U.S. patent application number 11/120165 was filed with the patent office on 2006-02-16 for ceria composition and process for preparing same.
Invention is credited to George P. Fotou, Cheng-Hung Hung, Kenneth C. Koehlert, Joseph D. Smith.
Application Number | 20060034745 11/120165 |
Document ID | / |
Family ID | 34525810 |
Filed Date | 2006-02-16 |
United States Patent
Application |
20060034745 |
Kind Code |
A1 |
Hung; Cheng-Hung ; et
al. |
February 16, 2006 |
Ceria composition and process for preparing same
Abstract
The present invention provides a cerium oxide particulate
composition and a process for preparing a cerium oxide particulate
composition comprising aggregates of approximately spherical
primary particles of cerium oxide. The method involves preparing a
solution of a cerium oxide precursor, aerosolizing the cerium oxide
precursor solution, and heating the aerosol to provide the cerium
oxide particle composition.
Inventors: |
Hung; Cheng-Hung;
(Champaign, IL) ; Smith; Joseph D.; (Owasso,
OK) ; Fotou; George P.; (Champaign, IL) ;
Koehlert; Kenneth C.; (Carlisle, MA) |
Correspondence
Address: |
Cabot Corporation;Law Department
157 Concord Road
Billerica
MA
01821
US
|
Family ID: |
34525810 |
Appl. No.: |
11/120165 |
Filed: |
May 2, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
09715634 |
Nov 17, 2000 |
6887566 |
|
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11120165 |
May 2, 2005 |
|
|
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60165955 |
Nov 17, 1999 |
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Current U.S.
Class: |
423/263 |
Current CPC
Class: |
C01P 2004/62 20130101;
C01P 2006/13 20130101; C01P 2004/32 20130101; C01P 2004/50
20130101; C01P 2004/04 20130101; C09G 1/02 20130101; C01P 2004/45
20130101; B82Y 30/00 20130101; C01P 2004/64 20130101; C09K 3/1409
20130101; C01P 2004/61 20130101; Y10T 428/2982 20150115; C01F
17/235 20200101; Y10T 428/29 20150115 |
Class at
Publication: |
423/263 |
International
Class: |
C01F 17/00 20060101
C01F017/00 |
Claims
1. A method of preparing a cerium oxide particle composition
comprising preparing a solution consisting essentially of a cerium
oxide precursor, converting the cerium oxide precursor solution
into an aerosol having droplets with a diameter of about 100 .mu.m
or less, passing the aerosol through a high temperature reaction
zone so that the cerium oxide precursor is converted to aggregates
consisting essentially of approximately spherical primary particles
of cerium oxide, and recovering the resulting aggregates as a
cerium oxide particle composition.
2. The method of claim 1, wherein the high temperature reaction
zone is a flame.
3. The method of claim 2, wherein the aggregates are a mixture of
aciniform aggregates and cenospherical aggregates.
4. The method of claim 3, wherein about 90% or more (by weight) of
the aggregates are cenospherical aggregates.
5. The method of claim 4, wherein about 95% or more (by weight) of
the aggregates are cenospherical aggregates.
6. The method of claim 5, wherein about 98% or more (by weight) of
the aggregates are cenospherical aggregates.
7. The method of claim 1, wherein the solution of a cerium oxide
precursor is an aqueous solution.
8. The method of claim 7, wherein the aqueous solution comprises
alcohol.
9. The method of claim 8, wherein the alcohol is methanol or
ethanol.
10. The method of claim 7, wherein the solution of a cerium oxide
precursor is acidic.
11. The method of claim 1, wherein the solution of a cerium oxide
precursor is about 5 wt. % or more of the cerium oxide
precursor.
12. The method of claim 11, wherein the solution of a cerium oxide
precursor is about 10 wt % or more of the cerium oxide
precursor.
13. The method of claim 1, wherein the droplets have a diameter of
about 10-100 .mu.m.
14. The method of claim 1, wherein the high temperature reaction
zone has a temperature of about 700-2000 K.
15. The method of claim 14, wherein the high temperature reaction
zone has a temperature of about 700-1100 K.
16. The method of claim 1, wherein the cerium oxide precursor is
selected from the group consisting of cerium acetate, cerium
acetylacetonate, cerium chloride, cerium nitrate, cerium oxalate,
and cerium perchlorate.
17. The method of claim 16, wherein the cerium oxide precursor is
cerium acetate.
18. The method of claim 16, wherein the cerium oxide precursor is
cerium acetylacetonate.
19. The method of claim 1, wherein the primary particles are of
crystalline cerium oxide.
20. The method of claim 19, wherein the crystalline cerium oxide
comprises cubic phase crystalline cerium oxide.
21. The method of claims 1, wherein the primary particles have an
average diameter (by number) of about 30 nm or less.
22. The method of claim 21, wherein the primary particles have an
average diameter (by number) of about 20 nm or less.
23. The method of claim 22, wherein the primary particles have an
average diameter (by number) of about 10 nm or less.
24. The method of claim 1, wherein the aggregates have a density of
about 6 g/cm.sup.3 or more.
25. The method of claim 24, wherein the aggregates have a density
of about 6-7 g/cm.sup.3.
26. The method of claim 3, wherein the cenospherical aggregates
have an average particle diameter (by weight) of about 1-20
.mu.m.
27. The method of claim 26, wherein the cenospherical aggregates
have an average particle diameter (by weight) of about 5-10
.mu.m.
28. The method of claims 3, wherein the aciniform aggregates have
an average particle diameter (by number) of about 500 nm or
less.
29. The method of claim 28, wherein the aciniform aggregates have
an average diameter (by number) of about 200 nm or less.
30. The method of claim 29, wherein the aciniform aggregates have
an average diameter (by number) of about 100 nm or less.
31. The method of claim 1, wherein the aggregates have a surface
area of about 50 m.sup.2/g or more.
32. The method of claim 31, wherein the aggregates have has a
surface area of about 70 m.sup.2/g or more.
33. The method of claim 1, wherein the aggregates are not
calcined.
34. A cerium oxide particulate composition comprising aggregates
consisting essentially of approximately spherical primary particles
of cerium oxide, wherein at least some of the aggregates are
cenospherical aggregates.
35. The composition of claim 34, wherein the remainder of the
aggregates are aciniform aggregates.
36. The composition of claim 35, wherein about 90% or more (by
weight) of the aggregates are cenospherical aggregates.
37. The composition of claim 36, wherein about 95% or more (by
weight) of the aggregates are cenospherical aggregates.
38. The composition of claim 37, wherein about 98% or more (by
weight) of the aggregates are cenospherical aggregates.
39. The composition of claim 34, wherein the primary particles have
an average diameter (by number) of about 30 nm or less.
40. The composition of claim 39, wherein the primary particles have
an average diameter (by number) of about 20 nm or less.
41. The composition of claim 40, wherein the primary particles have
an average diameter (by number) of about 10 nm or less.
42. The composition of claim 34, wherein the aggregates have a
density of about 6 g/cm.sup.3 or more.
43. The composition of claim 42, wherein the aggregates have a
density of about 6-7 g/cm.sup.3.
44. The composition of claim 34, wherein the cenospherical
aggregates have an average diameter (by weight) of about 1-20
.mu.m.
45. The composition of claim 44, wherein the cenospherical
aggregates have an average diameter (by weight) of about 5-10
.mu.m.
46. The composition of claim 35, wherein the aciniform aggregates
have an average diameter (by number) of about 500 nm or less.
47. The composition of claim 46, wherein the aciniform aggregates
have an average diameter (by number) of about 200 nm or less.
48. The composition of claim 47, wherein the aciniform aggregates
have an average diameter (by number) of about 100 nm or less.
49. The composition of claim 34, wherein the aggregates have a
surface area of about 50 m.sup.2/g or more.
50. The composition of claim 49, wherein the aggregates have a
surface area of about 70 m.sup.2/g or more.
51. The composition of claim 1, wherein the primary particles have
a crystallite size of about 1-30 nm.
52. The composition of claim 51, wherein the primary particles have
a crystallite size of about 5-15 nm.
53. The method of claim 9, wherein the solution of a cerium oxide
precursor is acidic.
54. The method of claim 10, wherein the solution of a cerium oxide
precursor contains nitric acid.
55. The method of claim 53, wherein the solution of a cerium oxide
precursor contains nitric acid.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This patent application claims priority to provisional U.S.
Patent Application No. 60/165,955 filed on Nov. 17, 1999.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to a cerium oxide particulate
composition and a process for preparing a cerium oxide particulate
composition.
BACKGROUND OF THE INVENTION
[0003] A substantial demand for cerium oxide compositions has
developed over the last 10-15 years. Cerium oxide compositions are
used in diverse industries such as the automobile and semiconductor
industries. In the automobile industry cerium oxide is included in
catalytic converter coatings where it helps to oxidize incomplete
combustion products. In the semiconductor industry, cerium oxide is
used as an abrasive composition for polishing semiconductor wafers.
Cerium oxide also is used for polishing glass, as an absorber for
ultraviolet light, in cosmetics, in mixtures for petroleum-refining
catalysts, in nickel-hydride batteries, as glass additives, in
structural ceramics, in televisions, as part of oxygen sensors, and
as an iron and steel additive.
[0004] Various methods have been disclosed for the production of
pyrogenic cerium oxide compositions. For example, U.S. Pat. No.
5,851,507 (Pirzada) discloses a method of producing pyrogenic
cerium oxide by processing powdered cerium oxide at very high
temperatures using a plasma arc reactor. However, there continues
to be a need for other cerium oxide compositions and methods for
their production. The present invention provides such a composition
and method. These and other advantages of the present invention, as
well as additional inventive features, will be apparent from the
description of the invention provided herein.
BRIEF SUMMARY OF THE INVENTION
[0005] The present inventive method of preparing a cerium oxide
particle composition comprises preparing a solution consisting
essentially of a cerium oxide precursor, converting the cerium
oxide precursor solution into an aerosol having droplets with a
diameter of about 100 .mu.m or less, heating the aerosol by passing
the aerosol through a high temperature reaction zone so that the
cerium oxide precursor is converted to aggregates consisting
essentially of approximately spherical primary particles of cerium
oxide, and recovering the resulting aggregates as a cerium oxide
particle composition.
[0006] The cerium oxide particulate composition of the present
invention comprises aggregates consisting essentially of
approximately spherical primary particles of cerium oxide, wherein
at least some of the aggregates are cenospherical aggregates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a transmission electron micrograph (40,000.times.)
of a cerium oxide particulate composition produced in accordance
with the present invention.
[0008] FIG. 2 is a scanning electron micrograph (1000.times.)
illustrating the cenospherical aggregates of the cerium oxide
particulate composition produced in accordance with the present
invention.
[0009] FIG. 3 is a schematic representation of a process for
producing cerium oxide particulate compositions in accordance with
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0010] The process of the present invention involves preparing a
solution consisting essentially of a cerium oxide precursor,
converting the solution to an aerosol having droplets of about 100
.mu.m or less, passing the aerosol through a high temperature
reaction zone so that the cerium oxide precursor is converted into
aggregates consisting essentially of (or even consisting of)
approximately spherical primary particles of cerium oxide, and
recovering the resulting aggregates as a cerium oxide particulate
composition. The cerium oxide product prepared according to the
process of this invention is typically substantially free of
contamination. Thus, additional processing steps, such as
calcination, are not generally necessary, although they may provide
certain advantages in some applications as will be appreciated by
those of skill in the art.
[0011] The cerium oxide precursor can be any suitable compound that
can be converted into cerium oxide in accordance with the present
invention. Suitable cerium oxide precursors include cerium
alkoxides, such as cerium isopropoxide, cerium acetate, cerium
acetylacetonate, cerium oxalate, and cerium carboxylate, cerium
nitrate, cerium chloride, cerium perchlorate, and cerium sulfate,
and mixtures thereof. The cerium oxide precursors can be in any of
the various possible hydration states. Cerium acetate and cerium
acetylacetonate are preferred because of their stability and
availability. Use of cerium chloride precursors can result in
cerium oxide particles that contain some amount of chloride. Thus,
in applications where chloride contamination of the cerium oxide
particulate composition can be a problem, it may be necessary to
utilize additional processing steps to minimize or eliminate such
contamination. This may be accomplished using methods generally
known in the art.
[0012] The solution of the cerium oxide precursor can be prepared
in any suitable manner. Generally, the cerium oxide precursor
solution is prepared by mixing a cerium oxide precursor with a
suitable solvent therefor. Suitable solvents include water and
organic solvents. A suitable organic solvent does not leave
residual contaminants in the cerium oxide particulate composition.
As with the cerium oxide precursor, solvents containing chloride
are not desirable where chloride contamination of the cerium oxide
particulate composition can be a problem. The use of a
chloride-free cerium oxide precursor solution avoids the need to
remove chloride from the cerium oxide particulate composition by an
additional calcining step that would add to the cost and complexity
of production.
[0013] Desirably, the solvent is water, alone or in combination
with an organic solvent. A preferred organic solvent for use in
combination with water as the solvent is both volatile and
combustible and improves the aerosol forming ability (i.e., by
reducing the surface tension) of the precursor solution, such as an
alcohol, particularly methanol or ethanol. Generally, the precursor
solution does not contain more than about 10 wt. % of such an
organic solvent in combination water. The organic solvent (e.g.,
alcohol) can be added directly to the feedstock cerium oxide
precursor solution, or can be combined with the precursor solution
at any time prior to reaching the high temperature reaction
zone.
[0014] The cerium oxide precursor can have any suitable
concentration in the solution thereof. A suitable concentration is,
for example, any concentration at which the cerium oxide precursor
can be aerosolized. Higher cerium oxide precursor concentrations
generally are preferred to lower cerium oxide precursor
concentrations in order to maximize production rates. Cerium oxide
precursor concentrations that approach saturation in the solution
thereof are particularly preferred. As will be appreciated by those
of ordinary skill in the art, the saturation point of the cerium
oxide precursor solution will depend upon the particular solvent
and cerium oxide precursor used, as well as external factors such
as pH, temperature and pressure. Thus, in some preparations, the
concentration of the cerium oxide precursor in the solution
typically will be about 5 wt. % or more, preferably about 10 wt. %
or more, more preferably about 12 wt. % or more, and most
preferably about 14 wt. % or more. In other preparations, the
cerium oxide precursor in the solution will be about 20 wt. % or
more, preferably about 30 wt. % or more, most preferably about 40
wt. %.
[0015] The cerium oxide precursor solution can have any suitable
pH, which can be adjusted with any suitable pH adjuster. The cerium
oxide precursor solution preferably has an acidic pH (e.g., pH less
than about 7), more preferably a pH of about 3-6 or even a pH of
about 4-6 (e.g., pH of about 4.5-5.5). Any suitable acid can be
used to adjust the pH of the cerium oxide precursor solution. A
desirable acid adjusts the pH of the precursor solution without
significantly diluting the precursor solution or contaminating it
with compounds that will carry through to the cerium oxide particle
composition. Although a variety of acids are suitable, nitric acid
typically is used. Hydrochloric acid may be used for certain
applications; however, the added chloride can be carried through
into the cerium oxide particulate composition. Thus, in
applications where chloride contamination is a concern, use of
hydrochloric acid may necessitate additional processing steps to
minimize or eliminate such contamination. This may be accomplished
by methods generally known in the art.
[0016] The cerium oxide precursor solution can contain additional
components, such as surfactants. Desirable surfactants reduce the
surface tension of the precursor solution so that aerosols
generated from the solution have smaller droplet sizes.
[0017] After the components of the precursor solution are combined,
the cerium oxide precursor solution desirably is mixed thoroughly,
and any undissolved components and particulate matter are removed
therefrom. The removal of undissolved components and particulate
materials can be accomplished by any suitable means, such as by
filtration.
[0018] The cerium oxide precursor solution is aerosolized by any
suitable means. In general, the precursor solution is used as a
feedstock for an aerosol generator or atomizer, which converts the
solution into a fine aerosol. Any suitable aerosol generator can be
used. Suitable aerosol generators are capable of converting the
precursor solution into an aerosol having an average droplet
diameter or size of about 100 .mu.m or less (e.g., about 10-100
.mu.m or even about 10-50 .mu.m). Suitable aerosol generators
include ultrasonic atomizers, high-pressure atomizers, gas
atomizers, and liquid jet atomizers using cross-current flow
streams. Suitable liquid-jet atomizers are described, for example,
in Ingebo, R. D. and H. H. Foster, NACA TN-4087, 1957, "Drop Size
Distribution for Crosscurrent Breakup of Liquid Jets in Air
Streams" and Weiss, M. A. and L. H. Worsham, ARS J., 29 (4), April,
1959, pp. 252-259, "Atomization in High Velocity Air Streams."
[0019] The aerosol of the cerium oxide precursor solution is passed
through a high temperature reaction zone such as a flame, hot gas
stream, oven, furnace or similar high temperature area Preferably,
the aerosol of the cerium oxide precursor solution is injected
through the flame or into the hot gas stream located downstream of
the flame. The flame can be produced by any suitable source that
can generate sufficient heat to quantitatively convert the aerosol
of the cerium oxide precursor solution to cerium oxide. Suitable
flame sources provide a uniform and highly controlled reaction
environment. Flames having suitable temperatures can be produced,
for example, from fuels such as H.sub.2, CH.sub.4, and
H.sub.2/CH.sub.4 mixtures and oxidants such as air or
oxygen-nitrogen mixtures. The supply of fuel and oxidant to the
flame can be adjusted in known ways by one of skill in the art to
obtain the appropriate reaction temperature. Preferably, the amount
of oxidant is sufficient to provide a ratio of oxidant to cerium
oxide precursor solution of about 10-16 Nm.sup.3/kg.
[0020] The reaction temperature is selected so that the resulting
cerium oxide particulate composition has a suitable surface area.
Some applications require higher surface areas, while for other
applications, lower surface areas are suitable. To obtain higher
surface areas, the reaction temperature desirably is about 700-2000
K. Preferably, the reaction temperature in a production scale
reactor is about 700-1100 K, more preferably 700-925 K. Reaction
temperatures that are much lower can cause the cerium oxide
precursor solution to be incompletely converted to cerium oxide,
which can result in lower surface areas and the introduction of
impurities. Reaction temperatures that are much higher also tend to
generate products having lower surface areas. The reaction
temperature in a flame can be measured by methods known in the art,
such as by thermocouples as described in Hung et al., J. Mater.
Res., 7, 1861-1869 (July 1992).
[0021] When the high temperature reaction zone is provided by a
flame or a hot gas stream, the temperature of the flame or hot gas
stream is, preferably, about 700-2000 K, more preferably about
1100-1900 K, such as about 1200-1400 K. The primary flame
temperature, or temperature of a hot gas stream, can be determined
by any suitable method known in the art. For example, the primary
flame temperature of gas-flames can be calculated from the
mainstream gas flow rate. In small-scale operations, the primary
flame temperature is expected to approximate the reaction
temperature. However, in production scale operations, the primary
flame temperature is expected to be lower than the reaction
temperature.
[0022] As the aerosol of the cerium oxide precursor solution passes
into the high temperature reaction zone, the solvent therein
rapidly evaporates and the cerium oxide precursor is directly
exposed to the reaction temperature. In the combustion process, the
cerium oxide precursor is converted into particles of pure cerium
oxide. The properties of the particles can vary in response to
process parameters (e.g., reaction temperature, aerosol droplet
diameter, precursor composition, precursor concentration, etc.).
Typically, the cerium oxide particulate composition comprises,
consists essentially of, or consists of two distinct particle
morphologies. Some particles are branched, three-dimensional,
chain-like aggregates of essentially spherical primary particles
having an aciniform structure (hereinafter referred to as
"aciniform aggregates") (see, e.g., FIG. 1(10)). Other particles
have an approximately spherical structures (see, e.g., FIG. 1(11)
and FIG. 2) having at least one hole (FIG. 2 (20)) that is visible
via electron microscopy (hereinafter referred to as "cenospherical
aggregates"). Without wishing to be bound by any particular theory,
it is believed that the cenospherical aggregates are hollow and
comprise, consist essentially of, or consist of, primary cerium
oxide particles joined together to form the cenospherical
aggregates. Generally, the method of the present invention can be
used to provide a mixture of cenospherical and aciniform
aggregates.
[0023] The cerium oxide particulate product exits the reaction zone
and is cooled by any suitable means. The product can be cooled
directly, for example, by quenching with a cooling gas or atomized
liquid, and/or indirectly, for example, by passing the product
through cooling tubes. Preferably, the product is quenched about
20-90 ms after passing through the high temperature reaction zone.
After the aggregate particles are cooled, the cerium oxide
particulate product is recovered by any suitable means. For
example, the cerium oxide particulate composition can be separated
from a cooling gas stream using a precipitator, cyclone separator,
bag filter, or other means known to those skilled in the art.
[0024] The cerium oxide particulate composition produced in
accordance with the present invention comprises, consists
essentially of, or consists of aggregates consisting essentially of
approximately spherical primary particles of cerium oxide, wherein
at least some of the aggregates are cenospherical aggregates.
Preferably the aggregates are a mixture of cenospherical and
aciniform aggregates. Preferred cerium oxide particulate
compositions prepared in accordance with the present invention
comprise aggregates at least about 90% (by weight) or more of which
are cenospherical aggregates. More preferably about 95% (by weight)
or more, or even about 98% (by weight) or more of the aggregates
are cenospherical aggregate particles. Typically, the remaining
aggregates (e.g., about 10% (by weight) or less, about 5% (by
weight) or less, or about 2% (by weight) or less of the aggregates)
are aciniform aggregates. It is further believed that the reaction
conditions used to produce the cerium oxide particulate composition
can be varied to change the ratio of cenospherical aggregates to
aciniform aggregates, as desired.
[0025] The force necessary to break the aciniform aggregates is
considerable and often considered irreversible because of the
fusion of those particles. The cenospherical aggregates are friable
and are believed to breakdown into aggregates resembling the
aciniform structure.
[0026] Desirably, no metal oxide other than cerium oxide is present
in the aggregates. Of course, one of skill in the art can
appreciate that allowance is made for the trace amounts of
impurities present in suitable ingredients (e.g., commercially
available ingredients) of the cerium oxide precursor solution.
[0027] As indicated above, the aggregates (e.g., aciniform and
cenospherical aggregates) are each comprised of a large number of
very small primary (generally spherical) particles, which are
nearly uniform in size. The particle size of the primary particles
and aggregate particles can be determined by conventional methods,
for example, by using standard scanning electron microscopy (SEM)
or transmission electron microscopy (TEM) techniques, or by
calculating particle size based on the weight and density of the
particles. Average particle size can be expressed as a function of
the number of particles measured (average particle size "by
number") or as a function of the weight of the particles measured
(average particle size "by weight"). The terms "particle size" and
"particle diameter" are used herein interchangeably to refer to the
spherical diameter of a three-dimensional particle.
[0028] The primary particles are typically about 30 nm or less in
average diameter (by number). In certain preparations, the primary
particles have an average diameter (by number) of about 20 nm or
less, and in other preparations the average primary particle
diameter (by number) is about 15 nm or less, preferably about 10 nm
or less. Thus, the primary particles in a given preparation can
range in size from about 2-100 nm, preferably about 5-50 nm, more
preferably about 5-25 nm. The primary particles typically are
composed primarily of crystalline cerium oxide in the cubic phase
and are nonporous. In preferred preparations, the primary particles
are between 50-99% crystalline, more preferably between about
75-99% crystalline, most preferably between about 90-99%
crystalline. The cerium oxide particulate composition has a
crystallite size ranging from about 1 nm to 30 nm, preferably about
2-20 nm, more preferably about 5-15 nm, as measured by x-ray
diffraction peak broadening. In preferred compositions, each
primary particle consists of a single cerium oxide crystal.
[0029] Preferably, the cerium oxide particulate composition
exhibits a bimodal distribution of cenospherical and aciniform
aggregates. Cenospherical aggregates typically range in size
between about 0.5 and about 20 .mu.m and have an average particle
size (by weight) of about 1-20 .mu.m, preferably about 5-10 .mu.m.
As mentioned, the cenospherical aggregates are believed to be
hollow and can be further characterized by wall structures (e.g.,
microporous or porous wall structures) of between 0.1 and 2 .mu.m
in thickness.
[0030] The aciniform aggregates are typically about 500 nm or less
in average diameter (by number). In certain preparations, the
aciniform aggregates have an average diameter (by number) of about
200 nm or less, and in other preparations the average aciniform
aggregate diameter (by number) is about 100 nm or less.
[0031] The surface area of the cerium oxide particulate composition
generally is related to the size of the primary particles that
comprise the cerium oxide aggregates. Preferred cerium oxide
particulate compositions have a surface area, as calculated from
the method of S. Brunauer, P. H. Emmet, and I. Teller, J. Am.
Chemical Society, 60, 309 (1938), and commonly referred to as BET,
of at least about 10 m.sup.2/g (e.g., at least about 20 m.sup.2/g).
The surface area of the cerium oxide particulate composition
preferably is at least about 50 m.sup.2/g (e.g., about 50-150
m.sup.2/g), more preferably at least about 70 m.sup.2/g (e.g.,
about 70-150 m.sup.2/g). Most preferably, the surface area of the
cerium oxide composition is at least about 80 m.sup.2/g (e.g. about
80-140 m.sup.2/g).
[0032] The density of the cerium oxide particulate composition
typically will be at least about 6 g/cm.sup.3 (e.g., about 6-7
g/cm.sup.3). As used herein, the term "density" refers to true
density and may be measured, for example, using a helium
pycnometer. In some preparations, the density will be at least
about 6.5 g/cm.sup.3, and in certain preparations the density can
be substantially the density of pure cerium oxide (e.g., about 7
g/cm.sup.3).
[0033] While not wishing to be bound to any particular theory, it
is believed that, as a result of being made in a high temperature
reaction zone (e.g., a high temperature flame) from a fine aerosol
precursor solution, the cerium oxide particulate composition of the
present invention has a more stable microstructure and a more pure
form (as compared to, for example, cerium oxides prepared by
certain alternative methods such as wet chemistry processes),
thereby resulting in superior characteristics for many end-uses. In
addition, the cerium oxide particulate composition prepared in
accordance with the present invention generally has minimal
contamination (in many cases, less than 100 ppm impurities), such
that, typically, no additional purification or treatment is
required prior to use of the cerium oxide particulate composition
in many end-uses. Such additional purification or treatment steps,
i.e., heat treatment steps, can increase the cost and complexity of
the process and can have undesirable effects on the product, such
as decreasing the surface area of the ceria particle.
[0034] As those of skill in the art will recognize, the cerium
oxide particulate compositions of the present invention can have
many uses. As mentioned, such compositions can be used in catalytic
converter coatings, as an absorber for ultraviolet light, as a flow
additive or thickening agent, in cosmetics, in mixtures for
petroleum refining catalysts, in nickel-hydride batteries, as a
glass additive, in structural ceramics, in televisions, as part of
oxygen sensors, and as an iron or steel additive. The cerium oxide
of the present invention can also be used as a polishing agent, for
example, to polish substrates such as glass, metal, or ceramic
substrates. The cerium oxide of the present invention can also be
used to polish the surface of semiconductor substrates, for
example, semiconductor substrate surfaces comprising metal (e.g.,
copper, aluminum, tungsten, tantalum, and the like), dielectrics
(e.g., silica, silicon nitrides, and silicon composites), or
mixtures of metals and dielectrics. Generally, when used to polish
such substrates, the cerium oxide composition can be incorporated
into a liquid carrier, such as water or a solution comprising
chemical reagents (e.g., oxidizers, film forming agents, acids,
bases, surfactants, complexing agents, and the like) to form a
slurry that can be used to polish the surface of the substrate
using, for example, a polishing pad. Such slurries are useful, for
example, in conjunction with shallow trench isolation (STI) and
interlevel dielectric layer (ILD) processing of semiconductor
substrates. Other suitable uses for the cerium oxide particulate
composition of the present invention are generally known in the
art.
[0035] The following examples further illustrate the present
invention but, of course, should not be construed as in any way
limiting its scope.
EXAMPLE 1
[0036] This example illustrates a method of preparing a cerium
oxide particulate composition in accordance with the present
invention. A cerium oxide precursor solution, along with combustion
air and fuel, is fed into a high-pressure atomizer. The
high-pressure atomizer comprises a central tube encased in a burner
tube. The central tube extends past the end of the burner tube and
is configured with an outlet restriction nozzle. The cerium oxide
precursor solution passes through the central tube and exits from
the restricting outlet nozzle. As the precursor solution passes
through the tube, it is heated by a burning fuel/air mixture that
passes through the surrounding burner tube. Upon exiting the
restriction nozzle, the solution is converted into an aerosol spray
consisting of suitably sized droplets. The aerosol of the cerium
oxide precursor solution is directed through a flame and is
converted into a cerium oxide particulate composition.
EXAMPLE 2
[0037] This example illustrates an alternative method of preparing
a cerium oxide particulate composition in accordance with the
present invention. A cerium oxide precursor solution is subjected
to gas atomization by use of a gas atomization device having three
concentrically arranged tubes. Either air or a fuel/nitrogen
mixture is passed through the inner tube; the cerium oxide
precursor solution is passed through the middle tube, which is
sandwiched, between the inner and outer tubes, and a burning
fuel/air mixture is passed through the outer tube. The flow rate of
each mixture through the tubes is controlled so that, as the
precursor solution exits the device, it is converted to an aerosol
spray having suitably sized droplets. The aerosol of the cerium
oxide precursor solution is directed through a flame and is
converted into a cerium oxide particulate composition.
EXAMPLE 3
[0038] This example illustrates the effect of flame temperature on
a cerium oxide particulate composition prepared in accordance with
the present invention. A solution of 7 wt. % cerium acetylacetonate
was prepared containing 10 wt. % methanol and 7 wt. % acetic acid
(remaining wt. % water). The solution was aerosolized and passed
through flames at two different temperatures, namely at 1150 K and
1400 K, as calculated based on the main stream gas flow rates. The
solution was fed into the 1150 K flame at a rate of 55 l/min, using
a combustion air flow rate of 250 l/min, hydrogen flow rate of 15
l/min, and natural gas flow rate of 4 l/min. The cerium oxide
particulate composition formed in the reaction temperature of 1150
K had a BET surface area of 35 m.sup.2/g, while the cerium oxide
particulate composition formed in the reaction temperature of 1400
K had a BET surface area of 60 m.sup.2/g.
EXAMPLE 4
[0039] This example illustrates an alternative method of preparing
a cerium oxide particulate composition in accordance with the
present invention, and is further illustrated in the schematic
representation provide by FIG. 3. A cerium oxide precursor solution
(30) is prepared containing 13.5 wt. % cerium acetate
sesquihydrate, 10 wt. % methanol, and 0.4 wt. % nitric acid
(remaining wt. % water). Natural gas (38) and an excess of air (32)
are ignited in a burner (31) to produce a high temperature
flame/gas stream (1200 K). The hot gas stream is accelerated by
passing through a venturi restriction (33), creating a high
temperature, high shear environment. The cerium oxide precursor
solution (30) is introduced into this environment through a liquid
jet stream (34) and is atomized via the high shear in the venturi
(33). A suitable combustion air/cerium oxide precursor solution
ratio is 15 Nm.sup.3 air/kg solution. The high temperature,
combusting environment rapidly evaporates the precursor solution
solvents, allowing for conversion of the cerium oxide precursor to
form the cerium oxide particulate composition as the precursor
passes from the high shear environment of the venturi (33) into the
reactor (35). Measured reaction temperatures after addition of the
cerium oxide precursor solution is approximately 875K. After a
suitable reaction time (25 ms), the reaction mixture (cerium oxide
particulate composition and combustion off gasses) is cooled by
introduction of a finely atomized water spray (36) sufficient to
allow collection of the cerium oxide particulate composition
through the use of a bag filter (37). The cerium oxide particulate
composition was of good quality and had a surface area of 90
m.sup.2/g.
[0040] All of the references cited herein, including patents,
patent applications, and publications, are hereby incorporated in
their entireties by reference.
[0041] While this invention has been described with an emphasis
upon preferred embodiments, it will be obvious to those of ordinary
skill in the art that variations of the preferred embodiments may
be used and that it is intended that the invention may be practiced
otherwise than as specifically described herein. Accordingly, this
invention includes all modifications encompassed within the spirit
and scope of the invention as defined by the following claims.
* * * * *