RE: [TCML] Capacitor encapsulent

["Lau, Gary" <Gary.Lau@xxxxxx>]

Hi Bert,

Thanks for the reply.  I knew that glass had no definitive melting point, but this is the first I've heard that glass is ever anything but an excellent insulator.  Given its composition, it makes perfect sense.  

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Bert Hickman
> Sent: Saturday, May 23, 2009 8:55 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Capacitor encapsulent
> 
> 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@xxxxxxxxxx
> >> [mailto:tesla-bounces@xxxxxxxxxx] 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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> 
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