There are metal, alloys etc. which behaves as perfect conductor below a critical temperature and critical magnetic fields.There do not show any resistance in super conducting stage. This is because resistance is produced due to scatterings of electrons. But I super conducting stage electrons get bound by forming cooper-pair. Therefore, no resistance will be developed due to electronic collisions. In this stage a current remains persistent for longer time without decaying.

Super conductivity was discovered by Kammerling Onnes when he cooled Mercury below 4k at which Mercury lost its resistance completely behaving as super-conductor.

Types of Superconductor

• Low temperature super conductors:In these temperature is lowered upto 40K & less for superconducting effect. It is found in alloys metals.
• High temperature Super conductor:These show superconducting behaviour at relatively high temperature i.e. at a temperature near 75k. Corprates of lanthanum & Barium shown high temperature super conductivity.

Advantages

Super conductors have high electrical & thermal conductivity: High Optical reflectivity. They are able to strongly repeal magnetic fields and surface friction of super conductors is extremely negligible.

Because of these properties, super conductors are highly energy-savers and can be utilised in many fields.   

• Super Computers:For enhancing speed of super-computers to peta flops (i.e. performing 1000 trillion operations per second).
• Defense:for detecting underground mines, submarines, small size motors etc.in manufacturing e-Bomb which generate high electromagnetic impulse to disable enemy’s electronic equipment.
• Medical: For manufacturing MRI machines (magnetic Resonance Imaging)
• SQUID (Super-conducting Quantum Interference Device) which an observe very small change in human body’s magnetic field. Therefore, will be helpful in assessing Brain functioning.
• Energy – Super conductors have no resistance and hence negligible transmission losses. Therefore, super conductors use energy efficiently and helps in reducing cost of electricity transmission.
• Transport – Super conductors refuel magnetic fields. Therefore, these can levitate strong magnetic fields. This property is utilized in floating trains e.g. Maglev trains in Japan.
• Electronics:In Ultra-high performance frequency filters which allow only desirable frequency to pass through. It can be used in cellular phones.In blocking undesirable frequency for decongesting radio frequency signals.
• Space: In cryogenic technology.In Gyroscope which stabilizes momentum of wheels.