This disclosure relates to the field of single crystalline boron nitride nanosheet (SC-BNNS) and single crystalline boron carbon nitride nanosheets (SC-BCNNS) production, and particularly to a scalable SC-BNNS and SC-BCNNS production.
Boron nitride nanosheets have been produced by various methods such as chemical vapor deposition (CVD) and pulsed laser deposition (PLD). Plasma enhanced chemical vapor deposition (PECVD) has been used with epitaxial growth for generating vertically aligned boron nitride nanosheets (BNNS) films on various types of substrates. Free-standing BNNS have also been synthesized through batch processes like ball milling, ultrasonic exfoliation, unzipping of boron nitride nanotubes (BNNT), pyrolysis, chemical blowing and by the micro-fluidization techniques. Unfortunately, these techniques have some common drawbacks that limit harnessing advantages BNNS may have in many of the prospective applications. For example, they (i) generate free-standing BNNS or single crystalline BNNS (SC-BNNS) of a low quality, (ii) require catalysts, (iii) are time-consuming, and/or (iv) involve contaminants and/or solvents, examples being ball milling and ultrasonic exfoliation. Moreover, when free-standing BNNS are produced according to the methods of the prior art, the BNNS will have low crystallinity or produce thicker and larger nanosheets which are undesirable for many applications. To fully utilize BNNS, it is particularly important to produce single crystalline BNNS which are few-layered, and have smaller spatial sizes in their 2-dimensional planar directions.
Recently, the production of boron nitride nanotubes (BNNTs) has been described in US20170197832 (Fathalizadeh et al) and US20170253485A2 (Kim et al). However, although the chemical composition of BNNTs is similar to that of BNNS, the nanostructure greatly differs and the BNNT methods does not prove to produce single crystalline BNNS. Indeed, both Fathalizadeh et al and Kim et al demonstrate a method for the controlled production of BNNTs, and Fathalizadeh et al also indicates boron nitride nanoribbons generated from flattened BNNTs. However, these two materials are one-dimensional and quasi-one dimensional nanostructures respectively with very high aspect ratios in contrast with SC-BNNS which are two-dimensional structures with a lower aspect ratio. BNNS are not produced by these two methods. Further, the two methods of Fathalizadeh et al and Kim et al do not show the possibility of synthesizing single crystalline boron nitride nanosheets (SC-BNNS), these two methods generating rather the 1-dimentional BNNT structures.
On the other hand, graphene nanosheets have a similar nanostructure with wholly different chemistries compared to SC-BNNS. Advancements in the field of graphene nanosheets, such as U.S. Ser. No. 10/329,156, do not translate to the field of BNNS due to very different and often opposite physicochemical and optical properties. The lack of processes that result in highly crystalline BNNS, having limited-layers, small and controllable in-plane sizes, and made readily available in scalable quantities in a powder form remain elusive.
Two-dimensional boron carbon nitride nanosheets (BCNNS) are ternary advanced materials composed of B, C, and N atoms having hexagonal a structure resembling that of graphene nanosheets and boron nitride nanosheets. Along the plane of a BCNNS sheet, the atoms are in an sp2-hypridized bonding. Compared to graphene and BNNS, BCNNS have strong interlayer interactions but an inferior in-plane stability caused by the tendency of B—N and C—C bonds to segregate. However, the conventional top-down methods used for graphene and BNNS synthesis are usually not effective for generating BCNNS due to their relatively low in-plane stability. A few bottom-up techniques have demonstrated the capability of fabricating BCNNS and thin BCN films such techniques include the catalytic chemical vapour deposition (CVD) method, the molten salt assembly growth technique, laser pulse deposition and magnetron sputtering. These techniques require catalysts or substrates, produce various contaminants (such as oxygen) and by-products and yields poor BCNNS morphology. There is thus a need to improve the quality and scalability of SC-BNNS and BCNNS production such that these materials become cost effective for real-life applications and in industrial settings.
In one aspect there is provided a method for producing single crystalline boron nitride nanosheet. A thermal plasma is provided at a plasma zone of a reaction chamber, the reaction chamber having an outlet opposite the plasma zone, a condensation zone and a growth zone downstream of the thermal plasma, wherein gas flows have a laminar flow in the reaction chamber wherein the laminar flow provides a controlled residence time in a nucleation temperature field. The pressure in the reaction chamber is between 20 to 200 kPa. A plasma-source gas flow is provided comprising a plasma-source gas for the thermal plasma, and a sheath gas flow at the plasma zone of the reaction chamber comprising nitrogen-containing gas to provide an excess of nitrogen in the reaction chamber. A boron source is provided to the thermal plasma through a probe into the thermal plasma to provide boron. The boron thus reacts with the nitrogen to form the single crystalline boron nitride nanosheets, the reaction comprising quenching in the condensation zone followed by two-dimensional nucleation downstream in the growth zone.
In one embodiment, the method further comprises providing a carbon precursor before the step of reacting to obtain single crystalline boron carbon nitride nanosheets (SC-BCNNS).
In one aspect there is provided a method for producing single crystalline boron carbon nitride nanosheet. A thermal plasma is provided at a plasma zone of a reaction chamber, the reaction chamber having an outlet opposite the plasma zone, a condensation zone and a growth zone downstream of the thermal plasma, wherein gas flows have a laminar flow in the reaction chamber wherein the laminar flow provides a controlled residence time in a nucleation temperature field. The pressure in the reaction chamber is between 20 to 200 kPa. A plasma-source gas flow is provided comprising a plasma-source gas for the thermal plasma, and a sheath gas flow at the plasma zone of the reaction chamber comprising nitrogen-containing gas to provide an excess of nitrogen in the reaction chamber. A carbon source is provided to the reaction chamber to provide atomic carbon. A boron source is provided to the thermal plasma through a probe into the thermal plasma to provide atomic boron. The boron thus reacts with the nitrogen and carbon to form the single crystalline boron carbon nitride nanosheets, the reaction comprising quenching in the condensation zone followed by two-dimensional nucleation downstream in the growth zone.
In one embodiment, the carbon precursor is methane.
In one embodiment, the nucleation temperature field is between 2000 to 5000 K.
In one embodiment, the laminar flow is a laminar expansion flow.
In one embodiment, the method further comprises the step of collecting the single crystalline boron nitride nanosheets or the single crystalline boron carbon nitride nanosheets.
In one embodiment, the reaction chamber has a cross sectional surface area that increases downstream from the plasma zone.
In one embodiment, the reaction chamber has a conical geometry.
In one embodiment, the reaction chamber is cylindrical and includes peripheral inlets.
In one embodiment, the boron source is in a solid, liquid, or gaseous state.
In one embodiment, the probe is a cooled probe.
In one embodiment, the cooled probe is a water cooled probe.
In one embodiment, the pressure in the reaction chamber is between 40 to 75 kPa.
In one embodiment, the pressure in the reaction chamber is between 60 to 64 kPa.
In one embodiment, the method further comprises cooling or heating walls of the reaction chamber.
In one embodiment, the plasma-source gas is selected from the group consisting of Ar, He, Ne, Xe, and N2.
In one embodiment, the boron source is selected from the group consisting of ammonia borane, boron particles, boron carbide, boron trioxide, diborane, boron trichloride and boric acid.
In one embodiment, the thermal plasma is an inductively coupled thermal plasma powered by radio frequency.
In one embodiment, where the method further comprises the step of modifying a residence time in the reaction chamber to control a lateral size and thickness of the single crystalline boron nitride nanosheets or the single crystalline boron carbon nitride nanosheets.
In one embodiment, the method is free of any catalyst.
In one embodiment, the single crystalline boron nitride nanosheets or the single crystalline boron carbon nitride nanosheets have an atomic B:N ratio of between 0.95:1.05 to 1.05:0.95.
In one embodiment, the single crystalline boron nitride nanosheets or the single crystalline boron carbon nitride nanosheets have a thickness of between 1 to 50 atomic layers.
In one embodiment, the single crystalline boron nitride nanosheets or the single crystalline boron carbon nitride nanosheets have a surface area of between 10 to 1500 nm2.
In one embodiment, the single crystalline boron nitride nanosheets or the single crystalline boron carbon nitride nanosheets have a crystallinity of at least 95%.
Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.
Single crystalline Boron nitride nanosheets (SC-BNNS) demonstrate an interesting range of properties. Some of these properties can be similar to those of graphene but some can be completely different such as large energy bandgaps making it a good insulator (i.e. a wide bandgap semiconductor), and excellent thermal and chemical stabilities. Importantly, SC-BNNS may be produced to also have fine-tuned properties such as hydrophobicity. The term “single crystalline” as used herein refers to sheet-like particles each forming one mono-crystal made of superposed and aligned atomic layers. This is in contrast to particles having a large number of defects and misalignments forming a multi-crystalline 2-dimensional sheet structure.
A boron nitride nanosheet is a two dimensional nanomaterial having a low aspect ratio and resembles graphene with alternating boron (B) and nitrogen (N) instead of carbon atoms. Along the plane of a multilayer nanosheet, B3—N3 make graphitic hexagons of atoms covalently bonded by sp2 strong bonds with an interatomic distance of ˜1.45 Å.
SC-BNNS display excellent chemical stability, tunable hydrophobicity, strong resistance to thermal oxidation and high thermal conductivity (around 2000 W m−1 K−1). These properties make SC-BNNS well-suited candidates for applications involving automotive and aerospace devices, as well as for protective antioxidation and/or anticorrosion coatings. In addition, SC-BNNS display a wide energy band gap (˜5.5 eV) rendering them electrically nonconductive (wide-bandgap semiconducting) materials, contrasting the electrical properties of graphene. SC-BNNS exhibit ultraviolet photoluminescence which qualifies for applications involving photoelectronic devices, data storage and in the high precision manufacturing industry. Owing to their enhanced biocompatibility and lower toxicity compared to graphene, SC-BNNS are promising materials for biomedical applications such as anticancer drug transport and delivery. In addition, SC-BNNS show exceptional mechanical properties (e.g. elastic modulus ˜850 GPa) and can thus be used as reinforcement agents in applications such as synthetic bone tissues. Furthermore, the polarity of B—N bonds combined with SC-BNNS high surface area also provide a strong affinity as well as a large capacity for adsorbing organic pollutants and contaminants or as an effective storage media for gases like hydrogen.
SC-BCNNS are boron nitride nanosheets doped with carbon (or graphene doped with boron nitride). The boron, carbon and nitrogen atoms are arranged in a hexagonal structure to form a two dimensional planar nanosheet thereby producing a SC-BCNNS layer. As previously mentioned, SC-BCNNS have strong interlayer interactions but have layers with an inferior internal stability when compared to graphene. SC-BCNNS combine the semi-metallic and dielectric properties of graphene (a good electrical conductor) and SC-BNNS (a wide bandgap semiconductor), which renders the SC-BCNNS semiconducting with a tunability of the bandgap characterised by a high degree of freedom. Accordingly, the electrical properties of SC-BCNNS can be optimized based on the atomic ratios of B/C/N present which can result in various band-gap values ranging from 0 eV (for pure graphene) to 5.5 eV (for pure boron nitride). By doping SC-BNNS with carbon, a SC-BCNNS that can act as a semiconductor is obtained. The semiconductor has a wide tunable bandgap that can be modified by changing the C concentration. SC-BNNS are dielectric materials whereas graphene is conductive. Therefore, SC-BCNNS provides useful electrical properties that are not obtained with these other two types of nanosheets. SC-BCNNS are useful in a variety of semiconductor applications since the bandgap can be tuned to the particular application needed. Uses of SC-BCNNS can be the same as the uses for SC-BNNS but will however require optimizing the bandgap of SC-BCNNS to be tailored to the specific application in question. SC-BCNNS in their powder form can be used for paints having specific electronic properties. For example, (i) paints that could act as solar collectors for energy harvesting applications. Bandgap matching could be optimized for solar radiation for example, or infrared radiation from heat sources; (ii) paints having electrochemical interaction in a fluid, potentially mitigating corrosion problems, (iii) composite structure or paints that can sustain very high temperatures, similar to SC-BNNS, but with an electronic component (example: thermo-electric current generation, transferring a cooling surface into an energy generating surface). The ability to adjust the bandgap means a possibility to adjust the material to the temperature in the specific cooling process, enhancing the efficiency. Additional examples include (iv) biomedical applications and (v) SC-BNNS and SC-BCNNS can both be useful to replace boron nitride nanotubes in composites for space applications. In fact, a mix of SC-BNNS and SC-BCNNS could provide specificity to radiation shielding. Further applications for SC-BCNNS include but are not limited to hydrogen storage, energy storage and conversion, wastewater purification, water splitting, CO2 reduction and other areas, adsorption of harmful pollutants owing to their additional merits such as good thermal and chemical stability, high specific surface area, relatively low cost and little secondary pollution, and finally electronic devices thanks to their ferromagnetic properties.
The present disclosure provides a scalable bottom-up approach utilizing a thermal plasma method, and boron-containing precursors that are injected into the thermal plasma in solid form and go through melting and/or evaporation. The term “thermal plasma” as used herein, refers to a plasma state where collisional equilibrium is attained between the free electrons and the heavy species (ions, atoms, and molecules). In a thermal plasma, a single temperature value or a proximal (±) range of temperature values may be used to describe the local plasma state. Making reference to
The thermal plasma 2 requires a flow of a plasma-source gas. The system 1 thus comprises a plasma-source gas compartment 7 comprising typically a gas that is in fluid communication with the plasma zone 3 via lines 8. The lines 8 thus comprise a flow of plasma-source gas. In one embodiment, the plasma-source gas may be provided centrally at the plasma zone 3 with or without a vortex generating mechanism. Examples of the plasma-source gas include but not limited to Ar, He, Ne, Xe, N2, H2, NH3 and combinations thereof. The plasma-source gas can be fed to the reaction chamber at a flow rate of between about 10 to about 60 standard liter per minute (slpm), between about 10 to between about 14 slpm or between about 10 to about 20 slpm. The flow rate of the plasma-source gas will vary depending on the scale of the thermal plasma 2 requirements. The values provided are appropriate for a laboratory scale SC-BNNS production. In an industrial setting the flow rate of the plasma-source gas can be from less than 1 slpm (for example for a microwave plasma) to about 500 slpm (for example a DC plasma). In one example, the flow rate is around 300 slpm for an industrial setting high power ICP system such as 100 kW or more.
To produce SC-BNNS or SC-BCNNS, the present method may require a nitrogen rich environment in the reaction chamber 4. To provide nitrogen, the system includes a nitrogen compartment 9 containing nitrogen-source gas for example nitrogen gas (N2) and/or ammonia (NH3). The nitrogen compartment 9 is in fluid communication with the plasma zone 3 via lines 10. The lines 10 comprises a sheath gas flow that comprises nitrogen, examples include a mixture of nitrogen and another inert gas such as Ar. In one embodiment, the sheath gas flow further comprises the inert gas. The flow rate of nitrogen in the sheath gas side may be between about 5 to about 25 slpm, between about 8 to about 15 slpm, or about 10 slpm. If the sheath gas flow comprises the inert gas, the flow rate of the inert gas may be between about 35 to about 120 slpm, between about 35 to about 100 slpm, between about 40 to about 80 slpm, between about 38 to about 42 slpm, or about 40 slpm. It is essential to maintain an excess of nitrogen in the reaction chamber in order to obtain a product of adequate quality, therefore as the method is scaled up the nitrogen flow rate can be increased to ensure an excess of nitrogen. In one embodiment, the nitrogen compartment comprises N2 gas. In a further embodiment, the sheath gas flow can be supplied peripherally at the plasma zone 3. In industrial scale the flow rate and power has to scale accordingly. Indeed the residence time depends on the plasma power. Generally, a high power will induce a shorter residence time and a low power induces a higher residence time. In one embodiment, the flow rate of the sheath gas flow can be between less than 1 slpm (for example microwave plasma) to about 500 slpm (DC plasma). In industrial ICP system can operate a flow rate of up to 300 slpm.
The present method employs a source of boron, for example a powder. The system 1 therefore comprises a boron compartment 11 that includes a source of boron-containing material that in some embodiments can be in a solid, liquid or gas form or any combination thereof. The boron-containing material may be selected from the group consisting of, for example, ammonia borane, amorphous boron particles, crystalline boron particles, boron carbide, boron trioxide, boric acid, boron trichloride and diborane or combinations of any of boron-containing materials. The boron compartment 11 is in communication with the plasma zone 3 through line 12 that leads into probe 13. The probe 13, which may be a cooled probe for example a water-cooled probe, reaches directly into the thermal plasma 2 to deliver the boron source. The feeding rate of boron to the reaction chamber 4 can optionally be controlled by a control system coupled to the boron compartment 11 or the line 12. The feeding rate of boron at the laboratory scale can be, for example, between about 1 to about 10 mg/min or between about 1 to about 5 mg/min. For a scale up to an industrial level process, the boron feed can be increased to higher suitable flow rates. The boron-containing material can be flowing to the plasma zone 3 by for example the effect of gravity, for example can be carried to the plasma zone 3 using a carrier gas or a carrier liquid. A carrier gas for example is nitrogen gas, and a carrier liquid for example a nitrogen-containing solvent.
To produce SC-BCNNS, a carbon source has to be provided. The carbon source can be a solid carbon source, a liquid carbon source or a gaseous carbon source. The system 1 thus further comprises a carbon compartment 11a that includes a source of carbon containing material. In some embodiments, the carbon source is methane, ethane, propane, melamine and dicyandiamide, carboxyphenylboronic acid, boron carbide, amines, carboxyphenylboronic acid, and trimethylamine borane. The carbon compartment 11a is in communication with the plasma zone 3 through the line 12a. The feeding rate of carbon to the reaction chamber 4 can optionally be controlled by a control system coupled to the boron compartment 11a or the line 12a. The BCNNS structure is generated upon providing atomic carbon in the plasma stream within the window of homogeneous nucleation of the nanosheet structure. Conditions can be set in order to make available atomic carbon within the nucleation and growth window of SC-BNNS, which is a complex determination in view of the specific temperatures involved for the vaporization and dissociation of the various B, C, and N precursors
In a preferred embodiment, the temperature is in the order of 10,000 K in the plasma volume (hot zone) at the plasma zone 3. Once injected to the hot zone of the plasma, the boron-containing precursors melt and/or vaporize in the nitrogen-rich environment to make boron nitride forming species, the building blocks of SC-BNNS and SC-BCNNS. Indeed, the thermal plasma 2 breaks down, to some extent, the nitrogen and boron into their respective various atomic species. In embodiments where SC-BCNNS are produced, the carbon source is provided and dissociates, to some extent, in the hot zone to provide elementary carbon. The created boron, nitrogen and optionally carbon mixtures are then transported into the reaction chamber 4 where controlled quenching takes place causing the melts/vapors to undergo supersaturation/supercooling, resulting in homogeneous, respectively heterogeneous nucleation pathways of new nanostructures. Under very specific nucleation conditions, fine solid products having specific structures can form by this process. This is achieved through a strong control of the thermal history of particle nucleation and growth that limits recirculation fields in the reaction chamber 4. This thermal history should provide uniform residence times for reactants in the hot zone of the thermal plasma 2, condensation zone 14 and the growth zones 15. The semi-continuous nature of this process allows easy scale-up to produce large quantities owing to the nature of the thermal plasma 2 processes which creates atmospheres of high temperature, high energy density and large densities of reactive species. The thermal plasma 2 produces a temperature gradient in the reaction chamber 4 which dictates where the condensation zone 14 and the growth zone 15 will form. The temperature is the highest at the plasma zone 3 where the thermal plasma 2 is and decreases gradually to the condensation zone 14, the growth zone 15, then the outlet 16 of the reaction chamber 4.
It is important to have the gases coming into the reaction chamber 4 from the plasma zone 3 take on a laminar flow in the reaction chamber 4. A laminar flow can be defined as a flow where recirculation and turbulence are substantially eliminated in the reaction chamber 4. Alternatively, the laminar flow can be defined as a flow where recirculation and turbulence are eliminated in the reaction chamber 4.
A characteristic of thermal plasmas is their high viscosity because of electrical cohesion of the species. Their viscosity may be estimated to about two thirds of that of water. Thermal plasmas generally exhibit in the radial direction a transition between a viscous plasma and a turbulent gas where each have vastly differing physics in terms of the atomic or molecular interactions. The turbulent zones of thermal plasmas are avoided in the present methods using a laminar flow. The flow direction can be controlled using the geometry of the reaction for example a conical reaction chamber (
A further important characteristic that promotes the two-dimensional nucleation and the formation of SC-BNNS or SC-BCNNS is the controlled residence time in a nucleation temperature field. In one embodiment the temperature is between about 2000 to about 5000 K, about 2000 to about 5500 K, about 2000 to about 4000 K, about 2200 to about 3500K, about 3000 to about 5000 K, or about 3000 to about 4000 K.
The geometry of the reaction chamber 4 can be used to control and create the laminar flow. In the case of
Without wishing to be bound by theory, a controlled nucleation temperature field (NTF) provides the means for setting first the nucleation of a stable critical cluster that is highly crystalline for the production of SC-BNNS or SC-BCNNS. Such high crystallinity is achieved because of the extreme temperatures in the thermal plasma providing the means for removing crystalline defects and minimizing internal energy. The initial critical cluster is in the shape of a single crystal. Once formed, the controlled size extension of the NTF enables diffusion of the B and N precursor species to the surface of the single crystal. This initial BN crystal has a stacked layer organization with superposed BN sheets as shown in
The geometry of the reaction chamber 4 as depicted in
The pressure inside the reaction chamber is an essential parameter in order to obtain highly crystalline and highly pure product. The pressure in the chamber is varied to be between about 20 kPa to about 200 kPa, between 20 kPa to about 100 kPa, between about 40 kPa to about 75 kPa, between about 50 kPa to about 70 kPa, between about 60 kPa to about 64 kPa or about 62 k Pa. Furthermore, pressure plays a significant role in the size and the thickness of SC-BNNS and SC-BCNNS.
The formed SC-BNNS or SC-BCNNS then exit the reaction chamber through the outlet 16. In some embodiments, the SC-BNNS and the SC-BCNNS are produced in powder form. Powder is a desirable form for easier dispersion in paints and nanocomposites as well as for avoiding contamination. Indeed, in some embodiments, the method of the present disclosure has limited or avoids entirely oxygen contamination. The system 1 may optionally comprise a collecting plate 19 downstream of the outlet 16 to collect the SC-BNNS or SC-BCNNS produced. The collecting plate may form a closed environment with the reaction chamber as shown in
The SC-BNNS obtained by the present method exhibit a highly crystalline structure and are of high purity. No BNNT are observed to be generated by the present method, meaning that no separation processes are needed to separate other BN-based nanomaterials from the SC-BNNS generated. The crystallinity of the SC-BNNSs is at least 95%, 96%, 97%, 98% or 99% and the purity is at least 95%, 96%, 97%, 98% or 99%. The SC-BNNS also have an excellent chemical composition characterized by an atomic ratio of B to N being approximately 1:1. For example, the atomic ratio B:N of the SC-BNNS according to the present disclosure is between about 0.95:1.05 to about 1.05:0.95, between about 0.97:1.03 to 1.03:0.97, between about 0.98:1.02 to about 1.02:0.92 or about 1:1. The SC-BNNS of the present disclosure can also be characterized as thin because each sheet of it contains between 1 atomic layer to 50 atomic layers. Indeed, in one embodiment the SC-BNNS have a thickness of 1 to 40 atomic layers or 2 to 30 atomic layers. In a further embodiment, the SC-BNNS have a thickness of between about 3 to about 8 nm. An individual SC-BNNS nanosheet obtained by the present method is characterized as having a small surface area. Indeed, in one embodiment the surface area of a single sheet is between about 10 to about 1500 nm2, between about 10 to about 1000 nm2, between about 20 and 200 nm2 or between about 10 to about 100 nm2. Therefore, the single crystal boron nitride nanosheets (SC-BNNS) of the present disclosure correspond to single crystal two dimensional structures of stacked atomic layers. The SC-BNNS of the present disclosure can also be characterized by a symmetry of planar atomic stacking from a group of boron nitride atomic planes (
In the synthesis of SC-BCNNS other nanostructures/nanomaterials may be produced as by-product. For example, ‘pure’ graphene, SC-BNNS, carbon black, carbon black coated with SC-BNNS or SC-BCNNS, amorphous boron coated with SC-BNNS may be formed. Improved control over the temperature inside the reactor can help reduce the formation of such by-products. Additionally, optimizing the various velocities (hence cooling rates) can also modify the clusters nucleation and growth conditions of the 2D/3D materials (2D being the nanosheets and 3D being the coated and uncoated carbon and boron spherical particles thereby reducing by-product formation.
An inductively coupled plasma system using a TEKNA PL-35 plasma torch and a conical reaction chamber was used to melt/vaporize/decompose the boron-containing precursor which was ammonia borane. The half-angle of the cone of expansion was about 7 degrees. The power generator that was used supplies an alternative current (AC) at 4 MHz. The inductively coupled plasma (ICP) torch was equipped with three inlets. The first inlet is used to provide the plasma-source gas which was Ar fed at 15 slpm at room temperature. The second inlet was used for the sheath gas injection which was a combination of Ar fed at 40 slpm and N2 at 10 slpm. The reactor operating pressure was 62 kPa. The ICP torch plate power was maintained at about 29 kW to ensure plasma stability, resulting in a power coupled to the plasma of around 14.5 kW. A fine solid powder of ammonia borane (NH3BH3, assay 97%) was fed at a rate of 1-2 mg/min to the ICP torch through a water-cooled probe. Once in the hot zone, the solid powders melted and vaporized in the nitrogen-rich environment to make BN-forming species, the building blocks of SC-BNNS. The formed SC-BNNS (and by-products) then accumulated on the water-cooled collecting plate.
Energy-Dispersive X-ray Spectroscopy (EDX) was used to investigate the chemical composition of the formed SC-BNNS and the contamination present within the formed powders. The results deduced from the EDX spectrum are shown in Table 2. The B:N atomic ratio was ˜1:1 as seen in table 2, confirming that stoichiometric boron nitride nanosheets were effectively generated. The SC-BNNS were dispersed on an aluminum-alloy scanning electron microscope (SEM) stub then coated with a 2 nm-layer of gold to enhance the SEM imaging and remove the surface charging effect of the electron beam on the insulative SC-BNNS. This explains the presence of the other elements (e.g. oxygen, carbon, copper, and gold). The signals of the heavy elements i.e. Al, Cu and Au were removed from the calculations to minimize the error associated with B and N atomic ratios. The chemical composition of SC-BNNS was also confirmed using Electron Energy Loss Spectroscopy (EELS).
Generally, the formation of nanoparticles in thermal plasmas can be affected by many interrelated variables. For example, the power coupling efficiency between the induction coil and the plasma strongly influences the temperature of the plasma core, while the pressure affects the axial velocity fields as well as the temperature profiles along the reactor. When these variables change, the thermal evolution of the various species and the quenching rates of the vaporized precursors are also modified. This results in various morphologies and sizes of the nanoparticles when condensed. In Example 1, the power was maintained constant at 29 kW while the relation between operating pressure and nitrogen loading and its impact on the process outcomes are observed. The pressure was either decreased or increased relative to 62 kPa while marinating the flow rate of N2 at 10 slpm in the sheath gas. Deviating from 62 kPa was found to have a major impact on the SC-BNNS sheet dimensions and the overall purity of the product. At 48 kPa, SC-BNNS tend to show smaller in-plane sizes and sheet thicknesses as seen in
Computational fluid dynamics (CFD) modeling was used to show a typical trend of the effect of the operating pressure on the formation of SC-BNNS in the reactor in argon plasma conditions and a sheath gas containing 10 slpm N2 (
The effect of N2 flow rate on SC-BNNS growth was evaluated while maintaining the operating pressure at 62 kPa. The flow rate of N2 gas also plays a significant role. At a low flow rate of 0-5 slpm N2, results (SEM/TEM images not shown) reveal boron particles being more predominant compared to SC-BNNS. This is expected since lowering N2 gas would reduce the amount of N species needed for BN formation, which is a precursor for SC-BNNS formation.
Without wishing to be bound by scientific theory, a possible model for the homogeneous growth of SC-BNNS is described herein. When ammonia borane is injected in the hot argon plasma zone, it melts and vaporizes to form mixtures of BxNyHz and, B, N, H, and BN species. These vapors are then transported axially into the reaction chamber 4 in a steady state manner and undergo supercooling by a well-controlled rapid quenching. This step induces BxNyHz species and N radicals to form BN species (and polyaminoborane networks) to nucleate into polyborazylene particles of critical spatial 2-D structures through a homogeneous nucleation scheme. Once stable, these nuclei serve as matrices for further condensation of the incoming flux of BN species. These species are transported by convective forces that are temperature dependent. On the surface of stable nuclei, BN/BxNyHz species condensate to laterally propagate SC-BNNS while releasing hydrogen gas. This possible mechanism is depicted in
Another SC-BNNS synthesis route in argon plasma using a different precursor was explored, namely amorphous boron particles instead of ammonia borane as used in Example 1. The boron particles sizes ranged from 100 nm to 1.5 μm in diameter and the powder was fed at a rate of 5 mg/min. The plasma-source and sheath gases were maintained the same as in Example 1, the plasma-source being argon fed at 15 slpm and the sheath gas being a mixture of argon and nitrogen fed at 40 and 10 slpm, respectively. The operating pressure was maintained at 62 kPa. A possible growth mechanism is summarized in
To shed light on controlling phenomena that affect the growth of SC-BNNS, the operating pressure and nitrogen flow rate in the sheath gas were varied to deviate from the conditions that resulted in SC-BNNS of
The possible heterogeneous growth mechanism of SC-BNNS as
Without wishing to be bound by scientific theory, the following is thought to explain the SC-BNNS growth process. When boron particles are injected into the plasma zone at 62 kPa, they have enough residence time to mostly melt forming liquid droplets while being transported downstream to the reaction chamber. By the effect of surface tension of boron in the liquid state, particles undergo spheroidization to assume spherical shapes.
Some fraction of the nitrogen fed in the sheath gas dissociates in the hot plasma zones into atomic nitrogen and gets exited or ionized to form active N species. Then, the active N species bind with B on the surface of the liquid boron particle to form a solid phase of boron nitride nanowall (BNNW) that vertically grow in all directions in a base-growth process leading to a corona-like structure. Local concentration gradients of N on the surface of the particle create a driving force for convective mass transfer of N from the surroundings to the surface of the particle. Upon the presence of more active N on the surface, BNNW propagate further taking the shape SC-BNNS until the boron liquid particle is depleted, which represents an ideal case in which pure SC-BNNS are formed. SC-BNNS can cease to grow when the concentration of active N becomes insufficient due to recombination reactions that form N2 gas (i.e. 2N→N2). Moreover, SC-BNNS can cease to grow when the boron particle falls below its melting point.
In conditions where pressure is decreased from 62 kPa to 27 kPa while keeping the same N2 flow rate (
In contrast, if pressure is increased from 62 kPa to 90 kPa at the same N2 flow rate (
Finally, increasing the nitrogen load in the sheath gas from 10 to 25 slpm while maintaining the optimum pressure of 62 kPa provides excess active N for the SC-BNNS formation (
From the two Examples, it is apparent that SC-BNNS growth favors an operating pressure of 62 kPa in the homogenous (Example 1) and heterogenous (Example 2) processes for the specific reactor geometry used (conical) as well as the laboratory scale flow rates. This pressure provides a residence time necessary for ammonia borane to decompose into BxNyHz and BN that allow SC-BNNS growth. For the boron particles precursor, 62 kPa is important for melting the boron powder in a plasma spheroidization process. In both cases, this pressure is essential to limit N species from recombining thus facilitating further growth in the SC-BNNS growth zone. Thus, it is vital to enrich that zone with N species by increasing the nitrogen flow rate.
Accordingly the present disclosure has demonstrated the homogeneous and the heterogenous synthesis of single crystalline flakes of boron nitride nanosheets in a powder form by a bottom-up approach using inductively coupled plasma (RF-ICP). The wording “bottom-up” is understood as the assembly of nanosheets from smaller basic units at the atomic/molecular level into the more complex nanosheets. The synthesis is strongly controlled through the laminar flow path-lines and optionally the geometry which allows for further control over uniform residence times in specific nucleation zones and prohibits re-circulations in the reactor.
The operating pressure in both examples is vital for controlling axial velocities of precursors vapors (in the homogenous nucleation, Example 1) and droplets (in the heterogenous nucleation, Example 2) and for limiting recombination reactions of N species which in both cases is an important aspect to minimize impurities/by-products. In line with that, it is found that N2 loading played a significant role as to compensate for recombined N species. Furthermore, in the heterogenous growth of Example 2, liquid boron particle size is an important parameter because large particles tend to only be partially depleted in the SC-BNNS formation process while small ones tend to be fully depleted.
Remarkably, it was found possible to control the lateral sizes and the thickness of SC-BNNS by merely opting for either homogenous or heterogenous growth processes. This is an important advantage for applications involving controlled SC-BNNS sizes. It is also important to note that the present method is catalyst-free and time-effective which significantly contribute to the scalability of the method.
As seen therefore, the two examples described above and illustrated are intended to be exemplary only. Practitioners of thermal plasma systems can transfer the ICP-thermal plasma processes described herein by changing the plasma source to other thermal plasma generation devices such as a DC plasma torch or a microwave plasma torch. In Example 1, ammonia borane was used as a precursor which contains boron and nitrogen in its chemical composition. Thus, a person skilled in the art would appreciate that injecting nitrogen gas into the system is not always necessary to obtain the SC-BNNS. However, adding nitrogen gas to the reaction can result in an increase in the overall purity/quality of the product because of a reduced formation of boron and an increase in the formation of SC-BNNS. In Example 2, the precursor was boron particles which contains only boron in its chemical composition. For this reason, injecting nitrogen is essential to form SC-BNNS. Injecting a low amount of nitrogen results in little to no formation of SC-BNNS, while injecting more nitrogen enhanced the yield of SC-BNNS. In general, achieving an abundance of nitrogen be it in the environment or provided with the boron source improves the purity and quality of the SC-BNNs produced.
SC-BCNNS were produced by introducing ammonia borane solid particles, nitrogen gas and methane gas in the argon plasma conditions to produce free-standing SC-BCNNS powders without the use of catalysts, solvents or substrates. The conditions are listed below in Table 3. The argon plasma provided high enthalpy heats to melt, vaporize and dissociate the solid precursor to obtain atomic boron and nitrogen. The injected methane dissociated to provide atomic carbon necessary for SC-BCNNS. Both ammonia borane and methane produced hydrogen gas that left the system as a gaseous by-product. Powders were collected downstream of the flow on the product collecting plate.
The powder was collected from the product collecting plate. The as-collected material contained various solid phases including SC-BNNS, graphene, and carbon particles coated with 2D BNNS. SC-BCNNS was found to form as one of the primary materials.
A high-angle annular dark-field (HAADF) image was taken and elemental mapping was obtained using energy dispersive X-ray spectroscopy (EDX) for the BCNNS structure shown in
The chemical composition of the BCN nanosheets was further characterized using electron energy loss spectroscopy (EELS). The EELS spectrum is presented in
In conclusion, the present example therefore demonstrated a bottom-up synthesis route for boron carbon nitride nanosheets (BCNNS). The material was generated using radio frequency inductively coupled plasma using ammonia borane, nitrogen, and methane. The scanning and transmission electron microscopy images clearly showed two-dimensional structures with slightly non-uniform layer stacking that is not characteristic to SC-BNNS or graphene. The presence of in-plane dislocation line defects generating the wavy structure in
The scope is indicated by the appended claims.
Filing Document | Filing Date | Country | Kind |
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PCT/CA2021/051601 | 11/10/2021 | WO |