Transient Flux For Alumina

[StevenRS11]

So, I got tired of purchasing ceramic crucibles and then abusing them in the name of science, so I decided to make them! I've been using a mullite, kaolin and alumina composition that so far has served me quite well for slip casting. Exceedingly refractory, and I have yet to break one from thermal shock.

 

The thing is, they are not particularly thermally conductive. Better than most ceramics considering all the alumina, but I think I can do better. Does anyone have any experience with cryolite? I know it is one of the few lowish temperature fluxes for alumina, but what makes it really interesting is that it can be oxidized. I *think* I should be able to decompose the cryolite to more alumina and HF, destroying it's fluxing ability in the process. Has anyone heard of this type of process?

 

The end result I am imagining is a vitrified alumina bonded ceramic, where the cryolite temporarily fluxes the alumina (probably from alumina hydrate) and then decomposes upon further heating, causing the fluxed alumina to rapidly re-crystallize. Perhaps I could use silicon carbide as a filler, or a lower mesh calcined alumina that will stay crystalline so I don't end up with a excessively glassy, brittle ceramic.

 

Two problems I can see are the left over sodium and the free fluorine. Ideally, I would like the two to form sodium fluoride which I can vaporize out at 1800C, but that seems unlikely. The HF will probably vaporize instantly, leaving the sodium behind to ruin dream of a 100% flux-less ceramic. Unless, of course, I sublimate it out at 2000C

 

 

Any feedback/ideas?

[MatthewV]

I suspect pressure is the way to go. If theses were heated under a few tons of pressure they should fuse without much flux. This would create a denser more conductive material.

[Tyler Miller]

This article, and others on "sintered alumina" represent where the research is going.  No need to mess with cryolite, fine grinds/colloidal preparations and tiny additions of flux will get you there.  http://onlinelibrary.wiley.com/doi/10.1111/j.1151-2916.1991.tb07825.x/abstract

 

 

Unless you're going into business, or have a specific application in mind, it will always be cheaper to just buy crucibles.

[StevenRS11]

They are for a very specific application. I make jewelry and other smallish but detailed metal parts (like clever little locking boxes, miniature trebuchets, physics demos for schools, etc) as a hobby.

 

Its a sort of modification of traditional investment casting where you coat wax with ceramic, fire it, and pour metal into it, except I combine the melting crucible with the casting mold. What I have works pretty well, except that it would be really cool if I could make them totally non-porous so keeping an inert atmosphere would be easier. Imagine casting titanium at home!

 

Honestly, I just love experimenting and finding novel ways to do things. People always say don't reinvent the wheel, but honestly, the wheel has been reinvented countless times for endless different applications.

 

Thanks for the article btw. Not sure how I could apply pressure to these, but the particle size stuff looks promising. I wonder if extended ball milling could get me there.

[Tyler Miller]

Argon atmosphere melting furnace and a graphite crucible wouldn't work?

[Tim T]

Hi Steven,

I've been playing around with Cryolite a bit recently, used in glazes on earthenware, fired at 1000 - 1100C. I'm a long way from understanding it fully (I'm not sure if anyone does for its use in Ceramics), but what I think is happening is that the Cryolite decomposes and then SiF4 is given off as a gas, which then decomposes to HF, and then on exposure to moisture (i.e. outside the kiln) decomposes to hydroflouric acid (which can be pretty nasty, so check out the health and safety side and make sure you are happy with this before playing).

Things I have found are that the glaze is very runny and very reactive - I've had pots melt through and collapse, and it has penetrated at least 6mm into kiln shelves. So now I put all pieces on supports smaller than the foot, and have a fully fired stoneware sacrificial plate underneath - I think the iron in the clay affects the reactivity, as it is lower if I use a layer of slip between the glaze and the clay. Also, I've found that the glaze penetrates the body of the clay and results in something very weak and brittle - 6mm thick clay that you can break as easily as a biscuit.

Have you read up on Cryolite as used in refining aluminium? They melt the aluminium ore in the Cryolite, and then use electrolysis to get the aluminium out. The aluminium/cryolite solution appears to be stable. Here's a couple of references that may be useful: http://www.chempap.org/file_access.php?file=276a737.pdfand http://www.diva-portal.org/smash/get/diva2:125541/FULLTEXT01.pdf

 

I'm interested in what you're doing with the casting - my other half makes jewellery and we've been discussing making ceramic moulds for casting, but haven't got as far as trying anything yet.

 

Tim

[PeterH]

Tyler Miller

This article, and others on "sintered alumina" represent where the research is going.  No need to mess with cryolite, fine grinds/colloidal preparations and tiny additions of flux will get you there.  http://onlinelibrary...7825.x/abstract

 

Very interesting article Tyler.

For those having problems with access, looks like there is a legitimate public domain version at the authors university website.

http://deepblue.lib.umich.edu/bitstream/handle/2027.42/66046/j.1151-2916.1991.tb07825.x.pdf?sequence=1&isAllowed=y

[Tyler Miller]

Thanks for finding that, Peter!

 

I feel I should add that I misinterpreted the pressures in the article. I misread and assumed 150 Kpa, which isn't huge. 150 MPa is like 21K psi. You need a ram press to get that kind of pressure.

[StevenRS11]

Hi Steven,

I've been playing around with Cryolite a bit recently, used in glazes on earthenware, fired at 1000 - 1100C. I'm a long way from understanding it fully (I'm not sure if anyone does for its use in Ceramics), but what I think is happening is that the Cryolite decomposes and then SiF4 is given off as a gas, which then decomposes to HF, and then on exposure to moisture (i.e. outside the kiln) decomposes to hydroflouric acid (which can be pretty nasty, so check out the health and safety side and make sure you are happy with this before playing).

Things I have found are that the glaze is very runny and very reactive - I've had pots melt through and collapse, and it has penetrated at least 6mm into kiln shelves. So now I put all pieces on supports smaller than the foot, and have a fully fired stoneware sacrificial plate underneath - I think the iron in the clay affects the reactivity, as it is lower if I use a layer of slip between the glaze and the clay. Also, I've found that the glaze penetrates the body of the clay and results in something very weak and brittle - 6mm thick clay that you can break as easily as a biscuit.

Have you read up on Cryolite as used in refining aluminium? They melt the aluminium ore in the Cryolite, and then use electrolysis to get the aluminium out. The aluminium/cryolite solution appears to be stable. Here's a couple of references that may be useful: http://www.chempap.org/file_access.php?file=276a737.pdfand http://www.diva-portal.org/smash/get/diva2:125541/FULLTEXT01.pdf

 

I'm interested in what you're doing with the casting - my other half makes jewellery and we've been discussing making ceramic moulds for casting, but haven't got as far as trying anything yet.

 

Tim

 

I actually got around to trying some mixtures with cryolite today. It is, as your results suggest, an unbelievably powerful flux. In any percentage above 1 or 2 in the clay body gave me crumbly, porous ruins. I think it first fluxes everything, then forms bubbles as it decomposes into a gas. Said bubbles promptly freeze because, well, the flux is gone now. As a final parting insult, it steals some silica as it leaves. I could see this being useful to make a foamy, insulating ceramic refractory though.

 

In fractional percentages, it seems to behave it bit more like I envisioned. Ill need to measure porosity and shrinkage to really see, but the crucibles with .3 percent total cryolite hold water better. The simple 10% kaolin, 20% mullite, and 70% alumina crucibles when filled with water wet a paper towel in about 45 seconds, while the ones with cryolite took at least 5 minutes (I got bored watching and left, came back in 30).

 

As for the the casting, Ill post some details about that when I get a sec. Its an established industrial process, and works really well for high melting or tricky to cast metals because the mold is heated to above the melting point of the metal before pouring. Hence why I just attach the crucible to the top of the mold and plop the whole thing in the furnace. Id like to design some sort of stopper for a bottom feed crucible, but right now I just tilt it.

[Tyler Miller]

Maybe try a more predictable, less aggressive (and dangerous) flux in small fractions of a percent and see what happens.  Like borate.

 

There's something to be said for all the money that's gone into the R&D of conventional metallurgical wisdom.

[StevenRS11]

Argon atmosphere melting furnace and a graphite crucible wouldn't work?

 

In college, our jewelry shop (which I spend wayyy to much time in- something about making just simple silver rings and filing patterns in them just entranced me) we had one of those induction vacuum casting things. I really never took advantage of it, but some of the stuff people managed to cast in there boggles my mind. We even had a few people from the engineering department come in to cast things occasionally, like this artificial knee joint implant thing. Obviously just as a project, but still cool stuff.

 

What I would do for one of those

 

Maybe try a more predictable, less aggressive (and dangerous) flux in small fractions of a percent and see what happens.  Like borate.

 

There's something to be said for all the money that's gone into the R&D of conventional metallurgical wisdom.

 

 

Yea, make a good point. I guess if its in small enough amounts it will become so diluted that it will not appreciatively decrease hot strength. Am I right say that firing it for a long period of time will increase hot strength as more and more of the unfluxed alumina diffuses into the flux, decreasing its relative concentration?

 

I don't have any insoluble borates lying around, so Ill see if I can get my hands on some. Somehow I feel that adding raw borax to my slip will at best make a self-glazing clay, and at worst Ill manage to discover aneutronic fusion in my garage and you will hear about me (and the crater) in the news.

 

I do have some potash feldspar, which I have read encourages the formation of mullite crystals in clay bodies. Think that might be worth trying as well?

 

Btw, the only experience I have with ceramics is a bit of research, a HS pottery class a few years ago, and the last 6 days when I got my kiln for firing the ceramic shells finally working right. If I say anything that makes no sense whatsoever, feel free to call me out on it.

[Tyler Miller]

The potash feldspar might actually do well also.  It's a historical component (or a tolerated contaminate) of the old hessian crucibles, the composition of which isn't too far off what you're playing with.  I've goofed around a little with crucibles, but I made mine out of a high fire, thermal shock resistant clay body I buy and use for raku (PSH's Sheba raku body).  I also only used them for water casting copper and shibuichi--not high demand applications.  I've tried making the hessian style, but to little success.

 

If I'm remembering correctly, you only need to get the body up above cone 4 and hold it there for a long while to get mullite formation, if your body's formulated properly.  Repeated firings are also a way to get there.  Kyanite's a nice precursor to mullite that can be useful as well.  Graphite additions will help with keeping the atmosphere neutral or reducing in the crucible.

 

I'd recommend checking out publications that have instructions on making your own kiln furniture, since that's basically what you're doing.  Cardew's "Pioneer Pottery" (NOT "Pioneer Potter" which is a biography) might have something on it, and floating around the net is a pdf copy of the series "the Self-reliant Potter."  I think it's associated with a group that helps developing areas build up local industry.