Properties of Superconductors

Hi friends,   In this article, am discussing the properties of superconductors and the applications of superconductors. This article will provide you sufficient information about superconductors.   The resistivity of most metals increases with increase in temperature and vice-versa. There are some metals and chemical compounds whose resistivity becomes zero when their temperature is brought near 0° Kelvin (— 273°C). At this stage such metals or compounds are said to have attained superconductivity.   For example, Mercury becomes superconducting at approximately 4.5 Kelvin (-268.5°C). The transition from normal conductivity to superconductivity takes place almost suddenly; it occurs over a very narrow range of temperature—about 0.05 K.   Superconductivity was discovered by Heike Kamerlingh Onnes at the University of Leiden in the Netherlands in 1911.

Transition Temperature

The temperature at which the transition takes place from the state of normal conductivity to that of superconductivity is called transition temperature.

Types of Superconductors

There are two types of superconductors commonly known as Type I and Type II superconductors.   Type I superconductors are soft superconductors. They are usually pure specimens of some elements i.e. metals. They have very little use in technical applications.   Type II superconductors are hard superconductors. They are usually alloys of metals with high value of resistivity in normal state. These are very useful as compared to Type I materials.   Experiments have shown that if a current is induced in a mercury ring at a temperature of 4.5 K, it will continue to flow for years without taking any power from the source of supply.   Similarly a lead ring carried a current of several hundred amperes over a year with no change. Lead becomes superconducting at 7.22 K.

Superconducting Materials

Many metals and compounds have superconducting property at very low temperatures. Superconductivity has been observed to occur in poorer metallic conductors such as tin, lead and tantalum rather than in good conductors such as gold, silver and copper. It has been found that superconductors may not only be pure metals but various alloys and chemical compounds as well.   At present about 30 superconductor metals and more than 600 superconductor alloys are already known. The highest temperature at which, until now, superconductivity has been observed to occur is 20 K (-253°C) for a compound consisting of Niobium, Aluminium and Germanium.

Effect of Magnetic Field on Superconductivity

Application of a sufficiently strong magnetic field to super-conductors causes the destruction of their superconductivity, i.e., the restoration of their normal conducting state.   The critical value of the magnetic field for the destruction of superconductivity is denoted by Hc and is functionally related to temperature as:   HC = HC(o)[1 – (T2/Tc2)]   Where HC(o) is the critical field at 0 K, and has a specific value for each material.   Note that the lower the temperature, the higher the value of Hc, and the highest critical temperature occurs when there is no magnetic field.   Thus we find that the superconducting state is stable only in some definite ranges of magnetic fields and temperatures. For higher fields and temperatures, the normal state is more stable.

Properties of Superconductors

Some of the important properties of superconductors are as follows :
  • The current in the superconductors persists for a very long time. This is demonstrated by placing a loop of the superconductor in a magnetic field, lowering its temperature below transition temperature Tc, and then removing the field. The current which is setup is found to persist over a period longer than a year without any attenuation.
  • The magnetic field does not penetrate into the body of the superconductor. The property known as the Meissner effect, is the fundamental characterization of superconductivity. However, when the magnetic field H is greater than a critical field Hc, the superconductor becomes a normal conductor.
  • When a current through the superconductor is increased beyond critical value Ic(T), the superconductor again becomes a normal conductor,   i.e., the magnetic field which causes a superconductor to become normal from a superconducting state is not necessarily an external magnetic field, it may arise as a result of electric current flow in the conductor.   The superconductivity may be destroyed when the current exceeds the critical value which at the surface of the wire will produce a critical field Hc, given by   Ic = 2 πrHc   This is known as Silsbee’s rule.
  • The specific heat of the material shows an abrupt change at T = Tc jumping to a large value for T < Tc.
  • In all cases involving transition metals, the variation of Tc, with number of valence electrons shows sharp maxima for Z = 3, 5 and 7.
  • A rather striking correlation exists between Tc and Z2 for elements Hg, La, Pb, Nb, Zn, in, In, Sn, V, Tc, Cd, Ga and Al.
  • For a given value of Z, certain crystal structures seem more favourable than others, e.g., β-tungsten and α-Mn structures are conducive to the phenomenon of superconductivity.
  • Tc increases with a high power of the atomic volume and inversely as the atomic mass and is known as isotope effect.
  • Superconductivity occurs in materials having high normal resistivities.   The condition nρ > 106 is a good criterion for the existence of superconductivity, where ‘n’ is number of valence electrons per cc and ‘ρ’ is the resistivity in use at 20°C.
properties of superconductors pdf
If one observes the total list of superconducting materials the general features to be noted are :
  • Monovalent metals are generally not superconductors.
  • Ferromagnetic and antiferromagnetic metals are not superconductors.
  • Good conductors at room temperature are not superconductors and superconducting metals are not good conductors at room temperature as the normal metals.
  • Amorphous thin films of Be, Bi and Fe show superconductivity.
  • Bi, Te and Sb become superconductor under high pressure.

Meissner Effect

Meissner and Ochsenfeld in 1933 found that if a long superconductor is cooled in a longitudinal magnetic field to below the value of critical temperature corresponding to that field, then the lines of induction ‘B’ are pushed out of the body of the superconductor at the transition.This phenomenon is called the Meissner effect.   Such flux exclusion is also observed if the superconductor is first cooled below Tc, and then placed in the magnetic field.   The effect is of fundamental importance as it shows that a bulk superconductor behaves in a external magnetic field ‘H’ as if inside the specimen B = µo(H + M) = 0 or X = – 1;   that is, a superconductor exhibits perfect diamagnetism.   We shall now show that the perfect diamagnetism of superconductors is an independent property, not at all related to zero resistivity.   Let us try to relate the two properties together and see what happens. From Ohm’s Law E = ρj , we see that if the resistivity ‘ρ’ goes to zero while ‘j’ is held infinite, then ‘E’ must be zero. Using the Maxwell’s equation
properties of superconductor
we obtain B = constant. This concludes that the flux through the specimen cannot change on cooling through the transition.   This means that when a perfect conductor (ρ = 0) is cooled in the magnetic field until its resistance becomes zero, the magnetic field in the material gets frozen in and cannot change subsequently irrespective of the applied field.   This is obviously in contradiction to the Meissner effect. Thus, perfect diamagnetism and zero resistivity are the two independent essential properties of the superconducting state.

Applications of Superconductors

Some important applications of superconductors are:
  • Superconductors are used for producing very strong magnetic field of about 20 – 30 T which is much larger than the field obtained from an electromagnet and such high magnetic fields are required in power generators.
  •   Efforts are being made at present to develop electrical machines and transformers utilizing superconductivity. Calculations show that if we could use superconductors as conducting material, in addition to superconducting magnets, which are already being produced, it is possible to manufacture electrical generators and transformers in exceptionally small size, having an efficiency as high as 99.99%.  
  • Magnetic energy can be stored in large superconductors and drawn as required to counter the voltage fluctuations during peak loading.
  • The superconductors can be used to perform logic and storage functions in computers.
  • A superconductor material can be suspended in air against repulsive force from permanent magnet. The levitation can be used in transportation.
  • As there is no heat loss in superconductors (i.e. I2R loss is zero), so power can be transmitted through the superconducting cables.
  • Superconducting materials if used for power cables enable transmission of power over very long distances using a diameter of a few centimeters without any significant power loss or drop in voltage. Superconducting solenoids which do not produce any heat during operations have been produced.   However, it must be noted that superconductivity can be destroyed if the magnetic field exceeds a critical value. It has been possible to design electromagnets using superconductivity for use in laboratories and for low temperature devices like the maser.
  Future Prospects: It must be realized that the above applications require conductors to be maintained at temperatures very close to 0 K. This may often mean that the whole equipment associated with the conductor has to be kept at near 0 K. This is a great challenge facing the scientists today.   Indeed a new technology, known as Cryogenics, has been developed to tackle this problem. Presently Helium is used to achieve low temperatures required for superconductivity. Helium being an expensive gas, efforts are being made to develop compounds which exhibit superconductivity at temperatures possible to be obtained by the more easily available and cheaper hydrogen gas.   Thanks for reading about “properties of superconductors” and “applications of superconductors”.  

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