Ph.D. Dissertation Defense by Scott Bondi
Friday, August 9, 2004
(Dr. Jack Lackey, Chair)
"LCVD Synthesis of Carbon Nanotubes and Their Characterization"
Among the most significant findings in the field of material science in the past two decades has been the advancement of research in the area of nanomaterials, more specifically that of the carbon nanotube. This material possesses unusual and extraordinary mechanical, electrical, and thermal properties which have potential to replace and improve upon several existing technologies. Commonly used synthesis methods of this material have been limited to processes which traditionally do not allow for direct and selective deposition on most materials; an important feature necessary for use in many applications.
The primary goal of this research was to develop the laser chemical vapor deposition (LCVD) process to be able to directly deposit carbon nanotubes onto substrates selectively. LCVD has traditionally been used to directly deposit complex geometries of other materials, including many metals and ceramics. Carbon nanotube deposits were formed using codeposition and other techniques. Multiwall carbon nanotubes as small as 7 nm were synthesized. Utilizing electron microscopy, deposits were characterized to determine the effects of laser power, catalyst and hydrocarbon concentration, time, pressure, and other variables on the number of nanotubes formed, their size, and their spatial location. The most important variables were shown to be hydrocarbon and catalyst concentration and laser power. These results were analyzed and statistics based models were developed to express these trends. Additionally, the process was also used successfully to deposit linear patterns of carbon nanotubes. Carbon nanotube deposits were also carried out in the presence of an electric field. It was demonstrated that a field of sufficient strength could be used to orient tube growth.
LCVD is a thermally driven process and a thermal feedback and control system is typically employed to allow for real time control of the reaction zone temperatures. The current thermal imaging system installed on the LCVD reactor is limited to operation at temperatures above which nanotube deposition occurs. A heat and mass transport model was therefore developed to simulate deposition temperatures and provide an estimate of the desired laser power needed to achieve a desired reaction temperature. This model included all significant modes of heat transport including conduction, natural convection and radiation. Temperature dependant material properties were also employed to help achieve greater accuracy. Additionally, the model was designed to be able to simulate a scanning laser beam which was used to deposit linear patterns of carbon nanotubes. Modeling calculations of laser heating compared favorably with experimental data.
The results of this work show that LCVD has potential for use in the commercial market for selective direct deposition of patterns of aligned carbon nanotubes on multiple substrate materials.