Boron-nitride fibres

Abstract

A method for the production of boron nitride fibre, the method comprising (i) providing an electrospinning solution comprising a precursor polymer, wherein the precursor polymer is an N-C1-6 alkyl or N,N-di(C1-6 alkyl) poly(aminoborane) having a molecular weight (MW) of at least 20,000 gmol-1; (ii) electrospinning the precursor polymer to provide precursor polymer fibres; (iii) curing the precursor polymer fibres to provide cured precursor polymer fibres; and (iv) thermolysis of the cured precursor polymer fibres in an ammonia-containing atmosphere.

Description

[0001] BORON-NITRIDE FIBRES

[0002] Field of the Invention

[0003] The invention relates to methods for producing boron nitride fibres (BNF), boron nitride fibres which may be obtained by such methods and composites and structural constructs of the boron nitride fibres.

[0004] Background of the Invention

[0005] Pure hexagonal boron nitride (h-BN) fibres are much sought after for ultra-lightweight applications that need to perform and survive in extreme environments, e.g., typically found in space, aerospace, and other means of transportation. Hexagonal Boron nitride fibres (h-BNF) are of particular interest due to their isoelectronic relationship with carbon-based materials, closely related structures, impressive mechanical properties, and superior thermal stability and chemical inertness. h-BNF present certain advantages over its carbon counterparts. Unlike carbon fibres, h-BNF are electrically insulating. This property, coupled with high Young's modulus of fibrous BN, makes h-BNF potentially the strongest known insulating material to date. BNF also possess the exceptionally rare combination of being both a dielectric and a thermal conductor. It outperforms carbon fibres in thermal stability in oxidizing and inert atmospheres and exhibits much greater chemical resistance to extreme conditions.

[0006] Despite these outstanding properties and the existing demand, the availability of h- BNF are limited as their synthesis is not straightforward. Therefore, only very few reports on the synthesis of h-BNF exist to date. Among the key bottlenecks are the chemistry, properties, and processability of the BN precursor systems into fibres. Further, the reported fibre sizes, crystal structures, morphologies, and quality of h-BNF prepared by existing processes are insufficient to meet present demand. Processes for manufacture of h-BNF are presently challenging to scale to commercial levels.

[0007] Polyborazylenes (PBZs) have been studied for the last three decades as possible materials for making boron nitrides (BN), especially fibrous BN via a polymer derived ceramic route. Thus, fibres of PBZ may be created by spinning and the fibres subsequently annealed to convert to ceramic BNF. PBZs have a structure already containing units of hexagonal B-N rings which prevents complicated changes during the process of turning them into ceramics, resulting in a high yield. However, PBZs have disadvantages, such as sensitivity to moisture, difficulty in electrospinning into fibres, and the fact that PBZs are not soluble in the solvents commonly used in the spinning process. Spinning can therefore only be achieved by meltspinning, rather than solution spinning. This limits their further development and use. Another challenge with PBZs is the complex production process, mainly because borazylene, the starting material, is challenging to synthesize and isolate from the reaction mixture, with a typical yield of only 10%.

[0008] Accordingly, there is a need for improved processes for obtaining boron-nitride fibres, in particular processes that provide greater consistency in the fibres produced. There is also a need for boron nitride fibres that have improved physical properties.

[0009] Summary of the Invention

[0010] The present invention provides a method for the production of boron nitride fibre, the method comprising

[0011] (i) providing an electrospinning solution comprising a precursor polymer, wherein the precursor polymer is an N-CI-6 alkyl or N,N-di(Ci-6 alkyl) poly(aminoborane) having a molecular weight (Mw) of at least 20,000 gmoT1;

[0012] (ii) electrospinning the precursor polymer to provide precursor polymer fibres;

[0013] (iii) curing the precursor polymer fibres to provide cured precursor polymer fibres; and

[0014] (iv) thermolysis of the cured precursor polymer fibres in an ammonia-containing atmosphere.

[0015] Preferred methods involve the use of an electrospinning solution comprising a precursor polymer having a molecular weight of at least 40,000 or at least 70,000 gmol'1.

[0016] Also provided is a method for producing a composite material, comprising producing boron nitride fibre by a method as described herein, and combining the boron nitride fibre with an additional substance to produce a composite material. Also provided is a method for producing a boron nitride fibre structural construct, which method comprises producing boron nitride fibre or a composite material as described herein, and forming the boron nitride fibre or composite material into a structural construct.

[0017] The present methods enable the fabrication of high-quality and high ceramic yield boron nitride fibres (BNFs), which are similar to carbon fibres but with higher oxidation resistance and better mechanical properties including in extreme conditions, e.g., high temperature, corrosive environments. These BNFs can be produced according to the invention with high levels of control over their composition and structure, something that is challenging with traditional ceramic-making methods, especially for nitride compounds. Accordingly, the invention also relates to boron nitride fibre or a boron nitride fibre structural construct, obtainable or obtained by the methods described herein. In particular, the invention provides boron nitride fibre characterised in that the boron nitride fibre comprises hexagonal boron nitride having an atomic percentage of B of from 50-56%; an atomic percentage of N of from 43 to 49% and an atomic percentage of impurities of no more than 2%, and wherein the elemental ratio of nitrogen to boron is from 1: 1.1 to 1:1.3.

[0018] Description of the Figures

[0019] Figure 1 shows the morphology of electrospun precursor polymer fibres of the invention using various polymer precursor Mw and concentrations.

[0020] Figure 2 shows viscosity effects on the spinnability of polymer precursor solutions.

[0021] Figure 3 shows polymer concentration and Mw effects on the diameter of the electrospun polymer precursor fibres.

[0022] Figure 4 shows SEM images of the cross-section of a precursor polymer fibres of the invention (a) before curing, (b) after curing, and (c) after thermolysis (i.e. a BN fibre of the invention).

[0023] Figure 5 shows representative solid-state "B NMR spectra of precursor polymer powder and of electrospun precursor polymer fibres of the invention before and after curing.

[0024] Figure 6 shows an XPS survey spectrum of BNF according to the invention, and the inset shows calculated element content of B, C, N, and O.

[0025] Figure 7g shows the XRD pattern of a synthesized BNF according to the invention (synthesized BNF) and exfoliated h-BN. Figure 7h shows an enlarged view of the (002) peak

[0026] Figure 8 shows HRTEM line profiles of BNF showing d-spacing of 0.334 nm for (002) planes and 0.214 nm for (100) planes. Detailed Description of the Invention

[0027] Definitions

[0028] A precursor polymer refers to a polymer used as a precursor to the final BN fibre products of the invention. The precursor polymer is a polymeric substance capable of being converted to polymeric fibres via electrospinning.

[0029] An electrospinning solution as referred to herein is a solution of the precursor polymer in a solvent and is used in the electrospinning step.

[0030] A Ci-6 alkyl group contains from 1 to 6 carbon atoms and may be a linear or branched alkyl group. Typically, a Ci-6 alkyl group is a linear Ci-6 alkyl alkyl group. Examples of Ci-6 alkyl groups include methyl, ethyl, iso-propyl, n-propyl, n-butyl. Preferred Ci-6 alkyl groups are methyl and ethyl, in particular methyl.

[0031] References to molecular weight herein are weight average molecular weight (Mw). Molecular weight can be measured using known techniques, for example by gel permeation chromatography (GPC).

[0032] The term microfibre as used herein indicates a fibre having a diameter of at least I pm, typically from 1 to 10pm. The term nanofibre as used herein indicates a fibre having a diameter of less than 1 m. Fibre diameter is determined from SEM imaging of fibres. Suitable instrumentation for carrying out SEM imaging is provided in the Examples herein. Microfibre diameter is typically indicated as a mean value.

[0033] The curing step of the present process is typically a reduced pressure curing step. Reduced pressure curing refers to a process of heating under a reduced pressure, typically a pressure of 10,000Pa or less. Thermolysis as used herein refers to the thermal decomposition of the precursor polymer fibres which is carried out at elevated temperatures, typically at 900°C or higher. Together, the curing and thermolysis steps are referred to herein as the ceramic conversion process, or as the process of conversion to BN fibre.

[0034] Precursor Polymers

[0035] The BNF is produced via electrospinning of a precursor polymer. The precursor polymer is an N-CI-6 alkyl poly(aminoborane) or N,N-di(Ci-6 alkyl) poly(aminoborane). Such poly aminoboranes typically have the structure (I):

[0036] H-[NR1R2-BH2]n-H (I) wherein R1and R2are each independently selected from H and Ci-6 alkyl, wherein at least one of Rxand R2is Ci-6 alkyl; and n is a positive integer.

[0037] Typically n is a value which provide an overall molecular weight of the polymer of 20,000 to 400,000. Suitable values for n are from 400 to 10,000, for instance from 500 to 10,000, from 1500 to 10,000, from 2,000 to 10,000, from 2,500 to 10,000, preferably from 3,500 to 10,000, from 4,500 to 7,000 or from 6,000 to 7,000.

[0038] Polymers are substances which contain a series of individual polymer molecules, where the value of n will vary amongst the individual polymer molecules. Accordingly, a precursor polymer as referred to herein is a polymeric substance having defined R1, R2and R3groups, but where the value of n will vary within the polymeric substance.

[0039] Typically, R1, R2and R3are each independently selected from H and Ci-4 alkyl, for example H, methyl and ethyl, preferably from methyl and ethyl, more preferably R1, R2and R3are each independently selected from H and methyl.

[0040] Typically, one of R1and R2is H and the other is Ci-6 alkyl. In one embodiment, R1is H and R2is Ci-4 alkyl, preferably R2is methyl or ethyl. In a preferred embodiment, R1is H and R2is methyl.

[0041] Therefore, the N-CI-6 alkyl or N,N-di(Ci-6 alkyl) poly(aminoborane) are typically N-CI-4 alkyl or N,N-di(Ci-4 alkyl) poly(aminoborane), in particular N-C1-2 alkyl or N,N-di(Ci-2 alkyl) poly(aminoborane), preferably N-methyl or N,N-dimethyl poly(aminoborane).

[0042] Preferred precursor polymers are N-C1-6 alkyl poly(aminoborane), in particular N-C1-4 alkyl poly(aminoborane), N-C1-2 alkyl poly(aminoborane). Shorter alkyl chains are beneficial as this increases the B-N content of the precursor. This assists in the removal of C during the thermolysis step and provides a BNF having a greater purity. The most preferred precursor polymer is accordingly N-methyl polyaminoborane, which has the lowest C content of the N-alkyl polyaminoborane series.

[0043] A single precursor polymer may be used (e.g. N-methyl polyaminoborane). Thus, the precursor polymer may consist of, or consist essentially of, one precursor polymer. Alternatively, a mixture of two or more precursor polymers may be used. A reference herein to the precursor polymer is intended to encompass a single precursor polymer, or a mixture of two or more precursor polymers.

[0044] The precursor polymers used in the present invention are linear polymers. Linear polymers offer several advantages over their circular counterparts such as PBZs, including better chain entanglement, improved processability, and greater control over the molecular weight. Further, linear polymers may be more soluble in various solvents, leading to more stable solutions. This in turn provides improved consistency of the electrospinning process and thus a more consistent end product.

[0045] The inherent chain flexibility of linear polymers also enables the production of fine fibres. From a mechanical perspective, this flexibility results in superior properties like tensile strength and elongation at break, making them particularly suitable for electrospun fibre applications.

[0046] The precursor polymer molecular weight (Mw) is at least 20,000 gmol'1and is typically no more than 400,000 gmol'1. The precursor polymer preferably has a molecular weight (Mw) of at least 40,000 gmol'1, for example at least 70,000 gmol'1, preferably at least 90,000 gmol'1, or at least 100,000 gmol'1, at least 150,000 gmol'1, at least 200,000 gmol'1or at least 250,000 gmol'1. The precursor polymer may have a molecular weight of no more than 300,000 gmol'1. The precursor polymer may therefore have a molecular weight (Mw) of from 20,000 to 400,000 gmol'1, for example from 40,000 to 400,000 gmol'1, from 70,000 to 400,000 gmol'1, from 90,000 to 400,000 gmol'1, from 100,000 to 400,000 gmol'1, 150,000 gmol'1to 400,000 gmol'1, at least 200,000 gmol'1to 400,000 gmol'1or from 250,000 to 400,000 gmol'1.

[0047] Higher molecular weight precursor polymers are better substrates for electrospinning, leading to more consistent fibres with good polymer chain entanglement. The increased entanglement of chains in turn provides a more stable and continuous jet during electrospinning and more uniform fibre formation. Thus the higher molecular weight precursors may provide improved fibre morphology and reduced defects in the fibres. Accordingly, preferred polymer precursors have a molecular weight of at least 70,000 gmol'1, preferably at least 90,000 gmol'1and most preferably at least 100,000 gmol'1. Polymers having a molecular weight (Mw) in the range of 250,000 to 400,000 gmol'1, in particular from 250,000 to 300,000 gmol'1have been found to produce consistent BNF with beneficial physical properties.

[0048] Production of Precursor Polymers

[0049] N-CI-6 alkyl or N,N-di(Ci-6 alkyl) poly(aminoborane)PMeAB is formed by a polymerization method. Suitable polymerization methods are known in the art and can be found, for example, in Staubitz, A. et al. Catalytic dehydrocoupling / dehydrogenation of N- methylamine-borane and ammonia-borane: synthesis and characterization of high molecular weight polyaminoboranes. J. Am. Chem. Soc. 132, 13332-13345 (2010), Jurca, T. et al. Stepgrowth titanium-catalysed dehydropolymerisation of amine-boranes. Chem. Sci. 9, 3360-3366 (2018), or De Albuquerque Pinheiro, C. A. et al. Solventless and Metal -Free Synthesis of High- Molecular-Mass Polyaminoboranes from Diisopropylaminoborane and Primary Amines. Agnew. Chem. Int. Ed. 57, 1519-1522 (2018).

[0050] Electrospinning

[0051] The polymer precursor is dissolved in a solvent to provide an electrospinning solution. The skilled person will be familiar with suitable solvents used in electrospinning processes, and any such standard solvents can be used. For example, one or a mixture of two or more solvents selected from CHCh, CH2CI2, dimethylformamide, tetrahydrofuran, acetonitrile, dimethylsulfoxide, dimethyl ether may be used. CHCh is a preferred solvent. The precursor polymers described herein typically have good solubility and stability in such solvents.

[0052] The concentration of the precursor polymer in the electrospinning solution is typically from 1 to 50 wt%, preferably from 5 to 30 wt%, in particular from 15 to 30 wt%. The concentration of the precursor polymer in the electrospinning solution may influence the electrospinning behaviour and resulting fibre morphology. For instance, if the concentration of precursor polymer is too low, this may result in the formation of droplets rather than fibres in the electrospinning step, as insufficient chain entanglement occurs. Increasing the polymer concentration may also reduce the formation of beads on the resulting fibres and lead to more homogeneous fibres.

[0053] On the other hand, if the concentration of polymer is too high, the solution may become viscous and more difficult to process in the electrospinning step. Accordingly, concentrations of from 5 to 30 wt% preferably 15 to 30 wt% are useful and are preferred where the solvent is CHCI3. Such concentrations have been found to provide high quality, bead-free fibres.

[0054] Higher molecular weight precursor polymers typically provide greater chain entanglement during electrospinning and therefore can be effectively spun at lower concentrations. However, higher concentrations of precursor polymer may be preferred in the case of a lower molecular weight precursor polymer. For instance, the concentration of precursor polymer may be from 15 to 50 wt% in the case of a precursor polymer having a molecular weight of less than 100,000 gmol'1, or from 5 to 30 wt% in the case of a precursor polymer having a molecular weight of 100,000 gmol'1or more. One or more additives may be present in the electrospinning solution. For instance, additives such as polyacrylonitrile (PAN), polyvinyl acetate (PVA), polyethylene oxide (PEO) and / or polyvinyl pyrrolidine (PVP) are sometimes added to electrospinning solutions in order to increase the stability of the solution and enhance the electrospinning properties. Such additives are typically carbon-rich materials which can be incorporated into the final fibre as impurities. Accordingly, to reduce impurities and limit the C content of the final ceramic fibre product, it is preferred that the electrospinning solution contains carbon -containing additives in an amount of no more than 10 wt%, preferably no more than 5 wt%, 2 wt%, 1 wt% or 0.5 wt%, relative to the total weight of the electrospinning solution. More preferably, carbon-containing additives are present in an amount of no more than 0.1% or 0.01 wt% relative to the total weight of the electrospinning solution. Preferably, the electrospinning solution is substantially free of, or is free of, additives. Thus, the electrospinning solution typically comprises one or more solvents and one or more precursor polymers in a total amount of at least 90 wt%, preferably at least 95 wt%, relative to the total weight of the electrospinning solution. More preferably, the electrospinning solution consists essentially of, or consists of, one or more solvents and one or more precursor polymers. As used herein, an electrospinning solution which consists essentially of one or more solvents and one or more precursor polymers typically contains the solvent(s) and the precursor polymer(s) in a total amount of at least 98 wt%, preferably at least 99%, 99.5% or 99.9 wt% relative to the total weight of the electrospinning solution.

[0055] Typically, the viscosity of the electrospinning solution is from 0.02 to 1 Pa.s. A viscosity of at least 0.02 Pa.s enables convenient electrospinning. However, where the viscosity is above 1 Pa.s, the solution has very high cohesion and electrospinning becomes difficult.

[0056] Typically, the surface tension of the electrospinning solution is at least 26 mN / m. The surface tension of the electrospinning solution results from the intermolecular interactions between the polymer chains and the solvent molecules. Above a surface tension of 26 mN / m, electrospinning can be effectively carried out and fibres having good quality, high homogeneity and continuity can be achieved.

[0057] Electrospinning processes are well-known in the art and the electrospinning step of the present process may be carried out using known techniques. Individual electrospinning parameters may be varied by the skilled person. Typically, a voltage of from 10 to 35kV, for instance form 20 to 35kV is applied. Typically, the electrospinning distance, i.e. the distance from the needle tip from which precursor polymer solution is provided, to the collector on which formed fibre is gathered, is from 10 to 50cm, for example from 20 to 40cm, e.g. around 30cm. Electrospinning may be carried out at a temperature around room temperature (for example from 15 to 35°C, for example from 18 to 22 °C or about 20 °C) and / or at a humidity of from 10 to 30%, for example 10 to 20% or about 15%.

[0058] The electrospinning step produces fibres formed of the precursor polymer (precursor polymer fibres). The fibres may be microfibres or nanofibres. The precursor polymer fibres typically have a diameter of from 0.1 to 10 pm, preferably from 1 to 5 pm. Increasing the concentration of the precursor polymer, and / or the molecular weight of the precursor polymer, generally causes an increase in the diameter of the fibre, due to more extensive polymer chain entanglement during spinning.

[0059] The fibres may be aligned or random or a combination of aligned and random. They may have a core-shell, porous or hollow morphology, or a combination thereof.

[0060] The macrostructure of the electrospun fibres may vary by control of the fibre collection. For instance, the fibres may take the forms of a mat (i.e. a 2D mat). The thickness of the mat may be tailored to meet the desired requirements. Alternatively, 3D structures may be formed.

[0061] BN Fibres

[0062] To produce BN fibres from the precursor polymer fibres, the precursor polymer fibres are first cured under reduced pressure conditions (reduced pressure curing), to provide cured precursor polymer fibres, and the cured precursor polymer fibres are then subject to thermolysis to produce the desired BN fibres. This process has been found to preserve the structure of the fibres produced in the previous step.

[0063] Curing is typically carried out at reduced pressure and elevated temperature.

[0064] Preferably, the curing pressure is 10,000 Pa or less and the temperature is at least 80 °C. More preferably, the curing pressure is from 100 to 10,000Pa and the temperature is 80 to 150 °C. In embodiments, curing may be carried out under one or more, preferably two or more, more preferably all, of the following conditions: a pressure of 10,000Pa or less, for example from 100 to 10,000Pa or from 1,000 to 10,000 Pa; a temperature of 80 to 150 °C, preferably about 100°C; a curing time of at least 24 hours, preferably from 36 to 72 hours or from 36 to 60 hours, for example for about 48 hours.

[0065] Cross-linking of the precursor polymer chains may occur during the reduced-pressure curing process, leading to a structure comprising cross-linked fibres of precursor polymer. The cross-linking stage provides a stable cross-linked structure which may assist in preserving the macro- and micro-structure of the fibres during the conversion to BN fibre.

[0066] Volatile substances may be released during the reduced pressure curing step, including, for example, solvent molecules. The removal of volatile substances in a controlled manner during the curing step enables a controlled release of substances during the later thermolysis step and may reduce overall weight loss from the precursor polymer fibres during the conversion to BN fibre.

[0067] Thermolysis is carried out in a gaseous atmosphere containing ammonia, typically in combination with an inert gas. The inert gas may be argon. The volume ratio of ammonia to inert gas is from 1:4 to 1:1, preferably from 1:3 to 2:3, more preferably about 1:2. Thermolysis in the presence of ammonia helps to reduce the carbon content of the BN fibre product. The reducing nature of the ammonia environment not only facilitates further release of amine and alkane (typically methane, CH4) from the precursor fibres but also provides an additional N source to react with the B-rich intermediates.

[0068] The thermolysis temperature is typically at least 900°C, preferably at least 1200°C or at least 1300°C. The thermolysis temperature may be up to 1800°C, for example up to 1500°C. Suitable ranges for the thermolysis temperature are therefore from 900°C to 1800°C, from 1200°C to 1800°C or from 1300°C to 1800°C. A benefit of the present process is that effective thermolysis, whilst preserving the structure of the precursor polymer fibres can be achieved at lower temperatures, e.g. up to 1700 °C, up to 1600 °C, up to 1500 °C or up to 1400 °C. Such lower temperatures have also been found to successfully remove carbon content from the precursor polymer fibres, thus leading to a high purity BN fibre product. Therefore, preferred thermolysis temperatures are from 900°C to 1600°C, from 1200°C to 1600°C or from 1300°C to 1600°C and preferably from 900°C to 1500°C, from 1200°C to 1500°C, particularly from 1300°C to 1500°C. Thermolysis may alternatively be carried out at from 1200°C to 1400°C or from 1300°C to 1400°C. Thermolysis is typically carried out for at least 1 hour, preferably from 1 to 5 hours. The ceramic conversion process as described herein substantially maintains both the micro- and macro-structure of the precursor polymer fibres. Thus, the fibres retain a smooth, dense surface.

[0069] It is preferred that the fibre diameter reduces by a minimum amount, e.g. by no more than 20%, compared with the diameter of the precursor polymer fibres, during the ceramic conversion process, i.e. on conversion from precursor polymer fibre to BN fibre. Thus, the present process allows for substantial maintenance of fibre diameter, for example leading to a reduction of no more than 15%, preferably no more than 10% or more preferably no more than 8% compared to the precursor polymer fibre diameter. The reduction in fibre diameter may, therefore, be from 0.1 to 15%, 0.1 to 10% or 0.1 to 8% of the original fibre diameter.

[0070] The BN fibres may be microfibres or nanofibres. The BN fibres typically have a diameter of from 0.1 to 10 pm, preferably from 1 to 5 pm.

[0071] The ceramic conversion process typically substantially removes all carbon content from the fibres. Thus, the BN fibres are substantially free of C. Substantially free of C indicates a C content of less than 3 wt%, preferably less than 2 wt%, more preferably less than 1 wt%, based on the total weight of the BN fibre.

[0072] Typically, the elemental ratio of nitrogen to boron in the boron nitride fibre is from 1:1.1 to 1:1.3. The atomic percentage of B is typically at least 45%, preferably at least 50%, for example from 50-57%, from 50 to 56% or from 51 to 54%. The atomic percentage of N is typically from 40 to 50%, for example from 43 to 49%. Preferably, the atomic percentage of atoms other than B and N is no more than 3%, more preferably no more than 2% or no more than 1%, i.e. 0 to 3, 0 to 2 or 0 to 1%.

[0073] For instance, the BN fibre may have an atomic percentage of B of from 50 to 57%, an atomic percentage of N of from 40 to 50% and an impurity content of 0 to 3%. In one embodiment, the BN fibre may have an atomic percentage of B of from 50 to 56%, an atomic percentage of N of from 43 to 49% and an impurity content of 0 to 2%.

[0074] The BN fibre typically comprises hexagonal BN. In one embodiment, the BN fibre may comprise at least 80wt%, more preferably at least 90wt% or at least 95wt% hexagonal BN. In one embodiment, the BN fibre consists of hexagonal BN. Hexagonal BN materials have the advantage of both ultra-high strength to weight ratio and low density.

[0075] Alternatively, the BN fibre may be a polycrystalline form comprising two or more different crystalline forms. Preferably a polycrystalline form is made up of hexagonal BN and one or more further crystalline forms. The one or more further crystalline forms typically include turbostratic BN. Thus, the BN fibre may be a polycrystalline form comprising at least hexagonal and turbostratic forms.

[0076] BN Fibre composites and constructs

[0077] The BN fibre of the present invention may be used in the production of a variety of different composites and / or structural constructs. For instance, the BN fibre may be combined with an additional substance to provide a composite material. Suitable additional substances which can be combined with the BN fibres include polymers, natural fibres, carbon fibres, other ceramics, metals and combinations thereof.

[0078] The additional substances may form covalent or non-covalent chemical bonds or physical interactions with the BN fibre.

[0079] The boron nitride fibre may take any desired structural form. The form of the BN fibre may be determined by the electrospinning and ceramic conversion processes. Further structural modification of the BN fibre may be carried out after ceramic conversion to form a structural construct. For example, BN fibre mats may be cut and / or shaped.

[0080] BN fibre materials may be used in a variety of products. An exemplified use is as a heat sink, for instance as a cell separator in a lithium ion battery.

[0081] The present invention provides the following embodiments:

[0082] 1. A method for the production of boron nitride fibre, the method comprising

[0083] (i) providing an electrospinning solution comprising a precursor polymer, wherein the precursor polymer is an N-CI-6 alkyl or N,N-di(Ci-6 alkyl) poly(aminoborane) having a molecular weight (Mw) of at least 20,000 gmol’1;

[0084] (ii) electrospinning the precursor polymer to provide precursor polymer fibres;

[0085] (iii) curing the precursor polymer fibres to provide cured precursor polymer fibres; and

[0086] (iv) thermolysis of the cured precursor polymer fibres in an ammonia-containing atmosphere.

[0087] 2. A method according to embodiment 1, wherein the precursor polymer has a molecular weight (Mw) of at least 40,000 gmol’1. 3. A method according to embodiment 1 or embodiment 2, wherein the precursor polymer is an N-CI-6 alkyl poly(aminoborane).

[0088] 4. A method according to embodiment 3, wherein the precursor polymer is N-methyl polyaminoborane.

[0089] 5. A method according to any one of the preceding embodiments, wherein the precursor polymer has a molecular weight (Mw) of at least 70,000 gmol'1, preferably at least 90,000 gmol'1.

[0090] 6. A method according to any one of the preceding embodiments, wherein the precursor polymer has a molecular weight (Mw) of from 100,000 to 400,000 gmol'1, preferably from 250,000 to 400,000 gmol'1.

[0091] 7. A method according to any one of the preceding embodiments, wherein the electrospinning solution comprises one or more solvents selected from CHCh, CH2CI2, dimethylformamide, tetrahydrofuran, acetonitrile, dimethylsulfoxide and dimethyl ether, preferably CHCh.

[0092] 8. A method according to embodiment 7, wherein the solvent is CHCh, and wherein the concentration of precursor polymer in the electrospinning solution is from 5 to 30 wt%, preferably from 15 to 30 wt%.

[0093] 9. A method according to any one of the preceding embodiments, wherein the precursor polymer is N-methyl polyaminoborane having a molecular weight (Mw) of from 250,000 to 400,000 gmol'1, and wherein the concentration of precursor polymer in the electrospinning solution is from 15 to 30 wt%.

[0094] 10. A method according to any one of the preceding embodiments, wherein the electrospinning solution has one or more of the following features:

[0095] (i) a viscosity of from 0.02 to 1 Pa.s;

[0096] (ii) a surface tension of at least 26 mN / m. 11. A method according to any one of the preceding embodiments wherein the electrospinning solution consists essentially of the precursor polymer and one or more solvents.

[0097] 12. A method according to any one of the preceding embodiments, wherein step (ii) comprises electrospinning the precursor polymer to provide precursor polymer fibres which are microfibres and / or nanofibres.

[0098] 13. A method according to embodiment 12, wherein the fibres have a morphology which is aligned, random, core-shell, porous, hollow or a combination thereof.

[0099] 14. A method according to embodiment 12 or 13, wherein step (ii) provides precursor polymer fibres in the form of a mat or a 3D structure.

[0100] 15. A method according to any one of the preceding embodiments, wherein the precursor polymer fibres have a diameter of from 0.1 to 10 pm, preferably from 1 to 5pm.

[0101] 16. A method according to any one of the preceding embodiments, wherein curing is carried out at a pressure of 10,000Pa or less.

[0102] 17. A method according to any one of the preceding embodiments, wherein curing is carried out at a temperature of 80 to 150 °C, preferably about 100°C.

[0103] 18. A method according to any one of the preceding embodiments, wherein curing is carried out for at least 24 hours, preferably from 36 to 72 hours.

[0104] 19. A method according to any one of the preceding embodiments, wherein thermolysis is carried out in an atmosphere comprising ammonia and an inert gas, preferably wherein the inert gas is argon.

[0105] 20. A method according to embodiment 19, wherein the volume ratio of ammonia to inert gas is from 1:4 to 1:1, preferably from 1:3 to 2:3, more preferably about 1:2. 21. A method according to any one of the preceding embodiments, wherein thermolysis is carried out at a temperature of at least 900°C, preferably at a temperature of from 1200°C to 1800°C, more preferably from 1300°C to 1500°C.

[0106] 22. A method according to any one of the preceding embodiments, wherein thermolysis is carried out for at least 1 hour, preferably from 1 to 5 hours.

[0107] 23. A method according to any one of the preceding embodiments, wherein the fibre diameter reduces by no more than 15% on conversion from precursor polymer fibre to boron nitride fibre.

[0108] 24. A method according to any one of the preceding embodiments, wherein the boron nitride fibre has a diameter of from 1 to 5 pm.

[0109] 25. A method according to any one of the preceding embodiments, wherein the elemental ratio of nitrogen to boron in the boron nitride fibre is from 1: 1.1 to 1:1.3.

[0110] 26. A method according to any one of the preceding embodiments, wherein the carbon content of the boron nitride fibre is less than 2 wt%, preferably less than 1 wt%.

[0111] 27. A method according to any one of the preceding embodiments, wherein the boron nitride fibre is a polycrystalline form comprising at least hexagonal and turbostratic forms.

[0112] 28. A method for producing a composite material, comprising producing boron nitride fibre by a method according to any one of the preceding embodiments, and combining the boron nitride fibre with an additional substance to produce a composite material.

[0113] 29. A method for producing a boron nitride fibre structural construct, which method comprises producing boron nitride fibre or a composite material according to any one of the preceding embodiments, and forming the boron nitride fibre or composite material into a structural construct. 30. Boron nitride fibre, composite material or boron nitride fibre structural construct obtainable or obtained by a method as defined in any one of embodiments 1 to 29.

[0114] 31. Boron nitride fibre characterised in that the boron nitride fibre comprises hexagonal boron nitride having an atomic percentage of B of from 50-56%; an atomic percentage of N of from 43 to 49% and an atomic percentage of impurities of no more than 2%, and wherein the elemental ratio of nitrogen to boron is from 1: 1.1 to 1:1.3.

[0115] 32. Boron nitride fibre according to embodiment 31, wherein the boron nitride fibre consists of hexagonal boron nitride.

[0116] Examples

[0117] Abbreviations:

[0118] PMeAB: Polymethylaminoborane

[0119] PMeABF: Poly methyl aminoborane fibre

[0120] BNF: boron nitride fibre

[0121] Materials:

[0122] Chloroform (CHCh, 99+%) was obtained from Fisher Scientific. Ammonia gas (NH3, 99.98%) and argon gas (Ar, 99.998%) were purchased from BOC. All reagents were used without further purification unless otherwise specified.

[0123] Samples of PMeAB having various molecular weights were used in the examples which follow. The molecular weights and dispersity of the samples of PMeAB were as set out in Table 1.

[0124] Characterisation Techniques:

[0125] All polymeric materials were analysed by gel permeation chromatography (GPC) measured on a Malvern Viskotec GPCmax together with a Viskotec TDA 305 RI detector. Polymer Mnis referenced to polystyrene standards between Mn474 - 476,800 g-rnol"1. All samples were passed through 3 columns consisting of a porous styrene divinylbenzene copolymer (2 x T5000 and 1 x T4000 Malvern columns). The eluent used was GPC grade THF containing 0.1% w / w [NBu4]Br and the flow rate was 1 cm3min-1. All polymer samples were dissolved in GPC grade THF [NBu4]Br (2 mg mL1) and filtered through a PTFE filter (pore size: 45 pm).

[0126] The viscosity of the PMeAB solutions was measured using a Brookfield DV-II viscometer with a cone spindle CP-41. A fixed volume of 2 mL of the solution was loaded into the sample cup, and a ramping of rotation speed was employed from 50 to 120 rpm, corresponding to shear rates of 100-240 s ' . For highly viscous PMeAB solutions, the rotation speed was reduced to 10 rpm to maintain the % torque between 10 and 100%. The viscosity measurement was conducted twice, and the averaged viscosity values were reported.

[0127] Surface tension measurements are performed using an Ossila contact angle goniometer in ambient conditions.

[0128] Scanning Electron Microscopy (SEM) images were acquired using a Zeiss Merlin SEM and a JEOL JSM-840F SEM operating at an accelerating voltage of 3 kV. Prior to imaging, the samples were coated with a 10 nm layer of platinum (Pt).

[0129] Fibre diameters were determined by analysing at least 200 unbiased counts from the SEM images and fitting the resulting histogram.

[0130] Energy -Dispersive X-ray Spectroscopy (EDX) line scanning and elemental mapping were performed using a Zeiss Merlin SEM operating at an accelerating voltage of 3 kV.

[0131] Transmission Electron Microscopy (TEM) images were obtained using a JEOL JEM-21 OOF TEM operating at an acceleration voltage of 200 kV.

[0132] X-ray Photoelectron Spectroscopy (XPS) analysis was conducted using a Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer System. An ion pumped VG Microtech CLAM 4 MCD analyser system equipped with unmonochromated Mg Ka X-ray radiation of 1253.6 eV was used.

[0133] Fourier Transform Infrared (FT-IR) Spectroscopy attenuated total reflection (ATR) spectra were recorded using a Varian Excalibur FTS 3500 FT-IR spectrometer in the range of 600 to 4000 cm

[0134] X-ray Diffraction (XRD) analysis was performed at room temperature using a Siemens D5000 powder diffractometer with copper Ka radiation ( = 0.15406 nm) and a secondary monochromator. The samples were continuously rotated during data collection, and a step size of 0.05° 29 was used in the range of 10-100° 29 with a count time of 12 s per step.

[0135] Thermogravimetric Analyser with Mass Spectrometer (TGA-DSC-MS) measurements were conducted using a STA 449 F3 Jupiter® instrument coupled with a 403 Aeolos Quadro quadrupole mass spectrometer. The samples were heated in Ar atmosphere from room temperature to 1400°C at a heating rate of 10°C / min. The quadrupole was used to scan all m / z ratios from m / z = 1 to m / z = 150 approximately every 30 s. nB magic angle spinning-nuclear magnetic resonance (nBMAS-NMR) measurements were carried out at 128.39 MHz using a Varian VNMRS spectrometer and 4 mm (rotor o.d.) probe. Spectra were acquired at a spin rate of 12 kHz. All direct excitationnB spectra were acquired with a 1 ps 30° solid pulse which was determined from a 6 ps solution pulse determined on BFs / OEt?. The spectra were acquired with a recycle delay of 1 s determined on the sample. Boron spectral referencing is relative to BFs / OEt?.

[0136] Example 1: Electrospinning PMeAB Fibres

[0137] To prepare the electrospinning solutions, PMeAB described in Table 1 was used.

[0138] Table 1

[0139] 1 PMeABl 27,900 46,700 1.7

[0140] 2 PMeAB2 47,900 71,800 1.5

[0141] 3 PMeAB3 74,200 110,500 1.5

[0142] 4 PMeAB4 76,600 112,800 1.5

[0143] 5 PMeAB5 101,200 148,600 1.5

[0144] 6 PMeAB6 115,400 172,000 1.5

[0145] 7 PMeAB7 131,800 205,200 1.6

[0146] 8 PMeAB8 162,300 250,500 1.5

[0147] 9 PMeAB9 186,300 290,500 1.6

[0148] 10 PMeABlO 196,100 318,300 1.5

[0149] PMeAB was dissolved in 2 mL of CHCh of desired concentration. Concentration was varied from 5 to 30wt% and spinning solutions are labelled as PMeABX-Y%, wherein X corresponds to the PMeAB sample of Table 1 and Y corresponds to the weight percentage of PMeAB in CHCh. The dissolution process involved continuous stirring for 1 hour at room temperature.

[0150] Once the PMeAB was completely dissolved, the solution was transferred immediately to a 1 mL syringe equipped with a 23 -gauge stainless steel needle. The syringe was connected to a high voltage supply (Genvolt High Voltage Power Supply). The solution was gravity -fed without the use of a syringe pump. The tip-to-collector distance was maintained at 30 cm throughout the electrospinning experiments. An applied voltage ranging from 20 to 35 kV was used to ensure optimal electrospinning continuity. The electrospinning process was conducted in a glove box with controlled temperature (20±l°C) and humidity (15±1%). Finally, the PMeAB precursor fibres were collected on a piece of aluminium foil attached to a grounded metal substrate and could be easily peeled off into free-standing fibres mat. Both PMeAB molecular weight and its concentration determine the spinning behavior and resulting fibre morphology (Figure 1). Electrospinning of PMeAB with lower Mw, including PMeABl, PMeAB2, PMeAB3, PMeAB4, PMe B5, at concentrations below 10 wt.% typically resulted in the formation of droplets. At polymer concentrations between 15 and 30 wt.%, the solutions were more easily spinnable.

[0151] While electrospinning of PMeAB5-15% led to fibres with beads, Figure la(V), increasing the concentration to PMeAB5-20% and PMeAB5-25% partially suppressed the formation of beads, e.g. Figure la(VI). Further increasing the concentration to PMeAB5-30% makes the solution very viscous and electrospinning becomes more difficult. Comparing PMeABl, 2, 3, 4, and PMeAB5 to higher MwPMeAB, such as PMeAB6 and PMeAB7, using at the same polymer concentration, revealed that PMeAB6 and PMeAB7 form more homogeneous fibres with significantly fewer beads, see Figures 2a(III) vs (V) and Figure la(IV) vs (VI). Notably, electrospinning PMeAB6-20% yields fibres without any beads. The further increase of the up to PMeAB8 or higher results in high-quality bead-free fibres, when a concentration of 15 wt.% is used.

[0152] Figure 2 shows the viscosities providing the ideal spinnable window, i.e. 0.02 to 1 Pa.s. Viscosity effects on the spinnability of polymer precursor solutions. Figure 3 shows polymer concentration and Mw effects on the diameter of the electrospun polymer precursor fibres, showing that, as the concentration or molecular weight of the polymer increases, the average diameter of the fibres tends to increase.

[0153] Example 2: Conversion of PMeAB Fibres to BN Fibres

[0154] The conversion of PMeAB fibres to BN fibres involved two essential steps: curing and annealing. The curing process was carried out in a vacuum oven at a temperature of 100°C for 48 h, maintaining a vacuum pressure below 10'1bar. Subsequently, the cured PMeAB fibres were annealed in a high-temperature tube furnace. The fibres were heated directly to 1400°C and held at that temperature for 2 h in an atmosphere consisting of a mixture of NEE and Ar gases in a volume ratio of 1 :2. The samples were then allowed to cool naturally to room temperature. The heating rate from room temperature to 1400°C was set at 10°C / min.

[0155] Figure 3a-c shows SEM images for the cross-section of (a) a raw PMeABF (i.e. prior to curing), (b) a cured PMeABF, and (c) a BNF. In each case the PMeAB are fibres made from PMeAB7 as set out in Table 1. The raw PMeABF has an average diameter of 3.635pm. After annealing (thermolysis), the BNF retains its well-defined fibrous morphology, and the surface remains dense without visible defects or pores. Crystallization is observable at the cross-section, indicating the conversion from PMeAB polymer to BN ceramic. The average diameter of BNF is 3.378 pm, which is not significantly different from PMeABF. The main difference between raw and cured PMeABF is that the latter cannot be dissolved in any common solvents, such as CHCh, THF, or DMF. The solid-statenB NMR spectra for precursor PMeAB powder and as-spun PMeABF showed no differences between them, with the relatively sharp signal characteristic of the [FEBNMeH],, motif observed at 8 — 8. The NMR spectrum of the cured PMeABF shows a notable decrease in the intensity of the 8 — 8 signal, a new broad signal at 8 -10, and a sharper one at 8 ~1 (Figure 5). The linewidths and chemical shifts of these new signals are characteristic of three coordinate, trigonal, BN3 environments and four coordinate BN4 environments.

[0156] The elemental ratio for BNF produced through annealing PMeABF at 1400°C in NH3 is 1:1.2, which suggests a nearly stoichiometric composition ofB and N elements, with a negligible carbon impurity of 1.31%.

[0157] Example 3: Characterization of BNF

[0158] The produced BNF (obtained from PMeAB7) was analysed by XPS. The XPS analysis shows the presence of B Is (190.4 eV), C Is (284.8 eV), N Is (398.0 eV), and O Is (533.1 eV) peaks confirming the intrinsic BN nature (Figure 6). The presence of weaker signals at approximately 188.7 eV and 396.6 eV for B-C and N-C, respectively, indicates less than 2% of C impurities. We did not observe B-0 (-192.5 eV) and N-0 (-400.4 eV) modes for the BNF. The presence of minor O impurities in the sample could potentially be attributed to negligible oxidation or moisture adsorption occurring under ambient atmosphere. The atomic percentages of B and N elements are 52.34% and 44.73%, respectively. Elemental mapping analysis provided evidence of the homogeneous distribution of B and N elements within the BNF.

[0159] The X-ray diffraction pattern of the synthesized BNF was compared with a corresponding pattern obtained from exfoliated h-BN (Figure 7g, h), which was prepared as a reference material using liquid phase exfoliation. h-BN platelets obtained from 3M and IPA were shear mixed to enable individual hOBN flakes to separate from the platelets and stay well dispersed in the IPA solution. The thus produced exfoliated h-BN flakes were then centrifuges to remove IPA. The XRD spectra display two broad peaks at approximately 29 ~ 25° and 43°, corresponding to the (002) and (100) crystal planes of h-BN, respectively. Compared with BNNS, the peaks show reduced intensity and broadened width [FWHM: BNF<oo2) peak = 5.25° and BNF(ioo) peak = 4.06° vs BNNS<oo2) peak = 0.29° and BNNS(ioo) peak = 0.29°], indicating the turbostratic structure of BN with less extended / ordered stacking along both a- and c-axes. The calculated d<oo2) layer spacing for the BNF is 0.352 nm, slightly larger than that of BNNS (0.333 nm), which is generally observed in ID-structured BN materials due to the combination of turbostratic ordering and wall curvature. The calculated average crystallite size of BNF is 2.16 nm, indicating the poly crystalline structure. While the turbostratic poly crystalline structure is evident, the presence of higher resolved peaks, e.g. (101), (102), (004), (110), in the XRD pattern of BNF indicates that a certain level of ordering is still present in the material.

[0160] TEM along with fast Fourier transform (FFT) pattern were used to further quantify the crystal information of BNF. From the HRTEM image, the measured d-spacing is 0.334 nm for (002) planes and 0.214 nm for (100) planes, in agreement with values reported in literatures (Figure 8). The FFT reveals the partially ordered structure: ring-like diffraction patterns dominated (corresponds to turbostratic or polycrystalline regions) with several visible diffraction spots (correspond to hexagonal or crystalline regions). By measuring the distance between two symmetric spots in FFT, we were able to assign the diffraction spots to the (002) and (100) planes, consistent with the results obtained from HRTEM and XRD analyses. This confirms that our method enables the fabrication of high purity polycrystalline hexagonal / turbostratic form of BNF.

Claims

CLAIMS1. A method for the production of boron nitride fibre, the method comprising(i) providing an electrospinning solution comprising a precursor polymer, wherein the precursor polymer is an N-CI-6 alkyl or N,N-di(Ci-6 alkyl) poly(aminoborane) having a molecular weight (Mw) of at least 20,000 gmol'1;(ii) electrospinning the precursor polymer to provide precursor polymer fibres;(iii) curing the precursor polymer fibres to provide cured precursor polymer fibres; and(iv) thermolysis of the cured precursor polymer fibres in an ammonia-containing atmosphere.

2. A method according to claim 1, wherein the precursor polymer has a molecular weight (Mw) of at least 40,000 gmol'1, preferably at least 70,000 gmol'1, more preferably at least 90,000 gmol'1.

3. A method according to claim 1 or claim 2, wherein the precursor polymer is an N-CI-6 alkyl poly(aminoborane).

4. A method according to claim 3, wherein the precursor polymer is N-methyl polyaminoborane.

5. A method according to any one of the preceding claims, wherein the precursor polymer has a molecular weight (Mw) of from 100,000 to 400,000 gmol'1, preferably from 250,000 to 400,000 gmol'1.

6. A method according to any one of the preceding claims, wherein the electrospinning solution comprises one or more solvents selected from CHCh, CH2CI2, dimethylformamide, tetrahydrofuran, acetonitrile, dimethylsulfoxide and dimethyl ether, preferably CHCh; optionally wherein: the solvent is CHCh, and the concentration of precursor polymer in the electrospinning solution is from 5 to 30 wt%, preferably from 15 to 30 wt%.

7. A method according to any one of the preceding claims, wherein the precursor polymer is N-methyl polyaminoborane having a molecular weight (Mw) of from 250,000 to 400,000 gmol’1, and wherein the concentration of precursor polymer in the electrospinning solution is from 15 to 30 wt%.

8. A method according to any one of the preceding claims, wherein the electrospinning solution has one or more of the following features:(i) a viscosity of from 0.02 to 1 Pa.s;(ii) a surface tension of at least 26 mN / m.

9. A method according to any one of the preceding claims wherein the electrospinning solution consists essentially of the precursor polymer and one or more solvents.

10. A method according to any one of the preceding claims, wherein step (ii) comprises electrospinning the precursor polymer to provide precursor polymer fibres which are microfibres and / or nanofibers; optionally wherein the fibres have a morphology which is aligned, random, core-shell, porous, hollow or a combination thereof.

11. A method according to claim 10, wherein step (ii) provides precursor polymer fibres in the form of a mat or a 3D structure.

12. A method according to any one of the preceding claims, wherein the precursor polymer fibres have a diameter of from 0.1 to 10 pm, preferably from 1 to 5pm.

13. A method according to any one of the preceding claims, wherein curing is carried out at a pressure of 10,000Pa or less; and / or wherein curing is carried out at a temperature of 80 to 150 °C, preferably about 100°C; and / or wherein curing is carried out for at least 24 hours, preferably from 36 to 72 hours.

14. A method according to any one of the preceding claims, wherein thermolysis is carried out in an atmosphere comprising ammonia and an inert gas, preferably wherein the inert gas is argon; optionally wherein the volume ratio of ammonia to inert gas is from 1:4 to 1:1, preferably from 1:3 to 2:3, more preferably about 1:2.

15. A method according to any one of the preceding claims, wherein thermolysis is carried out at a temperature of at least 900°C, preferably at a temperature of from 1200°C to 1800°C, more preferably from 1300°C to 1500°C.

16. A method according to any one of the preceding claims, wherein thermolysis is carried out for at least 1 hour, preferably from 1 to 5 hours.

17. A method according to any one of the preceding claims, wherein the fibre diameter reduces by no more than 15% on conversion from precursor polymer fibre to boron nitride fibre.

18. A method according to any one of the preceding claims, wherein the boron nitride fibre has a diameter of from 1 to 5 pm.

19. A method according to any one of the preceding claims, (i) wherein the elemental ratio of nitrogen to boron in the boron nitride fibre is from 1:1.1 to 1: 1.3; and / or (ii) wherein the carbon content of the boron nitride fibre is less than 2 wt%, preferably less than 1 wt%; and / or (iii) wherein the boron nitride fibre is a poly crystalline form comprising at least hexagonal and turbostratic forms.

20. A method for producing a composite material, comprising producing boron nitride fibre by a method according to any one of the preceding claims, and combining the boron nitride fibre with an additional substance to produce a composite material.

21. A method for producing a boron nitride fibre structural construct, which method comprises producing boron nitride fibre or a composite material according to any one of the preceding claims, and forming the boron nitride fibre or composite material into a structural construct.

22. Boron nitride fibre, composite material or boron nitride fibre structural construct obtainable by a method as defined in any one of claims 1 to 21.

23. Boron nitride fibre characterised in that the boron nitride fibre comprises hexagonal boron nitride having an atomic percentage of B of from 50-56%; an atomic percentage of N of from 43 to 49% and an atomic percentage of impurities of no more than 2%, and wherein the elemental ratio of nitrogen to boron is from 1: 1.1 to 1:1.3.

24. Boron nitride fibre according to claim 23, wherein the boron nitride fibre consists of hexagonal boron nitride.

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