实验测定碳纳米管“真实”机械性质
[color=blue]For more than 15 years, carbon nanotubes (CNTs) have been the flagship material of nanotechnology. Researchers have conceived applications for nanotubes ranging from microelectronic devices to cancer therapy. Their atomic structure should, in theory, give them mechanical and electrical properties far superior to most common materials.[/color][color=#0000ff][/color]
Unfortunately, theory and experiments have failed to converge on the true mechanical properties of CNTs. Researchers at Northwestern University recently made the first experimental measurements of the mechanical properties of carbon nanotubes that directly correspond to the theoretical predictions.
Carbon nanotubes are cylindrical structures usually less than 30 nanometers in diameter and several microns long. Their small size makes them very strong but at the same time quite difficult to test individually; as a result, experiments typically deviate widely from predictions based on quantum mechanics.
"Imaging and measurement resolutions as well as atomic structural ambiguities (defects) obscured the results of most experiments and provided unreliable mechanical predictions," said Horacio Espinosa, a professor of mechanical engineering at Northwestern's McCormick School of Engineering and Applied Science.
Espinosa and his group at Northwestern have resolved these issues using a nanoscale material testing system based on microelectromechanical system (MEMS) technology. This system allows electronic measurements of load and displacement during a test, which is performed inside a transmission electron microscope to provide real-time atomic imaging.
"This method removes all ambiguity from testing results," Espinosa said. "We can be certain of all the quantities we have measured, and the results match quantum mechanics predictions very well."
Espinosa collaborated with George Schatz, Morrison Professor of Chemistry in Northwestern's Weinberg College of Arts and Sciences, as well as with Peter Zapol, a physicist at Argonne National Laboratory. This work is published online in Nature Nanotechnology and will appear in print in the journal's October issue.
Further research also was reported in the same article regarding the effect of electron irradiation on these materials. One would think that irradiation would degrade the atomic structure of the material, but the researchers found the opposite.
"Irradiating a multiwalled carbon nanotube with an intense electron beam actually forms bonds among the shells of the tube. This is like combining multiple nanotubes into one to form a stronger structure," said lead author Bei Peng, who recently received his doctoral degree from Northwestern under Espinosa's supervision.
This phenomenon also has been theorized in the past, and the research confirms that the properties of multiwalled nanotubes can easily and controllably be altered by electron irradiation.
The irradiation work was supplemented by detailed atomistic modeling. Using computer simulations of the atomic structure of the nanotubes, the team of researchers was able to isolate the mechanism of strengthening due to irradiation.
"The same procedure used to strengthen individual multiwalled nanotubes by irradiation may also be used to link together individual nanotubes into a bundle," said Mark Locascio, a doctoral student co-author of the paper.
This mechanism of crosslinking is a promising method for creating much larger nanotube-based structures. When nanotubes are packed together, they typically have very weak interactions along their surfaces; a spun nanotube rope would not be nearly as strong as its nanoscale constituents. However, irradiation may be the key to improving these interactions by inducing covalent bonds between tubes. If the properties of nanotubes can be scaled up to macroscale ropes and fibers, they may become a viable option for any high-strength application. This could include large cables for applications in industry or infrastructure, as well as smaller threads for lightweight woven fabrics, ballistic armors or composite reinforcement.
The Nature Nanotechnology paper was authored by Espinosa, Peng, Locascio, Zapol and Schatz as well as Steven Mielke, a postdoctoral researcher, and Shuyou Li, an electron microscopist, both at Northwestern.
Aug 15, 2008
Northwestern University
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实验测定碳纳米管“真实”机械性质
[table=98%,#f8f9f4][tr][td]作者：任霄鹏 来源：[url=http://www.sciencenet.cn/][color=#0000ff]科学网 [url]www.sciencenet.cn[/url][/color][/url] 发布时间：2008-8-18 16:8:53[/td][/tr][tr][td][align=left][table=98%][tr][td][/td][/tr][tr][td][/td][/tr][tr][td]实验测定碳纳米管“真实”机械性质[/td][/tr][tr][td][b]相关论文在线发表于《自然—纳米技术》[/b][/td][/tr][/table]
15年来，碳纳米管已经成为纳米技术领域的旗舰材料。科学家们深信，纳米管可以应用于从微电子设备到癌症治疗的多个方面。理论研究发现，纳米管的原子结构赋予了它们远超过普通材料的机械和电特性。然而实验研究却一直无法得到相统一的结果。美国科学家的一项最新研究，首次用实验方法测定了碳纳米管的机械性质，结果直接符合理论预言。相关论文8月10日在线发表于《自然—纳米技术》（[i]Nature Nanotechnology[/i]）上。
碳纳米管通常是长几微米、直径小于30纳米的圆柱形结构。微小的尺度令碳纳米管十分结实，但同时也带来了难以单个检测的问题。正因为如此，实验结果一般都与量子力学的理论预言大相径庭。
在最新研究中，美国西北大学的机械工程教授Horacio Espinosa和他的小组以及美国Argonne国家实验室的科学家，利用一种基于微电机系统（MEMS）的纳米尺度材料检测系统，成功解决了上述问题。在透射电子显微镜提供实时原子成像的基础上，该系统能够对实验中的载荷和位移进行电子测量。
Espinosa说，“成像和测量分辨率以及原子结构缺陷令此前大部分实验的结果模糊不清。而新方法消除了检测结果中所有的不确定性，我们十分确信测定的全部数量，而且得到的结果也与量子力学语言十分吻合。”
科学家进一步研究了碳纳米管的电子辐射效应。论文第一作者Bei Peng表示，与此前通常认为的电子辐射会令破坏碳管的原子结构相反，“用强电子束照射多壁碳纳米管后，组成纳米管的壳与壳之间会形成化学键。这就好比将多重纳米管结合成一个更强大的结构。”
此前，科学家已经针对这种现象建立了理论。而最新的研究利用计算机原子建模模拟了碳纳米管的原子结构，找到了由辐射引起的“交联”（crosslinking）加强机制，证实了多壁碳纳米管的特性可以由电子辐射轻易、可控地改变。
论文合著者Mark Locascio表示，“利用辐射加强单个多壁碳纳米管的手段同样可以用于将单个碳纳米管连接成束”，从而创造出大得多的基于纳米管的结构。（科学网 任霄鹏/编译）
（《自然—纳米技术》（[i]Nature Nanotechnology[/i]），doi:10.1038/nnano.2008.211，Bei Peng, Horacio D. Espinosa）
[b]更多阅读（英文）[/b]
[url=http://www.nature.com/nnano/journal/vaop/ncurrent/abs/nnano.2008.211.html][color=#800000]《自然—纳米技术》论文摘要[/color][/url]
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[url=http://clifton.mech.northwestern.edu/~espinosa/][color=#800000]Horacio Espinosa实验室主页 [/color][/url]
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