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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  48
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 main idea of this passage is that ______.
选项 A、nitinol is a highly versatile metal
B、nitinol helped make the space program a success
C、nitinol has revolutionized dental care
D、most alloys have "memories"
答案A
解析 主旨大意题。A项：镍钛诺是一种用途很广的金属；B项：镍钛诺有利于太空计划的成功实现；C项：镍钛诺带来牙科护理领域的革命，D项：大部分金属都具有“记忆力”。还是按照处理主旨大意题的一贯做法，我们先来回顾一下全文；文中先讲到意外地获得镍钛诺合金，而后谈到其奇异的特i生，因为此种特性，它被广泛地应用于宇宙飞船、天线，探测器、防护材料等领域，文末又盲及，科学家而今又发现诙种合金可以有效地用于固牙齿，正在一步步得到推广，所以，本文重在介绍镍钛诺合金的特性及其广泛用途，A项正确。
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