Carbon Nanotubes — cylindrical molecules that consist of rolled-up sheets of single-layer carbon atoms, the same basis for graphene — goes back to 1991, which, weirdly, is long before the discovery of how to isolate graphene itself. The appeal of Carbon Nanotubes is their potential to dramatically improve energy storage and renewable energy… a topic that scientists have been attempting to tackle for decades. As it turns out, Carbon nanotubes can be easily mass-produced already, so why haven’t we seen the carbon nanotube revolution yet? Will they really be game changers?
In the early 2000’s, Carbon Nanotubes were all over the media, touted as being a breakthrough in energy potential and a solution for what sounded like endless applications. Everything from composite materials for large metallic parts like bikes, boats, and cars, to highly efficient transistors, nano-inks, bio tech, and even space elevators.1 2
But after years of expected market roll-outs, carbon nanotubes still haven’t made the splash in the tech world that they had promised. As usual, the hype set expectations too high … almost as high as the space elevator … so excitement dampened as time went on. But the key problem wasn’t production of Carbon Nanotubes themselves — that had been available long before — instead it was with the precision and separation of the different types during manufacturing. To understand that, let’s take a step back.
Carbon nanotubes are essentially molecular sheets of carbon atoms, also known as graphene, which are arranged in a hexagonal lattice or honeycomb arrangement – a bit like chicken wire. These sheets are organized in layers and manipulated into a cylindrical shape. So basically, graphene is the basis for Carbon Nanotubes. You can’t have Carbon Nanotubes without graphene.
Now, these nanotubes can be designed and arranged in many ways:
- Single-Walled Carbon Nanotubes (SWNTs) are made of, as you might imagine, a single layer of graphene. The way in which they are rolled can impact their conductivity. I’ll get to that in a minute.
- Multi-Walled Carbon Nanotubes (MWNTs) are an extension of Single-Walled. They’re made-up of multiple layers of graphene which better insulates their thermal and chemical properties compared with Single-Walled tubes.
- Double-Walled Carbon Nanotubes (DWNTs) are a combination of Single-Walled and Multi-Walled – with thermal, chemical and conductivity properties that are intermediate compared to the other two.3
Depending on how the hexagonal lattice is rolled, it affects what’s called the chiral angle4 of the final tube. It’s how that hexagonal pattern spirals along the tube. If you follow the pattern across the diameter of the tube, you’ll see three basic types: armchair, zig-zag, and chiral nanotubes.5 The orientation can make it act as either a metallic or semiconducting material.6 7
When manufacturing nanotubes, you usually end up with a mixture of types. The nanotubes have a tendency to clump into an entangled mess.1 And separating them apart has been the big challenge for mass-scale manufacturing. Remember, they’re only a few nanometers in diameter, or about 50,000 times smaller than the width of a human hair. It’s only when Carbon Nanotubes are grouped by purity, metallic or semiconducting, that they can be used effectively … which will ultimately advance all of those big promises that were made.
Over the past few years there have been some breakthroughs on how to effectively isolate carbon nanotubes after manufacturing, which has helped to push Carbon Nanotubes back into mainstream technology. Researchers at McMaster University adapted a polymer process, which had been popularly used to reduce carbon nanotubes to only metallic components. The problem with that process was that it dissolved and washed away the semiconducting nanotubes. The researchers found a way to reverse the process, leaving semiconducting nanotubes behind and intact.8
A team of researchers from Northwestern University used the common chemical, cresol, to isolate nanotubes.9 10 Some methods use chemicals and chemical reactions to modify the nanotubes to force them to separate, but that can sometimes leave a residue that alters their abilities. When they used cresol they not only separated the nanotubes, but formed them into a thick, moldable gel. As one of the researches pointed out:
“Essentially, this solvent system now makes nanotubes behave just like polymers…” “It is really exciting to see cresol-based solvents make once hard-to-process carbon nanotubes as usable as common plastics.” -Jiaxing Huang
It’s these types of breakthroughs that are going to take us from bulk manufacturing, all the nanotube types tangled together, to isolated mass-scale manufacturing of Carbon Nanotubes. If we’re ever going to make that space elevator, we’re going to need A LOT of isolated nanotubes. But where does that leave us with actual products that we may see in our daily lives not too far from now?
There are a lot of examples to pull from in just the past few years. One recent one is Vantablack, which is one of the darkest materials on the planet,11 absorbing up to 99.965% of visible light.12 Developed by Surrey NanoSystems in the UK, VANTA stands for Vertically Aligned Nanotube Arrays. It’s that alignment that traps light that hits the surface from bouncing back out, which makes it look like a deep, dark void. It may not seem like it has a lot of value, but it can have a big benefit for keeping stray light from entering a telescope and improving the performance of infrared cameras. On a less practical level, I don’t know about you, but I’d love to see a car painted with this in person.
Just this June, MIT researchers demonstrated how commercial silicon-based transistor foundries could transition to carbon nanotube transistors, and save an immense amount of time.13 The fabrication process is up to 1,100 times faster than today’s silicon process. Part of the reason for that is silicon transistors are manufactured at around 450-500 degrees Celsius, but carbon nanotubes can be manufactured at room temperatures. This will allow for the layering of circuits for a three-dimensional chip since you can build right on top of a previously fabricated layer. If you tried that with silicon process you’d melt the previous layer. 3D chip designs are expected to outperform 2D versions.
But one of the more active areas for Carbon Nanotubes research is around solar panels, and there’s been a lot of interesting advances there. Just last year Rice University had two interesting advancements. One team of researches showed that double-walled nanotubes could have a dramatic impact on solar panel efficiency because of the double-wall’s efficiency at separating positive and negative charges to create current.14
Another team has come up with a method to improve solar panel efficiency by up to 80%.15 16 Right now an efficient solar panel would fall somewhere between 20-30% efficiency. The fascinating part of this discovery is not that the nanotubes are actually improving the panel efficiency directly, but more in how it helps to capture unused potential … heat. To be more specific, infrared heat from the sun. Current panels are only capturing light and converting that into electricity. The research team designed an array of cavities patterned into a film of aligned carbon nanotubes. They’re able to absorb and channel thermal photons and emit them as light … essentially converting heat into a form that solar panels can then convert into electricity.
Related to heat is another innovation that came up just recently. You may be aware that solar panels lose efficiency when they’re too hot. Well, researchers at the King Abdullah University of Science and Technology (KAUST)17 have come up with a system that keeps panels cool with no moving parts or excess energy drain. In their project, KAUST researchers developed a polymer that contains calcium chloride. When the material is exposed to humid air it absorbs the moisture and expands in size. In the latest research they combined that polymer with carbon nanotubes to reverse that cycle and release the trapped water. When the gel was applied to the back of a solar panel, it’s able to absorb moisture from the humid air at night and then slowly release it during the hottest parts of the day. The team saw panel temperatures reduced by 10 degrees Celsius, which improved efficiency up to 20%. But this doesn’t just apply to solar panels. Renyuan Li, one of the researchers, said:
“We believe this cooling technology can fulfill the requirements of many applications because water vapor is everywhere and this cooling technology is easy to adapt to different scales.” ”The technology could be made as small as several millimeters for electronic devices, hundreds of square meters for a building, or even larger for passive cooling of power plants.” -Renyuan Li
Another area where we’re seeing carbon nanotube’s making an impact is energy storage. The French company, Nawa, has created an ultracapacitor that’s going into mass production.18 While batteries, like lithium ion, offer far more specific energy storage, they charge and discharge far slower than ultracapacitors. Capacitors can charge and discharge in an instant, so they’re perfect for quick bursts of huge power. They can also last for a million charge cycles. But again, the downside is the amount of energy they can store, and for how long. Nawa’s new ultracapacitor sits somewhere in between a capacitor and battery. It can store about 5 times more energy than competing ultracapacitors, but charges in seconds. It’s using vertically aligned carbon nanotubes that are grown with a patented method for coating two sides of aluminum or copper foil. Some of the first places they see this technology getting used is in power tools, automated vehicles in factories and warehouses, and electric vehicles. All areas that require high energy use with limited charging downtime.
The team at the Advanced Materials and BioEngineering Research Center (AMBER) at Trinity College in Dublin, have created a carbon nanotube composite electrodes for use in lithium ion batteries.19 They’re able to build cells with a specific energy of 480 Wh/kg, which is around twice of a normal lithium ion cell. Companies like Nokia Bell Labs, which is a partner of the research center, are interested in the breakthrough for powering 5G phones and the broader network with more efficient batteries. But this type of breakthrough could easily make an impact in grid storage and electric cars as well.
Carbon nanotubes may have fallen out of the limelight for a while, but there has been a lot of interesting advances over the past few years that’s reigniting interest. In my case, I’m really excited to see how the advancements impact solar and energy storage. But as cool as some of this is, we’re still not getting a space elevator anytime soon.
- Carbon Nanotubes – The unfulfilled Promise ↩︎
- Long, Stretchy Carbon Nanotubes Could Make Space Elevators Possible ↩︎
- Numerical Study of Quantum Transport in Carbon Nanotube Based Transistors ↩︎
- Chirality and symmetry of Nanotube ↩︎
- Structure of CNTs ↩︎
- A History of Carbon Nanotubes ↩︎
- Semiconducting and Metallic Carbon Nanotubes ↩︎
- Researchers resolve a problem that has been holding back a technological revolution ↩︎
- Important carbon nanotube breakthrough found to have surprising twist ↩︎
- Mass Production of Carbon Nanotubes made Easy ↩︎
- How carbon nanotubes built this bizarre ultrablack material ↩︎
- Vantablack ↩︎
- Carbon Nanotube Transistors Make the Leap From Lab to Factory Floor ↩︎
- Two Walls May Beat One for Solar-Panel Nanotubes ↩︎
- Rice device channels heat into light ↩︎
- Carbon Nanotubes Could Increase Solar Efficiency to 80 Percent ↩︎
- Moisture-sucking gels give solar panels the chills ↩︎
- Nawa’s carbon nanotube ultra-capacitors are going into mass production ↩︎
- Carbon nanotubes breakthrough doubles energy density of batteries ↩︎