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  • 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
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
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  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L99/00—Compositions of natural macromolecular compounds or of derivatives thereof not provided for in groups C08L89/00 - C08L97/00
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
  • C04B2235/48—Organic compounds becoming part of a ceramic after heat treatment, e.g. carbonising phenol resins

Definitions

• the present invention relates generally to the field of graphitic materials and, more particularly, to a highly oriented humic acid film and a graphitic film derived therefrom.
• This new thin-film material exhibits an unprecedented combination of exceptionally high thermal conductivity, high electrical conductivity, and high tensile strength.
• 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 graphene planes in one grain are parallel to one another, typically the graphene planes in one grain and the graphene planes in an adjacent grain are inclined at different orientations. In other words, the orientations of the various grains in a graphite particle typically differ from one grain to another.
• a graphite single crystal (crystallite) per se is anisotropic with a property measured along a direction in the basal plane (crystallographic a- or b-axis direction) being dramatically different than if measured along the crystallographic c-axis direction (thickness direction).
• the thermal conductivity of a graphite single crystal can be up to approximately 1,920 W/mK (theoretical) or 1,800 W/mK (experimental) in the basal plane (crystallographic a- and b-axis directions), but that along the crystallographic c-axis direction is less than 10 W/mK (typically less than 5 W/mK).
• the multiple grains or crystallites in a graphite particle are typically all oriented along different directions. Consequently, a natural graphite particle composed of multiple grains of different orientations exhibits an average property between these two extremes (i.e. typically not much higher than 5 W/mK).
• the present invention is directed at a new materials science approach to designing and building a new class of materials, herein referred to as the highly oriented humic acid film (HOHA film), from humic acid alone or a combination of humic acid and graphene (including graphene oxide, graphene fluoride, nitrogenated graphene, hydrogenated graphene, boron-doped graphene, other types of doped graphene, and other types of chemically functionalized graphene).
• HOHA film highly oriented humic acid film
• 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 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.
• NGPs are typically obtained by intercalating natural graphite particles 100 with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound 102 (GIC) or graphite oxide (GO).
• GIC graphite intercalation compound
• GO graphite oxide
• the presence of chemical species or functional groups in the interstitial spaces between graphene planes serves to increase the inter- graphene spacing (i3 ⁇ 4o 2 , 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 in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium perchlorate).
• the resulting GIC (102) is actually some type of graphite oxide (GO) particles.
• This GIC or GO is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion, which contains discrete and visually discernible graphite oxide particles dispersed in water.
• 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” (104), which are each a collection of exfoliated, but largely un-separated graphite flakes that remain interconnected.
• expanded graphite flakes 1028 which contain mostly graphite flakes or platelets thicker than 100 nm (hence, not a nano material by definition). These expanded graphite flakes may be made into a paper-like graphite mat (110).
• 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, 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.
• Graphene sheets or platelets may then be made into a graphene paper or membrane (114).
• Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating/isolating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation bas been increased from 0.3354 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separated, isolated, or discrete graphene oxide (GO) sheets.
• GO graphene oxide
• RGO reduced graphene oxides
• NGPs include discrete 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.
• flexible graphite foils obtained by compressing or roll-pressing exfoliated graphite worms
• electronic device thermal management applications e.g. as a heat sink material
• flexible graphite (FG) foils exhibit a relatively low thermal conductivity, typically ⁇ 500 W/mK and more typically ⁇ 300 W/mK.
• the resulting composite exhibits an even lower thermal conductivity (typically « 200 W/mK, more typically ⁇ 100 W/mK).
• Flexible graphite foils, without a resin impregnated therein or coated thereon are of low strength, low rigidity, and poor structural integrity. The high tendency for flexible graphite foils to get torn apart makes them difficult to handle in the process of making a heat sink.
• the flexible graphite sheets typically 50-200 ⁇ thick
• solid NGPs including discrete sheets/platelets of pristine graphene, GO, and RGO
• a film, membrane, or paper sheet (114) of non-woven aggregates using a paper-making process typically do not exhibit a high thermal conductivity unless these sheets/platelets are closely packed and the film/membrane/paper is ultra-thin (e.g. ⁇ 1 ⁇ , which is mechanically weak).
• ultra-thin film or paper sheets ( ⁇ 10 ⁇ ) are difficult to produce in mass quantities, and difficult to handle when one tries to incorporate these thin films as a heat sink material.
• a paper-like structure or mat made from platelets of graphene, GO, or RGO exhibit many defects, wrinkled or folded graphene sheets, interruptions or gaps between platelets, and non- parallel platelets (e.g. SEM image in FIG. 3(B)), leading to relatively poor thermal conductivity, low electric conductivity, and low structural strength.
• These papers 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.
• Another prior art graphitic material is the pyrolytic graphite film, typically thinner than 100 ⁇ .
• the process begins with carbonizing a polymer film (e.g. polyimide) at a carbonization temperature of 400-1, 500°C under a typical pressure of 10-15 Kg/cm 2 for 10-36 hours to obtain a carbonized material, which is followed by a graphitization treatment at 2,500-3, 200°C under an ultrahigh pressure of 100-300 Kg/cm 2 for 1-24 hours to form a graphitic film. It is technically utmost challenging to maintain such an ultrahigh pressure at such an ultrahigh temperature. This is a difficult, slow, tedious, energy-intensive, and extremely expensive process.
• a polymer film e.g. polyimide
• pyrolytic graphite film thinner than 10 ⁇ or thicker than 100 ⁇ from a polymer such as polyimide.
• This thickness-related problem is inherent to this class of materials due to their difficulty in forming into an ultra-thin ( ⁇ 10 ⁇ ) and thick film (> 100 ⁇ ) while still maintaining an acceptable degree of polymer chain orientation and mechanical strength that are required of proper carbonization and graphitization.
• a second type of pyrolytic graphite is produced by high temperature decomposition of hydrocarbon gases in vacuum followed by deposition of the carbon atoms to a substrate surface. This vapor phase condensation of cracked hydrocarbons is essentially a chemical vapor deposition (CVD) process.
• highly oriented pyrolytic graphite is the material produced by subjecting the CVD-deposited pyro-carbon to a uniaxial pressure at very high temperatures (typically 3,000-3,300°C).
• very high temperatures typically 3,000-3,300°C.
• This entails a thermo-mechanical treatment of combined and concurrent mechanical compression and ultra-high temperature for an extended period of time in a protective atmosphere; a very expensive, energy-intensive, time-consuming, and technically challenging process.
• the process requires ultra-high temperature equipment (with high vacuum, high pressure, or high compression provision) that is not only very expensive to make but also very expensive and difficult to maintain.
• the resulting HOPG still possesses many defects, grain boundaries, and mis- orientations (neighboring graphene planes not parallel to each other), resulting in less-than- satisfactory in-plane properties.
• the best prepared HOPG sheet or block typically contains many poorly aligned grains or crystals and a vast amount of grain boundaries and defects.
• the most recently reported graphene thin film ( ⁇ 2 nm) prepared by catalytic CVD of hydrocarbon gas (e.g. C 2 H 4 ) on Ni or Cu surface is not a single-grain crystal, but a poly- crystalline structure with many grain boundaries and defects.
• hydrocarbon gas e.g. C 2 H 4
• 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 that is poly-crystalline.
• the grains are typically much smaller than 100 ⁇ in size and, more typically, smaller than 10 ⁇ in size.
• HA is an organic matter commonly found in soil and can be extracted from the soil using a base (e.g. KOH). HA can also be extracted, with a high yield, from a type of coal called leonardite, which is a highly oxidized version of lignite coal. HA extracted from leonardite contains a number of oxygenated groups (e.g. carboxyl groups) located around the edges of the graphene-like molecular center (SP 2 core of hexagonal carbon structure).
• HA graphene oxide
• GO graphene oxide
• HA has a typical oxygen content of 5% to 42% by weight (other major elements being carbon and hydrogen). HA, after chemical or thermal reduction, has an oxygen content of 0.01% to 5% by weight.
• humic acid (HA) refers to the entire oxygen content range, from 0.01% to 42% by weight.
• the reduced humic acid (RHA) is a special type of HA that has an oxygen content of 0.01% to 5% by weight.
• the present invention provides a new class of graphene-like 2D materials (i.e. humic acid) that surprisingly can be used alone or in a combination with graphene to form a graphitic film.
• another object of the present invention is to provide a cost-effective method of producing such a humic acid or humic acid-graphene hybrid film-derived graphitic films in large quantities. This method or process does not involve the use of an environmentally unfriendly chemical.
• the humic acid- or humic acid/graphene-derived graphitic films exhibit a thermal conductivity, electrical conductivity, elastic modulus, and/or strength comparable to or greater than those of the conventional highly oriented pyrolytic graphite films. This process is capable of producing a highly oriented graphitic film of practically any desired thickness, from several nanometers (nm) to several hundred micrometers ( ⁇ ).
• Another object of the present invention is to provide products (e.g. devices) that contain graphitic films of the present invention and methods of operating these products.
• the product can be a heat dissipation element in a smart phone, tablet computer, digital camera, display device, flat-panel TV, LED lighting device, etc.
• Such a thin film exhibits a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, and elastic modulus unmatched by any material of comparable thickness range.
• the highly oriented graphitic film can exhibit an electrical conductivity greater than 12,000 S/cm, a thermal conductivity greater than 1,500 W/mK, a physical density greater than 2.1 g/cm 3 , a tensile strength greater than 120 MPa, and/or an elastic modulus greater than 120 GPa. No other material is known to exhibit this set of outstanding properties.
• the invention also provides a highly conducting graphitic film derived from the highly oriented humic acid film stated above through a heat treatment, wherein the graphitic film has hexagonal carbon planes with an inter-planar spacing doo 2 less than 0.4 nm and an oxygen content or non-carbon element content less than 2% by weight, a physical density no less than 1.6 g/cm 3 , an in-plane thermal conductivity greater than 600 W/mK, an in-plane electrical conductivity greater than 2,000 S/cm, a tensile strength greater than 20 MPa.
• the highly oriented humic acid film may further comprise graphene sheets or molecules that are parallel to said HA or CHA sheets, wherein a HA-to-graphene or CHA-to-graphene ratio is from 1/100 to 100/1 and said graphene is selected from pristine graphene, graphene oxide, reduced graphene oxide, graphene fluoride, graphene bromide, graphene iodide, boron-doped graphene, nitrogen-doped graphene, chemically functionalized graphene, or a combination thereof.
• the highly conducting graphitic film may further comprise graphene sheets, wherein the graphitic film has hexagonal carbon planes with an inter-planar spacing doo 2 less than 0.4 nm and an oxygen content or non-carbon element content less than 2% by weight, a physical density no less than 1.6 g/cm 3 , an in-plane thermal conductivity greater than 600 W/mK, an in-plane electrical conductivity greater than 2,000 S/cm, a tensile strength greater than 20 MPa.
• the highly oriented humic acid film may further comprise a polymer wherein said HA or CHA sheets are dispersed in or bonded by said polymer.
• the process comprises (a) preparing a dispersion of humic acid (HA) or chemically functionalized humic acid (CHA) having HA or CHA sheets dispersed in a liquid medium, wherein the HA sheets contain an oxygen content higher than 5 % by weight or the CHA sheets contain non-carbon element content higher than 5% by weight; (b) dispensing and depositing the HA or CHA dispersion onto a surface of a supporting substrate to form a wet layer of HA or CHA, wherein the dispensing and depositing procedure includes subjecting the dispersion to an orientation-inducing stress; (c) partially or completely removing the liquid medium from the wet layer of HA or CHA to form a dried HA or CHA layer having hexagonal carbon planes and an inter-planar spacing d 0 02 of 0.4 nm to 1.3 nm as determined by X-ray diffraction; and (d) heat-treating the dried HA or CHA layer at a first heat treatment temperature higher than 80°C
• the process can further comprise a step (e) of further heat-treating the humic acid film of reduced HA or CHA at a second heat treatment temperature higher than the first heat treatment temperature for a sufficient period of time to produce a graphitic film having an inter-planar spacing doo 2 less than 0.4 nm and an oxygen content or non-carbon element content less than 5% by weight; and (f) compressing said graphitic film to produce a highly conducting graphitic film.
• the HA or CHA dispersion further contains graphene sheets or molecules dispersed therein and the HA-to-graphene or CHA-to-graphene ratio is from 1/100 to 100/1 and these graphene sheets are selected from pristine graphene, graphene oxide, reduced graphene oxide, graphene fluoride, graphene bromide, graphene iodide, boron-doped graphene, nitrogen-doped graphene, chemically functionalized graphene, or a combination thereof.
• HA or CHA sheets are in an amount sufficient to form a liquid crystal phase in said liquid medium.
• the dispersion contains a first volume fraction of HA or CHA dispersed in the liquid medium that exceeds a critical volume fraction (V c ) for a liquid crystal phase formation and the dispersion is concentrated to reach a second volume fraction of HA or CHA, greater than the first volume fraction, to improve a HA or CHA sheet orientation.
• the first volume fraction may be equivalent to a weight fraction of from 0.05% to 3.0% by weight of HA or CHA in the dispersion.
• the dispersion may be concentrated to contain higher than 3.0% but less than 15% by weight of HA or CHA dispersed in said liquid medium prior to said step (b).
• the dispersion does not contain any other polymer than the HA or CHA itself.
• the dispersion may further contain a polymer dissolved in the liquid medium or attached to the HA or CHA.
• CHA or the externally added graphene sheets (if any), or both contains a chemical functional group selected from a polymer, S0 3 H, COOH, H 2 , OH,
• the second heat treatment temperature may be higher than 1,500°C for a length of time sufficient for decreasing an inter-plane spacing d 0 o 2 to a value less than 0.36 nm and decreasing the oxygen content or non-carbon element content to less than 0.1% by weight.
• the second heat treatment temperature is preferably from 1,500°C to 3,200°C.
• the liquid medium may contain water and/or an alcohol.
• the liquid medium may contain a non-aqueous solvent selected from polyethylene glycol, ethylene glycol, propylene glycol, an alcohol, a sugar alcohol, a polyglycerol, a glycol ether, an amine based solvent, an amide based solvent, an alkylene carbonate, an organic acid, or an inorganic acid.
• the dried layer of HA or CHA has a thickness from 10 nm to 200 ⁇ or the resulting highly conductive graphitic film has a thickness from 10 nm to 200 ⁇ .
• the process is a roll-to-roll or reel-to-reel process, wherein step (b) includes feeding a sheet of a solid substrate material from a roller to a deposition zone, depositing a layer of HA or CHA dispersion onto a surface of the sheet of solid substrate material to form the wet layer of HA or CHA dispersion thereon, drying the HA or CHA dispersion to form the dried HA or CHA layer deposited on the substrate surface, and collecting the HA or CHA layer-deposited substrate sheet on a collector roller.
• step (b) includes feeding a sheet of a solid substrate material from a roller to a deposition zone, depositing a layer of HA or CHA dispersion onto a surface of the sheet of solid substrate material to form the wet layer of HA or CHA dispersion thereon, drying the HA or CHA dispersion to form the dried HA or CHA layer deposited on the substrate surface, and collecting the HA or CHA layer-deposited substrate sheet on a collector roller.
• the first heat treatment temperature can contain a temperature in the range of 100°C-1,500°C and the highly oriented graphene film has an oxygen content less than 2.0 %, an inter-planar spacing less than 0.35 nm, a thermal conductivity of at least 800 W/mK, and/or an electrical conductivity no less than 2,500 S/cm.
• the first heat treatment temperature contains a temperature in the range of 1,500°C- 2, 100°C and the highly oriented humic acid film has an oxygen content less than 1.0 %, an inter-planar spacing less than 0.345 nm, a thermal conductivity of at least 1,000 W/mK, and/or an electrical conductivity no less than 5,000 S/cm.
• the first and/or second heat treatment temperature contains a temperature greater than 2, 100°C and the highly oriented humic acid film has an oxygen content no greater than 0.001%, an inter-graphene spacing less than 0.340 nm, a mosaic spread value no greater than 0.7, a thermal conductivity of at least 1,300 W/mK, and/or an electrical conductivity no less than 8,000 S/cm.
• the second heat treatment temperature can contain a temperature no less than 2,500°C and the highly conducting graphitic film has an inter-graphene spacing less than 0.336 nm, a mosaic spread value no greater than 0.4, a thermal conductivity greater than 1,600 W/mK, and/or an electrical conductivity greater than 10,000 S/cm.
• the process results in the highly oriented humic acid film exhibiting a degree of graphitization no less than 80% and/or a mosaic spread value less than 0.4.
• the highly oriented humic acid film contains chemically bonded hexagonal carbon planes that are parallel to one another.
• the starting HA or CHA sheets have a maximum original length and the resulting highly oriented humic acid film contains HA or CHA sheets having a length larger than the maximum original length.
• the highly oriented humic acid film is a poly-crystal graphene structure having a preferred crystalline orientation as determined by said X-ray diffraction method.
• step (e) of heat-treating induces chemical linking, merging, or chemical bonding of HA or CHA sheets (with other HA or CHA sheets or with graphene sheets), and/or re-graphitization or reorganization of a graphitic structure.
• HA/CHA sheets or molecules are capable of reacting or merging with other HA/CHA sheets or molecules and, further surprisingly, these HA/CHA sheets or molecules are capable of reacting or merging with externally added graphene sheets, provided all these HA/CHA sheets/molecules and graphene sheets are well-aligned and packed together so that their molecular planes are essentially parallel to each other.
• the highly oriented graphitic film has an electrical conductivity greater than 5,000 S/cm, a thermal conductivity greater than 800 W/mK, a physical density greater than 1.9 g/cm 3 , a tensile strength greater than 80 MPa, and/or an elastic modulus greater than 60 GPa.
• the highly oriented graphitic film has an electrical conductivity greater than 8,000 S/cm, a thermal conductivity greater than 1,200 W/mK, a physical density greater than 2.0 g/cm 3 , a tensile strength greater than 100 MPa, and/or an elastic modulus greater than 80 GPa.
• the highly oriented graphitic film has an electrical conductivity greater than 12,000 S/cm, a thermal conductivity greater than 1,500 W/mK, a physical density greater than 2.1 g/cm 3 , a tensile strength greater than 120 MPa, and/or an elastic modulus greater than 120 GPa.
• the present invention also provides a highly oriented graphitic film produced by the presently invented process (with or without externally added graphene sheets in the dispersion).
• the invention also provides a microelectronic device containing a highly oriented graphitic film of present invention as a heat-dissipating or heat-spreading element.
• the microelectronic device can be a smart phone, tablet computer, flat-panel display, flexible display, electronic watch, a wearable electronic device, a TV, or a microelectronic communications device.
• FIG.1 A flow chart illustrating various prior art processes for producing exfoliated graphite products (flexible graphite foils and flexible graphite composites) and pyrolytic graphite (bottom portion), along with a process for producing isolated graphene sheets and aggregates of graphene or graphene oxide sheets in the form of a graphene paper or membrane.
• FIG.2 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 plane and also showing many defects, kinked or folded flakes.
• FIG.3 A SEM image of a HA liquid crystal-derived HOGF, wherein multiple hexagonal carbon planes seamlessly merged into continuous-length graphene-like sheets or layers that can run for tens of centimeters wide or long (only a 50 ⁇ width of a 10-cm wide
• FIG.3(B) A SEM image of a cross-section of a conventional graphene paper prepared from
• discrete reduced graphene oxide 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.3(C) Schematic of a film of highly oriented humic acid molecules being chemically merged together to form a highly ordered graphitic film.
• FIG.4(A) Thermal conductivity values of the (HA+GO)-derived HOGF, GO-derived HOGF,
• FIG.4(B) Thermal conductivity values of the (HA+GO)-derived HOGF, HA-derived HOGF, and polyimide-derived HOPG, all plotted as a function of the final HTT;
• FIG.4(C) Electric conductivity values of the (HA+GO)-derived HOGF, GO-derived HOGF, HA- derived HOGF, and FG foil plotted as a function of the final heat treatment temperature.
• FIG.5(A) Inter-graphene plane spacing in HA-derived HOGF measured by X-ray diffraction
• FIG.5(C) The correlation between inter-graphene spacing and the oxygen content
• FIG.5(D) Thermal conductivity values of the (HA+GO)-derived HOGF, GO-derived HOGF, HA-derived HOGF, and FG foil plotted as a function of the final heat treatment temperature.
• FIG.6 Thermal conductivity of HOGF samples plotted as a function of the proportion of GO sheets in a HA/GO suspension.
• FIG.7(A) Tensile strength values of (HA+GO)-derived HOGF, GO-derived HOGF, HA-derived HOGF, flexible graphite foil, and reduced graphene oxide paper, all plotted as a function of the final heat treatment temperature;
• FIG.7(B) Tensile modulus of the (HA+GO)-derived HOGF, GO-derived HOGF, and HA- derived HOGF, plotted as a function of the final heat treatment temperature.
• Non-aqueous solvents for humic acid include polyethylene glycol, ethylene glycol, propylene glycol, an alcohol, a sugar alcohol, a polyglycerol, a glycol ether, an amine based solvent, an amide based solvent, an alkylene carbonate, an organic acid, or an inorganic acid.
• the present invention provides a process for producing a highly oriented humic acid film (with or without externally added graphene sheets) and humic acid-derived graphitic film with a thickness from 5 nm to 500 ⁇ (more typically and preferably from 10 nm to 200 ⁇ , even more typically from 100 nm to 100 ⁇ , further more typically from 1 ⁇ to 50 ⁇ ) and a physical density no less than 1.6 g/cm 3 (up to 2.2 g/cm 3 ).
• the process comprises:
• HA humic acid
• CHA chemically functionalized humic acid
• the HA or CHA dispersion further contains graphene sheets or molecules dispersed therein and the HA-to-graphene or CHA-to-graphene ratio is from 1/100 to 100/1.
• step (e) includes heat-treating the highly oriented humic acid film at a second heat treatment temperature higher than the first heat treatment temperature (typically > 300°C) for a length of time sufficient for decreasing an inter-plane spacing doo 2 to a value of from 0.3354 nm to 0.36 nm and decreasing the oxygen content or non-carbon content to less than 0.5% by weight.
• the second (or final) heat treatment temperature includes at least a temperature selected from (A) 100-300°C, (B) 300 - 1,500°C, (C) 1,500- 2,500°C, and/or (D) 2,500-3, 200°C.
• the second heat treatment temperature includes a temperature in the range of 300 - 1,500°C for at least 1 hour and then a temperature in the range of 1,500-3,200°C for at least another hour.
• the highly oriented humic acid (HOHA) film still contains planar molecules that are characteristic of humic acid molecules.
• the highly oriented humic acid (HOHA) film contains chemically bonded and merged hexagonal carbon planes, which are HA/CHA or combined HA/CHA-graphene planes. These planes (hexagonal structured carbon atoms having a small amount of oxygen- containing group) are parallel to one another.
• This HOHA film if exposed to a heat treatment temperature (HTT) of 1,500°C or higher for a sufficient length of time, typically no longer contains any significant amount of humic acid molecules and essentially all HA/CHA sheets/molecules have been converted to graphene- or graphene oxide-like hexagonal carbon planes that 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 dimensions (length/width) of the starting HA/CHA sheets.
• the presently invented HOHA is essentially a "giant hexagonal carbon crystal" or "giant planar graphene-like layer" having all constituent graphene-like 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.
• Step (a) entails dispersing HA/CHA sheets or molecules in a liquid medium, which can be water or a mixture of water and an alcohol, for certain HA or CHA molecules that contain a significant amount of -OH and/or -COOH groups at the edges and/or on the planes of the HA/CHA sheets (e.g. having an oxygen content between 20% and 47% by weight, preferably between 30% and 47%).
• a liquid medium which can be water or a mixture of water and an alcohol
• the HA/CHA suspension contains an initial volume fraction of HA/CHA sheets that exceeds a critical or threshold volume fraction for the formation of a liquid crystal phase prior to step (b).
• a critical volume fraction is typically equivalent to a HA/CHA weight fraction in the range of from 0.2% to 5.0% by weight of HA/CHA sheets in the dispersion.
• such a range of low HA/CHA contents is not particularly amenable to the formation of the desired thin films using a scalable process, such as casting and coating.
• the HA/CHA sheets in a liquid crystal state containing 4% to 16% by weight of HA/CHA sheets 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 HA/CHA suspension is formed into a thin-film layer preferably under the influence of a shear stress that promotes a laminar flow.
• a shearing procedure is casting or coating a thin film of HA/CHA suspension using a slot-die coating machine. This procedure is similar to a layer of polymer solution being coated onto a solid substrate.
• the roller, "doctor's blade", or wiper creates a shear stress when the film is shaped, or when there is a relative motion between the roller/blade/wiper and the supporting substrate at a sufficiently high relative motion speed.
• such a shearing action enables the planar HA/CHA sheets to well align along, for instance, a shearing direction.
• a molecular alignment state or preferred orientation is not disrupted when the liquid components in the HA/CHA suspension are subsequently removed to form a well-packed layer of highly aligned HA/CHA sheets that are at least partially dried.
• the dried layer has a high birefringence coefficient between an in-plane direction and the normal-to- plane direction.
• the present invention includes the discovery of a facile amphiphilic self-assembly approach to fabricate HA/CHA-based thin films with desired hexagonal plane orientation.
• HA containing 5-46% by weight of oxygen may be considered a negatively charged amphiphilic molecule due to its combination of hydrophilic oxygen-containing functional groups and a hydrophobic basal plane.
• the functional groups can be made to be hydrophilic or hydrophobic.
• the successful preparation of the HA/CHA films with unique hexagonal, graphene-like plane orientations does not require complex procedures. Rather, it is achieved by tailoring HA/CHA synthesis and manipulating the liquid crystalline phase formation and deformation behaviors to enable the self-assembly of HA/CHA sheets in a liquid crystalline phase.
• the HA/CHA suspension was characterized using atomic force microscopy (AFM),
• HA or CHA sheets feature high anisotropy, with monatomic or few-atom thickness (t) and normally micrometer-scale lateral width (w). According to Onsager's theory, high aspect ratio 2D sheets can form liquid crystals in dispersions, when their volume fraction exceeds a critical value: ⁇ c ⁇ 4t/w (Eq. 1)
• HA or CHA can be made to exhibit good dispersibility in water and polar organic solvents, such as alcohol, ⁇ , ⁇ -dimethyl formamide (DMF) and NMP, due to the numerous oxygen-containing functional groups attached to its edges.
• Naturally occurring HA e.g. that from coal
• non-aqueous solvents for humic acid include polyethylene glycol, ethylene glycol, propylene glycol, an alcohol, a sugar alcohol, a polyglycerol, a glycol ether, an amine based solvent, an amide based solvent, an alkylene carbonate, an organic acid, an inorganic acid, or a mixture thereof.
• HA/CHA samples were prepared using a pH-assisted selective sedimentation technique. The lateral sizes of HA/CHA sheets were assessed by dynamic light scattering (DLS) via three different measurement modes, as well as AFM.
• DLS dynamic light scattering
• HA/CHA liquid crystals During the investigation of HA/CHA liquid crystals we made an unexpected but highly significant discovery: The liquid crystalline phase of HA/CHA sheets in water and other solvents can be easily disrupted or destroyed with mechanical disturbances (e.g. mechanical mixing, shearing, turbulence flow, etc.). The mechanical stability of these liquid crystals can be significantly improved if the concentration of HA/CHA sheets is gradually increased to above 5% (preferably from 5% to 16% by weight) by carefully removing (e.g. vaporizing) the liquid medium without mechanically disturbing the liquid crystalline structure.
• HA/CHA weight fraction in this range of 5-16%, HA/CHA sheets are particularly amenable to forming desired orientations during casting or coating to form thin films.
• HA/CHA aspect ratio effect could be the structural corrugation of HA/CHA sheets in solvent as the restoring force originated from bending the sheets is much weaker than that along the sheet. It was found that the degree of HA/CHA corrugated morphology in solvent could be further enhanced if its aspect ratio is increased. This corrugated configuration will significantly affect both the intra and intermolecular interactions of HA/CHA in suspension. To achieve long-range ordering in an aqueous dispersion, well-exfoliated HA/CHA sheets with strong long-range electrostatic repulsion are required.
• the chemical composition of HA/CHA plays an important role in tailoring the electrostatic interaction in an aqueous or organic solvent dispersion.
• the increase of surface charge density will lead to an increase in the strength of the electrostatic repulsion against the attractive forces.
• the ratio of the aromatic and oxygenated domains can be easily tuned by the level of hexagonal carbon plane oxidation or chemical modification.
• the Fourier transform infrared spectroscopy under attenuated total reflectance mode (FTIR-ATR) results of the HA/CHA indicate that oxidized species (hydroxyl, epoxy, and carboxyl groups) exist on the HA/CHA surfaces.
• Thermogravimetric analysis (TGA) in nitrogen was used to probe the oxygen functional group density on the HA/CHA surface.
• a mass loss of -28 % by weight is found at around 250°C and is attributed to the decomposition of labile oxygen- containing species. Below 160°C, a mass loss of -16 wt % is observed, corresponding to desorption of physically absorbed water.
• the X-ray photoelectron spectroscopy (XPS) result of HA shows that an atomic ratio of C/O is about 1.9. This suggests that the HA has a relatively high density of oxygen functional groups.
• HA containing a lower density of oxygen functional groups by simply varying the thermal or chemical reduction time and temperature of heavily oxidized HA (e.g. from leonardite coal). We have observed that liquid crystals can be found with oxygen weight fractions preferentially in the range of 5%-40%, more preferably 5%-30%, and most preferably 5%-20%.
• the colloidal interaction between HA sheets can be significantly influenced by the ionic strength, because the Debye screening length ( ⁇ -1) can be effectively increased by reducing the concentration of free ions surrounding HA sheets.
• the electrostatic repulsion of the HA liquid crystal in water could decrease as the salt concentration increases. As a result, more water is expelled from the HA interlamellar space with an accompanying reduction in d spacing.
• ionic impurities in the HA dispersions should be sufficiently removed, as it is a crucial factor influencing the formation of HA liquid crystal structure.
• the dried HA/CHA layer may then be subjected to heat treatments.
• a properly programmed heat treatment procedure can involve at least two heat treatment temperatures (first temperature for a period of time and then raised to a second temperature and maintained at this second temperature for another period of time), or any other combination of at least two heat treatment temperatures (HTT) that involve an initial treatment temperature (first temperature) and a final HTT, higher than the first.
• HTT heat treatment temperatures
• the first heat treatment temperature is for chemical linking and thermal reduction of HA/CHA and is conducted at the first temperature of > 80°C (can be up to 1,000°C, but preferably up to 700°C, and most preferably up to 300°C). This is herein referred to as Regime 1 :
• HA/CHA sheets are packed and chemically bonded together side by side and edge to edge to form an integrated layer of graphene oxide-like entity.
• a HA/CHA layer primarily undergoes thermally-induced reduction reactions, leading to a reduction of oxygen content to approximately 5% or lower.
• This treatment results in a reduction of inter-graphene spacing from approximately 0.8-1.2 nm (as dried) down to approximately 0.4 nm, and an increase in in-plane thermal conductivity from approximately 100 W/mK to 500 W/mK. Even with such a low temperature range, some chemical linking between HA/CHA sheets occurs.
• the HA/CHA sheets remain well-aligned, but the inter-graphene plane spacing remains relatively large (0.4 nm or larger). Many O-containing functional groups survive.
• the oxygen content is essentially eliminated, typically 0.01% - 0.1%.
• the inter-graphene spacing is reduced to down to approximately 0.3354 nm (degree of graphitization from 80% to nearly 100%), corresponding to that of a perfect graphite single crystal.
• the graphene poly-crystal has all the graphene planes being closely packed and bonded, and all the planes are aligned along one direction, a perfect orientation.
• Such a perfectly oriented structure has not been produced even with the HOPG that was produced by subjecting pyrolytic graphite concurrently to an ultra-high temperature (3,400°C) under an ultra-high pressure (300 Kg/cm 2 ).
• the highly oriented graphene structure can achieve such a highest degree of perfection with a significantly lower temperature and an ambient (or slightly higher compression) pressure.
• the structure thus obtained exhibits an in- plane thermal conductivity from 1,500 up to slightly >1,700 W/mK, and in-plane electrical conductivity to a range from 15,000 to 20,000 S/cm.
• the presently invented highly oriented HA-derived structure can be obtained by heat-treating the HA/CHA layer with a temperature program that covers at least the first regime (typically requiring 1-24 hours in this temperature range), more commonly covers the first two regimes (1- 10 hours preferred), still more commonly the first three regimes (preferably 0.5-5 hours in Regime 3), and most commonly all the 4 regimes (Regime 4, for 0.5 to 2 hour, may be implemented to achieve the highest conductivity).
• a temperature program that covers at least the first regime (typically requiring 1-24 hours in this temperature range), more commonly covers the first two regimes (1- 10 hours preferred), still more commonly the first three regimes (preferably 0.5-5 hours in Regime 3), and most commonly all the 4 regimes (Regime 4, for 0.5 to 2 hour, may be implemented to achieve the highest conductivity).
• the HOHA having a i3 ⁇ 402 higher than 0.3440 nm reflects the presence of oxygen-containing functional groups (such as -OH, >0, and -COOH on graphene-like plane surfaces) that act as a spacer to increase the inter-graphene spacing.
• oxygen-containing functional groups such as -OH, >0, and -COOH on graphene-like plane surfaces
• mosaic spread which is expressed by the full width at half maximum of a rocking curve (X- ray diffraction intensity) of the (002) or (004) reflection. This degree of ordering characterizes the graphite or graphene crystal size (or grain size), amounts of grain boundaries and other defects, and the degree of preferred grain orientation.
• a nearly perfect single crystal of graphite is characterized by having a mosaic spread value of 0.2-0.4. Most of our HOHA-derived graphitic samples have a mosaic spread value in this range of 0.2-0.4 (if produced with a heat treatment temperature (HTT) no less than 2,500°C).
• HA or graphene may be functionalized through various chemical routes.
• the resulting functionalized HA or functionalized graphene (collectively denoted as Gn) may broadly have the following formula(e): [Gn] ⁇ R m
• R is selected from S0 3 H, COOH, H 2 , OH, R'CHOH, CHO, CN, COCl, halide, COSH, SH, COOR, SR, SiR 3 , Si(-OR-) y R 3 -y, Si(-0-SiR 2 ⁇ )OR, R", Li, A1R 2 , Hg-X, T1Z 2 and Mg-X;
• R is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R" is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate or trifluoroacetate.
• a polymer such as epoxy resin
• HA or graphene sheets can be combined to make a coating composition
• the function group -NH 2 is of particular interest.
• a commonly used curing agent for epoxy resin is diethylenetriamine (DETA), which can have 2 or more -NH 2 groups.
• DETA diethylenetriamine
• One of the -NH 2 groups may be bonded to the edge or surface of a graphene sheet and the remaining un-reacted -NH 2 groups will be available for reacting with epoxy resin later.
• DETA diethylenetriamine
• One of the -NH 2 groups may be bonded to the edge or surface of a graphene sheet and the remaining un-reacted -NH 2 groups will be available for reacting with epoxy resin later.
• Such an arrangement provides a good interfacial bonding between the HA (or graphene) sheet and the resin additive.
• Other useful chemical functional groups or reactive molecules may be selected from the group consisting of amidoamines, polyamides, aliphatic amines, modified aliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA), polyethylene polyamine, polyamine epoxy adduct, phenolic hardener, non-brominated curing agent, non-amine curatives, and combinations thereof.
• These functional groups are multi-functional, with the capability of reacting with at least two chemical species from at least two ends. Most importantly, they are capable of bonding to the edge or surface of graphene or HA using one of their ends and, during subsequent curing stage, are able to react with a resin at one or two other ends.
• the above-described [Gn] ⁇ R m may be further functionalized.
• the HA and/or graphene sheets may also be functionalized to produce compositions having the formula: [Gn]--[R— A] m
• compositions of the invention also include CHAs upon which certain cyclic compounds are adsorbed. These include compositions of matter of the formula: [Gn]--[X--RJ m
• X is a polynuclear aromatic, polyheteronuclear aromatic or metallopolyheteronuclear aromatic moiety and R is as defined above.
• Preferred cyclic compounds are planar. More preferred cyclic compounds for adsorption are porphyrins and phthalocyanines. The adsorbed cyclic compounds may be functionalized. Such compositions include compounds of the formula: [Gn]— [X— A a ] m
• the functionalized HA or graphene of the instant invention can be directly prepared by sulfonation, electrophilic addition to deoxygenated GO surfaces, or metallation.
• the graphene or HA sheets can be processed prior to being contacted with a functionalizing agent. Such processing may include dispersing the graphene or HA sheets in a solvent. In some instances the sheets may then be filtered and dried prior to contact.
• One particularly useful type of functional groups is the carboxylic acid moieties, which naturally exist on the surfaces of HAs if they are prepared from acid intercalation route discussed earlier. If an additional amount of carboxylic acid is needed, the HA sheets may be subjected to chlorate, nitric acid, or ammonium persulfate oxidation.
• Carboxylic acid functionalized graphene sheets are particularly useful because they can serve as the starting point for preparing other types of functionalized graphene or HA sheets.
• alcohols or amides can be easily linked to the acid to give stable esters or amides. If the alcohol or amine is part of a di- or poly-functional molecule, then linkage through the O- or H- leaves the other functionalities as pendant groups.
• These reactions can be carried out using any of the methods developed for esterifying or aminating carboxylic acids with alcohols or amines as known in the art. Examples of these methods can be found in G. W. Anderson, et al., J. Amer. Chem. Soc. 96, 1839 (1965), which is hereby incorporated by reference in its entirety.
• Amino groups can be introduced directly onto graphitic fibrils by treating the fibrils with nitric acid and sulfuric acid to obtain nitrated fibrils, then chemically reducing the nitrated form with a reducing agent, such as sodium dithionite, to obtain amino-functionalized fibrils.
• a reducing agent such as sodium dithionite
• the aforementioned functional groups can be attached to HA or graphene sheet surfaces or edges for one or several of the following purposes: (a) for improved dispersion of graphene or HA in a desired liquid medium; (b) enhanced solubility of graphene or HA in a liquid medium so that a sufficient amount of graphene or HA sheets can be dispersed in this liquid that exceed the critical volume fraction for liquid crystalline phase formation; (c) enhanced film-forming capability so that thin film of otherwise discrete sheets of graphene or HA can be coated or cast; (d) improved capability of graphene or HA sheets to get oriented due to modifications to the flow behaviors; and (e) enhanced capability for graphene or HA sheets to get chemically linked and merged into larger or wider graphene planes.
• Humic acid can be extracted from leonardite by dispersing leonardite in a basic aqueous solution (pH of 10) with a very high yield (in the range of 75%). Subsequent acidification of the solution leads to precipitation of humic acid powder.
• leonardite was dissolved by 300 ml of double deionized water containing 1 M KOH (or NH 4 OH) solution under magnetic stirring. The pH value was adjusted to 10. The solution was then filtered to remove any big particles or any residual impurities.
• the resulting humic acid dispersion containing HC alone or with the presence of graphene oxide sheets (GO prepared in Example 3 described below), was cast onto a glass substrate to form a series of films for subsequent heat treatments.
• the neutral mixture was then filtered through a 0.45-mm
• Natural graphite from Ashbury Carbons was used as the starting material.
• GO was obtained by following the well-known modified Hummers method, which involved two oxidation stages.
• the first oxidation was achieved in the following conditions: 1100 mg of graphite was placed in a 1000 mL boiling flask. Then, 20 g of K2S2O8, 20 g of P 2 0 5 , and 400 mL of a concentrated aqueous solution of H 2 S0 4 (96%) were added in the flask. The mixture was heated under reflux for 6 hours and then let without disturbing for 20 hours at room temperature. Oxidized graphite was filtered and rinsed with abundant distilled water until a pH value > 4.0 was reached. A wet cake-like material was recovered at the end of this first oxidation.
• the previously collected wet cake was placed in a boiling flask that contains 69 mL of a concentrated aqueous solution of H 2 S0 4 (96%).
• the flask was kept in an ice bath as 9 g of KMn0 4 was slowly added. Care was taken to avoid overheating.
• the resulting mixture was stirred at 35°C for 2 hours (the sample color turning dark green), followed by the addition of 140 mL of water. After 15 min, the reaction was halted by adding 420 mL of water and 15 mL of an aqueous solution of 30 wt % H 2 O 2 . The color of the sample at this stage turned bright yellow.
• the mixture was filtered and rinsed with a 1 : 10 HC1 aqueous solution.
• the collected material was gently centrifuged at 2700g and rinsed with deionized water.
• the final product was a wet cake that contained 1.4 wt % of GO, as estimated from dry extracts. Subsequently, liquid dispersions of GO platelets were obtained by lightly sonicating wet-cake materials, which were diluted in deionized water.
• water suspensions containing mixtures of GO and humic acid at various GO proportions were prepared and slot-die coated to produce thin films of various compositions, as illustrated in FIG. 3(C).
• the suspension after ultrasoni cation contains pristine graphene sheets dispersed in water and s surfactant dissolved therein. Humic acid was then added into the suspension and the resulting mixture suspension was further ultrasoni cated for 10 minutes to facilitate uniform dispersion and mixing.
• FHEG FHEG
• an organic solvent methanol and ethanol, separately
• an ultrasound treatment 280 W
• 280 W ultrasound treatment
• the dispersions were then made into thin films using comma coating.
• the highly oriented HA films were then heat-treated to various extents to obtain highly conducting graphitic films.
• EXAMPLE 6 Preparation of HOHA containing nitrogenataed graphene sheets and humic acid Graphene oxide (GO), synthesized in Example 3, was finely ground with different proportions of urea and the pelletized mixture heated in a microwave reactor (900 W) for 30 s. The product was washed several times with deionized water and vacuum dried. In this method graphene oxide gets simultaneously reduced and doped with nitrogen.
• the products obtained with graphene : urea mass ratios of 1 : 0.5, 1 : 1 and 1 : 2 are designated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogen contents of these samples were 14.7, 18.2 and 17.5 wt% respectively as found by elemental analysis.
• Example 7 Preparation of nematic liquid crystals from humic acid sheets
• a low-concentration dispersion typically 0.05-0.6 wt.% immobilized for a sufficiently long time (usually more than 2 weeks) macroscopically phase-separated into two phases.
• HA sheets form a liquid crystal phase when HA sheets occupy a weight fraction of 1.1%, and the liquid crystals can be preserved by gradually increasing the concentration of HA to the range of from 6% to 16%.
• the prepared humic acid dispersion exhibited an inhomogeneous, chocolate-milk-like appearance to the naked eye. This milky appearance can be mistaken for aggregation or precipitation of the graphene oxide but, in fact, it is a nematic liquid crystal.
• PET polyethylene terephthalate
• Each film was then subjected to different heat treatments, which typically include a chemical linking and thermal reduction treatment at a first temperature of 80°C to 300°C for 1- 10 hours, and at a second temperature of 1,500°C-2,850°C for 0.5-5 hours.
• heat treatments typically include a chemical linking and thermal reduction treatment at a first temperature of 80°C to 300°C for 1- 10 hours, and at a second temperature of 1,500°C-2,850°C for 0.5-5 hours.
• HOHA film was transformed into a highly conducting graphitic film (HOGF).
• the internal structures (crystal structure and orientation) of several dried HA layers (HOHA films), and the HOGF at different stages of heat treatments were investigated.
• X-ray diffraction curves of a layer of dried HOHA prior to a heat treatment, a HOHA film thermally reduced at 150°C for 5 hours, and the resultant HOGF were obtained.
• the dried film With some heat treatment at 150°C, the dried film exhibits the formation of a hump centered at 22°, indicating that it has begun the process of decreasing the inter-planar spacing, indicating the beginning of chemical linking and ordering processes. With a heat treatment temperature of 2,500°C for one hour, the d 0 o 2 spacing has decreased to approximately 0.336, close to 0.3354 nm of a graphite single crystal.
• the doo 2 spacing is decreased to approximately to 0.3354 nm, identical to that of a graphite single crystal.
• the (004) peak intensity relative to the (002) intensity on the same diffraction curve, or the 7(004)//(002) ratio, is a good indication of the degree of crystal perfection and preferred orientation of graphene planes.
• the (004) peak is either non- existing or relatively weak, with the 7(004)//(002) ratio ⁇ 0.1, for all conventional graphitic materials heat treated at a temperature lower than 2,800°C.
• the 7(004)//(002) ratio for the graphitic materials heat treated at 3,000-3,250°C is in the range of 0.2-0.5.
• the 7(004)//(002) ratio for all tens of flexible graphite foil compacts investigated are all « 0.05, practically non-existing in most cases.
• the 7(004)//(002) ratio for all graphene paper/membrane samples prepared with a vacuum-assisted filtration method is ⁇ 0.1 even after a heat treatment at 3,000°C for 2 hours.
• FIG. 5(A) The inter-graphene spacing values of both the HA liquid crystal suspension-derived HOGF samples obtained by heat treating at various temperatures over a wide temperature range are summarized in FIG. 5(A). Corresponding oxygen content values are shown in FIG. 5(B). In order to show the correlation between the inter-graphene spacing and the oxygen content, the data in FIG. 5(A) and 5(B) are re-plotted in FIG. 5(C). A close scrutiny of FIG. 5(A)-(C) indicate that there are four HTT ranges (100-300°C; 300-l,500°C; 1,500-2,000°C, and > 2,000°C) that lead to four respective oxygen content ranges and inter-graphene spacing ranges.
• the resulting highly oriented HA film exhibits a thermal conductivity of 756 W/mK (from HA alone) and 1, 105 W/mK (from a HA-GO mixture), respectively. This is in stark contrast to the observed 268 W/mK of the flexible graphite foil with an identical heat treatment temperature.
• the flexible graphite foil only shows a thermal conductivity lower than 600 W/mK.
• the presently invented HOGF layer delivers a thermal conductivity of 1,745 W/mK for a layer derived from a mixture of HA and GO (FIG.
• SEM scanning electron microscopy
• TEM transmission electron microscopy
• SAD selected-area electron diffraction
• BF bright field
• DF dark-field
• FIG. 2 A close scrutiny and comparison of FIG. 2, FIG. 3(A), and FIG. 3(B) indicates that the graphene-like layers in a HOGF are substantially oriented parallel to one another; but this is not the case for flexible graphite foil and graphene oxide paper.
• the inclination angles between two identifiable layers in the highly conducting graphitic film are generally less than 10 degrees and 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. 2).
• the mis- orientations between graphene platelets in NGP paper (FIG. 3(B)) are also high and there are many gaps between platelets.
• the HOGF entity is essentially gap-free.
• FIG.4 (A) shows the thermal conductivity values of the HA/GO-derived film, GO- derived film, HA suspension-derived HOGF, and FG foil, respectively, all plotted as a function of the final HTT.
• the HA/GO liquid crystal suspension-derived HOGF appears to be superior to the GO gel- derived HOGF in thermal conductivity at comparable final heat treatment temperatures.
• the heavy oxidation of graphene sheets in GO gel might have resulted in high defect populations on graphene surfaces even after thermal reduction and re-graphitization.
• the presence of HA molecules seem to be capable of helping to heal the defects or bridging the gaps between GO sheets.
• HA is one order of magnitude less expensive than natural graphite (a raw material for GO) and 2-4 orders of magnitude less expensive than GO.
• HOPG polyimide
• FIG. 4(B) shows the thermal conductivity values of the HA/GO suspension-derived HOGF, the HA suspension-derived HOGF, and the polyimide-derived HOPG, all plotted as a function of the final graphitization temperature.
• the conventional HOPG produced by using the carbonized polyimide (PI) route, exhibits a consistently lower thermal conductivity as compared to the HA/GO-derived HOGF, given the same HTT for the same length of heat treatment time.
• the HOPG from PI exhibits a thermal conductivity of 820 W/mK after a graphitization treatment at 2,000°C for 1 hour.
• the HA/GO-derived HOGF exhibits a thermal conductivity value of 1,586 W/mK. It may be noted that PI is also orders of magnitude more expensive than HA and the production of PI involves the use of several environmentally undesirable organic solvents.
• the highly oriented HA film (including highly oriented HA/GO film), and the subsequently heat-treated versions are fundamentally different and patently distinct from the flexible graphite (FG) foil, graphene/GO/RGO paper/membrane, and pyrolytic graphite (PG) in terms of chemical composition, crystal and defect structure, crystal orientation, morphology, process of production, and properties.
• FG flexible graphite
• PG pyrolytic graphite
• conductivity values of the HA/GO suspension-derived and HA suspension-derived HOGF HOGF are far superior to those of the FG foil sheets over the entire range of final HTTs investigated.
• Example 8 The effect of graphene addition on the properties of HA-based HOHA and highly oriented graphitic film films
• FIG. 7(A) For comparison, some tensile strength data of RGO paper and flexible graphite foil are also summarized in FIG. 7(A).
• the HA/GO and HA dispersion contains highly oriented/aligned, chemically active HA/GO and HA sheets/molecules that are capable of chemical linking and merging with one another during the heat treatment, while the graphene platelets in the conventional GO paper and the graphite flakes in the FG foil are essentially dead platelets.
• the HA or HA/GO-based highly oriented films and the subsequently produced graphitic films is a new class of material by itself.
• the film obtained by simply spraying HA-solvent solution onto a glass surface and drying the solvent, does not have any strength (it is so fragile that you can break the film by simply touch the film with a finger). After heat treating at a temperature > 100°C, this film became fragmented (broken into a huge number of pieces).
• the highly oriented HA film (wherein all HA molecules or sheets are highly oriented and packed together), upon heat treatment at 150°C for one hour, became a film of good structural integrity, having a tensile strength > 24 MPa.
• Example 10 Synthesis of polyacrylonitrile-grafted HA (HA-g-PAN)
• PAN was grafted onto HA sheets via the in situ free radical polymerization procedure.
• 100 mg of HA and 80 mL of dimethylformamide (DMF) were added to a 150 mL round-bottom flask, and a well-dispersed solution was obtained by sonicating in a 40 kHz sonic bath for 10 min.
• a well-dispersed solution was obtained by sonicating in a 40 kHz sonic bath for 10 min.
• AN 200 mmol
• initiator of AIBN 0.5 mmol
• the resultant mixture was precipitated in methanol, and the resulting gray precipitate was collected and re-dissolved in 200 mL of DMF.
• the solution was then centrifuged at the speed of 15 000 rpm (23,300 G) for 0.5-1 h to remove free polymers that were not covalently attached to HA.
• the resultant creamlike fluid was thoroughly washed with DMF for eight times until the upper layer appeared colorless. Then the resulting black colloidal product of HA-g-PAN was dispersed in 50 mL of DMF ready for use.
• the polymer-modified HA sheets were found to undergo transition from an isotropic phase to a liquid crystalline phase at a higher threshold volume fraction (V c ), which seems to be a little disadvantage, but since coating or casting was conducted with a dispersion of significantly higher concentration (e.g. > 3% by weight far exceeding V c ), this high V c is not a concern.
• V c threshold volume fraction
• this polymer component has made it easier to form thin films with good mechanical integrity and improved ease of handling, which are highly desirable features.
• a HA- g-PAN dispersion was cast to produce a wet film, which was dried and thermally treated, at 300°C for 5 hours, 1,000°C for 3 hours, and then 2,500°C for 2 hours.
• the density of HA-g-PAN liquid crystal-derived film is 2.13 g/cm 3 , exhibiting a thermal conductivity of 1,566 W/mk.
• the paper of HA-g-PAN was prepared by vacuum-assisted filtration of DMF dispersion with concentration of 5 mg/mL, followed by drying at 50°C in vacuum for 12 h. The paper sheet was compressed and then subjected to the same thermal treatments.
• the density of HA-g-PAN paper-derived film is 1.70 g/cm 3 , exhibiting a thermal conductivity of 805 W/mk.
• HOGF materials have the best combination of excellent electrical conductivity, thermal conductivity, mechanical strength, and stiffness (modulus). These HOGF materials can be used in a wide variety of thermal management applications. For instance, due to its exceptional thermal conductivity, a HOGF structure can be part of a thermal management device, such as a heat dissipation film used in a smart phone, tablet computer, flat-panel TV display, or other microelectronic or communications device.

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