1. Superconductivity Research Laboratory, Shibaura 1-16-25, Minato-ku, Tokyo 105-0023, Japan
2. Railway Technical Research Institute, 2-8-38 Hikari-cho, Kokubunji-shi, Tokyo 185-8540, Japan

Correspondence to: Masaru Tomita^1,2 Correspondence and requests for materials should be addressed to M.T. (e-mail: Email: tomita@rtri.or.jp)

Abstract

Large-grain high-temperature superconductors of the form RE-Ba-Cu-O (where RE is a rare-earth element) can trap magnetic fields of several tesla at low temperatures, and so can be used for permanent magnet applications^{1, 2}. The magnitude of the trapped field is proportional to the critical current density and the volume of the superconductor^{3, 4}. Various potential engineering applications for such magnets have emerged^{5, 6, 7, 8, 9, 10, 11, 12, 13}, and some have already been commercialized^{7, 8, 9, 10}. However, the range of applications is limited by poor mechanical stability and low thermal conductivity of the bulk superconductors^{14, 15, 16, 17}; RE-Ba-Cu-O magnets have been found to fracture during high-field activation, owing to magnetic pressure^{14, 15, 16}. Here we present a post-fabrication treatment that improves the mechanical properties as well as thermal conductivity of a bulk Y-Ba-Cu-O magnet, thereby increasing its field-trapping capacity. First, resin impregnation and wrapping the materials in carbon fibre improves the mechanical properties. Second, a small hole drilled into the centre of the magnet allows impregnation of Bi-Pb-Sn-Cd alloy into the superconductor and inclusion of an aluminium wire support, which results in a significant enhancement of thermal stability and internal mechanical strength. As a result, 17.24 T could be trapped, without fracturing, in a bulk Y-Ba-Cu-O sample of 2.65 cm diameter at 29 K.