Nanocomposites; including nano-materials such as nano-particles, nanoclays, nanofibers, nanotubes, and nanosheets; are of significant importance in the rapidly developing field of nanotechnology. Due to the nanometer size of these inclusions, their physicochemical characteristics differ significantly from those of micron size and bulk materials. The field of nanocomposites involves the study of multiphase materials where at least one of the constituent phases has one dimension less than 100 nm. This is the range where the phenomena associated with the atomic and molecular interaction strongly influence the macroscopic properties of materials. Since the building blocks of nanocomposites are at nanoscale, they have an enormous surface area with numerous interfaces between the two intermix phases. The special properties of the nanocomposite arise from the interaction of its phases at the interface and/or interphase regions. By contrast, in a conventional composite based on micrometer sized filler such as carbon fibers, the interfaces between the filler and matrix constitutes have a much smaller surface-to-volume fraction of the bulk materials, and hence influence the properties of the host structure to a much smaller extent. The optimum amount of nanomaterials in the nanocomposites depends on the filler size, shape, homogeneity of particles distribution, and the interfacial bonding properties between the fillers and matrix. The promise of nanocomposites lies in their multifunctionality, i.e., the possibility of realizing unique combination of properties unachievable with traditional materials. The challenges in reaching this promise are tremendous. They include control over the distribution in size and dispersion of the nanosize constituents, and tailoring and understanding the role of interfaces between structurally or chemically dissimilar phases on bulk properties. While the properties of the matrix can be improved by the inclusions of nanomaterials, the properties of the fibers can also be improved by the growth of nanotubes on the fibers. The combination of the two will produce super-performing materials, not currently available. Since the improvement of fiber starts with carbon nanotube grown on micron-size fibers (and matrix with a nanomaterial) to give the macro-composite, this process is a bottom-up “hierarchical” advanced manufacturing process, and since the resulting nanocomposites will have “multifunctionality” with improve properties in various functional areas such as chemical and fire resistance, damping, stiffness, strength, fracture toughness, EMI shielding, and electrical and thermal conductivity, the resulting nanocomposites are in fact “multifunctional hierarchical nanocomposites.” In this paper, the current state of knowledge in processing, performance, and characterization of these materials are addressed.

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