Super Conductivity

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                                  Super Conductivity 


https://en.m.wikipedia.org/wiki/File:Periodic_table_with_superconducting_temperatures.jpg




                              
Metals are good conductors of electricity, and their conductivity increases as the temperature is lowered. In 1911 the Dutch scientist Heike Kamerlingh Oimes discovered that metals such as Hg and Pb became superconductors at temperatures near absoiute zero.


 A superconductor has zero or almost zero electrical resistance: It can therefore carry an electric current without losing energy, and in principle the current can flow for ever. There is a critical temperature. 


That which the resistance drops sharply and superconduction occurs. Later, Meissner and Ochsen feld found that some superconducting materials will not permit a magnetic field to penetrate their bulk. 


This is now called the Meissner effect, and gives rise to 'levitation'. Levitation occurs when objects float on air. This can be achieved by the mutual repulsion between a permanent magnet and a superconductor. 


A superconductor also expels all internal magnetic fields (arising from unpaired electrons), so superconductors are diamagnetic. In many cases the change in magnetic properties is easier to detect than the increased electrical conductivity, since the passage of high currents or strong magnetic fields may destroy the superconductive state. 


Thus there is also a critical current and critical magnetization which are linked to Tc. A superconducting alloy of niobium and titanium, which has a Tc of about 4 K and requires liquid helium to cool it, has been known since the 1950s. 


Considerable effort has been put into finding alloys which are superconductors at higher temperatures. Alloys 6f Nb3Sn, Nb3Ge, Nb3AI and V 3Si all show superconductivity and have Tc values of about 20 K. It is interesting that these alloys all have the same tungsten structure.


The Nb3Sn and and Nb3Ge alloys have Tc values of 22 K and 24 K respectively. These alloys are used to make the wire for extremely powerful electro magnets. These magnets have a variety of uses: 


l. In linear accelerators used as atom smashers for high energy particle physics research

2. In nuclear fusion research to take powerful magnetic fieids .which act as a magnetic bottle for a plasma 

3. For nuclear magnetic resonance imaging (which is used in diagnostic medicine). 



An extremely high current can be passed thtough a very fine wire made of a superconductor. Thus small coils with a large number Of turns can be used to make extremely pbwerful high field electromagnets. 


Because the superconductor has effectively zero resistance. Since there is no current loss, once the current  is flowing in the coil it continues indefinitely. 


For example, in large superconducting magnets used in plasma research, the current used by a Nb. If a superconducting alloy at 4 K was only 0.3% of the current used in an electromagnet of similar power using copper wire for the metal turns. 


A major obstacle to the widespread use of these low temperature superconductors has been the very low value of the transition temperature. The only way of attaining these low temperatures was to use liquid helium, which is very expensive.


The first non-metallic superconductor was found in 1964. This was a metal oxide with a perovskite crystal structure and is a different type of superconductor from the alloys. It was of 60 prattical use since the Tc is only 0.01 K. 


The perovskite structure is formed by compounds of formula AB03 , where the oxidation states of A and B add up to 6. Examples include BaTi03, Ca1'i03 and NaNb vo3. 


The perovskite crystal structure is cubic. A Ca2+ ion is located at the body-centred position (at the centre of the cube), the smaller Ti4+ ions are located at each corner.


Metals are good conductors of electricity, and their conductivity increases as the temperature is lowered. In 1911 the Dutch scientist Heike Kamerlingh Oimes discovered that metals such as Hg and Pb became superconductors at temperatures near absoiute zero.


 A superconductor has zero or almost zero electrical resistance: It can therefore carry an electric current without losing energy, and in principle the current can flow for ever. There is a critical temperature. 


That which the resistance drops sharply and superconduction occurs. Later, Meissner and Ochsen feld found that some superconducting materials will not permit a magnetic field to penetrate their bulk. 


This is now called the Meissner effect, and gives rise to 'levitation'. Levitation occurs when objects float on air. This can be achieved by the mutual repulsion between a permanent magnet and a superconductor. 


A superconductor also expels all internal magnetic fields (arising from unpaired electrons), so superconductors are diamagnetic. In many cases the change in magnetic properties is easier to detect than the increased electrical conductivity, since the passage of high currents or strong magnetic fields may destroy the superconductive state. 


Thus there is also a critical current and critical magnetization which are linked to Tc. A superconducting alloy of niobium and titanium, which has a Tc of about 4 K and requires liquid helium to cool it, has been known since the 1950s. 


Considerable effort has been put into finding alloys which are superconductors at higher temperatures. Alloys 6f Nb3Sn, Nb3Ge, Nb3AI and V 3Si all show superconductivity and have Tc values of about 20K. It is interesting that these alloys all have the same tungsten structure.


The Nb3Sn and and Nb3Ge alloys have Tc values of 22 K and 24 K respectively. These alloys are used to make the wire for extremely powerful electromagnets. These magnets have a variety of uses: 


l. In linear accelerators used as atom smashers for high energy particle physics research

2. In nuclear fusion research to take powerful magnetic fieids .which act as a magnetic bottle for a plasma 

3. For nuclear magnetic resonance imaging (which is used in diagnostic medicine). 



An extremely high current can be passed thtough a very fine wire made of a superconductor. Thus small coils with a large number Of turns can be used to make extremely pbwerful high field electromagnets. 


Because the superconductor has effectively zero resistance, the wire does Mt get hbt. Since there is no current loss, once the current  is flowing in the coil it continues indefinitely. 


For example, in large superconducting magnets used in plasma research, the current used by a Nb. If a superconducting alloy at 4 K was only 0.3% of the current used in an electromagnet of similar power using copper wire for the metal turns. 


A major obstacle to the widespread use of these low temperature superconductors has been the very low value of the transition temperature. The only way of attaining these low temperatures was to use liquid helium, which is very expensive.


The first non-metallic superconductor was found in 1964. This was a metal oxide with a perovskite crystal structure and is a different type of superconductor from the alloys. It was of 60 prattical use since the Tc is only 0.01 K. 


The perovskite structure is formed by compounds of formula AB03, where the oxidation states of A and B add up to 6. Examples include BaTi03, Ca1'i03 and NaNb vo3. 


The perovskite crystal structure is cubic. A Ca2+ ion is located at the body-centred position (at the centre of the cube), the smaller Ti4+ ions are located at each corner.


References - 








https://en.m.wikipedia.org/wiki/File:Meissner_effect_p1390048.jpg
















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