TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to the field of graphite, and more particularly, to compositions and methods of making ultra-low oxidized thin graphite flakes.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is described in connection with composite materials.

Carbon Black is a common additive used to enhance strength, electrical and thermal conductivity of a host. In fact, carbon is used as an additive to pig iron to create steal. Graphite (poly-crystalline Carbon; "non-crystalline" graphite is generally made up of very small crystals) has superior electrical, thermal and mechanical properties relative to carbon black. The abundance of low cost carbon black has been the driver for the electrical, thermal and mechanical specifications as a host. Due to the cost differential between crystalline graphite and carbon black graphite, crystalline graphite has not been widely adopted as an additive. The emergence of portable electronic devices that operate at high frequencies and high power has driven companies to look for a higher preforming inexpensive additive. These higher performing inexpensive additives are intended to prevent the electronic devices from being disrupted by the internal or external RF electronic devices and components. The hosts in the portable electronic applications are commonly a plastic or resin for cellular phone applications. The amount of carbon black used to achieve the conductivity to create an effective EMI (Electromagnetic Interference) shield for these applications is driving companies to consider graphene and graphene oxide as an additive.

Crystalline graphite is the fundamental starting material for the creation of substantially single- crystal graphene-oxide flakes (graphene containing 20 to 35% oxygen by weight). Despite these mono-layer mechanical properties, Graphene has not been able to transition to a macro-scale mechanical structure. Various research institutes have loaded plastic/polymer/epoxy with carbon tubes (CNT), graphene flakes (GF), and graphene oxide (GO), and seen up to a 200% increase in tensile strength in addition to improved electrical and thermal properties in the host plastic/polymer/epoxy. The Hummers based process that produces non-planar highly-oxidized graphene oxide sheets has a price of hundreds of dollars per gram but has shown superior electrical and thermal conductivity, and the potential to reduce the amount of material required for additive applications by 5 to 10 times relative to carbon black. Existing methods of producing high quality crystalline graphene, generally, exfoliated into flakes and oxidized with the use of very strong oxidizers. These acids create an extremely caustic environment that requires careful purification as well as disposal of waste products. The oxidizers are typically strong acids used in high concentrations at elevated temperatures requiring complicated purification processes to yield oxidized graphene non-planar particles. The thinner of these forms of materials are referred to as graphene or graphene oxide (GO). Graphene oxide can cost up to $ 12,000.00/gram.

SUMMARY OF THE INVENTION

The present invention includes a method of making planar crystalline graphite oxide flakes, comprising: mechanically milling crystalline graphite in a controlled oxygen-level fluid; and controlling the oxygen-level in the fluid to low levels that provide planar crystalline thin graphite oxide flakes that can be spontaneously oxidizable in air.

Generally, the edges of the thin graphite oxide flake are only partially oxidized, leaving apristine portion that enhances suspension of the thin graphite oxide flakes in hosts such as liquids and plastics (both polar and non-polar) and promote flake-to-flake bonding when solidified. The easily observed spontaneous oxidation of these thin graphite oxide flakes in air, e.g., of surface flakes in a vessel, is believed to demonstrate that the flake edges have pristine portions.

In some embodiments, the oxidation state of the crystalline graphite oxide is less than 8%; in some others, the oxidation state of the crystalline graphite oxide is less than 5%; and in others the oxidation state of the crystalline graphite oxide is less than 2%. In some embodiments, the crystalline graphite oxide is used to provide high electrical and/or thermal conductivity.

Generally, the crystalline thin graphite oxide has partially pristine edges. In some embodiments, the oxidation of the crystalline thin graphite oxide occurs in air, moist air, oxygen or ozone. In some embodiments, the fluid is a gas that occurs in air. In some embodiments, the thin graphite oxide flakes are made at room temperature.

The present invention can also be a method of making planar crystalline thin graphite oxide flakes, comprising: mechanically milling crystalline graphite in a controlled oxygen-level fluid, wherein the oxidation state of the crystalline thin graphite oxide is controlled to less than 8%, 7%, 6%, 5%, 4%, 3%, 2% or 1%.

The mechanical milling into smaller pieces can use oxygen in the fluid, e.g., air in the milling vessel to partially oxidize newly exposed edges created by the milling. No special oxidizer (gaseous, liquid, or solid) need be added. The thin graphite oxide planar flakes of the present invention have better electrical, thermal, and structural properties than the prior non-planar (wrinkled) sheets. The pristine edges can provide better bonding to other flakes or other material(s).

Existing methods of producing multi-layer thin graphite oxide sheets have generally been accomplished by thermal or chemical exfoliating. The chemical exfoliating is done by using strong oxidizers under extreme conditions and requires careful purification. The chemicals used for the oxidation process are strong acids, used in high concentrations, and experience elevated temperatures as a result of the chemical reaction. The resulting product requires complicated purification processes to yield non-planar sheets of oxidized graphene with high oxidation states.

This method uses the controlled spontaneous oxidation creation of very low oxidized crystalline thin graphite oxide planar flakes without using special oxidizers.

The inability to translate the prior technology to a viable composite structure is a combination of technical issues and cost factors. The technical limitation includes stochastic processes in the curing of the plastic/polymer/epoxy that results in random shrinkage phenomena that is exacerbated in larger composite structures/devices. The distribution of the additive material is non-uniform, creating mechanically weak or electrically/thermally resistive regions resulting in failure points in the loaded plastic/polymer/epoxy host. The superior properties of crystalline thin graphite oxide flakes compared to polymers are also reflected in polymer composites. Improvement in the physicochemical properties of the composites or in use in an electrode structure depends on the distribution of crystalline thin graphite oxide layers in the polymer matrix as well as interfacial and alignment of the crystalline thin graphite oxide layers and polymer matrix.

Crystalline thin graphite can be oxidized by a number of processes including thermal, chemical, chemical-mechanical, or by epitaxial deposition. However, our experiments have shown that once the crystalline thin graphite machined to a flake size about 360,000 sq. nm in area and less than 250 nm thick can be spontaneously self-oxidizing in the right gaseous environment creating a Low Level Crystalline Graphite Oxide (LLCGO). This spontaneous oxidation for the LLCGO flakes range between 0.1% and 8% depending on the flake size and gaseous environment. The lower oxidation range, less than 5%, is virtually impossible to achieve using an aqueous oxidizer route. In the presence of other materials, such as iron contamination, the spontaneous oxidation can be flammable instead of resulting in a mild oxidation. Oxidation is a desirable feature in that it reduces agglomeration that is common in carbon allotropes when used as an additive. A mild or an oxidation less than 20% also functionalizes the crystalline graphene to be hydrophilic allowing it to be suspendable in polar fluids/solvents such as water. Hummers based Graphene or Graphene Oxide can range up to 45% oxidation by mass. The GO produced from the Hummer's process can be thermally reduced but is difficult to convert to have hydrophobic surfaces.

The spontaneous oxidation can occur by exposing the crystalline thin graphite through exposing the flakes/particles to dry air, moist (humid) air, oxygen, or ozone. The crystalline thin graphite is machined to this flake size and then exposed to one of these mild oxidizing agents. The process has the additional advantage of being dry and not requiring drying or further processing to produce the crystalline thin graphite oxide.

Research programs have shown chemical reactions on a micro scale are such that the surface area of the container relative to the volume of liquid contained could not support a self sustaining reaction, as the surface area of the container transported all of the thermal energy generated by the chemical reaction. A meso-scale chemical reactor was shown to have the optimal surface to volume ratio to maintain the thermal chemical balance and sustain the reaction. On the macro- or large-scale, the inner portion of the chemical reaction volume creates a reaction that is too large and is independent of the surface area and effects of the container, and thus, requires elaborate mixing and stirring to obtain a uniform reaction. As large scale processing of LLCGO is developed, the kinetics of the thermal dynamic chemo-mechanical process change dramatically. The large-scale production of LLCGO can use air to drive the oxidation process resulting in lightly edge-oxidized LLCGO with little to no surface oxidation. This eliminates the distortion and permanent damage to the LLCGO when reduced to produce crystalline thin graphite (CG). Ball milling can be a method of making LLCGO, comprising: putting crystalline thin graphite in a mill, wherein the mechanical forces of the ball mill both exfoliates and reduces the flake diameter of the crystalline thin graphite. The gas carrying oxygen is then added while the mill slowly mixes the media to allow uniform mixing with the CG to produce LLCGO.

Graphene's structure is one-atom-thick planar sheets of carbon atoms that are densely packed in a honeycomb or hexagonal crystal lattice. The carbon-carbon bond length in graphene is about 1.42A. Graphene sheets stack to form graphene with an inter-planar spacing of 3.35 A. Multiple graphene sheets/flakes are bonded together by van der Waals forces to form crystalline graphene.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.

To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as "a", "an" and "the" are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims.

Production of thin graphite oxides requires the use of an oxidizing agent in conjunction with mechanical energy (milling) for synthesis oxidization. Mechanical processing can be energy efficient and prevents the decomposition of the chemical functionalities that can occur during thermal processing.

The milling of the crystalline thin graphite to a flake size that can achieve spontaneous oxidation can be accomplished in a wide number of mechanical milling apparatuses that create mechanical energy with controlled thermal management/thermal isolation. A grinding mill can contain internally agitated balls in a variety of methods. It has been generically referred to as a "stirred ball mill."

A useful and simple equation describing the grinding momentum is M x V (mass x velocity), which enabled the inventors to determine how the attritor fits into the family of mills. For example, ball mills use balls, normally 0.25" or larger, and run at a low (10-500) rpm. The other mills, such as sand, bead, and horizontal, use smaller media from 0.3 mm to 2 mm, but run at a very high rpm (roughly 800-1,200). High-speed dispersers with no media run at an even faster rpm (1,200- 4,000). An attrition mill directly agitates the media to achieve grinding.

For efficient fine grinding, both impact action and shearing force are required. The grinding media's random movement and spinning at different rotation energies exert shearing forces and impact forces on the adjacent crystalline thin graphite. In general, the shearing force facilitates exfoliation and the impact facilitates size reduction. Depending on the grade of the starting crystalline thin graphite material determines the milling time and kinetics. The milling time can range from 2 to 12 hours. The grade of the crystalline thin graphite material indicates the purity, diameter and thickness. As a result, both shearing force and media impact force are present. Such combined shearing and impact results in size reduction, exfoliation and narrow size dispersion. Once the milling is accomplished, air is introduced to generate the spontaneous oxidation of the crystalline thin graphite material. The mild oxidation reduces the self-attraction phenomena associated with carbon materials that can result in agglomeration. The spontaneous oxidation can range from 0.1% to 8%, while the flake size is less than 360,000 nm in area and less than 250 nm thick (while graphene is generally single crystal and the thicker flakes of this method in particular may have more than one crystal, they are still referred to as thin graphite herein).

An additive can be added into the mill to reduce the potential of agglomeration. A surfactant additive such as polyisobutylene succinimide (PBSI) has been shown to reduce agglomeration of carbon black. The suspension is mixed for 20 minutes or less. The suspension can then be dried or concentrated through evaporation of the water. The concentrated aqueous or dried LLCGO can then be stored for long periods. The PBSI LLCGO material can then be readily dispersed in a high purity water carrier or other compatible solvent. This can be a technique for low cost, mass- production of mildly oxidized single-crystal thin graphite planar flakes using a ball mill for the processing of the material.

Milling of crystalline thin graphite powder to a flake size that can spontaneously oxidize producing a mildly oxidized material is ideal for use as an additive. These environmental conditions can exist in the ball mill process where amorphous carbon material has been shown to produce larger areas of oriented crystalline structure. The very mild oxidation produced from the spontaneous oxidization of the crystalline thin graphite produces flakes or particles with no distortion or texturing of the surface and a low resistant edge. Excessive edge oxidation can result in an electrical and thermal resistive layer between the host and the pristine sp ² orbitals on the surface of the flake. The oxidation region reduces the performance of that material for electrical and thermal applications.

Larger scale milling experiments were performed in an Attritor with 6 lbs (or -2,600 stainless steel balls) of 0.25" diameter stainless steel balls weighing lg each. Typically, graphite (TC306, 30g) was milled for 120-minutes at 350 rpm for a planetary mill or a 150g to 200g at 700 rpm in a ball mill. The mechanical agitation supplied by the attritor is sufficient to prevent the crystalline thin graphite from agglomerating, adhering to the milling media or adhering to the tank during milling and spontaneous oxidation. Mills where the tank is agitated (such as a shaker mill, planetary mill, or pebble mill), in general, do not effectively agitate to achieve the desired results. The inner chamber of the mill can be evacuated to control the potential for oxidation during the mill process. The region between the inner and outer vessel can be thermally controlled by using a heating element (heat tape, heat lamp, thermal management liquid or other heating device) to provide thermal stability, input and control to the inner chamber or by using a thin walled high strength inner chamber such as titanium inner chamber. Although a thin walled chamber would work, it is expensive and does not provide additional energy into the inner milling chamber. Another method is to use a combination of thermal isolation and thermal input into the inner milling chamber to control the oxidation of the exfoliated crystalline thin graphite to produce LLCGO. In one aspect, the amount of oxygen can be varied to achieve an oxidation state of the crystalline thin graphite oxide of 8%, 7%, 6%, 5%, 4%, 3%, 2% or 1%.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

It is contemplated that any embodiment discussed in this specification can be implemented with respect to any method, kit, reagent, or composition of the invention, and vice versa. Furthermore, compositions of the invention can be used to achieve methods of the invention.

It will be understood that particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention can be employed in various embodiments without departing from the scope of the invention. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this invention and are covered by the claims.

All publications and patent applications mentioned in the specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. The use of the word "a" or "an" when used in conjunction with the term "comprising" in the claims and/or the specification may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one." The use of the term "or" in the claims is used to mean "and/or" unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and "and/or." Throughout this application, the term "about" is used to indicate that a value includes the inherent variation of error for the device, the method being employed to determine the value, or the variation that exists among the study subjects.

As used in this specification and claim(s), the words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. In embodiments of any of the compositions and methods provided herein, "comprising" may be replaced with "consisting essentially of or "consisting of. As used herein, the phrase "consisting essentially of requires the specified integer(s) or steps as well as those that do not materially affect the character or function of the claimed invention. As used herein, the term "consisting" is used to indicate the presence of the recited integer (e.g., a feature, an element, a characteristic, a property, a method/process step or a limitation) or group of integers (e.g., feature(s), element(s), characteristic(s), propertie(s), method/process steps or limitation(s)) only.

The term "or combinations thereof as used herein refers to all permutations and combinations of the listed items preceding the term. For example, "A, B, C, or combinations thereof is intended to include at least one of: A, B, C, AB, AC, BC, or ABC, and if order is important in a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing with this example, expressly included are combinations that contain repeats of one or more item or term, such as BB, AAA, AB, BBC, AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan will understand that typically there is no limit on the number of items or terms in any combination, unless otherwise apparent from the context.

As used herein, words of approximation such as, without limitation, "about", "substantial" or "substantially" refers to a condition that when so modified is understood to not necessarily be absolute or perfect but would be considered close enough to those of ordinary skill in the art to warrant designating the condition as being present. The extent to which the description may vary will depend on how great a change can be instituted and still have one of ordinary skilled in the art recognize the modified feature as still having the required characteristics and capabilities of the unmodified feature. In general, but subject to the preceding discussion, a numerical value herein that is modified by a word of approximation such as "about" may vary from the stated value by at least ±1, 2, 3, 4, 5, 6, 7, 10, 12 or 15%.

Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a "Field of Invention," such claims should not be limited by the language under this heading to describe the so-called technical field. Further, a description of technology in the "Background of the Invention" section is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the "Summary" to be considered a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to "invention" in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.

All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.