Conduction in polymer and ionic material

Conduction in polymer and ionic materials:

• In ionic materials the sum of electronic conductivity and ionic conductivity(ions and electron both moves)
• Mobility of ion is smaller than the mobility of electrons due to larger mass of ions.
• The band gap in ionic material is larger than other materials. So there is poor conduction in ionic material.
• Thermal function or thermal energy decreases the conductivity for ions whereas increases electrons.
• High temperature in ionic materials produce frankel and schottky effect which results in higher ionic conductivity.

Conduction in polymer

1. Polymers are insulator at normal temperature. Some of the polymers can be made conducting by addition of impurity atom.
2. A few of polymers have very high electrical conductivity in which there is insufficient or free covalent bonds.

For examples: long polymer molecules having dangling or loose chain on the surface.

Dielectric materials:

A dielectric material is an insulator. The dielectric material is made up of non polar and polar molecules. If dielectric material is made up of polar molecules like HCl,\(H_2\)O, NaCl, \(H_2SO_4\) etc. then the dielectric is said to be polar dielectric. If the dielectric material made up of non-polar molecules (whose dipole moment is zero) such as \(H_2,N_2,CO_2\) etc. Then dielectric is known as non-polar dielectric. When dielectric material is placed inside electric field, two type of effect appears in dielectric.

1. Dipolar moment is induced in non-polar dielectric due to partial moment of negative and positive charge.(c.g. of negative and positive charge separates).

2. The polar molecules experienced torque due to the presence of permanent dipole moments. The medium polarized due to orientation of molecules in such a way that \(p_m\) tries to parallel to E. this phenomena of increases in dipole moment of medium is known as polarization. In the mechanism of polarization the total polarization vector is due to contribution from

$$P=P_e+P_i+P_\circ$$

Electric polarization measured in terms of net dipole moment per unit electric field.

Dielectric strength:

The maximum value if electric strength in which the electrons in insulator jumps from the valence band to conduction band and avalanche of electrons moves in conduction band. There is electric discharge that strength of electric field is known or dielectric strength. It depends upon the nature of material.

Ferroelectricity:

Ferroelectricity is the property of certain materials that have a spontaneous electric polarization (presence of electric field in the absence of external electric field). The direction of polarization can be reversed by the application of external electric field. It was discovered in 1920. The prefix ferro meaning iron was to describe the property of the ferroelectric material. Most of the ferroelectric materials do not contain iron.

Ferromagnetic material obeys law of hysteresis i.e. there is spontaneous polarization of ferromagnetic materials. Due to hysteresis effect of ferromagnetic material they can be used as a memory function and ferroelectric capacitor. They are used to make ferromagnetic RAM for computers.

The internal electric fields of ferromagnetic materials are coupled to material lattice so anything that changes the lattice will change the strength of electric dipole and spontaneous polarization. The two cause that will change the lattice dimension of material are force and temperature. The generation of surface charge due to application of external stress (force) to material is called piezoelectricity. A change in spontaneous polarization of material in response to change in temperature is called piezoelectricity.

There is ferroelectricity phase transition which are often characterized by displacive or order-disorder structure of material. One of the material which shows ferroelectric behavior is \(BaTiO_3\) and another material is \(NaNO_2\).

In Barium titanate spontaneous polarization is a consequences of positioning of \(Ba^2+\), \(Ti^4+\) and \(O^2-\) ion within the unit cell as shown in figure. \(Ba^2+\) ions are at corners of unit cell which is of tetragonal symmetry. The dipole moment results from the relative displacement of \(O^2-\) and \(Ti^4+\) ion from their symmetrical position. The \(O^2-\) ions are located near but slightly below each of six face where as \(Ti^4+\) ion is displaced upward from the unit cell centre due to which spontaneous polarization appears.

The spontaneous polarization of this group of material as a result of interaction between adjacent permanent dipoles create ferroelectric behavior. Other materials that display ferroelectric behavior are Rochelle salt (\(NaKC_4H_4O_6.4H_2O\). potassium Niobate (\(KNBO_3\)), lead zincronate titanate .

References:

Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

Patton, W.J. Materials in industry . New Delhi : Prentice hall of India, 1975.

Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004.