Preparation of Carbon Nanotube Encapsulated Copper Nanowires and Their Use as a Reinforcement for Y-Ba-Cu-O Superconductors Gan Lin Hwang, † Kuo Chu Hwang,* ,† Yeong-Tarng Shieh,* ,§ and Su-Jien Lin ‡ Departments of Chemistry and Material Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, and Department of Chemical Engineering, National Yunlin University of Science and Technology, Yunlin 640, Taiwan Received July 23, 2002. Revised Manuscript Received November 14, 2002 Copper nanowires with multiwall carbon nanotubes (MWNTs) encapsulated inside were prepared via surface modification of MWNTs followed by chemical deposition of copper on surfaces. These MWNTs-encapsulated copper nanowires were investigated as a reinforcement to improve the mechanical strength of the Y-Ba-Cu-O superconductor. TEM and SEM reveal that MWNTs are uniformly coated by copper and the diameters of the prepared MWNTs-encapsulated Cu nanowires range from 50 to 150 nm and the lengths are up to several micrometers. These MWNTs-containing Cu nanowires could effectively improve the hardness of the Y-Ba-Cu-O without sacrificing its superconductivity. The Vicker’s hardness of the Y-Ba-Cu-O is reinforced by ∼20% after 5 wt % of MWNTs incorporation. This study presents the first result that demonstrates the feasibility of the reinforcement of brittle YBaCuO superconductor by using MWNTs. Introduction Since Iijima discovered multiwall carbon nanotubes (MWNTs) in 1991, 1 MWNTs have drawn much research attention to their application potentials. MWNTs have many uncommon physical properties, such as high mechanical strength and toughness, and are thus sug- gested to be a promising reinforcement additive for composite materials. 2-4 To be used effectively as a reinforcement for composite materials, MWNTs should be homogeneously dispersed in the composite matrix and the interfacial adhesion between MWNTs and the matrix should be strong. 5 Unlike some other reinforce- ments, MWNTs are not soluble in solvents that can help disperse MWNTs in the matrix, and thus MWNTs tend to form bundles or aggregate together during the preparation of the composite. In addition, those liquids with surface tensions higher than 100-200 mN/m do not wet MWNTs. 6 This excludes the use of most metal elements as a wetting agent for MWNTs to be dispersed homogeneously in a ceramic matrix 7 and to increase the adhesion (or compatibility) between MWNTs and the matrix. 8 It has been found that some metal elements such as Ti and Ni having many or few d-vacancies were able to effectively deposit on the single-wall nanotubes (SWNTs) while other metal elements such as Au and Al having no d-vacancies were not. 8,9 This was at- tributed to the existence of strong interaction between Ti (and Ni) and SWNTs leading to partial covalent bonding between the metals and carbon atoms. Au and Al with negligible affinity for carbon interact weakly with SWNTs through van der Waals forces, resulting in an ineffective deposition on SWNTs. Cu chosen as a compatibilizer in this study has no d-vacancy and is not expected to have a strong interaction with carbon according to previous work. 9,10 To serve as an effective compatibilizer, thus, Cu needs a different way of de- positing on the outer surfaces of MWNTs. Our approach now is to deposit Cu on MWNTs 11 through a chemical reaction to address the bonding and wetting issues. The Y-Ba-Cu-O compound system having a high superconducting temperature (T c ) at 93 K was first discovered by Chu and Wu and co-workers in 1987. 12,13 This superconductor, however, is hard but brittle, and its applications are restricted. To improve the mechan- ical strength of the compound and develop its applica- tions, reinforcement additives may be applied. The * Corresponding authors. E-mail: kchwang@mx.nthu.edu.tw (K.-C. Hwang) and shiehy@yuntech.edu.tw (Y.-T. 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