Phase transformations in smart materials

One of the qualities that distinguishes living systems from inanimate matte r is the ability to adapt to changes in the environment. Smart materials ha ve the ability to perform both sensing and actuating functions and are, the refore, capable of imitating this rudimentary aspect of life. Four of the m ost widely used smart materials are piezoelectric Pb(Zr, Ti)O-3, electrostr ictive Pb(Mg,Nb)O-3, magnetostrictive (Tb, Dy)Fe-2 and the shape-memory all oy NiTi. All four are ferroic with active domain walls and two phase transf ormations, which help to tune the properties of these actuator materials. P b(Zr, Ti)O-3 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 whic h enhances the piezoelectric coefficients. Terfenol, (Tb, Dy)Fe-2, is also cubic at high temperature and then becomes magnetic on cooling through its Curie temperature. At room temperature, it too is poised on a rhombohedral- tetragonal transition which enhances its magnetostriction coefficients. Pb( Mg, Nb)O-3 and nitinol (NiTi) are also cubic at high temperatures and on an nealing transform to a partially ordered state. On further cooling, Pb(Mg, Nb)O-3 passes through a diffuse phase transformation at room temperature wh ere it exhibits very large dielectric and electrostrictive coefficients. Ju st below room temperature, it transforms to a ferroelectric rhombohedral ph ase. The partially ordered shape-memory alloy NiTi undergoes an austenitic (cubic) to martensitic (monoclinic) phase change just above room temperatur e. It is easily deformed in the martensitic state but recovers its original shape when reheated to austenite. The structural similarities between thes e four superb actuator materials are remarkable, and provide a key to the d evelopment of future smart materials.