Published on October 23, 2014
1. - Dielectrics are materials which can be polarised in the presence of an electric field. - In the presence of the field, there is NO flow of charges, but the charges slightly shift from their equilibrium position. - The positive ends align towards the negative and the negative ends align towards the positive of the field. This is called dielectric polarisation. - Dielectrics and insulators are the same, but the words are used in different situations.
2. - The measure of polarisation of a material is for a given electric field is called ‘electric susceptibility’ , χe - Given by the formula Where εo is permittivity of free space, P is polarisation density and E is the strength of the applied field. Note: Polarisation density P is defined as the average dipole moment per unit volume of the material P=εoχeE
3. Applications of dielectrics They are mainly used in capacitors as the charge storage material between the metallic plates. This also prevents the plates from coming in direct contact. Used in dielectric resonator antenna, for producing and receiving microwave signals. Mineral oil is used as a dielectric and as a cooling material in transformers. Used as insulator coating for wires and conducting materials.
4. Paraelectrics Materials which get polarised in the presence of an electric field but in a non linear pattern. For a dielectric the polarisation density baries linearly with the field, which is not the case as in paraelectrics. When placed under an electric field the electron cloud in molecules of the paraelectric gets distorted. The level of this distortion is non linear w.r.t the field. E.g. SiO2, Al2O3 etc
5. In paraelectrics like Tantalum oxide (Ta2O5), when the eletric field is applied, the central Ta ion moves from its eqbm. position (resulting in polarisation) but returns back when the field is removed. Above: Comparison of dielectric (left) and paraelectric polarisation (right). P is polarisation density and E is electric field strength
6. Applications of paraelectrics Used in timer devices and as signal filter materials. Used for refrigeration applications since it has been found that loss of polarisation when the field is removed leads to cooling.
7. Ferroelectric Materials Materials that get polarized in the presence of a field and retain the polarization even after the removal of the field. E.g BaTiO3 , where the central Ti ion moves from its eqbm. position on applying a field, but stays at the changed position even after the field is removed. This results in the retention of polarization. The magnitude of the change in position is much more than that of the change seen in paraelectrics.
8. Ferroelectric Polarisation - Left: The polarisation curve with changing electric field for a ferroelectric material. Note that the material retains the polarisation even when the E is zero. - The direction of polarisation can be reversed just by applying an electric field in the opposite direction. - Such a type of a curve is called a Hysterisis curve. - Ferroelectrics become a paraelectric when heated above a certain temperature called the Curie Temperature. (This idea is similar to that in ferromagnetics)
9. Applications of ferroelectrics Used to make capacitors with tunable capacitance. Used in ferroelectric RAM, which stores a lot more data in lesser space and is also more reusable. Used in RFID tags (Radio Frequency Identity) to tag various items especially in retail businesses
10. Presentation by Joel Cornelio email@example.com
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