Superconductivity and Superconducting Devices

Summary

Superconductivity is an electrical phenomenon in which current flows without resistance in certain metals, alloys, and ceramics at very low temperatures. Low-temperature superconductors exhibit their characteristic zero electrical resistance and perfect diamagnetism at temperatures close to absolute zero, and high-temperature superconductors manifest these properties at temperatures from 23 Kelvin (K) to more than 135 K. Major applications of superconductors include generating powerful magnetic fields for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) machines. They have been used in making extremely sensitive magnetometers that are able to measure magnetic fields a hundred billion times weaker than the Earth's. Superconducting magnets have appeared in transportation (“levitating” trains), particle accelerators, and a variety of industrial and military applications. Possible future applications include quantum computing, electric power generation and transmission, refrigeration, and various nanotechnology devices.

Definition and Basic Principles

Unlike most natural phenomena, superconductivity can be defined in terms of an absolute: Electric currents flow through superconductors with absolutely no resistance. This resistless flow is what warranted the name “superconductivity” because, in traditional electrical behavior, electrons traveling in wires lose energy in the form of heat to the atomic array of the wire. In superconductivity, in defiance of a long-held understanding, the resistance of certain metals and alloys did not simply decrease to a residual value as the material was cooled but precipitously fell to zero. This abrupt transition to the superconducting state took place at a specific temperature called the critical temperature, which is different for each superconducting material. The phenomenon of zero resistance applies only to superconductors through which direct electrical current flows. For alternating current, higher frequencies lead to greater resistance in the superconductor.

Unlike most substances, which allow magnetic field lines to pass through them (though certain other substances can become magnetized by strong fields), superconductors reject a magnetic field. For example, if a magnet's north or south pole is brought near a superconductor, each pole is repelled, which is what diamagnetism means. Studies have shown that a superconductor is not only a perfect conductor but also a perfect diamagnet, and these two properties are often used to define superconductivity.

Superconducting devices have made use of these unique electrical and magnetic properties. For example, superconductors provide a way to circulate direct electric currents with no resistive loss. Even though alternating electric currents generate resistance in superconductors, careful choice of material and frequency for conveying these currents can be done with minor resistive losses. SQUIDs (superconducting quantum interference devices) have become the principal achievement of superconductor electronics and have been used to precisely measure voltage, electrical currents, and gravity. SQUIDS can measure subtle magnetic fields. Superconductors have had numerous applications in medicine, including magnetoencephalography, magnetocardiography, and magnetoneurography.

Background and History

Dutch physicist Heike Kamerlingh Onnes was the first to discover superconductivity as an outgrowth of his efforts to reach extremely low temperatures—he is sometimes called the “father of cryogenics.” In 1908, he succeeded in liquefying helium, but he also wanted to investigate how specific substances behaved at very low temperatures. Three years later he found, to his surprise, that mercury superconducted at 4 degrees above absolute zero (4 K). Scientists around the world were exuberant about his discovery, and Kamerlingh Onnes won the 1913 Nobel Prize in Physics. During the decades after this discovery, many more superconductors were found; it turned out that about a quarter of the natural elements are superconductors. Further research revealed that hundreds of alloys and compounds also superconducted, but most did so only at very low temperatures. By the 1980s, despite seventy-five years of research, the highest temperature achieved for superconductivity was only 23 K.

Most physicists searching for the elusive high-temperature superconductor had studied metals and alloys, but in 1986 Swiss physicist Karl Müller and German physicist J. Georg Bednorz decided to study a ceramic material composed of lanthanum, barium, copper, and oxygen. They were working at the IBM research laboratory in Switzerland, where they found that their ceramic material superconducted at 35 K, a temperature much higher than any known substance. After they published their results, thousands of scientists in many countries began searching for new high-temperature superconductors. Within six months of the publication by Bednorz and Müller, more than 800 papers appeared on the chemical and physical properties of various new superconductors along with some theories to explain them. Particularly important was the discovery by American researchers of ceramic material YBCO, which superconducted at temperatures above 77 K. This meant that inexpensive liquid nitrogen could be used to study this superconductor rather than expensive and hard-to-handle helium. By the first decade of the twenty-first century, physicists and chemists had created substances that superconducted at temperatures in excess of 135 K, and many scientists and engineers were racing to develop commercial applications of these new superconductors.

In 2018, a high-pressure compound of hydrogen and lanthanum was superconductive at 286 K. Two years later, in 2020, a compound of three elements—hydrogen, carbon, and sulfur—superconducted at 288 K, which was close to room temperature. This discovery was important because it could lead to the creation of electronics that can run at very fast speeds without overheating.

How It Works

After a century of research on superconductivity, scientists have deepened their understanding about how this new and exciting phenomenon occurs, but a complete theory accounting for all superconductors has yet to be formulated to the satisfaction of a majority of scientists. The first theory to explain superconductivity actually drew from an explanation of the electrical properties of metals developed before Kamerlingh Onnes made his discovery. Dutch physicist Hendrik A. Lorentz proposed in 1900 that a crystalline metallic solid with no imperfections would actually conduct electricity without any resistance. However, real crystals have edges, faces, and missing atoms in their interiors, creating obstacles to passing electrons. Furthermore, high temperatures produce jiggling of the atoms in the lattice, thus impeding electron flow. Consequently, this old theory was unsatisfactory in its explanation of both conductivity and superconductivity.

Quantum Mechanics. By the mid-1920s physicists had developed quantum mechanics, a powerful new theory explaining the behavior of electrons in atoms. Different forms of quantum mechanics emphasized electrons as particles (matrix mechanics) and electrons as waves (wave mechanics), and these theories were eventually shown to be equivalent. Quantum mechanics proved very successful for understanding ionic and covalent crystals, organic chemical molecules, and many other physical and chemical phenomena, but it proved unable to unlock the mysteries of superconductivity. However, in 1933 German physicist Walther Meissner discovered a superconductor's ability to repel magnetism, which provided a clue to understanding superconductivity, since study of the Meissner effect showed how transitions from normal to superconducting states are thermodynamically reversible. Other studies helped explain some of the electromagnetic properties of superconductors. Nevertheless, theoretical physicists were still unable to explain superconductivity in terms of basic physical laws.

Josephson Effect. Another major development in understanding how superconductivity works came in 1962 when Welsh physicist Brian Josephson, a twenty-two-year-old graduate student, predicted the tunneling of electrons and Cooper pairs between linked superconductors. Within a year, experiments proved that pairs could travel across a barrier as easily as single electrons. In 1973 Josephson shared the Nobel Prize in Physics with physicists Leo Esaki and Ivar Giaever, who had also worked on tunneling.

High-Temperature Superconductivity. During the twenty-five years after the discovery of the first high-temperature superconductor by Bednorz and Müller, various physicists tried to develop an appropriate theory that explained the superconductivity of both low-temperature and high-temperature superconductors. BCS theory was able to explain low-temperature superconductivity, but, even with clever modifications, it failed to account for the properties of high-transition-temperature ceramics. One physicist remarked that there were nearly as many theories about these new superconductors as there were theorists. Some tried to explain particular categories of high-temperature superconductors, while others used superstring and gauge theories to try to resolve the mysteries. By the twenty-first century, some physicists were describing high-temperature superconductivity as one of the great, unexplained mysteries of condensed matter physics.

Applications and Products

In the twenty-first century, superconductivity was a century old, and, from the beginning, Kamerlingh Onnes, its discoverer, foresaw practical applications for this unique and propitious phenomenon, particularly in the resistance-free generation and distribution of electricity. However, for the next seventy-five years, commercial applications and products that he and other pioneers envisioned were few and far between. After the discovery of high-temperature superconductivity in 1986, practical uses for superconductors multiplied in a variety of fields, from scientific research and electronics to medicine and the military. Books, journals, newsletters, government programs, and professional organizations proliferated, urging the commercialization of low- and high-temperature superconductors for small-, medium-, and large-scale applications. Despite concrete evidence of multifarious applications, some critics point out that superconductivity, with its hundred years of research and development, has had much less commercial success than what followed: the discovery of electromagnetic waves, which spawned such phenomenally successful businesses as wireless telegraphy, radio, television, and myriad other electronic devices. However, with the discovery of a high-temperature superconductive three-element compound in 2020, physicists regained hope of new applications, such as the possibility of ultra-strong magnetic field generation for high-resolution MRI systems. Physicists at the University of Rochester in New York noted that including a third element in a superconductor reduces operational pressure, making the development of new applications more likely.

Scientific Research. Scientists have used superconductors to expand and deepen their understanding of the natural world, from the microcosm of nuclei, subatomic particles, and atoms to the macrocosm of stars and galaxies. High-energy physicists were among the first to grasp how superconducting magnets could facilitate their research on nuclei, protons, and electrons. Fermilab, the largest high-energy facility in the United States, changed from conventional electromagnets to superconducting magnets because they provided a superior means of controlling proton beams. Had the Superconducting Super Collider been realized (it was stopped because of its $8 billion cost), physicists believe it would have provided a way to discover new elementary particles and create a new understanding of the universe's fundamental forces. Despite this setback, physicists have used superconducting techniques in particle accelerators such as the electron synchrotron and in plasma research in an attempt to develop a practical nuclear fusion reactor. Research is also underway to develop superconductors for use in spacecraft, artificial satellites, and launch vehicles because superconductors provide maximum performance with minimum electrical power input.

Metrology. Superconductors first became famous because of their electrical properties, but they first made money because of their magnetic properties. SQUIDs, the best known of the metrological devices, were the result of research and development in the mid-1950s at the Ford Motor Company Scientific Laboratory in Michigan. Because SQUIDs were extraordinarily sensitive to very weak magnetic fields, they led to several magnetometric applications. For example, they helped create detailed magnetic maps of geological formations, which were valuable in locating ore deposits. SQUID magnetometers have also helped oil geologists to penetrate rock layers to locate possible petroleum resources. Initially, field use of SQUID magnetometers was limited because of the necessity of working with liquid-helium-cooled devices, but with the advent of high-temperature superconductors, liquid-nitrogen-cooled SQUIDs have proved more convenient for use in geophysical explorations.

Electronics and Computers. Particularly after the discovery of high-temperature superconductivity, an explosion of research occurred as scientists in a variety of fields, from physics and chemistry to computer science and engineering, worked to find practical applications. Many researchers concentrated on developing superconducting wire, cable, and thin films. With officials in nations around the world aware of the growing “energy crisis,” more efficient ways of generating, storing, transmitting, and using electrical energy would be most welcome. During the period of research and development, certain problems surfaced, such as the refrigeration costs involved in keeping transmission cables cool enough to superconduct. Nevertheless, companies in the twenty-first century began producing high-temperature superconducting wire for such applications as generators, motors, and cables.

Computer scientists and engineers have long recognized that a principal impediment to smaller, faster, and more energy-efficient computers has been waste heat produced by closely packed integrated circuits. Even before the discovery of high-temperature superconductivity, IBM invested more than $100 million to develop a quantum computer using superconducting elements. Other companies, such as Google, had a similar goal. In 2021, IBM built a quantum computer using high-temperature superconductors that was double the size of its predecessors, one of which was created by Google.

Medicine. Superconducting devices have been used in medicine principally in assisting physicians to make faster, more detailed, more accurate, and less discomfiting diagnoses. For example, a superconducting magnetometric device has been used to detect the difficult-to-diagnose disease, hemochromatosis, which leads to excessive iron buildup in many tissues. Superconductors have led to new technologies that allow physicians to diagnose problems noninvasively in brains and hearts. For example, magnetoencephalography provides doctors with the means to pinpoint sites deeply within the brain responsible for epileptic seizures. Magnetocardiography has proved useful in mapping cardiac arrhythmias so that catheters can precisely ablate the source of the problem. As advanced high-temperature superconductors become part of various magnetic resonance imaging (MRI) systems, the ability to generate detailed pictures of structures (and their functioning) within the human body is expected to improve and costs decline.

Transportation. Trains based on traditional wheels-on-rails technologies have insurmountable limits on very fast speeds, but these limitations can be overcome by superconducting magnets, new kinds of track, and the levitation of the train. From the 1970s to the 1990s, Japanese engineers developed the superconducting maglev (for magnetic levitation) train, which was capable of speeds in excess of 300 miles per hour (483 kilometers per hour). With the success of the bullet train operating between Osaka and Tokyo, other countries, including the United States, have been exploring more advanced systems that will make use of high-temperature superconductors. These have also been used for sea transportation, as in the Jupiter II, a ship powered by superconducting motors.

Military. The U.S. Department of Defense and other agencies have been the chief supporters of the research and development of high-temperature superconducting devices for military uses. For example, the U.S. Air Force is developing superconductors that will increase the efficiency of electrical systems in jet engines for fighters and bombers. Other applications include superconducting magnetic detection systems that offer distinct advantages over sonar for submarines. Both surface ships and submarines will also be able to employ SQUID magnetometers for mine detection.

Careers and Course Work

Those advising students on how to prepare for jobs in superconductor technology emphasize matching desired careers with education. If a student wants to install and maintain superconductor cables, an associate's degree in electrical technology from a technical school would be sufficient. If a student wants to work in the research, development, and manufacture of superconductors, a bachelor's, master's, or doctoral degree in electrical engineering, physics, chemistry, or materials science may be required, depending on how advanced a rank in the profession one wishes to attain. Excellent programs in superconductor technology exist at such institutions as Stanford University; University of California, Berkeley; Massachusetts Institute of Technology; Florida State University; and the University of Houston. After taking introductory and advanced courses in physics, chemistry, mathematics, and electrical engineering, students may then specialize in such courses as engineering thermodynamics, fluid mechanics, cryogenics, and the theory and applications of superconductors.

Many opportunities exist for graduates with degrees in superconductivity. The field has been growing, and prognosticators predict accelerated future growth so that successful careers can be pursued in government agencies, academic institutions, and a variety of well-established or new companies. After the discovery of high-temperature superconductors, some enthusiasts foresaw an accelerated need for those with expertise in this new field with an expected rapid introduction of applications in electric power, transportation, and medicine. Because of the modest increase in commercialized applications during the twenty-five years since this breakthrough discovery, early overly optimistic estimates of employment growth have had to be tempered with the more modest job opportunities that actually exist.

Social Context and Future Prospects

Before 1986, most scientists were skeptical that a room-temperature superconductor would ever be discovered, but in the twenty-first century such a discovery seems increasingly likely. If such a superconductor could be efficiently and economically manufactured, then most industries of modern society would be affected, and a superconductor revolution would occur, similar to the computer revolution of the second half of the twentieth century. Room-temperature superconductivity would transform the electrical-power, transportation, and consumer-electronics industries. The era of copper's dominance would end, leading to a new generation of smaller and more efficient home appliances, such as refrigerators, washing machines, and air conditioners. Societies whose technologies make use of these room-temperature superconductors would be quieter, cleaner, and more energy efficient. For example, in 2023, scientists at MIT created a superconductor device prototype that would significantly cut the amount of energy used in high-power computing systems. This advancement could impact energy efficiencies in other industries and could also cause positive environmental impacts.

Though this idealistic view of the future motivates many researchers, realists caution that, based on the previous century of superconductivity research and development, the path to the elusive room-temperature superconductor will not be smooth and straight. Throughout the beginning of the twenty-first century, there have been a series of uncorroborated, retracted, and unreliable studies claiming to have achieved room-temperature superconduction. Thus, research and studies continue.

Despite slow progress, superconductivity remains a major area of research, and breakthroughs have been made in the twenty-first century, such as a high-temperature superconductor compound consisting of hydrogen, carbon, and sulfur.

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