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Author: Subject: Conductive Sodium Silicate?
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[*] posted on 21-9-2021 at 08:31
Conductive Sodium Silicate?


I've been attempting to make a stainless retort for making K, Rb, and Cs metals. Instead of using a tube furnace, I wanted to wind some Nichrome wire around the pipe itself. I first slathered a layer of sodium silicate furnace cement on the pipe and baked it, to electrically insulate the Nichrome from the stainless pipe. After winding the heating element, I added another layer of furnace cement to protect the wire.

During testing, I noticed that once it reached maybe 400C, the wattage would start to climb. I could turn down the variac to correct the wattage, but it would still keep climbing. Eventually I'd see a dramatic power spike, and the element would quickly burn out. I re-built and tested this thing 3 times, with the same results. The highest temperature I measured was about 650C.

So this all leads to my question: Is it possible the sodium silicate cement is becoming conductive at a high enough temperature, and shorting out the coils? This seems strange, but I honestly can't think of any other explanation.
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Morgan
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[*] posted on 21-9-2021 at 13:05


There was this pdf article/title that maybe has something. Soda lime glass isn't very good for high voltage, borosilicate better and pure quartz a good insulator. So maybe the sodium ion is "freeing up" even for your relatively low voltage and high heat. Seems like you have a very thin layer of brittle silicate and the expansion of heated steel if the COE doesn't match, maybe along with the sodium, could be creating a pathway?
But I'm only guessing.

Selected properties of sodium silicate glasses and their structural significance - CORE
"The electrical resistivities of sodium silicate glasses ranging from 7.9 to 40.0 mole % Na20 were determined at 50 hz. and tempera tures fraa 360 to 480*C by Haven and Verkerk"
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[*] posted on 21-9-2021 at 13:54


Hot glass is a fairly good conductor.
https://www.youtube.com/watch?v=xwEQZw3KPWg
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[*] posted on 21-9-2021 at 15:03


Thank you both! That pretty much confirms it then. I'll need to find a different cement, or change the design.
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Morgan
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[*] posted on 21-9-2021 at 19:46


It's interesting unlike metals, glass becomes more conductive the hotter it gets. There was this article too using a lightbulb that lights when heated glass starts to slump.

"Insulating glass becomes a conductor of electricity when heated red-hot with a blowtorch."
https://sciencedemonstrations.fas.harvard.edu/presentations/...
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[*] posted on 22-9-2021 at 07:16


I did something quite similar using sodium silicate and talcum, it lasted for years at temps up to 900°C. This came from an old CRC handbook claiming it could handle temperatures up to 1000°C.



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[*] posted on 22-9-2021 at 08:41


Sodium ions are mobilized at high temps in a silica matrix.
This leads to the ability to put a light bulb in salt and produce sodium inside the bulb.
This is a classic experiment: http://www.sciencemadness.org/talk/viewthread.php?tid=148098
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