Molecular Mechanisms in Smart Materials
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Introduction It is a great honor to be named the Turnbull Lecturer of the Materials Research Society. Thirty years ago, David Turnbull was a professor at Harvard University, and I was a young professor at Massachusetts Institute of Technology. He and I share a common interest in phase transformations, an underlying theme of this lecture. Professor Turnbull's papers and the Solid State Physics Series he edited with Frederick Seitz are classics in the field. I refer you to his review entitled "Phase Changes"1 for a deeper understanding of today's subject. The outline of my talk is as follows: I will begin with a brief introduction of what is meant by smart, very smart, and intelligent materials, differentiating the role of sensing, actuating, and signal processing. I will then cover a few of the applications, followed by a discussion of the atomic mechanisms important in piezoelectrics, electrostrictors, magnetostrictors, and shape-memory alloys. I then summarize the common themes for these mechanisms, and end with a few thoughts about biomimetic design and functional composites, as well as a glimpse of the future as typified by thinfilm sensors and actuators. The concept of a smart material is exemplified by a videotape-head positioner (Figure 1). Bimorph piezoelectric ceramics are usually made from lead zirconate 20
titanate (PZT). The positioner has large actuator electrodes that move the material, as well as smaller electrodes to sense the position and orientation of the tape head. This combination of sensing and actuating mimics two of the functions of a living system—namely being aware of the surroundings and being able to respond to that signal with a useful response, usually in the form of a motion. A smart material is not simply a sensor. A sensor receives a stimulus and responds with a signal. A smart material is not simply an actuator either. An actuator produces a useful motion or action. Smart materials function as both
sensors and actuators. Smart materials sometime have a control system and sometimes not. Some function like our spinal cord in that they are passively smart and respond without thought- or signal-processing. They are analogous to the reflex responses of the human body. In other cases, smart materials analyze the sensed signal, perhaps for its frequency components, and then make a choice as to what type of response to make. We call that an actively smart system. We have also been developing a family of very smart materials—I am not willing to call them intelligent— which have a learning or tuning function, making it possible for them to get slightly smarter with age.2 Nonlinear properties like electrostriction or higher order elastic constants are used in these materials. Looking ahead to the use of thin-film microelectromechanical systems (MEMS), more intelligent materials are coming on the scene that integrate the control system with the sensors and actuators, all in one common piece of material. At this stage, I think the system deserves the name intelligent. Perhaps someday we will m
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