I frequently seen that people check the frequency dependence of dielectric constant of materials in frequency range of 1 kHz to 1 MHz. What is the purpose of this frequency range or simply in which applications this frequency range is important. please tell different frequency ranges for different applications.
Dear Pramod
Due to electrical polarization in the dielectric material, electric field inside the material decreases. The factor by which it decreases is called dielectric constant. There are various possibilities of electric polarization popularly known as space charge polarization, orientational or dipolar polarization, ionic or atomic polarization and electronic polarization. It is not necessary that all of them are present in a particular material. Each polarization has its own relaxation time which may be different in different materials. Atomic and electronic contribution occurs at higher frequencies more than MHz range. In the frequency range mHz toto MHz, space charge polarization and orientational polarization are effective. In solids, these contributions are important to understand the dielectric relaxation which sometimes may be quite important to understand the structural defects and structure in solids. The instruments used in audio and radio frequency range are also not very sophisticated. Hence this frequency range is most favourite choice. Dielectric loss occurs due to lag in polarization with the a.c. field frequency and is highly frequency dependent. A maxima normally is observed at a particular frequency which is related to relaxation time. Knowing dielectric loss, frequency dependence of a.c. conductivity can also be obtained. In conclusionconclusion, dielectric relaxation in audio and radio frequency range is quite important in solids. I have published many papers on dielectric relaxation in chalcogenide glasses. You can go through my research gate profile for details.
Dear Dr. Nobre and Dr. A. Kumar:
Thank you for your helpful answers
Please guide for one more thing.. In a perfectly dielectric material when we increase the frequency the dielectric constant decreases due to the mismatching in the relaxation time and field frequency and concurrently what should be happen to dissipation factor? (it should be decrease as I think)...
In polymer composites filled with conductive particles, the dissipation factor increases with increasing frequency below a critical filler concentration (below this the composite shows dielectric nature), but beyond this critical concentration (here the composite becomes conductive) the dissipation factor decreases almost linearly, Why?
Dear Pramod
In a dielectric material when dielectric dispersion occurs, as you have mentioned, dielectric constant decreases with frequency. In the same frequency range, dielectric loss first increases with frequency , becomes maximum at a particular frequency and then decreases with frequency. This dielectric loss peak is sometimes not sharp or may be absent when your system is not having single relaxation time, i.e., a distribution of relaxation times exists. In your polymer sample, at low concentration of additives, you may be on increasing side of maxima and on higher concentration, you may be on decreasing side of loss maxima. Please check the d.c.conductivity also. It may be increasing with additive. The instrument measures total loss, you have to subtract, d.c. conduction loss if this component is high. Normally, in low conductive samples this contribution is small.
if we do from 1kHz -few GHz, we can observe various polarization mechanisms.
The fatal synergy of 1)the experimental errors and 2)the higher electrodes' relative (side effect) contribution, is the main "purpose", the main constrain for low (sub-kHz) frequencies.
For most dielectric samples[1] the experimental errors in sub-kHz frequencies are higher than most commercial system's recommended specs for reliable Z-measurements (of the material's[1] properties), either in Amplitude or./and in Phase margins.
1. and electrodes' multi-component polarization for dielectric samples, prevailing (our scope, the dielectric material properties) in low (sub-kHz) frequencies.
Dear Pramod
This frequency window is used to measure contribution of orientation (dipolar) and ionic polarizability. In most of the materials we are interested in these two polarizability. Moreover in this frequency range we can use parallel plate capacitor technique for permittivity measurement. This technique is very easy and versatile.
- Badges
- Science method