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SIR C R REDDY COLLEGE OF ENGINEERING, ELURU
Department of Electronics & Communications Engineering
PAPER PRESENTATION ON
HIGH TEMPERATURE SUPERCONDUCTORS[EC 01]
T. U. S. S. S. Asha kiranmai
SIR C. R. R. COLEEGE OF ENGINEERING
In traditional conductors like copper or aluminium the transmission is asscoiated with the wastage of energy due to resistance drop.Typical copper wire generators are only fifty percent efficient in generating electricity. Due to this there is loss of billions of dollars. Super conductivity is a solution to all those problems because the electrical resistance is zero. If the national grid were made of super conductors rather than Al, savings would be enormous. Super conducting electricity generators are about 99% efficient. Cellular transmission involving super conductors provides clearer signals as it improves range, receiver sensitivity and frequency stability and also decreases the costs of cellular phone service. Computers, which became the major part of human life, if use high temperature super conductors will become 500 times faster than at present. It has many applications in fields like MRI, Maglev trains, electricity transportation, antennas etc. The US department of energy actively increases the use of super conductors as energy efficient devices.
An element inter - metallic alloy, or compound that will conduct electricity without resistance below a certain temperature. It is advantageous as resistance is undesirable because it produces losses in the energy flowing through the material. The super conductivity is referred as a "macroscopic quantum phenomenon. In earlier days, it exists at low temperatures. Now it has crossed halfway from room temperature. Since these superconductors occur at high temperature than 0 degree K, these are called high temperature superconductors. Below Tc, superconducting materials exhibit two characteristic properties.
1. Zero electrical resistance
2. Perfect diamagnetism (The Meissner effect)
Zero electrical resistance means that no electricity. This has many applications. Energy is lost as heat as material conducts. Due to zero electrical resistance there is no wastage of energy. The second of these properties, perfect diamagnetism, means that the superconducting material will exclude a magnetic field-this is known as Meissner effect and can be used to display extraordinary physical effects.
The Meissner Effect
Superconductors are actually perfect diamagnets and not perfect conductors. Perfect diamagnetism implies zero resistance that we have measured plus and added effect called "Flux Expulsion". The difference is quite subtle but can be readily seen by cooling the superconducting sample down while the magnet is sitting on its surface. When the sample is warm and the electrons are not "paired up", it is easy to place the magnet on the surface. Doing so causes the magnetic field from the magnet to penetrate into the sample. Then the sample is cooled and the electrons undergo the phase change. If the sample was a perfect conductor, nothing at all should happen. This is due to the fact that conductors do not like any form of change in magnetic fields. So where the magnet sits it should sit forever. But the superconductor will actually manage to remove the now present magnetic field from its interior. It accomplishes this by spontaneously running electric currents on the surface where no currents existed a moment before. The direction of the currents will be such as to create an opposing magnetic field to cancel the one present. As a result, the magnetic field coming from the sample will interact with that of the permanent magnet creating enough repulsion force to levitate the magnet again. The effect is shown in the frame below. If you look closely you can see Liquid Oxygen creeping up over the sample as its temperature drops. The oxygen is condensing out of the air as the sample temperature drops below about 85 K. There are two types of superconductors. They are type1 and type2 superconductors
Type 1 superconductors
Type 1 superconductors - characterized as the "soft" superconductors - were discovered first and require the coldest temperatures to become superconductive. They exhibit a very sharp transition to a superconducting state and "perfect" diamagnetism - the ability to repel a magnetic field completely. The Type 1 category of superconductors is mainly comprised of metals and metalloids that show some conductivity at room temperature. They require incredible cold to slow down molecular vibrations sufficiently to facilitate unimpeded electron flow in accordance with what is known as BCS theory. BCS theory suggests that electrons team up in "Cooper pairs" in order to help each other overcome molecular
obstacles - much like race cars on a track drafting each other in order to go faster. Scientists call this process phonon- mediated coupling because of the sound packets generated by the flexing of the crystal lattice
Type 2 Superconductors
Type 2 superconductors - also known as the "hard" superconductors - differ from Type 1 in that their transition from a normal to a superconducting state is gradual across a region of "mixed state" behavior. Since a Type 2 will allow some penetration by an external magnetic field into its surface, this creates some rather novel mesoscopic phenomena like superconducting "stripes" and "flux- lattice vortices".
The first superconducting Type 2 compound, an alloy of lead and bismuth, was fabricated in 1930 by W. de Haas and J. Voogd. But, was not recognized as such until later, after the Meissner effect had been discovered. This new category of super conductors was identified by L.V. Shubnikov
FUNDAMENTALS OF SUPERCONDUCTORS
Superconductors have the ability to conduct electricity without the loss of energy. When current flows in an ordinary conductor, for example copper wire, some energy is lost. In a light bulb or electric heater, the electrical resistance creates light and heat. In metals such as copper and aluminum, electricity is conducted as outer energy level electrons migrate as individuals from one atom to another. These atoms form a vibrating lattice within the metal conductor; the warmer the metal the more it vibrates. As the electrons begin moving through the maze, they collide with tiny impurities or imperfections in the lattice. When the electrons bump into these obstacles they fly off in all directions and lose energy in the form of heat.
Inside a superconductor the behavior of electrons is vastly different. The impurities and lattice are still there, but the movement of the superconducting electrons through the obstacle course is quite different. As the superconducting electrons travel through the conductor they pass unobstructed through the complex lattice. Because they bump into nothing and create no friction they can transmit electricity with no appreciable loss in the current and no loss of energy The ability of electrons to pass through superconducting material unobstructed has puzzled scientists for many years. The warmer a substance is the more it vibrates. Conversely, the colder a substance is the less it vibrates.
The understanding of superconductivity was advanced in 1957 by three American physicists-John Bardeen, Leon Cooper, and John Schrieffer, through their Theories of Superconductivity, know as the BCS Theory. The BCS theory explains superconductivity at temperatures close to absolute zero. Cooper realized that atomic lattice vibrations were directly responsible for unifying the entire current. They forced the electrons to pair up into teams that could pass all of the obstacles which caused resistance in the conductor. These teams of electrons are known as Cooper pairs. Electrons normally repel one another must feel an overwhelming attraction in superconductors. The answer to this problem was found to be in phonons, packets of sound waves present in the lattice as it vibrates. Although this lattice vibration cannot be heard, its role as a moderator is indispensable.
According to the theory, as one negatively charged electron passes by positively charged ions in the lattice of the superconductor. This in turn causes phonons to be emitted which forms a trough of positive charges around the electron. Before the electron passes by and before the lattice springs back to its normal position, a second electron is drawn into the trough. It is through this process that two electrons, which should repel one another, link up. The forces exerted by the phonons overcome the electrons' natural repulsion. The electron pairs are coherent with one another as they pass through the conductor in unison. The electrons are screened by the phonons and are separated by some distance. When one of the electrons that make up a Cooper pair and passes close to an ion in the crystal lattice, the attraction between the negative electron and the positive ion cause a vibration to pass from ion to ion until the other electron of the pair absorbs the vibration. The net effect is that the electron has emitted a phonon and the other electron has absorbed the phonon. It is this exchange that keeps the Cooper pairs together. It is important to understand, however, that the pairs are constantly breaking and reforming. Because electrons are indistinguishable particles, it is easier to think of them as permanently paired.
High Temperature Superconductivity
The History of Superconductivity Superconductivity was discovered in 1911 by the Dutch physicist, Heike Kammerlingh Onnes. Onnes first decided to pass an electrical current through a pure wire of mercury while measuring its resistance as he steadily lowered the temperature. there was total disappearance of resistance at 4.2 Kelvinâ„¢s. He called this newly discovered state "Superconductivity". In another experiment, Onnes started a current in a superconducting wire which was he kept cold in liquid helium for one year. After one year, Onnes found that the current was still flowing with no measurable losses without any assistance from a battery. He named these currents persistence currents. In a third experiment, Onnes measured the resistance of a lead wire in the superconducting state as he slowly increased the current. the wire showed no resistance initially but suddenly developed a high resistance as the current exceeded a critical value. This maximum current was termed the critical current density and it typically exceeds 10 million amps per square centimeter at 4 Kelvinâ„¢s! Due to these important discoveries, Kammerlingh Onnes was awarded the Nobel Prize in Physics in 1913.
Typical Superconductor Resistance vs Temperature Critical Current Density vs Temperature of a High Quality YBa2Cu3O7 Film Superconducting Antennas
The recent discovery of high-temperature superconductivity at liquid nitrogen temperatures (77 Kelvinâ„¢s) brings us a giant step closer to the vision of early scientists. The use of superconductors in transportation has already been proven using liquid helium as a refrigerant. Prototype levitating trains have been constructed in Japan using low-temperature superconducting magnets. Superconducting magnets are already crucial components of several technologies. Magnetic resonance imaging (MRI) is playing an increasing important role in medicine. Furthermore, particle accelerators used in high energy physics studies are very dependent on the use of high field superconducting magnets. The recent controversy surrounding the continued funding for the Superconducting Super Collider (SSC) illustrates the political ramifications of the applications of these new technologies. As liquid nitrogen based higher- temperature superconducting magnets become available, more industries will be able to afford these magnets because the cheaper cost of the higher operating temperature.
High- temperature superconductors could improve the communications industry. The telecommunications industry already uses high-temperature superconducting films to coat the inside of their microwave waveguides to reduce losses in their system. Furthermore, as superconducting transistors are developed, perhaps longer lasting and smaller "finals" could be developed for transceivers.
A more immediate application could perhaps be in the antenna system. Theoretically, superconductors could be employed to reduce the resistive losses in an antenna. . The short wavelengths used by many world wide broadcast stations, dramatic improvements are more likely at very long wavelengths because of the severe space limitations of the antenna. It is well known that an antenna needs to be a minimum of 1/8 wavelength in length to be reasonably efficient. Unlike the short wave frequencies employed in most world wide communications, this constraint is not severe. Due to salt water penetrating ability, submarines utilize 40 km wavelengths; therefore, an efficient antenna needs to be several miles long in order to have a reasonable efficiency. These long wires do pose obvious difficulties in the operation of submarines; it will be shown below how superconductivity could provide significant reductions in the antenna length while keeping nearly a 100% radiation efficiency.
Antenna efficiency E (%) is calculated by the equation E(%) = 100% x Rr / (Rr + Rg + Rc cos2 h), where Rr = radiation resistance, Rg = ground- loss resistance (approx. 0 in horiz. dipoles), Rc = loading coil resistance (~0 in superconducting coils assuming the superconducting ac- losses are negligible), and h = distance between the loading coil and the feed point in fractions of wavelength expressed in degrees. For example, if the loading coil placement is 1/8 wave from the feed point, h = 360 degrees/8 or 45 degrees. To perform this calculation, the radiation resistance must be known. From electromagnetic theory, the radiation resistance (Rr) of a center- fed dipole is given by Rr = 197 D2 / L2, where D = antenna length and L = wavelength. From this equation, it can be seen that a half-wave antenna has a radiation resistance of approximately 50 ohms. Most important, we see that as an antenna becomes short compared to the wave length, the coil resistance exceeds the radiation resistance thereby dissipating the power in the form of heat. Superconductivity will eliminate this coil loss while allowing the desired 100% radiation efficiencies in extremely short antenna systems.
Loading coils are often described by their Q-factors or "quality factors". The Q of a coil is given by Q = X1 / Rc where Xl = loading coil reactance; Rc = loading coil resistance. As the Q-factor increases, the resonance band width becomes narrower. In a superconducting antenna, one might expect the Q- factor to approach infinity implying it will only resonate at one specific frequency. Fortunately, this is not the case because all materials contain microscopic defects; in superconductors, these microscopic defects cause each electron to follow a slightly different path in order to avoid these defective, non- superconducting, regions. As a result, each electron has its own specific electrical length from one tip to the other tip of the antenna and this phenomenon, also known as percolation, prevents the antenna from developing the undesired infinite Q-factor while maintaining the desired zero resistance.
World's First Superconducting Antennas
In the spring of 1995, the Fusion Energy Division of the Oak Ridge National Laboratory built a 2m VHF BSCCO antenna. Using a Hewlett-Packard 8753A Network Analyzer, the principle investigators, E.C. Jones and D.O. Sparks, discovered that the resonance frequency dropped by approximately 5% as the superconducting tape was cooled below the superconducting transition temperature. The change in resonance frequency was believed to be the result of the rf current redistributing from the silver matrix in the normal state to the superconducting filaments as the tapes were cooled to their superconducting state with liquid nitrogen. Since these tapes had twisted filaments, the current had a 5% longer conduction path, i.e., "longer effective wavelength", at these superconducting temperatures. This was the first VHF antenna.
Uses of Superconductors
Efficient Electricity Transportation
Superconductors have many uses - the most obvious being as very efficient conductors; if the national grid were made of superconductors rather than aluminium, then the savings would be enormous - there would be no need to transform the electricity to a higher voltage (this lowers the current, which reduces energy loss to heat) and then back down again.
Superconducting magnets are also more efficient in generating electricity than conventional copper wire generators - in fact, a superconducting generator about half the size of a copper wire generator is about 99% efficient; typical generators are around 50% efficient. The US Department of Energy actively encourages the use of superconductors as energy efficient devices.
At the moment, the problem lies with the critical temperature - unless a material is found that can super conduct above 300K, some sort of cooling system needs to be employed, which would be expensive.
The magnetically levitated (Maglev) train is a super-high-speed non-adhesive transport system with a combination of superconducting magnets (SCMs) and linear motor technology. The concept was developed at the Japanese National Railways in 1970 The Maglev system applies the superconducting technology of low-temperature superconductors, Nb-Ti wires, and SCMs that require liquid helium as a coolant. In addition to these well-developed technologies, high-critical temperature superconductors that show superconductivity at liquid nitrogen are also prospective components for the Maglev system. Rare-earth barium-copper-oxide (REBCO) bulk superconductors are being considered for a superconducting magnetic bearing, a flywheel, a motor, high- field magnetic shielding, and a superconducting bulk magnet.
So-called 'MagLev' trains such as the Yamanashi MLX01 train show above have been under development in Japan for the past two decades - the train floats above the track using superconducting magnets; this eliminates friction and energy loss as heat, allowing the train to reach such high speeds.
Magnetic Resonance Imaging (MRI)
MRI is a technique developed in the 1940s that allows doctors to see what is happening inside the body without directly performing surgery. The development of superconductors has improved the field of MRI as the superconducting magnet can be smaller and more efficient than an equivalent conventional magnet.
Synchrotrons and Cyclotrons (Particle Colliders)
Particle Colliders like CERN's Large Hadron Collider (LHC) are like very large running tracks that are used to accelerate particles (i.e. electrons, positrons, hadrons and more) to speeds approaching the speed of light before they are collided with one another or other atoms, usually to split them (this was how many sub- nuclear particles such as neutrinos were discovered).They do this by cycling the particle using magnetic fields, continually increasing the speed of the particle. The first project and implimentation to use superconducting magnets was the proton-antiproton collider at Fermilab.
Fast Electronic Switches
Type 2 superconductors can be used to as very fast electronic switches (as they have no moving parts), due to the way in which a magnetic field can penetrate into the superconductor - this has allowed Japanese researchers to build a 4-bit computer microchip (compared to today's 32-bit and 64-bit processors) operating at about 500 times the speed of current processors, where heat output is currently a major problem with typical speeds approaching the 1GHz mark.
Using High-Temperature Superconductivity to Improve Cellular Phone Transmission Extending and Improving Cellular Phone Service To provide cellular phone or PCS service, a communications company using a land- based approach must place base stations - towers and reception/transmission equipment - at regular intervals throughout its service area. In deciding where to locate these base stations, the company considers the strength and clarity of its communications signals and how customer service will be affected when a signal shifts from one station to the next while the customer is traveling. All these factors depend on how well the station's equipment handles the communications signals. And that depends on how well each component of the equipment works as it attempts to distinguish the user's cellular phone signal from the surrounding electronic noise.
A compact one- box enclosure for Range Master and Spectrum Master. A High-Temperature Superconductivity Solution Superconducting components offer great benefits to cellular phone communications, including improvements in range, receiver sensitivity and frequency stability. These improvements, in turn, will extend the range of base stations, reducing the number needed to cover a given area and decreasing the costs of cellular phone service. Cellular phone users will receive clearer signals and suffer fewer dropped calls as their signals move from one base station site to the next. Focus on Preselector Receive Filters The goal of the ATP project and implimentation was to develop and demonstrate consistently performing RF superconducting components in a prototype base station. During the ATP project and implimentation, however, ISC narrowed its focus (with ATP approval) to preselector receive filters, which remove all extraneous RF signals and leave only those within the cellular spectrum allotted to that particular operator. Investigation of the cellular market indicated that the superconducting preselector receive filter was of greatest interest to customers in terms of improving system performance. Given the limited resources available to ISC, the company decided to focus on this component as an initial goal and to integrate others later. The new HTS technology is useful for other RF equipment and has potential applications in antennas, magnetic resonance imaging machines and other components of communications systems.
Superconductivity at room temperature has remained a dream. Critical current densities in HTS materials also tend to be naturally too low for technological applications, while there are persistent problems with poor mechanical properties which are related to the ceramic, granular, anisotropic nature of the HTS materials. They need to be formed at high temperatures in the presence of oxygen. HTS materials are very brittle and very difficult to shape and handle, while long, flexible, superconducting wires are necessary. Large super currents can only flow along the CuO2 planes, and only a small fraction is likely to be correctly oriented.
Are Superconductors the Future?
Are superconductors the future? Supercomputers, SQUIDS, electric power transmission, motors, and magnetically levitated trains are just some of the things superconductors can do; without wasting any energy. The Department of Energy is using much of its money for the research of high temperature superconductors. Some of the superconductor uses in our day-to-day life are:
Transmission cables that carried electricity without any loss of energy would mean more electricity could be transferred than before. Regular transmission lines lose about 3% of the energy transferred. This would also mean saving money and not much amount of space would be needed.
Motors made of superconductive wire would mean they would be smaller and more efficient. These could be especially used in submarines and ships.
Generators will be able to replace iron cores with superconducting wire. This will make it lighter and get more power from less fuel. Superconducting Magnetic Energy Storage (SMES) stores electricity for long periods of time in superconductive coils. SMES will be used by electrical utilities some day. SMES will be used for manufacturing plants by reducing power interruptions which cost American companies 12 billion dollars every year. Computers
If computers used superconducting parts they would be much more faster than the computers today. They would much smaller because no space for heat would be required. Computers of today need a great deal of space for cooling.
Superconducting Quantum Interference Device (SQUID) is able to detect magnetic fields less than ten billionth that of Earth. SQUIDs are used by physicist to search for gravitational waves. Geologists use SQUIDs to find oil and mineral deposits. Oil and mineral deposits distort the Earthâ„¢s magnetic field, letting the SQUID easily detect. Scientists use SQUIDs on airplanes or helicopters to check out the terrain of areas.
Superconductors , the advanced technology has enabled us to develop very high speed Maglev trains, MRI etc. and this concept can be extended to much more immense and numerous fields. They are the future of us or the generations to come. Some day superconductors will replace the conductors of electricity we use today and can be used at room temperature.We hope that the applications emerged from this technology are more scientific and economical.
1. Are superconductors the future by Jacob Eapen
2. High temperature superconductors by M. Marakami
3. Advances in superconductivity by H. Kamijo, K. Nemoto
4. NIST special publications 950-1.
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