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Unusual capacitor dielectrics
I've noticed that pure water has a dielectric constant of 80 (about 40 times
higher than that of polyethylene). Would it be practical to build capacitors
that use water as the dielectric? Such a capacitor could probably be built by
taking pieces of sheet metal, drilling holes in their corners, and then
bolting
them together, using nylon washers or other flat insulators to separate the
plates.
Guessing that water has a breakdown voltage of 250 volts/mil, A 0.0864 uF
7.5 kV
capacitor could be built by simply taking two 12x12 inch pieces of sheet metal
and using 30 mil seperators. Stacking 9 such plates (making 8 capacitors in
series) would yield 0.0108 uF at 60 kVDC, suitable for small-to-medium Tesla
coils. Several of these would then be wired in parallel and placed
vertically in
a rectangular container, with ceramic blocks to prevent the plates from
touching
the edges of the container, and with very pure water, very slowly so air
bubbles
don't get trapped.
If two plates very large (8 by 8 foot) plates are used with a separation of
0.5", then the capacitor would be 0.332 uF -at- 125 kV -- that's 2600 joules!
Increasing the spacing to 1.5 inch would make it 0.111 uF -at- 375 kV -- that's
7750 joules! Increasing the spacing still farther to 4 inches would make the
capacitor 0.0415 uF -at- an amazing 1 MV -- that's 20,750 joules!
Of course, for this to work, the water would have to be very pure. I doubt
that
the distilled water available at the grocery store would work. Aldrich has
some
water marked "Water, A.C.S. reagent" for 17.55 for 2L or 55.00 for 8L. I might
also be possible to build a distillation system and then triple-distill the
standard grocery-store water.
I don't have any idea what the RF loses of water are; they might prohibit its
use in Tesla coils. However, it might still be useful for pulse discharge
applications.
Also, what about barium titanate capacitors? Barium tatanate has a *huge* 1200
dielectric constant. It isn't exactly easy to find, but "barium titanate,
powder, < 3 micron, 99%" can be purchased from Aldrich for 22.80 for 500g or
66.90 for 2kg. I might be possible to make it into a sheet by building a open
box out of high-temperature steel, sprinkling a uniform coating of barium
titanate power inside it, melting the barium titanate with a hot torch
until it
forms a uniform sheet, and finding some way to remove the sheet without
breaking
it. I have no idea what barium titanate is like, but I'll bet it's brittle.
Does anyone know the puncture voltage and RF losses of pure water and barium
titanate???
--
-- Jacob Roberto
"Some people dream about being famous and having fancy cars. I dream about
1000KVA transformers and 1000KJ pulse caps!"