| Carbon nanofibers (CNFs), a kind of discontinuous graphite fiber prepared by pyrolysis of vapor-phase hydrocarbon, is a new type submicron reinforcing material arisen in recent years. As a kind of materials with quasi-one-dimensional structure, CNFs possess properties of vapor grown carbon fibers and carbon nanotubes, and their structures and applications are similar to that of carbon nanotubes. They are promising candidates in preparation of high-performance polymer composites and functional composites due to their unique physical and chemical properties. However, the applications of CNFs have been greatly limited by their poor dispersion in the polymer induced by the strong interaction between the fibers. In this dissertation, starting from the surface treatment of CNFs, the mechanical properties, reinforcing and toughening mechanism, electrical properties and the resistance- strain properties of CNFs composites were studied systematically. And the main results in this dissertation are summed as follows:(1) Concentrated acidification (vitriol / nitric acid, 1:3) , plasma oxidation and 60Coγirradiation technology were applied to the surface treatment of CNFs. All of the three methods could reduce tangling degree of the CNFs, decrease the quantities of the residual catalyst particles and impurities, and introduce a mount of hydroxyl and carboxyl functional groups onto the surface of CNFs, which benefits to disperse them into the matrix and enhance the interface bonding between the carbon nanofibers and matrix. As a result, the critical fracture toughness factors of composites were greatly improved. When the CNFs content was 0.5wt%, the toughness of the composite containing plasma oxidation treated CNFs was improved by 44.6% compared with that of pure epoxy resin, and by 25.3% compared with that of untreated CNFs/epoxy composites.(2) Base on the inspiration from the microstructure of abalone, multi-wall carbon nanotubes (MWCNTs) and organophilic montmorillonite(oMMT) were used to toughen epoxy resin. We found that MWCNTs and oMMT exhibited synergistic effect on improving the critical stress intensity factor of the pure epoxy. X-ray diffraction analysis and SEM observations showed that MWCNTs were dispersed well in the matrix and a certain degree of platelet interlocks structure was achieved. The fracture surface micrographs prove that micro-cracks, cavities and fiber pull-out are responsible for improving the toughness of MWCNTs -oMMT / epoxy resin composites.(3) Four electrodes and constant current method was used to measure the electrical conductivity of CNFs/epoxy composite. It showed that the resistivities of CNFs/epoxy nanocomposites decreased in geometric progression with the increasing of CNFs, and the threshold ranged between 0.1%～0.2wt%. The resistivities of CNFs/epoxy nanocomposites observed the Ohm's law well when the current ranges between 0.2-1.2μA. Both positive temperature coefficient (PTC) effect and negative temperature coefficient (NTC) effect could be found at elevated temperature, and good stability and repeatability were shown.(4) Four points bending method and periodic tensile experiment method were carried to investigate the resistivity-strian relationships of CNFs/epoxy composites sensors. The results showed that composite sensors had good static strain-sensing abilities. With the increasing of CNFs content, the linearity of the CNFs/epoxy composites sensor rose while the gauge factor descended. And incorporating graphite into the, system at the same time improved the linearity of the sensor but decreased the gauge factor. Except that above, both acid treatment andγ-ray irradiation treatment could improve the stability and repeatability of CNFs/epoxy composites sensors, and better results were shown for acid treated samples.
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