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Re: best dielectric?
-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Friday, February 05, 1999 6:27 AM
Subject: best dielectric?
Hello Robert,
>Original Poster: "ROBERT CRESSLER" <59CREROB-at-menasha-dot-com>
> I am trying to determine if i should purchase a sizable quantity of
>.010 film 10"wide x 1000ft roll of one of the following: $150
>(pp,hdpe) $250.00(uhmwpe) or.015 x 26" x 60" sheets of ps -at-.75/ea no
>price on the pc yet.
> Given a choice of using:Polypropylene,HDPE,UHMW PE,Polystyrene or
>Polycarbonate for Caps which would be best?
>( the PS will have a white pigment in it,others are unfilled)
>any thoughts from the list? anyone want to go "halves" for a roll?
>they seem to have similar properties.....
Here are a few interesting specs about the dielectric losses for each
material.
I am guessing you want to use it for (TESLA) cap construction.
Frequency = 10^6Hz
--------------------------
PP: <5*10^-4
HDPE: 2*10^-4
UHMWPE: Sorry, no info
PC: 1*10^-2
PS: 0.5-4*10^-4
Kv/mm:
----------
PP: 650
HDPE: 450
UHMWPE: Sorry, no info
PC: 380
PS: 300-700
Looking at the table, I would exclude PC right away because it is
verrrrrrry lossy (PVC has a very similar value: 0.015) and the kv/mm rating
is low compared to the others. I would also exclude the PS because this
stuff gets very brittle in oil (cracks, warps, etc). So, aside from the fact
that I have no info about UHMWPE, you have two real choices left. Either PP
or HDPE.
I am presently using polypropylene caps with very, very impressive results.
However, these are (multi-mini-caps) commercial caps. I couldnīt get hold of
PP in sheet form for homebrews.
PE does have one very big disadvantage: It melts verrrrrry easily. This
means if you get corona inside of your cap (yes, possible even under oil.
Seeing is believing), you will soften the PE and it will puncture in this
area. Heating usually shows up as yellow discoloring in the PE. All of my
homebrew PE caps that were run longer lengths of time (dry, however) had
internal heating problems, probably due to corona.
Unless UHMWPE has some great advantages in comparison to either PP or PE I
would not use this stuff, simply because it is almost twice as expensive as
the PE. If I were you, I would get small amounts of PP, PE and UHMWPE and
build a cap with them (if that is what you are planning to do). Try to get
the capacitance, etc. pretty much equal for all three materials. After
building these caps, go out and torture them any way you can (no safety
gaps, out of oil, high voltage, high current discharges, put them under oil
and overvolt them, etc). Take some notes during the experiments and
then decide on which material is best for you.
If, for example, you find that the UHMWPE caps will take only slightly more
punishment, than the PE caps, I would go for the PE or PP, etc.
Hope this was of some help.
Coiler greets from germany,
Reinhard