WO2018236312A2 - Method for production of high purity, high crystallinity bn structures at moderate temperatures - Google Patents

Method for production of high purity, high crystallinity bn structures at moderate temperatures Download PDF

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WO2018236312A2
WO2018236312A2 PCT/TR2017/050679 TR2017050679W WO2018236312A2 WO 2018236312 A2 WO2018236312 A2 WO 2018236312A2 TR 2017050679 W TR2017050679 W TR 2017050679W WO 2018236312 A2 WO2018236312 A2 WO 2018236312A2
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substance
mixture
gas
substrate
nanostructures
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PCT/TR2017/050679
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French (fr)
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WO2018236312A3 (en
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Mustafa Baysal
Kaan BILGE
Melih Papila
Yuda Yürüm
Cinar ÖNCEL
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Sabanci Üniversitesi
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Priority to EP17910509.3A priority Critical patent/EP3562587A2/en
Publication of WO2018236312A2 publication Critical patent/WO2018236312A2/en
Publication of WO2018236312A3 publication Critical patent/WO2018236312A3/en

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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
    • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
    • C23C16/30—Deposition of compounds, mixtures or solid solutions, e.g. borides, carbides, nitrides
    • C23C16/34—Nitrides
    • C23C16/342—Boron nitride
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/02—Boron or aluminium; Oxides or hydroxides thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
    • B01J23/76—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
    • B01J23/78—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36 with alkali- or alkaline earth metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/20—Catalysts, in general, characterised by their form or physical properties characterised by their non-solid state
    • B01J35/23—Catalysts, in general, characterised by their form or physical properties characterised by their non-solid state in a colloidal state
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration
    • B01J35/58—Fabrics or filaments
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/0238—Impregnation, coating or precipitation via the gaseous phase-sublimation
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/064—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with boron
    • C01B21/0641—Preparation by direct nitridation of elemental boron
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C01P2004/03—Particle morphology depicted by an image obtained by SEM
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C01P2004/04—Particle morphology depicted by an image obtained by TEM, STEM, STM or AFM
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/10—Particle morphology extending in one dimension, e.g. needle-like
    • C01P2004/13—Nanotubes

Definitions

  • the present invention relates to a catalyst for use in production of elongate microstructures and/or nanostructures substantially comprising BN, and a method for obtainment of such microstructures and/or nanostructures at low temperature.
  • boron nitride nanotubes have gained greatest attention after carbon nanotubes.
  • boron nitride nanotubes are analogue to carbon nanotubes.
  • boron nitride nanotubes have a higher chemical stability and higher thermal stability (resistant to oxidation up to 1100°C).
  • Boron nitride (BN) is piezoelectric, neutron shielding, and incomparably more hydrophobic compared to carbon nanotubes.
  • boron nitride nanotubes with their partial ionic characteristics are electrically non-conducting materials having a broad bandwidth of 5-6 eV, unlike to metallic or semi-conductive carbon nanotubes. Due to these unique characteristics, boron nitride nanotubes (BNNT) find potential application in nanoelectronic applications (nano-insulator material, nanosensors), optical applications (Deep-blue and infrared lasers), magnetic applications (targeted drug delivery), biomedical applications (tissue scaffolds, biosensors), energy applications (hydrogen storage), ceramic-glass composites, and polymeric nanocomposite applications.
  • Known methods for production of micro-/ or nanostructures including BN include arc-discharge, laser ablation, atomic stacking, plasma-jet, displacement reaction, reactive ball milling, pressurized vapor condensation and chemical vapor deposition. Temperature values of these methods and disadvantages thereof are summarized in the Table 1.
  • Chemical Vapor Deposition is a highly important method which is mostly employed in the synthesis of carbon nanotubes. This method, however, has some disadvantages including difficult arrangement of system parameters specific to the catalyst and purification.
  • Chemical Vapor deposition can be divided into different variations thereof, which include Catalytic Chemical Vapor Deposition, Non-Catalytic Chemical Vapor Deposition, and Conventional Chemical Vapor Deposition.
  • Catalytic Chemical Vapor Deposition has two sub-classes: namely Boron Oxide and Thermal Chemical Vapor Deposition method.
  • Each line of the Table 2 below shows various known alternatives within the boron oxide chemical vapor deposition sub-class.
  • Temperature range in production of micro-/ or nanostructures including BN (e.g. BNNT) by means of catalytic chemical vapor deposition is generally between 1100°C and 1400°C. MgO- and FeO-based catalyst materials are used in such methods.
  • EP 2 393 966 Al describes production of nanostructures and more particularly a method for production of boron nitride nanotube fibers. Yurum et al. (Ind. Eng. Chem. Res., 2012, DOI:10.1021/ie301605z) describes that reaction temperature and the type of catalyst have an effect on BNNT obtained using CVD.
  • the method used in said article is conventional CVD, and Fe2C>3, Fe 3+ -MCM-41, Fe 3+ -MCM-41/Fe2C>3 are used as catalysts.
  • a decrease occurs in boron nitride nanostructures in accordance with a temperature gradient from 600°C to 800°C.
  • the resulting boron nitride nanostructures do have rather poor crystallinity and poor purity. Furthermore, it is difficult to separate nanotubes from the catalysts.
  • Primary object of the present invention is to overcome the drawbacks encountered in the prior art.
  • Another object of the present invention is provision of a method enabling production of elongate microstructures and/or nanostructures including Boron nitride, with low temperature, high crystallinity and high purity.
  • a further object of the present invention is provision of a method enabling production of elongate microstructures and/or nanostructures including Boron nitride, with low cost and high yield.
  • a further object of the present invention is provision of an entity including a substrate which is provided with elongate microstructures and/or nanostructures including Boron nitride, even though said substrate is not as temperature resistant as those used in the prior art techniques.
  • the present invention relates to a method for preparation of a catalyst for use in production of elongate microstructures and/or nanostructures substantially comprising Boron nitride (BN), the method includes reacting a first substance selected from oxides or nitrates of Na, K or Li; with a second substance selected from Fe 2 0 3 , Al 2 0 3 , SiO, Fe(N0 3 )3, AI(N0 3 )3, and SiH 4 (N0 3 ) 4 ; such that if the first substance is an oxide, the second substance is selected from the list consisting of Fe 2 0 3 , Al 2 0 3 or SiO; and if the first substance is a nitrate, the second substance is selected from Fe(N0 3 ) 3 , AI(N0 3 ) 3 , and SiH 4 (N0 3 ) 4 ; wherein the molar amount of the second substance is at least twice of the molar amount of the first substance.
  • the present invention further proposes a first substance
  • Figure 1 is schematic view of an exemplary setup for conducting a version of the method according to the present invention, with reactive vapor-trapping chemical vapor deposition
  • Figure 2 shows a scanning electron microscopy (SEM) image of a field of elongate microstructures and/or nanostructures substantially comprising BN (here: boron nitride nanotube, BNNT) with 2000X magnification.
  • SEM scanning electron microscopy
  • Figure 3 shows a SEM image of such field with 10000X magnification.
  • Figure 4 shows a transmission electron microscopy (TEM) image emphasizing the wall structure of a single elongate microstructure and/or nanostructure substantially comprising BN (here: boron nitride nanotube, BNNT).
  • TEM transmission electron microscopy
  • Figure 5 shows a TEM image emphasizing the wall-gap-wall structure of a conventional BNNT.
  • Figure 6 is a list of steps in an exemplary version of the method according to the present invention, enabling elongate microstructure and/or nanostructure substantially comprising BN.
  • Figure 7 is a list of steps in an exemplary version of the method for obtaining a potassium ferrite catalyst according to the present invention.
  • the present invention proposes a method for preparation of a catalyst for use in production of elongate microstructures and/or nanostructures substantially comprising Boron nitride (abbreviated as BN), the method includes reacting a first substance selected from oxides or nitrates of Na, K or Li, with a second substance selected from Fe 2 0 3 , Al 2 0 3 , SiO, Fe(N0 3 )3, AI(N0 3 )3, and SiH 4 (N0 3 )4; such that
  • the second substance is selected from the list consisting of Fe20 3 , A 0 3 or SiO;
  • the second substance is selected from Fe(N0 3 ) 3 , AI(N0 3 ) 3 , and SiH 4 (N0 3 ) 4 , wherein the molar amount of the second substance is at least twice of the molar amount of the first substance.
  • the first substance can include K2O or Na20 or L12O, and the second substance includes Fe 2 0 3 .
  • the first substance can include KN0 3 or NaN0 3 or LiN0 3 and the second substance includes Fe(N0 3 ) 3 .
  • the first substance comprises an oxide of K
  • a reaction between the K2O and Fe20 3 to form KFeC>2 occurs with equi molar convertions of both K 2 0 and Fe 2 0 3 into KFeC> 2 according to the reaction stoichiometry below:
  • the first substance includes K 2 0 or Na20 and the second substance includes Fe2C>3; or alternatively, the first substance includes KNO3 or NaNC>3 and the second substance includes Fe(NC>3)3.
  • a respective reaction product includes potassium ferrite KFeC>2 or sodium ferrite NaFeC>2)
  • the present invention further proposes use of the reaction product as a catalyst in production of elongate microstructures and/or nanostructures substantially comprising BN.
  • the present invention further proposes a method for production of elongate microstructures and/or nanostructures substantially comprising BN, including the following steps: a) preparation of a first mixture (catalytic mixture) according to any one of the claims 1 or 2, b) mixing the first mixture with a Boron source to obtain a second mixture, c) arranging a substrate in vicinity of said second mixture, d) arranging a gas comprising a substance including nitrogen atoms (thereby acting as a nitrogen-source), in contact with both of the substrate and the second mixture, and e) heating the second mixture to a temperature corresponding to that at which the vapor pressure of an alkali metal in the first substance in its elementary form is at least 0.5 bar.
  • Said vapor pressure can be e.g. within the range between 0.5 bar and 1.2 bar, preferably within the range between 0.9 bar and 1.1 bar.
  • Such vapor pressure substantially corresponds to rapidly evaporating or boiling of an alkali metal desorbed from the reaction product of the first mixture, and enable conversion into a reaction product from the constituents of the first mixture due to the temperature and the presence of the nitrogen-source. Therefore the above-described temperature enables the reaction mechanism for formation of elongate microstructures and/or nanostructures substantially comprising BN.
  • the Boron source can comprise elementary Boron.
  • Elementary Boron gets oxidized rather easily at lower temperatures when compared to other Boron sources, e.g. Boron oxides or Boron minerals.
  • the first substance can preferably include an oxide of Na, K or Li, and the steps (a) and (b) described above can be performed in a substantially dry atmosphere; in the case where the first substance includes an oxide of Na or K.
  • Such alkali metal oxides are less stable in presence of water, and this provision enables a high yield in the desired reaction mechanism, without disturbances due to humidity.
  • the molar amount of B atoms in the boron source can be substantially equal to the molar amount of 0 atoms in the second mixture.
  • the gas can substantially comprise dry ammonia (NH3).
  • NH3 decomposes easily thus it is considered a high efficiency reactant in this method.
  • the step (b) includes: preparation of the second mixture and providing the second mixture on or in a holder for supporting the second mixture against gravity, into a cabinet comprising a gas inlet and a gas outlet, arranged to allow a gas stream in a gas flow direction from an upstream direction towards a downstream direction;
  • the step (c) includes placing a substrate into the cabinet; e.g. at a position for allowing gas diffusion communication between the holder and the substrate;
  • the step (d) includes arranging a gas which comprises a substance including nitrogen atoms thereby acts as a nitrogen-source, in contact with both of the substrate and the second mixture.
  • the gas can be substantially stagnant inside the cabinet, or can be streamed from the gas inlet towards the gas outlet as a gas stream.
  • the gas can be streamed from the gas inlet towards the gas outlet as a stream flowing from an upstream direction towards a downstream direction.
  • the holder can be formed substantially from a ceramic material, preferably including BN which facilitates nucleation of the micro-/nanostructures comprising BN.
  • the holder and the substrate can be placed in a receptacle having an opening, said receptacle being placed into the cabinet such that the opening is directed to the downstream direction.
  • This facilitates nucleation without disturbance under gas stream.
  • the holder can be substantially in the form of a vessel for holding liquids. Such functional form of the holder is useful in supporting any extensive amounts of molten alkali metal.
  • any air inside the receptacle can be replaced with a further gas which is inert at the temperature defined in the step (e).
  • a further gas which is inert at the temperature defined in the step (e).
  • the further gas can be e.g. N2 or Argon.
  • the replacement of air can be also performed by applying reduced pressure into the receptacle, then supplying the gas into the receptacle and heating the receptacle; said reduced pressure preferably corresponds to an absolute pressure of 10 Torr or less.
  • the vapor pressure of Na has a value of 0.5 bar at 799.95°C, 0.9 bar at 857.55°C, 1 bar at 868.54°C, and 1.1 bar at 878.66°C.
  • a substrate which is resistant against thermal decomposition only up to a temperature range between 857.55°C and 878.66°C can be used without damage, to form nanostructures or microstructures substantially including BN.
  • the vapor pressure of K has a value of 0.5 bar at 672.5°C, 0.9 bar at 726.75°C, 1 bar at 737.13°C, and 1.1 bar at 746.70°C.
  • a substrate which is resistant against thermal decomposition only up to a temperature range between 672.5°C and 746.70°C can be used without damage, to form nanostructures or microstructures substantially including BN.
  • the vapor pressure of Li has a value of 0.5 bar at 1223.2°C, 0.9 bar at 1298°C, 1 bar at 1312.2°C, and 1.1 bar at 1325,30°C.
  • a substrate which is resistant against thermal decomposition only up to a temperature range between 1223.2°C and 1325.30°C can be used without damage, to form nanostructures or microstructures substantially including BN.
  • the present invention further proposes an entity comprising a substrate provided with elongate microstructures and/or nanostructures which substantially comprise BN, provided that the substrate is resistant to thermal decomposition only at temperatures up to around 1000°C.
  • the method according to the present invention provides formation of such microstructures and/or nanostructures at lower temperatures, without compromising the extent of crystallinity and purity of said structures.
  • the alkali metal in the first mixture is Na
  • provision of the substrate with elongate microstructures and/or nanostructures is enabled even at temperatures starting from around 800°C.
  • the substrate with elongate microstructures and/or nanostructures is enabled even at temperatures starting from around 670°C, more particularly starting from around 725°C, even more particularly starting from around 750°C.
  • step (e) In processing a substrate which is resistant against thermal decomposition at temperatures higher than those specified above, higher temperatures can be employed in the step (e). Even though the method according to the present invention enables formation of elongate microstructures and/or nanostructures substantially comprising BN and provision thereof on a substrate; it is evident that the reaction rates are higher at higher temperatures. Accordingly, with higher reaction temperatures, better results are available: high values of reaction yield and greater extent in growth of microstructures and/or nanostructures are available in a shorter time. For instance, heating the second mixture to a temperature corresponding to that at which the vapor pressure of an alkali metal in the first substance in its elementary form can also have even significantly higher values than 0.5 bar.
  • the vapor-trapping chemical vapor deposition assembly (1) used for obtaining boron nitride nanotube (BNNT) structures according to the invention can comprise a furnace (10) for adjusting temperature of a reaction medium.
  • the furnace (10) can be positioned along a horizontal axis (x) which is substantially perpendicular to the gravity vector (g).
  • the temperature of the furnace (10) can be increased to a temperature value between 700°C and 1000°C after a second tube (101) is placed in the furnace.
  • Said furnace (10) can comprise a first tube (100) in which the reaction takes place and the first tube (100) can be positioned substantially along the horizontal axis (x).
  • a second tube (101) can be positioned inside the first tube (100) and one end of the second tube (101) can include an opening, through which a gas (102) can be supplied for being employed in the reaction mechanism.
  • the first tube (100) can be opened at both ends.
  • the carrier gas can be ammonia (NH3). Ammonia serves as a nitrogen source in the formation of boron nitride.
  • the carrier gas (102) can be introduced into the first tube (100) from a gas inlet (A) and released at a gas outlet (B). In other words, a flow direction of the carrier gas (102) can be defined from the gas inlet (A) towards the gas outlet (B).
  • the use of such second tube (101) corresponds to a variation of the method including chemical vapor deposition (CVD).
  • the second tube (101) has a smaller diameter than the first tube (100), and it may be easily positioned in the first tube (100).
  • the second tube (101) may be formed from e.g. quartz or alumina resistant to high temperatures.
  • the first tube (100) may also be made of quartz or alumina.
  • the second tube (101) can comprise a carrier member (1010) in which boron source and catalyst are positioned, and a cover (1011) for closing the carrier member (1010).
  • the cover (1011) may be formed from silicon wafer or another substrate material.
  • the carrier member (1010) can be formed from alumina or boron nitride (BN) or another ceramic material. In another embodiment, the carrier member (1010) may comprise mullite.
  • the carrier member (1010) can be loaded with a second mixture (1015) including process precursors, boron source and catalyst (e.g. potassium ferrite).
  • the carrier member (1010) can be positioned in the vicinity of a closed end of the second tube (101), which can be positioned in a central zone of the furnace (10).
  • the closed end of the second tube (101) can be positioned at upstream direction relative to the gas stream (102). As a result, a reactive steam formed can be utilized with high efficiency until a steam pressure allowing nucleation is achieved.
  • a catalyst is formed inside the second mixture; and under the above described conditions the catalyst can be potassium ferrite (KFeC>2).
  • a version of such catalyst preparation method (600) can include the following steps:
  • the molar ratio of K and Fe can be 50%.
  • a dry powder mixture obtained at the end of 602, can be calcinated at 700°C.
  • the molar ratio of K and Fe can be also within the range between 1% and 20%.
  • the potassium ferrite catalyst ensures that boron nitride nanotubes (BNNT) are obtained with high crystallinity, in pure form and at low temperatures. Said catalyst disintegrates into K and K 2 0 with temperature.
  • a further stage of an exemplary version of the method according to the present invention includes synthesis (500) of boron nitride nanotubes (BNNT) which can be performed using a vapor-trapping chemical vapor deposition assembly (1).
  • Said stage can include the following steps:
  • reaction product including boron nitride nanotubes (BNNT) with impurities and the transferring impurities to said liquid as a result of sonification, and thereby obtaining purified boron nitride nanotubes (BNNT).
  • BNNT boron nitride nanotubes
  • said catalyst can be potassium ferrite (KFeC>2).
  • the boron source can be elementary boron in the form of powder.
  • Elementary boron evaporates under rather moderate temperatures when compared to its compounds as boron source. Thereby the reaction is facilitated and the efficiency is increased for the method according to the present invention.
  • molar ratio of boron to oxygen second mixture can be within a range between 1:1 and 4:1, and preferably around 1:1 to ensure stoichiometric ratios for B2O2 steam as an intermediate phase.
  • molar ratio of potassium to iron in the first mixture to form KFeC>2 catalyst can be within the range between 1:1 and 1.2:1.
  • the respective ingredients are also partly converted into potassium ferrate (K 2 Fe0 4 ) as a further catalyst.
  • molar ratio of boron to KFeC>2 catalyst can be e.g. around 2:1 higher. This ratio corresponds to a molar ratio of boron to oxygen of 1:1 or higher at reaction conditions.
  • the carrier member (1010) can be partly covered with a cover (1011). Said cover (1011) can be a silicon wafer. With the partially covered carrier member (1010), a delay can be arranged in discharge of the nitrogen source (gas, 102, e.g. ammonia). The gas (cf. step 507) interacts with the second mixture which includes boron and catalyst.
  • boron nitride nanotubes can be formed/grown on outer surfaces (walls and/or base) of the carrier member (1010).
  • the impurities can substantially include metallic material which further can be ferromagnetic.
  • the impurities can be separated from fluid by means of magnets or using an acidic fluid for dissolving metallic material.
  • the method can be considered substantially on the basis of vapor-liquid-solid (VLS) mechanism.
  • VLS vapor-liquid-solid
  • the alkali metal e.g. potassium
  • the alkali metal e.g. potassium
  • K and K2O desorption occurs from the second mixture including KFeC>2 catalyst due to high steam pressures. This phenomenon is useful in formation of elongate micro-/nanostructures including BN by catalytic chemical vapor deposition.
  • boron nitride nanotubes Several characteristics of boron nitride nanotubes (BNNT) including their wall morphology, crystalline form, height, width and elemental composition are respectively characterized by TEM, SEM, EELS and RAMAN spectroscopy techniques.
  • Figure 2 and Figure 3 show scanning electron microscopy (SEM) images of boron nitride nanotubes (BNNT) on a carrier member (1010).
  • the mean diameter of the resulting boron nitride nanotubes (BNNT) is 30-100 nanometers and the lengths thereof are mostly above lOijm.
  • TEM transmission electron microscopy
  • Micro-/or nanostructures including BN (here: BNNT) with smooth nanotube wall structure and high crystallinity can be obtained in low temperatures with the method according to the present invention.
  • Raman spectroscopy result of boron nitride nanotubes (BNNT) obtained according to this exemplary method according to the present invention is given in Fig.9.
  • the method according to the present invention decreases cost and energy consumption in production of elongate microstructures and/or nanostructures including BN.
  • the elongate microstructures and/or nanostructures including BN obtainable with the method according to the present invention are applicable to a wide scale of entities including:
  • - biomedical applications e.g. nanoscale skin scaffolds, bone tissue regeneration, medicament administration
  • - radiation shielding applications especially neutron and UV shielding applications
  • piezo-electric applications especially light-weight and nontoxic piezoelectric systems, airless human vehicle sensors, energy collectors
  • a First Tube suitable for defining a gas flow direction from an upstream direction towards a downstream direction can be considered as a cabinet
  • Second Tube (as a receptacle)
  • BNNT elongate microstructure or nanostructure including Boron nitride, e.g. Boron nitride nanotube 500.

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Abstract

The present invention relates to a method for preparation of a catalyst for use in production of elongate microstructures and/or nanostructures substantially comprising Boron nitride (BN), the method includes reacting a first substance selected from oxides or nitrates of Na, K or Li; with a second substance selected from Fe2O3, Al2O3, SiO, Fe(NO3)3, Al(NO3)3, and SiH4(NO3)4; such that if the first substance is an oxide, the second substance is selected from the list consisting of Fe2O3, Al2O3 or SiO; and if the first substance is a nitrate, the second substance is selected from Fe(NO3)3, Al(NO3)3, and SiH4(NO3)4; wherein the molar amount of the second substance is at least twice of the molar amount of the first substance. The present invention further proposes a method for production of elongate microstructures and/or nanostructures substantially comprising BN, and an entity which is obtainable through such method.

Description

SPECIFICATION
METHOD FOR PRODUCTION OF HIGH PURITY, HIGH C RYSTALLI N ITY
BN STRUCTURES AT MODERATE TEMPERATURES
Technical Field of the Invention
The present invention relates to a catalyst for use in production of elongate microstructures and/or nanostructures substantially comprising BN, and a method for obtainment of such microstructures and/or nanostructures at low temperature.
Background of the Invention
Among many types of inorganic micro-/ or nanostructures, boron nitride nanotubes (BNNT's) have gained greatest attention after carbon nanotubes. Structurally, boron nitride nanotubes (BNNT) are analogue to carbon nanotubes. Despite several structural similarities, boron nitride nanotubes (BNNT) have a higher chemical stability and higher thermal stability (resistant to oxidation up to 1100°C). Boron nitride (BN) is piezoelectric, neutron shielding, and incomparably more hydrophobic compared to carbon nanotubes. Furthermore, boron nitride nanotubes (BNNT) with their partial ionic characteristics are electrically non-conducting materials having a broad bandwidth of 5-6 eV, unlike to metallic or semi-conductive carbon nanotubes. Due to these unique characteristics, boron nitride nanotubes (BNNT) find potential application in nanoelectronic applications (nano-insulator material, nanosensors), optical applications (Deep-blue and infrared lasers), magnetic applications (targeted drug delivery), biomedical applications (tissue scaffolds, biosensors), energy applications (hydrogen storage), ceramic-glass composites, and polymeric nanocomposite applications. Known methods for production of micro-/ or nanostructures including BN (e.g. BNNT) include arc-discharge, laser ablation, atomic stacking, plasma-jet, displacement reaction, reactive ball milling, pressurized vapor condensation and chemical vapor deposition. Temperature values of these methods and disadvantages thereof are summarized in the Table 1.
Table 1
Figure imgf000004_0001
Chemical Vapor Deposition (CVD) is a highly important method which is mostly employed in the synthesis of carbon nanotubes. This method, however, has some disadvantages including difficult arrangement of system parameters specific to the catalyst and purification.
Chemical Vapor deposition can be divided into different variations thereof, which include Catalytic Chemical Vapor Deposition, Non-Catalytic Chemical Vapor Deposition, and Conventional Chemical Vapor Deposition. Among them, Catalytic Chemical Vapor Deposition has two sub-classes: namely Boron Oxide and Thermal Chemical Vapor Deposition method. Each line of the Table 2 below shows various known alternatives within the boron oxide chemical vapor deposition sub-class. Temperature range in production of micro-/ or nanostructures including BN (e.g. BNNT) by means of catalytic chemical vapor deposition is generally between 1100°C and 1400°C. MgO- and FeO-based catalyst materials are used in such methods.
Table 2
Figure imgf000005_0001
As seen in Table 1 and Table 2, a wide range of reaction temperatures are used in obtainment of micro-/or nanostructures including BN (e.g. BNNTs). However, it is not possible with the known catalytic methods to achieve high purity, uniform morphology (in terms of crystallinity) at a low temperature, with high reaction rate. Said methods result in high costs, rather insufficient energy efficiency, and thus difficult to provide industrial use.
EP 2 393 966 Al describes production of nanostructures and more particularly a method for production of boron nitride nanotube fibers. Yurum et al. (Ind. Eng. Chem. Res., 2012, DOI:10.1021/ie301605z) describes that reaction temperature and the type of catalyst have an effect on BNNT obtained using CVD. The method used in said article is conventional CVD, and Fe2C>3, Fe3+-MCM-41, Fe3+-MCM-41/Fe2C>3 are used as catalysts. In said study, it is reported that a decrease occurs in boron nitride nanostructures in accordance with a temperature gradient from 600°C to 800°C. The resulting boron nitride nanostructures do have rather poor crystallinity and poor purity. Furthermore, it is difficult to separate nanotubes from the catalysts.
Objects of the present invention
Primary object of the present invention is to overcome the drawbacks encountered in the prior art.
Another object of the present invention is provision of a method enabling production of elongate microstructures and/or nanostructures including Boron nitride, with low temperature, high crystallinity and high purity.
A further object of the present invention is provision of a method enabling production of elongate microstructures and/or nanostructures including Boron nitride, with low cost and high yield.
A further object of the present invention is provision of an entity including a substrate which is provided with elongate microstructures and/or nanostructures including Boron nitride, even though said substrate is not as temperature resistant as those used in the prior art techniques.
Summary of the Invention The present invention relates to a method for preparation of a catalyst for use in production of elongate microstructures and/or nanostructures substantially comprising Boron nitride (BN), the method includes reacting a first substance selected from oxides or nitrates of Na, K or Li; with a second substance selected from Fe203, Al203, SiO, Fe(N03)3, AI(N03)3, and SiH4(N03)4; such that if the first substance is an oxide, the second substance is selected from the list consisting of Fe203, Al203 or SiO; and if the first substance is a nitrate, the second substance is selected from Fe(N03)3, AI(N03)3, and SiH4(N03)4; wherein the molar amount of the second substance is at least twice of the molar amount of the first substance. The present invention further proposes a method for production of elongate microstructures and/or nanostructures substantially comprising BN, and an entity which is obtainable through such method.
Brief Description of the Figures
Figure 1 is schematic view of an exemplary setup for conducting a version of the method according to the present invention, with reactive vapor-trapping chemical vapor deposition
Figure 2 shows a scanning electron microscopy (SEM) image of a field of elongate microstructures and/or nanostructures substantially comprising BN (here: boron nitride nanotube, BNNT) with 2000X magnification.
Figure 3 shows a SEM image of such field with 10000X magnification.
Figure 4 shows a transmission electron microscopy (TEM) image emphasizing the wall structure of a single elongate microstructure and/or nanostructure substantially comprising BN (here: boron nitride nanotube, BNNT).
Figure 5 shows a TEM image emphasizing the wall-gap-wall structure of a conventional BNNT. Figure 6 is a list of steps in an exemplary version of the method according to the present invention, enabling elongate microstructure and/or nanostructure substantially comprising BN.
Figure 7 is a list of steps in an exemplary version of the method for obtaining a potassium ferrite catalyst according to the present invention.
Detailed Description of the Invention
The present invention proposes a method for preparation of a catalyst for use in production of elongate microstructures and/or nanostructures substantially comprising Boron nitride (abbreviated as BN), the method includes reacting a first substance selected from oxides or nitrates of Na, K or Li, with a second substance selected from Fe203, Al203, SiO, Fe(N03)3, AI(N03)3, and SiH4(N03)4; such that
• if the first substance is an oxide, the second substance is selected from the list consisting of Fe203, A 03 or SiO; and
• if the first substance is a nitrate, the second substance is selected from Fe(N03)3, AI(N03)3, and SiH4(N03)4, wherein the molar amount of the second substance is at least twice of the molar amount of the first substance.
For instance, the first substance can include K2O or Na20 or L12O, and the second substance includes Fe203. Alternatively the first substance can include KN03 or NaN03 or LiN03 and the second substance includes Fe(N03)3.
In the case where the first substance comprises an oxide of K, a reaction between the K2O and Fe203 to form KFeC>2 occurs with equi molar convertions of both K20 and Fe203 into KFeC>2 according to the reaction stoichiometry below:
K20 + Fe203 2 KFe02 and therefore the ratio between molar amounts of K20 and Fe2C>3 respectively, remains within the same range, when compared to a ratio between initial molar amounts of K20 and Fe2C>3, respectively.
In an embodiment according to the present invention, the first substance includes K20 or Na20 and the second substance includes Fe2C>3; or alternatively, the first substance includes KNO3 or NaNC>3 and the second substance includes Fe(NC>3)3. In these cases it is enabled that a respective reaction product includes potassium ferrite KFeC>2 or sodium ferrite NaFeC>2)
Accordingly, the present invention further proposes use of the reaction product as a catalyst in production of elongate microstructures and/or nanostructures substantially comprising BN.
The present invention further proposes a method for production of elongate microstructures and/or nanostructures substantially comprising BN, including the following steps: a) preparation of a first mixture (catalytic mixture) according to any one of the claims 1 or 2, b) mixing the first mixture with a Boron source to obtain a second mixture, c) arranging a substrate in vicinity of said second mixture, d) arranging a gas comprising a substance including nitrogen atoms (thereby acting as a nitrogen-source), in contact with both of the substrate and the second mixture, and e) heating the second mixture to a temperature corresponding to that at which the vapor pressure of an alkali metal in the first substance in its elementary form is at least 0.5 bar. Said vapor pressure can be e.g. within the range between 0.5 bar and 1.2 bar, preferably within the range between 0.9 bar and 1.1 bar.
Such vapor pressure substantially corresponds to rapidly evaporating or boiling of an alkali metal desorbed from the reaction product of the first mixture, and enable conversion into a reaction product from the constituents of the first mixture due to the temperature and the presence of the nitrogen-source. Therefore the above-described temperature enables the reaction mechanism for formation of elongate microstructures and/or nanostructures substantially comprising BN.
The Boron source can comprise elementary Boron. Elementary Boron gets oxidized rather easily at lower temperatures when compared to other Boron sources, e.g. Boron oxides or Boron minerals.
The first substance can preferably include an oxide of Na, K or Li, and the steps (a) and (b) described above can be performed in a substantially dry atmosphere; in the case where the first substance includes an oxide of Na or K. Such alkali metal oxides are less stable in presence of water, and this provision enables a high yield in the desired reaction mechanism, without disturbances due to humidity.
In the step (b), the molar amount of B atoms in the boron source can be substantially equal to the molar amount of 0 atoms in the second mixture. Thereby, stoichiometric B and 0 amounts are secured for obtainment B2O2 vapors.
In the step (d), the gas can substantially comprise dry ammonia (NH3). NH3 decomposes easily thus it is considered a high efficiency reactant in this method.
In an embodiment according to the present invention,
- the step (b) includes: preparation of the second mixture and providing the second mixture on or in a holder for supporting the second mixture against gravity, into a cabinet comprising a gas inlet and a gas outlet, arranged to allow a gas stream in a gas flow direction from an upstream direction towards a downstream direction; - the step (c) includes placing a substrate into the cabinet; e.g. at a position for allowing gas diffusion communication between the holder and the substrate; and
- the step (d) includes arranging a gas which comprises a substance including nitrogen atoms thereby acts as a nitrogen-source, in contact with both of the substrate and the second mixture.
The gas (nitrogen-source) can be substantially stagnant inside the cabinet, or can be streamed from the gas inlet towards the gas outlet as a gas stream. Thus the gas can be streamed from the gas inlet towards the gas outlet as a stream flowing from an upstream direction towards a downstream direction.
The holder can be formed substantially from a ceramic material, preferably including BN which facilitates nucleation of the micro-/nanostructures comprising BN.
The holder and the substrate can be placed in a receptacle having an opening, said receptacle being placed into the cabinet such that the opening is directed to the downstream direction. This facilitates nucleation without disturbance under gas stream. An exemplary schematic drawing of such setup is given in the Fig.l.
The holder can be substantially in the form of a vessel for holding liquids. Such functional form of the holder is useful in supporting any extensive amounts of molten alkali metal.
Upon placement of the holder, any air inside the receptacle can be replaced with a further gas which is inert at the temperature defined in the step (e). By sweeping with said inert gas, air is removed from the vicinity of reactants and substrate. The further gas can be e.g. N2 or Argon.
The replacement of air can be also performed by applying reduced pressure into the receptacle, then supplying the gas into the receptacle and heating the receptacle; said reduced pressure preferably corresponds to an absolute pressure of 10 Torr or less.
The vapor pressure of Na has a value of 0.5 bar at 799.95°C, 0.9 bar at 857.55°C, 1 bar at 868.54°C, and 1.1 bar at 878.66°C. Thus, with the method according to the present invention, in the case where Na is used as alkali metal in preparation of the first mixture, a substrate which is resistant against thermal decomposition only up to a temperature range between 857.55°C and 878.66°C can be used without damage, to form nanostructures or microstructures substantially including BN.
Similarly, the vapor pressure of K has a value of 0.5 bar at 672.5°C, 0.9 bar at 726.75°C, 1 bar at 737.13°C, and 1.1 bar at 746.70°C. Thus, with the method according to the present invention, in the case where K is used as alkali metal in preparation of the first mixture, a substrate which is resistant against thermal decomposition only up to a temperature range between 672.5°C and 746.70°C can be used without damage, to form nanostructures or microstructures substantially including BN.
Since the vapor pressure of Li has a value of 0.5 bar at 1223.2°C, 0.9 bar at 1298°C, 1 bar at 1312.2°C, and 1.1 bar at 1325,30°C. Thus, with the method according to the present invention, in the case where Li is used as alkali metal in preparation of the first mixture, a substrate which is resistant against thermal decomposition only up to a temperature range between 1223.2°C and 1325.30°C can be used without damage, to form nanostructures or microstructures substantially including BN.
The above mentioned temperature values related to the use of Li are significantly higher when compared to those at the cases where Na or K is used as alkali metal in preparation of the first mixture. Therefore preferably said alkali metal includes Na and/or K, more preferably includes K.
Accordingly, the present invention further proposes an entity comprising a substrate provided with elongate microstructures and/or nanostructures which substantially comprise BN, provided that the substrate is resistant to thermal decomposition only at temperatures up to around 1000°C. Even though a more extensive growth in structures (i.e. elongate microstructures and/or nanostructures comprising BN) is achievable with higher temperatures, the method according to the present invention provides formation of such microstructures and/or nanostructures at lower temperatures, without compromising the extent of crystallinity and purity of said structures.
In the case where the alkali metal in the first mixture is Na, provision of the substrate with elongate microstructures and/or nanostructures is enabled even at temperatures starting from around 800°C.
In the case where the alkali metal in the first mixture is K, provision of the substrate with elongate microstructures and/or nanostructures is enabled even at temperatures starting from around 670°C, more particularly starting from around 725°C, even more particularly starting from around 750°C.
In processing a substrate which is resistant against thermal decomposition at temperatures higher than those specified above, higher temperatures can be employed in the step (e). Even though the method according to the present invention enables formation of elongate microstructures and/or nanostructures substantially comprising BN and provision thereof on a substrate; it is evident that the reaction rates are higher at higher temperatures. Accordingly, with higher reaction temperatures, better results are available: high values of reaction yield and greater extent in growth of microstructures and/or nanostructures are available in a shorter time. For instance, heating the second mixture to a temperature corresponding to that at which the vapor pressure of an alkali metal in the first substance in its elementary form can also have even significantly higher values than 0.5 bar. In exemplary studies according to the present invention, in which Na and/or K were employed as alkali metal in the first mixture, higher temperatures resulted in higher growth in the elongate structures: a high extent in growth of microstructures and/or nanostructures was achieved at 800°C, an even higher extent in the same terms was achieved 1000°C, and an even higher extent in the same terms was achieved 1100°C.
The gist of the present invention and implementation thereof is exemplified over the further disclosures hereinbelow. The invention is not limited to said further disclosures, and a person skilled in the relevant art may easily implement different embodiments according to the present invention.
With reference to the Fig.l, the vapor-trapping chemical vapor deposition assembly (1) used for obtaining boron nitride nanotube (BNNT) structures according to the invention can comprise a furnace (10) for adjusting temperature of a reaction medium. The furnace (10) can be positioned along a horizontal axis (x) which is substantially perpendicular to the gravity vector (g). The temperature of the furnace (10) can be increased to a temperature value between 700°C and 1000°C after a second tube (101) is placed in the furnace.
Said furnace (10) can comprise a first tube (100) in which the reaction takes place and the first tube (100) can be positioned substantially along the horizontal axis (x). A second tube (101) can be positioned inside the first tube (100) and one end of the second tube (101) can include an opening, through which a gas (102) can be supplied for being employed in the reaction mechanism. The first tube (100) can be opened at both ends. In a preferred embodiment of the invention, the carrier gas can be ammonia (NH3). Ammonia serves as a nitrogen source in the formation of boron nitride. The carrier gas (102) can be introduced into the first tube (100) from a gas inlet (A) and released at a gas outlet (B). In other words, a flow direction of the carrier gas (102) can be defined from the gas inlet (A) towards the gas outlet (B). The use of such second tube (101) corresponds to a variation of the method including chemical vapor deposition (CVD).
The second tube (101) has a smaller diameter than the first tube (100), and it may be easily positioned in the first tube (100). The second tube (101) may be formed from e.g. quartz or alumina resistant to high temperatures. The first tube (100) may also be made of quartz or alumina.
The second tube (101) can comprise a carrier member (1010) in which boron source and catalyst are positioned, and a cover (1011) for closing the carrier member (1010). The cover (1011) may be formed from silicon wafer or another substrate material. The carrier member (1010) can be formed from alumina or boron nitride (BN) or another ceramic material. In another embodiment, the carrier member (1010) may comprise mullite. The carrier member (1010) can be loaded with a second mixture (1015) including process precursors, boron source and catalyst (e.g. potassium ferrite). The carrier member (1010) can be positioned in the vicinity of a closed end of the second tube (101), which can be positioned in a central zone of the furnace (10). The closed end of the second tube (101) can be positioned at upstream direction relative to the gas stream (102). As a result, a reactive steam formed can be utilized with high efficiency until a steam pressure allowing nucleation is achieved.
A catalyst is formed inside the second mixture; and under the above described conditions the catalyst can be potassium ferrite (KFeC>2). A version of such catalyst preparation method (600) can include the following steps:
- dissolving (601) KNO3, iron nitrate (e.g. with crystal water:
Fe(N03)3.9H20) and stoichiometrically excess amount of citric acid in water and mixing same, wherein K and Fe are present at a predetermined molar ratio,
- obtaining (602) potassium ferrite by evaporation of water and calcination of the mixture at a predetermined temperature.
The molar ratio of K and Fe can be 50%.
A dry powder mixture obtained at the end of 602, can be calcinated at 700°C. The molar ratio of K and Fe can be also within the range between 1% and 20%.
The potassium ferrite catalyst ensures that boron nitride nanotubes (BNNT) are obtained with high crystallinity, in pure form and at low temperatures. Said catalyst disintegrates into K and K20 with temperature.
With reference to the Fig.6, a further stage of an exemplary version of the method according to the present invention includes synthesis (500) of boron nitride nanotubes (BNNT) which can be performed using a vapor-trapping chemical vapor deposition assembly (1). Said stage can include the following steps:
- mixing (501) of a boron source and the first mixture (catalyst) at a substantially moisture-free environment, in a predetermined molar ratio between the boron in the boron source and the oxygen in the first mixture, to obtain a second mixture;
- introducing (502) the second mixture into a holder (carrier member, 1010),
- covering (503) the carrier member (1010),
- positioning (504) the carrier member (1010) into a receptacle (second tube, 101) at a substantially moisture-free atmospheric environment,
- positioning (505) the second tube (101) into a first tube (100), a first end and a second end of which being respectively arranged to function as a gas inlet and a gas outlet, to determine a gas stream direction; thereby the first tube forming a cabinet;
- applying (506) vacuum to the assembly in order to remove air from the environment, - introducing (507) a gas (carrier gas, 102) into the first tube (100) and thus also into the second tube (102), and heating the assembly; to grow (508) boron nitride nanotubes (BNNT) inside the carrier member (1010) as a result of reactions,
- introducing (509) the carrier member (1010) into a further vessel and providing it with a certain amount of liquid (e.g. water),
- sonificating (510) the further vessel, and dipping (penetration, 511) of reaction product including boron nitride nanotubes (BNNT) with impurities and the transferring impurities to said liquid as a result of sonification, and thereby obtaining purified boron nitride nanotubes (BNNT).
At step 501, said catalyst can be potassium ferrite (KFeC>2).
At step 501, the boron source can be elementary boron in the form of powder. Elementary boron evaporates under rather moderate temperatures when compared to its compounds as boron source. Thereby the reaction is facilitated and the efficiency is increased for the method according to the present invention.
At step 501, molar ratio of boron to oxygen second mixture can be within a range between 1:1 and 4:1, and preferably around 1:1 to ensure stoichiometric ratios for B2O2 steam as an intermediate phase.
At step 501, molar ratio of potassium to iron in the first mixture to form KFeC>2 catalyst can be within the range between 1:1 and 1.2:1. In the case where the molar ratio is above 1:1, the respective ingredients are also partly converted into potassium ferrate (K2Fe04) as a further catalyst.
At step 501, molar ratio of boron to KFeC>2 catalyst can be e.g. around 2:1 higher. This ratio corresponds to a molar ratio of boron to oxygen of 1:1 or higher at reaction conditions. At step 503, the carrier member (1010) can be partly covered with a cover (1011). Said cover (1011) can be a silicon wafer. With the partially covered carrier member (1010), a delay can be arranged in discharge of the nitrogen source (gas, 102, e.g. ammonia). The gas (cf. step 507) interacts with the second mixture which includes boron and catalyst.
At step 508, boron nitride nanotubes (BNNT) can be formed/grown on outer surfaces (walls and/or base) of the carrier member (1010).
At step 511, the impurities can substantially include metallic material which further can be ferromagnetic. The impurities can be separated from fluid by means of magnets or using an acidic fluid for dissolving metallic material.
In a version of the method, the following reactions may take place:
2K20(s) +2B(s) B202& +
2Fe203 + 6B -» 3B202(g) + 4¥e(k)
W2(g) + 2NH3 2BN (e.g. nanotube) + 1W20(g) + W2(g)
The method can be considered substantially on the basis of vapor-liquid-solid (VLS) mechanism.
When the alkali metal (e.g. potassium) reaches onto the surface of the catalyst, an active region on the catalyst is formed, which in turn increases NH3 interaction and enhances the efficiency of boron nitride production.
An X-Ray Diffractogram (XRD image) of the KFeC>2 is shown in Fig.8.
At high temperatures and reducing atmosphere (including the gas which functions as Nitrogen source), K and K2O desorption occurs from the second mixture including KFeC>2 catalyst due to high steam pressures. This phenomenon is useful in formation of elongate micro-/nanostructures including BN by catalytic chemical vapor deposition.
Several characteristics of boron nitride nanotubes (BNNT) including their wall morphology, crystalline form, height, width and elemental composition are respectively characterized by TEM, SEM, EELS and RAMAN spectroscopy techniques. Figure 2 and Figure 3 show scanning electron microscopy (SEM) images of boron nitride nanotubes (BNNT) on a carrier member (1010). The mean diameter of the resulting boron nitride nanotubes (BNNT) is 30-100 nanometers and the lengths thereof are mostly above lOijm. In parallel, transmission electron microscopy (TEM) images of a single boron nitride nanotube (BNNT) are given in Figures 4 and 5. Micro-/or nanostructures including BN (here: BNNT) with smooth nanotube wall structure and high crystallinity can be obtained in low temperatures with the method according to the present invention. Raman spectroscopy result of boron nitride nanotubes (BNNT) obtained according to this exemplary method according to the present invention is given in Fig.9.
The method according to the present invention decreases cost and energy consumption in production of elongate microstructures and/or nanostructures including BN.
The elongate microstructures and/or nanostructures including BN obtainable with the method according to the present invention are applicable to a wide scale of entities including:
- polymeric composites, especially armor applications, thin film coatings, batteries, industrial parts for aircraft/spacecraft;
- ceramic composites for e.g. parts of jet motors, dentistry applications;
- biomedical applications, e.g. nanoscale skin scaffolds, bone tissue regeneration, medicament administration; - radiation shielding applications, especially neutron and UV shielding applications; piezo-electric applications, especially light-weight and nontoxic piezoelectric systems, airless human vehicle sensors, energy collectors;
- flame retardant cable applications, especially high thermal conductive and electrically insulated battery applications, and electronic cooling systems.
Reference signs
For a better understanding of the invention, individual reference numbers are assigned to the elements in the accompanying drawings, and the elements corresponding to these numbers are given below.
1. Assembly
10. Furnace
100. a First Tube suitable for defining a gas flow direction from an upstream direction towards a downstream direction (can be considered as a cabinet)
101. Second Tube (as a receptacle)
1010- a Carrier member
1015- a Material (as a second mixture)
1011- a Cover
102. Gas stream in the downstream direction
BNNT. elongate microstructure or nanostructure including Boron nitride, e.g. Boron nitride nanotube 500. method for obtaining elongate microstructure or nanostructure including Boron nitride, e.g. Boron nitride nanotube
600. Method for obtaining a first mixture (catalyst, catalytic mixture)
A. Gas inlet
B. Gas outlet

Claims

1. A method for preparation of a catalyst for use in production of elongate microstructures and/or nanostructures substantially comprising BN, the method includes reacting a first substance selected from oxides or nitrates of Na, K or Li; with a second substance selected from Fe2C>3, AI2O3, SiO, Fe(NC>3)3, AI(N03)3, and SiH4(N03)4; such that
if the first substance is an oxide, the second substance is selected from the list consisting of Fe2C>3, AI2O3 or SiO; and if the first substance is a nitrate, the second substance is selected from Fe(N03)3, AI(N03)3, and SiH4(N03)4,
wherein the molar amount of the second substance is at least twice of the molar amount of the first substance.
2. The method according to the claim 1, wherein the first substance includes K2O or Na20 and the second substance includes Fe2C>3, or the first substance includes KNO3 or NaNC>3 and the second substance includes Fe(N03)3.
3. Use of the reaction product as a catalyst in production of elongate microstructures and/or nanostructures substantially comprising BN.
4. A method for production of elongate microstructures and/or nanostructures substantially comprising BN, including a) preparation of a first mixture according to any one of the claims 1 or 2, b) mixing the first mixture with a Boron source to obtain a second mixture, c) arranging a substrate in vicinity of said second mixture, d) arranging a gas comprising a substance including nitrogen atoms in contact with both of the substrate and the second mixture, e) heating the second mixture to a temperature corresponding to that at which the vapor pressure of an alkali metal in the first substance in its elementary form is at least 0.5 bar.
5. The method according to the claim 4, wherein the Boron source comprises elementary Boron.
6. The method according to any one of the claims 4 or 5, wherein the first substance includes an oxide of Na, K or Li, and the steps (a) and (b) are performed in a substantially dry atmosphere; preferably the first substance includes an oxide of Na or K.
7. The method according to any one of the claims 4 to 6, wherein in the step (b), the molar amount of B atoms in the boron source are substantially equal to the molar amount of 0 atoms in the second mixture.
8. The method according to any one of the claims 4 to 7, wherein in the step (d), the gas substantially comprises dry NH3.
9. The method according to any one of the claims 4 to 8, wherein the step (b) includes preparation of the second mixture and providing the second mixture on or in a holder for supporting the second mixture against gravity, into a cabinet comprising a gas inlet and a gas outlet, arranged to allow a gas stream in a gas flow direction from an upstream direction towards a downstream direction; the step (c) includes placing a substrate into the cabinet; and the step (d) includes arranging a gas which comprises a substance including nitrogen atoms thereby acts as a nitrogen-source, in contact with both of the substrate and the second mixture.
10. The method according to the claim 9, wherein the holder and the substrate are placed in a receptacle having an opening, said receptacle being placed into the cabinet such that the opening is directed to the downstream direction.
11. The method according to the claim 10, wherein the gas is streamed from the gas inlet towards the gas outlet as a stream flowing from an upstream direction towards a downstream direction.
12. The method according to any one of the claims 9 to 11, wherein the holder is substantially in the form of a vessel for holding liquids.
13. The method according to any one of the claims 9 to 12, wherein upon placement of the holder, any air inside the receptacle is replaced with a further gas which is inert at the temperature defined in the step (e).
14. The method according to the claim 13, wherein the replacement of air is performed by applying reduced pressure into the receptacle, then supplying the gas into the receptacle and heating the receptacle; said reduced pressure preferably corresponding to an absolute pressure of 10 Torr or less.
15. An entity comprising a substrate provided with elongate microstructures and/or nanostructures which substantially comprise BN, provided that the substrate is resistant to thermal decomposition only at temperatures up to 1000°C.
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YURUM ET AL., IND. ENG. CHEM. RES., 2012

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* Cited by examiner, † Cited by third party
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WO2023016145A1 (en) * 2021-08-09 2023-02-16 中国科学院大学 Organic sulfur hydrolysis catalyst suitable for claus process, preparation method therefor, and application thereof

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