U.S. patent application number 10/904237 was filed with the patent office on 2005-05-12 for coated copper-containing powders, methods and apparatus for producing such powders, and copper-containing devices fabricated from same.
This patent application is currently assigned to CABOT CORPORATION. Invention is credited to Caruso, James, Chandler, Clive D., Hampden-Smith, Mark J., Kodas, Toivo T., Powell, Quint H., Skamser, Daniel J..
Application Number | 20050097987 10/904237 |
Document ID | / |
Family ID | 34557096 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050097987 |
Kind Code |
A1 |
Kodas, Toivo T. ; et
al. |
May 12, 2005 |
COATED COPPER-CONTAINING POWDERS, METHODS AND APPARATUS FOR
PRODUCING SUCH POWDERS, AND COPPER-CONTAINING DEVICES FABRICATED
FROM SAME
Abstract
Copper powder batches including coated copper-containing
particles and methods for producing the same. The coated
copper-containing particles having have a small particle size,
narrow size distribution and a spherical morphology. The present
invention is also directed to devices incorporating the coated
copper-containing particles.
Inventors: |
Kodas, Toivo T.;
(Albuquerque, NM) ; Hampden-Smith, Mark J.;
(Albuquerque, NM) ; Caruso, James; (Albuquerque,
NM) ; Powell, Quint H.; (Albuquerque, NM) ;
Chandler, Clive D.; (Portland, OR) ; Skamser, Daniel
J.; (Albuquerque, NM) |
Correspondence
Address: |
MARSH, FISCHMANN & BREYFOGLE LLP
3151 SOUTH VAUGHN WAY
SUITE 411
AURORA
CO
80014
US
|
Assignee: |
CABOT CORPORATION
Two Seaport Lane Suite 1300
Boston
MA
|
Family ID: |
34557096 |
Appl. No.: |
10/904237 |
Filed: |
October 29, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10904237 |
Oct 29, 2004 |
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10758866 |
Jan 16, 2004 |
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10758866 |
Jan 16, 2004 |
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09586151 |
Jun 2, 2000 |
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6679937 |
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09586151 |
Jun 2, 2000 |
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09030051 |
Feb 24, 1998 |
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Current U.S.
Class: |
75/332 |
Current CPC
Class: |
B22F 2998/00 20130101;
B22F 2998/10 20130101; B22F 2999/00 20130101; B22F 1/0062 20130101;
B22F 1/0059 20130101; B22F 1/02 20130101; B22F 1/0062 20130101;
B22F 9/026 20130101; B22F 1/02 20130101; B22F 9/026 20130101; B22F
2202/01 20130101; B22F 2999/00 20130101; B22F 2998/00 20130101;
B22F 2998/10 20130101 |
Class at
Publication: |
075/332 |
International
Class: |
B22F 009/06 |
Claims
What is claimed is:
1. A method for making coated copper-containing particles, the
method comprising: preparing particles comprising a
copper-containing material, as prepared the particles are dispersed
in a flowing aerosol stream; and after the preparing and while the
particles are in the aerosol stream, processing the particles,
wherein the processing comprises forming a coating on the
particles, the coating comprising a coating material that is
different than the copper-containing material.
2. The method of claim 1, wherein the coating has an average
thickness of not greater than 100 nanometers.
3. The method of claim 1, wherein the coating has an average
thickness of not greater than 50 nanometers.
4. The method of claim 3, wherein the coating has an average
thickness of at least 5 nanometers.
5. The method of claim 1, wherein the coated copper-containing
particles have a weight average particle size in a range of from
0.1 .mu.m to 5 .mu.m.
6. The method of claim 1, wherein the coating material is an
inorganic compound.
7. The method of claim 1, wherein the coating material is a metal
phase.
8. The method of claim 7, wherein the metal phase is an elemental
metal.
9. The method of claim 7, wherein the metal phase comprises a noble
metal.
10. The method of claim 9, wherein the noble metal is platinum.
11. The method of claim 9, wherein the noble metal is gold.
12. The method of claim 7, wherein the metal phase is selected from
the group consisting of elemental silver and silver alloys.
13. The method of claim 1, wherein the coating material is a metal
oxide.
14. The method of claim 13, wherein the metal oxide is selected
from the group consisting of ZrO.sub.2, SiO.sub.2, B.sub.2O.sub.5,
TiO.sub.2, Cu.sub.2O, CuO, Bi.sub.2O.sub.3, V.sub.2O.sub.5 and
Al.sub.2O.sub.3.
15. The method of claim 1, wherein the coating material is a
dielectric compound.
16. The method of claim 15, wherein the dielectric compound is
selected from the group consisting of titanates, silicates,
aluminates and tantalates.
17. The method of claim 15, wherein the dielectric compound is
selected from the group consisting of barium titanate, neodymium
titanate, magnesium titanate, calcium titanate, lead titanate and
strontium titanate.
18. The method of claim 15, wherein the dielectric compound is
selected from the group consisting of a zirconate and a
niobate.
19. The method of claim 15, wherein the dielectric compound is
selected from the group consisting of magnesium zirconate and
calcium zirconate.
20. The method of claim 1, wherein the coating material is a
non-metallic compound.
21. The method of claim 20, wherein the non-metallic compound is a
boride.
22. The method of claim 1, wherein the coating material is an
organic compound.
23. The method of claim 1, wherein the coating material is
polymethylmethacrylate.
24. The method of claim 1, wherein the coating material is
polystyrene.
25. The method of claim 1, wherein the coating material is a
surfactant.
26. The method of claim 1, wherein the coating material is
hydrophobic.
27. The method of claim 1, wherein the coating material is
hydrophilic.
28. The method of claim 1, wherein the coating is a monolayer
coating.
29. The method of claim 1, wherein the forming comprises chemical
vapor deposition.
30. The method of claim 1, wherein the forming comprises physical
vapor deposition.
31. The method of claim 1, wherein the forming comprises
gas-to-particle conversion.
32. The method of claim 1, wherein the forming comprises contacting
the copper-containing particles with a reactive gas
composition.
33. The method of claim 1, wherein the forming comprises reaction
at elevated temperature of a precursor selected from the group
consisting of metal acetates, metal chlorides, metal alkoxides and
metal halides.
34. The method of claim 1, wherein the forming comprises reaction
of SiCl.sub.4.
35. The method of claim 1, wherein the forming comprises reaction
of Si(OEt).sub.4.
36. The method of claim 1, wherein the forming comprises reaction
of Mg(O.sub.2CCH.sub.3).sub.2.
37. The method of claim 1, wherein the forming comprises reacting
an organic or inorganic molecule with a surface of the particles to
form the coating.
38. The method of claim 1, wherein the forming comprises reacting a
surface of the particles with a functionalized organo silane
compound.
39. The method of claim 38, wherein the functionalized organo
silane compound is a halo-silane.
40. The method of claim 38, wherein the functionalized organo
silane compound is an amino-silane.
41. The method of claim 38, wherein the functionalized organo
silane compound is hexamethyidisilazane.
42. The method of claim 38, wherein the functionalized organo
silane compound is trimethylsilylchloride.
43. The method of claim 1, wherein the forming comprises condensing
a volatile coating material on the particles.
44. The method of claim 43, wherein the volatile coating material
is selected from the group consisting of PbO, MoO.sub.3 and
V.sub.2O.sub.5.
45. The method of claim 1, wherein the preparing comprises forming
the particles in a thermal reactor.
46. The method of claim 45, wherein the thermal reactor is a
furnace reactor.
47. The method of claim 45, wherein the thermal reactor is a flame
reactor.
48. The method of claim 45, wherein the thermal reactor is a plasma
reactor.
49. The method of claim 1, comprising: prior to the preparing,
generating the aerosol stream, the aerosol stream as generated
comprising droplets of flowable medium comprising liquid and a
copper-containing precursor; and the preparing comprising removing
at least a portion of the liquid from the droplets.
50. The method of claim 49, wherein the precursor is dissolved in
the liquid in the droplets.
51. The method of claim 50, wherein the precursor is a copper
salt.
52. The method of claim 49 wherein, during the generating, the
droplets are formed by a spray nozzle.
53. The method of claim 49, wherein: the generating comprises
sweeping away with carrier gas the droplets from a reservoir of the
flowable medium ultrasonically energized by a plurality of
ultrasonic transducers underlying the reservoir.
54. The method of claim 53, comprising: after the forming, cooling
the aerosol stream, the cooling comprising passing the aerosol
through a perforated conduit while introducing a quench gas into
the perforated conduit through openings in a wall of the perforated
conduit.
55. A method for making a copper-containing product feature, the
method comprising: making the coated copper-containing particles
according to claim 1; and after the making, processing the coated
copper-containing particles to make a feature of a product
comprising copper from the coated copper-containing particles.
56. The method of claim 55, wherein the product is a
microelectronic device.
57. The method of claim 55, wherein the product is a multilayer
ceramic.
58. The method of claim 57, wherein the feature is a conductive
trace.
59. The method of claim 58, wherein the conductive trace has a line
width of less than 25 .mu.m.
60. The method of claim 55, wherein the product is a multilayer
ceramic capacitor and the feature is an internal electrode of the
multi-layer ceramic capacitor.
61. The method of claim 60, wherein the internal electrode has an
average thickness of not greater than 2 .mu.m.
62. The method of claim 55, wherein the product is a flat panel
display.
63. The method of claim 62, wherein the flat panel display is a
plasma display panel.
64. The method of claim 63, wherein the feature is an electrode of
the plasma display panel.
65. The method of claim 63, wherein the feature is a bus line of
the plasma display panel.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
patent application Ser. No.10/758,866 filed Jan. 16, 2004, which is
a divisional application of U.S. patent application Ser. No.
09/586,151 filed on Jun. 2, 2000, now U.S. Pat. No. 6,679,937,
which is a divisional application of U.S. patent application Ser.
No. 09/030,051, filed on Feb. 24, 1998, which claims priority to
U.S. Provisional Application Ser. No. 60/038,258 filed Feb. 24,
1997 and to U.S. Provisional Application Ser. No. 60/039,450 filed
Feb. 24, 1997. The foregoing applications are incorporated herein
by reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to copper metal powders and to
methods for producing such powders, as well as devices
incorporating the powders. In particular, the present invention is
directed to powder batches of copper metal particles that can have
a well controlled average particle size, well controlled particle
size distribution, spherical morphology and high crystallinity.
[0004] 2. Description of Related Art
[0005] Many product applications require metal-containing powders
with one or more of the following properties: high purity; high
crystallinity; small average particle size; narrow particle size
distribution; spherical particle morphology; controlled surface
chemistry; reduced agglomeration; and high density (low porosity).
Examples of metal powders requiring such characteristics include,
but are not limited to, those useful in microelectronic
applications, such as for the internal electrodes and external
terminations of multi-layer ceramic capacitors (MLCC's), for
conductive traces on hybrid integrated circuits, multilayer
ceramics or multichip modules, and for resistors and other
devices.
[0006] Electronic devices such as capacitors, and in particular
MLCC's, have traditionally used electrodes fabricated from noble
metals such as silver palladium and mixtures/alloys thereof. MLCC's
are fabricated by stacking alternate layers of a ceramic dielectric
and a conductive metal and then sintering (heating) the stack to
densify the layers and obtain a monolithic device. Most ceramic
dielectric compounds are oxides that must be sintered at an
elevated temperature in an oxygen-containing atmosphere to avoid
reduction of the ceramic and the loss of the dielectric properties.
Noble metals such as palladium advantageously resist oxidation
under these conditions. However, noble metals are relatively
expensive and significantly increase the cost of such devices.
Therefore, it would be advantageous to utilize less costly base
metals for such applications. Base metals such as copper are
generally at least an order of magnitude less costly than noble
metals. But most base metals tend to oxidize when held in an
oxygen-containing atmosphere at elevated sintering temperatures,
thereby ruining the electrical properties of the metal and creating
other problems in the device, such as delamination of the stacked
layers.
[0007] Various methods have been disclosed for the production of
copper metal powder. U.S. Pat. No. 3,881,914 by Heidelberg
discloses a liquid precipitation process for the production of
electronic grade copper useful for thick film conductive circuits.
Copper sulfate or copper acetate is reacted with hypophosphorous
acid to form elemental copper which is separated, washed with an
organic and dried in a vacuum. It is disclosed that the copper
powders have particle sizes of less than about 1 to 2 .mu.m.
[0008] U.S. Pat. No. 4,645,532 by Mackiw et al. discloses a process
for the production of copper powder having particles of less than
about 5 .mu.m. An ammoniacal cuprous salt solution is acidified in
a substantially oxygen-free environment to produce the fine copper
powder, which is substantially spherical. It is disclosed that the
copper powder has an oxygen content of less than about 1 percent by
weight.
[0009] Fievet et al. in an article entitled "Controlled Nucleation
and Growth of Micrometre-size Copper Particles Prepared by the
Polyol Process", (J. Mater. Chem., Vol. 3, pgs. 627-632, 1993)
disclose the preparation of copper particles by dissolving a
precursor in a liquid polyol to nucleate and grow copper metal. In
one example, the powder had a mean particle size of 0.46 .mu.m with
a standard deviation (.sigma.) of 0.26 .mu.m.
[0010] Hsu et al. in an article entitled "Preparation and
Characterization of Uniform Particles of Pure and Coated Metallic
Copper" (Powder Technology, Vol. 63, pgs. 265-275, 1990) disclose a
liquid precipitation and reduction process for the production of
copper powder. The powder has a narrow size distribution in the 1
to 3 .mu.m size range and the particles are substantially
spherical.
[0011] U.S. Pat. No. 4,778,517 by Kopatz et al. discloses a process
for producing finely divided spherical copper powder. An acidic
solution of copper is evaporated, the compounds are milled and than
reduced by heating. At least a portion of the compounds are then
sprayed into a high temperature zone to melt the particles and form
molten droplets. The droplets are cooled to form essentially
spherical metal particles of copper or copper alloys having a size
of less than about 20 .mu.m.
[0012] Other methods have been proposed to produce copper powders.
For example, Champion et al. in an article entitled "Preparation
and Characterization of Nanocrystalline Copper Powders" (Scripta
Materialia, Vol. 35, No. 4, Pages 517-522, 1996) discloses the
production of nanocrystalline copper powders having an average
particle size of about 35 nanometers with a standard deviation of
16 nanometers. The powders are formed by cryogenic melting, that
is, overheating the molten metal in contact with a cryogenic
liquid. Significant levels of Cu.sub.2O are present on the surface
of the particles.
[0013] Spray pyrolysis is not commonly used for the production of
metal powders, particularly those containing small particles, for
example, having an average size of less than about 5 .mu.m. This is
believed to be due to the high processing costs and low production
rates typically associated with spray pyrolysis. Further, spray
pyrolysis methods often produce hollow particles that are not
sufficiently densified for most applications.
[0014] Nagashima et al. in an article entitled "Preparation of
Fine, Spherical Copper Particles by Spray-Pyrolysis Technique"
(Nippon Kagaku Kaishi, Vol. 1, Pgs. 1 7-24, 1990) disclose the
preparation of copper particles by a spray pyrolysis technique. Two
types of copper particles were observed, those having a spherical
morphology and those having an irregular shape. Spherical copper
particles were formed when the particles were heated above the
melting point of copper for 0.1 seconds or longer. The copper
particles were utilized to form copper films having a low
resistivity.
[0015] In addition to substantially pure copper metal powders,
copper metal powders having modifications such as copper alloys,
metal-ceramic composites or coated copper powders have been
disclosed in the prior art.
[0016] For example, U.S. Pat. No. 4,600,604 by Siuta discloses a
metal oxide coated copper powder. The copper powder has an average
particle size of 1 to 5 .mu.m and the oxide layer is substantially
continuous with a thickness of 1 to 20 nanometers. The oxide
coating, which is formed from an organometallic coating deposited
by solution, controls the sintering and shrinkage characteristics
of the particles when used in connection with ceramic
substrates.
[0017] U.S. Pat. No. 4,781,980 by Yoshitake et al. discloses a
coated copper powder for use in a conductive paste composition. An
antioxidation film of an organic acid salt is formed on the surface
of the copper powder using a liquid route. The coating provides
good humidity resistance and thermal resistance to the powder by
reducing surface oxidation.
[0018] Properties of copper metal powder can also be altered by
additives included with the copper metal. For example, U.S. Pat.
No. 5,470,373 by Edelstein et al. discloses oxidation resistant
copper nanoparticles that include an additive that is phase
separated from the copper. The additive can be selected from
nickel, cobalt, iron, manganese, cadmium, zinc, tin, magnesium,
calcium and chromium. The copper metal powder is produced by a
liquid precipitation route.
[0019] There remains a need for copper powders having a small
particle size, narrow size distribution, high crystallinity (large
crystals) and spherical morphology. It would be particularly
advantageous if such metal powders could be produced in large
quantities on a continuous basis.
SUMMARY OF THE INVENTION
[0020] According to one embodiment of the present invention, the
powder batch comprising copper metal particles is provided. The
metal particles are substantially spherical, have a weight average
particle size of not greater than about 5 .mu.m and a narrow
particle size distribution and high crystallinity.
[0021] According to another embodiment of the present invention, a
powder batch of metal alloy particles comprising copper metal is
provided wherein the particles have a small particle size and a
narrow particle size distribution. According to yet another
embodiment of the present invention, a powder batch of coated
copper metal particles is provided. According to yet another
embodiment of the present invention, a powder batch of metal
composite particles which include copper metal and a non-metallic
phase is provided.
[0022] The present invention also provides thick film paste
compositions including copper metal particles, such as coated
copper metal particles and composite copper metal particles. The
present invention also provides green bodies suitable for sintering
to form microelectronic devices wherein the green bodies include a
thick film paste composition comprising copper metal particles.
[0023] The present invention is also directed to microelectronic
devices which incorporate the copper metal particles of the present
invention.
[0024] The present invention is further directed to a method for
the production of copper metal particles which generally includes
generating an aerosol of droplets including copper metal precursor
and moving the droplets to a heating zone to form copper metal
particles. The method of the present invention is applicable to the
formation of metal alloy particles, composite particles and coated
particles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a process block diagram showing one embodiment of
the process of the present invention.
[0026] FIG. 2 is a side view of a furnace and showing one
embodiment of the present invention for sealing the end of a
furnace tube.
[0027] FIG. 3 is a view of the side of an end cap that faces away
from the furnace shown in FIG. 2.
[0028] FIG. 4 is a view of the side of an end cap that faces toward
the furnace shown in FIG. 2.
[0029] FIG. 5 is a side view in cross section of one embodiment of
aerosol generator of the present invention.
[0030] FIG. 6 is a top view of a transducer mounting plate showing
a 49 transducer array for use in an aerosol generator of the
present invention.
[0031] FIG. 7 is a top view of a transducer mounting plate for a
400 transducer array for use in an ultrasonic generator of the
present invention.
[0032] FIG. 8 is a side view of the transducer mounting plate shown
in FIG. 7.
[0033] FIG. 9 is a partial side view showing the profile of a
single transducer mounting receptacle of the transducer mounting
plate shown in FIG. 7.
[0034] FIG. 10 is a partial side view in cross-section showing an
alternative embodiment for mounting an ultrasonic transducer.
[0035] FIG. 11 is a top view of a bottom retaining plate for
retaining a separator for use in an aerosol generator of the
present invention.
[0036] FIG. 12 is a top view of a liquid feed box having a bottom
retaining plate to assist in retaining a separator for use in an
aerosol generator of the present invention.
[0037] FIG. 13 is a side view of the liquid feed box shown in FIG.
8.
[0038] FIG. 14 is a side view of a gas tube for delivering gas
within an aerosol generator of the present invention.
[0039] FIG. 15 shows a partial top view of gas tubes positioned in
a liquid feed box for distributing gas relative to ultrasonic
transducer positions for use in an aerosol generator of the present
invention.
[0040] FIG. 16 shows one embodiment for a gas distribution
configuration for the aerosol generator of the present
invention.
[0041] FIG. 17 shows another embodiment for a gas distribution
configuration for the aerosol generator of the present
invention.
[0042] FIG. 18 is a top view of one embodiment of a gas
distribution plate/gas tube assembly of the aerosol generator of
the present invention.
[0043] FIG. 19 is a side view of one embodiment of the gas
distribution plate/gas tube assembly shown in FIG. 18.
[0044] FIG. 20 shows one embodiment for orienting a transducer in
the aerosol generator of the present invention.
[0045] FIG. 21 is a top view of a gas manifold for distributing gas
within an aerosol generator of the present invention.
[0046] FIG. 22 is a side view of the gas manifold shown in FIG.
21.
[0047] FIG. 23 is a top view of a generator lid of a hood design
for use in an aerosol generator of the present invention.
[0048] FIG. 24 is a side view of the generator lid shown in FIG.
23.
[0049] FIG. 25 is a process block diagram of one embodiment in the
present invention including an aerosol concentrator.
[0050] FIG. 26 is a top view in cross section of a virtual impactor
that may be used for concentrating an aerosol according to the
present invention.
[0051] FIG. 27 is a front view of an upstream plate assembly of the
virtual impactor shown in FIG. 26.
[0052] FIG. 28 is a top view of the upstream plate assembly shown
in FIG. 27.
[0053] FIG. 29 is a side view of the upstream plate assembly shown
in FIG. 27.
[0054] FIG. 30 is a front view of a downstream plate assembly of
the virtual impactor shown in FIG. 26.
[0055] FIG. 31 is a top view of the downstream plate assembly shown
in FIG. 30.
[0056] FIG. 32 is a side view of the downstream plate assembly
shown in FIG. 30.
[0057] FIG. 33 is a process block diagram of one embodiment of the
process of the present invention including a droplet
classifier.
[0058] FIG. 34 is a top view in cross section of an impactor of the
present invention for use in classifying an aerosol.
[0059] FIG. 35 is a front view of a flow control plate of the
impactor shown in FIG. 34.
[0060] FIG. 36 is a front view of a mounting plate of the impactor
shown in FIG. 34.
[0061] FIG. 37 is a front view of an impactor plate assembly of the
impactor shown in FIG. 34.
[0062] FIG. 38 is a side view of the impactor plate assembly shown
in FIG. 37.
[0063] FIG. 39 shows a side view in cross section of a virtual
impactor in combination with an impactor of the present invention
for concentrating and classifying droplets in an aerosol.
[0064] FIG. 40 is a process block diagram of one embodiment of the
present invention including a particle cooler.
[0065] FIG. 41 is a top view of a gas quench cooler of the present
invention.
[0066] FIG. 42 is an end view of the gas quench cooler shown in
FIG. 41.
[0067] FIG. 43 is a side view of a perforated conduit of the quench
cooler shown in FIG. 41.
[0068] FIG. 44 is a side view showing one embodiment of a gas
quench cooler of the present invention connected with a
cyclone.
[0069] FIG. 45 is a process block diagram of one embodiment of the
present invention including a particle coater.
[0070] FIG. 46 is a block diagram of one embodiment of the present
invention including a particle modifier.
[0071] FIG. 47 shows cross sections of various particle
morphologies of some composite particles manufacturable according
to the present invention.
[0072] FIG. 48 shows a side view of one embodiment of apparatus of
the present invention including an aerosol generator, an aerosol
concentrator, a droplet classifier, a furnace, a particle cooler,
and a particle collector.
[0073] FIG. 49 is a block diagram of one embodiment of the process
of the present invention including the addition of a dry gas
between the aerosol generator and the furnace.
[0074] FIG. 50 illustrates a schematic view of a microelectronic
device according to an embodiment of the present invention.
[0075] FIG. 51 illustrates a top view of a microelectronic device
according to an embodiment of the present invention.
[0076] FIG. 52 illustrates a schematic view of a multilayer ceramic
capacitor according to an embodiment of the present invention.
[0077] FIG. 53 illustrates a schematic view of a flat panel display
according to an embodiment of the present invention.
[0078] FIG. 54 illustrates another view of a flat panel display
according to an embodiment of the present invention.
[0079] FIG. 55 illustrates a photomicrograph of a copper metal
powder according to an embodiment of the present invention.
[0080] FIG. 56 illustrates a photomicrograph of a copper metal
powder according to an embodiment of the present invention.
[0081] FIG. 57 illustrates a photomicrograph of a copper metal
powder according to an embodiment of the present invention.
[0082] FIG. 58 illustrates a photomicrograph of a copper metal
powder according to an embodiment of the present invention.
[0083] FIG. 59 illustrates a photomicrograph of a copper metal
composite powder according to an embodiment of the present
invention.
[0084] FIG. 60 illustrates a photomicrograph of a copper metal
composite powder according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0085] The present invention is generally directed to copper metal
powders and methods for producing the powders. The present
invention is also directed to novel intermediate products and
devices fabricated using the copper metal powders. As used herein,
copper powders or copper particles are those that include copper or
a copper-based compound such as pure copper metal, copper metal
alloys, other copper compounds, composite particles, coated
particles, and the like.
[0086] In one aspect, the present invention provides a method for
preparing a particulate product. A feed of liquid-containing,
flowable medium, including at least one precursor for the desired
particulate product, is converted to aerosol form, with droplets of
the medium being dispersed in and suspended by a carrier gas.
Liquid from the droplets in the aerosol is then removed to permit
formation in a dispersed state of the desired particles. Typically,
the feed precursor is pyrolyzed in a furnace to make the particles.
In one embodiment, the particles are subjected, while still in a
dispersed state, to compositional or structural modification, if
desired. Compositional modification may include, for example,
coating the particles. Structural modification may include, for
example, crystallization, recrystallization or morphological
alteration of the particles. The term powder is often used herein
to refer to the particulate product of the present invention. The
use of the term powder does not indicate, however, that the
particulate product must be dry or in any particular environment.
Although the particulate product is typically manufactured in a dry
state, the particulate product may, after manufacture, be placed in
a wet environment, such as in a paste or slurry.
[0087] The process of the present invention is particularly well
suited for the production of particulate products of finely divided
particles having a small weight average size. In addition to making
particles within a desired range of weight average particle size,
with the present invention the particles may be produced with a
desirably narrow size distribution, thereby providing size
uniformity that is desired for many applications.
[0088] In addition to control over particle size and size
distribution, the method of the present invention provides
significant flexibility for producing particles of varying
composition, crystallinity and morphology. For example, the present
invention may be used to produce homogeneous particles involving
only a single phase or multi-phase particles including multiple
phases. In the case of multi-phase particles, the phases may be
present in a variety of morphologies. For example, one phase may be
uniformly dispersed throughout a matrix of another phase.
Alternatively, one phase may form an interior core while another
phase forms a coating that surrounds the core. Other morphologies
are also possible, as discussed more fully below.
[0089] Referring now to FIG. 1, one embodiment of the process of
the present invention is described. A liquid feed 102, including at
least one precursor for the desired particles, and a carrier gas
104 are fed to an aerosol generator 106 where an aerosol 108 is
produced. The aerosol 108 is then fed to a furnace 110 where liquid
in the aerosol 108 is removed to produce particles 112 that are
dispersed in and suspended by gas exiting the furnace 110. The
particles 112 are then collected in a particle collector 114 to
produce a particulate product 116.
[0090] As used herein, the liquid feed 102 is a feed that includes
one or more flowable liquids as the major constituent(s), such that
the feed is a flowable medium. The liquid feed 102 need not
comprise only liquid constituents. The liquid feed 102 may comprise
only constituents in one or more liquid phase, or it may also
include particulate material suspended in a liquid phase. The
liquid feed 102 must, however, be capable of being atomized to form
droplets of sufficiently small size for preparation of the aerosol
108. Therefore, if the liquid feed 102 includes suspended
particles, those particles should be relatively small in relation
to the size of droplets in the aerosol 108. Such suspended
particles should typically be smaller than about 1 .mu.m in size,
preferably smaller than about 0.5 .mu.m in size, and more
preferably smaller than about 0.3 .mu.m in size and most preferably
smaller than about 0.1 .mu.m in size. Most preferably, the
suspended particles should be able to form a colloid. The suspended
particles could be finely divided particles, or could be
agglomerate masses comprised of agglomerated smaller primary
particles. For example, 0.5 .mu.m particles could be agglomerates
of nanometer-sized primary particles. When the liquid feed 102
includes suspended particles, the particles typically comprise no
greater than about 25 to 50 weight percent of the liquid feed.
[0091] As noted, the liquid feed 102 includes at least one
precursor for preparation of the particles 112. The precursor may
be a substance in either a liquid or solid phase of the liquid feed
102. Frequently, the precursor will be a material, such as a salt,
dissolved in a liquid solvent of the liquid feed 102. The precursor
may undergo one or more chemical reactions in the furnace 110 to
assist in production of the particles 112. Alternatively, the
precursor material may contribute to formation of the particles 112
without undergoing chemical reaction. This could be the case, for
example, when the liquid feed 102 includes, as a precursor
material, suspended particles that are not chemically modified in
the furnace 110. In any event, the particles 112 comprise at least
one component originally contributed by the precursor.
[0092] The liquid feed 102 may include multiple precursor
materials, which may be present together in a single phase or
separately in multiple phases. For example, the liquid feed 102 may
include multiple precursors in solution in a single liquid vehicle.
Alternatively, one precursor material could be in a solid
particulate phase and a second precursor material could be in a
liquid phase. Also, one precursor material could be in one liquid
phase and a second precursor material could be in a second liquid
phase, such as could be the case when the liquid feed 102 comprises
an emulsion. Different components contributed by different
precursors may be present in the particles together in a single
material phase, or the different components may be present in
different material phases when the particles 112 are composites of
multiple phases. Specific examples of preferred precursor materials
are discussed more fully below.
[0093] The carrier gas 104 may comprise any gaseous medium in which
droplets produced from the liquid feed 102 may be dispersed in
aerosol form. Also, the carrier gas 104 may be inert, in that the
carrier gas 104 does not participate in formation of the particles
112. Alternatively, the carrier gas may have one or more active
component(s) that contribute to formation of the particles 112. In
that regard, the carrier gas may include one or more reactive
components that react in the furnace 110 to contribute to formation
of the particles 112. Preferred carrier gas compositions are
discussed more fully below.
[0094] The aerosol generator 106 atomizes the liquid feed 102 to
form droplets in a manner to permit the carrier gas 104 to sweep
the droplets away to form the aerosol 108. The droplets comprise
liquid from the liquid feed 102. The droplets may, however, also
include nonliquid material, such as one or more small particles
held in the droplet by the liquid. For example, when the particles
112 are composite, or multi-phase, particles, one phase of the
composite may be provided in the liquid feed 102 in the form of
suspended precursor particles and a second phase of the composite
may be produced in the furnace 110 from one or more precursors in
the liquid phase of the liquid feed 102. Furthermore the precursor
particles could be included in the liquid feed 102, and therefore
also in droplets of the aerosol 108, for the purpose only of
dispersing the particles for subsequent compositional or structural
modification during or after processing in the furnace 110.
[0095] An important aspect of the present invention is generation
of the aerosol 108 with droplets of a small average size, narrow
size distribution. In this manner, the particles 112 may be
produced at a desired small size with a narrow size distribution,
which are advantageous for many applications.
[0096] The aerosol generator 106 is capable of producing the
aerosol 108 such that it includes droplets having a weight average
size in a range having a lower limit of about 1 .mu.m and
preferably about 2 .mu.m; and an upper limit of about 10 .mu.m;
preferably about 7 .mu.m, more preferably about 5 .mu.m and most
preferably about 4 .mu.m. A weight average droplet size in a range
of from about 2 .mu.m to about 4 .mu.m is more preferred for most
applications, with a weight average droplet size of about 3 .mu.m
being particularly preferred for some applications. The aerosol
generator is also capable of producing the aerosol 108 such that it
includes droplets in a narrow size distribution. Preferably, the
droplets in the aerosol are such that at least about 70 percent
(more preferably at least about 80 weight percent and most
preferably at least about 85 weight percent) of the droplets are
smaller than about 10 .mu.m and more preferably at least about 70
weight percent (more preferably at least about 80 weight percent
and most preferably at least about 85 weight percent) are smaller
than about 5 .mu.m. Furthermore, preferably no greater than about
30 weight percent, more preferably no greater than about 25 weight
percent and most preferably no greater than about 20 weight
percent, of the droplets in the aerosol 108 are larger than about
twice the weight average droplet size.
[0097] Another important aspect of the present invention is that
the aerosol 108 may be generated without consuming excessive
amounts of the carrier gas 104. The aerosol generator 106 is
capable of producing the aerosol 108 such that it has a high
loading, or high concentration, of the liquid feed 102 in droplet
form. In that regard, the aerosol 108 preferably includes greater
than about 1.times.10.sup.6 droplets per cubic centimeter of the
aerosol 108, more preferably greater than about 5.times.10.sup.6
droplets per cubic centimeter, still more preferably greater than
about 1.times.10.sup.7 droplets per cubic centimeter, and most
preferably greater than about 5.times.10.sup.7 droplets per cubic
centimeter. That the aerosol generator 106 can produce such a
heavily loaded aerosol 108 is particularly surprising considering
the high quality of the aerosol 108 with respect to small average
droplet size and narrow droplet size distribution. Typically,
droplet loading in the aerosol is such that the volumetric ratio of
liquid feed 102 to carrier gas 104 in the aerosol 108 is larger
than about 0.04 milliliters of liquid feed 102 per liter of carrier
gas 104 in the aerosol 108, preferably larger than about 0.083
milliliters of liquid feed 102 per liter of carrier gas 104 in the
aerosol 108, more preferably larger than about 0.167 milliliters of
liquid feed 102 per liter of carrier gas 104, still more preferably
larger than about 0.25 milliliters of liquid feed 102 per liter of
carrier gas 104, and most preferably larger than about 0.333
milliliters of liquid feed 102 per liter of carrier gas 104.
[0098] This capability of the aerosol generator 106 to produce a
heavily loaded aerosol 108 is even more surprising given the high
droplet output rate of which the aerosol generator 106 is capable,
as discussed more fully below. It will be appreciated that the
concentration of liquid feed 102 in the aerosol 108 will depend
upon the specific components and attributes of the liquid feed 102
and, particularly, the size of the droplets in the aerosol 108. For
example, when the average droplet size is from about 2 .mu.m to
about 4 .mu.m, the droplet loading is preferably larger than about
0.15 milliliters of aerosol feed 102 per liter of carrier gas 104,
more preferably larger than about 0.2 milliliters of liquid feed
102 per liter of carrier gas 104, even more preferably larger than
about 0.2 milliliters of liquid feed 102 per liter of carrier gas
104, and most preferably larger than about 0.3 milliliters of
liquid feed 102 per liter of carrier gas 104. When reference is
made herein to liters of carrier gas 104, it refers to the volume
that the carrier gas 104 would occupy under conditions of standard
temperature and pressure.
[0099] The furnace 110 may be any suitable device for heating the
aerosol 108 to evaporate liquid from the droplets of the aerosol
108 and thereby permit formation of the particles 112. The maximum
average stream temperature, or reaction temperature, refers to the
maximum average temperature that an aerosol stream attains while
flowing through the furnace. This is typically determined by a
temperature probe inserted into the furnace. Preferred reaction
temperatures according to the present invention are discussed more
fully below.
[0100] Although longer residence times are possible, for many
applications, residence time in the heating zone of the furnace 110
of shorter than about 4 seconds is typical, with shorter than about
2 seconds being preferred, shorter than about 1 second being more
preferred, shorter than about 0.5 second being even more preferred,
and shorter than about 0.2 second being most preferred. The
residence time should be long enough, however, to assure that the
particles 112 attain the desired maximum stream temperature for a
given heat transfer rate. In that regard, with extremely short
residence times, higher furnace temperatures could be used to
increase the rate of heat transfer so long as the particles 112
attain a maximum temperature within the desired stream temperature
range. That mode of operation, however, is not preferred. Also, it
is preferred that, in most cases, the maximum stream temperature
not be attained in the furnace 110 until substantially at the end
of the heating zone in the furnace 110. For example, the heating
zone will often include a plurality of heating sections that are
each independently controllable. The maximum stream temperature
should typically not be attained until the final heating section,
and more preferably until substantially at the end of the last
heating section. This is important to reduce the potential for
thermophoretic losses of material. Also, it is noted that as used
herein, residence time refers to the actual time for a material to
pass through the relevant process equipment. In the case of the
furnace, this includes the effect of increasing velocity with gas
expansion due to heating.
[0101] Typically, the furnace 110 will be a tube-shaped furnace, so
that the aerosol 108 moving into and through the furnace does not
encounter sharp edges on which droplets could collect. Loss of
droplets to collection at sharp surfaces results in a lower yield
of particles 112. More important, however, the accumulation of
liquid at sharp edges can result in re-release of undesirably large
droplets back into the aerosol 108, which can cause contamination
of the particulate product 116 with undesirably large particles.
Also, over time, such liquid collection at sharp surfaces can cause
fouling of process equipment, impairing process performance.
[0102] The furnace 110 may include a heating tube made of any
suitable material. The tube material may be a ceramic material, for
example, mullite, silica or alumina. Alternatively, the tube may be
metallic. Advantages of using a metallic tube are low cost, ability
to withstand steep temperature gradients and large thermal shocks,
machinability and weldability, and ease of providing a seal between
the tube and other process equipment. Disadvantages of using a
metallic tube include limited operating temperature and increased
reactivity in some reaction systems.
[0103] When a metallic tube is used in the furnace 110, it is
preferably a high nickel content stainless steel alloy, such as a
330 stainless steel, or a nickel-based super alloy. As noted, one
of the major advantages of using a metallic tube is that the tube
is relatively easy to seal with other process equipment. In that
regard, flange fittings may be welded directly to the tube for
connecting with other process equipment. Metallic tubes are
generally preferred for making particles that do not require a
maximum tube wall temperature of higher than about 1100.degree. C.
during particle manufacture.
[0104] When higher temperatures are required, ceramic tubes are
typically used. One major problem with ceramic tubes, however, is
that the tubes can be difficult to seal with other process
equipment, especially when the ends of the tubes are maintained at
relatively high temperatures, as is often the case with the present
invention.
[0105] One configuration for sealing a ceramic tube is shown in
FIGS. 2, 3 and 4. The furnace 110 includes a ceramic tube 374
having an end cap 376 fitted to each end of the tube 374, with a
gasket 378 disposed between corresponding ends of the ceramic tube
374 and the end caps 376. The gasket may be of any suitable
material for sealing at the temperature encountered at the ends of
the tubes 374. Examples of gasket materials for sealing at
temperatures below about 250.degree. C. include silicone,
TEFLON.TM. and VITON.TM.. Examples of gasket materials for higher
temperatures include graphite, ceramic paper, thin sheet metal, and
combinations thereof.
[0106] Tension rods 380 extend over the length of the furnace 110
and through rod holes 382 through the end caps 376. The tension
rods 380 are held in tension by the force of springs 384 bearing
against bearing plates 386 and the end caps 376. The tube 374 is,
therefore, in compression due to the force of the springs 384. The
springs 384 may be compressed any desired amount to form a seal
between the end caps 376 and the ceramic tube 374 through the
gasket 378. As will be appreciated, by using the springs 384, the
tube 374 is free to move to some degree as it expands upon heating
and contracts upon cooling. To form the seal between the end caps
376 and the ceramic tube 374, one of the gaskets 378 is seated in a
gasket seat 388 on the side of each end cap 376 facing the tube
374. A mating face 390 on the side of each of the end caps 376
faces away from the tube 374, for mating with a flange surface for
connection with an adjacent piece of equipment.
[0107] Also, although the present invention is described with
primary reference to a furnace reactor, which is preferred, it
should be recognized that, except as noted, any other thermal
reactor, including a flame reactor or a plasma reactor, could be
used instead. A furnace reactor is, however, preferred, because of
the generally even heating characteristic of a furnace for
attaining a uniform stream temperature.
[0108] The particle collector 114, may be any suitable apparatus
for collecting particles 112 to produce the particulate product
116. One preferred embodiment of the particle collector 114 uses
one or more filter to separate the particles 112 from gas. Such a
filter may be of any type, including a bag filter. Another
preferred embodiment of the particle collector uses one or more
cyclone to separate the particles 112. Other apparatus that may be
used in the particle collector 114 includes an electrostatic
precipitator. Also, collection should normally occur at a
temperature above the condensation temperature of the gas stream in
which the particles 112 are suspended. Also, collection should
normally be at a temperature that is low enough to prevent
significant agglomeration of the particles 112.
[0109] Of significant importance to the operation of the process of
the present invention is the aerosol generator 106, which must be
capable of producing a high quality aerosol with high droplet
loading, as previously noted. With reference to FIG. 5, one
embodiment of an aerosol generator 106 of the present invention is
described. The aerosol generator 106 includes a plurality of
ultrasonic transducer discs 120 that are each mounted in a
transducer housing 122. The transducer housings 122 are mounted to
a transducer mounting plate 124, creating an array of the
ultrasonic transducer discs 120. Any convenient spacing may be used
for the ultrasonic transducer discs 120. Center-to-center spacing
of the ultrasonic transducer discs 120 of about 4 centimeters is
often adequate. The aerosol generator 106, as shown in FIG. 5,
includes forty-nine transducers in a 7.times.7 array. The array
configuration is as shown in FIG. 6, which depicts the locations of
the transducer housings 122 mounted to the transducer mounting
plate 124.
[0110] With continued reference to FIG. 5, a separator 126, in
spaced relation to the transducer discs 120, is retained between a
bottom retaining plate 128 and a top retaining plate 130. Gas
delivery tubes 132 are connected to gas distribution manifolds 134,
which have gas delivery ports 136. The gas distribution manifolds
134 are housed within a generator body 138 that is covered by
generator lid 140. A transducer driver 144, having circuitry for
driving the transducer discs 120, is electronically connected with
the transducer discs 120 via electrical cables 146.
[0111] During operation of the aerosol generator 106, as shown in
FIG. 5, the transducer discs 120 are activated by the transducer
driver 144 via the electrical cables 146. The transducers
preferably vibrate at a frequency of from about 1 MHz to about 5
MHz, more preferably from about 1.5 MHz to about 3 MHz. Frequently
used frequencies are at about 1.6 MHz and about 2.4 MHz.
Furthermore, all of the transducer discs 110 should be operating at
substantially the same frequency when an aerosol with a narrow
droplet size distribution is desired. This is important because
commercially available transducers can vary significantly in
thickness, sometimes by as much as 10%. It is preferred, however,
that the transducer discs 120 operate at frequencies within a range
of 5% above and below the median transducer frequency, more
preferably within a range of 2.5%, and most preferably within a
range of 1%. This can be accomplished by careful selection of the
transducer discs 120 so that they all preferably have thicknesses
within 5% of the median transducer thickness, more preferably
within 2.5%, and most preferablywithin 1%.
[0112] Liquid feed 102 enters through a feed inlet 148 and flows
through flow channels 150 to exit through feed outlet 152. An
ultrasonically transmissive fluid, typically water, enters through
a water inlet 154 to fill a water bath volume 156 and flow through
flow channels 158 to exit through a water outlet 160. A proper flow
rate of the ultrasonically transmissive fluid is necessary to cool
the transducer discs 120 and to prevent overheating of the
ultrasonically transmissive fluid. Ultrasonic signals from the
transducer discs 120 are transmitted, via the ultrasonically
transmissive fluid, across the water bath volume 156, and
ultimately across the separator 126, to the liquid feed 102 in flow
channels 150.
[0113] The ultrasonic signals from the ultrasonic transducer discs
120 cause atomization cones 162 to develop in the liquid feed 102
at locations corresponding with the transducer discs 120. Carrier
gas 104 is introduced into the gas delivery tubes 132 and delivered
to the vicinity of the atomization cones 162 via gas delivery ports
136. Jets of carrier gas exit the gas delivery ports 136 in a
direction so as to impinge on the atomization cones 162, thereby
sweeping away atomized droplets of the liquid feed 102 that are
being generated from the atomization cones 162 and creating the
aerosol 108, which exits the aerosol generator 106 through an
aerosol exit opening 164.
[0114] Efficient use of the carrier gas 104 is an important aspect
of the aerosol generator 106. The embodiment of the aerosol
generator 106 shown in FIG. 5 includes two gas exit ports per
atomization cone 162, with the gas ports being positioned above the
liquid medium 102 over troughs that develop between the atomization
cones 162, such that the exiting carrier gas 104 is horizontally
directed at the surface of the atomization cones 162, thereby
efficiently distributing the carrier gas 104 to critical portions
of the liquid feed 102 for effective and efficient sweeping away of
droplets as they form about the ultrasonically energized
atomization cones 162. Furthermore, it is preferred that at least a
portion of the opening of each of the gas delivery ports 136,
through which the carrier gas exits the gas delivery tubes, should
be located below the top of the atomization cones 162 at which the
carrier gas 104 is directed. This relative placement of the gas
delivery ports 136 is very important to efficient use of carrier
gas 104. Orientation of the gas delivery ports 136 is also
important. Preferably, the gas delivery ports 136 are positioned to
horizontally direct jets of the carrier gas 104 at the atomization
cones 162. The aerosol generator 106 permits generation of the
aerosol 108 with heavy loading with droplets of the carrier liquid
102, unlike aerosol generator designs that do not efficiently focus
gas delivery to the locations of droplet formation.
[0115] Another important feature of the aerosol generator 106, as
shown in FIG. 5, is the use of the separator 126, which protects
the transducer discs 120 from direct contact with the liquid feed
102, which is often highly corrosive. The height of the separator
126 above the top of the transducer discs 120 should normally be
kept as small as possible, and is often in the range of from about
1 centimeter to about 2 centimeters. The top of the liquid feed 102
in the flow channels above the tops of the ultrasonic transducer
discs 120 is typically in a range of from about 2 centimeters to
about 5 centimeters, whether or not the aerosol generator includes
the separator 126, with a distance of about 3 to 4 centimeters
being preferred. Although the aerosol generator 106 could be made
without the separator 126, in which case the liquid feed 102 would
be in direct contact with the transducer discs 120, the highly
corrosive nature of the liquid feed 102 can often cause premature
failure of the transducer discs 120. The use of the separator 126,
in combination with use of the ultrasonically transmissive fluid in
the water bath volume 156 to provide ultrasonic coupling,
significantly extending the life of the ultrasonic transducers 120.
One disadvantage of using the separator 126, however, is that the
rate of droplet production from the atomization cones 162 is
reduced, often by a factor of two or more, relative to designs in
which the liquid feed 102 is in direct contact with the ultrasonic
transducer discs 102. Even with the separator 126, however, the
aerosol generator 106 used with the present invention is capable of
producing a high quality aerosol with heavy droplet loading, as
previously discussed. Suitable materials for the separator 126
include, for example, polyamides (such as Kapton.TM. membranes from
DuPont) and other polymer materials, glass, and plexiglass. The
main requirements for the separator 126 are that it be
ultrasonically transmissive, corrosion resistant and
impermeable.
[0116] One alternative to using the separator 126 is to bind a
corrosion-resistant protective coating onto the surface of the
ultrasonic transducer discs 120, thereby preventing the liquid feed
102 from contacting the surface of the ultrasonic transducer discs
120. When the ultrasonic transducer discs 120 have a protective
coating, the aerosol generator 106 will typically be constructed
without the water bath volume 156 and the liquid feed 102 will flow
directly over the ultrasonic transducer discs 120. Examples of such
protective coating materials include platinum, gold, TEFLON.TM.,
epoxies and various plastics. Such coating typically significantly
extends transducer life. Also, when operating without the separator
126, the aerosol generator 106 will typically produce the aerosol
108 with a much higher droplet loading than when the separator 126
is used.
[0117] One surprising finding with operation of the aerosol
generator 106 of the present invention is that the droplet loading
in the aerosol may be affected by the temperature of the liquid
feed 102. It has been found that when the liquid feed 102 includes
an aqueous liquid at an elevated temperature, the droplet loading
increases significantly. The temperature of the liquid feed 102 is
preferably higher than about 30.degree. C., more preferably higher
than about 35.degree. C. and most preferably higher than about
40.degree. C. If the temperature becomes too high, however, it can
have a detrimental effect on droplet loading in the aerosol 108.
Therefore, the temperature of the liquid feed 102 from which the
aerosol 108 is made should generally be lower than about 50.degree.
C., and preferably lower than about 45.degree. C. The liquid feed
102 may be maintained at the desired temperature in any suitable
fashion. For example, the portion of the aerosol generator 106
where the liquid feed 102 is converted to the aerosol 108 could be
maintained at a constant elevated temperature. Alternatively, the
liquid feed 102 could be delivered to the aerosol generator 106
from a constant temperature bath maintained separate from the
aerosol generator 106. When the ultrasonic generator 106 includes
the separator 126, the ultrasonically transmissive fluid adjacent
the ultrasonic transducer disks 120 are preferably also at an
elevated temperature in the ranges just discussed for the liquid
feed 102.
[0118] The design for the aerosol generator 106 based on an array
of ultrasonic transducers is versatile and is easily modified to
accommodate different generator sizes for different specialty
applications. The aerosol generator 106 may be designed to include
a plurality of ultrasonic transducers in any convenient number.
Even for smaller scale production, however, the aerosol generator
106 preferably has at least nine ultrasonic transducers, more
preferably at least 16 ultrasonic transducers, and even more
preferably at least 25 ultrasonic transducers. For larger scale
production, however, the aerosol generator 106 includes at least 40
ultrasonic transducers, more preferably at least 100 ultrasonic
transducers, and even more preferably at least 400 ultrasonic
transducers. In some large volume applications, the aerosol
generator may have at least 1000 ultrasonic transducers.
[0119] FIGS. 7-24 show component designs for an aerosol generator
106 including an array of 400 ultrasonic transducers. Referring
first to FIGS. 7 and 8, the transducer mounting plate 124 is shown
with a design to accommodate an array of 400 ultrasonic
transducers, arranged in four subarrays of 100 ultrasonic
transducers each. The transducer mounting plate 124 has integral
vertical walls 172 for containing the ultrasonically transmissive
fluid, typically water, in a water bath similar to the water bath
volume 156 described previously with reference to FIG. 5.
[0120] As shown in FIGS. 7 and 8, four hundred transducer mounting
receptacles 174 are provided in the transducer mounting plate 124
for mounting ultrasonic transducers for the desired array. With
reference to FIG. 9, the profile of an individual transducer
mounting receptacle 174 is shown. A mounting seat 176 accepts an
ultrasonic transducer for mounting, with a mounted ultrasonic
transducer being held in place via screw holes 178. Opposite the
mounting receptacle 176 is a flared opening 180 through which an
ultrasonic signal may be transmitted for the purpose of generating
the aerosol 108, as previously described with reference to FIG.
5.
[0121] A preferred transducer mounting configuration, however, is
shown in FIG. 10 for another configuration for the transducer
mounting plate 124. As seen in FIG. 10, an ultrasonic transducer
disc 120 is mounted to the transducer mounting plate 124 by use of
a compression screw 177 threaded into a threaded receptacle 179.
The compression screw 177 bears against the ultrasonic transducer
disc 120, causing an o-ring 181, situated in an o-ring seat 182 on
the transducer mounting plate, to be compressed to form a seal
between the transducer mounting plate 124 and the ultrasonic
transducer disc 120. This type of transducer mounting is
particularly preferred when the ultrasonic transducer disc 120
includes a protective surface coating, as discussed previously,
because the seal of the o-ring to the ultrasonic transducer disc
120 will be inside of the outer edge of the protective seal,
thereby preventing liquid from penetrating under the protective
surface coating from the edges of the ultrasonic transducer disc
120.
[0122] Referring now to FIG. 11, the bottom retaining plate 128 for
a 400 transducer array is shown having a design for mating with the
transducer mounting plate 124 (shown in FIGS. 7-8). The bottom
retaining plate 128 has eighty openings 184, arranged in four
subgroups 186 of twenty openings 184 each. Each of the openings 184
corresponds with five of the transducer mounting receptacles 174
(shown in FIGS. 7 and 8) when the bottom retaining plate 128 is
mated with the transducer mounting plate 124 to create a volume for
a water bath between the transducer mounting plate 124 and the
bottom retaining plate 128. The openings 184, therefore, provide a
pathway for ultrasonic signals generated by ultrasonic transducers
to be transmitted through the bottom retaining plate.
[0123] Referring now to FIGS. 12 and 13, a liquid feed box 190 for
a 400 transducer array is shown having the top retaining plate 130
designed to fit over the bottom retaining plate 128 (shown in FIG.
1l), with a separator 126 (not shown) being retained between the
bottom retaining plate 128 and the top retaining plate 130 when the
aerosol generator 106 is assembled. The liquid feed box 190 also
includes vertically extending walls 192 for containing the liquid
feed 102 when the aerosol generator is in operation. Also shown in
FIGS. 12 and 13 is the feed inlet 148 and the feed outlet 152. An
adjustable weir 198 determines the level of liquid feed 102 in the
liquid feed box 190 during operation of the aerosol generator
106.
[0124] The top retaining plate 130 of the liquid feed box 190 has
eighty openings 194 therethrough, which are arranged in four
subgroups 196 of twenty openings 194 each. The openings 194 of the
top retaining plate 130 correspond in size with the openings 184 of
the bottom retaining plate 128 (shown in FIG. 11). When the aerosol
generator 106 is assembled, the openings 194 through the top
retaining plate 130 and the openings 184 through the bottom
retaining plate 128 are aligned, with the separator 126 positioned
therebetween, to permit transmission of ultrasonic signals when the
aerosol generator 106 is in operation.
[0125] Referring now to FIGS. 12-14, a plurality of gas tube
feed-through holes 202 extend through the vertically extending
walls 192 to either side of the assembly including the feed inlet
148 and feed outlet 152 of the liquid feed box 190. The gas tube
feed-through holes 202 are designed to permit insertion
therethrough of gas tubes 208 of a design as shown in FIG. 14. When
the aerosol generator 106 is assembled, a gas tube 208 is inserted
through each of the gas tube feed-through holes 202 so that gas
delivery ports 136 in the gas tube 208 will be properly positioned
and aligned adjacent the openings 194 in the top retaining plate
130 for delivery of gas to atomization cones that develop in the
liquid feed box 190 during operation of the aerosol generator 106.
The gas delivery ports 136 are typically holes having a diameter of
from about 1.5 millimeters to about 3.5 millimeters.
[0126] Referring now to FIG. 15, a partial view of the liquid feed
box 190 is shown with gas tubes 208A, 208B and 208C positioned
adjacent to the openings 194 through the top retaining plate 130.
Also shown in FIG. 15 are the relative locations that ultrasonic
transducer discs 120 would occupy when the aerosol generator 106 is
assembled. As seen in FIG. 15, the gas tube 208A, which is at the
edge of the array, has five gas delivery ports 136. Each of the gas
delivery ports 136 is positioned to divert carrier gas 104 to a
different one of atomization cones that develop over the array of
ultrasonic transducer discs 120 when the aerosol generator 106 is
operating. The gas tube 208B, which is one row in from the edge of
the array, is a shorter tube that has ten gas delivery ports 136,
five each on opposing sides of the gas tube 208B. The gas tube
208B, therefore, has gas delivery ports 136 for delivering gas to
atomization cones corresponding with each of ten ultrasonic
transducer discs 120. The third gas tube, 208C, is a longer tube
that also has ten gas delivery ports 136 for delivering gas to
atomization cones corresponding with ten ultrasonic transducer
discs 120. The design shown in FIG. 15, therefore, includes one gas
delivery port per ultrasonic transducer disc 120. Although this is
a lower density of gas delivery ports 136 than for the embodiment
of the aerosol generator 106 shown in FIG. 5, which includes two
gas delivery ports per ultrasonic transducer disc 120, the design
shown in FIG. 15 is, nevertheless, capable of producing a dense,
high-quality aerosol without unnecessary waste of gas.
[0127] Referring now to FIG. 16, the flow of carrier gas 104
relative to atomization cones 162 during operation of the aerosol
generator 106 having a gas distribution configuration to deliver
carrier gas 104 from gas delivery ports on both sides of the gas
tubes 208, as was shown for the gas tubes 208A, 208B and 208C in
the gas distribution configuration shown in FIG. 14. The carrier
gas 104 sweeps both directions from each of the gas tubes 208.
[0128] An alternative, and preferred, flow for carrier gas 104 is
shown in FIG. 17. As shown in FIG. 17, carrier gas 104 is delivered
from only one side of each of the gas tubes 208. This results in a
sweep of carrier gas from all of the gas tubes 208 toward a central
area 212. This results in a more uniform flow pattern for aerosol
generation that may significantly enhance the efficiency with which
the carrier gas 104 is used to produce an aerosol. The aerosol that
is generated, therefore, tends to be more heavily loaded with
liquid droplets.
[0129] Another configuration for distributing carrier gas in the
aerosol generator 106 is shown in FIGS. 18 and 19. In this
configuration, the gas tubes 208 are hung from a gas distribution
plate 216 adjacent gas flow holes 218 through the gas distribution
plate 216. In the aerosol generator 106, the gas distribution plate
216 would be mounted above the liquid feed, with the gas flow holes
positioned to each correspond with an underlying ultrasonic
transducer. Referring specifically to FIG. 19, when the ultrasonic
generator 106 is in operation, atomization cones 162 develop
through the gas flow holes 218, and the gas tubes 208 are located
such that carrier gas 104 exiting from ports in the gas tubes 208
impinge on the atomization cones and flow upward through the gas
flow holes. The gas flow holes 218, therefore, act to assist in
efficiently distributing the carrier gas 104 about the atomization
cones 162 for aerosol formation. It should be appreciated that the
gas distribution plates 218 can be made to accommodate any number
of the gas tubes 208 and gas flow holes 218. For convenience of
illustration, the embodiment shown in FIGS. 18 and 19 shows a
design having only two of the gas tubes 208 and only 16 of the gas
flow holes 218. Also, it should be appreciated that the gas
distribution plate 216 could be used alone, without the gas tubes
208. In that case, a slight positive pressure of carrier gas 104
would be maintained under the gas distribution plate 216 and the
gas flow holes 218 would be sized to maintain the proper velocity
of carrier gas 104 through the gas flow holes 218 for efficient
aerosol generation. Because of the relative complexity of operating
in that mode, however, it is not preferred.
[0130] Aerosol generation may also be enhanced through mounting of
ultrasonic transducers at a slight angle and directing the carrier
gas at resulting atomization cones such that the atomization cones
are tilting in the same direction as the direction of flow of
carrier gas. Referring to FIG. 20, an ultrasonic transducer disc
120 is shown. The ultrasonic transducer disc 120 is tilted at a
tilt angle 114 (typically less than 10 degrees), so that the
atomization cone 162 will also have a tilt. It is preferred that
the direction of flow of the carrier gas 104 directed at the
atomization cone 162 is in the same direction as the tilt of the
atomization cone 162.
[0131] Referring now to FIGS. 21 and 22, a gas manifold 220 is
shown for distributing gas to the gas tubes 208 in a 400 transducer
array design. The gas manifold 220 includes a gas distribution box
222 and piping stubs 224 for connection with gas tubes 208 (shown
in FIG. 14). Inside the gas distribution box 222 are two gas
distribution plates 226 that form a flow path to assist in
distributing the gas equally throughout the gas distribution box
222, to promote substantially equal delivery of gas through the
piping stubs 224. The gas manifold 220, as shown in FIGS. 21 and
22, is designed to feed eleven gas tubes 208. For the 400
transducer design, a total of four gas manifolds 220 are
required.
[0132] Referring now to FIGS. 23 and 24, the generator lid 140 is
shown for a 400 transducer array design. The generator lid 140
mates with and covers the liquid feed box 190 (shown in FIGS. 12
and 13). The generator lid 140, as shown in FIGS. 23 and 24, has a
hood design to permit easy collection of the aerosol 108 without
subjecting droplets in the aerosol 108 to sharp edges on which
droplets may coalesce and be lost, and possibly interfere with the
proper operation of the aerosol generator 106. When the aerosol
generator 106 is in operation, the aerosol 108 would be withdrawn
via the aerosol exit opening 164 through the generator cover
140.
[0133] Although the aerosol generator 106 produces a high quality
aerosol 108 having a high droplet loading, it is often desirable to
further concentrate the aerosol 108 prior to introduction into the
furnace 110. Referring now to FIG. 25, a process flow diagram is
shown for one embodiment of the present invention involving such
concentration of the aerosol 108. As shown in FIG. 25, the aerosol
108 from the aerosol generator 106 is sent to an aerosol
concentrator 236 where excess carrier gas 238 is withdrawn from the
aerosol 108 to produce a concentrated aerosol 240, which is then
fed to the furnace 110.
[0134] The aerosol concentrator 236 typically includes one or more
virtual impactors capable of concentrating droplets in the aerosol
108 by a factor of greater than about 2, preferably by a factor of
greater than about 5, and more preferably by a factor of greater
than about 10, to produce the concentrated aerosol 240. According
to the present invention, the concentrated aerosol 240 should
typically contain greater than about 1.times.10.sup.7 droplets per
cubic centimeter, and more preferably from about 5.times.10.sup.7
to about 5.times.10.sup.8 droplets per cubic centimeter. A
concentration of about 1.times.10.sup.8 droplets per cubic
centimeter of the concentrated aerosol is particularly preferred,
because when the concentrated aerosol 240 is loaded more heavily
than that, then the frequency of collisions between droplets
becomes large enough to impair the properties of the concentrated
aerosol 240, resulting in potential contamination of the
particulate product 116 with an undesirably large quantity of
over-sized particles. For example, if the aerosol 108 has a
concentration of about 1.times.10.sup.7 droplets per cubic
centimeter, and the aerosol concentrator 236 concentrates droplets
by a factor of 10, then the concentrated aerosol 240 will have a
concentration of about 1.times.10.sup.8 droplets per cubic
centimeter. Stated another way, for example, when the aerosol
generator generates the aerosol 108 with a droplet loading of about
0.167 milliliters liquid feed 102 per liter of carrier gas 104, the
concentrated aerosol 240 would be loaded with about 1.67
milliliters of liquid feed 102 per liter of carrier gas 104,
assuming the aerosol 108 is concentrated by a factor of 10.
[0135] Having a high droplet loading in aerosol feed to the furnace
provides the important advantage of reducing the heating demand on
the furnace 110 and the size of flow conduits required through the
furnace. Also, other advantages of having a dense aerosol include a
reduction in the demands on cooling and particle collection
components, permitting significant equipment and operational
savings. Furthermore, as system components are reduced in size,
powder holdup within the system is reduced, which is also
desirable. Concentration of the aerosol stream prior to entry into
the furnace 110, therefore, provides a substantial advantage
relative to processes that utilize less concentrated aerosol
streams.
[0136] The excess carrier gas 238 that is removed in the aerosol
concentrator 236 typically includes extremely small droplets that
are also removed from the aerosol 108. Preferably, the droplets
removed with the excess carrier gas 238 have a weight average size
of smaller than about 1.5 .mu.m, and more preferably smaller than
about 1.mu.m and the droplets retained in the concentrated aerosol
240 have an average droplet size of larger than about 2 .mu.m. For
example, a virtual impactor sized to treat an aerosol stream having
a weight average droplet size of about three .mu.m might be
designed to remove with the excess carrier gas 238 most droplets
smaller than about 1.5 .mu.m in size. Other designs are also
possible. When using the aerosol generator 106 with the present
invention, however, the loss of these very small droplets in the
aerosol concentrator 236 will typically constitute no more than
about 10 percent by weight, and more preferably no more than about
5 percent by weight, of the droplets originally in the aerosol
stream that is fed to the concentrator 236. Although the aerosol
concentrator 236 is useful in some situations, it is normally not
required with the process of the present invention, because the
aerosol generator 106 is capable, in most circumstances, of
generating an aerosol stream that is sufficiently dense. So long as
the aerosol stream coming out of the aerosol generator 102 is
sufficiently dense, it is preferred that the aerosol concentrator
not be used. It is a significant advantage of the present invention
that the aerosol generator 106 normally generates such a dense
aerosol stream that the aerosol concentrator 236 is not needed.
Therefore, the complexity of operation of the aerosol concentrator
236 and accompanying liquid losses may typically be avoided.
[0137] It is important that the aerosol stream (whether it has been
concentrated or not) that is fed to the furnace 110 have a high
droplet flow rate and high droplet loading as would be required for
most industrial applications. With the present invention, the
aerosol stream fed to the furnace preferably includes a droplet
flow of greater than about 0.5 liters per hour, more preferably
greater than about 2 liters per hour, still more preferably greater
than about 5 liters per hour, even more preferably greater than
about 10 liters per hour, particularly greater than about 50 liters
per hour and most preferably greater than about 100 liters per
hour; and with the droplet loading being typically greater than
about 0.04 milliliters of droplets per liter of carrier gas,
preferably greater than about 0.083 milliliters of droplets per
liter of carrier gas 104, more preferably greater than about 0.167
milliliters of droplets per liter of carrier gas 104, still more
preferably greater than about 0.25 milliliters of droplets per
liter of carrier gas 104, particularly greater than about 0.33
milliliters of droplets per liter of carrier gas 104 and most
preferably greater than about 0.83 milliliters of droplets per
liter of carrier gas 104.
[0138] One embodiment of a virtual impactor that could be used as
the aerosol concentrator 236 will now be described with reference
to FIGS. 26-32. A virtual impactor 246 includes an upstream plate
assembly 248 (details shown in FIGS. 27-29) and a downstream plate
assembly 250 (details shown in FIGS. 25-32), with a concentrating
chamber 262 located between the upstream plate assembly 248 and the
downstream plate assembly 250.
[0139] Through the upstream plate assembly 248 are a plurality of
vertically extending inlet slits 254. The downstream plate assembly
250 includes a plurality of vertically extending exit slits 256
that are in alignment with the inlet slits 254. The exit slits 256
are, however, slightly wider than the inlet slits 254. The
downstream plate assembly 250 also includes flow channels 258 that
extend substantially across the width of the entire downstream
plate assembly 250, with each flow channel 258 being adjacent to an
excess gas withdrawal port 260.
[0140] During operation, the aerosol 108 passes through the inlet
slits 254 and into the concentrating chamber 262. Excess carrier
gas 238 is withdrawn from the concentrating chamber 262 via the
excess gas withdrawal ports 260. The withdrawn excess carrier gas
238 then exits via a gas duct port 264. That portion of the aerosol
108 that is not withdrawn through the excess gas withdrawal ports
260 passes through the exit slits 256 and the flow channels 258 to
form the concentrated aerosol 240. Those droplets passing across
the concentrating chamber 262 and through the exit slits 256 are
those droplets of a large enough size to have sufficient momentum
to resist being withdrawn with the excess carrier gas 238.
[0141] As seen best in FIGS. 27-32, the inlet slits 254 of the
upstream plate assembly 248 include inlet nozzle extension portions
266 that extend outward from the plate surface 268 of the upstream
plate assembly 248. The exit slits 256 of the downstream plate
assembly 250 include exit nozzle extension portions 270 extending
outward from a plate surface 272 of the downstream plate assembly
250. These nozzle extension portions 266 and 270 are important for
operation of the virtual impactor 246, because having these nozzle
extension portions 266 and 270 permits a very close spacing to be
attained between the inlet slits 254 and the exit slits 256 across
the concentrating chamber 262, while also providing a relatively
large space in the concentrating chamber 262 to facilitate
efficient removal of the excess carrier gas 238.
[0142] Also as best seen in FIGS. 27-32, the inlet slits 254 have
widths that flare outward toward the side of the upstream plate
assembly 248 that is first encountered by the aerosol 108 during
operation. This flared configuration reduces the sharpness of
surfaces encountered by the aerosol 108, reducing the loss of
aerosol droplets and potential interference from liquid buildup
that could occur if sharp surfaces were present. Likewise, the exit
slits 256 have a width that flares outward towards the flow
channels 258, thereby allowing the concentrated aerosol 240 to
expand into the flow channels 258 without encountering sharp edges
that could cause problems.
[0143] As noted previously, both the inlet slits 254 of the
upstream plate assembly 248 and the exit slits 256 of the
downstream plate assembly 250 are vertically extending. This
configuration is advantageous for permitting liquid that may
collect around the inlet slits 254 and the exit slits 256 to drain
away. The inlet slits 254 and the exit slits 256 need not, however,
have a perfectly vertical orientation. Rather, it is often
desirable to slant the slits backward (sloping upward and away in
the direction of flow) by about five to ten degrees relative to
vertical, to enhance draining of liquid off of the upstream plate
assembly 248 and the downstream plate assembly 250. This drainage
function of the vertically extending configuration of the inlet
slits 254 and the outlet slits 256 also inhibits liquid build-up in
the vicinity of the inlet slits 248 and the exit slits 250, which
liquid build-up could result in the release of undesirably large
droplets into the concentrated aerosol 240.
[0144] As discussed previously, the aerosol generator 106 of the
present invention produces a concentrated, high quality aerosol of
micro-sized droplets having a relatively narrow size distribution.
It has been found, however, that for many applications the process
of the present invention is significantly enhanced by further
classifying by size the droplets in the aerosol 108 prior to
introduction of the droplets into the furnace 110. In this manner,
the size and size distribution of particles in the particulate
product 116 are further controlled.
[0145] Referring now to FIG. 33, a process flow diagram is shown
for one embodiment of the process of the present invention
including such droplet classification. As shown in FIG. 33, the
aerosol 108 from the aerosol generator 106 goes to a droplet
classifier 280 where oversized droplets are removed from the
aerosol 108 to prepare a classified aerosol 282. Liquid 284 from
the oversized droplets that are being removed is drained from the
droplet classifier 280. This drained liquid 284 may advantageously
be recycled for use in preparing additional liquid feed 102.
[0146] Any suitable droplet classifier may be used for removing
droplets above a predetermined size. For example, a cyclone could
be used to remove over-size droplets. A preferred droplet
classifier for many applications, however, is an impactor. One
embodiment of an impactor for use with the present invention will
now be described with reference to FIGS. 34-38.
[0147] As seen in FIG. 34, an impactor 288 has disposed in a flow
conduit 286 a flow control plate 290 and an impactor plate assembly
292. The flow control plate 290 is conveniently mounted on a
mounting plate 294.
[0148] The flow control plate 290 is used to channel the flow of
the aerosol stream toward the impactor plate assembly 292 in a
manner with controlled flow characteristics that are desirable for
proper impaction of oversize droplets on the impactor plate
assembly 292 for removal through the drains 296 and 314. One
embodiment of the flow control plate 290 is shown in FIG. 35. The
flow control plate 290 has an array of circular flow ports 296 for
channeling flow of the aerosol 108 towards the impactor plate
assembly 292 with the desired flow characteristics.
[0149] Details of the mounting plate 294 are shown in FIG. 36. The
mounting plate 294 has a mounting flange 298 with a large diameter
flow opening 300 passing therethrough to permit access of the
aerosol 108 to the flow ports 296 of the flow control plate 290
(shown in FIG. 35).
[0150] Referring now to FIGS. 37 and 38, one embodiment of an
impactor plate assembly 292 is shown. The impactor plate assembly
292 includes an impactor plate 302 and mounting brackets 304 and
306 used to mount the impactor plate 302 inside of the flow conduit
286. The impactor plate 302 and the flow channel plate 290 are
designed so that droplets larger than a predetermined size will
have momentum that is too large for those particles to change flow
direction to navigate around the impactor plate 302.
[0151] During operation of the impactor 288, the aerosol 108 from
the aerosol generator 106 passes through the upstream flow control
plate 290. Most of the droplets in the aerosol navigate around the
impactor plate 302 and exit the impactor 288 through the downstream
flow control plate 290 in the classified aerosol 282. Droplets in
the aerosol 108 that are too large to navigate around the impactor
plate 302 will impact on the impactor plate 302 and drain through
the drain 296 to be collected with the drained liquid 284 (as shown
in FIG. 34).
[0152] The configuration of the impactor plate 302 shown in FIG. 33
represents only one of many possible configurations for the
impactor plate 302. For example, the impactor 288 could include an
upstream flow control plate 290 having vertically extending flow
slits therethrough that are offset from vertically extending flow
slits through the impactor plate 302, such that droplets too large
to navigate the change in flow due to the offset of the flow slits
between the flow control plate 290 and the impactor plate 302 would
impact on the impactor plate 302 to be drained away. Other designs
are also possible.
[0153] In a preferred embodiment of the present invention, the
droplet classifier 280 is typically designed to remove droplets
from the aerosol 108 that are larger than about 15 .mu.m in size,
more preferably to remove droplets larger than about 10 .mu.m in
size, even more preferably to remove droplets of a size larger than
about 8 .mu.m in size and most preferably to remove droplets larger
than about 5 .mu.m in size. The droplet classification size in the
droplet classifier is preferably smaller than about 15 .mu.m, more
preferably smaller than about 10 .mu.m, even more preferably
smaller than about 8 .mu.m and most preferably smaller than about 5
.mu.m. The classification size, also called the classification cut
point, is that size at which half of the droplets of that size are
removed and half of the droplets of that size are retained.
Depending upon the specific application, however, the droplet
classification size may be varied, such as by changing the spacing
between the impactor plate 302 and the flow control plate 290 or
increasing or decreasing aerosol velocity through the jets in the
flow control plate 290. Because the aerosol generator 106 of the
present invention initially produces a high quality aerosol 108,
having a relatively narrow size distribution of droplets, typically
less than about 30 weight percent of liquid feed 102 in the aerosol
108 is removed as the drain liquid 284 in the droplet classifier
288, with preferably less than about 25 weight percent being
removed, even more preferably less than about 20 weight percent
being removed and most preferably less than about 15 weight percent
being removed. Minimizing the removal of liquid feed 102 from the
aerosol 108 is particularly important for commercial applications
to increase the yield of high quality particulate product 116. It
should be noted, however, that because of the superior performance
of the aerosol generator 106, it is frequently not required to use
an impactor or other droplet classifier to obtain a desired absence
of oversize droplets to the furnace. This is a major advantage,
because the added complexity and liquid losses accompanying use of
an impactor may often be avoided with the process of the present
invention.
[0154] Sometimes it is desirable to use both the aerosol
concentrator 236 and the droplet classifier 280 to produce an
extremely high quality aerosol stream for introduction into the
furnace for the production of particles of highly controlled size
and size distribution. Referring now to FIG. 39, one embodiment of
the present invention is shown incorporating both the virtual
impactor 246 and the impactor 288. Basic components of the virtual
impactor 246 and the impactor 288, as shown in FIG. 39, are
substantially as previously described with reference to FIGS.
26-38. As seen in FIG. 39, the aerosol 108 from the aerosol
generator 106 is fed to the virtual impactor 246 where the aerosol
stream is concentrated to produce the concentrated aerosol 240. The
concentrated aerosol 240 is then fed to the impactor 288 to remove
large droplets therefrom and produce the classified aerosol 282,
which may then be fed to the furnace 110. Also, it should be noted
that by using both a virtual impactor and an impactor, both
undesirably large and undesirably small droplets are removed,
thereby producing a classified aerosol with a very narrow droplet
size distribution. Also, the order of the aerosol concentrator and
the aerosol classifier could be reversed, so that the aerosol
concentrator 236 follows the aerosol classifier 280.
[0155] One important feature of the design shown in FIG. 39 is the
incorporation of drains 310, 312, 314, 316 and 296 at strategic
locations. These drains are extremely important for
industrial-scale particle production because buildup of liquid in
the process equipment can significantly impair the quality of the
particulate product 116 that is produced. In that regard, drain 310
drains liquid away from the inlet side of the first plate assembly
248 of the virtual impactor 246. Drain 312 drains liquid away from
the inside of the concentrating chamber 262 in the virtual impactor
246 and drain 314 removes liquid that deposits out of the excess
carrier gas 238. Drain 316 removes liquid from the vicinity of the
inlet side of the flow control plate 290 of the impactor, while the
drain 296 removes liquid from the vicinity of the impactor plate
302. Without these drains 310, 312, 314, 316 and 296, the
performance of the apparatus shown in FIG. 39 would be
significantly impaired. All liquids drained in the drains 310, 312,
314, 316 and 296 may advantageously be recycled for use to prepare
the liquid feed 102.
[0156] With some applications of the process of the present
invention, it may be possible to collect the particles 112 directly
from the output of the furnace 110. More often, however, it will be
desirable to cool the particles 112 exiting the furnace 110 prior
to collection of the particles 112 in the particle collector 114.
Referring now to FIG. 40, one embodiment of the process of the
present invention is shown in which the particles 112 exiting the
furnace 110 are sent to a particle cooler 320 to produce a cooled
particle stream 322, which is then feed to the particle collector
114. Although the particle cooler 320 may be any cooling apparatus
capable of cooling the particles 112 to the desired temperature for
introduction into the particle collector 114, traditional heat
exchanger designs are not preferred. This is because a traditional
heat exchanger design ordinarily directly subjects the aerosol
stream, in which the hot particles 112 are suspended, to cool
surfaces. In that situation, significant losses of the particles
112 occur due to thermophoretic deposition of the hot particles 112
on the cool surfaces of the heat exchanger. According to the
present invention, a gas quench apparatus is provided for use as
the particle cooler 320 that significantly reduces thermophoretic
losses compared to a traditional heat exchanger.
[0157] Referring now to FIGS. 41-43, one embodiment of a gas quench
cooler 330 is shown. The gas quench cooler includes a perforated
conduit 332 housed inside of a cooler housing 334 with an annular
space 336 located between the cooler housing 334 and the perforated
conduit 332. In fluid communication with the annular space 336 is a
quench gas inlet box 338, inside of which is disposed a portion of
an aerosol outlet conduit 340. The perforated conduit 332 extends
between the aerosol outlet conduit 340 and an aerosol inlet conduit
342. Attached to an opening into the quench gas inlet box 338 are
two quench gas feed tubes 344. Referring specifically to FIG. 43,
the perforated tube 332 is shown. The perforated tube 332 has a
plurality of openings 345. The openings 345, when the perforated
conduit 332 is assembled into the gas quench cooler 330, permit the
flow of quench gas 346 from the annular space 336 into the interior
space 348 of the perforated conduit 332. Although the openings 345
are shown as being round holes, any shape of opening could be used,
such as slits. Also, the perforated conduit 332 could be a porous
screen. Two heat radiation shields 347 prevent downstream radiant
heating from the furnace. In most instances, however, it will not
be necessary to include the heat radiation shields 347, because
downstream radiant heating from the furnace is normally not a
significant problem. Use of the heat radiation shields 347 is not
preferred due to particulate losses that accompany their use.
[0158] With continued reference to FIGS. 41-43, operation of the
gas quench cooler 330 will now be described. During operation, the
particles 112, carried by and dispersed in a gas stream, enter the
gas quench cooler 330 through the aerosol inlet conduit 342 and
flow into the interior space 348 of perforated conduit 332. Quench
gas 346 is introduced through the quench gas feed tubes 344 into
the quench gas inlet box 338. Quench gas 346 entering the quench
gas inlet box 338 encounters the outer surface of the aerosol
outlet conduit 340, forcing the quench gas 346 to flow, in a
spiraling, swirling manner, into the annular space 336, where the
quench gas 346 flows through the openings 345 through the walls of
the perforated conduit 332. Preferably, the gas 346 retains some
swirling motion even after passing into the interior space 348. In
this way, the particles 112 are quickly cooled with low losses of
particles to the walls of the gas quench cooler 330. In this
manner, the quench gas 346 enters in a radial direction into the
interior space 348 of the perforated conduit 332 around the entire
periphery, or circumference, of the perforated conduit 332 and over
the entire length of the perforated conduit 332. The cool quench
gas 346 mixes with and cools the hot particles 112, which then exit
through the aerosol outlet conduit 340 as the cooled particle
stream 322. The cooled particle stream 322 can then be sent to the
particle collector 114 for particle collection. The temperature of
the cooled particle stream 322 is controlled by introducing more or
less quench gas. Also, as shown in FIG. 41, the quench gas 346 is
fed into the quench cooler 330 in counter flow to flow of the
particles. Alternatively, the quench cooler could be designed so
that the quench gas 346 is fed into the quench cooler in concurrent
flow with the flow of the particles 112. The amount of quench gas
346 fed to the gas quench cooler 330 will depend upon the specific
material being made and the specific operating conditions. The
quantity of quench gas 346 used, however, must be sufficient to
reduce the temperature of the aerosol steam including the particles
112 to the desired temperature. Typically, the particles 112 are
cooled to a temperature at least below about 200.degree. C., and
often lower. The only limitation on how much the particles 112 are
cooled is that the cooled particle stream 322 must be at a
temperature that is above the condensation temperature for water as
another condensible vapor in the stream. The temperature of the
cooled particle stream 322 is often at a temperature of from about
50.degree. C. to about 120.degree. C.
[0159] Because of the entry of quench gas 346 into the interior
space 348 of the perforated conduit 322 in a radial direction about
the entire circumference and length of the perforated conduit 322,
a buffer of the cool quench gas 346 is formed about the inner wall
of the perforated conduit 332, thereby significantly inhibiting the
loss of hot particles 112 due to thermophoretic deposition on the
cool wall of the perforated conduit 332. In operation, the quench
gas 346 exiting the openings 345 and entering into the interior
space 348 should have a radial velocity (velocity inward toward the
center of the circular cross-section of the perforated conduit 332)
of larger than the thermophoretic velocity of the particles 112
inside the perforated conduit 332 in a direction radially outward
toward the perforated wall of the perforated conduit 332.
[0160] As seen in FIGS. 41-43, the gas quench cooler 330 includes a
flow path for the particles 112 through the gas quench cooler of a
substantially constant cross-sectional shape and area. Preferably,
the flow path through the gas quench cooler 330 will have the same
cross-sectional shape and area as the flow path through the furnace
110 and through the conduit delivering the aerosol 108 from the
aerosol generator 106 to the furnace 110. In one embodiment,
however, it may be necessary to reduce the cross-sectional area
available for flow prior to the particle collector 114. This is the
case, for example, when the particle collector includes a cyclone
for separating particles in the cooled particle stream 322 from gas
in the cooled particle stream 322. This is because of the high
inlet velocity requirements into cyclone separators.
[0161] Referring now to FIG. 44, one embodiment of the gas quench
cooler 330 is shown in combination with a cyclone separator 392.
The perforated conduit 332 has a continuously decreasing
cross-sectional area for flow to increase the velocity of flow to
the proper value for the feed to cyclone separator 392. Attached to
the cyclone separator 392 is a bag filter 394 for final clean-up of
overflow from the cyclone separator 392. Separated particles exit
with underflow from the cyclone separator 392 and may be collected
in any convenient container. The use of cyclone separation is
particularly preferred for powder having a weight average size of
larger than about 1 .mu.m, although a series of cyclones may
sometimes be needed to get the desired degree of separation.
Cyclone separation is particularly preferred for powders having a
weight average size of larger than about 1.5 .mu.m. Also, cyclone
separation is best suited for high density materials. Preferably,
when particles are separated using a cyclone, the particles are of
a composition with specific gravity of greater than about 5.
[0162] In an additional embodiment, the process of the present
invention can also incorporate compositional modification of the
particles 112 exiting the furnace. Most commonly, the compositional
modification will involve forming on the particles 112 a material
phase that is different than that of the particles 112, such as by
coating the particles 112 with a coating material. One embodiment
of the process of the present invention incorporating particle
coating is shown in FIG. 45. As shown in FIG. 45, the particles 112
exiting from the furnace 110 go to a particle coater 350 where a
coating is placed over the outer surface of the particles 112 to
form coated particles 352, which are then sent to the particle
collector 114 for preparation of the particulate product 1116.
Coating methodologies employed in the particle coater 350 are
discussed in more detail below.
[0163] With continued reference primarily to FIG. 45, in a
preferred embodiment, when the particles 112 are coated according
to the process of the present invention, the particles 112 are also
manufactured via the aerosol process of the present invention, as
previously described. The process of the present invention can,
however, be used to coat particles that have been premanufactured
by a different process, such as by a liquid precipitation route.
When coating particles that have been premanufactured by a
different route, such as by liquid precipitation, it is preferred
that the particles remain in a dispersed state from the time of
manufacture to the time that the particles are introduced in slurry
form into the aerosol generator 106 for preparation of the aerosol
108 to form the dry particles 112 in the furnace 110, which
particles 112 can then be coated in the particle coater 350.
Maintaining particles in a dispersed state from manufacture through
coating avoids problems associated with agglomeration and
redispersion of particles if particles must be redispersed in the
liquid feed 102 for feed to the aerosol generator 106. For example,
for particles originally precipitated from a liquid medium, the
liquid medium containing the suspended precipitated particles could
be used to form the liquid feed 102 to the aerosol generator 106.
It should be noted that the particle coater 350 could be an
integral extension of the furnace 110 or could be a separate piece
of equipment.
[0164] In a further embodiment of the present invention, following
preparation of the particles 112 in the furnace 110, the particles
112 may then be structurally modified to impart desired physical
properties prior to particle collection. Referring now to FIG. 46,
one embodiment of the process of the present invention is shown
including such structural particle modification. The particles 112
exiting the furnace 110 go to a particle modifier 360 where the
particles are structurally modified to form modified particles 362,
which are then sent to the particle collector 114 for preparation
of the particulate product 116. The particle modifier 360 is
typically a furnace, such as an annealing furnace, which may be
integral with the furnace 110 or may be a separate heating device.
Regardless, it is important that the particle modifier 360 have
temperature control that is independent of the furnace 110, so that
the proper conditions for particle modification may be provided
separate from conditions required of the furnace 110 to prepare the
particles 112. The particle modifier 360, therefore, typically
provides a temperature controlled environment and necessary
residence time to effect the desired structural modification of the
particles 112.
[0165] The structural modification that occurs in the particle
modifier 360 may be any modification to the crystalline structure
or morphology of the particles 112. For example, the particles 112
may be annealed in the particle modifier 360 to densify the
particles 112 or to recrystallize the particles 112 into a
polycrystalline or single crystalline form. Also, especially in the
case of composite particles 112, the particles may be annealed for
a sufficient time to permit redistribution within the particles 112
of different material phases. Particularly preferred parameters for
such processes are discussed in more detail below.
[0166] The initial morphology of composite particles made in the
furnace 110, according to the present invention, could take a
variety of forms, depending upon the specified materials involved
and the specific processing conditions. Examples of some possible
composite particle morphologies, manufacturable according to the
present invention are shown in FIG. 47. These morphologies could be
of the particles as initially produced in the furnace 110 or that
result from structural modification in the particle modifier 360.
Furthermore, the composite particles could include a mixture of the
morphological attributes shown in FIG. 47.
[0167] Referring now to FIG. 48, an embodiment of the apparatus of
the present invention is shown that includes the aerosol generator
106 (in the form of the 400 transducer array design), the aerosol
concentrator 236 (in the form of a virtual impactor), the droplet
classifier 280 (in the form of an impactor), the furnace 110, the
particle cooler 320 (in the form of a gas quench cooler) and the
particle collector 114 (in the form of a bag filter). All process
equipment components are connected via appropriate flow conduits
that are substantially free of sharp edges that could detrimentally
cause liquid accumulations in the apparatus. Also, it should be
noted that flex connectors 370 are used upstream and downstream of
the aerosol concentrator 236 and the droplet classifier 280. By
using the flex connectors 370, it is possible to vary the angle of
slant of vertically extending slits in the aerosol concentrator 236
and/or the droplet classifier 280. In this way, a desired slant for
the vertically extending slits may be set to optimize the draining
characteristics off the vertically extending slits.
[0168] Aerosol generation with the process of the present invention
has thus far been described with respect to the ultrasonic aerosol
generator. Use of the ultrasonic generator is preferred for the
process of the present invention because of the extremely high
quality and dense aerosol generated. In some instances, however,
the aerosol generation for the process of the present invention may
have a different design depending upon the specific application.
For example, when larger particles are desired, such as those
having a weight average size of larger than about 3 .mu.m, a spray
nozzle atomizer may be preferred. For smaller-particle
applications, however, and particularly for those applications to
produce particles smaller than about 3 .mu.m, and preferably
smaller than about 2 .mu.m in size, as is generally desired with
the particles of the present invention, the ultrasonic generator,
as described herein, is particularly preferred. In that regard, the
ultrasonic generator of the present invention is particularly
preferred for when making particles with a weight average size of
from about 0.2 .mu.m to about 3 .mu.m.
[0169] Although ultrasonic aerosol generators have been used for
medical applications and home humidifiers, use of ultrasonic
generators for spray pyrolysis particle manufacture has largely
been confined to small-scale, experimental situations. The
ultrasonic aerosol generator of the present invention described
with reference to FIGS. 5-24, however, is well suited for
commercial production of high quality powders with a small average
size and a narrow size distribution. In that regard, the aerosol
generator produces a high quality aerosol, with heavy droplet
loading and at a high rate of production. Such a combination of
small droplet size, narrow size distribution, heavy droplet
loading, and high production rate provide significant advantages
over existing aerosol generators that usually suffer from at least
one of inadequately narrow size distribution, undesirably low
droplet loading, or unacceptably low production rate.
[0170] Through the careful and controlled design of the ultrasonic
generator of the present invention, an aerosol may be produced
typically having greater than about 70 weight percent (and
preferably greater than about 80 weight percent) of droplets in the
size range of from about 1 .mu.m to about 10 .mu.m, preferably in a
size range of from about 1 .mu.m to about 5 .mu.m and more
preferably from about 2 .mu.m to about 4 .mu.m. Also, the
ultrasonic generator of the present invention is capable of
delivering high output rates of liquid feed in the aerosol. The
rate of liquid feed, at the high liquid loadings previously
described, is preferably greater than about 25 milliliters per hour
per transducer, more preferably greater than about 37.5 milliliters
per hour per transducer, even more preferably greater than about 50
milliliters per hour per transducer and most preferably greater
than about 100 millimeters per hour per transducer. This high level
of performance is desirable for commercial operations and is
accomplished with the present invention with a relatively simple
design including a single precursor bath over an array of
ultrasonic transducers. The ultrasonic generator is made for high
aerosol production rates at a high droplet loading, and with a
narrow size distribution of droplets. The generator preferably
produces an aerosol at a rate of greater than about 0.5 liter per
hour of droplets, more preferably greater than about 2 liters per
hour of droplets, still more preferably greater than about 5 liters
per hour of droplets, even more preferably greater than about 10
liters per hour of droplets and most preferably greater than about
40 liters per hour of droplets. For example, when the aerosol
generator has a 400 transducer design, as described with reference
to FIGS. 7-24, the aerosol generator is capable of producing a high
quality aerosol having high droplet loading as previously
described, at a total production rate of preferably greater than
about 10 liters per hour of liquid feed, more preferably greater
than about 15 liters per hour of liquid feed, even more preferably
greater than about 20 liters per hour of liquid feed and most
preferably greater than about 40 liters per hour of liquid
feed.
[0171] Under most operating conditions, when using such an aerosol
generator, total particulate product produced is preferably greater
than about 0.5 gram per hour per transducer, more preferably
greater than about 0.75 gram per hour per transducer, even more
preferably greater than about 1.0 gram per hour per transducer and
most preferably greater than about 2.0 grams per hour per
transducer.
[0172] One significant aspect of the process of the present
invention for manufacturing particulate materials is the unique
flow characteristics encountered in the furnace relative to
laboratory scale systems. The maximum Reynolds number attained for
flow in the furnace 110 with the present invention is very high,
typically in excess of 500, preferably in excess of 1,000 and more
preferably in excess of 2,000. In most instances, however, the
maximum Reynolds number for flow in the furnace will not exceed
10,000, and preferably will not exceed 5,000. This is significantly
different from lab-scale systems where the Reynolds number for flow
in a reactor is typically lower than 50 and rarely exceeds 100.
[0173] The Reynolds number is a dimensionless quantity
characterizing flow of a fluid which, for flow through a circular
cross sectional conduit is defined as:
Re=(pvd)/.mu.
[0174] where: p=fluid density;
[0175] v=fluid mean velocity;
[0176] d=conduit inside diameter; and
[0177] p=fluid viscosity.
[0178] It should be noted that the values for density, velocity and
viscosity will vary along the length of the furnace 110. The
maximum Reynolds number in the furnace 110 is typically attained
when the average stream temperature is at a maximum, because the
gas velocity is at a very high value due to gas expansion when
heated.
[0179] One problem with operating under flow conditions at a high
Reynolds number is that undesirable volatilization of components is
much more likely to occur than in systems having flow
characteristics as found in laboratory-scale systems. The
volatilization problem occurs with the present invention, because
the furnace is typically operated over a substantial section of the
heating zone in a constant wall heat flux mode, due to limitations
in heat transfer capability. This is significantly different than
operation of a furnace at a laboratory scale, which typically
involves operation of most of the heating zone of the furnace in a
uniform wall temperature mode, because the heating load is
sufficiently small that the system is not heat transfer
limited.
[0180] With the present invention, it is typically preferred to
heat the aerosol stream in the heating zone of the furnace as
quickly as possible to the desired temperature range for particle
manufacture. Because of flow characteristics in the furnace and
heat transfer limitations, during rapid heating of the aerosol the
wall temperature of the furnace can significantly exceed the
maximum average target temperature for the stream. This is a
problem because, even though the average stream temperature may be
within the range desired, the wall temperature may become so hot
that components in the vicinity of the wall are subjected to
temperatures high enough to undesirably volatilize the components.
This volatilization near the wall of the furnace can cause
formation of significant quantities of ultrafine particles that are
outside of the size range desired.
[0181] Therefore, with the present invention, it is preferred that
when the flow characteristics in the furnace are such that the
Reynolds number through any part of the furnace exceeds 500, more
preferably exceeds 1,000, and most preferably exceeds 2,000, the
maximum wall temperature in the furnace should be kept at a
temperature that is below the temperature at which a desired
component of the final particles would exert a vapor pressure not
exceeding about 200 millitorr, more preferably not exceeding about
100 millitorr, and most preferably not exceeding about 50
millitorr. Furthermore, the maximum wall temperature in the furnace
should also be kept below a temperature at which an intermediate
component, from which a final component is to be at least partially
derived, should also have a vapor pressure not exceeding the
magnitudes noted for components of the final product.
[0182] In addition to maintaining the furnace wall temperature
below a level that could create volatilization problems, it is also
important that this not be accomplished at the expense of the
desired average stream temperature. The maximum average stream
temperature must be maintained at a high enough level so that the
particles will have a desired high density. The maximum average
stream temperature should, however, generally be a temperature at
which a component in the final particles, or an intermediate
component from which a component in the final particles is at least
partially derived, would exert a vapor pressure not exceeding about
100 millitorr, preferably not exceeding about 50 millitorr, and
most preferably not exceeding about 25 millitorr.
[0183] So long as the maximum wall temperature and the average
stream temperature are kept below the point at which detrimental
volatilization occurs, it is generally desirable to heat the stream
as fast as possible and to remove resulting particles from the
furnace immediately after the maximum stream temperature is reached
in the furnace. With the present invention, the average residence
time in the heating zone of the furnace may typically be maintained
at shorter than about 4 seconds, preferably shorter than about 2
seconds, more preferably shorter than about 1 second, still more
preferably shorter than about 0.5 second, and most preferably
shorter than about 0.2 second.
[0184] Another significant issue with respect to operating the
process of the present invention, which includes high aerosol flow
rates, is loss within the system of materials intended for
incorporation into the final particulate product. Material losses
in the system can be quite high if the system is not properly
operated. If system losses are too high, the process would not be
practical for use in the manufacture of particulate products of
many materials. This has typically not been a major consideration
with laboratory-scale systems.
[0185] One significant potential for loss with the process of the
present invention is thermophoretic losses that occur when a hot
aerosol stream is in the presence of a cooler surface. In that
regard, the use of the quench cooler, as previously described, with
the process of the present invention provides an efficient way to
cool the particles without unreasonably high thermophoretic losses.
There is also, however, significant potential for losses occurring
near the end of the furnace and between the furnace and the cooling
unit.
[0186] It has been found that thermophoretic losses in the back end
of the furnace can be significantly controlled if the heating zone
of the furnace is operated such that the maximum stream temperature
is not attained until near the end of the heating zone in the
furnace, and at least not until the last third of the heating zone.
When the heating zone includes a plurality of heating sections, the
maximum average stream temperature should ordinarily not occur
until at least the last heating section. Furthermore, the heating
zone should typically extend to as close to the exit of the furnace
as possible. This is counter to conventional thought which is to
typically maintain the exit portion of the furnace at a low
temperature to avoid having to seal the furnace outlet at a high
temperature. Such cooling of the exit portion of the furnace,
however, significantly promotes thermophoretic losses. Furthermore,
the potential for operating problems that could result in
thermophoretic losses at the back end of the furnace are reduced
with the very short residence times in the furnace for the present
invention, as discussed previously.
[0187] Typically, it would be desirable to instantaneously cool the
aerosol upon exiting the furnace. This is not possible. It is
possible, however, to make the residence time between the furnace
outlet and the cooling unit as short as possible. Furthermore, it
is desirable to insulate the aerosol conduit occurring between the
furnace exit and the cooling unit entrance. Even more preferred is
to insulate that conduit and, even more preferably, to also heat
that conduit so that the wall temperature of that conduit is at
least as high as the average stream temperature of the aerosol
stream. Furthermore, it is desirable that the cooling unit operate
in a manner such that the aerosol is quickly cooled in a manner to
prevent thermophoretic losses during cooling. The quench cooler,
described previously, is very effective for cooling with low
losses. Furthermore, to keep the potential for thermophoretic
losses very low, it is preferred that the residence time of the
aerosol stream between attaining the maximum stream temperature in
the furnace and a point at which the aerosol has been cooled to an
average stream temperature below about 200.degree. C. is shorter
than about 2 seconds, more preferably shorter than about 1 second,
and even more preferably shorter than about 0.5 second and most
preferably shorter than about 0.1 second. In most instances, the
maximum average stream temperature attained in the furnace will be
greater than about 800.degree. C. Furthermore, the total residence
time from the beginning of the heating zone in the furnace to a
point at which the average stream temperature is at a temperature
below about 200.degree. C. should typically be shorter than about 5
seconds, preferably shorter than about 3 seconds, more preferably
shorter than about 2 seconds, and most preferably shorter than
about 1 second.
[0188] Another part of the process with significant potential for
thermophoretic losses is after particle cooling until the particles
are finally collected. Proper particle collection is very important
to reducing losses within the system. The potential for
thermophoretic losses is significant following particle cooling
because the aerosol stream is still at an elevated temperature to
prevent detrimental condensation of water in the aerosol stream.
Therefore, cooler surfaces of particle collection equipment can
result in significant thermophoretic losses.
[0189] To reduce the potential for thermophoretic losses before the
particles are finally collected, it is important that the
transition between the cooling unit and particle collection be as
short as possible. Preferably, the output from the quench cooler is
immediately sent to a particle separator, such as a filter unit or
a cyclone. In that regard, the total residence time of the aerosol
between attaining the maximum average stream temperature in the
furnace and the final collection of the particles is preferably
shorter than about 2 seconds, more preferably shorter than about 1
second, still more preferably shorter than about 0.5 second and
most preferably shorter than about 0.1 second. Furthermore, the
residence time between the beginning of the heating zone in the
furnace and final collection of the particles is preferably shorter
than about 6 seconds, more preferably shorter than about 3 seconds,
even more preferably shorter than about 2 seconds, and most
preferably shorter than about 1 second. Furthermore, the potential
for thermophoretic losses may further be reduced by insulating the
conduit section between the cooling unit and the particle collector
and, even more preferably, by also insulating around the filter,
when a filter is used for particle collection. The potential for
losses may be reduced even further by heating of the conduit
section between the cooling unit and the particle collection
equipment, so that the internal equipment surfaces are at least
slightly warmer than the aerosol stream average stream temperature.
Furthermore, when a filter is used for particle collection, the
filter could be heated. For example, insulation could be wrapped
around a filter unit, with electric heating inside of the
insulating layer to maintain the walls of the filter unit at a
desired elevated temperature higher than the temperature of filter
elements in the filter unit, thereby reducing thermophoretic
particle losses to walls of the filter unit.
[0190] Even with careful operation to reduce thermophoretic losses,
some losses will still occur. For example, some particles will
inevitably be lost to walls of particle collection equipment, such
as the walls of a cyclone or filter housing. One way to reduce
these losses, and correspondingly increase product yield, is to
periodically wash the interior of the particle collection equipment
to remove particles adhering to the sides. In most cases, the wash
fluid will be water, unless water would have a detrimental effect
on one of the components of the particles. For example, the
particle collection equipment could include parallel collection
paths. One path could be used for active particle collection while
the other is being washed. The wash could include an automatic or
manual flush without disconnecting the equipment. Alternatively,
the equipment to be washed could be disconnected to permit access
to the interior of the equipment for a thorough wash. As an
alternative to having parallel collection paths, the process could
simply be shut down occasionally to permit disconnection of the
equipment for washing. The removed equipment could be replaced with
a clean piece of equipment and the process could then be resumed
while the disconnected equipment is being washed.
[0191] For example, a cyclone or filter unit could periodically be
disconnected and particles adhering to interior walls could be
removed by a water wash. The particles could then be dried in a low
temperature dryer, typically at a temperature of lower than about
50.degree. C.
[0192] In one embodiment, wash fluid used to wash particles from
the interior walls of particle collection equipment includes a
surfactant. Some of the surfactant will adhere to the surface of
the particles. This could be advantageous to reduce agglomeration
tendency of the particles and to enhance dispersibility of the
particles in a thick film past formulation. The surfactant could be
selected for compatibility with the specific paste formulation
anticipated.
[0193] Another area for potential losses in the system, and for the
occurrence of potential operating problems, is between the outlet
of the aerosol generator and the inlet of the furnace. Losses here
are not due to thermophoresis, but rather to liquid coming out of
the aerosol and impinging and collecting on conduit and equipment
surfaces. Although this loss is undesirable from a material yield
standpoint, the loss may be even more detrimental to other aspects
of the process. For example, water collecting on surfaces may
release large droplets that can lead to large particles that
detrimentally contaminate the particulate product. Furthermore, if
accumulated liquid reaches the furnace, the liquid can cause
excessive temperature gradients within the furnace tube, which can
cause furnace tube failure, especially for ceramic tubes.
[0194] One way to reduce the potential for undesirable liquid
buildup in the system is to provide adequate drains, as previously
described. In that regard, it is preferred that a drain be placed
as close as possible to the furnace inlet to prevent liquid
accumulations from reaching the furnace. The drain should be
placed, however, far enough in advance of the furnace inlet such
that the stream temperature is lower than about 80.degree. C. at
the drain location.
[0195] Another way to reduce the potential for undesirable liquid
buildup is for the conduit between the aerosol generator outlet and
the furnace inlet be of a substantially constant cross sectional
area and configuration. Preferably, the conduit beginning with the
aerosol generator outlet, passing through the furnace and
continuing to at least the cooling unit inlet is of a substantially
constant cross sectional area and geometry.
[0196] Another way to reduce the potential for undesirable buildup
is to heat at least a portion, and preferably the entire length, of
the conduit between the aerosol generator and the inlet to the
furnace. For example, the conduit could be wrapped with a heating
tape to maintain the inside walls of the conduit at a temperature
higher than the temperature of the aerosol. The aerosol would then
tend to concentrate toward the center of the conduit due to
thermophoresis. Fewer aerosol droplets would, therefore, be likely
to impinge on conduit walls or other surfaces making the transition
to the furnace.
[0197] Another way to reduce the potential for undesirable liquid
buildup is to introduce a dry gas into the aerosol between the
aerosol generator and the furnace. Referring now to FIG. 49, one
embodiment of the process is shown for adding a dry gas 118 to the
aerosol 108 before the furnace 110. Addition of the dry gas 118
causes vaporization of at least a part of the moisture in the
aerosol 108, and preferably substantially all of the moisture in
the aerosol 108, to form a dried aerosol 119, which is then
introduced into the furnace 110.
[0198] The dry gas 118 will most often be dry air, although in some
instances it may be desirable to use dry nitrogen gas or some other
dry gas. If sufficient a sufficient quantity of the dry gas 118 is
used, the droplets of the aerosol 108 are substantially completely
dried to beneficially form dried precursor particles in aerosol
form for introduction into the furnace 110, where the precursor
particles are then pyrolyzed to make a desired particulate product.
Also, the use of the dry gas 118 typically will reduce the
potential for contact between droplets of the aerosol and the
conduit wall, especially in the critical area in the vicinity of
the inlet to the furnace 110. In that regard, a preferred method
for introducing the dry gas 118 into the aerosol 108 is from a
radial direction into the aerosol 108. For example, equipment of
substantially the same design as the quench cooler, described
previously with reference to FIGS. 41-43, could be used, with the
aerosol 108 flowing through the interior flow path of the apparatus
and the dry gas 118 being introduced through perforated wall of the
perforated conduit. An alternative to using the dry gas 118 to dry
the aerosol 108 would be to use a low temperature thermal
preheater/dryer prior to the furnace 110 to dry the aerosol 108
prior to introduction into the furnace 110. This alternative is
not, however, preferred.
[0199] Still another way to reduce the potential for losses due to
liquid accumulation is to operate the process with equipment
configurations such that the aerosol stream flows in a vertical
direction from the aerosol generator to and through the furnace.
For smaller-size particles, those smaller than about 1.5 .mu.m,
this vertical flow should, preferably, be vertically upward. For
larger-size particles, such as those larger than about 1.5 .mu.m,
the vertical flow is preferably vertically downward.
[0200] Furthermore, with the process of the present invention, the
potential for system losses is significantly reduced because the
total system retention time from the outlet of the generator until
collection of the particles is typically shorter than about 10
seconds, preferably shorter than about 7 seconds, more preferably
shorter than about 5 seconds and most preferably shorter than about
3 seconds.
[0201] For the production of copper metals according to the present
invention, the liquid feed 102 includes at least one copper metal
precursor for preparation of the particles 112. The copper metal
precursor may be a substance in either a liquid or solid phase of
the liquid feed 102. Typically, the copper metal precursor will be
a copper-containing compound, such as a salt, dissolved in a liquid
solvent of the liquid feed 102. The copper metal precursor may
undergo one or more chemical reactions in the furnace 110 to assist
in production of the particles 112. Alternatively, the copper metal
precursor may contribute to formation of the particles 112 without
undergoing chemical reaction. This could be the case, for example,
when the liquid feed 102 includes suspended particles as a
precursor material.
[0202] The liquid feed 102 thus includes the chemical components
that will form the copper metal particles 112. For example, the
liquid feed 102 can comprise a solution containing nitrates,
chlorides, sulfates, hydroxides, or oxalates of copper. A preferred
precursor to copper metal according to the present invention is
cupric nitrate, Cu(NO.sub.3).sub.2.xH.sub.2O. Cupric nitrate is
highly soluble in water and the solutions maintain a low viscosity,
even at high concentrations of nitrate. The solution preferably has
a copper precursor concentration that is unsaturated to avoid the
formation of precipitates in the liquid. The solution preferably
includes a soluble precursor to yield a copper metal concentration
of from about 0.3 to about 25 weight percent, more preferably from
about 1 to 15 weight percent. A particularly preferred
concentration is from about 2.5 to about 7.5 weight percent for the
production of particles having a size of from about 0.3 to about
0.8 .mu.m. The final particle size of the particles 112 is
influenced by the precursor concentration. Generally, lower
precursor concentrations will produce particles having a smaller
average particle size.
[0203] Preferably, the solvent is aqueous-based for ease of
operation, although other solvents, such as toluene, may be
desirable. The use of organic solvents can lead to carbon in the
metal particles. The pH of the aqueous-based solutions can be
adjusted to alter the solubility characteristics of the precursor
in the solution.
[0204] The precursor solution can also include other additives. For
example, a reducing agent to facilitate the reaction of the
precursor to a metal particle can advantageously be included in the
precursor solution. The use of a reducing agent may eliminate the
need for a reducing gas, such as hydrogen, in the carrier gas. A
preferred reducing agent according to the present invention is
hydrazine (H.sub.2NNH.sub.2), which can be included in the
precursor solution in an amount of, for example, from about 1 to
about 15 weight percent. Other reducing agents, such as
borohydrides (MBR.sub.4-xH.sub.x, where x=1 to 4, R is an alkyl or
aryl, such as methyl or ethyl, and M is Li, Na, K, or NH.sub.4) may
also be useful.
[0205] In addition to the foregoing, the liquid feed 102 may also
include other additives that contribute to the formation of the
particles. For example, a fluxing agent can be added to this
solution to increase the crystallinity and/or density of the
particles. For example, the addition of urea to metal salt
solutions, such as copper nitrate, can increase the density of
particles produced from the solution. In one embodiment, up to
about 1 mole equivalent urea is added to the precursor solution.
Further, if the particles are to be coated copper particles, as is
discussed in more detail below, soluble precursors to both the
copper particle and the coating can be used in the precursor
solution wherein the coating precursor is an involatile or volatile
species.
[0206] A carrier gas 104 under controlled pressure is introduced to
the aerosol generator to move the droplets away from the generator.
The carrier gas 104 may comprise any gaseous medium in which
droplets produced from the liquid feed 102 may be dispersed in
aerosol form. Also, the carrier gas 104 may be inert in that the
carrier gas 104 does not participate in the formation of the
particles 112. Alternatively, the carrier gas 104 may have one or
more active component(s) that contribute to formation of the
particles 112. In that regard, the carrier gas may include one or
more reactive components that react in the furnace 110 to
contribute to formation of the particles 112. Examples of preferred
carrier gases for the production of copper metal particles are
carrier gases that include a reducing gas such as hydrogen
(H.sub.2) or carbon monoxide (CO) and inert carrier gases such as
argon or nitrogen. Forming gas compositions, such as those
including 7% H.sub.2/93% N.sub.2 are particularly preferred, and
can be mixed with an inert gas such as N.sub.2.
[0207] According to the present invention for the production of
copper metal particles, it is preferred to use a carrier gas
including hydrogen as the reducing gas. Preferably the hydrogen
content of the carrier gas is in excess of the amount theoretically
required to convert all of the copper species to copper metal. The
preferred hydrogen content, expressed as moles H.sub.2 per mole of
Cu in the system, is at least about 7.5 moles H.sub.2 per mole Cu,
more preferably at least about 10 moles H.sub.2 per mole of Cu. The
hydrogen gas can be mixed with, for example, nitrogen gas.
Substantially pure and dense copper metal particles that are
substantially free of the copper oxides can be formed using these
levels of hydrogen.
[0208] According to the present invention, the stream temperature
(reaction temperature) in the heating zone is preferably at least
about 700.degree. C., more preferably is at least about 900.degree.
C. and even more preferably is at least about 1100.degree. C. Below
about 700.degree. C., the metal particles tend to be incompletely
reacted and may contain some impurities or have a low density. The
reaction temperature should not exceed about 1400.degree. C. and
preferably is less than about 1350.degree. C. One preferred range
according to the present invention is from about 1100.degree. C. to
about 1300.degree. C. It will be appreciated, however, that the
reaction temperature needed to form dense particles will vary with
the residence time of the particles in the heating zone. Increasing
the residence time of the particles in the heating zone will enable
the reaction temperature to be reduced. What is required is that
the droplets are exposed to a sufficient temperature for a
sufficient time to form particles 112. The reacted particles 112
exit the heating zone and are then separated from the gas in a
particle collector 114.
[0209] To form metal composite particles, the liquid feed can
include colloids, for example boehmite particles or silica
particles. Particles as large as about 0.3 .mu.m can be suspended
in the aerosol droplets using an ultrasonic nebulizer. The
suspended colloids can also coat the outer surface of the metal
particles (forming a particulate coating), depending on the process
conditions and the selected materials. The particles can also be
formed such that a metal phase uniformly coats a core of a
non-metallic phase.
[0210] To form substantially uniform coatings on the surface of the
copper metal particles such as those discussed above, a reactive
gas composition can be contacted with the copper metal particles at
an elevated temperature after the particles have been formed. For
example, the reactive gas can be introduced into the heated
reaction chamber at the distal end so that the desired compound
deposits on the surface of the particle.
[0211] More specifically, the droplets can enter the heated
reaction zone at a first end such that the precursor droplets move
through the heated zone and form the copper metal particles. At the
opposite end of the heated reaction zone, a reactive gas
composition can be introduced such that the reactive gas
composition contacts the copper metal particles at an elevated
temperature. Alternatively, the reactive gas composition can be
contacted with the heated particles in a separate heating zone
located downstream from the heated reaction zone.
[0212] For example, precursors to metal oxide coatings can be
selected from volatile metal acetates, chlorides, alkoxides or
halides. Such precursors are known to react at high temperatures to
form the corresponding metal oxides and eliminate supporting
ligands or ions. For example, SiCl.sub.4 can be used as a precursor
to SiO.sub.2 coatings where water vapor is present:
SiCl.sub.4(g)+2H.sub.2O.sub.(g).fwdarw.SiO.sub.2(s)+4HCl.sub.(g)
[0213] SiCl4 also is highly volatile and is a liquid at room
temperature, which makes transport into the reactor more
controllable.
[0214] Metal alkoxides can be used to produce metal oxide films by
hydrolysis. The water molecules react with the alkoxide M-O bond
resulting in clean elimination of the corresponding alcohol with
the formation of M-O-M bonds:
Si(OEt).sub.4+2H.sub.2O.fwdarw.SiO.sub.2+4EtOH
[0215] Most metal alkoxides have a reasonably high vapor pressure
and are therefore well suited as coating precursors.
[0216] Metal acetates are also useful as coating precursors since
they readily decompose upon thermal activation by acetic anhydride
elimination:
Mg(O.sub.2CCH.sub.3).sub.2.fwdarw.MgO+CH.sub.3C(O)OC(O)CH.sub.3
[0217] Metal acetates are advantageous as coating precursors since
they are water stable and are reasonably inexpensive.
[0218] Coatings can be generated on the particle surface by a
number of different mechanisms. One or more precursors can vaporize
and fuse to the hot particle surface and thermally react resulting
in the formation of a thin-film coating by chemical vapor
deposition (CVD). Preferred coatings deposited by CVD include metal
oxides and elemental metals. Further, the coating can be formed by
physical vapor deposition (PVD) wherein a coating material
physically deposits on the surface of the particles. Preferred
coatings deposited by PVD include organic materials and elemental
metals. Alternatively, the gaseous precursor can react in the gas
phase forming small particles, for example less than about 5
nanometers in size, which then diffuse to the larger particle
surface and sinter onto the surface, thus forming a coating. This
method is referred to as gas-to-particle conversion (GPC). Whether
such coating reactions occur by CVD, PVD or GPC is dependent on the
reactor conditions, such as temperature, precursor partial
pressure, water partial pressure and the concentration of particles
in the gas stream. Another possible surface coating method is
surface conversion of the surface of the particles by reaction with
a vapor phase reactant to convert the surface of the particles to a
different material than that originally contained in the
particles.
[0219] In addition, a volatile coating material such as PbO,
MoO.sub.3 or V.sub.2O.sub.5 can be introduced into the reactor such
that the coating deposits on the particles by condensation. Highly
volatile metals, such as silver, can also be deposited by
condensation. Further, the particles can be coated using other
techniques. For example, a soluble precursor to both the copper
powder and the coating can be used in the precursor solution
wherein the coating precursor is involatile, (e.g.
Al(NO.sub.3).sub.3 or volatile (e.g. Sn(OAc).sub.4 where Ac is
acetate). In another embodiment, a colloidal precursor and a
soluble copper precursor can be used to form a particulate
colloidal coating on the copper particle.
[0220] It will be appreciated that multiple coatings can be
deposited on the surface of the copper metal particles if such
multiple coatings are desirable.
[0221] The coating methodologies of the present invention allow
control over the time and temperature of the coating process and
therefore control over the coating properties. Fluidized bed
coating processes utilized in the prior art are generally limited
to particles having a size of greater than about 5 .mu.m and lack
adequate control over deposition times and temperatures. Further,
the present process permits the use of higher temperatures than is
believed to be possible using conventional fluidized beds.
[0222] The coatings are preferably as thin as possible while
maintaining conformity about particle such that the metal is not
substantially exposed. For example, the coatings can have an
average thickness of not greater than about 200 nanometers,
preferably not greater than about 100 nanometers, and more
preferably not greater than about 50 nanometers.
[0223] The present invention is directed to copper powder batches
including copper particles wherein the copper particles
constituting the powder batch preferably have a spherical
morphology, a small average particle size and a narrow particle
size distribution. The powders of the present invention offer
numerous advantages over conventional copper powders and are
particularly useful in a number of applications including the
fabrication of microelectronic devices, where the powders are
dispersed in thick film pastes used to form electrically conductive
layers or paths in devices such as multilayer capacitors and
multi-chip modules. Similar pastes are also useful in other
applications, such as for the formation of electrodes in flat panel
display devices.
[0224] The copper powder batches according to the present invention
include a commercially useful quantity of individual copper
particles. According to one embodiment, the copper particles
preferably include a metal phase having at least about 50 weight
percent copper metal, and depending upon the application, the
particles preferably include at least about 80 weight percent
copper metal and even more preferably at least about 90 weight
percent copper metal.
[0225] For many applications, the copper metal particles can be
copper metal alloy particles wherein copper metal is alloyed with
one or more alloying elements including, but not limited to,
palladium (Pd), silver (Ag), gold (Au), nickel (Ni), tungsten (W),
molybdenum (Mo), aluminum (Al), zinc (Zn), magnesium (Mg), tin
(Sn), beryllium (Be) and platinum (Pt). Zinc is a particularly
preferred alloying element for increasing the oxidation resistance
of the copper metal. As used herein, metal alloy particles includes
intermetallic compounds of copper and another metal(s).
[0226] The copper metal alloy particles according to this
embodiment of the present invention are preferably homogeneous,
well-mixed on the atomic level, and have substantially no phase
segregation of the copper metal and the alloying element. However,
it may be desirable for some applications that the particles can
consist of distinct metal phases that are segregated (see FIG.
47e). Depending on the intended application, the alloying element
can preferably be included in an amount of from about 0.1 to about
40 weight percent, such as from about 1 to about 15 weight percent,
based on the total amount of metal.
[0227] Such alloying elements can modify the properties of the
metal particles in several ways, as compared to pure copper
particles. These modifications can include an increased or
decreased sintering temperature, which is the temperature at which
individual particles begin to coalesce due to softening and
diffusion. The melting temperature can also be increased or
decreased. The vaporization of metal at the synthesis temperature
can be inhibited, which reduces the formation of ultrafine
particles from the vapors. Ultrafine particles can be detrimental
to the dispersion properties of the powder. Further, the alloying
element can improve the rheological properties of the particles for
better dispersion of the particles in organic and water-based
pastes. The oxidation resistance can be improved such as by
increasing the temperature at which oxidation begins or by reducing
the total amount of metal that will oxidize at a given temperature
and partial pressure of oxygen. Adhesion of the metal with ceramics
can also be improved by alloying the particles. The alloyed
particles can also be useful as a catalyst material.
[0228] The metal alloy particles can be formed in accordance with
the methodology described above. Typically, the metal alloy will be
formed from a liquid solution which includes both a copper metal
precursor and a precursor for the alloying element. The alloying
level can easily be adjusted by adjusting the relative ratios of
copper metal precursor and alloying element precursor in the liquid
solution. For example, copper/zinc alloy particles can be formed
from a solution of copper nitrate and zinc nitrate.
[0229] The copper metal powder batches according to the present
invention include particles having a small average particle size.
Although the preferred average size of the particles will vary
according to the particular application of the copper metal powder,
the weight average particle size of the metal particles is
preferably at least about 0.1 .mu.m and preferably is not greater
than about 5 .mu.m. For most applications, the weight average
particle size is more preferably not greater than about 3 .mu.m and
even more preferably is not greater than about 2 .mu.m, such as
from about 0.3 .mu.m to about 1.5 .mu.m.
[0230] A particularly preferred weight average particle size for
the copper powder batches according to the present invention is
from about 0.3 .mu.m to about 0.8 .mu.m. Copper powders having such
an average particle size are particularly useful in microelectronic
applications wherein conductive metal powders are dispersed in a
thick film paste which is applied to a substrate and heated to form
a copper metal film or line. Utilizing copper metal powder having
such a small average particle size enables the formation of
conductive traces having a narrower width and films having a
decreased thickness. Such powder batches are particularly useful
for the internal electrodes of multilayer ceramic capacitors, which
require a thin and uniform, defect-free conductive film.
[0231] As is discussed in more detail hereinbelow, copper metal
powders are also particularly useful for forming the external
electrodes on devices such as multi-layer ceramic capacitors. In
this regard, a preferred average particle size for the external
electrodes (terminations) of an MLCC is from about 1 .mu.m to about
3 .mu.m.
[0232] According to a preferred embodiment of the present
invention, the powder batch of copper particles has a narrow
particle size distribution, such that the majority of particles are
about the same size. Preferably, at least about 90 weight percent
of the particles and more preferably at least about 95 weight
percent of the particles are not larger than twice the weight
average particle size. Thus, when the average particle size is
about 1 .mu.m, it is preferred that at least about 90 weight
percent of the particles are not larger than 2 .mu.m and it is more
preferred that at least about 95 weight percent of the particles
are not larger than 2 .mu.m. Further, it is preferred that at least
about 90 weight percent and more preferably at least about 95
weight percent of the particles are not larger than about 1.5 times
the weight average particle size. Thus, when the average particle
size is about 1 .mu.m, it is preferred that at least about 90
weight percent of the particles are not larger than 1.5 .mu.m and
it is more preferred that at least about 95 weight percent of the
particles are not larger than 1.5 .mu.m.
[0233] It is also possible according to the present invention to
provide a copper powder batch having a bimodal particle size
distribution. That is, the powder batch can include copper
particles having two distinct and different average particle sizes,
each with a narrow size distribution as discussed above. Such
bimodal distributions can enhance the packing efficiency of the
powder in a variety of applications.
[0234] The copper metal particles of the present invention can be
substantially single crystal particles or may be comprised of a
number of crystallites. Copper metal particles having a high
crystallinity, i.e. large average crystallite size, enhance the
electrical properties of devices formed from the powder. Highly
crystalline particles will also increases the oxidation resistance
of the powder.
[0235] According to one embodiment of the present invention, it is
preferred that the average crystallite size is close to the average
particle size such that the particles are mostly single crystals or
are composed of only a few large crystals. Accordingly, the average
crystallite size of the particles is preferably at least about 40
nanometers, more preferably is at least about 60 nanometers, even
more preferably is at least about 80 nanometers, and most
preferably is at least about 100 nanometers. In one embodiment, the
average crystallite size is at least about 200 nanometers. As the
average crystallite size relates to the average particle size
disclosed above, the average crystallite size is preferably at
least about 20 percent of the average particle size, more
preferably is at least about 30 percent of the average particle
size and even more preferably is at least about 40 percent of the
average particle size. Copper metal powders having such high
crystallinity advantageously have improved electrical properties
and also have improved oxidation resistance as compared to copper
metal powders having lower crystallinity, i.e., a smaller average
crystallite size. As the average crystallite size approaches the
average particle size, the copper metal particles remain
substantially spherical but can appear faceted on the outer surface
of the particle.
[0236] The copper metal particles produced according to the present
invention also have a high degree of purity and it is preferred
that the particles include not greater than about 0.1 atomic
percent impurities and more preferably not greater than about 0.01
atomic percent impurities. Since no milling of the particles is
required to achieve the small average particle sizes disclosed
herein, there are substantially no undesired impurities such as
alumina, zirconia or high carbon steel in the powder batch.
[0237] The copper metal particles according to the present
invention are also preferably dense (e.g. not hollow or porous), as
measured by helium pycnometry. Preferably, the copper metal
particles according to the present invention have a particle
density of at least about 80% of the theoretical density (at least
about 7.1 g/cm.sup.3 for pure copper), more preferably at least
about 90% of the theoretical density (at least about 8.0 g/cm.sup.3
for pure copper) and even more preferably at least about 95% of the
theoretical density (at least about 8.5 g/cm.sup.3 for pure
copper). In one embodiment, the particle density is at least about
99% of the theoretical density. The theoretical density can be
easily calculated for multi-phase compositions, including alloys
and composites, based upon the relative percentages of each
component. High density particles provide many advantages over
porous particles, including reduced shrinkage during sintering.
[0238] The copper metal particles according to a preferred
embodiment of the present invention are also substantially
spherical in shape. That is, the particles are not jagged or
irregular in shape. Spherical particles are particularly
advantageous because they disperse more readily in a paste or
slurry and impart advantageous flow characteristics to paste
compositions. Although the particles are substantially spherical,
the particles can become faceted as the crystallite size increases
and approaches the average particle size.
[0239] The copper metal powder according to the present invention
also preferably has a low surface area. As is discussed above, the
particles are substantially spherical, which reduces the total
surface area for a given mass of powder. Further, the elimination
of larger particles from the powder batch eliminates the porosity
that is associated with open pores on the surface of such larger
particles. Due to the substantial elimination of the larger
particles with open porosity and the spherical shape of the powder,
the powder advantageously has a lower surface area. Surface area is
typically measured using the BET nitrogen adsorption method which
is indicative of the gas-accessible surface area of the powder,
including the surface area of accessible surface pores. For a given
particle size distribution, a lower value of surface area per unit
mass of powder generally indicates solid and non-porous particles.
According to one embodiment of the present invention, the powder
preferably has a specific surface area of not greater than about 3
m.sup.2/g, more preferably not greater than about 2 m.sup.2/g.
Decreased surface area reduces the susceptibility of the powders to
adverse surface reactions, such as oxidation of the metal. This
characteristic can advantageously extend the shelf-life of such
powders.
[0240] The surfaces of the copper particles according to the
present invention are typically smooth and clean and preferably
have a minimal deposition of ultrafine particles (e.g., particles
less than about 40 nanometers in size) on the particle surface. It
is believed that such ultrafine particles can inhibit the ability
of the particles to adequately disperse in a thick film paste
composition. Further, the surface of the copper metal particles is
substantially free of surfactants or other organic contaminants.
Metal particles produced by liquid precipitation routes are often
contaminated with residual surfactants from the manufacturing
process. Such surfactants can hinder the dispersibility of the
powder in a paste. The powders can also be produced with an
extremely thin layer of a copper oxide such that further oxidation
of the particles cannot occur and the shelf-life of the particles
is thereby increased since they do not oxidize appreciably upon
exposure to air.
[0241] The powder batches of copper metal particles according to
the present invention are preferably also substantially
unagglomerated, that is, they include substantially no hard
agglomerates of particles. Hard agglomerates are physically
coalesced lumps of two or more particles that behave as one larger,
irregularly-shaped particle. Agglomerates are disadvantageous in
most applications. For example, when agglomerated metal powders are
used in a thick film paste, the sintered metal film that is formed
can contain lumps that lead to a defective product. Accordingly, it
is preferred that no more than about 0.5 weight percent of the
copper metal particles in the powder batch of the present invention
are in the form of hard agglomerates and more preferably no more
than about 0.1 weight percent of the particles are in the form of
hard agglomerates.
[0242] According to one embodiment of the present invention, the
copper metal particles are metal composite particles wherein the
individual particles include a metal phase and at least one
non-metallic phase associated with the metal phase, such as one
that is dispersed throughout the metal phase. For example, the
metal composite particles can include a metal oxide dispersed
throughout a copper metal phase. Preferred simple metal oxides can
include, but are not limited to, NiO, SiO.sub.2, Cu.sub.2O, CuO,
B.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2, A1.sub.2O.sub.3, ZnO, SnO,
SnO.sub.2, BeO, V.sub.2O.sub.5 MoO. Also, the metal composite
particles can include a metal phase and a non-metallic phase
comprising carbon. Such a metal composite can be formed by
dispersing a particulate carbon precursor in a copper precursor and
forming the particles as described above.
[0243] Metal oxides can advantageously modify the characteristics
or other properties of the copper metal particles, such as
increasing the sintering temperature of the powder or modifying the
thermal expansion characteristics of the powder. Metal oxides can
also improve the adhesion of the metal to a ceramic substrate.
Further, oxides can provide an inexpensive filler material,
reducing the volume of the metal that is used without substantially
effecting conductivity. Metal oxides can also increase the
oxidation resistance of the particles.
[0244] Depending upon the application of the powder, the metal
composite particles preferably include at least about 0.1 weight
percent of the non-metallic phase and more preferably from about
0.2 to about 50 weight percent of the non-metallic phase and even
more preferably from about 0.2 to about 35 weight percent of the
non-metallic phase. For some applications, such as in MLCC
capacitors, it is preferred to incorporate from about 0.2 to about
5 weight percent of the non-metallic phase, such as from about 0.5
to about 2 weight percent. More than one non-metallic phase can be
included in the particles. The morphology and distribution of the
metal and non-metallic phases can vary, but it is preferred that
the non-metallic phase is homogeneously dispersed throughout the
metal phase.
[0245] For some applications, such as MLCC's discussed in more
detail hereinbelow, it is particularly advantageous to provide
metal composite particles including a metal phase and a
non-metallic phase of a ceramic dielectric compound, preferably
from about 0.5 to about 2 weight percent of a dielectric compound.
Such a composite particle is particularly useful for the internal
electrodes of an MLCC. Such metal composite powders advantageously
provide improved adhesion between the ceramic dielectric layers and
the metal layers as well as improved thermal expansion
characteristics during sintering of the MLCC. That is, the thermal
expansion characteristics of the powder will closely match that of
the dielectric. This property will advantageously result in fewer
rejections of the devices due to delamination, cracks or
camber.
[0246] Preferred dielectric compounds for incorporation into the
copper metal particles include titanates, zirconates, silicates,
aluminates, niobates and tantalates. Particularly preferred
dielectric compounds are titanates such as barium titanate,
neodymium titanate, magnesium titanate, calcium titanate, lead
titanate and strontium titanate. Also preferred are zirconates such
as magnesium zirconate or calcium zirconate and niobate compounds,
commonly referred to as relaxor dielectrics. Those skilled in the
art will recognize that many dielectric compounds are a combination
of the foregoing and/or are non-integral stoichiometry compounds,
such as BaTi.sub.0.903Zr.sub.0.097O.sub.3.
[0247] When the particles are to be used to form a conductive film
on a ceramic, it is often preferred to include a ceramic compound
dispersed in the metal composite particles that is the same or has
similar thermal expansion characteristics as the ceramic used to
form the ceramic substrate. For example, a particularly preferred
embodiment for the fabrication of MLCC's utilizes barium titanate
as the ceramic substrate layer and metal composite particles
including copper metal and barium titanate.
[0248] According to another embodiment of the present invention,
the copper metal particles are coated particles that include a
particulate coating (FIG. 47d) or non-particulate (film) coating
(FIG. 47a) that substantially encapsulates an outer surface of the
particles. The coating can be a metal or a non-metallic compound.
Preferably, the coating is very thin and has an average thickness
of not greater than about 200 nanometers, more preferably not
greater than about 100 nanometers, and even more preferably not
greater than about 50 nanometers. While the coating is thin, the
coating should substantially encapsulate the entire particle such
that substantially none of the original particle surface is
exposed.
[0249] As is discussed above, the coating can be a metal, metal
oxide or other inorganic compound, or can be an organic compound.
For example, copper particles can be coated with a metal, such as a
more costly noble metal, to obtain the surface properties of the
noble metal at a reduced cost. Thus, the copper particles can be
coated with platinum or gold to obtain an oxidation resistant
powder at a reduced cost. According to one preferred embodiment,
the copper particles are coated with silver or a silver alloy.
[0250] Alternatively, a metal oxide coating can advantageously be
used, such as a metal oxide selected from the group consisting of
ZrO.sub.2, NiO, SiO.sub.2, TiO.sub.2, Cu.sub.2O, CuO,
Bi.sub.2O.sub.3, Al.sub.2O.sub.3, ZnO, SnO, SnO.sub.2 and
V.sub.2O.sub.5. Among these, SiO.sub.2 and Al.sub.2O3 are often
preferred. Metal oxide coatings can inhibit the sintering of the
metal particles and also improve the dispersibility of the
particles in a paste. The coatings can also increase the oxidation
resistance of the metal particles and increase the corrosion
resistance in a variety of conditions. The copper metal particles
can include more than one coating, if multiple coatings are
desirable.
[0251] In a further preferred embodiment, the copper particles can
be coated with a ceramic dielectric compound, such as those
discussed hereinabove with respect to metal composite
particles.
[0252] The copper metal particles of the present invention can be
coated with an organic compound, for example to provide improved
dispersion which will result in smoother prints having lower lump
counts when applied as a paste. The organic coating can
advantageously be placed onto a previously formed metal oxide
coating encapsulating the metal particle, as is discussed
above.
[0253] The organic compound for coating the metal particles can
also be selected from a variety of organic compounds such as PMMA
(polymethylmethacrylate), polystyrene or the like. The particles
can also be coated with a surfactant to aid in the dispersion of
the particles in a flowable medium such as a paste. The organic
coating preferably has an average thickness of not greater than
about 100 nanometers, more preferably not greater than about 50
nanometers, and is substantially dense and continuous about the
particle. The organic coatings can advantageously reduce corrosion
of the particles and also can improve the dispersion
characteristics of the particles in a paste or slurry.
[0254] The coating can also be comprised of one or more monolayer
coatings, such as from about 1 to 3 monolayer coating. A monolayer
coating is formed by the reaction of an organic or an inorganic
molecule with the surface of the particles to form a coating layer
that is essentially one molecular layer thick. In particular, the
formation of a monolayer coating by reaction of the surface of the
metal particle with a functionalized organo silane such as halo- or
amino-silanes, for example hexamethyidisilazane or
trimethylsilylchloride, can be used to modify the hydrophobicity
and hydrophilicity of the powders. Such coatings allow for greater
control over the dispersion characteristics of the powder in a wide
variety of thick film paste compositions.
[0255] The monolayer coatings can also be applied to copper metal
powders that have already been coated with an organic or inorganic
coating thus providing better control over the corrosion
characteristics (through the use of the thicker coating) as well as
dispersibility (through the monolayer coating) of the
particles.
[0256] The copper metal powder batches according to the present
invention, including powder batches comprising composite particles
and coated particles, are useful in a number of applications and
can be used to fabricate a number of novel devices and intermediate
products. Such devices and intermediate products are included
within the scope of the present invention.
[0257] One preferred class of intermediate products according to
the present invention are thick film paste compositions, also
referred to as thick film inks. These pastes are particularly
useful in the microelectronics industry for the application of
conductors, resistors and dielectrics onto a substrate and in the
flat panel display industry for applying conductors, dielectrics
and phosphors onto a substrate.
[0258] In the thick film process, a viscous paste that includes a
functional particulate phase (metals, dielectrics, metal oxides,
etc . . . ) is screen printed onto a substrate. A porous screen
fabricated from stainless steel, polyester, nylon or similar inert
material is stretched and attached to a rigid frame. A
predetermined pattern is formed on the screen corresponding to the
pattern to be printed. For example, a UV sensitive emulsion can be
applied to the screen and exposed through a positive or negative
image of the design pattern. The screen is then developed to remove
portions of the emulsion in the pattern regions.
[0259] The screen is then affixed to a printing device and the
thick film paste is deposited on top of the screen. The substrate
to be printed is then positioned beneath the screen and the paste
is forced through the screen and onto the substrate by a squeegee
that traverses the screen. Thus, a pattern of traces and/or pads of
the paste material is transferred to the substrate. The substrate
with the paste applied in a predetermined pattern is then subjected
to a drying and sintering treatment to solidify and adhere the
functional phase to the substrate.
[0260] Thick film pastes have a complex chemistry and generally
include a functional phase, a binder phase and an organic vehicle
phase. The functional phase include the copper metal powders of the
present invention which can provide conductivity paths and surfaces
for electrical transmission and are useful in components such as
capacitors. The particle size, size distribution, surface chemistry
and particle shape of the copper particles all influence the
rheology of the paste, as well as the characteristics of the
sintered film.
[0261] The binder phase is typically a mixture of inorganic binders
such as metal oxide or glass frit powders. For example, PbO based
glasses are commonly used as binders. The function of the binder
phase is to control the sintering of the film and assist the
adhesion of the functional phase to the substrate and/or assist in
the sintering of the functional phase. Reactive compounds can also
be included in the paste to promote adherence of the functional
phase to the substrate. For example, 0.1 to 1 percent CuO or CdO
can be included in a metal paste applied to an alumina substrate.
The CuO or CdO reacts to form an aluminate which provides improved
adhesion of the metal film.
[0262] Thick film pastes also include an organic vehicle phase that
is a mixture of solvents, polymers, resins or other organics whose
primary function is to provide the appropriate rheology (flow
properties) to the paste. The liquid solvent assists in mixing of
the components into a homogenous paste and substantially evaporates
upon application of the paste to the substrate. Usually the solvent
is a volatile liquid such as methanol, ethanol, terpineol, butyl
carbitol, butyl carbitol acetate, aliphatic alcohols, esters,
acetone and the like. The other organic vehicle components can
include thickeners (sometimes referred to as organic binders),
stabilizing agents, surfactants, wetting agents and the like.
Thickeners provide sufficient viscosity to the paste and also acts
as a binding agent in the unfired state. Examples of thickeners
include ethyl cellulose, polyvinyl acetate, resins such as acrylic
resin, cellulose resin, polyester, polyamide and the like. The
stabilizing agents reduce oxidation and degradation, stabilize the
viscosity or buffer the pH of the paste. For example,
triethanolamine is a common stabilizer. Wetting agents and
surfactants are well known in the thick film paste art and can
include triethanolamine and phosphate esters. In addition to the
foregoing, oxidation resistant additives can be included in the
paste to reduce oxidation of copper metal. For example, the
incorporation of boron-containing additives, including borate
glasses, is known to inhibit the oxidation of some base-metals.
[0263] The different components of the thick film paste are mixed
in the desired proportions in order to produce a substantially
homogenous blend wherein the functional phase is well dispersed
throughout the paste. Typically, the thick film paste will include
from about 5 to about 95 weight percent such as from about 60 to 85
weight percent, of the functional phase, including the copper metal
powders of the present invention.
[0264] Some applications of thick film pastes require higher
tolerances than can be achieved using standard thick-film
technology, as is described above. As a result, some thick film
pastes have photo-imaging capability to enable the formation of
lines and traces with decreased width and pitch. In this type of
process, a photoactive thick film paste is laid down substantially
as is described above. The paste is then dried and exposed to
ultraviolet light through a photomask and the exposed portions of
the paste are developed to remove unwanted portions of the paste.
This technology permits higher density interconnections and
conductive traces to be formed. The combination of the foregoing
technology with the copper powders of the present invention permits
the fabrication of devices with increased circuit density and
tolerances as compared to conventional technologies using
conventional powders.
[0265] Examples of thick film pastes are disclosed in U.S. Pat.
Nos. 4,172,733; 3,803,708; 4,140,817; and 3,816,097 all of which
are incorporated herein by reference in their entirety.
[0266] The copper metal powders of the present invention are
particularly advantageous for many applications of thick film
pastes. Copper is significantly less expensive than the noble
metals such as palladium and silver-palladium alloys that are used
in pastes for many applications. Copper metal is also advantageous
because copper metal resists leaching when soldered. Some other
metals, in particular silver-based metals, can leach significantly
when the metal is soldered. Copper metal is also advantageous
because of its high conductivity.
[0267] One of the disadvantages of copper metal that has limited
its widespread use is that copper must be fired in a substantially
reducing atmosphere due to the strong tendency of copper to oxidize
at relatively low temperatures (e.g. at about 600.degree. C. or
lower) in oxygen-containing atmospheres, such as air. This presents
a significant problem, particularly in the manufacture of
multi-layer ceramic capacitors that include an oxide dielectric
(e.g. BaTiO.sub.3) that must be sintered at high temperatures in an
oxygen-containing atmosphere to avoid reduction of the oxide
ceramic.
[0268] Metal powders for use in thick-film pastes should have good
dispersibility and flow properties. As is discussed above, the
copper metal powders according to the present invention are
substantially spherical in shape, have a narrow particle size
distribution and have a clean particle surface. Due to this unique
combination of properties, the metal powders disperse and flow in a
paste better than conventional copper metal powders which are not
small and spherical.
[0269] One of the limitations for the application of thick-film
pastes by screen printing is the difficulty creating lines of
narrow width and fine pitch (distance between lines from center to
center), and of reduced thickness. The continuing demand for
microelectronic components having a reduced size has made these
limitations critical in component design. One of the obstacles to
screen-printing surfaces having these properties is that
conventional metal powders include an undesirable percentage of
large particles and also include agglomerates of particles. Either
of these conditions can produce conductive traces having an uneven
width and an uneven thickness profile. The unpredictable width and
thickness of the conductive traces forces manufacturers to design
microelectronic devices to account for such variations, which can
needlessly occupy valuable space on the surface of the device and
waste considerable amounts of paste.
[0270] One use for such thick film pastes is in the manufacture of
multilayer ceramics, sometimes referred to as multichip modules.
The packaging of integrated circuits (IC's) typically utilizes a
chip carrier or module to which one or more integrated circuit
chips are attached. The module can then be joined to a printed
circuit board which is placed into a device, such as a computer.
Such modules can advantageously incorporate multiple wiring layers
within the module itself. Multilayer ceramic modules are typically
formed by laminating and sintering a stack of ceramic sheets that
have been screened with thick film pastes.
[0271] Typically, unfired (green) ceramic sheets are punched with
via holes, screened with a thick film metal paste, laminated into a
three dimensional structure and sintered in a furnace. The ceramic
and metal both densify simultaneously in the same sintering cycle.
Alternatively, the multilayer ceramic can be built sequentially
wherein alternate layers of metallurgy and dielectric are deposited
on the substrate and fired.
[0272] A schematic illustration of a multichip module is
illustrated in FIG. 50. The module includes two integrated circuit
devices 391 and 392. A number of electrically conductive traces,
such as traces 393 and 394, are formed on and through the various
layers of the device. Interconnection between the two integrated
circuit devices 391 and 392 or exterior devices is made by the
conductive traces and vias, which can terminate, for example, at
wire bonding pads such as pad 395 or at conductive pins, such as
pin 396. The multichip module can include any number of layers, and
many such modules include 20 or more such layers for
interconnection. The layers are typically formed from a dense
ceramic substrate, such as an alumina substrate.
[0273] A top view of a multichip module is illustrated in FIG. 51.
Electrically conductive traces 401 and 402 are printed on a ceramic
substrate 403 in parallel spaced relation. The conductive traces in
such a relation have a design pitch, that is, an average
center-to-center spacing between the conductive traces.
Manufacturers of such devices desire the linewidth and pitch to be
as small as possible to conserve available space on the surface of
the module. However, presently available powders for forming the
conductive traces which contain agglomerates and/or a wide particle
size distribution of particles inhibit the reliable manufacture of
conductive traces having a narrow linewidth and pitch. For example,
the linewidth and pitch for such traces is typically not smaller
than about 100 .mu.m. There is a demand in the industry to
significantly reduce the linewidth and pitch to significantly lower
levels, such as less than about 25 .mu.m and even less than about
15 .mu.m.
[0274] To achieve such narrow linewidths and fine pitch, thick film
pastes will have to be modified to have improved rheology and more
reliable characteristics. The thick film pastes incorporating the
copper metal powders of the present invention can consistently and
reliably produce a finer width line and pitch due to the spherical
morphology, small particle size and narrow particle size
distribution of the copper metal powders, as well as the
unagglomerated state of the powder. These properties will
advantageously permit the design of microelectronic devices with
conductive traces having a narrower pitch, and thus reduce the
overall size of the devices. Thick film pastes utilizing the copper
metal powders of the present invention can be used to produce
conductive traces having a significantly reduced linewidth and
pitch, such as less than about 25 .mu.m and even less than about 15
.mu.m.
[0275] The copper metal powders of the present invention are also
advantageous for use in thick-film pastes due to the increased
oxidation resistance of the copper metal powders. The increased
oxidation resistance of the copper metal powders is due to a number
of factors. The high crystallinty of the powders reduces the volume
of grain boundaries within the particle and thereby reduces the
ability of oxygen to diffuse along the grain boundaries and oxidize
the metal. The powders can also be easily coated or otherwise
modified to increase the oxidation resistance.
[0276] As is discussed above, thick-film pastes contain a number of
organics used as a vehicle to apply the paste to a substrate. These
pastes are then heated to a low temperature in order to volatilize
these organics. The organics are preferably volatilized in an
oxidizing atmosphere so that the organics are removed, such as in
the form of carbon dioxide. The temperature at which oxidation of
the copper metal particles of the present invention begins to occur
can be increased, and therefore, the binder and other organics can
be burned out at an increased temperature and/or under a higher
partial pressure of oxygen. The copper metal alloy powders and
metal composite powders of the present invention, as discussed
above, can also increase the oxidation resistance of the
powders.
[0277] The copper metal powders and pastes according to the present
invention are particularly useful for the fabrication of multilayer
ceramic capacitors (MLCC's). FIG. 52 illustrates an example of a
multilayer ceramic capacitor according to the present invention.
The MLCC 410 includes a plurality of ceramic dielectric layers 412
separated by internal electrodes 414. The ceramic dielectric layers
can be fabricated from a variety of materials such as titanates
(e.g., BaTiO.sub.3, Nd.sub.2Ti.sub.2O.sub.7, SrTiO.sub.3,
PbTiO.sub.3 or CaTiO.sub.3), zirconates (e.g. CaZrO.sub.3 or
MgZrO.sub.3) or niobate relaxors (e.g. lead magnesium niobate).
Many modifications of these compounds are known to those skilled in
the art. Terminations 416 and 418 for electrical connection are
typically fabricated using copper metal, and are included at
opposing sides of the MLCC. Alternating electrodes are connected to
each termination. Such devices typically utilize metals such as
palladium and silver-palladium for the internal electrodes, which
are co-fired with the ceramic dielectric. Copper is a significantly
less expensive alternative to these metals.
[0278] Copper metal powders according to the present invention, are
also useful for forming the external electrodes, or terminations,
of the multilayer ceramic capacitor, particularly when the internal
electrodes are formed using nickel metal. The external electrodes
are applied to the opposed sides of the MLCC after the formation of
the laminated ceramic dielectric/internal electrode structure.
Copper metal is particularly preferred for use as the external
electrode when the internal electrodes are fabricated using copper
metal.
[0279] Designers of MLCC's prefer the internal electrodes to be as
thin as possible to maximize capacitance, reduce cost and reduce
total volume, without sacrificing electrical integrity. Therefore,
the powders within the paste should disperse well, have a small
particle size and contain substantially no agglomerates or large
particles. Powders that do not meet these criteria force MLCC
manufacturers to design the devices with thick internal electrodes
to account for the variability. The copper metal powders according
to the present invention are particularly well-suited to permit the
design of MLCC's with thinner internal electrodes. Preferably, the
average thickness of the internal electrodes is not be greater than
about 2 .mu.m and more preferably is not greater than about 1.5
.mu.m.
[0280] Another problem typically associated with MLCC's fabricated
with copper electrodes is the sintering of the multilayer
structure. Because of the different sintering characteristics of
the copper metal and the dielectric material, many defects can
arise in the device such as cracks, delaminations and camber. In
order to alleviate some of these defects, thick film paste
manufacturers incorporate dispersed metal oxide powders in the
thick film paste. However, this is not always sufficient to
eliminate the foregoing problems. The composite metal particles of
the present invention, as is discussed hereinabove, provide a
unique solution to this problem and can significantly increase the
yield of devices. Copper metal particles that are composite
particles comprising an intimate mixture of the metal phase and a
non-metallic second phase can advantageously reduce the mismatch in
sintering characteristics between the metal layer and the
dielectric layer. Preferred non-metallic second phases include the
metal oxides and it is particularly preferred that a material
similar to the dielectric material can be used. For example, where
the dielectric layer comprises BaTiO.sub.3, it is preferred that
the copper metal particles include a copper metal phase intimately
mixed with a BaTiO.sub.3.
[0281] An MLCC such as that illustrated in FIG. 52 is typically
fabricated by first forming a green body, that is, an unsintered
structure which is adapted to be sintered to form the MLCC. Thus,
the green body includes a plurality of green ceramic sheets with a
thick film paste composition disposed between alternating sheets.
For example, sheets of tape cast ceramic can be screen-printed with
the metal electrode paste and a multilayer structure built by
alternating layers. Individual MLCC green bodies can than be cut
from the laminated sheets. The stacked and laminated structure is
then heated to remove organics from the thick film paste and
ceramic green sheets and sinter and densify the MLCC. An MLCC
including a titanate dielectric is typically sintered at about
1100-1300.degree. C. External electrodes can then be applied to
complete the device.
[0282] Another technology where the copper metal pastes according
to the present invention provide significant advantages is for flat
panel displays, such as plasma display panels. Operating under the
same basic principle as a fluorescent lamp, a plasma display panel
consists of millions of pixel regions on a transparent substrate
that mimic individual fluorescent tubes. The light emitted by each
region is controlled to form a video display. Plasma displays can
be produced in a very large size, such as 20 to 60 inch diagonal
screen size, with a very thin profile, such as less than about 3
inches.
[0283] Copper metal is particularly useful for forming the
electrodes for a plasma display panel. A cross-section of a plasma
display device as illustrated in FIG. 53. The plasma display
comprises two opposed panels 502 and 504 in parallel opposed
relation. A working gas is disposed and sealed between the two
opposing panels 502 and 504. The rear panel 504 includes a backing
plate 506 on which are printed a plurality of electrodes 508
(cathodes) which are printed parallel to one another. An insulator
510 covers the electrodes and spacers 512 are utilized to separate
the rear panel 504 from the front panel 502.
[0284] The front panel 502 includes a glass face plate 514 which is
transparent when observed by the viewer. Printed onto the rear
surface of the glass face plate 514 are a plurality of electrodes
516 (anodes) in parallel spaced relation. An insulator 518
separates the electrode from the pockets of phosphor powder
520.
[0285] A schematic view of the electrode configuration in such a
plasma display panel is illustrated in FIG. 54. The plasma display
includes a front panel 502 and a rear panel 504 printed on the
front panel are a plurality of electrodes 516 in parallel spaced
relation. Printed on the rear panel 504 are a plurality of
electrodes 508 which intersect the front panel electrodes 516 thus
forming an addressable XY grid of electrodes.
[0286] Thus, each pixel of phosphor powder can be activated by
addressing an XY coordinate defined by the intersecting electrodes
516 and 508. Plasma display panels can have a large surface area,
such as greater than 50 diagonal inches, and therefore the
uniformity and reliability of the addressing electrodes is critical
to the proper function of the plasma display device.
[0287] The copper powder according to the present invention can
advantageously be used to form the electrodes, as well as the bus
lines, for the plasma display panel. Copper metal advantageously
has a high conductivity and can be fired in air at the temperatures
typically used to form the electrode pattern. Typically, a copper
paste is printed onto a glass substrate and is fired in air at from
about 450-600.degree. C. Copper metal is also advantageously
resistant to corrosion (etching) from the plasma gas. Additives,
such as boron or boron compounds, can also be included in the
electrode thick-film paste to enhance the oxidation resistance of
the copper metal.
[0288] The copper metal powder of the present invention has a small
average particle size and a narrow size distribution to provide
high resolution lines which lead to a high pixel density and
precision pattern over a large area. For most flat panel displays,
a resolution of at least about 25 to 30 .mu.m is desirable. That
is, the average line width and spacing should be no greater than
about 30 .mu.m. For higher resolution displays, the resolution
should be even higher, such as a resolution of less than 20 .mu.m
or even less than 10 .mu.m. The copper metal powders of the present
invention enable such high resolutions over a large area, while
maintaining an acceptable yield.
[0289] The copper metal powders according to the present invention
are also useful for a number of other applications. For example,
the powders could be used as a source for spray coatings wherein
the powder is sprayed onto a heated substrate to form a coating of
copper metal. In this application, the particles would be
agglomerated to form the larger particles adapted for spraying.
[0290] Further, the powders could be used for powder metallurgy
wherein an intricate parts are formed using conditionally powder
metallurgy fabrication techniques such as die casting or the like.
The powders are also useful in conductive adhesive compositions,
which are commonly used for surface mount devices in portable
electronics. Such adhesives can replace solders in many
applications. The conductivity and adhesive strength of the
adhesive is improved through use of the powders of the present
invention due to the high crystallinity, spherical morphology and
surface properties of the powders.
EXAMPLES
[0291] Examples were prepared in accordance with the various
embodiments of the present invention. These examples are summarized
in Tables I through VI.
[0292] Examples 1-7 (Table I) illustrate the effect of varying the
reaction temperature on the characteristics of the copper metal
powder. In each of Examples 1-7, a precursor solution of cupric
nitrate (Cu(NO.sub.3).sub.2.2.5H.sub.2O) was formed such that the
concentration of copper metal in the precursor solution was about
7.5 weight percent. The solution was atomized using an ultrasonic
nebulizer operating at a frequency of about 1.6 MHZ. The aerosol of
atomized precursor solution was carried through a heating zone in a
gas mixture of forming gas (7% H.sub.2/N.sub.2) and N.sub.2. The
reaction temperature in the heating zone ranged from 700.degree. C.
to 1400.degree. C., as indicated in Table I. A Tuffryn filter
maintained at 60.degree. C. to 80.degree. C. was used to collect
the powder.
[0293] In all instances, x-ray diffraction analysis indicated that
substially phase pure copper metal particles were formed. The
powder density was best at a reaction temperature range of about
1200.degree. C. to 1300.degree. C. SEM photomicrographs of the
powder produced according to Example 5 are illustrated in FIGS. 55
and 56. The morphology of the particles is generally spherical and
substantially no agglomerated particles are apparent. The powder
has a narrow particle size distribution.
1TABLE I Effect of Reactor Temperature Powder Percent of Reactor
Density Standard Theoretical Example Temp. (.degree. C.)
(g/cm.sup.3) Deviation (g/cc) Density 1 700 8.37 0.09 93.8 2 900
8.32 0.06 93.3 3 1000 8.84 0.4 99.1 4 1100 8.26 0.06 92.6 5 1200
9.14* 0.05 102.5 6 1300 9.14* 0.38 102.5 7 1400 8.24 0.08 92.4
*Theoretical density for copper is 8.92 g/cm.sup.3. Errors are
possibly due to calibration or absorbed H.sub.2O
[0294] Further examples were produced to determine the affect of
the hydrogen concentration in the carrier gas on the formation of
the particles. Table II summarizes Examples 8-16 which illustrate
experiments conducted at different concentrations of hydrogen in
the carrier gas at different reactor temperatures.
2TABLE II Effect of Hydrogen Concentration Reactor Total Hydrogen
Stoichiometric Temp. Flow Flow Rate Hydrogen Detected Example
(.degree. C.) Rate (lpm) Ratio Excess Phases 8 900 0.5 0.7 5.8 Cu +
Cu.sub.2O 9 900 1 1.4 11.3 Cu + Cu.sub.2O 10 900 1.25 1.75 15.2 Cu
11 1200 0.5 0.7 6.6 Cu + Cu.sub.2O 12 1200 0.75 1.05 9.9 Cu 13 1200
1 1.4 13.2 Cu 14 1400 0.5 0.7 5.8 Cu + Cu.sub.2O 15 1400 1 1.4 11.7
Cu + Cu.sub.2O 16 1400 1.25 1.75 15.7 Cu
[0295] When the concentration of hydrogen is less than about 0.7
volume percent of the carrier gas, the copper has a tendency to
form copper oxide. For Hydrogen concentrations greater than about
1.75 percent, only copper metal is observed by x-ray diffraction.
Based on this data, it was concluded that the amount of hydrogen
required to maintain phase pure copper is at least about 10 to 15
times the theoretically calculated stoichiometric amount. Fir
example, if one mole of hydrogen is required to reduce CU.sub.2O to
2 moles of Cu based on the predominant chemical reaction, the
process of the present invention requires about 10 to 15 moles to
reliably produce phase pure Cu.
[0296] Table III illustrates the ability to use air as a quench gas
for the copper metal powder. The powder produced according to
Example 18 was pure copper, so that it is evident that an air
quench can be used. The ability to use air as a quench gas is
particularly advantageous since air is significantly less costly
than other gases, such as nitrogen.
3TABLE III Air as a Quench Gas Stoichiometric Example Hydrogen
Excess Collection Gas Phases 17 9.9 none Cu 18 6.4 air Cu
[0297] Table IV illustrates the use of an impactor and its effect
on the size and size distribution of the copper metal particles. An
SEM photomicrograph of the particles produced using an impactor
(Example 19) is illustrated in FIGS. 57 and 58 and particles
produced without an impactor (Example 5) is illustrated in FIG. 55.
The use of the impactor narrowed the particle size distribution and
the largest observed particle size decreased from about 2 .mu.m to
about 1 .mu.m. The particles are spherical with clean, smooth
surfaces.
4TABLE IV Effect of Impactor on Particle Size Largest Observed
Example Impactor Particle 5 no 2 .mu.m 19 yes 1 .mu.m
[0298] Table V illustrates the effect of precursor concentration on
the collection rate of particles.
5TABLE V Effect of Precursor Concentration Precursor Concentration
Collection Example (weight % Cu) Rate (g/hr) Phase 20 7.5 1.80 Cu
21 15 3.24 Cu 22 30 0.20 Cu
[0299] When the solution concentration was about 15 weight percent
(Example 21), the collection rate was about 3.24 g/hr or 1.8 times
higher than for a 7.5 weight percent solution (Example 20), under
otherwise identical conditions. Solution concentrations up to about
15 weight percent are advantageous for increasing production rate.
However, solution concentrations of about 30 weight percent
(Example 22) do not appear advantageous since the collection rate
decreases significantly.
[0300] Table IV illustrates alloys of copper made using zinc and
tin as alloying elements. Such alloying additions can
advantageously increase the oxidation resistance of the copper
metal. For example, the addition of 30 weight percent Zn (Example
26) lowered the onset of oxidation from about 450.degree. C. to
about 250.degree. C. The 10 percent tin alloy shows a rougher
surface, but the particles remain spherical.
6TABLE VI Copper Metal Alloys Example Element Weight Percent 23 Zn
1 24 Zn 5 25 Zn 10 26 Zn 30 27 Sn 1 28 Sn 5 29 Sn 10
[0301] Metal composite particles were produced using copper metal
and the ferroelectric ceramic dielectric compounds barium titanate
and neodymium titanate. The precursor for the dielectric compounds
was prepared by adding either barium or neodymium nitrate to a
water solution containing titanium tetraisopropoxide. A fine
precipitate was formed and additions of nitric acid to the solution
caused the precipitate to decompose and from a soluble
solution.
[0302] The precursor solutions contained about 25 weight percent of
the ferroelectric precursor along with the copper nitrate. The
powders produced are illustrated in FIGS. 59 and 49
(Cu/Nd.sub.2Ti.sub.2O.sub.7). X-ray diffraction indicated that the
powders included both phase pure ferroelectric material and copper
metal. The morphology of the particles is smooth nodules of copper
connected to rough surface particles of the titanate.
[0303] While various embodiments of the present invention have been
described in detail, it is apparent that modifications and
adaptations of those embodiments will occur to those skilled in the
art. However, it is to be expressly understood that such
modifications and adaptations are within the spirit and scope of
the present invention.
* * * * *