SENSORS AND ACTUATORS: SMART CYRSTALS. R. E. Newnham, Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA.

One of the qualities that distinguishes living systems from inanimate matter is the ability to adapt to changes in the environment. Smart materials have the ability to perform both sensing and actuating functions and are, therefore, capable of imitating this rudimentary aspect of life. Four of the most widely used smart materials are piezoelectric Pb(Zr,Ti)O₃, electrostrictive Pb(Mg,Nb)O₃, magnetostrictive (Tb,Dy)Fe₂, and the shape memory alloy NiTi. All four are ferroic with active domain walls, and two phase transformations which help tune the properties of these active materials. Pb(Zr,Ti)O₃ is a ferroelectric ceramic which is cubic at high temperature and becomes ferroelectric on cooling through the Curie temperature. At room temperature, it is poised on a rhombohedral-tetragonal phase boundary which enhances the piezoelectric coefficients. Terfenol, (Tb,Dy)Fe₂, is also cubic at high temperature and then becomes magnetic on cooling through its Curie temperature. At room temperature, it too, is poised on rhombohedral-tetragonal transition which enhances its magnetostriction coefficients. Pb(Mg,Nb)0₃ and Nitinol (NiTi) are also cubic at high temperatures, and on annealing, undergo an order-disorder transformation to a different cubic space group. On further cooling, the partially ordered Pb(Mg,Nb)0₃ structure goes through a diffuse phase transformation at room temperature where it exhibits very large dielectric and electrostrictive coefficients. Just below room temperature, it transforms to a ferroelectric rhombohedral phase. The ordered shape memory alloy NiTi undergoes an austenitic (cubic) to martensitic (monoclinic) phase change just above room temperature. It is easily deformed in the martensitic state but recovers its original shape when reheated to austenite. The structural similarities between these four superb actuator materials is remarkable. A review of the applications and structure-property relationships in these and other smart materials will be presented.