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SUPER CONDUCTIVITY

SUPER CONDUCTIVITY

SUPER CONDUCTIVITY


SUPER CONDUCTIVITY

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HISTORY :

The history of superconductivity, the property exhibited by certain substances lacking electrical resistance at temperatures close to absolute zero, began at the end of the 19th century and culminated in Heike Kamerlingh Onnes’s 1911 discovery. The theory surrounding the property of superconductivity was further developed over the course of the 20th century.

James Dewar initiated research into electrical resistance at low temperatures. Zygmunt Florenty Wroblewski conducted research into the electrical properties at low temperatures, though his research ended early due to his accidental death. Around 1864, Karol Olszewski and Wroblewski predicted the electrical phenomena in ultra-cold temperatures of dropping resistance levels. Olszewski and Wroblewski documented evidence of this in the 1880s.

Dewar and John Ambrose Fleming predicted that at absolute zero, pure metals would become perfect electromagnetic- conductors (though, later, Dewar altered his opinion on the disappearance of resistance believing that there would always be some resistance). Walther Hermann Nernst developed the third law of thermodynamics and stated that absolute zero was unattainable. Carl von Linde and William Hampson, both commercial researchers, nearly at the same time filed for patents on the Joule-Thomson effect. Linde’s patent was the climax of 20 years of systematic investigation of establish facts, using a regenerative counterflow method. Hampson’s designs was also of a regenerative method. The combined process became known as the Linde-Hampson liquefaction process.

Onnes purchased a Linde machine for his research. On March 21, 1900, Nikola Tesla was granted a US patent for the means for increasing the intensity of electrical oscillations by lowering temperature, which was caused by lowered resistance, a phenomenon previously observed by Olszewski and Wroblewski. Within this patent it describes the increase intensity and duration of electric oscillations of a low temperature resonating circuit. It is believed that Tesla had intended that Linde’s machine would be used to attain the cooling agents.

A milestone was achieved on 10 July 1908 when Heike Kamerlingh Onnes at the Leiden University in Leiden for the first time liquified helium.

WHAT IS SUPER CONDUCTIVITY

Superconductivity is a phenomenon observed in several metals and ceramic materials. When these materials are cooled to temperatures ranging from near absolute zero (-459 degrees Fahrenheit, 0 degrees Kelvin, -273 degrees Celsius) to liquid nitrogen temperatures (-321 F, 77 K, -196 C), they have no electrical resistance. The temperature at which electrical resistance is zero is called the critical temperature (Tc) and varies with the individual material. For practical purposes, critical temperatures are achieved by cooling materials with either liquid helium or liquid nitrogen. The following table shows the critical temperatures of various superconductors:

Material                        Type                    Tc (K)

Zinc                                 metal                   0.88

Aluminium                     metal                   1.19

Tin metal                        3.72

Mercury                          metal                   4.15

YBa2Cu307                  ceramic               90

TIBaCaCuO                  ceramic               125

Because these materials have no electrical resistance, meaning electrons can travel through them freely, they can carry large amounts of electrical current for long periods of time without losing energy as heat. Superconducting loops of wire have been shown to carry electrical currents for several years with no measurable loss. This property has implications for electrical power transmission, if transmission lines can be made of superconducting ceramics, and for electrical-storage devices. Another property of a superconductor is that once the transition from the normal state to the superconducting state occurs, external magnetic fields can’t penetrate it. This effect is called the Meissner effect and has implications for making high speed, magnetically-levitated trains. It also has implications for making powerful, small, superconducting magnets for magnetic resonance imaging (MRI).

How do electrons travel through superconductors with no resistance? Lets’s look at this more closely.

The atomic structure of most metals is a lattice structure, much like a window screen in which the intersection of each set of perpendicular wires is an atom. Metals hold on to their electrons quite loosely, so these particles can move freely within the lattice ~ this is why metals conduct heat and electricity very well. As electrons move through a typical metal in the normal state, they collide with atoms and lose energy in the form of heat. In a superconductor, the electrons travel in pairs and move quickly between the atoms with less energy loss.

As a negatively-charged electron moves through the space between two rows of positively-charged atoms (like the wires in a window screen), it pulls inward on the atoms. This distortion attracts a second electron to move in behind it. This second electron encounters less resistance, much like a passenger car following a truck on the freeway encounters less air resistance. The two electrons form a weak attraction, travel together in a pair and encounter less resistance overall. In a superconductor, electron pairs are constantly forming, breaking and reforming, but the overall effect is that electrons flow with little or no resistance. The low temperature makes it easier for the electrons to pair up.

TYPES I AND II SUPERCONDUCTORS

There are thirty pure metals which exhibit zero resistivity at low temperatures and have the property of excluding magnetic fields from the interior of the superconductor (Meissner effect). They are called Type I superconductors. The superconductivity exists only below their critical temperatures and below a critical magnetic field strength. Starting in 1930 with lead-bismuth alloys, a number of alloys were found which exhibited superconductivity; they are called Type II superconductors. They were found to have much higher critical fields and therefore could carry much higher current densities while remaining in the superconducting state.

The variations on barium-copper-oxide ceramics which achieved the superconducting state at much higher temperatures are often just referred to as high temperature superconductors and form a class of their own

Type I Superconductors

The thirty pure metals listed at right below are called Type I superconductors. The identifying characteristics are zero electrical resistivity below a critical temperature, zero internal magnetic field (Meissner effect), and a critical magnetic field above which superconductivity ceases.

The superconductivity in Type I superconductors is modelled well by the BCS theory which relies upon electron pairs coupled by lattice vibration interactions. Remarkably, the best conductors at room temperature (gold, silver, and copper) do not become superconducting at all. They have the smallest lattice vibrations, so their behaviour correlates well with the BCS Theory. While instructive for understanding superconductivity, the Type I superconductors have been of limited practical usefulness because the critical magnetic fields are so small and the superconducting state disappears suddenly at that temperatures are sometimes called “soft” superconductors while the Type II are “hard”, maintaining the superconducting state to higher temperatures and magnetic fields.

The superconductivity in Type I superconductors is modelled well by the BCS theory which relies upon electron pairs copper) do not become superconducting at all. They have the smallest lattice vibrations, so their behaviour correlates well with the BCS Theory. While instructive for understanding superconductivity, the Type I superconductors have been of limited practical usefulness because the critical magnetic fields are so small and the superconducting state disappears suddenly at that temperature. Type I superconductors are sometimes called “soft” superconductors while the Type II are “hard”, maintaining the superconducting state to higher temperatures and magnetic fields.

Type II Superconductors

Superconductors made from alloys are called Type II superconductors. Besides being mechanically harder than Type I superconductors, they exhibit much higher critical magnetic fields. Type II superconductors such as niobiumtitanium (NbTi) are used in the construction of high field superconducting magnets.

Type-II superconductors usually exist in a mixed state of normal and superconducting regions. This is sometimes called a vortex state, because vortices of superconducting current surround filaments or cores of normal material.

Model of Type-I Superconductors

The superconductivity in Type I superconductors is modelled well by the BCS theory which relies upon electron pairs coupled by lattice vibration interactions. Remarkably, the best conductors at room temperature (gold, silver, and copper) do not become superconducting at all. They have the smallest lattice vibrations, so their behaviour correlates well with the BCS Theory.

While instructive for understanding superconductivity,

the Type I superconductors have been of limited practical usefulness because the critical magnetic fields are so small and the superconducting state disappears suddenly at that temperature. Type I superconductors are sometimes called “soft” superconductors while the Type II are “hard”, maintaining the superconducting state to higher temperatures and magnetic fields.

   Mat.               Tc                      Mat.                       Tc

   Be                  0                        Al                            1.2

Rh                  0                        Pa                           1.4

W                   0.015                Th                           1.4

Ir                    0.1                     Re                           1.4

Lu                  0.1                     TI                            2.39

Hf                  0.1                     In                            3.408

Ru                  0.5                     Sn                           3.722

Os                   0.7                     Hg                           4.153

Mo                 0.92                   Ta                           4.47

Zr                   0.546                V                             5.38

Cd                  0.56                   La                           6.00

U                    0.2                     Pb                           7.193

Ti                    0.39                   Tc                           7.77

Zn                  0.85                   Nb                           9.46

Ga                  1.083

   Material                  Transiti on         Critical Field (T)

                                      Temp (K)

   NbTi                          10                          15

PbMoS                     14.4                      6.0

V3Ga                        14.8                      2.1

NbN                          15.7                      1.5

V3Si                          16.9                      2.35

Nb3Sn                      18.0                      24.5

Nb3AI                      18.7                      32.4

Nb3 (AIGe)              20.7                      44

Nb3Ge                      23.2                      38

High-Temperature Superconductors

HighTemperature Superconductors (abbreviated HTS) are materials that are have a superconducting transition temperature (Tc) above 30 K, which was thought (1960-1980) to be the highest theoretically allowed Tc. The first, high-Tc superconductor was discovered in 1986 by Karl Muller and Johannes Bednorz, for which they were awarded the Nobel Prize in Physics in 1987. The term high-temperature superconductor was used interchangeably j with cuprate superconductor until Fe-based superconductors were discovered in 2008. The best known high- temperature superconductors are bismuth strontium calcium copper oxide, BSCCO and yttrium barium copper oxide, YBCO.


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