IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 22, NO. 5, OCTOBER 2012 7701605 Sol-gel-Derived Al 2 O 3 −SiO 2 Composite Coating for Electrical Insulation in HTS Magnet Technology Hom Kandel, Jun Lu, Jianyi Jiang, Ke Han, Scott Gundlach, Youri Viouchkov, William Denis Markiewicz, and Hubertus Weijers Abstract—Electrically insulating Al 2 O 3 −SiO 2 thin coatings have been deposited on long-length 316 stainless steel (SS) tape using a reel-to-reel continuous solgel dip coating process for cowinding insulation into YBCO pancake coils, a high- temperature superconductor magnet technology. Coatings with a thickness of ∼5 μm are achieved after just two dips with a tape withdrawal speed of ∼8 mm/s (0.5 m/min) and a calcination at 700 ◦ C. The coatings are measured to have a room-temperature breakdown voltage of ∼200 V, corresponding to a dc dielectric strength of about 40 MV/m. Consequently, this process has low cost and high throughput and produces a thin electrical insulation with excellent thermal, dielectric, and mechanical properties. A new technique has been developed in the coating process to miti- gate Al 2 O 3 particulate aggregation and coating buildup along the edges of the SS tape. Index Terms—Al 2 O 3 −SiO 2 composite coatings, HTS magnet technology, insulation, sol-gel, YBCO coated conductors. I. I NTRODUCTION E LECTRICAL insulation is required in superconducting magnet systems such as the typical high-temperature superconductor (HTS) magnet to prevent the electrical short circuits within the winding of the conductor coils in the magnet solenoid. However, there is a great challenge to develop an effective insulation in HTS conductors for many reasons. First, the insulation must have an excellent dielectric, mechanical, and thermal properties to operate at cryogenic temperatures under large Lorentz forces in high magnetic fields. Second, the insulation has to be very thin to maximize the engineering cur- rent density in the magnet coil. Third, the insulation technique has to be not only feasible in itself but also compatible with the other aspects of magnet design such as high throughput and low cost. Ceramic coatings on metal provide viable option for such insulation requirements because of their very good thermal and electrical (dielectric) properties [1], [2]. Brinker and Scherer Manuscript received January 15, 2012; revised March 8, 2012 and June 1, 2012; accepted June 6, 2012. Date of current version August 28, 2012. This work was supported in part by the U.S. National Science Foundation under Grant DMR-0654118 and in part by the State of Florida. This paper was recommended by Editor in Chief J. Schwartz. The authors are with the National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310 USA (e-mail: kandel@asc. magnet.fsu.edu; junlu@magnet.fsu.edu; jjiang@asc.magnet.fsu.edu; han@ magnet.fsu.edu; gundlach@magnet.fsu.edu; Viouchkov@magnet.fsu.edu; markwcz@magnet.fsu.edu; weijers@magnet.fsu.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2012.2204745 produced silica films having quite high breakdown strengths (up to 600 MV/m) and insulating properties approaching those of thermally grown silica [3]. Schmidt and Wolter reported ormocers for dielectric applications [4]. Similarly, ZrO 2 - and Al 2 O 3 -based ceramic insulation coatings have been reported in the literature [5]–[8]. There are several techniques for coating metals with ceramic layers: physical vapor deposition, chemical vapor deposition, plasma spraying, electrophoresis, and sol-gel. However, sol- gel technology is very simple, easy for operation, cost effec- tive, and a low-temperature open-atmosphere-solution-based coating method. Using this process, it is possible to deposit films and coatings with a thickness from ∼10 nm to several micrometers with advantages of good homogeneity and ease of composition control in the films. The principle of the tech- nology is that a film of the precursor solution is distributed onto a substrate, where it undergoes the sol-to-gel transforma- tion. The gel layer is calcined to remove the volatile organic components and densify the coating. The coatings obtained after the calcination provide a good adherence with metallic substrates. Large planar or axially symmetric substrates may be uniformly coated with a batch or a continuous process, and the system can be easily scaled up for coating kilometers of tapes or wires using a reel-to-reel continuous process. This technology has been successfully used for many applications including the preparation of fine glass and ceramics, organic–inorganic and hybrid coatings, oxide coatings for optical and electronic applications, and thin layers of ceramic insulations for super- conducting magnet applications [6]–[12]. The National High Magnetic Field Laboratory (NHMFL) has started the construction of the 32-T all-superconducting user magnet using a YBCO-coated conductor as high-field insert coil. The design of the insert coil in the 32-T magnet takes advantage of dry-wound double-pancake configuration [13]. The coil will be wound with stainless steel (SS) cowinding tapes which not only provide additional reinforcement but also allow the application of very thin insulation on the SS cowinding tapes for turn-to-turn insulation. The advantage of this scheme is that there is no restriction on insulation process temperature such as in the direct coating on the surface of the YBCO-coated conductors with possible damage by high-temperature coating process. We have coated the 316 SS tape with Al 2 O 3 −SiO 2 through a solgel process where the SS tape is dip coated with a solgel solution prepared by dispersing Al 2 O 3 fine ceramic powders in SiO 2 sol. The coating is then dried at 300 ◦ C and calcined at 700 ◦ C for densification. The coating after calcination is 1051-8223/$31.00 © 2012 IEEE