The fuselage of a modern airplane is usually made of multiple pieces of different composite materials, like many layers in a foliar dough. After these sheets are stacked and formed, these structures are pushed into warehouse-sized ovens and autoclaves, and fused to form a resilient pneumatic shell.

MIT postdoctoral fellow Jeonyoo Lee Photo credit: Melanie Gonick of MIT

Engineers at the Massachusetts Institute of Technology have recently developed a method for producing aviation-grade composite materials without the need for huge ovens and pressure vessels. New technologies may help speed up the manufacture of aircraft and other large, high-performance composite structures, such as wind turbine blades.

The results of the study were published in "Advanced Materials Interface", and the thesis introduced in detail the new method for the production of aviation-grade composite materials with carbon nanotube films.

If you want to manufacture a main structure such as a fuselage or wing, you need to build a pressure vessel or autoclave, the size of a two- or three-story building, which itself requires time and money to pressurize, now Primary structural materials can be manufactured without the pressure of an autoclave, so we can get rid of all these huge infrastructures.

Get out of the oven and cover it with a blanket

In 2015, Lee led the team with Wardle Laboratories to create a method for manufacturing aerospace-grade composite materials without using an oven to fuse the materials together.

The researchers did not put the material layers in the oven for curing, but wrapped them in ultra-thin films of carbon nanotubes (CNTs). When they apply electrical current to the film, CNTs, like nano-scale electric blankets, quickly generate heat, which causes the materials in them to solidify and fuse together.

Using this outside-of-oven sterilization technology, the research team used only 1% of the energy to produce composite materials that were as strong as traditional aircraft-made ovens.

Next, the researchers looked for a method of high-performance composite materials that does not use large autoclaves. Large containers generate enough pressure to press the materials together to squeeze out any voids or air pockets at the interface.

Each layer of the material has a microscopic surface roughness, and the rough area between the two layers will be filled with air, which is the main source of voids and weak points in the composite. Autoclave can push these gaps to the edge and eliminate

"Autoclave" technology allows composite materials to be manufactured without the use of large machines. However, most of the produced composite materials have nearly 1% of the materials containing voids, which will damage the strength and life of the material. In contrast, aviation-grade composite materials made with autoclaves are of high quality, and any voids contained are negligible and difficult to measure.

The problem with the autoclave method is that the materials are specially formulated, and none of the materials are suitable for main structures such as wings and fuselages. They have made some progress in auxiliary structures (such as flaps and doors), and the product still has gaps.

The cross-section of the composite shows that the nanoporous film with morphologically controlled nanocapillary provides the required pressure at the interface in the layered polymer structure. Image source: MIT

Part of the researcher's work focuses on the development of nanoporous networks, that is, ultra-thin films made of aligned microscopic materials (such as carbon nanotubes), which can be specially designed to include color, strength, and capacitance.

The researchers wanted to know whether these nanoporous membranes could replace the huge autoclave to squeeze the gap between the two material layers, which seems unlikely.

The carbon nanotube film is a bit like a dense tree forest, and the space between the trees can function like a nanotube or a capillary. Capillaries such as straw can generate pressure based on their geometry and surface energy, or the ability of carbon nanotubes to attract liquids or other materials.

The researchers suggest that if the thin film of carbon nanotubes is sandwiched between two materials, as the material is heated and softened, the capillary between the carbon nanotubes should have surface energy and geometry so that they can attract the material to each Kinds of materials other than leaving gaps between them.

The capillary pressure should be greater than the pressure applied by the autoclave.

The researchers grew vertically aligned carbon nanotube films in the laboratory by using previously developed techniques, and then placed the film between layers of materials commonly used in the manufacture of major autoclave-based aircraft structures. They wrapped these layers in a second layer of carbon nanotube film and then applied electric current to heat them. They observed that as the materials were heated and softened, they were pulled into the capillary of the middle CNT film.

The resulting composite material has no voids, similar to the aerospace-grade composite material produced in the autoclave. The researchers conducted a strength test on the composite material, trying to separate the layers, predicting that if there are gaps, it will make the layers easier to separate.

In laboratory research and testing, they found that autoclaved composites are as strong as the gold standard autoclaved composites used in major aerospace structures.

In their experiments, they used a sample a few centimeters wide. The sample was large enough to prove that the nanopore network can pressurize the material and prevent the formation of voids. In order for this process to be used to manufacture entire wings and fuselages, researchers will have to find ways to mass produce CNTs and other nanoporous membranes.

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