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Carbon nanotubes (CNTs) are hollow cylindrical tubes formed by rolling a sheet of carbon atoms arranged in a hexagonal ring, as in a sheet of graphite, either in monolayer (single walled nanotube, SWCNT) or multilayer (multi-walled nanotube, MWCNT) form. Their diameter may range from 0.7 (SWCNT) to 50 naometers (MWCNT) and few tens of micron in length.
CNT being a hollow tube comprised entirely of carbon, they are also extremely light weight. They exhibit extraordinary strength and unique electrical properties, at the individual tube level: 200X the strength and 5X the elasticity of steel; 5X the electrical conductivity, 15X the thermal conductivity and 1,000X the current capacity of copper; at almost half the density of aluminum.
The unique physical, electrical, and molecular properties of carbon nanotubes (CNTs) cause them to be the focus of research for a wide range of applications. Due to their high strength and elastic modulus, CNTs are being finding application as reinforcements in composites like polymer matrix composites (PMCs). CNT fibre and thread are being envisaged for applications as reinforcements in composites or woven for bullet proof vests and high strength ropes.
They also provide large specific surface area making it a potential material for sensing applications. It can be made highly selective for a range of gases / vapours by modifying the CNT surface by functionalisation (attaching group of atoms or molecules on the graphitic structure). Their hollow structure also acts as a storage space for hydrogen, drug etc required for certain applications like energy storage and targeted drug delivery.
UC professor Vesselin Shanov co-directs UC’s Nanoworld Laboratories with research partner and UC professor Mark Schulz. Together, they harness their expertise in electrical, chemical and mechanical engineering to craft “smart” materials that can power electronics. “The major challenge is translating these beautiful properties to take advantage of their strength, conductivity and heat resistance,” Shanov said.
Defense industry is also investing heavily in CNT research initiatives and manufacturers. And the technology is already finding its way into several military applications. UC’s College of Engineering and Applied Science has a five-year agreement with the Air Force Research Laboratory to conduct research that can enhance military technology applications. Schulz said manufacturing is at the cusp of a carbon renaissance. Carbon nanotubes will replace copper wire in cars and planes to reduce weight and improve fuel efficiency. Carbon will filter our water and tell us more about our lives and bodies through new biometric sensors. Carbon will replace polyester and other synthetic fibers. And since carbon nanotubes are the blackest objects found on Earth, absorbing 99.9 percent of all visible light, you might say carbon is the new black. “In the past, metals dominated manufacturing goods,” Schulz said. “But I think carbon is going to replace metals in a lot of applications. “There’s going to be a new carbon era — a carbon revolution,” Schulz said.
Scientists at Florida State University’s High-Performance Materials Institute are looking into using carbon nanotubes as a construction material for hypersonic weapons. Carbon nanotubes are a synthetic material consisting of carbon tubes with a diameter as small as one nanometer. Carbon nanotubes are stronger than steel and insulate against heat. Now, researchers have discovered that soaking carbon nanotubes in phenol can increase their ability to disperse heat by one-sixth, allowing less nanomaterials to be used for the same job.
While new applications for CNTs continually arise, the primary reason that CNTs have not yet fully lived up to their potential is the difficulty in manufacturing them with the “right” properties for the desired application.
CNT applications
CNTs are well-suited for virtually any application requiring high strength, durability, electrical conductivity, thermal conductivity and lightweight properties compared to conventional materials. Carbon nanotubes (CNTs)’s unique combination of properties including high mechanical strength, large aspect ratios, high surface area, distinct optical characteristics, high thermal and electrical conductivity, which make them suitable for a wide range of applications in areas from electronics (transistors, energy production and storage) to biotechnology (imaging, sensors, actuators and drug delivery) and other applications (displays, photonics, composites and multi-functional coatings/films).
CNTs exhibit different electronic properties based on the way these graphene layers are rolled into a cylinder. Nanotubes could either be single-walled structures, called single-walled carbon nanotubes (SWCNTs) or could have many walls, called multi-walled carbon nanotubes (MWCNTs). SWCNTs can be further categorized electrically into semiconducting and metallic SWCNTs (s-SWCNTs and m-SWCNTs), while MWCNTs mainly display metallic behavior. The novel and useful properties of CNTs, such as low-cost, light-weight, high aspect ratios and surface area, distinct optical characteristics, high thermal and electrical conductivity and high mechanical strength make them suitable and of interest for a wide range of electronic, biomedical and other industrial applications.
Carbon nanotube enabled nanocomposites have received much attention as a highly attractive alternative to conventional composite materials due to their mechanical, electrical, thermal, barrier and chemical properties such as electrical conductivity, increased tensile strength, improved heat deflection temperature, or flame retardancy.
These materials promise to offer increased wear resistance and breaking strength, antistatic properties as well as weight reduction. For instance, it has been estimated that advanced CNT composites could reduce the weight of aircraft and spacecraft by up to 30%.
These composite materials already find use in
- Sporting goods(bicycle frames, tennis rackets, hockey sticks, golf clubs and balls, skis, kayaks; sports arrows)
- Yachting (masts, hulls and other parts of sailboats)
- Textiles (antistatic and electrically conducting textiles (‘smart textiles’); bullet-proof vests, water-resistant and flame-retardant textiles)
- Automotive, aeronautics and space (light-weight, high-strength structural composites)
- Industrial engineering (e.g. coating of wind-turbine rotor blades, industrial robot arms)
- Electrostatic charge protection (for instance, researchers have a developed electrically conducting and flexible CNT film specifically for space applications) and radiation shielding with CNT-based nanofoams and aerogels.
Post-Silicon Future with Carbon Nanotube Electronics
Despite the rise of graphene and other two-dimensional (2D) materials, semiconducting single-walled carbon nanotubes are still regarded as strong candidates for the next generation of high-performance, ultra-scaled and thin-film transistors as well as for opto-electronic devices to replace silicon electronics. So far researchers have achieved only promising experimental results and at this point there remain numerous challenges related to integrating CNT transistors into industrial-scale chip manufacturing.
The most matured technology to take over silicon is the single-walled carbon nanotube after the Moore’s Law, which stated that the number of transistors on a chip will double approximately every two years expected to come to an end in the beginning of the 2020s. Carbon nanotube chips could greatly improve the capabilities of high performance computers, enabling Big Data to be analyzed faster, increasing the power and battery life of mobile devices and the Internet of Things, and allowing cloud data centers to deliver services more efficiently and economically. Now, for the first time, University of Wisconsin–Madison materials engineers have created carbon nanotube transistors that outperform state-of-the-art silicon transistors. The team’s CNT transistors have successfully achieved currents that are 1.9 times higher than silicon semiconductors.
Depending on the diameter and the way they are rolled (chirality), electron energy band gap changes that makes them either metallic (rolled along its length – arm chair structure) or semiconducting (rolled askew –zig-zag structure) behaviour. They have large electrical conductivity and have good electron emission characteristics. These properties enable them for applications in shielding against electrostatic charges and EMI pulses; energy efficient, high resolution display devices and miniature electronics. They are replacing copper wire and cable because it’s much lighter weight.
On major issue with carbon nanotube transistors is that their performance has been below that of their silicon-based counterparts. However, researchers at the University of Wisconsin-Madison recently announced the development of a carbon nanotube transistor that is capable of outperforming a similar silicon transistor.
Led by Michael Arnold and Padma Gopalan, UW–Madison professors of materials science and engineering, the team’s carbon nanotube transistors achieved current that’s 1.9 times higher than silicon transistors. The nanotube’s ultra-small dimension makes it possible to rapidly change a current signal traveling across it, which could lead to substantial gains in the bandwidth of wireless communications devices. The new transistors are particularly promising for wireless communications technologies that require a lot of current flowing across a relatively small area. “This breakthrough in carbon nanotube transistor performance is a critical advance toward exploiting carbon nanotubes in logic, high-speed communications, and other semiconductor electronics technologies,” team member Michael Arnold said in a news release.
Carbon nanotube transistors should be able to perform five times faster or use five times less energy than silicon transistors, according to extrapolations from single nanotube measurements. “Making carbon nanotube transistors that are better than silicon transistors is a big milestone. This breakthrough in carbon nanotube transistor performance is a critical advance toward exploiting carbon nanotubes in logic, high-speed communications, and other semiconductor electronics technologies.”
Unlike its two-dimensional cousin, graphene, the carbon nanotube can be a natural semiconductor, which means it can be turned on and off to make high speed and energy-efficient switches, which could result in considerable increases in the bandwidth of wireless communications devices. A carbon-nanotube transistor looks much the same as a silicon transistor. The main difference is that the channel is made of carbon nanotubes instead of silicon.
But researchers have struggled to isolate purely carbon nanotubes, which are crucial, because metallic nanotube impurities act like copper wires and disrupt their semiconducting properties — like a short in an electronic device. The UW–Madison team used polymers to selectively sort out the semiconducting nanotubes, achieving a solution of ultra-high-purity semiconducting carbon nanotubes.
Electrodes
Carbon nanotubes have been widely used as electrodes for chemical and biological sensing applications and many other electrochemical studies. With their unique one-dimensional molecular geometry of a large surface area coupled with their excellent electrical properties, CNTs have become important materials for the molecular engineering of electrode surfaces where the development of electrochemical devices with region-specific electron-transfer capabilities is of paramount importance.
Displays
Given their high electrical conductivity, and the incredible sharpness of their tip (the smaller the tips’ radius of curvature, the more concentrated the electric field, the higher field emission), carbon nanotubes are considered the most promising material for field emitters and a practical example are CNTs as electron emitters for field emission displays (FED).
Field emission display (FED) technology makes possible a new class of large area, high resolution, low cost flat panel displays. However, FED manufacturing requires CNT to be grown in precise sizes and densities. Height, diameter and tip sharpness affect voltage, while density affects current.
Flexible Electronics
For emerging applications like real-time analytics and Internet of Things (IoT), high-performance logic circuits and sensors made on flexible or unconventional substrates are needed in order to enable the true computation at the edge. These are several examples of growing areas where flexible nanomaterials, like carbon nanotubes (CNTs), could offer many appealing advantages over rigid silicon, such as low cost, low power, large-area fabrication or even roll-to-roll production.
Carbon nanotubes (CNTs) hold great promise for high-performance flexible electronics due to their extremely high carrier mobility, superior mechanical flexibility, and stability. They could also be used to make wide-variety of applications such as flexible circuits, flexible displays, flexible solar cells, skin-like pressure sensors, and conformable RFID tags.
Apple has been granted the patent for a portable device that is “bendable” in half. According to Patently Apple, the technology behind the capability of the phone materials to be flexible is through carbon nanotubes. Carbon nanotubes can form conductive paths for printed circuits or other flexible substrates that are flexible and resistant to cracking such as substrates associated with touch sensors and displays and can form structural components in an electronic device.
The low-cost and large-area manufacturing of flexible and stretchable electronics using printing processes could have potential applications ranging from personalized wearable electronics to large-area smart wallpapers and from interactive bio-inspired robots to implantable health/medical apparatus.
However, till recently flexible CNT integrated circuits typically exhibit low-speed operation with logic gate delays of over 1 microseconds. However, this situation could be changed with the new advances in IBM Research.
In a recent journal article, Flexible CMOS integrated circuits based on carbon nanotubes with sub-10 ns stage delays, published on Nature Electronics, IBM reported demonstration of high-performance CNT TFTs and complementary integrated circuits fabricated on flexible substrates. IBM, has addressed several key challenges in the fabrication of high-performance flexible CNT electronics, including purity and density of semiconducting CNTs, reliable n-type doping technique for complementary logic, as well as process yield and variation on flexible substrates. Overall, the fabricated flexible CNT TFTs have shown state-of-the-art performance, highlighted by the high current densities (>17 mA/mm), large current ON/OFF ratios (>106), small subthreshold slopes (<200 mV/dec), high mobilities (~50 cm2/Vs) and also excellent flexibility—when wrapping on a finger, the flexible TFTs can still work with no performance degradation.
Integrating all the pieces together, we then took one step further to demonstrate high-speed CMOS ring oscillator—a standard benchmark circuit in any logic technology. The functional 5-stage CMOS ring oscillator exhibits stage delays down to only 5.7 nanoseconds, showing nearly 1000X improvement over previous carbon nanotube work. It also represents the fastest flexible ring oscillator ever made with any nanomaterials including CNTs, organic polymers, oxide semiconductors, and nanocrystals. The superior performance and integration-level demonstration here highlight the potential of using CNTs for future applications such as IoT, edge computing, flexible displays and sensors, where our work provides a useful approach to build scalable, low-cost, and high-speed flexible electronics.
Carbon nanotubes have been explored in light-harvesting and photovoltaic devices because of their unique optoelectronic properties.
Solar Cells
Researchers at Northwestern University enhanced the energy efficiency of solar cells based on CNTs to over 3 percent, twice as efficient as its predecessors. Hersam and his team used a mixture of multiple chirality CNTs to maximize the amount of photocurrent produced by absorbing a broader range of solar-spectrum wavelengths. The Northwestern team will be looking to create polychiral CNT solar cells that have multiple layers with each layer being optimized for a particular solar spectrum.
Solar thermophotovoltaics
A team of MIT researchers has for the first time demonstrated a device based on a method that enables solar cells to break through a theoretically predicted ceiling on how much sunlight they can convert into electricity. They have developed a solar thermophotovoltaic device which involves pairing conventional solar cells with added layers of high-tech materials, that could more than double the theoretical limit of efficiency, potentially making it possible to deliver twice as much power from a given area of panels. Those standard solar cells can only absorb energy from a fraction of sunlight’s color spectrum, mainly the visual light from violet to red.
But the MIT scientists added an intermediate component made up of carbon nanotubes and nanophotonic crystals that together function sort of like a funnel, collecting energy from the sun and concentrating it into a narrow band of light. One of the key high-tech materials is called nanophotonic crystals, which can be made to emit precisely determined wavelengths of light when heated.
The nanotubes capture energy across the entire color spectrum, including in the invisible ultraviolet and infrared wavelengths, converting it all into heat energy. As the adjacent crystals heat up to high temperatures, around 1,000 °C, they reëmit the energy as light, but only in the band that photovoltaic cells can capture and convert.
The researchers suggest that an optimized version of the technology could one day break through the theoretical cap of around 30 percent efficiency on conventional solar cells. In principle at least, solar thermophotovoltaics could achieve levels above 80 percent, though that’s a long way off, according to the scientists. But there’s another critical advantage to this approach. Because the process is ultimately driven by heat, it could continue to operate even when the sun ducks behind clouds, reducing the intermittency that remains one of the critical drawbacks of solar power. If the device were coupled with a thermal storage mechanism that could operate at these high temperatures, it could offer continuous solar power through the day and night.
Nanotube Device Channels Heat Into Light
Rice University scientists are designing arrays of aligned single-wall carbon nanotubes to channel mid-infrared radiation (aka heat) and greatly raise the efficiency of solar energy systems. Gururaj Naik and Junichiro Kono of Rice’s Brown School of Engineering introduced their technology in ACS Photonics.
Their invention is a hyperbolic thermal emitter that can absorb intense heat that would otherwise be spewed into the atmosphere, squeeze it into a narrow bandwidth and emit it as light that can be turned into electricity. “Thermal photons are just photons emitted from a hot body,” Kono said. “If you look at something hot with an infrared camera, you see it glow. The camera is capturing these thermally excited photons.”
Infrared radiation is a component of sunlight that delivers heat to the planet, but it’s only a small part of the electromagnetic spectrum. “Any hot surface emits light as thermal radiation,” Naik said. “The problem is that thermal radiation is broadband, while the conversion of light to electricity is efficient only if the emission is in a narrow band. “The challenge was to squeeze broadband photons into a narrow band,” he said.
The nanotube films presented an opportunity to isolate mid-infrared photons that would otherwise be wasted. “That’s the motivation,” Naik said. “A study by (co-lead author and Rice graduate student) Chloe Doiron found that about 20% of our industrial energy consumption is waste heat. That’s about three years of electricity just for the state of Texas. That’s a lot of energy being wasted.
“The most efficient way to turn heat into electricity now is to use turbines, and steam or some other liquid to drive them,” he said. “They can give you nearly 50% conversion efficiency. Nothing else gets us close to that, but those systems are not easy to implement.” Naik and his colleagues aim to simplify the task with a compact system that has no moving parts.
The aligned nanotube films are conduits that absorb waste heat and turn it into narrow-bandwidth photons. Because electrons in nanotubes can only travel in one direction, the aligned films are metallic in that direction while insulating in the perpendicular direction, an effect Naik called hyperbolic dispersion. Thermal photons can strike the film from any direction, but can only leave via one.
“Instead of going from heat directly to electricity, we go from heat to light to electricity,” Naik said. “It seems like two stages would be more efficient than three, but here, that’s not the case.”
Naik said adding the emitters to standard solar cells could boost their efficiency from the current peak of about 22%. “By squeezing all the wasted thermal energy into a small spectral region, we can turn it into electricity very efficiently,” he said. “The theoretical prediction is that we can get 80% efficiency.”
Nanotube films suit the task because they stand up to temperatures as high as 1,700 degrees Celsius (3,092 degrees Fahrenheit). Naik’s team built proof-of-concept devices that allowed them to operate at up to 700 C (1,292 F) and confirm their narrow-band output. To make them, the team patterned arrays of submicron-scale cavities into the chip-sized films.
“There’s an array of such resonators, and each one of them emits thermal photons in just this narrow spectral window,” Naik said. “We aim to collect them using a photovoltaic cell and convert it to energy, and show that we can do it with high efficiency.”
Filtration
High-flow membranes are an important part of future energy-efficient water purification. Already, researchers have demonstrated efficient water transport in carbon nanotubes with openings of less than one nanometer. When embedded in fatty membranes, the nanotubes squeeze entering water molecules into a single file chain, which leads to very fast transport. The flow was 10 times faster than in wider carbon nanotubes and 6 times faster than in the best biological membrane, a protein called aquaporin (read more: “Filtering water better than nature”).
Carbon nanotubes also have been used to demonstrate protective textiles with ultra breathable membranes. These membranes provide rates of water vapor transport that surpass those of commercial breathable fabrics like GoreTex, even though the CNT pores are only a few nanometers wide. Crucially, they also provide protection from biological agents due to their very small pore size, less than 5 nanometers wide. Biological threats like bacteria or viruses are much larger and typically more than 10-nm in size.
Nanotubes to reduce CO2 emissions
Russian President Vladimir Putin said during his speech at the United Nations Climate Change Conference in Paris on November 30, 2015 that new technologies for producing carbon nanotubes will help reduce carbon dioxide emissions in Russia by roughly 160 to 180 million tons by 2030. Nanotubes shall improve the qualities of 70 percent of materials known to mankind; that is, they enhance a material’s durability. This helps increase the lifetime of metals, rubber, and other materials by two or three times. And since all sorts of items will last longer, there will be a significant reduction in energy spent for producing new materials, as well as less energy spent to recycle waste.
“Nanotubes not only provide an indirect positive effect in electronics and industry that leads to the reduction of CO2 emissions,” remarked Professor Albert Nasibulin of the Skolkovo Institute of Science and Technology, and who is also a specialist on nanomaterials. “It will also be possible to directly convert CO2 into carbon nanotubes.”
The new method of creating nanotubes from CO2 was suggested this year by scientists at George Washington University in Washington, D.C. The essence of the technology is that a high-temperature electrochemical reaction helps break down CO2 into carbon nanotubes and oxygen
Carbon nanotubes shown to protect metals against radiation damage
One of the main reasons for limiting the operating lifetimes of nuclear reactors is that metals exposed to the strong radiation environment near the reactor core become porous and brittle, which can lead to cracking and failure. Now, a team of researchers at MIT and elsewhere has found that for reactors built using aluminium, adding a tiny quantity of carbon nanotubes to the metal can dramatically slow this breakdown process.
Aluminum is currently used in not only research reactor components but also nuclear batteries and spacecraft, and it has been proposed as material for storage containers for nuclear waste. So, improving its operating lifetime could have significant benefits, says Ju Li, who is the Battelle Energy Alliance Professor of Nuclear Science and Engineering and a professor of materials science and engineering.
The researchers showed that by adding only tiny quantities of carbon nanotubes (CNTs) — about 1 percent by weight added to the meta, the 1-D structure was able to survive up to 70 DPA of radiation damage. (DPA is a unit that refers to how many times, on average, every atom in the crystal lattice is knocked out of its site by radiation.)
The metal with carbon nanotubes uniformly dispersed inside “is designed to mitigate radiation damage” for long periods without degrading, says Kang Pyo So. The nanotubes can form a percolating, one-dimensional transport network, to provide pathways for the helium to leak back out instead of being trapped within the metal, where it could continue to do damage. Helium from radiation transmutation takes up residence inside metals and causes the material to become riddled with tiny bubbles along grain boundaries and progressively more brittle, the researchers explain. The team says the method may also be usable in the higher-temperature alloys used in commercial reactors.
Nanomedicine and biotechnology
Carbon nanomaterials such as nanotubes or graphene not only are widely researched for their potential uses in industrial applications, they also are of great interest to biomedical engineers working on nanotechnology applications.There is considerable interest in using CNTs for various biomedical applications. The physical properties of CNTs, such as mechanical strength, electrical conductivity, and optical properties, could be of great value for creating advanced biomaterials.
Carbon nanotubes can also be chemically modified to present specific moieties (e.g., functional groups, molecules, and polymers) to impart properties suited for biological applications, such as increased solubility and biocompatibility, enhanced material compatibility and cellular responsiveness. Nitrogen-doped carbon nanotubes for instance have been developed for drug delivery applications (“Nanoparticle-corked carbon nanotubes as drug delivery vehicles”).
However, the issue of cytotoxicity of CNTs is an area that has already attracted much research interest and has not resulted in a definitive answer yet. Given the inconclusive state of these nanotoxicology studies researchers says that more systematic biological evaluations of CNTs having various chemical and physical properties are warranted in order to determine their precise pharmacokinetics, cytotoxicity, and optimal dosages.
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