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: interconnected units including 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 2D h-BN tubes coated with SiO.sub.2, wherein at least one of the second 2D h-BN tubes is connected to at least one of the first 2D h-BN tubes.

2. The microstructure of 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 of claim 1, wherein the interconnected units of 2D h-BN tubes form a rigid structure.

4. The microstructure of claim 1, wherein the interconnected units form a microlattice.

5. The microstructure of claim 1, wherein the 2D h-BN tubes are hollow.

Description

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

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

(2) FIG. 1B is a flowchart for the fabrication process depicted schematically in FIG. 1A.

DETAILED DESCRIPTION

(3) 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.

(4) 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.

(5) 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). As illustrated schematically in FIG. 1A, an interconnected 3D photopolymer lattice may be produced upon exposure of an appropriate liquid photomonomer 16 to collimated UV light 12 through a specifically designed (e.g using a computer-aided design model 10) digital mast 14 that contains openings with certain spacing and size. The fabricated organic polymer template microlattice 18 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.

(6) FIG. 1A schematically illustrates an exemplary fabrication process of organic polymeric microlattices (scaffolds) 18 prior to coating with electroless plating.

(7) 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.

(8) 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.

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

(10) 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.

(11) Collimated UV light 12 through a photomask 14 or multi-photon lithography may be used in a photo-initiated polymerization to produce a polymer microlattice 18 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.

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

(13) 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.

(14) 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.

(15) 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.

(16) Activation may be done with an immersion into a sensitizer/activator solution—for example, a mixture of palladium chloride, tin chloride, and hydrochloric acid. In the case of non-metallic substrates, a proprietary solution is often used.

(17) 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.

(18) 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.

(19) 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.

(20) A benefit of this approach is that the technique can be used to plate diverse shapes and types of surfaces.

(21) As illustrated in FIG. 1B, the organic polymeric microlattice may be electrolessly plated 20 with metal followed by dissolving out 22 the organic polymer scaffold. The resulting metal-based microlattice may be used in several applications 24—e.g. it 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.

(22) 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.

(23) 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.

(24) Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.