WO2018236847A1

WO2018236847A1 - Boron nitride foam, methods of manufacture thereof, and articles containing the boron nitride foam

Info

Publication number: WO2018236847A1
Application number: PCT/US2018/038249
Authority: WO
Inventors: Qiaochu Han; Li Zhang
Original assignee: Rogers Corporation
Priority date: 2017-06-19
Filing date: 2018-06-19
Publication date: 2018-12-27

Abstract

A boron nitride foam includes a structure defined by a three-dimensional network of interconnected open cells defined by cell walls, wherein the cell walls include a plurality of hexagonal boron nitride sheets, and have a thickness defined by a thickness of the plurality of hexagonal boron nitride sheets.

Description

BORON NITRIDE FOAM, METHODS OF MANUFACTURE THEREOF, AND ARTICLES CONTAINING THE BORON NITRIDE FOAM

BACKGROUND

[0001] Boron nitride can exist in different lattice structures. Common lattice structures include a hexagonal, cubic, and wurtzite (i.e., turbostratic) structure. Of these, hexagonal boron nitride has great chemical stability and softness, and therefore, has been broadly used in a variety of applications. The hexagonal lattice structure includes stacked layers of interconnected hexagons comprising covalently bonded boron and nitrogen atoms. The positions of the B and N atoms alternate from layer to layer, resulting in a sheet structure analogous to graphene. Generally, covalent bonds are relatively strong. The strong covalent bonding within each boron nitride layer leads to high mechanical strength and very good thermal conductivity in the x-y plane of the sheet material. On the other hand, adhesion between the boron nitride layers is relatively weak. This weak bonding results in lower mechanical strength and thermal conductivity perpendicular to the x-y plane, i.e., in the direction of the thickness of the material, or z-axis.

[0002] To address at least some of the drawbacks associated with hexagonal boron nitride sheets, there is a need for new boron nitride materials. It would be particularly advantageous if the materials had one or more of improved thermal conductivity, mechanical strength, or elasticity. It would be especially desirable for to have materials with one or more of these properties improved in the z-axis of hexagonal boron nitride sheets.

BRIEF DESCRIPTION

[0003] A boron nitride foam comprises a structure defined by a three-dimensional network of interconnected open cells defined by cell walls, wherein the cell walls comprise a plurality of hexagonal boron nitride sheets, and have a thickness defined by a thickness of the plurality of hexagonal boron nitride sheets.

[0004] A method of preparing the above-described boron nitride foam comprises providing a dispersion comprising a plurality of hexagonal boron nitride sheet and a freeze castable medium, disposing the dispersion in a mold; freezing the dispersion to form foam in the medium; and removing the medium from the foam.

[0005] Further described herein is a boron nitride foam made by the above-described methods. [0006] A composition comprises the above-described boron nitride foam, and an additional material.

[0007] An article comprises the above-described boron nitride foam, or the above- described composition.

[0008] The above described and other features are exemplified by the following Figures and Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The following Figures are exemplary embodiments.

[0010] FIG. 1 illustrates a cross-section of a boron nitride foam cut across the short axis of the cells.

[0011] FIG. 2 illustrates a section of the boron nitride foam of FIG. 1 cut across the longer axis of the cells.

[0012] FIG. 3 illustrates a boron nitride foam having a honeycomb structure.

[0013] FIGS. 4A-4D illustrates cross-sections of foams having different types of honeycomb structures.

[0014] FIGS. 5 A to 5C are a schematic illustration of a method of preparing a boron nitride foam.

DETAILED DESCRIPTION

[0015] Described herein is a boron nitride foam formed of hexagonal boron nitride sheets. This three-dimensional boron nitride foam is especially useful for thermal

management systems. Advantageously, the boron nitride foam can provide improved thermal conductivity and mechanical strength in thermal management applications. In a further advantage, the boron nitride foam can be elastic, in that it returns to its original thickness upon release of pressure, with good hysteresis with respect to its elasticity.

[0016] The boron nitride foam has a structure defined by a three-dimensional network of interconnected open cells, which in some embodiments are further ordered. With respect to foam structures, those skilled in the art will appreciate that a cell within a foam defines a pore or opening within the foam structure. The cells of the boron nitride foam are open cells, in that the pore or opening which the cells define is not fully encased by a cell wall. In other words, the cell has an opening in it through which matter such as gas or liquid can pass.

Usually the cells will have two separate openings through which matter such as gas or liquid can pass. [0017] In some embodiments, the foam comprises greater than 40% of open cells, for example at least 60% open cells, or at least 70% open cells, or at least 80% open cells, or at least 90% open cells. The percentage of open cells can be based on a volume percent based on a total volume of the foam. In some embodiments, substantially all cells in the boron nitride foam are open cells.

[0018] The open cells of the boron nitride foam are interconnected. As used herein, the term "interconnected" cells means that a cell wall that defines a given cell also defines at least part of an adjacent cell. In other words, the open cells in the foam share common cell walls. A cell at the edge of a foam structure can of course have a cell wall that is not common with an adjacent cell. Thus, a feature of the open cells within the boron nitride foam is that they are defined by the cell walls. The term "cell wall(s)" as used herein refers to a structural feature that defines the volume of the cells. In certain embodiments, the cells are at least partially enclosed, despite being interconnected, which can also be referred as being alveolar or capsular in character.

[0019] Each open cell of the boron nitride foam has a pore size. The size of the individual cells defined by the cell walls can vary widely, even within the same sample. The diameter of an individual cell in a sample can be determined, for example, by scanning electron microscopy (SEM) of a cross-section of the sample. As used herein, the term "pore size" refers to the distance presented by the largest diameter of a given cell in a cross- sectional sample of the foam. As the cross sectional shape of a given cell may not be circular, reference to the cross sectional "diameter" is intended to mean the largest cross sectional distance between the cell walls. As there may be slight variation in pore size of a given boron nitride foam, it is often more convenient to refer to the average pore size. In an embodiment the open cells have an average pore size of 0.1 micrometer (μπι) to 1 millimeter (mm), or 0.5 μπι to 1 mm, depending on the manufacturing conditions. In some

embodiments the average pore size can be 1 to 100 μπι, or 10 to 80 μπι, or 20 to 60 μπι.

[0020] In addition to the cell walls being interconnected, the open cells of the boron- nitride foam can be essentially random or ordered to some degree. As used herein, the term "ordered" means that as viewed in any cross-section, the cells are not all randomly oriented relative to each other. In other words, upon viewing a collection of adjoining cells it is apparent that the cells are arranged in a non-random fashion. The degree of order can be lesser or greater. It is also possible for the cells to appear random in one cross-section, but ordered in another. For example, in a foam sample containing cells that are elongated in one direction, a cross-section across the short axis of the cells shows an essentially random (anisotropic) orientation, as shown in FIG. 1. However, a cross-section across the long axis of the cells shows a low degree of ordering along the longer axis of the cells, as shown in FIG. 2. Accordingly, reference to open cells being "ordered" is intended to mean a collection of adjacent cells are oriented in a similar direction along a given axis. Where the cells are ordered, their general orientation can progressively vary throughout the foam. Nevertheless, regardless of a progressive change in orientation of cells relative to each other, it will still be apparent that they are present in an ordered fashion.

[0021] With further reference to FIG. 1 and FIG. 2, it can be seen that the degree of ordering in a foam can further affect the apparent pore size of the foam. Thus, the apparent average pore size of the foam measured from the cross-section shown in FIG. 1 is less than the apparent average pore size determined from the cross section shown in FIG. 2. Because the pore sizes in an ordered foam can vary depending on the cross-section analyzed, as used herein, "longest axis pore size" of an ordered foam as used herein refers to the dimension of the longest axis of the cells, i.e., the dimensions determined from a cross-section as shown in FIG. 2. In an embodiment, in an ordered foam, the cells have an average longest axis pore size of 0.1 μπι to 1 mm, or 0.5 μιη to 1 mm, or 1 to 100 μπι, or 10 to 80 μπι, or 20 to 60 μιη depending on the manufacturing conditions.

[0022] In some embodiments the cell structure present in the boron nitride foams can be more ordered, or highly ordered, for example as shown in FIG. 3. FIG. 3 shows a highly ordered honeycomb structure of hexagonal cells. It is to be understood that this structure is not limiting, and that foams having a honeycomb structure can have various cross-sectional shapes and sizes as shown in FIGS. 4A-4D. The edge connectivity (i.e., the number of cell wall edges that intersect together) can accordingly vary, where in some embodiments, the open cells have an edge connectivity of 3, 4, 5, 6, or more. In some embodiments, the square cells shown in FIG. 4A have an edge connectivity of 4 and the triangular cells as shown in FIG. 4B have an edge connectivity of 6. Further as shown in FIG. 4C and 4D, different cell sizes can be present in the same structure. It is further to be understood that in any ordered structure, there can be degrees of structural irregularities that deviate from the generally ordered structure.

[0023] As described above, the open cells of the boron nitride foam are defined by cell walls. The cell walls in turn comprise a plurality of hexagonal boron nitride sheets. The hexagonal boron nitride sheets that form the cell walls can include a functional group, preferably a carboxyl group, aldehyde group, ketone group, hydroxyl group, thiol group, amino group, amide group, sulfate group, sulfonate group, phosphate group, phosphonate group, halogen, (meth)acryloxy group, vinyl group, allyl group, tri(Ci-6 alkyl)silyl group, or a combination comprising at least one of the foregoing. In some embodiments, the cell walls are formed from only hexagonal boron nitride sheets.

[0024] Generally, hexagonal boron nitride sheets include boron and nitrogen atoms forming interconnected hexagons. Each hexagon includes three boron atoms and three nitrogen atoms. Boron and nitrogen alternate in the hexagonal ring. Each of these atoms is trivalent and is covalently bonded to its neighbor. This arrangement results in stacked layers of interconnected hexagons. A layer of such covalently interconnected boron nitride hexagons is usually referred to as a sheet. Without wishing to be limited by theory, the formation of the cells as described above is believed to stem at least in part from the presence of weak van der Waals forces between hexagonal boron nitride sheets.

[0025] The cell walls have a thickness defined by the thickness of a plurality of hexagonal boron nitride sheets. The boron nitride sheets that form the cell walls can be present in a layered formation. For example, at least a section of a cell wall can be formed of three to five layered hexagonal boron nitride sheets, with these sheets being oriented to form the cell wall such that the thickness of the three to five layered sheets define the thickness of the cell wall. In some embodiments, the cell walls have a thickness defined by the thickness of a plurality of layered hexagonal boron nitride sheets. Thus, as used herein, reference to the "thickness of a plurality of layered hexagonal boron nitride sheets" means the cumulative sheet thickness of the plurality of layered boron nitride sheets.

[0026] The boron nitride sheets are not only present as a plurality of layers to define the thickness of the cell wall, but some of the layered sheets can also only partially overlap. Accordingly, the cell walls can be constructed of a plurality of layered boron nitride sheets, some of which can only partially overlap. Despite some of the boron nitride sheets only partially overlapping within the layered structure, the thickness of the cell wall will nevertheless be defined by at least two layered boron nitride sheets. As there may be slight variations in cell wall thickness across each cell in a given foam, it is usually more convenient to refer to the average cell wall thickness. The average cell wall thickness can vary widely, for example from 2 nm (nanometer) to 5 mm. In some embodiments the average cell wall thickness is 0.1 micrometer to 5 millimeters, or 1 micrometer to 1 millimeter. In other embodiments, the average cell wall thickness can be 2 to 10,000 nm, or 2 to 700 nm, or 2 to 500 nm, or 2 to 250 nm, or 2 to 100 nm, or 2 to 50 nm, or 2 to 30 nm. The cell walls can comprise any number of hexagonal boron nitride sheets more than one. For example, the cell walls can comprise 2 to 1,000 hexagonal boron nitride sheets, or 2 to 100 hexagonal boron nitride sheets, or 2 to 50 hexagonal boron nitride sheets.

[0027] In some embodiments, the average cell wall thickness of the foam is less than the average pore size of a random foam, or less than the longest axis pore size of an ordered foam. For example, the ratio of the average wall thickness to the average pore size can be 1 : 50 to 1 :25,000. In some embodiments, the ratio of the average wall thickness to the average pore size of a random foam can be 1 : 100 to 1 : 10,000, or 1 :500 to 1 :8,000, or 1 : 1,000 to 1 : 8,000. In other embodiments, the ratio of the average wall thickness to the longest axis pore size of an ordered foam can be 1 : 100 to 1 : 10,000, or 1 : 500 to 1 : 8,000, or 1 : 1,000 to 1 :8,000.

[0028] It will be appreciated that structural features of the foam such as the pore size and cell wall thickness can influence the overall density of the boron nitride foam. These foams can advantageously be prepared to exhibit a variety of densities including a very low density. For example, the foams can be prepared having a density of only 0.5 milligrams per centimeter cubed (mg/cm³). Surprisingly, even at such low densities the foams can still exhibit improved properties such as excellent elasticity. In an embodiment, the density of the boron nitride foam is 0.5 to 1,000 mg/cm³, or 0.5 to 700 mg/cm³, or 0.5 to 500 mg/cm³, or 0.5 to 100 mg/cm³. In other embodiments the density of the boron nitride foam is 0.5 to 50 mg/cm³, or 0.5 to 10 mg/cm³, or 0.5 to 7 mg/cm³, or 0.5 to 5 mg/cm³. In some instances, the boron nitride foam has a density of 0.5 to 2 mg/cm³.

[0029] The boron nitride foams can advantageously exhibit improved mechanical properties. For example, the foams or compositions comprising the foams can exhibit excellent structural elasticity. The structural elasticity of the foams can be observed when measuring their compression set. As used herein, the term "compression set" means a measurement of the permanent deformation remaining after release of a compressive stress that is applied to the foam or composition. Compression set is expressed as the percentage of the original deflection (i.e., a constant deflection test). Accordingly, a test specimen of the boron nitride foam or composition comprising the boron nitride foam is compressed at a nominated % for one minute at 25 degrees Celsius (°C). Compression set is taken as the percentage of the original deflection after the specimen is allowed to recover at standard conditions for 30 minutes. The compression set value C can be calculated using the formula [(to-ti)/(to-tn)] ^x 100, where to is the original specimen thickness, ti is the specimen thickness after testing, and t_n is the spacer thickness which sets the % compression that the foam is to be subjected. For comparative results, the specimens to be tested should have the same dimensions, e.g., where the diameter is 12 mm, and the height is 8 mm. The compression set measurement is based on that outlined in ASTM D395, for example, ASTM D395-16el .

[0030] In some embodiments, the boron nitride foam has a compression set at 15% compression of 20% or less, or 15%> or less, or 10%> or less, or 5 to 20%. In other words, the foam specimen can be compressed 15%> of its volume or height and upon release of the compressive stress the 15%> deflection in the foam recovers by at least 97%, or at least 97.8%, or at least 98.5%, or 97 to 99.5%.

[0031] In some embodiments, the boron nitride foam has a compression set at 30% compression of 20% or less, or 15% or less, or 10% or less, or 5 to 20%. In some

embodiments, the boron nitride foam has a compression set at 50% compression of 15% or less, or 10% or less, or 7% or less, or 2 to 15%. In some embodiments, the boron nitride foam has a compression set at 70% compression of 15% or less, or 10% or less, or 7% or less, or 2 to 15%). In some embodiments, the boron nitride foam has a compression set at 80% compression of 15% or less, or 10% or less, or 5% or less, or 2 to 15%. In some

embodiments, the boron nitride foam has a compression set at 90% compression of 15% or less, or 10% or less, or 5% or less, or 2 to 15%. In some embodiments, the boron nitride foam has a compression set at 95% compression of 15% or less, or 10% or less, or 5% or less, or 2 to 15%.

[0032] In practical terms, the elastic properties of the boron nitride foams enable the foam to be highly compressed and yet have the ability to return into its original shape.

[0033] The boron nitride foams can advantageously exhibit improved thermal conductivity and dielectric properties. Accordingly, the boron nitride foams can have a thermal conductivity of 1 Watt per meter Kelvin (W/m K) or more, or 2 W/m K or more, or 4 W/m K or more, for example a thermal conductivity of 1 to 600 W/m K, or 10 to 200 W/m K, determined according to ASTM E1461, for example, ASTM E1461-13.

[0034] The foam can have a dielectric constant of 1.5 to 15, or 3 to 12, or 4 to 10. The dielectric constant can be determined at a temperature of 25°C and a frequency of 5.75 x 10⁹ Hertz. These properties can be attained at a very low foam density.

[0035] The boron nitride foams can be prepared by a method comprising fireeze- casting a dispersion of a hexagonal boron nitride sheet in a freeze castable medium. Freeze casting is a known wet shaping technique that can include preparing a dispersion of a material, disposing the dispersion into a mold, freezing the dispersion while it is in the mold, and then removing the freeze castable medium. Here, the dispersion of the material can comprise a plurality of boron nitride sheets, in a freeze castable medium. A review of freeze casting of porous materials is provided in International Materials Reviews 2012, vol 57, No. 1 page 37-60.

[0036] The boron nitride sheets used to form the boron nitride foams can be in the form of a platelet, flake, whisker, fiber, tube, or a combination comprising at least one of the foregoing. Accordingly, hexagonal boron nitride particles, flakes, whiskers, fibers, tubes, or a combination comprising at least one of the foregoing can be used. In an embodiment, the hexagonal boron nitride sheets are at least partially or fully exfoliated before freeze-casting. The boron nitride sheets can be functionalized as described above, or contain additional materials, for example, as dopants.

[0037] Those skilled in the art will appreciate that various liquids can be used as the freeze castable medium. Examples of freeze castable mediums include an aqueous medium, an ethanol medium, a dimethylformamide (DMF) medium, a N-methyl-2-pyrrolidone (NMP) medium, an acetone medium, a dimethylsulfoxide (DMSO) medium, or a combination comprising at least one of the foregoing.

[0038] Without wishing to be limited by theory, it is believed that upon freezing the dispersion, the hexagonal boron nitride sheets are rejected from the forming frozen particles, e.g., ice particles when an aqueous freeze castable medium is present, and entrapped in channels that develop between the growing frozen particles to form a continuous network. The ultimate structure of the continuous network is determined by the so-formed frozen particles. Accordingly, the boron nitride sheets are forced to align along the advancing freezing particle walls while at the same time they are concentrated and squeezed at the forming solid crystal boundaries. This concentration and compaction effect is believed to provide the ordered and layered cell wall construction. Again without being bound by theory, the ability of the boron nitride sheets to align along advancing freezing particles and be suitably compressed at the forming crystal boundaries is believed to be enhanced if the so- formed freezing particles are anisotropic in shape. Accordingly, in an embodiment the freeze castable medium provides anisotropic frozen particles during freeze casting.

[0039] Those skilled in the art of freeze casting will appreciate the general parameters for controlling the morphological features of porous materials. For example, the density of the boron nitride foam can be varied by adjusting the concentration of the dispersion that is to be subjected to the freeze casting. For example, a boron nitride foam having a density of up to 1,000 mg/cm³ can be formed from a very concentrated dispersion, whereas a boron nitride foam having as low as 0.5 mg/cm³ can be formed from a very dilute dispersion. In some embodiments, it is possible that the concentration of the dispersion can correspond closely to the achieved density, such that a boron nitride foam having a density of 7 mg/cm³ can be prepared using a dispersion comprising 7 milligrams per milliliter (mg/ml) of boron nitride, or a boron nitride foam having a density of 5 mg/cm³ can be prepared using a dispersion comprising 5 mg/ml of boron nitride, and so on.

[0040] The pore size of the foam can be varied by controlling parameters such as the freezing temperature, the concentration of the boron nitride, or a combination comprising at least one of the foregoing. For example, a boron nitride foam with 20 μιη pore size can be obtained by freeze casting of 5 mg/ml boron nitride dispersion in dry ice.

[0041] The cell wall thickness can be varied by controlling the density of boron nitride foam. For example, a boron nitride foam with density of 0.5 mg/cm³ can have an average wall thickness of 2 nm, or a boron nitride foam with density of 1.0 mg/cm³ can have an average wall thickness of 4 nm, or a boron nitride foam with density of 5 mg/cm³ can have an average wall thickness of 8 nm.

[0042] The freeze castable medium can be removed after freeze casting by freeze- drying, sublimation, supercritical drying, oven drying, air drying, or a combination comprising at least one or more of the foregoing.

[0043] The methods herein advantageously result in little shrinkage of the foam. Accordingly, the methods offer excellent control over the morphological features of the foam. If desired, boron nitride foam can be subjected to crosslinking via functional groups on the hexagonal boron nitride sheets. Such crosslinking can impart improved mechanical and electrical properties to the graphene foams.

[0044] A variety of compositions can be formulated that include the boron nitride foams. In some embodiments, the composition includes the boron nitride foam and an additional material, for example, a polymer, a metal, a non-metal, a ceramic, a glass, or a combination comprising at least one of the foregoing. Manufacture of the boron nitride foam and the polymer, metal, ceramic, glass, or other material can be by methods known in the art. For example, the foam can be impregnated or infiltrated with a prepolymer, melted polymer, polymer solution, melted metal, metal precursor, metal precursor solution, ceramic precursor, or the like. Alternatively, the foam can be present in the form of particulates, and used as an additive to any polymer, metal, nonmetal, ceramic, or glass composition.

[0045] The polymer can include a thermoplastic polymer. As used herein, the term "thermoplastic" refers to a material that is plastic or deformable, melts to a liquid when heated, and freezes to a brittle, glassy state when cooled sufficiently. Examples of thermoplastic polymers that can be used include polyacetals (e.g., polyoxyethylene and polyoxymethylene), poly(Ci-6 alkyl)acrylates, polyacrylamides (including unsubstituted and mono-N- and di-N-(C_1-8 alkyl)acrylamides), polyacrylonitriles, polyamides (e.g., aliphatic polyamides, polyphthalamides, and polyaramides), polyamideimides, polyanhydrides, polyarylene ethers (e.g., polyphenylene ethers), polyarylene ether ketones (e.g., polyether ether ketones (PEEK) and polyether ketone ketones (PEKK), polyarylene ketones, polyarylene sulfides (e.g., polyphenylene sulfides (PPS)), polyarylene sulfones (e.g., polyethersulfones (PES), polyphenylene sulfones (PPS), and the like), polybenzothiazoles, polybenzoxazoles, polybenzimidazoles, polycarbonates (including homopolycarbonates and polycarbonate copolymers such as polycarbonate-siloxanes, polycarbonate-esters, and polycarbonate-ester-siloxanes), polyesters (e.g., polyethylene terephthalates, polybutylene terephthalates, polyarylates, and polyester copolymers such as polyester-ethers),

polyetherimides (including copolymers such as polyetherimide-siloxane copolymers), polyimides (including copolymers such as polyimide-siloxane copolymers), poly(Ci-6 alkyl)methacrylates, polymethacrylamides (including unsubstituted and mono-N- and di-N- (Ci-₈ alkyl)acrylamides), cyclic olefin polymers (including polynorbornenes and copolymers containing norbornenyl units, for example copolymers of a cyclic polymer such as norbornene and an acyclic olefin such as ethylene or propylene), polyolefins (e.g., polyethylenes, polypropylenes, and their halogenated derivatives (such as

polytetrafluoroethylenes), and their copolymers, for example ethylene-alpha-olefin copolymers, polyoxadiazoles, polyoxymethylenes, polyphthalides, polysilazanes,

polysiloxanes (silicones), polystyrenes (including copolymers such as acrylonitrile-butadiene- styrene (ABS) and methyl methacrylate-butadiene-styrene (MBS)), polysulfides,

polysulfonamides, polysulfonates, polysulfones, polythioesters, polytriazines, polyureas, polyurethanes, vinyl polymers (including polyvinyl alcohols, polyvinyl esters, polyvinyl ethers, polyvinyl halides (e.g., polyvinyl fluoride), polyvinyl ketones, polyvinyl nitriles, polyvinyl thioethers, and polyvinylidene fluorides), or the like. A combination comprising at least one of the foregoing thermoplastic polymers can be used.

[0046] The polymer can also be a thermoset. Thermosets are derived from

thermosetting prepolymers (resins) that can irreversibly harden and become insoluble with polymerization or cure, which can be induced by heat or exposure to radiation (e.g., ultraviolet light, visible light, infrared light, or electron beam (e-beam) radiation). Thermoset polymers include alkyds, bismaleimide polymers, bismaleimide triazine polymers, cyanate ester polymers, benzocyclobutene polymers, diallyl phthalate polymers, epoxies,

hydroxymethylfuran polymers, melamine-formaldehyde polymers, phenolics (including phenol-formaldehyde polymers such as novolacs and resoles), benzoxazines, polydienes such as polybutadienes (including homopolymers and copolymers thereof, e.g. poly(butadiene- isoprene)), polyisocyanates, polyureas, polyurethanes, silicones, triallyl cyanurate polymers, triallyl isocyanurate polymers, certain silicones, and polymerizable prepolymers (e.g., prepolymers having ethylenic unsaturation, such as unsaturated polyesters, polyimides), or the like. The prepolymers can be polymerized, copolymerized, or crosslinked, e.g., with a reactive monomer such as styrene, alpha-methylstyrene, vinyltoluene, chlorostyrene, acrylic acid, (meth)acrylic acid, a (Ci-6 alkyl)acrylate, a (Ci-6 alkyl) methacrylates, acrylonitrile, vinyl acetate, allyl acetate, triallyl cyanurate, triallyl isocyanurate, acrylamide, or a combination comprising at least one of the foregoing.

[0047] Where the composition also includes a polymer, a liquid or solvated form of the polymer can be introduced into the cell structure of the foam. Alternatively, a monomer can be introduced into cell structure of the foam and subsequently polymerized to form a polymer. Accordingly, methods for preparing a composition comprising boron nitride foam and a polymer are also disclosed, the methods comprising introducing a liquid or solvated form of the polymer into the cell structure of the foam. In some embodiments, a method for preparing a composition comprising boron nitride foam and a polymer includes introducing a prepolymer composition into the cell structure of the foam and polymerizing the monomer to form the polymer.

[0048] There is no particular limitation on the other materials that can be used to form the compositions. In particular, a metal or metal alloy comprising at least one of aluminum, nitrogen, gallium, indium, phosphorus, arsenic, antimony, bismuth, lithium, silicon, copper, or a combination comprising at least one of the foregoing can be used.

[0049] In some embodiments, a composition comprising the boron nitride foam has a compression set at 15% compression of 20% or less, or 15% or less, or 10% or less, or 5 to 20%). In these instances, the composition specimen can be compressed 15%> of its volume or height and upon release of the compressive stress the 15%> deflection in the foam recovers by at least 97%, or at least 97.8%, or at least 98.5%, or 97 to 99.5%.

[0050] In some embodiments, the composition has a compression set at 30% compression of 20% or less, or 15% or less, or 10% or less, or 5 to 20%. In some embodiments, the composition has a compression set at 50% compression of 15% or less, or 10%) or less, or 7% or less, or 2 to 15%. In some embodiments, the composition has a compression set at 70% compression of 15% or less, or 10% or less, or 7% or less, or 2 to 15%). In some embodiments, the composition has a compression set at 80% compression of 15% or less, or 10% or less, or 5% or less, or 2 to 15%. In some embodiments, the composition has a compression set at 90% compression of 15% or less, or 10% or less, or 5% or less, or 2 to 15%. In some embodiments, the composition has a compression set at 95% compression of 15% or less, or 10% or less, or 5% or less, or 2 to 15%.

[0051] The porous nature of the foams also makes them particularly effective absorbent materials. For example, the foams are effective at absorbing organic liquids such as oil or fat.

[0052] The boron nitride foams are useful in a wide variety of applications, in particular applications that involve thermal management material, such as thermal pads, electrodes for energy storage, and in conversion devices such as supercapacitors, fuel cells, and batteries, in capacitive desalination devices, in thermal and acoustic insulators, specifically thermal insulation composites, in chemical or mechanical sensors, in biomedical applications, in actuators, in adsorbents, as catalyst supports, in field emission, in mechanical dampening, as filters, in three dimensional flexible electronic components, circuit materials, integrated circuit packages, printed circuit boards, electronic device, cosmetic products, wearable electronics, high efficiency flexible electronics, power electronics, high frequency materials and energy storage materials.

EXAMPLE: Preparation of boron nitride foam

[0053] The preparation scheme is shown in the FIG. 5A-5C. With reference to FIG. 5A-5C , a well-dispersed hexagonal boron nitride flake dispersion in water (FIG. 4A) is frozen at the temperature of dry ice (-78°C). It is believed that the hexagonal boron nitride sheets are concentrated at the boundary of ice crystals and then aligned along the growth direction of ice due to a squeezing effect (FIG. 4B). This network retains its connectivity when the ice is thawed by gentle heating to room temperature (FIG. 4C). The resultant foam has desirable thermal conductivity, dielectric properties, and mechanical properties (i.e., tensile strength and compression set).

[0054] The disclosure is further illustrated by the following non-limiting aspects.

[0055] Aspect 1 : A boron nitride foam, the foam comprising a structure defined by a three-dimensional network of interconnected open cells defined by cell walls, wherein the cell walls comprise a plurality of hexagonal boron nitride sheets, and have a thickness defined by a thickness of the plurality of hexagonal boron nitride sheets.

[0056] Aspect 2: The boron nitride foam of aspect 1, wherein the interconnected open cells have a random structure. [0057] Aspect 3 : The boron nitride foam of aspect 1, wherein the interconnected open cells have an ordered structure.

[0058] Aspect 4: The boron nitride foam of aspect 3, wherein the ordered structure comprises a honeycomb structure.

[0059] Aspect 5 : The boron nitride foam of aspect 4, wherein honeycomb structure has an edge connectivity of 3, 4, or 6.

[0060] Aspect 6: The boron nitride foam of any one or more of aspects 1 to 5, wherein the cell walls have an average thickness of 2 nanometers to 5 millimeters; or 0.1 micrometer to 5 millimeters, or 1 micrometer to 1 millimeter, or 2 nanometers to 0.01 millimeters, or 2 nanometers to 1,000 micrometers.

[0061] Aspect 7: The boron nitride foam of any one or more of aspects 1 to 6, wherein the cell walls comprise 2 to 1,000 hexagonal boron nitride sheets, or 2 to 100 hexagonal boron nitride sheets, or 2 to 50 hexagonal boron nitride sheets.

[0062] Aspect 8: The boron nitride foam of any one or more of aspects 1 to 7, wherein the hexagonal boron nitride sheets comprise a functional group, preferably a carboxyl group, aldehyde group, ketone group, hydroxyl group, thiol group, amino group, amide group, sulfate group, sulfonate group, phosphate group, phosphonate group, halogen, (meth)acryloxy group, vinyl group, allyl group, tri(Ci-6 alkyl)silyl group, or a combination comprising at least one of the foregoing.

[0063] Aspect 9: The boron nitride foam of any one or more of aspects 1 to 8, having a density of 0.5 to 1,000 mg/cm³.

[0064] Aspect 10: The boron nitride foam of any one or more of aspects 1 to 9, having a compression set at 50% compression of 15% or less.

[0065] Aspect 11 : The boron nitride foam of any one or more of aspects 1 to 10, having a thermal conductivity of 1 W/m K or more, specifically 1 to 600 W/m K, determined according to ASTM E1461.

[0066] Aspect 12: A method of preparing a boron nitride foam of any one or more of aspects 1 to 11, the method comprising providing a dispersion comprising a plurality of hexagonal boron nitride sheet and a freeze castable medium; disposing the dispersion in a mold; freezing the dispersion to form foam in the medium; removing the medium from the foam; optionally adding an additional material to a void space in the foam.

[0067] Aspect 13 : A boron nitride foam made by the method of aspect 12.

[0068] Aspect 14: A composition comprising the boron nitride foam of any one or more of aspects 1 to 13, and an additional material. [0069] Aspect 15: The composition of aspect 14, wherein the additional material comprises a polymer, a metal, a non-metal, a ceramic, a glass, or a combination comprising at least one of the foregoing.

[0070] Aspect 16: The composition of aspect 15, wherein the additional material is a polymer.

[0071] Aspect 17: The composition of aspect 16, wherein additional material is a metal or a metal alloy comprising aluminum, nitrogen, gallium, indium, phosphorus, arsenic, antimony, bismuth, lithium, silicon, copper, or a combination comprising at least one of the foregoing.

[0072] Aspect 18: An article comprising the boron nitride foam of any one or more of aspects 1 to 13, or the composition of any one or more of aspects 14 to 18.

[0073] Aspect 19: The article of aspect 18, wherein the article is a thermal management material, a thermal pad, an electrode for energy storage, a supercapacitor, a fuel cell, a battery, a capacitive desalination device, an acoustic insulator, a thermal insulation composite, a chemical sensor, a mechanical sensor, a biomedical device, an actuator, an adsorbent, a catalyst support, a field emission device, a mechanical dampening device, a filter, a three-dimensional flexible electronic component, a circuit material, an integrated circuit package, a printed circuit board, an electronic device, a cosmetic product, a wearable electronic, a high efficiency flexible electronic device, a power electronics device, a high frequency device, or an energy storage device.

[0074] In general, the compositions, articles, and methods described here can alternatively comprise, consist of, or consist essentially of, any components or steps herein disclosed. The articles and methods can additionally, or alternatively, be manufactured or conducted so as to be devoid, or substantially free, of any ingredients, steps, or components not necessary to the achievement of the function or objectives of the present claims.

[0075] "Alkyl" as used herein means a straight or branched chain saturated aliphatic hydrocarbon having the specified number of carbon atoms. "Aryl" means a cyclic moiety in which all ring members are carbon and at least one ring is aromatic, the moiety having the specified number of carbon atoms. More than one ring can be present, and any additional rings can be independently aromatic, saturated or partially unsaturated, and can be fused, pendant, spirocyclic or a combination comprising at least one of the foregoing.

[0076] All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. "Combination" is inclusive of blends, mixtures, alloys, reaction products, and the like. "Or" means "and/or." The terms "a" and "an" and "the" do not denote a limitation of quantity, and are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various aspects.

[0077] Every value herein is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. The term "combination" is inclusive of blends, mixtures, alloys, reaction products, and the like. Also, "combinations comprising at least one of the foregoing" means that the list is inclusive of each element individually, as well as

combinations of two or more elements of the list, and combinations of at least one element of the list with like elements not named. All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference. Unless specified to the contrary herein, all test standards are the most recent standard in effect as of the filing date of this application, or, if priority is claimed, the filing date of the earliest priority application in which the test standard appears.

[0078] While particular aspects have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications, variations, improvements, and substantial equivalents.

Claims

CLAIMS What is claimed is:

1. A boron nitride foam, the foam having

a structure defined by a three-dimensional network of interconnected open cells defined by cell walls;

wherein the cell walls comprise a plurality of hexagonal boron nitride sheets;

wherein the cell walls have a thickness defined by a thickness of the plurality of hexagonal boron nitride sheets.

2. The boron nitride foam of claim 1, wherein the interconnected open cells have a random structure.

3. The boron nitride foam of claim 1, wherein the interconnected open cells have an ordered structure.

4. The boron nitride foam of claim 3, wherein the ordered structure comprises a honeycomb structure.

5. The boron nitride foam of claim 4, wherein honeycomb structure has an edge connectivity of 3, 4, or 6.

6. The boron nitride foam of any one or more of claims 1 to 5, wherein the cell walls have an average thickness of 2 nanometers to 5 millimeters, or 0.1 micrometer to 5 millimeters, or 1 micrometer to 1 millimeter, or 2 nanometers to 0.01 millimeters, or 2 nanometers to 1,000 micrometers.

7. The boron nitride foam of any one or more of claims 1 to 6, wherein the cell walls comprise 2 to 1,000 hexagonal boron nitride sheets, or 2 to 100 hexagonal boron nitride sheets, or 2 to 50 hexagonal boron nitride sheets.

8. The boron nitride foam of any one or more of claims 1 to 7, wherein the hexagonal boron nitride sheets comprise a functional group, preferably a carboxyl group, aldehyde group, ketone group, hydroxyl group, thiol group, amino group, amide group, sulfate group, sulfonate group, phosphate group, phosphonate group, halogen, (meth)acryloxy group, vinyl group, allyl group, tri(Ci-6 alkyl)silyl group, or a combination comprising at least one of the foregoing.

9. The boron nitride foam of any one or more of claims 1 to 8, having a density of 0.5 to 1,000 mg/cm³.

10. The boron nitride foam of any one or more of claims 1 to 9, having a compression set at 50% compression of 15% or less.

11. The boron nitride foam of any one or more of claims 1 to 10, having a thermal conductivity of 1 W/m K or more, specifically 1 to 600 W/m K, determined according to ASTM E1461.

12. A method of preparing a boron nitride foam of any one or more of claims 1 to 11, the method comprising

providing a dispersion comprising a plurality of hexagonal boron nitride sheets and a freeze castable medium;

disposing the dispersion in a mold;

freezing the dispersion to form the foam in the medium;

removing the medium from the foam; and

optionally adding an additional material to a void space in the foam.

13. A boron nitride foam made by the method of claim 12.

14. A composition comprising the boron nitride foam of any one or more of claims 1 to 13, and an additional material.

15. The composition of claim 14, wherein the additional material comprises a polymer, a metal, a non-metal, a ceramic, a glass, or a combination comprising at least one of the foregoing.

16. The composition of claim 15, wherein the additional material is a polymer.

17. The composition of claim 16, wherein additional material is a metal or a metal alloy comprising aluminum, nitrogen, gallium, indium, phosphorus, arsenic, antimony, bismuth, lithium, silicon, copper, or a combination comprising at least one of the foregoing.

18. An article comprising the boron nitride foam of any one or more of claims 1 to 13, or the composition of any one or more of claims 14 to 18.

19. The article of claim 18, wherein the article is a thermal management material, a thermal pad, an electrode for energy storage, a supercapacitor, a fuel cell, a battery, a capacitive desalination device, an acoustic insulator, a thermal insulation composite, a chemical sensor, a mechanical sensor, a biomedical device, an actuator, an adsorbent, a catalyst support, a field emission device, a mechanical dampening device, a filter, a three- dimensional flexible electronic component, a circuit material, an integrated circuit package, a printed circuit board, an electronic device, a cosmetic product, a wearable electronic, a high efficiency flexible electronic device, a power electronics device, a high frequency device, or an energy storage device.