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HIGH TEMPERATURE SUPER CONDUCTORS
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Presented By
CH.SUDHARSHAN RAO
04H75A0404
J.VIJAY KUMAR
03H71A0457
IV / IV E.C.E
D.V.R&Dr H.S MIC COLLEGE OF TECHNOLOGY,
KANCHIKACHERLA.
Krishna district, AP .
HIGH TEMPERATURE SUPER CONDUCTORS
ABSTRACT :
A substantial fraction of electricity is lost in the form of heat through resistance associated with the traditional conductors such as copper or aluminum. More than 15% of the world's generated electricity is lost as heat-this corresponds to trillions of US dollars that could be saved. A.P.Telecom has announced that the electricity loss as heat is worth of 1000Crores.
The loss of electricity as heat is due to the resistance of transmission lines .Super conductors are those in which the electrical resistance is called equal to zero. But in earlier days, it exists at very low temperatures .But after sometime the temperature at which they act as superconductor has increased. Now it has crossed halfway from room temperature. Since these super conductors occur at high temperatures than 0ok, these are called High temperature super conductors.These have many applications in communications field (antennas..), electrical field (generators , trains..) etc.
HIGH TEMPERATURE SUPER CONDUCTORS
INTRODUCTION:
Superconductivity is a phenomenon displayed by some materials when they are cooled below a certain temperature, These materials are known as the superconducting critical temperature, Tc. Below Tc,, superconducting materials exhibit two characteristic properties: Zero electrical resistance Perfect diamagnetism (the Meissner effect) Zero electrical resistance means that no electricity .This has many applications energy is lost as heat as the 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 the Meissner effect and can be used to display extraordinary physical effects Superconducting materials can be categorised into one of two types: Type I Superconductors - which totally exclude all applied magnetic fields. Most elemental superconductors are Type I. Type II Superconductors - which totally exclude low applied magnetic fields, but only partially exclude high applied magnetic fields; their diagmagnetism is not perfect but mixed in the presence of high fields. Niobium is an example of an elemental Type II superconductor.
Both types exhibit perfect electrical conductivity, and can be restored to 'normal' conductors in the presence of a sufficiently strong magnetic field.
The Meissner Effect :
A superconducting material cooled below its critical temperature in a magnetic field excluded the magnetic flux. This effect is known as the Meissner effect . Superconductors will not allow a magnetic field to penetrate into their interior. This finding is due to the generation of currents on the surface of the superconductor which exactly cancel the magnetic field in the superconductor's interior. This new property was called the Meissner Effect and it is this property that may someday allow the development of high speed levitating trains. Due to a limiting critical current density, Meissner discovered that if the magnetic field placed on a superconductor is increased beyond a critical value, the superconducting state suddenly disappears and resistance returns. This maximum magnetic field is now called the critical field. The limit of external magnetic field strength at which a superconductor can exclude the field is known as the critical field strength, Bc. Type II superconductors have two critical field strengths; Bc1, above which the field penetrates into the superconductor, and Bc2, above which superconductivity is destroyed, as per Bc for Type I superconductors.
Theory of Superconduction
BCS theory was proposed by J. Bardeen, L. Cooper and J. R. Schrieffer in 1957 .BCS suggests the formation of so-called 'Cooper pairs' .The BCS theory (1957) of Bardeen, Cooper and Schrieffer was a detailed microscopic theory that was quickly accepted as an explanation for the condensate in the superconductors that were known at the time. Such a condensate requires that the particles composing it be bosons, that is, have integral spin. Bosons obey Bose-Einstein statistics. Below the critical temperature the bosons in a superconductor can all gather together in the lowest possible energy state to form the condensate, and the greater the number that have accumulated, the harder it is for one of them to leave. Electrons are not bosons because they do not have integral spin. BCS theory explained how the interaction between the electrons and the phonons or lattice vibrations in the metal causes an electron-electron attraction. Some of the electrons form so-called Cooper pairs where the spins and momentum are opposite and therefore cancel out. Because the Cooper pairs have zero spin, they can participate in Bose condensation. It appeared that superconductivity was well explained and only possible at very low temperatures. However, BCS theory does not account well for high temperature superconduction, which is still not fully understood.
Applications:
The discovery of superconductivity soon generated interest in practical applications, mainly because of its potential to save energy. Indeed the replacement of copper or other normal conductors by superconducting materials avoids heat dissipation and other energy losses due to finite resisitance. In some types of equipment such as magnetic separators, these losses may account for most of the energy consumed in the device. Early prototypes for motors, transmission lines and energy storage magnets were developed, but they were never widely accepted.
There were important reasons for this, apart from the tremendous investment in existing technology. In most superconducting metals and alloys the superconductivity tends to fail in self- generated magnetic fields when the current densities through them are increased to practical levels. A second problem was the cost and complexity of operating refrigeration equipment near liquid helium temperatures (4 K, -269°C). Removing one watt of heat generated at 4 K demands about 1000 W of refrigeration power at room temperature.
High temperature superconductor
Introduction
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. Applications currently being pursued are mostly extensions of current technology used with the low-temperature superconductors such as powerful magnets used in MRI scanners. Additional applications include magnetic shielding devices, extremely sensitive medical imaging systems, infrared sensors, analog signal devices, and microwave communication devices, and waveguides. As our knowledge of the properties of high- temperature superconducting materials increase, more efficient power transmission lines, smaller and more efficient generators, energy storage devices, particle accelerators, and levitating trains will become more practical.
High Temperature Superconduction :
The highest known temperature at which a material went superconducting increased slowly as scientists found new materials with higher values of Tc, but it was in 1986 that a Ba-La-Cu-O system was found to superconductor at 35K - by far the highest then found. This was interesting as BCS theory had predicted a theoretical limit of about 30-40K to Tc (due to thermal vibrations). Soon, materials were found that would superconduct above 77K - the melting point of liquid nitrogen, which is far safer and much less expensive than liquid helium as a refrigerant. Although high temperature superconductors are more useful above 77K, the term technically refers to those materials that superconduct above 30-40K. In 1994, the record for Tc was 164K, under 30GPa of pressure, for HgBa2Ca2Cu3O8+x. In February 1987, a still higher superconducting record of 92 K was made by scientists at the University of Houston and the University of Alabama in Huntsville who substituted yttrium for lanthanum bringing superconductivity into the liquid nitrogen range. This increase in temperature is significant because liquid nitrogen (which boils at 77 K) is as cheap as coffee. Because of this large increase in operating temperature, these new materials are now called "High Temperature Superconductors". Since then scientists have found additional materials that superconduct at temperatures exceeding 133 K-nearly half way to room temperature (or 290 K)! Currently, many governments, corporations and universities are investing huge sums of money in the study of High Temperature Superconductivity, particularly in the development of commercial applications. The higher operating range of these new materials has influenced vast efforts in the development of these compounds, and changing the theory of the behavior of superconductors at these relatively higher temperatures.
Disadvantages :
These grandiose expectations inevitably led to disappointment. Room temperature superconductivity 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. These problems are both related to the ceramic, granular, anisotropic nature of the HTS materials (other than MgB2, which behaves as a brittle metal). They need to be formed at high temperatures in the presence of oxygen. Like all ceramics, HTS materials are very brittle and very difficult to shape and handle, while long, flexible, superconducting wires are necessary for many large-scale applications. Large supercurrents can only flow along the CuO2 planes, and only a small fraction of the material in a completed device is likely to be correctly oriented. The grain boundaries attract impurities, leading to weak links, which reduce the inter- grain current density and provide an easy path for flux vortices to enter the material. Flux creep or vortex penetration into HTS material is unusually rapid. The coherence length or diameter of a vortex core tends to be very small. This is a problem because pinning is most effective if the defect or impurity is of the same 'size' as the coherence length.
Magnetically Leviated Train:
The magnetically levitated (Maglev) train is a super-high-speed nonadhesive transport system with a combination of superconducting magnets (SCMs) and linear motor technology. The concept was developed at the Railway Technical Research Institute of the Japanese National Railways in 1970. In 1990, construction of the Maglev test line in the Yamanashi prefecture started, for the final confirmation of the Maglev train for practical use; running tests have been carried out since April 1997.
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.3 The magnetization of rare earth (RE)Ba2Cu3O7-x superconductors with a high critical-current density (Jc) results in a strong bulk magnet with liquid nitrogen refrigeration.4 The trapped magnetic field of the superconducting bulk magnet with a large single domain has been reported to be superior to that of a conventional permanent magnet. The superconducting bulk magnet can generate a higher magnetic field with increasing Jc and volume. Further, a large light rare earth (LRE)Ba2Cu3O7-x (e.g., Nd, Sm, Eu, or Gd) bulk superconducting magnet is believed to trap very high magnetic fields†more than 5 T at 77 K. Therefore, a superconducting bulk magnet or the superconducting quasi-permanent magnet for the Maglev train is possible.
MAGLEV Train :
Superconducting Antennas :
By now, you are probably wondering how can these new high- temperature superconductors could improve the communications industry. Only the mind can set limits to the potential number of improvements that can be envisioned. 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. However, since one "S" unit of signal strength corresponds to a change of 6 dB, a substantial increase in efficiency will be required for a target station to notice any improvement. Although less likely at 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.
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.
In addition, the Q- factor increased only slightly as already discussed above. 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. To the best of our knowledge, this was the first VHF antenna of its kind to have ever been built and to my disappointment, the Oak Ridge National Laboratory (who owned my superconductor patent rights) and the Department of Energy decided not to pursue this line of work or file any patents. * In 1997, on my personal time, I built a 2 foot tall 160m superconducting antenna for use near 1.86 MHz. I found that as the antenna was cooled with liquid nitrogen, the signal strength meter of the transceiver used to test the antenna increased from an S1 to S9 indicating the clear feasibility of these materials for long-wavelength communications. Also, to the best of my knowledge, I am unaware of any one else who has ever built a superconducting antenna for long-wave communications. In contrary, other research groups have used superconducting antennas to reduce the ac-losses found at the higher microwave frequencies. The first microwave superconducting antenna is credited to the Electronic Materials and Devices Research Group at the University of Birmingham (United Kingdom) and this group was recently presented with the International IEE Premium Award for their work.
Conclusions:
The ability of superconductors to conduct electricity with zero resistance can be exploited in the use of many electronic applications.High temperature super conductors are used in many fields .These have many applications in many sectors . Sceintists are making many experiments to take the high temperature super conductors to room temperature which make many revolutions . We hope that High temperature super conductors will come to room temperature and serve our application.
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