Memory materials, the flexible friends

A shape memory material is a material that can be persuaded to change its size or shape to one previously remembered, in response to some applied stimulus. The stimulus is usually temperature, though it can also be an applied electric or magnetic field. The concept of shape memory in engineering is familiar from the humble thermostat. If you take two strips of metal that have different coefficients of thermal expansion, and fix them together, then the resulting bimetallic strip will change shape with temperature as one of the metals changes length with temperature faster than the other. Since they are fixed together, this change in length is exaggerated as change in the shape. In this case the change of shape manifests itself as a curvature, which can be used to activate either a switch (as in a domestic room thermostat) or a valve (as in a car cooling system).

Shape memory materials enable such change to be achieved within a single material, with a much wider choice of responses possible. I first encountered shape memory materials when making loudspeaker cabinets as a child. You could cover the front of the completed cabinet with a special fabric which would shrink slightly, and hence tighten when you heated it in front of a fire.

Anyone who works with electrical or electronic circuits is familiar with polymer tubing that will shrink to a tight fit when it is heated with the industrial equivalent of a hair drier. These are examples of shape memory polymers. They work by building the final shape that you want into the equilibrium configuration of the polymer chains, and then somehow pinning the chains into some other configuration in a way that is released when heating to a higher temperature. Such materials have a one-way shape effect: you cannot recover the original shape by subsequent heat treatment.

One-way shape memory effects are also used with metal alloys. The first large scale application was established in 1971 for connectors for hydraulic tubing in aircraft. This was not introduced as a cost-saving measure: rather it provided a reliable coupling system to enable the weight savings associated with titanium tubing to be exploited. Couplings may still represent the largest tonnage use of shape memory alloys, but medical applications probably now dominate the value.

This book brings together contributions in a symposium that summarises the understanding and applications of shape memory materials. It begins with two chapters by the editors that introduce the theory of the martensitic transformation, and how this is the basis of shape memory effects in a range of alloys. As well as irreversible shape memory, these can include reversible effects, and also super-elasticity, which is finding widespread applications for mobile phone aerials and spectacle frames, allowing elastic recovery from much greater deformations than would otherwise be sustainable. In the following three chapters this is elucidated for three important classes of shape memory alloys: Ti-Ni, Cu-based, and ferrous. There are then two chapters on the fabrication and characteristics of shape memory alloys.

Shape memory ceramics and shape memory polymers each receive a chapter. These non-metallic materials are becoming increasingly important, and are the subject of rapid and fascinating developments. The shape memory ceramics provide a new class of materials that can respond to an applied electric field. These differ from traditional ceramic transducers in the amount of hysteresis that they exhibit. Whereas transducers generally rely on the poling of a stable ferroelectric phase, shape memory ceramics use an electrically switched transition between ferroelectric and anti-ferroelectric phases. The strains involved are generally small (typically 0.1% or less), but can be mechanically magnified to offer a whole new class of electro-mechanical actuators. The final three chapters describe general applications, the design and application of shape memory actuators, and finally medical and dental applications.

The symposium is of a remarkably high and uniform standard. There is a strong Japanese background: of the 13 contributors, three are from Europe, eight are from Japan, and only two are from the USA (of those one has a Japanese name). You can read the chapters in any order; indeed some people may prefer to read the book in reverse, with reference to earlier chapters where necessary.

The applications in the final three chapters provide the motivation for understanding the materials. The ceramic and polymer materials are each very exciting, and require scarcely any knowledge of martensitic transformations. And even those whose primary interest lies in the shape memory alloys will find their appetite for the tensor mathematics, which starts on page five whetted by wanting to understand the mechanisms for specific alloys.

Andrew Briggs is reader in materials, University of Oxford.