WO2017053089A1 - Film monolithique de graphène halogéné intégré hautement orienté - Google Patents

Film monolithique de graphène halogéné intégré hautement orienté Download PDF

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WO2017053089A1
WO2017053089A1 PCT/US2016/051058 US2016051058W WO2017053089A1 WO 2017053089 A1 WO2017053089 A1 WO 2017053089A1 US 2016051058 W US2016051058 W US 2016051058W WO 2017053089 A1 WO2017053089 A1 WO 2017053089A1
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graphene
graphene oxide
layer
halogenated
sheets
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PCT/US2016/051058
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English (en)
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Aruna Zhamu
Bor Z. Jang
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Nanotek Instruments, Inc.
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Priority claimed from US14/756,591 external-priority patent/US9809459B2/en
Priority claimed from US14/756,592 external-priority patent/US10553357B2/en
Application filed by Nanotek Instruments, Inc. filed Critical Nanotek Instruments, Inc.
Priority to CN201680055565.7A priority Critical patent/CN108137415B/zh
Priority to JP2018514865A priority patent/JP7050667B2/ja
Priority to KR1020187011426A priority patent/KR102399147B1/ko
Publication of WO2017053089A1 publication Critical patent/WO2017053089A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
    • C04B35/52—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbon, e.g. graphite
    • C04B35/522—Graphite
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C01B32/184—Preparation
    • C01B32/19—Preparation by exfoliation
    • C01B32/192—Preparation by exfoliation starting from graphitic oxides
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C01B32/194—After-treatment
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/20—Graphite
    • C01B32/21—After-treatment
    • C01B32/23—Oxidation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G4/00—Fixed capacitors; Processes of their manufacture
    • H01G4/002—Details
    • H01G4/018—Dielectrics
    • H01G4/06—Solid dielectrics
    • H01G4/08—Inorganic dielectrics
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2204/00—Structure or properties of graphene
    • C01B2204/04—Specific amount of layers or specific thickness
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2204/00—Structure or properties of graphene
    • C01B2204/20—Graphene characterized by its properties
    • C01B2204/22—Electronic properties
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
    • C04B2235/72—Products characterised by the absence or the low content of specific components, e.g. alkali metal free alumina ceramics
    • C04B2235/723—Oxygen content
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
    • C04B2235/72—Products characterised by the absence or the low content of specific components, e.g. alkali metal free alumina ceramics
    • C04B2235/724—Halogenide content
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G4/00—Fixed capacitors; Processes of their manufacture
    • H01G4/002—Details
    • H01G4/018—Dielectrics
    • H01G4/06—Solid dielectrics

Definitions

  • the present invention relates generally to the field of graphene materials and, more particularly, to a new form of halogenated graphene film composed of originally separated multiple halogenated graphene sheets or molecules that are oriented and chemically merged and integrated together to form a monolithic integral layer.
  • Carbon is known to have five unique crystalline structures, including diamond, fullerene (0-D nano graphitic material), carbon nano-tube or carbon nano-fiber (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material).
  • the carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall.
  • Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs) have a diameter on the order of a few nanometers to a few hundred nanometers.
  • Their longitudinal, hollow structures impart unique mechanical, electrical and chemical properties to the material.
  • the CNT or CNF is a one- dimensional nano carbon or 1-D nano graphite material.
  • the constituent graphene planes of a graphite crystallite in a natural or artificial graphite particle can be exfoliated and extracted or isolated to obtain individual graphene sheets of carbon atoms provided the inter-planar van der Waals forces can be overcome.
  • An isolated, individual graphene sheet of carbon atoms is commonly referred to as single-layer graphene.
  • a stack of multiple graphene planes bonded through van der Waals forces in the thickness direction with an inter-graphene plane spacing of approximately 0.3354 nm is commonly referred to as a multilayer graphene.
  • a multi-layer graphene platelet has up to 300 layers of graphene planes ( ⁇ 100 nm in thickness), but more typically up to 30 graphene planes ( ⁇ 10 nm in thickness), even more typically up to 20 graphene planes ( ⁇ 7 nm in thickness), and most typically up to 10 graphene planes (commonly referred to as few-layer graphene in scientific community).
  • Single-layer graphene and multi-layer graphene or graphene oxide sheets are collectively called “nano graphene platelets" (NGPs).
  • NGPs are a new class of carbon nano material (a 2-D nano carbon) that is distinct from the 0-D fullerene, the 1-D CNT, and the 3-D graphite.
  • Isolated or separated graphene or graphene oxide sheets are typically obtained by intercalating natural graphite particles with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound (GIC) or graphite oxide (GO), as illustrated in FIG. 5(A) (process flow chart) and FIG. 5(B) (schematic drawing).
  • GIC graphite intercalation compound
  • GO graphite oxide
  • FIG. 5(A) process flow chart
  • FIG. 5(B) schematic drawing.
  • the presence of chemical species or functional groups in the interstitial spaces between graphene planes serves to increase the inter- graphene spacing (d 02 , as determined by X-ray diffraction), thereby significantly reducing the van der Waals forces that otherwise hold graphene planes together along the c-axis direction.
  • the GIC or GO is most often produced by immersing natural graphite powder (20 in FIG.
  • Route 1 involves removing water from the suspension to obtain "expandable graphite,” which is essentially a mass of dried GIC or dried graphite oxide particles.
  • expandable graphite essentially a mass of dried GIC or dried graphite oxide particles.
  • the GIC undergoes a rapid volume expansion by a factor of 30-300 to form “graphite worms" (24 or 104), which are each a collection of exfoliated, but largely un- separated graphite flakes that remain interconnected.
  • graphite worms 24 or 104
  • these graphite worms can be re-compressed to obtain flexible graphite sheets or foils (26 or 106) that typically have a thickness in the range of 0.1 mm (100 ⁇ ) - 0.5 mm (500 ⁇ ).
  • flexible graphite sheets or foils 26 or 106
  • Exfoliated graphite worms, expanded graphite flakes, and the recompressed mass of graphite worms are all 3-D graphitic materials that are fundamentally different and patently distinct from either the 1-D nano carbon material (CNT or CNF) or the 2-D nano carbon material (graphene sheets or platelets, NGPs).
  • Flexible graphite (FG) foils can be used as a heat spreader material, but exhibiting a maximum in-plane thermal conductivity of typically less than 500 W/mK (more typically ⁇ 300 W/mK) and in-plane electrical conductivity no greater than 1,500 S/cm.
  • the exfoliated graphite is subjected to high-intensity mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill) to form separated single-layer and multi-layer graphene sheets (collectively called NGPs, 33 or 112), as disclosed in our US Application No. 10/858,814.
  • Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm, but more typically less than 20 nm.
  • Route 2 entails ultrasoni eating the graphite oxide suspension for the purpose of separating/isolating individual graphene oxide sheets from graphite oxide particles.
  • graphene or NGPs include discrete (isolated or separated) sheets/platelets of single-layer and multi-layer pristine graphene, graphene oxide, or reduced graphene oxide (RGO).
  • Pristine graphene has essentially 0% oxygen.
  • RGO typically has an oxygen content of 0.001%-5% by weight.
  • Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.
  • Isolated solid NGPs i.e. discrete and separate sheets/platelets of pristine graphene, GO, and RGO having a typical length/width of 100 nm to 10 ⁇
  • a paper-making process typically do not exhibit a high thermal conductivity, as an example of useful physical properties.
  • these sheets/platelets are typically poorly oriented and many types of defects are formed in the film/membrane/paper.
  • a paper-like structure or mat made from platelets of graphene, GO, or RGO e.g.
  • those paper sheets prepared by vacuum-assisted filtration process exhibit many defects, wrinkled or folded graphene sheets, interruptions or gaps between platelets, and non-parallel platelets (e.g. SEM image in FIG. 7(B)), leading to relatively poor thermal conductivity, low electric conductivity, low dielectric breakdown strength, and low structural strength.
  • These paper-like structures or aggregates of discrete NGP, GO or RGO platelets alone (without a resin binder) also have a tendency to get flaky, emitting conductive particles into air; but the presence of a binder resin significantly reduces the conductivity of the structure.
  • Graphene thin films ( ⁇ 5 nm and most typically ⁇ 2 nm) can be prepared by catalytic chemical vapor deposition CVD of hydrocarbon gas (e.g. C 2 H 4 ) on Ni or Cu surface.
  • hydrocarbon gas e.g. C 2 H 4
  • Ni or Cu being the catalyst
  • carbon atoms obtained via decomposition of hydrocarbon gas molecules at 800-l,000°C are deposited onto Ni or Cu foil surface to form a sheet of single-layer or few-layer graphene (2-5 layers in this case) that is poly-crystalline.
  • These ultra-thin graphene films being optically transparent and electrically conducting, are intended for applications such as the touch screen (to replace indium-tin oxide or ITO glass) or semiconductor devices (to replace silicon, Si).
  • the Ni- or Cu-catalyzed CVD process does not lend itself to the deposition of more than 5- 10 graphene planes (typically ⁇ 5 nm and more typically ⁇ 2 nm) beyond which the underlying Ni or Cu catalyst can no longer provide any catalytic effect. There has been no experimental evidence to indicate that CVD graphene film thicker than 5 nm is possible. Furthermore, the CVD process is known to be extremely expensive.
  • multi-layer graphene sheets are a metallic or conductor material and single-layer graphene sheets are a semi-metal.
  • the single-layer pristine graphene lacks an energy band gap because its valence and conduction bands touch each other and, hence, it is labeled as a semimetal.
  • Si a semiconductor, has an energy band gap of 1.1 eV between the conduction band and valence band of its electronic configuration.
  • the lack of a band gap limits usage of graphene in contemporary electronic devices.
  • the band structure of single-layer graphene can be modified to open the band gap by many strategies, e.g., halogenation, oxidation, hydrogenation or noncovalent attachment of various molecules and species.
  • processing temperature usually exceeds 1,000°C), high density, and low breakdown strength.
  • polymers are more applicable in higher electric fields.
  • polymers possess the following advantages over inorganic ceramics: low weight, low cost, ease in processing, and self-healing.
  • low operation temperatures restrict the further development of polymer dielectrics.
  • Commercial capacitors are only used in limited applications such as cell phones, video/audio systems, and personal computers. For example, biaxial oriented polypropylene polymer-based capacitors can only be operated at temperatures below 105°C.
  • halogenated graphene is a group of graphene derivatives, in which some carbon atoms are covalently linked with halogen atoms. The carbon atoms linked with halogens have sp 3 hybridization and other carbon atoms have sp 2
  • halogenated graphene also referred to as graphene halide
  • thicker graphene halide films > 10 nm, preferably > 100 nm, further preferably > 1 ⁇ , and more preferably > 10 ⁇
  • ultra-thin films e.g. « 10 nm
  • thicker graphene fluoride films having a combination of desired physical and chemical properties have not been available.
  • the present invention provides a process for producing an integrated layer of highly oriented halogenated graphene sheets or molecules, wherein the film has a thickness from 10 nm to 500 ⁇ .
  • the process comprises: (a) preparing either a graphene oxide dispersion having graphene oxide sheets dispersed in a fluid medium or a graphene oxide gel having graphene oxide molecules dissolved in a fluid medium, wherein the graphene oxide sheets or graphene oxide molecules contain an oxygen amount higher than 5 % by weight; (b) dispensing and depositing a layer of graphene oxide dispersion or graphene oxide gel onto a surface of a supporting substrate under a shear stress condition, wherein the dispensing and depositing procedure includes shear-induced thinning of graphene oxide dispersion or gel and shear-induced orientation of graphene oxide sheets or molecules, to form a wet layer of graphene oxide on the supporting substrate; (c) either (i) introducing a halogenating agent into the wet layer of graph
  • inter-planar spacing range of 0.350-1.0 nm is due to the presence of halogen elements or halogen-containing chemical groups (plus some residual O or O-containing groups in some cases) on graphene planes that push the neighboring planes apart.
  • the halogenating agent can be introduced before, during, or after removal of the liquid medium from the wet layer of GO.
  • the graphene halide sheets in the dried integrated layer of graphene halide are substantially parallel to one another along one direction and the average deviation angle of these sheets is less than 10 degrees.
  • graphene sheets or platelets are inclined with respect to one another at a very large angle (e.g. mis-orientation of 20-40 degrees).
  • the average deviation angle from the desired orientation angle is greater than 10°, more typically > 20°, and often > 30°.
  • step (b) can occur during or after step (b).
  • graphene halide sheets in the dried integrated layer of graphene halide are substantially parallel to one another along one direction and the average deviation angle of these sheets is less than 10 degrees.
  • the integrated layer has multiple constituent graphene halide planes that are substantially parallel to one another along one direction having an average deviation angle of these graphene halide planes less than 10 degrees (more typically less than 5 degrees.
  • the starting graphene oxide sheets contain single-layer graphene oxide or few-layer graphene oxide sheets each having 2-10 oxidized graphene planes.
  • the fluid medium can be water, an alcohol, a mixture of water and alcohol, or an organic solvent.
  • the fluorinating agent preferably contains a chemical species in a liquid, gaseous, or plasma state containing a halogen element selected from F, CI, Br, I, or a combination thereof.
  • the fluorinating agent contains a chemical species in a liquid, gaseous, or plasma state containing a halogen element selected from F, CI, Br, I, or a combination thereof.
  • the fluorinating agent is selected from hydrofluoric acid, hexafluorophosphoric acid or HPF 6 , XeF 2 , F 2 gas, F 2 /Ar plasma, CF 4 plasma, SF 6 plasma, HC1, HPCle, XeCl 2 , Cl 2 gas, Cl 2 /Ar plasma, CC1 4 plasma, SC1 6 plasma, HBr, XeBr 2 , Br 2 gas, Br 2 /Ar plasma, CBr 4 plasma, SBr 6 plasma, HI, Xel 2 , 1 2 , 1 2 /Ar plasma, CI 4 plasma, S3 ⁇ 4 plasma, or a combination thereof.
  • the step of dispensing and depositing can include a printing, spraying, coating and/or casting procedure, which is in combination with a shear stress procedure.
  • the coating process can include a slot die coating or comma coating procedure. More preferably, the dispensing and depositing step includes a reverse roll transfer coating procedure.
  • the step of dispensing and depositing includes dispensing the layer of graphene oxide dispersion or graphene oxide gel onto a surface of an application roller rotating in a first direction at a first line velocity to form an applicator layer of graphene oxide, wherein the application roller transfers the applicator layer of graphene oxide to a surface of a supporting film driven in a second direction opposite to the first direction at a second line velocity, to form the wet layer of graphene oxide on the supporting film.
  • the supporting film may be driven by a counter-rotating supporting roller disposed at a working distance from the application roller and rotating in the second direction opposite to the first direction.
  • the velocity ratio defined as (said second line velocity )/(said first line velocity), is preferably from 1/5 to 5/1, more preferably is greater than 1/1 and less than 5/1. If the external surface of the application roller moves at the same speed as the linear movement speed of the supporting film, then the velocity ratio is 1/1 or unity. If, as an example, the external surface of the application roller moves at a speed three times as fast as the linear movement speed of the supporting film, then the velocity ratio is 3/1. In certain embodiments, the velocity ratio is greater than 1/1 and less than 5/1. Preferably, the velocity ratio is greater than 1/1 and up to 3/1.
  • the step of dispensing graphene oxide dispersion or graphene oxide gel onto the surface of the application roller includes using a metering roller and/or a doctor's blade to provide a desired thickness of the applicator layer of graphene oxide on the application roller surface.
  • the process can include operating 2, 3, or 4 rollers.
  • the supporting film is driven by a counter-rotating supporting roller disposed at a working distance from the application roller and rotating in the second direction opposite to the first direction. The speed at the external surface of this supporting roller dictates the second line velocity (of the supporting film).
  • the supporting film is fed from a feeder roller and the dried layer of graphene halide supported by the supporting film is wound on a winding roller and the process is conducted in a roll-to-roll manner.
  • the invented process further comprises a step of aging the wet layer of graphene oxide after step (b), aging the wet layer of halogenated graphene after step (c), or aging the integrated layer of halogenated graphene after step (d), in an aging room at an aging temperature from 25°C to 100°C and humidity level from 20% to 99% for an aging time of 1 hour to 7 days.
  • the process may further comprise a compression step, during or after said step (d), to reduce a thickness of said integrated layer.
  • the process may further comprise a step (e) of heat treating the integrated layer of oriented halogenated graphene at a first heat treatment temperature higher than 100°C but no greater than 3,200°C for a desired length of time to produce a graphitic film having an inter- planar spacing d 0 02 less than 0.4 nm and a combined oxygen/halogen content less than 1% by weight.
  • the process may further comprise a compression step, during or after the heat treatment step, to reduce a thickness of the graphitic film.
  • the graphene oxide sheets in the graphene oxide dispersion preferably occupy a weight fraction of 0.1% to 25% based on the total weight of graphene oxide sheets and liquid medium combined. More preferably, the graphene oxide sheets in the graphene oxide dispersion occupy a weight fraction of 0.5% to 15%. In some embodiments, graphene oxide sheets occupy a weight proportion from 3% to 15% based on the total weight of graphene oxide sheets and liquid medium combined. In certain embodiments, the graphene oxide dispersion or graphene oxide gel has greater than 3% by weight of graphene oxide dispersed in the fluid medium to form a liquid crystal phase.
  • the graphene oxide dispersion or graphene oxide gel may be prepared by immersing a graphitic material in a powder or fibrous form in an oxidizing liquid in a reaction vessel at a reaction temperature for a length of time sufficient to obtain said graphene oxide dispersion or said graphene oxide gel wherein said graphitic material is selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.
  • the graphene oxide dispersion or graphene oxide gel may be obtained from a graphitic material having a maximum original graphite grain size and the resulting halogenated graphene film is a poly-crystal graphene structure having a grain size larger than this maximum original grain size.
  • This larger grain size is due to the notion that heat-treating of GO sheets, GO molecules, halogenated molecules, or halogenate sheets induces chemical linking, merging, or chemical bonding of graphene oxide/halide sheets or graphene oxide/halide molecules in an edge-to-edge manner. It may be noted that such an edge-to-edge linking significantly increases the length or width of graphene sheets or molecules.
  • a graphene halide sheet 300 nm in length if merged with a graphene halide sheet 400 nm in length could result in a sheet of approximately 700 nm in length.
  • Such an edge-to-edge merging of multiple graphene halide sheets enables production of graphene films having huge grain sizes that could not be obtained otherwise.
  • the graphene oxide dispersion or graphene oxide gel is obtained from a graphitic material having multiple graphite crystallites exhibiting no preferred crystalline orientation as determined by an X-ray diffraction or electron diffraction method and the resulting halogenated graphene film is a single crystal or a poly-crystal graphene structure having a preferred crystalline orientation as determined by said X-ray diffraction or electron diffraction method.
  • All the coating procedures capable of inducing shear stresses to the GO sheets or halogenated GO sheets may be implemented in the presently invented process; e.g. slot-die coating, comma coating, and reverse roll transfer coating.
  • the reverse roll procedure is particularly effective in enabling the GO sheets or GO molecules to align themselves along a particular direction (e.g. X-direction or length-direction) or two particular directions (e.g. X- and Y-directions or length and width directions) to produce preferred orientations. Further surprisingly, these preferred orientations are preserved and often further enhanced during the subsequent heat treatment of the GO layers.
  • such preferred orientations are essential to the eventual attainment of exceptionally high dielectric breakdown strength, dielectric constants, elastic modulus, and tensile strength of the resulting halogenated graphene film (even for thick films; e.g. from 10 nm to even > 500 ⁇ ) along a desired direction.
  • the thickness of the coated or cast films (layers) cannot be too high (e.g. greater than 50 ⁇ ), otherwise a high degree of GO or halogenated sheet orientation cannot be achieved.
  • the coated or cast films must be sufficiently thin so that when they become dried, they form a dried layer of graphene oxide having a thickness no greater than 50 ⁇ , more typically no greater than 20 ⁇ , and most typically no greater than 10 ⁇ .
  • the integrated layer of oriented halogenated graphene herein produced typically has a dielectric constant greater than 4.0 (more typically greater than 5.0, often greater than 10, or even greater than 15), an electrical resistivity typically from 10 8 ⁇ -cm to 10 15 ⁇ -cm, and/or a dielectric breakdown strength greater than 5 MV/cm (more typically greater than 10 MV/cm, some greater than 12 MV/cm, and others even greater than 15 MV/cm) when measured at a layer thickness of 100 nm.
  • the present invention also provides a microelectronic device containing the integrated layer of halogenated graphene as a dielectric component.
  • This new class of materials i.e., highly oriented GO-derived graphene halide films, GOGH
  • This GOGH film is an integrated graphene halide (GH) entity that is a poly-crystalline structure composed of well-aligned, interconnected multiple grains with exceptionally large grain sizes.
  • the HOGH has all the graphene planes in all the grains being essentially oriented parallel to one another (i.e., the crystallographic c-axis of all grains essentially pointing in an identical direction).
  • the GOGH is an integrated graphene entity that is not a simple aggregate or stack of multiple discrete platelets of graphene/GO/RGO/GH (graphene halide), and does not contain any discernible or discrete flake/platelet derived from the original GO sheets. These originally discrete flakes or platelets have been chemically bonded or linked together to form larger grains (grain size being larger than the original platelet/flake size).
  • This GOGH is not made by using a binder or adhesive to glue discrete flakes or platelets together. Instead, under select aging or heat treatment conditions, well-aligned GO/GH sheets or GO/GH molecules are capable of chemically merging with one another mainly in an edge-to-edge manner to form giant 2-D graphene grains, but possibly also with adjacent GO/GH sheets below or above to form 3-D networks of graphene chains.
  • the GO/GH sheets are adhered into an integrated graphene entity, without using any externally added linker or binder molecules or polymers.
  • crystallographic c-axis is derived from GO, which is in turn obtained from moderate or heavy oxidation of natural graphite or artificial graphite particles each originally having multiple graphite crystallites that are randomly oriented.
  • GO dispersion moderate-to-heavy oxidation of graphite
  • GO gel graphite gel
  • these starting or original graphite crystallites Prior to being chemically oxidized to become GO dispersion (moderate-to-heavy oxidation of graphite) or GO gel (heavy oxidation for a sufficiently long oxidation time to achieve fully separated GO molecules dissolved in water or other polar liquid), these starting or original graphite crystallites have an initial length (L a in the crystallographic a-axis direction), initial width (J b in the b-axis direction), and thickness (L c in the c-axis direction).
  • the resulting GOGH typically has a length or width significantly greater than the L a and L b of the original graphit
  • This process for producing a monolithic, integrated layer of highly oriented GH can be conducted on a continuous roll-to-roll basis and, hence, is a scalable, cost-effective process.
  • FIG.1 Schematic of a reverse roll-based GO/GH layer transfer apparatus for producing highly oriented GO/GH films.
  • FIG.2 Schematic of another reverse roll-based GO/GH layer transfer apparatus for producing highly oriented GO/GH films.
  • FIG.3 Schematic of yet another reverse roll-based GO/GH layer transfer apparatus for
  • FIG.4 Schematic of still another reverse roll-based GO/GH layer transfer apparatus for
  • FIG.5(A) A flow chart illustrating various prior art processes of producing exfoliated graphite products (flexible graphite foils and flexible graphite composites) and pyrolytic graphite
  • FIG.5(B) Schematic drawing illustrating the processes for producing conventional paper, mat, film, and membrane of simply aggregated graphite or NGP flakes/platelets. All processes begin with intercalation and/or oxidation treatment of graphitic materials (e.g. natural graphite particles).
  • graphitic materials e.g. natural graphite particles
  • FIG.6(A) A SEM image of a graphite worm sample after thermal exfoliation of graphite
  • GICs intercalation compounds
  • graphite oxide powders graphite oxide powders
  • FIG.6(B) An SEM image of a cross-section of a flexible graphite foil, showing many graphite flakes with orientations not parallel to the flexible graphite foil surface and also showing many defects, kinked or folded flakes.
  • FIG.7(A) A SEM image of a GO-derived film, wherein multiple graphene planes (having an initial length/width of 30 nm - 300 nm in original graphite particles) have been oxidized, exfoliated, re-oriented, and seamlessly merged into continuous-length graphene sheets or layers that can run for tens of centimeters wide or long (only a 50 ⁇ width of a 10-cm wide graphitic film being shown in this SEM image).
  • FIG.7(B) A SEM image of a cross-section of a conventional graphene paper/film prepared from discrete graphene sheets/ platelets using a paper-making process (e.g. vacuum-assisted filtration).
  • the image shows many discrete graphene sheets being folded or interrupted (not integrated), with orientations not parallel to the film/paper surface and having many defects or imperfections.
  • FIG.7(C) Schematic drawing and an attendant SEM image to illustrate the formation process of a HOGF that is composed of multiple graphene planes that are parallel to one another and are chemically bonded in the thickness-direction or crystallographic c-axis direction.
  • FIG.7(D) One plausible chemical linking mechanism (only 2 GO molecules are shown as an example; a large number of GO molecules can be chemically linked together to form a graphene layer).
  • FIG.8 Dielectric breakdown strength of GO film, GO-derived graphene fluoride film (made by the reverse roll transfer coating process), and polyimide film plotted as a function of the film thickness.
  • FIG.9 Dielectric breakdown strength of GO-derived fluorinated graphene films and chlorinated graphene films (both prepared by a reverse roll transfer procedure and a casting procedure) plotted as a function of the degree of fluorination (atomic ratio, F/(F+0)) or the degree of chlorination (atomic ratio, Cl/(Cl+0)).
  • FIG.10 Dielectric breakdown strength of GO-derived fluorinated graphene films (prepared by a reverse roll transfer procedure) and GO-derived fluorinated graphene paper prepared by the conventional paper-making procedure (vacuum-assisted filtration) plotted as a function of the degree of fluorination in terms of the atomic ratio, F/(F+0).
  • FIG.11 Dielectric constants of GO-derived fluorinated graphene films and brominated graphene films plotted as a function of the degree of fluorination (atomic ratio, F/(F+0)) or the degree of bromination (atomic ratio, Br/(Br+0)).
  • the preparation of this integrated layer begins with graphene oxide (GO) in a suspension (dispersion) or gel form.
  • the process begins with (a) preparing either a graphene oxide (GO) dispersion having graphene oxide sheets dispersed in a fluid medium or a graphene oxide gel having graphene oxide molecules dissolved in a fluid medium, wherein the graphene oxide sheets or graphene oxide molecules contain an oxygen amount higher than 5 % by weight (typically from 5% to 46%, but preferably from 10% to 46% and more preferably from 20% to 46%).
  • a graphene oxide (GO) dispersion having graphene oxide sheets dispersed in a fluid medium or a graphene oxide gel having graphene oxide molecules dissolved in a fluid medium wherein the graphene oxide sheets or graphene oxide molecules contain an oxygen amount higher than 5 % by weight (typically from 5% to 46%, but preferably from 10% to 46% and more preferably from 20% to 46%).
  • an oxygen amount higher than 5 % by weight typically from 5% to 46%, but preferably from 10% to 46% and more preferably from 20% to 46%).
  • step (b) which entails dispensing and depositing a layer of graphene oxide dispersion or graphene oxide gel onto a surface of a supporting substrate under a shear stress condition, wherein the dispensing and depositing procedure includes shear-induced thinning of graphene oxide dispersion or gel and shear-induced orientation of graphene oxide sheets or molecules, to form a wet layer of graphene oxide on the supporting substrate.
  • This step includes spraying, printing, extruding, casting, and/or coating of a wet layer of GO onto a solid substrate surface (e.g. a PET film, Al foil, glass surface, etc.) that contains or is followed by a shearing procedure.
  • a shear stress is essential to aligning GO sheets or molecules along a desired direction.
  • the third step, Step (c), is then conducted to chemically replace O or oxygen-containing functional group with a halogen element or halogen-containing group.
  • Halogen herein refers to F, CI, Br, I, or a combination thereof.
  • the fluorinating agent may contain a chemical species in a liquid, gaseous, or plasma state containing a halogen element selected from F, CI, Br, I, or a combination thereof.
  • Halogenated graphene is a group of graphene derivatives, in which some carbon atoms are covalently linked with halogen atoms. The carbon atoms linked with halogens have sp 3 hybridization and other carbon atoms have sp 2 hybridization.
  • the physical and chemical properties of halogenated graphene (also referred to as graphene halide) are strongly dependent on the degree of halogenation.
  • Step (c) is then followed by step (d) of removing the fluid medium from the wet layer of halogenated graphene to form the integrated layer of halogenated graphene having an inter- planar spacing d 0 02 of 0.35 nm to 1.2 nm as determined by X-ray diffraction.
  • the removal of liquid fluid may be conducted before, during, or after the halogenating reaction.
  • hydrofluoric acid or hexafluorophosphoric acid (FIPF 6 ) liquid may be injected into the GO suspension or GO gel stream before, during, or after the dispensing/depositing stage.
  • F 2 gas, Cl 2 gas, Br 2 gas, and/or I 2 gas (vapor) may be introduced to a chamber where the wet GO layer is contained, enabling the halogen gas molecules to permeate into the wet GO layer and reacting with GO therein and thereon.
  • one may choose to remove the liquid medium from the wet layer of GO to form a dried layer of GO prior to introducing the halogenating agent to react with GO.
  • the dried layer of GO may preferably be treated with halogen-containing gas or plasma.
  • the fluorinating agent may be selected from hydrofluoric acid,
  • hexafluorophosphoric acid or FIPF 6 XeF 2 , F 2 gas, F 2 /Ar plasma, CF 4 plasma, SF 6 plasma, HC1, HPC1 6 , XeCl 2 , Cl 2 gas, Cl 2 /Ar plasma, CC1 4 plasma, SC1 6 plasma, HBr, XeBr 2 , Br 2 gas, Br 2 /Ar plasma, CBr 4 plasma, SBr 6 plasma, HI, Xel 2 , 1 2 , 1 2 /Ar plasma, CI 4 plasma, S3 ⁇ 4 plasma, or a combination thereof.
  • a highly preferred dispensing and depositing procedure is the reverse roll transfer coating, which intrinsically induces high shear stresses to the suspension or gel coated on the rollers.
  • the process of producing the monolithic, integrated layer of highly oriented graphene halide (HOGH) begins with preparation of a graphene oxide dispersion (GO dispersion) or graphene oxide gel (GO gel) that is delivered to a trough 208.
  • GO dispersion graphene oxide dispersion
  • GO gel graphene oxide gel
  • the rotational motion of an application roller 204 in a first direction enables the delivery of a continuous layer 210 of GO dispersion or gel onto the exterior surface of the application roller 204.
  • An optional doctor's blade 212 is used to regulate the thickness (amount) of an applicator layer 214 of graphene oxide (GO).
  • This applicator layer is continuously delivered to the surface of a supporting film 216 moving in a second direction (e.g. driven by a counter-rotating roller 206, rotating in a direction opposite to the first direction) to form a wet layer 218 of graphene oxide.
  • This wet layer of GO is then subjected to a liquid removal treatment (e.g. under a heating environment and/or being vacuum-pumped).
  • the process begins with preparation of either a graphene oxide dispersion having graphene oxide sheets dispersed in a fluid medium or a graphene oxide gel having graphene oxide molecules dissolved in a fluid medium, wherein the graphene oxide sheets or graphene oxide molecules contain an oxygen content higher than 5 % by weight.
  • the graphene oxide dispersion or graphene oxide gel is then dispensed and delivered onto a surface of an application roller rotating in a first direction at a first line velocity (the line speed at the external surface of the application roller) to form an applicator layer of graphene oxide and transferring this applicator layer of graphene oxide to a surface of a supporting film driven in a second direction opposite to the first direction at a second line velocity, forming a wet layer of graphene oxide on the supporting film.
  • the supporting film is driven by a counter-rotating supporting roller (e.g. 206 in FIG. 1) disposed at a working distance from the application roller and rotating in the second direction opposite to the first direction.
  • the speed at the external surface of this supporting roller dictates the second line velocity (of the supporting film).
  • the supporting film is fed from a feeder roller and the dried layer of graphene oxide supported by the supporting film is wound on a winding roller and the process is conducted in a roll-to-roll manner.
  • the GO dispersion/gel trough 228 is naturally formed between an application roller 224 and a metering roller 222 (also referred to as a doctor's roller).
  • the relative motion or rotation of the application roller 224, relative to the metering roller 222, at a desired speed generates an applicator layer 230 of GO on the exterior surface of the application roller 224.
  • This applicator layer of GO is then transferred to form a wet layer 232 of GO on the surface of a supporting film 234 (driven by a supporting roller 226 counter-rotating in a direction opposite to the rotational direction of the applicator roller 224).
  • the wet layer may then be subjected to halogenating and drying treatments.
  • the GO dispersion/gel trough 244 is naturally formed between an application roller 238 and a metering roller 236.
  • the relative motion or rotation of the application roller 238, relative to the metering roller 236, at a desired speed generates an applicator layer 248 of GO on the exterior surface of the application roller 238.
  • a doctor's blade 242 may be used to scratch off any GO gel/dispersion carried on the exterior surface of the metering roller 236.
  • This applicator layer 248 of GO is then transferred to form a wet layer 250 of GO on the surface of a supporting film 246 (driven by a supporting roller 240 counter-rotating in a direction opposite to the rotational direction of the applicator roller 238).
  • the wet layer may then be subjected to halogenating and drying treatments.
  • the GO dispersion/gel trough 256 is naturally formed between an application roller 254 and a metering roller 252.
  • the relative motion or rotation of the application roller 254, relative to the metering roller 252, at a desired speed generates an applicator layer 260 of GO on the exterior surface of the application roller 254.
  • This applicator layer 260 of GO is then transferred to form a wet layer 262 of GO on the surface of a supporting film 258, driven to move in a direction opposite to the tangential rotational direction of the applicator roller 254.
  • This supporting film 258 may be fed from a feeder roller (not shown) and taken up (wound) on a winding roller (not shown), which may also be a driving roller. There would be at least 4 rollers in this example.
  • the wet layer may then be subjected to halogenating and drying treatments.
  • halogenating and drying treatments there can be a heating zone after the wet layer of GO is formed to at least partially remove the liquid medium (e.g. water) from the wet layer to form a dried layer of GO.
  • the step of dispensing the graphene oxide dispersion or graphene oxide gel onto the surface of the application roller includes using a metering roller and/or a doctor's blade to provide a desired thickness of the applicator layer of graphene oxide on the application roller surface.
  • the process includes operating 2, 3, or 4 rollers.
  • the process includes a reverse roll coating procedure.
  • the velocity ratio defined as (the second line velocity )/(first line velocity), is from 1/5 to 5/1. If the external surface of the application roller moves at the same speed as the linear movement speed of the supporting film, then the velocity ratio is 1/1 or unity. If, as an example, the external surface of the application roller moves at a speed three times as fast as the linear movement speed of the supporting film, then the velocity ratio is 3/1. As a consequence, the transferred wet layer of GO would be approximately 3 -fold in thickness as compared to the applicator layer of GO. Quite unexpectedly, this enables the production of much thicker layer yet still maintaining a high degree of GO orientation in the wet layer and the dried layer.
  • the velocity ratio is greater than 1/1 and less than 5/1.
  • the velocity ratio is greater than 1/1 and equal to or less than 3/1.
  • the slot-die coating or comma coating is also capable of applying a shear stress to induce the required orientation of GO or GH sheets or molecules.
  • the process further comprises a step of aging the wet or dried layer of graphene oxide in an aging room at an aging temperature from 25°C to 200°C (preferably from 25°C to 100°C and more preferably from 25°C to 55°C) and humidity level from 20% to 99% for an aging time of 1 hour to 7 days to form an aged layer of graphene oxide.
  • an aging temperature from 25°C to 200°C (preferably from 25°C to 100°C and more preferably from 25°C to 55°C) and humidity level from 20% to 99% for an aging time of 1 hour to 7 days to form an aged layer of graphene oxide.
  • this aging procedure enables some chemical linking or merging of GO sheets or molecules in an edge-to-edge manner, as manifested by the observation by microscopy that the average length/width of the GO sheets is significantly increased (by a factor of 2-3) after aging. This would make it possible to maintain the sheet orientation and accelerate subsequent edge-to-edge linking to huge grans or crystal domains
  • the process further comprises a step of heat treating the dried or dried and aged layer of graphene oxide at a first heat treatment temperature higher than 100°C but no greater than 3,200°C (preferably no greater than 2,500°C) for a desired length of time to produce a film having an inter-planar spacing doo 2 less than 0.4 nm and an oxygen and/or halogen content less than 5% by weight.
  • the process can further comprise a compression step, during or after this heat-treating step, to reduce the thickness of the graphene film.
  • the graphene oxide sheets in the graphene oxide dispersion preferably occupy a weight fraction of 0.1% to 25% based on the total weight of graphene oxide sheets and liquid medium combined. More preferably, the graphene oxide sheets in the graphene oxide dispersion occupy a weight fraction of 0.5% to 15%. In some embodiments, graphene oxide sheets occupy a weight proportion from 3% to 15% based on the total weight of graphene oxide sheets and liquid medium combined. In certain embodiments, the graphene oxide dispersion or graphene oxide gel has greater than 3% by weight of graphene oxide dispersed in the fluid medium to form a liquid crystal phase.
  • the monolithic integrated halogenated graphene film contains chemically bonded and merged graphene planes. These planar aromatic molecules or graphene planes (hexagonal structured carbon atoms having a desired amount of oxygen- and/or halogen-containing group) are parallel to one another. The lateral dimensions (length or width) of these planes are huge, typically several times or even orders of magnitude larger than the maximum crystallite dimension (or maximum constituent graphene plane dimension) of the starting graphite particles.
  • the presently invented halogenated graphene film is a "giant graphene crystal" or "giant planar graphene particle" having all constituent graphene planes being essentially parallel to one another. This is a unique and new class of material that has not been previously discovered, developed, or suggested to possibly exist.
  • the dried graphene halide (GH) layer has a high birefringence coefficient between an in- plane direction and the normal-to-plane direction.
  • the oriented graphene oxide and/or halide layer is itself a very unique and novel class of material that surprisingly has great cohesion power (self-bonding, self-polymerizing, and self-crosslinking capability). These characteristics have not been taught or hinted in the prior art.
  • the GO is obtained by immersing powders or filaments of a starting graphitic material in an oxidizing liquid medium (e.g. a mixture of sulfuric acid, nitric acid, and potassium permanganate) in a reaction vessel.
  • an oxidizing liquid medium e.g. a mixture of sulfuric acid, nitric acid, and potassium permanganate
  • the starting graphitic material may be selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso- carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano- tube, or a combination thereof.
  • the resulting slurry is a heterogeneous suspension and appears dark and opaque.
  • the reacting mass can eventually become a suspension that appears slightly green and yellowish, but remain opaque.
  • the degree of oxidation is sufficiently high (e.g. having an oxygen content between 20% and 50% by weight, preferably between 30% and 50%) and all the original graphene planes are fully oxidized, exfoliated and separated to the extent that each oxidized graphene plane (now a graphene oxide sheet or molecule) is surrounded by the molecules of the liquid medium, one obtains a GO gel.
  • the GO gel is optically translucent and is essentially a homogeneous solution, as opposed to a heterogeneous suspension.
  • This GO suspension or GO gel typically contains some excess amount of acids and can be advantageously subjected to some acid dilution treatment to increase the pH value (preferably > 4.0).
  • the GO suspension (dispersion) preferably contain at least 1% by weight of GO sheets dispersed in a liquid medium, more preferably at least 3% by weight, and most preferably at least 5% by weight. It is advantageous to have an amount of GO sheets sufficient for forming a liquid crystalline phase. We have surprisingly observed that GO sheets in a liquid crystal state have the highest tendency to get readily oriented under the influence of a shear stress created by a commonly used casting or coating process.
  • the graphene oxide suspension may be prepared by immersing a graphitic material (in a powder or fibrous form) in an oxidizing liquid to form a reacting slurry in a reaction vessel at a reaction temperature for a length of time sufficient to obtain GO sheets dispersed in a residual liquid.
  • this residual liquid is a mixture of acid (e.g. sulfuric acid) and oxidizer (e.g. potassium permanganate or hydrogen peroxide).
  • This residual liquid is then washed and replaced with water and/or alcohol to produce a GO dispersion wherein discrete GO sheets (single-layer or multi-layer GO) are dispersed in the fluid.
  • the dispersion is a heterogeneous suspension of discrete GO sheets suspended in a liquid medium and it looks optically opaque and dark
  • the GO sheets contain a sufficient amount of oxygen-containing functional groups and the resulting dispersion (suspension or slurry) is mechanically sheared or
  • GO gel a material state in which all individual GO molecules are surrounded by the molecules of the liquid medium.
  • the GO gel looks like a homogeneous solution which is translucent and no discernible discrete GO or graphene sheets can be visibly identified.
  • Useful starting graphitic materials include natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.
  • the GO sheets in such a GO dispersion or the GO molecules in such a GO gel are in the amount of 1%-15% by weight, but can be higher or lower. More preferably, the GO sheets are 2%-10% by weight in the suspension. Most preferably, the amount of GO sheets is sufficient to form a liquid crystal phase in the dispersing liquid.
  • the GO sheets have an oxygen content typically in the range from 5% to 50% by weight, more typically from 10% to 50%, and most typically from 20% to 46% by weight.
  • a graphite particle (e.g. 100) is typically composed of multiple graphite crystallites or grains.
  • a graphite crystallite is made up of layer planes of hexagonal networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another in a particular crystallite.
  • These layers of hexagonal-structured carbon atoms commonly referred to as graphene layers or basal planes, are weakly bonded together in their thickness direction (crystallographic c-axis direction) by weak van der Waals forces and groups of these graphene layers are arranged in crystallites.
  • the graphite crystallite structure is usually characterized in terms of two axes or directions: the c-axis direction and the a-axis (or b-axis) direction.
  • the c- axis is the direction perpendicular to the basal planes.
  • the a- or ⁇ -axes are the directions parallel to the basal planes (perpendicular to the c-axis direction).
  • a highly ordered graphite particle can consist of crystallites of a considerable size, having a length of L a along the crystallographic a-axis direction, a width of L b along the crystallographic ⁇ -axis direction, and a thickness L c along the crystallographic c-axis direction.
  • the constituent graphene planes of a crystallite are highly aligned or oriented with respect to each other and, hence, these anisotropic structures give rise to many properties that are highly directional.
  • the thermal and electrical conductivity of a crystallite are of great magnitude along the plane directions ⁇ a- or ⁇ -axis directions), but relatively low in the perpendicular direction (c-axis).
  • c-axis perpendicular direction
  • different crystallites in a graphite particle are typically oriented in different directions and, hence, a particular property of a multi-crystallite graphite particle is the directional average value of all the constituent crystallites.
  • natural graphite can be treated so that the spacing between the graphene layers can be appreciably opened up so as to provide a marked expansion in the c-axis direction, and thus form an expanded graphite structure in which the laminar character of the carbon layers is substantially retained.
  • the process for manufacturing flexible graphite is well-known in the art.
  • flakes of natural graphite e.g. 100 in FIG. 5(B)
  • the GICs are washed, dried, and then exfoliated by exposure to a high temperature for a short period of time.
  • the exfoliated graphite flakes are vermiform in appearance and, hence, are commonly referred to as worms 104.
  • These worms of graphite flakes which have been greatly expanded can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as "flexible graphite" 106) having a typical density of about 0.04-2.0 g/cm 3 for most applications.
  • FIG. 5(A) shows a flow chart that illustrates the prior art processes used to fabricate flexible graphite foils and the resin-impregnated flexible graphite composite.
  • the processes typically begin with intercalating graphite particles 20 (e.g., natural graphite or synthetic graphite) with an intercalant (typically a strong acid or acid mixture) to obtain a graphite intercalation compound 22 (GIC).
  • intercalant typically a strong acid or acid mixture
  • the GIC After rinsing in water to remove excess acid, the GIC becomes “expandable graphite.”
  • the GIC or expandable graphite is then exposed to a high temperature environment (e.g., in a tube furnace preset at a temperature in the range of 800- 1,050°C) for a short duration of time (typically from 15 seconds to 2 minutes).
  • This thermal treatment allows the graphite to expand in its c-axis direction by a factor of 30 to several hundreds to obtain a worm-like vermicular structure 24 (graphite worm), which contains exfoliated, but un-separated graphite flakes with large pores interposed between these interconnected flakes.
  • graphite worms An example of graphite worms is presented in FIG. 6(A).
  • the exfoliated graphite (or mass of graphite worms) is re- compressed by using a calendaring or roll-pressing technique to obtain flexible graphite foils (26 in FIG. 5(A) or 106 in FIG. 5(B)), which are typically 100-300 ⁇ thick.
  • An SEM image of a cross-section of a flexible graphite foil is presented in FIG. 6(B), which shows many graphite flakes with orientations not parallel to the flexible graphite foil surface and there are many defects and imperfections.
  • the exfoliated graphite worm 24 may be impregnated with a resin and then compressed and cured to form a flexible graphite composite 28, which is normally of low strength as well.
  • the electrical and thermal conductivity of the graphite worms could be reduced by two orders of magnitude.
  • the exfoliated graphite may be subjected to high-intensity mechanical shearing/separation treatments using a high-intensity air jet mill, high-intensity ball mill, or ultrasonic device to produce separated nano graphene platelets 33 (NGPs) with all the graphene platelets thinner than 100 nm, mostly thinner than 10 nm, and, in many cases, being single-layer graphene (also illustrated as 112 in FIG. 5(B)).
  • An NGP is composed of a graphene sheet or a plurality of graphene sheets with each sheet being a two-dimensional, hexagonal structure of carbon atoms.
  • graphite worms tend to be separated into the so-called expanded graphite flakes (108 in FIG. 5(B) having a thickness > 100 nm.
  • These flakes can be formed into graphite paper or mat 106 using a paper- or mat-making process.
  • This expanded graphite paper or mat 106 is just a simple aggregate or stack of discrete flakes having defects, interruptions, and mis-orientations between these discrete flakes.
  • the NGP is described as having a length (the largest dimension), a width (the second largest dimension), and a thickness.
  • the thickness is the smallest dimension, which is no greater than 100 nm, preferably smaller than 10 nm in the present application.
  • the length and width are referred to as diameter.
  • both the length and width can be smaller than 1 ⁇ , but can be larger than 200 ⁇ .
  • a mass of multiple NGPs may be made into a graphene film/paper (34 in FIG.
  • FIG. 7(B) shows a SEM image of a cross-section of a graphene paper/film prepared from discrete graphene sheets using a paper-making process.
  • the image shows the presence of many discrete graphene sheets being folded or interrupted (not integrated), most of platelet orientations being not parallel to the film/paper surface, the existence of many defects or imperfections.
  • a heat spreader in many electronic devices is normally required to be thicker than 10 ⁇ but thinner than 35 ⁇ ).
  • FIG. 5(A) Another graphene-related product is the graphene oxide gel 21 (FIG. 5(A)).
  • This GO gel is obtained by immersing a graphitic material 20 in a powder or fibrous form in a strong oxidizing liquid in a reaction vessel to form a suspension or slurry, which initially is optically opaque and dark.
  • This optical opacity reflects the fact that, at the outset of the oxidizing reaction, the discrete graphite flakes and, at a later stage, the discrete graphene oxide flakes scatter and/or absorb visible wavelengths, resulting in an opaque and generally dark fluid mass.
  • this graphene oxide gel is optically transparent or translucent and visually homogeneous with no discernible discrete flakes/platelets of graphite, graphene, or graphene oxide dispersed therein.
  • the GO molecules are uniformly "dissolved” in an acidic liquid medium.
  • suspension of discrete graphene sheets or graphene oxide sheets in a fluid look dark, black or heavy brown in color with individual graphene or graphene oxide sheets discernible or recognizable even with naked eyes or using a low-magnification light microscope (100X-1,000X).
  • a fluid e.g. water, organic acid or solvent
  • Chopped graphite fibers with an average diameter of 12 ⁇ and natural graphite particles were separately used as a starting material, which was immersed in a mixture of concentrated sulfuric acid, nitric acid, and potassium permanganate (as the chemical intercalate and oxidizer) to prepare graphite intercalation compounds (GICs).
  • the starting material was first dried in a vacuum oven for 24 h at 80°C. Then, a mixture of concentrated sulfuric acid, fuming nitric acid, and potassium permanganate (at a weight ratio of 4: 1 :0.05) was slowly added, under appropriate cooling and stirring, to a three-neck flask containing fiber segments.
  • the acid-treated graphite fibers or natural graphite particles were filtered and washed thoroughly with deionized water until the pH level of the solution reached 6. After being dried at 100°C overnight, the resulting graphite intercalation compound (GIC) or graphite oxide fiber was re-dispersed in water and/or alcohol to form a slurry.
  • GIC graphite intercalation compound
  • halogenated graphene films For making halogenated graphene films, several GO films were subjected to aging and halogenating treatments that typically involve an aging temperature of 30-100°C for 1-8 hours, followed by halogenating treatment at 25-250°C for 1-24 hours. Typical morphology is shown in Fig. 7(C).
  • Example 2 Preparation of single-layer graphene oxide sheets from meso-carbon micro-beads (MCMBs)
  • MCMBs Meso-carbon microbeads
  • This material has a density of about 2.24 g/cm 3 with a median particle size of about 16 ⁇ .
  • MCMB (10 grams) were intercalated with an acid solution (sulfuric acid, nitric acid, and potassium permanganate at a ratio of 4: 1 :0.05) for 48-96 hours. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The intercalated MCMBs were repeatedly washed in a 5% solution of HC1 to remove most of the sulphate ions.
  • the sample was then washed repeatedly with deionized water until the pH of the filtrate was no less than 4.5.
  • the slurry was then subjected ultrasonication for 10-100 minutes to produce GO suspensions.
  • TEM and atomic force microscopic studies indicate that most of the GO sheets were single-layer graphene when the oxidation treatment exceeded 72 hours, and 2- or 3-layer graphene when the oxidation time was from 48 to 72 hours.
  • the GO sheets contain oxygen proportion of approximately 35%-47% by weight for oxidation treatment times of 48-96 hours.
  • the suspension was then coated onto a PET polymer surface using a reverse roll transfer coating and separately, a comma coating procedure to form oriented GO films.
  • the resulting GO films after removal of liquid, have a thickness that can be varied from approximately 0.5 to 500 ⁇ .
  • Halogenation treatments were conducted before (using HF acid) and after the dispensing step (e.g. F 2 and Br 2 plasma, further discussed later).
  • Graphite oxide was prepared by oxidation of graphite flakes with an oxidizer liquid consisting of sulfuric acid, sodium nitrate, and potassium permanganate at a ratio of 4: 1 :0.05 at 30°C.
  • an oxidizer liquid consisting of sulfuric acid, sodium nitrate, and potassium permanganate at a ratio of 4: 1 :0.05 at 30°C.
  • the reacting mass was rinsed with water 3 times to adjust the pH value to at least 3.0.
  • a final amount of water was then added to prepare a series of GO-water suspensions. We observed that GO sheets form a liquid crystal phase when GO sheets occupy a weight fraction > 3% and typically from 5% to 15%.
  • SEM scanning electron microscopy
  • TEM transmission electron microscopy
  • SAD selected-area electron diffraction
  • BF bright field
  • DF dark-field
  • FIG. 6(A), FIG. 6(A), and FIG. 7(B) A close scrutiny and comparison of FIG. 6(A), FIG. 6(A), and FIG. 7(B) indicates that the graphene layers in a halogenated GO film herein produced are substantially oriented parallel to one another; but this is not the case for flexible graphite foils and GO or GH paper.
  • the inclination angles between two identifiable layers in the integrated film of halogenated graphene are mostly less than 5 degrees.
  • there are so many folded graphite flakes, kinks, and mis-orientations in flexible graphite that many of the angles between two graphite flakes are greater than 10 degrees, some as high as 45 degrees (FIG. 6(B)).
  • the mis-orientations between graphene platelets in graphene paper (FIG. 7(B)) are also high (average » 10-20°) and there are many gaps between platelets.
  • the integrated halogenated graphene film is essentially gap-free.
  • Example 4 Halogenating treatments of GO after deposition of a GO layer
  • Chlorination of GO platelets was conducted with chloroform (CF) and, separately, with chlorobenzene (CB) at a temperature of 50-100°C for 1-10 hours.
  • the extent of chlorination of GO was evaluated by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS).
  • XPS X-ray photoelectron spectroscopy
  • the fluorination of reduced graphene oxide sheets can be conducted in plasmas containing CF 4 , SF 6 , XeF 2 , fluoropolymers, or Ar/F 2 as fluorinating agents.
  • the fluorine content of the resulting fluorinated graphene can be varied by changing the plasma treatment time as well as the fluorinating agent type.
  • XeF 2 is a strong fluorinating agent for graphene oxide without etching, thereby providing a facile route for graphene halogenation. Characterization of this process via X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy reveals room-temperature fluorination saturates 25-50% coverage (corresponding to a formula C 4 F-C 2 F) for single-sided exposure and CF for double- sided exposure. Due to its high electronegativity, fluorine induces strong chemical shifts in the carbon Is binding energy allowing the use of X-ray photoelectron spectroscopy (XPS) to quantify composition and bonding type.
  • XPS X-ray photoelectron spectroscopy
  • Fluorination was also conducted in a plasma-enhanced chemical vapor deposition
  • PECVD PECVD
  • a PECVD chamber was evacuated to approximately 5 mTorr, and the temperature was increased from room temperature to 200°C.
  • the CF 4 gas was then introduced into the chamber at controlled gas flow rates and pressures. The degree of
  • fluorination of the GO sample was adjusted by varying the exposure time.
  • a suitable CF 4 plasma exposure time was found to be from 3 to 7 minutes per nm of Go layer thickness. For instance, for a GO layer 10 nm thick, the exposure time was 30 to 70 minutes.
  • Graphene fluoride suspensions were obtained by chemically etching graphite fluoride particles (commercially available) via sonication in the presence of sulfolane, dimethyl- formamide (DMF), or N-methyl-2-pyrrolidone ( MP).
  • the solvent molecules intercalate within the inter-graphene layers, weakening the van der Waals forces between neighboring layers and facilitating the exfoliation of graphite fluoride into graphene fluoride suspension.
  • the GF suspension could be directly coated onto the surface of a PET film using comma coating, slot-die coating, or, preferably, reverse roll transfer coating under a high shear condition (e.g. a higher line speed ratio, 2/1 to 5/1).
  • Graphene fluoride with different fluorine contents could be readily obtained by the chemical reaction of graphene oxide with hydrofluoric acid. Fluorination of graphene oxide can be done by exposing graphene oxide to anhydrous FIF vapors at various temperatures or photochemically at room temperature using FIF solution. These procedures were conducted on GO sheets or molecules prior to the step of dispensing and depositing.
  • Both single-layer and few-layer graphene can be chlorinated up to 56-74 wt. % by irradiation with UV light in a liquid chlorine medium.
  • the bromination of both single-layer and few-layer graphene can be conducted under comparable conditions.
  • GO 15 mg was mixed in 30 mL of carbon tetrachloride and sonicated for 20 min using a tip-style ultrasonicator.
  • the suspension obtained was transferred to a 500 mL quartz vessel which was fitted with a condenser maintained at 277°C.
  • the reaction chamber was purged with high purity nitrogen for 30 min and chlorine gas was passed through the chamber.
  • the gaseous chlorine condensed in the quartz vessel.
  • the quartz vessel containing around 20 mL of liquid chlorine was heated to 250°C with simultaneous irradiation of UV light (250 Watt high pressure Hg vapor lamp) for 1.5 h. The solvent and excess chlorine was removed, leaving a transparent film on the wails of the quartz vessel.
  • the solid was dispersed in absolute alcohol under ultrasoni cation, filtered and washed with distilled water and absolute alcohol .
  • the filtrate was then re-dispersed in 40 ml of distilled water, ultrasonicated for 2 min and centrifuged.
  • the black supernatant obtained was separated and filtered using a PVDF membrane (200 nm pore size).
  • the yield from the 15 mg GO sample was around 7-10 mg.
  • the chlorinated graphene sheets can be dispersed in a solvent (e.g. CC1 4 ) to form a suspension for dispensing and depositing.
  • a desired amount of halogenated GO sheets/molecules can be added to a GO suspension or GO gel to produce a GO/halogenated GO mixture suspension or gel prior to the dispensing/ depositing step.
  • a halogenated GO-to-GO weight ratio is from 10/1 to 1/ 10 (in selected liquid medium, such as DMF and MP), more typically from 1/1 to 1/10 (if the liquid medium is water).
  • FIG. 8 shows the dielectric breakdown strength of GO film, presently invented GO- derived integrated graphene fluoride film (prepared by the reverse roll transfer coating process), and prior art polyimide film plotted as a function of the film thickness.
  • the dielectric breakdown strength of GO-derived fluorinated graphene films and chlorinated graphene films are plotted as a function of the degree of fluorination (atomic ration, F/(F+0) or the degree of chlorination (atomic ratio, Cl/(Cl+0) in FIG. 9.
  • the casting procedure did not involve any significant amount of shear stress, but the reverse-roll coating process includes high shear stresses in orienting GO and halogenated GO films.
  • the shear induced orientation of fluorinated or un-fluorinated GO sheets or molecules enables the integrated film to endure a significantly higher dielectric breakdown strength (2.6-22.2 MV/cm) than those (1.1 -7.2 MV/cm) of the un- oriented or less oriented counterparts prepared by conventional casting.
  • integrated films of highly oriented chlorinated GO sheets or molecules also deliver much higher dielectric strength as compared to their cast counterparts wherein the sheets/molecules are not properly oriented. This is a highly significant discovery, which provides a versatile strategy for achieving exceptional dielectric strength of graphene materials.
  • FIG. 10 Summarized in FIG. 10 are dielectric breakdown strength of GO-derived fluorinated graphene films (prepared by a reverse roll transfer procedure) and GO-derived fluorinated graphene paper prepared by the conventional paper-making procedure (vacuum-assisted filtration) plotted as a function of the degree of fluorination in terms of the atomic ratio, F/(F+0). These data show that the dielectric strengths (1.1-1.45 MV/cm) of GO-based paper membranes prepared by the conventional method of vacuum-assisted filtration of multiple fluorinated GO sheets are relatively poor and are relatively independent of the degree of fluorination. Although some degree of orientation is achieved with this method, the dielectric strength remains very low.
  • halogenated graphene increases, reaches a maximum, and then begins to decrease after the degree of fluorination or bromination exceeds 0.6.
  • the integrated films of halogenated graphene (highly oriented GO-derived graphene halide films, GOGH) prepared from original graphene oxide suspension or GO gel using an orientation-controlling shear stress-based process have the following characteristics:
  • the integrated halogenated graphene films are an integrated halogenated
  • graphene oxide or essentially oxygen-free graphene halide structure that is typically a poly- crystal having large grains.
  • the film has wide or long chemically bonded graphene planes that are all essentially oriented parallel to one another.
  • the crystallographic c- axis directions of all the constituent graphene planes in all grains are essentially pointing in the same direction.
  • the integrated layer has multiple constituent graphene halide planes that are substantially parallel to one another along one direction having an average deviation angle of these graphene halide planes less than 10 degrees (more typically less than 5 degrees.
  • the GOGH film is a fully integrated, essentially void-free, single graphene entity or
  • the paper or membrane of graphene halide or GO platelets are a simple, un-bonded aggregate/stack of multiple discrete platelets of GO or halogenated GO.
  • the platelets in these paper/membranes are poorly oriented and have lots of kinks, bends, and wrinkles. Many voids or other defects are present in these paper/membrane structures, leading to poor dielectric breakdown strength.
  • the preparation of the presently invented integrated films of halogenated graphene involves heavily oxidizing the original graphite particles, to the extent that practically every one of the original graphene planes has been oxidized and isolated from one another to become individual graphene planes or molecules that possess highly reactive functional groups (e.g. -OH, >0, and -COOH) at the edge and on graphene plane surfaces.
  • These individual hydrocarbon molecules containing elements such as O and H, in addition to carbon atoms
  • a liquid medium e.g. mixture of water and alcohol
  • the molecules completely lose their own original identity and they no longer are discrete sheets/platelets.
  • This integrated GOGH film is not made by gluing or bonding discrete flakes/platelets
  • GO or halogenated GO sheets (molecules) in the dispersion or gel are merged through joining or forming of covalent bonds with one another, into an integrated graphene entity, without using any externally added linker or binder molecules or polymers.
  • These GO or halogenated GO molecules are "living" molecules capable of linking with one another in a way similar to living polymers chains undergoing "recombination" (e.g. a living chain of 1,000 monomer units and another living chain of 2,000 monomer units combine or join to become a polymer chain of 3,000 units).
  • a 3,000-unit chain can combine with a 4,000-unit chain to become a giant chain of 7,000 units, and so on.
  • This integrated film is typically a poly-crystal composed of large grains having incomplete grain boundaries, typically with the crystallographic c-axis in all grains being essentially parallel to each other.
  • This entity is derived from a GO suspension or GO gel, which is in turn obtained from natural graphite or artificial graphite particles originally having multiple graphite crystallites. Prior to being chemically oxidized, these starting graphite crystallites have an initial length (L a in the crystallographic a-axis direction), initial width (X b in the b- axis direction), and thickness (L c in the c-axis direction).
  • the resulting unitary graphene entity typically has a length or width significantly greater than the L a and Jb of the original crystallites.
  • the length/width of this graphitic film is significantly greater than the L a and L b of the original crystallites.
  • Even the individual grains in a poly-crystalline graphitic film have a length or width significantly greater than the L a and J of the original crystallites.
  • the highly oriented graphene oxide-derived halogenated GO film has a unique combination of outstanding dielectric constants and dielectric breakdown strength.

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Abstract

La présente invention concerne une couche intégrée (de 10 nm à 500 μm) de graphène halogéné hautement orienté ayant la formule C6ZxOy, dans laquelle Z représente un élément halogène choisi parmi F, Cl, Br, I, ou une combinaison de ceux-ci, x = de 0,01 à 6,0, y = de 0 à 5,0, et x + y ≤ 6,0. Ladite couche intégrée comprend des cristaux de graphène halogéné présentant un espacement inter-planaire d002 de 0,35 nm à 1,2 nm (plus généralement, de 0,4 à 1,0 nm) tel que déterminé par diffraction des rayons X. Ladite couche intégrée présente de multiples plans de constituant d'halogénure de graphène qui sont sensiblement parallèles l'un à l'autre le long d'une direction ayant un angle de déviation moyen de ces plans d'halogénure de graphène inférieur à 10 degrés.
PCT/US2016/051058 2015-09-23 2016-09-09 Film monolithique de graphène halogéné intégré hautement orienté WO2017053089A1 (fr)

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