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Author: Subject: Silicon (and Boron)
Cyrus
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biggrin.gif posted on 17-5-2004 at 21:05
Silicon (and Boron)


On a completely random note, who can make elemental silicon? It is shiny:D and hard to make, and those are plenty of reasons for a mad scientist.

Of course you can do

SiO2 + C -> Si + CO2

but that takes 3000 C! Can this be achieved by thermite, burning coal/coke, microwave ovens, lasers, etc...? Any suggestions besides expensive electric arc furnaces?

[Edited on 19-5-2004 by Cyrus]
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Lestat
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[*] posted on 17-5-2004 at 21:34


Just a suggestion, what i've done in the past is to find various electronic garbage and hack up the chips inside, there's a little sillicon inside but not very much.

The Si is pretty damn pure, but remember there will be dopants in it i cant remember for sure what tho.

What do u want sillicon for anyway? I've always thought of it as a pretty boring element really.

[Edited on 18-5-2004 by Lestat]




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[*] posted on 17-5-2004 at 22:35
Aluminum Alloys


Commercial production of aluminum-silicon superalloys start with pure aluminum (molten) with addition of SiO2. The Al both reduces the SiO2 to Si / Al2O3 and also acts as a solvent for the formed Si which normally does not melt until a ridiculous temperature.

Cooling of the molten alloy results in Si precipitation into an ordered matrix, starting before the aluminum itself solidifies.

Im not quite sure how a process such as this could be exploited for pure Si, but it is information nonetheless.
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[*] posted on 18-5-2004 at 07:16


The Al can be dissolved with a mix of HCL and H2O2, since NaOH is able to react with Si.
Or the Al/Si mix can be electrolised in order to disolve Al.

This is preatty much the same way of obtaining an alotrope of phosphorus (violet I guess) from phosphorus dissolved in lead .

I think that the Si would be in a very fine powder form.

Recently at a flea market I saw a mineral sample labeled as Silicium (?). It was very hard to break a crystal and the density was small aswell. I didn't buy it, and afterward I saw that native silicon is really rare. Damn...
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[*] posted on 18-5-2004 at 08:45


I once visited a steel production plant and they had a huge (about 3m tall) pile of elemental silicon they used to make alloys. Surface looks like glass, has the color and density of graphite.

I believe is a relatively cheap material. It may be hard to find because it has few uses except for steel alloys. In fact, I can't think of any use for it.

Superpure silicon cristals for electronics is a totally diferent matter.
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[*] posted on 18-5-2004 at 09:03


Some dealers at Mineral and Gem shows carry pieces of Silicon (along with Bismuth crystals). That's where I got a piece just 'cause I thought it looked (and felt) neat.
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[*] posted on 18-5-2004 at 14:05


I've made silicon from the reaction of Mg with SiO2, I dissolved the mass I ended up with in HCl, it exploded :(

The reaction makes appreciable quanities of magnesium silicide which will react with acids to produce silane. If you try this add your solid to water then slowly drip in acid with stirring.




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[*] posted on 18-5-2004 at 14:37


I've done the same reaction as Bromic Acid has done. You need really fine silicon dioxide - sand will not work. I bought "silica powder" for my experiment. If you use magnesium, especially if in excess, you get magnesium silicide. This substance produces silane (SiH4) gas, which ignites spontaneously in air, in acid solution. I believe if you use Al instead of Mg you don't get any silane.

The silicon I produced was a dark gray powder that looked very much like mud or dirt. However, it had a shine on the surface which was visible where it contacted the glass it was drying in.

KC
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thumbup.gif posted on 18-5-2004 at 15:52
A full preparation of crystalline silicium


According to Jander & Blasius, prep. inorg. chem. (which I am coincidentially scanning), Si is made the following way:

90 g of fine, well-dried sand, 100 g of Al grains (0.1-0.5 mm), and 120 g of sulphur are mixed well, and set off with the various methods described first (KNO3/Al, NaClO3/Al, Mg ribbon, and my fav. sparklers stuck right into the mix).
After the reaction, let it cool down, and remove the product. Polverise/make chunks with a hammer or similar, and add a large amount of water. Large amounts of H2S are evolved (so do it outside, it's a toxic gas), and get rid off Al(OH3)/Al2O3 by flowing water accross it (the term is 'abschlaemmen', for which I don't have a translation), to remove the less dense Al(OH3)/Al2O3 suspension.
Separate the Si spheres mechanically from Al2O3 etc (hammer once again), and treat this with hand-warm medium concentrated HCl for several days (replacing used HCl a few times).
At last, cook the insoluble remainder with concentrated HCl (37%), and decant.
Then, to remove any residual SiO2, treat the silicon crystals with 40% H2F2 for one hour (warm again).


Clean Boron is made the same way, although one always gets Al containing boron (AlB12).


In terms of feasibility - I guess the only problem is the H2F2 - but, ammonium hydrofluoride can be obtained relatively easily in some hardware stores. Dont remember what it's used for though.
I am sure though this can be used just as well.

For clean crystalline Silicon- thats the way to go! :)




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[*] posted on 18-5-2004 at 16:04


Why do you want sillicon anyway?
(just curious)




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[*] posted on 18-5-2004 at 16:11


Thanks a lot, I will definitely try some of these. However, my propane burner barely melts aluminum, maybe I will make a larger one. Does anyone have some temperatures for these reactions?

As for uses, this is Mad science, not practical science. ;)
(No practical use whatsoever for me)

Quote:
Originally posted by Turel

Cooling of the molten alloy results in Si precipitation into an ordered matrix,



Do you mean that it will form one large lump? Why do the other posts describe the silicon produced as a powder?

Just saw your post, chemoleo. Would the
reaction do this, or is the sulfur involved?
B2O3 + 2Al -> B + Al2O3

On the subject of boron, I have tried getting boric acid from borax to no avail.
Of course you can buy boric acid from the drugstore, but that's cheating! I want my boron to come from borax! (Not that I have anything to do with it. It is the second hardest element and I bet most people are deprived of the pleasure of seeing it in their whole life. Two good reasons.)

I add conc. HCl to
sat. borax soln. and all that happens is the solution looks wavy for a while, I guess different refraction of light from different concentrations of solutes.
I have also tried sulfuric- (from drain cleaner), and acetic acid- (5% vinegar) with no results, and I have tried adding borax powder (decahydrate) to the above acids,
nothing happened except in the HCl.
The borax sank to the bottom and the solution became murky. After several minutes the bottom of the test tube was filled with a much finer white ppt. than the coarse white borax I added.

What happened? I can't even balance the reaction.

[Edited on 19-5-2004 by Cyrus]
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[*] posted on 18-5-2004 at 16:14


I don't know about other countries, but in england, boric acid is easily bought OTC from pharmacies, I bought some one but never found a use for it.



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[*] posted on 18-5-2004 at 16:57
Boron


Although it's slightly or considerably off-topic - but yes, the preparation is done similar to the silicon prep. With B2O3 of course.
Amounts are
50 g water-free B2O3, 75 g of S, and 100 g of Al. The cleaning and processing is done as above, i.e. first heating in conc HCl, then H2F2.

Now I don't see why you had trouble making boric acid. I made quite a large batch some time ago. Make sure your solutions are saturated with borax, and only then add HCl. If you cool, B(OH)3 should precipated. This, when you melt it, forms a glass like mass, B2O3. You can make very fine threads with it, which sting (because they are so thin) if you handle it improperly.

If you want to discuss boron properly, have a look in the forum first for existing threads, if not, then create one. Refer to this one to bring some sense to it :)

Lestat - sometimes you make compounds/elements for the sheer joy of it. That's what amateur experimentalism is about - not to make things to eat/inject/inhale or blow up, but to make things because you can! Who could possibly deny the sense of achievement of having made silicone or boron??




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[*] posted on 18-5-2004 at 21:22
Silicon (and Boron)


Hehe, Boron is off topic no longer!:D
-am I allowed to do that?

I did have a look at the other threads, but found them insufficient...
The reason I did not get any B(OH)3 is because I never knew you cooled the soln. I should have tried it.
Do you understand the reaction though?

Na2B4O7 + HCl -> B(OH)3 + ???

Tomorrow, I make Boron.

Chemoleo, I assume you meant making silicon, not silicone, though silicone is nice too.

[Edited on 19-5-2004 by Cyrus]
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[*] posted on 18-5-2004 at 23:19
Cyrus


No, precipitation into an ordered matrix refers to the formation of an ordered structure within the still liquid aluminum. The basic structure while the Al is liquid is of pure elemental silicon, but as the Al solidifes, solid aluminum silicides in lattices form the alloy.
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thumbup.gif posted on 19-5-2004 at 03:42


Aluminium-silicon alloys do make silane...

On the upside, you could melt your aluminium, add a little bit of sand, let it react, let it cool and then dissolve in acid. Due to silane, the hydrogen produced will self-ignite... Cool.
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[*] posted on 19-5-2004 at 15:23


Could you grab out this ordered silicon structure with pliers or filter the molten aluminum somehow, leaving pure Si behind?



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[*] posted on 20-5-2004 at 11:04


Does anyone know how much SiO2 there is in common sand. Could someone please name a cheap source of SiO2, which is available in somewhat large quantities. Thanks.



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[*] posted on 20-5-2004 at 11:46
No


Unfortunately I do not think you can seperate out the Si physically from the suspension. Perhaps with the aid of centrifugation. I do not know. But you won't ever catch me centrifugating molten alloys in a home lab.....

White sand is nearly pure SiO2. Common beach sands are a healthy mix of SiO2 (vast majority) and group 1 and 2 metal silicates. There are some trace compounds, but the real majority is actually SiO2 and silicates. The whiter the sand when dry, the more SiO2 in it.
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[*] posted on 20-5-2004 at 13:27


Quote:

Does anyone know how much SiO2 there is in common sand. Could someone please name a cheap source of SiO2, which is available in somewhat large quantities. Thanks.

My pottery supplier sells it in pure form as a very fine dust for about 4 Euro / kg. Try a pottery supplier. :)




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[*] posted on 20-5-2004 at 16:29


For small amounts of glass powder, could you take sand and several steel balls, and shake them around in a film canister?

Well, I tried making boron yesterday.

First I added conc. HCl to warm conc. borax soln., in about 1 to 1 proportions,
I think this is the right reaction.
Na2B4O7 + 2HCl + 5 H20 -> 4B(OH)3 + NaCl
I then cooled the solution to near 0 C, and a semitransparant precipitate formed,
but there was so LITTLE of it that I was disgusted, I used about ten scoops of borax, and got less than one scoop of B(OH). Borax is a decahydrate though, would that increase the volume enormously? I will do the reaction again with better measurements later.

Then I took a small amount and dried it in the microwave making about 0.5 ml
of the boric acid powder, which smelled strongly of HCl, and heated it to 600+ C in a piece of Al foil, it turned black, bubbled, and ate a hole in the Al foil. (which was probably because of the HCl) I dissolved the mass in HCl, and a few black specks remain today. I claim that these are boron;)
Now to see if they are green in a flame test... more to follow.




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[*] posted on 20-5-2004 at 18:56


You heated it in a piece of aluminum foil? You need to mix it with aluminum powder or likewise to achieve measurable results and dehydrate your boric acid to the oxide. Magnesium works much better though. Plus just because you get black insoluble doesn't mean boron, a strongly inert refectory material, aluminum boride results from this reaction as a competing side product and is very difficult to separate. And yes, the decahydrate increases the volume tremendosly. If you take your equation and put the atomic weights in place of the reactants you will see that only a small precentage of your borax is the actual boric acid expecially in the case of the decahydrate.

By the way, my reduction of boric oxide with aluminum on a large scale was propagated by the addition of about .05% KClO4 which started the mix going nicely. The total charge was 350g total and it went up like a jet engine, huge green gout of flame shooting up with enough force to make it audible, simply beautiful.




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[*] posted on 20-5-2004 at 19:10


Yes, I know the Al foil was not sufficient for a great reaction, but I wanted to see how it dehydrated...

This time I used at least 1 ml of boric acid, which I heated in a copper endcap... ahem crucible, it dehydrated nicely, giving off BEAUTIFUL green light. I then finely ground up the B2O3 and mixed it with Al powder in a 1 to 2 ratio. Back it went
into the crucible, and I cooked it from all sides for a couple minutes, then took out the red-brown mass, crunched it up,
and put the junk in a watch glass.
How can I determine if it is boron?
Did the reaction you did have sulfur in it?
I don't have any on hand right now:(




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[*] posted on 21-5-2004 at 12:38


I don't have much to say, but I found this in my chemistry book:

Na2B4O7*10H2O + 2 HCl ---> 4H3BO3 + 2NaCl + 5H2O

Boric acid is used as a mild antiseptic and is such a weak acid that it may safely be used as an EYEWASH (wow?).

When small amounts of boric acid are dissolved in ethanol and the vapour ignited, a flame tinged with green results.

and I found this equation:

2H3BO3 --heat--> B2O3 + 3H2O


Might not be of much use to you, but I thought I'd post it anyway!

[Edited on 22-5-2004 by thunderfvck]




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[*] posted on 21-5-2004 at 20:28


Yes, it is an antiseptic, contact solutions have boric acid in them. It also is good at killing ants and termites.

I like the green flame idea... All of my future pyro experiments are in grave danger of having B2O3 added to them:D. The color is so magnificent!

On a lesser note, perhaps candles could be made with wax impregnated with B2O3
for a neat effect.




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