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南京理工大学结课论文 课程名称： Materials Physics 论文题目：High Temperature Superconductors 姓 名： 周红云 学 号： 113116001166 授课教师： 徐锋 ContentsAbstract31 Introduction42 Development of HTS Materials62.1 HTS Bulk62.2HTS Wires82.2.1 First-Generation HTS Wires82.2.2 Second-Generation HTS Wires102.3 HTS Thin Films113 Theory and Mechanism of High Tc Superconductivity123.1 Magnetic Resonance Theory123.2 Electron-Lattice Interactions123.3 Circulating Currents Theory133.4 Pseudogap143.5 Electron Spin Theory164 Applications of HTS Materials175 Conclusions18References19Abstract Twenty years after the discovery of hightemperature superconductors (HTSs), the HTS materials now have been well developed. Meanwhile the mechanism of superconductivity is still one of the topical interests in physics. At present, the focus on the applied HTS technology has been moving to the industrial preparations from the laboratory research stage, and the technology has been well verified for practical applications from small to large scales. The fabrication techniques of engineering HTS materials are being industrialized; and various HTS devices are also on the way towards practical applications. This paper provides a comprehensive summary on the applied high temperature superconductivity with regard to various applicable HTS materials, their preparation techniques and characterization, and applications in a wide range.Index Terms High temperature superconductivity, high temperature superconductor, high temperature superconducting theory1 Introduction When the temperature was 23K(minus 250), human first discovered superconducting effect. Later the effect was discovered at a higher temperature ,it is called “high-temperature superconductors”. After the discovery of the high-temperature superconductors (HTSs) by Bednorz and muller1 and the succeeding discovery of superconductivity above liquid nitrogen temperatures by Wu et al., many scientists and engineers believed that a breakthrough for broad commercial applications of superconducting components was likely to occur in the following years. Primarily ,this discovery initiated a gold rush in research and development on this new material class leading to the discovery of a large number of oxide superconducting compounds with Tc up to 164 K2. However ,when the very peculiar physical properties of the oxide superconductors became more and more enlightened, material researchers and engineers were confronted with many hurdles which had to be overcome before any useful could be envisaged.The development of applicable HTS materials has progressed on several routes. HTS single crystals are not suitable for applications due to their small size and their low critical current density (Jc) as a consequence of a low density of pinning centers. However, the “quick and dirty”version melt-textured HTS bulk material shows superb magnetic pinning properties and are already applied as high-field permanent magnets, e.g. in magnetic bearings, motors, large current leads and magnetic separation3.Superconducting material has two main advantages: (1) Flow capacity is large , wire current density up to 106 A/cm2 number. The wound coil can prepare very strong magnetic ,when applicated into electrical equipment, it can greatly reduce the weight and volume of the equipment , improve equipment capacity density ; ( 2 ) zero-resistance properties, can completely solve the problem of transmission losses and improve the efficiency of the equipment significantly. The development of applicable HTS materials has progressed on several routes. HTS single crystals are not suitable for applications due to their small size and their low critical current density(Jc) as a consequence of a low density of pinning centers. However, the”quick and dirty” version melt-textured HTS bulk material shows superb magnetic pinning properties and are already applied as high-field permanent magnets, e.g. in magnetic bearings, motors, large current leads and magnetic separation.In spite of the ceramic nature of the cuprate oxides, flexible HTS wires or tape are obtained either by embedding HTS as thin filaments in a silver matrix or by HTS coating of metal carrier tapes. HTS wires are moving into the second generation (2G) of their development. The first generation (1G) relied on bismuth strontium calcium copper oxide (BSCCO), specifically Bi-2212 and Bi-2223. 1 km class long Bi-2223/Ag wires have already been fabricated and several applications have been demonstrated intensively6. The 2G is based on yttrium barium copper oxide (YBCO), which has the potential to be less expensive and to perform better. The development of YBCO-coated conductor technology is making possible the design and fabrication of smaller, lighter, and more efficient power devices such as transmission cables, motors, generators, transformers, and fault-current limiters that can be operated at temperatures approaching that of liquid nitrogen.Epitaxial HTS thin films achieve excellent superconducting properties that are well-suited for superconductive electronics. HTS Josephson junctions based either on Josephson tunneling through ultra-thin artificial barriers or on the Josephson junction behavior of HTS grain boundaries have become available; they can be used for the construction of highly sensitive magnetic field sensors of superconducting quantum interference devices (SQUIDs)4 HTS thin films are increasingly applied for the manufacturing of diverse microwave devices for the mobile and satellite communication. Intensive researches on these microwave applications include passive microwave devices and active microwave devices. Moreover, HTS films can be applied into digital circuit, voltage standard and so on.2 Development of HTS Materials2.1 HTS BulkDevelopment of bulk superconducting materials with superior electromagnetic properties is one of the most promising ways to exploration of HTSs in practice. Over the past 20 years, there are many kinds of HTS bulks having got great development, such as: YBa2Cu3O7-(Y-123), LRE-B2C3Oy “LRE-123” (RE=Sm, Gd, Nd, Eu···) and BSCCO (Bi-2223 and Bi-2212).HTS Y-123 bulk growth techniques have experienced 4 stages. 1) Solid state sintering (1987): the bulks have small coherence length and large anisotropy, high-angle grain boundaries in the solid act as weak links for supercurrents. 2) Melt processing (1989): the main fabrication methods include: melt texture growth (MTG), quench melt growth (QMG liquid phase process (LPP), powder melt process (PMP), melt powder melt growth (MPMG), solid-liquid melt growth (SLMG), and melt-quenched pressurized partial-melt growth (MQPPMG). All these variations of the melt-growth process for Y-123 involve the slow cooling of a mixture of Y-211 phases and a liquid phase through the peritectic temperature. 3) Top-seeded melt-texture growth(TSMTG): the method enables the growth of very large single-grained YBCO sample up to several centimeters in diameter and thickness, and the bulk can resolve weak-link problem of grain boundary and the weak flux pinning problem. In these methods, Y-123 crystal is difficult to form core in solution and cannot grow steady, so it is very difficult to prepare single crystal. Fig.1 illustrates the top-seeded melt-texture growth method. 4) Solute rich liquid crystal pulling (SRLCP)17: after 1993, the method of preparing single crystal HTS bulk has been studied. One of the useful methods for obtaining large scale single crystals under controlled crystal growth is the crystal pulling method and its modified SRLCP method more suitable for continuous growth5.Fig. 1. Schematic illustration of the TSMTG method.Recently, the Y-123 has enabled trapping of magnetic field above 17 T. It was realized by a treatment which improves the mechanical properties as well as thermal conductivity of a bulk Y-Ba-Cu-O magnet, thereby increasing its field-trapping capacity. First, resin impregnation and wrapping the materials in carbon fiber improve the mechanical properties. Second, a small hole drilled into the centre of the magnet allows impregnation of Bi-Pb-Sn-Cd alloy into the superconductor and inclusion of an aluminum wire support, which results in a significant enhancement of thermal stability and internal mechanical strength. As a result, 17.24 T was trapped, without fracturing, in a bulk YBCO sample of 2.65 cm diameter at 29 K.The LRE-Ba2Cu3Oy (LRE-123) family different from Y-123 shows that the replacement of Y by Nd, Sm, Gd, Eu,and some other light rare earth (LRE) results in a particular pinning even in an ideally oxygenated state, namely in the absence of oxygen-deficient clusters. When melting in air, a substitution of trivalent LRE for bivalent Ba takes place,leading to a depression of a hole or carrier concentration and thereby resulting in a low Tc. To avoid this undesirable effect, the oxygen controlled melt growth (OCMG) process can be developed6. Under melt processing of the LRE-123 compound in a reduced oxygen atmosphere, a LRE/Ba substitution is largely suppressed and Tc values of these systems reached 96 K, which has never been achieved before in the Y-123 system.MgO seeds proved to be efficient in the growth orientation control of large grain LRE-123 pellets when a small quantity of ZnO was added, accompanied by a substantial reduction of liquid phase loss. Perfect facet lines growing up to the pellet bottom demonstrated excellent melt growth. Field distribution contour maps at the top and bottom surfaces were nearly identical. Magnetization measurements indicated that flux pinning performance improved continuously with increasing ZnO content up to 0.035 mol%, with critical current density reaching 105 A/cm2 at 3 T and 77 K.For bismuth strontium calcium copper oxide (BSCCO) bulks, there are two systems for engineering applications:Bi2Sr2Ca2Cu3O10 (Bi-2223) with Tc of 110 K and Bi2Sr2Ca1Cu2O8 (Bi-2212) with Tc of 80 K to 90 K. Unlike yttrium barium copper oxide (YBCO) or REBCO, flux pinning of BSCCO is extremely weak, mainly due to a strong anisotropy. The main bulk-type applications of BSCCO for current leads and fault current limiters are in rod, tubular, and plate forms. Bi-2223 rod is usually prepared by normal sintering, hot forging process, and cold isostatic pressing method7. Due to extremely large anisotropy, textured structure is easy to obtain.Bi-2212 rod can be made by a partial melting process, and more textured structure is obtained than that of Bi-2223.One of the more successful techniques for fabricating Bi-2212 tubes is the melt cast process, these are manufactured by cutting these tubes to bifilar coils. At the operation temperature of 65 K, a current density of 4 kA/cm2 was achieved. Due to the progress in material development, BSCCOs superconducting bulk materials show very good prospects for technological and economical viability, such as use in resistive type current limiters 8.2.2HTS Wires HTS wires (tapes) are moving into the second generation (2G) of their development. The first generation (1G) relied on BSCCO102-104, whereas the 2G is based on YBCO, which has the potential to be less expensive and to perform better. One of the main challenges in developing high performance superconductors has been the brittleness of many of the most promising materials to be drawn into wires that can carry current, yet recent developments allow fabrication of superconductor wires and tapes.2.2.1 First-Generation HTS WiresThe most commonly used materials for the HTS wires in the early stage are bismuth-based, specifically Bi-2212 and Bi-2223. These materials and their wire products, especially the Bi-2223 Ag clad multifilament wires, are known as the 1G of HTS wires and have been used to demonstrate a variety of HTS heavy current devices.The oxide powder in tube (OPIT) technology is the well established route for the commercial fabrication of Bi-2223 Ag clad ribbon type conductors9, as shown in Fig.2. At the beginning of the process, rods of isostatically pressed oxide powder are inserted into a silver or silver alloy tube. This tube is then sealed with Cu or silver plugs and evacuated to remove air and moisture. After vacuum closing the billets are drawn down to a final diameter determined by the number of filaments and ultimate tape dimensions, the mono filamentary wire is then cut and re-bundled into an alloy tube. Subsequently, the billet is evacuated again, closed and cold worked to a final diameter around 2 mm, and the single filament thickness of this multifilament wire is around 100 m for example. The wire fabrication is followed by the thermo-mechanical treatment, a repeated process of rolling and heat treatments. This process is designed to establish phase compositions and to engineer the microstructure. Its Jc has reached near 105 A/cm2(77 K, 0T), and engineering current density (Je)104/cm2. The most influential parameters to accomplish a high performance product are the phase content of the precursor powder, the deformation parameters of the composite during the cold working process, and the heat treatment parameters, i.e. time, atmosphere, temperature. The practical performance of the 1G HTS wires, i.e. The current density, long length with flexible winding capability,and production maturity, has arrived at the level of industrialization10.Fig. 2. Flow chart of the OPIT process of the multi-filamentary composite HTS wire.2.2.2 Second-Generation HTS Wires Due to the higher cost of 1G wire and the intrinsic properties of YBCO, the researchers have shifted their efforts toward the development of second-generation (2G) HTS wires as a so-called YBCO coated conductor to replace the 1G Bi-2223 OPIT tape. The important advantages of 2G wires over 1G wire are that YBCO has better in-field electrical performance at higher temperatures, a potentially lower-cost fabrication process,and low alternating current(ac) losses.YBCO has useful magnetic properties at 77 K, but is highly susceptible to magnetic field degradation in the transport current due to weak-links resulting from highangle grain boundaries. The solution to this grain boundary problem for YBCO is to produce “textured” substrate that will provide a template for growing biaxially textured films of YBCO with low-angle boundaries.Typically, 2G HTS wires have three components: flexible metal substrate, buffer layers, and YBCO superconductor layers. Several methods were developed to obtain biaxially textured metal substrates suitable for fabricating high-performance YBCO coated conductors.They are ion-beam assisted deposition (IBAD), rollingassistedbi