Questions about melting Aluminum Oxide

[bobulus31]

Hello everyone, I have a few questions. First of all, I'm looking to melt Al2O3 in a flux in a crucible. I was wondering what would be the best combination of crucible lining and flux to do so at the lowest possible temperature. I know that it can be done in a platinum crucible quite easily but i don't exactly have 5 grand to throw down on a crucible so if anyone knows if this is possible or even theoretically possible in any less expensive apparatuses i would love to hear from you. Thanks a ton in advance.

[John Cuthber]

"I know that it can be done in a platinum crucible"

Not really.

The melting point of alumina is higher than that of platinum.

Dissolving it in a molten flux might work.

Perhaps sodium hydroxide in a nickel crucible would work, but it's horribly corrosive stuff to work with.

 

 

What are you trying to do?

[bobulus31]

I'm trying to crystallize different corundum gems. The corrosive properties wont bother me too much as long as the corundum is able to crystallize properly. I'll definitely look into the sodium hydroxide and nickel combo tho. Thanks for the info. If anyone else has other possible ideas id love to hear them.

The process I have heard that works is dissolving the alumina in molybdenum oxide inside a platinum crucible and heat to about 2100 degrees F. then slowly lower the temperature by a couple degrees per hour to get the alumina to crystallize into corundum such as ruby.

[John Cuthber]

Unfortunately, the sodium hydroxide dissolves the alumina by reacting with it. You won't get alumina back when it cools.

 

It's an interesting idea but I wonder if it isn't easier to make artificial ruby the way they "usually" do.

http://en.wikipedia.org/wiki/Verneuil_process

[bobulus31]

Ahhh darn. And I have looked into that and the Czochralski process but the gems they create end up more like a really hard glass than an actual ruby. I have heard that growing the gems in flux or hydro-thermally creates crystals that shine much more brilliantly which is what I'm looking for. I've been looking into tungsten and Inconel crucibles since they can stand the heat required, i just don't know for certain whether either one would react with the alumina or whatever flux I use.

[rktpro]

In metallurgical process they use Na3AlF6 or CaF2 to reduce melting point. They use this when they carry electrolysis, though.

[bobulus31]

Hmmmm. Any idea if Molybdenum Trioxide would react with a tungsten crucible? If not, then I would imagine the same process that is done in the platinum crucible could be done in a tungsten one.

[rktpro]

You can check that through an Ellingham diagram online. Any element whose curve is lying below the curve of particular oxide would reduce it at that temperature. However, only that knowledge wouldn't be enough to actually carry out the process.

Edited February 27, 2014 by rktpro

[StringJunky]

Is 'Glassy Carbon' worth looking into? It can be taken to 2500c with the appropriate grade and 3000c ( I read elsehere) in a vacuum or Helium atmosphere.

 

http://www.2spi.com/catalog/mounts/spi-glas-glassy-carbon-crucibles.php

 

__www.2spi.com_catalog_mounts_spi-glas-glassy-carbon-cruci.pdf

Edited February 27, 2014 by StringJunky

[rktpro]

Is 'Glassy Carbon' worth looking into? It can be taken to 2500c with the appropriate grade and 3000c ( I read elsehere) in a vacuum or Helium atmosphere.

 

http://www.2spi.com/catalog/mounts/spi-glas-glassy-carbon-crucibles.php

Somehow, the link is not working for me. The crucible should have non-reacting components,besides having higher melting point than oxide.

[StringJunky]

Somehow, the link is not working for me. The crucible should have non-reacting components,besides having higher melting point than oxide.

I've converted the page to a .pdf and added it to the previous post for you to download.

[bobulus31]

Well the crucible should really only need to handle about 1200c since the molybdenum trioxide lowers the melting point of alumina to about 1100c. I would like to not have to use a vacuum if possible but I suppose that is the least of my worries. The glassy carbon looks promising as i doubt it is going to react with any of the chemicals but it only works in open air up to 500c according to the site. That is why I was hoping to use tungsten, however from what I have seen it may react to the molybdenum trioxide. I do not know that for certain though.

 

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422010000500018 (sorry i could only find the full document in Portuguese)

 

This is the main document I have been working off of in my efforts. I just do not have the funds currently to purchase even the smallest platinum crucible.

[StringJunky]

Well the crucible should really only need to handle about 1200c since the molybdenum trioxide lowers the melting point of alumina to about 1100c. I would like to not have to use a vacuum if possible but I suppose that is the least of my worries. The glassy carbon looks promising as i doubt it is going to react with any of the chemicals but it only works in open air up to 500c according to the site. That is why I was hoping to use tungsten, however from what I have seen it may react to the molybdenum trioxide. I do not know that for certain though.

 

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422010000500018 (sorry i could only find the full document in Portuguese)

 

This is the main document I have been working off of in my efforts. I just do not have the funds currently to purchase even the smallest platinum crucible.

Sorry, I missed the note about the maximum temperature in open air is only 500c. It's inert properties are good though aren't they?

Edited February 28, 2014 by StringJunky

[bobulus31]

Yeah everything else about it seems perfect. Price is right there with the tungsten though, so if tungsten would do it in open air without reacting i would rather use that. Thank you guys for all the information by the way, its been a huge help.

[John Cuthber]

MO3 is quite volatile at 1100C

[bobulus31]

https://www.jstage.jst.go.jp/article/cl/34/12/34_12_1620/_article

 

Stumbled across this last night. Looks like an aluminum oxide crucible might work fine with a seed crystal. I might end up breaking the crucible getting it out but i can get those all day for 30 bucks.