據報道，麻省理工學院航天工程師于近日設計了碳納米管“針”，它可以“穿針引線”使復合材料層間實現良好粘合，從而有助于制造出質量更輕、抗損傷性能更強的航天飛機。

　　此前，空客和波音公司最新的載人航天飛機機身主要是由先進的復合材料構成的，譬如用質量極輕且使用性能優異的碳纖維增強塑料代替飛機的鋁基材料，可以使其重量減輕約20%。復合材料在飛機上的主要應用優勢就在于通過減輕重量以節省燃油消耗。但是復合材料抗損傷性能較差：與鋁基材料在斷裂前可以承受較大的沖擊相比，復合材料的多層結構在較小沖擊下就很容易發生斷裂。

　　研究人員使用碳納米管將每一層復合材料“栓”在一起。碳納米管中的薄卷狀碳原子雖然“身形”微小，但是強度極高。他們在類膠狀聚合物基體中嵌入碳納米管 “森林”，然后“壓緊”碳纖維復合材料層間的聚合物基體。納米管就像是細小的豎直排列的“針”，充當多層結構的支架，在層間部位進行“縫合”。

　　測試結果表明，與現有復合材料相比，經碳納米管“縫合”的復合材料強度可提升30%，在斷裂前能承受更大的作用力。

　　此項研究的首席研究員，MIT航空航天系博士后Roberto Guzman認為，性能提升的復合材料可以用于制造強度更高、質量更輕的飛機零部件，尤其是那些使用傳統復合材料制造的因包含螺釘或螺栓而容易斷裂的零部件。

　　“尺寸是關鍵”

　　當前，復合材料由層狀的橫向碳纖維組成，通過膠粘劑粘接。此項研究參與者，MIT航空航天系教授Wardle指出，“層間粘合處是非常薄弱、存在問題的區域”。許多學者嘗試采用“Z釘扎”方法固定或通過“三維編制”復合材料層的碳纖維束以增強結合性能，類似于釘子和針線所起的作用。

　　Wardle 表示，“釘子或針的尺寸是碳纖維的幾千倍，所以如果在碳纖維中加入這些物質，將會破壞成千上萬的碳纖維，對復合材料本身的損傷不言而喻。”而碳納米管直徑約10納米，只有碳纖維尺寸的百萬分之一。

　　“尺寸的問題很重要，正因為納米管進入復合材料內部而不會影響大尺寸的碳纖維，才使復合材料的性能得以保持，” Wardle解釋說，“碳納米管擁有的表面積達到碳纖維的1000倍，這使它們與聚合物基體結合良好。”

　　Guzman和Wardle采用的新技術即可使碳納米管嵌入聚合物膠內部。首先，他們獲得豎直排列的碳納米管森林，然后將納米森林置于粘稠的、未固化的復合層之間，重復此過程一直到16層(典型的復合材料疊層結構)，實現碳納米管在層與層之間粘結。

　　Wardle認為，“隨著大多數新型飛機的重量超過一半來自于復合材料，提升當前復合材料的綜合性能對拓寬其在航空結構中的應用將起到很大的推動作用。”

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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