Researchers from MIT have found a cheaper way to make strong composite materials, such as those used for aeroplane wings, by using carbon nanotubes.

Current methods for producing high-grade raw materials for aeroplane parts and wind turbine blades are expensive. It involves layering and stacking multiple sheets of different materials on top of one another and then baking them in ovens and pressurized chambers (autoclaves) that can be as large as warehouses. In the pressurized oven the layers fuse together to form a strong and resilient shell.

By using nanomaterials, the researchers have made composites of comparable strength without the need to construct, heat, and power such large pressure-ovens.

“If you’re making a primary structure like a fuselage or wing,” explains Brian Wardle, a professor of aeronautics and astronautics at MIT and the study’s lead author, “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. These things are massive pieces of infrastructure. Now we can make primary structure materials without autoclave pressure, so we can get rid of all that infrastructure.”

The research began some years ago, when Wardle, working alongside MIT postdoc Jeonyoo Lee, and Seth Kessler of Metis Design Corporation found a way to make composites without the oven.

As the industry journal Nanowerk, reports, “In 2015, Lee led the team, along with another member of Wardle’s lab, in creating a method to make aerospace-grade composites without requiring an oven to fuse the materials together. Instead of placing layers of material inside an oven to cure, the researchers essentially wrapped them in an ultrathin film of carbon nanotubes (CNTs). When they applied an electric current to the film, the CNTs, like a nanoscale electric blanket, quickly generated heat, causing the materials within to cure and fuse together.”

Significantly, the report notes that this out-of-oven (OoO) technique was able to produce composites, “… as strong as the materials made in conventional airplane manufacturing ovens, using only 1 percent of the energy.”

However, the composites still needed squeezing at high pressure to remove any air pockets or empty spaces in the layers that would weaken the materials.

As Wardle explains, “There’s microscopic surface roughness on each ply of a material, and when you put two plys together, air gets trapped between the rough areas, which is the primary source of voids and weakness in a composite. An autoclave can push those voids to the edges and get rid of them.”

To resolve this the team began looking at out-of-autoclave (OoA) techniques.

Earlier studies had already had some success, but most still produced composites where 1% or more of the material contained tiny air pockets - a level that is unacceptable for aeroplane parts.

“The problem with these OoA approaches,” says Wardle, “is also that the materials have been specially formulated, and none are qualified for primary structures such as wings and fuselages. They’re making some inroads in secondary structures, such as flaps and doors, but they still get voids.”

This is where the team decided to look again at nanomaterials, in particular carbon nanotube films, which although incredibly thin are actually composed of densely packed forests of tubes where the spaces between the tubes can work as capillaries.

Capillaries function like drinking straws, using pressure to draw liquids (and even gels and microscopic solids) upwards. This effect is due to the surface energy on the internal sides of the straw which attracts material, so pulling it up.  

Considering this force, the researchers hypothesized that placing a thin film of carbon nanotubes between the layers would create a capillary action when heated. This would pull the layers together and squeeze out any voids.

Testing this theory, the MIT press release describes how, “… by growing films of vertically aligned carbon nanotubes using a technique they previously developed, then 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, to which they applied an electric current 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 result was a composite material of aerospace-quality with hardly any pockets of air. While strength tests found that the nanomaterial bonding method had produced materials that were, “… just as strong as the gold-standard autoclave process composite used for primary aerospace structures,” says Wardle.

The team have now published their findings in the journal Advanced Materials, where they outline how, “Manufacturing of aerospace‐grade advanced carbon fiber composites is performed for the first time without utilizing pressure from an autoclave. Combined with a conductive curing approach, this work allows advanced composites to be manufactured without costly oven or pressure vessel infrastructure.”

While at present, the team have only been able to manufacture material samples just a few centimetres in size, they are now working on how to scale up the process to allow for industrial production. Work will also be needed to produce sufficient quantities of carbon nanotube films to fit between the layers that make up aeroplane wings and fuselages.

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

The team is also looking into how different sizes, lengths, and thicknesses of carbon nanotube films effect the way the capillary force works. Possibly creating even stronger bonding for other high-performance materials.

“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.”

In fact, this approach opens a whole new industry for applying pressure, squeezing, pushing, and pulling objects of any size without the need for pressure chambers.

As Wardle notes, “Now we have this new [carbon nanotube] material solution, we can provide on-demand pressure wherever you need it.”

Photo credit: MIT, Nanowerk, AZdailysun, Science, Teamtrade, MIT, aeroastro & MIT