- Permittivity And Dielectric Loss
It has been known that because of its non-polar nature, the dielectric properties of PTFE were of an ideal character. Careful study by Ehrlich in the USA showed that the fall in permittivity from 2.0 to 1.8 in the temperature range 24 to 314°C (75 to 597°F) could be accounted for entirely in terms of density changes by the Clausius-Mossotti formula. No changes of permittivity with frequency were detected.
In 1955 Mikhailovand co-workers in the USSR found a loss peak in the-80 to -40°C (-112 to-40°F) range at audio and radio frequencies which was correlated in its temperature/frequency location with dynamic mechanical loss behavior. From studies of the effect of changes of crystal) inity by quenching and slow cooling they concluded that the relaxation losses were attributable to amorphous regions of the polymer. In 1959 Krum and Mliller (of Marburg) reported higher dielectric loss values than those found by earlier workers and found more detailed correlation with mechanical properties and effects of crystallinity changes. Eby and Sinnott in the USA have suggested, however, that these higher loss values must be due to polar impurities.
The dielectric loss of PTFE is sufficiently low to allow the permittivity to be calculated with an accuracy of better than 0.5% using the Clausius-Mossotti formula e-1/e+2= Kd.
where
e = permittivity
d = relative density
K = constant, 0.119
From which it can been seen that e = (1+2Kd)/(1-Kd)
thus, at a relative density of 2.174 the permittivity is 2.05
Measurements of permittivity and dielectric loss in the audio frequency range (178 Hz—31.6 kHz) were made using a fully-shielded, three-terminal conjugate Sobering bridge; a resonance substitution method, based on that published by Hartshorn and Ward but suitably modified to give a higher resolution. Was used for the radio frequency region (105 to 108 Hz). A modified version of the re-entrant cavity resonator method of Parry was used for the 108 –109 Hz range; an Hoi cavity resonator was used at 9 x 109 Hz: this made use of the Bleaney, Loubser and Penrose method of avoiding unwanted modes.
Graph:
Loss angle vs. temperature for sintered granular PTFE (60 % crystallinity) using evaporated gold electrodes.
Loss angle vs. frequency for PTFE at room temperature.
- D.C. Conduction Behaviour
In attempting to study the d.c. conduction behaviour of PTFE, the current measured was that which occurs after the application of a d.c. voltage step. The results for a typical sintered sample, using evaporated gold electrodes, are expressed as log (apparent volume resistivity) as a function of time of polarisation. It will be seen that steady state conduction was not established clearly in the time of the experiments which was 15 minutes. On the diagram are shown lines of constant loss angle, which can be calculated by means of a Fourier, transform assuming a constant permittivity of 2.0.
The short time values are consistent with the low frequency values (=2C) At radians measured by a.c. methods. In fact it is considered that such d.c. step response results are equivalent to a low frequency extension of the a.c. frequency range. The apparent resistivity is to be thought of as a very low frequency relaxation loss phenomenon rather than a steady state charge transport phenomenon, although the onset of conduction may be apparent above 160°C(320°F).
Graph
Effect of different electrode material and preconditioning for sintered granular PTFE.
This graph shows such currents may be removed by heat treatment in the presence of electrodes; apparent resistivity values > 10-8 ohm m have been obtained from such experiments without shoeing evidence of steady state condition.
- High Voltage Uses of PTFE
With regard to high voltage applications it has been known for a long time that in the presence of surface discharges failure occurs by erosion, as PTFE is a non-tracking material.
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