The carbon nanotube industry has a promising future. Founded on a strong atomic structure with firm chemical bonds, carbon nanotubes (CNTs) add stiffness and rigidity, as well as being highly electrical and thermal conductive.
These exceptional properties make it an ideal industrial raw ingredient for numerous materials and composites for use in tennis rackets, car tyres, pharmaceuticals, and countless other products.
However, these properties are greatly dependant on the purity and quality of the carbon nanotubes. Low-grade products, with numerous flaws and weaknesses contain significantly less potency and value.
As the scientific journal Phys.org, reports, “The first studies of carbon nanotubes were made nearly 30 years ago, without yet meeting the high expectations for applications. The reason for this is largely the failure to scale up the manufacturing process while retaining a high quality. As the properties of the material depend on the crystal structure, the quality of the material is absolutely crucial to the performance of the final product.”
“Carbon atoms must sit perfectly in a well-organised crystal structure at precise distances, but they don't in the commercially available materials on the market today,” notes Krister Svensson, an associate professor of physics at Karlstad University in Sweden.
“The problem is complicated by the fact that it is difficult to analyse crystal structure and that there are no established standard methods for classifying the materials. This has led to a kind ‘Wild West’ situation on the market in terms of prices and quality of the materials on sale.”
Other industries, such as manufacturers of carbon fibres have a defined grading system to characterise and help give a value to their products. The challenge for carbon nanotube (CNT) producers, suppliers, and buyers is to establish a formal system of protocols and classification.
Whereas one carbon nanotube producer may spend the time and money to produce high-quality CNTs, there is no official way to acknowledge his superior product. This has led to a general fear that a large amount of inferior carbon nanotubes on the market may ruin the industry as a whole, as the perceived value of the product is diminished. This in turn will detract further investment and lead to a sub-standard industry of missed opportunities.
However, Prof. Svensson has developed a method of classification that may avoid this situation, allowing for a universal system of gradation and valuation that ensures high-quality products are rewarded with premium prices.
While a classification system may appear straight-forward, grading something as small as a carbon nanotube is a highly technical operation. As Svensson explains in his recently published paper, Quantifying Crystallinity in Carbon Nanotubes. Here he states that, “There are essentially three available experimental techniques; Raman spectroscopy, X-ray diffraction (XRD), and Transmission electron microscopy (TEM).”
But while these techniques are well-suited to evaluate graphene, each of has serious flaws when assessing the quality of a carbon nanotube, often (in layman’s terms) due to their curved nature.
For graphite one can use Raman, “… to quantify the level of crystallinity in terms of interlayer spacing d002, crystallite sizes, Lc and La (see image below), and the level of graphitisation.”
However, “In multiwalled carbon nanotubes (MWCNTs) the graphene layers are now forming cylinders and the parameters Lc and the standard measure of graphitisation lose their importance.”
Similarly, the curvature of CNTs distort analysis using X-ray diffraction.
While the third option of using Transmission Electron Microscopy (TEM), both in imaging and electron diffraction modes has been shown to highlight defects, the study explains that this method is still not suitable as, “For CNTs this traditional type of analysis would be hampered by the streaking of diffraction spots, similar to the issues encountered in XRD.”
However, Svensson now believes he has identified a superior method of classification. He suggests using a highly coherent electron beam on, “… straight sections of individual tubes with detailed analysis of diffraction spots in positions and directions that are not influenced by the curvature of the tube.”
Comparing the two images below gives an idea of the wide differences in the quality of carbon nanotubes on the market.
By providing quantitative information about the crystallinity of CNTs, not only has the study provided a benchmark for comparison of carbon nanotube samples, but it has also, “… demonstrated the detrimental effects that low crystallinity has on mechanical properties.”
Furthermore, Svensson is convinced that, if applied, his new technique will stabilise the entire industry, improving standards, and putting unethical manufacturers and suppliers out of business. Stating that, “It is quite obvious that [some] commercially available materials fail to live up to expectations, it's simply a different material. Vigorous efforts to put a stop to 'fake' materials and develop standardised measuring methods and classifications of materials are required. Not until then can the market be ready to develop new products for the various material classes.”