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Author: Subject: Silicon electrode for electrolysis
chief
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[*] posted on 9-2-2009 at 14:04
Silicon electrode for electrolysis


Since in the diamond-thread nobody answered:

Has anyone tried to use Si as an electrode in a chlorate cell ? Could it stand the chemistry ? Finally it is contained in solar-cells, old power-transistors, etc. ...

It has, at least, the very same structure as diamond, also the same sort of bonding ... ; only that carbon (diamond) has a much smaller atom and a much stronger bond ...
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dann2
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[*] posted on 9-2-2009 at 17:27


Hello Chief,
First it would have to be doped Si, as pure Si is non-conducting.
I tried a piece of pure Si once only to discover is is non-conducting.
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not_important
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[*] posted on 9-2-2009 at 20:16


Something like this :
Silicon a New Electrode for Electrochemical Reduction of Nitrates: Comparison with Boron-doped Diamond

Attachment: ise081249.pdf (33kB)
This file has been downloaded 400 times

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[*] posted on 10-2-2009 at 06:22


Of course it has to be doped ; usually it's easier to get doped material, since any electronics-Si is either p- or n- doped, at least at the surfaces ... ; maybe someone could try this ? Some slaughtered power-mosfet or power-transistor or solar-cell ?

If it works for nitrate-reduction, as in the paper mentioned by not_important, then maybe it will for chlorate ??? Though there the chemistry would be more agressive ...

Maybe even that a higher current-density would be achieved (limited by the electrolyte), since the surface of a power-mosfet is small (but it can conduct 100 A), and the cooling is effectively in place by the electrolyte ...

[Edited on 10-2-2009 by chief]

[Edited on 10-2-2009 by chief]
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[*] posted on 10-2-2009 at 09:15


Chief: Why not just buy a doped wafer on ebay? They're pretty cheap.

http://cgi.ebay.ca/8-inch-Blanket-Black-Diamond-Wafer-on-sil...

http://cgi.ebay.ca/12-inch-Silicon-Germanium-Wafer_W0QQitemZ...
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chief
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[*] posted on 10-2-2009 at 12:52


I just want to find out before I try ; probably someone has it around and will try it anyhow, then report ... ... . One fine day I might abuse one of my old 2N3055-Power-Transistors for that, or one of my burned diodes ...
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[*] posted on 10-2-2009 at 15:32


The paper is about reduction of nitrates. You are talking about oxidizing chlorine. Quite a different beast.
The silicon crystal lattice is mmuch weaker than the carbon varieties. It is also much more reactive (you can find essentially pure carbon in nature, but pure silicon has to be made). It also forms polymers (silicates), from Si(OH)4 units. A carbon analogue for that does not exist.
In my opinion, silicon would not survive a combination of water, oxigen and chorine. But you are free to try, nonetheless.
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[*] posted on 10-2-2009 at 17:11


Well, Si is certainly resistant to HCl, and it reacts with Cl2 gas only above 400 deg C. I would certainly give it a try!
Titanium is also oxidised in nature, yet it is used as an electrode.




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[*] posted on 10-2-2009 at 17:26


Titanium is covered with a layer of oxide, thus forming a quasi-inert barrier. Much like the passivation of aluminum in acids. On the other hand, aluminum reacts energically with water in basic media. Anyway, Ti is slowly consumed. The best materials used as anodes are metal oxides, which can not be further oxidised.
Silicon doesn't react with HCl because oxidising conditions are not present. The choride ion is not an oxidizer. Electric current, on the other hand, is the best oxidizer known to man. The fact that oxigen and chlorine are also generated at the anode and are good oxidisers too further complicates the issue.
The oxidation of Si would form Si-OH bonds in macro-mollecular compounds, which can then rearrange to the practically inert and highly insulated SiO2 with the help of heat. In water, however, a silicagel of some sort would form, or maybe Si-Cl compounds.
The main difference from the reduction of the nitro group is that in the reduction scenario, the Si can not be further reduced, whereas in the oxidation scenario oxidation products can be formed and passivation is unlikely.
Hell, I'd like it to work, because it would be quite useful. I have a large assortment of big old transistors, too. But I think it's an unfounded hope.
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[*] posted on 10-2-2009 at 18:21


Silicon is mentioned in a few patents such as this one as having been "suggested" for use as an anode in a perchlorate cell.

And in the archives of some ancient discussions on the newsgroup rec.pyrotechnics, someone claimed to have had promising results with silicon, although it wasn't clear whether the cell in question was for chlorate or perchlorate.

At one time I tried to find a suitable piece of silicon for an experiment, but I couldn't find any and subsequently gave up on it.
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[*] posted on 10-2-2009 at 22:15


Hello,

The Silicon is being used for Nitrates reduction (as pointer out by others), ie. it is a Cathode in the cell.

When it is stated that Ti is use as an Electrode (Anode anyways) the writer is always referring to the substrate. Often MMO (Ti substrate) is referred to as a 'Titanium' Anode. This is very misleading. The MMO is making all the difference.
Bare Ti can be used as a Cathode of course.

I have some Si wafers with and without circuits on them. The pure Si is non conducting.
I put a meter on the stuff with circuits and got resistances of approx. 15 ohms between some die sites (chips before they are cut out). Resistances on the same die site are as low as 1 ohm. It depends on where on the die site you put the probes. Some adjacent die sites do not seem to have any connection to each other at all.
A single die site a approx. 11mm by 2mm BTW!.

Will try in a cell and see what happens.
I have very very little hope of anything useful happening.

The conducting layer on a Silicon wafer is very very thin. A pure Si wafer is taken and exposed to heat and a gas (containing P say). The P diffuses into the wafer to create N-Type semiconductor. Other methods are also used.
The layer of conducting stuff is very very thin, Microns?. This (if it works) will wear away quickly and you are left with pure Si (non conducting). Also the Micron thick coating (if the die sites don't have it cut up in the first place) is going to be useless for carrying large Chlorate or Perchlorate cell currents.
The fact the (say) a 2N3055 can carry 30(?) amps is not an indication of the amperage that could be run through its guts as an anode, not to talk about corrosion of connectios.

You would need a thick piece of Silicon that has NOT been produced from the electronics business, ie. with some impurities in it to make it conductive throughout its (reasonably substantial) mass.

Dann2
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[*] posted on 11-2-2009 at 03:32


Another thought: If the Si could be consumed electrolytically, then this could be a cheap way to make the expensive "aerosil", which is nanocrystalline SiO2, for reactions (wanted for the large surface-area).

So eiher way it might be worth trying it out (and just hereby making it unpatentable ...)
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[*] posted on 11-2-2009 at 06:28


You might end up with some chlorine in your product, instead of -OH, but it can be tried. Something useful for you has a really high combined probability of occurring :cool:
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[*] posted on 11-2-2009 at 09:17


I should see if my foundry grade silicon is conductive.. Next time I run across the damn chunks, guys. I think I know where they are.
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[*] posted on 11-2-2009 at 12:10


I might throw in here, that gemstone-quality opal is nothing else byt nanospheres of silicon-dioxide, regularly arranged ...
It may be synthesized, and a well known Pierre Gilson made it since decades, but he keeps the process secret ...

[Edited on 12-2-2009 by chief]
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[*] posted on 11-2-2009 at 16:25


Hello Folks,

I checked the piece of wafer below with an Ohm meter. Most of the die were isolated. The resistance accross individual die was one or two Ohm. It depended where you put the probes. In places there was no connection. There was no hope of simply placing a connection at the top of the wafer and having currrent going to the rest of the die, because, (as I said they are electrically isolated).
I connected the piece of Si wafer to the + of a power supply and placed into a NaCl solution together with a Cathode.
About 200mA flowed at 12Volts from a single die but it was hard to tell if it was the wet wafer that carried some of the current. Each die measures 10mm by 2 mm approx. It is a hopeless Anode IMHO.

Dann2

Si_Waf.jpg - 16kB
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[*] posted on 11-2-2009 at 16:52


Quote:
Originally posted by chief
I might throw in here, that gemstone-quality opal is nothing else byt nanospheres of silicon, regularly arranged ...
It may be synthesized, and a well known Pierre Gilson made it since decades, but he keeps the process secret ...


Silica, not silicon.
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[*] posted on 12-2-2009 at 04:32


I corrected it ; That's what I meant, sorry for the typo and thaks for the hint.

Besides this gemstone-quality opal is quite something to get _very_ rich upon, one of my would-be projects, in case I had the time ... ; but I have other projects as well, ... ...

@dann: Such a piece of waver is chemically resistent: I once dissolved several pentiums (133 pro, mmx etc.) in aqua-regia (including all the gold): The waver was untouched ! Maybe it has a surface-layer for isolation (maybe also just for the dielectric constan of the layer) ...

Thats why I would propose an old powerdiode: It just has 2 plain regions: 1 p and 1 n ; with just a piece of silicon one might get all sorts of rectifying-effect etc.

[Edited on 12-2-2009 by chief]
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[*] posted on 12-2-2009 at 12:01


As already said, those wafers are nonconducting because they are too pure. If they were made of crude industrial silicon (like, maybe, 99,5% pure) they would conduct perfectly fine.

The answer to this problem is making your own silicon.
Two methods are viable for the amateur, one is reduction of SiO2 with Al, the other one SiO2 with Mg.
The reaction of the first one is not self-sustaining because it is not exothermic enough. It is neccesary to add an oxidiser and additional aluminium to provide the heat.
A standard laboratory preparation of silicon, found in several german textbooks, uses a mix of SiO2 and Al with sulfur:
120g sulfur
100g aluminum
90g silica flour
This reaction gives melted globules of silicon and lots of aluminum sulfide as byproduct (produces H2S with HCl, this can be useful and is worth collecting or absorbing with NaOH).
The silicon can be freed of adhering SiO2 by means of treatment with warm HF.

With Mg instead of Al and no sulfur (!), the reaction is self-sustaining and produces Si as a medium fine grey powder. It can be melted together in a quartz ampule using an oxy-propane blowtorch

Si is resistant to oxidation in air at up to 1100°C and is not attacked by HF, in contrast to SiO2.
With NaOH solution, it reacts to give hydrogen and sodium silicate solution (here's a "cold" method of producing sodium silicate!).

[Edited on 12-2-2009 by garage chemist]




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[*] posted on 12-2-2009 at 13:00


SiC (silicon carbide) is a semiconductor too ; it industrially is made by laying out a trace of coal (several meters long), covering it with a lot of sand (several tons), and then passing directly some 220/380 Volts through the coal, which thereby, insulated by the sand, heats up nicely and reacts with somewhat sand to SiC .

That SiC might be even closer to diamond ... in some ways ...

By the way: Just braking a power-diode in 2 will yield some well-contacted ("bonded") and doped Si, for the quick test. Insulate the bonding-material by some glue or paraffine ... and there you go !

[Edited on 12-2-2009 by chief]
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[*] posted on 10-5-2012 at 13:51


I'm thinking about starting a research on this... I can't find a reliable method to analyse the reduced silicon (described as a fine grey powder) and prove the content of the crystal. I'm also thinking about doping it with two other metals.
BTW, can anyone recommend some publications on reduction methods for silicon carbide and silicon oxide?
Thanks.
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