Nano-engineering can produce substances with unique properties that will give renewable energy a boost

    納米工程技術能制造出具有獨特性質的物質，這將促進可再生能源的開發(fā)

    New materials for renewable energy:The power of being made very small

    Big improvements in the production of energy, especially from renewable sources, are expected over the coming years. Safer nuclear-power stations, highly efficient solar cells and the ability to extract more energy from the wind and the sea are among the things promised. But important breakthroughs will be needed for these advances to happen, mostly because they require extraordinary new materials.

    The way researchers will construct these materials is now becoming clear. They will engineer them at the nanoscale, where things are measured in billionths of a metre. At such a small size materials can have unique properties. And sometimes these properties can be used to provide desirable features, especially when substances are formed into a composite structure that combines a number of abilities. A series of recent developments shows how great that potential might be.

    Grand designs

    Researchers have already become much better at understanding how the structure of new nano-engineered materials will behave, although the process remains largely one of trial and error because different samples have to be repeatedly manufactured and tested. Michael Demkowicz of the Massachusetts Institute of Technology is developing a model that he hopes will address the problem from a different direction: specifying a set of desired properties and then trying to predict the nanostructures needed to deliver them.

    Dr Demkowicz is working with a team based at the Los Alamos National Laboratory, one of a number of groups being funded under a new $777m five-year programme by the American government to accelerate research into energy technologies. The material Dr Demkowicz is looking for will be good at resisting damage from radiation. It could be used instead of stainless steel to line a nuclear reactor, which would extend the reactor's working life and allow it to be operated more efficiently by burning a higher percentage of nuclear fuel. At present, says Dr Demkowicz, reactors burn only around 1% of their fuel, so even a modest increase in fuel burn would leave less radioactive waste.

    The reason why the linings of nuclear reactors degrade is that metals can become brittle and weak when they are exposed to radiation. This weakness is caused by defects forming in their crystal-lattice structure, which in turn are caused by high-energy particles such as neutrons bumping into individual atoms and knocking them out of place. When these displaced atoms collide with other atoms, the damage spreads. The result is holes, or "vacancies", and "interstitials", where additional atoms have squeezed into the structure.

    Dr Demkowicz says it is possible to design nanocomposites with a structure that resists radiation damage. This is because they can be made to exhibit a sort of healing effect in the areas between their different layers. The thinner these layers are, the more important these interfaces become because they make up more of the total volume of the material. Depending on how the nanocomposites are constructed, both the vacancies and the interstitials get trapped at the interfaces. This means there is a greater chance of their meeting one another, allowing an extra atom to fill a hole and restore the crystal structure. In some conditions the effect can appear to show no radiation damage at all, he adds.

    The ideal nanocomposite would not only resist radiation damage. It would also not itself become radioactive by absorbing neutrons. Dr Demkowicz has used his modelling techniques to come up with some candidates; iron-based ones for fission reactors and tungsten-based ones for those that may one day use nuclear fusion. It could still take years before such materials are approved for use, but the modelling methods will greatly speed up the process.

    Across the spectrum

    Nano-engineered materials will also play an important role in a more efficient generation of solar cells, according to an exhibition by researchers at Imperial College, London, called "A Quantum of Sol", which opened this week at the Royal Society Summer Science Exhibition, also in London. Again, the desired effects are obtained by using combinations of material produced at extremely small sizes. In this case, they are used to make "multi-junction" solar cells, in which each layer captures energy from a particular colour in the spectrum of sunlight. Overall, this is more efficient than a conventional solar cell which converts energy from only part of the spectrum.

    Whereas conventional solar cells might turn 20% or so of the energy in sunlight into electricity, multi-junction solar cells already have an efficiency of just over 40% and within a decade that could reach 50%, predicts Ned Ekins-Daukes, a researcher at Imperial. Until nano-engineering costs come down with economies of scale, multi-junction solar cells will remain expensive. The researchers expect that electricity-generation costs can still be cut in the meantime by using mirrors to concentrate sunlight on the cells.

    Through the glass

    Solar cells could also be incorporated into the structure of buildings, including windows. Researchers at the Fraunhofer Institute for Mechanics of Materials are looking for suitable transparent materials to make them. They too are using computer models to explore atomic structures and then to simulate how electrons will behave in them. With the right combination of conductive and transparent material, says Wolfgang K?rner, from the German institute, it should be possible to produce completely see-through electronics.

    The nanostructure of composites can also provide great mechanical strength in relatively light materials. Composites such as fibreglass and carbon fibre bonded in a plastic resin are already widely used to replace metal in making, for instance, cars and aircraft. But by controlling the direction and the tension of the fibres during their construction it is possible to produce a morphing composite, which adjusts its shape under certain conditions. The change can be instigated by an external control or it can be automatic, for instance in response to variations in heat, pressure or velocity.

    These morphing composites could be used to produce more efficient turbine blades in wind and tidal generators, a seminar at Bristol University's Advanced Composites Centre for Innovation and Science was told this week. A bistable composite capable of altering its aerodynamic profile rapidly when wind and current conditions changed would help to remove unwanted stresses in the blades. That would increase the efficiency of the blades and extend the working life of the generator systems they power, says Stephen Hallett, a member of the Bristol team. Morphing composites would mean, for instance, that tidal generators could be made smaller and would last longer, which would make them more viable commercially. In this way many tiny changes in the science of materials could generate a big future for renewable energy.

    利用納米新材料促進可再生能源的開發(fā)：納米的力量

    在未來的幾年里，能源生產(chǎn)有望得到大幅改進，尤其是來自可再生能源的生產(chǎn)。其中包括更加安全的核電站、高效率的太陽能電池以及對風能和潮汐的利用。但是這些進步如果要得以實現(xiàn)的話，需要有一些重要的突破，主要是因為這些進步需要極其新穎的材料。

    研究者們構造這些材料的方法目前變得越來越清晰。他們將在納米尺度上加工這些材料，在這種尺度上，測量物質是按照10億分之米來進行的。材料處于這樣小的尺寸的時候會有一些獨特的性質。有時候，這些性質能提供一些有用的材料特性，尤其是當物質形成復合結構的時候，這種復合結構把許多性能結合在一起。目前的一系列進展表明這種技術前途極其巨大。

    宏偉設計

    研究者們對新型納米材料結構的行為方式已經(jīng)有了更好的了解，盡管他們對其中的過程仍然處于摸索階段，這是因為不同的樣品必須反復地重新制備和測試。來自麻省理工學院（Massachusetts Institute of Technology）的Michael Demkowicz目前正在開發(fā)一種模型，他希望這種模型能從不同的方向來解決上述問題：具體列出一些希望得到的性質，然后嘗試預測出能提供這些性質的納米結構。

    Demkowicz博士正和洛斯阿拉莫斯國家實驗室（Los Alamos National Laboratory）的一個研究小組一起工作。美國政府為了加快新能源技術的研究，新設立了一個為期5年、資助金額為7.77億美元的研究項目，許多科研小組都在這個項目資助下開展工作。

    Demkowicz博士所在的研究小組就是其中之一。Demkowicz正在尋找的材料在抗輻射方面有良好的性能。這種材料可以替代核反應堆的不銹鋼襯里，這將會延長反應堆的工作壽命，這種材料還能提高核燃料的燃燒效率，從而使核反應堆的效率提高。Demkowicz說，目前核反應堆僅僅只燃燒1%的核燃料，因此，即使只是稍微提高核燃料的燃燒效率，這也會減少放射性核廢料。

    核反應堆襯里為什么損害的原因在于，當金屬暴露于輻射之下的時候，它們會變得脆弱。金屬的這種變弱是由于它們的晶格結構中形成了缺陷，這種缺陷是由于高能粒子，比如中子撞擊到金屬的單個原子，從而把它們從晶格中撞出。當這些"錯位"的原子與其它原子碰撞到一起的時候，這種損害就會傳播開。這種結果導致了空穴（或者"空位"）以及"間隙",在這里其它的原子就嵌入進來。

    Demkowicz說，設計出能抵抗輻射的納米復合材料是可能的。這是因為，這種材料可以在不同層之間的區(qū)域里表現(xiàn)出一種"修復"效果。這些層越薄，這些層間界面就越重要，這是因為層間界面構成了整個材料體積的大部分。取決于納米復合材料的構成情況，"空位"和"間隙"都能在界面被俘獲。這就意味著，它們碰到一起的機會很大，這就可以讓另外一個額外的原子去填充空穴，從而恢復晶體結構。Demkowicz補充說，在某些條件下，這樣的結果看起來就好像沒有任何輻射損害發(fā)生一樣。

    理想的納米復合材料應該不僅僅能抵抗輻射損害。它應該在吸收了中子之后，自身也不變成具有輻射性質的物質。Demkowicz博士已經(jīng)利用他的模型技術設計出了幾種候選材料；基于鐵的納米復合材料可能用于核裂變反應堆，基于鎢的納米復合材料可能用于核聚變反應堆。這些材料的真正投入使用可能還需要好幾年的時間，但是這種模型方法將會極大地加快這種過程。

    利用所有太陽能

    根據(jù)倫敦帝國理工學院（Imperial College, London）研究者們的一個展覽，納米材料也將會在高效率太陽能電池中扮演重要的角色，這個展覽叫做"太陽量子",本周在"皇家學會夏季科技展"（Royal Society Summer Science Exhibition）上展出，后者也在倫敦舉辦。研究者們也是通過把極小尺寸的材料組合起來，得到了他們期望的結果。在這里，納米復合技術用于制造"多節(jié)點"太陽能電池，在這種電池中，每一層捕獲太陽光譜中某種特定的顏色的能量。總起來，這比傳統(tǒng)太陽能電池的效率要更加高，因為傳統(tǒng)太陽能電池僅僅只轉化太陽光光譜中的一部分能量。

    傳統(tǒng)的太陽能電池能把約20%的太陽光的能量轉化為電能，不過多節(jié)點（multi-junction）太陽能電池的效率已經(jīng)超過了40%,Ned Ekins-Daukes（倫敦帝國理工學院的一名研究者）預測，在十年內，其效率將會達到50%.除非納米材料因規(guī)模經(jīng)濟而降低成本，多節(jié)點太陽能電池將會仍然很昂貴。研究者們認為，使用玻璃鏡把陽光聚集在電池上也能降低太陽能電池產(chǎn)電的成本。

    透過玻璃

    太陽能電池也可以嵌入到建筑物種，包括窗戶。來自德國弗勞恩霍夫材料力學研究所（Fraunhofer Institute for Mechanics of Materials）的研究者們正在尋求合適的透明材料來制備這種太陽能電池。他們也使用計算機模型來研究原子結構，然后模擬電子在原子中的行為。來自該研究所的Wolfgang K?rner說，通過導電材料和透明材料合適的組合應該能制備出完全透明的太陽能電池。

    對于質量相對較輕的材料，復合材料的納米材料也能提供很好的機械強度。結合在塑料樹脂中的玻璃纖維和碳纖維復合材料已經(jīng)廣泛用于汽車和飛機的生產(chǎn)中，它們取代了金屬材料。另外，在制造過程中，通過控制纖維的方向和張力，有可能生產(chǎn)出變型復合材料（morphing composite）,這種材料在某些特定的條件下能調整其形狀。這種變化可以來自于外部的控制，也可以是自動的，比如在對熱、壓力或者速度的變化做出響應的時候。

    本周在布里斯托爾大學創(chuàng)新與科學高級復合材料中心（Bristol University's Advanced Composites Centre for Innovation and Science）的研討會上，有報道稱，這些變型復合材料能用于生產(chǎn)效率更高的風能和潮汐渦輪機片。當風和水流條件改變的時候，雙穩(wěn)復合材料能迅速改變它的空氣動力學形式，這有利于消除渦輪機片上多余的壓力。來自布里斯托爾大學的Stephen Hallett說，這能提高渦輪機片的效率，還能延長發(fā)電機系統(tǒng)（由渦輪機提供能量）的工作壽命。變型復合材料將意味著潮汐發(fā)電機可以制造的更小巧，而且更耐用，這使其更有商業(yè)前景。材料科學領域的這類許多微小的變化能為可再生能源帶來美好的未來。

    Vocabulary:

    Unique:獨特的

    Renewable:可再生的

    Boost:增進；提高

    Breakthrough: 突破

    Desirable: 可取的；值得的

    Specify:詳述；具體說明

    Radiation:輻射

    Lining:襯里

    Brittle:脆的

    Defect:缺陷

    Neutron:中子

    Fission:聚合

    Tungsten:（化學元素）鎢

    Exhibition:展覽

    Convert:轉化

    Spectrum:光譜

    Incorporate:嵌入

    Transparent:透明的

    Simulate:模擬

    Mechanical:機械的

    Adjust:調整

    Instigate:激起

    Velocity:速度

    Aerodynamic:空氣動力學的