| Carbon nanotubes (CNTs) with unique structure and properties are in the vanguard of technological development of the time, and are predicted to be the best candidate as the multifunctional component of the next generation of composite materials. To fully utilize the unique mechanical and physical properties of carbon nanotubes, it is essential to assemble them into macroscopic structures. Thus, this dissertation aims to achieve two main goals: 1) demonstrating the growth of tall, uniform and dense vertically aligned CNT arrays and 2) fabricating CNT composites with ideal structural features and excellent mechanical, thermal and electrical performances, and increasing the understanding of the structure-property relationship in these materials.;Vertically aligned CNT arrays containing a high density of long and aligned nanotubes were successfully grown from chemically-synthesized catalyst Fe 3O4 nanoparticles by chemical vapor deposition. In order to improve the thermal stability of catalyst nanoparticles and preserve their monodisprese sizes at the growth temperature, an ultrathin layer of Al 2O3 was coated on the nanoparticles prior to CNT growth. The atomic layer deposited Al2O3 coating served as an encapsulating layer for the catalysts preventing them from agglomeration at the growth temperature, which resulted in highly uniform vertically aligned CNT arrays. The adoption of chemically-synthesized nanoparticles provides opportunity for large-scale synthesis of vertically aligned CNT arrays. In addition, the improved uniformity and quality of the arrays due to the Al 2O3 coating will improve their performance in real world applications.;A novel one-step stretch-winding approach was developed to fabricate ultrastrong, stiff and multifunctional CNT/bismaleimide (BMI) composites. The unidirectional composite films, with controlled sizes, were prepared by incorporating superaligned CNTs into BMI matrix by coupling the spraying of a BMI solution with the continuous winding of CNT sheets from an array onto a rotating spool. The CNT sheets were mechanically stretched to improve alignment and straightness before being embedded into the polymer matrix. The resultant composites contain critical structural characteristics including long CNTs, high packing density of nanotubes, high degree of CNT alignment and straightness, and intimate nanotube-matrix integration. The obtained CNT composite films with ∼50-55 wt% CNTs exhibited a record high tensile strength (3.8 GPa), high Young's modulus (293 GPa), electrical conductivity (1230 S/cm) and thermal conductivity (41 W/m˙K). This fabrication approach also has potential for large-scale fabrication. This study shows the importance of aligning and straightening CNTs for maximizing composite properties.;Similar phenomena were observed in CNT -- thermoplastic polymer composites. The mechanical and electrical properties of CNT/nylon 6,6 composites were improved through reducing nanotube waviness by a drawing and stretching method. The tensile strength, Young's modulus and electrical conductivity of the 7%-stretched composite showed 191%, 294% and 207% increase, respectively, comparing to the unstretched composite.;Finally, based on the development of unidirectional CNT composites composed of long and aligned CNTs, the nanotube length effect at millimeter scale on composite properties was investigated. Longer CNTs with larger tubular diameter and higher aspect ratio resulted in higher thermal and electrical conductivities of the composites. However, the CNT length had no appreciable effect on mechanical properties of the composites. We propose that longer CNTs with a larger tubular diameter and higher aspect ratio result in a more efficient conduction channel for phonons or electrons, and thicker nanotubes are more densely packed in the composite structure, which reduces the contact interfacial thermal or electrical resistance. However, the mechanical properties of the composites seem to be governed by the interplay of many factors, such as CNT aspect ratio, diameter, alignment and defects, which should be isolated in future study. |
