SAMPLE 20

[物理學(xué)類]

　　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.

　　1. The text is primarily concerned with discussing

　　[A] a way to isolate electrons.

　　[B] the characteristics of a new kind of crystal.

　　[C] the structure of an ionic salt.

　　[D] commercial uses for electrides.

　　2. In the first paragraph, the author is primarily concerned with

　　[A] introducing a variant on the standard atomic theory.

　　[B] describing how chlorine atoms can become negatively charged.

　　[C] describing some early research at Michigan State University.

　　[D] providing background for the technical discussion to follow.

　　3. According to the text, the defining characteristic of an electride is which of the following?

　　[A] Its positive are of particularly low mass.

　　[B] Its ions possess identical electrical charges.

　　[C] It contains a framework of regularly stacked ions.

　　[D] Its negative ions consist solely of electrons.

　　4. It can be inferred from the text that anions behaving as “simple charged spheres” (line 2, paragraph 3) could be expected to

　　[A] readily lose electrons and become positively charged.

　　[B] move freely in and out of their cavities.

　　[C] respond to photons by liberating electrons.

　　[D] remain fixed relative to their cations.

　　5. With which of the following statements regarding electrides would the author most likely agree?

　　[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.

　[答案與考點(diǎn)解析]

　　1. 【答案】B

　　【考點(diǎn)解析】本題是一道中心主旨題。本文的中心主旨句在第二段的第一句，如果抓住了這句話并且聯(lián)系每段的中心主旨句，就可得出本題的正確選項(xiàng)B。其實(shí)本文第二段就是在講一種新的晶體的與眾不同的“characteristics”(特點(diǎn))�？忌�在解題時(shí)一定要首先找出全文的中心主旨句，并與上下文聯(lián)系。

　　2. 【答案】D

　　【考點(diǎn)解析】這是一道段落關(guān)系題。從第二段到尾段是本文作者闡述其中心思想的段落。在展開中心思想之前，作者為中心思想的展開進(jìn)行了鋪墊(講述一般晶體的特點(diǎn))，從而為第二段到尾段的陳述(講述一種新晶體的特點(diǎn))提供明確的對(duì)比。考生在解題時(shí)一定要注意段落之間的關(guān)系。

　　3. 【答案】D

　　【考點(diǎn)解析】這是一道審題定位題。題干中沒有給出明確的審題定位信息。破解此題需要考生平時(shí)的閱讀積累，這是一道較難的題目。題干中所涉及的是“the defining characteristic of an electride”(電子晶體的特點(diǎn))。在第二段作者第一次在文章中談到了“electride”，所以其特點(diǎn)也應(yīng)該在第二段向讀者交待，故本題的答案信息來源應(yīng)該在第二段。綜合第二段第二、三句的內(nèi)容可推導(dǎo)出本題的正確選項(xiàng)D�？忌�在解題時(shí)一定要具備出色的審題定位能力。

　　4. 【答案】D

　　【考點(diǎn)解析】本題是一道句間關(guān)系與反推題。題干中的信息以將本題的答案信息確定在第三段。根據(jù)第三段所陳述的內(nèi)容進(jìn)行反推可得出本題的正確選項(xiàng)D。在進(jìn)行反推時(shí)一定要注意第一句中的“do not behave as if…”以及第二句中的“cannot be pinned down”所起的作用�？忌�在解題時(shí)一定要具備反推即逆向推理的能力。

　　5. 【答案】B

　　【考點(diǎn)解析】這是一道審題定位題。本題的題干沒有提供任何審題定位的信息。但是善于閱讀的考生應(yīng)該知道每篇文章尾段的重要性。在本題到來之前本文的尾段還沒有涉及任何一道題，故考生應(yīng)該首先從尾段中尋找本題的答案信息。尾段的第二句第四句表明新的晶體在商業(yè)方面的可利用性;尾段的第五句表明這種新的晶體的不足之處�？梢姳绢}的正確選項(xiàng)應(yīng)該是B�？忌�在解題時(shí)一定要對(duì)文章結(jié)構(gòu)以及出題點(diǎn)有所認(rèn)識(shí)，最好對(duì)全文有整體的把握。

　　[參考譯文]

　　在各種最簡單和最常見的晶體中，離子鹽是其中一例，它典型的實(shí)例就是氯化鈉，或普通的食鹽。離子鹽的基本組成成份便是離子，也就是那些由于得到或失去一個(gè)或多個(gè)電子而帶有電荷的原子或分子。例如，在形成氯化鈉的過程中，鈉原子釋放一個(gè)電子(從而帶有正電荷)而氯電子得到一個(gè)電子(從而帶有負(fù)電荷)。這些離子由于帶有相反的電荷而相互吸引，它們緊密地堆砌在一起就象緊密擠壓在一起的球體。

　　最近，密西根州立大學(xué)的科學(xué)家們研制出一種新的被稱為電子晶體的晶體。在電子晶體中，陰離子(即帶負(fù)電荷的離子)全部被電子替代，這些電子被困在自然形成的空穴中，而空穴位于規(guī)則堆砌的陽離子(即帶正電荷的離子)的框架內(nèi)。電子晶體是第一種陰離子空間全被電子占據(jù)的離子鹽。

　　和其它類型的陰離子不同，作為陰離子的電子不像那些簡單的帶電球體。特別地，由于它們的質(zhì)量很小而且有與遠(yuǎn)距離的離子進(jìn)行相互作用的傾向，它們不會(huì)被固定在任何一個(gè)位置。相反，它們會(huì)到處游離，或者緊靠那些在空穴邊上的原子，或者在這些原子之間游離，并且還和附近空中的電子發(fā)生相互作用，甚至于可能與它們交換位置。

　　電子晶體的特征很大程度上依賴于空穴間的距離，而這些空穴便是容納被捕獲電子的地方。當(dāng)這些被捕獲的電子相距很遠(yuǎn)時(shí)，它們之間的相互作用不強(qiáng)，因而在某種程度上表現(xiàn)為一組相互孤立的負(fù)電荷。但當(dāng)它們間的距離很近時(shí)，它們便表現(xiàn)出一些特征來，很像大量相同離子的集合。當(dāng)它們間的距離更近時(shí)，群體集合屬性更占主導(dǎo)地位，而且電子也出現(xiàn)“離位”現(xiàn)象：它們不再被束縛于單個(gè)的空穴之上，而是或多或少地可以在正離子的構(gòu)架之間穿越。

　　通過從不同的材料來合成電子晶體，科學(xué)家們可以改變陰離子空穴的幾何結(jié)構(gòu)，以及改變它們同周圍陰離子之間的關(guān)系。這些隨之而來的特征使得電子晶體可能成為經(jīng)濟(jì)實(shí)用的新材料和新設(shè)備的基礎(chǔ)。例如，由于在某些電子晶體中，電子結(jié)合得非常微弱，從而使得這些晶體可以有效地用作光敏探測(cè)器。在這種探測(cè)器中，一束光子可以釋放一個(gè)電子，從而形成一股小電流。這種結(jié)合微弱的特性同樣可使電子晶體對(duì)太陽能轉(zhuǎn)換器和電池的陰極很有用�？呻娮泳�體的一個(gè)缺陷在于它易于與空氣和水發(fā)生反應(yīng)而分解。目前，研究人員正設(shè)法尋找提高電子晶體穩(wěn)定性的方法。