U.S. patent application number 10/464208 was filed with the patent office on 2004-04-08 for nano-engineered phosphors and related nanotechnology.
Invention is credited to Pfaffenbach, Karl, Yadav, Tapesh.
Application Number | 20040067355 10/464208 |
Document ID | / |
Family ID | 26672930 |
Filed Date | 2004-04-08 |
United States Patent
Application |
20040067355 |
Kind Code |
A1 |
Yadav, Tapesh ; et
al. |
April 8, 2004 |
NANO-ENGINEERED PHOSPHORS AND RELATED NANOTECHNOLOGY
Abstract
Dispersed phosphor powders are disclosed that comprise nanoscale
powders dispersed on coarser carrier powders. The composition of
the dispersed fine powders may be oxides, carbides, nitrides,
borides, chalcogenides, metals, and alloys. Such powders are useful
in various applications such as lamps, cathode ray tubes, field
emission displays, plasma display panels, scintillators, X-ray
detectors, IR detectors, UV detectors and laser detectors.
Nano-dispersed phosphor powders can also be used in printing inks,
or dispersed in plastics to prevent forgery and counterfeiting of
currency, original works of art, passports, credit cards, bank
checks, and other documents or products.
Inventors: |
Yadav, Tapesh; (Longmont,
CO) ; Pfaffenbach, Karl; (Boulder, CO) |
Correspondence
Address: |
HOGAN & HARTSON LLP
ONE TABOR CENTER, SUITE 1500
1200 SEVENTEENTH ST
DENVER
CO
80202
US
|
Family ID: |
26672930 |
Appl. No.: |
10/464208 |
Filed: |
June 18, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10464208 |
Jun 18, 2003 |
|
|
|
10004387 |
Dec 4, 2001 |
|
|
|
6652967 |
|
|
|
|
60310967 |
Aug 8, 2001 |
|
|
|
Current U.S.
Class: |
428/323 ;
428/325 |
Current CPC
Class: |
H01M 8/1246 20130101;
B01J 12/005 20130101; B01J 2219/0018 20130101; C01G 53/006
20130101; H01C 7/112 20130101; C01B 19/007 20130101; C01G 41/02
20130101; H01G 4/33 20130101; B82Y 25/00 20130101; C01P 2006/12
20130101; H01M 4/9066 20130101; Y02P 70/50 20151101; C01B 3/001
20130101; C01B 21/062 20130101; C08K 2201/013 20130101; C04B 35/00
20130101; H01M 4/90 20130101; B01J 23/08 20130101; B82Y 5/00
20130101; H01G 4/12 20130101; B82Y 30/00 20130101; C09K 11/7774
20130101; H01F 1/0045 20130101; B01J 2219/00135 20130101; B29C
70/58 20130101; H01M 8/1253 20130101; B01J 2219/00094 20130101;
C01F 11/06 20130101; B29B 2009/125 20130101; C01P 2002/60 20130101;
C01P 2004/51 20130101; C01B 32/991 20170801; C04B 41/89 20130101;
Y10T 428/25 20150115; C01B 32/956 20170801; C01P 2006/40 20130101;
B01J 19/24 20130101; B22F 9/12 20130101; C01B 13/145 20130101; B01J
2219/0894 20130101; C01P 2006/10 20130101; C01G 23/006 20130101;
Y02E 60/50 20130101; Y10T 428/2991 20150115; B01J 37/18 20130101;
B29B 9/12 20130101; Y02E 60/32 20130101; B01J 12/02 20130101; C08K
2201/011 20130101; A61L 27/06 20130101; C01F 5/06 20130101; C01B
35/04 20130101; C04B 41/52 20130101; C04B 2/10 20130101; Y10T
428/252 20150115; B01J 2219/00177 20130101; B82B 1/00 20130101;
C04B 20/0004 20130101; C01P 2002/72 20130101; B29B 9/08 20130101;
B01J 35/0013 20130101; B22F 1/0003 20130101; B22F 2999/00 20130101;
H01M 4/02 20130101; B01J 23/002 20130101; B01J 23/14 20130101; C01P
2004/64 20130101; H01M 8/1213 20130101; C04B 35/62222 20130101;
B22F 2999/00 20130101; B22F 9/12 20130101; B22F 2202/13 20130101;
B22F 2999/00 20130101; B22F 1/0003 20130101; B22F 1/054 20220101;
B22F 2999/00 20130101; B22F 1/0003 20130101; B22F 1/054
20220101 |
Class at
Publication: |
428/323 ;
428/325 |
International
Class: |
B32B 005/16 |
Claims
We claim:
1. A phosphor composition comprising dispersed powder comprising: a
sub-micron carrier particle having a first composition; and
nanoscale particles of a second composition that are dispersed on
and attached to the surface of the carrier particle in a
mechanically stable state, wherein the attached particles are
smaller than the carrier particle and wherein at least two
neighboring attached particles on the surface of said carrier
particle do not touch each other at 300 Kelvin.
2. The composition of matter of claim 1, wherein the distance
between the at least two neighboring attached particles on the
surface of the carrier that do not touch each other is at least 2
Angstroms.
3. The composition of matter of claim 1, wherein said distance is
greater than 5 Angstroms.
4. The composition of matter of claim 1, wherein said distance is
greater than 10 Angstroms.
5. The composition of matter of claim 1, wherein said distance is
greater than 50 Angstroms.
6. The composition of matter of claim 1, wherein the ratio of the
average diameter of the carrier particles and the average diameter
of the attached particles is greater than or equal to 2.
7. The composition of matter of claim 1, wherein said ratio is
greater than 10.
8. The composition of matter of claim 1, wherein the surfaces of
the attached particle and carrier particle interact physically,
chemically, electrochemically.
9. The composition of matter of claim 1, wherein the carrier
particle comprise nanoscale particle.
10. The composition of matter of claim 1, wherein the composition
of the attached particle is selected from the group consisting of
rare earths, metals, and alloys.
11. The composition of matter of claim 1, wherein the attached
particle comprises a multi-metal composition.
12. The composition of matter of claim 1, wherein a composition of
the carrier particle is selected from the group consisting of
oxides, sulfides, aluminate, silicate, halide and rare earth
comprising compositions.
13. The composition of matter of claim 1, wherein the phosphor
comprises a Stoke phosphor.
14. The composition of matter of claim 1, wherein the phosphor
comprises an anti-Stoke phosphor.
15. A product containing the composition of matter of claim 1.
16. The composition of matter of claim 1, wherein the carrier
particles have an aspect ratio greater than 1.
17. A phosphor comprising dispersed powder comprising: a carrier
particle having a first composition; nanoscale particles of a
second composition that are dispersed on and attached to a surface
of the carrier particle in a mechanically stable state, wherein the
attached particles are smaller than the carrier particle and
wherein at least two neighboring attached particles on the surface
of said carrier particle do not touch each other at 300 Kelvin;
wherein the carrier particles have a size less than 25 microns;
wherein the dispersed particles have a size less than 100
nanometers; wherein the first composition is selected from the
group consisting of oxides, chalcogenides, borides, carbide,
nitrides, metals, alloys and polymers, wherein the second
composition is a phosphor powder; and wherein the dispersed powder
prepared from the carrier particles and the dispersed particles
exhibits a light emitting efficiency that is higher by at least 5%
than the phosphor powder that is not dispersed.
18. A product containing the phosphor of claim 17.
Description
RELATED APPLICATIONS
[0001] The present application is a divisional of Copending U.S.
patent application Ser. No. 10/004,387 filed on Dec. 4, 2001
entitled "Nano-dispersed powders and methods for their manufacture"
which claims the benefit of provisional application No. 60/310,967
filed Aug. 8, 2001 all of which are assigned to the assignee of the
present invention and which are incorporated herein by
reference.
[0002] The present application is also a divisional of copending
U.S. patent application Ser. No. 10/150,722 filed on May 17, 2002
entitled "Nanotechnology for Inks and Dopants" which claims the
benefit of provisional application No. 60/111,442 filed Dec. 8,
1998 and is a divisional of U.S. patent application Ser. No.
09/274,517 filed on Mar. 23, 1999 entitled "MATERIALS AND PRODUCTS
USING NANOSTRUCTURED NON-STOICHIOMETRIC SUBSTANCES" now U.S. Pat.
No. 6,344,271 which claims the benefit of provisional application
No. 60/107,318, filed Nov. 6, 1998, entitled "Materials and
Products Using Nanostructured Non-stoichiometric Materials," all of
which are assigned to the assignee of the present invention and
which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates, in general, to nano-dispersed
phosphor powders, and, more particularly, to nano-dispersed,
complex composition fine powders and methods to produce such
powders.
[0005] 2. Background of the Invention
[0006] Powders are used in numerous applications. They are the
building blocks of catalytic, electronic, telecommunication,
electrical, magnetic, structural, optical, biomedical, chemical,
thermal and consumer goods. On-going market demands for more
efficient, reliable, smaller, faster, superior and more portable
products have demanded miniaturization of numerous products. This,
in turn, has demanded miniaturization of the building blocks, i.e.
the powders. Sub-micron and nanoscale (or nanosize, ultra-fine)
powders, with a size 10 to 100 times smaller than conventional
micron size powders, enable quality improvement and differentiation
of product characteristics at scales currently unachievable by
commercially available micron-sized powders.
[0007] Nanopowders in particular, and sub-micron powders in
general, are a novel family of materials whose distinguishing
features include that their domain size is so small that size
confinement effects become a significant determinant of the
materials' performance. Such confinement effects can, therefore,
lead to a wide range of commercially important properties.
Nanopowders, therefore, are an extraordinary opportunity for
design, development and commercialization of a wide range of
devices and products for various applications. Furthermore, since
they represent a whole new family of material precursors where
conventional coarse-grain physiochemical mechanisms are not
applicable, these materials offer unique combination of properties
that can enable novel and multifunctional components of unmatched
performance. Bickmore, et al. in U.S. Pat. No. 5,984,997, which
along with the references contained therein is incorporated herein
by reference, teach some applications of sub-micron and nanoscale
powders.
[0008] Conventional dispersed powders comprise powders of a first
composition (e.g. metal) dispersed on the surface of a carrier
which may be of a second composition (e.g. carbon). The dispersed
powder structure enables greater and more effective availability of
the first composition. It also provides a cost reduction because
the second composition can be a low-cost carrier. Additionally, the
dispersed powder structure improves the stability and enhances the
performance synergistically.
[0009] Dispersed powders are desired in a number of applications
such as catalysis. The junctions provide active sites for useful
chemical reactions. Dispersed powders are often produced using
chemical precipitation techniques. These techniques fail to provide
a fine and uniform distribution of the dispersed particles on the
surfaces of the carrier. Furthermore, the challenge becomes even
more difficult when complex compositions need to be dispersed on a
carrier powder. Chemical precipitation techniques also leave
chemical residues on the surfaces that sometimes are not desirable.
Given the difficulty in their production, few dispersed powders are
known in the literature and these have found only limited
applications.
[0010] Phillips in U.S. Pat. No. 5,989,648 (which, along with its
references, is specifically incorporated herein by reference)
teaches a plasma-based method for preparing metal supported
catalysts from an aerosol comprising a mixture of at least one
metal powder and at least one support powder. Phillips reports the
unusual benefits as catalysts of the metal supported powders so
prepared. However, Phillips does not offer motivation for or
methods of utilizing fluid precursors to form dispersed powders.
Phillips also does not teach nano-dispersed sub-micron powders,
motivations for their use, or their benefits to various
applications.
SUMMARY OF THE INVENTION
[0011] Briefly stated, the present invention involves
nano-dispersed powders comprising powders that have been
morphologically engineered. More specifically, the term
nano-dispersed powders according to this invention refers to
powders that have been arranged to provide a desired morphological
distribution (dispersion) at nanoscale levels (e.g., sub-100 nm
levels). As described in the definition section, nano-dispersed
powders comprise carrier particles and attached particles dispersed
on the surface of the carrier particles.
[0012] The carrier particles may be spherical, non-spherical,
porous, tubular, planar, crystallites, amorphous, or any other
useful form. The nanoparticles may similarly be one-dimensional,
two-dimensional, or three-dimensional, spherical, non-spherical,
porous, tubular, planar, crystallites, or amorphous forms, or any
other useful form. The attached nano-dispersed particles may be
free flowing, agglomerated, porous, coated, or hollow forms or any
other useful form. The same carrier may have nanoparticles of more
than one composition attached to its surface. In addition, various
nano-dispersed particles of different compositions may be blended
to achieve useful compositions.
[0013] The invention provides nano-dispersed powders with unusually
engineered morphology. The unusual morphology provides a high
density of multi-phasic points (i.e. points where two or more
distinct phases interact with each other and/or species in the gas
phase). These morphologically engineered nano-dispersed powders
offer benefits to numerous applications. Some illustrative, but
non-limiting applications include (a) catalytic transformation of
less valuable chemicals and material feed stocks into more valuable
chemicals and materials; (b) catalytic transformation of more
hazardous chemicals and materials into less hazardous or
non-hazardous forms of substances; (c) unusual phosphor, photonic,
and optical materials for display, photonic, and optical
applications; (d) unusual carriers, tracers, drug delivery
vehicles, and markers for biomedical and genomic applications; (e)
unusual building blocks for batteries, sensors, and electrochemical
products; (f) fillers for polymers, ceramics, and metal matrix
composites; and (g) dopants for electronic, magnetic, thermal,
piezo, electrical, tooling, structural, inks, paints, and topical
health products.
[0014] The concept of dispersed powders disclosed and their methods
of manufacture may be applied to produce commercially useful
submicron and micron dispersed powders as well.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows an example of a sub-micron powder comprising
nanopowders discretely dispersed on and attached to the surface of
the submicron powder.
[0016] FIG. 2 shows an example of a nanotube carrier having
nanoparticles dispersed on and attached to its surface, wherein at
least two of the nanoparticles are not in contact with each
other.
[0017] FIG. 3 shows one embodiment for producing nano-dispersed
powders by combining a metal precursor and carrier particles.
[0018] FIG. 4 shows a schematic concentric flame approach to
improve the uniformity of particle size distribution.
[0019] FIG. 5 shows an alternate embodiment for producing
nano-dispersed particles in which both the nano-sized powders and
the carrier particles are prepared in-situ during the thermal
processing.
[0020] FIG. 6 shows an alternate embodiment for producing
nano-dispersed powders by combining a metal precursor and carrier
particles.
[0021] FIG. 7 shows an alternate embodiment for producing
nano-dispersed powders by combining a metal precursor and carrier
particles.
[0022] FIG. 8 shows an alternate embodiment for producing
nano-dispersed powders by combining a metal precursor and carrier
particles.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention is directed to dispersed powders in
general and dispersed nanoscale powders in particular. In a broad
sense, dispersed powders provide a structure having a particle size
that is largely determined by the size of a carrier particle, and
surface behavior that is largely determined by dispersed particles
attached to the carrier particle. This somewhat oversimplifies
dispersed particle structures in that both the size and ultimate
surface behavior may be affected by each component, however the
simplification is useful for understanding. With respect to
dispersed nanoscale powders in particular, the composite structure
can be engineered to have some benefits (e.g., cost, material
handling, and the like) associated with larger particle sizes while
exhibiting behaviors, particularly surface-related behaviors, of
the nanoscale powders dispersed on the carrier.
[0024] FIGS. 1 and 2 show two non-limiting examples of
nano-dispersed sub-micron powders and nano-dispersed nanopowders,
respectively. For example, FIG. 1 shows an example of a sub-micron
powder comprising nanopowders 200 discretely dispersed on and
attached to the surface of a submicron carrier 102. By "discretely"
it is meant that the particles 200 do not touch or overlap. In one
sense means particles do not physically overlap. In another sense
means that they are sufficiently separate that the solid states of
atoms within adjacent particles 200 have a level of interaction
determined by their separation. FIG. 2 shows an example of a
nanotube carrier 203 having nanoparticles 200 dispersed on and
attached to its surface, wherein at least two of the nanoparticles
are not in contact with each other.
[0025] Definitions
[0026] Certain terms used to describe the invention herein are
defined as follows:
[0027] "Fine powders" as used herein, refers to powders that
simultaneously satisfy the following criteria:
[0028] (1) particles with mean size less than 100 microns,
preferably less than 10 microns; and
[0029] (2) particles with aspect ratio between 1 and 1,000,000.
[0030] "Submicron powders" as used herein, refers to fine powders
that simultaneously satisfy the following criteria:
[0031] (1) particles with mean size less than 1 micron; and
[0032] (2) particles with aspect ratio between 1 and 1,000,000.
[0033] The terms "dispersed powders," "morphologically-engineered
powders," "decorated powders," and "surface dispersed powders" are
used interchangeably and refer to powders that simultaneously
satisfy the following criteria:
[0034] (1) they comprise at least a first composition that serves
as a carrier particle;
[0035] (2) they comprise particles of at least a second composition
that are attached to the surface of the carrier particle in a
mechanically stable state, where the second composition can be the
same as or different from the first composition;
[0036] (3) the surfaces of the attached particle and carrier
particle interact physically, chemically, or electrochemically, but
the attached particles exhibit properties that are distinct from
the carrier particles;
[0037] (4) at least two neighboring attached particles on the
surface of the carrier are not in contact with each other at
ambient temperature (300 K);
[0038] (5) the average separation distance between the center of
gravity of the at least two neighboring attached particles on the
surface of the carrier that are not in contact with each other is
at least 1.05 times the average diameter of the attached particles,
preferably greater than 2.5 times the average diameter of the
attached particles, more preferably greater than 5 times the
average diameter of the attached particles, and most preferably
greater than 10 times the average diameter of the attached
particles; and (6) the attached particle is smaller than the
carrier particle. More particularly, the ratio of the average
diameter of the carrier particles and the average diameter of the
attached particles is greater than or equal to 2, preferably
greater than 10, more preferably greater than 25, and most
preferably greater than 100. In one embodiment, the carrier powder
is less than 1000 microns, preferably less than 100 microns, more
preferably 10 microns, and most preferably 1 micron.
[0039] The terms "nanopowders," "nanosize powders," and "nanoscale
powders" are used interchangeably and refer to fine powders that
simultaneously satisfy the following criteria:
[0040] (1) particles having a mean size less than 250 nanometers,
preferably less than 100 nanometers; and
[0041] (2) particles with an aspect ratio between 1 and 1,
000,000.
[0042] "Pure powders" as used herein, refers to powders that have a
composition purity of at least 99.9%, preferably 99.99% by metal
basis.
[0043] "Nano-dispersed powders" as used herein refers to dispersed
powders in which the attached particle is a nanopowder.
[0044] "Nano-dispersed sub-micron powders" as used herein refers to
dispersed powders in which the attached particle is a nanopowder
and the carrier particle is a sub-micron powder.
[0045] "Nano-dispersed nanopowders" as used herein refers to
dispersed powders where the attached particle is a nanopowder and
the carrier particle is also a nanoscale powder.
[0046] The terms "powder," "particle," and "grain" are used
interchangeably and encompass oxides, carbides, nitrides, borides,
chalcogenides, halides, metals, intermetallics, ceramics, polymers,
alloys, and combinations thereof. The term includes single metal,
multi-metal, and complex compositions. These terms further include
hollow, dense, porous, semi-porous, coated, uncoated, layered,
laminated, simple, complex, dendritic, inorganic, organic,
elemental, non-elemental, composite, doped, undoped, spherical,
non-spherical, surface functionalized, surface non-functionalized,
stoichiometric, and non-stoichiometric forms or substances.
Further, the term powder in its generic sense includes
one-dimensional materials (fibers, tubes), two-dimensional
materials (platelets, films, laminates, planar), and
three-dimensional materials (spheres, cones, ovals, cylindrical,
cubes, monoclinic, parallelolipids, dumbbells, hexagonal, truncated
dodecahedron, irregular shaped structures, etc.).
[0047] The term "aspect ratio" refers to the ratio of the maximum
to the minimum dimension of a particle.
[0048] The definitions provided above are intended to be applied in
the interpretation and understanding of the present invention, and
are not necessarily applicable to interpretation of prior art and
conventional processes. Some inventive features of the present
invention are implicitly expressed in the definitions provided
above, and are not to be interpreted as admissions that the defined
term is prior art. To the extent these definitions are inconsistent
with or more specific than a similar term used in the prior art, it
is to be understood that the definition provided herein is
preferred in the interpretation of the invention.
[0049] The present invention is directed to dispersed powders in
general and dispersed nanoscale powders in particular. Dispersed
powders preferably simultaneously satisfy the following
criteria:
[0050] (1) they comprise a carrier particle with at least a first
composition;
[0051] (2) they comprise particles of at least a second composition
that are dispersed on and attached to the surface of the carrier
particle in a mechanically stable state (i.e., sufficiently
attached to prevent undesired physical mobility during normal use),
where the composition of the attached particles may be the same as
or different than the carrier particle;
[0052] (3) the surfaces of the attached particle and carrier
particle interact physically, chemically, or electrochemically with
each other, but the attached particles exhibit properties (e.g.,
electrical properties, chemical properties, solid state properties,
size-confinement properties, surface properties and/or the like)
that are distinct from the carrier particle;
[0053] (4) at least two neighboring attached particles on the
surface of the carrier are not in contact with each other at
ambient temperature (300 Kelvin);
[0054] (5) the average separation distance between the center of
gravity of the at least two neighboring attached particles that are
not in contact with each other is at least 1.05 times the average
diameter of the attached particles, preferably greater than 2.5
times the average diameter of the attached particles, more
preferably greater than 5 times the average diameter of the
attached particles, and most preferably greater than 10 times the
average diameter of the attached particles; and
[0055] (6) the attached particle is smaller than the carrier
particle. More particularly, the ratio of the average diameter of
the carrier particles and the average diameter of the attached
particles is greater than or equal to 2, preferably greater than
10, more preferably greater than 25, and most preferably greater
than 100.
[0056] In one embodiment, the carrier particle is a ceramic
composition (oxide, carbide, nitride, boride, chalcogenide) or an
intermetallic composition (aluminide, silicide) or an elemental
composition. Examples of ceramic composition include, but are not
limited to (a) simple oxides such as aluminum oxide, silicon oxide,
zirconium oxide, cerium oxide, yttrium oxide, bismuth oxide,
titanium oxide, iron oxide, nickel oxide, zinc oxide, molybdenum
oxide, manganese oxide, magnesium oxide, calcium oxide, and tin
oxide; (b) multi-metal oxides such as aluminum silicon oxide,
copper zinc oxide, nickel iron oxide, magnesium aluminum oxide,
calcium aluminum oxide, calcium aluminum silicon oxide, indium tin
oxide, yttrium zirconium oxide, calcium cerium oxide, scandium
yttrium zirconium oxide, barium titanium oxide, barium iron oxide
and silver copper zinc oxide; (c) doped oxides such as zirconium
doped cerium oxide, antimony doped tin oxide, boron doped aluminum
oxide, phosphorus doped silicon oxide, and nickel doped iron oxide;
(d) carbides such as silicon carbide, boron carbide, iron carbide,
titanium carbide, zirconium carbide, hafnium carbide, molybdenum
carbide, and vanadium carbide; (e) nitrides such as silicon
nitride, boron nitride, iron nitride, titanium nitride, zirconium
nitride, hafnium nitride, molybdenum nitride, and vanadium nitride;
(f) borides such as silicon boride, iron boride, titanium diboride,
zirconium boride, hafnium boride, molybdenum boride, and vanadium
boride; (g) complex ceramics such as titanium carbonitride,
titanium silicon carbide, zirconium carbonitride, zirconium
carboxide, titanium oxynitride, molybdenum oxynitride, and
molybdenum carbonitride; and (h) non-stoichiometric ceramics. Other
preferred specifications for the carrier particles are provided in
Table 1.
1TABLE 1 Specifications for the carrier particles Parameter Desired
Range Preferred Range Average particle 5 nm-5 mm 50 nm-5 microns
size Standard deviation 1 nm-10 micron 1 nm-1000 nm of the Size
distribution Purity, by wt % Dependant on the >99.99% needs of
the application and cost (normally, greater than 90%) Surface Area
>1 m.sup.2/gm >10 m.sup.2/gm XRD crystallite Amorphous, 1 nm
to <1000 nm size >1 micron Porosity Dependant on the High
needs of the application and cost Composition Ceramics, Single
metal and elements, alloys multi-metal oxide ceramics
[0057] Preferably, the dispersed particles that are attached to the
carrier particle are elemental, ceramic, intermetallic or polymer
compositions. The composition of the attached particles can be the
same as or different than the composition of the carrier particle.
The particles are preferably separated from each other either
uniformly or non-uniformly across the surface of the carrier
particle. In a particular example, the distance between two
neighboring attached particles on the surface of the carrier that
do not touch each other is at least 2 Angstroms, but may be greater
than 5 Angstroms, 10 Angstroms, 50 Angstroms or more to meet the
needs of a particular application.
[0058] Examples of elemental compositions for the dispersed,
attached particles include, but are not limited to, (a) precious
metals such as platinum, palladium, gold, silver, rhodium,
ruthenium and their alloys; (b) base and rare earth metals such as
iron, nickel, manganese, cobalt, aluminum, copper, zinc, titanium,
samarium, cerium, europium, erbium, and neodymium; (c) semi-metals
such as boron, silicon, tin, indium, selenium, tellurium, and
bismuth; (d) non-metals such as carbon, phosphorus, and halogens;
(e) clusters such as fullerenes (C.sub.60, C.sub.70, C.sub.82),
silicon clusters, and nanotubes of various compositions; and (f)
alloys such as steel, shape memory alloys, aluminum alloys,
manganese alloys, and superplastic alloys.
[0059] Examples of ceramic compositions for the dispersed, attached
particles include, but are not limited to, (a) simple oxides such
as aluminum oxide, silicon oxide, zirconium oxide, cerium oxide,
yttrium oxide, bismuth oxide, titanium oxide, iron oxide, nickel
oxide, zinc oxide, molybdenum oxide, manganese oxide, magnesium
oxide, calcium oxide, and tin oxide; (b) multi-metal oxides such as
aluminum silicon oxide, copper zinc oxide, nickel iron oxide,
magnesium aluminum oxide, calcium aluminum oxide, calcium aluminum
silicon oxide, indium tin oxide, yttrium zirconium oxide, calcium
cerium oxide, scandium yttrium zirconium oxide, barium titanium
oxide, and silver copper zinc oxide; (c) doped oxides such as
zirconium doped cerium oxide, antimony doped tin oxide, boron doped
aluminum oxide, phosphorus doped silicon oxide, and nickel doped
iron oxide; (d) carbides such as silicon carbide, boron carbide,
iron carbide, titanium carbide, zirconium carbide, hafnium carbide,
molybdenum carbide, and vanadium carbide; (e) nitrides such as
silicon nitride, boron nitride, iron nitride, titanium nitride,
zirconium nitride, hafnium nitride, molybdenum nitride, and
vanadium nitride; (f) borides such as silicon boride, iron boride,
titanium diboride, zirconium boride, hafnium boride, molybdenum
boride, and vanadium boride; (g) complex ceramics such as titanium
carbonitride, titanium silicon carbide, zirconium carbonitride,
zirconium carboxide, titanium oxynitride, molybdenum oxynitride,
and molybdenum carbonitride; and (h) non-stoichiometric
ceramics.
[0060] The nano-dispersed powders of this invention may further
comprise carrier particles having dispersed particles of more than
one composition dispersed on and attached to their surfaces. In
addition, the dispersed powders may comprise multiple layers of the
attached particles, where the layers may be concentric or
non-concentric. Other preferred specifications for the carrier
particles are provided in Table 2.
2TABLE 2 Specifications for dispersed, attached particles Parameter
Desired Range Preferred Range Average particle Less than 5 micron 1
nm-250 nm size Standard deviation 1 nm-750 nm 1 nm-50 nm of the
Size distribution Purity, by wt % Dependant on the >99.99% needs
of the application and cost (normally, greater than 90%) Surface
Area >1 m.sup.2/gm >100 m.sup.2/gm XRD crystallite Amorphous,
1 nm to <250 nm size 1 micron Mechanical Dependant on the High
Stability needs of the application and cost
[0061] The distinctive features that make nano-dispersed powders of
this invention commercially desirable result in part from (a) the
separation between the attached nanoparticles during their use, (b)
the unusual properties of attached nanoparticles, (c) the useful
interaction between the carrier composition and the dispersed
attached particles, and (d) the morphologically induced interaction
of dispersed attached particle interfaces and the carrier particle
interface with the chemical, electromagnetic, electrochemical,
photonic, magnetic, charges, and thermodynamic environment around
the dispersed particles.
[0062] More specifically, the distinct usefulness of nano-dispersed
powders is in part a result of the separation between the dispersed
nanoparticles attached to the surface of the carrier particle,
which in turn reduces the potential sintering of the particles at
higher temperatures. It is known in the art that closely packed
small particles in general, and nanoscale particles in particular,
sinter faster as the temperature of use increases. This limits the
time during which the useful performance of the particle is
available. Many applications, particularly those that operate at
high temperatures (e.g. catalysis), require that the surface and
bulk properties of the material in use do not vary or that they
vary only slightly with time. This is difficult to accomplish with
closely packed nanoparticles, because such nanoparticles sinter
(diffuse and grow) across the grain boundaries as a function of
temperature and time. By dispersing the nanoparticles on the
surface of the carrier particle, the surfaces of the dispersed
nanoparticles are kept from touching each other. This reduces or
eliminates the interaction and consequent sintering between the
nanoparticles, even at high temperatures. As a result, the
interaction at the grain boundary is eliminated, and consequently
the time and temperature based variances are eliminated. Thus,
dispersing the nanoparticles solves an outstanding problem that
confronts attempts to utilize the beneficial properties of
nanoscale powders.
[0063] The distinct usefulness of nano-dispersed powders is also in
part a result of the unusual inherent properties of nano-scaled
particles. Nano-scaled materials are a family of materials whose
distinguishing feature is that their mean grain size is less than
100 nm. Nanopowders, because of their nanoscale dimensions
(near-molecular), feature a variety of confinement effects that
significantly modify the properties of the material. The physics
behind this has been aptly conjectured to be the following: a
property will be altered when the entity or mechanism responsible
for that property is confined within a space smaller than the
critical length associated with that entity or mechanism. Such
confinement effects lead to very desirable properties. For
example:
[0064] (a) nanopowders have a very high surface area which leads to
enhanced interfacial diffusivities and thus enables rapid, low
temperature formation of materials that are typically difficult to
process;
[0065] (b) nanopowders are isomorphic because of dimensional
confinement. Furthermore, enhanced solubilities are observed
leading to non-equilibrium compositions. This leads to catalysts
and reactants with extremely high surface areas, high selectivity
and activity;
[0066] (c) nanopowders have grain sizes that are too small for
Frank-Read dislocation to operate in the conventional yield stress
domain; consequently, enhancement in strengths and hardness of 100%
to 500% are observed in films and pellets made from
nanopowders;
[0067] (d) the size of the nanopowder is less than the wavelength
of visible light; consequently unique optical materials with grain
sizes tailored for excitonic interactions with particular
wavelengths can be prepared;
[0068] (e) nanopowders are confined to a dimension less than the
mean free path of electrons; consequently, unusual electrical and
electrochemical properties can be observed;
[0069] (f) nanopowders are confined to dimension less than the
domain size of magnetic materials; consequently, nanopowders are
precursors to magnetic materials exhibiting Giant Magnetoresistive
(GMR) and superparamagnetic effects; and
[0070] (g) nanopowders feature quantum confinement to dimensions
less than Debye length. This leads to electrochemical properties
with order of magnitude higher sensitivities to chemical
species.
[0071] Nanopowders in general, and nano-dispersed powders in
particular, thus provide an extraordinary opportunity for design,
development and commercialization of a wide range of structural,
electrochemical, electrical, optical, electronic, magnetic and
chemical applications. Furthermore, since nanopowders represent a
whole new family of material precursors for which conventional
coarse-grain physiochemical mechanisms are not performance
determining, nanomaterials in general and nano-dispersed powders in
particular offer unique combination of properties that can enable
novel and multifunctional components of unmatched performance.
[0072] Yet another source of distinct usefulness of nano-dispersed
powders results in part from the useful interaction between the
dispersed attached nanoparticles and the carrier particles.
Dimensionally confined nanomaterials have properties that are
determined in part by the interface thermodynamics and
characteristics. These interfaces in turn are influenced by
neighboring atoms. By dispersion, the nanoparticles interact with
the interface of the carrier particles. This interaction can induce
a novel performance that is not exhibited by either of the carrier
particle or nanoparticle materials in isolation.
[0073] Yet another source of distinct usefulness of nano-dispersed
powders results in part from the high concentration of triple
points. Triple points are the points where three or more phases
meet and lead to useful interaction between the dispersed
particles, the carrier particles, and the fluid environment around
the junction of dispersed and carrier particles. The nanoscale size
of dispersed particles significantly increases the density of
triple points. These are points where useful chemical,
electrochemical, physical, electronic, photonic and electrical
interactions can occur.
[0074] 1. Methods of Producing Nano-Dispersed Powders
[0075] FIG. 3 shows one embodiment of a system for producing
dispersed powders in accordance with the present invention. This
method can be used to produce dispersed powders that are coarse and
pure, but is particularly useful for nano-dispersed sub-micron and
nano-dispersed nanoscale powders.
[0076] The process shown in FIG. 3 begins at 100 with a
metal-containing precursor such as an emulsion, fluid,
particle-containing liquid slurry, or water-soluble salt. The
precursor may be a gas, a single-phase liquid, a multi-phase
liquid, a melt, fluid mixtures, or combinations thereof. The
metal-containing precursor comprise a stoichiometric or a
non-stoichiometric metal composition wherein at least some portion
is in a fluid phase. Fluid precursors are preferred in this
invention over solid precursors because fluids are easier to
convey, evaporate, and thermally process, and the resulting product
is more uniform.
[0077] In one embodiment of this invention, the precursors are
preferably environmentally benign, safe, readily available,
high-metal loading, lower cost fluid materials. Examples of
metal-containing precursors suitable for purposes of this invention
include, but are not limited to, metal acetates, metal
carboxylates, metal ethanoates, metal alkoxides, metal octoates,
metal chelates, metallo-organic compounds, metal halides, metal
azides, metal nitrates, metal sulfates, metal hydroxides, metal
salts soluble in organics or water, and metal-containing
emulsions.
[0078] In another embodiment, multiple metal precursors may be
mixed if complex nano-dispersed powders are desired. For example, a
barium precursor and iron precursor may be mixed to prepare high
purity barium ferrite powders. As another example, a yttrium
precursor, barium precursor, and copper precursor may be mixed in
correct proportions to yield a high purity YBCO powder for
superconducting applications. In yet another example, an aluminum
precursor and silica precursor may be mixed to yield aluminum
silicate powders. Such complex nano-dispersed powders can help
create materials with surprising and unusual properties not
available through the respective single metal oxides or a simple
nanocomposite formed by physical blending powders of different
compositions. To illustrate, nanoscale powders formed from blending
two or more metals can create materials with a hardness, refractive
index, or other property or a combination of such properties that
have values that are intermediate to the properties of the
respective single metal oxide forms. As an example, complex powders
may be prepared from aluminum and silicon precursors to create
novel aluminum silicate nanomaterials with refractive index that is
intermediate to the refractive index of the alumina and silica.
[0079] In all embodiments of this invention, it is desirable to use
precursors of a higher purity to produce a nano-dispersed powder of
a desired purity. For example, if purities greater than x % (by
metal basis) is desired, one or more precursors that are mixed and
used have purities greater than or equal to x % (by metal basis) to
practice the teachings herein.
[0080] With continued reference to FIG. 3, the metal-containing
precursor 100 (containing one or a mixture of metal-containing
precursors) is mixed with carrier particles 102 of desired size,
composition, and characteristics. Carrier particles 102 may
comprise micron-sized particles, sub-micron particles, or
nanostructured particles. The resultant slurry precursor 104 is the
preferred feed material for producing nano-dispersed powders. The
relative concentrations of the metal-containing precursors 100 and
the carrier particles 102 should be substantially equivalent to
that desired in the final product.
[0081] Upon formation of the slurry precursor 104, the slurry
precursor 104 is fed into a high temperature process 106
implemented using a high temperature reactor, for example. In one
embodiment, a synthetic aid such as a reactive fluid 108 can be
added along with the slurry precursor 104 as it is being fed into
the reactor 106. For example, when the object is to prepare a
nano-dispersed powder comprising a dispersed oxide, a preferred
embodiment of this invention is to use a precursor 100 in which the
oxygen-to-carbon elemental ratio in the precursor molecule is high.
Alternatively, or in addition, a reactive fluid 108 that provides
excess oxygen may be added along with the slurry precursor 104 to
the reaction zone 106. Examples of such reactive fluids include,
but are not limited to, oxygen gas and air.
[0082] As another example, when the object is to prepare a
nano-dispersed powder comprising a dispersed carbide, a preferred
embodiment of this invention is to use a precursor 100 in which the
oxygen-to-carbon elemental ratio is less than 0.1, more preferably
less than 1.0, and most preferably less than 2.0. Alternatively, or
in addition, a reactive fluid 108 that provides excess carbon or
reduces excess oxygen may be added along with the slurry precursor
104 to the reaction zone 106. Examples of such reactive fluids
include, but are not limited to, methane, ethylene, acetylene,
ethane, natural gas, benzene, naphtha, and hydrogen.
[0083] As another example, when the object is to prepare a
nano-dispersed powder comprising a dispersed nitride, a preferred
embodiment of this invention is to use a precursor 100 in which the
oxygen-to-nitrogen elemental ratio in the precursor molecule less
than 0.1, more preferably less than 1.0, and most preferably less
than 2.0. Alternatively, or in addition, a reactive fluid 108 that
provides excess nitrogen or reduces excess oxygen may be added
along with the slurry precursor 104 to the reaction zone 106.
Examples of such reactive fluids include, but are not limited to,
amines, ammonia, hydrazine, nitrogen, and hydrogen.
[0084] As another example, when the object is to prepare a
nano-dispersed powder comprising a dispersed boride, a preferred
embodiment of this invention is to use a precursor 100 in which the
oxygen-to-boron elemental ratio in the precursor molecule less than
0.1 and more preferably less than 1.0, and most preferably less
than 1.5. Alternatively, or in addition, a reactive fluid 108 that
provides excess boron or reduces excess oxygen may be added along
with the slurry precursor 104 to the reaction zone 106. Examples
include, but are not limited to, boranes, boron, and hydrogen.
[0085] As another example, when the object is to prepare a
nano-dispersed powder comprising a dispersed carbonitride, a
preferred embodiment of this invention is to use a precursor 100 in
which the (a) oxygen-to-carbon elemental ratio in the precursor
molecule less than 0.1 and more preferably less than 1.0, and most
preferably less than 2.0, and (b) the oxygen-to-nitrogen elemental
ratio in the precursor molecule less than 0.1, more preferably less
than 1.0, and most preferably less than 2.0. Alternatively, or in
addition, a reactive fluid 108 may be added along with the slurry
precursor 104 to the reaction zone 106. Examples of such reactive
fluids include, but are not limited to, methane, ethylene,
acetylene, ethane, natural gas, benzene, naphtha, amines, ammonia,
hydrazine, nitrogen, and hydrogen.
[0086] While the above examples specifically teach methods of
preparing dispersed powders of oxides, carbides, nitrides, borides,
and carbonitrides, the teachings may be readily extended in an
analogous manner to other compositions such as chalcogenides. While
it is preferred to use high temperature processing, a moderate
temperature processing or a low/cryogenic temperature processing
may also be employed to produce high purity nano-dispersed
powders.
[0087] The precursor 100 may be also pre-processed in a number of
other ways before the high temperature thermal treatment. For
example, the pH may be adjusted to ensure stable precursor.
Alternatively, selective solution chemistry such as precipitation
may be employed to form a sol or other state of matter. The
precursor 101 may be pre-heated or partially combusted before the
thermal treatment.
[0088] The slurry precursor 104 may be injected axially, radially,
tangentially, or at any other angle into the high temperature
region 106. As stated above, the slurry precursor 104 may be
pre-mixed or diffusionally mixed with other reactants. The slurry
precursor 104 may be fed into the thermal processing reactor by a
laminar, parabolic, turbulent, pulsating, sheared, or cyclonic flow
pattern, or by any other flow pattern. In addition, one or more
metal-containing precursors 100 can be injected from one or more
ports in the reactor 106. The feed spray system may yield a feed
pattern that envelops the heat source or, alternatively, the heat
sources may envelop the feed, or alternatively, various
combinations of this may be employed. A preferred embodiment is to
atomize and spray the feed in a manner that enhances heat transfer
efficiency, mass transfer efficiency, momentum transfer efficiency,
and reaction efficiency. The reactor shape may be cylindrical,
spherical, conical, or any other shape. Methods and equipment such
as those taught in U.S. Pat. Nos. 5,788,738, 5,851,507, and
5,984,997 (each of which is specifically incorporated herein by
reference) can be employed in practicing the methods of this
invention.
[0089] With continued reference to FIG. 3, after the slurry
precursor 104 has been fed into reactor 106, it is processed at
high temperatures to form the product nano-dispersed powder. The
thermal treatment is preferably done in a gas environment with the
aim to produce a product such as powders that have the desired
porosity, strength, morphology, dispersion, surface area, and
composition. This step produces by-products such as gases. To
reduce costs, these gases may be recycled, mass/heat integrated, or
used to prepare the pure gas stream desired by the process.
[0090] The high temperature processing is conducted at step 106
(FIG. 3) at temperatures greater than 1500.degree. C., preferably
2500.degree. C., more preferably greater than 3000.degree. C., and
most preferably greater than 4000.degree. C. Such temperatures may
be achieved by various methods including, but not limited to,
plasma processes, combustion, pyrolysis, electrical arcing in an
appropriate reactor, and combinations thereof. The plasma may
provide reaction gases or just provide a clean source of heat.
[0091] An outstanding problem with conventional nanopowder
synthesis methods is broad size distribution. This may happen
because of non-uniformities in heat, mass, and/or momentum
transfer. One reason for such non-uniformities is the sudden drop
in temperature and reacting species at the outer edge of the
reaction front such as a combustion flame. An illustration of outer
edge of a flame would be the outer perimeter of the cross section
of the flame, i.e. plane perpendicular to the direction of flame
flow. At these edges, the reaction pathways are influenced by the
heat and mass and momentum boundary conditions. This zone,
therefore, yields conditions where the product produced is
non-uniform and therefore different than those produced inside the
boundary. Such conditions apply to all sorts of high temperature
flow fields including flames and are created by various types of
burners or reactor system. Some illustrations of such burners are
taught by R. M. Fristrom (in Flame Structure and Processes, Oxford
University Press, New York, 1995, which along with references
contained therein is specifically incorporated herein by
reference).
[0092] One feature of this invention is the ability to reduce this
non-uniformity by eliminating or reducing the above-described
source of non-uniformity. This can be accomplished in many ways. As
an illustration, the reaction zone (such as a combustion flame) can
be surrounded by a fully or a partially concentric zone of a medium
with a thermal, mass and momentum profile that reduces such
non-uniformity. For example, FIG. 4 shows a primary combustion
burner 420, over which useful particle producing flame chemistry
occurs, is preferably surrounded by a concentric secondary burner
421 where a fuel is burned to maintain the outer edge temperatures
in region 422 as close to the primary flame's highest temperature
in region 423 as possible. To the extent possible, the mass and
momentum profile of the medium created by the concentric secondary
burner 421 should be similar in one or more respects to the mass
and momentum profile of the medium created by the primary burner
420. Such concentric burners can assist in a more uniform thermal,
mass, and momentum profile for the reaction space created by the
primary burner 420. A non-limiting illustration of such concentric
burners is discussed and referenced by Howard et al., Carbon
30(8):1183-1201 (1992), which along with references contained
therein is specifically incorporated herein by reference.
[0093] In another embodiment the slurry precursor 104 is
pre-treated to minimize non-uniformity in heat, mass, and/or
momentum transfer. This can be achieved through techniques such as
(a) axially, radially, or tangentially surrounding the high
temperature processing zone 106 with an inert gas plasma, (b)
axially, radially, or tangentially surrounding the high temperature
processing zone 106 with a complete combustion flame, preferably of
high temperature, or (c) axially, radially, or tangentially
surrounding the high temperature processing zone 106 with an
electrical arc or high temperature radiation source. The concentric
flame's adiabatic temperature (or concentric thermal zone) is
preferably greater than 500.degree. C., more preferably greater
than 1000.degree. C., and most preferably greater than 2000.degree.
C. The minimal requirement of this technique is that the high
temperature processing zone temperature at the outer edges be
higher when the concentric high temperature thermal zone is present
than when it is absent.
[0094] This principle of concentric thermal zones can be applied to
any method of producing dispersed powders. Illustrative examples of
processes where this principle can be used include one-dimensional
combustion flames, diffusion flames, turbulent flames, pre-mixed
flames, flat flames, plasma, arcing, microwave, sputtering,
pyrolysis, spray evaporation, laser and hydrothermal
processing.
[0095] In the embodiment shown in FIG. 3, carrier particles 102 are
present in the high temperature process. The carrier particles 102
may be substantially inert during high temperature process 106, or
they may be transformed by physical, chemical, or solid state
reactions. High temperature processing is performed in a manner
such that the end product of high temperature process 106 includes
carrier particles 102 of desired size, composition and uniformity.
Alternatively, the carrier particles can be added at a later stage
of the high temperature process.
[0096] The high temperature process 106 results in a vapor
comprising fine powders and carrier particles. After the thermal
processing, this vapor is cooled at step 110 to nucleate dispersion
of fine powders, preferably nanopowders, onto the surface of the
carrier particles. Preferably, the cooling temperature at step 110
is high enough to prevent moisture condensation. The dispersed
particles are formed from the precursor because of the
thermokinetic conditions in the process. By engineering the process
conditions such as pressure, residence time, flow rates, species
concentrations, diluent addition, degree of mixing, momentum
transfer, mass transfer, and heat transfer, the morphology of the
dispersed powders can be tailored. It is important to note that the
focus of the process is on producing a dispersed powder product
that excels in satisfying the end application requirement and
customer needs. In some cases, this may be achieved with uniformly
dispersed particles and in others it may be non-uniformly
distributed particles that best meet the customer needs.
[0097] After cooling, the nano-dispersed powder is preferably
quenched to lower temperatures at step 116 to minimize and
preferably prevent agglomeration or grain growth. Suitable
quenching methods include, but are not limited to, methods taught
in U.S. Pat. No. 5,788,738. It is preferred that quenching methods
be employed which can prevent deposition of the powders on the
conveying walls. These methods may include electrostatic means,
blanketing with gases, the use of higher flow rates, mechanical
means, chemical means, electrochemical means, or
sonication/vibration of the walls.
[0098] In one embodiment, the high temperature processing system
includes instrumentation 112, 114 that can assist in the quality
control of the process by analyzing the quality of the product
either between steps 116 and 118, or between steps 118 and 120. The
data collected after analysis of the product can provide
information on how to adjust the process variables to adjust the
quality of the product.
[0099] It is preferred that the high temperature processing zone
106 is operated to produce fine powders 120 (FIG. 3), preferably
submicron powders, and most preferably nanopowders. The gaseous
products from the process may be monitored for composition,
temperature and other variables to ensure quality at 112 (FIG. 3).
The gaseous products may be recycled to be used in process 108
(FIG. 3), or used as a valuable raw material when dispersed powders
120 have been formed. Following quenching step 116 (FIG. 3) the
nano-dispersed powders are cooled further at step 118 and then
harvested at step 120.
[0100] The product nano-dispersed powders 120 may be collected by
any method. Suitable collection means include, but are not limited
to, bag filtration, electrostatic separation, membrane filtration,
cyclones, impact filtration, centrifugation, hydrocyclones,
thermophoresis, magnetic separation, and combinations thereof.
[0101] FIG. 5 shows an alternate embodiment for producing dispersed
particles according to this invention. The embodiment shown in FIG.
5 begins with nano-scale powders 200 produced by any technique.
These nanoscale powders 200 are mixed with desired coarser carrier
particles 202 into a slurry precursor 204. The slurry precursor 204
is mixed with a fluid such as a fuel and then used as precursor to
make nano-dispersed particles following steps 204-210 in a manner
similar to that described for steps 104-112 of FIG. 3.
[0102] Alternatively, precursors may be blended into or emulsified
into a commercially available nanoparticulate sol, such as
NALCO.RTM. silica sols or NYACOL.RTM. alumina sol. This
multi-phased feed is then used to make particles by the process
described by FIG. 5.
[0103] FIG. 6 shows yet another embodiment for producing dispersed
particles according to this invention. In this method, both the
nano-scale particles and the carrier particles are formed in-situ
during the thermal processing step. More specifically, a
metal-containing precursor 300 (containing one or a mixture of
metal-containing precursors) and optional dopants 301 are combined
to form a precursor batch 302. The dopants may be added to modify
or enable the performance of the dispersed powders suitably for a
particular application. Such dopants include, but are not limited
to, transition metals, rare earth metals, alkali metals, alkaline
earth metals, semi-metals, and non-metals. It is preferred that,
like other metal precursors, precursors for such dopants are
intimately mixed with the metal-containing precursor 300. It is
also preferred that dopant precursors are fluids. The precursor
batch is then feed into a high temperature reactor 306. In one
embodiment, one or more synthetic aids such as a reactive fluid 308
can be added along with the precursor batch 302 as it is being fed
into the reactor 306. Examples of synthetic aids include, but are
not limited to, oxygen, methane, nitrogen, feed gases, oxidants, or
reactants.
[0104] With continued reference to FIG. 6, the precursor batch 302
is then fed into a thermal reactor 306 where the precursors are
partially transformed, or preferably completely transformed, into
the vapor form. The source of thermal energy in the preferred
embodiments is plasma generator 305. Plasma gas 307, which may be
inert or reactive, is supplied to plasma generator 305.
Alternatively, the source of thermal energy may be internal energy,
heat of reaction, conductive, convective, radiative, inductive,
microwave, electromagnetic, direct or pulsed electric arc, laser,
nuclear, or combinations thereof, so long as it is sufficient to
cause the rapid vaporization of the powder suspension being
processed.
[0105] The high temperature process 306 results in a vapor
comprising both fine powders and carrier particles formed in-situ
from the precursors 300. In order to produce both the dispersed and
carrier particles, the thermokinetic conditions and ratio of the
precursor to the synthetic aid are controlled. Alternatively, the
precursors can be fed at different locations in the reactor to
engineer the residence time experienced by each feed location. A
change in residence time or thermokinetic condition or process
variable produces powders of different characteristics (size,
shape, composition, etc.). This method can therefore be employed to
produce both carrier and attached particles. After the thermal
processing, this vapor is cooled at step 310 to nucleate dispersion
onto the surface of the carrier particles. Preferably, the cooling
temperature at step 310 is high enough to prevent moisture
condensation.
[0106] With continued reference to FIG. 6, after cooling step 310
the nano-dispersed powder is preferably quenched as described above
to lower temperatures at step 316 to prevent agglomeration or grain
growth. It is preferred that quenching methods be employed which
can prevent deposition of the powders on the conveying walls.
Following quenching step 316 the nano-dispersed powders are cooled
further at step 318 and then harvested at step 320. The product of
this process is a dispersed powder, such as nano-scale particles
dispersed on larger nano-scale particles or nano-scale particles
dispersed on sub-micron particles.
[0107] In yet another embodiment (not shown), the nano-dispersed
powders are produced by first combining nano-scale powders produced
by any method with carrier particles. The relative concentrations
of the nano-scale powder and the carrier particles should be
substantially equivalent to that desired in the final product. The
mixture is then mechanically milled by methods known in the art to
produce the nano-dispersed powders. The milling may be done in gas
or liquid environment. If a liquid environment is employed, the
liquid may comprise acids, alkalis, oxidizers, dispersants, metal
containing precursors, or other suitable constituents.
[0108] FIG. 7 shows an alternative flow diagram of a thermal
process for the synthesis of nano-dispersed powders. In this
method, precursors 404 such as metal containing emulsions, fluid,
or water-soluble salt, are combined with carrier particles 405.
Although a single precursor storage tank 404 is shown in FIG. 7, it
is to be understood that multiple precursor tanks may be provided
and used with or without premixing mechanisms (not shown) to premix
multiple precursors before feeding into reactor 401.
[0109] In one embodiment, a feed stream of precursor material from
storage tank 404 and carrier particles 405 is atomized in mixing
apparatus 403. Alternatively, a precursor storage 404 may be
implemented by suspending the precursor in a gas, preferably in a
continuous operation, using fluidized beds, spouting beds, hoppers,
or combinations thereof, as best suited to the nature of the
precursor.
[0110] The resulting suspension is advantageously preheated in a
heat exchanger (not shown), preferably with the exhaust heat, and
is then fed into a thermal reactor 401 where the atomized
precursors are partially transformed, or preferably completely
transformed, into the vapor form. The source of thermal energy in
the preferred embodiments is plasma generator 402 powered by power
supply 206. Plasma gas 407, which may be inert or reactive, is
supplied to plasma generator 402. Alternatively, the source of
thermal energy may be internal energy, heat of reaction,
conductive, convective, radiative, inductive, microwave,
electromagnetic, direct or pulsed electric arc, laser, nuclear, or
combinations thereof, so long as it is sufficient to cause the
rapid vaporization of the precursor being processed. The peak
temperature in the thermal reactor 401 is greater than 1500.degree.
C., preferably greater than 2500.degree. C., more preferably
greater than 3000.degree. C., and most preferably greater than
4000.degree. C. Optionally, in order to prevent contamination of
the vapor stream caused by partial sublimation or vaporization, the
walls of reactor 401 may be pre-coated with the same material being
processed.
[0111] The vapor next enters an extended reaction zone 411 of the
thermal reactor which provides additional residence time as needed
to complete the processing of the feed material and to provide
additional reaction and forming time for the vapor (if necessary).
As the stream leaves the reactor, it passes through a zone 409
where the thermokinetic conditions favor the nucleation of solid
powders from the vaporized precursor. These conditions are
determined by calculating the supersaturation ratio and critical
cluster size required to initiate nucleation. Rapid quenching leads
to high supersaturation which gives rise to homogeneous nucleation.
The zones 401, 411, and 409 may be combined and integrated in any
manner to enhance material, energy, momentum, and/or reaction
efficiency.
[0112] As soon as the vapor has begun nucleation, the process
stream is quenched in a heat removal apparatus within nucleation
zone 409 comprising, for example, a converging-diverging
nozzle-driven adiabatic expansion chamber at rates at least
exceeding 1,000 K/sec, preferably greater than 1,000,000 K/sec, or
as high as possible. A cooling medium (not shown) may be utilized
for the converging-diverging nozzle to prevent contamination of the
product and damage to the expansion chamber.
[0113] The quenched gas stream is filtered by appropriate
separation equipment in harvesting region 413 to remove the
nano-dispersed product from the gas stream. As is well understood
in the art, the filtration can be accomplished by single stage or
multistage impingement filters, electrostatic filters, screen
filters, fabric filters, cyclones, scrubbers, magnetic filters, or
combinations thereof. The filtered nano-dispersed product is then
harvested from the filter, either in batch mode or continuously,
using screw conveyors or gas-phase solid transport, and the product
stream is conveyed to powder processing or packaging unit
operations (not shown).
[0114] The process is preferably operated at near ambient pressures
and more preferably at pressures that are less than 750 mm Hg
absolute (i.e. vacuum). Such a low pressure can be achieved using
any type of vacuum pump, compressor, and more preferably using
compressed fluid-based eductor operating on a venturi principle
given the lower cost, simplicity and environmental benefits. Vacuum
generating equipment may be placed at any stage of the overall
process. The product stream from the vacuum generating equipment
may be utilized elsewhere in the process to achieve heat and mass
integration and thereby to reduce costs. For example, in one
embodiment a suspension or dispersion may be prepared in a liquid
directly if the liquid were to be utilized as the high pressure
driving fluid for the eductor.
[0115] In an alternate embodiment shown in FIG. 8, rather than
harvesting the nano-dispersed product, the nano-dispersed product
is deposited directly on a substrate 601 to form a coating or film
or near-net shape structural part. In this embodiment, the fluid
precursor 504 and carrier particles 505 are fed into mixing
apparatus 503 and then fed into a thermal reactor 501 where the
atomized precursors are partially transformed, or preferably
completely transformed, into the vapor form.
[0116] The preferred source of thermal energy in the embodiment
illustrated in FIG. 8 is plasma generator 502 powered by power
supply 506. The mixture is thermally heated in reactor 501 to high
temperatures to yield a hot vapor. A substrate 601 having an
exposed surface is provided within or in communication with
reaction chamber 501 on, for example, a thermally controlled
substrate holder. The hot vapor is then contacted with the exposed
substrate surface and coats the exposed surface. The hot vapor may
be cooled or quenched before the deposition step to provide a
stream that has fine liquid droplets or hot particulate matter. The
substrate 601 may be cooled or heated using a substrate thermal
control 514 to affect the quality of the coating.
[0117] The substrate 601 may be mounted on a turn-table or drum to
rotate the substrate 601 parallel, perpendicularly, tangentially
(or at any other angle) relative to the gas stream comprising of
nanoparticles. The rotation can help achieve different thickness, a
conformal form, or a curved form. The substrate 601 to be coated
may be continuously fed and removed over rotating cylinders to
substrate 601. By controlling the substrate feed rate, the coating
thickness can be controlled. Such coating method can employ
suitable in-situ instrumentation to control the quality of the
coating formed.
[0118] The deposition approach in accordance with the present
invention is different from thermal spray technology currently in
used in many ways such as: (a) the feed in conventional methods is
a solid micron sized powder in thermal spray processes, whereas in
this invention the feed is a fluid precursor; and (b) the
conventional thermal spray process is considered to yield a powder
with molten surface which then sticks to the substrate, whereas in
this invention, as the hot vapor cools it is anticipated to yield a
molten droplet or soft particulate that forms the coating. The
advantage of forming a coating or film according to this invention
is the fine to nanoscale microstructure of the resultant coating or
film.
[0119] Furthermore, it is contemplated that the present invention
will yield additional benefits in the ability to easily transport
fluids within the process, the ability to form coatings, and the
ability to form wide range of compositions (oxides, carbides,
nitrides, borides, multimetal compositions, composites, etc.) from
a limited collection of precursors through mixing and other methods
as taught herein.
[0120] A coating, film, or component may also be prepared by
dispersing the dispersed nanopowder, followed by applying various
known methods such as, but not limited to, electrophoretic
deposition, magnetophoretic deposition, spin coating, dip coating,
spraying, brushing, screen printing, ink-jet printing, toner
printing, and sintering. The nanopowders may be thermally treated
or reacted to enhance their electrical, optical, photonic,
catalytic, thermal, magnetic, structural, electronic, emission,
processing, or forming properties before such a step.
[0121] The powder may be post-processed to further improve its
performance or characteristics such as flowability. For example,
the post-processing of the dispersed powder may be include one or
more of the following steps in any order: air classification,
sieving, drying, reduction, chemical reaction with liquid, chemical
reaction with gases, humidification, surface treatment, coating,
pyrolysis, combustion, casting, dispersion, dissolution,
suspension, molding, hipping, pressing, milling, composite forming,
coarsening, mixing, agglomeration, de-agglomeration, weighing, and
packaging. A non-limiting illustration of such post-processing
would be where the dispersed powder are dissolved in a media
selected such that the carrier particle dissolves in the media
while the attached particles do not dissolve in the media. This
post-processing can produce hollow nanostructured or sub-micron
particles. Similarly, if the dispersed particles comprise of a
polymeric carrier powders and the attached particles are ceramic,
pyrolysis or combustion can also be utilized to make hollow
particles. Such hollow particles are anticipated to have unusual
properties such as lower effective density, low effective
dielectric constant, lower effective thermal conductivity.
[0122] Uses
[0123] Dispersed powders have numerous applications in industries
such as, but not limited to, catalysis, biomedical,
pharmaceuticals, sensor, electronic, telecom, optics, electrical,
photonic, thermal, piezo, magnetic and electrochemical
products.
[0124] Biomedical implants and surgical tools can benefit from
dispersed powders. It is expected that nano-dispersed powders can
enable implants with modulus and other properties that match the
part being replaced. The match is expected to be within 10% of the
target properties. The surgical tools produced using nano-dispersed
powders are expected to offer strength that is at least 10% higher
than that achieved using powders without nano-dispersion.
[0125] Powdered marker, drug carriers and inhalation particulates
that reduce side effects can benefit from nano-dispersed powders.
For inhalation product applications, carrier particles with a size
range of 500 nm to 50 microns are preferred, and carrier particles
with a geometric diameter of 1-50 .mu.m and an aerodynamic diameter
of 1-10 .mu.m are most preferred. The nanoscale dispersed particle
can be a drug or a carrier of the drug. The carrier particle can be
engineered to favor prolonged release. For injectable product
applications, carrier particles with a size range of 100 nm to 25
microns are preferred, and carrier particles with a geometric
diameter of 0.1-5 .mu.m and an aerodynamic diameter of 0.1-1 .mu.m
are most preferred. The nanoscale dispersed particles can be
markers, tracers, drug vehicles or target carriers.
[0126] Another category of application of nano-dispersed powders is
phosphors. Phosphors emit light when exposed to radiation.
Not-limiting illustrations of phosphors include
Zn.sub.2SiO.sub.4:Mn, ZnS:Ag, ZnO:Zn, CaSiO.sub.3:Mn,
Y.sub.3Al.sub.5O.sub.12:Ce, Y.sub.2O.sub.3:Eu, Y.sub.2SiO.sub.5:Ce,
Y.sub.3(Al,Ga).sub.5O.sub.12:Tb, BaO.6Al.sub.2O.sub.3:Mn,
BaMg.sub.2Al.sub.16O.sub.27:Eu, CsI:Na, and CaS:Ce,Sm. It is
expected that the methods of manufacture and other teachings of
this invention can be applied wherein the major phase of the
phosphor is the carrier particle and the minor phase is the
nano-dispersed particle. As a non-limiting example, for
Y.sub.3Al.sub.5O.sub.12:Ce, Y.sub.3Al.sub.5O.sub.12 can be the
carrier particle while Ce is the nano-dispersed phase on the
surface of the carrier. For phosphor product applications, carrier
particles with a size range of 50 nm to 25 microns are preferred,
and carrier particles with a geometric diameter of 0.5-10 microns
are preferred. The dispersed particles with a size range of 1 nm to
0.5 microns are preferred, and dispersed particles with a geometric
diameter of 1-100 nanometers are preferred. It is anticipated that
the light emitting efficiency of nano-dispersed phosphor powders
will be higher by 5% or more than phosphor powder of equivalent
composition that is not dispersed. The scope of this invention
includes Stoke and anti-Stoke phosphors. Nano-dispersed phosphor
powders can be used in lamps, cathode ray tubes, field emission
displays, plasma display panels, scintillators, X-ray detectors, IR
detectors, UV detectors and laser detectors. Nano-dispersed
phosphor powders can also be used in printing inks, or dispersed in
plastics to prevent forgery and counterfeiting of currency,
original works of art, passports, credit cards, bank checks, and
other documents or products. The nano-dispersed powders can also be
used to prepare optical networking components such as detectors,
emitters, photodiodes, and phototransistors.
[0127] Another broad use of nano-dispersed powders is in electrical
and electronic components such as capacitors, inductors, resistors,
thermistors, sensors and varistors. Nano-dispersed particles can
increase the reliability of these components by 10% or more when
used as electroceramic dopants. Furthermore, nano-dispersed
particles can enable miniaturization of these components by
enabling ceramic layer thicknesses below 500 nm and electrode layer
thicknesses below 400 nm.
[0128] Electrochemical capacitors prepared from nano-dispersed
powders are expected to have charge densities that are 10% higher
than those prepared from non-dispersed powders of equivalent
composition. The electrochemical capacitors are also expected to
offer high volumetric efficiencies, and longer mean times between
failures. Batteries prepared from nano-dispersed powders can offer
power densities that are 5% higher than those prepared from
non-dispersed powders of equivalent composition. Chemical sensors
prepared from nano-dispersed powder are expected to offer
sensitivities that are at least 10% higher than those prepared from
non-dispersed powders of equivalent composition.
[0129] A major application area for nano-dispersed powders produced
using the high temperature process of this invention is in chemical
catalysis. Catalytic materials that are prepared from
nano-dispersed powders are expected to last 10% or more longer and
give superior yields and selectivity than catalytic materials
prepared from non-dispersed powders of equivalent composition. They
are also expected to offer turn over rates that are 5% higher than
those prepared from non-dispersed powders of equivalent
composition. For this application, the process of this invention
for producing nano-dispersed powders can additionally offer
desirable porosity, structural strength, and uniformity. Examples
of such applications include (a) catalytic transformation of less
valuable chemicals and material feed stocks into more valuable
chemicals and materials and (b) catalytic transformation of more
hazardous chemicals and materials into less hazardous or
non-hazardous forms of substances.
[0130] Other applications of nano-dispersed powders include (a)
fillers for polymers, ceramics, and metal matrix composites and (b)
dopants for electronic, magnetic, thermal, piezo, electrical,
tooling, structural, inks, paints, and topical health products.
[0131] Magnetic devices prepared from dispersed powders are
expected to offer superior magnetic performance. In general,
nano-dispersed powders offer a means of improving the value-added
performance of existing products that are produced from
non-dispersed powders.
[0132] In some applications where material cost is a critical
parameter, affordability can be achieved by combining low cost
carrier powders with highly functional but somewhat more expensive
attached nanoparticles thereby yielding more affordable yet high
performance nano-dispersed powders on a per unit weight basis. As
an added benefit, improved ability to process micron size carrier
powders can accelerate the adoption of nano-dispersed powders in
commerce.
[0133] Other embodiments of the invention will be apparent to those
skilled in the art from a consideration of the specification or
practice of the invention disclosed herein. It is intended that the
specification and examples be considered as exemplary only, with
the true scope and spirit of the invention being indicated by the
following claims.
* * * * *