Note: Descriptions are shown in the official language in which they were submitted.
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CARBON AND METAL NANOMATERIAL COMPOSITION AND SYNTHESIS
FIELD OF THE INVENTION
The invention relates generally to nanopowder synthesis processes, and more
particularly to the controlled use of a
precursor material (such as a precursor gas) to assist in the formation of
unagglomerated nanoparticles of the
powder. It also relates to novel nanomaterials comprised of carbon and metals
produced by the process along with
the fundamental processes the novel nanomaterials enable.
BACKGROUND
In the area of material powders, metal is used in many applications including
electrically and themially conductive
pastes, photographic films, antibacterial agents and conductive inks. Most of
the current applications use micron
or sub micron powders. Recently, several processes have demonstrated
commercial scale of nanopowders, some
including metals. Nanopowders exhibit unique properties that are different
than their micron counter-parts such as
lower melting/sintering temperatures, higher hardness, increased optical
transparency and increased reactivity.
Many applications would like to benefit by exploiting these properties. Until
recently, the commercial availability
of nanopowders has been limited to a few materials such as silica, carbon
black and alumina. Several new
processes are now producing nanopowders in commercial scale and have the
ability to make a wide range of
materials including silver, copper, gold, platinum, titanium and iron as well
as others.
An important aspect of these powders is that the particles generally need to
be unagglomerated. This aids in
preserving the properties unique to the nanoscale and allows easier
incorporation of the powder into most
applications. Many of the new processes, especially with metals, cannot
produce unagglomerated particles. Metal
particles at this size have high surface energies and are consequently
unstable. When two particles contact one
another, the particles form a neck to decrease both the local curvature and
surface area, consequently lowering the
total surface energy. The result is the formation of hard agglomerates,. or
aggregates, of the nanopowder which are
nearly impossible to break apart. Since the particles are fused to one
another, they begin to act like a much larger
particle and lose many of the desired characteristics of nanoparticles. In
reality, the particles form a nanostructured
material instead of a true nanopowder. When this happens it is nearly
impossible to process the particles down to
their original primary particle size either by chemical or mechanical means.
This is especially true in gas phase
condensation synthesis techniques in which the particles are formed at
elevated temperatures, meaning that the
particles have even higher energies and are colliding with one another while
at an elevated temperature. Nano-
sized particles have been shown to have reduced melting and sintering
temperatures relative to the bulk material
properties. This makes nanopowders very prone to aggregation at elevated
temperatures.
Several methods have been tried to eliminate this problem. Some processes such
as Sol-Gel chemistry can produce
lab scale quantities of nano-metal particles in a solution which fonn
discrete, unagglomerated particles by
incorporating specific surfactants or ligands that bond to the particle's
surface to prevent the particles from
contacting one another while in solution. However, when the solvent is
evaporated to isolate the particles from the
solution, the particles typically form aggregates. Other researchers at the
University of Bologna, Italy reported
dodecanethiol coating of silver nanoparticles in an aqueous solution to avoid
the agglomeration. These methods
have the limitation that the particles form hard agglomerates when the
solution is dried to extract the powder;
therefore they are limited to applications where the modified particle surface
chemistry, the chemistry of the
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particle-solution and the chemistry of the application solutions are
compatible. Additionally, these processes are
not amiable to large-scale production due to the high cost and difficulties
associated with scaling the batch process.
Another method uses a sodium/halide flame and encapsulation technology (SFE)
to form discrete nanoparticle
powders. This process uses a three-inch long flame inside a four-foot long
tubular flow reaction furnace for sodium
reduction of metal halides, such as boron trichloride and titanium
tetrachloride, to produce metal and ceramic
nanoparticles. The particles produced are 10 to 100 nm in diameter with a salt
encapsulation. This system is an
open loop process that requires continuous feed of the salt encapsulation
solution and the combustion gases into the
reaction furnace. Hence, it uses considerable gases and is not very efficient.
Lastly, for most applications, this
material requires an additional step to remove the salt encapsulation. The
salt encapsulation can present chemical
compatibility issues, especially in applications where ionic contamination is
not well tolerated, even when the
encapsulation is removed.
The Harima Electronic Material division of Harima Chemicals based in Tokyo,
Japan uses a gas evaporation
process to produce a nano-silver paste containing particles with an average
size of 7 nm coated with an organic
dispersing agent. This material has much of the same issues as Sol-Gel
produced material in that the dispersant
agent that is bonded to the particle's surface must be removed from the silver
to have the silver reactive.
Additionally, if the paste is dried to form a powder, the particles become
aggregated.
Another metliod described in the publication "Production of carbon-coated
aluminum nanopowders in pulsed
microarc discharge" published September 16, 2002, in Nanotechnology 13 (2002)
638-643 describes the use of a
1-50 V and 30-150A, 200 microsecond, microarc discharge between closely spaced
electrodes (0.01-0.1mm) of
aluminum and copper in a 1 atm natural gas or methane environment to produce
microscopic quantities of 23 nm
aluminum particles with a 1 nm carbon coating. This process contained no gas
controls and is an open loop system
requiring working gases and carrier gas. It produced metals with inconsistent
morphologies. Generally, the
particles have a metastable amorphous morphology. Amorphous morphology is
generally not desirable for metal
particles because the particles will crystallize over time and/or at
temperature resulting in unstable reactivity of the
particles. Lastly, in this process, the microscopic quantities of particles
were collected 3mm from the arc by
drifting onto a substrate, again further demonstrating that the technology is
not commercially feasible.
Another method for producing unagglomerated nano-particles is described in
United States Patent Application
Serial No. 10/669,858 ("the '858 Patent Application"), which patent
application is commonly owned by the
Applicant of the Application and the invention disclosed therein is referred
hereinafter as "the Solenoid process" or
"the '858 Patent Application process." In the Solenoid process, a pulsed
solenoid is used in conjunction with a high
power, pulsed plasma (500-5000+V, 10,000-100,000+A, 0.1- lOms) process to
produced unagglomerated nano-
particles in'commercial quantities. In this application, the liner of the
solenoid provides an uncontrolled precursor
for coating the particles. In operation, the plasma created from the metal
precursor materials used to make the
nanopowders evaporates the liner. The amount of material removed from the
liner is not controlled.
Additionally, the gas species evolved by the vaporization of the liner is not
controlled and is dependent upon the
liner composition and production conditions. The liner is restricted to
materials that are compatible with this
process and limits the choice of particle coating materials to a very short
list of high strength, plasma tolerant and
insulating materials. Hence, it is impossible to control the coating precursor
concentration within this process.
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The material produced from the Solenoid process consisted of discrete metal
particles surrounded by carbonaceous
material. Because the silver is not tightly bound to the carbon material and
there is no surface chemistry attached to
the silver particles, they are very active. Specifically, 25nm silver
nanoparticles were produced that have been
shown to have good bacterial efficacy in a commercial topical wound dressing.
So while chemistry methods are capable of producing discrete, unagglomerated
nanometals in solutions, they
generally are not commercially viable and the particles contain surface
chemistry that is often not compatible with
formulations and can adversely affect the uniqueness of the nano-properties.
Other processes can produce dry nano
metal particles, however they contain surface chemistries that are
undesirable, the particles are aggregated or the
processes are not commercially viable. Therefore, there remains a need to
produce commercial quantities of
unagglomerated nanometals in dry powder form.
SUMMARY OF THE INVENTION
The current invention overcomes the previous art problems and difficulties, by
producing dry, unagglomerated
coated nanopowders in commercial volumes in a controllable process. The
particles are stable at room temperature
and remain discrete. The new process can use a siniilar high-powered, pulsed
plasma process as disclosed and
described in United States Patent No. 6,777,639 ("the '639 Patent") and the
'858 Patent Application, but without
the complexity of the pulsed solenoid used in the Solenoid process.
Additionally, unlike the Solenoid process, the
current invention provides a high level and wide range of control of coating
properties and coating precursors. The
current invention produced far-reaching results and produced both non-
agglomerated nanoparticles and novel
nanomaterial compositions.
The invention in the broad extent provides a novel method for synthesizing
nanometals as well as a method for
producing novel nano-materials. In some embodiments, the synthesis process
incorporates a system for
automatically controlling the coating precursor material within the synthesis
process. The controlled coating
precursor system can be in multiple forms including a controlled gas, liquid
or solid feed system or combination
therein. The coating precursor may interact with the plasma, the particles or
combinations therein. By using these
methods of controlled coating precursor, a wide range of particle sizes and
coatings can be achieved.
In an embodiment of the invention, the control of the coating precursor
material is accomplished by using a gas
injection control system to provide a controlled hydrocarbon precursor
material that interacts with the synthesis
process to produce highly unagglomerated nanometal particles. The hydrocarbon
gas interacts with the plasma and
nanomaterial precursor material to form carbonaceous materials that assists in
keeping the nanoparticles
unagglomerated. Additionally control of the agglomeration level is
accomplished by control of the hydrocarbon
gas species and quantity.
In another embodiment of the invention, a gas evolving system is used to
introduce the hydrocarbon precursor into
the system to control the amount of particle agglomeration. In this
embodiment, a solid or liquid precursor is used
to evolve gas in a controlled manner into the synthesis process. The gas
evolution may occur by interaction with
the plasma or by an independent source such as heating the solid or liquid.
For instance, a solid hydrocarbon
precursor rod can be fed into the process in a controlled manner to evolve the
hydrocarbon gas.
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In another embodiment of the invention, the hydrocarbon gas is created by
controlled injection of a liquid
hydrocarbon precursor into the process to interact with the plasma. The
hydrocarbon gas may also be created by
controlled evaporation of the liquid precursor material.
The process of the current invention produces novel materials. In some
embodiments, the novel materials are a
composite of unagglomerated nanometals and a carbonaceous material. The
carbonaceous material has been shown
to contain a carbyne form of carbon.
Furthermore, the material produced by the process has been shown to be
effective against a wide range of bacteria.
For instance, the silver material embodiment of the present invention has been
shown to have bacterial efficacy
against both gram positive and gram-negative bacteria.
LIST OF DRAWINGS
Figure 1 is a diagram of the pulsed power synthesis system embodiment of the
present invention that is configured
with an automated gas control system for the coating precursor material.
Figure 2 is a TEM image of 77nm silver produced without any coating precursor.
Figure 3 is a TEM image of a composition embodiment of the present invention
(45nm silver produced using 44
ppm of acetylene gas).
Figure 4 is a TEM image of another composition embodiment of the present
invention (28mn silver produced using
440 ppm of acetylene gas).
Figure 5 is a TEM image of another composition embodiment of the present
invention (22nm silver produced using
4,400 ppm of acetylene gas).
Figure 6 is a TEM image of another composition embodiment of the present
invention (9nm silver produced using
44,000 ppm of acetylene gas).
Figure 7 is a TEM image of another composition embodiment of the present
invention (30nm silver produced using
8800 ppm of methane gas).
Figure 8 are the XRD plots of 25nm material produced by the Solenoid process.
Figure 9 are the XRD plots of another composition embodiment of the present
invention (25nm material produced
using 4400 ppm acetylene).
Figure 10 are the XRD plots of another composition embodiment of the present
invention (lOnm material produced
using 44,000 ppm acetylene).
Figure 11 are the XRD plots of the composition embodiment of the present
invention of Figure 7 (the 30nm
material produced using 8800 ppm methane).
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Figure 12 is a TEM image of the composition embodiment of the present
invention of Figure 9(25nxn silver
produced using 4,400 ppm acetylene), which shows the carbyne structures.
Figure 13 is the EELS data of the composition embodiment of the present
invention of Figures 9 and 12 (25mn
silver produced using 4,400 ppm acetylene), which confirms the presence of
carbynes and discrete silver particles.
Figure 14 is a TEM image of the composition embodiment of the present
invention of Figure 10 (l Onm silver
produced using 44,000 ppm acetylene), which shows the carbyne structures and
discrete silver particles.
Figure 15 is a TEM image of the carbyne structures of the composition
embodiment of the present invention of
Figures 10 and 14 (lOnm silver produced using 44,000 ppm acetylene), which
shows the presence of carbynes.
Figure 16 is a TEM image of a prior art carbon/silver composite.
Figure 17 is another TEM image of the same prior art carbon/silver composite
of Figure 16.
Figure 18 is another TEM image of the same prior art carbon/silver composite
of Figures 16 and 17, which shows
crystalline particles.
Figure 19 is another TEM image of the same prior art carbon/silver composite
of Figures 16-18, which shows
crystalline particles.
Figure 20 is a TEM image of the same prior art carbon/silver composite
material of Figures 16-19, which shows
only the carbon material.
Figure 21 is a TEM image of the prior art carbon/silver composite material of
Figures 16-20, which shows only the
carbon material.
Figures 22A-C are TEM images of a composition embodiment of the present
invention (silver/carbon composite
material made using 8800 ppm methane), which shows the presence of carbyne.
Figure 23 shows EELS analysis of the composition embodiment of the present
invention of Figures 22A-C
(silver/carbon composite material made using 8800 ppm methane), which confirms
the presence of carbyne.
Figures 24A-D are TEM images of another composition embodiment of the present
invention (copper/carbon
composite produced using 44,000 ppm acetylene), which shows the presence
graphitic and fullerene carbon.
Figure 25 is the EELS data of the composition embodiment of the present
invention of Figures 24A-D
(copper/carbon composite produced using 44,000 ppm acetylene), which confirms
the presence graphitic and
fullerene carbon.
Figures 26A-B are TEM images of another composition embodiment of the present
invention (iron/carbon
composite produced using 4,400 ppm acetylene), which shows the presence
graphitic and fullerene carbon.
Figure 27 is the EELS data of the composition embodiment of the present
invention of Figures 26A-B (iron/carbon
composite produced using 4,400 ppm acetylene), which confirms the presence
graphitic and fullerene carbon.
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Figures 28A-B are TEM images of another composition embodiment of the present
invention (iron/silver/carbon
composite/alloy material using acetylene).
Figures 29A-B are TEM images of another composition embodiment of the present
invention (carbon material
using carbon precursor material and acetylene gas).
Figure 30 is a diagram of another pulsed power synthesis system of the present
invention that is configured with an
automated liquid control system for the coating precursor material.
Figures 31A-B are TEM images of another composition embodiment of the present
invention (silver/carbon
composite material using 10 gm heptaethiol).
Figure 32 is a TEM image of another composition embodiment of the present
invention (silver/carbon composite
material using 20 gm heptaethiol).
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The current invention alleviates the problems of the previous systems and
provides a unique system that has the
ability to control the agglomeration of the particles and is versatile enough
to handle different coating precursors.
Figure 1 shows a detailed schematic of the current invention. The current
invention uses a gas injection system with
a continuous closed loop feedback concentration control system to control the
hydrocarbon precursor to assist in
forming the unagglomerated nanometals.
The system is composed of the radial gun synthesis process 100 described in
the '639 Patent, of which is
incorporated by reference and which details have been omitted. The reaction
vessel 101 is connected to the cyclone
102 via a collection pipe 103. The cyclone is used to remove larger particles,
typically greater than 0.5 micron,
which are collected in the cyclone hopper 104. The cyclone is connected to the
dust collector 105 by a stainless
steel pipe 106. Located within the dust collector is a filter 107 used to
separate the powder from the gas stream.
The bottom of the dust collector contains a packaging valve 108 which is
connected to a packaging container 109
used to collect the powder. The outlet of the dust collector is connected to
the inlet of a sealed blower 110. The
outlet of the sealed blower is then connected to the reaction vessel 101 to
form a closed loop system.
Additionally, gas bottles 120, typically helium and nitrogen, are connected to
a gas injection manifold, 121. The
new invention incorporates a bottle of particle coating precursor gas 150,
such as a hydrocarbon gas like acetylene
or methane, connected to the gas injection valve 154. While the preferred
embodiment uses hydrocarbon gases it is
not limited to hydrocarbon gases and other gases such as silane can be used. A
gas sensor 151 is connected to the
outlet of the reaction vessel and pulls gas samples out of the reaction
vessel. The gas sensor contains a set point
controller 152 that uses the data from the gas sensor to maintain a predefmed
gas concentration.
In operation, the system is vacuumed to remove any oxygen from the system and
is then filled with the inert gases.
The inert gases may be, but are not limited to argon, helium, nitrogen, and
neon. The blower 110 is turned on and
the gases are recirculated. The set point controller 152 is set to maintain a
specific gas concentration, typically in
the range of 1-500,000 ppm and more specifically in the range of 50-50,000
ppm, and then the hydrocarbon gas is
injected into the system. The gas sensor 151 monitors the gas concentration
and the set point controller causes the
gas injection valve 154 to inject the hydrocarbon gas to maintain the
specified concentration. The material
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synthesis is started and powder is produced. As powder is produced, the
coating precursor material is consumed
and additional coating precursor material is automatically added to the
system. As the powder is produced, the
blower 110 is continuously recirculating the gas within the closed loop. The
powder moves with the gas through
the cyclone 102 where larger particles are removed. The remaining powder
continues to the dust collector 105
where it collects on the surface of the filter 107. The filter is back pulsed
with compressed gas (not shown) to
remove the powder and allow it to fall into the packaging container 109. The
packaging valve 108 can be closed to
seal the system and allow the packaging container with the powder to be
removed. The gas that passes through the
filter then flows into the blower and is sent back to the reaction vessel.
During the synthesis process, the hydrocarbon gas interacts with the plasma.
As the gas quenches, it may form
solids, react with the metal vapor or may catalytically interact with the
metal vapor and particles. The resulting
product of the plasma and gas quench is the formation of highly unagglomerated
metal nanoparticles. Additionally
carbon structures, including amorphous carbon (soot), graphite, fullerenes,
carbon nanotubes, diamond like carbon
structures and carbyne structures and combinations thereof may be formed. The
carbon may interact with the inert
gases to form other compounds such as cyano derivatives in the case of
nitrogen. Additionally, for some metals
such as aluminum, compounds may be formed that contain carbide compounds.
Consequently the hydrocarbon gas
is being consumed and must be adjusted to maintain a specific gas
concentration.
While the current examples show materials produced using acetylene and
methane, other gases such as alkanes
(methane CH4, ethane C2H6, propane C3H8, butane C4H10, pentane C5H12, heptane
C6H14, etc.), alkenes,
alkynes at an appropriate but non-explosive vapor pressure could be used with
the current invention. While the
current examples use hydrocarbon gas, it is not necessarily limited to them.
For example silane gas could be used
to form a silicon matrix or borane gas could be used to form a boron matrix
when a portion of the matrix gas does
not form a compound with the metal precursor. One skilled in the art will also
recognize that mixture of gases
could also be used. In some cases it may be possible to form combinations of
the metal particles, compounds of
the metal and matrix gas and the matrix. Other gases such as organo-metallic
gases such as ferrocene could also be
used.
The current invention has broad capabilities and demonstrated the ability to
produce a wide array of material sizes,
morphologies and compositions. In producing materials, the new invention was
able to produce materials in the
range of 8-100nm with precise and consistent control, far greater than the
solenoid process. For comparison
purposes, the solenoid process was able to produce material down to 25nm;
however, it could not produce this
material consistently. The new process was also able to produce new materials
that contained higher carbyne
contents and compositions that were pyrophoric and had different
dispersibility characteristics. Additionally, the
new process is capable of producing various materials including but not
limited to metals, metal alloys and
combinations of metals and metal alloys. More specifically, silver, copper,
aluminum, iron, nickel, combinations
thereof, alloys thereof and combinations of the metals and alloys thereof were
produced. One skilled in the art will
recognize that other metals such as zirconium, niobium, gold, platinum,
cobalt, titanium, zinc, hafnium, tantalum,
tungsten, alloys thereof and mixtures thereof can be used in the process. The
following shows some examples of
materials produced by the current process.
Example 1
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The following tests were performed using the system invention of the present
Application. The radial gun synthesis
technique as described above was used to produce the material. A commercial
dead-band feedback controller,
Omega CNI 3222-C24, and hydrocarbon sensor, VIG industries FID Model 20, were
integrated into the gas control
system. Acetylene and methane were used for the hydrocarbon gas. All size
measurements are computed based on
BET measurements and an equivalent sphere diameter model.
The tests were conducted using the feedback controller to maintain specific
levels of hydrocarbon including 0 ppm,
44 ppm, 440 ppm, 4400 ppm and 44,000 ppm in an atmosphere otherwise composed
of helium and nitrogen at a
total pressure of 1 atmosphere. All the tests were performed with the same
production conditions with only the
hydrocarbon and make-up gas concentrations being varied. Results are shown in
Table 1.
Table 1
Gas Concentration Hydrocarbon BET (nm) Agglomeration
(ppm) Gas
0 None 77 Highly necked
44 Acetylene 45 Slightly necked
440 Acetylene 28 Mostly
unagglomerated
4400 Acetylene 22 Unagglomerated
44,000 Acetylene 10 Unagglomerated
8800 Methane 30 Unagglomerated
Figure 2 shows the 77 mn silver produced with no hydrocarbon gas. It shows
extensive necking between particles.
Figure 3 shows the 45 nni silver produced by adding only a 44-ppm
concentration of acetylene. The amount of
agglomeration is substantially reduced. Figure 4 shows 28 nm silver produced
by adding 440-ppm acetylene.
Figures 5 and 6 show material produced at 4400 ppm and 44,000 ppm levels of
acetylene. The material size
produced was 22 mn and 9 nm, respectively. This shows a clear trend of
decreasing particle size with increasing
gas concentration. Additionally the particles are discrete and unagglomerated.
Even smaller silver, such as having a material size as low as 8 nm, was
achieved by further increasing the acetylene
concentration. Thus, embodiments of the invention have an average size of
material in the range between about
8nm and about 45nm, and more specifically, can have a size of material in the
range between about 8 mn and about
25nm, and even more specifically, can have a size of material in the range
between about 8nm and 15 nm.
Additional tests were performed using methane at 8800-ppm concentration using
the same production conditions as
above. The results of the material produced are shown in Figure 7. This
material is also unagglomerated.
The material from the current process has been analyzed to determine its
uniqueness. TEM, Static light scattering
(SLS) and dynamic light scattering (DLS) have been used to determine the
extent of agglomeration and to quantify
the dispersibility of the material. Inductively coupled plasma Optical
Emission Spectroscopy (ICP), X-ray
diffraction (XRD), LECO and Electron Energy Loss Spectrometry (EELS) were used
to determine the material
composition and morphology. Fourier Transform Infrared (FTIR) and gas
chromatography/mass spectroscopy
(GC/MS) were used to identify hydrocarbons.
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The Table 2 summarizes some of the powder characteristics as determined by
various tests. The mean particle size
is computed from an equivalent sphere diameter model based on the surface area
measurements from the Monosorb
BET. The Brunauer, Emmett and Teller (BET) method for particle measurement
uses gas adsorption to determine
the specific surface area that is then used to compute a particle size based
on an equivalent sphere model. The mean
particle size in various liquids was determined by a Horiba LA9 10 SLS. In
each SLS test, a 20 gm, 0.1 % solution
was prepared in a beaker and sonicated using a Misonix Sonicator 3000 with a
0.5-in probe for 2 minutes (4 minute
elapsed) at 90% power, half duty cycle. The carbon content of each powder was
measured based on LECO
analysis.
Table 2
BET Carbon SLS D50 (nm)
(nm) Gas Conc. (ppm) Mass % Water IPA Ethanol
Solenoid 25 - 3 328 257 148
Acetylene 25 4,400 3 155 205 243
Acetylene 10 44,000 30 392 900 2185
Methane 30 8,800 1.5 215 220 171
From these results, the material from the new process is distinguishable from
the solenoid process. Additionally,
for a given gas the carbon content increases with the amount of gas in the
reaction chamber. Interestingly, for a
given ambient precursor gas carbon content (4,400 ppm acetylene vs. 8,800 ppm
methane) the carbon content in the
materials is not the same. This indicates that the coating composition is
different for the different gases.
The composition of the produced material was determined from a host of tests
including XRD, ICP and LECO. X-
ray diffraction tests were performed to determine the material composition and
crystal structure. The results are
summarized in Table 3. The results confirm that the material is predominantly
crystalline silver with a small
amount of carbon. The crystallite size of the silver was also estimated from
the XRD analysis and shows good
correlation with the BET results indicating that the particles are discrete.
The XRD analysis showed small amounts
of carbon that was interpreted to be Fullerite structures. Figures 8-11 show
the XRD data for the various materials.
In peaks indicated by 801a-1101a (thin lines) in each sample indicate the Face
Centered Cubic (FCC) form of silver
whereas peaks 803a-1103a (thick lines) indicate the primitive hexagonal form
of silver. These results confirm the
material is highly crystalline. The peaks indicated by 805b-1105b (solid
shaded areas) can be interpreted as
fullerite. Later analysis using Scanning Transmission Electron Microscope
(STEM), EELS and EDS indicate that
this is a mixture of sp2-bonded (graphitic) and sp1-bonded (carbyne) carbon.
Table 3
Gas Hydrocarbon Gas BET Crystal Size Metal Crystal Carbon
Concentration (nm) Structure Structure
(ppm)
Solenoid None 25 22 FCC (3C) / Fullerite
Hexa onal (4H)
4400 Acetylene 22 18 FCC (3C) / Fullerite
Hexa onal (4H)
44,000 Acetylene 10 9 FCC (3C) / Fullerite
Hexagonal (4H)
8800 Methane 30 22 FCC (3C) / Fullerite
Hexagonal (4H)
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The metal content was also verified using ICP analysis which shows in excess
of 99% silver on a metal basis. This
means that of the metal in the sample, 99% is silver. It does not tell the
total amount of metal in the sample. From
the XRD analysis, the only materials present were carbon and silver. LECO
analyses were performed to determine
the carbon content in each of the samples. This is shown in the Table 4.
Table 4
Gas BET C N 0
None 77 0.0198 0.217 1.34
44 m C2112 45 0.959 0.194 0.983
440 m C2H2 28 1.63 0.129 1.36
4400 ppm C2H2 22 3.85 0.442 1.74
44000 ppm C2H2 9 27.9 0.480 1.93
8800 ppm CH4 30 1.38 0.209 0.618
It appears that carbon content increases with gas concentration and the size
decreases. Additional testing was
performed using GC/MS and FTIR analyses to determine if the carbon is in the
form of a hydrocarbon. The
GC/MS found only trace amounts of volatile and semi-volatile hydrocarbons. The
FTIR results did not show any
hydrocarbons confirming the GC/MS results. The FTIR did show that the carbon
might be in the form of a cyano
derivative. This would indicate that the carbon is partially reacting with the
nitrogen.
Analysis of the material was performed using an ultrahigh resolution STEM in
conjunction with EELS and Energy
Dispersive Spectroscopy (EDS) analysis to quantify the composition of the
material. Initial analysis using EELS
shows that the particles are silver and that the linear structure is carbon.
Notably, the EELS analysis shows that under certain conditions the carbon has
an sp2 and/or sp 1 bonding structure.
Carbon bonding structure with spl is referred to as carbyne. The carbyne
structure is elemental carbon in a triply
bonded form; rod-like molecule comprised mostly of alkyne (C=C) groups, more
commonly referred to as sp1-
bonded chains of carbon atoms. A more generic term is carbyloid which refers
to individual types of carbon
compounds collectively referred to as carbynes. The two predominate forms of
carbyloid are cumulene and
polyyne. Cumulene consists of a series of double bonds between carbon atoms
(=C=C=C=). Also referred to as
allenic carbyne or (3-carbyne. Polyyne consists of series of alternating
single and triple carbon-carbon bonds (-
C=C-C C-). Also referred to as acetylenic carbyne or a-carbyne. The carbyne
form of carbon is extremely
difficult to produce and has only been produced in laboratories under very
specialized conditions. It is generally
considered unstable and hence it has been difficult to study.
One embodiment of the invention produced by this unique process is a composite
of metal particles interspersed
within a carbon structure that appears to contain carbyne bonding (sp1). This
material can be produced in
significant cominercial scale quantities, more specifically, silver particles
inter-dispersed within a carbon matrix
containing carbyne structures. Interestingly, the morphology of the carbon
structure appears to change based on the
production conditions. One production condition using 4,400-ppm acetylene
produces material with a specific
surface area of 22 m2/g (BET) that is 97wt.% of silver and 3 wt. % carbon is
shown in Figure 12. It shows discrete
silver particles interspersed within a low-density carbon matrix. Figure 13
shows the EELS data, specifically the
Carbon K-edge spectra, of the sample made using 4400ppm of acetylene. The
spectra shows equivalent heights for
the ~*, 1301, and ~*, 1305, peaks indicating the presence of spl bonding or
carbynes. By increasing the acetylene
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concentration to 44,000 ppm, the same production conditions yielded a material
with a specific surface area of 60
m2/g that is 70 wt.% silver and 30 wt.% carbon. The carbon structure in this
material has a different morphology
which appear to be "layers" of carbon deposited on the curved surface of the
metal nanoparticles as shown in
Figure 14a, 14b and 14c. The dark elements in figure 14a show discrete silver
particles while the high crystallinity
of the silver particles is shown in Figure 14c as evident by the presence of
the lattice planes. For the most part, the
silver particles are discrete and are interspersed within the carbon
structure. Additionally, the TEM images show
that the silver particles are much smaller than the material produced using
4,400 ppm of acetylene. Figure 15
shows a TEM of the carbynes structures produced using silver and 44,000 ppm of
acetylene. When this composite
material is exposed to the electron TEM beam, the silver particles are excited
and expelled from the carbon matrix.
This indicates that the silver particles are not tightly bond to the carbon.
For comparison purposes, the material
produced with the solenoid process is also shown in Figures 16-21. This
material appears very different than the
material produced with the 44,000 ppm of acetylene.
The silver/carbon composite material was also produced using methane at a
concentration that has approximately
the same amount of carbon as one of the previous conditions. This material
gave similar results exhibiting the
intertwined layers of carbon with interspersed silver particles as shown in
Figures 22A-C. The specific surface area
was 19 m2/g and with a silver and carbon mass content of 98.5/1.5,
respectively. Figure 23 shows the low loss
EELS spectra for the carbon material in the methane silver sample. The peaks
2301 at 4.85 eV and 2305 at 19.5 eV
indicate the presence of carbynes. The silver produced using methane did have
one notable difference in that when
larger quantities were exposed to air it was pyrophoric. The TEM images were
taken from a small sample that was
isolated before the material ignited. The material produced with acetylene gas
was also pyrophoric but only at
concentrations below about 500ppm. For comparison purposes, the solenoid
material was not pyrophoric.
While the above example demonstrates the capability of the new invention with
silver, it can also be applied to
other materials. The following examples show several different materials that
were produced using silver, copper,
iron, graphite, gold and combinations thereof for the metal precursor. For the
most part acetylene gas at various
concentrations was used as the hydrocarbon gas. Other hydrogen carbon gases
could be used. The following
describes the unique materials that were produced.
Example 2. Copper/Carbon composite using acetylene
The same production conditions were used to make a copper and carbon composite
material. At an acetylene gas
concentration of 44,000 ppm, a material with a specific surface area of 44
m2/g and 20 wt% copper and 80 wt%
carbon was produced as shown in Figures 24A-D. The EELS K-edge spectra is
shown in Figure 25. The high ~*
peak, 2505, relative to the ~* peak, 2501, indicates the material contains a
high presence of sp3 carbon (diamond
like carbon or fullerene) structures. The ~* peak, 2501, also indicates that
there is some sp2 carbon or graphitic
carbon.
When nanocopper is produced in an inert atmosphere, it will readily oxidize
and turn from black to a brownish
green when exposed to even small amounts of oxygen. This has been confirmed by
XRD analysis. The current
nanocopper does not exhibit this feature and XRD analysis confirms that the
copper remains copper when exposed
to air.
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Example 3. Iron/Carbon composite using acetylene
The same production conditions were used to make an iron and carbon composite
material. At an acetylene gas
concentration of 4,400 ppm, a material with a specific surface area of 65 m2/g
was produced as shown in Figures
26A-B. TEM images show graphitic structures, 2601, as well as other carbon
structures. The EELS K-edge spectra
is shown in Figure 27. The relative heights of ~* peak, 2701 and the ~* peak,
2705, indicate the particles are
interspersed in an sp2-bonded or graphitic carbon matrix. This material is
attracted to a magnet and appears to be
paramagnetic.
Example 4. Iron/Silver/Carbon Composite using acetylene
A novel material was synthesized using silver and iron as the precursor metals
and acetylene gas as shown in
Figures 28A-B. The resulting material demonstrated some unique properties, one
of which is that the material is
magnetic and appears to be paramagnetic. The other unique property is that at
certain production conditions the
material is pyrophoric when exposed to air.
Example 5. Graphite precursor material
Material was produced using graphite precursor rods and acetylene gas as shown
in the TEM images shown in
Figures 29A-B.
Example 6. Other materials
Other materials made with the current invention include a nickel and carbon
composite and a nickel/silver and
carbon composite. One skilled in the art recognizes that other materials can
be fabricated using the gas injection
process. Other precious metals such as gold, palladium and platinum can also
be used to produce metal/carbon
composite materials. These materials are of particular interest because of
their catalytic nature, which is similar to
silver but generally stronger. This would probably produce more of the carbyne
structure. Other metals such as
cobalt, aluminum, and other metals can also be used.
For the most part, the material produced with the new process will bum or
oxidize when exposed to elevated
temperatures, radiation or a flame. It is not clear if this is a pure
oxidation reaction of the metal or a chemical
reaction involving the carbon matrix.
ALTERNATE EMBODIMENTS
Solid
An alternative embodiment of the current invention involves feeding rods of
polycarbonate or other solid material
in the vicinity of the arc. In this manner, varying the size, composition,
position relative to the arc and the feed rate
of the rod can control the amount of material that is removed. Materials such
as polycarbonate, thennoplastics such
as polyethylene, polypropylene, poly (vinyl chloride), polystyrene, acrylics,
nylons and cellulosics, thermoset
plastics such as polyamide, polybutadiene, polyether block amide (PEBA),
polyetherimide, polyimide, polyurea,
polyurethane (PUR), silicone and vinyl ester. Phenolic, melamine and urea
formaldehyde could also be used.
Fluropolymers such as polytetrafluorethylene (PTFE) and polyvinylidene
fluoride (PVDF) may also be used.
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It is also possible to control the spatial location where the solid materials
are introduced to again control at the point
in the quench process where the coating is applied. However, the degree of
spatial control would be much more
restricted in the case of solids if one depends on the arc plasma to vaporize
the material. Supplemental heating can
be applied to a solid to induce vaporization at the desired location. An
alternative would be to melt, vaporize, or
decompose solids external to the reaction chamber and introduce droplets,
sprays, or jets in the liquid or gas phase.
Another means of introducing solids into the arc region is the use of a pellet
injector. This could either be a simple
gravity or mechanically driven injector, or it could be a more sophisticated
light gas gun (or similar). It would be
possible to use helium as the propellant in a light gas gun to avoid
introducing any contaminants.
Liquid
In yet another embodiment, liquid evaporation is used to control the amount of
hydrocarbon gas in the system. A
test was conducted by the addition of 10 grams and 20 grams, respectively, of
heptanethiol directly into the reaction
vessel using the apparatus shown in Figure 30.
In the process, the system is vacuumed and then filled with the inert gas. The
ball valve 3025 isolating the liquid
port 3026 is closed and the cap from the liquid port is removed. The liquid
coating precursor, heptanethiol, is
added into the liquid port 3026 and the cap 3027 replaced. The 2-in ball valve
3025 is opened and allows the
heptanethiol to enter into the reactor 3001. The blower 3010 is turned on and
run for 5-10 minutes to evaporate
some of the liquid coating precursor. The reactor 3001 is then operated as
normal. By controlling the amount of
liquid in the system, the amount of vapor is consequently controlled.
The results of the 10 gm and 20 gm heptanethiol experiments are shown Figures
31A-B and Figure 32,
respectively. The materials show that the agglomeration could be reduced,
however, there where several aspects
that were undesirable. Analysis of the material shows small silver particles
coated in carbon or hydrocarbons which
then form into agglomerates. Additionally, during collection of the particles,
the heptanethiol would condense out
and wet the filter resulting in collection problems. Another undesirable
aspect was that the amount of vapor could
not be as accurately controlled as with the direct gas injection. Lastly, the
heptanethiol is difficult to remove from
the silver particles. While this system does have some drawbacks, it would be
beneficial in a system that uses a wet
collection technique. I
While the existing test used gas vaporization due to convection and a liquid
pool, there are numerous possible
secondary material injection systems possible. For example it is possible to
use a liquid spray, mist, jet or
automated dropper. The choice of system would depend on the molecular weight,
vapor pressure, and boiling point
of the liquid used. This is only a partial list of the technologies possible
for admitting coating liquids.
An alternative type of gas or liquid injection system consists of precisely
located pulsed or continuous jets of
secondary coating material(s) in the expansion region of the arc synthesized
material. This allows controlling the
point during the synthesis and quench that the coating material or materials
are introduced. This allows partial
decoupling of the condensation and coating processes, which allows coating
nanoparticles of one material with a
material with a higher melting temperature before any agglomeration could
occur.
Combination
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The gas and liquid phase systems and applications demonstrated to date barely
begin to explore the range of
possibilities afforded by this technology. This technology enables the
production of an entirely new class of
nanomaterials with the surface chemical properties of one material and the
bulk properties of another material.
The tests have been limited to a narrow range of hydrocarbons and a limited
range of concentrations. This
technology allows controlling the secondary matrix gas precursor concentration
from less than a part per million to
100%. It is possible to introduce more than one material simultaneously by
using multiple matrix precursor gases
and liquids, as well as electrode materials. By controlling both the arc
synthesis parameters and the gas
concentration it is possible to independently control the primary particle
size, secondary coating thickness, and
degree of agglomeration.
It is possible to introduce material into the arc region with a plasma
injector. This could include Marshall guns,
electrothermal injectors, and other means. In short, coatings could be
introduced in solid, liquid, gas, or plasma
form resulting in the ability to produce nanomaterials with various coating
properties.
Another embodiment includes the simultaneous or staged injection of similar or
dissimilar materials by any of the
distinct means described above. For example, it is possible to inject a liquid
spray of one coating material directly
into the arc region while controlling a fixed background concentration of a
second coating gas.
ANTIBACTERIAL USES
In yet another embodiment the silver nanopowder is used as an antibacterial
agent. Silver has long been known to
have antimicrobial effects that are a function of the ions released by
material. Silver ions can kill bacteria by
interfering with respiration (Bragg, P.D. and Rainnie, D.J., "The effect of
silver ions on the respiratory chain of
Escerhichia coli.," Can. J. Microbiol. 20, 883-889 (1974)), or by interacting
with bacterial DNA (Modak K., and
Fox C., "Binding of silver sulfadiazine in the cellular components of
Pseudomonas aeruginosa," Biochem. Pharm.
22: 2392-2404 (1973)). Because of the unique nature of the material, in that
the carbon does not appear to be
bonded to the surface of the silver as evidence by the particle expulsion in
the TEM electron beam, it was
speculated that the silver particles would be highly active. Tests were
conducted using the lOnm and 25nm silver
produced using acetylene and 35nm silver produced using methane. During the
testing it was found that the silver
had efficacy against several bacteria.
A series of antibacterial efficacy tests were conducted on the new silver
using ASTM Standard E2315-03 "Standard
Guide for Assessment of Antimicrobial Activity Using a Time-Kill Procedure"
protocol to test the new
nanopowders against Escherichia coli 4157:H7, ATCC 43895 and Staphylococcus
aureus, ATCC 6538. The
testing incorporated the recommendations described in the "Manual of Clinical
Microbiology," 5th ed., edited by A.
B. Balows, ASM, Washington and is directed by the Federal Register, June 1994.
Each sample was prepared by
mixing the nanopowder in de-ionized water and sonicated using the Misonix
Sonicator 3000 for two cycles of 2
minutes at 50% power and 50% duty cycle. The silver was prepared at the higher
concentration. The sample at the
lower concentration was prepared by dilution of a portion of the higher
concentration and then re-sonicated. The
results of the test are shown in Table 5 and Table 6.
Table 5
Test Results for E. coli O 157:H7
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Results Expressed as Average Colony Forming Units (CFU) per mL and Percent
Reduction
Initial counts: 5.9 x 106
Lot No. Conc. Contact Time Replicate Avg. CFU/mL Percent Reduction
m
100 30 seconds 1 >3.0 x 105 <94.9153
25 nm Silver, 100 2 >3.0 x 105 <94.9153
Acetylene 100 1 Hour 1 1.6 x 102 99.9973
100 2 1.5 x 102 99.9975
1000 30 seconds 1 >3.0 x 105 <94.9153
25 nm Silver, 1000 2 >3.0 x 105 <94.9153
Acetylene 1000 1 Hour 1 <5.0 x 10 >99.9999
1000 2 <5.0 x 10 >99.9999
100 30 seconds 1 >3.0 x 105 <94.9153
35 nm Silver, 100 2 >3.0 x 105 <94.9153
Methane 100 1 Hour 1 5.5 x 101 99.9991
100 2 1.1 x 10z 99.9981
1000 1 >3.0 x 105 <94.9153
35 nm Silver, 1000 30 seconds 2 >3.0 x 105 <94.9153
Methane 1000 1 <5.0 x 10 >99.9999
1 Hour
1000 2 <5.0 x 10 >99.9999
Table 6
Test Results for S. aureus
Results Expressed as Average Colony Forming Units (CFU) per mL and Percent
Reduction
Initial counts: 6.2 x 106
Lot No. Conc. (pp ) Contact Time Replicate Avg. CFU/mL Percent Reduction
m
100 30 seconds 1 >3.0 x 105 <95.1613
25 nm Silver, 100 2 >3.0 x 105 <95.1613
Acetylene 100 1 Hour 1 8.0 x 101 99.9987
100 2 1.5 x 101 99.9998
1000 30 seconds 1 >3.0 x 105 <95.1613
25 nm Silver, 1000 2 >3.0 x 105 <95.1613
Acetylene 1000 1 Hour 1 <5.0 x 10 >99.9999
1000 2 <5.0 x 10 >99.9999
100 30 seconds 1 >3.0 x 105 <95.1613
35 nm Silver, 100 2 >3.0 x 105 <95.1613
Methane 100 1 Hour 1 3.5 x 101 99.9994
100 2 2.0 x 101 99.9997
1000 30 seconds 1 >3.0 x 105 <95.1613
35 nm Silver, 1000 2 >3.0 x 105 <95.1613
Methane 1000 1 Hour 1 <5.0 x 100 >99.9999
1000 2 <5.0 x 10 >99.9999
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The results from these tests show that both silver nanopowders had an
immediate kill of the bacteria and at one
hour most had a complete kill. In industry, antibacterial static agents are
considered to be materials which prevent
growth. These materials typically have at least a Log 0 reduction. Materials
are generally considered to have
antibacterial properties if there is at least a Log 2 reduction and preferably
a Log 3 reduction. Depending on
standards, a "complete kill" is defmed as between at least a Log 4 or at least
a Log 6 percent reduction. While the
current tests were performed for one hour, one skilled in the art will
recognize the time sensitivity of these tests.
Often, additional kill of the bacteria will occur with longer exposure times
to the antibacterial agent.
The two bacteria that were tested were chosen because they represent gram
positive, S. aureus, and gram negative,
E. coli, forms of bacteria. Traditionally silver ions have been shown to be
effective against gram-negative bacteria
but have been shown to ineffective against gram-positive bacteria. This effect
is attributed to the thicker
peptidoglycan wall of the gram-positive bacteria. A unique property of the new
silver materials is that they had
good efficacy against both gram positive and gram-negative bacteria.
In a separate test, the l Omn silver produced using acetylene gas was tested
against E. coli. Concentrations of 100
ppm and 500 ppm were tested and results measured after 5 hours of inoculation.
Both concentrations showed 9-log
percent reduction in CFU's.
The antibacterial efficacy of the new material can be exploited in many
applications. Some areas and products that
the new material can be used to give antibacterial properties are wound
dressing such as topical wound dressings,
creams and ointments. It can be incorporated into various consumer
electronics, such as cell phone screens,
telephone receivers and keyboards; athletic gear such as shoes, clothing,
underwear, protection pads, sweat bands,
handle grips, tent surfaces or any other area where there is high moisture
exposure such as sweating or water
exposure; personal hygiene products such as soaps, deodorant, feminine hygiene
pads and personal wipes; dental
products such as toothbrushes and dental floss; water filters; humidifiers and
wipes to give the item antibacterial
properties. It can also be incorporated into various coatings and textiles
such architectural epoxies and paints, wood
decking and preservation products, awnings, roof covers and pool covers. It
can also be used as a biocide for
paints, cleaning supplies, pulp and paper, plastics and food products.
The high-current pulsed arc discharge with adjustable feedback-controlled
concentration of coating precursor
mixture has significant and far-reaching advantages over the competing
processes. Either the arc discharge plasma
pyrolyses the background gas or liquid vapor, or the hot particles or droplets
of the primary material produced
during condensation accomplish the same end. This secondary material gas then
co-condenses with the primary
material. Depending on the relative melting point of the materials, the point
at which the secondary material is
pyrolysed, and other factors, one or the other material will primarily or
entirely reside on the surface of particles
composed of the alternate material. By varying the gas concentration and
composition, the level of agglomeration
and coating thickness can be controlled.
When compared with the previously cited microarc discharge, the current
invention delivers roughly 5,000 times
more energy to the arc. This allows the current invention to produce orders of
magnitude more material per unit
time. The higher energy also produces a much hotter plasma with a more rapid
quench which enables the synthesis
of a much wider range of nanomaterials with superior properties. The new
process results in co-condensed particles
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rather than forming a compound (e.g. metal carbide). It also allows precise
control of gas concentration as well as
gas mixing.
Comparatively, the microarc experiments were performed with 1 atmosphere of
pure methane, whereas the current
invention controls the concentration to as low as 1 ppm in an inert gas
background. The new nanoparticle coating
process produces particles which are far less agglomerated than those produced
with the microarc process and are
much more crystalline and hence stable.
Compared to the flame synthesis nano-encapsulation processes, the much higher
temperature and rapid quench of
the new process again allows it to work with a wider range of materials. The
flame synthesis process is
fundamentally a different process than the current invention and is much more
restricted in choice of coating
materials and doesn't afford nearly the ability to control the size of
particles produced or the range of coating
thickness.
Another pre-existing competitor to the new process is the use of chemistry
methods such as Sol-Gel to coat one
nanomaterial with another. In general, these methods are known to produce soft
agglomerates and once the
nanoparticles are in solution, it is not possible to dry them without forming
aggregates and clumps.
Applicant believes the closest process to the invention of the Application is
the Solenoid process. The coating
process is an artifact of the solenoid protection armor and is restricted to
materials which are compatible with this
application. This fact constrains the choice of coating materials to a very
short list of high strength, plasma tolerant,
insulating, arc resistant materials. Additionally, the decomposition of the
liner material results in a wide variety of
gas species, the composition of the coating precursor cannot be accurately
controlled. Control of the coating
process is not possible with the solenoid process. Also the solenoid process
introduces significant additional cost
and complexity to the synthesis process because it requires a sophisticated
solenoid magnet and a large pulsed
power supply and control system. Additionally, the solenoid liner needs to be
replaced on a periodic basis because
it is being consumed. This is a time consuming and labor intensive process.
This compares to the addition of a
very inexpensive automated gas injection system in the present invention.
Finally the solenoid presently used with
the solenoid system has a fairly restricted lifetime which limits it to
relatively low production applications
compared to the current invention.
Significant differences exist between the silver of the current invention and
that produced using the solenoid
process. First the size range of material exceeded the size range of material
produced with the solenoid process.
The material also had different dispersion characteristic and the amount of
carbyne present was larger. Lastly the
material when compared to the other nanosilvers appears to have higher
efficacy against bacteria, particularly
gram-positive bacteria.
The above Examples are included to demonstrate particular embodiments of the
present invention. It should be
appreciated by those of skill in the art that the methods disclosed in the
examples that follow merely represent
exemplary embodiments of the present invention. However, those of skill in the
art should, in light of the present
disclosure, appreciate that many changes can be made in the specific
embodiments described and still obtain a like
or similar result without departing from the spirit and scope of the present
invention.
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All patents and publications referenced herein are hereby incorporated by
reference. It will be understood that
certain of the above-described structures, functions, and operations of the
above-described embodiments are not
necessary to practice the present invention and are included in the
description simply for completeness of an
exemplary embodiment or embodiments. In addition, it will be understood that
specific structures, functions, and
operations set forth in the above-described referenced patents and
publications can be practiced in conjunction with
the present invention, but they are not essential to its practice. It is
therefore to be understood that the invention
may be practiced otherwise than as specifically described without actually
departing from the spirit and scope of the
present invention as defmed by the appended claims.
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