U.S. patent application number 10/265449 was filed with the patent office on 2004-04-08 for method for producing nano-structured materials.
Invention is credited to Huang, Wayne, Wu, L. W..
Application Number | 20040065170 10/265449 |
Document ID | / |
Family ID | 32042451 |
Filed Date | 2004-04-08 |
United States Patent
Application |
20040065170 |
Kind Code |
A1 |
Wu, L. W. ; et al. |
April 8, 2004 |
Method for producing nano-structured materials
Abstract
A method for synthesizing a nano-structured material, including
four primary steps: (A) providing a reaction chamber wherein the
nano-structured material is generated from at least a starting
material selected from the group consisting of a metal, a metal
alloy, a metal compound, and a ceramic; (B) operating a twin-wire
arc nozzle, comprising two wires and a working gas being
controllably fed into the chamber, to form an arc between two
converging leading tips of the two wires to heat and melt the
starting material at the leading tips for providing a stream of
liquid droplets traveling in a predetermined direction (preferably
vertically downward); (C) operating at least a second high energy
source for producing a vaporizing zone adjacent to the arc and
inside the chamber wherein the liquid droplets are vaporized to
form vapor species; and (D) cooling the vapor species for forming
the nano-structured material. The second high energy source is
selected from the group consisting of a laser beam, an electron
beam, an ion beam, a flame, an induction plasma, and combinations
thereof. The second high energy source may also be a plurality of
arc plasmas. The nano-structured material produced can be in the
form of a nanometer-sized powder particles or a coating (or thin
film) containing nanometer-sized phases deposited on a
substrate.
Inventors: |
Wu, L. W.; (Fargo, ND)
; Huang, Wayne; (Fargo, NC) |
Correspondence
Address: |
Wayne Huang
2902, 28TH AVE, SW
FARGO
ND
58103
US
|
Family ID: |
32042451 |
Appl. No.: |
10/265449 |
Filed: |
October 7, 2002 |
Current U.S.
Class: |
75/10.13 ;
75/10.14; 75/10.19 |
Current CPC
Class: |
B01J 2219/0813 20130101;
C01P 2004/64 20130101; C01G 19/00 20130101; B01J 2219/0871
20130101; B82Y 30/00 20130101; B01J 19/085 20130101; B01J 2219/0894
20130101; C01P 2004/84 20130101; C01G 1/02 20130101; B01J 19/088
20130101; B01J 2219/0822 20130101; B01J 2219/0886 20130101; C23C
4/131 20160101; B01J 2219/0843 20130101; B22F 1/054 20220101; C01B
13/20 20130101; B01J 19/121 20130101; B22F 9/12 20130101; B01J
2219/0828 20130101; B01J 2219/0841 20130101; B22F 1/16 20220101;
B22F 2999/00 20130101; C01B 21/06 20130101; C22B 4/005 20130101;
B22F 2999/00 20130101; B22F 9/12 20130101; B22F 2202/11 20130101;
B22F 2202/13 20130101 |
Class at
Publication: |
075/010.13 ;
075/010.14; 075/010.19 |
International
Class: |
C22B 004/00 |
Goverment Interests
[0001] The present invention was based on the research results of a
project supported by the U.S. National Science Foundation SBIR
Program. The U.S. Government has certain rights on this patent.
Claims
What is claimed:
1. A method for synthesizing a nano-structured material,
comprising: (A) providing a chamber wherein said nano-structured
material is generated from at least a starting material selected
from the group consisting of a metal, a metal alloy, a metal
compound, and a ceramic; (B) operating a twin-wire arc nozzle,
comprising two wires and a working gas being controllably fed into
said chamber, to form an arc between two converging leading tips of
the two wires to heat and melt said at least a starting material at
said leading tips for providing a stream of liquid droplets
traveling in a predetermined direction; (C) operating at least a
second high energy source for producing a vaporizing zone adjacent
to said arc and inside said chamber with said liquid droplets
traveling into said vaporizing zone and being vaporized therein to
form vapor species; and (D) operating heat treatment means to cool
said vapor species for forming said nano-structured material.
2. The method as defined in claim 1, wherein said second high
energy source is selected from the group consisting of a laser
beam, an electron beam, an ion beam, a flame, a high-frequency
induction plasma, and combinations thereof.
3. The method as defined in claim 1, wherein said nano-structured
material comprises nanometer-sized powder particles.
4. The method as defined in claim 1, wherein step (D) comprises a
sub-step of directing said vapor species to impinge upon a
substrate and deposit thereon to form said nano-structured material
that is characterized by a coating containing nanometer-sized
phases deposited on said substrate.
5. The method as defined in claim 1, wherein said second high
energy source comprises a plurality of arc plasmas.
6. The method as defined in claim 1, further comprising an
additional step (E), after step (C) and before step (D), said step
(E) comprises introducing a stream of reactive gas into said
chamber to impinge upon said vapor species and exothermically react
therewith to produce said nano-structured material.
7. The method as defined in claim 1, wherein step (B) includes:
operating wire feeding and control means to either continuously or
intermittently feed said two wires into said chamber in such a
fashion that the two leading tips are maintained at a desired
separation; and operating power supply means to provide currents
through said two wires to form said arc with a temperature
sufficient for melting said at least a starting material at said
leading tips.
8. The method as defined in claim 6, wherein step (E) includes
operating means for controlling the flow rate of the reactive gas,
thereby enabling change of particle size of the nanometer-scaled
powder material.
9. The method as defined in claim 6, wherein said reactive gas is
selected from the group consisting of nitrogen, phosphorus,
arsenic, oxygen, sulfur, selenium, tellurium, fluorine, chlorine,
bromine, iodine, a carbon-containing gas, and mixtures thereof.
10. The method as defined in claim 1, wherein said working gas is
selected from the group consisting of nitrogen, hydrogen, noble
gases and mixtures thereof.
11. The method as defined in claim 1, wherein said vapor species
are cooled in step (D) to become nanometer-sized powder particles
and step (D) is followed by a step (F) which comprises operating a
powder collector for collecting said powder particles.
12. The method as defined in claim 3, wherein said step (D)
includes a sub-step of operating means for injecting a cooling gas
into said vapor species, thereby minimizing agglomeration of said
nanometer-sized powder material.
13. The method as defined in claim 1, wherein said working gas flow
direction is arranged to be approximately vertically downward.
14. The method as defined in claim 1, wherein said at least a
starting material comprises two different materials.
15. The method as defined in claim 14, wherein said two different
materials make up the two wires in such a manner that the two wires
have different material compositions.
16. The method as defined in claim 14, wherein said two different
materials include indium and tin.
17. The method as defined in claim 14, wherein said two different
materials include antimony and tin.
18. The method as defined in claim 1, further including a step of
positioning a reservoir at the bottom portion of said twin-wire arc
or a distance below said twin-wire arc in such a fashion that said
reservoir receives said stream of liquid droplets from the wires
and exposes said liquid to said second energy source to vaporize at
least a portion of said liquid.
19. The method of claim 1, wherein said step (D) includes a
sub-step of passivating said vapor species to stabilize said
nano-structured material.
Description
FIELD OF THE INVENTION
[0002] The present invention relates to a method for producing
nano-structured material, including nanometer-sized powder
particles and a coating or thin film containing nanometer-sized
phases from a starting material such as a metal, metal alloy, metal
compound, and ceramic. More particularly, it relates to a method
for producing nano-structured materials at a high production rate
using a twin-wire arc based device for material feeding, heating
and melting along with at least a second energy source for heating
and vaporizing.
BACKGROUND
[0003] Nanometer-sized particles and phases (d.ltoreq.100 nm) of
metals, semiconductors and ceramics exhibit unique processing
characteristics as well as performance properties. The novel
properties of nano-structured materials are due to their small
residual pore sizes (small intrinsic defect sizes), limited
dimensions of grains, phases or domains, unique Bohr radius, and
large fraction of atoms residing at the interfaces or grain
boundaries. Specifically, ceramics fabricated from ultra-fine
particles are known to possess high strength and toughness because
of the ultra-small intrinsic defect sizes and the ability for grain
boundaries to undergo a large plastic deformation. In a multi-phase
material, limited phase dimensions could imply a limited crack
propagation path if the brittle phase is surrounded by ductile
phases so the cracks in a brittle phase would not easily reach a
critical crack size. In addition, dislocation movement distances in
a metal could be limited in ultra fine metallic phases or grains,
leading to unusually high strength and hardness. Even with only one
constituent phase, a nano-structured crystalline material may be
considered as a two-phase material, composed of distinct interface
and crystalline phases. Further, the possibilities for reacting,
coating, and mixing various types of nano materials create the
potential for fabricating new composites with nanometer-sized
phases and novel properties. Commercial applications of nano
particles have included health care/cosmetics, chemical catalysts,
microelectronic devices, polishing slurries, light-emitting
devices, structural ceramics, and biomedical devices.
[0004] The techniques for the generation of nanometer-sized
particles and nano-phased films or coatings may be divided into
three broad categories: vacuum, gas-phase, and condensed-phase
synthesis. Vacuum synthesis techniques include sputtering, laser
ablation, and liquid-metal ion sources. Gas-phase synthesis
includes inert gas condensation, oven sources (for direct
evaporation into a gas to produce an aerosol or smoke of clusters),
laser-induced vaporization, electron beam-induced vaporization,
laser pyrolysis, aerosol decomposition and flame hydrolysis.
Condensed-phase synthesis includes reduction of metal ions in an
acidic aqueous solution, liquid phase precipitation of
semiconductor clusters, and decomposition-precipitation of ionic
materials for ceramic clusters. Other methods include high-energy
milling, mix-alloy processing, chemical vapor deposition (CVD), and
sol-gel techniques.
[0005] All of these techniques have one or more of the following
problems or shortcomings:
[0006] (1) Most of these prior-art techniques provide extremely low
production rates. It is not unusual to find a production rate of
several grams a day. Vacuum sputtering, for instance, only produces
small amounts of particles at a time. Laser ablation and
laser-assisted chemical vapor deposition techniques are well-known
to be excessively slow processes. The high-energy ball milling
method, known to be a "quantity" process, is capable of producing
only several kilograms of nano-scaled powders in approximately 100
hours. These low production rates, resulting in high product costs,
have severely limited the utility value of nano-phase materials.
There is, therefore, a clear need for a faster, more cost-effective
method for preparing nanometer-sized powder materials and
nano-phased coatings.
[0007] (2) Condensed-phase synthesis such as direct reaction of
metallic silicon with nitrogen to produce silicon nitride powder
requires pre-production of metallic silicon of high purity in
finely powdered form. This reaction tends to produce a silicon
nitride powder with a broad particle size distribution.
Furthermore, this particular reaction does not yield a product
powder finer than 100 nm except with great difficulty. Due to the
limited availability of pure metallic silicon in finely powdered
form, the use of an impure metallic powder necessarily leads to an
impure ceramic product. These shortcomings are true of essentially
all metallic elements, not just silicon.
[0008] (3) Some processes require expensive precursor materials to
ceramic powders and could result in harmful gas that has to be
properly disposed of. For instance, the reaction scheme of
3SiCl.sub.4+4NH.sub.3=Si.sub.3N.- sub.4+12HCl involves the
utilization of expensive SiCl.sub.4 and produces dangerous HCl
gas.
[0009] (4) Most of the prior-art processes are capable of producing
a particular type of metal, metal compound, or ceramic powder at a
time, but do not permit the preparation of a uniform mixture of two
or more types of nano-scaled powders at a predetermined
proportion.
[0010] (5) Most of the prior-art processes require heavy,
complicated, and/or expensive equipment, resulting in high
production costs.
[0011] (6) The conventional mechanical attrition and grinding
processes have the disadvantages that powders can only be produced
up to a certain fineness and with relatively broad particle-size
distribution. As a matter of fact, with the currently familiar
large-scale process for manufacturing powders it is rarely
possible, or only possible with considerable difficulty, to produce
powders having average particle sizes of less than 0.5 .mu.m
(microns).
[0012] (7) Aerosol processes provide the advantages of small
particle size, narrow size distribution, nearly spherical particles
and high purity. Aerosol processes also are energy efficient and
avoid the treatment of large liquid volumes associated with
traditional wet chemistry processes. However, conventional aerosol
processes are very complex and involve many physicochemical
phenomena and mechanisms, such as chemical reaction, particle
nucleation, condensation, coagulation, aggregation, heat and mass
transfer, and thermophoresis. The fundamentals of these processes
are not well understood and, consequently, the processes are
difficult to control precisely. This makes design, operation and
control of industrial reactors to carry out these processes more of
an art than a science, relying heavily on experience and
empiricism. The most serious problem associated with aerosol
decomposition is the formation of impurities from the precursor
chemical species (e.g., SiCl.sub.4, NH.sub.3, TiCl.sub.4, etc.).
The final impure products cannot be used in health care or
cosmetics markets, for instance.
[0013] Ultra-fine particles of metals, metallic compounds, and
ceramics can be produced by heating and vaporizing a starting
material with a plasma or a multiplicity of plasmas. The thermal
plasma approach has a major advantage in that ultra-high
temperatures can be readily achieved in relatively large energy
amounts. However, in the case of using a direct-current (DC) plasma
alone (e.g., in a flame spray apparatus), melting is possible but
vaporization of the whole starting material is difficult. In the
case of using a high-frequency induction plasma alone, there have
been encountered several difficulties: incomplete vaporization,
difficulty in maintaining a stable plasma when the starting
material is being added to the plasma, and difficulty in feeding
the starting powder material into the central portion of the plasma
without powder particles being scattered around or dispersed
outwardly to adhere to the walls of the plasma chamber. With a
hybrid plasma consisting of one DC plasma and one high-frequency
plasma, entry of a starting powder material to the DC plasma and
then the high-frequency plasma has been difficult.
[0014] In order to overcome this difficulty, Saiki, et al. (U.S.
Pat. No. 4,812,166, Mar. 14, 1989) developed a method that involved
vaporizing a starting material by supplying this material into a
plurality of DC plasma currents combined at the central axis of a
work coil for generating a high frequency induction plasma
positioned below the DC plasma-generated zone. One major
shortcoming of this process is the need to use a complicated
configuration of multiple plasmas for heating and vaporizing the
incoming material. We have found that, even with such a
configuration, it remains difficult for the starting material to
enter and stay in both the DC and high-frequency plasmas, leaving a
significant portion of the starting material un-vaporized and
scraped. Furthermore, this process is difficult to control and is
not energy efficient since a significant portion of the plasma heat
is not utilized.
[0015] Another example of plasma arc-based apparatus is disclosed
by Araya, et al. (U.S. Pat. No. 4,732,369, Mar. 22, 1988 and U.S.
Pat. No. 4,610,718, Sep. 9, 1986). The apparatus for producing
ultra-fine particles by arc energy comprises a generating chamber
for generating ultra-fine particles therein, an electrode
positioned opposite to a base material so as to generate an
electric arc, a suction opening for sucking the particles generated
in the chamber, a trap for collecting the particles sucked from the
suction opening, and a cooler positioned between the suction
opening and the trap for cooling the sucked ultra-fine particles.
The process involves the utilization of dissociable oxygen in the
working gas which tends to cause erosion of the non-consumable
tungsten electrode used in the apparatus and generates tungsten
impurities in the final product. Furthermore, it takes a long time
for the base material to vaporize and, hence, the vaporization
procedure is a bottle neck in this process, making the whole nano
powder production process very slow. Araya, et al. (U.S. Pat. No.
4,619,691, Oct. 28, 1986 and U.S. Pat. No. 5,168,097) used laser
beam irradiation or combined laser beam irradiation and arc heating
to produce a plume from a base material. Since the base material
has to be replenished frequently, this process is non-continuous
and not suitable for mass production of nano-scaled powder or
nano-phased coating.
[0016] Still another example of a plasma arc-based process for
synthesizing nano particles was disclosed by Parker, et al. (U.S.
Pat. No. 5,460,701, Oct. 24, 1995 and U.S. Pat. No. 5,514,349, May
7, 1996). The system used in this process includes a chamber, a
non-consumable cathode shielded against chemical reaction by a
working gas (including an inert gas, but not oxidizing gas), a
consumable anode vaporizable by an arc formed between the cathode
and the anode, and a nozzle for injecting at least one of a quench
and a reaction gas in the boundaries of the arc. This system has
several drawbacks. Firstly, the configuration of having a
non-consumable electrode and a consumable electrode being paired up
to form an arc does not provide efficient vaporization of the
consumable electrode. Just like in the apparatus disclosed by
Araya, et al. (U.S. Pat. No. 4,732,369), vaporization of the base
material takes a long time. Second, the configuration does not
permit an efficient use of the plasma arc energy with most of the
energy being wasted. Third, since the ionic or plasma arc
environment is highly erosive to the non-consumable electrode, it
is difficult to maintain a stable arc and the operator has to
replace the electrode periodically.
[0017] Glazunov, et al. (U.S. Pat. No. 3,752,610, Aug. 14, 1973)
disclosed a powder-producing device that includes a rotatable,
consumable electrode and a non-consumable electrode. In a method
proposed by Clark (U.S. Pat. No. 3,887,667, Jun. 3, 1975), an arc
is struck between a consumable electrode and a second electrode to
produce molten metal which is collected, held and homogenized in a
reservoir and subsequently atomized to produce powdered metals.
Akers (U.S. Pat. No. 3,975,184, Aug. 17, 1976) developed a method
for powder production, which entails striking an electric arc
between an electrode and the surface of a pool of molten material.
The arc rotates under the influence of a magnetic field to thereby
free liquid particles from the surface of the pool. The liquid
particles are then quenched to become a solid powder material. Uda,
et al. (U.S. Pat. No. 4,376,740, Mar. 15, 1983; U.S. Pat. No.
4,482,134, Nov. 13, 1984; U.S. Pat. No. 4,642,207, Feb. 10, 1987;
and U.S. Pat. No. 4,889,665, Dec. 26, 1989) taught a process for
producing fine particles of a metal or alloy. The process involves
contacting a molten metal or alloy with activated hydrogen gas
thereby releasing fine particles of the metal or alloy. The method
disclosed by Ogawa, et al. (U.S. Pat. No. 4,610,857, Sep. 9, 1986)
entails injecting a powder feed material into a plasma flame
created in a reactive gas atmosphere. The powder injection rate is
difficult to maintain and, with a high powder injection rate, a
significant portion of the powder does not get vaporized by the
plasma flame.
[0018] In summary, the above prior art plasma-based methods exhibit
one or more of the following shortcomings: (1) The powder particles
or grains/phases produced tend to be on the micrometer-scaled and
particles of 20 nm or smaller are difficult to obtain with some of
these prior-art methods; (2) Most of the prior-art systems or
apparatus are not energy efficient with most of the plasma arc
energy being wasted; (3) It is difficult to feed the starting
material into the plasma zone and maintain the material in the
plasma zone for complete vaporization, (4) Systems or apparatus
that involve a non-consumable electrode are unstable and difficult
to control due to the arc-induced erosion on the non-consumable
electrode; (5) In most cases, it takes a long time to achieve
complete vaporization of the starting material if at all; and (6)
In many cases, the need to have a high vacuum and/or the batch-wise
material-feeding mechanism make them non-continuous processes that
are not amenable to the mass production of nano-structured
materials.
[0019] Accordingly, one object of the present invention is to
provide an improved method for producing nano-structured metal,
metal compound and ceramic materials.
[0020] A specific object of the present invention is to provide a
method that is more reliable (e.g., in which it is easier to feed
starting materials) and/or more energy-efficient for producing
nano-scale powder materials or nano-phased films/coatings.
[0021] Another specific object of the present invention is to
provide a method for producing nano-structured materials at a high
throughput rate.
[0022] Another object of the present invention is to provide a
method for producing ultra fine metal, metal compound, and ceramic
powder materials from a wide range of starting materials.
[0023] A further object of the present invention is to provide a
method for producing a mixture of ultra fine powder materials which
are well mixed at a predetermined proportion.
SUMMARY OF THE INVENTION
[0024] One embodiment of the present invention is a method for
synthesizing a nano-structured material. The method includes four
primary steps: (A) providing a reaction chamber wherein the
nano-structured material is generated from at least a starting
material selected from the group consisting of a metal, a metal
alloy, a metal compound, and a ceramic; (B) operating a twin-wire
arc nozzle, comprising two wires and a working gas being
controllably fed into the chamber, to form an arc between two
converging leading tips of the two wires to heat and melt the
starting material at the leading tips for providing a stream of
liquid droplets traveling in a predetermined direction (preferably
vertically downward); (C) operating at least a second high energy
source for producing a vaporizing zone adjacent to the arc and
inside the chamber wherein the liquid droplets are vaporized to
form vapor species; and (D) operating heat treatment means to cool
the vapor species for forming the nano-structured material. The
second high energy source is selected from the group consisting of
a laser beam, an electron beam, an ion beam, a flame and
combinations thereof. The second high energy source can be a
plurality of arc plasmas. The nano-structured material produced can
be in the form of a nanometer-sized powder particles or a coating
(or thin film) containing nanometer-sized phases deposited on a
substrate.
[0025] The twin-wire arc nozzle is arranged in supplying relation
to the receiving reaction chamber for providing melted droplets of
the starting material in the chamber. In step (B), this twin-wire
arc nozzle includes two wires made up of at least a starting
material, each wire having a leading tip and each wire being
continuously or intermittently fed into the chamber in such a
fashion that the two leading tips are maintained at a desired
separation. The nozzle also includes means for providing electric
currents and a working gas for creating an ionized arc between the
two converging leading tips for melting the starting material to
generate a stream of liquid droplets.
[0026] In one preferred embodiment of the present invention, the
method further includes an additional step (E), which is carried
out after step (C) and before step (D). The step (E) entails
introducing a stream of reactive gas into the reaction chamber to
impinge upon the vapor species and exothermically react therewith
to produce the nano-structured material. In this embodiment, the
nano-structured material can include a metal compound or ceramic
material. Step (E) could include operating control means for
regulating the flow rate of the reactive gas, thereby enabling
variations of the particle size or phase size of the
nano-structured material. The reactive gas may be selected from the
group consisting of nitrogen, phosphorus, arsenic, oxygen, sulfur,
selenium, tellurium, fluorine, chlorine, bromine, iodine, a
carbon-containing gas, and mixtures thereof. The working gas may be
selected from the group consisting of nitrogen, hydrogen, noble
gases and mixtures thereof.
[0027] In another preferred embodiment, the method contains cooling
the vapor species in step (D) to become nanometer-sized powder
particles, followed by a step (F) which comprises operating a
powder collector for collecting the powder particles. The powder
collector may include a powder classifier (e.g., cyclones) and a
filter. In the case of producing nanometer-sized powder particles,
step (D) could include a sub-step of operating means for injecting
a cooling gas into the vapor species, thereby minimizing
agglomeration of the nanometer-sized powder particles.
[0028] In the presently invented method, the starting material
could include two different materials in a wire. The two different
materials could make up the two wires in such a fashion that the
two wires have different material compositions. The two different
materials could include indium and tin and, if oxygen is used as a
reactive gas, the resulting nano-structured material would be
indium-tin oxide, a material commonly coated onto a glass substrate
for liquid crystal display applications. The two different
materials could include antimony and tin.
[0029] In another preferred embodiment, the method further includes
a step of positioning a reservoir at the bottom portion of the
twin-wire arc or a distance below the twin-wire arc in such a
fashion that the reservoir receives the stream of liquid droplets
from the wires and further exposes the liquid to a second energy
source to vaporize at least a portion of the liquid. Preferably,
the second energy source is such that the liquid is completely
vaporized.
[0030] The presently invented method is applicable to essentially
all metallic materials (including pure metals and metal alloys),
metal compounds, and ceramic materials. As used herein, the term
"metal" refers to an element of Groups 2 through 13, inclusive,
plus selected elements in Groups 14 and 15 of the periodic table.
Thus, the term "metal" broadly refers to the following
elements:
[0031] Group 2 or IIA: beryllium (Be), magnesium (Mg), calcium
(Ca), strontium (Sr), barium (Ba), and radium (Ra).
[0032] Groups 3-12: transition metals (Groups IIIB, IVB, VB, VIB,
VIIB, VIII, IB, and IIB), including scandium (Sc), yttrium (Y),
titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium
(Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W),
manganese (Mn), technetium (Tc), rhenium (Re), iron (Fe), ruthenium
(Ru), osmium (Os) cobalt (Co), rhodium (Rh), iridium (Ir), nickel
(Ni), palladium (Pd), platinum (Pt), copper (Cu), silver (Ag), gold
(Au), zinc (Zn), cadmium (Cd), and mercury (Hg).
[0033] Group 13 or IIIA: boron (B), aluminum (Al), gallium (Ga),
indium (In), and thallium (TI).
[0034] Lanthanides: lanthanum (La), cerium (Ce), praseodymium (Pr),
neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu),
gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho),
erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).
[0035] Group 14 or IVA: germanium (Ge), tin (Sn), and lead
(Pb).
[0036] Group 15 or VA: antimony (Sn) and bismuth (Bi).
[0037] When high service temperatures are not required, the
component metal element (in an alloy, compound, or ceramic) may be
selected from the low melting point group consisting of bismuth,
cadmium, cesium, gallium, indium, lead, lithium, rubidium,
selenium, tellurium, tin, and zinc. When a high service temperature
is required, a metallic element may be selected from the
high-melting refractory group consisting of tungsten, molybdenum,
tantalum, hafnium and niobium. Other metals with intermediate
melting points such as copper, zinc, aluminum, iron, nickel and
cobalt may also be selected.
[0038] Some gases may be selected to passivate the nano-scaled
clusters to produce un-agglomerated fine metal or ceramic powders
and to prevent oxidation or other undesirable reaction from taking
place; e.g., Se vapor may be used to passivate the surface of
telluride clusters. The other gases may be used to react with the
metal clusters to form nanometer-scale compound or ceramic powders
of hydride, oxide, carbide, nitride, chloride, fluoride, boride,
iodide, sulfide, phosphide, arsenide, selenide, and telluride, and
combinations thereof.
[0039] Specifically, a reactive gas can rapidly react with the
metal vapor species to form nanometer-sized ceramic particles
(e.g., oxides). If the reactive gas contains a mixture of two or
more reactive gases (e.g., oxygen and nitrogen), the resulting
product will contain a mixture of two compounds or ceramics (e.g.,
oxide and nitride). If the metal wire is a metal alloy or mixture
(e.g., containing both indium and tin elements) and the reactive
gas is oxygen, the resulting product will contain ultra-fine
indium-tin oxide particles.
[0040] The reactive gas can undergo a reaction with vaporized metal
species at high temperatures in a substantially spontaneous and
self-sustaining fashion. The reaction heat released is effectively
used to sustain the reactions in an already high temperature
environment.
[0041] Advantages of the present invention may be summarized as
follows:
[0042] 1. A wide variety of nano-structured metal, metal compound,
and ceramic materials can be readily produced using the present
method. The metallic element(s) in a starting feed material can be
selected from any element in the periodic table that is considered
to be metallic. The corresponding partner gas reactants may be
selected from, for instance, hydrogen, oxygen, carbon, nitrogen,
chlorine, fluorine, boron, and sulfur to form respectively metal
hydrides, oxides, carbides, nitrides, chlorides, fluorides,
borides, and sulfides and combinations thereof. No known prior-art
technique is so versatile in terms of readily producing so many
different types of nano-structured metallic, compound, and ceramic
materials at a high production rate.
[0043] 2. The wire material may contain an alloy of two or more
elements which are uniformly dispersed. When vaporized, these
elements remain uniformly dispersed and are capable of reacting
with selected reactive gases to form uniformly mixed compound or
ceramic powder particles. No post-fabrication mixing is necessary
for the purpose of making a nano-phased composite material.
[0044] 3. The method allows a spontaneous reaction to proceed
between a metallic element and a reactive gas such as oxygen. The
reaction heat released is spontaneously used to maintain the
reacting medium in a sufficiently high temperature so that the
reaction can be self-sustaining until completion for the purpose of
producing a compound or ceramic material. The reaction between a
metal and certain reactive gas (e.g., oxygen) can rapidly produce a
great amount of heat energy, which can be used to drive other
reactions that occur concurrently or subsequently when other
reactant elements (e.g., carbon or nitrogen) are introduced.
[0045] 4. The method permits an uninterrupted feed of wires or
rods, which can be of great length. This feature makes the process
fast and effective and now makes it possible to mass produce
nano-structured metal, compound, and ceramic materials
cost-effectively.
[0046] 5. The method is simple and easy to operate. It does not
require the utilization of heavy and expensive equipment. The
over-all product costs are very low.
[0047] 6. The present method fundamentally differs from the method
used in U.S. Pat. No. 4,610,718 (Sep. 9, 1986 to Araya, et al.). As
indicated earlier, the process of Araya, et al. involves the
utilization of dissociable oxygen in the working gas which tends to
cause erosion of the non-consumable tungsten electrode used in the
apparatus and generates tungsten impurities in the final product.
The Araya's apparatus design does not allow an efficient use of the
arc energy and it takes an excessively long time for the arc to
fully vaporize the feed material. In contrast, the presently
invented method only requires the twin wire to be melted (not
necessarily vaporized, although partial vaporization is possible,
which is a good feature), the produced melt droplets are then
substantially vaporized completely when passing through the second
heating/vaporizing stage. The apparatus of Araya, et al. is not
equipped with a quench gas for preventing particle agglomeration,
nor is it supplied with a reactive gas to react with a metal
element in the electrode for producing a compound or ceramic
material through a self-propagating reaction.
[0048] 7. The present method has several advantages over the method
of Parker, et al. (U.S. Pat. No. 5,514,349, May 7, 1996), which is
essentially a variant of the method proposed by Araya, et al. In
Parker's method, the configuration of having a non-consumable
electrode and a consumable electrode being paired up to form an arc
again does not provide efficient vaporization of the consumable
electrode. A significant portion of the consumable electrode is
just melted and drips down to the surface of a supporting substrate
to form a "weld pool" thereon. The consumable electrode (typically
a thick rod) cannot be advanced (fed) into the arc zone until most
of the material in this pool of molten metal is vaporized, which
takes a long time. This bottleneck severely limits the rod-feeding
rate and the over-all powder production rate with this system is
very low. Further, since the ionic or plasma arc environment is
highly erosive to the non-consumable electrode in the Parker's
system, it is difficult to maintain a stable arc and the operator
has to replace the electrode periodically. In contrast, in our
present invention, the two wires can be continuously fed into the
heating/melting/vaporization chamber with the leading tips of the
wires continuously melted (and partially vaporized) at a high
feeding rate for producing nano-structured materials continuously
without interruption and at a high throughput rate. The
un-vaporized melt droplets are completely vaporized during the
second-stage heating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] FIG. 1A schematic of a preferred embodiment of a twin-wire
arc system for producing nano-structured metallic, metal compound,
and ceramic materials.
[0050] FIG. 2A system similar to that in FIG. 1, but has a
provision for introducing a reactive or cooling gas to impinge upon
the vapor species.
[0051] FIG. 3 The same system as in FIG. 1, but with a melt
reservoir 77 to receive or trap melt 63 that drips out of the arc
66 and a laser beam 44 to vaporize the melt 63.
[0052] FIG. 4 The same system as in FIG. 1, equipped with a powder
classifier-filter system for collecting the produced nano
powders.
[0053] FIG. 5 The same system as in FIG. 1, but has a solid
substrate to receive nano-scaled species (cooled vapor species) for
forming a coating.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0054] In order to illustrate the basic steps involved in the
presently invented method, please refer to FIG. 1, which
schematically shows one example of the systems that can be used to
produce a nano-structured material. This system includes four major
functional component sub-systems:
[0055] (a) a chamber (e.g., 83) in which a starting material is
melted and then vaporized;
[0056] (b) a twin-wire electrode device (arc nozzle) in supplying
relation to the chamber. This electrode device includes: (i) two
wires 50,52 each made up of a desired starting material, each wire
having a leading tip 50a or 52a and each wire being continuously or
intermittently fed (e.g., through rotating rollers 54) into the
chamber in such a fashion that the two leading tips 50a,52a are
converged toward each other and maintained at a desired separation;
and (ii) means (e.g., power source 70 through two respective
conductive jackets 72 and electrodes 56,58) for providing electric
current and a working gas 64 (e.g., from a gas source such as a
bottle designated by 62 and through a pipe means 60) for creating
an ionized arc 66 between the two leading tips for melting the
material at the tips to generate a stream of melt droplets in the
chamber;
[0057] (c) a second energy source (e.g., a high-frequency induction
plasma 67 produced through electric coils 69) to completely
vaporize the liquid droplets for generating vapor species,
preferably flowing along a predetermined direction (e.g.,
vertically downward);
[0058] (d) optional means (e.g. through a pipe 73 in FIG. 2) for
injecting a reactive gas into a reaction zone inside the chamber at
a point inside the tail end of the plasma zone or downstream from
the plasma zone. The reactive gas reacts with the vapor species to
produce compound or ceramic particles; and
[0059] (e) means to heat treat the vaporized species (e.g., a
cooling chamber 90 with cooling water jackets 92 wrapped around the
cooling chamber 90 in FIG. 4) to, for instance, allow for
condensation of the vapor species, resulting in the formation of
nano-sized powder particles. Alternatively, as shown in FIG. 5, a
solid substrate 42 may be positioned down stream from the vapor
species, which impinge upon and deposit onto the substrate to form
a thin film or coating. The temperature in the direct vicinity of
the substrate surface may be regulated in such a fashion that vapor
species condense and form a coating containing nano-scaled phases;
and
[0060] (f) For the production of nano-sized powder particles, a
powder collector system (e.g., including a cyclone or powder
classifier, from 94 to 105 in FIG. 4) may be used to collect the
nano-scaled powder material.
[0061] Based on this system, the method consists of the following
four primary steps: (A) providing a reaction chamber wherein the
nano-structured material is generated from at least a starting
material selected from the group consisting of a metal, a metal
alloy, a metal compound, and a ceramic; (B) operating a twin-wire
arc nozzle, comprising two wires and a working gas being
controllably fed into the chamber, to form an arc between two
converging leading tips of the two wires to heat and melt the
starting material at the leading tips for providing a stream of
liquid droplets traveling in a predetermined direction (preferably
vertically downward); (C) operating at least a second high energy
source for producing a vaporizing zone adjacent to the arc and
inside the chamber wherein the liquid droplets are vaporized to
form vapor species. The second high energy source may be a laser
beam, an electron beam, an ion beam, a flame, a high-frequency
induction plasma, and combinations thereof. The second high energy
source may also be a plurality of arc plasmas; and (D) operating
heat treatment means to cool the vapor species for forming the
nano-structured material, which can be in the form of a
nanometer-sized powder particles or a coating (or thin film)
containing nanometer-sized phases deposited on a substrate.
[0062] For the purpose of clearly defining the claims, the word
"wire" means a wire of any practical diameter, e.g., from several
microns (a thin wire or fiber) to several centimeters (a long,
thick rod). A wire can be supplied from a spool, which could
provide an uninterrupted supply of a wire as long as several miles.
This is a very advantageous feature, which makes the related powder
production process a continuous one.
[0063] In a preferred embodiment of the presently invented method,
as indicated in FIG. 1, the twin-wire electrode device used in the
method basically contains a twin-wire electric arc spray nozzle,
which is mainly comprised of two feed wires 50, 52, a feeding
mechanism (e.g., including motorized rollers 54), and a working gas
passage or pipe means 60 which directs the flow of a working gas
from the supply into a cell near the two respective leading tips of
the wires. The two metal wires 50,52 are supplied with a DC voltage
or current (one "+" and the other "-") or a pulsed power through
the electrodes 56,58 to form an arc 66. This arc 66, being at an
ultra-high temperature (up to 6,000.degree. C.), functions to melt
the wire tips to form a stream of liquid droplets. The stream 64 of
working gas passes through the passage means 60 into the arc
chamber not only to help generate and maintain the arc 66 but also
to carry the liquid droplets downward toward the vaporization zone
induced by a second energy source such as an high-frequency
induction plasma or a laser beam. The power of this energy source
is chosen in such a fashion that the stream of liquid, when passing
through the vaporization zone, is completely vaporized. The
produced vapor species are preferably directed to flow in a desired
direction, vertically downward as shown in FIG. 1. The creation of
a plasma zone is well-known in the art. The ultra-high temperature
in the plasma (up to as high as 32,000.degree. K) rapidly vaporizes
the melt droplets that pass through the plasma zone.
[0064] The vapor species may then be allowed to react with a stream
of reactive gas introduced into the reaction chamber through a pipe
73 in FIG. 2. The reaction products are metal compounds or ceramics
that are different from the starting material in chemical
composition. The reaction products, in vapor and/or liquid states,
are nano-scaled clusters. It may be noted that, if the gas coming
through pipe 73 in FIG. 2 contains a highly reactive gas such as
oxygen, the vapor species can quickly react with oxygen to form
nano-sized oxide clusters. Since the oxidation of a metal is
normally a highly exothermic process, a great amount of reaction
heat is released which can in turn be used to activate, maintain,
or accelerate the oxidation reactions of other metal vapors,
clusters or droplets. Such a self-sustaining reaction rapidly
converts the liquid metal droplets or vapor clusters into
nano-scaled metal compound or ceramic particles. Other reactive
gases that can be selected include hydrogen, carbon-containing gas
(e.g., CO), nitrogen, chlorine, fluorine, boron, iodine, sulfur,
phosphorus, arsenic, selenium, tellurium and combinations
thereof.
[0065] The vapor species or nano-scaled clusters thus generated
could remain in a vapor or liquid state in which individual
clusters could aggregate or stick together if left alone. It is
therefore desirable to operate a heat treatment means to help the
clusters solidify and remain separated from one another if nano
powder particles are the desired final product. In this case, the
operation of this heat treatment means may include blowing a
quenching gas (e.g., cool, inert gas) to impinge upon the vapors or
clusters immediately or soon after the vapors or clusters are
formed. The cooling gas may contain a small amount of passivating
gas to help stabilize the nano particles against any undesired
oxidation or other side reaction. These cooling means may include
copper or steel tubing 92 or channels, containing cooling water,
that are jacketed around the cooling chamber 90. These powders,
along with the residual working gas and cooling gas are transferred
through a conduit 20 to a powder collector/separator system (e.g.,
schematically shown in the lower portion of FIG. 4.
[0066] Alternatively, if the desired final product is a
nano-grained or nano-phased thin film or coating on a substrate (42
in FIG. 5), then this substrate may be disposed in a position
downstream from the vaporization zone 67. The vapor species or
their reaction products with a reactive gas are directed to strike
the substrate and get deposited thereon to form a thin film or
coating. Prior to impinging upon the substrate these vapor species
or nano-scaled clusters may go through a heat treatment zone to
reach a desired temperature, which dictates the grain or phase size
of the resulting coating.
[0067] The twin-wire arc spray nozzle, originally developed for use
in a spray coating process, can be adapted in the present method
for providing a continuous stream of liquid droplets flowing into
the vaporization zone 67. This low-cost device is capable of
readily melting and, if so desired, partially vaporizing the metal
wire to a temperature as high as 6,000.degree. C.
[0068] Schematically shown in FIG. 1 is an open-style twin-wire arc
spray nozzle that can be used in the practice of the presently
invented method. Two metal wires 50,52 are driven by powered
rollers 54 to come in physical contact with two respective
conductive jackets 72 which are supplied with "+" and "-" voltage
or pulsed power through electrically conductive blocks 56 and 58,
respectively. The voltage polarity may be reversed; i.e., "-" and
"+" instead of "+" and "-". The voltages come from a DC or pulsed
power source 70. The lower ends of the two wires approach each
other at an angle of approximately 30-60.degree.. The two ends are
brought to contact each other for a very brief period of time. Such
a "short circuit" contact creates an ultra-high temperature due to
a high current density, leading to the formation of an ionized arc
66. A stable arc can be maintained provided that the current is
constantly supplied, a certain level of gas pressure is maintained,
and the wires are fed at a constant or pulsating speed. A stream 64
of compressed air, introduced through a gas passage 60 from a gas
source (e.g., a compressed air bottle), also serves to carry the
stream of liquid downward into the vaporization zone.
[0069] For some materials with a relatively high vaporization
temperature or boiling point, and if the working gas flow rate is
relatively high, a certain amount of the wire tip material may not
be exposed to the high temperature environment for a sufficiently
long duration of time needed for a full vaporization. As a
consequence, small melt droplets may drip downward toward the
cooling chamber in FIG. 4. One way to overcome this difficulty is
to make use of a reservoir (77 in FIG. 3a or 3b) positioned just
below the twin-wire arc 66 (FIG. 3a) or near the edge of the
high-frequency induction plasma 67 (FIG. 3b) to trap or accommodate
the un-vaporized material 63, which will continue to receive heat
from the plasma 67 (FIG. 3b) or a laser beam 44 (FIG. 3a) for
further vaporization.
[0070] In the actual practice of the present method, the cooling
chamber 90 is preferably further connected to a powder collector
and separator system, as shown in FIG. 4. As an example, the
chamber 90 is connected to a collector chamber 94, commonly
referred to as an expansion chamber. The lower part of this
expansion chamber 94 has an outlet being communicated to a
removable powder collection container 102 through a valve 95. The
valve 95 is open during production of the clusters so that powder
separated and collected by the chamber 94 can be received and
collected in the container 102. The expansion chamber may be
allowed to communicate through conduits 96,99 with a series of
cyclones (only one cyclone 98 being shown) and a filter device
(e.g., including a wet scrubber 100). The finely divided metallic,
compound, or ceramic powder product is suspended in reaction
product gases as well as excess working gas, hereinafter
collectively referred to as product gases or product fluids. The
product fluids are removed from the chamber 90 through conduit
96,99 and introduced into cyclones 98 and filer/separator device,
in order to separate the solid powder from the product fluids. The
nano-sized particles are formed completely in the chamber and since
the cluster effluent is rapidly cooled to below the powder forming
temperatures substantially immediately, little or no additional
ceramic or metal solid formation or individual particle growth
occurs outside the chamber.
[0071] A cyclone 98 is normally cooled (e.g., externally water
chilled) to cool the powder product. As the product fluids travel
through cyclones 98, the powder drops into receiver 104 with the
valve 105 being open, while gaseous effluent leaves cyclone 98
through conduit 99 into a solid separation chamber (e.g., a wet
scrubber 100). The wet scrubber can be a caustic water scrubber,
containing packing of balls, saddles, etc. for greater contact. The
scrubber separates the fine solid particles from the gas stream and
possibly neutralizes acidic species therein before the gas is
discharged to the atmosphere or to a flue. Any additional filtering
device such as a bag filter, electrostatic precipitator or other
equivalent means for separating suspended solids from a gas may be
used. While only one cyclone and one solid separator are shown,
more than two can be used. Alternatively, other types of powder
collector and/or separator devices may be used. Solid powder
collector and solid-gas separator systems are widely available in
industry.
[0072] The starting material can be an alloy of two or more
elements which are uniformly dispersed. When vaporized, these
elements remain uniformly dispersed and are capable of reacting
with selected reactive gas to form uniformly mixed ceramic powder
particles. No post-fabrication mixing is necessary for the
preparation of a hybrid or composite material.
[0073] The reactive gas can contain vapor, liquid, or solid
particles suspended in a carrier gas. A solid reactant in fine
powder form requires a carrier gas to carry it into the arc cell.
An example is fine carbon powders suspended in either an inert gas
(e.g., helium) or reactive gas (e.g., oxygen), depending upon the
types of intended ceramic powders to be produced. In the former
example, a metal carbide will be produced. The helium gas is used
only as a carrier medium. In the latter example, oxygen gas is used
to react with metal vapor clusters. If more than two reactant
elements are used (e.g., carbon particles suspended in oxygen gas,
or a mixture of CO and O.sub.2) more complicated reactions can
occur. Under favorable conditions, oxidation of a metal occurs,
resulting in the release of a great amount of heat, which can be
used to promote the reaction between a metal element (if still
available) and carbon. The supply of a vaporized metal element and
a mixture of two reactant gases can lead to the formation of a
mixture of two compounds or ceramics.
[0074] If the method is used to produce a uniform mixture of
ceramics from a metallic alloy, this alloy can be introduced as two
wires of identical alloy composition into the twin-wire arc spray
nozzle. Alternatively, the two wires may be made up of different
metal compositions. For example, a technologically important oxide
mixture is indium-tin oxides. This product can be used in a flat
panel display technology. In one instance, a tin wire and an indium
wire were fed into an arc sprayer nozzle and induction plasma and
vaporized. An oxygen flow at a rate of 200 scfm under a gas
pressure of approximately 200 psi was used to mix and react with
the mixture of metal vapor clusters. Nano-scaled indium-tin oxide
particles with an average diameter of 12 nm were obtained. A
production rate of 20 kilograms per hour was achieved with a
lab-scale apparatus.
[0075] Table 1 gives a list of examples of the nano-structured
materials produced by the presently invented method.
1TABLE 1 List of nanometer-sized powder (NP) and nano-phased
coating (NC) materials produced with selected vacuum or reactive
gas condition (working gas = argon). Vacuum Starting (NP &
Oxygen gas Nitrogen gas Material Group NC) Air (NP) (NP & NC)
(NP) Cu IA Cu Cu Zn IIA Zn Zn + ZnO ZnO Zn Al III Al Al +
A1.sub.2O.sub.3 Al.sub.2O.sub.3 Al + AlN Sn IVA Sn Sn + SnO
SnO.sub.2 Sn Ti IVB Ti Ti + TiO.sub.2 TiO.sub.2 TiN Nb VB Nb Nb
Nb.sub.2O.sub.5 Nb.sub.4N.sub.3 Mo VIIB Mo Mo MoO.sub.3 Mo Mn VIIB
Mn Mn Mn.sub.2O.sub.3 Mn + Mn.sub.4N Fe VIIIB Fe Fe +
Fe.sub.2O.sub.3 Fe.sub.2O.sub.3 Fe ZnO ZnO ZnO ZnO ZnO ZrO.sub.2
ZrO.sub.2 ZrO.sub.2 ZrO.sub.2 ZrO.sub.2 + ZrN Fe-Ti Fe-Ti
Fe.sub.2O.sub.3 + Fe + TiN TiO.sub.2 Ni-Ti Ni-Ti NiO + TiO.sub.2 Ni
+ TiN In-Sn SnO + In.sub.2O.sub.3
* * * * *