
Recently published in the journal ACS Nano, research reports that MXenes, a class of two-dimensional materials originally discovered at Drexel University in 2011, demonstrate the rare combination of high electrical conductivity and low thermal conductivity. While MXene materials have proven exceptional among two-dimensional materials in a number of ways — including their strength, ability to selectively block and trap radiation, and filter chemicals — their performance as an ultrathin thermal insulator could be their most promising trait for future applications, according to the research team.
“Thermal insulation of this magnitude, that is also 100 to 1,000 times thinner than a human hair, would simply have been unimaginable until now,” said Yury Gogotsi, Ph.D., Professor at Drexel’s College of Engineering, who was a leader of the thermally-insulating MXene development. “This could change the way we insulate buildings and industrial equipment, and make thermal clothing, just to name a few exciting possibilities.”
Here is an exclusive Tech Briefs interview, edited for length and clarity, with Gogotsi and Lead Author Pascal Gehring, Ph.D., from UCLouvian (Belgium).
Tech Briefs: What was the biggest technical challenge you faced while making this discovery about MXene?
Gogotsi: This is a very long story because it happened in 2010; it was serendipitous. We were trying to make anodes for lithium-ion batteries. We etched max phases trying to make room for lithium there and layered ceramic fell apart, forming two-dimensional flakes. I have spent the following years researching and expanding this family of materials.
Tech Briefs: What was the catalyst for this project?
Gogotsi: About five years ago, we found that MXene films have a very low infrared signature. So, if you look with an infrared camera, they seem cold. They felt cold in spite of being metallic. It was an unusual finding, and we tried to understand it at Drexel. We filed for a patent. We found that it does not emit infrared, so it doesn't emit heat. And this is one of the major ways of transferring heat. That's why you see, for example, aluminum foil inside boxes of coffee. That's also why you have a polished, mirror-like surface inside thermos flasks.
But, we could not explain sufficiently with this low emissivity, why different MXenes have such low thermal conductivity. And that was when we approached Pascal to measure its thermal conductivity, to understand to what extent it conducts heat through the body of the film. It's known that two-dimensional materials like graphene and also bulk metal carbides, which are structural elements of MXenes, are typically very good heat conductors. And this is where our collaboration started, because we were not able to measure the thermal conductivity of these materials ourselves. And there are very few experts in the world who can do it for nanoscale materials.
Tech Briefs: Can you just explain in simple terms how it works? How everything works?
Gehring: The principle is a bit like if you would touch a material to estimate how much heat flows into that material. For example, if you, with your finger, touch a piece of wood or a piece of metal, then the metal will feel cold while the wood feels warm. The reason is that your finger is a few degrees warmer than the metal or the wood, and heat is transferred from your finger into the material. And the more heat that flows into the material, the better it's thermal conductance, the cooler it feels. But we can't use our finger because it's nanoscale material. So, we put a little thermometer on an atomic-force microscope probe, and then with a resolution of just a few tens of nanometers, we can raster over a surface and measure how much heat flows into that surface and then basically estimate the thermal conductivity of that surface.
Tech Briefs: The article I read says, “While additional investigation is needed to verify the exact mechanisms involved, the team suggests that the material’s low thermal activity is likely due to its structure.” My question is, do you have any plans for additional investigation?
Gogotsi: We do. One thing that's very important to understand is that at least 80 different MXene compositions and structures have been reported in the literature. Theoretically, we can make an almost infinite number of these one-hundred billionth of a meter-thin flakes with different metals: carbon, nitrogen; and different groups on the surface, like oxygen or chlorine, or others. And they all have different properties.
So, the fact that we measured one material, and it showed great promise, doesn't mean that it is the best material. And we also don't know exactly why it has such a low thermal conductivity — there are several hypotheses. So, what we want to do, we want Pascal to measure more materials and also design experiments that will help us to understand what matters. Is it the number of atomic layers, is it the metal elements, is it defects in the lattice? Is it that the surface of MXene is oxygen, or ceramic, or chlorine. We will try to find out because if we understand the mechanism, we can better predict how to make materials with even better thermal insulation properties.
Tech Briefs: Do you have any updates you can share? If not, what are your next steps?
Gehring: In principle, there are many more compounds to study, and we're eagerly waiting for them in our lab.
Tech Briefs: How soon could we see this technology commercialized?
Gogotsi: There are already quite a few companies that manufacture MXenes. My former Ph.D. students from Drexel recently started a company called MXene, Inc., which will provide colloidal dispersion — like a paint — with MXene that can be sprayed onto any surface, anywhere. That means the material is available.
There are companies now already in Asia that can manufacture kilogram quantities and hundreds of meters of MXene foil, like aluminum foil. So, it is there, but it's still a fairly expensive high-tech material because it's produced in small quantities. If you wanted to insulate a satellite or spaceship, it's available today. If you want to insulate every building, it will still be too expensive.