Introduction |
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| Superconductor is a material with no electrical resistivity at a range of temperature and pressure. There are many types of compounds which have this property. | |
| An element, inter-metallic alloy, or compound that will conduct electricity without resistance below a certain temperature. However, this applies only to direct current (DC) electricity and to finite amounts of current. All known superconductors are solids. None are gases or liquids. And all require extreme cold to enter a superconductive state. Once set in motion, current will flow forever in a closed loop of superconducting material - making it the closest thing to perpetual motion in nature. Scientists refer to superconductivity as a "macroscopic quantum phenomenon". In addition to being classified Type 1 and Type 2, superconductors can be categorized further by their dimensionality. Most are 3-D. But some compounds, like surface-doped NaWO3 and some organic superconductors are 2-D. Li2CuO2 and single-walled carbon nano-tubes have shown rare 1-D superconductivity. In addition to repelling magnetic fields, enhanced thermal conductivity and higher optical reflectivity are also properties of superconductors. From : | |
| H. Kamerlingh Onnes, after having successfully liquified helium in 1908, investigated the low temperature resistivity of mercury in 1911. The mercury could be made very pure by distillation, and this was important because the resistivity at low temperatures tends to be dominated by impurity effects. He found that the resistivity suddenly dropped to zero at 4.2K, a phase transition to a zero resistance state. This phenomenon was called superconductivity, and the temperature at which it occurred is called its critical temperature. | |
| Lead is a Type I superconductor
with a critical temperature of 7.2 K. Although such
superconductors can conduct currents with zero
resistance, their usefulness is limited because of low
critical magnetic fields. Above a certain current, the
magnetic field created by the current drives the material
into a normal resistive state. If a current is generated in a superconducting lead ring, it will persist because of the zero resistivity. Currents have been maintained in lead rings for several years to test the zero resistance condition. An induced current in an ordinary metal ring would decay rapidly from the dissipation of ordinary resistance, but superconducting rings had exhibited a decay constant of over a billion years! An exactly zero resistance implies a quantum effect - an energy gap. If the charge carriers do not interact with their environment to reduce their energy even a little bit, it must be because they can't - they are forbidden to by conservation of energy. This implies that there are no available quantum states within reach of the energy they have. The evidence for an energy gap was one of the steps which led to the BCS theory of superconductivity. |
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http://hyperphysics.phy-astr.gsu.edu/hbase/solids/scdis.html |
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| If mercury is
cooled below 4.1 K, it loses all electric resistance.
This discovery of superconductivity by H.
Kammerlingh Onnes in 1911 was followed by the
observation of other metals which exhibit zero
resistivity below a certain critical temperature. The
fact that the resistance is zero has been demonstrated by
sustaining currents in superconducting lead rings for
many years with no measurable reduction. An induced
current in an ordinary metal ring would decay rapidly
from the dissipation of ordinary resistance, but
superconducting rings had exhibited a decay constant of
over a billion years! One of the properties of a superconductor is that it will exclude magnetic fields, a phenomenon called the Meissner effect. The disappearance of electrical resistivity was modeled in terms of electron pairing in the crystal lattice by John Bardeen, Leon Cooper, and Robert Schrieffer in what is commonly called the BCS theory. A new era in the study of superconductivity began in 1986 with the discovery of high critical temperature superconductors. |
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http://hyperphysics.phy-astr.gsu.edu/hbase/solids/scond.html |
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| The flow of
current in normal conductors dissipates energy (producing
heat). This "frictional" dissipation is
measured by the resistivity of the material and is the
origin of Ohm's Law. At lower temperatures some materials called superconductors lose measurable electrical resistivity. Without any driving voltage, superconducting currents will flow indefinitely with no discernible decay. This phenomenon was originally discovered in 1911 by the Dutch physicist Heike Kamerlingh-Onnes (Nobel Prize: 1913) .. The superconducting transition occurs over a relatively small change in temperature. The temperature at which it occurs, which is called the critical temperature Tc , depends on the material. Until recently, the known superconductors had a Tc below 23K. The new ceramic superconductors (Nobel Prize: 1987) used in this experiment have much higher critical temperatures ( 90-100K) than those of the original metallic superconductors. Since easily available liquid nitrogen boils at 77K ,these materials make superconducting phenomenon easily observable above 77K. The theoretical understanding of the phenomena of superconductivity in these materials is not entirely understood. Before the discovery of the ceramic superconductors in 1986, the accepted model was the BCS theory (Bardeen, Cooper and Schrieffer. Noble Prize 1957). According to this theory a superconducting current is carried by bound pairs of electrons (Cooper pairs) which move through a metal without energy dissipation. The mechanism for the new ceramic superconductors remains unknown and is the subject of present day research. Another striking property of superconductors is that they are perfectly diamagnetic. When superconducting, a superconductor will not allow any magnetic field to penetrate it. This is the Meisner Effect!( named after the person who discovered this effect in 1933). Any external magnetic field which would penetrate the superconducting material is cancelled by an opposing field produced by electric currents induced in the superconductor. If this external field comes from a magnet, then the expulsion results in a repulsive force on the magnet The magnet, if above the superconductor ,will be levitated. This magnetic repulsion phenomenon is a manifistation of the the Meissner Effect . The purpose of this laboratory is to introduce students to superconductivity and demonstrate the characteristic properties of superconductors. With the use of the ceramic high temperature superconductors provided, you will observe the Meissner effect in which a magnetic field is excluded from the interior of a superconductor. Also you will observe that the electrical resistance vanishes when the material becomes superconducting, and you will measure the critical temperature and determine the Resistance vs. Temperature characteristic. In the process you will learn the principle (based on Ohm's law) of the use of a four point probe to measure the resistance of a material (in spite of the interfering effects of contact resistance) by making voltage and current measurements. From : |
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http://teacher.nsrl.rochester.edu/phy_labs/Superconductivity/Superconductivity.html |
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