Title: Method for the production of metal-carbon composite powders
Abstract: Metal-carbon composite powders and methods for producing metal-carbon composite powders. The powders have a well-controlled microstructure and morphology and preferably have a small average particle size. The method includes forming the particles from an aerosol of powder precursors. The invention also includes novel devices and products formed from the composite powders.
Patent Number: 6,841,099 Issued on 01/11/2005 to Kodas, et al.
| Inventors:
|
Kodas; Toivo T. (Albuquerque, NM);
Hampden-Smith; Mark J. (Albuquerque, NM);
Caruso; James (Albuquerque, NM);
Skamser; Daniel J. (Albuquerque, NM);
Powell; Quint H. (Albuquerque, NM)
|
| Assignee:
|
Cabot Corporation (Boston, MA)
|
| Appl. No.:
|
209343 |
| Filed:
|
July 31, 2002 |
| Intern'l Class: |
B29B 009/10 |
| Field of Search: |
264/5,346,9,13,14
75/331,335,336,340,341,351
|
References Cited [Referenced By]
U.S. Patent Documents
| 6338809 | Jan., 2002 | Hampden-Smith et al. | 264/7.
|
Primary Examiner: Theisen; Mary Lynn
Attorney, Agent or Firm: Marsh Fischmann & Breyfogle LLP
Parent Case Text
This is a continuation of application of U.S. patent application Ser. No.
09/636,732 filed on Aug. 10, 2000 now abandoned, which is a divisional
application of U.S. patent application Ser. No. 09/141,397 filed Aug. 27,
1998, now U.S. Pat. No. 6,103,393, which is a continuation-in-part
application of U.S. patent application Ser. Nos. 09/028,029 now abandoned,
09/028,277 now U.S. Pat. No. 6,277,169 and 09/030,057 now U.S. Pat. No.
6,388,809, each filed on Feb. 24, 1998.
Claims
What is claimed is:
1. A method for the production of metal-carbon composite particles
comprising the steps of:
a) forming a liquid precursor including at least a first metal phase
precursor and carbon;
b) drying the liquid precursor; and
c) heating the dried precursor for not greater than about 10 seconds to
form metal-carbon composite particles.
2. A method as recited in claim 1, wherein said carbon is in the form of
suspended carbon particles having an average size of not greater than
about 0.1 .mu.m.
3. A method as recited in claim 1, wherein said carbon is in the form of
agglomerates of smaller primary particles.
4. A method as recited in claim 1, wherein said heating step comprises
heating the dried precursor for not greater than about 4 seconds.
5. A method as recited in claim 1, wherein said heating step comprises
heating the dried precursor for not greater than 2 seconds.
6. A method as recited in claim 1, wherein said composite particles have an
average particle size of not greater than about 20 .mu.m.
7. A method for making metal-carbon composite particles comprising a metal
alloy phase dispersed on a support phase, wherein said particles are at
least partially annealed while suspended in a gas for a time sufficient to
permit redistribution of different metal phases and form said metal alloy.
8. A method as recited in claim 7, wherein said metal alloy phase comprises
platinum.
9. A method as recited in claim 7, wherein said support phase comprises
carbon.
10. A method as recited in claim 7, wherein said composite particles are
formed by a spray process.
11. A method for making metal-carbon composite particles comprising at
least a first metal phase and a second metal phase, wherein said particles
are at least partially annealed while suspended in a gas for a time
sufficient to permit redistribution of said first and second metal phases.
12. A method as recited in claim 11, wherein at least one of said first and
second metal phases comprises platinum metal.
13. A method as recited in claim 11, wherein said particles are formed by a
spray process.
14. A method for producing metal-carbon composite particles, comprising the
steps of:
a) providing a liquid precursor comprising a platinum metal precursor and a
particulate carbon precursor;
b) generating an aerosol from said liquid precursor; and
c) carrying said aerosol in a carrier gas comprising air;
d) heating said aerosol to convert said platinum metal precursor to a
platinum-containing metal dispersed on said carbon.
15. A method as recited in claim 14, wherein said heating step comprises
heating for less than about 10 seconds.
16. A method as recited in claim 14, wherein said particulate carbon
precursor comprises particulate carbon having an average size of from
about 5to about 100 nanometers.
17. A method as recited in claim 14, wherein said aerosol has as droplet
loading of at least about 0.04 ml/l.
18. A method as recited in claim 14, wherein said aerosol has a droplet
loading of at least about 0.083 ml/l.
19. A method for producing metal-carbon composite particles, comprising the
steps of:
a) providing a flowable liquid feed comprising at least a first metal
precursor and a second metal precursor and a particulate carbon precursor;
b) generating an aerosol from said liquid precursor; and
c) heating said aerosol to convert at least one of said metal precursors to
a metallic phase dispersed on said carbon.
20. A method as recited in claim 19, wherein at least one of said metal
precursors comprises metal particulates.
21. A method as recited in claim 20, wherein at least one of said metal
precursors is a platinum precursor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to metal-carbon composite powders and to
methods for producing such powders, as well as products and devices
incorporating the composite powders. The powders are preferably produced
by a spray conversion process.
2. Description of Related Art
Many product applications require metal-carbon composite powders. Such
composite powders should have one or more of the following properties:
high purity; controlled crystallinity; small average particle size; narrow
particle size distribution; spherical particle morphology; controlled
surface chemistry; controlled surface area; and little or no agglomeration
of particles. Examples of metal-carbon composite powders requiring such
characteristics include, but are not limited to, those useful in
electrocatalyst applications such as fuel cells and batteries, as well as
in conductive pastes and inks.
With the advent of portable and hand-held electronic devices and an
increasing demand for electric automobiles due to the increased strain on
natural resources there is a need for rapid development of high
performance, economical power systems. Such power systems require improved
means for both energy storage, achieved by use of batteries, and energy
generation, achieved by use of fuel cells. Batteries can be subdivided
into primary (non-rechargeable) and secondary (rechargeable) batteries.
Fuel cells are electrochemical devices which are capable of converting the
energy of a chemical reaction into electrical energy. The electrical
energy is produced without combustion and creates virtually no pollution.
Fuel cells are unlike batteries because fuel cells convert chemical energy
to electrical energy as the chemical reactants are continuously delivered
to the fuel cell. When the fuel cell is off, it has zero electrical
potential. As a result, fuel cells are typically used to produce a
continuous source of electrical energy and compete with other forms of
continuous electrical energy production such as the combustion engine,
nuclear power and coal-fired power stations. Different types of fuel cells
are categorized by the electrolyte used in the fuel cell. The five main
types of fuel cells are alkaline, molten carbonate, phosphoric acid, solid
oxide and proton exchange membrane (PEM) or solid polymer fuel cells.
In fuel cells, gases are often used as a source of chemical energy which is
converted to electrical energy. One of the critical requirements for these
energy devices is the efficient catalytic conversion of the reactants to
electrical energy. A significant obstacle to the wide-scale
commercialization of such devices is the need for superior electrocatalyst
materials for this conversion process.
A PEM fuel cell stack is comprised of hundreds of membrane electrode
assemblies (MEA's). An MEA includes a cathode and anode, each constructed
from, for example, carbon cloth. The anode and cathode sandwich a proton
exchange membrane which has a catalyst layer on each side of the membrane.
Power is generated when hydrogen is fed into the anode and oxygen (air) is
fed into the cathode. In a reaction catalyzed by a platinum-based catalyst
in the catalyst layer, the hydrogen ionizes to form protons and electrons.
The protons are transported through the proton exchange membrane to a
catalyst layer on the opposite side of the membrane where another
catalyst, typically platinum or a platinum alloy, catalyzes the reaction
of the protons with oxygen to form water.
Anode: 2H.sub.2.fwdarw.4H.sup.+ +4e.sup.-
Cathode: 4H.sup.+ +4e.sup.- +O.sub.2.fwdarw.2H.sub.2 O
Overall: 2H.sub.2 +O.sub.2.fwdarw.2H.sub.2 O
The electrons formed at the anode are routed to the cathode through an
electrical circuit which provides the electrical power.
The critical issues that must be addressed for the successful
commercialization of fuel cells are cell cost, cell performance and
operating lifetime. In terms of fuel cell costs, current fuel cell stacks
employ MEA's containing unsupported platinum black electrocatalysts with a
loading of about 4 milligrams of platinum per square centimeter on each of
the anode and cathode. When this loading is compared to a typical cell
performance of 0.42 watts per square centimeter, then 19 grams of platinum
per kilowatt is required. It is clear that a significant cost reduction in
the electrocatalyst is necessary for these cells to become economically
viable. However, reducing the amount of precious metal is not a suitable
solution because there is also a strong demand for improved cell
performance. For automotive applications, improved power density is
critical whereas for stationary applications, higher voltage efficiencies
are necessary. The major technical challenge continues to be improved
cathode electrocatalyst performance with air as the oxidant.
A type of battery which utilizes a similar principle is the zinc-air
battery, which relies upon the redox couples of oxygen and zinc. Zinc-air
batteries are advantageous since they consume oxygen from the air as a
fuel, contain no toxic or explosive constituents and operate at one
atmosphere of pressure. Zinc-air batteries typically operate by adsorbing
oxygen from the air where it is reduced using an oxygen reduction
catalyst. As the oxygen is reduced, zinc metal is oxidized. The two
half-reactions of a zinc-air battery during discharge are:
Cathode: O.sub.2 +2H.sub.2 O+4e.sup.-.fwdarw.4OH.sup.-
Anode: 2Zn.fwdarw.2Zn.sup.2+ +4e.sup.-
Overall: 2Zn+O.sub.2 +2H.sub.2 O.fwdarw.2Zn(OH).sub.2
Zinc-air batteries can be primary batteries or secondary batteries.
Although zinc-air batteries consume oxygen as a fuel, they are typically
not considered fuel cells because they have a standing potential without a
fuel source. Zinc-air cells absorb oxygen from the air on the air
electrode during discharge and release air out of the cell during
recharge.
Typically, air electrodes (cathodes) are alternatively stacked with zinc
electrodes (anodes) which are packaged in a container that is open to the
air using small holes or ports. When the battery cell discharges, oxygen
is reduced to O.sup.2- while zinc metal is oxidized to Zn.sup.2+. When all
of the zinc has been oxidized, the secondary battery can be recharged
where Zn.sup.2+ is reduced back to zinc metal.
The advantages of zinc air batteries over other rechargeable battery
systems are safety, long run time and light weight. The batteries contain
no toxic materials and can run as long as 10 to 14 hours, compared to 2 to
4 hours for most lithium-ion batteries. Zinc-air batteries are also very
light weight, leading to good power density (power per unit of weight or
volume), which is ideal for portable applications. The two major problems
associated with zinc-air batteries, however, are limited total power and
poor rechargeability/cycle lifetime.
In particular, power is becoming a major area of attention for battery
manufacturers trying to meet the increased demands of modern electronics.
Current zinc-air batteries can deliver sufficient power to permit the
batteries to be used in specific low-power laptops and other portable
devices that have relatively low power requirements. Most laptops and
other portable electronic devices, however, require batteries that are
able to provide a level of power that is higher than the capabilities of
current zinc-air batteries.
The main reason for the low power of zinc-air batteries is believed to be
related to the inefficiency of the catalytic reactions in the air
electrodes. In zinc-air batteries, metal-carbon composite powders are used
at the cathode to reduce the oxygen from the air to O.sup.2-. It is
believed that poor accessibility of the catalyst and the local
microstructural environment around the catalyst and adjoining carbon is
important in the efficiency of oxygen reduction. See, for example, P. N.
Ross et al., Journal of the Electrochemical Society, Vol. 131, pg. 1742
(1984).
Rechargeability is also a problem with zinc-air batteries. Current zinc-air
technology can deliver safe, non-toxic and light weight batteries with
very long run times. However, the batteries degrade in performance after a
number of recharging cycles and therefore have a short cycle life. The
short cycle life of zinc-air batteries is believed to be related to the
catalyst used in the air electrodes. Specifically, it is believed that
corrosion of the carbon used in these systems leads to a loss in capacity
and hence, a decreasing discharge time. Control over the powder properties
such as crystallinity, surface area and metal dispersion can enhance the
performance of these batteries.
Methods for preparing noble metal electrocatalyst materials are known in
the art. U.S. Pat. No. 4,052,336 by VanMontfoort et al. discloses a
process for preparing an active noble metal catalyst on a carbon carrier,
such as palladium on carbon, by adsorbing a salt of the metal onto the
carbon, forming an oxide or hydroxide from the metal salt and reducing the
oxide or hydroxide to a metal. The carbon support comprises porous active
carbon particles having a widely varying particle size of less than 1
.mu.m up to 60 .mu.m. The catalyst comprises from about 0.1 to about 15
percent by weight of the noble metal. It is disclosed that the noble metal
is deposited on the carbon carrier in the form of very small crystallites
which have a high degree of catalytic activity per gram of noble metal.
U.S. Pat. No. 4,136,059 by Jalan et al. discloses a method for the
production of electrochemically active platinum particles for use in fuel
cell electrodes. The particles are formed by mixing chloroplatinic acid
and sodium dithionite in water to provide a colloidal dispersion which is
absorbed onto a support material (e.g. carbon black).
U.S. Pat. No. 4,482,641 by Wennerberg discloses a high surface area porous
active carbon matrix containing a uniform dispersion of a metal. The
material is formed by spray drying a carbon precursor and a metal
precursor to form particles and then pyrolyzing the spray dried particles
under an inert gas and in the presence of an alkali metal hydroxide. A
preferred heating method for the pyrolyzation step is to heat using
microwave heating. It is disclosed that the metal crystals have a size
from about 5 to 30 angstroms and are disposed on active carbon having a
cage-like structure.
U.S. Pat. No. 4,569,924 by Ozin et al. discloses a carbon-metal catalyst
having an active metal such as silver deposited on the carbon substrate in
a zero-valent, small cluster form. The catalyst is produced by vaporizing
the metal under low vapor pressure conditions in an organic liquid solvent
such that the metal dissolves in the solvent. The solvent is then
contacted with carbon so that the complex diffuses onto the surface of the
carbon and into the pores of the carbon. The carbon particles have a metal
loading of 0.1 to 15 weight percent.
U.S. Pat. No. 4,652,537 by Tamura et al. discloses a process for producing
a catalyst useful for converting carbon monoxide into carbon dioxide. The
process includes contacting activated carbon with an aqueous solution of
chloroplatinic acid, reducing the absorbed chloroplatinic acid to platinum
with a reducing agent and decomposing the excess reducing agent. The
catalyst preferably contains at least about 6 milligrams of platinum per
gram of activated carbon. The activated carbon particles have an average
grain size of from about 0.4 to about 10 millimeters.
U.S. Pat. No. 4,970,128 by Itoh et al. discloses a supported platinum alloy
electrocatalyst for an acid electrolyte fuel cell. The platinum alloy
includes platinum, iron and copper. The electrocatalyst has better initial
activity and lifetime than conventional platinum or other multi-component
alloy electrocatalysts. U.S. Pat. No. 5,489,563 by Brand et al. discloses
a platinum/cobalt/chromium catalytic alloy which is precipitated onto a
carbon support from nitrate salts.
U.S. Pat. No. 4,970,189 by Tachibana discloses a porous, metal-containing
carbon material which includes fine particles of a metal having an average
particle size of 1 .mu.m or less dispersed in a carbonaceous body. The
method includes mixing a metal oxide with an organic, carbonizing and
converting the oxide to metal particles. The catalyst includes from about
5 to 50 weight percent metal.
U.S. Pat. No. 5,068,161 by Keck et al. discloses an electrocatalytic
material suitable for use in phosphoric acid fuel cells. The material
includes an alloy of platinum with another element such as titanium,
chromium, manganese, iron, cobalt, nickel, copper, gallium, zirconium or
hafnium. The platinum alloy loading is 20 to 60 weight percent and the
electrochemical area of the alloy is greater than about 35 m.sup.2 /g.
U.S. Pat. No. 5,120,699 by Weiss et al. discloses a catalyst containing
from 0.01 to 5 weight percent platinum on a graphite support. The graphite
support has a particle size distribution of from about 1 to 600 .mu.m. The
catalyst material has good longevity when used for hydrogenation
reactions.
U.S. Pat. No. 5,453,169 by Callstrom et al. discloses an electrocatalytic
material including glassy carbon which contains graphite crystals having a
size of from about 1 to 20 nanometers.
U.S. Pat. No. 5,501,915 by Hards et al. discloses a porous electrode
suitable for use in a solid polymer fuel cell which includes highly
dispersed precious metal catalyst on particulate carbon which is
impregnated with a proton conducting polymer.
The foregoing methods generally result in poor control over the composition
and microstructure of the electrocatalytic materials, as well as the
dispersibility and surface area of the metal on the carbon surface.
Further, alloy compositions such as platinum/ruthenium used for oxygen
reduction in a fuel cell are not made in a reproducible fashion. The
inability to control the fundamental powder characteristics is a major
shortcoming for the future development of the electrocatalyst materials.
In addition to electrocatalyst applications metal-carbon composite powders
are also useful for electrically and thermally conductive traces in
microelectronic applications. Such traces are typically formed using a
thick-film paste. The resulting traces have good flexibility when fired at
low temperatures and are useful for many applications, including touch
screens and similar devices.
It would be advantageous to provide a flexible production method capable of
producing metal-carbon composite powders which would enable control over
the powder characteristics as well as the versatility to accommodate
metal-carbon compositions which are either difficult or impossible to
produce using existing production methods. It would be advantageous to
provide control over the particle size, particle size distribution, weight
loading of the metal and carbon, surface area of the powder, pore
structure of the powder and compositional uniformity. It would be
particularly advantageous if such metal-carbon composite powders could be
produced in large quantities on a substantially continuous basis.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a process block diagram showing one embodiment of the process of
the present invention.
FIG. 2 is a side view in cross section of one embodiment of aerosol
generator of the present invention.
FIG. 3 is a top view of a transducer mounting plate showing a 49 transducer
array for use in an aerosol generator of the present invention.
FIG. 4 is a top view of a transducer mounting plate for a 400 transducer
array for use in an ultrasonic generator of the present invention.
FIG. 5 is a side view of the transducer mounting plate shown in FIG. 4.
FIG. 6 is a partial side view showing the profile of a single transducer
mounting receptacle of the transducer mounting plate shown in FIG. 4.
FIG. 7 is a partial side view in cross-section showing an alternative
embodiment for mounting an ultrasonic transducer.
FIG. 8 is a top view of a bottom retaining plate for retaining a separator
for use in an aerosol generator of the present invention.
FIG. 9 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.
FIG. 10 is a side view of the liquid feed box shown in FIG. 9.
FIG. 11 is a side view of a gas tube for delivering gas within an aerosol
generator of the present invention.
FIG. 12 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.
FIG. 13 shows one embodiment for a gas distribution configuration for the
aerosol generator of the present invention.
FIG. 14 shows another embodiment for a gas distribution configuration for
the aerosol generator of the present invention.
FIG. 15 is a top view of one embodiment of a gas distribution plate/gas
tube assembly of the aerosol generator of the present invention.
FIG. 16 is a side view of one embodiment of the gas distribution plate/gas
tube assembly shown in FIG. 15.
FIG. 17 shows one embodiment for orienting a transducer in the aerosol
generator of the present invention.
FIG. 18 is a top view of a gas manifold for distributing gas within an
aerosol generator of the present invention.
FIG. 19 is a side view of the gas manifold shown in FIG. 18.
FIG. 20 is a top view of a generator lid of a hood design for use in an
aerosol generator of the present invention.
FIG. 21 is a side view of the generator lid shown in FIG. 20.
FIG. 22 is a process block diagram of one embodiment of the process of the
present invention including a droplet classifier.
FIG. 23 is a top view in cross section of an impactor of the present
invention for use in classifying an aerosol.
FIG. 24 is a front view of a flow control plate of the impactor shown in
FIG. 23.
FIG. 25 is a front view of a mounting plate of the impactor shown in FIG.
23.
FIG. 26 is a front view of an impactor plate assembly of the impactor shown
in FIG. 23.
FIG. 27 is a side view of the impactor plate assembly shown in FIG. 26.
FIG. 28 is a process block diagram of one embodiment of the present
invention including a particle cooler.
FIG. 29 is a top view of a gas quench cooler of the present invention.
FIG. 30 is an end view of the gas quench cooler shown in FIG. 29.
FIG. 31 is a side view of a perforated conduit of the quench cooler shown
in FIG. 29.
FIG. 32 is a side view showing one embodiment of a gas quench cooler of the
present invention connected with a cyclone.
FIG. 33 is a process block diagram of one embodiment of the present
invention including a particle coater.
FIG. 34 is a block diagram of one embodiment of the present invention
including a particle modifier.
FIG. 35 shows cross sections of various particle morphologies of some
composite particles manufacturable according to the present invention.
FIG. 36 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.
FIGS. 37a and b illustrate a schematic of a zinc-air battery according to
an embodiment of the present invention.
FIG. 38 illustrates a schematic of a membrane electrode assembly for use in
a proton exchange membrane fuel cell according to an embodiment of the
present invention.
FIG. 39 illustrates an SEM photomicrograph of a metal-carbon composite
powder according to the present invention.
FIG. 40 illustrates an TEM photomicrograph of a metal-carbon composite
powder according to an embodiment of the present invention.
FIG. 41 illustrates a particle size distribution for a metal-carbon
composite powder according to an embodiment of the present invention.
FIG. 42 illustrates an x-ray diffraction pattern of a metal-carbon
composite powder according to an embodiment of the present invention.
FIG. 43 illustrates an x-ray diffraction pattern of a metal-carbon
composite powder according to an embodiment of the present invention.
FIG. 44 illustrates an x-ray diffraction pattern of a metal-carbon
composite powder according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is generally directed to metal-carbon composite
powders and methods for producing metal-carbon composite powders. The
invention is also directed to novel products and devices fabricated using
the composite powders. As used herein, metal-carbon composite powders or
metal-carbon composite particles are those that include within the
individual particles at least a first metal phase, such as a pure metal or
a metal alloy, and a carbon phase. The particles preferably include more
than a trace amount of carbon, such as at least about 3 weight percent
carbon. The powders of the present invention are not mere physical
admixtures of metal particles and carbon particles, but are comprised of
particles that include both a metal phase and a carbon phase.
In one aspect, the present invention provides a method for preparing a
particulate product including a metal-carbon composite. 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 composite particles. In one
embodiment, the particles can be subjected, while still in a dispersed
state, to compositional or structural modification such as
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.
The process of the present invention is particularly well suited for the
production of finely divided composite particles having a small weight
average size. In addition to making particles within a desired range of
weight average particle size, the particles may advantageously be produced
with a narrow size distribution, thereby providing size uniformity that is
desired for many applications.
In addition, the method of the present invention provides significant
flexibility for producing composite particles of varying composition,
crystallinity, morphology and microstructure. For example, the metal phase
may be uniformly dispersed throughout a matrix of carbon. Other
morphologies and microstructures are also possible.
Referring now to FIG. 1, one embodiment of the process of the present
invention is described. A liquid feed 102, including the 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.
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, such as suspended carbon particles, those particles should be
relatively small in relation to the size of droplets in the aerosol 108.
Such suspended particles should typically be not larger than about 1 .mu.m
in size, preferably smaller than about 0.5 .mu.m in size, and more
preferably not larger than about 0.3 .mu.m in size and most preferably not
larger than about 0.1 .mu.m in size. Most preferably, the suspended
particles should be colloidal. 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 carbon particles, the carbon particles
typically comprise from about 3 to 15 weight percent of the liquid feed.
As noted, the liquid feed 102 includes one or more precursors 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 a substantial chemical
reaction. This could be the case, for example, when the liquid feed 102
includes, as a precursor material, suspended carbon particles that are not
substantially chemically modified in the furnace 110. In any event, the
particles 112 comprise at least one component originally contributed by
the precursor.
For the production of metal-carbon composite powders, the liquid feed 102
will 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. Examples of such precursor solutions and the reactions to
form metal-carbon composites include:
aM(NO.sub.3).sub.n +b(C.sub.x H.sub.y O.sub.z).sub.m.fwdarw.M.sub.a C.sub.b
The use of a liquid carbon precursor typically results in amorphous carbon,
which may not be desirable for many applications. Alternatively, one
precursor material could be in a solid particulate phase (e.g. particulate
carbon) and a second precursor material could be in a liquid phase (e.g. a
metal salt). Advantageously, highly crystalline carbon can be selected to
yield metal-carbon composite particles having a highly crystalline
(graphitic) carbon 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.
The carrier gas 104 may comprise any gaseous medium in which droplets
produced from the liquid feed 102 may be dispersed in aerosol form. 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 gases according to the
present invention include mixtures of hydrogen and nitrogen.
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 also include nonliquid material, such as one or more
small particles held in the droplet by the liquid. For example, one phase
of the composite particles may be provided in the liquid feed 102 in the
form of suspended precursor particles and a second phase of the composite
particles 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.
An important aspect of the present invention is generation of the aerosol
108 with droplets of a small average size and, preferably, a narrow size
distribution. In this manner, the particles 112 may be produced at a
desired small size with a narrow size distribution, which is advantageous
for many applications.
The aerosol generator 106 is preferably 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 20 .mu.m; preferably about 10 .mu.m, more preferably
about 7 .mu.m and most preferably about 5 .mu.m. A weight average droplet
size in a range of from about 2 .mu.m to about 4 .mu.m is preferred for
many applications. The aerosol generator is also capable of producing the
aerosol 108 such that it includes droplets having 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.
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.
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 0.25
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.
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.
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.
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. Further, 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.
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. Quartz tubes can also be advantageous. 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. According to one embodiment of the present invention,
the tube is a metal tube coated on the interior with a refractory material
such as alumina.
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.
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.
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.
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 the 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. Further, collection should normally be at
a temperature that is low enough to prevent significant agglomeration of
the particles 112.
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. 2, 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. 2, includes forty-nine
transducers in a 7.times.7 array. The array configuration is as shown in
FIG. 3, which depicts the locations of the transducer housings 122 mounted
to the transducer mounting plate 124.
With continued reference to FIG. 2, 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.
During operation of the aerosol generator 106, as shown in FIG. 2, 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 preferably within 1%.
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.
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.
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. 2
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.
Another important feature of the aerosol generator 106, as shown in FIG. 2,
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 extends 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.
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.
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 discs 120 are preferably also at
an elevated temperature in the ranges discussed for the liquid feed 102.
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.
FIGS. 4-21 show component designs for an aerosol generator 106 including an
array of 400 ultrasonic transducers. Referring first to FIGS. 4 and 5, the
transducer mounting plate 124 is shown with a design to accommodate an
array of 400 ultrasonic transducers, arranged in four subarrays 170 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. 2.
As shown in FIGS. 4 and 5, 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. 6 the pro
