Electrical Properties of Materials.Introduction:Materials are important in everyday life because of their versatile structure and properties. The properties of materials doesnt depend its shape and side. Some prime properties of materials include electrical, mechanical thermal and optical properties. Electrical behaviors of materials are diverse so are uses in electrical applications.Classifications:Materials based on electrical properties classified into following types: Conductors Insulators Semi-ConductorsElectronic & Ionic Conduction:In solid materials a current arise due to flow of electron called electronic conduction. Also in ionic material, the net positive charge is possible for current called ionic conduction.Energy band structures: The energy corresponding to the highest filled state at 0 K is called the Fermi energy, Eve. Four types of band structures are possible at 0 K as shown in the following figure

In the first, one outermost band is partially filled with electrons. This energy band structure is typified by some metals. e.g.: Copper Second band structure also found in metals, there is an overlapping of an empty and filled band. Magnesium has this band structure. The final two bands are similar, one band (valence band) completely filled with electrons is separated from an empty band that is conduction band by an energy band gap lies between them.The difference between two band structures is the magnitude of energy gap for Insulator the energy band gap is wide whereas for Semiconductors it is narrow. Electrical Conduction: Electrical conductivity of a material is defined in terms of ease of charge flow through it. Electrical conductivity of a material is defined in terms of ease of charge flow through it.Ohms law relates the current and applied voltage:V = IR whereV applied voltage (volts) I current (amperes) R Resistance (ohms) Materials electric resistance is NOT an intrinsic-propertyI.e. it depends on object geometry. Electrical resistivity, defined as follows, is an intrinsic property, inverse of which called conductivity. = RA/ A= Area of an object = length of an object Or, From Ohms Law: = VA/IAlso, = 1/ = /RA, = conductivity of materials Power loss is given by, P=IV=I2R Either conductivity / resistivity can be used to classify materials.Classification Electrical Conductivity:Conductivity of solid engineering material is observed to vary over 27 orders of power. Based on their conductivity, materials are classified as: Conductors , Semiconductors, Insulators.

Conductors > 107 (-m)-1Semiconductors 10-6 -104 Insulators < 10-10 Resistivity in Metals:Lattice vibrations and scattering center play a role in disrupting the mean free path of electrons. In addition, crystalline defects and impurity atom affect the conductivity. These scattering mechanisms act independently on one another. Thus the effective resistivity of metals can be represented as follows: = t + i + d , t, i, d are the individual thermal, impurity and the deformation resistivity respectively. This equation also known as MATHIESSENs EQUATION.With increase of temperature, thermal vibrations increase so the resistivity, and vice versa. In the same manner, with increase of either defects or impurities, resistivity increases. For pure metals, the resistivity approaches zero at absolute zero temperature.Electron Mobility:When an electric field applied, all the free electron should accelerate in the direction according to the direction opposite to the field, which would give rise in electric current that is continuously increasing with time. However, some frictional force counter this acceleration from an external field and constant the value of current. Because of this force the electron scattered by imperfection in the crystal lattice.This scattering phenomenon called resistance to the passage of an electric current. Several parameters are used to describe the extent of this scattering, these includes drift velocity and the electron mobility. The drift velocity d is directly proportional to electric field. d=e EfThe constant of proportionality e called electron mobility.The conductivity of most materials can be expressed as: = n|e| e , n= number of free electrons per unit volume.|e|= Absolute magnitude of the electric charge on an electron.Semi Conductivity: Electrical properties of semiconductors are unique, in the sense that their electrical properties are extremely sensitive to even minute concentrations of impurities. Two kinds of semiconductors intrinsic and extrinsic. For intrinsic semiconductors, their electrical behavior is based on inherent electronic structure of the pure material. On the other hand, if the electrical properties are dominated by impurities, they are called extrinsic semiconductors. In semiconductors, the valence and conduction bands do not overlap as in metals, but they possess enough electrons in the valence band those can be promoted to the conduction band at a certain temperature.Intrinsic Semi-Conduction: Conduction is due to promoted electrons, and charged hole left behind by these electrons. This occurs at elevated temperatures. At still higher temperatures, the concentration of thermally excited electrons in the conduction band becomes so high that the semiconductor behaves more like a metal. Ex.: Si, Ge, Sn, Pb, etc.

Extrinsic Semi-Conduction: The charge carrier density can also be increased by adding impurities of either higher or lower valence to intrinsic semiconductors. This addition of impurities is known as doping, and impure atoms in the element are called donor atoms. n-type semiconductor uses higher valence elements as donors, while p-type semiconductors uses lower valence elements. Donor atoms increases number of charge carriers in form negatively charged electrons (n-type) or positively charged holes (p-type). Doping also results in altering the Fermi energy level.Electrical Conduction In Polymers: Polymers are, in general, insulators. They can be made conductors in two ways:1. Introducing an additive to the polymer to improve conductivity.Such as: Carbon black2. Creating polymers with inherent conductivity by doping.Such as: polyparaphynylene, polypyrole,polyaniline,acetal polymers. Some other polymers such as polyphthaocyanine can be cross-linked by special curing processes to raise its conductivity.Electrical Conduction In Ionic Ceramics: Charge can also be conducted via ions - called ionic conduction. This may occur either in conjunction with or separately from electronic conduction. Several types of compounds show exceptionally high ionic conductivity. Such phases fall into three broad categories: halide and chalcogenides of silver and copper; oxides with -alumina structure; and oxides of fluorite structure.Ex.: La2CuO4 (Tc = 30 K), YBC compounds yttrium doped perovskite structure, YBa2Cu3O7 (Tc = 92 K). By properly engineering the point defects, it is possible to convert ceramics into semiconductors. Ex.: Indium tin oxide (ITO).