The future of metals

PERSPECTIVES

Cray-2 supercomputer used liquid immer-sion cooling ( 10).

Recently, chilled-liquid cooling was rein-troduced in high-end mainframes and densely packed servers to cope with the high heat ? uxes. Y et, liquid cooling can be taken further if we consider a micro? uidic heat sink ( 11) (see the ? gure, panel C). Microchannel heat sinks can be designed such that the thermal resistance between the transistor and the ? uid is reduced to the extent that even cooling-water temperatures of 60° to 70°C ensure no overheating of the microprocessors. This hot-water cooling has compelling advantages. First, chillers are no longer required year-round and thus the data-center energy con-sumption plummets by up to 50%. Second, and perhaps most important, direct utiliza-tion of the collected thermal energy becomes

feasible, either using synergies with district

heating or specific industrial applications.

With such an appealing waste-heat recovery

system, the green diligence of data centers

would be upped substantially.

Reducing the energy consumption of

data centers and concomitantly restrain-

ing costs, while curtailing carbon emis-

sion, is achievable. Despite power dissipa-

tion in microprocessors continuing to be

a source of severe concern, liquid cooling

and deploying waste heat appear to become

imperative in the drive for improving the

data-center energy ef? ciency.

References

1. International Data Corporation, Document No. 221346

(2009), https://www.wendangku.net/doc/8c14741608.html,.

2. U.S. Environmental Protection Agency, Report to

Congress on Server and Data Center Energy Ef? ciency

(2007).

3. European Commission, Code of Conduct on Data Centres

Energy Ef? ciency (2008).

4. International Technology Roadmap for Semiconduc-

tors, 2009 Edition, Executive Summary; https://www.wendangku.net/doc/8c14741608.html,/

Links/2009ITRS/Home2009.htm.

5. K. Mistry et al., IEEE IEDM 2007 Tech. Digest, 10.2

(2007).

6. M. Chudzik et al., IEEE VLSI 2007 Tech. Digest, 11A-1

(2007).

7. N. A. Kurd et al., IEEE ISSCC 2010 Tech. Digest, 5.1

(2010).

8. M. Ware et al., IEEE HPCA 2010 Tech. Digest, 6.4 (2010).

9. L. A. Barroso, U. H?lzle, The Datacenter as a Computer

(Morgan and Claypool, 2009); https://www.wendangku.net/doc/8c14741608.html,/

corporate/green/datacenters.

10. S. R. Cray Jr., U.S. Patent 4590538 (1986).

11. D. B. Tuckerman, R. F. W. Pease, Electron Device Lett.2,

126 (1981).

10.1126/science.1182769

The Future of Metals MATERIALS SCIENCE

K. Lu Despite advances made in composite materials, metals remain irreplaceable in many important applications.

O n 15 December 2009, the world’s most fuel-ef? cient commercial jet-

liner—the Boeing 787 Dreamliner—completed its ? rst ? ight. The airliner is mostly made from carbon fiber–reinforced poly-meric composites (50% by weight, up from 12% in the Boeing 777) ( 1). Traditional met-als are substantially replaced by composites with higher strength/weight ratios; aluminum usage has dropped to 20% (versus 50% in the 777). Ever since the 1950s, when “engi-neering materials” mainly meant metals ( 2), the share of metals in engineering materials has been diminishing. What are the reasons behind this trend, and which applications are likely to stay in the domain of metals?

The main property limitation of metals as structural materials is their low speci? c strength (the strength/weight ratio). Most engineering designs call for structural materi-als that have high strength, fracture toughness (a measure of the energy required for propa-gating cracks), and stiffness while minimiz-ing weight. Most metals have high strength and stiffness, but because they are dense (steels are several times as dense as ceram-ics and polymers), their strength/weight and stiffness/weight ratios are low relative to competing materials (see the ? gure). This is a key reason for replacing metals in aircraft and sporting goods, where weight is a primary concern. Some metals such as aluminum and

magnesium are light, but they are too soft for

many applications and have low toughness

and stiffness. Titanium alloys partly over-

come these problems: They are about half as

dense as steels, have higher strength, and are

very tough. Titanium was ? rst used in airlin-

ers in the 1960s in the Boeing 707 and its use

has increased to 15% in the Boeing 787 ( 1).

Metals can be strengthened through con-

trolled creation of internal defects and bound-

aries that obstruct dislocation motion ( 3).

But such strategies compromise ductility and

toughness, in contrast to the increasing tough-

ness at higher strength seen with polymeric

composites (see the figure). Strengthening

may also compromise other metal properties,

such as conductivity and corrosion resistance.

One method for strengthening metals without

losing toughness is grain re? nement (grain

size reduction), but when the grain sizes fall

below ~1 μm, strengthening is usually accom-

panied by a drop in ductility and toughness

( 4). A recent study points the way to overcom-

ing this problem: In a low-alloy steel contain-

ing ultra? ne elongated ferrite grains strength-

ened with nanosized carbides, toughness and

strength both rose when temperature was low-

ered from 60° to –60°C ( 5). In contrast, con-

ventional metals become strong but brittle at

lower temperatures. The authors attributed the

observed toughening to the unique hierarchi-

cal anisotropic nanostructures in their steels.

Nanotwinned metals are another exam-

ple of hierarchical nanostructured metals

with extraordinary mechanical properties

( 3). When a high density of twin boundaries

(highly symmetrical interfaces between two

grains of the same lattice structure) is incor-

porated into polycrystalline copper grains,

with boundary spacing in the nanometer scale,

the material becomes stronger than coarse-

grained copper by a factor of 10; it is also

very ductile. The ultrastrong nanotwinned

copper has an electrical conductivity com-

parable to that of high-conductivity copper

( 6) and a much enhanced resistance against

electromigration ( 7). It has great potential for

applications in microelectronics.

Corrosion is another headache for metals

( 8). To protect metals from corrosion, they

are commonly coated with a layer of corro-

sion-resistant material. The Hangzhou Bay

Bridge in China is an outstanding example of

this technique. This 36-km-long bridge—the

world’s longest to date, with a design life of

100 years—is supported by several thousand

pillars made of concrete-? lled steel tubes ~80

m in length. The tubes are protected against

corrosion in the harsh ocean environment

by a coating of novel polymeric composites

combined with cathode attachments.

Metal corrosion can also be resisted by

forming a continuous protective passiva-

tion layer on the metal surface. For example,

Y amamoto et al. ( 9) have added 2.5% Al to

conventional austenitic stainless steels, result-

ing in the formation of a protective aluminum

oxide layer that can resist further oxidation at

elevated temperatures. Given their enhanced

Shenyang National Laboratory for Materials Science, Insti-

tute of M etal Research, Chinese Academy of Sciences,

Shenyang 110016, China. E-mail: lu@https://www.wendangku.net/doc/8c14741608.html,

Published by AAAS o n A p r i l 1 5 , 2 0 1 0 w w w . s c i e n c e m a g . o r g D o w n l o a d e d f r o m

https://www.wendangku.net/doc/8c14741608.html, SCIENCE VOL 328 16 APRIL 2010319

16 APRIL 2010 VOL 328 SCIENCE https://www.wendangku.net/doc/8c14741608.html,

320

PERSPECTIVES

oxidation resistance, these steels could be

used at the high temperatures and aggres-sive oxidizing conditions of energy conver-sion systems to replace the currently used expensive nickel-based superalloys (metals speci? cally developed for high-temperature applications). Another route to enhancing the corrosion resistance of metals is to modify the chemical composition of surface layers. But most such processes require high tem-peratures that may cause serious deteriora-tion of the metal substrates. The processing temperatures can be lowered substantially if the grains of surface layers are re? ned into the nanoscale via surface mechanical attri-tion treatments ( 10).

Metals also g et soft at elevated tem-peratures; they can rarely be used above 1000°C with useful strengths. Superalloys have higher operating temperatures (up to ~1150°C for nickel-based superalloys), enabling their use in hig h-temperature applications such as jet turbine engines. Superalloys for higher operating tempera-tures, based on metals with higher melting points such as molybdenum and niobium, are under investigation ( 11).

Despite these limitations, metals are still the major workhorse of our society and will remain so in the future, thanks to unique properties that make them irreplaceable.

First, metals have a much higher fracture toughness than other materials (see the ? g-ure); steels are the toughest known materials. Therefore, metals are usually used for key components with the highest requirements for reliability and durability, such as bridge cables, concrete reinforcement in buildings, and vehicle body frames. The Boeing 787 contains a considerable amount of metals (45%), mainly in critical parts with the high-est reliability requirements, such as engines, wings, and alighting carriage.

Second, the properties of metals are uni-form in all directions, and their strength is the same in tension and compression. Also, the strengths of metals are usually predict-able. These features are critically impor-tant for predicting fracture in eng ineer-ing structures. In contrast, it remains very dif? cult to predict fracture of composites and ceramics; their fracture is often cata-strophic (the material fails all at once) and may cause serious economic loss and even loss of life. Hence, many advanced technol-ogies continue to rely on high-performance metals. For instance, only extremely tough metal alloys can survive in the harsh irra-diation and temperature conditions in the next generation of nuclear power stations. Body implants must sustain severe corro-sive and loading conditions that also call for advanced tough metals.

Third, most metals are more conductive than ceramics and polymers. Copper and alu-minum remain the best materials for overhead electricity transmission lines. Conducting lines and thermal spreaders used in informa-tion technology are mostly made from cop-per and its alloys. Metals also have unique

magnetic properties that are not easily reproduced in other materials.

Fourth, metals have the best over-all mechanical properties at tempera-tures up to a few hundred degrees. This window covers most of the operation temperatures in chemical engineering processes, power stations, and various engines. Finally, most metals are recy-clable, making metals more competi-tive for quantity applications.

Modern technolog ies not only strongly rely on these unique prop-erties of metals, but urgently call for even better metals. Increasing the strength of metals without sacri? cing other properties is critical for their competitiveness. Multiscale hierarchical structures provide a possible route to opti-mizing overall properties. Metals may also be mixed with other materials in a controlled way to form composite structures. Assem-bling metals with other components in this way—for example, in novel reinforcements or hierarchical assemblies—may shift their strength/toughness ratios toward the upper right corner in the ? gure. Developments in different material families may thus bene? t from each other.

References and Notes

1. 787 Dreamliner Program Fact Sheet (Boeing Corp.,

2007; see https://www.wendangku.net/doc/8c14741608.html,/commercial/787family/programfacts.html).

2. M. F. Ashby, Materials Selection in Mechanical Design

(Elsevier, Oxford, ed. 3, 2005).

3. K. Lu, L. Lu, S. Suresh, Science 324, 349 (2009).

4. M. A. Meyers, A. Mishra, D. Benson, Prog. Mater. Sci. 51,

427 (2006).

5. Y. Kimura, T. Inoue, F. Yin, K. Tsuzaki, Science 320, 1057

(2008).

6. L. Lu, Y. Shen, X. Chen, L. Qian, K. Lu, Science 304, 422

(2004).

7. K.-C. Chen, W.-W. Wu, C.-N. Liao, L.-J. Chen, K. N. Tu,

Science 321, 1066 (2008).

8. A. King, G. Johnson, D. Engelberg, W. Ludwig, J. Marrow,

Science 321, 382 (2008).

9. Y. Yamamoto et al ., Science 316, 433 (2007).

10. X. Si, B. N. Lu, Z. B. Wang, J. Mater. Sci. Technol. 25, 433

(2009).

11. J. H. Perepezko, Science 326, 1068 (2009).

12. I thank C. W. Che, R. B. Tan, and R. Yang for their critical

comments; Y. Zhang for assistance in ? gure preparation; and the National Natural Science Foundation of China, the Chinese Academy of Sciences, and the Danish-Chinese Center for Nanometals for ? nancial support.

10.1126/science.1185866

300

250

200

150

100

50

100

200

300

400

Strength-to-weight ratio (MPa cm 3 g –1)

F r a c t u r e t o u g h n e s s , K I c (M P a m 1/2)

1000

2000

Carbon fibers

Ceramics

Engineering polymeric composites Polymers

Mg alloys

Al alloys

Cu

alloys

Ni alloys Ti alloys

Steels

Competing materials. Steels have the highest toughness, whereas carbon ? bers have the highest strengths. Titanium alloys and polymer composites are increasingly used in aircraft and sporting goods because of their outstanding combination of prop-erties. Research on advanced materials for structural applications aims toward the upper right corner. [Data from ( 2)]

Published by AAAS

o n A p r i l 15, 2010

w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m

中考英语语法-连词与状语从句

中考英语词汇语法专项训练篇 连词与状语从句 一、单项选择（共25小题；共25分） 1. The foreign woman can't speak French Japanese. A. so B. nor C. but D. or 2. It's four years I came to Shanghai. A. before B. when C. since D. as 3. It was a nice meal, a little expensive. A. though B. whether C. as D. since 4. John fell asleep he was listening to the music. A. after B. before C. while D. as soon as 5. You'd better do it your mother did. A. when B. as C. like D. because 6. We have been good friends we joined the same ping-pong team. A. after B. before C. since D. until 7. Help others whenever you can you'll make the world a nicer place to live in. A. and B. or C. unless D. but 8. I hardly knew anything about it you told me. A. since B. after C. until D. when 9. --- How do you like this skirt? --- It's beautiful and it fits me well, I like it very much. A. but B. for C. so D. since 10. --- What do you think of the book Harry Potter? --- I like it very much. It's interesting exciting. A. neither; nor B. not; but C. not only; but also D. either; or 11. Mary was riding her bicycle to the park she was hit by a car. A. when B. after C. until D. since 12. Study hard, you won't catch up with your friends. A. but B. or C. and D. so 13. You should make a good plan you do anything important. A. after B. though C. before D. until 14. My grandfather is very old, he never stops learning. A. than B. though C. but D. unless 15. Henry's mother his father speaks English. They both speak Chinese.

如何区分定语从句、宾语从句和状语从句

如何区分定语从句、宾语从句和状语从句？（附习题）| 虫虫讲英语 2018-12-02 14:52 「虫虫讲英语」老少咸宜的英语学习号——有时候，语法换一种方式讲，就听懂了。 如何区分定语从句、宾语从句和状语从句，是学生最常问我的问题之一。今天，我们通过青铜、白银、黄金、王者四级难度的例句，一起研究下怎样一眼辨别英语3大从句。 01 概念 3大从句的区别均在于前面两个字：定语、宾语和状语： 知道了这几个概念，这三种从句就很好理解了： >> 定语从句：作定语的从句，做adj.修饰先行词；

在这里，dog “狗子” 是先行词，即“走在定语从句前面的名词”；定语从句 that shits a lot 其中的 that，指 代了前面的 dog，告诉我们这是一条怎样的狗子：拉很多的狗子。 >> 宾语从句：作宾语的从句，放在动词或介词后面； 第一句，宾语从句为普通的陈述句，放在 think 这个动作后面，由连接词 that 引导。 第二句，宾语从句已经改成陈述句语序，原来人讲的话则是一般疑问句Do you let it go “你丫放不放手” ？该从句放在 depend on 的介词 on 后面，一般疑问句由 whether/ if 引导。 >> 状语从句：作状语的从句，给主句增加信息量。 状语从句，是3大从句中比较好记的一种：有个完整的主句，从句是提供更多信息的，比如上述例句告诉我们他为什么养狗。只要熟悉九大状语从句的引导词（← 戳可查看），几乎一眼就能辨别出状语从句。 02 当堂练习 现在，我们一起看几个句子找找感觉。请判断下列句子是定语从句，宾语从句还是状语从句？ >> 青铜 1. If it is fine tomorrow, I will visit you. 2. I helped an old man who lost his way.

创新引领未来InnovationCreatestheFuture

https://www.wendangku.net/doc/8c14741608.html, Sponsor 主办单位承办单位Organizers CPCIF

关于 ICIF China 展品范围 基础化学品 基础化学品、基本有机、无机化工原料等 石油化工及新能源 原油、溶剂油、石蜡及石油产品添加剂等 精细与专用化学品 化工原料、汽车化学品、电子化学品、染料、防御性化学品、农用化学品、皮革和纺织化学品、塑料添加剂、纸浆和造纸化学品、表面活性剂复印化学品/油墨化学品、稳定剂等 化学品包装与储运 包装：金属包装、塑料包装、柔性包装、纸包装、 包装检测设备等 储运：化工物流公司、存储设备、储罐设计与工程等 化工安全生产技术、化工环境保护技术与设备、危险化学品存储及运输等 化工安全与环保 实验室分析仪器、工程仪器、流程机械、热处理装置、泵、压缩机、制药机械、阀门、工业和劳动安全、自动化控制技术、仪器仪表等 化工技术与装备 中国国际化工展览会（ICIF China）是亚洲化工行业顶级展会。展会规模约为90,000平方米，参展企业近2,000家，展品范围涵盖了基础化学品、精细与专用化学品、化学品包装与储运、石油化工及新能源、化工技术与装备和智慧化工创新产品等。展会将吸引超过90,000名的观众前来参观。自1992年以来，ICIF China已经成为政府机构以及企业决策者交流和贸易对接的首选平台。 ICIF China 2018(第十七届)中国国际化工展览会将于2018年9月19-21日在上海新国际博览中心举办，届时，展会将与第十三届中国国际橡胶技术展、第二十一届中国国际胶粘剂及密封剂展览会同期举办，期待您的加入！ 中国化工行业顶级贸易平台

英语让步状语从句用法总结

英语让步状语从句用法总结（一） 让步状语从句的用法一（表示“虽然，即使，尽管”） 1. although，though，even though，even if都表示“虽然，即使，尽管”之意，though 和although 语气较弱(其中的though 比although 通俗, 不如although 正式)，even if和even though 带有强调意味而显得语气更强。例如：Although they are poor, they are happy. 虽然他们很穷，但很快乐。天气虽然很冷, 但他没有穿大衣就出去了。即使他们可能不会成功，但他们仍努力尝试。那篇文章虽然很短，但很重要。他好一点了，虽仍未痊愈。即使下雨，她也是走着去工作的。今晚我将去拜访他，即使我只能停留一小会儿。 注意：though 引导的让步状语从句的用法比较特殊，它可以倒装，也可以不倒装，具体用法参见as引导让步状语从句的用法。 2. as引导让步状语从句表示“虽然、即使、尽管”，其引导的让步状语从句必须采用倒装结构，as引导的倒装形式的让步状语从句的基本结构为：形容词/副词/名词(单数可数名词前一般不带冠词，有时也可见单数可数名词前形容词与不定冠词连用、形容词放在不定冠词前)/动词/过去分词＋as＋主语＋谓语的其他部分。注意体会下列典型例句：Great as the author was, he proved a bad model. 这位作者尽管了不起，到头来却成了一个坏榜样。这个故事看似荒唐，却是真的。虽然他很富有，但他并不幸福。倒装时要省去单个名词前的冠词)尽管还是个孩子，他却能分辨黑白。他尽管勇敢，可见到蛇还是发抖。尽管伤势很重，他仍然十分乐观。虽然尽了最大努力，但是他仍然不能搬动那块石头。 ［考题1］We had to wait half an hour ____ we had already booked a table. A. since B. although C. until D. before ［答案］B ［解析］下划线处之后所述的“我们已经预订了一桌”与前面所述的“我们已经不得不等了半个小时”之间语意相反，四个选项中although表示“即便……”，引导让步状语从句最为合适。 ［考题2］____ most of the earth’s surface is covered by water, fresh water is very rare and precious. A. As B. Once C. If D. Although ［答案］D ［解析］句意为：虽然地球表面覆盖着水，但是纯净水仍然很稀有、珍贵。下划线处应引导让步状语从句，填入although最为合适。 ［考题3］____ he has limited technical knowledge, the old worker has a lot of experience. A. Since B. Unless C. As D. Although ［答案］D ［解析］下划线处应引导让步状语从句，填入although最为合适。 ［考题4］Although he is considered a great writer, ____. A. his works are not widely read B. but his works are not widely read C. however his works are not widely read D. still his works are not widely read ［答案］A ［解析］英语不允许在连词although、though引导的让步状语从句之后的主句句首重复使用连词，如果要在复合句中强调某种特别的意义，可以在主句的句首加上yet、still等副词。例如：Although she has a lot of money, (yet/still) she is not happy. 虽然她很有钱，但是她并不快乐。

状语从句连词总结

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