Synthesis Of Carbon Nanotube In Aluminum Powders By Polymer Pyrolysis Chemical Vapor Deposition

Carbon nanotubes (CNTs), tubular structures of coaxially woundgraphite nanosheets, exhibit good stability and excellent mechanical,electrical and thermal properties. They are the strongest fibers everknown and have been regarded as the most ideal reinforcement forcomposite materials with metallic, ceramic and polymeric matrices.In recent years, CNT-reinforced metal matrix composites (MMCs)have been studied intensively as the next generation of lightweightstructural material for applications in aerospace, defense and automotiveindustries. Many methods have been developed to incorporate CNTs intometal powders to supply raw materials for powder metallurgy (PM)fabrication of CNT-reinforced MMCs, either by mechanical mixingmethods such as high-energy ball milling, friction stir processing,accumulative roll bonding, or by in situ synthesizing CNTs by means ofchemical vapor deposition (CVD). While structural damage to CNTs isinevitably induced by mechanical impaction during mechanical mixing, direct growth of CNTs in metal powders is able to achieve uniformdistribution of undamaged CNTs, thus better property enhancement couldbe expected. However, the traditional CVD processes have twodrawbacks in the case that reactive metal powders such as Al are used assubstrate to grow CNTs, one is the hydrolysis of Al when impregnatedwith the catalyst precursor solution, and the other is the risk of explosionwhen flushed with the flammable gases of carbon source. Therefore, newfabrication route which can not only guarantee homogeneous dispersionof CNTs in Al matrix, but maintain the morphology of Al and CNTsneeds to be explored.To overcome the disadvantages of traditional CVD, a novel polymerpyrolysis CVD was explored to synthesize CNTs in Al nanoflakes usingthe pyrolysis gases of polyethylene glycol (PEG) as the carbon source. Inaddition, PEG also acts as corrosion inhibitor to restrict the hydrolysis ofAl during solution impregnation, and steric hindrance to restrict thecoarsening of Co catalytic nanoparticles. Moreover, by integrating severalsub-models, a model of polymer pyrolysis CVD for the growth of CNTsin Al powders within a closed batch reactor has been established.Through demonstrating the effect of processing parameters, the CNTgrowth mechanism by polymer pyrolysis CVD has been clarified.Compared with the existing in situ method, the batch reaction modeand the relatively lower synthesis temperature (typically600oC) qualify the PPCVD method as a safe and easy way to scale up the synthesis ofCNT/Al composite powders for industrial production. Moreover, thepyrolytic products of PEG served as the reducing agent for Co catalystformation and the carbon source for CNT growth, thus avoiding the directusage of explosive and flammable hydrogen and hydrocarbon as in CVD.Furthermore, this method can not only be applied in Al matrix but also inother metal matrix to in situ fabricate CNTs, like copper and magnesium.