What is superconductivity?
As we know resistivity of the materials increases with the increase in temperature. And it decreases with the decrease in temperature. At some temperature near 0 K, the resistivity becomes zero. In this state, The material is said to be in the superconducting state. The change of the state of a material from the nonconductive state to superconductive state is not gradual but it is abrupt. The temperature at which this sudden or abrupt fall in resistivity occurs is called the transition temperature. The figure shows the graph of resistivity v/s temperature in K.
Near 3.5 K the resistivity falls to zero value from 10*10-11 Ω m. It is found that good conducting metals like gold, silver etc do not exhibit superconducting property. Whereas metals and compounds which are poor conductors at room temperature exhibit this property. Also, monovalent metal, ferromagnetic and antiferromagnetic metals are not superconducting.
Application of the superconductivity:
At present, about 30 superconductor elements and more than 600 superconductor compounds are known. A compound consisting of niobium, aluminum, and germanium is found to exhibit superconducting property ar highest temperature of 20 K. The superconducting property of material will find many applications. however, this is in the research in the laboratory stage. Commercial application is yet to come out. This is because of very low temperatures to be produced and maintained.some of the possible useful applications are as below.
Lossless power cables:
The resistance of the material becomes zero when it becomes superconductive. Then if power transmitted through the cable with its conductor in zero resistance state, there will be no voltage drop and no power loss. Hence a small cross-section of a conductor can transmit power with efficiency.
Another important application is to produce electromagnets with the high magnetic field intensities. In this, a ferromagnetic core is used. Winding used is of superconducting material. With such electromagnet, The magnetic field intensity of 8*106 A/m is produced with the solenoid of 1 m diameter. This requires 104 times less power than an ordinary electromagnet.
Electrical machines like a generator, a transformer can be designed with the windings made from the superconducting material. This will reduce the size of the machine and the high efficiency can be achieved. Efforts are made to design generators up to 20 MV A rating.
Computer switching elements:
Film cryotrons are used as computer switching elements.
It is used to design modulators conversion of weak constant current to audio frequency.
Detection of frequency modulated signals:
use of the nonlinear property at the transition temperature is made in the detection of high frequency modulated signals.
It can be used in producing memory elements.
It can be used in a fuse, circuit breakers etc.
It can also be used to produce frictionless bearings. In this bearings and rotor are made of superconducting material. Magnetic poles and magnetic fields are produced by passing an electric current through both of them. There will be repulsion between the bearings and rotor due to similar poles produced in them. So the two surfaces will be away from each other. Hence the rotor can rotate without friction.