Fabrication and Characterization of Aluminum Nano-Carbon Composites and Investigation of Their Electrical Conductivity
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Incorporating nano-carbon phases into metal-matrix composites is a promising strategy for simultaneously enhancing electrical conductivity and mechanical properties of metals. Here, we describe the manufacture of novel nano-carbon-aluminum composites by an electro-charge-assisted process (EAP) that show 5.7% increase in electrical conductivity and 8.2% enhancement in hardness compared to the base metal alloy. In this method, a DC current is applied to molten Al metal containing activated carbon particles in an enclosed inert environment. The high current density first facilitates ionization of the carbon atoms and migration of the carbon ions similar to “electromigration in metals”. Then, the current further induces polymerization and formation of graphitic chains and ribbons along preferential directions of the Al lattice, and propagates the graphitic structures inside the Al matrix. The EAP induces crystallization of the amorphous carbon into graphitic nanocarbon network contrary to other methods which start by mixing the metal with either graphene of carbon nanotubes. The EAP incorporates the nanocarbon structures into the metal matrix with strong interaction which is not the case in most other methods, such as friction stir processing, ball milling and others. Raman mapping measurements, X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscope (TEM) and Electron Energy Loss Spectroscopy (EELS) have confirmed that carbon is incorporated more uniformly and forming ordered network of graphitic structures in Aluminum matrix. Both enhancements are attributed to nano-graphitic structures that extend through the lattice of the metal. Through n-type doping from the metal to the nano-structures the electron density at the interface of nano-crystalline graphite and the metal lattice increases thereby enhancing the bulk electrical conductivity. We identify the important fabrication parameters of the EAP for a reaction system employing a tapered graphite cathode. A high current density of 100 A/cm2 causes ionization and crystallization of the carbon in the liquid metal. The increase in electrical conductivity and hardness of the composite are directly related to the incorporation of the nanocrystalline carbon in the metal lattice. The superior performance of these nano-carbon aluminum composites makes them promising candidates for power transmission lines and other applications.