Keywords: superconducting technology, high-temperature superconducting materials, MgB2, yttrium-barium-copper-oxygen complexes, YBCO
I. Generation and development of superconducting technology
Superconducting technology, as a new technology for energy conservation and its environmentally friendly characteristics, will become the core technology of the twenty-first century. Its development has gone through three stages:
1, the first stage is the basic understanding of superconductivity, exploration and BCS theory.
In 1911, Onnes found that the resistance of Hg at 4.15K suddenly dropped to the extent that the precision of the instruments at that time had been unable to measure, that is, Hg in a determined critical temperature Tc = 4.15K below will lose its resistance. Subsequently, people in the Pb and other materials also found this property: in the critical conditions (critical temperature Tc, critical current Ic, I critical magnetic field Hc) when the resistance of the material suddenly disappeared, that is, for the superconductivity of the zero resistance phenomenon. Another fundamental property of superconductors is that they are completely antimagnetic. This means that a superconductor can completely exclude the entry of magnetic lines of force when it is in a superconducting state. [2] This phenomenon was discovered by Meissner and Oschenfeld in 1933, so it is called the Meissner effect. These are the two basic properties of superconductors.The discovery of Nb3Sn alloy superconducting materials by the B. T. Matthias group at Bell Labs in 1954 led to the dawn of superconductivity in the 1960s, but it required very low temperatures (in liquid helium) for superconductivity to manifest itself. E. Kunzler with an excess of tin silver, tin mixed powder filled with silver tubes processed into wires, after heat treatment at 4.2K, 8.8T Ic up to 1.5 × 105A/cm2. Since then a long period of time Tc = 23.3K Nb3Ge is regarded as the limit value.
3, the third stage is the discovery of high-temperature copper oxide in 1986, opened the human superconductivity technology development. Zurich scientists J.G. Bendnorz and others in 1986 found the lanthanum silver-copper composite oxides reached 30K, a breakthrough in the traditional BCS theory caused a huge worldwide response. [3] People again began to look for higher critical temperature superconducting materials. Subsequently, in 1987, Zhu Jingwu of the University of Houston found that yttrium barium copper oxygen composite oxide (YBCO) superconducting critical temperature (Tc = 93K) exceeds the temperature of liquid nitrogen (b.p = 77K), causing a world sensation; because most of the previous practical application of superconductors is the use of liquid helium as a coolant, liquid helium is very expensive, which hinders the application of superconducting technology. Liquid nitrogen, on the other hand, is cheap and easily available (it is a by-product of oxygen preparation). [4] Further superconducting transition temperatures of 110 K and 125 K for bismuth-strontium-calcium-copper-oxygen and thallium-barium-calcium-copper-oxygen superconductors were discovered in 1988.In 1993, mercury-barium-calcium-copper-oxygen was discovered with a superconducting critical transition temperature of 133 K. The superconducting transition temperature of 133 K for mercury-barium-calcium-copper-oxygen was discovered in 1993.
Two, superconducting high-temperature copper oxide (YBCO) and magnesium diboride (MgB2) preparation and properties
1, now high-temperature copper oxide has been more research on superconducting materials, in the study of its superconducting at the same time people on the preparation and processing of superconductors for the study in detail. Research within the foreign country shows that to produce high-performance YBCO, it is necessary to prepare YBCO nanopowder to obtain about 100 nm of ultrafine powder, which will greatly improve the dispersion and uniformity of YBCO materials, from the essence of improving the performance of YBCO materials. [4]
YBCO powders are prepared by Sol-Gel and chemical pyrolysis. Y2O3 (99.99 %), BaCO3 (99.9 %), CuO (99.9 %), etc. are mixed in the sintering molding with the participation of auxiliary materials so that the metal reaches the ionic mixing, and the oxides form a homogeneous single-phase after the combustion, resulting in a homogeneous powder. This method is reproducible, is currently a relatively simple and effective preparation of YBCO nanopowder technology, and then the nano-powder can be pressed and molded to obtain the initial superconductor. However, YBCO has its own shortcomings: the chemical elements that make up the oxide high-temperature superconductors are expensive, and the synthesized superconducting materials are brittle and difficult to be processed into wires, which greatly limits their applications.
2. Another new type of high-temperature superconducting material that should emerge is magnesium diboride. [5] In 2001, Japan's Aoyama College (Aoyama Gakuin) Akimitsu pure professor (Jun Akimitsu) held in Sendai, Japan, "transition metal oxides" conference announced the discovery of the high-temperature superconducting properties of MgB2, its critical temperature Tc = 39 K, which stirred the entire superconducting materials and condensed matter physics, and set off a wave of research on the superconducting properties of simple compounds. It is a simple binary compound with hexagonal crystal system and simple hexagonal structure of AlB2 type. Prof. Akimitsu pure is the purity of 99.19 % of magnesium powder and purity of 99 % of amorphous boron powder mixed at a ratio of 1:2, pressed into small balls in the high-pressure nitrogen heating reaction to get MgB2. can also be used to use the heat of combustion reaction of titanium and boron triggered by the combustion synthesis reaction of magnesium (b.p. 650 ° C) and boron (b.p. 2080 ° C) in a vacuum in a very short period of time to generate MgB2 which minimizes the oxidation and evaporation of magnesium and simplifies the generation of MgB2 superconducting materials. The composition ratio of magnesium to boron is roughly stabilized and is expected to improve the superconducting properties of the material, and the short time-consuming production of the MgB2 superconducting material using this method is expected to further reduce the cost. [6]
MgB2 is a simple, stable metal compound superconducting material with the highest critical temperature found so far, and a more promising superconducting material for practical use. Research on the properties of magnesium diboride superconductors has progressed very rapidly, and the understanding of the mechanism of magnesium diboride superconductors has been deepened.
Theoretical calculations have shown that there is more than one energy band spanning the Fermi surface in MgB2, and that the Fermi surface destabilization caused by electroacoustic coupling may produce an energy gap at the Fermi surfaces of the two energy bands, which is a very different point between MgB2 superconductors and the conventional superconductors. First, MgB2 superconductors can carry large superconducting currents at temperatures around 20 K and at 80,000 times the Earth's magnetic field with very low energy consumption. Second, magnesium diboride materials are very inexpensive and far easier to mold than oxide high-temperature superconductors with ceramic properties. Also, the most important characteristics of magnesium diboride-based superconducting materials are that they are easy to synthesize, easy to process, and have good application prospects. Unlike oxide high-temperature superconductors, magnesium diboride-based superconducting materials are easy to make films or wires.
Third, the application of superconductors
The above briefly introduces the preparation methods and properties of two important superconducting materials - YBCO and MgB2 [7]. They have been used in many fields, such as superconducting materials made of superconducting magnets with extremely strong magnetism, superconductors produce magnetic fields to study the structure of living organisms and for the treatment of various complex human diseases. In practical terms, the United States, Japan, and other countries have spared no effort to carry out research in this area and achieved significant results, has now entered the practical application of the development and research stage.
1, superconducting magnet magnetism [8]
Superconducting magnetic levitation train is the most successful example of the application of superconducting technology. Compared with the normal conduction type maglev train, low temperature superconducting maglev train has many advantages, one, the superconductor can flow through a very large current, superconducting magnets magnetic field is stronger than the conventional electromagnet; two, the superconductor has almost no resistance, the loss is very small. Once the current is used to excitation, can be removed from the power supply, only to maintain its low-temperature operating environment to ensure that it does not lose super. From the perspective of long-term use, superconducting magnets have low energy consumption and low cost, making them an ideal magnet. Superconducting magnets due to its zero resistance characteristics, in a superconducting state almost no heat, so in the case of no loss of super, the current through the superconducting magnets can be very large without energy consumption, to achieve the requirements of a strong magnetic field of low-energy consumption; Third, light weight, small size, low pollution, strong climbing ability.
Superconducting magnets another important application is in the nuclear fusion reactor " magnetic closure body ". [9] nuclear fusion reaction, the internal temperature is as high as 100 million to 200 million ℃, there is no conventional material can contain these substances. The superconductor produces a strong magnetic field can be used as a "magnetic closure", the thermonuclear reactor in the ultra-high-temperature plasma surrounded, constrained, and then slowly released, so that the controlled fusion energy to become a new energy in this century with broad prospects.
2, superconducting computer [10]
Josephson devices made of superconducting tunneling effect for a variety of high-precision instrumentation has become possible. Most of the current computer using semiconductor technology, silicon integrated circuit technology plays a big role, such as Intel and AMD processors using high-purity silicon. However, to continue to improve the performance of computers and computing speed, energy consumption is a limiting factor, if the silicon integrated circuits to increase the speed of computation, will inevitably result in the heat of the chip, these heat will have an adverse effect on the semiconductor material. [10]
Superconducting tunnel junctions (also known as Josephson devices) can resolve this conflict. In superconductors, it takes only 10-10 seconds to express two states with zero and non-zero voltages, which can increase the speed of computer operations by more than an order of magnitude. In this way superconducting computers operate efficiently without resistance or heat, and their operating speed can reach billions of times per second. Secondly, its output voltage is high, which means that it outputs a strong signal, which can be more stable, clearer images and data, so that the current use of computers in the image quality, clarity and stability dwarf. There is also superconducting computer power loss is small, it is estimated that a fast switch during the energy consumption of less than 10-13 Joule, so that the computer is almost no internal heat, which is very important to improve the stability of the computer and extend the life of the computer core. It is conceivable that in this century, whoever develops a superconducting technology computer first will dominate the computer industry and even the world economy.
3, superconductivity in the military field
Superconducting high temperature copper oxide with superconductivity made of superconducting magnetic field meter can distinguish 10-14 -10-15 Tesla such a weak magnetic field. Its measurement accuracy is 3-4 orders of magnitude higher than other common electromagnetic instruments, so it can measure extremely weak magnetic fields and tiny changes in magnetic fields, and can be used to measure landmines and mines, so that the accuracy of the measurement is greatly improved. In addition, we can install superconducting magnetometers as trackers on mines. The military calls this kind of mines as superconducting magnetic mines, and its hit rate will be much higher than other mines. In the national defense can also use superconducting magnetometer to detect all kinds of ships along the coast, especially submarine movements, when the submarine is close to the coast, destroying the geomagnetic distribution, then the superconducting magnetometer can immediately show the change of the magnetic field, this anti-submarine method is much more accurate than the other methods, one is the high accuracy of the measurement, and the second is that this method is passive , it can find the submarine and the submarine can not find it.
Now the United States, Britain and other countries have superior performance of superconducting motor as a ship power to promote the ideal power equipment, respectively, invested a lot of energy in the development and research, successfully carried out 2200KW and 1000KW superconducting single DC electric propulsion system of the actual ship test, at the same time carried out the 30MW and 50MW large-capacity superconducting single in large destroyers and icebreakers on the design of the detailed. . Superconductors applied to ships, the biggest advantage is a substantial increase in power density to reduce the weight of the motor, reducing the space occupied by the power equipment can be used to place more other combat equipment to improve combat maneuverability and capacity. [11] In addition superconducting motors emit voltages that do not contain harmonics and will not be detected by other ships or submarines.
Superconducting materials have a wide range of applications in other areas, such as the development and application of superconducting energy storage magnets, synchrotrons with superconducting magnets, and superconducting nuclear magnetic *** vibration tomography imagers.
Four, summarize
In 2001, the World Bank's International Superconductivity Industry Summit predicted that by 2020 the world's superconducting products sales will total $244 billion. Superconducting materials, if further breakthroughs can be achieved at room temperature, will have no less an impact than another industrial revolution. [12] One of the tasks now facing inorganic chemistry is the quest for room-temperature superconductors to achieve superconductivity at room temperature. It is reasonable to believe that in the next few decades, superconducting materials will not only be an important means of solving the energy crisis, enabling controlled fusion to become a new clean energy source while dramatically reducing the amount of power lost due to the electrical resistance of the conductor. [13] At the same time, superconducting materials are the platform for the creation and development of new technologies and emerging disciplines. It can be said that superconductors will profoundly affect and change our lives.
References:
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[2], Ren Qing commendation, Zhu Weihao superconductivity and its application of the current state of research and prospects JOUBNAIJ 0F IJISHUI TEACHEBS COIJIJEGE 2002 Oct. Oct. 2002 (8): 30
[3], Miao Ruiping1 , Qi Xiuzhen1 , Zhao Yong2 Journal of Fuzhou University (Natural Science) Vol. 33 No. 1 Feb. 2005 : 94-99
[4], Zhu Wen detailed Intermediate Inorganic Higher Education Press 2004 265-268
[5], Huang Y., Liu X. Y., Zeng C. M. Progress in MgB2-based superconducting materials 2002, 25(1): 31-36
[6], Wang S. F., Zhou Y. L., Zhu Y. B., Liu Z. Zhen, Zhang Q., Chen Z. H., Lv H. B., Yang G. B. Preparation of superconducting thin films of MgB2 by chemical vapor deposition Low Temperature Physics Letters 2003, 25 Suppl. 233
[7], Wang Mei, Xu Zhijie, Su Xiyu Doping properties of magnesium boride superconductors QuFu Normal University Journal 2003, 26(2): 52-54
[8], Wang Jingrong, Wu Xiaozu, Zhou Lian High-temperature superconducting magnetic levitation and flywheel energy storage Tsinghua Tongfang Optical Disc Co. 233
[9], Bill Lee Superconductivity Technology and Its Applications New Technology Enlightenment 2005
[10], High Temperature Superconducting YunDian YingNa Superconducting Cable Network 2004
[11], ShaoDong Tang High Temperature Superconducting AC Synchronous Motors Shipboard Power Technology 2004, 1: 4-9
[12], Suguru Paik Most Important Energy Saving for the Electric Power of the 2lth Century Tree Material Energy Newsletter 2002, 2: 10
[13], Wu Ou 2003 Nobel Prize in Physics - Unimpeded Streams Nanfang.com Comprehensive