Aiming for high-temperature applications (1200-2000°C), traditional carbon/carbon (C/C) composites are composed of carbon fibers (CFs) and amorphous pyrolytic carbon (PyC) matrix. Although still heterogeneous in the nature, both the reinforcement and the matrix are considered homogeneous with very similar chemical composition, which minimizes the interfacial tension and stress. As CFs possess superior mechanical properties resulting from the highly crystalline and condensed graphitic structure, the brittleness remains an issue with the failure strain less than 1%. The toughness difference between CFs and PyC matrix explains the interfacial peeling as the typical and principal failure mode of CF-based C/C (CF/C) composites. In comparison, carbon nanotubes (CNTs) fall into the category of nanomaterials with tens of nanometers in diameter. Being much more flexible with ultra-high surface areas, the CNT-based C/C (CNT/C) composites are expected to dramatically improve the material toughness, also with excellent mechanical and electrical properties maintained at high temperature. Additionally, CNTs could offer the same theoretical strength with only one tenth of CFs' weight, which significantly reduces the overall composite weight. In this work, we optimized the manufacturing process of CNT/C composites in three aspects: the precursor infiltration, the CNT quality, and the CNT bundle orientation. The continuous CNT synthesis and collection process with floating catalyst chemical vapor deposition (FCCVD) were explained in Chapter 3, where the catalyst solution composed of ferrocene (catalyst), thiophene (promoter), and acetone (carbon source) was carried through the 1250 ºC reaction zone by a flowing gas mixture of argon and hydrogen. Differing from the conventional water-assisted synthesis in which water vapor is one part of the carrier gas mixture, we included de-ionized water into the catalyst system, which achieved a more uniform and controlled distribution. Accordingly, a transition from double- to multi-walled CNTs were observed with elevated Raman Iɢ/Iᴅ ratio and improved mechanical and electrical properties. In Chapter 4, we developed a novel methodology applying topological data analysis (TDA) to the scanning electron micrographs to detect and quantify the CNT bundle orientation. The CNT bundle extensions in certain directions were summarized algebraically and expressed as visible barcodes. The barcodes were then calculated and converted into the total spread function V(X,θ), from which the alignment fraction and the preferred direction could be determined. The results showed high consistency (R2=0.975) compared to the Herman's orientation factors derived from the polarized Raman spectroscopy and wide-angle X-ray scattering. Additionally, the TDA method presented great robustness with varying SEM acceleration voltages and magnifications, which might alter the scope. In Chapter 5, we applied the chemical vapor infiltration (CVI) process and cyclic polymer infiltration pyrolysis (PIP) processes onto the commercialized CNT sheets (buckypapers, BPs), inspired by the conventional fabrication procedures of the CF/C composites. By controlling the parameters such as gas flow rate of carbon source and deposition time, pyrolytic carbon (PyC) as a carbon matrix precursor was deposited within the CNT network structure to achieve a near fully densified CNT/C composite structure. PIP process involves precursor impregnation, mechanical stretching (optional) and heat treatment steps, and polyacrylonitrile (PAN) and phenolic resins were investigated. To further enhance the precursor infiltration, in Chapter 6, we proposed an efficient integrated fabrication route combining the continuous CNT synthesis, collection, and precursor infiltration into one single step. One of the prominent advantages lies in the significant reduction of the penetration pathway, which is critically beneficial for the densification process, especially in thick components.
