Lawrence Joseph G, Berhan Lesley M, Nadarajah Arunan
Department of Chemical & Environmental Engineering, University of Toledo, Toledo, Ohio 43606, USA.
ACS Nano. 2008 Jun;2(6):1230-6. doi: 10.1021/nn7004427.
The structural complexity of vapor-grown carbon nanofibers means that they require a method that determines both their elastic properties and their corresponding morphology. A three-point bending test method was developed combining atomic force microscopy, transmission electron microscopy (TEM) and focused ion beam techniques to suspend individual nanofibers and measure their deflection coupled with accurate determinations of inner and outer diameters and morphology using high resolution TEM. This resulted in much improved accuracy and reproducibility of the measured values of the elastic modulus which ranged from 6 to 207 GPa. The data showed two distinct trends, with higher values of the modulus when the outer wall thickness of the nanofibers is larger than that of the inner wall, with the values decreasing with the overall wall thickness. These results suggest that the more ordered layers of the outer wall, closest to the inner wall, are mostly responsible for the nanofiber strength. For large nanofiber wall thicknesses of greater than 80 nm, the elastic modulus becomes independent of the thickness with a value of approximately 25 GPa. The results also demonstrate that this technique can be a standardized one for the detailed study of mechanical properties of nanofibers and their relationship to morphology.
气相生长碳纳米纤维的结构复杂性意味着它们需要一种既能确定其弹性性能又能确定其相应形态的方法。结合原子力显微镜、透射电子显微镜（TEM）和聚焦离子束技术开发了一种三点弯曲测试方法，以悬浮单个纳米纤维并测量其挠度，同时使用高分辨率TEM精确测定其内径和外径以及形态。这使得弹性模量测量值的准确性和可重复性有了很大提高，弹性模量范围为6至207 GPa。数据显示出两种不同的趋势，当纳米纤维的外壁厚度大于内壁厚度时，模量值较高，且模量值随总壁厚减小。这些结果表明，最靠近内壁的外壁更有序的层对纳米纤维强度起主要作用。对于大于80 nm的大纳米纤维壁厚，弹性模量变得与厚度无关，其值约为25 GPa。结果还表明，该技术可以成为用于详细研究纳米纤维力学性能及其与形态关系的标准化技术。
