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Introduction: Second generation (2G) Y-Ba-Cu-O coated conductor

The second generation (2G) wire is a fundamentally different technology than 1G, the key element being a high-performance 1-2 micron thin YBCO epitaxial layer deposited on a bi-axially textured oxide buffered metal tape. The 2G technology has two key components: (1) a textured template that enables the growth of the biaxially aligned YBCO and (2) the superconducting YBCO layer. Here YBCO stands for the well-known high temperature superconductor YBa2Cu3O7 with Tc = 91 K. Two major approaches used for the textured template include (1) the IBAD process based on the ion beam assisted deposition of a textured oxide layer (i.e., YSZ, GZO, MgO) on a metallic substrate, (2) the RABiTS process based on the deformation and recrystallization texturing of a metal substrate followed by deposition of oxide buffer. The two primary approaches for deposition of the YBCO layers include (1) metal organic deposition (MOD); (2) metal organic chemical vapor deposition (MOCVD). 2G tape is currently produced in U.S. by American Superconductor Corporation who is using RABiTS substrate and deposits YBCO layer by MOD, and SuperPower Inc. (now subsidiary of Phillips Corp.) who is using IBAD (substrate) -MOCVD (YBCO layer) combination.










Structure of YBCO material

Architecture of 2G wire produced American Superconductor Corporation. YBCO layer is deposited on buffered Ni alloy tape by metal-organic deposition.

2G wire contains no precious elements and uses very little superconductor, so the price can be made, at least theoretically, very low. Also YBCO material is 10 times less anisotropic that Bi-2223 resulting in a more robust critical current in strong magnetic fields; the best YBCO layers are still superconducting at 10 Tesla, 77 Kelvin. Better field performance of YBCO-based wires opens a multitude of high-field 77 Kelvin applications, such as motors, transformers, magnets, etc., which were inaccessible to 1G technology:

Critical currents of 1G and 2G wires in an external magnetic field. Note that 2G conductor significantly out-performs 1G wire in field over 1 Tesla. Source: American Superconductor Corp.

A shortcoming of 2G conductors is relatively low engineering current density, Je. Current values of 200500 A/mm2 at 0 Tesla, 77 Kelvin are explained by the small cross-section of conductor occupied by the YBCO layer, 1-2 microns, compared to the metal substrate, ~50 microns, and copper stabilizer, ~50 microns:

Cross-section of a production sample of 2G wire (left) optical micrograph of the substrate-YBCO-copper stabilizer interface (right) . Note that a very small part part (<1%) of the cross-section is occupied by YBCO superconductor (right picture). Source: American Superconductor Corp.

Current 2G materials research is directed towards improving engineering current density Je of the conductor. One obvious way to raise Je is to increase the thickness of the YBCO layer. Unfortunately, simply raising the thickness does not proportionally increase Je. This is because thick, > 1 microns, YBCO layers tend to have inferior structural quality and lower critical current density, Jc, than sub-micron thick films. This trend was clearly illustrated in a recent review of the coated conductor development (Foltyn et al. Nature Materials 6 (2007) p.631):

Thickness dependence of the critical current in YBCO layers synthesized by a variety of methods. Adapted from (Foltyn et al. Nature Materials 6 (2007) p. 631).

Our group effort is focused on enhancing performance of 2G wires by improving structure of YBCO thick layers.

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Last Modified: February 3, 2010
Please forward all questions about this site to: Robert Sundling