Smart materials

Materials science is entering a whole new realm. As Philip Ball puts it: 'Smart materials represent the epitome of the new paradigm of materials science whereby structural materials are being superseded by functional ones'. Smart materials carry out their tasks as a result of their intrinsic properties. In many situations they will replace mechanical operations. We will see 'smart devices in which the materials themselves do the job of levers, gears and even electronic circuitry'. There is even the prospect of 'A house built of bricks that change their thermal insulating properties depending on the outside temperature so as to maximise energy efficiency' (op. cit., p. 104).

Smart materials are already on the market, like thermochromic or electrochromic glass. At present electrochromic glass is a sandwich construction with a gel which changes its light emission properties in response to an electric current. The current is required to change its state not to maintain that state. Pilkington is developing a solid state version of this glass which should make both cheaper and available in much larger sizes. This will dispense with the need for mechanical blinds and solar shades and will give individuals much greater control over their immediate environment. Such materials come into the general category of passive smart materials. The really exciting advances are in active smart materials. An active system is controlled not only be external forces but also by some internal signal. In smart systems an active response usually involves a feedback loop that enables the system to 'tune' its response and thus adapt to a changing environment rather than be passively driven by external forces. An example is a vibrating-damping smart system. Mechanical movement triggers a feedback loop into providing movement that stabilises the system. As the frequency or amplitude of the vibrations changes so the feedback loop modifies the reaction to compensate.

One useful class of smart materials are 'shape memory alloys' (SMAS) alternatively called 'solid state phase transformations'. These are materials which, after deformation, return completely to their former shape. They function by virtue of the fact that the crystal structures of SMAS change when heated. An application already being exploited are thermostats where bimetal strips are replaced by alloys. They can be incorporated into mechanisms for operating ventilation louvres or ventilation/heating diffusers.

In general smart systems can be divided into sensors and actuators. Sensors are detection devices to respond to changes in the environment and warn accordingly. Actuators make things happen; they are control devices that close or open an electrical circuit or act as a valve in a pipe.

For example, they may perform a dual role extracting heat from low grade sources like ground water or geothermal reservoirs and serve as mechanical pumps to deliver the warmed water to the heating system of a building. No moving parts; no possibility of mechanical breakdown and all at low cost; it seems 'such stuff as dreams are made of'.

In principle SMAS can be used for any application which requires heat to be converted into mechanical action.

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