| dc.description.abstract | An X-band microwave bridge-interferometer is constructed for measuring
the permittivity of solids. Using simple theory, the system is first tested
with several materials of known permittivity (e.g. ertalon, plexiglas and
teflon). It is found that the results agreed well with published data for
lossy materials only. Loss factors of very low-loss materials are irregular
rather than linear with the sample lengths as is expected. This is due
to the fact that the usual simple theory neglects internal multiple
reflections which are predominant inside low-loss materials. The problem
does not exist in lossy materials because the transmitted signal is
sufficiently dissipated through one traverse of the sample. Theoretical correction for low-loss materials is made but is found too tedious to be
used. On the other hand, multiple-reflection effects could be eliminated
if the samples are cut exactly to multiples of half-wavelengths. Samples of
arbitrary lengths have to be used first to obtain a preliminary measurement
of the real part of the permittivity which is related to the wavelength
of the sample by a simple equation. Fine adjustment of the klystron frequency is also made to ensure minimum reflections by the samples.
The precision method is then employed to study the dielectric properties
of pure polystyrene and polystyrene doped with diluted amounts (0.1~2.0%)
of polar molecules (viz. 8-Quinolinol) at various temperatures (20°-80°C).
It is anticipated that the new data obtained here will provide useful
information for the study of dielectric relaxation mechanisms in these
substances. | |
