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Author: Subject: A true “gel” silica gel binder for graphite (or PbO2?) electrodes.
Tacho
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[*] posted on 21-7-2005 at 15:39
A true “gel” silica gel binder for graphite (or PbO2?) electrodes.


I just tested an electrode made of powdered graphite binded with silica gel (following the discussion with Cyrus in the Geopolymers thread). I tested it as an anode for 1 hour in a NaCl solution, with a current of about 70mA/cm2 and the electrode didn’t show any deterioration. It even retained the glossy surface.

The electric resistance of the material tested with my multimeter (leads 2cm apart) is 30 ohms/cm. Low enough to make it a good electrode for electroforming (finally!). Interestingly, before I dryied it in the warmplate, the resistance was much lower.

I have prepared silica gel (gelatin like, not the cristalyzed thing you get with mineral acids) mixing a solution “A” made with 1g boric acid in 20ml water with solution “B” made of 10ml of commercial sodium silicate solution (I don’t know its concentration) with 20 ml of water. Mixing equal parts promote gellification (sp?) within minutes. Adding more water can retard gellification for days.

I added powdered graphite to the mix, while liquid, until it became a slurry. I left it covered for a day to prevent drying and complete gellification. Then I let it dry completely. Took a couple of days. It shrank a bit, but retained the shape of the mould in fine detail, including the glossy surface. Finally I heated it in my warmplate (70°C?) for an hour.

The resulting electrode was quite fragile to mechanical stress but, as I said, didn’t show deterioration in the test.
I don’t make perchlorate, but anodes for making it are clearly a problem for those who do. If someone have PbO2, some sodium silicate and boric acid at hand, please, give it a try and post your results. First learn how to make the gel, then test the conductive filler. Since silica gel is inorganic, won’t oxidize like plastic subtracts.



[Edited on 21-7-2005 by Tacho]
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Twospoons
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[*] posted on 21-7-2005 at 17:16


It might be possible to improve the mechanical properties by adding chopped carbon fibre, or glass fibre, to the mix. Carbon fibre has the advantage that it is also conductive.
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Lambda
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[*] posted on 21-7-2005 at 17:38


Nice work Tacho !. Maybe your electrodes can be made more resilliant to mechanical stress by inserting a Glassfiber cloth used in automobile repaire.

I would however like to make a remark about your heater. It would be safer to allways use a transformer to galvanically sepperate the mains supply from your heating element. Using an autotransformer has the same danger attached to it, for there is only one coil. Seeing the low power consumption your heater uses, it would make a cheap transformer feasable. Not only the danger of electrocution becomes a possibility when your heater has become moist/wet by spillage of fluid, but allso the chance of causing even more havoc when you jump back, fall or knock over other maybe very dangerouse chemicals.
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Lambda
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[*] posted on 21-7-2005 at 17:51


Ah,....Twospoons, we were thinking the same. Your idee of using Carbon fibre is good though. You can buy this in model-airplaine/race care or Boot-building stores.
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jpsmith123
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[*] posted on 21-7-2005 at 19:35


Interesting.

One of my many ideas was to try to mix PbO2 with a castable ceramic like this stuff:

http://www.cotronics.com/vo/cotr/cm_castable.htm

It would be nice to make a reliable PbO2 anode without the hassle of having to plate something using soluble lead salts.
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Twospoons
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[*] posted on 21-7-2005 at 23:53


Yes, but to get a nice conductive anode you really should plate solid PbO2 over the top.
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darkflame89
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[*] posted on 22-7-2005 at 04:26


You mentioned in the other thread that borax solution and ascorbic acid would "gellify", though the process takes a long time to solidy. Would the same be able to subsitute this sodium silicate and boric acid gel? Hmm..because sodium silicate solution is a bit diffcult to get down here...The only commercial ones that I have seen are those drying silica gels, those that come in little blue beads. So I'm thinking might borax work just as well?



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Tacho
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[*] posted on 22-7-2005 at 05:26


Sorry, in that thread I meant "borax solution + sodium silicate solution" and "ascorbic acid solution + sodium silicate solution". I'll edit the post.

I don't think there is easy method of making silica gel without sodium silicate.

By the way, native english speakers, how do I call the process of becoming a gel? "gellification"?
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12AX7
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[*] posted on 22-7-2005 at 10:28


Quote:
Originally posted by darkflame89Hmm..because sodium silicate solution is a bit diffcult to get down here...


Did you check the cleaning aisle? I get sodium metasilicate (Na2O.SiO2 = Na2SiO3) as phosphate-free TSP (..*blink*..). Approx. 60-70 parts per 100 solution should do it for "water glass". It seems to dissolve slow so you'll want to heat it.

If you still can't get that, you can always try fusing sand and baking soda in a tin can - you'll need a furnace to do at least 800C.

Tim




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