Ph.D. thesis (pdf) - dirac

5.1. Dielectric spectroscopy 73
with a thickness of 0.5 mm in order to ensure that the pressure is well transmitted
from the compression liquid to the sample liquid.
Figure 5.1: The experimental setup for dielectric spectroscopy under pressure at
Orsay. The Teflon parts shown in grey to distinguish from the metal. The length
of the autoclave is ∼ 30 cm, the Teflon cell is 5 cm long. The parts are shown
separately in appendix C.
The advantage of this setup, compared to other setups for dielectric spectroscopy
under pressure, is that it ensures a hydrostatic pressure because the sample is compressed
from all sides. It is moreover possible to take spectra both under compression
and decompression, and it can hence be verified that there is no hysteresis in the
system.
The capacitor used for the measurement is composed by two gold coated electrodes
separated by small Teflon spacers. The distance between the capacitor plates is
adjustable and the area is 5.44 cm 2 . We report data measured with a plate distance
of 0.3 mm giving an empty capacitance of 16 pF. The strength of the measured
signal is proportional to the area and inversely proportional to the distance between
the plates. It is therefore an advantage, especially for measurements on systems
with small dipole moments, to maximize the area and minimize the plate distance.
Although the area is limited by the dimensions of the pressure cell, the distance
could in principle be smaller; however it is crucial that the plates do not touch, even
when the pressure is applied, as this would lead to an electrical short cut.