HEXAGONAL BORON NITRIDE STRUCTURES

Abstract

A microstructure comprises a plurality of interconnected units wherein the units are formed of hexagonal boron nitride (h-BN) 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 an h-BN precursor on the metal microlattice, converting the h-BN precursor to h-BN, and removing the metal microlattice.

Claims

1. A microstructure comprising: a plurality of interconnected units including at least a first unit formed of first two-dimensional hexagonal boron nitride (2D h-BN) tubes; and a second unit formed of second two-dimensional hexagonal boron nitride (2D h-BN) tubes, wherein one or more of the second two-dimensional hexagonal boron nitride (2D h-BN) tubes are connected to one or more of the first two-dimensional hexagonal boron nitride (2D h-BN) tubes.

2. The microstructure recited in claim 1 wherein the 2D h-BN 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 interconnected units of 2D h-BN 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 2D h-BN tubes are hollow.

6. A method of forming a 2D h-BN 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 2D h-BN precursor on the metal microlattice; converting the 2D h-BN precursor to 2D h-BN; and removing the metal microlattice.

7. The method recited in claim 6 wherein photo-initiating the polymerization of the monomer comprises passing collimated light through a photomask.

8. The method recited in claim 6 wherein photo-initiating the polymerization of the monomer comprises multi-photon lithography.

9. The method recited in claim 6 wherein coating the polymer microlattice with a metal comprises the electroless deposition of nickel.

10. The method recited in claim 6 wherein the polymer microlattice comprises polystyrene or poly(methyl methacrylate).

11. A 2D h-BN 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 a 2D h-BN precursor on the metal microlattice; converting the 2D h-BN precursor to 2D h-BN; and removing the metal microlattice.

12. The 2D h-BN microstructure recited in claim 11 wherein photo-initiating the polymerization of the monomer comprises passing collimated light through a photomask.

13. The 2D h-BN microstructure recited in claim 11 wherein photo-initiating the polymerization of the monomer comprises multi-photon lithography.

14. The 2D h-BN microstructure recited in claim 11 wherein coating the polymer microlattice with a metal comprises the electroless deposition of nickel.

15. The 2D h-BN microstructure recited in claim 11 wherein the polymer microlattice comprises polystyrene.

16. The 2D h-BN microstructure recited in claim 11 wherein the polymer microlattice comprises poly(methyl methacrylate).

17. A microstructure comprising: a plurality of interconnected units including at least a first unit formed of first two-dimensional hexagonal boron nitride (2D h-BN) tubes coated with SiO.sub.2; and a second unit formed of second two-dimensional hexagonal boron nitride (2D h-BN) tubes coated with SiO.sub.2, wherein one or more of the second tubes formed of two-dimensional hexagonal boron nitride (2D h-BN) tubes coated with SiO.sub.2 are connected to one or more of the first tubes formed of two-dimensional hexagonal boron nitride (2D h-BN) tubes coated with SiO.sub.2.

18. The microstructure recited in claim 17 wherein the interconnected units consist essentially of parallel tubes.

19. The microstructure recited in claim 18 wherein the parallel tubes are hollow.

20. A method of forming a metal/2D h-BN 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 2D h-BN precursor on the metal microlattice; and converting the 2D h-BN precursor to 2D h-BN.

Description

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

[0019] FIG. 1A is a schematic drawing of a fabrication process for a metal-based microlattice template in accordance with an example.

[0020] FIG. 1B is a flowchart for the fabrication process depicted schematically in FIG. 1A.

DETAILED DESCRIPTION

[0021] Hexagonal boron nitride may be essentially flat or 2D. Forming h-BN in one regular repeating structure has not been reported in the prior art. Superstructures of these may provide very strong, light, highly thermally conductive structures. Attempts have been made to fabricate h-BN sponges, however these structures are irregular and exhibit properties that vary with position.

[0022] Growth of regular 3D superstructures using h-BN could address the shortcomings of the flexible h-BN for 3D applications given that h-BN is strong, chemically inert, and an electrical insulator. These new superstructures may then be used for many applications from packaging, thin optically transparent strong thin films, and many more.

[0023] 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 h-BN 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 then coated by electroless nickel or other suitable metal (e.g. Cu, Co, Au, Ag, Pt, Ir, Ru 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 h-BN. 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.

[0024] FIG. 1A schematically illustrates an exemplary fabrication process of organic polymeric microlattices (scaffolds) prior to coating with electroless plating.

[0025] The present disclosure is of a periodically structured h-BN nanostructure. The h-BN nanostructures of the prior art are irregular and have much larger dimensions than those which may be achieved using the methodology disclosed herein.

[0026] 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.

[0027] There are several aspects of this procedure that are noteworthy: [0028] it provides a regular structure with defined dimensions; [0029] it can form very thin metal (e.g. Ni, Co, Cu, Ag, Au) microlattices; [0030] it enables the formation of h-BN on very thin metals by a surface-limited process for very thin h-BN wires or tubes.

[0031] 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 borazine (BH).sub.3(NH).sub.3 to form h-BN, followed by etching of the metal from under the h-BN using appropriate etchants such as, for example, FeCl.sub.3 or potassium permanganate.

[0032] 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.

[0033] The polymer structure (polymer scaffold) may then be plated with a suitable metal using an electroless plating process.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] A benefit of this approach is that the technique can be used to plate diverse shapes and types of surfaces.

[0042] 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. The fabricated metal-based microlattice may be used as a template to synthesize an h-BN superstructure. The metal may then be etched out to produce an h-BN microstructure comprising a plurality of interconnected units wherein the units are formed of h-BN tubes. The tubes that form the h-BN microstructure may be arranged in an ordered structure and form symmetric patterns that repeat along the principal directions of three-dimensional space

[0043] In another example, SiO.sub.2 may be deposited around the h-BN tubes. Such SiO.sub.2-coated h-BN tubes may have application in the fabrication of integrated circuits having enhanced heat dissipation characteristics.

[0044] In yet another example, the metal microlattice may be retained. A process for forming such a metal/2D h-BN 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; removing the polymer microlattice to leave a metal microlattice; depositing 2D h-BN precursor on the metal microlattice; and converting the 2D h-BN precursor to 2D h-BN.

[0045] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.