superconductor n kegunaannye..........

emiliahani

WHAT IS LOW-TEMPERATURE SUPERCONDUCTIVITY?

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In a normal metal, the electrons act relatively independently of both each other and the lattice of ions. Under certain conditions, however, the delocalised cloud can cause indirect bonding between electrons, thereby giving current enough coherence to resist any collisions or electrostatic interactions that individual electrons may experience. In this way, the only influence on the electrons is electron potential, a gradient of which is provided by an electromotive force. The result of this is a conductor without any resistance at all--a superconductor.

The temperature of the material is absolutely critical for electron bonding. The ion lattice must have such low energy that most of the valence electrons remain with their associated atoms, and the vibration of the lattice is only very slight. As a result, it is impossible to produce this state in temperatures above about 25K. The boiling point of liquid helium is around 4K, so this is ideal for use with low-temperature superconductors.

Under these conditions, when an electron is liberated from an atom's orbital, the energy of the lattice is low enough that the moving electron exerts a significant attractive force on the surrounding ion lattice, leaving a region of low electron potential in its wake. This region then attracts other electrons causing them to follow the path of the first. As a result, an indirect attraction between the two electrons is formed, pairing them over a fixed distance--about a thousand times the spacing between adjacent ions in the lattice. This attraction is only possible if both electrons have opposed spins, which reduces the electrostatic repulsion between them. In fact, the two electrons in an orbital always have opposed spins and very similar energies, and the valence orbital is destabilised under these extremely low temperatures. This means that most often the pairs will consist of electrons originating form the same orbital within a very short time of each other. This interaction between electrons is called Cooper Pairing, named for Leon Cooper who received the 1972 Nobel Prize for Physics along with John Bardeen and John Schreiffer.

  
(Figure 6: Effect of Electrons on Metallic Ion Lattice) (Animated version. Not to scale)
Image created by the author for this website Taken from Superconductors.org

It is preferable for the electrons to remain in this state, separated by a distance much larger than would be the case if their energy levels dropped and they re-entered the orbital they had just left. The result is that virtually all of the delocalised cloud consists of paired electrons with energy only slightly higher than they would have in the ground state.

Not only constantly interacting with each other, but also being in the same quantum state gives the whole cloud of electrons a high degree of coherence despite every electron not being paired to every possible other. This means that even if one pair of electrons independently receive enough energy to break the Cooper pair, they will not dissociate with each other unless they also reach the next quantum energy level, bringing a far greater degree of stability to the superconductive state than might otherwise be expected.

However, the paired state is still relatively fragile, because the amount of energy required to raise the quantum state is fairly trivial. As a result, there are several considerations which ensure that a superconductor remains in this state.

emiliahani

High-temperature superconductors (abbreviated high Tc) are a family of superconducting ceramic materials largely containing copper-oxide planes as a common structural feature. For this reason, the term was (before 2008) often used interchangeably with cuprate superconductors. "High" temperature in this context just means above 30 K, which was thought (1960-1980) to be the highest possible Tc and was well above the 1973 record of 23 K.

High-Tc superconductivity was discovered in 1986; until then it was thought that BCS theory ruled out superconductivity at temperatures above 30 K. The experimental discovery of the first high-Tc superconductor by Karl M黮ler and Johannes Bednorz was immediately recognized by the Nobel Prize in Physics in 1987.

High-temperature superconductivity allows some materials to support superconductivity at temperatures above the boiling point of liquid nitrogen (77 K or −196 癈). Indeed, they offer the highest transition temperatures of all superconductors. The ability to use relatively inexpensive and easily handled liquid nitrogen as a coolant has increased the range of practical applications of superconductivity.

The best known HTS are BSCCO and YBCO.

The critical magnetic field that destroys superconductivity tends to be higher for materials with a high Tc and in magnet applications this may be more valuable than the high Tc itself. Some cuprates have an upper critical field around 100 tesla.

Although cuprate compounds in the normal superconducting state share many characteristics with each other, there is as of 2008 no widely accepted theory to explain their properties. The search for a theoretical understanding of high-temperature superconductivity is widely regarded as one of the most important unsolved problems in physics, and it continues to be a topic of intense experimental and theoretical research, with over 100,000 published papers on the subject.[1] Cuprate superconductors (and other unconventional superconductors) differ in many important ways from conventional superconductors, such as elemental mercury or lead, which are adequately explained by the BCS theory. There has been much debate as to high-temperature superconductivity coexisting with magnetic ordering in several ruthenocuprates and other exotic superconductors, and the search continues for other families of materials. In February 2008, researchers at the Tokyo Institute of Technology announced that the quaternary compound LaOFeAs (an oxypnictide), when doped with F for O, is a new non-cuprate high-temperature superconductor[2]. and already hints at an upper critical field around 64 T.

akinari

aku suka tgk maglev...teori yg mengatakan roda bulat lagi laju tak leh pakai dah...

jebeng

dlm maglev...roda hnya sbgai safety precaution jer...x der fungsi dlm acceleration..huhu

jebeng

kalo dlm research cuprate mmg leh dikategorikan sbgi HTS...but dlm applikasi ssh nk pki since seramik ni ssh nk lentur n dibuat wayar cam kabel yg sedia ade....issue yg len kene ade pump utk cecair cyrogenic ni...theory is good but practically right now ade byk prob...