Advanced Energy Materials Group
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
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
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
A shortcoming of 2G conductors is relatively
low engineering current density, Je. Current values of 200–500 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.
Last Modified: February 3, 2010
Please forward all questions about this site to: