(377a) From Academic Soot Research to Commercial Synthesis of Single-Walled Carbon Nanotubes

    Single-walled carbon nanotubes (SWCNT) are expected to contribute significantly to sustainable growth of U.S. industry and to help meet societal needs. Their potential applications range from selective gas sensors pertinent for counter-terrorism to the reinforcement of a new generation of high-strength, high-stiffness polymers, drug delivery and electronics. However, expansion of the commercial use of these materials requires cost-efficient, reliable, reproducible and environmentally friendly production. Due to its exothermic character and its ease of scalability, combustion synthesis is expected to become a major source of SWCNT and therefore to play a major role in the ongoing nanotechnology revolution.

Ground-laying Academic Research

    As part of a long-term academic research effort, fuel-rich premixed hydrocarbon combustion has been investigated over several decades, involving, among many others, the authors of the present contribution. Most of the work was conducted by means of public funding obtained from federal agencies such as DOE Office of Basic Energy Sciences, EPA, NIH and NSF. Initially, this work was mainly motivated by health concerns associated with the release of airborne species such as polycylic aromatic hydrocarbons (PAH) and soot from large scale combustion processes used in transportation, incineration or power generation. Well-defined laboratory systems such as well-mixed or plug-flow reactors and premixed flames have been used for the detailed analysis of the chemical processes involved in hydrocarbon oxidation, formation, and depletion of PAH and soot. To assess formation pathways leading to carbon structures of increasing size, a large range of experimental techniques such as on-line mass spectrometry, optical methods or probe sampling followed by chemical or optical analysis has been applied. More recently, kinetic models aiming for the quantitative description of combustion processes and demonstrating encouraging predictive capabilities have been developed. The internal structure of growing soot particles, correlated to macroscopic properties of the material, has been investigated by means of transmission electron microscopy (TEM). Triggered by the discovery of close-shelled fullerenes in 1985 and their synthesis in macroscopic quantities in 1990 followed by a significant increase of interest in carbon nanomaterials, activities in combustion research at MIT began to emphasize the potential materials synthesis aspect of controlled combustion processes. In 1991, premixed low-pressure combustion of aromatic hydrocarbons was identified in the Department of Chemical Engineering at MIT as a powerful tool for the synthesis of C₆₀ and C₇₀ fullerenes. In comparison with other methods, high concentrations of larger fullerenes up to C₁₁₆ were observed. Whereas high-pressure liquid chromatography (HPLC) was used for the fast and unambiguous detection of fullerenes accessible by solvent extraction, TEM allowed for identification of carbon shells including carbon nanotubes present in combustion-generated condensed materials. Further development of high-resolution transmission electron microscopic techniques allowed for the visualization of individual fullerenes.

The founding of Nano-C

    The performance and scalability of combustion synthesis of fullerenes led to the founding of Nano-C in 2001 by Prof. Jack B. Howard, responsible for the corresponding research activities at MIT. Nano-C has been developing the technology and contributed significantly to the emergence of a market for their industrial use. Another major step in the academic research at MIT has been the demonstration of the formation of single-walled carbon nanotubes after addition of catalyst precursors such as iron pentacarbonyl to the fresh gas mixture of premixed hydrocarbon flames. This work was part of the Ph.D. thesis of Murray J. Height, conducted under the supervision of Prof. Howard. The intellectual property of all major innovations in the work at MIT has been protected by patent applications. MIT has been extremely supportive in the development of Nano-C and has licensed all pertinent technology to the company. Initial funding from private investors and licensing activities was used for the improvement of first-generation combustion synthesis of fullerenes as initially developed at MIT allowing for a further, approximately 10-fold, increase in cost efficiency. Due to the limited amount of available private funding, NSF SBIR support was needed and has been used successfully for enabling activities in the synthesis of single-walled carbon nanotubes. Further development of the synthesis of single-walled carbon nanotubes is now a major thrust of Nano-C activities, covering all fullerenic materials from spherical fullerene molecules to nanostructured carbon (?fullerene black?) and SWCNT.

Nano-C's Benefits in Interacting with MIT

    Success of the SBIR work has been significantly facilitated by previously established collaborative research activities between the researchers involved. Nano-C has largely benefited from expertise and equipment available at MIT, particularly in the Department of, and Center for, Materials Science and Engineering. Raman spectroscopy has been a major tool for the identification of SWCNT and the determination of the diameter distribution of analyzed samples whereas high resolution electron microscopy (HRTEM) has allowed for the more detailed investigation of structural features. Visualization of larger sample areas has been achieved by means of scanning electron microscopy (SEM). Thermogravimetric analysis (TGA) has been conducted for a semi-quantitative assessment of impurities, particularly iron or iron oxide and amorphous carbon. During the Phase I of the discussed SBIR project, Prof. Vander Sande acted as consultant, conducted the electron microscopic work and facilitated access to other instrumentation such as Raman spectroscopy at MIT. Regular meetings between Prof. Vander Sande and a Nano-C representative discussing characteristics of samples generated at Nano-C, allowed for meaningful adjustments of the synthesis process. Examples of TEM and SEM images generated during Phase I of the SBIR work are shown in Figs. 1 and 2.

    Taking into account the increased volume and depth of the work, in the Phase II of this SBIR project, materials characterization was subcontracted to Prof. Vander Sande's laboratory at MIT. This arrangement, instead of the previous consulting agreement, allows for the use of additional human resources such as a postdoctoral researcher and eases sample characterization with additional techniques. For instance, scanning transmission electron microscopy (STEM) will be used on samples of particular interest in order to establish structural and compositional maps of specimen. In addition to information about most efficient purification strategies, insight in the formation mechanism of SWCNT will be gained. Other, complementary, techniques such as Atomic Force Microscopy (AFM), will be also used for the visualization and further characterization of SWCNT.

MIT's Benefits in Interacting with Nano-C

    SBIR funding subcontracted to MIT allows participation of an academic laboratory in research at the forefront of technology development in industry. Involvement of undergraduate, graduate and postdoctoral researchers contributes to an efficient training of workforce by providing an industry-relevant experience. A project duration of at least 2 and up to four years (including Phase IIB options) is sufficiently long to conduct research meeting highest academic standards while satisfying all expectations of the industrial partner. In addition to the joint work on characterization of SWCNT, the SBIR activities at Nano-C have facilitated two projects in the Department of Chemical Engineering at MIT. A postdoctoral researcher (John Wen) has received a Canadian NSERC PDF fellowship for the development of a first kinetic model describing catalytic formation of carbon nanotubes in flames. His model will be validated by means of data generated at Nano-C. A student (Joanna Yu), in the PhD in Chemical Engineering Practice program, organized in collaboration with the Sloan School of Management at MIT, will investigate in her ?Integrative Perspective Paper? correlations between Nano-C's technological innovations and the evolution of the single-walled carbon nanotube market.