WO2014130687A1 - Methods of modifying boron nitride and using same - Google Patents

Abstract

The present disclosure is directed to the preparation and use of boron nitride materials. The present disclosure provides advantageous systems/methods of modifying (e.g., functionalizing and exfoliating) boron nitride, and improved systems/methods for using the same. More particularly, the present disclosure provides improved systems/methods for modifying hexagonal boron nitride and using same in filler materials. In general, the present disclosure provides for the preparation/use of boron nitride based polymer fillers. In some embodiments, the present disclosure provides systems/methods for exfoliating hexagonal boron nitride for use as a filler (e.g., nanofiller) in polymer composites. This non-flammable, transparent, high-surface area material can be utilized as a flame retardant. Furthermore, the boron nitride materials (e.g., polymer composites including hexagonal boron nitride) of the present disclosure can be utilized for a range of applications. Furthermore, the present disclosure provides for the functionalization of BN for application as a composite component.

Description

METHODS OF MODIFYING BORON NITRIDE AND USING SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/767,099 filed February 20, 2013, all of which is herein incorporated by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to systems and methods of functionalizing and exfoliating boron nitride (BN) and using same and, more particularly, to methods for functionalizing and exfoliating hexagonal boron nitride (hBN) and using same in filler materials (e.g., polymer-based filler materials).

2. Background Art

Hexagonal boron nitride (hBN) is a thermally stable material with uses ranging from cosmetics to high temperature lubricants. BN does not occur naturally, but is manufactured industrially at high temperatures from boron sources such as boron oxide or boric acid and nitrogen sources such as melamine, urea, or ammonia. In some ways hBN resembles graphite; both consist of stacked sheets with the component atoms arranged in a honeycomb pattern, the boron/nitrogen pair of atoms is isoelectric to a pair of atoms in graphite, and both are good thermal conductors. The cubic form of boron nitride (cBN) is analogous to diamond, both in structure and hardness, with the hardness of cBN second only to diamond.

Unlike graphite, however, hBN is an electrical insulator with a band gap of about 5.2 eV. It also has a much higher thermal stability than graphite, with a melting temperature near 3,000°C. In addition, hBN has been shown to be a superior substrate to silicon for graphene- based electrical devices. Despite these advantageous properties, the number of reports of hBN composites is small compared to graphite. Recent reviews of the field describe examples of solvent based sonication techniques, successful for graphite exfoliation, capable of generating partially exfoliated hBN at mg/ml concentrations. However, while hBN has similarities to graphite, the large scale formation of a graphite oxide (GO) analog has not been achieved due to the resistance of hBN to the oxidation process used to prepare GO, and thus the large scale exfoliation of hBN has remained a challenge. Thus, an interest exists for improved systems and methods of modifying boron nitride, and related methods for using the same. These and other inefficiencies and opportunities for improvement are addressed and/or overcome by the systems, methods and assemblies of the present disclosure. SUMMARY

The present disclosure provides advantageous systems and methods of modifying (e.g., functionalizing and exfoliating) boron nitride, and improved methods/systems for using the same. More particularly, the present disclosure provides improved systems and methods for modifying hexagonal boron nitride and using same in filler materials (e.g., polymer-based filler materials). In general, the present disclosure is directed to the preparation and use of boron nitride materials. In certain embodiments, the present disclosure provides for the preparation and/or use of boron nitride based polymer fillers.

In exemplary embodiments, the present disclosure provides systems and methods for exfoliating hexagonal boron nitride (hBN) for use as a filler (e.g., as a nanofiller) in polymer composites. This non-flammable, transparent, high surface area material of the present disclosure can advantageously be utilized as a flame retardant. Furthermore, the improved boron nitride materials (e.g., polymer composites including hexagonal boron nitride) of the present disclosure can be utilized for a range of applications, as discussed further below. In certain embodiments, the present disclosure provides for the functionalization of BN for application as a composite component.

Hexagonal BN has similar structural properties of graphite, a known lubricant.

However, unlike conductive graphite, BN is an electrical insulator and is also an excellent thermal conductor. BN is useful in making flame retardant polymer composites and coatings. In general, a range of other fillers have been used for flame protection in plastics. These can include, for example, inorganic salts, fluoropolymers, and halogenated hydrocarbons.

Several aspects of the boron nitride filler disclosed herein provide unique and useful properties. For example, the high aspect ratio of the atomically thin sheets provides for very efficient use of the filler in a polymer. Additionally, the material is non-toxic. Moreover, the disclosed material is transparent and so can be used in applications such as, for example, aircraft windows. Furthermore, the material has a temperature stability that is nearly double that of other known fillers. Disclosed herein is a unique and highly advantageous process/method that successfully produces functionalized and exfoliated boron nitride. In exemplary

embodiments, the method includes a controlled thermal treatment step to partially oxidize the boron nitride material. A subsequent step includes water washing of the material, as discussed further below in connection with the thermal gravimetric analysis data for the partial decomposition of BN in air, nitrogen and argon. In some embodiments, the method produces atomically thin sheets of BN with lateral dimensions of several hundred

nanometers, as discussed further below in connection with the atomic force microscopy image.

The present disclosure provides for a method for modifying boron nitride including: a) providing a boron nitride material; b) thermally treating the boron nitride material under controlled conditions to partially oxidize the boron nitride material; c) water washing the partially oxidized boron nitride material; and d) allowing the boron nitride material to dry.

The present disclosure also provides for a method for modifying boron nitride wherein after step d), the boron nitride material includes boron nitride oxide. The present disclosure also provides for a method for modifying boron nitride wherein step b) includes thermally treating the boron nitride material at about 950°C to about 1000°C in air for a predetermined period of time.

The present disclosure also provides for a method for modifying boron nitride wherein step a) includes providing a hexagonal boron nitride material. The present disclosure also provides for a method for modifying boron nitride wherein after step d), the boron nitride material is substantially transparent. The present disclosure also provides for a method for modifying boron nitride wherein step b) includes thermally treating the boron nitride material at about 950°C to about 1000°C in air and holding at that temperature for about one hour.

The present disclosure also provides for a method for modifying boron nitride wherein after step d), the boron nitride material includes a plurality of single sheets of boron nitride material, and one or more sheets extend about 150 nm in their lateral dimension. The present disclosure also provides for a method for modifying boron nitride wherein the plurality of single sheets of the boron nitride material are water dispersible.

The present disclosure also provides for a method for modifying boron nitride wherein after step d), the dried boron nitride material is reacted with phenyl isocyanate. The present disclosure also provides for a method for modifying boron nitride wherein after step d), the boron nitride material is incorporated covalently into a polymer composite material. The present disclosure also provides for a method for modifying boron nitride wherein after step d), the boron nitride material is utilized as a filler in a polymer composite material.

The present disclosure also provides for a method for modifying boron nitride wherein after step d), the boron nitride material is utilized in combination with graphene sheets to form a substrate material. The present disclosure also provides for a method for modifying boron nitride wherein after step d), the boron nitride material is utilized as a flame retardant in a polymer composite material. The present disclosure also provides for a method for modifying boron nitride wherein the polymer composite material includes polycarbonate.

The present disclosure also provides for a method for modifying boron nitride wherein after step d), a polymer is grafted or attached to the boron nitride material. The present disclosure also provides for a method for modifying boron nitride wherein the polymer is poly(methyl methacrylate) or polyurethane.

The present disclosure also provides for a method for modifying boron nitride wherein the polymer composite material includes a material selected from the group consisting of poly(methyl methacrylate), polyurethane, polycarbonate and nylon. The present disclosure also provides for a method for modifying boron nitride wherein one or more sheets have a sheet thickness of about 0.69 nm. The present disclosure also provides for a method for modifying boron nitride wherein step d) includes spray-drying the boron nitride material.

The present disclosure also provides for a method for fabricating a filler material for a composite including: a) providing a hexagonal boron nitride material; b) thermally treating the hexagonal boron nitride material at about 900°C to about 1000°C in air for a

predetermined period of time; c) water washing the thermally treated boron nitride material; d) allowing the boron nitride material to dry; and e) utilizing the dried boron nitride material as a filler in a polymer composite material.

The present disclosure also provides for a method for fabricating a filler material for a composite including: a) providing a boron nitride material; b) thermally treating the boron nitride material under controlled conditions in air for a predetermined period of time; c) water washing the thermally treated boron nitride material; d) allowing the boron nitride material to dry; e) reacting the dried boron nitride material with phenyl isocyanate; and f) utilizing the boron nitride material as a filler in a polymer composite material.

Any combination or permutation of embodiments is envisioned. Additional advantageous features, functions and applications of the disclosed systems, methods and assemblies of the present disclosure will be apparent from the description which follows, particularly when read in conjunction with the appended figures. All references listed in this disclosure are hereby incorporated by reference in their entireties. BRIEF DESCRIPTION OF THE DRAWINGS

Features and aspects of embodiments are described below with reference to the accompanying drawings, in which elements are not necessarily depicted to scale.

Exemplary embodiments of the present disclosure are further described with reference to the appended figures. It is to be noted that the various steps, features and combinations of steps/features described below and illustrated in the figures can be arranged and organized differently to result in embodiments which are still within the scope of the present disclosure. To assist those of ordinary skill in the art in making and using the disclosed systems, assemblies and methods, reference is made to the appended figures, wherein:

Figure 1 is a TGA analysis of BN heated in air (bottom line when time = 0), argon

(middle line when time = 0) and nitrogen (top line when time = 0), and the measurements were done by quickly heating the BN to temperature, and then holding that temperature;

Figure 2 shows FTIR spectra of BN after different treatments. The top line is pristine BN with no heat treatment. The other lines are BN heated to about 1000°C in air and held at that temperature for increasing amounts of time. The second line from the top is BN held for less than about one minute. The third line from the top is BN held at temperature for about one hour. The fourth line from the top is BN held for about 3 hours 10 minutes. The fifth line from the top (the bottom line) is BN held for about six hours. The formation of B-0 bonds are observed as peaks arising at about 640 cm^"1. The peaks occurring at about 3200 cm^"1 arise from O-H stretching;

Figure 3 shows TGA traces of BN heated at different temperatures in air. The top line at time = 500 is heating at about 1000°C, the second line from the top at time = 500 is heating at about 900°C, and the third line from the top at time = 500 is heating at about 800°C. The bottom line at time = 500 is BN heated at about 1000°C in nitrogen;

Figures 4A and 4B are AFM images of BN exfoliated flakes. These flakes were imaged by spin coating a suspension of BN in the water used to wash the flakes after heat treatment. FIG. 4A is a 5 μιη by 5 μιη image, while FIG. 4B is a 2 μιη by 2 μιη image. The sheets are shown to be single sheets approximately 150 nm in lateral dimension;

Figures 5 A-F show FESEM images of BN at different stages of the exfoliation process. After heat and water treatment, the material imaged is a minority component. In FIG. 5A is the pristine hexagonal BN. FIGS. 5B and 5C show the oxidized BN before washing. FIG. 5D is the BN after washing, and FIGS. 5E and 5F is the BN suspended in the aqueous wash. The scale bars in each frame denote a length of 1 micron;

Figures 6A-D are FESEM images of oxidized BN before and after reaction with an isocyanate. FIGS. 6A and 6B are oxidized BN after reaction with isocyanate, FIGS. 6C and 6D are after sonication and dispersion in THF;

Figure 7 shows XRD traces of BN. The upper trace at 0.3 is pristine BN, the other lines are after oxidation. The insert shows a zoom- in view of two lower lines: the rounded peak is oxidized BN after reaction with the isocyanate;

Figure 8 shows a top row that is PC control sample with no BNO. From left to right (top row) is immediately after lighting, 3 seconds after lighting, and 8 seconds after lighting. The bottom row is PC containing about 1% BNO. From left to right (bottom row) is immediately after lighting, 3 seconds after lighting, and 8 seconds after lighting. The images are frame captures from video;

Figure 9 is a schematic diagram of a procedure of exfoliation and functionalization of BNNS, and using them as fillers;

Figure 10 displays an exemplary mechanism of oxidation of hBN;

Figure 11 displays a structure of boric acid and hydroxide groups on fBNNS;

Figure 12 shows an exemplary functionalization of BNO by phenyl isocyanate;

Figure 13 displays FTIR of pBN (second line from bottom at 3400), BNO (top line at 3400), hot water washed BNO (second line from top at 3400), and isocyanate fBNO (bottom line at 3400);

Figure 14 is an image showing filtration samples;

Figures 15A-C are images showing: A) Unfunctionalized hBN in water, B) functionalized and exfoliated hBN in water, and C) AFM image of oxidized and exfoliated hBN sheets;

Figures 16A-D show an analysis of hydroxylated hBN: A) FTIR traces of hBN at different stages of preparation: a. pristine hBN before heating, b. hBN after heating but before being placed in water, c. BNO after washing with water, and d. BNO that has been washed by water and reacted with phenyl isocyanate; B) TGA traces of hBN heated to 1000°C in air, argon and nitrogen; C) XRD of pristine hBN and isocyanate functionalized hBN with the functionalized material displaying a broader peak shifted to lower angle; and D) XPS spectra of hydroxylated BNO with the boron peak showing both a B ls(N) and a Bls(O) component (left) and the nitrogen peak showing only a Nls(B) component (right);

Figures 17A-D show FESEM images of hBN: A) pristine hBN, B) BNO precipitate after washing with water, C) image of BNO nanosheets from water suspension, and D) BNO precipitate after reaction with phenylisocyanate with the curling of the edges consistent with the XRD data in FIG. 16C;

Figure 18 shows an exemplary structure of hydroxylated hexagonal boron nitride;

Figure 19 shows a height histogram of FIG. 15C;

Figures 20A-D show TEM images of: (a) BNO sheets from water suspension, (b) lower magnification of BNO sheets, (c) BNO precipitate after addition of thermally treated sample to water with voids on the surface (marked by arrows), and (d) selected area

(electron) diffraction pattern from (b);

Figure 21 shows Raman spectra of pristine hBN and hydroxylated BNO nanosheets;

Figure 22 shows DLS result of hydroxylated BNO nanosheets water suspension;

Figure 23 shows FTIR of oxidation product (BNO without any treatment) of different reaction times; and

Figure 24 shows TGA traces of oxidation at 800°C, 900°C, 1000°C.

DETAILED DESCRIPTION

The exemplary embodiments disclosed herein are illustrative of advantageous boron nitride materials, and systems of the present disclosure and methods/techniques thereof. It should be understood, however, that the disclosed embodiments are merely exemplary of the present disclosure, which may be embodied in various forms. Therefore, details disclosed herein with reference to exemplary boron nitride materials/fabrication methods and associated processes/techniques of assembly and use are not to be interpreted as limiting, but merely as the basis for teaching one skilled in the art how to make and use the advantageous boron nitride materials/systems and/or alternative materials of the present disclosure.

In general, the present disclosure is directed to the preparation and use of boron nitride materials. The present disclosure provides improved systems and methods of modifying (e.g., functionalizing and/or exfoliating) boron nitride, and related

methods/systems for using the same. More particularly, the present disclosure provides advantageous systems and methods for modifying hexagonal boron nitride, and using the same in filler materials (e.g., polymer-based filler materials).

In exemplary embodiments, the present disclosure provides systems and methods for exfoliating hexagonal boron nitride for use as a filler (e.g., nanofiller) in polymer composites. It has been found that this non-flammable, transparent, high surface area material of the present disclosure can advantageously be utilized as a flame retardant. Moreover, the improved boron nitride materials (e.g., polymer composites including hexagonal boron nitride) of the present disclosure can be utilized for a wide range of applications. In certain embodiments, the present disclosure provides for the functionalization of boron nitride (e.g., for use as a composite component).

In exemplary embodiments, the disclosed method is scalable for production of the BN sheets. Also disclosed are methods of compounding BN sheets with various selected polymers to produce BN-polymer composites having desirable properties (e.g., with respect to dispersion and flame retardancy).

The relative ease of the disclosed manufacturing/modifying process contributes to its scalability in that the process can include heating in air, followed by exfoliation in water. In contrast, other solution based methods for making BN are not so easily scalable. Some conventional methods to make BN nanosheets include chemical vapor deposition. Such methods are not easily scalable and are significantly more costly than the systems/methods disclosed herein.

Exfoliated BN nanosheets produced by the disclosed process have a higher aspect ratio than is achievable by other methods. In exemplary embodiments, the process produces a signature on the material through the OH groups on the perimeter of the nanosheets. BN nanosheets with their high aspect ratio have advantages over the lower aspect ratio BN produced by other methods.

In exemplary embodiments, it has been found that the use of BN nanosheets in combination with graphene sheets forms novel, optically transparent, flexible conductive substrate materials.

As noted, current practice provides that some conventional methods to make BN nanosheets include chemical vapor deposition, and such methods are not easily scalable and are very costly. In exemplary embodiments, the present disclosure provides for improved systems and methods for modifying (e.g., functionalizing and/or exfoliating) boron nitride that includes the steps of heating in air, followed by exfoliation in water, thereby providing a significant commercial, manufacturing and/or operational advantage as a result. The present disclosure will be further described with respect to the following examples; however, the scope of the disclosure is not limited thereby. The following examples illustrate advantageous systems/methods for modifying (e.g., functionalizing and/or exfoliating) boron nitride (e.g., hBN) and using the same (e.g., in filler materials). EXAMPLE 1: Boron Nitride Exfoliation

In an exemplary embodiment, commercially obtained hexagonal boron nitride sheets were placed in a quartz tube in a tube furnace. The furnace was then heated to 1000°C, held at that temperature for half an hour in air, then the furnace was turned off and the tube was allowed to cool. The thermally treated boron nitride was then washed with water and dried. The reaction that occurred upon heating the boron nitride, followed by treatment with water, resulted in functionalized sheets of BN.

The reaction that occurred upon heating the boron nitride in air was oxidation that resulted in attachment of oxygen to the edges of the boron nitride sheets as well as the removal of smaller sheets from the larger boron nitride sheets. With the use of

thermogravimetric analysis (TGA), it has been shown that there was a weight gain of boron nitride when it was heated in air, in contrast to a weight loss when it was heated in nitrogen. There was no change in weight when BN was heated in argon. FIG. 1 shows the TGA traces.

As well as TGA, Fourier transform infrared spectroscopy (FTIR) was used to determine the change in functional groups on the BN as a result of heating for different lengths of time. FIG. 2 shows the results. What can be seen is that the BN stayed largely intact, but there was an additional peak. The top line of FIG. 2 is BN as received, e.g., before heat treatment. The other lines of FIG. 2 are BN after being heated for increasing amounts of time in air. The oxidation of the BN was shown to result in the formation of B-0 and O-H bonds that increase in density with increasing oxidation time, leveling off after several hours.

The temperature needed for this oxidation was determined by TGA. FIG. 3 shows the results. Heating at 800°C or less produced no observable oxidation. Heating at 900°C showed some oxidation, with heating at 1000°C being needed for extensive oxidation. For comparison, also included in FIG. 3 is a trace of BN heated at 1000°C in nitrogen, where substantially no oxidation occurs.

Imaging also showed the effects of heating in air on the morphology of the BN. FIG.

4 shows atomic force microscopy images of exfoliated sheets of BN.

These single sheets of BN are a result of the oxidation of larger sheets of pristine hexagonal BN. They represent the majority of the mass of the washed samples, but a small amount of larger BN remained and could be imaged. FIG. 5 shows field emission scanning electron microscopy (FESEM) images of this residue. Holes can be seen in these larger sheets that correlate with sizes of the exfoliated BN seen by AFM. It is also possible to see that the initially smooth edges of the BN have become rough.

It appears that the heating process introduces oxygen into the BN lattice. These oxygen functionalities are stable to further oxidation and the amount of oxygen incorporated reaches a saturation point. This is not the case when the BN is heated with nitrogen.

Nitrogen appears to react with the BN, but the result is formation of volatile molecules that break away from the BN and result in the loss of BN mass. When heated in an inert atmosphere such as argon (Ar), no mass loss or mass gain of the BN occurs.

When the oxidized BN is then placed in water, the oxygen functional groups react to form B-O-H groups at the edges of the BN. This leads to the larger BN flakes being reduced in size. The smaller BN sheets are water dispersible leading to exfoliation of the sheets as the introduced hydroxyl groups interfere with stacking.

EXAMPLE 2: Reaction and functionalization of exfoliated BN

It is possible to react the BN after heat and water treatment. In one embodiment, the oxidized BN is reacted with phenyl isocyanate. The larger oxidized BN samples were collected by filtration and reacted with phenyl isocyanate. FIG. 6 shows the change in morphology caused by this functionalization.

FIG. 7 shows the effect the reaction has on the stacking of the oxidized BN. X-ray diffraction (XRD) indicates that BN stacking is affected by the reaction with the isocyanate. BN prepared by the method disclosed herein is novel in that it can be functionalized with organic molecules. The method disclosed herein provides material having properties such that the BN sheets may be incorporated covalently into polymer composites.

Using exfoliated BN produced by the process disclosed herein, flame resistance has been demonstrated in an exemplary BN polymer composite. A polycarbonate (PC) sample was blended with 1% of oxidized boron nitride (BNO). As illustrated in FIG. 8, when ignited, a control sample of PC burns until entirely consumed. Due to the relatively small amounts of BNO sample, tests were run using small test samples. These small samples are demanding in flame retardant tests as their large surface areas make the samples extremely flammable. Nevertheless, a sample of the same size as a control sample, but which comprised 1% BNO, extinguished quickly upon lighting and would not relight. The above-described tests were conducted with five commercial samples of BN purchased from Saint-Gobain Advanced Ceramics. Samples were analyzed both before and after oxidation by FESEM and FTIR. A sample was chosen that appeared closest to a research-grade sample obtained from Aldrich. This sample was then oxidized, washed, collected and dried in numerous batches to provide data on process reproducibility and to provide enough sample for compounding. In a first series of tests BN was successfully compounded with poly (methyl methacrylate) (PMMA).

The exfoliated hexagonal BN produced by the method disclosed herein, also referred to as BN nanosheets (BNNS), is highly functionalized and chemically reactive. By using such nanosheets as fillers, hybrid composites are prepared that have excellent mechanical, thermal and surface properties.

As noted above, in some ways BN resembles graphite: the hexagonal BN (hBN) consists of stacked sheets similar to graphite. Prior to development of the methods disclosed herein there has been little if any experimental evidence that BN nanosheets can undergo chemical modifications, and there has been a lack of effective methods to prepare BN nanosheets on a large scale.

Disclosed herein are methods for the formation of the hBN that is an analog of graphite oxide (GO). As the use of GO has allowed for the utilization of graphitic material in numerous applications, the availability of large quantities of exfoliated, functionalized hBN allows the incorporation of hBN into composites and coatings to create materials with previously unattainable properties. There are a number of applications that can

advantageously utilize the thermal conductivity, chemical inertness, and electrical insulating properties of hBN nanosheets prepared by the methods disclosed herein.

EXAMPLE 3 : A versatile strategy for chemical peeling and functionalization of hBN For various applications of boron nitride nanosheets (BNNS), especially fillers for polymers, compatibility and dispersibility of nanosheets are of importance. In some cases, the problems of physical exfoliation, such as low yield and low functionality limit the use of this material in composites. Therefore, a chemical method may possess advantages, such as efficient, effective, highly reactive and better processibility, over other methods, which can significantly improve the interaction between BNNS and organic molecules. In this disclosure, the oxidation of boron nitride has been proven to be effective for exfoliation. Obtained boron nitride oxide is suitable for further functionalization reactions and better properties of composites.

Boron nitride is an inorganic compound with chemical formula BN. The hexagonal form corresponding to graphite, hexagonal boron nitride (hBN) is a used material with desirable properties. The lattice structure of hBN appears very similar to that of graphite in which alternating B and N atoms substitute for C atoms to form 2-dimensional layer structures. Nanosheets of graphite, graphene, the isoelectric analogue of hBN, has become one of the most exciting topics of research in the last several years. Presumably, hBN can also be exfoliated to form unique 2D crystal structures. Recently, fabrication of BNNS has been attempted by mechanical exfoliation, nanotube unwrapping, liquid phase sonication, reacting boric acid with urea, and chemical vapor deposition. Several techniques were attempted for the preparation of minute quantities of BNNS.

The unique properties of BNNS are primarily valuable as novel nanofillers in highly thermoconductive and electrically insulating polymeric composites, or as functional materials in electronic devices working in hazardous or high-temperature environments. However, most investigations in the literature were focused on BNNS in the solid state, because these nanosheets, like graphene, are generally insoluble in common organic and aqueous media. For graphene, the research efforts on their functionalization have stimulated and enabled the exploitation of the properties and applications that are not accessible in the solid state, such as the dispersion of graphene in polymeric nano-composites. Similarly, effects on the research of BNNS may be expected from the introduction of BNNS into solution or suspension.

Functionalization is one of the best solutions of solubilization. So far, BNNS have been modified through various interactions, including covalent or noncovalent bonding.

Recently, efforts have been made to functionalize BN (both in original crystal structure and 2D crystal structure). Some researchers functionalized ball-milled h-BN with a long alkyl chain amine via Lewis acid-base interactions between the amino groups and the boron atoms of h-BN to obtain soluble amine- attached nanosheets products. Later, the same group reported that, with the assistance of bath sonication, water can directly disperse and exfoliate BNNS and BN nanoribbons. These researchers pointed out that the sonication- assisted hydrolysis promoted the cutting of the pristine hBN sheet structures and yielded smaller and more exfoliated h-BN nanosheets. But this method is not without its limits: yield is relatively low since sonication is not efficient on promoting breaking the chemical bonds between boron and nitrogen atoms. As a result of low yield, functionality is also limited by the amount of newly produced edges. That may be the reason why no discussion about the reactivity of groups on edges is included. Based on the novel properties of BNNS, they may lead to useful fillers for composites with mechanical reinforcement, thermal properties improvement, etc. Some advantages of BNNS compared to other preexisting conventional fillers may be briefly summarized as follows: high thermal conductivity and electrical resistivity, superb anti-oxidation ability, and the lowest coefficient of friction versus all the other materials. Another researcher reported fabrication of PMMA/BNNSs transparent composites. Improvement in the elastic modulus, strength and thermal properties were obtained. Another researcher reported composite films of kinds of two-dimensional nanosheets, including BNNS. The resultant thermoplastic polyurethane films with 5% of BNNS as fillers show improved modulus, stress at low strain and ultimate tensile strength. Authors of both papers attribute the properties increases to efficient matrix interactions with the embedded BNNSs, which indicates that increasing interaction may result in better properties. BN nanoflakes are also reported to improve the thermally conductive of a photosensitive polyimide by up to three times when weight fraction equals to 30%. Their exfoliation process is based on a surfactant modified BN and the yield is not mentioned, which might not be higher than other similar methods. More important, matrix- filler interactions can be increased by introducing covalent bond instead of surfactants.

In this disclosure, we demonstrate an easy but efficient method for fabrication of highly functionalized BNNS, which possess high yield of exfoliation and remarkable improved reactivity with organic reagents. One approach of our method is shown in FIG. 9. The slow oxidation mechanism of BNNS and reactivity of produced boron nitride oxide (BNO) were investigated thoroughly and BNO was utilized for further reactions.

Subsequently, soluble and functionalized BNNS were fabricated successfully by chemical exfoliation and reaction. Dramatically improved dispersibility and remarkably improved reactivity of oxidized BNNS were demonstrated.

(TGA data) - The oxidation reaction of hBN was studied by thermogravimetric analysis (TGA) in air atmosphere, as shown in FIG. 3. In a typical experimental run, it was revealed that at about 1000°C the weight started to increase, which indicated that oxidation has no introducing period and the hBN began to have attached oxygen atoms on it. From 400 minutes, the weight stopped increasing, and this implies completion of oxidation. Therefore, it is suggested that for controllable oxidation of hBN, about 950 to about 1000°C is a suitable temperature range. The weight loss-temperature dependence curve revealed that the rate of oxidation increases from 900°C to 1000°C. But due to the anti-oxidation property of hBN at lower temperature, no weight change can be observed at about 800°C. Compared with oxidation of boron nitride nanotubes, pristine hBN is more thermally stable. FIG. 1 shows hBN's behavior in nitrogen and in argon atmosphere for comparison. A slight mass loss is observed in nitrogen atmosphere, which can be attributed to the formation of boron lattice vacancies.

(FTIR) - The relatively slow oxidation makes it possible to break the stubborn structure of the hBN in a controlled manner. Although it was revealed by TGA that oxidation happens at about 1000°C, Fourier transform infrared spectroscopy (FTIR) measurement was needed to study the structure of the product after oxidation. The appearance of B-0 bonds was monitored by FTIR (FIG. 2), which revealed that B-0 bonds start to appear from 30 minutes.

(SEM) - FESEM studies of products of oxidation show a significant change of morphology. In a typical image of pristine hBN, flakes with lateral sizes of a few microns are observed. After oxidation, BN flakes are still having similar sizes and smooth edges, which implies that the layered structure is not damaged during oxidation. B-N peaks in FTIR measurement also support this conclusion. Washing BNO can remove possible B2O₃ introduced by oxidation. In FIG. 5D, almost all flakes are bearing defective edges, and holes appear on surfaces. Size of holes on surfaces are about lOOnm. Because B2O₃ has a boiling point lower than 1000°C, it will spread out and leave the hBN matrix if it is produced. To confirm the origin of holes and defects, "clear" liquid on top of aqueous dispersions of washed BNO was collected and analyzed. In FIGS. 5E and 5F, much smaller flakes in dimension of a tenth of a micron are found, which are from holes on surfaces of larger flakes.

(mechanism) - It is proposed that pristine hBN is very difficult to be oxidized. In FTIR, peak at about 3400 cm-1 is attributed to N-H bond on edges and/or defects. The oxidation at about 1000°C starts from defect sites and/or edges of the sheets. At the beginning of oxidation, vacancies appear and exposed B atoms are reacted with oxygen. Weight rapidly increases from oxygen atoms. While reaction time is being extended, the reaction will go deeper, with small sheets becoming smaller. Normalized FTIR of hBN, BNO for different times, hot water washed BNO was obtained (e.g., 20 minutes, 1 hour, and 6 hours). It was found that the intensity of the B-0 bond increases during reaction. Due to the formation of B-0 bonds, further oxidation may occur from defects or edges and finally cut large flakes into smaller ones. But this process is controlled by diffusion of oxygen. So TGA shows the rate of increasing slows down. When size of tiny sheets is small enough, they will able to be washed away and filtered out by hot water, because they are soluble in it and small enough. B-0 bonds not only decrease Van der Waal force between layers but also increase interactions with polar solvent like water, so having relatively higher functionality smaller sheets are peeled and dispersed in solvent. It was found that, compared with BNO, washed BNO has lower intensity of B-0 bonds. When all edges and defects are reacted, the very inner sheets still cannot be oxidized because of isolation of air by the outer layers. This can explain why the weight stopped increasing in TGA and peaks of B-N bonds are intact in FTIR. BNO is not as unctuous as as-purchased hBN but the color of BNO is still pure white. The filtration that contains BNO sheets in it is cloudy.

(AFM) - AFM measurement in FIGS. 4 A and 4B was performed on hot water washed BNO. The sample was dispersed in water at about 0.1 mg/mL with a tip sonicator for about 30 mins. Tapping mode AFM shows very small (<100 nm diam.) single-layer sheets. Sheet heights are on average 0.77 nm. 5 x 5 μιη scan using tapping mode AFM with a sharp tip yields small, uniform particles less than 100 nm in diameter. 2 x 2 μιη scan using tapping mode AFM with a sharp tip shows more detailed view of sheets. Origin of these nanosheets has been explained before, but those sheets are not produced by sonication, like in other works. Small sheet size is not due to excessive sonication, as a sample dispersed in a bath sonicator for 5 minutes yielded similarly sized sheets. The only use of sonication is to help the nanosheets be well dispersed in water. So it is evidence that OH groups can significantly change interactions between layers and produce single layers. This method can be called "chemical peeling."

(reactivity) - Although hydroxide group was introduced on BNNS, the chemistry of it is not well understood. High yield and high functionality make it possible to study chemical property of fBNNS. Since hydroxide groups on BNNSs have similar structure with boric acid (FIG. 11), reactions that have been studied on boric acid might be good for fBNNS.

(reaction, factors affect it, control) - In several reactions of boric acid, reaction with isocyanate is one of the most promising ones. The nature of hydroxide groups on fBNNS should allow fBNNS to react with isocyanate. This reaction is proved by FTIR shown in FIG. 13. The B-0 bond disappeared after functionalization. FES EM shown in FIGS. 6A and 6B have peeled off and curved edges which are from decreased interactions between layers. And extended layer spacing is caused by inserting phenyl groups in between layers, which have a large volume than hydroxide groups. XRD shows the increase of d spacing and change of peak shape (FIG. 7). Using pristine to perform this reaction produces no change on FTIR. This is also evidence of reaction of hydroxide group on fBNNS. NMR can calculate average functionality of fBNNS. After optimizing the reaction condition, a scaled up reaction was performed.

(composites) - As mentioned above, BNNS can be a good nanofiller for a polymer matrix. The nature of hydroxide groups on fBNNS allows it to react with isocyanate, an essential reaction to synthesize polyure thanes. So by initiating polymerization from BNO, a new method to fabricate BNNS filled polymeric materials is introduced by the present disclosure. Thermoplastic polyurethanes have excellent mechanical and elastic properties, good hardness, high abrasion and chemical resistance. Then, fBNNS is added during the chain extension reaction and a composite will be obtained after the reaction is finished.

During the process of dispersing fBNNS into polymer/prepolymer, interactions between layers are decreased by the functional groups on nanosheets, while interactions between layer and matrix is increased. This effect not only prevents aggregation but also promotes a higher solubility and better dispersion. This strategy can produce polymeric composites with fBNNS fillers bonded to the matrix, which makes this filler very suitable in lots of systems and compatible with conventional routes toward composites.

Methods - pBN oxidized in air for about 40 minutes, then washed by 50ml * 3 hot water, and obtained BNO. lg BNO is tip sonicated in 50 ml water, then filtered and filtration was collected. Filtration was centrifuged at 3000g for 15min. Collect cloudy liquid, a (FIG. 14).

lg pristine BN is tip sonicated in 50ml water then filtered and filtration was collected. Filtration was centrifuged at 3000g for 15min. Collect cloudy liquid, b (FIG. 14).

Conclusion - A new, high efficient method of producing functionalized hBN nanosheets has been achieved by the exemplary systems and methods of the present disclosure. Some advantages of these nanosheets are that they are highly functionalized and chemically active. By using these nanosheets as fillers, hybrid composites, which may have excellent mechanical, thermal and surface properties may be prepared. This route can advantageously pave the way for boron nitride nanosheets used in engineering plastics.

EXAMPLE 4: Large Scale Thermal Exfoliation and Functionalization of Boron Nitride

The present disclosure demonstrates for the first time the formation of large quantities of functionalized exfoliated boron nitride sheets.

For fundamental studies, chemical vapor deposition (CVD) is used to produce single sheet hBN. However, for large-scale applications, such as nanofillers in polymer composites, exfoliation has been found to be a more economically attractive route to single sheet hBN. A previous attempt to produce hBN nanofillers using the sonication of hBN in N, N- dimethylformamide (DMF) produced layered hBN with the functionality of the sheets not determined due to difficulties in obtaining FTIR spectra. Other methods, not involving sonication, have included using graphene as a template for carbon substitution to form hBN, exfoliation by reaction with molten hydroxides at low temperature and the splitting of BN nanotubes to form ribbons, much like is done to form graphene ribbons from carbon nanotubes.

In addition to exfoliation, the functionalization of hBN has also drawn interest. Some methods for functionalizing hBN include a method to form associations of hBN nanosheets with alkyl amines by first using a ball mill to cleave the sheets and produce defect sites and the use of hydrazine, hydrogen peroxide, nitric acid and sulfuric acid heated under pressure, followed by sonication, to produce 0.3 g L^"1 suspensions of hBN. Although not using hBN sheets, it was shown that at high temperatures hBN nanotubes slowly form defect sites that can be used to break the tubes into smaller segments to aid in solvent dispersion. Recently, one researcher used oxygen radicals in sonicated NMP solutions to attach hydroxyl groups to hBN.

There has been no report of the formation of the hBN equivalent of GO: hydroxylated sheets that enable the aqueous suspension of single sheets in high yield. In this disclosure, such a system is advantageously described, and using atomic force microscopy (AFM), thermo-gravimetric analysis (TGA), FTIR, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, chemical functionalization, and dynamic light scattering (DLS), the functionalized single sheets and their mechanism of formation are described. The reactivity of the hydroxylated hBN by functionalization with phenyl isocyanate is also demonstrated in this disclosure.

The procedure used to form GO does not work with hBN as hBN is very resistant to oxidation. However, it was found that heating hBN in air results in a mass gain as oxygen is incorporated into the hBN lattice. Following the heat treatment, stirring the material in deionized water for several minutes results in the mixture thickening as the thermally treated material hydrolyzes and exfoliates to form hydroxylated boron nitride (BNO). These sheets form a suspension without the need for sonication, although mild bath sonication is normally used to increase the rate at which the suspension forms. Yields of water suspended sheets are about 65%, with the balance precipitating from the aqueous suspension as larger, less hydroxylated material that remains stacked.

Shown in FIG. 15A is a vial containing pristine hBN in deionized water after bath sonication. FIG. 15B shows BNO suspended in water with no sonication. There is no apparent water solubility with pristine hBN, while the hydroxylated material results in a cloudy suspension containing a small amount of precipitate on the bottom of the vial. FIG. 15C shows an AFM image of these suspended sheets drop cast on an HOPG surface. The sheets are single layers, with lateral dimensions on the hundreds of nanometers length scale. A height histogram (FIG. 19) gives the sheet thickness to be about 0.69 nm. No

centrifugation is employed to fractionate the sheets for AFM imaging, and thus the image represents the entire population of the suspended material. This is in stark contrast to other methods that rely on centrifugation to separate the samples into fractions with different numbers of hBN layers.

The BNO is also investigated by transmission electron microscopy (TEM), Raman spectroscopy, and dynamic light scattering (DLS). TEM images (FIGS. 20A-D) are in good agreement with the AFM analysis, and show single sheets with lateral dimensions of about 100-200 nm. Raman analysis (FIG. 21) indicates a blue shift of 2 cm^"1 in the E_2g phonon mode at 1369 cm^"1 for the BNO, and DLS (FIG. 22) analysis shows a mean diameter of about 360 nm.

To understand the mechanism of BNO sheet formation, FTIR and TGA are used to follow the reaction. FIG. 16A shows the FTIR spectra obtained at different stages of material synthesis. Prior to heating, the spectrum of pristine hBN exhibits only the characteristic peaks of B-N in-plane stretching at 1370 cm^"1 and B-N-B out of plane bending observed at 810 cm^"1, as shown Figure FIG. 16A(a). The formation of boron-oxygen bonds in the heated material (before adding to water) is indicated in the FTIR spectrum FIG. 16A(b) by the B-O- H peak at -3200 cm^"1 and the small in-plane peak near 1200 cm^"1. The peak at 1200 cm^"1 has been assigned previously to boron coordinated with three oxygen in an extended lattice. After washing with water (FIG. 16A(c)), the peak near 1200 cm^"1 disappears while B-O-H peak remains, with a shoulder at about 3400 cm^"1 indicating loss of the in-plane B-0 bonds. The reason for the decreased B-O-H peak intensity seen near 3200 cm^"1 before washing (FIG. 16A(b)), and after washing (FIG. 16A(c)), is not completely clear. It may be as simple as dryness of the samples, but storing in a vacuum oven prior to analysis did not appear to have an effect. Alternatively, the loss of peak area may be attributed to the loss of small, highly functionalized sheets during the washing process. A further explanation may be the presence of both N₂BOH and NB(OH)₂ groups in the initial sample, with the washing step leading to the decomposition of NB(OH)₂ to boric acid. That the peaks correspond to hydroxyl groups is supported by FIG. 16A(d) however, as they completely disappear upon treatment with phenylisocyanate. The isocyanate is very reactive towards hydroxyl groups and results in the disappearance of the O-H stretch.

In order to show that our material is not simply boric acid from the decomposition of hBN, the hydroxylated BNO is repeatedly washed with hot water to remove boric acid. This does not result in loss of the hydroxyl peaks in the FTIR. Additionally, boric acid melts at 171°C and boils at 300°C. As the oxidation is run open to the air at 1000°C, the retention of formed boric acid is unlikely and would be observed as a loss of mass in the TGA.

Additionally, FTIR spectra comparing boric acid with hydroxylated hBN (FIG. 23) indicates no significant formation of boric acid. Another possibility, the formation of boron trioxide, B2O₃, from boric acid is ruled out as no evidence of it is seen by AFM or FESEM, and our heating rate is not conducive to its formation. Even if a trace amount of B2O₃ is formed, it will be converted to boric acid during treatment with hot water and removed.

As mentioned previously, hBN is thermally stable, and we find that at 800°C, no significant oxidation is observed. As the temperature is raised, there is an increase of mass with time, and the rate of oxidation increases with increasing temperature (FIG. 24). TGA studies indicate that when heated in air at temperatures at 1000°C, hBN gains mass for a period of time, followed by a leveling off (FIG. 16B, top). In contrast, when heated in an argon atmosphere at the same temperature, no mass change is observed (FIG. 16B, middle). Interestingly, when heated in a nitrogen atmosphere at 1000°C, the hBN losses mass (FIG. 16B, bottom). While no reaction is seen in an argon atmosphere, a stable oxidation product is formed in air. In nitrogen, however, with no oxygen present, slow decomposition is observed. This is in agreement with the findings of others who have observed a loss of mass in a nitrogen atmosphere and mass gain in air. Unlike other studies however, we observe a plateau in the weight gain with oxidation, apparently caused by the complete oxidation of regions susceptible to oxidation at that temperature.

Oxidation occurs in the plane of the hBN, resulting in the insertion of oxygen into the hBN lattice, as has been observed in chemical vapor infiltration studies done at the same temperature used in our study. Our XRD studies indicate that the d-spacing of hBN and BNO are the same, suggesting the oxygen is inserted into the plane of the BN. Evidence for the formation of B-0 rather than N-0 bonds comes from X-ray photoelectron spectroscopy (XPS) shown in FIG. 16D. The XPS spectrum of BNO identifies the Bis and Nls peaks. Both values are in close agreement with values reported for pristine hBN. Due to the background oxygen content of the sample, which we speculate to result from trace amounts of oxygen absorbed on the nanosheets, we use fitted Is peaks of different species to find the oxygen bond. While boron atoms show both a B-N Is peak at 191 eV and a B-0 Is peak at 191.7 eV, nitrogen is found to be bound only to boron atoms. The ratio of B-OH to B-N peak areas is approximately 1:10. In pristine hBN, boron and nitrogen are bond only to each other, without any oxygen. The oxidation sensitivity of boron sites is consistent with results from other published work.

To study the morphology of the BNO, FESEM and TEM are employed. FIG. 17A shows pristine hBN with dimensions of microns and smooth surfaces and edges, while the hydroxylated BNO sample (FIG. 17B) clearly shows the rough edges and "divots" on the surface corresponding to the size of the single sheets imaged by AFM in FIG. 15C. Similar results are also obtained by TEM (FIG. 20C), where roughly circular thin spots are seen decorating the previous (prior to heating) surface. Electron-diffraction patterns of BNO reveal the typical six-fold symmetry structure of hBN. FIG. 17C shows BNO sheets obtained by drop casting an aqueous suspension on a SEM stub. The images of the sheets are consistent with those obtained by AFM, but with the high concentration of sheets leading to sheet overlap.

To demonstrate the reactivity of the hydroxyl groups on the material, we reacted the precipitated BNO (the larger, unexfoliated pieces) with phenyl isocyanate. From the FESEM image shown in FIG. 17D, the sheets appear to be pulled back and folded as a result of the added functional groups. This is analogous to what is observed when graphite is oxidized to GO. The well-known turbostratic structure, parallel layered but randomly stacked graphite, has an interlayer spacing (3.44 A), considerably greater than ideal graphite (3.35 A). The XRD spectrum of the functionalized BNO is shown in FIG. 16C, where the XRD trace of pristine hBN is superimposed with the trace of isocyanate functionalized BNO. After reaction with the isocyanate, the XRD profiles of the 002 reflection moves to a slightly lower angle of 26.41°, a larger d spacing of 3.37 A, and broadens compared with pristine hBN (26.79°, 3.32 A). The disappearance of the hydroxide group after functionalization is also confirmed by the FTIR in FIG. 16A(d). The reaction is believed to form new B-NHPh groups, substituting the hydroxide groups, in a manner similar to that observed with boric acid and isocynates. The phenyl group is not seen in the spectrum due to the low density of functional groups on precipitated BNO. In this disclosure, we have described the formation of the hBN analog of GO by heating in air followed by treatment with water. We have fully characterized the formed BNO and proposed a mechanism for its formation. Just as the use of GO has allowed for the utilization of graphitic material in numerous applications, the availability of large quantities of exfoliated, functionalized BNO is expected to allow for the incorporation of BNO as a multifunctional nanofiller as well as providing material for sensing and catalytic substrates. Other potential applications include the use of hBN in electronics, especially in conjunction with graphene. With this method it will be possible to more fully utilize the thermal conductivity, chemical inertness, and electrical insulating properties of this promising material.

Experimental - Samples were prepared by the thermal treatment of hexagonal boron nitride in air. In a typical experimental run, BN powder (about 10 g, 99.5%, Alfa Aesar, used as received) was placed in a quartz tube in a tube furnace. The furnace was heated to about 1000°C and held at that temperature for about one hour in air. After cooling, the material was washed with hot water and dried. Approximately 700 mg of BNO could be collected from 1 g washed hydroxylated BN powder after sonication and centrifugation.

For the functionalized material, 10 ml phenyl isocyanate (99%, Acros organics) and 3 drops triethyl amine was added to 20 mg hydroxylated BN after it had been water washed and completely dried. After reacting under nitrogen at 100°C for one day, the reaction product was repeatedly rinsed with THF and dried under vacuum.

FESEM measurements were performed using a JEOL JSM-6335F cold cathode field emission (12 kV) scanning electron microscope. TGA was carried out on a TA Instruments Q 500 Thermogravimetric Analyzer at a heating rate of about 10°C min^"1 in a platinum pan. IR transmission measurements were conducted on a Nicolet 560 instrument coupled with SpectraTech IR Plan 0044-003 microscope. For X-ray diffraction (XRD) measurements, a Bruker AXS D2 Phaser was utilized. XPS measurements were performed using a 595 Scanning Auger Electron Spectrometer.

The yield of the process was taken to be the amount of material suspended in water divided by the amount of precipitate. While the precipitate was hydroxylated as well, it was not exfoliated and thus did not suspend. A typical determination of yield: 15.2 mg of thermally treated material was placed in water and briefly bath sonicated. The suspension was then filtered, and about 5.0 mg of material was recovered from the filter, giving a yield of about 67%. The mass of the precipitate was utilized rather than the exfoliated material as sheets were retained in the filter material.

AFM - AFM measurements were conducted at room temperature using a NTEGRA

Prima Scanning Probe Laboratory (NT-MDT, Zelenograd, Russia). Scanning was carried out in tapping mode. The AFM probes were ACTA silicon cantilevers (APPNANO, Santa Clara, California) with a typical resonance frequency of f = 300 kHz, a radius of curvature of r < 10 nm, and a spring constant of k = 40 N/m. The biggest peak is the height at which most pixels of the image are the substrate. Therefore, the center of this peak is assigned height 0. The second highest height in the image represents the height of the sheets, about 0.7nm. The ratio of the areas under the peaks gives the ratio of substrate area vs. sheet area, about 15% of the substrate is covered with sheets. In this technique all sheets in the image are averaged, giving higher precision and being more representative than a single height profile.

TEM samples were prepared by drop casting the suspension with a low concentration, generally at about 0.1 mg/mL, onto a carbon grid and viewed in both transmission and diffraction mode on a FEI Tecnai T12 STEM.

Raman spectra were obtained by a Renishaw Ranascope System 2000 operating at

514 nm. Raman spectra of pristine hBN and hydroxylated BNO nanosheets from water suspensions show changes in integrated intensity and the position of peak.

DLS: dynamic light scattering (DLS) using a NICOMP 300 Submicron Particle Sizer - Dynamic light scattering (DLS) was utilized to determine the particle size in the cloudy suspensions obtained by using a NICOMP 300 Submicron Particle Sizer. The results shown below are in general agreement with the results obtained by AFM, with DLS indicating a somewhat larger size than the 100 nm length scale obtained by AFM. However, the parameters used for the calculation of size by DLS are only estimates, and thus the AFM data is expected to be more reliable.

Result summary:

Mean diameter = 359.6 nm

Chi squared = 1.748

Stnd deviation = 285.5nm (79.4 %)

Baseline Adj. = 0.024%

Coeff. Of Var'n = 0.794

Mean Diff. Coeff. = 1.29E-008 cm²/s

Cumulative result:

25% of distribution < 144.7nm

50% of distribution < 245.8nm

75% of distribution < 418.5nm

90% of distribution < 678. Onm

99% of distribution < 1523.3nm

FTIR of hBN during reaction with/without boric acid - Increasing the heating time of the hBN at about 1000°C in air confirmed the functionalization plateaus after approximately 3 hours (FIG. 23). Further, comparing the FTIR of the hydroxylated hBN with boric acid

(FIG. 23) gives a clear indication that boric acid is not the product we obtain when heating in air. Study of hBN oxidation at different temperatures by TGA - Thermal gravimetric analysis was done at different temperatures to determine the optimal conditions for oxidation. Shown in FIG. 24 are three TGA traces run at constant temperature in air. Based on these results, our standard oxidations were performed at about 1000°C. EXAMPLE 5: - BN as a Flame Retardant

Another goal of this disclosure is to utilize the presently developed method for the functionalization and exfoliation of boron nitride to fabricate and/or commercialize a new class of flame retardant (e.g., for use in the plastics industry), boron nitride oxide (BNO). Current flame retardants either require very high loadings, compromising the properties of the product, or are banned or about to banned in the US and Europe due to environmental and health concerns (they can be utilized only at factories grandfathered in that continue to use them). Boron nitride, a synthetic material used in cosmetics and as a high temperature lubricant, after conversion to BNO, has the potential to replace many of the current flame retardants in this big market.

Boron nitride is a layered material, much like graphite, that is thermally conductive, but electrically insulating. It is also very chemically inert and does not burn in air until approximately 900°C (1652°F). It is believed that, in order to utilize successfully, it should be exfoliated to increase its surface area, and functionalized in order to be well dispersed in the composite. The present disclosure provides some routes to make and fabricate functionalized exfoliated sheets in a commercially viable way.

In exemplary embodiments, this project/disclosure has been approached by two simultaneous directions: (i) making composite materials using material produced in the laboratory and (ii) using larger-scale manufacturing to produce pilot plant scale BNO material.

The effort to produce composites has involved polycarbonate (PC), poly(methyl methacrylate) (PMMA), polyurethane (PU) and nylon. Initial efforts focused on very small batches, in the order of 20 grams of polymer with up to 5% loading. The small scale was enough to demonstrate flame retardation. The composites thus formed by mixing the polymer with the BN do show some flame retardant ability.

Another approach to making the composites has been to graft polymers from the hydroxyl groups formed at the edges of the BNO sheets. This is expected to provide a filler material that will be well dispersed in the polymer, resulting in a more effective flame retardant at a much lower loading. Both PMMA and PU have been successfully attached to the edges of BN sheets.

The other direction, the larger-scale production of BNO, has been investigated. Some investigations have included discovering the equipment to heat the BN in air, add water to the resulting material, then spray dry the final product. For example, some investigations have looked at rotary heaters that will continually turn the BN in a tube as it moves through the hot zone. This can allow one to make much larger batches as the necessary contact with oxygen will be provided to the entire material.

Whereas the disclosure has been described principally in connection with boron nitride, such description has been utilized for purposes of disclosure and is not intended as limiting the disclosure. To the contrary, it is recognized that the disclosed systems, methods, techniques and assemblies are capable of use with other materials.

Although the systems and methods of the present disclosure have been described with reference to exemplary embodiments thereof, the present disclosure is not limited to such exemplary embodiments and/or implementations. Rather, the systems and methods of the present disclosure are susceptible to many implementations and applications, as will be readily apparent to persons skilled in the art from the disclosure hereof. The present disclosure expressly encompasses such modifications, enhancements and/or variations of the disclosed embodiments. Since many changes could be made in the above construction and many widely different embodiments of this disclosure could be made without departing from the scope thereof, it is intended that all matter contained in the drawings and specification shall be interpreted as illustrative and not in a limiting sense. Additional modifications, changes, and substitutions are intended in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the disclosure.

Claims

What is claimed is: CLAIMS

1. A method for modifying boron nitride comprising:

a) providing a boron nitride material;

b) thermally treating the boron nitride material under controlled conditions to partially oxidize the boron nitride material;

c) water washing the partially oxidized boron nitride material; and

d) allowing the boron nitride material to dry.

2. The method of claim 1, wherein after step d), the boron nitride material includes boron nitride oxide.

3. The method of claim 1, wherein step b) includes thermally treating the boron nitride material at about 950°C to about 1000°C in air for a predetermined period of time.

4. The method of claim 1, wherein step a) includes providing a hexagonal boron nitride material.

5. The method of claim 1, wherein after step d), the boron nitride material is substantially transparent.

6. The method of claim 1, wherein step b) includes thermally treating the boron nitride material at about 950°C to about 1000°C in air and holding at that temperature for about one hour.

7. The method of claim 1, wherein after step d), the boron nitride material includes a plurality of single sheets of boron nitride material, and one or more sheets extend about 150 nm in their lateral dimension.

8. The method of claim 7, wherein the plurality of single sheets of the boron nitride material are water dispersible.

9. The method of claim 1, wherein after step d), the dried boron nitride material is reacted with phenyl isocyanate.

10. The method of claim 1, wherein after step d), the boron nitride material is incorporated covalently into a polymer composite material.

11. The method of claim 1, wherein after step d), the boron nitride material is utilized as a filler in a polymer composite material.

12. The method of claim 1, wherein after step d), the boron nitride material is utilized in combination with graphene sheets to form a substrate material.

13. The method of claim 1, wherein after step d), the boron nitride material is utilized as a flame retardant in a polymer composite material.

14. The method of claim 13, wherein the polymer composite material includes polycarbonate.

15. The method of claim 1, wherein after step d), a polymer is grafted or attached to the boron nitride material.

16. The method of claim 15, wherein the polymer is poly(methyl methacrylate) or polyurethane.

17. The method of claim 11, wherein the polymer composite material includes a material selected from the group consisting of poly (methyl methacrylate), polyurethane, polycarbonate and nylon.

18. The method of claim 7, wherein one or more sheets have a sheet thickness of about 0.69 nm.

19. The method of claim 1, wherein step d) includes spray-drying the boron nitride material.

20. A method for fabricating a filler material for a composite comprising:

a) providing a hexagonal boron nitride material;

b) thermally treating the hexagonal boron nitride material at about 900°C to about

1000°C in air for a predetermined period of time;

c) water washing the thermally treated boron nitride material;

d) allowing the boron nitride material to dry; and

e) utilizing the dried boron nitride material as a filler in a polymer composite material.

21. A method for fabricating a filler material for a composite comprising:

a) providing a boron nitride material;

b) thermally treating the boron nitride material under controlled conditions in air for a predetermined period of time;

c) water washing the thermally treated boron nitride material;

d) allowing the boron nitride material to dry;

e) reacting the dried boron nitride material with phenyl isocyanate; and f) utilizing the boron nitride material as a filler in a polymer composite material.