Since their discovery, carbon nanotubes (CNTs) have been one of the most well-known and widely-researched materials in the field of nanotechnology. The discovery of their outstanding molecular properties led to a lot of excitement about the material (even bordering on hype) and a widespread hope that that CNTs would eventually be used in cutting edge products that could help address some of our biggest technological problems.
An individual CNT has exceptionally high strength, electrical conductivity, and thermal conductivity, and their hollow cylindrical structure allows them to form sturdy and stable materials while maintaining a low density – all fantastic properties for high-performance materials. Unfortunately, achieving the same high performance that CNTs exhibit on the nanoscale while building them into larger-scale materials has been a challenge for many years, and has been a barrier to the development of truly revolutionary CNT products. One of the keys to overcoming this challenge is achieving control over the arrangement and interconnections between CNT molecules, including the density of packing and degree of alignment and order. The quality of available CNT structures has determined what commercial applications have been achievable to date, and improvement in this area will determine which applications might be achieved in the near future.
Structure and Applications of Raw CNTs
Today, CNTs are commercially available in a few different forms. Most CNTs that are available for purchase come in a very raw form, a disordered and entangled state that arose from random growth of the molecules during synthesis. These often take the form of powders or (in the case of very long CNT molecules) mats of material made up of entangled, non-aligned CNT molecules, as illustrated in the images above. Another common form in which CNTs may be purchased is that of a vertically aligned array, in which the CNT molecules were grown from a solid substrate and developed a rough vertical alignment due to their common direction of growth.
These raw CNTs may be single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), or a combination of both. CNTs can have a wide variety of molecular diameters and lengths, and typically a range of both values are present in any commercially available sample; different sources of CNTs also vary in purity (the amount of non-nanotube material included in the sample) and in molecular quality (the frequency of defects in their molecular structure). Purified samples, very high molecular quality samples, or even samples that have been sorted to contain only certain selected CNT types can each be obtained for a higher price. Sorted CNTs may sometimes be sold in the form of a liquid solution, since fluid and chemical processing steps are used in the process of sorting CNT types from one another.
Given this variance in CNT type and quality, CNTs from a given commercial source may or may not be suited for a given application. The earliest commercial applications of CNTs focused on mixing particles of entangled, disorganized CNTs into other materials to increase the strength and flexibility of common commercial goods and coatings. CNTs have been used in this way to help strengthen composite materials like boat hulls or bicycle frames; some more recent incarnations of this type of product use CNT composites in additive manufacturing. Other applications for disorganized CNTs mixed into other materials include enhancement of the conductivity of thin film electronics and the formation of anti-corrosion coatings. These applications are often not very sensitive to the quality of the CNT material used, so they can be achieved through the use of less-expensive CNT products with low quality or purity; however, they do not harness very much of the potential of CNTs’ excellent mechanical or electrical properties.
Aligned arrays of CNTs can be used to take slightly better advantage of the shape and properties of the CNT molecules. The fact that the molecules are all oriented roughly in a single direction can be exploited to turn these arrays into field emission devices or directional heat transfer interfaces. Growing a CNT array on the surface of an underlying substrate is useful for creating electrodes or gas sensors. One of the most well-known applications of CNTs in recent years is their use as an ultra-black light absorbing coating (e.g. Vantablack); the array format is critical in this case, because the rough surface, formed by millions of CNT molecule tips aligned together, allows for such fantastic absorption of light.
Better Structure for Better Performance
Synthesizing CNTs in an aligned array is one way of imposing a useful structure or organization onto CNTs. Other ways to do this involve processing or molding raw CNTs into different formats to make a processed material. CNT molecules tend to have strong adhesion to one another through van der Waals forces because of their relatively large and smooth surface areas; this makes it challenging to re-arrange the molecules through processing, since it is difficult to break apart the clumps and bundles that are present in the raw material. On the other hand, these strong adhesive forces mean that once CNT molecules have been re-arranged into a new solid structure that structure will tend to be relatively stable without the need for molecular cross-linking. Aside from allowing the creation of new structures, there is an additional benefit to this: as long as solid CNT structures are not held together with permanent crosslinked bonds, they can potentially be recycled by disassembling and re-assembling the CNTs again into a new structure.
Examples of processed CNT structures include buckypaper, in which raw CNTs have been compressed together into a thin sheet, yarns that are formed from drawing and twisting entangled mats or arrays of long CNTs, or thin films that are formed by dispersing CNTs in solvents and surfactants and subsequently depositing them on a filter or a solid surface. The technology used at DexMat to produce Galvorn fibers and films falls into this broad category of techniques: it is a method of re-arranging CNTs into a more useful organized structure, specifically by dissolving them in acidic solutions and using fluid processing to align them and increase their packing density.
Once CNTs have been processed into different structures they can be used for a wider variety of applications appropriate to their new forms. For example, the long thin shape of CNT molecules can be exploited to create porous network of overlapping, randomly-aligned CNTs that would be perfect for transparent flexible electronics. A long chain of well-packed and well-aligned CNT molecules, on the other hand, can serve very well as a high-strength fiber, since this type of structure makes the best use of the combined mechanical strength of the molecules and the slip resistance of their long surfaces of contact to grant tensile strength to increase the tensile strength of the fiber. Maximizing alignment and density of packing also enhances electrical and thermal contact between the constituent molecules, creating CNT materials with high thermal and electrical conductivity.
Regardless of the particular property that is being sought, optimizing the structure of CNTs allows more and more of the inherent potential of the molecules to be used effectively. As a result, the properties of processed CNT materials can be maximized by ensuring that high purity and high quality CNT materials are used as the raw material to be processed. We have found that this is one of the keys to achieving excellent material performance in Galvorn fibers and films.
Many of the commercial applications of processed, high-performance CNT materials (e.g., transparent flexible electronics, high-strength conductive wire, etc.) are in the early stages of development or are still just on the horizon. Continued improvement in our techniques for manipulating and optimizing CNT structure, and improvements in the availability and price of high-quality CNTs, should help ensure that these applications become commercial realities. Next week’s post will highlight some of the potential applications in which processed CNT materials are expected to excel, including biomedical devices, consumer goods, and electrical distribution systems.
In the mean time you can learn more about Galvorn CNT fibers and films by following the link below!