Sciencemadness Discussion Board

How is alumina ceramic made?

garage chemist - 8-5-2008 at 09:51

I mean the type of alumina ceramic from e.g. www.sentrotech.com .
Especially what kind of alumina is used, the additives, and the sintering temperature.
I can get alumina hydrate (Al(OH)3, bayerite), this turns into gamma-alumina when heated in a crucible over a bunsen burner, with abundant evolution of steam. This readily re-adsorbs water vapor, hence the term active alumina.
At 1200°C, it becomes dead-burned, no longer adsorbing water vapor and losing its catalytic properties. But it still is a very fine dusty powder.
I have prepared both products.

But what really interests me is how to sinter it into a dense, nonporous and tough ceramic.
I have done simple experiments, like moistening, forming and drying some of the dead-burned alumina into a tiny slug, followed by firing inside a Kanthal wire coil surrounded by kaowool, up to temperatures where the wire melted (1550°C). It still was as powdery as before and crumbled at the slightest touch.

Does anybody know what it takes to sinter alumina into a ceramic? Just even more heat? I find that hard to believe.
Special additives? What's the 0,2 or 0,5% in the 99,5 or 99,8% alumina ceramics? How are they mixed with the alumina?

I was unable to find any information on this. The manufacture of ceramics that are composed from pure chemicals instead of natural products like clays seems to be a hidden science.
Anybody know something about it?

12AX7 - 8-5-2008 at 11:06

Metric fuckloads of heat and/or time, or HIP (Hot Isostatic Pressing). Liquid phase sintering (impurities, intentional or otherwise) is another option, but probably isn't as strong.

Sintering is a diffusion process, so it starts at roughly half the melting point (1000C?) and is fastest just below the melting point (1800C or more, depending on impurities). Very fine particles are helpful, micron sized if possible. Consider that clays are on the single digit to fractional micron scale.

With your equipment, you're probably better off working with mullite, which can be made from alumina and fireclay in suitable proportions (look up your choice clay's composition). The melting point is lower, but if all you're using is kanthal wire, which melts before this stuff anyway, that's not an issue.

How long did you leave that "slug" of powder at temperature, anyway?

Tim

Edit: P.S.: should move to Technochemistry.

[Edited on 5-8-2008 by 12AX7]

not_important - 8-5-2008 at 14:10

Quote:
Alumina ceramics are typically polycrystalline materials that are formed by sintering or hot pressing. Sintering is typically performed at about 1700 °C. A small amount of MgO may be added to control the grain size during sintering.


Besides intensionally added MgO or CaO, sodium is the most common additional element in alumina.

These might be of interest:

sintering wear parts with microwave heating
http://www.springerlink.com/index/j065820q286v7724.pdf

additives in coarse grain alumina ceramics for metallization
http://fizika.hfd.hr/fizika_a/av96/a5p085.pdf

12AX7 has a good idea with going for mullite, as it can be formed from materials at temperatures below the melting point of mullite - the crystals grow from the flux, consuming as they grow. You end up with intertwined mullite crystals with only a small amount of simicae based glass.

Baphomet - 12-5-2008 at 21:54

That's a co-incidence I was looking into the same thing this morning! I found a couple of abstracts that sounded useful.. especially TiO2 which is readily available from pottery suppliers.


INFLUENCE OF HYDROLYSIS--PRECIPITATION MEDIUM ON THE NATURE OF ALUMINIUM HYDROXIDE GELS AND Al203 POWDER CHARACTERISTICS:
M. Thiruchitrambalam, V.R. Palkar*, V. Gopinathan, P. Ramakrishnan, M.S. Multani*.
Dept. of Metallurgical Engineering and Materials Science, I.I.T, Bombay - 76; *Tata Institute of Fundamental Research, Bombay -5, India

Aluminium hydroxide gels have been widely used in applications like sorbents and catalyst supports. Thermal treatment of aluminium hydroxide gels i.e., calcination and/or sintering, first leads to dehydration and then a series of phase changes. Once dehydration is complete several transition aluminium oxides namely ZETA, SIGMA and OMEGA appear, and finally ALPHA Al2O3 is formed at about 1200°C. In the current investigation boehmite (AIOOH) a bayerite (Al(OH)3) have been prepared in the presence of water-glycerol solutions. The results indicate that the precipitation media have considerable influence on the nature of aluminium hydroxide precipitate. Aluminium hydroxide gels thus prepared were characterised by TEM and XRD. Calcined powders were examined by XRD for phase content, by SEM for morphology and BET method for specific surface area. Calcination of aluminium hydroxide gels prepared by Hot water hydrolysis-Controlled precipitation technique yielded agglomerate free, spherical ALPHA Al2O3 powder. Al2O3 was also prepared by calcining the aluminium hydroxide gels at 600°C and characterised by XRD, TEM and BET method.


Low-Temperature Sintering of Alumina with Liquid-Forming Additives
Liang A. Xue and I-Wei Chen
Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, Michigan 48109 and
C. A. Handwerker — contributing editor

Simultaneous application of colloidal processing and liquid-forming additives to alumina resulted in a sintered density of >99% in 1 h at a temperature as low as 1070°C for a commercial high-purity alumina powder at a total dopant level of 2 mol%. The additives were 0.9% CuO + 0.9% TiO2 + 0.1% B2O3 + 0.1% MgO. At higher temperatures or after prolonged sintering, the doped alumina ceramic developed a duplex microstructure containing large elongated grains and exhibited a relatively high fracture toughness of ~3.8 MPa · m1/2 as compared to a value of ~2.6 MPa · m1/2 for the undoped alumina.

Baphomet - 12-5-2008 at 21:58

By the way, see if you can grab a microwave 'art' furnace, they sell them on EBay (I think they are constructed from alumina foam actually)

These small furnaces are supposed to achieve very high temps (some claim >= 1000 degrees c) so it is worth a try, but only for very small samples

Another point to note is that your greenware needs to be fired for a long time. Even regular pottery is fired 6 hours or more so alumina should take as long, or longer.

garage chemist - 13-5-2008 at 02:28

Thanks for the comments so far.
I only held the temperature for a few minutes with the Kanthal wire coil, so it was certainly too short for any sintering process, which always take time.

1700°C as the sintering temperature for nearly pure alumina, that's extreme. Do the manufacturers all have production-size furnaces with MoSi2 heating elements?
And what kind of insulation would stand up to such a temperature?
I know of polycrystalline alumina fiber, that would probably be a suitable material, though expensive. But this doesn't provide structural integrity of a kiln- some dense refractory would have to be used on the inside.

An amateur could maybe use graphite rods as heating elements, and have the kiln filled with inert or reducing atmosphere to keep the graphite from burning up.

But this:
Quote:
Simultaneous application of colloidal processing and liquid-forming additives to alumina resulted in a sintered density of >99% in 1 h at a temperature as low as 1070°C for a commercial high-purity alumina powder at a total dopant level of 2 mol%. The additives were 0.9% CuO + 0.9% TiO2 + 0.1% B2O3 + 0.1% MgO. At higher temperatures or after prolonged sintering, the doped alumina ceramic developed a duplex microstructure containing large elongated grains and exhibited a relatively high fracture toughness of ~3.8 MPa · m1/2 as compared to a value of ~2.6 MPa · m1/2 for the undoped alumina.

would be much better, as it would allow Kanthal wire to be used as heating in the kiln. Thanks, Baphomet!
Remains the question on how to prepare the raw mass, and how to form it.
In Ullmann, I read that alumina for ceramics ("reactive alumina") is made by prolonged milling of calcined alumina in ball mills to get the particle size as small as possible.
I would have to build a suitable ball mill first, with hard porcelain balls as milling media.
I still don't know if the alumina for ceramics is the dead-burned alpha kind or some lower calcined gamma-form.

And then, how are those additives incorporated? Simply milling together with the alumina, or some solution process?

How are the green bodies formed? Wet forming with addition of organic binders?
Dry pressing?

[Edited on 13-5-2008 by garage chemist]

12AX7 - 13-5-2008 at 10:17

Quote:
Originally posted by garage chemist
1700°C as the sintering temperature for nearly pure alumina, that's extreme. Do the manufacturers all have production-size furnaces with MoSi2 heating elements?
And what kind of insulation would stand up to such a temperature?


Sure, and maybe alumina bubble or generic high-alumina (or zirconia or magnesia) firebricks, backed up with lower temperature materials (bricks, castable, etc.), and probably a blanket on the outside. A couple feet thick overall. Great efficiency for a couple thousand cubic feet of hot volume operated 24/7, and well worth the investment.

Not so practical for home use...

I would go with a hot face, coated with ITC-100, backed up with reasonably refractory ceramic blanket. I've heard of fifteen minute, oil-fired cast iron melts in such a furnace. Don't know about longevity, but you certainly can't argue the performance.
Quote:
...how are those additives incorporated? Simply milling together with the alumina, or some solution process?

How are the green bodies formed? Wet forming with addition of organic binders?
Dry pressing?


Sure, whatever works best. How doesn't matter, in the end the composition is homogeneous by diffusion (I mean, except for crystal growth and grain boundaries). How you get there (e.g., particle size, distribution of additives) is more of a thermodynamic thing, i.e. how long do you want to wait. Coprecipitation would probably give the fastest compound, while dispersing soluble additives by solution would be next.

Without silica, clay is out, so an organic binder would probably be the most convienient. I don't remember offhand any typical binders used for this, but it can't be too hard to find. I would suppose gums and whatnot, but I don't know if there are special types that leave matrix once burned out. (I do recall phenolic resin and asphalt are used to leave a glassy carbon matrix, which crystallizes into graphite on extended firing. For obvious reasons, this is only used for the most robust brick, like MgO-C used in the roughest environments, like steel ladles.)

Tim