据报道，麻省理工学院航天工程师于近日设计了碳纳米管“针”，它可以“穿针引线”使复合材料层间实现良好粘合，从而有助于制造出质量更轻、抗损伤性能更强的航天飞机。

　　此前，空客和波音公司最新的载人航天飞机机身主要是由先进的复合材料构成的，譬如用质量极轻且使用性能优异的碳纤维增强塑料代替飞机的铝基材料，可以使其重量减轻约20%。复合材料在飞机上的主要应用优势就在于通过减轻重量以节省燃油消耗。但是复合材料抗损伤性能较差：与铝基材料在断裂前可以承受较大的冲击相比，复合材料的多层结构在较小冲击下就很容易发生断裂。

　　研究人员使用碳纳米管将每一层复合材料“栓”在一起。碳纳米管中的薄卷状碳原子虽然“身形”微小，但是强度极高。他们在类胶状聚合物基体中嵌入碳纳米管 “森林”，然后“压紧”碳纤维复合材料层间的聚合物基体。纳米管就像是细小的竖直排列的“针”，充当多层结构的支架，在层间部位进行“缝合”。

　　测试结果表明，与现有复合材料相比，经碳纳米管“缝合”的复合材料强度可提升30%，在断裂前能承受更大的作用力。

　　此项研究的首席研究员，MIT航空航天系博士后Roberto Guzman认为，性能提升的复合材料可以用于制造强度更高、质量更轻的飞机零部件，尤其是那些使用传统复合材料制造的因包含螺钉或螺栓而容易断裂的零部件。

　　“尺寸是关键”

　　当前，复合材料由层状的横向碳纤维组成，通过胶粘剂粘接。此项研究参与者，MIT航空航天系教授Wardle指出，“层间粘合处是非常薄弱、存在问题的区域”。许多学者尝试采用“Z钉扎”方法固定或通过“三维编制”复合材料层的碳纤维束以增强结合性能，类似于钉子和针线所起的作用。

　　Wardle 表示，“钉子或针的尺寸是碳纤维的几千倍，所以如果在碳纤维中加入这些物质，将会破坏成千上万的碳纤维，对复合材料本身的损伤不言而喻。”而碳纳米管直径约10纳米，只有碳纤维尺寸的百万分之一。

　　“尺寸的问题很重要，正因为纳米管进入复合材料内部而不会影响大尺寸的碳纤维，才使复合材料的性能得以保持，” Wardle解释说，“碳纳米管拥有的表面积达到碳纤维的1000倍，这使它们与聚合物基体结合良好。”

　　Guzman和Wardle采用的新技术即可使碳纳米管嵌入聚合物胶内部。首先，他们获得竖直排列的碳纳米管森林，然后将纳米森林置于粘稠的、未固化的复合层之间，重复此过程一直到16层(典型的复合材料叠层结构)，实现碳纳米管在层与层之间粘结。

　　Wardle认为，“随着大多数新型飞机的重量超过一半来自于复合材料，提升当前复合材料的综合性能对拓宽其在航空结构中的应用将起到很大的推动作用。”

原文入下：

　　The newest Airbus and Boeing passenger jets flying today are made primarily from advanced composite materials such as carbon fiber reinforced plastic — extremely light, durable materials that reduce the overall weight of the plane by as much as 20 percent compared to aluminum-bodied planes. Such lightweight airframes translate directly to fuel savings, which is a major point in advanced composites’ favor.

　　Method could help make airplane frames lighter, more damage-resistant

　　But composite materials are also surprisingly vulnerable: While aluminum can withstand relatively large impacts before cracking, the many layers in composites can break apart due to relatively small impacts — a drawback that is considered the material’s Achilles’ heel.

　　Now MIT aerospace engineers have found a way to bond composite layers in such a way that the resulting material is substantially stronger and more resistant to damage than other advanced composites. Their results are published in the journal Composites Science and Technology.

　　The researchers fastened the layers of composite materials together using carbon nanotubes — atom-thin rolls of carbon that, despite their microscopic stature, are incredibly strong. They embedded tiny “forests” of carbon nanotubes within a glue-like polymer matrix, then pressed the matrix between layers of carbon fiber composites. The nanotubes, resembling tiny, vertically-aligned stitches, worked themselves within the crevices of each composite layer, serving as a scaffold to hold the layers together.

　　In experiments to test the material’s strength, the team found that, compared with existing composite materials, the stitched composites were 30 percent stronger, withstanding greater forces before breaking apart.

　　Roberto Guzman, who led the work as an MIT postdoc in the Department of Aeronautics and Astronautics (AeroAstro), says the improvement may lead to stronger, lighter airplane parts — particularly those that require nails or bolts, which can crack conventional composites.

　　“More work needs to be done, but we are really positive that this will lead to stronger, lighter planes,” says Guzman, who is now a researcher at the IMDEA Materials Institute, in Spain. “That means a lot of fuel saved, which is great for the environment and for our pockets.”

　　The study’s co-authors include AeroAstro professor Brian Wardle and researchers from the Swedish aerospace and defense company Saab AB.

　　“Size matters”

　　Today’s composite materials are composed of layers, or plies, of horizontal carbon fibers, held together by a polymer glue, which Wardle describes as “a very, very weak, problematic area.” Attempts to strengthen this glue region include Z-pinning and 3-D weaving — methods that involve pinning or weaving bundles of carbon fibers through composite layers, similar to pushing nails through plywood, or thread through fabric.

　　“A stitch or nail is thousands of times bigger than carbon fibers,” Wardle says. “So when you drive them through the composite, you break thousands of carbon fibers and damage the composite.”

　　Carbon nanotubes, by contrast, are about 10 nanometers in diameter — nearly a million times smaller than the carbon fibers.

　　“Size matters, because we’re able to put these nanotubes in without disturbing the larger carbon fibers, and that’s what maintains the composite’s strength,” Wardle says. “What helps us enhance strength is that carbon nanotubes have 1,000 times more surface area than carbon fibers, which lets them bond better with the polymer matrix.”

　　Stacking up the competition

　　Guzman and Wardle came up with a technique to integrate a scaffold of carbon nanotubes within the polymer glue. They first grew a forest of vertically-aligned carbon nanotubes, following a procedure that Wardle’s group previously developed. They then transferred the forest onto a sticky, uncured composite layer and repeated the process to generate a stack of 16 composite plies — a typical composite laminate makeup — with carbon nanotubes glued between each layer.

　　To test the material’s strength, the team performed a tension-bearing test — a standard test used to size aerospace parts — where the researchers put a bolt through a hole in the composite, then ripped it out. While existing composites typically break under such tension, the team found the stitched composites were stronger, able to withstand 30 percent more force before cracking.

　　The researchers also performed an open-hole compression test, applying force to squeeze the bolt hole shut. In that case, the stitched composite withstood 14 percent more force before breaking, compared to existing composites.

　　“The strength enhancements suggest this material will be more resistant to any type of damaging events or features,” Wardle says. “And since the majority of the newest planes are more than 50 percent composite by weight, improving these state-of-the art composites has very positive implications for aircraft structural performance.”

　　Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University, says advanced composites are unmatched in their ability to reduce fuel costs, and therefore, airplane emissions.

　　“With their intrinsically light weight, there is nothing on the horizon that can compete with composite materials to reduce pollution for commercial and military aircraft,” says Tsai, who did not contribute to the study. But he says the aerospace industry has refrained from wider use of these materials, primarily because of a “lack of confidence in [the materials’] damage tolerance. The work by Professor Wardle addresses directly how damage tolerance can be improved, and thus how higher utilization of the intrinsically unmatched performance of composite materials can be realized.”

　　This work was supported by Airbus Group, Boeing, Embraer, Lockheed Martin, Saab AB, Spirit AeroSystems Inc., Textron Systems, ANSYS, Hexcel, and TohoTenax through MIT's Nano-Engineered Composite aerospace STructures (NECST) Consortium and, in part, by the U.S. Army.

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