Superconducting materials have the property that the resistance is equal to zero, and the magnetic field lines are repelled at a certain low temperature. It has been found that 28 elements and thousands of alloys and compounds can become superconductors.
The properties of characteristic superconducting materials are quite different from those of conventional conductive materials. Mainly has the following performance. ① Zero resistance: When superconducting material is in superconducting state, its resistance is zero and it can transmit electric energy without loss. If a magnetic field is used to induce an induced current in the superconducting ring, the current can remain undiminished. This "continuous current" has been observed many times in experiments. ② Complete diamagnetism: When the superconducting material is in superconducting state, as long as the applied magnetic field does not exceed a certain value, the magnetic field lines cannot penetrate, and the magnetic field in the superconducting material is always zero. (3) Josephson effect: When a thin insulating layer (about 1nm thick) is formed between two superconducting materials to form a low-resistance connection, there will be electron pairs passing through the insulating layer to form a current, but there is no voltage on both sides of the insulating layer, that is, the insulating layer also becomes a superconductor. When the current exceeds a certain value, a voltage u appears on both sides of the insulation layer (voltage u can also be added). At the same time, the DC current becomes high-frequency alternating current, which radiates electromagnetic waves with a frequency of, where H is Planck constant and E is electron charge. These characteristics form the basis for the more and more remarkable application of superconducting materials in the field of science and technology.
Basic critical parameters include the following three basic critical parameters. ① Critical temperature: when the external magnetic field is zero, the temperature at which superconducting materials change from normal state to superconducting state (or vice versa) is expressed by Tc. Tc value varies with different materials. The lowest Tc of superconducting materials has been measured to be tungsten, which is 0.0 12K. By 1987, the maximum critical temperature has been raised to about 100k k(2) critical magnetic field: the magnetic field intensity required to destroy the superconducting state of superconducting materials and make it normal, expressed by Hc. The relationship between Hc and temperature t is Hc=H0[ 1-(T/Tc)2], where H0 is the critical magnetic field at 0K. (3) Critical current and critical current density: When the current passing through the superconducting material reaches a certain value, it will also break the superconducting state and make it become a normal state, which is represented by Ic. Ic generally decreases with the increase of temperature and external magnetic field. The Ic carried by unit cross-sectional area is called critical current density, which is expressed by Jc.
These parameters of superconducting materials limit the application conditions of materials, so finding new high-parameter superconducting materials has become an important topic for people. Take Tc as an example. Since the Dutch physicist H. Kemelin-Agnes discovered superconductivity (Hg, Tc=4.2K) in 19 1 year, the highest Tc discovered by people did not reach 23.2K(Nb3Ge, 1977) until 1986. 1986 Swiss physicist K.A. Miller and Federal German physicist J.G. Bednorz discovered the superconductivity of oxide ceramics, thus increasing the Tc to 35K. Only one year later, the Tc of the new material has increased to about 100K K, which opens up a broad prospect for the application of superconducting materials. Miller and Bedeneau Z won the 1987 Nobel Prize in physics.
Classification Superconducting materials can be divided into elemental materials, alloy materials, compound materials and superconducting ceramics according to chemical composition. ① Superconducting elements: There are 28 kinds of superconducting elements under normal pressure, among which the Tc of niobium (Nb) is the highest, 9.26 k k ... Nb and Pb (Pb, Tc=7.20 1K) are mainly used in electrical engineering and have been used to manufacture superconducting AC power cables and high-q resonant cavities. (2) Alloy materials: Adding some other elements into superconducting elements as alloy components can improve all properties of superconducting materials. For example, the earliest used Nb-75Zr alloy has a Tc of 10.8K and a Hc of 8.7 tex. Subsequently, niobium titanium alloy was developed. Although Tc is slightly lower, Hc is much higher and can carry more current under a given magnetic field. Its properties are Nb-33Ti, Tc=9.3K, HC =11.0tex; Nb-60Ti，Tc=9.3K，HC = 12te (4.2K)。 At present, Nb-Ti alloy is the main superconducting magnet material used in 7 ~ 8 special magnetic fields. The addition of tantalum ternary alloy further improves the properties of Nb-Ti alloy. The properties of Nb-60Ti-4Ta are: TC = 9.9K, HC =12.4te (4.2k); The properties of Nb-70Ti-5Ta are: Tc=9.8K, HC =12.8t. ③ Superconducting compounds: Superconducting elements often have good superconductivity when combined with other elements. For example, Nb3Sn, Tc= 18. 1K, Hc=24.5 has been widely used. Other important superconducting compounds are V3Ga, Tc= 16.8K, Hc = 24 TexNb3Al, Tc= 18.8K, and Hc=30. ④ Superconducting ceramics: In the early 1980s, Miller and Bednorz began to notice that some oxide ceramic materials may have superconductivity. Their group conducted experiments on some materials and found Tc=35K superconductivity in La-Ba-Cu- oxide of 1986. 1987, scientists in China, the United States, Japan and other countries found that Tc has superconductivity in the temperature range of liquid nitrogen in Ba-Y-Cu oxide, making superconducting ceramics a promising superconducting material.
The application of superconducting materials has shown attractive application prospects to mankind from the day it was discovered. However, the practical application of superconducting materials is restricted by a series of factors, first of all, its critical parameters, and then there are problems such as the manufacturing process of materials (for example, how to make brittle superconducting ceramics into flexible wires has a series of technological problems). By the 1980s, the applications of superconducting materials mainly include: ① Magnets can be made by using the superconductivity of materials, which can be used in motors, high-energy particle accelerators, magnetic levitation transportation, controlled thermonuclear reactions, energy storage, etc. Power cables can be made for large-capacity transmission (power can reach10000 MVA); ); Communication cables and antennas can be made, and their performance is superior to that of conventional materials. ② Using the complete diamagnetism of materials, frictionless gyroscopes and bearings can be made. (3) Josephson effect can be used to make a series of precision measuring instruments, radiation detectors, microwave generators, logic elements, etc. Josephson junction is used as the logic and storage element of computer, its operation speed is 10 ~ 20 times faster than that of high performance integrated circuit, and its power consumption is only a quarter.
19 1 1 year, Dutch physicist Anis (1853 ~ 1926) found that the resistivity of mercury did not gradually decrease with the decrease of temperature as expected, but when the temperature dropped to about 4. 15K. When the temperature of some metals, alloys and compounds drops to a certain temperature close to absolute zero, the phenomenon that their resistivity suddenly drops to unmeasurable is called superconductivity, and substances that can conduct superconductivity are called superconductors. The temperature at which a superconductor changes from a normal state to a superconducting state is called the transition temperature (or critical temperature) TC of this substance. It is found that most metal elements and thousands of alloys and compounds show superconductivity under different conditions. For example, the transition temperature of tungsten is 0.0 12K, zinc is 0.75K, aluminum is 1. 196K, and lead is 7. 193K.
The unique characteristics of superconductors make it possible to be widely used in various fields. However, the early superconductors existed at the extremely low temperature of liquid helium, which greatly limited the application of superconducting materials. People have been exploring high-temperature superconductors. From 19 1 1 to 1986, it increased from 4.2K of mercury to 23.22K of niobium and germanium in 75 years, and then increased to 19K.
1986, a major breakthrough has been made in the research of high temperature superconductors. The "superconductivity", which aims at studying metal oxide ceramic materials and finding high critical temperature superconductors, has been set off. More than 260 experimental groups from all over the world participated in the competition.
1986 65438+1October, scientists Bernoz and Miao Lei of IBM's laboratory in Zurich, Switzerland, first discovered that Ba-La-Cu oxide is a high-temperature superconductor, which raises the superconducting temperature to 30k；; Then, the superconducting temperature was raised to 37 K by the Faculty of Engineering, Tokyo University, Japan. On February 30th, 65438, the University of Houston announced that Zhu Jingwu, a Chinese-American scientist, had raised the superconducting temperature to 40.2 K again.
1987 65438+1At the beginning of October, Kawasaki National Institute of Molecular Science in Japan raised the superconducting temperature to 43k；; Soon, the Japanese Institute of Integrated Electronics raised the superconducting temperature to 46K and 53 K ... The research group led by Zhao Zhongxian and Chen Liquan of the Institute of Physics of the Chinese Academy of Sciences obtained the Sr-La-Cu-O superconductor of 48.6 K, and found the signs of phase transition of this kind of material at 70 K ... On February 15, the United States reported that Zhu Jingwu and Wu Maokun had obtained the superconductor of 98K. On February 20th, China also announced the discovery of superconductors above 100K. On March 3rd, Japan announced the discovery of 123K superconductor. On March 12, China Peking University successfully conducted the liquid nitrogen superconducting magnetic levitation experiment. On March 27th, Chinese-American scientists discovered superconducting signs with a transition temperature of 240K in oxide superconducting materials. Soon, the Department of Engineering of Kagoshima University in Japan found that ceramic materials composed of lanthanum, strontium, copper and oxygen showed signs of superconductivity at 14℃. A major breakthrough in high-temperature superconductors, using liquid nitrogen instead of liquid helium as superconducting refrigerant to obtain superconductors, has promoted the large-scale development and application of superconducting technology. Nitrogen is the main component of air, and the efficiency of liquid nitrogen refrigerator is at least 10 times higher than that of liquid helium, so the price of liquid nitrogen is actually only1100 of liquid helium. Liquid nitrogen refrigeration equipment is simple, so the existing high-temperature superconductors must be cooled by liquid nitrogen, but it is considered as one of the greatest scientific discoveries in the 20th century.
Superconducting scientific research
1. Flux dynamics and superconducting mechanism of unconventional superconductors
This paper mainly studies the mechanism of magnetic field line movement in mixed state region, the nature and cause of irreversible line and its relationship with magnetic field and temperature, the dependence of critical current density on magnetic field and temperature and anisotropy. The research on superconducting mechanism mainly focuses on magnetoresistance, Hall effect, fluctuation effect, Fermi surface properties and T.
2. Study on the characteristics of low-dimensional condensed matter under strong magnetic field.
The low dimension makes the low-dimensional system show the characteristics that the three-dimensional system does not have. Low dimensional instability leads to many ordered phases. Strong magnetic field is an effective means to reveal the characteristics of low-dimensional condensed matter. The main research contents include: the structure and source of organic ferromagnetism; Mechanism and magnetism of organic (including fullerene) superconductors; Special properties of nonlinear element excitation in two-dimensional electron gas under strong magnetic field: phase transition and magnetic interaction of low-dimensional magnetic materials; Transport and carrier characteristics of organic conductors in magnetic field: energy band structure and Fermi surface characteristics in magnetic field.
3. Photoelectric characteristics of semiconductor materials under strong magnetic field.
Strong magnetic field technology is becoming more and more important to the development of semiconductor science, because the external magnetic field is the only physical factor that changes the symmetry of momentum space while keeping the crystal structure unchanged, so the magnetic field plays a particularly important role in the study of semiconductor energy band structure, element excitation and their interaction. Through the experimental study on the optical and electrical properties of semiconductor materials under strong magnetic field, we can further understand and master the physical properties of semiconductors, thus making a basic exploration for manufacturing semiconductor devices with various functions and developing high technology.
4. Physical problems of very fine scale under strong magnetic field.
There are many new phenomena and strange characteristics in micro-scale system that conventional materials do not have, which are closely related to the microstructure of such materials, especially the electronic structure. Strong magnetic field provides a powerful means to study the electronic states and transport characteristics of micro-scale systems, which can not only further reveal the strange phenomena that are difficult to appear under conventional conditions, but also provide rich scientific information for a deeper understanding of their physical characteristics. This paper mainly studies the electron transport, electron localization and correlation characteristics of extremely fine-scale metals and semiconductors under strong magnetic fields. Quantum size effect, quantum confinement effect, small size effect and surface and interface effect; As well as the optical properties and energy gap fine structure of oxides, carbides and nitrides with very fine scale.
5. Strong magnetic field chemistry
The effect of strong magnetic field on electron spin and nuclear spin in chemical reactions can lead to the relaxation of corresponding chemical bonds, create favorable conditions for the formation of new bonds, induce physical and chemical changes that cannot be realized under general conditions, and obtain new materials and compounds that could not be prepared before. Strong magnetic field chemistry is a new field with strong application foundation, with a series of theoretical topics and broad application prospects. In the near future, we can study the magnetization and mechanism of water and organic solvents and new chemical reactions induced by strong magnetic fields.
6. Biological and biomedical research under magnetic field.
Magnet science and technology
The value of strong magnetic field lies in its important contribution to physical knowledge. An important concept development in 1980s was the discovery of quantum Hall effect and fractional quantum Hall effect. This was discovered when studying the transport phenomenon of two-dimensional electron gas under strong magnetic field (/kloc-0 won the Nobel Prize in 985). The discovery of quantum Hall effect and fractional quantum Hall effect has inspired physicists to explore their origins, which has shown great significance in the establishment of natural reference objects of resistance, accurate measurement of basic physical constants E, H and fine structure constants (= E2/H (0C)) and other applications. The ultimate revelation of the mechanism of HTS will also depend on people's exploration of the properties of HTS in strong magnetic field.
Anyone familiar with the history of physics knows that the important symbol of the evolution of solid physics to condensed physics lies in the expansion of its research objects, from periodic structure to aperiodic structure, from three-dimensional crystals to low-dimensional and high-dimensional systems, and even fractal systems. These new celestial bodies show many new characteristics and physical phenomena, and their physical mechanisms are quite different from the traditional ones. The appearance of these new celestial bodies and the explanation of new effects and phenomena have continuously enriched and developed condensed matter physics. In this process, extreme conditions have always played a vital role, because extreme conditions often highlight some factors and inhibit others, thus making the originally complicated process simpler and helping to directly understand the physical essence.
Compared with other extreme conditions, strong magnetic field has its own characteristics. The function of strong magnetic field is to change the physical state of a system, that is, to change the angular momentum (spin) and the orbital motion of charged particles, so it also changes the state of the physical system. It is at this point that the strong magnetic field is different from other expensive physical means such as neutron source and synchrotron, and does not change the physical state of the studied system. Magnetic field can create a new physical environment and lead to new characteristics. Without magnetic field, it would not exist. Low temperature can also lead to new physical states, such as superconductivity and phase transition, but the strong magnetic field is very different from low temperature, which is more effective than low temperature, because the magnetic field quantizes the remote control and energy of charged particles and magnetic particles, destroys the symmetry of time reversal, and makes them have more unique properties.
Strong magnetic field can change the symmetry of momentum space while keeping the crystal structure unchanged, which is very important for studying the energy band structure, element excitation and their interaction of solids. The complex Fermi surface structure of solid is proved by the principle that strong magnetic field makes electrons and holes move freely in a specific direction, resulting in magnetization and magnetoresistance oscillation. The study of Fermi surface structure and characteristics in solids has always been a frontier topic in condensed matter physics. At present, many important hot spots in the basic research of condensed matter physics are inseparable from the extreme condition of strong magnetic field, and even many of them are based on the research under strong magnetic field. If wave color condensation only occurs in momentum space, it is possible to observe this phenomenon in real space only in inhomogeneous strong magnetic field. Another example is the mechanism of high-temperature superconductivity, the study of quantum Hall effect, the physical problems in nano-materials and mesoscopic objects, the physical causes of giant magnetoresistance effect, the structure and source of organic ferromagnetism, the mechanism and magnetism of organic (including fullerene) superconductors, the phase transition and magnetic interaction of low-dimensional magnetic materials, the energy band structure and Fermi surface characteristics in solids, and the study of element excitation and their interaction. The research work under strong magnetic field will help to understand and reveal these problems correctly, thus promoting the further development of condensed matter physics.
The motion of charged particles such as electrons, ions and some polar molecules will change fundamentally in the magnetic field, especially in the strong magnetic field. Therefore, studying the influence of strong magnetic field on chemical reaction process, surface catalysis process, formation process of materials, especially magnetic materials, biological effect and liquid crystal formation process may lead to new discoveries and new interdisciplinary topics. The application of strong magnetic field in materials science has opened up a new road for the development of new functional materials, and this work has been paid attention to abroad and has begun to be required at home. It is precisely because of the immeasurable application prospect of high-temperature superconductors in the future high-voltage field that they have attracted great attention from scientific and technological circles and even governments all over the world. Therefore, no matter from the basic research or application point of view, the physical and chemical research under strong magnetic field has important scientific and technological significance. Through this research, it will not only help to deepen the contemporary basic research, but also play an important role in promoting the development of the national economy.