Porous aluminum oxide structures and processes for their...

Details Porous aluminum oxide structures and processes for their... Porous aluminum oxide structures and processes for their...

2001-03-05

2002-06-04

Marcantoni, Paul (Department: 1755)

Compositions: ceramic

Ceramic compositions

Pore-forming

C501S085000, C501S105000, C501S127000, C501S153000, C423S625000, C423S628000, C427S419200, C427S419300, C264S043000

Reexamination Certificate

active

06399528

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. §119 of European Application No. EP 00118972.9, filed on Sep. 1, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety. The present application also incorporates by reference herein in its entirety the disclosure of German Patent Application No. 199 43 075.6, filed on Sep. 3, 1999.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to technical ceramics and to porous aluminum oxide structures and processes for their production. These production processes also relate to those processes used for the production of mesoporous filtration membranes, more coarsely structured intermediate layers or filter substrates, as well as catalytic converter substrates.
2. Discussion of Background Information
Filter modules made of Al
2
O
3
have been available for some time and may typically comprise a construction of several layers having graduated pore sizes. Although coarsely porous ceramic oxide filter substrates usually comprise corundum (&agr;-Al
2
O
3
), and optionally glass as binding agents, predominantly solutions of transitional aluminas (&ggr;-, &dgr;-, or &kgr;-Al
2
O
3
) have been used for the separating layers and the transitional aluminas are usually deposited via a sol/gel process in the mesoporous range of 20-60 nm, which is advantageous for application techniques. The transitional aluminas used in the sol/gel process comprises precursors of Al
2
O
3
. However, a construction of pure corundum is desired due to the more disadvantageous chemical and thermal stability of transitional alumina as compared to &agr;-Al
2
O
3
.
It would be technologically advantageous to produce the more coarsely structured intermediate layers using the sol/gel process, that is, layers intermediate between a separating layer comprising 80 nm particles and the substrate having pore sizes of 1-2 &mgr;m; however, no previously used sol/gel process has been able to produce Al
2
O
3
. structures having a sufficient pore size of more than about 50-100 nm and a sufficiently high porosity of greater than about 30% by vol., and preferably greater than about 40% by vol. Typically, the known sol/gel processes produce only more finely grained, mesoporous structures, which comprise the above-mentioned transitional phases of Al
2
O
3
. When these structures are ignited at high temperatures in order to enlarge the pores, a considerable increase in pore size does not occur until the transition to the thermodynamically stable corundum phase and, therefore, it is connected to a sudden collapse of the porosity to low levels reducing its usability.
Further, powder technologies using Al
2
O
3
powder, instead of precursors of the Al
2
O
3
powder, cannot be used for producing porous layers of the aforementioned type, since layers of powder, which are formed by dip coating in powder slips, have a very high compacting density due to the grain sizes of 0.1-1 &mgr;m in the unsintered state of the powder and do not allow a connection of pores in a desired size range of 100-500 nm with porosities greater than about 40% by vol.
Heretofore, it has not been known how to produce mesoporous Al
2
O
3
structures of high porosity comprising corundum and having an average pore size of 20-60 nm.
Nor has it been known how to use in sol/gel-processes other solutions known for producing sintered, highly porous Al
2
O
3
structures having desired larger pore sizes from about 50 to about 1000 nm.
To make the sol/gel process usable for producing intermediate layers of commercial precursors (such as DISPERAL®, a boehmite made by Condea Chemie, Hamburg, Germany), a considerable material transport must be allowed at high porosity and must remain at a high porosity level during ignition. Since the boehmite has primary particle sizes of 2-7 nm and agglomerate sizes of 30-60 nm, usual annealing conditions will lead to smaller pores as desired here. On the other hand, sol/gel-processes originating from such boehmites are known for small particle sizes and a strong surface curvature of the particles leading to a high sintering activity and enhancing the dense sintering. Within the known theories for solid phase sintering, it cannot be expected, therefore, that it is possible to overcome the above-mentioned problems in the production of mesoporous corundum structures and of structures having pore sizes of 50-1000 nm, while maintaining an evenly high porosity. Thus, according to valid theories, considerable pore growth is related to grain growth which is always parallel to a considerable reduction of residual porosity. Thus, according to Coble, J. Appl. Physics, vol. 32(5), pp.787-792 (1961), which is incorporated by reference herein in its entirety, the sintering process comprises three stages:
An initial stage is characterized by the increase of the sintering necks from zero to an area equivalent to ½ of the cross section area of the particles. The process is accompanied by a small percentage of shrinkage which already represents a reduction of porosity from the originally typical 40-50% (at a relative initial density of 50-60%) to 30-40%, without a considerable growth of the particles even being possible. During annealing, the porosity already reduces in the initial stage of the sintering process, and without any grain or pore increase, to values that mark the limit of usability for many highly porous products.
An intermediate stage begins when the first moderate, grain growth and a change in the shape of the pores start the transformation into a structure having pores and larger amounts of grain limits. The overwhelming amount of porosity is open; during the sintering process, porosity reduction correlates with shrinking cylindrical pore channels with an overall low grain growth. See Johnson, J. A. Ceram. Soc. 53(10), pp. 574-577 (1970), which is incorporated by reference herein in its entirety, who assumes a constant particle size for his model of an intermediate stage. See also Greskovich u.a., J. Am. Ceram. Soc. 55 pp. 142-146 (1972), which is incorporated by reference herein in its entirety, whose measurements show in MgO-doped Al
2
O
3
a grain growth from 300 to 660-850 nm, while simultaneously cutting porosity in half.
A final stage starts with the transformation to closed porosity and corresponds with an increased grain growth and an enlargement of the average pore size, with a considerable reduction of porosity.
SUMMARY OF THE INVENTION
The present invention provides porous aluminum oxide structures comprising &agr;-Al
2
O
3
that, at a high porosity, have mesoporous pore structures having average pore sizes in the range from about 20 to about 60 nm. A mesoporous structure as referred to herein is a structure having a pore size between about 2 to about 60 nm. The present invention also provides porous aluminum oxide structures comprising &agr;-Al
2
O
3
that, at a high porosity, have pore structures in larger average pore sizes up to about 1000 nm. Here, pore sizes are defined as average “effective” pore diameters that result from conventional methods of mercury-porosimetrical measurements. Since real open pore structures cannot have ideal spheric or cylindrical forms, no real “diameter” is present in the pores. Thus, the pore diameter results as the effective value based on known geometrical models.
The porous aluminum oxide structures are producible by powder techniques as well as by sol-gel processes. Additionally, porous aluminum oxide structures are producible over the entire range of pore sizes of &agr;-Al
2
O
3
.
The present invention relates to a porous aluminum oxide structure comprising Al
2
O
3
and Zr, the structure having an open porosity greater than about 30% and an average pore size from about 20 to about 1000 nm, wherein the Zr has a concentration which, expressed as ZrO
2
based on Al
2
O
3
, constitutes less than about 5 weight % of the weight of the Al
2
O
3
.
The porous aluminum oxide structure preferably has an open porosity greater than about 40% and th

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