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

    纳米工程技术能制造出具有独特性质的物质，这将促进可再生能源的开发

    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.

    利用纳米新材料促进可再生能源的开发：纳米的力量

    在未来的几年里，能源生产有望得到大幅改进，尤其是来自可再生能源的生产。其中包括更加安全的核电站、高效率的太阳能电池以及对风能和潮汐的利用。但是这些进步如果要得以实现的话，需要有一些重要的突破，主要是因为这些进步需要极其新颖的材料。

    研究者们构造这些材料的方法目前变得越来越清晰。他们将在纳米尺度上加工这些材料，在这种尺度上，测量物质是按照10亿分之米来进行的。材料处于这样小的尺寸的时候会有一些独特的性质。有时候，这些性质能提供一些有用的材料特性，尤其是当物质形成复合结构的时候，这种复合结构把许多性能结合在一起。目前的一系列进展表明这种技术前途极其巨大。

    宏伟设计

    研究者们对新型纳米材料结构的行为方式已经有了更好的了解，尽管他们对其中的过程仍然处于摸索阶段，这是因为不同的样品必须反复地重新制备和测试。来自麻省理工学院（Massachusetts Institute of Technology）的Michael Demkowicz目前正在开发一种模型，他希望这种模型能从不同的方向来解决上述问题：具体列出一些希望得到的性质，然后尝试预测出能提供这些性质的纳米结构。

    Demkowicz博士正和洛斯阿拉莫斯国家实验室（Los Alamos National Laboratory）的一个研究小组一起工作。美国政府为了加快新能源技术的研究，新设立了一个为期5年、资助金额为7.77亿美元的研究项目，许多科研小组都在这个项目资助下开展工作。

    Demkowicz博士所在的研究小组就是其中之一。Demkowicz正在寻找的材料在抗辐射方面有良好的性能。这种材料可以替代核反应堆的不锈钢衬里，这将会延长反应堆的工作寿命，这种材料还能提高核燃料的燃烧效率，从而使核反应堆的效率提高。Demkowicz说，目前核反应堆仅仅只燃烧1%的核燃料，因此，即使只是稍微提高核燃料的燃烧效率，这也会减少放射性核废料。

    核反应堆衬里为什么损害的原因在于，当金属暴露于辐射之下的时候，它们会变得脆弱。金属的这种变弱是由于它们的晶格结构中形成了缺陷，这种缺陷是由于高能粒子，比如中子撞击到金属的单个原子，从而把它们从晶格中撞出。当这些"错位"的原子与其它原子碰撞到一起的时候，这种损害就会传播开。这种结果导致了空穴（或者"空位"）以及"间隙",在这里其它的原子就嵌入进来。

    Demkowicz说，设计出能抵抗辐射的纳米复合材料是可能的。这是因为，这种材料可以在不同层之间的区域里表现出一种"修复"效果。这些层越薄，这些层间界面就越重要，这是因为层间界面构成了整个材料体积的大部分。取决于纳米复合材料的构成情况，"空位"和"间隙"都能在界面被俘获。这就意味着，它们碰到一起的机会很大，这就可以让另外一个额外的原子去填充空穴，从而恢复晶体结构。Demkowicz补充说，在某些条件下，这样的结果看起来就好像没有任何辐射损害发生一样。

    理想的纳米复合材料应该不仅仅能抵抗辐射损害。它应该在吸收了中子之后，自身也不变成具有辐射性质的物质。Demkowicz博士已经利用他的模型技术设计出了几种候选材料；基于铁的纳米复合材料可能用于核裂变反应堆，基于钨的纳米复合材料可能用于核聚变反应堆。这些材料的真正投入使用可能还需要好几年的时间，但是这种模型方法将会极大地加快这种过程。

    利用所有太阳能

    根据伦敦帝国理工学院（Imperial College, London）研究者们的一个展览，纳米材料也将会在高效率太阳能电池中扮演重要的角色，这个展览叫做"太阳量子",本周在"皇家学会夏季科技展"（Royal Society Summer Science Exhibition）上展出，后者也在伦敦举办。研究者们也是通过把极小尺寸的材料组合起来，得到了他们期望的结果。在这里，纳米复合技术用于制造"多节点"太阳能电池，在这种电池中，每一层捕获太阳光谱中某种特定的颜色的能量。总起来，这比传统太阳能电池的效率要更加高，因为传统太阳能电池仅仅只转化太阳光光谱中的一部分能量。

    传统的太阳能电池能把约20%的太阳光的能量转化为电能，不过多节点（multi-junction）太阳能电池的效率已经超过了40%,Ned Ekins-Daukes（伦敦帝国理工学院的一名研究者）预测，在十年内，其效率将会达到50%.除非纳米材料因规模经济而降低成本，多节点太阳能电池将会仍然很昂贵。研究者们认为，使用玻璃镜把阳光聚集在电池上也能降低太阳能电池产电的成本。

    透过玻璃

    太阳能电池也可以嵌入到建筑物种，包括窗户。来自德国弗劳恩霍夫材料力学研究所（Fraunhofer Institute for Mechanics of Materials）的研究者们正在寻求合适的透明材料来制备这种太阳能电池。他们也使用计算机模型来研究原子结构，然后模拟电子在原子中的行为。来自该研究所的Wolfgang K?rner说，通过导电材料和透明材料合适的组合应该能制备出完全透明的太阳能电池。

    对于质量相对较轻的材料，复合材料的纳米材料也能提供很好的机械强度。结合在塑料树脂中的玻璃纤维和碳纤维复合材料已经广泛用于汽车和飞机的生产中，它们取代了金属材料。另外，在制造过程中，通过控制纤维的方向和张力，有可能生产出变型复合材料（morphing composite）,这种材料在某些特定的条件下能调整其形状。这种变化可以来自于外部的控制，也可以是自动的，比如在对热、压力或者速度的变化做出响应的时候。

    本周在布里斯托尔大学创新与科学高级复合材料中心（Bristol University's Advanced Composites Centre for Innovation and Science）的研讨会上，有报道称，这些变型复合材料能用于生产效率更高的风能和潮汐涡轮机片。当风和水流条件改变的时候，双稳复合材料能迅速改变它的空气动力学形式，这有利于消除涡轮机片上多余的压力。来自布里斯托尔大学的Stephen Hallett说，这能提高涡轮机片的效率，还能延长发电机系统（由涡轮机提供能量）的工作寿命。变型复合材料将意味着潮汐发电机可以制造的更小巧，而且更耐用，这使其更有商业前景。材料科学领域的这类许多微小的变化能为可再生能源带来美好的未来。

    Vocabulary:

    Unique:独特的

    Renewable:可再生的

    Boost:增进；提高

    Breakthrough: 突破

    Desirable: 可取的；值得的

    Specify:详述；具体说明

    Radiation:辐射

    Lining:衬里

    Brittle:脆的

    Defect:缺陷

    Neutron:中子

    Fission:聚合

    Tungsten:（化学元素）钨

    Exhibition:展览

    Convert:转化

    Spectrum:光谱

    Incorporate:嵌入

    Transparent:透明的

    Simulate:模拟

    Mechanical:机械的

    Adjust:调整

    Instigate:激起

    Velocity:速度

    Aerodynamic:空气动力学的