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  3. A metal with a built in memory is one of the very useful discoveries made during America’s space program. Nitinol was discovered

A metal with a built in memory is one of the very useful discoveries made during America’s space program. Nitinol was discovered

admin2010-04-30  54
问题     A metal with a built in memory is one of the very useful discoveries made during America’s space program. Nitinol was discovered by scientists working at the Naval Surface Weapons Center. They were looking for heat shield metals for missiles and satellites, but what they found was nitinol. This alloy can be formed into shapes just like any other metal, but it returns to its original form at a certain temperature.
    When this alloy of nickel and titanium is first formed, it is shaped into ingots, brick shaped masses of the metal. Then it is drawn out into a wire of varying thickness, depending on the job it is destined to do. It is set into its desired curve or shape and heated briefly. It is then cooled off by dipping the wire in cold water. This "sets" its memory. The metal can then be formed into any desired shape, but whenever its temperature exceeds its room temperature, ’ it returns to the shape it has memorized.
    This makes nitinol perfect, of course, for, spacecraft, for antennas, probes, shields, and folding "arms" made of nitinol can be stored safely inside a spacecraft, protecting them from damage during launching. Then, once safe in orbit, the spacecraft would suddenly "grow" antennas and other structures. The sun’s rays would have warmed up the folded and coiled nitinol, and it would have returned to its original shape.
    But scientists have found a more down to earth use for this wire: they now use it to strengthen teeth. Before nitinol, teeth that needed straightening have brackets attached to them; wires were attached to these to exert a pull in the right direction. But this pull must be constant, and the stainless steel arch wire cannot exert the correct force if it has been bent several times or if the tooth has actually moved toward it as planned. As a result, many people with dental braces must have the wire replaced monthly, tightened every few weeks, or adjusted painfully tight at the beginning of the treatment.
    But nitinol wires can be formed to exert force in a certain direction, cooled of and then reformed to fit the patient’s mouth. As the mouth warms the nitinol, it begins to try to assume its preset shape. This exerts a steadier, more constant pull on the tooth than could ever be achieved with stainless steel wire. Nitinol can be bent into such intricate shapes — while maintaining its pull — that very difficult treatments, such as moving one tooth from behind another, can be done now without the expense or pain of constant adjustment. Nitinol has cut tile time needed for these treatments by almost half, a real benefit for both patient and dentist.
The passage suggests that ______.
选项 A、nitinol is an organic metal
B、nitinol is stronger than any metal
C、nitinol’s sensitivity to temperature changes makes it impractical for most uses
D、nitinol is better than stainless steel in jobs that require constant stress on the metal
答案D
解析 细节定位题。A项：镍钛诺是一种有机金属；B项：镍钛诺比任何金属都坚固；C项：镍钍诺对温度反应灵敏性，这使得它的应用面不广泛，A项说有机金屑，文中根本没有提及这样的相关信息。B项说比任何金属都坚固，镍钛诺合金的特性就是对温度敏感，在温度达到要求时就恢复原状，不用任何压力就发生形状变化，从哪点来说都难以和坚固牵扯得上关系，所以B项错误。c项是完全错误的推断或者说结论，正因为该合金有对温度反应灵敏的特性，其用途广泛，而且用在很多特殊、重要的场合，而c项的意思刚好相反，意思是其特性局限了其用途。只有选项D：对于需要金屑长期稳固的场合来说，镍钛诺比不锈钢更为适用；由本文最后一段第4句可知D项正确。
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