[TCML] Capacitor encapsulent

[Bert Hickman]

Hi Gary,

I personally witnessed this with an glass plate capacitor in air about 
50 years ago with one of the first Tesla Coils I built. The coil used a 
10 kV NST and double-strength window glass plate capacitors. When 
operating the coil, there was virtually continuous corona around the 
edges of the aluminum foil plates. The system was, in fact, more of an 
ozone generator than a streamer generator.

During a longer run, one of these corona areas began to develop a more 
well-defined tracking path, and the discharges along the path began to 
change from purple, to blue, to bluish-white, then briefly to a 
yellowish-white before forming a small glowing hole through the glass 
plate.

The glass that most folks would use in bottle caps or window glass is 
basically soda-lime glass - a mixture of mainly SiO2 (~73-74%), Na2O 
(13-14%), CaO (10-13%), MgO (0-4%) with smaller portions of various 
other oxides of aluminum, iron, sulfur, etc. Although we think of glass 
as an insulator, it's actually an electrolyte whose electrical 
conductivity is strongly temperature dependent. Most glass formulations 
follow the law of Rasch and Hinrichsen (1908) that related electrical 
resistivity versus temperature:	

Log R = A/T + B
where T is absolute temperature and A and B are constants that define 
the slope and intercept of the resistivity function

In window and bottle glasses, conduction is primarily via electrolytic 
conduction via mobile sodium ions. The higher the temperature, the 
greater the ionic mobility. The decrease in resistivity, although 
dramatic, is continuous since there are no discrete phase changes along 
the way. For example, a soda-lime glass specimen with an initial 
resistivity of 10^19 ohm-cm at 25 C can smoothly decrease by _19 orders 
of magnitude_ to 1 ohm-cm at 1200 C.

Bert

Lau, Gary wrote:
> Hi Jim,
> 
> I've not personally experienced it, but I had assumed that plate
> glass failure in a cap was just due to localized heating from corona
> fracturing the glass.  How hot does glass have to get to become
> conductive?
> 
> One further point regarding the original plan - Using brass for the
> plates is expensive and unnecessary.  Aluminum foil or flashing is
> electrically perfectly adequate and a lot cheaper.  The only down
> side is that one can't solder directly to it.
> 
> Regards, Gary Lau MA, USA
> 
>> -----Original Message----- From: tesla-bounces at pupman.com
>> [mailto:tesla-bounces at pupman.com] On Behalf Of jimlux Sent:
>> Saturday, May 23, 2009 11:28 AM To: Tesla Coil Mailing List 
>> Subject: Re: [TCML] Capacitor encapsulent
>> 
>> 
>>> Hi Raymond,
>>> 
>> eading to degradation, tracking,
>>> and eventual failure of your capacitor. Any hot spots in the
>>> dielectric become increasingly electrically conductive, which can
>>> also lead to localized thermal runaway, punch through, and
>>> shorting of the dielectric. Bert
>>> 
>> I completely forgot about that.. hot glass is a conductor. 
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