A Comparison of Carbon Nanotubes and Carbon Nanofibers
While carbon nanotubes (CNT) and carbon nanofibers (CNF) are both hollow, nanometer in scale, and produced in a similar manner, there are distinct differences which significantly impact their performance and ability to be processed. The primary differences between the materials are morphology, size, ease of processing, and price. Carbon nanofibers, also known as Stacked-Cup Carbon Nanotubes (SCCNT), have a unique morphology in that graphene planes are canted from the fiber axis, resulting in exposed edge planes on the interior and exterior surfaces of the fiber. CNTs, on the other hand, typically resemble an assembly of concentric cylinders of graphene. To illustrate the difference in morphology, Figure 1 below shows a side by side comparison of A) Multi-walled carbon nanotubes and B) stacked cup carbon nanotubes.
Figure 1. A) Multi-wall CNTs are composed of concentric cylinders of graphene where the basal planes form an inert surface. B) The stacked-cup structure of CNF has exposed graphitic edge planes along its length, providing opportunities for chemical modification of the surface for covalent bonding directly with the matrix.
Carbon nanotubes typically feature fiber diameters between 1-30 nanometers. Carbon nanofibers, or SCCNTs, feature fiber diameters ranging from 50-200 nanometers depending upon the carbon nanofiber type. Both nanomaterials are available in varying lengths, up to several hundred micrometers, depending on the feedstock and production method. While the difference in diameter ranges between the nanomaterials appear to be modest, the real-world implications for using these materials are significant. In the case of carbon nanotubes, Van der Waals forces cause the nanotubes to form ropes or reassemble after being dispersed. Due to their smaller size, Van der Waals forces are stronger in carbon nanotubes, requiring the use of chemical dispersants or functionalization techniques to aid and maintain dispersion. Unlike carbon nanotubes, stacked-cup carbon nanotubes are less affected by Van der Waals forces and tend to stay dispersed for longer periods of time. This difference enables the stacked-cup carbon nanotubes to be dispersed through purely mechanical processing techniques without the need for additional, and costly, processing steps, making CNFs easier and cheaper to process.
Because CNFs feature exposed graphene edge planes on its surfaces, the surface state can be readily modified through chemical functionalization or thermal treatments, when necessary, to facilitate chemical bonding with any matrix. Both functionalizing and dispersing the CNFs are performed using traditional, readily scalable, processing methods and these steps can be performed quickly. On the other hand, CNT functionalization is performed by first creating defect sites along the side walls of the fibers, which can then be utilized for attaching functional groups. This requires several processing steps, and can be difficult and costly to scale-up.
Finally, the price of CNTs and CNFs vary depending on the producer but in general, the cost of using the CNFs is typically an order-of-magnitude lower than for CNTs. CNFs are available in large volumes (up to 70,000 pounds per year) and range in price from as low as $100 per pound to as much as $500 per pound. The price of CNTs also vary widely, and are very dependent on the quality and purity of the CNT, but can be found for as low as $100 per pound to as much as $750 per gram or more! Remember, this is the cost for just the raw material, which then need to be processed into a composite. In the case of CNTs, additional processing steps, such as purification, functionalization, and the addition of chemical dispersant are often required prior to dispersion; these additional steps significantly increase the cost and complexity of using nanotubes. Considering the final composite properties are many times equivalent or better for CNF-reinforced composites when compared to CNT-reinforced composites, carbon nanofibers often have a lower overall impact on the final cost of producing the nanocomposite.