Nitinol
Nitinol is one of the most extraordinary metals to be discoveredthis century: A simple alloy of nickel and titanium, nitinol has someperplexing properties. A metal with a memory, it can be made to remember anyshape into which it is fashioned, returning to that shape whenever it isheated.For example, a piece of nitinol wire bent to form a circle that is thenheated and quenched will remember this shape. It may then be bent or crumpled, but on reheating, will violently untwist,reforming its original shape. This remarkable ability is calledShape Memory Effect; other alloys, such asbrasses, are known to possess it to a limited extent. No one fully understands SME, and nitinol remains particularlyperplexing, for, whenever it performs this peculiar feat, it appearsto be breaking the laws of thermodynamics by springing back into shape withgreater force than was used to deform it in the first place.
But not only is nitinol capable of remembering, it also has the ability to learn. If theheating-cooling-crumpling-reheating process is carried out sufficiently often,and the metal is always crumpled in exactly the same way, the nitinol willnot only remember its original shape, but gradually it learns toremember its crumpled form as well, and will begin to return to the samecrumpled shape every time it is cooled.
Eventually, the metal will crumple and uncrumple,totally unaided, in response to changes in temperature and without anysign of metal fatigue. Engineers have produced prototype engines that aredriven by the force of nitinol springing from one shape to another as italternately encounters hot and cold water. The energy from theseremarkable engines is, however, not entirely free: heat energy is required to produce the temperature differences needed to runthe engine. But the optimum temperatures at which the metal reacts can becontrolled by altering the proportions of nickel to titanium; some alloys willeven perform at room temperature. The necessary temperature range between thewarm and the cold can be as little as twelve degrees centigrade.
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