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    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(SME); 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.
    全文翻譯：鎳鈦合金
    鎳鈦諾是這個(gè)世紀(jì)所發(fā)現(xiàn)的最不尋常的金屬之一，作為鎳和鈦的簡(jiǎn)單的一種合 金，鎳鈦諾具有一些令人驚嘆的特征。這是一種有記憶力的金屬，人們可以使它記住它被 塑成的任何形狀，并在加熱后恢復(fù)這一形狀。比如，一節(jié)鎳鈦諾線(xiàn)圈在加熱冷卻后會(huì)記住 圓圈這形狀。隨后它可能被彎折成其它形狀，但一旦再次加熱，就會(huì)迅速地自動(dòng)恢復(fù)成最初的圓圈狀。這種不尋常的能力被稱(chēng)為形狀記憶效果(SME)。
    其它一些合金如黃銅在一定程度上也具有這種特性。目前人們對(duì) SME 這一特性尚缺乏透徹的認(rèn)識(shí)，而鎳鈦諾尤其使 人驚奇，因?yàn)槊慨?dāng)它展現(xiàn)這一驚人的功能時(shí)，似乎都違背了熱力學(xué)原理。因?yàn)樗诨謴?fù)原 有形狀時(shí)所釋放的力比人們使它變形所施加的力大得多。
    鎳鈦諾不僅有記憶力，還能"學(xué)習(xí)"。如果加熱-冷卻-彎曲-再加熱這一過(guò)程重復(fù)一定次數(shù)，且每次冷卻后它都被絲毫不差地彎成同一形狀，它不僅可以記住最初的形狀，還能逐漸記住它被彎成的形狀，并開(kāi)始在每次冷 卻時(shí)恢復(fù)這一形狀。最終，它會(huì)自動(dòng)地隨溫度變化而彎曲和恢復(fù)這些形狀，并且沒(méi)有任何疲勞跡象。工程師們已制造出一些發(fā)動(dòng)機(jī)樣機(jī)，利用鎳鈦合金在交替遇熱水和冷水時(shí)迅速改變形狀所產(chǎn)生的力做推動(dòng)力。然而這些神奇的發(fā)動(dòng)機(jī)工作時(shí)并非完全不耗能，因?yàn)楸仨?有熱能來(lái)制造溫差才能使機(jī)器運(yùn)轉(zhuǎn)。但人們可以通過(guò)改變合金中鎳鈦的比例來(lái)控制使它反應(yīng)的最佳溫度。有的鎳鈦合金甚至能在室溫下做出反應(yīng)，冷暖之間的最小溫差只有 12℃。
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