Sciencemadness Discussion Board

Corundum Synthesis

Mechaton - 31-3-2006 at 10:42

I am interested in novel ways to synthesize corundum crystals (ruby/ sapphire). In particular, I'd like to undertake a synthesis similar to that used in commercial production of quartz- using an autoclave filled with raw material in solution. Trouble is, I can't find any good info on the solubility of alumina. Some places have reference to alumina dissolving in or reacting with strong acids or bases, but nothing clear. Surely, given that these crystals form away from igneous rock in nature, they have to be soluble in something under some condition. Does anybody have pointers on finding more info about alumina?

chromium - 31-3-2006 at 11:12

I do not know for sure but alumina may be soluble in molten metals. Acids or bases are probably no help here as from aqueous solutions alumina precipitates as amorphous mass. At least this is what i have experienced.

Edit: thinking more about it... It seems that alumina can not be very soluble in molten metals. If it were then thermit reaction would give us metals with alumina dissolved in it.

[Edited on 31-3-2006 by chromium]

chemoleo - 31-3-2006 at 11:21

Well, from the 'crystal growing' thread, garage chemist pointed out that SiO2 is soluble in H2O at 300 deg C (therefore under pressure). It is possible that a similar property is seen with Al2O3.
Alternatively, maybe the MP of Al2O3 is lowered due to the presence of natural melting-pt decreasing agents. For instance, Al2O3 is electrolysed in the presence of kryolite, which reduces the MP to managable temperatures.


[Edited on 31-3-2006 by chemoleo]

Mechaton - 31-3-2006 at 15:13

I didn't Know that about kryolite. I'll hare to look into that-I'm also sort of interested in making a little arc furnace to sinter objects from alumina composites.

The process Garage Chemist mentionsfor SiO2 is exactly what I'm thinking of for making corundum. There just seems to be a lot of information about using such techniques for crystals other than corundum. Maybe nobody has tried to grow it from solution since the flame fusion and pulling processes work so well. It's just like me to plan on throwing hundreds of dollars into a process that's obsolete from the start. It'd be neat though.

chemoleo - 31-3-2006 at 15:19

Quote:
There just seems to be a lot of information about using such techniques for crystals other than corundum.

If you have them in an electronic form I am sure many, including myself, would appreciate to have a look at this information, so post away!

Quote:
Maybe nobody has tried to grow it from solution since the flame fusion and pulling processes work so well.

How does this process work?

Mechaton - 31-3-2006 at 15:44

Here's all I happened to save, it's quartz only. My understanding is that the Chatham style process used to make emeralds is similar.

The flame fusion process (Vernuili SP?) uses an oxy hydrogen flame directed onto a wire or metal tube. Alumina powder and colorants (like titania for sapphires) are injected into the flame and a crystal forms an the metal surface.

The Czochralskipulling process involves creating a constant melt of alumina wherein a seed crystal is lowered just to the surface then very slowly withdrawn, usually while rotating. Large crystals can be pulled from the melt over the course of days. I believe this is how the silicon for chip wafers is made as well.

Attachment: e_01.mht (76kB)
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Mechaton - 31-3-2006 at 15:47

More into on quartz synthesis.

Attachment: synthetic.pdf (88kB)
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Mechaton - 31-3-2006 at 15:48

And more...

I need to figure out multiple attachments,

Attachment: Synthetic Quartz Crystals; Rockhounding Arkansas.mht (54kB)
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Mr. Wizard - 31-3-2006 at 15:52

Look under synthetic gems at this site:
http://encarta.msn.com/encyclopedia_761563821_2/Gemstones.ht...

and
http://encyclopedia.jrank.org/GAG_GEO/GEM_ARTIFICIAL.html

I have seen diagrams of very simple mechanisms to hold the boule or rod while it was being formed, while finely ground alumina was carried into the flame by the oxygen.

12AX7 - 31-3-2006 at 16:46

Alumina is usually grown from a melt. You might be able to use a basic solution, possibly comparable to the quartz process but maybe with more alkali. Cryolite (and other molten fluorides) have some solubility for alumina so a convection process inside a mostly sealed container might grow crystals. It has to be "mostly sealed" because molten salts usually have a low vapor pressure. An electric furnace would be the best route, well insulated and adjusted to give an even heat, with more heat at one end of the container to dissolve the alumina. It'll probably have to run for days or weeks on end to produce a reasonable amount of growth.

I wonder what the percent impurity is for fluoride, sodium, calcium and other ions in the corundum structure.

Tim

Mechaton - 31-3-2006 at 18:49

So I'm off to research cryolite, thanks for the lead!

Mechaton - 4-4-2006 at 15:42

FYI

Alumina can also be dissolved in Sodium Hydroxide and Hydrochloric acid. Apparently that's why they will affect aluminum metal, they dissolve the protective oxide coating.

12AX7 - 4-4-2006 at 16:01

Yeah, but that forms sodium aluminate or aluminum chloride, respectively. The aluminum oxide composition is lost in the reaction.

Tim

_cyan_89 - 19-5-2006 at 13:14

maybe you cold mix Cr2O3 (or K2Cr2O7) with Al-wer and iron oxide and light it for getting rubins:)

_cyan_89 - 19-5-2006 at 13:15

Al-powder sry ,my keyboard isn´t very well

BromicAcid - 20-5-2006 at 06:58

In one of the books in our library, "A course in inorganic preparations" on page 146 a process is described for a thermite reaction between aluminum and chromium oxide just as _cyan_89 suggested.
Quote:
The reduction between aluminum and chromium sesquioxide proceeds with much less heat evolution than with manganese oxide, and with small quantities of material the heat is sometimes insufficient to melt the slag of aluminum oxide. The addition of a little dichromate increases the heat and gives better results. For the charge, mix 70 g. of Cr<sub>2</sub>O<sub>3</sub> with about 1 g. less than the theoretical weight of aluminum. Place all of the mixture in the Hessian crucible, which should be about three-fourths full. Ignite as before with a starting mixture and magnesium fuse. Rubies may sometimes be found in the slag.
I think I quoted or at least mentioned this before but I always though it was somewhat interesting. I'm sure the procedure could be modified in some way, shape, or form in order to maximize the probability of making rubies although it would hardly be cost effective.