超伝導と超伝導デバイス

Superconductivity and Superconducting Devices | Physics | Research Starters | EBSCO Research Back Browse Subject Areas Copy URL Copy URL Superconductivity and Superconducting Devices Superconductivity is a remarkable electrical phenomenon characterized by the ability of certain materials—metals, alloys, and ceramics—to conduct electricity without resistance when cooled to very low temperatures. Superconductors can be categorized into low-temperature superconductors, which operate near absolute zero, and high-temperature superconductors, which function at temperatures above 23 Kelvin and up to approximately 288 Kelvin. This property not only allows for the efficient flow of electric current but also enables superconductors to exhibit perfect diamagnetism, meaning they can repel magnetic fields. Superconducting devices leverage these unique properties across various applications, such as in magnetic resonance imaging (MRI) machines, particle accelerators, and specialized magnetometers known as SQUIDs, which are capable of detecting extremely weak magnetic fields. Potential future applications of superconductors span diverse fields, including quantum computing, energy transmission, and advanced transportation systems, such as magnetic levitation trains. The scientific exploration of superconductivity began over a century ago with the work of Heike Kamerlingh Onnes, and significant advancements have been made since, particularly with the discovery of high-temperature superconductors in the 1980s. Despite the challenges of commercializing these materials, ongoing research continues to offer promising developments that could revolutionize technology in various sectors, enhancing efficiency and performance while reducing environmental impact. Authored By : Paradowski, Robert J. 1 of 3 Authored By : Paradowski, Robert J. Published In : 2024 2 of 3 Published In : 2024 Related Topics : Alloys ; Magnetic field ; Heike Kamerlingh Onnes ; Nobel Prize ; Computer Science ; Engineering ; Magnetic Resonance Imaging ; Jet engines ; Electrical Engineering Alloys ; Magnetic field ; Heike Kamerlingh Onnes ; Nobel Prize ; Computer Science ; Engineering ; Magnetic Resonance Imaging ; Jet engines ; Electrical Engineering 3 of 3 On This Page Full Article Full Article 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 superconducto