The superconducting materials of interest for
engineering applications have a transition temperature,
below which they become superconducting, that is at or
above 77 K, the boiling point of liquid nitrogen, which is
an inexpensive coolant.
According to our Special Topics Analysis of
High-Temperature Superconductors over the past decade, the
researcher at #15 by total number of papers is Professor
David Cardwell, who is credited with 81 papers that have
received 414 citations. He is Professor of
Superconductivity Research in the Department of Engineering
at the University of Cambridge.
In the interview below,
ScienceWatch.com's European correspondent Dr. Simon Mitton
converses with Professor Cardwell about his quest for
superconducting materials with industrial
Where did you do your academic
training, and what attracted you to research in
I was an undergraduate at the University of Warwick, located in the heart
of England. My bachelor degree is in physics, and I remained at Warwick for
a Ph.D., on the topic of inelastic gamma-ray scattering (nothing to do with
superconductivity!). After that, in 1986, I joined one of the UK’s
few remaining industrial laboratories (Plessey Caswell, now part of BAE
Systems) where "blue skies" research was still encouraged. I set to work on
ferroelectric materials, which was somewhat different to the subject area
of my Ph.D.
However, within six months of joining Plessey, the first high-temperature
superconductor (HTS), a cuprate, was discovered. Plessey had a number of
far-sighted individuals who realized the potential of these materials. In
view of my recent knowledge of physics and materials science, I was given
responsibility for starting research on HTS. Within three weeks I was
making my own superconductors, and I never looked back.
Would you like to describe your group in the
Department of Engineering at Cambridge?
I established the bulk superconductivity group in 1992 when the department
appointed me. Currently we are a modest group by any standards: a couple of
post-docs, a couple of doctoral students, and a technician. Our total
funding is relatively small: around $300,000 a year. We’ve always
worked closely with industry and we collaborate extensively throughout
Europe and the US. My philosophy is this: the better the people you work
with the better the research is going to be. I believe in collaboration,
not competition. In 2001 I established an international network spread
across 11 European countries. The majority of my papers are about making
superconducting materials that can generate large magnetic fields.
Why is that important?
Large magnetic fields are used to provide torque in electrical machines, to
enable magnetic suspension, and to improve magnetic resonance imaging.
These are examples of applications where the stronger the field, the bigger
the torque that can be exerted, or the more stable the magnetic levitation.
My group develops materials that can generate large, stable magnetic
So your many HTS papers are devoted to engineering
and materials science rather than the fundamental physics of
While that is correct, this is an area with a large overlap between physics
and engineering. One could define engineering as the application of science
to practical tasks that are perceived to be useful, whereas physics is the
quest to understand the physical world for its own sake. Yes, we have
fundamental physics challenges, but in solving those we aim to contribute
to developing useful devices as well.
To give an example, we investigate how magnetic flux behaves in materials:
in the case of superconductors, the physicists tend to ask about the
mechanism of superconductivity, and how high the critical temperature can
be pushed, whereas we engineers want to know how high an electrical current
the materials can carry, and the highest temperature at which we can get
those currents to flow. The current HTS material with the highest
superconducting transition temperature is pretty useless at carrying
current at a useful temperature!
Why is the development of a viable HTS industry so
Superconductivity is the ability of a material to carry an electric current
without the dissipation of energy. That is a very topical issue,
particularly now given that environmental considerations are so high.
Hitherto economic considerations have been driving the technology, but
environmental concerns are becoming increasingly important. With
superconductivity there are clear potential benefits to the environment. I
cannot see any other class of materials even coming close to the potential
environmental benefits of HTS.
What examples can you give of environmental
Up to 20% of the energy can be lost in the transmission electrical energy
from a power station to the point of use. If you consider energy storage
devices using superconductors, they enable the generated energy to be
managed better, enabling its release when and where it is needed.
Superconductors allow electrical motors to be made smaller. And guess what?
One of the big problems with electrical motors is that large machines
cannot be delivered to certain end-user locations in the UK: they are
simply too heavy to lift by helicopter, and by road are defeated either by
a low bridge or limited road strength.
With superconductors we can invoke an energy density argument: the amount
of energy per unit volume is greatly increased, with a corresponding
decrease in size and mass for a given quantity of energy. You could
potentially use very high-power, high-energy density, compact and
lightweight machines virtually anywhere you like by exploiting
superconductivity. Superconductors can carry about 100 times more current
than copper at 77 K, so if you think about laying superconducting cables it
is possible to increase the current-carrying capacity by a factor of 100,
although, to be fair, we are still some way off developing practical cables
of kilometer lengths.
Considering the HTS materials that are the subject
of your highly cited papers, are any of them now being
Yes. The earliest applications are energy storage flywheels. We have a
collaborative program with Boeing in the US, who is developing their own
energy storage flywheel using magnetic levitation. I should explain that if
you take two dipole permanent magnets, and try to balance north pole over
north pole, the top one inevitably flips over in order to minimize energy,
by pairing north and south poles. Bulk HTS materials generate the magnetic
field in a different way. Unlike permanent magnets, they do not consist of
aligned spins. In a superconductor, magnetic field is applied to the
sample, which induces a current via Faraday’s Law. Removal of the
field causes the current to reverse direction, and it continues to flow
perpetually in the material. Effectively this is a solenoid without a power
A property of a superconductor is that magnetic flux lines exist in the
form of individual filaments within its interior. Furthermore,
superconductors contain pinning centers, which prevent or resist the
movement of these flux lines. When flux penetrates a superconductor,
therefore, it is hard for it to escape. So once you get the magnetic flux
into these materials it wants to stay there. And that means superconductors
can be used for stable magnetic levitation, and no other materials can
achieve that. Bigger trapped flux actually equates to larger currents, so
this all ties together quite well.
"Hitherto economic considerations
have been driving the technology, but
environmental concerns are becoming
We are involved in making bulk samples of superconductors for Boeing in the
form of specialized geometries, or shapes that can be machined. Our aim is
to make large single grains of bulk HTS materials up to several cm in
diameter. The point is that grain boundaries limit current flow within the
material. Generation of a large magnetic field requires the current to flow
over a large area, and therefore we need to replace a large number of
micron-sized grains with just one giant grain.
Five of your top 10 papers in our survey have Dr.
Hari Babu as the first author. Can you tell me about that
Nadendla Hari Babu was a very successful post-doc in my group for eight
years. He joined us in 1998, having pursued a Ph.D. in superconductivity at
the University of Hyderabad, India. He now works on advanced solidification
technology at Brunel University in West London, but maintains an interest
in developing HTS materials for microwave engineering applications with us.
We remain close collaborators. He has been my most productive collaborator
over the years by some margin, as is reflected in your rankings.
Our joint research focuses on solidification, microstructural, processing,
and characterization studies of HTS materials. Among Hari’s many
achievements are the fabrication of a HTS (NdBCO) with an irreversibility
field in excess of 40 T at 77 K, which remains a world record at this
temperature, and the development of a practical processing method to
fabricate high-performance bulk superconductors. Together we have also
successfully developed the technology for fabrication of bulk
superconducting nano-composites that are suitable for high magnetic field
What key advance is reported in your highly cited
joint paper, "Processing and microstructure of single grain,
uranium-doped Y-Ba-Cu-O superconductor" (Babu NH, et al.,
Supercond. Sci. Tech. 15: 104-10, January 2002)?
We describe doping a YBCO superconductor with uranium dioxide to enhance
flux pinning. That paper reports the processing, microstructural features,
and magnetic properties of large-grain YBCO doped with, in this case,
depleted uranium. We succeeded in enhancing pinning for reasons that were
not immediately obvious. When we looked in detail at the material
microstructure, however, we found the presence of unique secondary phase
inclusions that have some remarkable properties that we have subsequently
patented. This phase has the potential to revolutionize the generation of
high magnetic fields by bulk HTS. It’s a phase that retains its
integrity when added to the melt process: it does not get bigger, it does
not react, and, most importantly, the properties of the HTS are not
changed. The phase is about the size of a magnetic flux line so it pins
very well. As I have already said, the better the magnetic flux is pinned
(nailed down!) the better the current-carrying capability of the material.
#1 reports a doubling of the current, just by introducing this phase.
What for you is the unifying thread to the papers
in this Special Topics analysis?
All of these papers are about processing and properties. They report
detailed understanding of the processing of rare earth barium copper oxides
in the form of large grains. For example, phase diagrams show the effect of
certain parameters on processing, which determine fundamentally the
integrity of the materials we produce, their electrical, magnetic, and
mechanical properties, and provide insight into how we can process
materials with better properties
Finally, Professor Cardwell, what are your
priorities right now?
Our goals are to continue to produce good materials but to do so in a
practical way that can be scaled up from laboratory demonstrations to
industrial production. That means developing batch-processing methods for
the manufacture of samples with uniform properties; samples that are
better, bigger, cheaper, and more reliable.
Professor David A. Cardwell
Department of Engineering
University of Cambridge
KEYWORDS: HIGH-TEMPERATURE SUPERCONDUCTORS, BULK
SUPERCONDUCTIVITY, LARGE STABLE MAGNETIC FIELDS, TORQUE, ENVIRONMENTAL
BENEFITS, ELECTRICAL ENERGY, ELECTRICAL MOTORS, CURRENT, APPLICATIONS,
ENERGY STORAGE FLYWHEELS, YBCO, NDBCO, MAGNETIC FLUX.