356
F. Bordry et al.
8.1.3.1.4 Cuprate Superconductors
These are the two HTS materials that seem to bear the largest potential for future
use in accelerator magnets, mainly because of the high critical field at low operating
temperature that makes them the ideal candidates to break the 20 T barrier. Both
BSCCO and REBCO are ceramic compounds obtained respectively by chemical
synthesis of B, Sr, Ca, Cu and O, or Y, Ba, Cu and O. As for all other HTS
materials, their production is a complex process that may involve assisted crystal
growth, at high temperature, and under very well controlled chemical conditions.
BSCCO and REBCO still have unresolved engineering issues, they are fragile, and
have comparatively high material and production costs. One of the main limitations
to critical current in HTS materials comes from the effect of grain boundaries.
Mis-aligned grain boundaries are a barrier to the free flow of the super-currents.
In principle, this means that HTS conductors would require solving the tantalizing
task of growing single crystals of km length. Different ways have been found to
deal with this limitation in BSCCO and REBCO. BSCCO is manufactured using a
Powder in Tube (PIT) technique, filling Ag tubes with a precursor BSCCO oxide
powder. Two different techniques are used to produce the 2212 and 2223 variants
of the BSCCO compound. In the case of BSCCO-2212, the tubes are stacked and
drawn to wires that can be wound, cable and finally reacted, much like Nb 3 Sn, albeit
at higher temperature (900 ◦ C) and under controlled oxygen atmosphere. BSCCO2212 undergoes a melt process, critical to produce the connected grain structure
required for high critical current. For BSCCO-2223 the common form is tape. An
initial liquid-mediated reaction of the precursor is used to form the 2223 compound.
In this case the required grain alignment cannot be obtained upon heat treatment,
but can be induced by mechanical deformation. This is why reacted BSCCO-2223
is mechanically deformed to the final tape dimension, and undergoes a final sintering
heat treatment. REBCO is also manufactured in the form of tape. A robust substrate
such as stainless steel or hastelloy, is polished and prepared with a series of buffer
layers that imprint a crystal texture to a thin layer of REBCO superconductor. This is
the crucial step to induce aligned crystal growth of the superconducting phase. The
superconductor growth can be achieved by various chemical or physical deposition
techniques. The tape is then capped with a sealing Ag layer, finally adding Cu as
a stabilizer. When cooled down to 4.2 K, REBCO and BSCCO attain exceptional
current densities at high-field, surpassing the performance of LTS materials at fields
of 10 to 15 T. In spite of the early phase of development, the HTS BSCCO-2223
already found a large-scale application in the current leads of the LHC, where the
gain in operating efficiency and margin has offset the additional investment.
8.1.3.2 Superconducting Cables
Wires and tapes manufactured with the LTS and HTS materials listed above carry
currents in the range of few hundreds of A, and are appropriate to wind small
magnets, where the magnet inductance and stored energy are not an issue (see later
F. Bordry et al.
8.1.3.1.4 Cuprate Superconductors
These are the two HTS materials that seem to bear the largest potential for future
use in accelerator magnets, mainly because of the high critical field at low operating
temperature that makes them the ideal candidates to break the 20 T barrier. Both
BSCCO and REBCO are ceramic compounds obtained respectively by chemical
synthesis of B, Sr, Ca, Cu and O, or Y, Ba, Cu and O. As for all other HTS
materials, their production is a complex process that may involve assisted crystal
growth, at high temperature, and under very well controlled chemical conditions.
BSCCO and REBCO still have unresolved engineering issues, they are fragile, and
have comparatively high material and production costs. One of the main limitations
to critical current in HTS materials comes from the effect of grain boundaries.
Mis-aligned grain boundaries are a barrier to the free flow of the super-currents.
In principle, this means that HTS conductors would require solving the tantalizing
task of growing single crystals of km length. Different ways have been found to
deal with this limitation in BSCCO and REBCO. BSCCO is manufactured using a
Powder in Tube (PIT) technique, filling Ag tubes with a precursor BSCCO oxide
powder. Two different techniques are used to produce the 2212 and 2223 variants
of the BSCCO compound. In the case of BSCCO-2212, the tubes are stacked and
drawn to wires that can be wound, cable and finally reacted, much like Nb 3 Sn, albeit
at higher temperature (900 ◦ C) and under controlled oxygen atmosphere. BSCCO2212 undergoes a melt process, critical to produce the connected grain structure
required for high critical current. For BSCCO-2223 the common form is tape. An
initial liquid-mediated reaction of the precursor is used to form the 2223 compound.
In this case the required grain alignment cannot be obtained upon heat treatment,
but can be induced by mechanical deformation. This is why reacted BSCCO-2223
is mechanically deformed to the final tape dimension, and undergoes a final sintering
heat treatment. REBCO is also manufactured in the form of tape. A robust substrate
such as stainless steel or hastelloy, is polished and prepared with a series of buffer
layers that imprint a crystal texture to a thin layer of REBCO superconductor. This is
the crucial step to induce aligned crystal growth of the superconducting phase. The
superconductor growth can be achieved by various chemical or physical deposition
techniques. The tape is then capped with a sealing Ag layer, finally adding Cu as
a stabilizer. When cooled down to 4.2 K, REBCO and BSCCO attain exceptional
current densities at high-field, surpassing the performance of LTS materials at fields
of 10 to 15 T. In spite of the early phase of development, the HTS BSCCO-2223
already found a large-scale application in the current leads of the LHC, where the
gain in operating efficiency and margin has offset the additional investment.
8.1.3.2 Superconducting Cables
Wires and tapes manufactured with the LTS and HTS materials listed above carry
currents in the range of few hundreds of A, and are appropriate to wind small
magnets, where the magnet inductance and stored energy are not an issue (see later