U.S. patent application number 11/526489 was filed with the patent office on 2010-09-02 for mass production of nano-scaled platelets and products.
Invention is credited to Jiusheng Guo, Bor Z. Jang, Aruna Zhamu.
Application Number | 20100222482 11/526489 |
Document ID | / |
Family ID | 42646608 |
Filed Date | 2010-09-02 |
United States Patent
Application |
20100222482 |
Kind Code |
A1 |
Jang; Bor Z. ; et
al. |
September 2, 2010 |
MASS PRODUCTION OF NANO-SCALED PLATELETS AND PRODUCTS
Abstract
Disclosed is a process for exfoliating a layered material to
produce nano-scaled platelets having a thickness smaller than 100
nm, typically smaller than 10 nm, and often between 0.34 nm and
1.02 nm. The process comprises: (a) charging a layered material to
an intercalation chamber comprising a gaseous environment at a
first temperature and a first pressure sufficient to cause gas
species to penetrate into the interstitial space between layers of
the layered material, forming a gas-intercalated layered material;
and (b) operating a discharge valve to rapidly eject the
gas-intercalated layered material through a nozzle into an
exfoliation zone at a second pressure and a second temperature,
allowing gas species residing in the interstitial space to
exfoliate the layered material to produce the platelets. The
gaseous environment preferably contains only environmentally benign
gases that are reactive (e.g., oxygen) or non-reactive (e.g., noble
gases) with the layered material. The process can additionally
include dispersing the platelets in a matrix material to form a
nanocomposite. The process also can include an additional process
of re-compressing the nana-scaled platelets into a product such as
a flexible graphite sheet.
Inventors: |
Jang; Bor Z.; (Centerville,
OH) ; Zhamu; Aruna; (Centerville, OH) ; Guo;
Jiusheng; (Centerville, OH) |
Correspondence
Address: |
Bor Z. Jang
9436 Parkside Drive
Centerville
OH
45458
US
|
Family ID: |
42646608 |
Appl. No.: |
11/526489 |
Filed: |
September 26, 2006 |
Current U.S.
Class: |
524/424 ;
423/327.1; 423/448; 423/509; 524/445; 977/700; 977/755 |
Current CPC
Class: |
C01B 2204/04 20130101;
C01B 32/22 20170801; C01B 2204/02 20130101; C01B 33/40 20130101;
C01B 32/19 20170801; C01B 32/23 20170801; B82Y 30/00 20130101; B82Y
40/00 20130101 |
Class at
Publication: |
524/424 ;
524/445; 423/448; 423/509; 423/327.1; 977/700; 977/755 |
International
Class: |
C08K 3/20 20060101
C08K003/20; C08K 3/34 20060101 C08K003/34; C01B 31/04 20060101
C01B031/04; C01B 19/04 20060101 C01B019/04; C01B 33/26 20060101
C01B033/26 |
Goverment Interests
[0001] This invention is based on the research result of a DoE SBIR
project. The US government has certain rights on this invention.
Claims
1. A process for exfoliating a layered material to produce
nano-scaled platelets having a thickness smaller than 100 nm, said
process comprising: a) charging a layered material, excluding
pre-intercalated graphite, to an intercalation chamber comprising a
gas at a first temperature and a first pressure sufficient to cause
gas species to penetrate into an interstitial space between layers
of the layered material, forming a gas-intercalated layered
material, wherein said gas is selected from hydrogen, helium, neon,
argon, nitrogen, oxygen, fluorine, carbon dioxide, water vapor, or
a combination thereof; and b) operating a discharge means to
rapidly eject said gas-intercalated layered material through a
nozzle into an exfoliation zone at a second pressure and a second
temperature, allowing gas species residing in the interstitial
space to exfoliate said layered material to produce the
platelets.
2. (canceled)
3. The process of claim 1 wherein said gas comprises a gas at a
supercritical fluid state.
4. The process of claim 1 further including a step of air milling,
ball milling, mechanical attrition, and/or sonification to further
separate said platelets and/or reduce a size of said platelets.
5. The process of claim 1 further comprising a step of
re-compressing said platelets into a sheet-like structure.
6. The process of claim 1 wherein said step of ejecting comprising
forcing gas species in said intercalation chamber to flow through a
tapered structure from a larger cross-section zone to said nozzle
with a smaller cross-section to eject all of said intercalated
layered material.
7. The process of claim 1 wherein said first temperature and said
first pressure are sufficiently high for said gaseous environment
to propel said gas-intercalated layered material out of said
intercalation zone.
8. The process of claim 1 wherein said step of charging a layered
material is preceded by a step of pre-pressurizing said layered
material in a pre-pressurization chamber.
9. The process of claim 8 wherein said pre-pressurization chamber
is at a temperature lower than said first temperature.
10. The process of claim 1 wherein said layered material comprises
particles with a dimension smaller than 1 .mu.m.
11. The process of claim 1 wherein said platelets have a thickness
smaller than 10 nm.
12. The process of claim 1 wherein said platelets have a thickness
smaller than 1 nm.
13. The process of claim 1 wherein said platelets comprise single
graphene sheets having a thickness of approximately 0.34 nm.
14. The process of claim 1 wherein said second pressure is lower
than said first pressure and said second temperature is lower than
said first temperature.
15. The process of claim 1 wherein said layered material comprises
graphite, graphite oxide, graphite fluoride, graphite or carbon
fiber, graphite nano-fiber, or a combination thereof.
16. The process of claim 1 wherein said layered material comprises
a layered inorganic compound selected from (a) clay; (b) bismuth
selenides or tellurides; (c) transition metal dichalcogenides; (d)
sulfides, selenides, or tellurides of niobium, molybdenum, hafnium,
tantalum, tungsten or rhenium; (e) layered transition metal oxides;
or a combination thereof.
17. The process of claim 1 wherein said layered material reacts
with said gas.
18. The process of claim 1 further including a step of dispersing
said platelets in a liquid to form a suspension or in a monomer- or
polymer-containing solvent to form a nanocomposite precursor
suspension.
19. The process of claim 18 further including a step of converting
said suspension to a mat or paper, or converting said nanocomposite
precursor suspension to a nanocomposite solid.
20. The process of claim 1 further including steps of mixing said
platelets with a monomer or polymer to form a mixture and
converting said mixture to obtain a nanocomposite solid.
21. (canceled)
22. (canceled)
23. The process of claim 1 wherein said layered material is placed
in said intercalation chamber and said gas is produced by
vaporizing a liquid inside said intercalation chamber or from a
different chamber.
24. The process of claim 1 wherein said first temperature or second
temperature is lower than 200.degree. C.
Description
FIELD OF THE INVENTION
[0002] The present invention relates generally to a process for
mass-producing nano-scaled plate-like or sheet-like structures,
such as nano-scaled graphene platelets (NGPs) and clay
nano-platelets, and products derived from these platelets.
BACKGROUND
[0003] Carbon is known to have four unique crystalline structures,
including diamond, graphite, fullerene and carbon nano-tubes. The
carbon nano-tube (CNT) refers to a tubular structure grown with a
single wall or multi-wall, which can be conceptually obtained by
rolling up a graphene sheet or several graphene sheets to form a
concentric hollow structure. A graphene sheet is composed of carbon
atoms occupying a two-dimensional hexagonal lattice. Carbon
nano-tubes have a diameter on the order of a few nanometers to a
few hundred nanometers. Carbon nano-tubes can function as either a
conductor or a semiconductor, depending on the rolled shape and the
diameter of the tubes. Its longitudinal, hollow structure imparts
unique mechanical, electrical and chemical properties to the
material. Carbon nano-tubes are believed to have great potential
for use in field emission devices, hydrogen fuel storage,
rechargeable battery electrodes, and composite reinforcements.
[0004] However, CNTs are extremely expensive due to the low yield
and low production and purification rates commonly associated with
all of the current CNT preparation processes. The high material
costs have significantly hindered the widespread application of
CNTs. Rather than trying to discover much lower-cost processes for
nano-tubes, we have worked diligently to develop alternative
nano-scaled carbon materials that exhibit comparable properties,
but can be produced in larger quantities and at much lower costs.
This development work has led to the discovery of processes for
producing individual nano-scaled graphite planes (individual
graphene sheets) and stacks of multiple nano-scaled graphene
sheets, which are collectively called "nano-scaled graphene plates
(NGPs)." NGPs could provide unique opportunities for solid state
scientists to study the structures and properties of nano carbon
materials. The structures of these materials may be best visualized
by making a longitudinal scission on the single-wall or multi-wall
of a nano-tube along its tube axis direction and then flattening up
the resulting sheet or plate. Studies on the structure-property
relationship in isolated NGPs could provide insight into the
properties of a fullerene structure or nano-tube. Furthermore,
these nano materials could potentially become cost-effective
substitutes for carbon nano-tubes or other types of nano-rods for
various scientific and engineering applications.
[0005] Direct synthesis of the NGP material had not been possible,
although the material had been conceptually conceived and
theoretically predicted to be capable of exhibiting many novel and
useful properties. Jang and Huang have provided an indirect
synthesis approach for preparing NGPs and related materials [B. Z.
Jang and W. C. Huang, "Nano-scaled Graphene Plates," U.S. Pat. No.
7,071,258 (Jul. 4, 2006)]. Another process developed by B. Z. Jang,
et al. ["Process for Producing Nano-scaled Graphene Plates," U.S.
Pat. No. pending, 10/858,814 (Jun. 3, 2004)] involves (1) providing
a graphite powder containing fine graphite particles (particulates,
short fiber segments, carbon whisker, graphitic nano-fibers, or
combinations thereof) preferably with at least one dimension
smaller than 200 .mu.m (most preferably smaller than 1 .mu.m); (2)
exfoliating the graphite crystallites in these particles in such a
manner that at least two graphene planes are either partially or
fully separated from each other, and (3) mechanical attrition
(e.g., ball milling) of the exfoliated particles to become
nano-scaled to obtain NGPs. The starting powder type and size,
exfoliation conditions (e.g., intercalation chemical type and
concentration, temperature cycles, and the mechanical attrition
conditions (e.g., ball milling time and intensity) can be varied to
generate, by design, various NGP materials with a wide range of
graphene plate thickness, width and length values. Ball milling is
known to be an effective process for mass-producing ultra-fine
powder particles. The processing ease and the wide property ranges
that can be achieved with NGP materials make them promising
candidates for many important industrial applications. The
electronic, thermal and mechanical properties of NGP materials are
expected to be comparable to those of carbon nano-tubes; but NGP
will be available at much lower costs and in larger quantities.
[0006] In this and other methods for making separated graphene or
other non-carbon inorganic platelets, the process begins with
intercalating lamellar flake particles with an expandable
intercalation compound (intercalant), followed by expanding the
intercalant to exfoliate the flake particles. Conventional
intercalation methods and recent attempts to produce exfoliated
products or separated platelets are given in the following
representative references: [0007] 1. J. W. Kraus, et al.,
"Preparation of Vermiculite Paper," U.S. Pat. No. 3,434,917 (Mar.
25, 1969. [0008] 2. L. C. Olsen, et al., "Process for Expanding
Pyrolytic Graphite," U.S. Pat. No. 3,885,007 (May 20, 1975). [0009]
3. A. Hirschvogel, et al., "Method for the Production of
Graphite-Hydrogensulfate," U.S. Pat. No. 4,091,083 (May 23, 1978).
[0010] 4. T. Kondo, et al., "Process for Producing Flexible
Graphite Product," U.S. Pat. No. 4,244,934 (Jan. 13, 1981). [0011]
5. R. A. Greinke, et al., "Intercalation of Graphite," U.S. Pat.
No. 4,895,713 (Jan. 23, 1990). [0012] 6. F. Kang, "Method of
Manufacturing Flexible Graphite," U.S. Pat. No. 5,503,717 (Apr. 2,
1996). [0013] 7. F. Kang, "Formic Acid-Graphite Intercalation
Compound," U.S. Pat. No. 5,698,088 (Dec. 16, 1997). [0014] 8. P. L.
Zaleski, et al. "Method for Expanding Lamellar Forms of Graphite
and Resultant Product," U.S. Pat. No. 6,287,694 (Sep. 11, 2001).
[0015] 9. J. J. Mack, et al., "Chemical Manufacture of
Nanostructured Materials," U.S. Pat. No. 6,872,330 (Mar. 29, 2005).
[0016] 10. Morrison, et al., "Forms of Transition Metal
Dichalcogenides," U.S. Pat. No. 4,822,590 (Apr. 18, 1989).
[0017] One common feature of these methods is the utilization of
liquid or solution-based chemicals to intercalate graphite or other
inorganic flake particles. These chemicals often comprise strong
acids (e.g., sulfuric or nitric acids), solvents, or other
undesirable species that can reside in the material. For instance,
Mack, et al. [Ref.9] intercalated laminar materials with alkali
metals (e.g. Li, Na, K, Rb, Cs), alkaline earth metals (e.g. Mg,
Ca, Sr, Ba), Eu, Yb, or Ti. Intercalation of these elements was
accomplished by five different routes: (1) intercalated
electrochemically using a non-aqueous solvent; (2) using an alkali
plus naphthalene or benzophenone along with a non-aqueous solvent
(usually an ether such as tetrahydrofuran); (3) using amalgams
(metal+mercury); (4) dissolving any of the afore-mentioned metals
in a liquid ammonia solution to create solvated ions; and (5) using
n-butyl lithium in a hydrocarbon solvent (e.g., hexane).
[0018] In addition to the utilization of undesirable chemicals, in
most of these methods of graphite intercalation and exfoliation, a
tedious washing step is required, which produces contaminated waste
water that requires costly disposal steps. Furthermore,
conventional exfoliation methods normally involve a very high
furnace temperature (typically between 500.degree. C. and
2,500.degree. C.) since the process depends on vaporization or
decomposition of a liquid or solid intercalant. Intercalation with
an alkali or alkaline earth metal normally entails immersing the
layered material in a metal compound solution (rather than pure
metal), allowing the metal ions to penetrate into the inter-layer
galleries (interstitial spaces). Typically, metal ion content is
relatively low compared to other elements in such a compound
solution (e.g., in a solution of 20% by weight lithium chloride in
water, lithium content is only 3.27% by weight). Hence, only a
small amount of ions from a relatively dilute solution penetrates
and stays sporadically in these spaces. The resulting exfoliated
product often exhibits platelets of widely varying thicknesses and
many incompletely delaminated layers.
[0019] In a co-pending application [Bor Z. Jang, Aruna Zhamu, and
Jiusheng Guo, "Process for Producing Nano-scaled Platelets and
Nanocomposites," U.S. Pat. No. Pending, 11/509,424 (Aug. 25,
2006)], we provided an environmentally benign process for
exfoliating a laminar or layered compound or element, such as
graphite, graphite oxide, and transition metal dichalcogenides,
without using undesirable intercalating chemicals. This was a
relatively low-temperature process and it produced nano-scaled
platelets with relatively uniform thicknesses. This process
comprises: (a) subjecting a layered material to a gaseous
environment at a first temperature and first pressure sufficient to
cause gas species to penetrate into the interstitial space between
layers of the layered material, forming a gas-intercalated layered
material; and (b) subjecting the gas-intercalated layered material
to a second pressure, or a second pressure and a second
temperature, allowing gas species to greatly pressurize the
interstitial space and thereby exfoliating the layered material to
produce partially delaminated or totally separated platelets. In a
preferred mode, step (a) of subjecting a layered material to a
gaseous environment comprises placing the material in a sealed
vessel containing a pressurized gas and step (b) comprises opening
the vessel to partially or totally release the gas. Upon pressure
release, the material is placed in a pre-heated furnace at a second
temperature (which is typically higher than the first temperature)
to help soften the intercalated material and instantaneously
increase the internal pressure of the interstitial space. This
earlier application did not address the issue of mass production.
In the present application, we describe, in detail, a specific
process that is capable of producing nano-scaled platelets on a
semi-continuous basis. This specific process typically involves a
higher first temperature and a lower second temperature with a much
higher first pressure and a lower second pressure.
SUMMARY OF THE INVENTION
[0020] In summary, the present invention provides a process for
exfoliating a layered (laminar) material to produce nano-scaled
platelets having a thickness smaller than 100 nm. The process
comprises: (a) charging a layered material to an intercalation
chamber comprising a gaseous environment at a first temperature
(typically 50.degree. C. to 200.degree. C., but could be higher)
and a first pressure (typically 2 atm to 10 atm) sufficient to
cause gas species to penetrate into the interstitial space between
layers of the layered material, forming a gas-intercalated layered
material; and (b) operating a discharge valve to rapidly eject the
gas-intercalated layered material through a nozzle into an
exfoliation zone at a second pressure and a second temperature,
allowing gas species residing in the interstitial space to
exfoliate the layered material to produce the platelets. The
gaseous environment preferably contains only environmentally benign
gases that are reactive (e.g., oxygen) or non-reactive (e.g., noble
gases) with the layered material. Step (a) may be preceded by a
step of pre-pressurizing or pre-intercalating the layered material
in a separate chamber to reduce the necessary intercalation time in
step (a).
[0021] The starting layered material preferably comprises small
particles with a dimension smaller than 10 [m and more preferably
smaller than 1 .mu.m. The gas preferably is selected from hydrogen,
helium, neon, argon, nitrogen, oxygen, fluorine, carbon dioxide, or
a combination thereof. The process may include an additional step
of applying air milling, ball milling, mechanical attrition, and/or
sonification to further separate the platelets and/or reduce a size
of the platelets. The resulting platelets typically have a
thickness smaller than 10 nm and many have a thickness smaller than
1 nm. For graphite flakes, the resulting graphene platelets
typically contain one to five layers of graphite planes or graphene
sheets with each layer of approximately 0.34 nm (3.4 .ANG.) thick.
For graphite oxide flakes, each layer or sheet is approximately
0.64 nm to 1.02 nm in thickness (depending upon the degree of
oxidation), but more typically close to 0.74 nm.
[0022] The layered material could be graphite, graphite oxide,
graphite fluoride, pre-intercalated graphite, pre-intercalated
graphite oxide, graphite or carbon fiber, graphite nano-fiber, or a
combination thereof. It could comprise a layered inorganic compound
selected from a) clay; b) bismuth selenides or tellurides; c)
transition metal dichalcogenides; d) sulfides, selenides, or
tellurides of niobium, molybdenum, hafnium, tantalum, tungsten or
rhenium; e) layered transition metal oxides; f) graphite or
graphite derivatives; g) pre-intercalated compounds, or a
combination thereof. In the case of graphite flakes, this layered
material can react with oxygen in the gaseous environment at an
elevated temperature (e.g., higher than 100.degree. C.) to form
partially oxidized graphite or graphite oxide.
[0023] Certain nano-scaled platelets (e.g., graphite oxides) are
hydrophilic in nature and, therefore, can be readily dispersed in
selected solvents (e.g., water). Hence, the invented process can
include an additional step of dispersing the platelets in a liquid
to form a suspension or in a monomer- or polymer-containing solvent
to form a nanocomposite precursor suspension. This suspension can
be converted to a mat or paper (e.g., by following a paper-making
process). The nanocomposite precursor suspension may be converted
to a nanocomposite solid by removing the solvent or polymerizing
the monomer. Alternatively, the platelets may be mixed with a
monomer or polymer to form a mixture, which can be converted to
obtain a nanocomposite solid. In the case of graphite oxide
platelets, the process may further include a step of partially or
totally reducing the graphite oxide (after the formation of the
suspension) to become graphite (serving to recover at least
partially the high conductivity that a pristine graphite would
have).
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 Schematic of an apparatus that can be used to produce
nano-scaled platelets such as nano-scaled graphene plates
(NGPs).
[0025] FIG. 2 Schematic of a nano platelet-producing apparatus that
can be used to produce platelets according to a preferred
embodiment of the presently invented process.
[0026] FIG. 3 Schematic of another apparatus for mass production of
nano platelets.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0027] One preferred embodiment of the present invention is a
process for mass-producing a nano-scaled graphene plate (NGP)
material that is essentially composed of a sheet of graphene plane
or multiple sheets of graphene plane stacked and bonded together.
Each graphene plane, also referred to as a graphene sheet or basal
plane, comprises a two-dimensional hexagonal structure of carbon
atoms. Each plate has a length and a width parallel to the graphite
plane and a thickness orthogonal to the graphite plane. The
thickness of an NGP is 100 nanometers (nm) or smaller. The length
and width of a NGP could exceed 1 .mu.m. Preferably, however, both
length and width are smaller than 1 .mu.m. Graphite is but one of
the many examples of laminar or layered materials that can be
exfoliated to become nano-scaled platelets. A layered inorganic
compound may be selected from (a) clay; (b) bismuth selenides or
tellurides; (c) transition metal dichalcogenides; (d) sulfides,
selenides, or tellurides of niobium, molybdenum, hafnium, tantalum,
tungsten or rhenium; (e) layered transition metal oxides; (f)
graphite or graphite derivatives; (g) pre-intercalated compounds,
or a combination thereof. For instance, both un-intercalated and
intercalated graphites are commercially available, which can be
exfoliated with the presently invented pressure reduction and
material ejection process. The presently invented process works for
all of these classes of laminar materials.
[0028] Generally speaking, a process has been developed for
exfoliating a layered or laminar material to produce nano-scaled
platelets having a thickness smaller than 100 nm. The process
comprises: (a) charging a layered material in a fine powder form to
a gaseous environment at a first temperature and first pressure
sufficient to cause gas species to penetrate into the interstitial
space between layers of the layered material, forming a
gas-intercalated layered material; and (b) operating a discharge
valve to rapidly eject the gas-intercalated layered material
through a nozzle into an exfoliation zone at a second pressure and
a second temperature, allowing gas species residing in the
interstitial space to exfoliate the layered material to produce the
platelets.
[0029] To illustrate the preferred embodiments of the presently
invented process, we begin by describing a preferred embodiment of
our co-pending application [U.S. patent application Ser. No.
11/509,424 (Aug. 25, 2006)]. As indicated in FIG. 1 (which is
identical to FIG. 1 of the co-pending application), a
platelet-producing process begins with step (a) of subjecting a
layered material to a gaseous environment. This step comprises
placing the layered material 10 in a sealed vessel 12 containing a
pressurized gas typically at a pressure greater than 1 atm and at a
first temperature, typically room temperature or slightly higher.
The vessel can be internally or externally heated to provide a
controlled first temperature. The process also includes step (b)
which comprises releasing the excess gas from the vessel to
suddenly reduce the vessel pressure and removing the
gas-intercalated material from the vessel, preferably into a
furnace or oven at a pre-set temperature, which is typically much
higher than the first temperature.
[0030] The pressurizing or intercalating gas may be supplied from a
gas cylinder 24 through a tubing 18, with the gas pressure
controlled by a gas regulator 22 and a pressure gauge 20. The gas
species can penetrate into the interstitial space between layers of
the laminar material and stay therein under a pressure. The amount
(solubility) of gas species that can reside in the interstitial
space at a given temperature increases with the increasing
pressure. After a duration of gas intercalation time, typically
from minutes to hours, the excess pressurized gas is released
(e.g., through a gas release valve 16) and the gas-intercalated
layered material is removed from the vessel (e.g., by removing the
cover 14 first). The gas-intercalated material is now at a second
pressure (e.g., 1 atm in room air), which is lower than the first
pressure of typically greater than 1 atm (typically up to 10 atm,
but could be higher). The material is quickly transferred to a
furnace pre-set at a second temperature of typically in the range
of 50.degree. C. to 1,500.degree. C., but more typically between
100.degree. C. to 500.degree. C., allowing the gas species to
exfoliate the layered material by way of pressurizing, expanding,
and escaping.
[0031] It is of great interest to note that prior art exfoliation
processes normally involve intercalating laminar materials with
liquid or solid intercalants, which are heated to pressurize the
interstitial space through vaporization as a result of chemical
decomposition or phase transition. Heating to a relatively high
temperature is absolutely required in these prior art processes to
achieve exfoliation. In contrast, as indicated in our co-pending
application, it was surprising to observe that by simply reducing
the surrounding pressure of the laminar material (containing
super-saturated gas species residing in the interlayer spaces) in
an abrupt or quick manner one could readily exfoliate layered
materials. Additional heat was not required. However, optionally
and preferably, this pressure reduction step was immediately
followed by a step to rapidly expose the gas-intercalated material
to a higher temperature. This higher temperature presumably
produced a high pressure in the interstitial space, leading to a
larger expansion ratio (final exfoliated sample thickness/original
sample thickness).
[0032] This co-pending application [U.S. patent application Ser.
No. 11/509,424 (Aug. 25, 2006)] did not address the issue of mass
production. Hence, we presented herein a semi-continuous process
that is capable of mass-manufacturing exfoliated platelets at a
relatively high production rate. The presently invented process
enables the availability of nano-scaled platelets in large
quantities and can significantly reduce the product cost. Low
production rates and high product costs have been the main
obstacles to successful commercialization of nano materials in
general and nano carbon materials (e.g., CNTs) in particular. The
presently invented process makes use of environmentally benign
gases as intercalating agents, resulting in nano-scaled platelets
containing no chemical residue.
[0033] One of the preferred embodiments of the present invention is
a process that involves intercalating and ejecting the layered
material at a relatively high temperature (but still typically
lower than 200.degree. C.) and relatively high pressure (typically
2-10 atm, but could be much higher). This process generally results
in fully separated, isolated platelets. However, with a relatively
low intercalation pressure (e.g., lower than 2 atm), the exfoliated
product may contain incompletely separated platelets. Hence, the
process may further include a step of air milling, ball milling,
mechanical attrition, and/or sonification to further separate the
platelets and/or reduce the size of the platelets.
[0034] Referring to FIG. 2, a layered material (e.g., fine graphite
powder 10) is charged into a chamber 26, which is pressurized by
gas molecules introduced from a gas cylinder 24 through a tubing
18, with the gas pressure controlled by a gas regulator 22 and a
pressure gauge 20. A pressurized gas may come from an air
compressor or other sources (e.g., vaporization of water). A cover
14 may be opened and closed for receiving the layered material
particles. The chamber temperature and pressure are such that a
sufficient amount of gas species enters the interstitial space
between layers. The maximum amount or solubility of interstitial
gas species in a layered material is proportional to both the
temperature and pressure, with pressure being a more important
factor than temperature in determining the solubility. However,
temperature appears to be the dominating factor in determining the
intercalation time.
[0035] The chamber 26 preferably has a tapered region 28 where the
diameter is decreased to a much smaller diameter at a nozzle 30.
Such a configuration allows the pressurized gas in the chamber to
rapidly propel the gas-intercalated layered material from the
intercalation chamber 26 into an exfoliation zone 34 once a valve
32 of the nozzle 30 is opened. The gas flow 36 carries the
intercalated layered material powder into the exfoliation zone
where the material experiences a sudden reduction in pressure that
puts the intercalated material in a supersaturated state. The
material undergoes an instantaneous exfoliation to become separated
nano-scaled platelets 39. The exfoliation zone may be a chamber
that has many holes to allow exhaust gas 38 to escape.
[0036] Using graphite as an example, the first step may involve
preparing a laminar material powder containing fine graphite
particulates (granules) or flakes, short segments of carbon fiber
(including graphite fiber), carbon or graphite whiskers, graphite
nano-fibers, or their mixtures. The length and/or diameter of these
graphite particles are preferably less than 0.2 mm (200 .mu.m),
further preferably less than 0.01 mm (10 .mu.m), and most
preferably smaller than 1 .mu.m. The graphite particles are known
to typically contain micron- and/or nanometer-scaled graphite
crystallites with each crystallite being composed of one sheet or
several sheets of graphite plane. Preferably, large graphite
particles are pulverized, chopped, or milled to become small
particles or short fiber segments before being charged into the
intercalation chamber 26. The reduced particle sizes facilitate
fast diffusion or migration of an intercalating gas into the
interstices between graphite planes in graphite crystallites. Other
layered compounds have similar inter-layer galleys to accommodate
intercalant gases.
[0037] The advantage of having small-sized starting materials may
be further illustrated as follows: The diffusion coefficient D of
an intercalant between two graphite planes is known to be typically
in the range of 10.sup.-12 to 5.times.10.sup.-9 cm.sup.2/sec at
room temperature [e.g., M. D. Levi and D. Aurbach, J. Phys. Chem.
B, 101 (1997) 4641-4647]. The required diffusion time .tau. to
achieve a desired diffusion path .lamda. is known to be given
approximately by .tau.=.lamda..sup.2/D. Assume that D=10.sup.-10
cm.sup.2/sec and .lamda.=100 .mu.m (graphite particle size=100
.mu.m), then the required diffusion time at room temperature will
be .tau.=(100.times.10.sup.-4 cm).sup.2/(10.sup.-10
cm.sup.2/sec)=10.sup.6 sec (277 hours or 11.5 days). This implies a
very lengthy gas intercalation time if the particle size is too
large (say, greater than 100 .mu.m). This issue may be addressed by
using two approaches: by increasing the chamber temperature and/or
decreasing the particle sizes. Specifically, the diffusion time can
be reduced if the diffusion temperature T is raised substantially
to increase the diffusion coefficient since D=D.sub.0 exp(-Q/RT),
where Q is the activation energy for the diffusion process and R is
the universal gas constant. Further, if the particle size is
.lamda.=1 .mu.m (rather than 100 .mu.m), then we have
.tau.=(1.times.10.sup.-4 cm).sup.2/(10.sup.-10 cm.sup.2/sec)=100
sec at room temperature. This is a very reasonable diffusion time,
which can be further reduced by increasing the intercalation
chamber temperature. In the worst case scenario, where D=10.sup.-12
cm.sup.2/sec (instead of 10.sup.10 cm.sup.2/sec), the required
intercalation time will be 10,000 sec=2.78 hours for graphite
particles with a lateral dimension of 1 .mu.m. Again, this
processing time of less than 3 hours can be further reduced by
increasing the pressure vessel temperature for interaction. For all
of the layered materials that we have studied so far, pure graphite
and graphite fibers have the lowest diffusion coefficients, likely
due to the smallest interlayer spacing (only 0.34 nm). Other
materials, such as clay; bismuth selenides or tellurides;
transition metal dichalcogenides; sulfides, selenides, or
tellurides of niobium, molybdenum, hafnium, tantalum, tungsten or
rhenium; layered transition metal oxides; and graphite oxides,
typically have an interlayer spacing between 0.6 and 1.5
nanometers. Hence, the diffusion coefficients of a gas species in
these laminar materials are much higher and the required
intercalation times are shorter. Furthermore, these larger
interstitial spaces can more readily accommodate larger gas
molecules.
[0038] The second step involves a phenomenon similar to the
discharge of gases from an internal combustion engine chamber. The
high pressure generated by the compressed gas and a high
temperature (typically in the range of 50.degree. C.-200.degree.
C.) serves to discharge the gas-intercalated laminar material out
of the intercalation chamber into an exfoliation zone, which is at
a much lower pressure (normally 1 atm). At such a reduced pressure,
the gas solubility in a laminar material (e.g., graphite flakes) is
much lower and the material is at a super-saturation state. Hence,
the excess gas species would want to expand or escape. The escaping
gas species appear to be capable of overcoming weak van der Waal's
forces between layers, thereby delaminating or fully separating
graphene planes in a graphite crystallite. This observation could
also be theorized as follows: When the laminar material is
subjected to a high gas pressure, gas molecules penetrate into the
interstitial spaces to the extent that the internal pressure
(inside the interstitial spaces) is balanced by the chamber
pressure of the sealed vessel. When the gas pressure surrounding
the material is suddenly reduced, the gas molecules inside the
interstitial spaces find themselves under a high pressure and
wanting to expand. This pressure is sufficient to overcome the
relatively weak van der Waal's forces between layers, producing
separated layers.
[0039] Another preferred embodiment of the present invention is a
process that entails the aforementioned step (a) and step (b), plus
a step that precedes step (a). This additional step comprises
pre-pressurizing the starting layered material in a separate
chamber hereinafter referred to as a pre-pressurization chamber (40
in FIG. 3). This chamber 40 is normally isolated from the
intercalation chamber 26 via a control valve 42. This valve is
normally closed until when it is ready to re-charge a desired
amount of layered material into the intercalation chamber 26 from
the pre-pressurization chamber 40. The two chambers may be
separately pressurized with a gas through valves 50, 46, tubings
52, 54, and regulators/gauges 20b, 20c, respectively. The gas in
the two chambers can be the same or different gas. The pressure in
the two chambers can be the same or different, but typically the
pressure in the pre-pressurization chamber is lower than that in
the intercalation chamber. The pre-pressurization chamber is
preferably equipped with a release valve 44 for safety. The two
chambers are shown to come from the same compressed gas source 24
regulated by a valve 22 and pressure gauge 20a; but they do not
have to come from the same source. This pre-pressurization step
serves to pre-intercalate the layered material to a desired extent
for the purpose of reducing the intercalation time of the laminar
material in the intercalation chamber 26. The pre-pressurization
chamber (really a pre-intercalation chamber) may be slightly heated
to accelerate the intercalation process. Each time after the
intercalation chamber 26 is emptied, a desired amount of
pre-intercalated layered material may be charged into chamber 26
from chamber 40. This amount of material is almost ready for
discharge, except for perhaps some additional heating if a higher
temperature is so desired. Such an arrangement can significantly
curtail the required interaction time in the intercalation chamber
26 and significantly reduce the total production cycle time. Such a
semi-continuous process makes it possible to mass-produce
nano-scaled platelets in large quantities at a high through-put
rate.
[0040] An optional third step includes subjecting the exfoliated
material to a mechanical attrition treatment to further reduce the
particle sizes (or fully separate those un-separated platelets) for
producing the desired nano-scaled platelets. With this treatment,
either the individual graphene planes (one-layer NGPs) or stacks of
graphene planes bonded together (multi-layer NGPs) are reduced to
become nanometer-sized in width and/or length. In addition to the
thickness dimension being nano-scaled, both the length and width of
these NGPs could be reduced to smaller than 100 nm in size if so
desired. In the thickness direction (or c-axis direction normal to
the graphene plane), there may be a small number of graphene planes
that are still bonded together through the van der Waal's forces
that commonly hold the basal planes together in a natural graphite
(when the intercalation pressure was relatively low, e.g., lower
than 2 atm). In these cases, typically, there are less than 20
layers (often less than 5 layers) of graphene planes, each with
length and width smaller than 100 nm, that constitute a multi-layer
NGP material produced after mechanical attrition.
[0041] Attrition can be achieved by pulverization, grinding,
ultrasonication, milling, etc., but the most effective method of
attrition is ball-milling. High-energy planetary ball mills were
found to be particularly effective in producing nano-scaled
graphene plates. Since ball milling is considered to be a mass
production process, the presently invented process is capable of
producing large quantities of NGP materials cost-effectively. This
is in sharp contrast to the production and purification processes
of carbon nano-tubes, which are slow and expensive.
[0042] The ball milling procedure, when down-sizing the particles,
tend to produce free radicals at peripheral edges of graphene
planes. These free radicals are inclined to rapidly react with
non-carbon elements in the environment. These non-carbon atoms may
be selected to produce desirable chemical and electronic
properties. Of particular interest is the capability of changing
the dispersibility of the resulting nano-scaled platelets in a
liquid or matrix material for the purpose of producing
nanocomposites. Non-carbon atoms typically include hydrogen,
oxygen, nitrogen, sulfur, and combinations thereof.
[0043] Another embodiment of the present invention is a process as
described above, but the pressurizing gas is produced by placing a
controlled amount of a volatile but benign liquid (liquid with a
low vaporization temperature such as water, ethanol, and methanol)
inside the vessel and implementing a heating element in the vessel
to heat and vaporize the liquid. The resulting water and/or alcohol
vapor is capable of intercalating interlayer galleries of a range
of laminar materials such as graphite oxide and transition metal
dichalcogenides. The vessel may be opened while the intercalant is
still in the vaporous state to discharge and exfoliate the
intercalated material.
[0044] In yet another preferred embodiment of the present
invention, the gas environment may comprise a supercritical fluid
(SCF). A SCF can be defined as a material that is maintained at a
temperature that exceeds a critical temperature and at a pressure
that exceeds a critical pressure so as to place the material in a
supercritical state. In such a state, the SCF has properties that
cause it to act, in effect, as both a gas and a liquid. Thus, in
the supercritical state, such a fluid has the solvent
characteristics of a liquid, but the surface tension thereof is
substantially less than that of a liquid so that the fluid can
diffuse much more readily into a solute material (i.e., a layered
material in the present context). For example, carbon dioxide
(CO.sub.2) can be transformed into a supercritical state when its
temperature exceeds 31.degree. C. and its pressure exceeds 74.83
atm (1,100 psi). The thermodynamic and physical properties of a SCF
vary with pressure. Some representative characteristics include
diffusivity, density, dynamic viscosity, cohesive energy density,
heat capacity, and thermal conductivity. The diffusivity and
dynamic viscosity of a SCF may be adjusted to maximize the
intercalation rate of the fluid into the interstitial space of a
layered material.
[0045] Carbon materials can assume an essentially amorphous
structure (glassy carbon), a highly organized crystal (graphite),
or a whole range of intermediate structures that are characterized
in that various proportions and sizes of graphite crystallites and
defects are dispersed in an amorphous matrix. Typically, a graphite
crystallite is composed of a number of graphene plates (sheets of
graphene planes or basal planes) that are bonded together through
van der Waals forces in the c-direction, the direction
perpendicular to the basal plane. These graphite crystallites are
typically micron- or nanometer-sized. The graphite crystallites are
dispersed in or connected by crystal defects or an amorphous phase
in a graphite particle, which can be a graphite flake,
carbon/graphite fiber segment, or carbon/graphite whisker or
nano-fiber. In the case of a carbon or graphite fiber segment, the
graphene plates may be a part of a characteristic "turbostratic
structure."
[0046] When a graphite flake sample is sealed in a vessel
containing oxygen at an elevated temperature (200-500.degree. C.),
chemical reactions between oxygen and graphite could occur,
resulting in the formation of a partially oxidized graphite or
graphite oxide. By releasing the pressurizing oxygen gas, one
obtains well-exfoliated graphite oxide platelets. This example
illustrates the potential of permitting a pressurizing gas to take
part in a benign chemical reaction to produce an exfoliated product
that is chemically different from the starting laminar
material.
[0047] In prior art processes, for the purpose of exfoliating
graphene plane layers, the chemical treatment of the graphite
powder involves subjecting particles of a wide range of sizes to a
chemical solution for periods of time ranging from about one minute
to about 48 hours. The chemical solution was selected from a
variety of oxidizing solutions maintained at temperatures ranging
from about room temperature to about 125.degree. C. Commonly used
intercalation compounds are H.sub.2SO.sub.4, HNO.sub.3,
KM.sub.nO.sub.4, and F.sub.eCl.sub.3, ranging from about 0.1 normal
to concentrated strengths. These strong acids are undesirable and,
hence, the resulting exfoliated material has to be thoroughly
washed, which is an expensive and lengthy process. In contrast, the
presently invented process involves the utilization of
environmentally benign intercalation gases such as hydrogen, inert
gases, or oxygen. No potentially hazardous chemical is required and
no intercalation compound remains in the resulting platelets as a
chemical residue.
[0048] Morrison, et al. [U.S. Pat. No. 4,822,590, Apr. 18, 1989]
have disclosed a method of preparing single-layer materials of the
form MX.sub.2, where MX.sub.2 is a transition metal layer-type
dichalcogenide such as MoS.sub.2, TaS.sub.2, WS.sub.2, and the
like. The process involved intercalating the MX.sub.2 with an
alkali metal (e.g., lithium or sodium) in a strictly dry
environment for a sufficient length of time to enable the lithium
or sodium to substantially intercalate the MX.sub.2. The lithium-
or sodium-intercalated MX.sub.2 is then immersed in water. The
water reacts with the intercalated lithium or sodium and forms
hydrogen gas between the layers of MX.sub.2. The pressure of the
evolved hydrogen gas causes the layers of MX.sub.2 to exfoliate
into single layers. This single layer MX.sub.2 material may be
useful as a coating and a lubricant. However, pure lithium and
sodium must be handled with extreme care in an absolutely dry
environment. With a melting point of 180.7.degree. C., lithium will
have to be intercalated into MX.sub.2 at a high temperature in a
completely water-free environment, which is not very conducive to
mass production of exfoliated products. In contrast, the presently
invented process does not involve a highly explosive chemical or a
violent chemical reaction such as
2Li+2H.sub.2O.fwdarw.H.sub.2+2Li.sup.++2OH.sup.-.
[0049] Once the nano platelets are produced, the platelets may be
subjected to further treatments to prepare useful products. For
instance, the platelets (e.g., graphene platelets) may be
re-compressed into a sheet-like structure commonly referred to as
flexible graphite. Additionally, the platelets may be dispersed in
a liquid to form a suspension or in a monomer- or
polymer-containing solvent to form a nanocomposite precursor
suspension. The process may include a step of converting the
suspension to a mat or paper, or converting the nanocomposite
precursor suspension to a nanocomposite solid. If the platelets in
a suspension comprise graphite oxide platelets, the process may
further include a step of partially or totally reducing the
graphite oxide after the formation of the suspension.
[0050] Alternatively, the resulting platelets may be mixed with a
monomer to form a mixture, which can be polymerized to obtain a
nanocomposite solid. The platelets can be mixed with a polymer melt
to form a mixture that is subsequently solidified to become a
nanocomposite solid.
Example 1
Nano-Scaled Graphene Platelets (NGPs) from Graphite Flakes
[0051] One hundred grams of natural graphite flakes ground to an
average size of approximately 2.3 .mu.m were sealed in a helium
gas-filled steel container (the intercalation chamber schematically
shown in FIG. 2) at 150.degree. C. and 16 atm for 20 minutes to
yield the desired gas-intercalated graphite (GIG). Subsequently,
the exit valve was opened to expel the GIG out of the chamber. The
GIG was exfoliated to a good extent with an expansion ratio
(exfoliated flake thickness/GIG flake thickness) of approximately
12/1 to 26/1. The thickness of individual platelets was found to
range from single graphene sheet to approximately 20 graphene
sheets. A small portion of the exfoliated graphite particles were
then ball-milled in a high-energy plenary ball mill machine for 24
hours to produce nano-scaled particles with reduced length and
width (now 0.5-2 .mu.m).
Example 2
NGPs from Short Carbon Fibers
[0052] The procedure was similar to that used in Example 1, but the
starting material was carbon fibers chopped into segments with 0.2
mm or smaller in length prior to the gas intercalation treatment.
The diameter of carbon fibers was approximately 12 .mu.m. An
exfoliation ratio of 6/1-8/1 was observed after being discharged
from the exit nozzle.
Example 3
NGPs from Graphitic Nano-Fibers (GNFs)
[0053] A powder sample of graphitic nano-fibers was prepared by
introducing an ethylene gas through a quartz tube pre-set at a
temperature of approximately 800.degree. C. A small amount of
Cu--Ni powder was positioned on a crucible to serve as a catalyst,
which promoted the decomposition of the hydrocarbon gas and growth
of GNFs. Approximately 2.5 grams of GNFs (diameter of 10 to 80 nm)
were intercalated with a mixture of hydrogen and helium gases at
approximately 12 atm and 150.degree. C. for 30 minutes and then to
200.degree. C. at 10 atm for additional 10 minutes. After discharge
from the chamber, the intercalated particles were found to be
exfoliated to a great extent (without an expansion ratio
measurement). A large number of single- or double-layer graphene
discs of 10-80 nm in diameter and 0.34 or 0.68 nm in thickness were
obtained.
Example 4
Synthesis of Molybdenum Diselenide Nanostructured Materials
[0054] The same sequence of steps was utilized to form nano
platelets from other layered compounds: gas intercalation and
exfoliation, followed by milling, attrition, or sonication. For
instance, MoSe.sub.2 consisting of Se--Mo--Se layers held together
by weak van der Waals forces can be exfoliated via the presently
invented process. Dichalcogenides, such as MoS.sub.2, have found
applications as electrodes in lithium ion batteries and as
hydro-desulfurization catalysts. Intercalation was achieved by
placing Mo Se.sub.2 powder in a sealed chamber with pressurized
oxygen gas, allowing oxygen to intercalate into the van der Waals
gap between Se--Mo--Se sheets at 250.degree. C. and 8 atm for less
than 10 minutes. After discharge from the exit nozzle, the
resulting MoSe.sub.2 platelets were found to have a thickness in
the range of approximately 1.4 nm to 13.5 nm with most of the
platelets being mono-layers or double layers.
[0055] Other single-layer platelets of the form MX.sub.2
(transition metal dichalcogenide), including MoS.sub.2, TaS.sub.2,
and WS.sub.2, were similarly intercalated and exfoliated, with the
intercalation pressure varied between 6 and 8 atm. Again, most of
the platelets were mono-layers or double layers. This observation
clearly demonstrates the versatility of the presently invented
process in terms of producing relatively uniform-thickness
platelets.
Example 5
Graphite Oxide Nano Platelets and Their Nanocomposites
[0056] Graphite oxide was prepared by oxidation of graphite flakes
with KM.sub.nO.sub.4/H.sub.2SO.sub.4 followed by a chemical removal
step according to the method of Lerf, et al. [J. Phys. Chem., B 102
(1998) 4477-4482]. The interlayer spacing of the resulting laminar
graphite oxide was determined by the Debey-Scherrer X-ray technique
to be approximately 0.73 nm (7.3 .ANG.), which was found to be
conducive to the intercalation by larger gas species such as oxygen
and nitrogen molecules.
[0057] Selected samples of graphite oxide (particle sizes of
approximately 4.2 .mu.m) were sealed in an oxygen-filled chamber at
a pressure of approximately 8 atm and 200.degree. C. for 30
minutes. The chamber was then isolated from the gas-supplying
cylinder with the exit valve being opened to propel the material
out of the intercalation chamber. Well exfoliated and separated
graphite oxide nano platelets were obtained.
[0058] When mixed with water and subjected to a mild ultrasonic
treatment after mixing, these nano platelets were well-dispersed in
water, forming a stable water dispersion (suspension). Upon removal
of water, the nano platelets settled to form an ultra-thin
nano-carbon film. Depending upon the volume fraction of nano
platelets, the film could be as thin as one to ten graphite oxide
layers (approximately 0.73 nm to 7.3 nm).
[0059] A small amount of water-soluble polymer (e.g., poly vinyl
alcohol) was added to the nano platelet-water suspension with the
polymer dissolved in water. The resulting nano platelet suspension
with polymer-water solution as the dispersing medium was also very
stable. Upon removal of water, polymer was precipitated out to form
a thin coating on nano platelets. The resulting structure is a
graphite oxide reinforced polymer nanocomposite.
[0060] A small amount of the nano platelet-water suspension was
reduced with hydrazine hydrate at 100.degree. C. for 24 hours. As
the reduction process progressed, the brown-colored suspension of
graphite oxides turned black, which appeared to become essentially
graphite nano platelets or NGPs.
Example 6
Clay Nano Platelets and Composites
[0061] Bentolite-L, hydrated aluminum silicate (bentonite clay) was
obtained from Southern Clay Products. Bentolite clay (5 g) was
subjected to intercalation by argon gas at 8 atm and 150.degree. C.
for 30 minutes. Exfoliation was achieved by rapidly discharging the
intercalated material from the interaction chamber into a chamber
with an open air environment at room temperature. The resulting
clay nano platelets have a thickness in the range of approximately
1 to 25 nm.
[0062] Subsequently, melt mixing was used to prepare nanocomposite
samples. The amounts of clay and epoxy were 0.1 g, and 0.9 g,
respectively. The mixture was manually stirred for 30 min. When
stirring, the sample was actually sheared or "kneaded" with a
spatula or a pestle. A well dispersed clay nano platelet-based
composite was obtained.
* * * * *