SP2-Bonded Carbon Structures

Abstract

A microstructure comprises a plurality of interconnected units wherein the units are formed of graphene tubes. The graphene tubes may be formed by photo-initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice, removing unpolymerized monomer, coating the polymer microlattice with a metal, removing the polymer microlattice to leave a metal microlattice, depositing graphitic carbon on the metal microlattice, converting the graphitic carbon to graphene, and removing the metal microlattice.

Claims

1. A microstructure comprising: a plurality of interconnected units including at least a first unit formed of first graphene tubes; and a second unit formed of second graphene tubes wherein one or more of the second graphene tubes are connected to one or more of the first graphene tubes.

2. The microstructure recited in claim 1 wherein the graphene tubes are arranged in an ordered structure and form symmetric patterns that repeat along the principal directions of three-dimensional space.

3. The microstructure recited in claim 1 wherein the graphene tubes form a rigid structure.

4. The microstructure recited in claim 1 wherein the plurality of interconnected units forms a microlattice.

5. The microstructure recited in claim 1 wherein the graphene tubes are hollow.

6. The microstructure recited in claim 1 wherein the graphene tubes are interconnected by chemical electronic bonds.

7. A method of forming a graphene microstructure comprising: photo-initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer; coating the polymer microlattice with a metal; removing the polymer microlattice to leave a metal microlattice; depositing graphitic carbon on the metal microlattice; converting the graphitic carbon to graphene; and removing the metal microlattice.

8. The method recited in claim 7 wherein photo-initiating the polymerization of the monomer comprises passing collimated light through a photomask.

9. The method recited in claim 7 wherein photo-initiating the polymerization of the monomer comprises multi-photon lithography.

10. The method recited in claim 7 wherein coating the polymer microlattice with a metal comprises the electroless deposition of copper or nickel.

11. The method recited in claim 7 wherein the polymer microlattice comprises polystyrene or poly(methyl methacrylate).

12. A graphene microstructure prepared by the process comprising the steps of: photo-initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer; coating the polymer microlattice with a metal; removing the polymer microlattice to leave a metal microlattice; depositing graphitic carbon on the metal microlattice; converting the graphitic carbon to graphene; and removing the metal microlattice.

13. The graphene microstructure recited in claim 12 wherein photo-initiating the polymerization of the monomer comprises passing collimated light through a photomask.

14. The graphene microstructure recited in claim 12 wherein photo-initiating the polymerization of the monomer comprises multi-photon lithography.

15. The graphene microstructure recited in claim 12 wherein coating the polymer microlattice with a metal comprises the electroless deposition of copper or nickel.

16. The graphene microstructure recited in claim 12 wherein the polymer microlattice comprises polystyrene or poly(methyl methacrylate).

17. A method of forming a metal microstructure comprising: photo-initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer; coating the polymer microlattice with a metal; and removing the polymer microlattice to leave a microlattice of interconnected metal tubes in a pattern of interconnected units.

18. A metal/graphene microstructure prepared by the process comprising the steps of: photo-initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer; coating the polymer microlattice with a metal; removing the polymer microlattice to leave a metal microlattice; depositing graphitic carbon on the metal microlattice; and converting the graphitic carbon to graphene.

19. A method of forming a metal/polymer microstructure comprising: photo-initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer; coating the polymer microlattice with a metal; removing the polymer microlattice to leave a metal microlattice; exposing a surface of the metal microlattice to a hydroxylated alkyl mercaptan to produce a hydroxylated metal surface; and reacting hydroxyl functional groups on the hydroxylated metal surface with reactive functional groups of a pre-polymer matrix.

20. A composite material comprising: a metal-based microlattice embedded within an organic polymeric matrix wherein the metal-based microlattice comprises a plurality of interconnected units including at least a first unit formed of first metal tubes, and a second unit formed of second metal tubes wherein one or more of the second metal tubes are connected to one or more of the first metal tubes and a surface of the metal-based microlattice is functionalized with functional groups that provide anchoring or reactions sites for interaction with the organic polymeric matrix.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0012] FIG. 1A is a schematic drawing of a fabrication process for a metal-based microlattice template in accordance with an example.

[0013] FIG. 1B is a flowchart for the fabrication process depicted schematically in FIG. 1A.

[0014] FIG. 2 is a schematic diagram of the fabrication of an electrically, thermally, or both thermally and electrically conductive composite by embedding a fabricated metal-based or graphene-based microlattice within an organic polymer matrix.

[0015] FIG. 3 is a schematic diagram of a surface functionalization methodology for functionalizing a fabricated metal-based microlattice.

DETAILED DESCRIPTION

[0016] It has been found that an organic/inorganic superstructure may be used as a template for the formation of a 3D metal superstructure that may then be used to grow graphitic carbon on the surface of the metal. The template may be fabricated through a self-propagating photopolymer waveguide technique (see, e.g., Xiaoyu Zheng et. al., Ultralight, Ultrastiff Mechanical Metamaterials; Science 344 (2014) 1373-1377 and T. A. Schaedler, et al., Ultralight Metallic Microlattices; Science 334 (2011) 962-965) in which an interconnected 3D photopolymer lattice may be produced upon exposure of an appropriate liquid photomonomer to collimated UV light through a specifically designed mask that contains openings with certain spacing and size. The fabricated microlattice may then be coated by electroless copper or other suitable metal (e.g. Ni, Co, Au, Ag, Cu, and alloys thereof) followed by etching away the organic polymeric matrix (scaffold). The resulting metal-based microlattice may be then used as a template to grow the graphitic carbon. The thickness of the electroless plated metal may be controlled in the range of nanometer to micrometer by adjusting the plating time, temperature, and/or plating chemistry.

[0017] FIG. 1A schematically illustrates an exemplary fabrication process of organic polymeric microlattices (scaffolds) prior to coating with electroless plating.

[0018] The present disclosure is of a periodically structured carbon nanostructure. The carbon nanostructures of the prior art are irregular and have much larger dimensions than those which may be achieved using the methodology disclosed herein.

[0019] The present process may be used to create a regular array, and the superstructure dimensions (unit) and structure may be optimized for strength, thermal and other fundamental properties.

[0020] There are several aspects of this procedure that are noteworthy: [0021] it provides a regular structure with defined dimensions; [0022] it can form very thin metal (e.g. Ni, Co, Cu, Ag, Au) microlattices; [0023] it enables the formation of graphitic carbon on very thin metals by a surface-limited process for very thin metal wires or tubes.

[0024] The present process uses a polymeric structure as a template for such fabrication with the subsequent formation of a metal superstructure that may then be exposed to a hydrocarbon (e.g. methane, ethylene, acetylene, benzene) to form graphitic carbon, followed by etching of the metal from under the graphitic carbon using appropriate etchants such as, for example, FeCl.sub.3 or potassium permanganate.

[0025] Collimated light through a photomask or multi-photon lithography may be used in a photo-initiated polymerization to produce a polymer microlattice comprised of a plurality of interconnected units. Exemplary polymers include polystyrene and poly(methyl methacrylate) (PMMA). Once polymerized in the desired pattern, the remaining un-polymerized monomer may be removed.

[0026] The polymer structure (polymer scaffold) may then be plated with a suitable metal using an electroless plating process.

[0027] Electroless nickel plating (EN) is an auto-catalytic chemical technique that may be used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid workpiece, such as metal, plastic, or ceramic. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO.sub.2H.sub.2.H.sub.2O) which reacts with the metal ions to deposit metal. Alloys with different percentages of phosphorus, ranging from 2-5 (low phosphorus) to up to 11-14 (high phosphorus) are possible. The metallurgical properties of the alloys depend on the percentage of phosphorus.

[0028] Electroless plating has several advantages over electroplating. Free from flux-density and power supply issues, it provides an even deposit regardless of workpiece geometry, and with the proper pre-plate catalyst, may deposit on non-conductive surfaces. In contradistinction, electroplating can only be performed on electrically conductive substrates.

[0029] Before performing electroless plating, the material to be plated must be cleaned by a series of chemicals; this is known as the pre-treatment process. Failure to remove unwanted soils from the part's surface results in poor plating. Each pre-treatment chemical must be followed by water rinsing (normally two to three times) to remove chemicals that may adhere to the surface. De-greasing removes oils from surfaces, whereas acid cleaning removes scaling.

[0030] Activation may be done with an immersion into a sensitizer/activator solutionfor example, a mixture of palladium chloride, tin chloride, and hydrochloric acid. In the case of non-metallic substrates, a proprietary solution is often used.

[0031] The pre-treatment required for the deposition of metals on a non-conductive surface usually consists of an initial surface preparation to render the substrate hydrophilic. Following this initial step, the surface may be activated by a solution of a noble metal, e.g., palladium chloride. Electroless bath formation varies with the activator. The substrate is then ready for electroless deposition.

[0032] The reaction is accomplished when hydrogen is released by a reducing agent, normally sodium hypophosphite (with the hydrogen leaving as a hydride ion) or thiourea, and oxidized, thus producing a negative charge on the surface of the part. The most common electroless plating method is electroless nickel plating, although silver, gold and copper layers can also be applied in this manner.

[0033] In principle, any hydrogen-based reducing agent can be used although the redox potential of the reducing half-cell must be high enough to overcome the energy barriers inherent in liquid chemistry. Electroless nickel plating most often employs hypophosphite as the reducer while plating of other metals like silver, gold and copper typically makes use of low-molecular-weight aldehydes.

[0034] A benefit of this approach is that the technique can be used to plate diverse shapes and types of surfaces.

[0035] As illustrated in FIG. 1B, the organic polymeric microlattice may be electrolessly plated with metal followed by dissolving out the organic polymer scaffold. The resulting metal-based microlattice may then be coated with a thin layer of immersion tin to prevent the metal from oxidizing during the subsequent process which may include a heat treatment. Alternatively, the surface of the metal-based microlattice may be functionalized with appropriate functional groups to provide anchoring or reactions sites for subsequent interaction with a polymer matrix. A copper/nickel super-lattice may be used. The fabricated and surface-treated metal-based microlattice may be embedded within an organic polymeric matrix to fabricate an electrically or thermally or both an electrically and thermally conductive composite (see FIG. 2). Alternatively, the fabricated metal-based microlattice may be used as a template to synthesize a graphitic carbon superstructure. The metal may then be etched out to produce a graphene microstructure comprising a plurality of interconnected units wherein the units are formed of graphene tubes that are interconnected by chemical electronic bonds (as distinguished from van der Waals forces which may cause carbon nanotubes to agglomerate).

[0036] FIG. 3 schematically illustrates a surface treatment methodology for functionalizing the fabricated metal-based microlattice. As it can be seen, the surface of the metal microlattice may be exposed to a mercaptan-based compound. The mercaptan-based compound may be a hydroxylated alkyl mercaptan such as 3-mercapto-1,2 dihydroxy propane or it may be based on an isocyanate function group such as isocyanate alkyl trialkoxy silane. The mercaptan-based compound bonds to the metal (such as e.g. copper, silver or gold) through the sulfur atom of the mercapto moiety resulting in a hydroxylated metal surface. The hydroxyl functional groups implanted on the metal surface may then be reacted with a reactive functional group from the pre-polymer matrix. In the case of isocyanate-based mercaptan compounds, the free isocyanate functional groups may subsequently be reacted with certain functional groups of the organic polymeric matrix such as OH, NH, etc. resulting in the formation of chemical bonds at the copper-organic polymer interface. The polymer for the composite may be selected for its mechanical properties and/or electronic properties. Exemplary polymers include fluorocarbons (such as polytetrafluoroethylene) and polybutadiene.

[0037] In an example, the growth of graphene on the metal tubes may be omitted to produce a mircolattice of interconnected metal tubes. A process for forming such a metal microstructure may comprise: photo-initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer; coating the polymer microlattice with a metal; and removing the polymer microlattice to leave a microlattice of interconnected metal tubes in a pattern of interconnected units.

[0038] In another example, a metal/graphene microstructure of graphene-coated metal tubes in a pattern of interconnected units may be prepared by the process comprising the steps of: photo-initiating the polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer; coating the polymer microlattice with a metal; removing the polymer microlattice to leave a metal microlattice; depositing graphitic carbon on the metal microlattice; and converting the graphitic carbon to graphene.

[0039] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.