Investigating Catalyst Composition, Doping, and Salt Treatment for Carbon Nanotube Sheets, and Methods to produce Carbon Hybrid Materials

Although relatively recently discovered, Carbon nanotubes (CNTs) have gained popularity as one of the most exotic materials known due to their extraordinary electronic, thermal, and mechanical properties. CNTs have a very high aspect ratio and they exhibit a very high electrical conductivity and a good mechanical strength. Individually at the nanotube scale, CNTs have extraordinary properties, however, it is challenging to attain these properties when CNTs are used to form macroscopic yarns, tapes, and sheets. Individual single-walled nanotubes (SWNTs) exhibit electrical conductivities and tensile strength of 10^6 S/cm and 30 GPa whereas multi-walled nanotubes (MWNTs) have corresponding values in the range of 10^4 S/cm and 30 GPa. For macroscopic CNT sheet assemblies, the reported electrical conductivity and tensile strength are in the range of 103 S/cm and 102 MPa respectively. This is primarily because of the non – woven nature of macroscopic CNT sheet assemblies along with defects within the individual CNTs that form these macroscale entities. Additionally, the continuous synthesis of CNT sheets itself introduces impurities like amorphous carbon and residual catalyst into the material. These defects and impurities could lead to weak inter-tube interactions and poor alignment of the CNTs and have a significant impact on the final properties. In this dissertation research, CNTs were synthesized using a combination of transition metal catalyst precursors. B and N doped CNTs were synthesized to study the influence of varying catalyst composition and doping on the structure, electronic properties, and purity of CNT sheets produced using the floating catalyst chemical vapor deposition (CVD) method. A second method involving a post processing salt treatment was also developed to improve the properties and purity of CNT sheets. Both the doping and salt treatment methods were successful in improving the electrical conductivity of CNT sheets. Various characterization techniques were used to study the effects of catalyst composition, doping, and salt treatments on the structure, morphology, quality, and purity of the CNT sheets. Additionally, initial experiments to produce carbon hybrid materials (CHMs) by combining CNTs with other materials were also performed. This can be done in two ways: either by utilizing a particle injector to introduce nanoparticles at the reactor inlet or by introducing a solvent/polymer at the outlet of the reactor system during the densification process. CHMs synthesized by combining CNTs with other materials using a particle injector at the inlet of the reactor produced CNT sheets decorated with different distributions of spherical particles and in some cases conical carbon structures were produced. CNT – Silicone hybrid materials produced during the densification process at the outlet of the rector are lightweight, durable, and offer excellent flame - resistant properties which can be used for potential firefighter garments and smart textiles. The additive materials approach allowed for the design and creation of unique and versatile CHMs in a one-step synthesis process. Overall, the synthesis methods and purification method developed in this dissertation produced bulk CNT sheet with improved and customizable properties which opens the design space for more commercial applications of carbon nanotube materials.