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Principle and applications of dielectric polarization

Introduction

Dielectric

Dielectrics are insulators, plain and simple. The two words refer to the same class of materials, but are of the different origin and are can be used preferentially in different contexts.

* Since charges tend not to move so easily in nonmetallic solids it's possible to have "islands" of charge in glass, ceramics, and plastics. In others word, charges in metallic solids tend to move easily — as if something was leading them. A material that provides a good and safe passage for electric charges is a conductor.

* Putting a layer of nonmetallic solid between the plates of a capacitor gains its capacitance. A line across the angles of a rectangle is a diagonal. The measurement across a circle is a diameter. The material placed across the plates of a capacitor like a little non-conducting bridge is a dielectric.

The plastic coating on a electrical cord is an insulator. The glass or ceramic plates that are used to support power lines and keep them from shorting out to the ground are insulators. Pretty much anytime a nonmetallic solid is used in an electrical device, and so it is called an insulator. Perhaps the only time the word dielectric is used is in reference to the non-conducting layer of a capacitor.

History

The concept of the macroscopic polarization is the basic one in electrostatics of dielectric materials, but for a long period of time this concept has evaded even a precise microscopic definition, and has severely challenged quantum-mechanical calculations. This concept as we know has undergone a genuine revolution in recent years (1992 onwards). It is now very clear clear that contrary to a widespread incorrect belief, the macroscopic polarization has nothing to do with the periodic charge distribution of the polarized crystal, the first one is essentially a property of th electronic wavefunction, where as the second one is a property of its modulus. An outline of the modern viewpoint is being presented. Experiments invariably address the polarization derivatives or polarization differences, and these differences are measured as an the integrated electrical current. The new theory addresses this same current, which is dominated by the phase of the electronic wave-functions. The First-principle calculations is based on this theory are in spectacular agreement with experiments and provide thorough understanding of the behavior of dielectric materials.

Principle of dielectric polarization

When the atom of a dielectric is placed in an external electric field, the nuclei are pushed away with the field resulting in an gained positive charge on one side while the electron clouds are pulled against it resulting in an gained negative charge on other side. This process is known as the polarization and dielectric material in such a state is said to be polarized. There are 2 principal methods by which we can polarize a dielectric, stretching and rotation.

Effect of polarization

If there are polar molecule in a material, they will be distributed in random orientations when there is no electric field.On the other hand an applied electric field will polarize the material by orienting the dipole moments of polar molecules.

This will lower the effective electric field in between the plates and will increase the capacitance of the parallel plate structure. The dielectric should be a good electric insulator in order to minimize any DC leakage current through a capacitor.

Effect on permittivity and capacitance.

Electric susceptibility

The electric susceptibility of the dielectric material is a measure of how easily it polarizes in response to an electric field. This, in turn, determines the electric permittivity of the material and thus it influences many other phenomena in that medium, from the capacitance of capacitors to the speed of the light.

It is defined as the constant of proportionality relating an electric field E to the induced dielectric polarization density P such that:

where is the electric permittivity of free space.

Therefore the susceptibility of a medium is related to its relative permittivity by

So in the case of a vacuum,

The electric displacement D is related to the polarization density P by

Applications

1. The physical mechanism which can produce the second order dielectric polarization are discussed on the basis of the simple extension of the theory of dispersion in ionic crystals. Four distinct mechanisms are described, three of which are related to an anharmonicity, second order moment, and Raman scattering of the lattice. These mechanisms are strongly frequency dependent, since they have ionic motions with resonant frequencies lower than the light frequency. The other mechanism is related to the electronic processes of greater frequency than the light, and, therefore, is essentially flat in the range of the frequencies of optical masers. Since this order lies an order of magnitude higher than the ionic resonances, the fourth mechanism may be the dominant one. On the other hand, a consideration of the linear electro optic effect shows that the lattice is strongly involved in this effect, and, therefore, may be less linear than the electrons. It has shown that the question of the mechanism involved in the second harmonic generation of the light from strong laser beams may be settled down by experiments which test the symmetry of the effect. The electronic mechanism is subjected to further symmetry requirements beyond those for piezoelectric coefficients. In most of the cases, this could largely reduce the number of independent constants describing the effect. In particular, for quartz and KDP there would be a single constant.

2. Less-frequency dielectric responses of carbon nanotubes are important for their manipulation, separation, and electronic applications. So we can report the first experimental measurement of dc polarization of anindividual carbon nanotubes by using modified scanning force microscopy techniques. The transverse polarizability of the carbon nanotubes is equivalent to the solid cylindrical media with the dielectric constant of about 10, irrespective of the tube diameter and chirality. The longitudinal polarization is also observed and used is to distinguish metallic from semiconducting nanotubes.

References

1. http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/dielec.html

2. http://physics.info/dielectrics/

3. http://prola.aps.org/abstract/PR/v126/i6/p1977_1

4. http://pubs.acs.org/doi/abs/10.1021/nl071208m

5. www.britanica.com

6. www.wikipedia.com