15-06-2012, 04:00 PM
HIGH HIGH TEMPERATURE SUPERCONDUCTORS SUPERCONDUCTORS
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I. INTRODUCTION
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)
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.
A. 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
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.
Disadvantages:-
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.