MIT engineers have developed a method to produce aerospace-grade composites without the enormous ovens and pressure vessels. The technique may help to speed up the manufacturing of airplanes and other large, high-performance composite structures, such as blades for wind turbines.

Testing Nano-porous Networks to Avoid Giant Autoclaves

The researchers look for ways to make high-performance composites without the use of large, high-pressure autoclaves, building-sized vessels that generate high enough pressures to press materials together, squeezing out any voids, or air pockets, at their interface.

Wardle’s work focuses on developing nano-porous networks, ultrathin films made from aligned, microscopic material, such as carbon nanotubes, that can be engineered with exceptional properties, including color, strength, and electrical capacity. The researchers wondered whether these nano-porous films could be used in place of giant autoclaves to squeeze out voids between two material layers.

They proposed that if a thin film of carbon nanotubes were sandwiched between two materials, then, as the materials were heated and softened, the capillaries between the carbon nanotubes should have a surface energy and geometry such that they would draw the materials in toward each other, rather than leaving a void between them. Lee calculated that the capillary pressure should be larger than the pressure applied by the autoclaves.

Heating Vertically Aligned CNT for Void-free Composites

The researchers tested their idea in the lab by growing films of vertically aligned carbon nanotubes and laying the films between layers of materials that are typically used in the autoclave-based manufacturing of primary aircraft structures. They wrapped the layers in a second film of carbon nanotubes, which they applied an electric current to for it to heat it up. They observed that as the materials heated and softened in response, they were pulled into the capillaries of the intermediate CNT film.

The resulting composite lacked voids, like aerospace-grade composites that are produced in an autoclave. The researchers subjected the composites to strength tests, attempting to push the layers apart, the idea being that voids, if present, would allow the layers to separate more easily.

In these tests, we found that our out-of-autoclave composite was just as strong as the gold-standard autoclave process composite used for primary aerospace structures,” Wardle says.

Scaling Up the Pressure-generating CNT Film Production

The team will next look for ways to scale up the pressure-generating CNT film. In their experiments, they worked with samples measuring several centimeters wide, large enough to demonstrate that nano-porous networks can pressurize materials and prevent voids from forming. To make this process viable for manufacturing entire wings and fuselages, researchers will have to find ways to manufacture CNT and other nano-porous films at a much larger scale.

There are ways to make really large blankets of this stuff, and there’s continuous production of sheets, yarns, and rolls of material that can be incorporated in the process,” Wardle says.

Further Research and Developments

Wardle plans to explore different formulations of nano-porous films, engineering capillaries of varying surface energies and geometries, to be able to pressurize and bond other high-performance materials.

Now we have this new material solution that can provide on-demand pressure where you need it,” Wardle says. “Beyond airplanes, most of the composite production in the world is composite pipes, for water, gas, oil, all the things that go in and out of our lives. This could make making all those things, without the oven and autoclave infrastructure.”

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