Since the first discovery of carbon nanotubes (CNTs) in 1991, a window to new technological areas has been opened. [1] One of the emerging applications of CNTs is the reinforcement of composite materials to overcome the performance limits of conventional materials. [2,3] Recent developments in CNT/polymer composites have shown the potential for improving the strength of polymers, [2] and this finding has encouraged researchers to use carbon nanotubes as reinforcements for metal and ceramic matrices.[4±7] However, because of the difficulties in distributing CNTs homogeneously in a metal or ceramic matrix by means of traditional composite processes, it has been doubted whether CNTs can really reinforce metals or ceramics.[4±7] Here we report on a CNTreinforced Cu matrix nanocomposite, fabricated by a novel fabrication process called ªmolecular-level mixingº; this nanocomposite shows extremely high strength, several times higher than the matrix. The novel process for fabricating CNT/Cu composite powders involves suspending CNTs in a solvent by surface functionalization, mixing Cu ions with the CNT suspension, drying, calcination, and reduction. This process produces CNT/Cu composite powders, whereby the CNTs are homogeneously implanted within the Cu powders. The CNT/Cu nanocomposite, consolidated by spark plasma sintering of CNT/Cu composite powders, is shown to possess three times the strength of the Cu matrix and to have twice the Young's modulus. This extraordinary strengthening effect of carbon nanotubes in metal is higher than that of any other reinforcement ever used for metal-matrix composites. Several researchers have attempted to fabricate CNT-reinforced metal-or ceramic-matrix composite materials by COMMUNICATIONS

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