Image: Institute of Physics |
In a study published in Superconductor Science and Technology,
researchers have examined bismuth strontium calcium copper oxide (Bi2Sr2CaCu2Ox,
Bi2212)—one of the most promising superconducting materials capable of creating
large magnetic fields way beyond the limit of existing magnets—and found that
its capabilities are limited by the formation of bubbles during its fabrication
process.
Bi2212 is the only high-temperature superconductor
capable of being made into round wire, providing the preferred flexibility in
magnet construction, and giving it potential uses in medical imaging and
particle accelerators, such as the Large Hadron Collider in Switzerland.
For magnet applications, these wires must exhibit a high
critical current density—the current density at which electrical resistance
develops—and sustain it under large magnetic fields. This remains a stumbling
block for utilising the huge potential of Bi2212 in the magnet technology as
compellingly high critical current densities have not yet been achieved.
Previous studies have shown that a critical current
varies widely between Bi2212 wire lengths—the critical current in wires that were
50 to 200 m long was 20 to 50% lower than in 5 to 10 cm long samples. This led
the researchers, from the Applied Superconductivity Centre and the National
High Magnetic Field Laboratory, Florida State Univ., to conclude that this
variability must be caused by the connectivity of Bi2212 grains within the
wires.
Bi2212 wires, made up of multiple filaments, are
fabricated using the powder-in-tube (PIT) method in which Bi2212 powder is
packed inside silver tubes and drawn to the desired size. The filaments of
Bi2212 powder must firstly be melted inside their silver sheath and then slowly
cooled to allow the Bi2212 to reform, greatly enhancing the critical current
density.
As the processes between the critical melt and
re-growth step is still largely unknown, the researchers decided to rapidly
cool samples at different times in the melting process in order to get a
snapshot of what occurs inside Bi2212 wires.
Using a scanning electron microscope and synchrotron x-ray
microtomography, the researchers observed that the small powder pores, inherent
to the PIT process, agglomerate into large bubbles on entering the melting
stage.
The consequences of this are major as the Bi2212
filaments become divided into discrete segments of excellent connectivity which
are then blocked by the residual bubbles, greatly reducing the long-range
filament connectivity, and strongly suppressing the flow of current.
The new findings suggest that a key approach to
improve the critical current density of the material would be to make it denser
before melting.
Lead author Dr Fumitake Kametani, of The Applied
Superconductivity Centre, Florida State Univ., said, “Our study suggested that
a large portion of bubbles originates from the 30 to 40% of empty space,
inevitable in any powder-in-tube process, which requires particle rolling to
allow deformation of the metal-powder composite wire.
“Densification of the filaments at final size—increasing
the powder-packing density from 60 to 70% to greater than 90%–is an excellent
way to reduce or eliminate the bubble formation. Various densification
processes are now being tested.”
