A layer of conductive, atom-thin graphene nanoribbons on the surface of an aircraft has been found to repel water and ice at temperatures above 7 degrees Fahrenheit. Below that level, a small voltage heats up the graphene nanoribbons and ice slides right off the surface through a process called Joule heating. The conductive nanoribbons, each just a few microns long, create a “resistive barrier” and heat whatever surface they’re attached to.
“Joule heating is what happens on your back windshield when you flip the switch,” James Tour, professor at Rice University, says. “You're applying a voltage across these wires and they're heating, that's how your back window in your car deices.”
Car manufacturers don’t use Joule heating on front windshields, obviously, because the wires obstruct the driver’s vision. The nanoribbon technology in the Rice studies creates a transparent film, they’ll even work on glass.
The electronics, Tour says, are dead simple. Two electrodes at the base of the film complete the circuit and the resistance from the graphene generates heat.
“These act as little resistors,” Tour says. “It's just that you have billions and billions of them across these junction points and you don't see them, but that's where the heat is generated.”
The nanoribbon film, which can be sprayed on or built into materials, requires no power as a passive deicing solution when temperatures are above 7 degrees Fahrenheit. No voltage is necessary. Theoretically, below that threshold, a ramp agent or pilot only has to flip a switch when the aircraft is connected to ground power to deice the entire aircraft.
“Applying 40 volts to the film brought it to room temperature, even if the ambient temperature was 25 degrees below zero,” Mike Williams, a senior media relations specialist at Rice University’s Office of Public Affairs, wrote announcing the latest developments in late May. “Ice allowed to form at that temperature melted after 90 seconds of resistive heating.”
Economic and Environmental Impact
The potential for graphene, a single-atom thick sheet of carbon, deicing has two main advantages for ground handlers.
First, the diminishing role of deicing trucks. The graphene coating is likely going to be built into aircraft, according to Tour. A sheet of nanoribbons can be applied in a number of ways including through polyethylene paint or in an epoxy, but the most likely solution is that the technology becomes a part of future aircraft designs.
“It would be part of the filler, part of the material that's used to make the composite,” Tour says. “So, as long as the composite is there the nanoribbons are there.”
If for some reason, Tour says, the wing was damaged and some of the composite was damaged or removed, an MRO can repair that with an epoxy as they normally would. The only difference would be the addition of the graphene nanoribbons to the epoxy and the overcoat. Good as new.
“We’re not putting nanoribbons in the entire structure,” Tour says.
“In Rice’s lab tests, nanoribbons were no more than 5 percent of the composite,” Williams wrote in a January update. “The researchers led by Rice graduate student Abdul-Rahman Raji spread a thin coat of the composite on a segment of rotor blade supplied by a helicopter manufacturer; they then replaced the thermally conductive nickel abrasion sleeve used as a leading edge on rotor blades. They were able to heat the composite to more than 200 degrees Fahrenheit.”
With deicing built into the aircraft, the equipment becomes less necessary. That also means a labor reduction on the ramp.
Fewer people means less ramp traffic. The decrease in ramp traffic just so happens to involve some of the largest and most expensive equipment on the ramp.
Additionally, the environmental comparison is staggering. There’s no need for deicing fluids and therefore no dangerous chemicals pouring onto or off of aircraft.
“So there is no environmental impact in the sense that there's nothing spilling off the wing,” Tour says. “It’s not like you have mountains of ethylene glycol or propylene glycol going down into the drainage systems.”
That’s Great, But Where Did This Come From?
Tour and his team originally developed the compound with Lockheed Martin to deice sensitive marine and airborne radar systems. The goal was to develop a coating that would protect radomes while maintaining radio frequency integrity.
“When we were doing radomes, we found that we could apply this with polyurethane,” Tour says. “But if this were going to be used in the aircraft industry, I suspect they just want to put it as part of the epoxy that's already used in the carbon-carbon composite that makes that aircraft. When you’re flying at high speeds, overcoats have a way of being braided off.”
With an atom-thin coating of the conductive carbon, RF radiation, which is what radars operate on, continued to work without any problem and could be deiced by just applying a small voltage across them.
The “R” part of R&D is done, Tour says. And they’ve demonstrated that it works on any number of surfaces and as a part of any number of coatings.
“But then there's the ‘D’ part, Tour says. “The D part is never inexpensive.”
Rice University is currently looking for a serious partner to develop the technology and go to “the next level.” That level isn’t limited to aircraft, though. Tour says he sees this coating protecting vehicles, homes, cables and power lines alike.
Lockheed’s involvement in the radome developments resulted in shared intellectual property for that segment. Budgetary issues, however, derailed the opportunity to continue development for aircraft.
“They were very interested but Lockheed has a volatile budget and it goes up and then it goes down and they lay off tens of thousands of people and then they rehire tens of thousands of people,” Tour says. “So, they went into a down cycle and all the people that were working in this area either were laid off or got reassigned to other things and it was just one of those cycle things.
“It's not going to happen until somebody buys it. We have the licenses. The licenses are available.”
