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Report of the Research Bnefing Panel on High-Temperature Superconductivity
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Research Beefing Panel on High-Temperature Superconductivity John K. Hulm (Chairman), Director, Corporate Research and R&D Planning, Westinghouse R&D Center, Pittsburgh, Pa. Neil W. Ashcroft, Professor of Physics, Cornell University Roger W. Boom, Professor and Director, Applied Superconductivity Center, Nuclear Engineering and Metallurgical Engineering, College of Engineering, University of Wisconsin, Madison H. Kent Bowen, Ford Professor of Engineering, Massachusetts Institute of Technology Robert l. Cava, Technical Staff, AT&T Bell Laboratories, Murray Hill, N.~. Paul C.W. Chu, Temple Chair in Science, University of Houston, Tex. John Clarke, Professor of Physics, University of California, Berkeley, and Principal investigator, Lawrence Berkeley Laboratory Marvin L. Cohen, Professor of Physics, University of California, Berkeley 2 lames S. Edmonds, Senior Project Manager, EPRT, Palo Alto Douglas K. Finnemore, Associate Director, Science and Technology Division, Ames Laboratory, Department of Energy Eric B. Forsyth, Chairman, Accelerator Development Department, Brookhaven National Laboratory Theodore H. Geballe, Professor of Applied Physics, Stanford University David C. Larbalestier, Associate Director, Applied Superconductivity Center, Department of Metallurgical Engineering, University of Wisconsin, Madison Charles Laverick, Private Consultant, Patchogue, N.Y. Alexis P. Malozemoff, Division Coorclinator for High-Temperature Superconductivity, T.~.Watson Research Center, IBM Corporation lames H. Parker, Private Consultant, Penn Hills, Pa. David Pines, Professor, Loomis Laboratory of Physics, University of Illinois, Urbana
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HIGlI- TEMPE - TURE S UPER COND UCTIVI~ Carl H. Rosner, President, Intermagnetics General Corporation, GuilclerIand, N.Y. John Rowell, Assistant Vice President, Solid State Science and Technology Research Laboratory, Bell Communications Research, Red Bank, N.~. Arthur Sleight, Research Leader, Central Research and Development Department, Experimental Station, A. duPont de Nemours & Company lames L. Smith, Senior Scientific Advisor, Center for Materials Science, Los Alamos National Laboratory Masaki Suenaga, Senior Metallurgist, Department of Applied Sciences, Brookhaven National Laboratory Maury Tigner, Director, SSC Central Design Group, Universities Research Association 3 Michael Tinkham, Rumford Professor of Physics and Gordon McKay Professor of Applied Physics, Harvard University John Williams, Head, Magnet Technology Division, National Magnet Laboratory, Massachusetts Institute of Technology Committee on Science, Engineering, and Public Policy Staff AlIan R. Hoffman, Executive Director John R. B. Clement, StaffOff'cer Barbara A. CandIand, Administrative Coordinator Cathy D. Dyson, Senior Secretary Nisha Govindani, Senior Secretary
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Report of the Research Bnefing Panel on [ligh-Temperature Superconductivity EXECUTIVE SUMMARY The recent discovery of superconductivity at temperatures up to 95 K is one of the more important scientific events of the past dec- ade. The sheer surprise of this discovery, as well as its potential scientific and commercial importance, largely underlie the degree of excitement in the field. Because our previous understanding of superconductivity has been so funciamentally challenged, a cloor has been opened to the possibility of super- conductivity at temperatures at or above room temperature. Such a development would represent a truly significant break- through, with implications for widespread application in modern society. While the base of experimental knowledge on the new superconductors is growing rap- idly, there is as yet no generally accepted the- oretical explanation of their behavior. Appli- cations presently being considered are largely extrapolations of technology already under investigation for Tower-temperature superconductors. To create a larger scope of applications, inventions that use the new materials will be required. The fabrication and processing challenges presented by the new materials suggest that the period of pre 5 commercial exploration for other applica- tions will probably extend for a decade or more. Near-term prospects for applications of high-temperature superconducting materi- als include magnetic shielding, the voltage standard, superconducting quantum inter- ference devices, infrared sensors, micro- wave crevices, and analog signal processing. T onger-term prospects inclucle large-scare applications such as microwave cavities; power transmission lines; and supercon- ducting magnets in generators, energy stor- age devices, particle accelerators, rotating machinery, medical imaging systems, levi- tated vehicles, and magnetic separators. in electronics, long-term prospects include computer applications with semiconduct- ing-superconducting hybrids, Josephson devices, or novel transistor-like supercon- ducting devices. The United States has a good competitive position in the science of this field, and U.S. researchers have contributed significantly to the worldwide expansion of scientific knowledge of the new materials. Interna- tional competition is intense. Several other leading industrialized countries have mounted substantial scientific and techno
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logical efforts, especially Japan, a number of ing on synthesis, processing, stability, and Western Euronean nations ~nr1 the I JEER methods for large-scale production; . _ ,~ , ~ ~ . ~ The short-term problems and long-term potential of high-temperature superconduc tivity may both be easily underestimatecl. Given this potential and the current limited understanding of the new superconducting materials and their properties, it is essential that government, academic institutions, and industry take a long-term, multidisciplinary view. Since science and technology in this field are strongly intertwined, progress must occur simultaneously in basic science, man ufacturing/processing science, and engi neering applications. It is also important to maintain an open and cooperative interna tional posture. The panel has identified eight major scien tific and technological objectives for a na tional program to exploit high-temperature superconductivity. They are: I. to improve understanding of the essen tial properties of current hi~h-temnerature . . C~ - r ~ superconducting materials (especially Tc' HC2' fc, and alternating current losses) through the acquisition of additional experi- mental data; 2. to develop an understanding of the ba- sic mechanisms responsible for supercon- ductivity in the new materials; 3. to search for additional materials exhib- iting superconductivity at higher tempera- tures by the synthesis of new compositions, structures, and phases; 4. to prepare thin films of controllable and reproducible quality from present high-tem- perature superconducting materials anc! to establish preferred techniques for growing films suitable for electronic device fabrica- tion; 5. to develop bulk conductors from cur- rent high-temperature superconducting ma- terials, with special emphasis on enhanced electric current-carrying capacity; 6. to advance the understanding of the chemistry, chemical engineering, and ce- ramic properties of the new materials, focus 6 7. to fabricate a range of prototype circuits and electronic devices based on su- perconducting microcircuits or hybrid superconductor/semiconductor circuits, as suitable thin film technologies become avail- able; and 8. to fabricate a range of prototype high- field magnets, alternating and direct current power devices, rotating machines, transmis- sion circuits, and energy storage devices, as suitable bulk conductors are developed. The panel recommends that the following actions be taken to carry out the objectives listed above: · The U.S. government should proceed with its plans to provide funding for high- temperature superconductivity research and development on the order of $100 mil- lion for fiscal year 1938. This funding level represents a good beginning in addressing the challenges and opportunities offered by the new materials. · Sufficient new money must be provided both to the science and the technology of high-temperature superconductivity so that other important and promising areas of re- search and development are not held back. · A mechanism should be established to monitor the potential demand for increased scientific and technical manpower if the promise of high-temperature superconduc- tivity is fully realized, and to make appropri- ate recommendations on the funding of U. S. graduate and postgraduate research pro- grams. · An interagency mechanism should be established to help coordinate planning for superconductivity programs among the var- ious federal agencies. · Given the anticipated rate of advance in high-temperature superconducting science and technology, the federal government should review progress in the field after 12 months as a guide to future resource alloca- tion.
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HIGH-TEMPERATURE S UPERCOND UCTIVITY · Through its agencies, the U. S. govern- ment must enhance the probability that U. S. industry gains a competitive advantage in this new fielcI. This could be accomplishec! by the close association of industry with the Engineering Research or Science and Tech- nology Center programs of the National Sci- ence Foundation, by cost-sharing between government and industry on proof-of-con- cept projects, and by other joint efforts. 7 · An important mechanism for enhancing U.S. industry's position is improved tech- nology transfer from the national laborato- ries to the private sector. Although a variety of means are already in place to encourage such transfer, the pane! is concerned about the effectiveness of past efforts anc! urges both government and industry to pursue linkages more aggressively.
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INTRODUCTION Perhaps the most remarkable feature of the discovery of high-temperature super- conductivity is the fact that it was so unex- pected. The sheer surprise of this discovery, as well as its potential scientific and commer- cial importance, largely underlie the degree of excitement and fervor of the field. Super- conductivity in the past has always been a challenging fundamental and technological problem, for which understancling and ap- plication have come slowly. Because our pre- vious understanding of superconductivity has been so fundamentally challengeci, there is hope that the progress that has been achieved so dramatically in the past IS months can be continued. High-temperature superconductivity of- fers an important opportunity for our na- tion's scientific and technological commu- nity. The opportunity merits a substantial thrust in fundamental research; at the same time, enough is already known to encourage commercial development efforts with the newly discovered materials. BACKGROUND Superconductivity was discovered by a Dutch scientist, Kamerlingh Onnes, in 1911. He found that the electrical resistance of frozen mercury (Hg) disappeared suddenly at 4.2 degrees Kelvin (K) (-269 degrees Cel- sius tCl), a temperature accessible only through immersion in liquid helium. In 1913 Onnes also found that weak magnetic fields (of a few hundred gauss) destroyed the ef- fect, with the metal reverting to its normally resistive state. Subsequently, other metals such as tin (Sn) and lead (Pb) were founc! to be superconductors at similarly low temper- atures. People soon began to invent applica- tions for superconductors for example, to reduce Tosses in electric power systems. However, in the 1920s it was found that su- perconductivity disappeared in these met als when rather Tow electric currents were passed through them. As a result, power ap- plications were abandonecl.* Significant progress in understanding the physical basis of superconductivity came in the 1950s. In the theory of Barcleen, Cooper, anc! Schrieffer, interaction between elec- trons and "phonons" (vibrational modes in the lattice of atoms making up the material) leads to a pairing of electrons. At Tow tem- peratures, these so-called Cooper pairs con- dense into an electrical superfluid, with en- ergy levels a discrete amount below those of normal electron states (known as the super- conducting energy gap). in the same period, new materials were discovered that dis- played superconductivity at temperatures as high as 20 K, almost 5 times higher than the temperature of superconductivity in mer- cury. These scientific discoveries had two im- portant consequences. First, in a direct ex- periment to verify the energy gap, Giaever at the General Electric Research Laboratories observed electron tunneling between super- conductors that is, electrons passing from one superconductor to another through a thin insulating barrier. The observation of normal electron tunneling lecl Josephson in England to speculate that Cooper pairs could also tunnel through a barrier, a prediction that was soon verified by Rowell and Ancler- son at Bell Laboratories. These discoveries laid the foundation for a whole new super- conclucting electronics technology. Second, Kunzler and coworkers at Bell T aboratories established that a group of su- perconducting compounds and alloys (the Type-2 superconductors) could carry ex- tremely high electric currents (up to a million amperes per cm2 of conductor cross-section) and remain superconducting in intense magnetic fields (up to 30 tesla iT] or 300,000 *Electric utility equipment typically carries thousands of amperes and has associated magnetic fields of up to 20,000 gauss or 2 tesla. 8
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HIGH- TEMPERATURE S UPER COND UCTIVITY gauss). These materials, offering the pros- pect of very high magnetic fielcls and cur- rent-carrying capacity at a much Tower cost than before, revived interest in super- conducting magnets and electric power components. In the years between 1960 and 1986, sev- eral hundred materials were found to be su- perconducting at sufficiently low tempera- tures. However, the highest critical tempera- ture (i.e., the temperature below which a material becomes superconducting, Tc) achieved in this period was 23 K, which still required either liquid helium or liquid hy- drogen cooling. The workhorse of the high-field magnet technology has been niobium-titanium (NbTi), a ductile alloy that can be made into wires. A second material of great promise, niobium-tin (Nb3Sn), can support even larger electric currents and remain supercon- ducting in higher magnetic fields, but it has found much less use because of its brittle na- ture. Other materials have found more lim- ited uses for example, pure niobium in ra- diofrequency cavities and niobium nitride (NbN) in electronics. RECENT DISCOVERIES Recently, new materials have been discov- ered that have substantially higher TCS. In January 1986 Bedsore and Muller, working at the IBM Laboratory in Zurich and search- ing for superconductivity in previously unexplored materials, determined that a lan- thanum-barium-copper ceramic oxide be- came superconducting at temperatures over 30K. Spurred on by this unexpected discovery, laboratories in the United States and else- where have since found materials with even higher Ts. The highest stable value to date that has been independently confirmed, 95 K (-178 C), was first achieved by Chu and colleagues at the University of Houston working with Wu and coworkers at the Uni 9 versity of Alabama. At this temperature, liq- uid nitrogen (which boils at 77 K at atmo- spheric pressure and is much cheaper than liquid helium) can be used for cooling. A large number of the so-called high-tem- perature superconductors are now known to exist, all of them variations of two basic types (the so-called 40 K and the 95 K for I-2-3] ma- terials). Those with TC greater than 77 K are based on only one structure, with copper (Cu) and oxygen (O) a constant feature. The new materials present an enormous sci- entific opportunity and open new vistas for potential applications. Because our under- standing of superconductivity has been chal- lenged in so fundamental a fashion, with the present theoretical understanding of super- conductivity being insufficient to explain the properties of the new materials, there is hope that what has been achieved in such a short time can be extended. The excitement surrounding the field has caught the imagi- nation of policymakers, the media, and the public at large. Research students are also at- tracted by this excitement, often being drawn from other areas of science by the prospect of careers in the field; but it is im- portant to note that they are drawn from a manpower pool that can only be expanded slowly. There have been several preliminary re- ports of superconductivity at still higher temperatures, but at present there is no con- sensus as to their validity. It is likely that room-temperature superconductivity would make possible a much broader range of ap- plications. A more immediate concern is whether the present high-temperature su- perconductors can be used to improve present electronic or power applications; on this question, researchers worldwide are cautiously optimistic. *Some of the manpower issues are discussed in Physics Through the 1990s: An Overview, Washington, D.C.: National Academy Press, 1986.
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CURRENT KNOWLEDGE OF THE NEW HIGH-TEMPERATURE SUPERCONDUCTORS The new high-temperature superconduc- tors are mixed metal oxides that display the mechanical and physical properties of ce- ramics. A key to the behavior of the new ma- terials appears to be the presence of planes containing copper and oxygen atoms chemi- cally bonded to each other. The special na- ture of the copper-oxygen chemical bonding gives rise to materials that conduct electricity well in some directions, in contrast to the majority of ceramics, which are electrically insulating. The first class of high-TC oxides discoverec! was based on the chemical alteration of the insulating ternary compound La2CuO4 by replacement of a small fraction of the ele- ment lanthanum (La) with one of the alkaline earths barium (Ba), strontium (Sr), or cal- cium (Ca). The substitution led to com- pounds with critical temperatures of up to 40 K. In these materials, an intimate relation be- tween superconductivity anc! magnetic or- der is presently uncler intensive study and has inspired one of the many classes of theo- ries that attempt to explain high-tempera- ture superconductivity. In a second class of compounds, based on YBa2Cu3Ox (where Y is yttrium, a rare earth), the metallic atoms occur in fixec! pro- portions. These are the so-called 1-2-3 compounds, which are highly sensitive to oxygen content, changing from semicon- ducting, at YBa2Cu306.5, to superconducting near 95 K at YBa2Cu307, without losing their crystalline integrity. The high sensitivity of their properties to oxygen content is due to the apparent ease with which oxygen can move in and out of the molecular lattice. The 40 K and 1-2-3 (or 95 K) materials have similar structures but differ significantly in other respects. In both compounds, the rare earth and alkaline earth atoms provide a structural framework within which the chains of copper and oxygen atoms may be hung. Surprisingly, the substitution of other rare earths, even magnetic ones, for yttrium in the 95 K compounds results in very little change in superconducting properties. Vari- ous substitutions are under study, both to understand the present materials and to achieve higher critical temperatures in new ones. STATUS OF THEORETICAE UNDERSTANDING In the microscopic theory of Bardeen- Cooper-Schrieffer, the presence of a net at- tractive interaction between conduction electrons, which would normally repel each other because of their like electrical charges, is essential to the occurrence of supercon- ductivity. In conventional superconductors this attraction originates in the dynamic mo- tion of the crystal lattice, which leads to an attractive ''electron-phonon-electron" in- teraction. But the recent appearance of su- perconductivity in a class of materials quite different from the conventional supercon- ductors, and with extremely high transition temperatures as well, has led physicists to explore a very wide spectrum of possible new pairing mechanisms involving, for ex- ample, spin fluctuations, acoustic plasmons, and excitonic processes. The physical origin of the pairing "glue" remains an open and to some extent crucial question. There is a wide range of theoretical possibilities, and the ul- timate explanation may involve a combina- tion of mechanisms. Indeed, some theorists have discarded conventional Bardeen- Cooper-Schrieffer theory and have sug- gested that there may not even be the tradi- tional close relationship between energy gaps and basic superconducting properties. Given the wealth of puzzling experimental features in a variety of different materials, it may take a considerable effort, with a diverse theoretical program, to unravel fully the se- crets of these compounds. In the meantime 10
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HIGH- TEMPERATURE S UPERCOND UCTIVITY the fact that they obviously do exist can form the basis of immediate commercial exploita- tion. But the prospect of even more promis- ing materials has led to a substantial theoreti- cal effort aimed at eTuciciating the principles underlying the phenomenon. Typical of the questions currently under active consider- ation are the role played by oxygen, the na- ture and scope of dynamic mechanisms and the resulting electron pairing, whether this coupling is weak or strong, and whether the anisotropic nature of the materials is a truly important feature. The appearance of super- conducting coherence lengths ~ or 2 orders of magnitude smaller than those previously encountered, the very low carrier concentra- tions, and the apparent importance of both copper and oxygen will probably require a considerable extension of our current undier- standing of superconductivity. The fact that the superconducting interaction mechanism in the new materials is likely to be very differ- ent from that in low-temperature supercon- ductors certainly enhances the prospect that other high-temperature superconducting materials may be discovered. PHYSICAL PROPERTIE S IMPORTANT FOR TECHNOLOGY The most important physical properties for applications are the superconducting critical temperature (Tc), the upper critical magnetic field (Had' and the maximum cur- rent-carrying capacity in the superconduct- ing state (/c) Also important are the mechan- ical, chemical, and electromagnetic proper- ties: physical and thermal stability, resis- tance to radiation, and alternating-current loss characteristics. Each is discussed more fully below. CriticaZ Temperature, Tc A rule of thumb for general applications is that materials must be operated at a tempera- ture of 3/4 Tc or below. At about 3/4 Tc critical 11 fielcis have reached roughly half their low- temperature limit, and critical current densi- ties roughly a quarter of their limit. Thus, to operate at liquicl nitrogen temperature (77 K), one would like Tc near 100 K, making the 95 K material just sufficient. To operate at room temperature (293 K) one requires a ma- terial with Tc greater than 400 K, well above the highest demonstrated value. Higher Tc materials would be superior across the board for applications, other properties being ac- ceptable, and materials with TCS above 400 K would have a truly revolutionary impact on technology. In this temperature domain one could consider mass market applications. Upper CriticaZ Magnetic FieZci, Hc2 YBa2Cu3O7 samples generally exhibit ex- tremely high upper critical fielcls. Prelimi- nary measurements indicate that for single crystals HC2 is anisotropic, that is, dependent upon field direction relative to the a-, b-, or c- axes of the orthorhombic lattice. Values ranging from 30 T (c-axis) to 150 T (a- or b- axes) are reported at 4.2 K. The mechanical stresses associated with the confinement of such high magnetic fields in typical compact geometries are frequently beyond the yield or crushing strengths of known materials. Hence, improving these intrinsic HC2 values is less important than increasing Tc or Ic val- ues. In fact, materials with higher TCS should exhibit higher HC2 values if the performance of known materials is any guide. However, developing materials that can practically be fabricated into magnets and that retain use- fuT lcs at fields approaching HC2 even at 77 K is an important challenge. CriticaZ Current Density, fc For practical applications, k values in ex- cess of 103 amperes per square millimeter (A/mm2), are desirable both in bulk conduc- tors for power applications and in thin film superconductors for microelectronics.
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the new materials. A better understanding of phase equilibria, solid solutions, and in- termetallic compounds is needed to find sta- ble ohmic contacts that do not degrade su- perconducting behavior. 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 de- vice that utilizes a superconductor with a TC of 95 K. Cryogenic systems that operate be- low 77 K should be investigated, and the compatibility of high-temperature super- conductors with refrigerants other than liq- uid nitrogen (e.g., liquid neon) should be tested. Further, and perhaps most impor- tantly, 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 detectecI at some future date in compounds not yet stuctied. CURRENT FIELDS OF APPLICATION AND THE LIKELY IMPACT OF THE NEW MATERIALS Virtually all of the applications currently envisioned for high-temperature supercon- ductors are extrapolations of devices already operated at liquid helium temperatures. The most important applications, however, may well involve devices that have yet to be con- templated, much less invented. As shown in Table 1, present and potential applications fall into several distinct classes. Present applications include high-field mag- nets, radiofrequency devices, and electron- ics. Superconductivity brings unique ad- vantages to high-field applications because resistive conductors such as copper dissipate large amounts of energy as heat when carry- ing large currents. Superconductors are also useful in high-Q cavities because of their Tow alternating current losses at high frequencies compared to those in normal metals. 14 TABLET PrincipalApplicationsof Superconductivitya PRESENT APPLICATIONS Magnets . Commercial and industrial uses Medical diagnostics and research (magnetic-resonance imaging and spectroscopy) Radiofrequency devices (gyrotrons) -Ore refining R&D magnets Magnetic shielding · Physics machines (colliders, fusion machines, radiofrequency cavities) Electronics · Sensitive accurate instrumentation (super- conducting quantum interference devices, infrared sensors) · Electromagnetic shielding POTENTIAL APPLICATIONS (Proven superconducting technology, but no current market adoption) Power utility applications Energy production (magnetohydrody- namics, magnetic fusion) Large turbogenerators -Energy storage Electrical power transmission Transportation High-speed trains (magnetic levitation) Ship drive systems Computers Semiconducting-superconducting hybrids Active superconducting elements aThere are many other potential applications' some of which are mentioned in the text. . Electronic applications for superconduc- tors usually involve Tow electric currents (al- though high current densities) and low mag- netic fielcls. The core element has been a unique bistable device, the Josephson junc- tion. Superconductors may also eliminate resistive current losses in electronic lines and device interconnections. In addition, vari- ous kinds of superconducting sensors have
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HIGH- TEMPERATURE S UPER COND UCTIVITY been produced. ATi of these applications, in- cluding the assembly of superconducting electronic components into larger crevices, will be reconsiderecT with the new com- pouncls. Potential applications of high-temperature superconductors divide into those relevant to currently available materials with critical temperatures near 95 K, and those relevant to possible future materials with higher criti- cal temperatures. The most exciting possibil- ities, of course, arise with materials with crit- ical temperatures above room temperature. H~GH-F~E~D AND ~ARGE-SCAEE APPLICATIONS Superconducting magnets using liquid helium technology have been successfully applied for a number of years in engineered systems and development projects in hospi- tals, mines, industrial plants, laboratories, and transportation systems. Most of these applications require multiple technologies, with superconductivity playing a critical role. In medicine, superconductors have been a significant factor in the clevelopment of a new market. In high-energy physics, superconductors have lee! to machines of un- precedented and previously inconceivable energy. Tn electric power, potential applica- tions in energy storage and power trans . . mission equipment promise to extend the capacity and range of current technology. Superconducting magnets are essential components in experimental systems for magnetic fusion and magnetohydrodynam- ics (MHD). The extremely high power-to- weight ratio possible for superconducting machines makes them particularly attractive for space applications. For magnet and power applications, the higher the critical temperature, the smaller will be the scale at which commercial viabil- ity will be achieved. As an example, the power level at which motors ancT generators become competitive will be much lower than 15 with the present low-temperature supercon- ductors, when compared to nonsupercon- clucting machines. A liquid nitrogen-cooled motor, for instance, operating at modest cur- rent and magnetic field, might well be smaller, more efficient, and more reliable for the same power output than many present- clay motors. For most applications the switch from liq- uid helium to liquid nitrogen technology is not revolutionary but will lead to improve- meets. The continued neec! for refrigeration is a disadvantage and will reduce market penetration. Of course, the reconsideration of applications held to be impractical at liq- uid helium temperatures might lead to new products. A hollow conductor cooled with liquic! nitrogen is easy to visualize in practi- cal use, for instance. It may not be necessary to demand that the technical specifications of new materials compete with the best com- mercial superconducting materials of toclay: a conductor of modest specifications may have value in a wicler context than conven- tional low-temperature superconductors. The new materials, in short, may not so much replace present-day superconductors as extend the applications of superconduc- tivity to a larger circle of users. Medical Applications Magnetic-resonance imaging (MRT) and spectroscopy (MRS) constitute radically new techniques in medical diagnosis and treat- ment, and their full impact is yet to be real- izecl. Much more widespread availability of MRT and MRS systems can be anticipated, with concomitant reductions in cost and en- hancement of features. The use of high-tem- perature superconducting materials would likely bring further small reductions in the costs of manufacture and operation. The re- design of MR] and MRS systems with liquid nitrogen cooling wouIcl also make them more user-friendly and reliable by reducing cooling system complexity.
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Superconducting Radiofrequency Cavities If microwave alternating current Toss char- acteristics are tolerable, the new supercon- cluctors may greatly improve the perfor- mance of superconducting radiofrequency cavities by allowing them to operate at higher fields. Tndeecl, the potential impacts embrace all of microwave power technology, especially in the promising millimeter-wave region. Accelerator technology might also be significantly advanced by the availability of ~liquicl nitrogen-cooled superconducting cav- ities. The applicability of superconducting technology to recirculating linear accelera- tors, on the other hand, is an accepted fact. In addition to providing high-quality beams for nuclear physics research, these machines are natural canctidates for continuous beam injectors used in free-electron lasers. As technology matures anct industrial applica- tions develop for high-power, high-effi- ciency tuneable lasers in biotechnology, fusion plasma heating, and other fields, superconducting radiofrequency crevices will proliferate. Transportation Ambitious attempts to apply supercon- ductivity to lane! and ocean transportation have been made over the years with some success in the United States, Europe, and la- pan. In fact, a Japanese magnetic levitation rail system is available for interested buyers anc7 is economically viable. Superconducting ship propulsion systems were studied in England in the 1960s. The U. S. Navy successfully installecl a prototype superconducting drive system on a small ship in 1980; the clevelopment of a 40,000- horsepower drive system continues. A sec- ond concept uses seawater as the working fluid in an MHD propulsion system; the drive scheme is known as an electromag- netic thruster (EMT). Ship models based on this propulsion principle were promoted in the United States in the 1960s and operated in Japan in the 1970s. Practical designs for a full-scare EMT ship have been proposed, and industrial collaboration is being sought. Because the present worldwide systems of air, sea, and land transportation are well es- tablished anc! represent a substantial invest- ment, society has not yet made use of the potential advantages that have been demon- strated in prototype transportation systems using low-temperature superconductors. In land-based systems the principal advantage is high speect. Ship-based systems are lighter, have better speed control, and per- mit the radical rearrangement of power drive systems within ship structures. The combi- nation of room-temperature superconduc- tors and the low specific volume of super- conclucting machines could revolutionize surface transportation. ELECTRONIC APPLICATIONS Some of the most promising applications of high-temperature superconductors are electronic systems involving thin film lines or Josephson elements. Applications in com- puters would have the largest commercial impact, but may take longer because of their complexity. Sensor and instrument applica- tions are simpler and are likely to be com- mercialized within a few years. Simplest of all is the use of high-temperature super- conductors for Tow-field electromagnetic shielding. Computers and Logic Devices Much work has already been carried out on computer subsystems based on liquid he- lium superconductors. Semiconductor tech- nology is still advancing rapidly, however, and continues to dominate the computer field. The discovery of high-temperature su- perconductors may change this situation. 16
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HIGH-TEMPERATURE SUPERCONDUCTIVITY One possible role of superconductors in such systems is simply to interconnect the semicondiucting devices with superconduct- ing microcircuit transmission lines. This pos- sibility is aIreacly interesting at 77 K, because certain semiconducting devices switch faster at this lower temperature. However, 77 K copper lines present significant competition because of their decreased resistivity com- pared to that at room temperature. The most exciting opportunities would use room temperature superconductors, offering compatibility with the entire line of semiconductors, including the highest per- formance bipolar devices. In the most prom- ising scenario, the use of room temperature superconductors could affect the full range of data processing systems, which form the largest high-technology industry in the world tociay. Although the implications of high-temper- ature superconductors for semiconducting computer systems have yet to be assessed, the reduced losses compared to normal con- ductors offer many possible advantages. System performance (i.e., switching speed) can be increased by reducing the RC time constant associated with the interconnect line. Narrower lines can be used, saving space on the chip. The elimination of power Tosses and voltage drops permits miniatur- ization of power busses and potentially, therefore, of the entire system. The high-temperature superconductors have also been proposed for computer appli- cations using Tosephson junctions. High- temperature superconductors may offer higher device switching speeds, higher bandwidth transmission, and the possibility of using semiconclucting memory to supple- ment ultra-high-speed superconducting logic. The disadvantages of high-tempera- ture superconductors are increased thermal noise and switching power losses at 77 K compared to earlier liquid-helium tempera- ture designs. A variety of superconducting transistor 17 like crevices have been proposed, among them superconducting fielct-effect transis- tors (FETs), several nonequilibrium devices, and optically switcher! FETs. These devices are at early stages of development, even us- ing the conventional low-temperature su- perconductors; but although there are still considerable materials and fabrication prob- lems, the potential performance of some of these devices might be enhanced by higher switching speeds and output voltage changes stemming from the larger energy gaps of the high-temperature superconduc- tors. SENSORS AND OTHER APPEICATIONS SQUlDs Superconducting quantum interference devices (SQUTDs), operating at liquid he- lium temperatures as sensitive magnetic field detectors, are already of value in many disciplines including medical diagnostics, geophysical prospecting, undersea commu- nications, and submarine detection. SQUTDs made with the new high-tempera- ture materials have been operated at liquid nitrogen temperatures. Relatively inexpen- sive SQUlD-based magnetometers operat- ing at 77 K or higher would be deployed in large numbers if electrical noise can be held to acceptably low levels. Radiation Detectors Superconducting microwave and far-in- frared radiation detectors (quasiparticle mix- ers, superconducting bolometers) already exist using conventional superconductors. in spite of a loss of sensitivity due to in- creased electrical noise at higher tempera- tures, the increased energy gap of high- temperature superconductors would offer sensitive detection in a largely inaccessible frequency range, and the simplified refriger
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ation allows increased ease of use. Other mi- crowave applications include high-Q wave- guides, phase shifters, and antenna arrays. Analog Signal Processors High-speed analog signal processors per- forming such functions as filtering, convolu- tion, correlation, Fourier transformation, and analog-to-digital (A-to-D) conversion are important for many applications. Vari- ous high-speed A-to-D converters have been tested successfully at 4.2 K. If high-quaTity Josephson junctions can be fabricated from the new superconductors, these devices shouIc! perform comparably at 77 K. At this temperature, integration of the supercon- ducting devices with some semiconclucting devices (for example, complementary metal- oxide semiconductors becomes feasible, and new hybrid systems may well result in the fastest A-to-D converters available. Magnetic Shielding Both superconducting wires and super- conducting sheets have been used for many years to create regions free from all magnetic fields or to shape magnetic fielcls. The ad- vent of high-temperature superconductors may extend the range of this application. Like niobium-tin, high-temperature super- conductors may be plasma-sprayed, permit- ting their use on surfaces of complex shape. Voltage Standard Many countries now maintain a voltage stanciard in terms of the voltage generated across a low-temperature superconducting Josephson junction irradiatect by micro- waves at a precise frequency. This standard could be more cheaply maintained and more widely available with no significant loss of accuracy by operating at 77 K with the new materials. THE PRESENT MARKET FOR SUPERCONDUCTORS AND LIKELY CHANGES ASSOCIATED WITH THE NEW MATERIALS The world superconductor industry is small, but superconducting devices are usu- ally components of larger systems whose gross annual sales volume is many times the value of the devices themselves. Annual de- vice sales total about $400 million, of which medical imaging machines and electronics instruments each account for approximately $150 million. Magnet coils represent 10 to 20 percent of device costs in MRI systems, and annual sales of basic materials such as alloy rod and sheet are on the order of $10-$20 mil- lion. It is difficult to estimate the potential eco- nomic impact of today's high-temperature superconductors because so little is known about them and much depends on improved understancling and technological develop- ment. Assuming that satisfactory concluc- tors can be manufactured, there are con- siderable advantages to operation in liquid nitrogen. Refrigeration units are simpler and cost less to operate. Conductor stability gen- erally improves as the temperature increases because of the higher heat capacity of materi- als; however, the protective effect of the shunting normal conductor is reduced slightly because of its increased resistivity. Structural materials are less brittle at higher temperatures; therefore, more conventional structures can be used. Cryogenic liquicTs 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 supply continuous refrigeration, or having to wait for refrigera- tion systems to reach operating tempera- tures. Assuming that some utility and heavy 18
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HIGH-TEMPERATURE SUPERCONDUCTIVI7 Y electric power applications can be competi- tively marketed using systems cooled by liq- uid nitrogen, the superconducting materials market may be substantially increased; the market for heavy electrical equipment, how- ever, would be mainly a replacement one, because few new systems are being built. For substantial business growth above that projected for low-temperature supercon- ductors, new technology developments are needed. There is little doubt that the new materials offer technological advantages, for they promise high-magnetic-field devices and new types of electronic sensors and switches at lower refrigeration costs than be- fore. The panel is unanimous in stating that advances are bound to result in new applica- tions and new economic growth. If room temperature superconductors become avail- able, we can confidently expect a truly revo- lutionarv expansion of superconducting ~ ~ 1 applications in electrotechnology. THE GLOBAL PICTURE: SUPERCONDUCTIVITY CAPABILITIES IN OTHER COUNTRIES On a global scale, today's world supercon- ductor industry is small but mature and prin- cipally confined to the developed countries. Basic research capabilities are more wide- spread. Although much of the early impetus for re- search and development came from the United States, technology transfer has not been unidirectional. National and interna- tional conferences on all aspects of low-tem- perature physics have become routine. Over the past 25 years, in several countries a wide variety of applications of supercon- ducting electrotechnology have been exam- ined in prototype development programs. No replacements for conventional applica- tions have reached the market, however. As a result the demand for superconducting materials has been relatively small and has lacked continuity, being largely oriented to 19 ward development. Nevertheless, in most countries, government programs have sup- ported a fledgling industry. In the United States, magnet development for high-energy physics machines has been carried out in the national laboratories. Fu- sion and MHD magnets have been built both in the national laboratories and in private in- dustry. There is also a rapidly growing com- mercial market based mainly on new medi- cal imaging systems. A small materials and wire industry serves magnet development efforts. Many U. S. firms have supported their own research and development efforts in superconductor technology, both for power and electronic applications. A few small, continuing ventures have succeeded in superconducting electronics; a large mar- ket for superconducting electronic devices or systems has not yet developed. Corporations in Europe and Japan have also fostered and maintained an expertise in superconductivity. In those nations, foreign governments have to some degree protected their superconductor industries by ensuring that equipment for government laboratories is built by domestic private industry; foreign bids are not accepted, a policy that ensures national industrial expertise. By compari- son, much of this work in the United States is carried out in the federal laboratories from which there is little transfer to industry. In addition, foreign superconductor firms are allowed to bid on equipment needed by the United States government. The USSR also has aggressive, long-term programs in energy conservation (including superconducting power transmission and storage), fabrication of superconducting wires and tapes, electronics, collider con- struction, magnetic fusion, magnetohydro- dynamics, and superconducting generators. Over the years, the United States has pro- vided world leadership in superconducting science and technology, and has generously shared its own technology with other na- tions. Current collaborative efforts include
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the International Large Coil Fusion Project at Oak Ridge National Laboratory and lapa- nese development of a very large detector magnet for the Fermilab collider interaction area. In the evaluation of conductors for the physics collider magnets (the Superconduct- ing Super Collider and the heavy-ion colli- der), material has been purchased fromiapa- nese and European firms. The United States, through Brookhaven National Laboratory, also provides test evaluations of cables ant! conductors for the Hadron Electron Ring Accelerator (HERA) collider under construc- tion in Hamburg, West Germany. Two pro- totype magnets for the Relativistic Heavy- Ton Collider (RHTC) have been purchased from the Brown Boveri Corporation (West Germany) because this was the cheapest and quickest way to obtain them (Brown Boveri had the necessary tooling because of their work for HERA). BASIC H~GH-TEMPERATURE SUPERCONDUCTING RESEARCH Basic research in high-temperature super- conductivity is being actively pursued in all of the developed nations mentioned above and in several developing nations. In most cases scientists have switched spontane- ously from other scientific activities into high-temperature superconductivity re- search. To this point, however, little new money has gone into basic research efforts. Plans are being prepared for 198S, but at present no major new government resources have been committed. The prevailing atti- tude appears to be that of waiting to see how the science progresses. In Japan the scientific and technical com- munity has responded vigorously, but aside from reprogramming there has been modest immediate additional action by government agencies. The latter have, however, been quite active in formulating plans for the next fiscal year (which begins in April 1988~. Pri- vate industrial corporations are said to be in- vesting their own funcis heavily in research 20 on high-temperature superconductors, with the government intervening to establish in- dustry consortia to pursue prototyping and other early development activities. Japan of- fers perhaps the strongest long-range com- petitive threat to the U. S. position. In Europe, historical strengths in basic re- search and industrial development are being applied to the new superconductors. Na- tional and cross-national efforts are in the early organizational stages at best (again, with the exception of the reprogramming of research funds), and major project goals to drive technical problem solving are not yet in place. On the other hand, a variety of indus- trial corporations are involved in research, and precompetitive collaborations appear to be at advanced planning stages. In the USSR, traditional scientific strengths in superconductivity theory and basic experimental approaches are being ap- plied to the new materials. In addition, work is being carried out on the susceptibility of high-temperature superconducting materi- als to radiation damage. SUMMARY OF PANEL VIEWS AND RECOMMENDATIONS STATUS OF SCIENCE AND TECHNOLOGY · The discovery of materials that exhibit superconductivity at temperatures up to 95 K is a major scientific event, certainly one of the more important of the last decade. Meeting the complex challenge of under- standing the phenomenon will improve fun- damental knowledge of the electronic prop- erties of solids. · Although a large number of promising theories are being explored, there is as yet no generally accepted theoretical explanation of the high critical temperature behavior. Cur- rent theoretical understanding does not pre- clude TCS above 95 K. · The base of experimental knowledge on the new superconductors is growing rap
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HIGH- TEMPERATURE S UPER COND UCTIVITY idly. The intrinsic properties that can guide theory are still being determined. A number of investigators have reported supercon- ducting-like transitions at temperatures above 95 K, in some cases even above room temperature; at present those effects have not been firmly established. · The prospect exists for applying the new superconductors to both electrical anct elec- tronic technology. The nature of the new ma- terials (quaternary ceramic oxides) suggests that a substantial materials engineering ef- fort will be required to develop bulk conduc- tors for power applications or thin films for electronic applications. · The applications currently being consid- ered are largely extrapolations of technology already under investigation for lower tem- perature superconductors. To create a larger scope of applications, inventions that use the new materials will be required. Given the materials engineering problems already mentioned, the period of precommercial ex- ploration of the new superconductors for other applications will probably last for a decacle or more. Although it is too early to make a sound engineering judgment about most of the possible high-temperature superconductivity applications, the poten- tial impact could be enormous, especially if operation at room temperature can be achieved. · Near-term prospects for high-tempera- ture-superconductivity applications inclucle magnetic shielding, the voltage standard, SQUTDs, infrared sensors, microwave de- vices, and analog signal processing. Longer- term prospects include large-scare applica- tions such as microwave cavities; power transmission lines; and superconducting magnets in generators, energy storage de- vices, particle accelerators, rotating machin- ery, medical imaging systems, levitated vehicles, anti magnetic separators. In elec- tronics, long-term prospects include com- puter applications with semiconductin~-su- perconducting hybrids, Josephson devices, or novel transistor-like superconducting de 21 vices. Several of these technologies will have military applications. · The complexity of the materials technol- ogy and of many of these applications makes a long-term view of research and develop- ment essential for success in commerciaTiza- tion. The infectious enthusiasm in the press and elsewhere may have contributed to pre- mature public expectations of revolutionary technology on a very short time scale. Over- reaction in either ctirection could be detri- mental to achieving the true long-term potential of high-temperature supercon- ductivity. THE UNITED STATES AND THE WORLD SITUATION · Although the initial high-temperature superconductivity discovery was made in the Swiss research laboratory of IBM, the next advances occurred very soon afterward in the United States. These advances were the synthesis of 1-2-3 compounds with TCS of up to 95 K. Before the constituents of the ma- terial were known, inclependent discoveries of the superconducting behavior of these compounds were also made in China and la- pan a few days later. · The United States has a good competi- tive position in the science of this field. it has also shown flexibility; scientists in universi- ties and industrial and government laborato- ries have spontaneously switched into this field from their previous endeavors and have contributed significantly to the worId- wide expansion of scientific knowledge on the new superconductors. Nevertheless, there is concern about the effectiveness of the nation's capabilities for translating this research strength into commercial products. · The U. S. government has already repro- grammed close to $30 million of research funds for high-temperature superconductiv- ity work in universities and industrial labo- ratories for fiscal year 1987. We estimate that at least an equivalent amount of private funds are being expended by U.S. corpora
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lions. Total annual funding levels will proba- bly be in the range of $100-$200 million by 1988, although firm figures for ind~trv ore not available. ~, · ~ nese expenditures will expand the U. S. graduate and postgraduate population working in areas relevant to high-tempera- ture superconductivity and will create pres- sures on the nation's scientific manpower pool. · The rapid dissemination of scientific results has occurred mainly through word of mouth, preprints, and press releases, re- flecting the close-knit global community of scientific endeavor. At present there are hardly any restrictions on the flow of infor- mation throughout the world. · international competition in high-tem- perature superconductivity is intense. The leading industrialized countries, especially the Unitecl States, Japan, several Western European countries, and the USSR have mounted substantial scientific and techno- logical efforts. · Should successful technologies emerge from the discoveries in high-temperature su- perconductivity, the Unitec! States has many of the ingredients needed to develop and commercialize those technologies. Whether this will result in business success for U.S. corporations in the global market will de- pend not only on technological factors but also on company business strategies and a range of government policies. The panel is not qualified to address these latter issues comprehensively but has made some recom- mendations, listed below, that should assist commercialization. FURTHER PROGRE S S: THE NEXT STEPS The short-term problems and Tong-term potential of high-temperature superconduc- tivity may both be easily underestimated. Given this potential and today's limited un- derstanding of the new superconducting materials and their properties, it is essential that government, academic institutions, and 22 industry take a long-term, multidisciplinary view. Because science and technology in this field are strongly intertwined, progress must occur simultaneously in basic science, man- ufacturing processing science, and engi- neering applications. It is also important to maintain an open and cooperative interna- tional posture. Scientific and Technological Objectives The panel has identified eight major scien- tific and technological objectives for a na- tional program to exploit high-temperature superconductivity. They are: I. to improve understanding of the essen- tial properties of current high-temperature superconducting materials (especially Tc, HC2, Jc, and alternating current Tosses) through the acquisition of additional experi- mental data; 2. to develop an understancling of the ba- sic mechanisms responsible for supercon- ductivity in the new materials; 3. to search for additional materials exhib- iting superconductivity at higher tempera- tures by the synthesis of new compositions, structures, and phases; 4. to prepare thin films of controllable and reproducible quality from present high-tem- perature superconducting materials, and es- tablish preferred techniques for growing films suitable for electronic device fabrica- tion; 5. to develop bulk conductors from cur- rent high-temperature superconducting ma- terials, with special emphasis on enhanced electric current-carrying capacity; 6. to advance the understanding of the chemistry, chemical engineering, and ce- ramic properties of the new materials, focus- ing on synthesis, processing, stability, and methods for large-scare production; 7. to fabricate a range of prototype circuits and electronic crevices based on supercon- ducting microcircuits or hybrid supercon
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HIGH-TEMPERATURE SUPERCONDUCTIVITY ductor/semiconductor circuits, as suitable thin film technologies become available; and 8. to fabricate a range of prototype high- field magnets, alternating- and direct-cur- rent power devices, rotating machines, transmission circuits, and energy storage devices, as suitable bulk conductors are de- veloped. PANEL RECOMMENDATIONS The panel recommends that the following actions be taken to carry out the objectives listed above: · The U.S. government should proceed with its plans to provide funding for high- temperature superconductivity research and development on the order of $100 mil- lion for fiscal year 1988. This funding level represents a good beginning in addressing the challenges and opportunities offered bv the new materials. 1 ~- - - ~ · Sufficient new money must be provided to both the science and the technology of high-temperature superconductivity so that other important and promising areas of re- search and development are not held back. · A mechanism should be established to monitor the potential demand for increased scientific and technical manpower in the event that the promise of high-temperature superconductivity is fully realized, and to 23 formulate appropriate recommendations on the funding of U. S. graduate and postgradu- ate research programs. · An interagency mechanism should be established to help coordinate planning for superconductivity programs among the var- ious federal agencies. · Given the anticipated rate of advance in high-temperature superconducting science and technology, the federal government should review progress in the field after 12 months as a guide to future resource alloca- tion. · Through its agencies the U.S. govern- ment must enhance the probability that U. S. industry gains a competitive advantage in this new field. This could be accomplished by the close association of industry with the Engineering Research or Science and Tech- nology Center programs of the National Sci- ence Foundation, by cost-sharing between government and industry on proof-of-con- cept projects, and by other joint efforts. · An important mechanism for enhancing U.S. industry's position is improved tech- nology transfer from the national laborato- ries to the private sector. Although a variety of means are already in place to encourage such transfers, the panel is concerned about the effectiveness of past efforts and urges both government and industry to pursue linkages more aggressively.
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Representative terms from entire chapter: