The dielectric spectroscopy consists in the study of the frequency dependent
complex dielectric permittivity of insulator materials. The frequency range
more commonly investigated is below a few GHz where the dielectric response
of the insulator materials is usually dominated by orientational polarization
processes. If the material under investigation contains permanent dipoles
with a molecular origin, the dielectric spectroscopy would allow to get
insight about the molecular motions.
A schematic diagram of the application of dielectric spectroscopy
to the investigation of molecular motions is shown below. In the simplest
case, the sample is a part of a parallel plate capacitor. In such a case
the frequency dependent complex dielectric permittivity is proportional
to the specimen complex capacitance, which can be measured with great accuracy
over an extremely wide frequency range. For an alternating voltage of very
high frequency the dipoles would not be able to follow the electric field
direction and the measured dielectric permittivity will be essentially determined
by the induced atomic and electronic material polarization. As the frequency
is continuously reduced the molecular dipoles try to orient in the field
direction. The level of orientation increases as the frequency is reduced
and consequently the dielectric permittivity increases. At sufficiently
low frequencies the molecular dipoles would be able to follow the field
direction and a plateau value of the dielectric permittivity would result.
In the frequency range where the dielectric permittivity increases, dispersion
range, there is and associated energy dissipation process which manifests
as a peak of the imaginary part of the complex dielectric permittivity.
The reciprocal of the angular frequency of the dielectric loss peak provides
a direct measure of the characteristic time of the molecular motions responsible
of the dipolar orientation.
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