This chapter discusses the concept of intelligent materials. Intelligent mechanisms involve self-diagnosis, self-adjustment, self-recovery, tuning (including recycling) capability, etc. These functions may be achieved by installing sensors and actuators within materials. If the structure is complicated, the material is no longer intelligent. One may believe the idea that the more complicated something is, the more advanced it is. This false recognition is sometimes called the “spaghetti syndrome”,—that is, many electric wires tangling the object. Once technologies suffer these syndromes, they become increasingly complicated. The need is to design intelligent materials to save complicated and unstable circuits. One of the important objectives for intelligent materials is to cure this syndrome. Another objective for intelligent materials is to develop technology—that is, friendly to the environment. Materials for technology must be designed to meet certain requirements: reasonable fabrication cost, high reliability during use, and capability for recycling. A PTC (positive temperature coefficient) thermistor is a good example of intelligent material with a simple structure. From the viewpoint of heater performance, the PTC thermistor is more intelligent than the NTC (negative temperature coefficient) one. The resistivity of NTC thermistor decreases with temperature. An increase in temperature gives rise to an increase in electric current. This is positive feedback and difficult to control. On the other hand, the PTC thermistor is a heater below the Curie temperature (Tc), it is a critical temperature sensor, and it is a Switch because above the temperature the resistivity increases tremendously. It is multifunctional material.
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