MIT engineers created a new method to produce aerospace-grade composites without ovens or pressure vessels. This would speed up the manufacture of planes and other large composite structures.

Researcher and MIT postdoc Jeonyoon Lee with the new CNT material. Source: Melanie Gonick, MIT

Airplanes are made of multiple sheets of layered composite materials. Once the layers are stacked and molded into shape, the structures are placed into a warehouse-sized oven or an autoclave. The layers form a resilient aerodynamic shell. Creating a primary structure requires a large pressure vessel and it can be a cumbersome and lengthy process.

The method uses layers of material wrapped in ultrathin carbon nanotubes (CNTs) to create a film. When an electric current was applied to the film, the CNTs act as a nanoscale electric blanket. This quickly generates heat and fuses the materials together. The technique is called the out of the oven technique (OOO). OOO produced components were just as strong as materials that were made with conventional airplane manufacturing, but used 1% of the energy to produce.

Next, the team looked for ways to make composites without autoclaves, or out of autoclave (OOA). Autoclaves press the materials together to squeeze out any air pockets, or voids. When two materials are put together, air gets trapped between the surfaces, creating a void. This makes the composite weak and unsuitable for aerospace.

Most of today's OOA techniques produced composites that are 1% voids. This makes them too weak and unstable to be used to create primary structures. Autoclave produced composites have voids so small they can't be measured.

The team created nonporous networks with ultrathin films made of carbon nanotubes that could be used in place of autoclaves to squeeze out any voids. These materials can be engineered to have exceptional qualities, like color, strength and electrical capacity.

Capillaries, or the spaces between carbon nanotubes, can generate pressure based on geometry and surface energy. The team believed that if a thin film of carbon nanotubes is placed between two materials, as the materials are heated and softened, the capillaries will draw the materials towards each other and get rid of any voids.

To test their idea, the team grew films of vertically aligned nanotubes and the films between layers of airplane manufacturing material. They wrapped the layers in the second film of carbon nanotubes and applied an electric current to heat it. As the materials warmed up and softened, they were pulled into the capillaries. This created a void-free composite similar to aerospace-grade composites.

Researchers then put the material through strength tests. The OOA composite tested just as well as the gold-standard autoclave composites.

The next step is to scale up the pressure generating CNT film. The team worked with samples that were a few centimeters wide. While this is good for testing the material, it must be produced at a larger scale to be used in aerospace.

A paper on this research was published in Advanced Materials Interfaces.