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Bulk Single-walled Carbon Nanotube Growth

A scalable vertical flow reactor is being developed to control parameters of Carbon Nanotube growth, such as diameter and chirality/electrical properties, in order to inexpensively produce Carbon Nanotubes in larger quantities.

Carbon nanotubes can play a variety of roles in future space systems, including wiring, high-strength lightweight composite materials, thermal protection and cooling systems and electronics/sensors. Successful development of these technologies is highly dependent on a reliable method to produce controlled carbon nanotubes.

Research Overview
We are constructing a scalable vertical flow reactor depicted schematically in Fig. 1. A co-flow of catalyst precursor (e.g. ferrocene) is delivered with hydrocarbon growth precursor (e.g. methane) into a quartz tube passing through a tube furnace heater. A three-zone heater allows the temperature profile to be varied across the length of the reactors and different optima temperatures may exist in controlling the important phases of nanotube growth: nanoparticle catalyst formation, nanotube growth and agglomeration. Large agglomerates may be collected at the bottom of this tube, while lighter weight nanotubes will be carried through the top of the reactor by the flow, then are collected by use of an electrostatic precipitator. Finally, the reactor output is passed through a bubbler to remove any materials and byproducts that are not captured in the precipitator.

Critical to the nucleation of catalyst nanoparticles is the method of delivery of catalyst precursor. Presently, material is being delivered by vaporizing the solid metal precursor into a carrier gas stream. Future iteration will explore use of other methods, such as conventional spray atomizers or an electrospray method. The electrospray approach may have the added benefit of charging nanoparticles and preventing agglomeration by electrostatic repulsion on the charged particles.

Schematics of a scalable vertical flow reactor Figure 1. Schematics of a scalable vertical flow reactor.

Materials produced in this reactor are characterized by many analytical instruments available at Ames - Scanning and Transmission Electron Microscopes (SEM/TEM), confocal Raman Spectroscopy, Thermo-Gravimetric Analysis (TGA) and Absorption Isotherm Analysis. In addition, we intend to develop in-situ optical diagnostics to our production reactor. Raman spectroscopy can be used to qualitatively characterize metallic versus semiconducting nanotubes, and recently individual features in the Raman spectrum have been assigned to metallic nanotubes of differing chirality. Researchers have also been able to use fluorescence to detect chirality of isolated semiconducting nanotubes in solution. Both of these techniques may be employed for real time monitoring of our reactor. In solution, it is necessary to debundle nanotube aggregates as metallic tubes in close vicinity to the semiconducting tube quench the fluorescence. Applying this to our reactor means we will only be able to detect nanotubes in the initial formation stage prior to aggregation. Therefore, this technique will also allow us to observe the aggregation process of nanotubes in the gas phase.

Finally, once successful application of the in-situ diagnostic to the growth process is demonstrated, we can use this ability to tune reactor conditions and examine means of controlling nanotube formation rates, diameters and chiralities.
Magnified Bulk Single-walled Carbon Nanotubes
Figure 2. Magnified Bulk Single-walled Carbon Nanotubes

Among the parameters to be varied are the catalyst/hydrocarbon flow ratio, reactor temperature, spatial temperature profile, atomizer/ electrospray settings, and catalyst/hydrocarbon chemistries. A design of experiments will be performed over this wide variety of parameters to determine the best attainable relationship. Following the design of experiments, single variable variations will be explored to improve scientific understanding of the growth process.

Carbon nanotubes (CNTs) exhibit remarkable mechanical and electrical properties which make them well-suited a wide variety of applications. The CNTs posses Young's modulus of 1 TPa, greater than diamond, making them suitable for structural/composite applications. The thermal conductivity of single-walled nanotubes (SWNTs) is also greater than that of diamond at 3000 W/m-K, making them suitable for heat removal applications. Electrically, CNTs demonstrate high current carrying capacities, as high as 10 13 A/m 2 . Current transport in CNTs is known to be ballistic, i.e. conductance is limited only by quantum mechanics and is independent of length. Thus, CNTs show a great potential in use as wiring, especially in small-scale circuits. Further, depending on the nanotube's chirality, it can be semiconducting or metallic. While the metallic nanotubes are promising as wiring, the semiconducting nanotubes show promise as next generation transistor materials. However, the use of nanotubes in applications requiring large quantities of nanotubes, particularly structural applications (e.g. composites, heat shields, high heat flux probes) is limited by the high price and low output of present nanotube production reactors. Presently, CNTs are produced only on a research scale, with output on the order of a few grams per day and costs near $500 per gram. This research scale effort, pioneered at Rice University and funded by NASA, uses a high pressure reactor (30 atm) comprising CO. Though this effort has been spun off into a first US-based company to manufacture SWNTs, the economic viability of this approach is questionable given the high pressure nature and associated safety problems. Other major efforts in this field are in Japan and China using undisclosed processes. While the limitation in structural applications is scalability, the limitation in electronics and sensor applications is the control of chirality (metallic vs. semiconductor) and diameter.