> However, one thing to be aware of, is when stacking films or sheets of
> dielectric between plates, you may have tiny air gaps in between the
> materials, even slight. Think of the capacitor then as a series of small
> capacitors with a total voltage applied at the ends. The voltage
> division across the capacitors will be in Cair+Cptfe+Cair. Since the
> PTFE is thick and has Er=2.1 and the air gaps are thin and have Er=1,
> you have the potential to develop some high voltages in those tiny air
> spaces. What happens is that it can make corona in there, and eventually
> eat the dielectric. Going to thinner Teflon (as voltage rating is high
> per mil of thickness as Gerald said) would exagerate the problem as the
> air gap would start approaching the Teflon thickness. If a single air
> space is 0.01 inches wide and the Teflon is the same (with no air gap on
> its other side, pressed tight against conductor), then the total voltage
> would be divided so that ~33% is across the Teflon and ~66% across the
> single air gap. This is because the Cptfe is 2.1 time Cair, or the
> reactance is about half. The air space may have microsparking over time,
> that can cause noise, ozone, and damage the dielectric and the conductor
> surface.
>
> A way around this is to have the electrodes tightly bonded to the
> dielectric, the way commercial capacitors are done. Unfortunately this
> doesn't work in variable meat-slicer style capacitors. As long as your
> air space on either side of the Teflon sheet is large, then the
> ionization should be minimal in the air space. Just something you should
> watch out for in homemade RF capacitors.
One thing which I have seen done commerically to try and avoid the creation of
air bubbles or pockets is to coat the metallic surfaces with a thin coating of
silicone grease which then sandwiches the PTFE film.
But I don't know if this was to specifically prevent corona or was just
considered "good engineering practise" - the voltage in question was only in
the low kV region which I wouldn't have thought to have been high enough for
corona.
David G4FTC
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