This paper describes the development of RE-Ba-Cu-O (RE:Y or rare earth elements) bulk superconductors (“bulks”). HTS bulks have a great potential to pioneer a new application field of superconductors. The high-quality HTS bulks developed by Nippon Steel Corporation are called “QMG”, which is a single-grained material of REBa2Cu3Ox (RE123) with RE2BaCuO5 (RE211) inclusions finely dispersed in the superconducting matrix. The name QMG originally comes from the initial process, “Quench and Melt-Growth”. However, no quench step is required in the advanced QMG process, and thus “Q” now means “Quality” instead of “Quench”. The key point of QMG technologies is the simultaneous control of the crystal growth and the micro-structure. Although it is believed to be very difficult to produce large single-grained HTS bulks exceeding 80 mm in diameter, we have succeeded in enlarging QMG up to 150 mm using the RE compositional gradient method. These ultra-large HTS bulks have been used for the superconducting magnetic bearing (SMB) of the flywheel energy storage system in Yamanashi, Japan, contributing to non-contact support of a heavy 4,000 kg rotating flywheel. In addition to SMB, other promising application are HTS bulk magnets for ship propulsion motors, wind or tidal power generators, and desktop cryogen-free NMR/MRI. HTS bulks are also an ideal material for current leads. Different bulk properties are required for different applications. We have therefore customized QMG bulks depending on application requirements: Dy-QMG for current leads, Eu-QMG for NMR/MRI, and Gd-QMG for other HTS bulk applications.
