U.S. patent application number 09/797848 was filed with the patent office on 2002-05-02 for porous aluminum oxide structures and processes for their production.
This patent application is currently assigned to Fraunhofer-Gesellschaft zur Foderung der Angewandt. Invention is credited to Buse, Frank, Krell, Andreas, Ma, Hongwei.
Application Number | 20020052288 09/797848 |
Document ID | / |
Family ID | 8169729 |
Filed Date | 2002-05-02 |
United States Patent
Application |
20020052288 |
Kind Code |
A1 |
Krell, Andreas ; et
al. |
May 2, 2002 |
POROUS ALUMINUM OXIDE STRUCTURES AND PROCESSES FOR THEIR
PRODUCTION
Abstract
The present invention relates to a porous aluminum oxide
structure comprising Al.sub.2O.sub.3 and Zr, the structure having
an open porosity greater than about 30% and an average pore size
from about 20 nm to about 1000 nm, wherein the Zr, expressed as
ZrO.sub.2, has a concentration less than about 5 weight % of the
weight of the Al.sub.2O.sub.3. The present invention also relates
to a process for producing a porous aluminum oxide structure
comprising Al.sub.2O.sub.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, through doping alumina or precursors thereof
with a doping effective amount of Zr, wherein the Zr, expressed as
ZrO.sub.2, has a concentration less than about 5 weight % of the
weight of the Al.sub.2O.sub.3, the process comprising: introducing
the Zr into the alumina or the precursors thereof, the Zr being
selected from at least one ZrO.sub.2 powder, a solution of a Zr
precursor, or mixtures thereof, by admixing the Zr with the alumina
or the precursors thereof, forming a green body, drying the green
body, and sintering the dried body to produce the porous aluminum
oxide structure.
Inventors: |
Krell, Andreas; (Dresden,
DE) ; Buse, Frank; (Dresden, DE) ; Ma,
Hongwei; (Dresden, DE) |
Correspondence
Address: |
GREENBLUM & BERNSTEIN, P.L.C.
1941 ROLAND CLARKE PLACE
RESTON
VA
20191
US
|
Assignee: |
Fraunhofer-Gesellschaft zur
Foderung der Angewandt
Munichen
DE
|
Family ID: |
8169729 |
Appl. No.: |
09/797848 |
Filed: |
March 5, 2001 |
Current U.S.
Class: |
501/105 |
Current CPC
Class: |
C04B 38/00 20130101;
C04B 38/00 20130101; C04B 38/0058 20130101; C04B 35/10 20130101;
C04B 35/48 20130101; C04B 38/0074 20130101; C04B 38/0054
20130101 |
Class at
Publication: |
501/105 |
International
Class: |
C04B 035/10; C04B
035/48 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 1, 2000 |
EP |
EP 00118972.9 |
Claims
What is claimed is:
1. A porous aluminum oxide structure comprising Al.sub.2O.sub.3 and
Zr, the structure having an open porosity greater than about 30%
and an average pore size between about 20 nm to about 1000 nm,
wherein the Zr, expressed as ZrO.sub.2, has a concentration less
than about 5 weight % of the weight of the Al.sub.2O.sub.3.
2. The porous aluminum oxide structure according to claim 1, having
an open porosity greater than about 40% and wherein the
Al.sub.2O.sub.3 comprises .alpha.-Al.sub.2O.sub.3.
3. The porous aluminum oxide structure according to claim 1, having
an average pore size between about 20 nm to about 60 nm.
4. The porous aluminum oxide structure according to claim 1, having
an average pore size between about 50 nm to about 1000 nm.
5. The porous aluminum oxide structure according to claim 1,
wherein the Al.sub.2O.sub.3 comprises .alpha.-Al.sub.2O.sub.3 and
wherein the structure has an open porosity greater than about 40%
and an average pore size between about 20 nm to about 60 nm.
6. The porous aluminum oxide structure according to claim 1, having
an open porosity greater than about 30% and an average pore size
from about 50 to about 1000 nm.
7. The porous aluminum oxide structure according to claim 6, having
an open porosity greater than about 40%.
8. The porous aluminum oxide structure according to claim 1,
wherein the Al.sub.2O.sub.3 comprises .alpha.-A.sub.2O.sub.3 and
the Zr, expressed as ZrO.sub.2, has a concentration from about 0.03
to about 1.5 weight %. of the weight of the Al.sub.2O.sub.3.
9. The porous aluminum oxide structure according to claim 8,
wherein the structure has an open porosity greater than about 40%
and an average pore size from about 20 to about 60 nm.
10. A porous aluminum oxide structure comprising
.alpha.-Al.sub.2O.sub.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, expressed as ZrO.sub.2, has a
concentration less than about 5 weight % of the weight of the
Al.sub.2O.sub.3.
11. The porous aluminum oxide structure according to claim 10,
having an average pore size from about 50 to about 1000 nm.
12. The porous aluminum oxide structure according to claim 10,
having an average pore size from about 20 to about 60 nm.
13. The porous aluminum oxide structure according to claim 12,
wherein the Zr has a concentration which, expressed as ZrO.sub.2
and based on Al.sub.2O.sub.3, constitutes from about 0.03 to about
1.5 weight % of the weight of the Al.sub.2O.sub.3.
14. A process for producing a porous aluminum oxide structure
comprising Al.sub.2O.sub.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, through doping alumina or precursors thereof
with a doping effective amount of Zr, wherein the Zr, expressed as
ZrO.sub.2, has a concentration less than about 5 weight % of the
weight of the Al.sub.2O.sub.3, the process comprising: introducing
the Zr into the alumina or the precursors thereof, the Zr being
selected from at least one ZrO.sub.2 powder, a solution of a Zr
precursor, or mixtures thereof, by admixing the Zr with the alumina
or the precursors thereof, forming a green body, drying the green
body, and sintering the dried body to produce the porous aluminum
oxide structure.
15. The process of claim 14, wherein the sintering takes place at a
temperature from about 700.degree. C. to about 1600.degree. C.
16. The process of claim 15, wherein the sintering takes place at a
temperature from about 850.degree. C. to about 1400.degree. C.
17. The process of claim 16, wherein the sintering takes place at a
temperature from about 900.degree. C. to about 1370.degree. C.
18. The process of claim 14, wherein the introducing the Zr into
the alumina or the precursors thereof comprises admixing the Zr in
the alumina or the precursors thereof.
19. The process of claim 14, wherein the introducing the Zr into
the alumina or the precursors thereof comprises introducing the at
least one ZrO.sub.2 powder into the alumina or the precursors
thereof through milling a suspension of the alumina or the
precursors thereof with ZrO.sub.2 balls.
20. The process of claim 14, wherein the green body is formed by
casting the admixture into a mold.
21. The process of claim 14, wherein the green body is formed by
dip coating a porous substrate to form a membrane layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority under 35 U.S.C.
.sctn. 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
[0002] 1. Field of the Invention
[0003] 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.
[0004] 2. Discussion of Background Information
[0005] Filter modules made of Al.sub.2O.sub.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
(.alpha.-Al.sub.2O.sub.3), and optionally glass as binding agents,
predominantly solutions of transitional aluminas (.gamma.-,
.delta.-, or .kappa.-Al.sub.2O.sub.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.sub.2O.sub.3. However, a construction of pure
corundum is desired due to the more disadvantageous chemical and
thermal stability of transitional alumina as compared to
.alpha.-Al.sub.2O.sub.3.
[0006] 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 .mu.m;
however, no previously used sol/gel process has been able to
produce Al.sub.2O.sub.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.sub.2O.sub.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.
[0007] Further, powder technologies using Al.sub.2O.sub.3 powder,
instead of precursors of the Al.sub.2O.sub.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
.mu.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.
[0008] Heretofore, it has not been known how to produce mesoporous
Al.sub.2O.sub.3 structures of high porosity comprising corundum and
having an average pore size of 20-60 nm.
[0009] Nor has it been known how to use in sol/gel-processes other
solutions known for producing sintered, highly porous
Al.sub.2O.sub.3 structures having desired larger pore sizes from
about 50 to about 1000 nm.
[0010] To make the sol/gel process usable for producing
intermediate layers of commercial precursors (such as
DISPERAL.RTM., 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:
[0011] An initial stage is characterized by the increase of the
sintering necks from zero to an area equivalent to 1/2 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.
[0012] 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.sub.2O.sub.3 a grain growth from
300 to 660-850 nm, while simultaneously cutting porosity in
half.
[0013] 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
[0014] The present invention provides porous aluminum oxide
structures comprising .alpha.-Al.sub.2O.sub.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 .alpha.-Al.sub.2O.sub.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.
[0015] 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 .alpha.-Al.sub.2O.sub.3.
[0016] The present invention relates to a porous aluminum oxide
structure comprising A.sub.2O.sub.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.sub.2 based on Al.sub.2O.sub.3, constitutes
less than about 5 weight % of the weight of the
Al.sub.2O.sub.3.
[0017] The porous aluminum oxide structure preferably has an open
porosity greater than about 40% and the Al.sub.2O.sub.3 comprises
.alpha.-Al.sub.2O.sub.3.
[0018] The porous aluminum oxide structure preferably can have an
average pore size from about 20 to about 60 nm or can have an
average pore size from about 50 to about 1000 nm.
[0019] In another aspect of the present invention, the porous
aluminum oxide structure comprises .alpha.-Al.sub.2O.sub.3 and has
an open porosity greater than about 30% and an average pore size of
from about 20 to about 60 nm, and preferably an open porosity
greater than 40%.
[0020] In yet another aspect of the present invention, the porous
aluminum oxide structure can have an open porosity greater than
about 30% and an average pore size of from about 50 to about 1000
nm, and preferably an open porosity greater than about 40%.
[0021] In a further aspect of the present invention, the porous
aluminum oxide structure comprises .alpha.-Al.sub.2O.sub.3 and the
Zr has a concentration which, expressed as ZrO.sub.2 and based on
Al.sub.2O.sub.3, constitutes less than about 5 weight % of the
weight of the Al.sub.2O.sub.3. The porous aluminum oxide structure
also can have an open porosity greater than about 30% and an
average pore size from about 20 to about 1000 nm, and preferably an
open porosity greater than 40%.
[0022] In a preferred aspect of the present invention, the porous
aluminum oxide structure can have an average pore size from about
50 nm to about 1000 nm, or can have an average pore size from about
20 nm to 60 nm.
[0023] The present invention also relates to a porous aluminum
oxide structure comprising .alpha.-Al.sub.2O.sub.3 and Zr, wherein
the Zr has a concentration which, expressed as ZrO.sub.2 and based
on Al.sub.2O.sub.3, from about 0.03 to about 1.5 weight %of the
weight of the Al.sub.2O.sub.3. The porous aluminum oxide structure
can also have an open porosity greater than about 40% and an
average pore size from about 50 to about 1000 nm.
[0024] The present invention also relates to a process for
producing a porous aluminum comprising Al.sub.2O.sub.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, through doping
alumina or precursors thereof with a doping effective amount of Zr,
wherein the Zr, expressed as ZrO.sub.2, has a concentration less
than about 5 weight % of the weight of the Al.sub.2O.sub.3, wherein
the process comprises: introducing the Zr into the alumina or the
precursors thereof, the Zr being selected from at least one
ZrO.sub.2 powder, a solution of a Zr precursor, or mixtures
thereof, by admixing the Zr with the alumina or the precursors
thereof, forming a green body, drying the green body, and
[0025] sintering the dried body to produce the porous aluminum
oxide structure.
[0026] In the process, the Zr can be introduced into the alumina or
the precursors thereof by admixing the Zr in the alumina or the
precursors thereof.
[0027] In another aspect of the present invention, the Zr can be
introduced into the alumina or the precursors thereof by
introducing the at least one ZrO.sub.2 powder into the alumina or
the precursors thereof through milling a suspension of the alumina
or the precursors thereof with ZrO.sub.2 balls.
[0028] In the process, the green body can be formed by casting the
admixture into a mold. In yet another aspect of the present
invention, the green body can be formed by dip coating a porous
substrate to form a membrane layer.
[0029] The sintering can take place at a temperature from about
700.degree. C. to about 1600.degree. C., preferably from about
850.degree. C. to about 1400.degree. C., and more preferably from
about 900.degree. C. to about 1370.degree. C.
[0030] Other exemplary embodiments and advantages of the present
invention may be ascertained by reviewing the present
disclosure.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0031] The particulars shown herein are by way of example and for
purposes of illustrative discussion of the embodiments of the
present invention only and are presented in the cause of providing
what is believed to be the most useful and readily understood
description of the principles and conceptual aspects of the present
invention. In this regard, no attempt is made to show structural
details of the present invention in more detail than is necessary
for the fundamental understanding of the present invention, the
description taken with the drawings making apparent to those
skilled in the art how the several forms of the present invention
may be embodied in practice.
[0032] The porous aluminum oxide structures according to the
present invention are preferably provided with a Zr dopant.
[0033] The process for producing porous aluminum oxide structures
according to the present invention comprises adding a Zr dopant
during production in such a concentration that it is less than
about 5 weight % expressed as ZrO.sub.2 based on the weight of
Al.sub.2O.sub.3.
[0034] The production process, according to the present invention,
provides porous aluminum oxide structures having an open porosity
greater than about 30%, preferably greater than about 35%, and more
preferably greater than about 40%. Open porosity is defined as
percentage of pores that are not completely closed, i.e., the pores
have passageways connected to each other.
[0035] The production process, according to the present invention,
provides porous aluminum oxide structures having an average pore
size from about 20 nm to about 1000 nm, and more preferably from
about 20 nm to about 500 nm.
[0036] The production process, according to the present invention,
provides porous aluminum oxide structures having a Zr content,
which, expressed as ZrO.sub.2 and based on the weight of
Al.sub.2O.sub.3, is less than about 10 weight % ZrO.sub.2,
preferably less than about 5 weight %, and more preferably less
than about 1.5 weight % ZrO.sub.2.
[0037] The Zr can be incorporated into the structures in the known
manner by using known ZrO.sub.2 materials or precursors. ZrO.sub.2
precursors are those compounds which are hydrolyzable in water and
ultimately form ZrO.sub.2 or optionally hydrated ZrO.sub.2. These
ZrO.sub.2 precursors include, but are not limited to ZrOCl.sub.2,
or Zr(OR).sub.4 where R is alkyl or lower alkyl. Preferably, the
lower alkyl can be from 1 to 7 carbon atoms, either straight chain
or branched alkyl, including isopropyl and isobutyl. More
preferably, R can be CH.sub.3, C.sub.2H.sub.5, C.sub.3H.sub.7 or
C.sub.4H.sub.9. A preferred Zr precursor is aqueous zirconium
oxychloride solution. To incorporate the Zr, the sol/gel processes
as well as the powder technological processes can be used.
[0038] A particularly advantageous high porosity can be maintained
up to high annealing temperatures by using from about 0.03 weight %
to about 1.5 weight % ZrO.sub.2. Smaller additions reduce the
desired effect of stabilization of high porosity in the product,
preferably exceeding 40 vol. %; higher concentrations can cause
undesired side effects, such as the modification of the chemical
behavior of the created products.
[0039] The effect of low Zr dopant (ZrO.sub.2 after conventional
air sintering) noted here cannot be explained and is not equivalent
to any other known effects of this addition during the sintering of
Al.sub.2O.sub.3. JP 62-21750, which is incorporated by reference
herein in its entirety, has reported that higher amounts of
ZrO.sub.2 enhance the sintering of corundum. However, K. Bhatia
u.a., "Pressure Assisted Sintering of Mixtures of Alumina/Zirconia
Powders", 23rd Annual Cocoa Beach Conference, The Am. Ceram. Soc.,
24-29.01.1999, Lecture S1-061-99, which is incorporated by
reference herein in its entirety, has reported that low additions
less than about 1% ZrO.sub.2 impede the sintering of corundum. Both
results only point, in one case, to a general diffusion
accelerating effect, and, in the other case, to a diffusion slowing
effect of different ZrO.sub.2-concentrations. GB 2,071,073 A and
B2, which is incorporated by reference herein in its entirety, is
analogous to one of the countless examples that describe the
impeding effect of low ZrO.sub.2 concentrations to the grain growth
of sintering corundum.
[0040] In contrast thereto, the present invention provides the
conditions for a surprisingly different effect that enables the
ignition with advanced pore growth at an almost constant porosity
(i.e., reduced densification).
[0041] It is believed that Zr or ZrO.sub.2 probably contaminates
the surface of the still highly porous Al.sub.2O.sub.3 in such a
way that diffusion processes driving the densification are largely
decelerated without stopping the leading diffusion processes
necessary for the pore growth. The present invention uses Zr or
ZrO.sub.2 for producing highly-porous Al.sub.2O.sub.3 sintering
products of a defined pore structure in such a way that it is
equally usable for powder technical processes using Al.sub.2O.sub.3
powder as well as for sol/gel processes using precursors of
Al.sub.2O.sub.3.
[0042] It should be noted that the influence of the Zr dopant
and/or the ZrO.sub.2 dopant for the phase transformations to be
performed can obviously be different depending on the existing
conditions of the sol/gel processes. Vereshtshagin et al., Zh.
Prikladnoy Khimii vol. 55(9), pp. 1946-51 (1982), which is
incorporated by reference herein in its entirety, reports that
Zr.sup.4+ has no effect on the corundum yield, i.e., with Zr.sup.4+
as well as without it, 88-90% corundum results at 1250.degree. C.
However, Xue, J. Mater. Sci. Lett. vol. 11(8), pp. 443-445 (1992),
which is incorporated by reference herein in its entirety, finds,
among other things, an aggravated corundum formation with an
increased temperature from 1216.degree. C. to 1291.degree. C. under
the influence of ZrO.sub.2 added as oxychloride.
[0043] On the other hand, the sol/gel process of the present
invention comprising dispersing the above-mentioned boehmite with
Zr and/or ZrO.sub.2 dopant provides a very high yield of corundum
at 1150.degree. C. and, starting at approximately 1200.degree. C.,
only corundum is produced which, in contrast to previously known
results, indicates a transformation accelerating effect of this
type of dopant.
[0044] A particular advantage of the present invention is the fact
that mesoporous aluminum oxide structures comprising corundum and
having intermediate pore sizes from about 20 to about 60 nm can be
produced with high porosity greater than about 30%, and preferably
greater than about 40%.
[0045] Another particular advantage of the present invention is
that the structures having the listed characteristics can be
produced equally well by a process using powders (the Zr compound
is in powder form) as well as by sol/gel processes which use a
hydrolyzable Zr precursor.
[0046] In accordance with the present invention, the sol/gel
processes or the powder processes can produce Al.sub.2O.sub.3
structures with coarser pore structures, i.e., more coarsely
structured intermediate layers or substrates, which are
characterized by an open porosity greater than about 30%, and
preferably greater than about 40%, by average pore sizes from about
50 to about 1000 nm, and preferably from about 100 to about 500 nm,
and by a Zr content of less than 5 weight %, expressed as ZrO.sub.2
and based on the weight of Al.sub.2O.sub.3.
[0047] The production of mesoporous Al.sub.2O.sub.3 structures,
particularly of chemically and thermodynamically highly stable
corundum (.alpha.-Al.sub.2O.sub.3), presents a particular
difficulty in relation to prior art. In the sol/gel-processes, the
formation of corundum requires the maintenance of certain minimum
annealing temperatures, while alternatively, a quick sintering must
be expected when highly sinteractive nanocorundum powder is being
used. This quick sintering is due to the high sintering activity,
which results from the small curvature radius of the particle
surfaces. In both cases, a quick reduction of the open porosity
during ignition (calcination, sintering) occurs. The dopant
according to the present invention counteracts the above and allows
the production of mesoporous structures comprising corundum and
having high porosity. Both sol/gel technologies and powder
technologies can be used in the present invention.
[0048] However, the advantage of a phase structure comprising
corundum need not be specifically stressed for more coarsely
structured intermediate layers or substrates with pore sizes
preferably from about 100 to about 500 nm. Although corundum is
also particularly advantageous here, it is readily employable
within the range of the present invention by using commercial
corundum powder and/or with by igniting sol/gel derived products
having dopant at temperatures above the temperatures of corundum
formation.
[0049] In this regard, in accordance with the present invention,
sol/gel processes are particularly advantageous since such
structures comprising corundum could not be produced using sol/gel
previously.
[0050] However, the advantage for many applications, particularly
of corundum products, does not limit the utilization of the dopant
according to the present invention for other purposes where, e.g.,
Al.sub.2O.sub.3 phases other than corundum might be more suitable
for catalytic converter substrates. It is within the scope of the
present invention to produce Al.sub.2O.sub.3 products having phase
compositions other than corundum, which are stable and have high
porosity and optimal pore sizes which can be controlled.
[0051] In the present invention, the raw materials can include
corundum itself, or corundum doped with Zr and/or ZrO.sub.2 or any
corundum precursor. A corundum precursor is any aluminum compound
which will form corundum. The specification also states that
numerous corundum precursors may be utilized in the present
invention. The precursors include, but are not limited to, aluminum
salts (such as aluminum nitrate, aluminum sulfate, alum, aluminum
chloride and the like), aluminum alcoholate (such as Al(OR).sub.3
where R is alkyl from 1 to 9 and preferably R can be CH.sub.3,
C.sub.2H.sub.5, C.sub.3H.sub.7 or C.sub.4H.sub.9), aluminum
hydroxides, (such as Al(OH).sub.3 or aluminum polyhydroxides),
boehmite, diaspore, transitional alumina (such as .gamma., .delta.,
.theta., or .kappa.-Al.sub.2O.sub.3 as well as corundum itself.
[0052] In accordance with the present invention, the dopant leads
to the maintenance of a high porosity during the annealing up to
the high temperatures necessary for production of porous products.
This results in an additional advantage in that the porous products
can be used in high temperature ranges.
[0053] The present invention is further described in detail for
sol/gel processes and powder technical processes using exemplary
embodiments. Here, the examples of powder technical processes are
described with a broad range of different corundum material having
intermediate particle sizes from about 50 nm to about 1.5
.mu.m.
[0054] The following preferred specific embodiments are, therefore,
to be construed as merely illustrative, and not limitative of the
remainder of the disclosure in any way whatsoever. In the following
examples, all temperatures are set forth uncorrected in degrees
Celsius; unless otherwise indicated, all parts and percentages are
by weight.
EXAMPLES
[0055] EXAMPLE 1
Sol/gel technology; 50-200 nm pore size
[0056] Boehmite (DISPERAL.RTM., a produced by Condea Chemie,
Hamburg, Germany) is used as the raw material for sol/gel
production of more coarsely structured filtration layers. This
boehmite is a colloidal aluminum monohydrate (AlOOH) whose specific
surface is 188 m.sup.2/g determined according to the BET method.
The primary particle size according to TEM images is approximately
2-7 nm while size distributions, measured in suspension, result in
average particle sizes from about 30 to about 60 nm depending on
the agglomeration.
[0057] Using an agitator and ultrasound, 100 g of boehmite is
dispersed for 30 minutes in 180 ml distilled water at pH of about
3; the pH value being adjusted by adding 10% nitric acid. After
adding 2.7 g ZrOCl.sub.2.cndot.8H.sub.2O as a solution in 20 ml
distilled water, the sol is agitated for another hour in the
ultrasound bath and subsequently is poured as a 5 mm thick layer
into a glass shell. After a 12-hour drying in the drying cabinet at
80.degree. C., the resulting gel is broken, granulated, and sifted
into a granulate with a particle size of 1-2 mm. The Zr content
expressed as an oxide is 1.2 weight % ZrO.sub.2 based on the weight
of Al.sub.2O.sub.3.
[0058] The dried samples are sintered in air in a pressure-free
fashion. The heating rate is 3 K/min up to about 550.degree. C. and
subsequently 5 K/min up to the respective sintering temperature,
which is followed by an isothermal holding time of 2 hours. The
open porosity and the pore size is determined by Hg
porosimetry.
[0059] Sintering at final temperatures between 1215 and
1370.degree. C. leads to a quadrupling of the average pore size in
the case of constant open porosity in the range of 40.65.+-.0.59
vol. %; radiographic phase analysis shows that all samples have
completely transformed into corundum (.alpha.-Al.sub.2O.sub.3).
1 Sintering temperature in 1215 1235 1260 1300 1350 1370 .degree.
C. Open porosity in % 41.2 40.6 40.2 40.0 41.5 40.5 Total porosity
in % 42.9 43.5 42.8 41.8 43.9 42.0 Average pore size in nm 56 63 73
96 196 199
[0060] For comparison purposes, an experiment is performed without
the addition of the ZrO.sub.2 but under otherwise identical
conditions. After sintering at 1000.degree. C., the open porosity
is still 49.7% (with an average pore size of 12 nm), but has
already shrunk to only 36% at 1200.degree. C. and a pore size of 13
nm. Further comparison experiments are conducted with dopants, such
as La, Si, Ca, and Mg, which are recommended in U.S. Pat. No.
5,837,634, which is incorporated by reference herein in its
entirety, for preventing decomposition of porosity at higher
temperatures. However, all of these dopants lead constantly to pore
sizes of less than 70 nm at porosities of less than or equal to 30%
and slightly higher porosities of less than or equal to 35% can be
attained with smaller pores less than 15 nm.
EXAMPLE 2
Ceramic with pores of a size of 100-350 nm, produced from coarser
aluminas
[0061] Commercially available aluminas, A16SG.RTM. (Alcoa Company,
USA) having an average grain size approximately 0.5 .mu.m and
CT1200.RTM. (Alcoa Company, USA) having a grain size of
approximately 1.4 .mu.m are dispersed by an agitator and ultrasound
for 30 minutes in distilled water with known dispersing agents
(HNO.sub.3, DOLAPIX.RTM. CE 64 available from Zschimmer &
Schwarz). An additional dispersion milling is performed in a
laboratory agitator ball mill using 3Y-TZP milling balls (Y
partially stabilized ZrO.sub.2, Tosoh Company, Japan) to add
ZrO.sub.2 dopant as abrasion from a milling ball; the running time
is 2 hours with the A16SG compositions having solid material
contents from about 60 to about 75 weight %, while compositions
with 60 weight % of the coarser alumina CT1200 are ground for 4
hours and those with 71 weight % of CT1200 are ground for 1 hour.
In this manner, suspensions having different concentrations of the
ZrO.sub.2 dopant are obtained. An undoped comparison sample of
A16SG alumina is irradiated with ultrasound without grinding for
only one hour.
[0062] All suspensions are poured in a glass shell with a 5 mm
layer thickness and are granulated and sifted as described in
Example 1 after a 24-hour drying at 60.degree. C. The sintering as
well as the determination of porosity and pore size is the same as
in Example 1.
[0063] The table below depicts the results with and without
ZrO.sub.2 dopant with respect to sintering at final temperatures of
1200 and 1300.degree. C. The ZrO.sub.2 dopant products are highly
porous corundum products with enlarged pores, as contrasted to the
ZrO.sub.2-free composition. The value for open porosity is in vol.
% followed by a slash and the value for average pore size in
nm.
2 ZrO.sub.2 concentration (added as abrasion from a milling ball;
Alumina A16SG Alumina A16SG Alumina CT1200 precision .+-. 0.1%)
sintered at 1200.degree. C. sintered at 1300.degree. C. sintered at
1300.degree. C. 0 22.1%/87 nm -- -- 0.8% 46.1%/126 nm 34.2%/121 nm
34.3%/276 nm 2.4% -- 44.6%/146 nm -- 4.5% -- -- 51.0%/315 nm
EXAMPLE 3
Corundum membrane with 50 nm pores, produced from commercial
corundum powder
[0064] Using ultrasound and an agitator, 100 g of fine grain
corundum alumina (TAIMICRON.RTM. TM-DA, obtained from
Bohringer-Taimai, Japan) having an average grain size of
approximately 0.2 .mu.m is dispersed for 30 minutes in 35 ml
distilled water at pH of about 4; the pH value is again adjusted by
adding nitric acid. After adding 2.7 g ZrOCl.sub.2.cndot.8H.sub.2O
as a solution in 5 ml of distilled water, the suspension is
agitated in an ultrasound bath for 2 hours and is poured out as a 5
mm thick layer in a glass shell, is dried for 12 hours at
80.degree. C. and is subsequently granulated and sifted as
described in Example 1. As in Example 1, the Zr content, expressed
as an oxide is 1.2 weight % ZrO.sub.2 based on the weight of
Al.sub.2O.sub.3. The sintering as well as the determination of
porosity and pore size is the same as in Example 1.
[0065] Using a commercial corundum powder as a raw material
guarantees a product structure comprising pure corundum. This has
also been radiographically verified. Using the Zr dopant in
accordance with the present invention results in a consistently
high open porosity and constant pore size over a larger range of
annealing temperatures. At 800.degree. C., the resulting product
has an open porosity of 41.2 vol. % and an average pore size of 48
nm. At 1000.degree. C., the resulting product has an open porosity
of 39.1 vol. % and an average pore size of 49 nm. At 1100.degree.
C., the resulting product has an open porosity of 39.3 vol. % and a
average pore size of 51 nm.
[0066] A comparison experiment without Zr dopant under otherwise
identical conditions results in structures which, despite their
limitation to only smaller attainable pores of less than or equal
to 40 nm, still have a low, unsatisfactory porosity of less than or
equal to 32%.
EXAMPLE 4
Porous alumina ceramic with 50 nm pores, produced from nano
alumina
[0067] A nano alumina powder is used as described in DE 199 22 492;
the particle size is from about 50 to about 100 nm. In order to
prepare the suspension, the alumina powder is dispersed in water at
pH of about 4 by grinding; the dopant according to the present
invention is added by grinding 3Y-TZP milling balls (Y partially
stabilized ZrO.sub.2, Tosoh Company, Japan). The original ground
suspensions have a solid material content of 33 weight %; after 3
hours grinding, ZrO.sub.2 is added by grinding to about 0.6 weight
% (based on the weight Al.sub.2O.sub.3 solids) to the suspension;
the suspension is diluted to about 20 weight %; and, with the pH
value being constantly maintained at about 4.0, the suspension is
agitated for another hour in the ultrasound bath. The suspension is
subsequently poured out in a glass shell as a 2-3 mm thick layer
and is dried for 12 hours at 90.degree. C. A 2-hour isothermal
sintering in air at 1100.degree. C. and 1200.degree. C. follows the
drying.
[0068] The porosity and pore size are determined as in example
1.
[0069] After annealing, the composition with ZrO.sub.2 in
accordance with the present invention exhibits the following pore
structures: At 1100.degree. C., the composition has an open
porosity of 43.7 vol. % (the total porosity being 51.4%) and an
average pore size of 47 nm. At 1200.degree. C., the composition has
an open porosity of 41.1 vol. % (the total porosity being 44.5%)
and an average pore size of 55 nm.
[0070] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention,
and without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
[0071] It is noted that the foregoing examples have been provided
merely for the purpose of explanation and are in no way to be
construed as limiting of the present invention. While the present
invention has been described with reference to an exemplary
embodiment, it is understood that the words which have been used
herein are words of description and illustration, rather than words
of limitation. Changes may be made, within the purview of the
appended claims, as presently stated and as amended, without
departing from the scope and spirit of the present invention in its
aspects. Although the present invention has been described herein
with reference to particular means, materials and embodiments, the
present invention is not intended to be limited to the particulars
disclosed herein; rather, the present invention extends to all
functionally equivalent structures, methods and uses, such as are
within the scope of the appended claims.
* * * * *