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  3. A metal with a built in memory is one of the very useful discoveries made du

A metal with a built in memory is one of the very useful discoveries made du

游客2024-06-09  39
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. [br] Which of the following is NOT a step in forming nitinol for space use?
选项 A、The metal is shaped into ingots.
B、The metal is separated into nickel and titanium.
C、The metal is drawn into a wire.
D、The metal is set in its desired curve or shape and heated briefly.
答案 B
解析 细节定位题。文中第2段第1句提到：it is shaped into ingots 与 A项“被铸成钢锭”一致。该段第2句又提到：Then it is drawn out into a wire of varying thickness与 C项“该种金属被拉成线”相符合。第3句同样提到：It is set into its desired curve or shape and heated briefly. 与D项“制成要求的曲型或任何形状，或者稍微加热”表达的意思相同。只有B项：该合金被分离成镍和钛，没有提到。且由第1段第4句得知，镍钛诺是一种合金，具有在一定温度下恢复原状的特性，如果把这种合金分离成镍和钛，则不具备这种特性了。故选择B项。
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