Second Generation Superconducting Cable with Exfoliated YBCO Filaments (ExoCable™)
The advent of second generation (2G) YBa2Cu3O7 (YBCO) wire technology has spawned impressive technological progress since the first meter of a 2G wire was manufactured in 1995. Further developments in the field have been driven by existing and emerging applications, such as fault current limiters, transformers, and wind turbines . The 2G wires have the record high upper critical field and critical temperature, potentially enabling design of high-temperature superconducting magnets, which could be cooled with inexpensive single-stage cryo-coolers. The core of the 2G wire technology can be described as a thin, <2 μm, YBCO layer deposited on a 50-100 μm thick metal substrate. In order to protect the substrate from oxidation a 30-40 nm, oxide stack is deposited on the substrate. The following figure shows the sequence of epitaxial layers in a RABiTS-based 2G wire (AMSC product) and the right panel shows the actual cross-section of an AMSC 344 conductor.
Cross-section of 344 2G conductor (© American Superconductor Corp.)
There are several problems with this architecture as far as magnet applications are concerned:
- High aspect ratio, > 1:1000, is the source of high magnetization (AC) losses, which can be as high as 10’s of J/m of wire.
- The superconducting layer is insufficiently stabilized because only the top stabilizer layer is in good electrical contact with the superconductor.
- Highly anisotropic mechanical properties. 2G wires are exceptionally strong in the direction along the tape, the tensile axial yield strength is approximately 500-600 MPa, however the c-axis pull (transverse) strength is > 10 times lower and the cleavage strength is almost negligible, < 1 MPa.
- The HTS tape allows only for pancake-type winding, which involves labor-intensive diagonal splicing.
- The geometry does not allow for a simple splicing of a multi-strand cable.
BTG is developing a novel symmetric high-temperature superconducting (HTS) YBCO cable. The technology uses the following steps: (i) exfoliation of epitaxial YBCO layer from the substrate, (ii) slicing (laser or mechanical) the filaments and (iii) bundling them into a narrow, 2-3 mm wide, cable. The new cable will feature: (i) 1:1 aspect ratio; (ii) a factor of 5 AC loss reduction, up to 20% fill factor and isotropic predictable mechanical strength (the same axial and transverse yield strength, the cleavage strength > 50 MPa). This is achieved by eliminating of the problematic YBCO-buffer interface, which is the weak point of the 2G wire architecture, both electrically and mechanically.
Comparison of the geometries of the traditional 2G conductor and BTG wire