PROPERTIES OF CRYSTALLINE SOLIDS
Crystalline solids exhibit an important property called conductivity. Conductivity is the property of a material by which it allows the flow of electric current. Conducting materials are generally classified into three types namely conductors, semi conductors and super conductors.
Typical metals are good conductors of electricity while elements like silicon and germanium are non conductors at ordinary temperature. However, they exhibit appreciable conductivity upon addition of impurities as Arsenic and Boron. The resulting materials are called semiconductors. Semi conductors which exhibit conductivity due to the flow of excess negative electrons are called n-type semiconductors. The conductivity due to the positive holes are called p-type semiconductors. Semiconductors find application in modern devices as recitifiers, transitors and solar cells.
The ability of certain ultra cold substances to conduct electricity without resistance is called super conductivity. This superconductivity state is a state in which a material has virtually zero electrical resistance. Substances having this property are called super conductors.
The super conducting transition temperature 'Tc' of a material is defined as a critical temperature at which the resistivity of the material is suddenly changed to zero. Thus at that temperature a material is changed from normal material to superconductor.
At the extremely low temperatures, vibration of the nuclei of certain atoms slow down so much and they synchronise with the passing waves of electrons in a flow of electric current. When this happens, resistance to electric current disappears.
Application of superconductors
1) It is a basis of new generation of energy saving power systems. Super conducting generators are smaller in size and weight when we compare with conventional generators. These generators consume very low energy and so we can save more energy.
2) High efficiency ore separating machines may be built using superconducting magnets.
3) Superconducting solenoids are used in Nuclear Magnetic Resonance Imaging equipment which is a whole body scan equipment.