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[*] posted on 23-2-2008 at 08:03
NaCl fusion to produce large single crystal?


Has anyone here tried to produce a large crystal of NaCl by melting it and slowly cooling the molten salt? I've done a little searching around, but haven't been able to find anything yet.

I was thinking that it <em>might</em> be possible to melt some pure sodium chloride (mp 801°C) in a small, say 15 mL, ceramic crucible supported by a clay triangle/ring/clamp/ring stand over a Meeker burner (propane, 2000 - 3200 BTUs?). Start with the crucible at a medium height to warm things up, lower it to melt the salt, then very gradually raise it to lower the temperature and allow the salt to solidify.

Does a Meeker burner output enough heat to do this? Would this be possible at all? If so, are there any hazards (like fumes) besides splashing the salt? Would a single crystal be produced? (Could it be used to make IR plates?)

[Edited on 2/23/08 by bfesser]




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microcosmicus
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[*] posted on 23-2-2008 at 09:30


To get really slow and uniform cooling, you might try adopting the blacksmith's
method for annealing steel. Start with a container having a rather large thermal mass ---
if your crucible has too thin walls, maybe encase it in a hollow carved out in in a
brick or just use a hole in a brick as a crucible and cover with another brick. Then
heat bothe the crucible and the brick until both are hot and the salt melts. Then encase
the hot stuff in some insulating material like perlite of glass wool and leave it that
way so it cools down over a period of hours, if not days.

The idea here is that the thermal mass serves to keep the temperature constant
whilst the insulation slows down the heat flow to a trickle. After all, what will
determine the rate of crystal growth is the rate of heat flow when the salt is
at its melting point; for large crystals, you want to make this as small as possible.

As for raising the crucible above the fire, I worry that the heating and cooling is
not going to be all that uniform. Sure, you could make the average rate of
temperature decrease as slow as you want by controlling the rate of raising, but
the fluctuations about this average would be about the same and I think that
these would be what limits the size of crystals obtainable.
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[*] posted on 23-2-2008 at 09:35


I tried this with Rochelle salt. Yes there's a big difference in mp, but I melted a single, large crystal as big as my thumb grown in water, slowly cooled it, and it retained its plastic, amorphous shape, resembling a pool of dried white glue. No crystals. It looked like melted plastic. I used a regular cooler that was preheated on the inside for the slow-cooling process. Maybe if using something with a much higher mp, the cooling process will be different.



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[*] posted on 23-2-2008 at 12:08


Why do you want a single crystal? If you do succeed, you will get a lump the shape of the crucible, not a perfect cube that you can sit on your shelf.

If you're going to try, I would love to see the results. You will need very pure salt, which you might recrystallize (by evaporation) once or twice from distilled R.O. water to get nice and clean. You will need a similarly clean crucible, one which doesn't react with the environment or melt. Porcelain is out. Nickel might work, but you'll need a reducing atmosphere. Platinum, expensive as it is, would be a good candidate.

You'll probably want more BTU's. Try something in the 40k BTU/hr range, neutral gas mixture (regular stove burners are low pressure and even lower air content, making a very cool, diffuse blue flame, just fine for cooking but no good for making stuff HOT), and use a lot of insulation. Some conductive thermal mass might be a good idea, as Microcosmicus suggests; I would however suggest conductive bricks if possible. The densest stuff you can find: yellow fireplace bricks, kiln hardware (posts/shelves), that kind of stuff. Both glass wool and perlite are excellent insulators

Prole: Rochelle salt is with an organic anion. It's my experience with acetates at least that they don't like to assemble into crystals very well, even in aqueous solution. It needs to be done very slowly. One big advantage to simple inorganic salts like this is, it can be annealed very easily because the ions aren't locked into position and can diffuse through the crystal; a huge tartrate ion would be hard pressed to rotate and move through a matrix locked at merely 100C, whereas spherical sodium or chloride ions at 700C have plenty of mobility to align crystal planes.

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[*] posted on 23-2-2008 at 15:12


The key is to grow the crystal slooooooooooooowly, which for fusion means keeping
the rate of heat flow as small as possible. Remember that, while it is solidifying, the
temperature of the NaCl is going to be constant, with only the rate of heat transfer
determining the rate of crystal growth.

As for what that rate might be, here are a few numerical figures. While I found it hard
to find good figures online, is seems that the heat of fusion is around 25 KJ per mole.
Since you are talking about a 15 ml crucible, that means something like 20 g of NaCl,
or a third of a mole, so a total of 8 KJ of energy. If we want to give the crystals at
least an hour to form so they get big, that means keeping the rate of heat transfer
underneath 2 Watts. Given that the temperature of the salt is 800C but room
temperature is at best 40C, that's quite a temperature gradient, so insulation alone
doesn't sound like it alone would keep the rate of heat transfer down to a Watt.
(To be sure, I should look up diffusion constants and solve the diffusion equation
with the appropriate boundary conditions, but I don't feel like doing that right now.)

That is where the thermal mass comes in. If you take something like a big honking
brick or a lump of iron at 800C and wrap it in insulation, it is quite easy to get
the temperature to drop at a few degrees per hour --- as I mentioned, blacksmiths
anneal steel this way and, depending on the dopants, steel needs to cool at a
rate of at most something between at most 5 to 25 degrees per hour. The idea is
the salt is losing heat to the thermal mass which loses it to the air through the
insulation. Since the thermal mass is cooling quite gradually, it will remain only a
few degrees cooler than the salt for an hour or more; since the thermal gradient
is only a few degrees, the heat will travel much slower, so it is relatively easy to
arrange that the loss rate is a Watt or so.

[Edited on 24-2-2008 by microcosmicus]
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[*] posted on 23-2-2008 at 17:35


Quote:
Originally posted by 12AX7
Why do you want a single crystal? If you do succeed, you will get a lump the shape of the crucible, not a perfect cube that you can sit on your shelf.


It could be cleaved into a cubic shape if needed, I suppose. I was more considering trying to just get two smooth polished sides on it, to tinker around with it's optical properties (transparency to IR).

Thanks to everyone for all of the great advice. It sounds like it's outside of my capabilities and definitely my budget at the moment. But hopefully I'll be able to try it someday, and I'll definitely post the results.
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[*] posted on 23-2-2008 at 19:38


Simple cooling, even very slow, isn't certain of producing large crystals. As an example, melt some sulfur and let it cool.

For many materials single crystals are produced using the Czochralski or Bridgeman processes. Both fully melt the material, then use a seed crystal and cooling to grow the single crystal. Czochralski touches the seed crystal to the surface of the melt and slowly withdraws it; the melt in contact with the crystal freezes on the crystal, enlarging it. Bridgeman melts the material, then using slow cooling with a controlled gradient and the introduction of a seed crystal at the cooler end of the melt container.

In both cases the melt must not be too hot or it will melt/dissolve the seed crystal. Too rapid cooling is likely to lead to polycrystalline growth.

Neither is beyond the reach of an amateur for lower melting point materials, but the high temperatures needed for NaCl and other similar salts would likely make it a real challenge taking a good deal of effort.
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[*] posted on 23-2-2008 at 20:02


I apologise in advance for ignorance - but why not just growing the crystal from solution?
How is an NaCl crystal different, when grown in solution? As far as I know it forms a regular (Na-Cl)n lattice, is it different from a crystal obtained from a melt? How?

As to Rochelle Salt - the reason why it is a plastic amorphous shape is that there was a) likely no crystallisation nucleus and b) not enough time allowed for recrystallisation.
I.e. glacial acetic acid is similar - you can cool it well below its MP, but it will remain liquid. ONce something provides a nucleus, the liquid 'freezes' almost instantly. I've seen this with other compounds too. Glass is also an amorphous mass that never crystallised, crystallised glass is in fact not transparent.

[Edited on 24-2-2008 by chemoleo]




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[*] posted on 23-2-2008 at 20:10


Growing crystals (other than plain recrystallizations) is not something I'm very familiar with, sorry. In my experience, I've never seen NaCl crystals from solution form much bigger than ordinary table salt size. How difficult is it to grow larger ones?
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[*] posted on 23-2-2008 at 20:13


Quote:
Originally posted by bfesser
If so, are there any hazards (like fumes) besides splashing the salt?

When I melted 10g or so of sodium chloride in a nickel(?, not sure it was free) crucible, there was a distinct odour in the air, as the vapour pressure of salt is not negligable at such a high temperature. I am unsure of the physical toxicity of this, but it was extremely unpleasant it produced a metallic taste in my mouth, it literally felt like I was chewing metal.

IR plates are usually made from large solution grown crystals which are then cut and polished.




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[*] posted on 23-2-2008 at 21:11


Quote:

How difficult is it to grow larger ones?


Not that hard. If you place the small crystal on a string in a saturated solution
and let the solution slowly evaporate over the course of a few days, it should
easily grow to a much larger crystals. Since the solubility of NaCl depends
little on temperature, you do not have to be too fussy about keeping the
temperature constant, just put the solution in a corner of the house with a
reasonably steady temperature where the crystal won't be disturbed as it grows.
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[*] posted on 23-2-2008 at 21:33


Quote:
Originally posted by 12AX7
If you're going to try, I would love to see the results. You will need very pure salt, which you might recrystallize (by evaporation) once or twice from distilled R.O. water to get nice and clean. You will need a similarly clean crucible, one which doesn't react with the environment or melt. Porcelain is out. Nickel might work, but you'll need a reducing atmosphere. Platinum, expensive as it is, would be a good candidate.
Tim


Platinum will not work, sodium chloride will react with it.




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[*] posted on 23-2-2008 at 22:15


Please see the 'crystal growing' thread. It was active recently.
In fact I remember reading crystal growing 'manuals', where it was to suggested to not ever bother with NaCl as it is so trivial, and as very large natural crystals can be obtained from ebay, mineral stores, hardware stores (lamps) you name it. Not all will be high quality of course.
Anyway microcosmicus is on the mark. Just string up a salt crystal on a nylon thread, and refill with more sat. NaCl as it evaporates.




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[*] posted on 24-2-2008 at 01:25


I remember when I was a kid I melted some kitchen salt in a crucible, but it did not crystallize during cooling (I did not cool it down slowly, just switched off the Bunsen burner). Instead it remained an amorphous glass-like solid which slowly cracked into pieces due to delayed crystallization. Maybe it would be different with a very slow cooling.

I would check the exact method used for preparing IR plates. It must be published somewhere. In school, all we were only told is that they are prepared by growing large crystals from extra pure NaCl from its saturated solutions in HCl(aq). Maybe there is some info in the patent literature?




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[*] posted on 24-2-2008 at 03:29


I've melted salt in bulk many times -- it freezes into a polycrystalline bulk which shrinks readily on cooling, thin pours often curling into huge arcs before cracking apart from the stress.

Salt grows unusually badly in solution! I have never seen crystals larger than 1/4" across in all the brine solutions I've left forgotten in corners. Dislocations (giving a white appearance in spots) often make them less than stellar, and obdurated moisture makes them dangerous to melt (I haven't met a single sample of salt that didn't explode like popcorn when being melted -- a lid is a must!).

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[*] posted on 3-3-2008 at 15:33


You will not grow a single crystal by melting it, of any ionic compound- this is simply beyond the reach of what is possible outside of a lab.

Czochralski and Bridgeman crystals must be done in an inert atmosphere, with a precise rate of pulling and simultaneous twisting for the former and a just as precise heat gradient and hot zone movement for the latter, and these both require an oriented seed crystal.

What is not outside the reach of amateur chemists is directional solidification of metals such as indium, gallium, bismuth, tin, antimony- if done correctly and with very pure starting materials, I've made pinky-sized crystal grains in my garage with minimal equipment.

I do have a bunch of old lab-grown Czochralski and flux-grown quartz, sapphire, NaCl, KCl, CdSe, and LaB6 I was thinking of putting on eBay soon ;)...




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[*] posted on 11-3-2008 at 19:07


Hi there all!

I have a BIG NaCl crystal which was brought to me from a salt mine. It is more or less a 3 cm side cube, embedded in an amorphous mass of smaller crystals. One day the cleaners saw it had dust on it, so guess what they did... YES: they washed it!!! so now it has a more or less rounded shape.. (F@kc 'em!). I agree with the fact that growing a crystal from molten salt will need a ridiculously slow cooling rate.
I have grown CuSO4 crystals by saturating an aqueous solution, and filtering it DURING A WARM MIDSUMMER AFTERNOON - and then let it rest until chilly winter comes (yep: 6 months!). CuSO4 has a very big solubility variation with temperature, something that NaCl does not have, so I am not quite sure how to maintain the small supersaturation needed to make the crystals grow. When the solution becomes saturated and it is at equilibrium with the solid, no further crystallization will occur unless the solubility drops with temperature. Perhaps adding minute quantities of pure NaCl as the crystal grows will enable it to keep on growing.

Cheers!
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[*] posted on 11-3-2008 at 20:12


Quote:

CuSO4 has a very big solubility variation with temperature, something that NaCl does not have, so I am not quite sure how to maintain the small supersaturation needed to make the crystals grow.


By letting the water evaporate slowly as the crystals grow.

Quote:

no further crystallization will occur unless the solubility drops with temperature.


Decreasing solubility is not the only way to make crystallization ---
keeping the solubility the same but removing solvent also works.

Quote:

Perhaps adding minute quantities of pure NaCl as the crystal grows will enable it to keep on growing.


Adding solid to the mix does not sound like a good idea because the
powder you add could act as seed crystals making additional crystals
in addition to the one you are trying to grow; rather, add more
supersaturated solution as it dries out.


[Edited on 12-3-2008 by microcosmicus]
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[*] posted on 12-3-2008 at 04:20


Hi Microcosmicus!

Yes, I agree on what you say. Evaporating solution should be done taking care not to allow dust particles to come in, which is why perhaps it did not come to my mind. Indeed the same problem would appear with the addition of the solute I mentioned before (which I am not aware of how to do it: the "perhaps" was bound to imply this, not sure if I made it... ). Yes, adding supersaturated solution little by little could also help.

Cheers!

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[*] posted on 12-3-2008 at 07:36


To grow from a melt and have decent results, you'll need not only highly pure NaCl but also probably a clean room environment. Even the tiniest particulates have to be filtered out of the lab atmosphere.

I used to know a couple guys who did that sort of thing industrially. As I understood it, the clean environment and temp control hardware cost a lot of money. The end result was something shaped like whatever container they used, although the clarity was impressive :D

I didn't see it pointed out in this thread, but you might want to avoid table salt. Sodium silico-aluminate and other additives will probably interfere with uniform crystallization; this could also explain why so many people have trouble growing anything but a mass of 1/8 inch crystals with NaCl.
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[*] posted on 12-3-2008 at 08:14


Quote:

Evaporating solution should be done taking care not to allow dust particles to come in,
which is why perhaps it did not come to my mind.


If dust is a problem, maybe use a ball of cotton or a piece of filter paper (or even
ordinary paper) to close off your jar --- this way the moisture can get out but
the dust cannot get in.

[Edited on 12-3-2008 by microcosmicus]
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