Carbon nanotubes (CNTs) exhibit exceptional mechanical, thermal, and electronic properties but are still very expensive to manufacture. The production volume of CNTs is currently growing at ~15%/year but the amount of CNTs produced today is miniscule (0.01%) compared to carbon black, a material commonly used today in many applications such as automotive tires. Because the current CNT growth processes are not cost effective, CNTs are not used in many applications despite their higher performance. This research project investigates and develops the underlying science and technology of new synthetic scalable processes to make ultralong CNTs by taking advantage of minimizing the amount of seed chemical (catalyst) required. Over 90% of carbon black in use today is for rubber reinforcement, mostly in automobile tires where the carbon provides a preferred trade-off between the rolling resistance/wet traction for improved gas mileage. Increased gas mileage of 3-5% could be achieved if CNTs were used instead of carbon black in tires, which in turn could reduce US greenhouse gas emissions by ~1%, providing significant societal and economic benefits for the US. This project is jointly funded by the Division of Civil, Mechanical and Manufacturing Innovation and the Established Program to Stimulate Competitive Research (EPSCoR).
The mechanism underlying the growth of ~1 mm long carbon nanotubes on exfoliated laminar mineral sheets, in a commercially scalable process, is investigated. The expected yield is 10-100 times higher than currently achieved and would eliminate the need to separate the catalyst/support from the nanotubes. Critical scientific and technical challenges include models for the improved catalyst dispersion within narrow lamellar catalyst layers and the reaction â€“ diffusion conditions required so that the reactants and co-fed water can reach all active sites and extend catalyst lifetimes. The understanding and control of reactant concentration gradients is critical as it impacts the nanotube length and uniformity. Long nanotubes are unsuitable for applications that involve compounding tubes with a polymer and hence the tubes will be milled to produce lengths similar to what is commercially used today (average lengths 1-10 microns). The relationship between length reduction and ball milling parameters, specifically impact energy of a single collision and cumulative impact energy, will be determined to allow scaling up of this process to commercial milling devices. Average lengths of 10-50 microns will also be produced to better explore how longer tubes perform in compounding operations. Tubes will be mixed in one thermoplastic (polycarbonate) and one tire formulation to determine nanotube performance. CNTs of three different diameters will be milled to the same aspect ratio to determine if aspect ratio is important in filled polymer properties.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.