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superconductivity and its applications


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this documents explains the clear view of superconductor applications.

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superconductivity and its applications

  1. 1. Applications of Super conductors Santhosh Kumar. S, Venkatesh. S, Karpagam institute of technology, Coimbatore.Abstract:As a consequence, the past decade or two of superconductor discovery can best besummarized by the phrase “expect the unexpected”. Superconductivity is an electricalresistance of exactly zero which occurs in certain materials below a characteristictemperature. It was discovered by Heike Kamerlingh Onnes in 1911. Like ferromagnetismand atomic spectral lines, superconductivity is a quantum mechanical phenomenon. Thebehavior of superconductor suggests that electron pairs are coupling over a range of hundredsof nanometers, three orders of magnitude larger than the lattice spacing, called cooper pairs.These coupled electrons can take the character of a boson and condense into the ground state.This clear explanation is given by BCS theory. hereby after reading this paper you will easilyget the clear ideas of Josephson Devices-BCS theory-applications such as super conductinggenerators, SEMS, superconducting cables.
  2. 2. Introduction: 1. London theory- 1935: Kamerlingh onnes, founder of This is the first macroscopic theorysuper conductivity observed that mercury to explain the concept of superacts as superconductor below 4.2 k conductivity London proposed that theretemperature. The resistance of his mercury are two types of conduction electronsample did not tend toward some constants namely super electrons, normal electrons.value as the temperature was reduced Here, the super electrons are not subjectedbelow 4.2k. At 4.15k, the DC resistance of to any lattice scattering and they movethe sample is completely vanished. freely over the lattices points. But the total conduction electron density is equal to In 1913, he wrote that no doubt sum of super electrons and normalwas left of the existence of a new state of electrons.mercury in its resistance has practicallyvanished superconducting state. 2. Ginsburg- Landau Theory-1950: The most striking and well known This theory gives the idea that theproperty of superconductor is their lack of super conducting state is characterized byelectrical resistance. In 1933, Meissner and a single complex wave function. ThisOchsen field found, most surprisingly, that theory describes the properties of superwhen a pure metal is cooled through its conducting state such as meissner effect,superconducting transition temperature in zero electrical resistance and Type II superthe presence of magnetic field all magnetic conductor.flux is expelled from within its bulk.Spontaneous exclusion of magnetic flux 3. Bardeen, Cooper and Schriefferfrom a superconductor could not be quite (BCS) theory:complete of course, even in principle This is the first microscopic theorycurrents in metals arises from the flow of based on quantum theory. They introducedelectrons and no physical current sheet can a new pair of electrons called cooper pairbe made indefinitely thin due to repulsion and they are responsible for the superforce between these electrons. Hence the conductivity.temperature at which a normal conductorloses its resistivity and become a Principle:superconductor is known as transitiontemperature (or) critical temperature. This theory states that the electrons experience a special type of attractive At the transition temperature, the interaction, overcoming the coulombfollowing physical changes are observed. forces of repulsion between them, as a result Cooper pairs (i.e.,) electrons pairs 1. The electrical resistance drops to are formed. At low temperature these zero. pair’s moves without scattering (i.e.,) 2. The magnetic flux lines are without any resistance through the lattice excluded from the material. points and the material becomes 3. There is a discontinuous change in superconductor. Here the electrons- lattice- specific heat. electrons interaction should be stronger 4. Further there are also a small than electron-electron interaction. change in thermal conductivity and the volume of the material. The superconducting state of a metal may be considered to be resulting With the help of reference given by from cooperative behavior of conductiononnes, the succeeding scholars proposed electrons. Such a cooperation or coherencethe following theories.
  3. 3. of electrons takes place when a number ofelectrons occupy the same question state.This, however, appears to be impossiblefor both statistical and dynamic reasons. The electrons attract each other in acertain energy range and forms pairs. Apair of electron behaves like a boson. Thusa number of pairs can occupy the samequantum state which causes coherenceamong electrons. In the electron – lattice - electron interaction, the electron will not be fixed, When an electron they move in opposite direction and theirmoves through a crystal, it produces lattice co-relation may persists over lengths ofdistortion and sets the heavier ions into maximum 10-6 m. this length is calledslow forced oscillations. Since the electron coherence length.moves very fast it leaves this region much Temperature:before the oscillations can die off.Meanwhile, if another electron happens to When the temperature (T) is lesspass to pass through this distorted region, than the critical temperature (Tc), theit experiences a force which is one of resistivity due to lattice vibrations will be less. If the electron – lattice- electron isattraction and is of the type of polarization stronger than electron-electron interactionforce. This attractive force lowers the than more number of cooper pairsenergy of the second electron. The electrons will be generated, these cooperrepulsive force between the electrons is pairs electrons will sail (move freely) oversmall since the Coulomb’s repulsion is the lattice points without any exchange ininstantaneous while the attraction energy. So, they will not be slowed down.meditated by lattice distortion is highly Hence, the material property and the conductivity becomes infinite andregarded in time. Therefore, the attraction exhibiting the phenomenon of as supercaused by even a weak lattice distortion conductivity.can overcome stranger Coulomb’srepulsion. Thus the net effect is the General properties of super conductors:attraction of two electrons via lattice 1. Electrical resistancedistortion to form a pair of electronsknown as the cooper pair. It is now The electrical resistivity drops to zeroobvious that the mass of an ion has an at the transition temperature. One canimportant role conclude that super conductors have virtually zero Electrical resistance and theyCooper pairs: can conduct Electricity without resistance. The pairs of electrons formed due 2. Diamagnetic propertyto electron – lattice- electron interaction(forces of attraction) by overcoming the The super conductors are a perfectelectron-electron interaction (forces of diamagnetic. As the material which isrepulsion) with equal and oppositemomentum spins called Cooper Pairs. placed in a uniform magnetic field (whose value is smaller than the critical magnetic
  4. 4. Hc), is cooled below Tc” the magnetic flux Correspondingly. The value of theinside material is excluded from the field will be different for differentmaterial. This is called Meissner effect. materials.Thus a material can behave as a 4. Effect of magnetic current:superconductors only when The superconducting properties of I. The resistivity of the material conductors disappear when a should be zero and sufficiently heavy current is passed II. The magnetic induction in the material should be zero when it is though them. placed in an uniform magnetic Since when current flows through field. conductor it will set up Both the conditions are The magnetic field which destroys the independent to each other and to super conducting state. According to get super conducting state, these Silsbee’s rule, for a superconducting two conditions should wire. simultaneously exist in the IC =2π r HC material. Where IC is the critical current and r is the radius of the wire. 3. Effect of magnetic field 5. Effect of pressure Below TC, Superconducting can By applying very high pressure, be destroyed by the application of we can bring TC of a material nearer to strong magnetic field. At the room temperature .if we increase the temperature, the minimum field pressure on the material, Tc also required to destroy superconductivity increases. TC is directly proportional is called critical field (Hc) of the to pressure at very pressures. material. Thus the value of the critical Researchers are going on to get field depends upon the temperature of superconducting state at room the super conducting material. temperature by applying very high HC=Ho {1-(T2 /Tc2)} pressures. Where Ho =critical field 0 K. 6. Isotope effect Maxwell found that the transition One can know that when the temperatures are inversely proportional temperature of the material increases, to the square roots of the atomic the value of the critical magnetic field weights of the isotopes of a single decreases superconductor .thus,
  5. 5. Mα Tc = a constant Then we learn something about Josephson Where α is a constant and it is devices, approximately equal to 0.5 .for The Josephson effects and tunneling: example the atomic weights of isotopes Josephson observered some remarkable of mercury are from 199.5 to 203.4 effects associated with the tunneling of atomic mass units . Therefore their super conducting electrons through a very thin insulator (1-5nm) sandwiched transition temperatures are also from between two superconductors. Such an 4.185k to 4.146k respectively. insulating layer forms a weak link between the super conductors which is referred toOther general properties: as the Josephson junction. The effects observed are given below1. Generally good conductors are not goodsuper conductors. Example: gold (i) The DC Josephson effect According to this effect, a dc2. The critical temperature and the criticalmagnetic field of a current flow across thesuperconductor changes slightly under the junction, even when there is noinfluence of an applied voltage is applied across it.stress. A stress increases the dimensions ofthe specimen increases (ii) The AC Josephson effectthe transition temperature and produces a If a dc voltage is applied acrosscorresponding changes in the critical the junction, rf current ofmagnetic field. oscillations of frequency3. The introduction of chemical impuritiesmodifies almost all the f=2eV/h are step across it. Bysuperconducting properties particularly the measuring the frequency andmagnetic ones. voltage, the value of e/h can be4. The elastic properties and the thermal determined. Hence this effectexpansion coefficient remainunaffected below and above Tc. has been utilized to measure e/h5. The superconducting state does not very precisely and may be usedexhibit any thermoelectric effect. as a means of establishing a6. No changes in photo electric propertiesare observed. voltage standard. Furthermore,7. No appreciable changes in the an application of rf voltagereflectivity are observed in the along with the dc voltage canvisible and infrared regions. result in flow of direct current8. The zero resistance of superconductors through the junction.changes slightly at veryhigh frequencies (above 10 MHz) of the (iii) Macroscopic quantumalternative current. interference
  6. 6. This effect explains the sensitive to very small changesinfluence of the applied in magnetic field, the SQUIDmagnetic field on the super can be used as galvanometer.current flowing through the The flow of current takes placejunction. According to this between any two points wherejunction =n if dc magnetic field the wave function has differentis applied to superconducting phases. The changes in phasescircuit containing two can also be brought about byjunctions, the maximum super the applied electric andcurrent shows interference magnetic fields. The stateseffect which depend on the having different phases can beintensity of magnetic field. super imposed by using theConsider the arrangement as arrangement. Thus theshow in the fig. which is Josephson effects exhibit theknown as the super conducting quantum interferencequantum interference device phenomenon on macroscopic(SQUIDS). It consists of ring scale. The interference dependsof superconducting material on the phase of the state whichhaving two side arms A and B can be changed by applyingwhich act as entrance and exit electric and magnetic fields,of super current respectively. and the states with differentThe insulating layers P and Q phases can produce interferencemay, in general they have effects.different thickness and currents Let us discuss about the new applicationsthrough the layers are I1 and I2 of super conductivityrespectively. This variation of What is a superconducting Generator?I1 and I2 versus the magnetic A common generator converts rotationalfield as obtained. Both I1 and I2 mechanical input energy, such as that fromvary periodically with the a steam or gas turbine, into electricity. It does this by rotating a rotor field, whichmagnetic field, the periodicity produces voltage in stationary armatureof I1 being greater than that of conductors. The generator field can beI2. The variation of I2 is the produced with copper windings or permanent magnets. In large machines,interference effect of the two mechanical considerations and the desirejunctions. Since the current is
  7. 7. to vary the level of field produced HTS generators will produce electrictypically favour the use of copper power with lower losses than theirwindings over permanent magnets. The conventional equivalents. A 1,000 MW superconducting generator (a typical sizedifference between the basic design of a in large power plants) could save as muchconventional and as $4 million per year in reduced lossessuperconducting generator will be better per generator. Even small efficiencyappreciated in the light of improvements produce big dollar savings.the fundamentals of generation. A half of one percent improvementIn the generator, mechanical energy is provides a utility or IPP with additionalconverted into electrical capacity to sell with a related value of nearly $300,000 per 100 MVA by rotating a conductor relative to The worldwide demand for additionalmagnetic field produced electrical generation is ever increasing.usually by an electromagnet. The resulting The National Energy Information Centerflow of current in forecasts that the world will requireconductor generates its own magnetic 500,000 MW of additional electricfield. The final useful generating capacity over the next tenelectrical output depends upon the years.interaction of these two magnetic fields. Superconducting magnetic energyBenefits of superconducting generators: storage system (SEMS):  50% reduction in size and weight. This CTW description focuses on Superconducting Magnetic Energy Storage  70% less in transportation cost. (SMES). This technology is based on three  Cheaper foundation and buildings. concepts that do not apply to other energy  1% higher electrical efficiency. storage technologies. First, some materials carry current with no resistive losses.  Higher stability due to lower machine reactance. Second, electric currents produce magnetic fields. Third, magnetic fields are a form ofEfficient gain of superconducting pure energy which can be stored.generators:Generators lose power in the rotor SMES combines these three fundamentalwindings and in the armature bars. By principles to efficiently store energy in ausing superconducting wire for the field superconducting coil. SMES waswindings, these losses can be practically originally proposed for large-scale, loadeliminated. The fields created in the leveling, but, because of its rapidarmature by the rotor are not limited by the discharge capabilities, it has beensaturation characteristics of iron and thearmatures are constructed without iron implemented on electric power systems forteeth. This removes the losses experienced pulsed-power and system stabilityin the armature teeth. The added space for applications. Operationally, SMES iscopper in the armature made possible by different from other storage technologiesthe removal of the armature teeth further in that a continuously circulating currentreduces losses.
  8. 8. within the superconducting coil produces reducing the storage time would increasethe stored energy. In addition, the only the economic viability of the technology.conversion process in the SMES system is Thus, there has also been considerablefrom AC to DC. As a result, there are none development on SMES for pulsed powerof the inherent thermodynamic losses systems.associated with conversion of one type of Power Conversion Systemenergy to another. The combination Charging and discharging a SMES coil isof the three fundamental principles different from that of other storage(current with no restrictive losses; technologies. The coil carries a current atmagnetic fields; and energy storage in a any state of charge. Since the currentmagnetic field) provides the potential for always flows in one direction, the powerthe highly efficient storage of electrical conversion system (PCS) must produce aenergy in a superconducting coil. positive voltage across the coil whenOperationally, SMES is different from energy is to be stored, which causes theother storage technologies in that a current to increase. Similarly, forcontinuously circulating current within the discharge, the electronics in the PCS aresuperconducting coil produces the stored adjusted to make it appear as a load acrossenergy. In addition, the only conversion the coil. This produces a negative voltageprocess in the SMES system is from AC to causing the coil to discharge. The productDC. As a result, there are none of the of this applied voltage and theinherent thermodynamic losses associated instantaneous current determine the power.with conversion of one type of energy to SMES manufacturers design their systemsanother (EPRI, 2002).The original so that both the coil current and thedevelopment of SMES systems was for allowable voltage include safety andload levelling as an alternative to pumped performance margins. Thus, the PCShydroelectric storage. Thus, large energy power capacity typically determines thestorage systems were considered initially. rated capacity of the SMES unit (EPRI,Research and then significant development 2002). The PCS provides an interfacewere carried out over a quarter century, between the stored energy (related to thebeginning in the early 1970s. In the U.S., direct current in the coil) and the AC in thethis effort was mainly supported by the power grid.Department of Defense, the Department of Control SystemEnergy, and Electric Power Research The control system establishes a linkInstitute (EPRI). Internationally, Japan had between power demands from the grid anda significant program for about 20 years, power flow to and from the SMES coil. Itand several European countries receives dispatch signals from the powerparticipated at a modest level. grid and status information from theAt several points during the SMES SMES coil. The integration of the dispatchdevelopment process, researchers request and charge level determines therecognized that the rapid discharge response of the SMES unit. The controlpotential of SMES, together with the system also measures the condition of therelatively high energy related (coil) costs SMES coil, the refrigerator, and otherfor bulk storage, made smaller systems equipment. It maintains system safety andmore attractive and that significantly sends system status information to the
  9. 9. operator. Modern SMES systems are tied perfectly efficient, so there is some loss ofto the Internet to provide remote liquid nitrogen needed to cool the line.observation and control. Typical values for cooling efficiency are estimated to be on the order of 10% [3].Super conducting transmission line Furthermore, there are losses due to thecables: imperfect efficiency of the liquid nitrogenThe obvious advantage of superconducting pumping system itself, as well as hydraulictransmission lines is they have no resistive losses due to the flow friction in thelosses in the bulk. If superconducting circulating liquid nitrogen.transmission lines had no other sources of Similar to conventional transmission lines,power dissipation, the choice between superconducting transmission lines alsotypes of transmission lines would be easy. have shield and dielectric losses, whichWe would simply calculate the cost of can be calculated using the same physicalconventional power lines and subtract the models. Unlike conventional lines,cost of the power that is dissipated in superconducting transmission lines havetransporting the electricity. Then, we conductor AC losses. There is no generallywould compare it to the cost of making accepted physical model to describe theseand cooling superconducting transmission losses; so much of the data is empirical.lines. There are also losses due to imperfectOf course, real superconducting cables thermal insulation of the superconductinghave other sources of loss which must also cable. The result is a thermal leak betweenbe factored in. There are a number of the cold liquid nitrogen and the warmmajor sources of losses in superconducting surroundings. The losses can be reducedtransmission lines, many of them but not eliminated by creating a vacuumfundamentally different from those in between the superconducting cable and theconventional transmission lines. There are thermal insulator. Finally, there are smalla number of relatively small losses due to losses due to joints and terminations ofneed to cool the line. No cooling system is cables.Conclusion:Superconductors are always amazing in this world. Developed and developing countries ofour world are constantly thinking about this super power. Production and transmission ofelectricity should be improved still more to conserve for future generation. Scientists areworking on designing superconductors that can operate at room temperature. In Tamil Naduwe suffer from frequent shut downs. In spite of using nuclear reactor in our country, whichcause fear to our people, our Indian researchers and scholars can take part in this wonderingsuper conducting power?