SUPERCONDUCTIVITY

INTRODUCTION TO SUPERCONDUCTIVITY

Superconductivity is a phenomenon observed in certain materials like metals & ceramic materials. When these materials are cooled to temperature ranging from near absolute zero (-459 Fahrenheit, -0 degree Kelvin, -273 Celsius) to liquid nitrogen temperatures (-321F, 77K,-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. Electrically resistivity of a metallic conductor decreases gradually as the temp. is lowered. However, in ordinary conductors such as copper & silver, impurities & other defects impose a lower limit. Even near absolute zero a real sample of copper shows a non-zero resistance.
An electrical current flowing in a loop of superconductivity wire can persist indefinitely with no power source. Superconductivity occurs in a wide variety of materials, including simple elements like aluminium & tin, various metallic alloys and some heavily doped superconductors. Superconductivity does not occur in noble metals like gold & silver, nor in most Ferro magnetism.
The following table shows the critical temp. of various superconductors
Material Types Tc (K)
Zinc Metal 0.88
Aluminium Metal 1.19
Tin Metal 3.72
Mercury Metal 4.15
YBa2Cu3O7 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. Superconductivity 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 superconductivity ceramic & for electrical storage devices.


HISTORY OF SUPERCONDUCTIVITY
Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes, who was studying the resistance of solid mercury at cryogenic temperatures using the recently-discovered liquid helium as a refrigerant. At the temperature of 4.2 K, he observed that the resistance abruptly disappeared. In subsequent decades, superconductivity was found in several other materials. In 1913, lead was found to super conduct at 7 K, and in 1941 niobium nitride was found to super conduct at 16 K. The next important step in understanding superconductivity occurred in 1933, when Meissner and Ochsenfeld discovered that superconductors expelled applied magnetic fields, a phenomenon which has come to be known as the Meissner effect. In 1935, F. and H. London showed that the Meissner effect was a consequence of the minimization of the electromagnetic free energy carried by superconducting current. After then many researcher had been explain different microscopic properties of superconductor. Ginzburg-Landau theory predicts the division of superconductors into the two categories now referred to as Type I and Type II. Maxwell and Reynolds et al. found that the critical temperature of a superconductor depends on the isotopic mass of the constituent element. The complete microscopic theory of superconductivity was finally proposed in 1957 by Bardeen, Cooper, and Schrieffer. This BCS theory explained the superconducting current as a super fluid of Cooper pairs, pairs of electrons interacting through the exchange of phonons. In 1959, Lev Gor'kov showed that the BCS theory reduced to the Ginzburg-Landau theory close to the critical temperature. According to theoretical prediction that a super current can flow between two pieces of superconductor separated by a thin layer of insulator. This phenomenon, now called the Josephson effect, is exploited by superconducting devices such as SQUIDs, used for the most accurate available measurements of the magnetic flux quantum. In that year, Bednorz and Müller discovered superconductivity in a lanthanum-based cuprate perovskite material, which had a transition temperature of 35 K. Many other cuprate superconductors have since been discovered, and the theory of superconductivity in these materials is one of the major outstanding challenges of theoretical condensed matter physics.
As of March 2007, the highest temperature superconductor is a ceramic material consisting of thallium, Hg, Cu, Ba, Ca, strontium and oxygen, with TC=138 K. Some evidence of superconductivity has been seen at an even higher temperature, 150K in the compound InSnBa4Tm4Cu6O18. A patent has been applied for this material.

ELEMENTARY PROPERTIES OF SUPERCONDUCTORS
Most of the properties of superconductors vary from material to material, such as the heat capacity and the critical temperature at which superconductivity is destroyed. On the other hand, for all superconductors have exactly zero resistivity to low applied currents when there is no magnetic field present.

THEORETICAL SUPERCONDUCTIVITY
The simplest method to measure the electrical resistance of a sample of some material is to place it in an electrical circuit in series with a current source I and measure the resulting voltage V across the sample is given by Ohm's law, i.e. V ≠IR. If the voltage is zero, this means that the resistance is zero and that the sample is in the superconducting state. Superconductors are also able to maintain a current with no applied voltage whatsoever, a property exploited in superconducting electromagnets such as those found in MRI machines. Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least 100,000 years, and theoretical estimates for the lifetime of a persistent current exceed the lifetime of the universe.
In a normal conductor, an electric current may be visualized as a fluid of electrons moving across a heavy ionic lattice. Electrons are constantly colliding with each other & producing energy which absorbed by lattice and converted into heat i.e. the energy carried by the current is constantly being dissipated. But in a conventional superconductor, the electronic fluid cannot be resolved into individual electrons. Instead, it consists of bound pairs of electrons known as Cooper pairs. This pairing is caused by an attractive force between electrons from the exchange of phonons. Due to quantum mechanics, the Cooper pair fluid is thus a super fluid; it can flow without dissipation of energy.

APPLICATIONS
1. Superconductivity electromagnets are some of the most powerful electromagnets are used in MRI & NMR machines.
2. They can also be used for magnetic separation, where weakly magnetic particles are extracted from a background of less or nonmagnetic particles, as in pigment industries.
3. SQUIDS (super conducting quantum interference devices) used for measurement of magnetic flux quantum.
4. Promising future applications include high performance transformers, power storage devices, electric power transmission, electric motors (e.g. vehicle propulsion, maglev trains), magnetic levitation devices etc.
5. Medical: MRI m/c, Biotechnical Engg.
Electronic: SQUIDS, transistors, Josephson junction devices etc.
Industry: Separation, magnets, sensor & transducers.

LIMITATIONS
1. Complicate Phenomenon
2. High Cost Manufacturing
 
REFERENCES
• Tinkham Michael
• Introduction to superconductivity
• Website:
http://www.google.com
Wikipedia, the encyclopedia
Teachers guide about superconductivity.