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Re: pure water capacitor?
The question of using pure water as a dielectric is full of difficulties.
Maintaining the purity is the first. Water is probably the best solvent of
all. For a start it dissolves gases from the air, in particular carbon
dioxide (try measuring the pH over a period of time). In addition it can
dissolve many materials in contact with it, either directly, or as a result
of forming salts due to the presence of minute amounts of acidic compounds
formed by dissolved gases. It is even able to dissolve plasticizers from
polyethylene and PVC (I did some research on this thirty or so years ago,
when we found that de-ionised water stored in polythene interfered with some
immunological reactions ( on the other hand if it was stored in soda-glass
that had been previously acid washed then there was no problem. Adding
various substances to the water can only increase the problem. The published
work that I have seen suggests that what is formed is a kind of electrolytic
capacitor, and they are certainly of no use for TCs.
Steve Cook
----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, February 17, 2000 3:17 AM
Subject: Re: pure water capacitor?
> Original Poster: "Bert Hickman" <bert.hickman-at-aquila-dot-com>
>
> Mark,
>
> As you've noted, pure water is a reasonably good insulator with a
> comparatively high dielectric constant. In fact, deionized water has
> been tried in experimental high voltage capacitors for high power pulsed
> power systems with limited success. And, it's often used to create low
> impedance, high-power, transmission lines or low-impedance pulse forming
> networks at many research facilities. However, water's insulating
> properties under high-voltage stress only exist for a short time
> (microseconds or tens of microseconds), and then the water breaks down.
>
> As you've also noted, keeping the "universal solvent" from partially
> disolving anything it's housed in is extremely difficult. And inserting
> metallic capacitor plates simply provides another ready source of
> metallic ions. Keeping the water to an acceptable level of deionization
> requires continual circulation through deionizing equipment. And, unless
> you also scrupulously de-gas the water, slight amounts of entrapped gas
> will come out of solution, forming microscopic gas bubbles on the
> plates. These then become ionization sites, promoting easier breakdown
> of the water.
>
> Kinda' makes even a LDPE/foil/mineral oil cap look mighty attractive in
> comparison... :^)
>
> Safe coilin' to you!
>
> -- Bert --
>
> Tesla List wrote:
> >
> > Original Poster: "Mark Broker" <broker-at-uwplatt.edu>
> >
> <SNIP>
> The only problem I can see is
> > contamination: everything has to be 110% clean before adding water.
This
> > is do-able.
> >
> > Are there any reasons why I shouldn't try to use this?
> >
> > For the record, I'm looking at a 15kV/120mA+ NST.
> >
> > Thanks
> >
> > Mark
>
>
>