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The recent application of nanotechnology to structural materials is a promising way to produce new strong materials that exceed current limitations. Grain-size refinement is considered as one of the most effective strengthening methods of materials. [1][2][3][4][5] At the same time, the addition of carbon nanotubes (CNTs) [6][7][8][9] in a material is known to be more effective than conventional reinforcements. Here, CNT/Co nanocomposites were fabricated by reinforcing the CNTs in ultrafine-grained Co matrix via a modified molecular-level mixing process. The CNT/Co nanopowders have a structure resembling a pearl necklace, consisting of Co nanoparticles penetrated by CNTs and were fabricated and then consolidated into CNT/Co nanocomposites using spark plasma sintering. The microstructure of the CNT/Co nanocomposite consisted of a three-dimensional (3D) network of CNTs in ultrafine-grained Co matrix with an average grain size of 300 nm. The CNT/Co nanocomposite showed an outstanding yield strength of 1.5 GPa. This indicates that the synergistic strengthening mechanism of homogeneously dispersed CNTs in an ultrafine-grained-metal matrix could improve the mechanical properties of materials.Since the strength is the most important property for structural metals, many attempts have been made throughout history to fabricate stronger metals. Advances in physics and chemistry have been applied continuously to overcome the limitations on the mechanical performance of metals. The recent emergence of nanotechnology has also contribut-ed to the development of strong structural metals. Grainsize refinement to fabricate ultrafine-grained or nanocrystalline metals [1][2][3][4][5] and reinforcement with CNTs [6][7][8][9] are known to be one of the most promising methods to strengthen the metals through nanotechnology. Among these, grain-size refinement has characteristic features. [1] The strength of metals increases with decreasing grain size according to the well-known Hall-Petch relationship. When the grain size of metals reaches a submicrometer-and nanoscale, [1][2][3][4][5] very high strengths can be obtained compared to coarse-grained metals with the same chemical composition and phase constitution. There are several ways to fabricate ultrafinegrained or nanocrystalline metals, including the sintering of nanoparticles and the severe deformation of bulk metals. [1][2][3][4][5] Compared to refinement of metal grain size into ultrafinegrained or nanocrystalline ranges, it is difficult to add CNTs to a metal matrix due to the strong aggregation of CNTs and the weak interfacial strength between the metal matrix and the CNTs. [6][7][8] There is therefore some doubt about the strengthening ability of CNTs in a metal matrix. However, with the development of the molecular-level mixing process, [9] in which CNTs are mixed with a metal matrix in the disperse state in a solvent to form CNT-implanted composite powders, it has been observed that the strengthening efficiency of CNTs in a Cu matrix is extraordinarily high. The CNTs have a ...

Pearl‐necklace‐structured CNT/Co nanocomposite powders, with spherical Co nanoparticles threaded by penetrating CNTs, have been fabricated using a molecular process. The nanocomposite powders have been screen‐printed and sintered into CNT‐implanted Co nanocomposite emitters, with the CNTs standing upright on the surface of the substrate. The CNT‐implanted Co nanocomposite emitters show outstanding field‐emission behavior, including a low turn‐on electric field and high current density.

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