华中科大校友陈刚 打破百年物理定律 麻省理工学院（MIT）30日宣布，该校动力工程学华裔教授陈刚与其团队的研究，首次打破「黑体辐射定律」公式，证实物体极接近的热力传导，可以高到定律所预测的千倍。研究报告将在8月号「NanoLetter」杂志上发表。 德国物理学家普朗克1900年创「黑体辐射定律」（blackbody radiation law），公认是物体间热力传导基本法则，虽有人怀疑定律在两物体极接近时不成立，但始终无法证明。 「黑体幅射定律」创定在不同温度下，在绝大多数情况下都成立，但在极微小距离中稳定控制物体以测试能量传导，极为困难。百多年来，科学家始终无法突破。普朗克本人对此定律在微距物体间是否仍成立，亦无把握。 陈刚出身中国华中科技大学、柏克莱加州大学，是知名纳米热电材料和流体学者。他的团队采用方位较易控制的小玻璃珠对着平面物体的方式，取代在纳米（10亿分之一米）距离中根本不可能不碰触的两平行平面体；并采用双金属臂梁（bi-metallic cantilever）科技的原子能动力显微镜，精准测量两物体间温度变化。 MIT表示，陈刚团队证实了科学家所预言但无法实证的理论，获国际间同领域学者喝采。不但让人们对基本物理有进一步了解，对改良电脑资料储存用的硬碟的「记录头」（recording head），以及发展储聚能源的新设计等工业应用上十分重要。 陈刚说，目前电脑使用的硬碟，记录头与硬碟表面约有5、6纳米距离，记录头容易发热，而研究员一直在寻找控制热力的方法。热力传导和控制是磁力储存（magnetic storage）领域十分重要的一环，此类应用也将因陈刚的发现而迅速发展。新的发现还有助于开发新一代能源转换装置，陈刚说，此研究也提供对基本物理进一步了解。 麻省理工当日首条新闻 http://web.mit.edu/newsoffice/2009/heat-0729.html MIT news Breaking the law, at the nanoscale Bringing notallowobjects close together can boost radiation heat transfer, acc ording to new study that shows breakdown in Planck's law David L. Chandler, MIT News Office July 29, 2009 A well-established physical law describes the transfer of heat between two not allowobjects, but some physicists have long predicted that the law should brea k down when the notallowobjects are very close together. Scientists had never been able to confirm, or measure, this breakdown in practice. For the first ti me, however, MIT researchers have achieved this feat, and determined that the heat transfer can be 1,000 times greater than the law predicts. The new findings could lead to significant new applications, including better design of the recording heads of the hard disks used for computer data storage , and new kinds of devices for harvesting energy from heat that would otherwis e be wasted. Planck's blackbody radiation law, formulated in 1900 by German physicist Max P lanck, describes how energy is dissipated, in the form of different wavelength s of radiation, from an idealized non-reflective black notallowobject, called a blackbody. The law says that the relative thermal emission of radiation at d ifferent wavelengths follows a precise pattern that varies according to the te mperature of the notallowobject. The emission from a blackbody is usually cons idered as the maximum that an notallowobject can radiate. The law works reliably in most cases, but Planck himself had suggested that wh en notallowobjects are very close together, the predictions of his law would b reak down. But actually controlling notallowobjects to maintain the tiny separ ations required to demonstrate this phenomenon has proved incredibly difficult . Planck was very careful, saying his theory was only valid for large systems, explains Gang Chen, MIT's Carl Richard Soderberg Professor of Power Engineeri ng and director of the Pappalardo Micro and Nano Engineering Laboratories. So he kind of anticipated this , but most people don't know this. Part of the problem in measuring the way energy is radiated when notallowobjec ts are very close is the mechanical difficulty of maintaining two notallowobje cts in very close proximity, without letting them actually touch. Chen and his team, graduate student Sheng Shen and Columbia University Professor Arvind Na rayaswamy, solved this problem in two ways, as described in a paper to be publ ished in the August issue of the journal Nano Letters (available now online). First, instead of using two flat surfaces and trying to maintain a tiny gap be tween them, they used a flat surface next to a small round glass bead, whose p osition is easier to control. If we use two parallel surfaces, it is very har d to push to nanometer scale without some parts touching each other, Chen exp lains, but by using a bead there is just a single point of near-contact, which is much easier to maintain. Then, they used the technology of the bi-metallic cantilever from an atomic-force microscope to measure the temperature changes with great precision. We tried for many years doing it with parallel plates, Chen says. But with t hat method, they were unable to sustain separations of closer than about a mic ron (one millionth of a meter). By using the glass (silica) beads, they were a ble to get separations as small as 10 nanometers (10 billionths of a meter, or one-hundredth the distance achieved before), and are now working on getting e ven closer spacings. Professor Sir John Pendry of Imperial College London, who has done extensive w ork in this field, calls the results very exciting, noting that theorists ha ve long predicted such a breakdown in the formula and the activation of a more powerful mechanism. Experimental confirmation has proved elusive because of the extreme difficult y in measuring temperature differences over very small distances, Pendry says . Gang Chen's experiments provide a beautiful solution to this difficulty and confirm the dominant contribution of near field effects to heat transfer. In today's magnetic data recording systems - such as the hard disks used in co mputers - the spacing between the recording head and the disk surface is typic ally in the 5 to 6 nanometer range, Chen says. The head tends to heat up, and researchers have been looking for ways to manage the heat or even exploit the heating to control the gap. It's a very important issue for magnetic storage, he says. Such applications could be developed quite rapidly, he says, and so me companies have already shown a strong interest in this work The new findings could also help in the development of new photovoltaic energy conversion devices to harness photons emitted by a heat source, called thermo phovoltaic, Chen says. The high photon flux can potentially enable higher eff iciency and energy density thermophovoltaic energy converters, and new energy conversion devices, he says. The new findings could have a broad impact, says Shen. People working with d evices using small separations will now have a clear understanding that Planck 's law is not a fundamental limitation, as many people now think, he says. B ut further work is needed to explore even closer spacings, Chen says, because we don't know exactly what the limit is yet in terms of how much heat can be dissipated in closely spaced systems. Current theory will not be valid once we push down to 1 nanometer spacing. And in addition to practical applications, he says, such experiments might pr ovide a useful tool to understand some basic physics. The work was funded by the U.S. Department of Energy and the Air Force Office of Scientific Research.
标签: 华中科技大学
