Frequency and temperature Dependence In Insulating Materials

Frequency dependence 
    
1. on application of an alternating field across the material, the polarization process occurs as a function of time

2. Electronic polarization is extremely rapid and is complete at any instant of time even when the frequency of the voltage is very high in the optical range thus it occurs at all frequencies.

3. Ionic polarization is slower and the ions do not respond at all when the voltage corresponds to visible optical frequencies. That is, the electric field here is changing too rapidly for the ions to reorient themselves in response to the field, each time it is reversed. So the ionic polarization does not occur at visible optical frequencies.

4. Orientation polarization is slower than the ionic polarization and occurs only at electrical frequencies which are smaller than the infrared frequencies.

5. Space- charge polarization is the slowest process and occurs only at power frequencies.

6. Thus at low frequencies, the value of the total polarization is very high and at high frequencies (optical frequencies) the value of the total polarization is very small.

7. Figure shows the frequency dependence on the occurrence of various polarization mechanisms and corresponding power losses at those frequencies.

Temperature dependence 
          
1. Usually orientation Polaris ability depends strongly on temperature where as the electronic and the ionic polarisabilites are practically independence of temperature fir normal temperature.

2. Orientational polarization is inversely proportional to temperature.

3. normal temperatures will oppose the permanent dipoles to align in the field direction but higher temperatures facilitate the movement of ions and molecules so that a given polarization process, which is not possible at higher frequencies at normal temperature can occur at higher frequencies at elevated temperatures. Thus at very high temperatures, ionic polarization can occur at infrared frequencies.