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High temperature superconductors: their promise and challenge
the industry-previews-research
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the industry-previews-research
High temperature superconductors" their promise and challenge*
The workhorse of high field magnet technology was, and still is, Nb-Ti, a ductile alloy that could be made into wires. A second material of great promise, Nb3Sn, can support larger electric currents and remain superconducting in higher magnetic fields, but has found much less use because of its brittle nature. Other materials have found specialized uses, for example, pure niobium in RF cavities and NbN in electronics. Recently, new materials have been discovered that have substantially higher upper critical temperatures, Tc: designated high temperature superconductors (HTSCs), they still become superconductors at very low temperatures, but at temperatures almost five times higher than Nb3Sn. In January 1986 Bednorz and MLiller at the IBM Laboratory in ZLirich, searching for superconductivity in previously unexplored materials determined thatthe ceramic La Ba CuO oxide became superconducting at temperatures over 30 K. Spurred on by this unexpected discovery, scientists throughout the world have since found materials with unexpectedly high TcS. The highest value achieved to date that has been independently confirmed is that achieved by Chu and colleagues at the University of Houston working with Wu and co-workers at the University of Alabama; about 98 K. At this temperature liquid nitrogen (which boils at 77 K atmospheric pressure and is much cheaper than liquid helium) can be used for cooling. Even before the constituents of Chu's material were known, independent discoveries of the superconducting behaviour of these compounds were also made in China and Japan a few days later. The rapid dissemination of scientific results has occurred mainly through word of mouth, preprints, press releases and even television news programmes, reflecting the close-knit global community of scientific endeavour and the intense interest of policy makers, the news media and the public at large. International competition in this field is intense as the leading industrialized countries strive to determine what the technological and commercial prospects for the new materials might be. A large number of the so-called high temperature superconductors, are now known to exist, all of them variations of two basic types (40 and 90 K, or 1-2 3 materials). Those with Tc > 77 K are based on only one structure, with copper and oxygen a constant feature. There have been many preliminary reports of higher temperature superconductors but at present there is no consensus as to their validity. Room temperature superconductivity (T c > 400 K) would make possible *Based on the Hulm Panel Briefing, Committee on Science and Pubfic Poficy, US Academy of Sciences, 1987
a range of applications that now can scarcely be envisioned. A more immediate question is whether the present high temperature superconductors can be used to improve present electronic and power applications; on this question researchers worldwide at present are cautiously optimistic.
Current
knowledge
The new high temperature superconductors are metal oxides, displaying the mechanical and physical properties of ceramics. The key to the behaviour of the new materials are the atoms of Cu and O chemically bonded to each other and arranged in long chains and planes that weave through the remainder of the structure. The special nature of the Cu O chemical bonding allows for the easy transfer of electrons, resulting in materials that conduct electricity well in these directions, whereas the majority of ceramics are electrically insulating. The first class of high Tc oxides discovered was based on the chemical alteration of the compound La2CuO 4 by replacement of a small fraction of La, by the alkaline earths Ba, Sr or Ca. These substitutions led to compounds with critical temperatures up to 40 K. In these materials, an intimate relation between superconductivity and magnetism is presently under intensive study and has inspired one of the many classes of theories that attempt to explain high temperature superconductivity. In a second class of materials, based on YBa2Cu3Ox, the metallic atoms occur in fixed proportions; these are the so-called 1-2-3 compounds. Such materials can be changed from semiconducting, at x = 6.5, to superconducting near 90 K at x ~ 7 , without losing their crystalline integrity. The high sensitivity of the material's properties to oxygen content is due to the ease with which oxygen can move in and out of the molecular lattice. The 40 K and 1 2 3 materials are similar in structure but otherwise differ significantly. In both basic materials, the rare earth and alkaline atoms play dual roles: they provide a structural framework within which the chains and planes of Cu and O atoms may be hung; and they provide a chemically favourable environment for the special Cu-O bonding which is required for the material to conduct electricity like a metal. Surprisingly, substitution of other rare earths, even magnetic ones, for Y in the 98 K compounds results in very little change in superconducting properties. Other substitutions are under study, both to understand the present materials and to achieve higher critical temperatures in new ones.
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t he industry- previews-research • the industry-previews-research Status of theoretical understanding Essential to the occurrence of superconductivity is the presence of an attractive interaction between conduction electrons, which would normally repel each other due to their like electrical charges. In conventional superconductors, this attraction originates in the dynamical motion of the crystal lattice which leads to an attractive electron-phonon interaction. However, the recent appearance of superconductivity in a class of materials quite different from the conventional superconductors, with extremely high transition temperatures as well, has led physicists to explore a very wide spectrum of possible new pairing mechanisms involving, for example, spin fluctuations, acoustic plasmons and excitonic processes. The physical origin of the pairing 'glue" remains an open question. Some theorists have discarded conventional BardeenCooper-Schrieffer theory and have suggested that there may not be a close relationship between energy gaps and basic superconducting properties. Hence, at present there are a variety of theoretical approaches, each needing experimental tests for their verification. Without knowledge of why superconductivity occurs at these high temperatures, it is impossible either to guess at the range of possible applications, or to estimate the maximum temperature at which superconductivity may occur. A long term experimental and theoretical programme will be necessary to unravel the secrets of superconductivity in these compounds. It might be that many effects are present simultaneously.
Physical properties important for technology The most important physical properties for applications are the critical superconducting temperature, To, the upper critical magnetic field, Hc2, and the maximum current carrying capacity in the superconducting state, Jc. Also important are the mechanical, chemical and electromagnetic properties: physical and thermal stability, resistance to radiation, alternating current loss characteristics and anisotropy. Each is discussed more fully below.
Critical temperature, T~. A rule of thumb for general applications, is that materials must be maintained at a temperature of 3/4Tc or below to ensure an error margin. At about 3/4Tc critical magnetic fields have reached roughly one half of their low temperature limit, and critical current densities roughly one quarter of their limit. Thus, to operate at liquid nitrogen temperature (77 K), one would like to use a material with T~ near 100 K, making the 95 K material just sufficient. To operate at room temperature (293 K) one must have a material with Tc greater than 400 K, well above the highest demonstrated (or even claimed) value. Higher Tc materials are superior across the board for applications, other properties being acceptable, and materials with Tc above 400 K would have a truly revolutionary impact on technology. In the
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domain of room temperature superconductivity one could consider mass market applications.
Upper critical magnetic field, Hc2. Existing singlecrystal HTSC materials have H~2 values at liquid helium temperatures estimated to be 30-150 T and even higher (depending on the orientation of the field with respect to the crystal axes). The mechanical stresses associated with confining such high magnetic fields in typical geometries are frequently beyond the yield and crushing strengths of known materials. Hence, improving these Hc2 values is less important than increasing Tc or Jc. In fact, materials with higher values of T~ should automatically have higher values of Hc2 if the performance of known materials is any guide. However, developing materials which are practical to fabricate into magnets and which retain useful Jc at fields approaching Hc2 and even at 77 K presents a great challenge.
Critical current density, Jc. Improvement of Jc values for conveniently fabricated material is perhaps the central problem for most applications. In practice values of Jc of 103 A mm 2 or more are desirable. This lower limit on current density applies both for microelectronics, where conductors may be 1 #m in width, and for high field magnets, where excessive size becomes an important liability. Based on early measurements, HTSCs appear to have intrinsic critical current densities of 106A mm 2 at zero field and 4.2 K, a value similar to those of conventional superconductors. Values achieved to date in typical multicrystalline ceramic samples, however, are around 10 A mm z at 77 K, while the best thin film samples deposited on crystalline SrTiO 3 substrates show over 10"A mm 2 at 77 K. In today's bulk materials, Jc s are reduced by orders of magnitude in very low fields (<0.01 T) and are quite useless for any application. A major complication is the anisotropy of Jc by up to a factor of 30 between parallel and perpendicular directions with respect to the material's crystalline c axis. The drastically reduced Jc levels, especially in a field, presumably reflect the granular nature of ceramics, interfaces between grains in the partially oriented films, or lack of correct stoichiometry near the granular boundary, all of which limit the supercurrent. Such problems do not exist for conventional superconductors. Achieving acceptable values for Jc in today's bulk HTSC compounds is a make or break proposition and must be the focus of research on fabrication processes. Based on current experience, a reasonable target specification for a commercial magnet conductor would be Jc= 103Amm 2 at 5T and 77 K, measured at an effective conductor resistivity of less than 10 -14 ~m with strain tolerance of 0.5 %, and available at prices comparable to or less than those of conventional low temperature superconductors. Mechanicalproperties. Present ceramic HTSC materials can be strong but are always brittle, hence it may be that HTSC wire will most likely be wound into magnets
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the industry-previews-research • the industry-previews-research prior to the final high temperature oxidation step in its fabrication after which step in the process it becomes very brittle. However, other conductor fabrication techniques might be feasible, for example, those used for Nb3Sn, such as flexible tapes. An elastic strain tolerance of 0.5 % may be induced in a multifilamentary conductor by fine filament size and induced compressive stresses. Presently available ceramic technology allows the fabrication of complicated pieces such as may be needed for RF cavities. The HTSC ceramics become deformable above 800°C and can then be shaped. However, the development of a mechanical forming process is constrained by the parallel need for the process to optimize Jc, both by aligning anisotropic crystal grains and by increasing the strength of the intergranular electrical coupling. Life testing will also be necessary to understand the performance of materials under realistic conditions, such as, temperature cycling and induced stresses due to transient fields. The adhesion of HTSCs to other materials, important in microelectronics, results in thermal expansion which causes stresses at the interface; this factor needs exploration.
Chemical stability. The new HTSC materials have been reported to degrade under exposure to moisture. However, these problems seem to be less severe as the purity and density of the materials is improved. Chemical stability is limited because oxygen diffuses from the structure under vacuum, even at room temperature. Surface protection techniques need to be developed to allow satisfactory performance and lifetime under various conditions of storage and operation. These concerns are heightened in thin films where, for some applications, the chemical composition of the outer 0.3#m or more of the surface must be maintained through many processing steps, and where interdiffusion into the substrate interface could degrade the superconducting properties. Radiation effects. HTSCs appear to be more sensitive to radiation than conventional superconductors. Conflicting data have been reported, but radiation damage in HTSCs may be from somewhat greater to 1000 times more severe. High sensitivity to radiation damage could pose a difficult but not insurmountable problem for application to magnetic fusion machines. The situation is different for semiconductor electronics, where the substitution of either conventional or high temperature superconducting devices for semiconductor ones would result in an improvement of several orders of magnitude in resistance to radiation damage. Alternating current losses. Conventional superconductors exhibit losses in a.c. applications, such as in 60 Hz power transmission or in microwave devices. Although little is known about the a.c. characteristics of the new HTSCs there is no reason to expect that the new materials will exhibit lower a.c. losses than other materials, whether conventional superconduc-
tors or ordinary conductors. Recent measurements on thin films in parallel applied fields show the presence of a large surface barrier for the entry of flux which indicates that hysteresis losses would be small. More direct measurements of such losses will be required.
Synthesis and fabrication Two steps are usually required to synthesize the 95 K superconducting materials. First, the basic structure must be formed at temperatures above 600 to 700°C. The tetragonal structure so formed is too deficient in oxygen to possess superconducting properties. The second synthesis step involves annealing under oxygen at a temperature below about 500°C. The arrangement of this additional oxygen in the lattice causes a conversion from tetragonal to orthorhombic symmetry that supports high temperature superconductivity. For the future development of HTSC materials, we require a much better understanding of how synthesis conditions relate to the structure of the 1-2-3 compounds on the atomic and nanometer scales. We need to know, further, how this structure relates to superconducting properties and to other important properties such as chemical stability and mechanical strength. It is important for ceramic processing that the powders employed to form ceramic pieces be free flowing. The production of this powder involves grinding of pre-reacted starting materials which can result in contamination from the grinding media. At the present time there is no evidence that impurities introduced by grinding degrade superconducting properties; however, further work is needed to optimize this process. Two recently announced fabrication techniques may change these processes. Staff members of both the Massachussetts Institute of Technology and the University of Arkansas have reported independently that materials in bulk can be made by melting the ingredients, thus making the manufacture of wires and specially shaped pieces much easier and eliminating the need for working with sintered materials and the associated grinding, compacting and sintering. Also enhancements of up to 39°C in Tc have been reported from the University of New South Wales when the powdered ingredients are processed in a nitrogen atmosphere. The 1-2 3 compounds readily react with the ambient atmosphere at typical ambient temperatures, Both H20 and CO2 participate in the degradation through the formation of hydroxides and carbonates. Further study on the nature of the degradation is necessary to develop handling procedures or protective coatings which will insure against degradation of superconducting properties by atmospheric attack. Processes are required for the commercial production of high quality thin films on useful substrates. What is needed is to compare the various ways which have been used to produce thin films electron beam,
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t he in dustry- p reviews-research • the industry-previews-research planar magnetron sputtering, pulsed laser evaporation, chemical vapour deposition - and establish the strength and weaknesses of each method. Epitaxial growth methods also need to be studied. Reliable low resistance ohmic contacts to the new materials are required. A better understanding of phase equilibria, solid solutions, and intermetallic compounds is needed to find stable ohmic contacts which do not degrade superconductivity.
New superconducting materials Finally, we must not neglect the search for new compounds with intrinsically superior superconducting properties. Operation at 77 K leaves little margin when running a device which utilizes a superconductor with a Tc of 95 K. Cryogenic systems which operate below 77 K should be investigated, and compatibility of HTSCs with refrigerants other than liquid nitrogen (e.g. liquid neon) should be tested. The events of the past year have shown that surprises do occur and it may be that superconductivity at or above room temperature may be detected at some future date in compounds not yet studied.
Current fields of application and the likely impact of the n e w materials Virtually all of the applications currently envisioned for high temperature superconductors are extrapolations of devices already operated at liquid helium temperatures. The most important applications, however, may well involve devices that have yet to be contemplated, much less invented. As shown in Table 1, present and potential applications fall into several distinct classes. The present applications include high field magnets and their applications, radio frequency devices and electronics. Superconductivity brings unique advantages to high field applications, since resistive conductors, such as copper, dissipate large amounts of energy as heat when carrying large currents. Superconductors can also be useful at high frequencies, including the microwave region, because the low a.c. losses, as compared with metals such as copper, lead to high Q behaviour. Electronic applications usually involve low electric currents and magnetic fields. The core element has been a unique bistable device, the Josephson junction. Superconductors may also eliminate resistive current losses in electronic lines and device interconnections. Various kinds of superconducting sensors have been produced. All of these applications, including the assembly of superconducting electronic components into larger devices, will be reconsidered with the new compounds. Potential applications of high temperature superconductors divide into those relevant to currently available materials with critical temperatures near 95 K, and those of possible future materials with higher critical
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Table 1 Principal applications of superconductivity Present applications Magnets Commercial and industrial uses: • medical diagnostics and research (magnetic resonance - imaging and spectroscopy) • radJotrequency(gyrotrons, etc.) • ore refining (magnetic separation) • researchand development • magneticshielding Physics machines (colliders, fusion machines, RF cavities) Electronics: • sensitiveaccurate instrumentation(SQUIDs, infrared sensors) Electromagneticshielding Potential applications (proven superconductingtechnology, no market adoption) Power utility applications: • energyproduction (magnetohydrodynamics,magnetic fusion) • large turbogenerators • energystorage • electrical power transmission Transportation: • high speed trains (Maglev) • ship drive systems Computers
temperatures. The most exciting possibilities, of course, arise with materials with critical temperatures above room temperature.
High field and large scale applications Superconducting magnets using liquid helium technology have been successfully applied in engineered systems and development projects for a number of years, in hospitals, mines, industrial plants, laboratories and transportation systems. In most of these applications, superconductivity has been an enabling technology rather than playing a central critical role in the parent application. In medicine, superconductors have been a significant factor in the development of a new market. In high energy physics, superconductors have led to machines of unprecedented and previously inconceivable energy. In electric power, potential applications in energy storage and power transmission equipment extend the capacity and range of current technology, while possible new sources of energy, such as magnetic fusion, and more efficient power plants using magnetohydrodynamic ( M H D ) converters, can be reconsidered. The extremely high power to weight ratio possible for superconducting machines makes them particularly attractive for space applications.
the industry-previews-research • the industry-previews-research For magnet and power applications, the higher the critical temperature, the smaller will be the scale at which commercial viability will be achieved. As an example, the power level at which motors and generators become competitive will be much lower than with the present low temperature superconductors, when compared to non-superconducting machines. A liquid nitrogen cooled motor, for example, at modest current and magnetic field might well be smaller, more efficient, and more reliable for the same power output than many present day motors. For most applications, the switch from liquid helium to liquid nitrogen technology will lead to improvements but is not revolutionary. The continued need for refrigeration is a disadvantage and will reduce market penetration. Of course, reconsideration of applications held to be impractical at liquid helium temperatures might lead to new products. A hollow conductor cooled with liquid nitrogen is easy to visualize in practical use, for example. It may not be necessary to demand that the technical specifications of new materials compete with the best commercial superconducting materials of today: a conductor of modest specifications may have value in a wider context than those of conventional low temperature superconductors. The new materials, in short, may not so much replace present day superconductors as extend the applications of superconductivity to a larger circle of users.
Medical applications. Magnetic resonance imaging (MRI) and spectroscopy (MRS) constitute radically new techniques in medical diagnosis and treatment. Their full impact is yet to be realized. Much more widespread availability of MRI and MRS systems can be anticipated, with concomitant reductions in cost and enhancement of features. The use of HTSC materials would likely bring further small reductions in cost of manufacture and operation. Redesign of systems with liquid nitrogen cooling would also make systems more user-friendly and reliable by reducing the complexity of cooling systems.
Superconducting radio frequency cavities. If microwave alternating loss characteristics are tolerable, the new superconductors may greatly improve the performance of superconducting radio frequency cavities by allowing them to operate at higher fields. Potential impacts embrace all of microwave power technology, especially in the promising millimetre wave region. Accelerator technology might be significantly advanced by the availability of liquid nitrogen cooled superconducting cavities. The applicability of superconducting technology to recirculating linear accelerators is an accepted fact. In addition to providing high quality beams for nuclear physics research, these machines are natural candidates for continuous beam injectors for free electron lasers. As technology matures and industrial applications develop for high power, high efficiency tuneable lasers in biotechnology, fusion
plasma heating, etc., superconducting radio frequency cavities will proliferate.
Transportation. Ambitious attempts to apply superconductivity to land and ocean transportation have been made over the years in the United States, Europe, and Japan, with some success; in fact, a Japanese magnetic levitation rail system is available for interested buyers and is economically viable. Its principal advantage is speeds approaching that of propeller aircraft and reduced noise. Because the current worldwide systems of air, sea, and land transportation are well established and represent a substantial investment, society has not yet made use of the potential advantages that have been demonstrated in prototype transportation systems using low temperature superconductors. Only time will tell if high temperature superconductors alter this situation. Superconducting ship propulsion systems were studied in England in the 1960s. In comparison to conventional electric ship drive systems they are lighter in weight. As in the conventional electric drive systems they also have better speed control and permit the radical rearrangement of power drive systems within ship structures. The US Navy successfully installed a prototype superconducting drive system on a small ship in 1980; development of a 40000 horsepower drive system continues. A second concept uses seawater as the working fluid in an MHD propulsion system; the drive scheme is known as an electromagnet thruster (EMT), and ship models based on this propulsion principle were promoted in the US in the 1960s and operated in Japan in the 1970s. Practical designs for a full scale EMT ship have been proposed and industrial collaboration is being sought. Electronic applications Devices being explored at present are usually based on very small amounts of superconductor in the form of very thin films or wires, or on the Josephson junction. The latter can detect the smallest measured quantities of magnetic flux, leading to very precise and sensitive instrumentation. We do not have any of these junctions with the new materials as yet. A serious obstacle to many of the applications mentioned here is the difficulty of making the two film junction when the second film must be heated to 900 or even 650°C.
Standard volt. In many countries the standard volt is maintained in terms of the voltage generated across a Josephson junction by microwave irradiation at a precisely known frequency. This standard could be more cheaply maintained (and therefore more widely disseminated) if it were operated in liquid nitrogen. There should be no loss in accuracy in operating the standard at liquid nitrogen rather than at liquid helium temperature but only when we have the new device will we be certain of this.
SQUIDs. Superconducting quantum interference de-
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the industry-previews-research vices at liquid helium temperatures are already of value in many disciplines, including medical diagnostics, geophysical prospecting, and undersea communications. Rather low performance SQUIDs made with the high temperature materials have already been operated at liquid nitrogen temperatures, but much more development is needed to make acceptable devices. Relatively inexpensive SQUID-based magnetometers operating at 77 K or higher would be deployed in large numbers if electrical noise can be held to acceptably low levels.
Infrared detection. One important application of superconductivity at 4 K is the detection of far infrared radiation. Although detectors at 77 K will inevitably be less sensitive, it is likely that they will outperform non-superconducting devices at this temperature. Microcircuit transmission lines. Superconducting transmission lines for microcircuits have been used in prototype devices of high speed analog signal processing circuitry. The extension of this technology to liquid nitrogen temperatures would lead to devices that perform certain specialized computation-intensive calculations at much faster rates than non-superconducting devices operating at 300 K, but not much faster than devices using aluminium or copper at 77 K. A/D converters. Various high speed analog to digital converters have been tested successfully at 4 K. If high quality Josephson tunnel junctions can be fabricated from the new superconductors, these devices should perform comparably at 77 K. At this temperature, integration of the superconducting devices with certain semiconducting devices (for example, complementary metal oxide semiconductors) becomes feasible, and new hybrid systems may well result in the fastest A/D converters available. Microbridges (as an alternative to tunnel junctions) may also be used in some versions of A/D converters. The microbridge has the advantage that it is made of one film rather than two. Computers. A great deal of promising development has been carried out on superconducting computers but advances in semiconductor technology proved more attractive for such machines. The promise of very high packing density for the switches and connectors, made possible by the absence of heat generation due to current flow, meant that very powerful computers and logic systems could be miniaturized and their reliability increased. The promise of liquid nitrogen cooled superconductors could lead to a re-examination of this possibility but, as yet, there are no suitable nitrogen cooled junctions. However, the promise is exciting and may lead to many new areas of application, including hybrid combinations of semiconductor and superconductor technologies. Magnetic shielding. Normal conductors and superconductors have been used for many years to create regions
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the industry-previews-research free from all magnetic fields or to shape magnetic fields. Both superconducting wires and sheets have been used for this purpose. The advent of the high temperature superconductors may extend the range of this application since, as with Nb3Sn, the possibility of plasma sprayed sheets or films permits the shielding of regions with surfaces of complex shape.
Present m a r k e t f o r s u p e r c o n d u c t o r s and likely changes associated w i t h t he n e w materials The world superconductor industry is small but superconducting devices are usually components of larger systems whose gross annual sales volume is many times the value of the devices themselves. Annual device sales total about US$400 million, of which medical imaging machines and electronic instruments account for about US$150 million each. Magnet coils account for 10-20 % of system costs and annual sales of basic materials such as alloy rod and sheet are of the order of US$10-20 million. It is difficult to estimate the potential economic impact of the present high temperature superconductors because so little is known about them and much depends on improved understanding and technological development. Assuming that satisfactory conductors can be produced, there are considerable advantages to operation in liquid nitrogen. Refrigeration units are simpler and cost less to operate. Conductor stability improves as the temperature increases, because of the higher heat capacity of materials are less brittle at higher temperatures and higher resistivity of the stabilizing matrix. Structural materials are less brittle at higher temperatures an cheaper less exotic materials can be used. Cryogenic liquids and systems, however, will still be needed. In comparing superconductor technology with present room temperature devices, the need for cooling is a serious economic and technological disadvantage. There is a great difference between switching on a machine as needed and having to wait for the refrigeration system to reach operating temperature. Assuming that some utility and heavy electrical power applications can be competitively marketed using systems cooled by liquid nitrogen, the superconducting materials market may be substantially increased; the market for electrical equipment used in the electric utility industry, however, would be mainly a replacement one, since few new systems are being built. For substantial business growth above that achieved with low temperature superconductors, new technology developments are needed. There is little doubt that the new materials offer technological advantages, as they promise high magnetic field devices and new types of electronic sensors and switches at lower costs in refrigeration than before. The Hulm Panel is unanimous in stating that advances are bound to result in new applications and new economic growth. If room
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the industry- previews-research temperature superconductors become available, we can confidently expect truly revolutionary expansion of superconducting applications in electrotechnology.
Superconductivity: the global picture On a global scale, today's world superconductor industry is small but mature and principally confined to the more industrialized countries. Over the past 25 years, in many such countries, a wide variety of applications of superconducting electrotechnology has been examined in prototype development programmes. However, no replacements for conventional applications have reached the market. As a result, the demand for superconducting materials has been relatively small and has lacked continuity, being largely oriented to development. Nevertheless, government programmes have supported this fledgling industry in each of these countries. Basic research capabilities are more widespread and national and international conferences on all aspects of low temperature physics have become routine. In the USA, magnet development for high energy physics machines has been carried out in the national laboratories. Fusion and MHD magnets have been built both in the national laboratories and in private industry. There is also a rapidly growing commercial market based mainly on new medical imaging systems. A small materials and wire industry serves magnet development efforts. Many US firms have supported their own research and development efforts in superconductor technology. Corporations in Europe and Japan have also fostered and maintained an expertise in superconductivity. Their governments have to some degree protected their superconductor industries by ensuring that equipment for government laboratories is built by private industry, with foreign bids not being accepted. This assures an 'in house' industrial expertise. The USSR also has agressive long term programmes in energy conservation (including power transmission and storage); the fabrication of superconducting wires and tapes; electronics; and in collider construction, magnetic fusion, magnetohydrodynamics, and superconducting generators. They also have a long tradition of theoretical and experimental strengths in superconductivity. Basic HTSC research Basic research in high temperature superconductivity is being actively pursued in all of the countries mentioned above and in several developing countries. In most cases, the scientists themselves have automatically switched from other scientific activities into HTSC research and caught up in the excitement of the moment, officialdom has condoned the switch. However, little new money has gone into the effort. Plans are being prepared for 1988 but at present no
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the industry-previews-research major new government resources have been committed. The prevailing attitude appears to be that of waiting to see how the science progresses. Japan. In Japan the scientific and technical community has responded vigorously but aside from reprogramming there has been minimal immediate additional action by government agencies. The latter have, however, been very active in formulating plans for the fiscal year which began in April 1988. Private industrial corporations are said to be investing their own funds heavily in research on high temperature superconductors, with the government intervening to establish industry consortia to pursue prototyping and other early development activities. Europe. In Europe there is a long tradition of basic research and industrial development. These strengths are being applied to the new superconductors. National and cross- national efforts are in the early organizational stages at best (again, with the exception of reprogramming of research funds), and major project goals to drive technical problem solving are not yet in place. However, a variety of industrial corporations are involved in research and pre-competitive collaborations appear to be at advanced planning stages. USSR. The extent of the effort in the USSR is not known to us in detail. There is, however, great enthusiasm, at least equal to that expressed in the USA.
Update This article, our condensation of the Hulm Panel Briefing, was prepared in November 1987. In December, Maeda et al. at the National Institute for Metals (NRIM) in Japan developed a material with the formula BJ2Sr3 xCaxCu208 + v which is more stable than the 1 -2-3 compounds, is easier to make, and has a critical temperature around 1 25 K. Later, Sheng and Hermann at the University of Arkansas, USA, obtained similar results by substituting thallium for the bismuth. The structure of these materials differs from that of previous compounds in that they are made up of alternating double copper-oxygen and double bismuth (or thallium)-oxygen sheets. While remarkable progress has been made in the last year in understanding the nature and structure of all these new superconducting ceramics, no theoretical explanation for their behaviour has, as yet, been agreed upon. Our comments on the technical difficulties and barriers to application are still valid f J. K. Hulm Westinghouse R & D Center, Pittsburgh, Pennsylvania, USA C. Laverick Independent Consultant, Patchogue, New York, USA
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High temperature superconductors" their promise and challenge*
The workhorse of high field magnet technology was, and still is, Nb-Ti, a ductile alloy that could be made into wires. A second material of great promise, Nb3Sn, can support larger electric currents and remain superconducting in higher magnetic fields, but has found much less use because of its brittle nature. Other materials have found specialized uses, for example, pure niobium in RF cavities and NbN in electronics. Recently, new materials have been discovered that have substantially higher upper critical temperatures, Tc: designated high temperature superconductors (HTSCs), they still become superconductors at very low temperatures, but at temperatures almost five times higher than Nb3Sn. In January 1986 Bednorz and MLiller at the IBM Laboratory in ZLirich, searching for superconductivity in previously unexplored materials determined thatthe ceramic La Ba CuO oxide became superconducting at temperatures over 30 K. Spurred on by this unexpected discovery, scientists throughout the world have since found materials with unexpectedly high TcS. The highest value achieved to date that has been independently confirmed is that achieved by Chu and colleagues at the University of Houston working with Wu and co-workers at the University of Alabama; about 98 K. At this temperature liquid nitrogen (which boils at 77 K atmospheric pressure and is much cheaper than liquid helium) can be used for cooling. Even before the constituents of Chu's material were known, independent discoveries of the superconducting behaviour of these compounds were also made in China and Japan a few days later. The rapid dissemination of scientific results has occurred mainly through word of mouth, preprints, press releases and even television news programmes, reflecting the close-knit global community of scientific endeavour and the intense interest of policy makers, the news media and the public at large. International competition in this field is intense as the leading industrialized countries strive to determine what the technological and commercial prospects for the new materials might be. A large number of the so-called high temperature superconductors, are now known to exist, all of them variations of two basic types (40 and 90 K, or 1-2 3 materials). Those with Tc > 77 K are based on only one structure, with copper and oxygen a constant feature. There have been many preliminary reports of higher temperature superconductors but at present there is no consensus as to their validity. Room temperature superconductivity (T c > 400 K) would make possible *Based on the Hulm Panel Briefing, Committee on Science and Pubfic Poficy, US Academy of Sciences, 1987
a range of applications that now can scarcely be envisioned. A more immediate question is whether the present high temperature superconductors can be used to improve present electronic and power applications; on this question researchers worldwide at present are cautiously optimistic.
Current
knowledge
The new high temperature superconductors are metal oxides, displaying the mechanical and physical properties of ceramics. The key to the behaviour of the new materials are the atoms of Cu and O chemically bonded to each other and arranged in long chains and planes that weave through the remainder of the structure. The special nature of the Cu O chemical bonding allows for the easy transfer of electrons, resulting in materials that conduct electricity well in these directions, whereas the majority of ceramics are electrically insulating. The first class of high Tc oxides discovered was based on the chemical alteration of the compound La2CuO 4 by replacement of a small fraction of La, by the alkaline earths Ba, Sr or Ca. These substitutions led to compounds with critical temperatures up to 40 K. In these materials, an intimate relation between superconductivity and magnetism is presently under intensive study and has inspired one of the many classes of theories that attempt to explain high temperature superconductivity. In a second class of materials, based on YBa2Cu3Ox, the metallic atoms occur in fixed proportions; these are the so-called 1-2-3 compounds. Such materials can be changed from semiconducting, at x = 6.5, to superconducting near 90 K at x ~ 7 , without losing their crystalline integrity. The high sensitivity of the material's properties to oxygen content is due to the ease with which oxygen can move in and out of the molecular lattice. The 40 K and 1 2 3 materials are similar in structure but otherwise differ significantly. In both basic materials, the rare earth and alkaline atoms play dual roles: they provide a structural framework within which the chains and planes of Cu and O atoms may be hung; and they provide a chemically favourable environment for the special Cu-O bonding which is required for the material to conduct electricity like a metal. Surprisingly, substitution of other rare earths, even magnetic ones, for Y in the 98 K compounds results in very little change in superconducting properties. Other substitutions are under study, both to understand the present materials and to achieve higher critical temperatures in new ones.
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t he industry- previews-research • the industry-previews-research Status of theoretical understanding Essential to the occurrence of superconductivity is the presence of an attractive interaction between conduction electrons, which would normally repel each other due to their like electrical charges. In conventional superconductors, this attraction originates in the dynamical motion of the crystal lattice which leads to an attractive electron-phonon interaction. However, the recent appearance of superconductivity in a class of materials quite different from the conventional superconductors, with extremely high transition temperatures as well, has led physicists to explore a very wide spectrum of possible new pairing mechanisms involving, for example, spin fluctuations, acoustic plasmons and excitonic processes. The physical origin of the pairing 'glue" remains an open question. Some theorists have discarded conventional BardeenCooper-Schrieffer theory and have suggested that there may not be a close relationship between energy gaps and basic superconducting properties. Hence, at present there are a variety of theoretical approaches, each needing experimental tests for their verification. Without knowledge of why superconductivity occurs at these high temperatures, it is impossible either to guess at the range of possible applications, or to estimate the maximum temperature at which superconductivity may occur. A long term experimental and theoretical programme will be necessary to unravel the secrets of superconductivity in these compounds. It might be that many effects are present simultaneously.
Physical properties important for technology The most important physical properties for applications are the critical superconducting temperature, To, the upper critical magnetic field, Hc2, and the maximum current carrying capacity in the superconducting state, Jc. Also important are the mechanical, chemical and electromagnetic properties: physical and thermal stability, resistance to radiation, alternating current loss characteristics and anisotropy. Each is discussed more fully below.
Critical temperature, T~. A rule of thumb for general applications, is that materials must be maintained at a temperature of 3/4Tc or below to ensure an error margin. At about 3/4Tc critical magnetic fields have reached roughly one half of their low temperature limit, and critical current densities roughly one quarter of their limit. Thus, to operate at liquid nitrogen temperature (77 K), one would like to use a material with T~ near 100 K, making the 95 K material just sufficient. To operate at room temperature (293 K) one must have a material with Tc greater than 400 K, well above the highest demonstrated (or even claimed) value. Higher Tc materials are superior across the board for applications, other properties being acceptable, and materials with Tc above 400 K would have a truly revolutionary impact on technology. In the
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domain of room temperature superconductivity one could consider mass market applications.
Upper critical magnetic field, Hc2. Existing singlecrystal HTSC materials have H~2 values at liquid helium temperatures estimated to be 30-150 T and even higher (depending on the orientation of the field with respect to the crystal axes). The mechanical stresses associated with confining such high magnetic fields in typical geometries are frequently beyond the yield and crushing strengths of known materials. Hence, improving these Hc2 values is less important than increasing Tc or Jc. In fact, materials with higher values of T~ should automatically have higher values of Hc2 if the performance of known materials is any guide. However, developing materials which are practical to fabricate into magnets and which retain useful Jc at fields approaching Hc2 and even at 77 K presents a great challenge.
Critical current density, Jc. Improvement of Jc values for conveniently fabricated material is perhaps the central problem for most applications. In practice values of Jc of 103 A mm 2 or more are desirable. This lower limit on current density applies both for microelectronics, where conductors may be 1 #m in width, and for high field magnets, where excessive size becomes an important liability. Based on early measurements, HTSCs appear to have intrinsic critical current densities of 106A mm 2 at zero field and 4.2 K, a value similar to those of conventional superconductors. Values achieved to date in typical multicrystalline ceramic samples, however, are around 10 A mm z at 77 K, while the best thin film samples deposited on crystalline SrTiO 3 substrates show over 10"A mm 2 at 77 K. In today's bulk materials, Jc s are reduced by orders of magnitude in very low fields (<0.01 T) and are quite useless for any application. A major complication is the anisotropy of Jc by up to a factor of 30 between parallel and perpendicular directions with respect to the material's crystalline c axis. The drastically reduced Jc levels, especially in a field, presumably reflect the granular nature of ceramics, interfaces between grains in the partially oriented films, or lack of correct stoichiometry near the granular boundary, all of which limit the supercurrent. Such problems do not exist for conventional superconductors. Achieving acceptable values for Jc in today's bulk HTSC compounds is a make or break proposition and must be the focus of research on fabrication processes. Based on current experience, a reasonable target specification for a commercial magnet conductor would be Jc= 103Amm 2 at 5T and 77 K, measured at an effective conductor resistivity of less than 10 -14 ~m with strain tolerance of 0.5 %, and available at prices comparable to or less than those of conventional low temperature superconductors. Mechanicalproperties. Present ceramic HTSC materials can be strong but are always brittle, hence it may be that HTSC wire will most likely be wound into magnets
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the industry-previews-research • the industry-previews-research prior to the final high temperature oxidation step in its fabrication after which step in the process it becomes very brittle. However, other conductor fabrication techniques might be feasible, for example, those used for Nb3Sn, such as flexible tapes. An elastic strain tolerance of 0.5 % may be induced in a multifilamentary conductor by fine filament size and induced compressive stresses. Presently available ceramic technology allows the fabrication of complicated pieces such as may be needed for RF cavities. The HTSC ceramics become deformable above 800°C and can then be shaped. However, the development of a mechanical forming process is constrained by the parallel need for the process to optimize Jc, both by aligning anisotropic crystal grains and by increasing the strength of the intergranular electrical coupling. Life testing will also be necessary to understand the performance of materials under realistic conditions, such as, temperature cycling and induced stresses due to transient fields. The adhesion of HTSCs to other materials, important in microelectronics, results in thermal expansion which causes stresses at the interface; this factor needs exploration.
Chemical stability. The new HTSC materials have been reported to degrade under exposure to moisture. However, these problems seem to be less severe as the purity and density of the materials is improved. Chemical stability is limited because oxygen diffuses from the structure under vacuum, even at room temperature. Surface protection techniques need to be developed to allow satisfactory performance and lifetime under various conditions of storage and operation. These concerns are heightened in thin films where, for some applications, the chemical composition of the outer 0.3#m or more of the surface must be maintained through many processing steps, and where interdiffusion into the substrate interface could degrade the superconducting properties. Radiation effects. HTSCs appear to be more sensitive to radiation than conventional superconductors. Conflicting data have been reported, but radiation damage in HTSCs may be from somewhat greater to 1000 times more severe. High sensitivity to radiation damage could pose a difficult but not insurmountable problem for application to magnetic fusion machines. The situation is different for semiconductor electronics, where the substitution of either conventional or high temperature superconducting devices for semiconductor ones would result in an improvement of several orders of magnitude in resistance to radiation damage. Alternating current losses. Conventional superconductors exhibit losses in a.c. applications, such as in 60 Hz power transmission or in microwave devices. Although little is known about the a.c. characteristics of the new HTSCs there is no reason to expect that the new materials will exhibit lower a.c. losses than other materials, whether conventional superconduc-
tors or ordinary conductors. Recent measurements on thin films in parallel applied fields show the presence of a large surface barrier for the entry of flux which indicates that hysteresis losses would be small. More direct measurements of such losses will be required.
Synthesis and fabrication Two steps are usually required to synthesize the 95 K superconducting materials. First, the basic structure must be formed at temperatures above 600 to 700°C. The tetragonal structure so formed is too deficient in oxygen to possess superconducting properties. The second synthesis step involves annealing under oxygen at a temperature below about 500°C. The arrangement of this additional oxygen in the lattice causes a conversion from tetragonal to orthorhombic symmetry that supports high temperature superconductivity. For the future development of HTSC materials, we require a much better understanding of how synthesis conditions relate to the structure of the 1-2-3 compounds on the atomic and nanometer scales. We need to know, further, how this structure relates to superconducting properties and to other important properties such as chemical stability and mechanical strength. It is important for ceramic processing that the powders employed to form ceramic pieces be free flowing. The production of this powder involves grinding of pre-reacted starting materials which can result in contamination from the grinding media. At the present time there is no evidence that impurities introduced by grinding degrade superconducting properties; however, further work is needed to optimize this process. Two recently announced fabrication techniques may change these processes. Staff members of both the Massachussetts Institute of Technology and the University of Arkansas have reported independently that materials in bulk can be made by melting the ingredients, thus making the manufacture of wires and specially shaped pieces much easier and eliminating the need for working with sintered materials and the associated grinding, compacting and sintering. Also enhancements of up to 39°C in Tc have been reported from the University of New South Wales when the powdered ingredients are processed in a nitrogen atmosphere. The 1-2 3 compounds readily react with the ambient atmosphere at typical ambient temperatures, Both H20 and CO2 participate in the degradation through the formation of hydroxides and carbonates. Further study on the nature of the degradation is necessary to develop handling procedures or protective coatings which will insure against degradation of superconducting properties by atmospheric attack. Processes are required for the commercial production of high quality thin films on useful substrates. What is needed is to compare the various ways which have been used to produce thin films electron beam,
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t he in dustry- p reviews-research • the industry-previews-research planar magnetron sputtering, pulsed laser evaporation, chemical vapour deposition - and establish the strength and weaknesses of each method. Epitaxial growth methods also need to be studied. Reliable low resistance ohmic contacts to the new materials are required. A better understanding of phase equilibria, solid solutions, and intermetallic compounds is needed to find stable ohmic contacts which do not degrade superconductivity.
New superconducting materials Finally, we must not neglect the search for new compounds with intrinsically superior superconducting properties. Operation at 77 K leaves little margin when running a device which utilizes a superconductor with a Tc of 95 K. Cryogenic systems which operate below 77 K should be investigated, and compatibility of HTSCs with refrigerants other than liquid nitrogen (e.g. liquid neon) should be tested. The events of the past year have shown that surprises do occur and it may be that superconductivity at or above room temperature may be detected at some future date in compounds not yet studied.
Current fields of application and the likely impact of the n e w materials Virtually all of the applications currently envisioned for high temperature superconductors are extrapolations of devices already operated at liquid helium temperatures. The most important applications, however, may well involve devices that have yet to be contemplated, much less invented. As shown in Table 1, present and potential applications fall into several distinct classes. The present applications include high field magnets and their applications, radio frequency devices and electronics. Superconductivity brings unique advantages to high field applications, since resistive conductors, such as copper, dissipate large amounts of energy as heat when carrying large currents. Superconductors can also be useful at high frequencies, including the microwave region, because the low a.c. losses, as compared with metals such as copper, lead to high Q behaviour. Electronic applications usually involve low electric currents and magnetic fields. The core element has been a unique bistable device, the Josephson junction. Superconductors may also eliminate resistive current losses in electronic lines and device interconnections. Various kinds of superconducting sensors have been produced. All of these applications, including the assembly of superconducting electronic components into larger devices, will be reconsidered with the new compounds. Potential applications of high temperature superconductors divide into those relevant to currently available materials with critical temperatures near 95 K, and those of possible future materials with higher critical
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Table 1 Principal applications of superconductivity Present applications Magnets Commercial and industrial uses: • medical diagnostics and research (magnetic resonance - imaging and spectroscopy) • radJotrequency(gyrotrons, etc.) • ore refining (magnetic separation) • researchand development • magneticshielding Physics machines (colliders, fusion machines, RF cavities) Electronics: • sensitiveaccurate instrumentation(SQUIDs, infrared sensors) Electromagneticshielding Potential applications (proven superconductingtechnology, no market adoption) Power utility applications: • energyproduction (magnetohydrodynamics,magnetic fusion) • large turbogenerators • energystorage • electrical power transmission Transportation: • high speed trains (Maglev) • ship drive systems Computers
temperatures. The most exciting possibilities, of course, arise with materials with critical temperatures above room temperature.
High field and large scale applications Superconducting magnets using liquid helium technology have been successfully applied in engineered systems and development projects for a number of years, in hospitals, mines, industrial plants, laboratories and transportation systems. In most of these applications, superconductivity has been an enabling technology rather than playing a central critical role in the parent application. In medicine, superconductors have been a significant factor in the development of a new market. In high energy physics, superconductors have led to machines of unprecedented and previously inconceivable energy. In electric power, potential applications in energy storage and power transmission equipment extend the capacity and range of current technology, while possible new sources of energy, such as magnetic fusion, and more efficient power plants using magnetohydrodynamic ( M H D ) converters, can be reconsidered. The extremely high power to weight ratio possible for superconducting machines makes them particularly attractive for space applications.
the industry-previews-research • the industry-previews-research For magnet and power applications, the higher the critical temperature, the smaller will be the scale at which commercial viability will be achieved. As an example, the power level at which motors and generators become competitive will be much lower than with the present low temperature superconductors, when compared to non-superconducting machines. A liquid nitrogen cooled motor, for example, at modest current and magnetic field might well be smaller, more efficient, and more reliable for the same power output than many present day motors. For most applications, the switch from liquid helium to liquid nitrogen technology will lead to improvements but is not revolutionary. The continued need for refrigeration is a disadvantage and will reduce market penetration. Of course, reconsideration of applications held to be impractical at liquid helium temperatures might lead to new products. A hollow conductor cooled with liquid nitrogen is easy to visualize in practical use, for example. It may not be necessary to demand that the technical specifications of new materials compete with the best commercial superconducting materials of today: a conductor of modest specifications may have value in a wider context than those of conventional low temperature superconductors. The new materials, in short, may not so much replace present day superconductors as extend the applications of superconductivity to a larger circle of users.
Medical applications. Magnetic resonance imaging (MRI) and spectroscopy (MRS) constitute radically new techniques in medical diagnosis and treatment. Their full impact is yet to be realized. Much more widespread availability of MRI and MRS systems can be anticipated, with concomitant reductions in cost and enhancement of features. The use of HTSC materials would likely bring further small reductions in cost of manufacture and operation. Redesign of systems with liquid nitrogen cooling would also make systems more user-friendly and reliable by reducing the complexity of cooling systems.
Superconducting radio frequency cavities. If microwave alternating loss characteristics are tolerable, the new superconductors may greatly improve the performance of superconducting radio frequency cavities by allowing them to operate at higher fields. Potential impacts embrace all of microwave power technology, especially in the promising millimetre wave region. Accelerator technology might be significantly advanced by the availability of liquid nitrogen cooled superconducting cavities. The applicability of superconducting technology to recirculating linear accelerators is an accepted fact. In addition to providing high quality beams for nuclear physics research, these machines are natural candidates for continuous beam injectors for free electron lasers. As technology matures and industrial applications develop for high power, high efficiency tuneable lasers in biotechnology, fusion
plasma heating, etc., superconducting radio frequency cavities will proliferate.
Transportation. Ambitious attempts to apply superconductivity to land and ocean transportation have been made over the years in the United States, Europe, and Japan, with some success; in fact, a Japanese magnetic levitation rail system is available for interested buyers and is economically viable. Its principal advantage is speeds approaching that of propeller aircraft and reduced noise. Because the current worldwide systems of air, sea, and land transportation are well established and represent a substantial investment, society has not yet made use of the potential advantages that have been demonstrated in prototype transportation systems using low temperature superconductors. Only time will tell if high temperature superconductors alter this situation. Superconducting ship propulsion systems were studied in England in the 1960s. In comparison to conventional electric ship drive systems they are lighter in weight. As in the conventional electric drive systems they also have better speed control and permit the radical rearrangement of power drive systems within ship structures. The US Navy successfully installed a prototype superconducting drive system on a small ship in 1980; development of a 40000 horsepower drive system continues. A second concept uses seawater as the working fluid in an MHD propulsion system; the drive scheme is known as an electromagnet thruster (EMT), and ship models based on this propulsion principle were promoted in the US in the 1960s and operated in Japan in the 1970s. Practical designs for a full scale EMT ship have been proposed and industrial collaboration is being sought. Electronic applications Devices being explored at present are usually based on very small amounts of superconductor in the form of very thin films or wires, or on the Josephson junction. The latter can detect the smallest measured quantities of magnetic flux, leading to very precise and sensitive instrumentation. We do not have any of these junctions with the new materials as yet. A serious obstacle to many of the applications mentioned here is the difficulty of making the two film junction when the second film must be heated to 900 or even 650°C.
Standard volt. In many countries the standard volt is maintained in terms of the voltage generated across a Josephson junction by microwave irradiation at a precisely known frequency. This standard could be more cheaply maintained (and therefore more widely disseminated) if it were operated in liquid nitrogen. There should be no loss in accuracy in operating the standard at liquid nitrogen rather than at liquid helium temperature but only when we have the new device will we be certain of this.
SQUIDs. Superconducting quantum interference de-
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the industry-previews-research vices at liquid helium temperatures are already of value in many disciplines, including medical diagnostics, geophysical prospecting, and undersea communications. Rather low performance SQUIDs made with the high temperature materials have already been operated at liquid nitrogen temperatures, but much more development is needed to make acceptable devices. Relatively inexpensive SQUID-based magnetometers operating at 77 K or higher would be deployed in large numbers if electrical noise can be held to acceptably low levels.
Infrared detection. One important application of superconductivity at 4 K is the detection of far infrared radiation. Although detectors at 77 K will inevitably be less sensitive, it is likely that they will outperform non-superconducting devices at this temperature. Microcircuit transmission lines. Superconducting transmission lines for microcircuits have been used in prototype devices of high speed analog signal processing circuitry. The extension of this technology to liquid nitrogen temperatures would lead to devices that perform certain specialized computation-intensive calculations at much faster rates than non-superconducting devices operating at 300 K, but not much faster than devices using aluminium or copper at 77 K. A/D converters. Various high speed analog to digital converters have been tested successfully at 4 K. If high quality Josephson tunnel junctions can be fabricated from the new superconductors, these devices should perform comparably at 77 K. At this temperature, integration of the superconducting devices with certain semiconducting devices (for example, complementary metal oxide semiconductors) becomes feasible, and new hybrid systems may well result in the fastest A/D converters available. Microbridges (as an alternative to tunnel junctions) may also be used in some versions of A/D converters. The microbridge has the advantage that it is made of one film rather than two. Computers. A great deal of promising development has been carried out on superconducting computers but advances in semiconductor technology proved more attractive for such machines. The promise of very high packing density for the switches and connectors, made possible by the absence of heat generation due to current flow, meant that very powerful computers and logic systems could be miniaturized and their reliability increased. The promise of liquid nitrogen cooled superconductors could lead to a re-examination of this possibility but, as yet, there are no suitable nitrogen cooled junctions. However, the promise is exciting and may lead to many new areas of application, including hybrid combinations of semiconductor and superconductor technologies. Magnetic shielding. Normal conductors and superconductors have been used for many years to create regions
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the industry-previews-research free from all magnetic fields or to shape magnetic fields. Both superconducting wires and sheets have been used for this purpose. The advent of the high temperature superconductors may extend the range of this application since, as with Nb3Sn, the possibility of plasma sprayed sheets or films permits the shielding of regions with surfaces of complex shape.
Present m a r k e t f o r s u p e r c o n d u c t o r s and likely changes associated w i t h t he n e w materials The world superconductor industry is small but superconducting devices are usually components of larger systems whose gross annual sales volume is many times the value of the devices themselves. Annual device sales total about US$400 million, of which medical imaging machines and electronic instruments account for about US$150 million each. Magnet coils account for 10-20 % of system costs and annual sales of basic materials such as alloy rod and sheet are of the order of US$10-20 million. It is difficult to estimate the potential economic impact of the present high temperature superconductors because so little is known about them and much depends on improved understanding and technological development. Assuming that satisfactory conductors can be produced, there are considerable advantages to operation in liquid nitrogen. Refrigeration units are simpler and cost less to operate. Conductor stability improves as the temperature increases, because of the higher heat capacity of materials are less brittle at higher temperatures and higher resistivity of the stabilizing matrix. Structural materials are less brittle at higher temperatures an cheaper less exotic materials can be used. Cryogenic liquids and systems, however, will still be needed. In comparing superconductor technology with present room temperature devices, the need for cooling is a serious economic and technological disadvantage. There is a great difference between switching on a machine as needed and having to wait for the refrigeration system to reach operating temperature. Assuming that some utility and heavy electrical power applications can be competitively marketed using systems cooled by liquid nitrogen, the superconducting materials market may be substantially increased; the market for electrical equipment used in the electric utility industry, however, would be mainly a replacement one, since few new systems are being built. For substantial business growth above that achieved with low temperature superconductors, new technology developments are needed. There is little doubt that the new materials offer technological advantages, as they promise high magnetic field devices and new types of electronic sensors and switches at lower costs in refrigeration than before. The Hulm Panel is unanimous in stating that advances are bound to result in new applications and new economic growth. If room
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the industry- previews-research temperature superconductors become available, we can confidently expect truly revolutionary expansion of superconducting applications in electrotechnology.
Superconductivity: the global picture On a global scale, today's world superconductor industry is small but mature and principally confined to the more industrialized countries. Over the past 25 years, in many such countries, a wide variety of applications of superconducting electrotechnology has been examined in prototype development programmes. However, no replacements for conventional applications have reached the market. As a result, the demand for superconducting materials has been relatively small and has lacked continuity, being largely oriented to development. Nevertheless, government programmes have supported this fledgling industry in each of these countries. Basic research capabilities are more widespread and national and international conferences on all aspects of low temperature physics have become routine. In the USA, magnet development for high energy physics machines has been carried out in the national laboratories. Fusion and MHD magnets have been built both in the national laboratories and in private industry. There is also a rapidly growing commercial market based mainly on new medical imaging systems. A small materials and wire industry serves magnet development efforts. Many US firms have supported their own research and development efforts in superconductor technology. Corporations in Europe and Japan have also fostered and maintained an expertise in superconductivity. Their governments have to some degree protected their superconductor industries by ensuring that equipment for government laboratories is built by private industry, with foreign bids not being accepted. This assures an 'in house' industrial expertise. The USSR also has agressive long term programmes in energy conservation (including power transmission and storage); the fabrication of superconducting wires and tapes; electronics; and in collider construction, magnetic fusion, magnetohydrodynamics, and superconducting generators. They also have a long tradition of theoretical and experimental strengths in superconductivity. Basic HTSC research Basic research in high temperature superconductivity is being actively pursued in all of the countries mentioned above and in several developing countries. In most cases, the scientists themselves have automatically switched from other scientific activities into HTSC research and caught up in the excitement of the moment, officialdom has condoned the switch. However, little new money has gone into the effort. Plans are being prepared for 1988 but at present no
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the industry-previews-research major new government resources have been committed. The prevailing attitude appears to be that of waiting to see how the science progresses. Japan. In Japan the scientific and technical community has responded vigorously but aside from reprogramming there has been minimal immediate additional action by government agencies. The latter have, however, been very active in formulating plans for the fiscal year which began in April 1988. Private industrial corporations are said to be investing their own funds heavily in research on high temperature superconductors, with the government intervening to establish industry consortia to pursue prototyping and other early development activities. Europe. In Europe there is a long tradition of basic research and industrial development. These strengths are being applied to the new superconductors. National and cross- national efforts are in the early organizational stages at best (again, with the exception of reprogramming of research funds), and major project goals to drive technical problem solving are not yet in place. However, a variety of industrial corporations are involved in research and pre-competitive collaborations appear to be at advanced planning stages. USSR. The extent of the effort in the USSR is not known to us in detail. There is, however, great enthusiasm, at least equal to that expressed in the USA.
Update This article, our condensation of the Hulm Panel Briefing, was prepared in November 1987. In December, Maeda et al. at the National Institute for Metals (NRIM) in Japan developed a material with the formula BJ2Sr3 xCaxCu208 + v which is more stable than the 1 -2-3 compounds, is easier to make, and has a critical temperature around 1 25 K. Later, Sheng and Hermann at the University of Arkansas, USA, obtained similar results by substituting thallium for the bismuth. The structure of these materials differs from that of previous compounds in that they are made up of alternating double copper-oxygen and double bismuth (or thallium)-oxygen sheets. While remarkable progress has been made in the last year in understanding the nature and structure of all these new superconducting ceramics, no theoretical explanation for their behaviour has, as yet, been agreed upon. Our comments on the technical difficulties and barriers to application are still valid f J. K. Hulm Westinghouse R & D Center, Pittsburgh, Pennsylvania, USA C. Laverick Independent Consultant, Patchogue, New York, USA
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