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  3. One of the simplest and best known kinds of crystal is the ionic salt, of which a typical example is sodium chloride, or ordinar

One of the simplest and best known kinds of crystal is the ionic salt, of which a typical example is sodium chloride, or ordinar

admin2015-01-31  65
问题     One of the simplest and best known kinds of crystal is the ionic salt, of which a typical example is sodium chloride, or ordinary table salt. The fundamental components of an ionic salt are ions: atoms or molecules that have become electrically charged by gaining or losing one more electrons. In forming sodium chloride, for example, sodium atoms give up an electron(thereby becoming positively charged)and chlorine atoms gain an electron(thereby becoming negatively charged). The ions are attracted to one another by their opposite charges, and they stack together compactly, like tightly packed spheres. Recently, scientists at Michigan State University created a new kind of crystal called an electride. In electrides, the anions(negative ions)are completely replaced by electrons, which are trapped in naturally formed cavities within a framework of regularly stacked cations(positive ions). Electrides are the first examples of ionic salts in which all these anionic sites are occupied solely by electrons.
    Unlike other types of anions, anionic electrons do not behave as if they were simple charged spheres. In particular, because of their low mass and their tendency to interact with one another over great distances, they cannot be "pinned down" to any one location. Instead, they wander close to and among the atoms lining the cavity and interact with electrons in nearby cavities, perhaps changing places with them.
    The properties of an electride depend largely on the distance between the cavities that hold trapped electrons. When the trapped electrons are far apart, they do not interact strongly, and so behave somewhat like an array of isolated negative charges. When they are closer together, they begin to display properties associated with large ensembles of identical particles. When they are still closer, the ensemble properties dominate and the electrons "delocalize": they are no longer tightly bound within individual cavities but are more or less free to pass through the spaces within the framework of positive ions.
    By synthesizing electrides from a variety of materials, one can vary the geometry of the anionic cavities and their relation to the surrounding cations. The resulting properties may make it possible for electrides to become a basis for economically useful new materials and devices. For instance, because the electrons in some electrides are very weakly bound, these crystals could be effective as photosensitive detectors, in which an impinging photon liberates an electron, resulting in a small electric current. The same weak binding could also make electrides useful in solar energy converters and as cathodes in batteries. One obstacle is the tendency of electrides to decompose through reaction with air and water. Researchers are seeking ways to increase their stability.
With which of the following statements regarding electrides would the author most likely agree?
选项 A、They have proven themselves to be of great commercial value.
B、Their future commercial value is promising but uncertain.
C、They are interesting but of no practical value.
D、They have commercial value mainly in solar energy applications.
答案B
解析 推断题。根据文章最后一段的The same weak binding could also makeelectrides useful in solar energy converters and as cathodes in batteries． One obstacle isthe tendency of electrides to decompose through reaction with air and water． Researchersare seeking ways to increase their stability．“这种结合微弱的特性同样可使电子晶体对太阳能转换器和电池的阴极很有用。可电子晶体的一个缺陷在于它易于与空气和水发生反应而分解。目前，研究人员正设法寻找提高电子晶体稳定性的方法。”可知电子晶体对于太阳能转换器和电池的阴极很有用，是存在商业价值的。但是由于本身的缺陷，研究人员正在努力改进，所以它的商业前景尚不明朗。和选项B的表述一致。所以答案选B。
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