically nonlinear, and exhibit huge electrostriction effects. Under electric bias, the piezoelectric coefficient increases dramatically from zero to a value far higher than lead zirconate titanate. Both the resonant frequency and electromechanical coupling coefficient can be tuned when electric fields and mechanical stress are applied simultaneously. Integration and miniaturization of electroceramic sensors and actuators is an ongoing process in the automotive and consumer electronics areas. Today's midrange vehicles have about 50 sensors and actuators, and even more sophisticated functional materials are under development for exhaust gas monitoring, active suspensions, and integrated traffic guidance systems. Multilayer packages containing signal processing layers made up of low-permittivity dielectrics and printed metal interconnections are in widespread production. Further integration with imbedded resistors and capacitors is under development and it seems likely that intelligent
MRS BULLETIN/APRIL 1993
Smart, Very Smart, and Intelligent Materials
systems will make use of this processing technology. Tape casting and screen printing are the processes used most often, and varistors, chemical sensors, thermistors, and piezoelectric transducers can all be fabricated in this way, opening up the possibility of multicomponent multifunction ceramics with both sensor and actuator capabilities. Sil
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