high temperature superconductors take off

8/2/2019 High Temperature Superconductors Take Off

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NEW APPROACHES

High temperature

superconductors take off

Colin GoughUniversity of Birmingham Superconductivity Research Group,Birmingham, UK

This article describes the progress made

towards real engineering applications ofhigh temperature superconductors (HTS)in the ten years following the Nobel Prizewinning discovery by Bednorz and Mullerin August 1986. Examples include HTSwires and tapes for more efficient andpowerful electric motors and forincreasing the electrical power into theheart of modern cities, HTS permanent

magnets for levitation, microwave filtersfor cellular telephone networks, SQUIDs(superconducting quantum interferencedevices) to monitor foetal heart and brainsignals, and a new generation of superfastlogic devices based on the flux quantum.

It is just over ten years since Bednorz and Muller,

at the IBM Research Laboratories in Ruschlikon

near Zurich, announced the discovery of high

temperature superconductivity. Within a year this

discovery had won them the 1987 Nobel Prize

for Physics and had triggered an explosion of

research activity worldwide that continues to this

day. This article gives an up-to-date account

of the remarkable progress that has been made

towards engineering applications of both HTS and

conventional superconductors.

Applications of superconductivity extend over

a very wide range of fields: from large-scale

power engineeringsuperconducting wires and

cables for the electrical power industry, super-

conducting magnets for more efficient electrical

motors, levitated trains in ultra-fast intercity

This article is based on a lecture given at the Annual British

Association Meeting held in September 1996 at Birmingham

University.

transport, and magnetic separation of minerals,to small-scale devicesSQUIDs (superconductingquantum interference devices) to monitor foetalheartbeats and magnetic signals from the brain,microwave filters to enhance the performance ofcellular telephone and microwave communicationsnetworks, and a new flux quantum logic with animprovement of almost three orders of magnitudein both speed and power consumption overexisting silicon-based logic.

Superconductivity

Superconductivity was first discovered 85 yearsago by Kamerlingh Onnes in Leiden andinvolves the complete loss of electrical resistancewhen certain metals are cooled below theircritical temperature Tc. Kamerlingh Onnesimmediately recognized the potential importanceof such materials for electrical engineering.Superconducting power cables could significantlyreduce the energy lost in distributing electricalpower around the country, superconducting coil

windings for electric motors and generators wouldmake them far lighter and more efficient, andelectrical power could even be stored in short-circuited superconducting coils, around whicha current could flow indefinitely. However,for over half a century superconductivity wasconfined to operating temperatures a few degreesabove absolute zero (273 C), requiring theuse of liquid helium as a coolant. Even worse,it was quickly discovered that the magneticfield produced by the current flowing througha superconducting wire was often sufficient todestroy its superconductivity altogether.

It was not until the late 1950s, when new kindsof superconductors mostly based on niobium al-loys were developed, which were far less sensitive

38 Phys. Educ. 33(1) January 1998


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to the effect of magnetic fields, that superconduc-

tivity became a viable technology. However, such

superconductors still had to be cooled with liquid

helium, which remains relatively expensive and

inconvenient. Nevertheless, these new alloy su-

perconductors were used extensively for supercon-

ducting magnets in research laboratoriesto study

the properties of electrons in metals, magnets and

semiconductorsand on an even larger scale in

elementary particle accelerators like the supercon-

ducting linear accelerator (SLAC) at Stanford, Cal-ifornia, and at CERN in Genevato investigate

the fundamental properties of matter itself. The

real commercialization of superconductors took

place around 15 years ago, with the development

and manufacture of superconducting magnets suf-

ficiently large to envelop a human body; these

are used for magnetic resonance imaging (MRI)

systemscurrently a $3 bn/year industry. When-

ever a large magnetic field is required over a large

volume (e.g. to contain the plasma in any future

fusion reactor), superconducting magnets are the

only viable option.

Within six months of Bednorz and Mullers

original discovery of the cuprate superconductors,

Paul Chu and his colleagues in the USA had

discovered a HTS compound YBa2Cu3O7 (YBCO)

with a superconducting transition well above

liquid nitrogen temperatures (196 C or 77 K).

This was excitedly heralded as a discovery

that would completely transform electronic and

electrical engineering by the turn of the century!

Unfortunately, the initial euphoria was rapidly

tempered by the realization that the new ceramic

superconductors were extremely brittle, so they

could not easily be turned into wires andcables. Worse still, they suffered from the same

problem as the first original superconductors to be

discoveredsuperconductivity was destroyed by

fields only a few times greater than the Earths

magnetic field.

Nevertheless, over the last ten years remark-

able progress has been made in overcoming

the problems of brittleness, with a million-fold

increase in superconducting properties and an

equally dramatic reduction in their field depen-

dence. The technology of making superconduct-

ing wires and tapes has advanced rapidly and sev-

eral companies in the USA and Japan are alreadymaking kilometre lengths of relatively high per-

formance, flexible, HTS conductors, with a perfor-

mance already sufficient to facilitate a large num-

ber of promising demonstrator projects illustrating

the potential applicability of these materials to real

engineering systems.

Targeted applications currently include super-

conducting windings to improve the efficiency of

electrical motors, flexible superconducting cables

to deliver electrical power into the heart of major

cities in the USA and Japan, and bulk HTS with

magnetic properties almost ten times stronger than

the most powerful permanent magnets known. Onthe device side, HTS SQUIDs have already been

on the market for over five years, and HTS mi-

crowave filters are being successfully developed

for several advanced information technology and

communications systems, including cellular tele-

phone systems in the USA.

The term high temperature superconductors,

used by Bednorz and Muller in their original 1986

paper, is of course something of a misnomer.

We still need to cool such superconductors to

around liquid nitrogen temperatures for most

practical applications. This even remains true

for the most recently discovered mercury-based

cuprate superconductors, which have a world

record Tc under ambient pressure of 135 K

(almost halfway to room temperature), rising to

165 K (108 C) under pressure. Figure 1 shows

the complex crystal structure of this material.

The superconducting properties are believed to be

largely associated with the parallel planes of CuO2within this structure.

Cryogenic refrigeration technology has also

advanced rapidly in recent years and electrically

driven refrigeration units to cool superconducting

magnets down to liquid helium temperatures arenow commercially available. Liquid nitrogen or

helium is no longer required for many applications

of superconductivity. In addition, one of the

most unglamorous properties of the new ceramic

HTStheir poor thermal conductivityhas led to

perhaps the most unexpected but possibly the most

important recent advance in cryogenic technology.

Simply replacing the copper current leads for a

conventional superconducting magnet operating at

liquid helium temperatures with HTS material can

reduce the heat input by a factor of about ten.

The magnets can then easily be cooled by an

electrically driven closed-cycle refrigerator unitinstead of having to use liquid helium. This

allows powerful superconducting magnets to be

Phys. Educ. 33(1) January 1998 39


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Figure 1. The structure of HgBaCaCuO which hasa Tc of 135 K at ambient pressure and 165 K athigh pressures.

used for the separation of magnetic ores in large-

scale mining operations, even in the depths of far-

off jungles, simply by driving the cooling systemby a diesel generator.

High temperature superconductors will un-

doubtedly play an important role in 21st century

technology: the only question is the extent to

which they will replace well-established existing

technology and penetrate the market.

High temperature superconductors

High temperature superconductors are all based

on perovskite cuprate structures, which contain

planes of CuO, like the Hg-based cupratesuperconductor illustrated in figure 1. Chemists

have already synthesized literally hundreds of such

superconductors. Nevertheless, early applications

are likely to be based on some of the first HTS

families to be discoveredYBCO with a Tc of

93 K and the BSCCO family of compounds (e.g.

Bi2Sr2Ca2Cu3O10) with Tcs greater than 100 K,

first reported by Maeda in February 1988 but also

patented at about the same time by researchers at

Hoechst in Germany.

Perovskite superconductors are by their very

nature brittle and are most easily grown as rather

granular ceramic materials, rather like a coarseblack china. The poor performance of early HTS

was entirely due to the highly granular nature

of such material. Technological progress has

resulted from the ability of materials scientists

to make use of the natural, often mica-like,

properties of the highly anisotropic perovskite

structures. Todays HTS are fabricated with

the crystal grains highly alignedeither as large

single-crystal grains or as highly aligned thick

and thin films. Superconducting currents can then

flow easily from one grain to the next along the

CuO planes. Flexible wires can be fabricated

by pressing the raw ceramic material into silver

tubes, which are then drawn out and rolled or

stamped to align the crystal grainsthe powder-

in-tube (PIT) method. The brittleness of the HTS

is no more serious a problem than the brittleness of

glassremember that our telephone conversations

are transmitted all around the world by a network

of flexible optical fibres which would stretch to

stretch from the Earth to the Moon if laid out

straight.

Very high performance epitaxial thin films

for device applications, carrying currents > 106

A cm2

at liquid nitrogen temperatures, can begrown relatively easily, simply by depositing the

appropriate composition of cations on a heated

substrate. Nature then takes over and, remarkably,

self-assembles the molecules into their complex

crystal structures. A wide variety of thin film

technologies can be used including pulsed laser

deposition and MOCVD (metal organic chemical

vapour deposition), which is particularly suitable

for large-scale manufacturing processes.

Two properties of superconductors are impor-

tant for practical applicationstheir zero resis-

tance and their wave-mechanical behaviour. The

latter is important for a number of device appli-cations. It arises because superconducting elec-

trons have to be described as waves rather than



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as single particles. Now a wave of superconduct-

ing electrons circulating in a ring has to join its

head to its tail exactly, otherwise it will interfere

destructively with itself on subsequent orbits. Just

as the essential wave nature of electrons leads to

the quantization of energy levels in an atom, the

current associated with superconducting electrons

circulating a ring can only assume specific values.

This results in the quantization of magnetic flux

passing through the ring (magnetic flux is mag-

netic field multiplied by the area enclosed). Thiswas first confirmed for HTS in Birmingham early

in 1987, within a few months of their discovery.

Flux is quantized in units of h/2e = 2 1015

T m2 (h is Plancks constant and e the electronic

charge), corresponding to about a millionth of the

Earths magnetic field passing through a wedding

ring.

Devices based on the quantum of flux

are called SQUIDs (superconducting quantum

interference devices). HTS SQUIDs are already

sufficiently sensitive to monitor foetal heart beats

and are the most sensitive sensor known to man, as

we will illustrate later. Another application basedon flux quantization that we will describe is a novel

logic systemSFQL (single flux quantum logic),

where the flux quantum is used as the primary bit

of information in a computing system.

Selected applications

As there are so many potential applications to

choose from, I have had to be very selective

and will confine the rest of this article to a few

examples in just a little more detailmagnetic

levitation and permanent magnets, wires formotors, magnets and power distribution systems,

microwave filters for cellular telephone networks,

SQUIDs as sensors of foetal heart beats and brain

signals, and Single Flux Quantum Logic.

Magnetic levitation and permanent magnets

The magnetic levitation of a superconductor above

a magnet remains one of the most potent symbols

of superconductivity, see figure 2. Photographs

of levitated HTS samples appeared on the front

of the Annual Report of very many Japanese

companies in the first years of HTSconsistentwith their general mission, Master materials

today and capture global markets tomorrow.

Figure 2. One of the first demonstrations of aliquid nitrogen-cooled YBCO HTS superconductingsample floating above a permanent magnet.

Murakami, at the Centre for Superconductivity(ISTEC) in Tokyo, currently has a contract worth>$3 M/annum from Japanese Railways to developHTS materials for magnetic levitation, ultimatelyfor mass transport systems. The performanceof bulk HTS materials for levitation continuesto improvein 1994 the Japanese succeeded in

levitating a goldfish (in its bowl of water) andlast year they levitated a sumo wrestler! Ifthe sumo wrestler were to be spun on his axis,a large amount of energy could be stored asrotational kinetic energy. This is the principleof the Levitated HTS Superconducting Flywheel,which has already been demonstrated by a Germancompany. This can be used instead of a battery tosupply electrical power in the case of temporarypower failure, already storing sufficient energy torun a 1 kW heater for about 20 minutes. Muchlarger flywheel storage systems are envisaged.

In Japan commercial MAGLEV trains are be-

ing developed using conventional superconduct-ing magnets to levitate trains travelling faster than300 mph (480 km h1) between Tokyo and Osaka,as illustrated in figure 3. The first two-carriagetrain and 26 1

2miles (42 km) of track (the Ya-

manashi Test Line) have already between built andsuccessfully operated. Each carriage carries fourvery large superconducting magnets cooled to liq-uid helium temperatures by an electrically poweredrefrigeration unit. As the train moves forward, theeddy currents set up by the magnets skimmingover the aluminium track cause the train to belevitated about 10 cm above it. Superconducting

magnets are essential for this, as iron-based per-manent magnets would not be sufficiently strongand would be far too heavy to be levitated.



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Figure 3. A Japanese Maglev train using superconducting magnets for levitation.

In addition to levitation, a HTS superconductor

can also be used to trap a magnetic field, which

allows one to suspend a magnet beneath it, ascan easily be demonstrated. The HTS essentially

behaves just like a very powerful permanent

magnet, but with the additional property that

it can support a levitated or suspended magnet

in a stable position. Researchers at Houston

have recently demonstrated a large-grained HTS

sample, irradiated with very energetic nuclei,

which can trap fields as high as 10 T at 23 K. Such

fields are almost ten times more powerful than can

currently be achieved with conventional magnets.

Introducing such material into electrical machines

would revolutionize the power to weight ratio ofelectrical motors and magnetic transducers.

Superconducting wires for electrical power

distribution and motors

HTS tapes can now be fabricated in kilometre

lengths for the field winding of magnets and

electrical motors. The best HTS wires now support

currents about 100 times larger than pass through

a typical 15 A mains cable. The major outstanding

technological challenge is to produce long lengths

of such wires with uniform properties and no weak

spots.The Reliance Electric and American Supercon-

ductor Corporation companies in the USA have

already demonstrated a 200-horsepower (150 kW)

electric motor using HTS coil windings. A $21 M

development programme expects to commercial-ize HTS motors with outputs greater than 1000

horsepower (750 kW) within the next four to five

years. The DoE in the USA have estimated that

about 58% of all electricity produced is used to run

electric motorspumping oil, gas, water and air,

running compressors and other industrial machin-

ery. About half of such pumps could be replaced

by lighter and more efficient HTS electrical mo-

tors. The savings in electrical power would be

immense.

Offices and shops in the centre of large cities

require an ever increasing amount of electricalpower. Electrical power is currently supplied by

oil-cooled copper pipes in special conduits buried

below ground. Unfortunately, the power handling

capacity of the existing cables into many American

and Japanese cities is already saturated, and it

would be enormously expensive to have to dig

up the roads and tunnel under buildings to lay

new cables. By replacing the existing oil-cooled

cables with nitrogen-cooled HTS cables, the power

capacity could be increased by a factor of three.

Pirelli, who collaborate with BICC in the

UK, and the Electrical Power Research Institute

in the USA have recently announced a world-record 50 m length of flexible high-voltage cable.

The cable already carries more than twice the



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Figure 4. Cross section of a liquid nitrogen-cooled flexible HTS power transmission cable.

energy of a comparatively sized conventionalcopper cable. A 6 km long superconducting HTS

tape was supplied by American Superconductor

Corp. (ASC) for counter-wound helical windings

on the liquid nitrogen-cooled inner support tube,see figure 4. Insulating and protective armoured

layers will eventually surround the cable, which

is designed to be a direct replacement for a

conventional power cable. HTS cables could alsobe viable whenever large amounts of power have

to be distributed over relatively short distances

from power stations to distribution systems oracross the English Channel to connect the Frenchand English distribution networks.

Microwave filters for cellular telephone

networks

The near-zero resistance of a superconductor

is also important in microwave applications.

High performance filters can be designed with

lower loss and sharper wings than can be

achieved using conventional metals like copper

or gold, even when they too are cooled to liquid

nitrogen temperatures. The characteristics of HTS

microwave filters can also be rapidly switched by

optical inputs, making them particularly useful forrapidly switched multiband operation. Switchable

HTS microwave filters have already being tested



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on F15 fighter planes in the USA, enabling them

to communicate on 32 channels instead of the four

channels used at present. The increased flexibility

and security that such filters can provide are likely

to be just as important for commercial and civil

information and communications technologies in

general.

HTS microwave filters are already being

used to increase the performance of cellular

telephone networks in the USA. For one of these

projects, involving Illinois Superconductors Inc.and AT&T, Birmingham University has supplied

400 microwave filters based on a specially

developed HTS thick film technology to coat torus-

shaped resonators. In another project, Conductus

has demonstrated that the combination of a HTS

thin film microwave resonator and a cryogenic low

noise amplifier can improve the sensitivity of the

network by a factor of two. The operating range

of a base-station near Green Bay in Wisconsin

was increased from 14 to 17 miles, in a situation

where line-of-sight range was limited by the

hilly terrain. The higher sensitivity achievable

with superconducting HTS filters and cryogenic

low-noise amplifiers in a city environment could

reduce the microwave power output needed from

hand-held telephones, which could have important

health and environmental implications. Cellular

telephone network systems are expected to be

particularly important for developing countries, as

such systems circumvent the need for setting up

an expensive infrastructure of individual telephone

lines. The better the filter performance, the

more telephone conversations that can be handled

simultaneously, the fewer transmitter stations that

need to be built, and the larger the profits!The use of superconductors to enhance

systems performance in IT and communications

technology is an obvious target area for HTS

applications. Such applications will become

increasingly important with the inexorable growth

in future IT requirements and the limited

bandwidth and speed of any communication

network.

SQUIDs

SQUIDs are the most sensitive sensors known to

man and are able to resolve minute changes inmagnetic field as small as 1011 of the Earths

magnetic field. This is already sufficient to detect

the minute magnetic fields produced from the

tiny currents associated with mental processes in

the brain. Helmets already exist containing over

100 helium-cooled SQUIDs, which can precisely

map the various regions of the brain associated

with certain sensory and motor activities. The

location of the centre which triggers epileptic

seizures can be accurately determined, providing a

valuable diagnostic tool to the surgeon, if surgery

is required.

SQUIDs can also be used to monitor both themothers and childs heartbeat during childbirth

a project currently being carried out by Professor

Gordon Donaldson at the University of Strathclyde

in collaboration with the Southern General

Hospital in Glasgow.

SQUIDs were the first HTS application to

be commercialized (early 1992), by Conductus in

the USA, in a project led by Mark Colclough,

who demonstrated the first crude HTS SQUID in

Birmingham in 1987. He has now returned to the

UK to develop a scanning SQUID microscope at

Birmingham University.

The performance of HTS SQUIDs is already

comparable with that of liquid helium devices, and

many potential applications in instrumentation are

being investigated. These include non-destructive

testing for cracks in aircraft bodies, geophysical

surveying, mine-detection, security surveillance

and ultra-sensitive scientific instrumentation.

SFQL (single flux quantum logic)

My last example is a unique application based on

the quantization of flux. A superconducting loop

enclosing a single flux quantum can be used torepresent the logic state 1. When no quantum is

present, it represents the state 0. Any change in

state of the ring is indicated by a very short voltage

pulse (a few mV in magnitude) proportional to the

rate that the flux quantum enters or leaves the ring

(in around 1012 s).

SFQL was first proposed by Terry Clark

in Sussex and independently by Likharev in

Moscow, who is now in the USA at SUNY

Stony Brook, leading developments in this area,

in a collaboration with IBM and AT&T Bell.

Using conventional liquid helium-cooled super-

conductors, SFQL has been demonstrated at360 GHza thousand times faster than the fastest

Pentium computer on your table-top. It is not



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Figure 5. A road map comparing the predictedperformance of silicon computing devices anbdsuperconducting single flux quantum logic (SFQL).(Courtesy of LikharevSUNY.) The numbers showthe necessary minimum feature size (in microns).

simply the speed that promises to outperform

silicon technology, it is the very low power

dissipation per gate operation. This is always an

important factor in the design of large, ultra-fast

computing systems, where the power requirement

per logic operation determines the density of logic

circuits that can be accommodated in a given

volume without excessive heating. A road map

comparing the current performance of silicon logic

devices and the projected performance of SFQL is

shown in figure 5.

Future prospects

Our examples demonstrate the potential forsuperconductivity in a number of strategic

technological areas that will continue to be as

important in the next century as they are todayenergy, communications, information technologyand health care.

Large-scale application of superconductivitywill require the continued improvement inperformance of wires for power applications andof thin films and junctions for devices. Progressin all these areas remains encouragingindeedonly a few months ago the US Departmentof Energy Laboratories at Argonne reported a

simple way of improving the performance ofpowder-in-tube processing of HTS wires by afurther factor of three (Jc > 100000 Acm

2)simply by adding a silver wire down the centreof the composite formed by the powder in asilver tube. We know from the performanceof epitaxially grown thin films on single-crystalsubstrates that current densities could, in principle,be increased by a further factor of ten. This isa challenge for materials scientists and we canconfidently anticipate continuing steady progressin superconducting wire and tape performance.However, as already demonstrated, existing

performance is already sufficient for many earlydemonstrator projects.

Another major challenge will be to overcomethe understandable conservatism of the electronicand electrical engineering industries and theircustomers, who will only convert to a newtechnology when its reliability and performancehave been firmly established to at least the samestandards as existing technology. However, aswe have argued, HTS is already making animpact on new technologies like cellular telephonecommunications, where a marginal improvementin performance can lead to very significant

increases in performance, competitiveness andprofit.

Of course, we would like to have discovereda room temperature superconductorthat reallywould revolutionize everyday technology. How-ever, we have probably already reached the lim-iting temperature for the cuprate family of per-ovskite superconductors. But the discovery of thecuprate HTS has awoken us to the realization thatsuperconductivity may well be a much more per-vasive property of matter than we had originallybelieved. Since the discovery of superconductiv-ity in the perovskite cuprate compounds, super-

conductivity has also been discovered at relativelyhigh temperatures in a number of other metal ox-ides and, even more excitingly, in alkali-doped



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high temperature superconductors take off

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