WO2022144085A1 - Non-catalytic sol-gel method for production of boron carbide nanofibers - Google Patents

Non-catalytic sol-gel method for production of boron carbide nanofibers Download PDF

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WO2022144085A1
WO2022144085A1 PCT/EP2020/088063 EP2020088063W WO2022144085A1 WO 2022144085 A1 WO2022144085 A1 WO 2022144085A1 EP 2020088063 W EP2020088063 W EP 2020088063W WO 2022144085 A1 WO2022144085 A1 WO 2022144085A1
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boron
acid
range
initial mixture
boron carbide
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PCT/EP2020/088063
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French (fr)
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Cengiz KAYA
Figen KAYA
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Yildiz Teknik Üniversitesi
Sabanci Üniversitesi
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Priority to PCT/EP2020/088063 priority Critical patent/WO2022144085A1/en
Publication of WO2022144085A1 publication Critical patent/WO2022144085A1/en

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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/90—Carbides
    • C01B32/914—Carbides of single elements
    • C01B32/991—Boron carbide
    • 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
    • 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
    • C01P2004/17—Nanostrips, nanoribbons or nanobelts, i.e. solid nanofibres with two significantly differing dimensions between 1-100 nanometer

Definitions

  • the present invention relates to a method for production of boron carbide nanofibers.
  • the present invention proposes a non-catalytic sol-gel method for obtainment of boron carbide nanofibers.
  • Boron carbide (B 4 C) nanostructures are considered useful in several technologies including electronics and supercapacitors.
  • Avcioglu et al. disclose a method for obtaining boron carbide nanopowder (Ceramics International, Volume 46, Issue 11, Part A, 1 August 2020, Pages 17938-17950, DOI: 10.1016/j.ceramint.2020.04.104).
  • Avcioglu et al. disclose the obtainment of LDPE composites reinforced with boron carbide nanostructures in the form of powder (Ceramics International, Volume 46 (2020) pp.343-352, DOI:
  • Shawgi et al. disclose a method for obtaining fiber shaped boron carbide powder starting from poly(vinyl alcohol) ("Synthesis of nano particles and fiber-like shape boron carbide powder from ploy (vinyl alcohol) and boric acid” (2006), J Sol-Gel Sci Technol, Springer, DOI: 10.1007/S10971-017-4320-4).
  • the structures which are characterized as having fiber-like shape appear to be highly non-linear and have a high length per diameter (L/D ratio) value.
  • the fiber-like shaped grains obtained with the method described in the publication do not include nanofibers.
  • a high-accuracy, low-cost and energy efficient method for production of boron carbide nanofibers is still sought in the related technical field.
  • the primary object of the present invention is to overcome the shortcomings in the prior art.
  • Another object of the present invention is to provide a high-accuracy, low-cost and energy efficient method for production of boron carbide nanofibers.
  • the present invention relates to a method for obtaining boron carbide nanofibers.
  • the method employs sol-gel technique and does not necessitate the use of catalytic particles.
  • the availed boron carbide nanofibers are characterized and it has been concluded that the nanofibers have a high potential for being employed as electrode material in supercapacitors.
  • the method proposes a low-cost technique.
  • the method allows or includes composition control in molecular level: the reactants can be homogeneously mixed in molecular level, which eliminates formation of residual carbon and boron oxide.
  • the method further allows the optimization of polymeric gel structure, thereby enabling the control of the L/D ratio of boron carbide nanofibers, rendering a high purity and/or crystallinity available with low cost.
  • the resulting boron carbide nanofibers can be employed as electrode material in supercapacitors which are the main feature of high efficiency power storage systems in electric vehicles.
  • the high performance of the product availed by the method proposed hereby is further useful in portable communication systems, aeronautics, unmanned aerial vehicles (UAVs), electronics and medicine.
  • Fig.l shows boron carbide nanofibers obtained with the method according to the present invention.
  • the present invention relates to a method for obtaining boron carbide nanofibers.
  • the method can be considered to include a non-catalytic sol-gel process.
  • the method proposed in the present application includes the following features: i. Preparation of an initial mixture comprising one or more carboxylic acids, glycerin and (nano-) elementary boron; ii. Gelling of the initial mixture, thereby obtaining a gel; iii. Pyrolysis of the gel at a temperature within the range between 450°C and 750°C, thereby obtaining a precursor; iv. Heat treatment at a temperature within the range between 1450°C and 1550°C, thereby obtaining a product which includes boron carbide nanofibers. v. The heat treatment can be preferably followed by physical separation of boron carbide nanofibers out of the product obtained with the heat treatment.
  • nanofibers are substantially separated based on geometric shape/characteristics from non-fiber boron carbide nanoparticles.
  • the physical separation can preferably include the following steps: dry vibration of the product in an ultrasonic bath, followed by preparation of an aqueous mixture which includes the product, ionized water, one or more alcohols and one or more surfactants, followed by subjecting the aqueous mixture to an ultrasonic probe, followed by centrifugation (for deposition of relatively high particle size structures in the product), followed by vacuum filtration (for filtering relatively high L/D ratio structures (here: nanofibers) and permeating relatively low L/D ratio structures in the product) (preferably using filter papers, more preferably a progressive vacuum filtration using a series of filter papers with gradually decreasing pore sizes), followed by drying.
  • the initial mixture of step (i) further comprises boric acid particles, or the method includes an introduction of boric acid (H 3 BO 3 ) particles and nano-boron oxide into the precursor obtained in step (iii) prior to the step (iv).
  • glycerin can be considered to serve as a solvent for said one or more carboxylic acids, and if present, also for boric acid.
  • the gelling (or "jel ling") can be performed for instance by mixing preferably at an elevated temperature which can be for instance within the range between 80°C and 150°C.
  • an ultrasonic probe is considered advantageous for achieving a high extent of homogeneity in a short time period such as 2 hours.
  • the extent of homogeneity corresponds to a high yield in obtainment of boron carbide as product of the method.
  • the pyrolysis can be performed for instance by placing the gel into a first vessel (e.g. in the form of a crucible) which can be for instance made of alumina, and by keeping the first vessel in a shuttle kiln heated to a temperature within the range between 450°C and 750°C.
  • the duration of pyrolysis can preferably be 6h or longer.
  • the heat treatment can be performed under an inert atmosphere (such as Argon).
  • the heat treatment can be performed using a tube furnace, e.g. by placing the precursor into a second vessel (e.g. in the form of a crucible) which can be for instance made of graphite, preferably enveloping the second vessel with carbon paper, and then keeping the second vessel in the tube furnace heated to a temperature within the range between 1450°C and 1550°C.
  • the duration of heat treatment can preferably be 6 hours or longer.
  • the initial mixture can be prepared by adding a first mixture comprising boric acid, elementary boron and a first carboxylic acid, with a second mixture comprising glycerin and a second carboxylic acid which is stronger than the first carboxylic acid.
  • the molar ratio of carbon to boron used can be arranged to be within the range between 3: 1 and 5: 1 (for the variation above, in the initial mixture). This molar ratio range eliminates the formation of residual C and B in the product, thereby achieving an enhanced purity and crystallinity.
  • the initial mixture of step (i) comprises one or more carboxylic acids, glycerin (as solvent for the carboxylic acids) and (nano-) elementary boron.
  • boric acid is introduced into the precursor upon the pyrolysis of step (iii), along with boron oxide (B 2 O 3 ) (preferably, in the form of boron oxide nanoparticles).
  • the boric acid can preferably be excluded/absent in the initial mixture prepared in step (i).
  • the initial mixture of step (i) can include a single carboxylic acid instead of a plurality/variety of carboxylic acids.
  • components of the initial mixture(s) of step (i) enable the formation of borat-ester and carbon-ester bonds upon gelling of step (ii).
  • step (ii) provides a nanometer-level homogeneity (in the matrix/network/lattice), thereby enhances/maximizes the efficiency in pyrolysis of step (iii) and heat treatment of step (iv).
  • Composition of the initial mixture in step (i) affects the geometric characteristics (i.e. length, diameter and L/D ratio) of the boron carbide nanofibers in the product.
  • the one or more carboxylic acids to be employed in the method according to the present invention can preferably be selected from the list consisting of citric acid, tartaric acid, tannic acid, glycolic acid, malic acid, mandelic acid, oxalic acid, and mixtures thereof.
  • the method as exemplified below can be considered to include a modified sol-gel process, in which the formation of free boron oxide, thereby enabling the synthesis of boron carbide fibers with arrangeable geometric characteristics (in particular, L/D ratio). None of the variations of the method necessitates the use of catalytic particles. The variations of the method enable the obtention of high purity and high crystallinity boron carbide nanofibers.
  • components of the initial mixture(s) of step (i) enabled the formation of borat-ester and carbon-ester bonds upon gelling of step (ii).
  • the use of an ultrasonic probe in the gelling of step (ii) provided a nanometer-level homogeneity (in the matrix/network/lattice), thereby enhancing/maximizing the efficiency in pyrolysis of step (iii) and heat treatment of step (iv). It is considered that the composition of the initial mixture in step (i) affected the geometric characteristics (i.e. length, diameter and L/D ratio) of the boron carbide nanofibers obtained with each variation of the method.
  • the one or more carboxylic acids to be employed in the method variations described below were (as preferred) selected from the list consisting of citric acid, tartaric acid, tannic acid, glycolic acid, malic acid, mandelic acid, oxalic acid, and mixtures thereof.
  • Example 1 i.
  • An "initial mixture” was prepared such that the C/B molar ratio was arranged to be within the range between 3: 1 and 5: 1. With this molar ratio range, the formation of residual carbon and boron (in particular, residual carbon) in the product was eliminated for achieving high purity and crystallinity.
  • the initial mixture comprised one or more carboxylic acids, boric acid (H 3 BO 3 ), glycerin as common solvent for carboxylic acids and boric acid and (nano-) elementary boron.
  • the initial mixture can be prepared by combining a first mixture comprising boric acid, elementary boron and a first carboxylic acid, with a second mixture comprising glycerin and a second carboxylic acid which is stronger than the first carboxylic acid. ii.
  • the initial mixture was subjected to gelling, preferably under heating to a temperature below the degradation temperature of carboxylic acid(s) in the initial mixture.
  • Degradation temperatures information for various carboxylic acids are available to a person skilled in chemistry, and can be exemplified as 230°C for citric acid, 257°C for tartaric acid, 190°C for tannic acid, 100°C for glycolic acid, 225°C for malic acid, 322°C for mandelic acid and 190°C for oxalic acid.
  • the selection of (one or more) carboxylic acid(s) is considered to have an effect in modifying the network structure of the gel obtained upon the gelling step.
  • the gelling was performed by mixing, using an ultrasonic probe; thereby a high extent of homogeneity can be availed. The mixing was continued for a time period of 2 hours.
  • the extent of homogeneity corresponds to a high yield in obtainment of boron carbide as product of the method. Borate ester and carbon ester bonds formed at the gelling step; thus the gelling step resulted in formation of a "gel”. iii.
  • the gel was subjected to a pyrolysis step, and thereby the gel was converted into a "precursor". The pyrolysis was performed by subjecting the gel to a temperature within the range between 450°C and 750°C.
  • the temperature used in the pyrolysis step can be selected based on the carboxylic acid(s) employed in the initial mixture: for instance, a relatively high temperature value within said range can be selected for achieving a relatively quick conversion (into precursor) of a gel formed from an initial mixture comprising one or more acids with relatively high degradation temperature(s).
  • An exemplary period of 6 hours for the pyrolysis step corresponds to a quick conversion within the context of the present application.
  • the pyrolysis was performed by placing the gel into a first vessel which was an alumina crucible and by keeping the first vessel for 6 hours in a shuttle kiln heated to the temperature within the range between 450°C and 750°C. iv.
  • the precursor was then subjected to a heat treatment at a temperature within the range between 1450°C and 1550°C, and thereby a "product" is obtained which includes boron carbide nanofibers.
  • the temperature used in the heat treatment step can be selected based on the carboxylic acid(s) employed in the initial mixture; such as in the gelling step (iii) (mutatis mutandis).
  • the heat treatment was performed under an inert atmosphere (here: Argon, fed at a flow rate of e.g. 200 mL/min).
  • the atmosphere being "inert” corresponds to that the precursor is protected from contacting with any reactive gases (such as air oxygen), to avoid the oxidation of the precursor.
  • the heat treatment was performed by placing the precursor into a second vessel which was a graphite crucible enveloped with carbon paper, and then keeping the second vessel in a tube furnace heated to the temperature within the range between 1450°C and 1550°C. The duration of heat treatment was 6h. v.
  • the product was then subjected to physical separation, for separating boron carbide nanofibers out of the product.
  • the physical separation included the following steps: dry vibration of the product in an ultrasonic bath, followed by preparation of an aqueous mixture which includes the product, ionized water, one or more alcohols and one or more surfactants, followed by subjecting the aqueous mixture to an ultrasonic probe, followed by vacuum filtration (preferably using filter papers, more preferably a progressive vacuum filtration using a series of filter papers with gradually decreasing pore sizes), followed by drying.
  • Example 2 The method variation of Example 2 was different from the variation described in Example 1 in that:
  • the initial mixture of step (i) comprised glycerin as solvent for the carboxylic acid(s) and (nano-) elementary boron.
  • the initial mixture of step (i) further comprised a single carboxylic acid (here: citric acid as an exemplary carboxylic acid) (instead of a plurality/variety of carboxylic acids).
  • boric acid (as preferred, in the form of boric acid particles) was introduced into the precursor upon the pyrolysis of step (iii), along with boron oxide (B 2 O 3 ) (as preferred, in the form of boron oxide nanoparticles).
  • the boric acid was (as preferred) excluded/absent in the initial mixture prepared in step (i).
  • the introduction of boric acid particles and boron oxide (preferably, nanoparticles) was performed by grinding/mixing into the precursor, resulting in a "modified precursor" which equals to a "precursor which contains boric acid particles and boron oxide (preferably, nanoparticles)".
  • the modified precursor is then subjected to heat treatment as described in step (iv) of Example 1.
  • 1500°C was selected as an exemplary heat treatment temperature.
  • the product of the heat treatment step was subjected to physical separation as described in step (v) of Example 1.
  • Fig.l shows SEM image of boron carbide nanofibers obtained with the method as described in the Example 2.
  • the method variation described in the Example 1 also results in boron carbide nanofibers which are highly comparable (or equivalent) with those shown in Fig.l.
  • the present invention proposes a method for obtaining boron carbide nanofibers, including the following steps: i. preparation of an initial mixture comprising one or more carboxylic acids, , glycerin and elementary boron nanoparticles; ii. gelling of the initial mixture to obtain a gel; iii. subjecting the gel to a pyrolysis step, to obtain a precursor; performed at a temperature within the range between 450°C and 750°C; iv. subjecting the precursor to a heat treatment at a temperature within the range between 1450°C and 1550°C, thereby obtaining a product which includes boron carbide nanofibers.
  • the initial mixture of step (i) further comprises boric acid particles, or alternatively, the method includes an introduction of boric acid particles along with boron oxide into the precursor obtained in step (iii) prior to the step (iv).
  • the heat treatment described in step (iv) can be followed by physical separation of boron carbide nanofibers out of the product obtained upon said heat treatment step (iv).
  • the physical separation can include dry vibration of the product in an ultrasonic bath, followed by preparation of an aqueous mixture which includes the product, ionized water, one or more alcohols and one or more surfactants, followed by subjecting the aqueous mixture to an ultrasonic probe, followed by centrifugation, followed by vacuum filtration, followed by drying.
  • the molar ratio of carbon to boron in the initial mixture can be preferably arranged to be within the range between 3: 1 and 5: 1.
  • the initial mixture can preferably be prepared by combining a first mixture comprising boric acid particles, elementary boron nanoparticles and a first carboxylic acid, with a second mixture comprising glycerin and a second carboxylic acid which is stronger than the first carboxylic acid.
  • the method includes an introduction of boric acid particles and boron oxide are into the precursor obtained in step (iii); the molar ratio of carbon to boron obtained upon the introduction of boric acid particles and boron oxide into the precursor, can preferably be arranged to be within the range between 3: 1 and 5: 1.
  • Said introduction can preferably be performed by grinding/mixing of the boron oxide nanoparticles and boric acid into the precursor obtained in step (iii).
  • the initial mixture of step (i) can be arranged to comprise a single carboxylic acid.
  • the one or more carboxylic acid can be for instance (or preferably) selected from the list consisting of citric acid, tartaric acid, tannic acid, glycolic acid, malic acid, mandelic acid, oxalic acid, and mixtures thereof;
  • the gelling in step (ii) can be performed by subjecting the initial mixture to a mixing by an ultrasonic probe, preferably for 2 hours or longer; the gelling in step (ii) can be performed at a gelling temperature below a degradation temperature of the one or more carboxylic acids; the one or more carboxylic acids can include (or be) citric acid and the gelling temperature can be 150°C
  • step (iv) the heat treatment of step (iv) can be performed at a temperature of e.g. 1500°C.
  • pure boron carbide (B 4 C) nano-fibers were produced using a non-catalytic sol-gel process.
  • the characterization of the boron carbide nano-fibers were also investigated in terms of their phase content, and their potential in application of supercapacitors as an electrode material was also assessed.
  • the effects of the chemical substances (i.e. starting materials or reactants) used in the method, temperature and time/periods for jelling, pyrolysis and heat treatment steps/operations for the final formation of B 4 C nanofiber on the purity, diameter and the length of the nanofiber are optimised.
  • a novel method which has the ability of composition control in molecular scale and preferential crystal growth without necessitating the assistance of any catalytic metal particles.
  • the method provides homogeneous mixing of reactants at molecular scale, therefore final nano-fibers are fully crystalline boron carbide free from residual carbon or boron oxides.
  • the method enables the formation of boron carbide nanofibers with a diameter within the range between 20 nanometers and 400 nanometers, and with a length within the range between 10.000 nanometers and 300.000 nanometers (i.e. within the range between 10 micrometers and 300 micrometers). Accordingly, the present invention proposes boron carbide nanofibers having a diameter within the range between 20 nanometers and 400 nanometers and a length within the range between 10.000 nanometers and 300.000 nanometers.
  • boron carbide nano-fibers with tunable cross section/length are synthesized.
  • the obtained products can be used in supercapacitors (e.g. for being employed in electrical vehicles with high efficiency), in defence, electronics as well as medicine industries.
  • the present invention further proposes the use of boron carbide nanofibers having a diameter within the range between 20 nanometers and 400 nanometers and a length within the range between 10.000 nanometers and 300.000 nanometers, in supercapacitors.

Abstract

The present invention relates to a method for obtaining boron carbide nanofibers, including the steps of: preparation of an initial mixture comprising one or more carboxylic acids, glycerin and elementary boron nanoparticles; gelling of the initial mixture to obtain a gel; subjecting the gel to a pyrolysis step, to obtain a precursor; performed at a temperature within the range between 450°C and 750°C; and subjecting the precursor to a heat treatment at a temperature within the range between 1450°C and 1550°C, thereby obtaining a product which includes boron carbide nanofibers; and in the method, the initial mixture of step (i) further comprises boric acid particles, or alternatively, the method includes an introduction of boric acid particles along with nano-boron oxide into the precursor obtained in step (iii) prior to the step (iv).

Description

SPECIFICATION
NON-CATALYTIC SOL-GEL METHOD FOR PRODUCTION OF BORON CARBIDE NANOFIBERS
Technical Field of the Invention
The present invention relates to a method for production of boron carbide nanofibers. In particular, the present invention proposes a non-catalytic sol-gel method for obtainment of boron carbide nanofibers.
Background of the Invention
Boron carbide (B4C) nanostructures are considered useful in several technologies including electronics and supercapacitors.
Avcioglu et al. disclose a method for obtaining boron carbide nanopowder (Ceramics International, Volume 46, Issue 11, Part A, 1 August 2020, Pages 17938-17950, DOI: 10.1016/j.ceramint.2020.04.104). In another publication, Avcioglu et al. disclose the obtainment of LDPE composites reinforced with boron carbide nanostructures in the form of powder (Ceramics International, Volume 46 (2020) pp.343-352, DOI:
10.1016/j.ceramint.2019.08.268).
Hadian and Bigdeloo (Journal of Materials Engineering and Performance, Volume 17(1) (2008) pp.44-49, DOI: 10.1007/S11665-007-9125-0) propose a method for obtaining non- fibrous boron carbide nanostructures. In the paper, skilled reader is led away from using elementary boron for economic reasons.
Shawgi et al. disclose a method for obtaining fiber shaped boron carbide powder starting from poly(vinyl alcohol) ("Synthesis of nano particles and fiber-like shape boron carbide powder from ploy (vinyl alcohol) and boric acid" (2006), J Sol-Gel Sci Technol, Springer, DOI: 10.1007/S10971-017-4320-4). Referring to SEM image given in Fig.8(b) of said publication, the structures which are characterized as having fiber-like shape appear to be highly non-linear and have a high length per diameter (L/D ratio) value. Thus, the fiber-like shaped grains obtained with the method described in the publication do not include nanofibers. A high-accuracy, low-cost and energy efficient method for production of boron carbide nanofibers is still sought in the related technical field.
Objects of the Invention
The primary object of the present invention is to overcome the shortcomings in the prior art.
Another object of the present invention is to provide a high-accuracy, low-cost and energy efficient method for production of boron carbide nanofibers.
Summary of the Invention
The present invention relates to a method for obtaining boron carbide nanofibers. The method employs sol-gel technique and does not necessitate the use of catalytic particles. The availed boron carbide nanofibers are characterized and it has been concluded that the nanofibers have a high potential for being employed as electrode material in supercapacitors. Unlike the prior art methods which employ high-cost catalysts such as Co or Ni and carbothermal reactions, the method proposes a low-cost technique. The method allows or includes composition control in molecular level: the reactants can be homogeneously mixed in molecular level, which eliminates formation of residual carbon and boron oxide. The method further allows the optimization of polymeric gel structure, thereby enabling the control of the L/D ratio of boron carbide nanofibers, rendering a high purity and/or crystallinity available with low cost. The resulting boron carbide nanofibers can be employed as electrode material in supercapacitors which are the main feature of high efficiency power storage systems in electric vehicles. The high performance of the product availed by the method proposed hereby is further useful in portable communication systems, aeronautics, unmanned aerial vehicles (UAVs), electronics and medicine.
Brief Description of the Figures
The figure, whose brief explanation is herewith provided, is solely intended for providing a better understanding of the present invention and are as such not intended to define the scope of protection or the context in which said scope is to be interpreted in the absence of the description.
The figure presented for supporting the specification, representatively shows a combination of separate features which are disclosed in the specification; and any alternative combinations of said features which are in consistency with the teaching of the specification are also within the targeted scope of protection related to the present invention.
Fig.l shows boron carbide nanofibers obtained with the method according to the present invention.
Detailed Description of the Invention
Referring to the drawings, short explanations of which being provided above, the present invention is described below in detail. With reference to the above information, the present invention relates to a method for obtaining boron carbide nanofibers. The method can be considered to include a non-catalytic sol-gel process.
The method proposed in the present application includes the following features: i. Preparation of an initial mixture comprising one or more carboxylic acids, glycerin and (nano-) elementary boron; ii. Gelling of the initial mixture, thereby obtaining a gel; iii. Pyrolysis of the gel at a temperature within the range between 450°C and 750°C, thereby obtaining a precursor; iv. Heat treatment at a temperature within the range between 1450°C and 1550°C, thereby obtaining a product which includes boron carbide nanofibers. v. The heat treatment can be preferably followed by physical separation of boron carbide nanofibers out of the product obtained with the heat treatment. With the physical separation, nanofibers are substantially separated based on geometric shape/characteristics from non-fiber boron carbide nanoparticles. The physical separation can preferably include the following steps: dry vibration of the product in an ultrasonic bath, followed by preparation of an aqueous mixture which includes the product, ionized water, one or more alcohols and one or more surfactants, followed by subjecting the aqueous mixture to an ultrasonic probe, followed by centrifugation (for deposition of relatively high particle size structures in the product), followed by vacuum filtration (for filtering relatively high L/D ratio structures (here: nanofibers) and permeating relatively low L/D ratio structures in the product) (preferably using filter papers, more preferably a progressive vacuum filtration using a series of filter papers with gradually decreasing pore sizes), followed by drying.
The initial mixture of step (i) further comprises boric acid particles, or the method includes an introduction of boric acid (H3BO3) particles and nano-boron oxide into the precursor obtained in step (iii) prior to the step (iv).
In step (i), glycerin can be considered to serve as a solvent for said one or more carboxylic acids, and if present, also for boric acid.
In step (ii), the gelling (or "jel ling") can be performed for instance by mixing preferably at an elevated temperature which can be for instance within the range between 80°C and 150°C. For the mixing, the use of an ultrasonic probe is considered advantageous for achieving a high extent of homogeneity in a short time period such as 2 hours. The extent of homogeneity corresponds to a high yield in obtainment of boron carbide as product of the method.
In step (iii), the pyrolysis can be performed for instance by placing the gel into a first vessel (e.g. in the form of a crucible) which can be for instance made of alumina, and by keeping the first vessel in a shuttle kiln heated to a temperature within the range between 450°C and 750°C. The duration of pyrolysis can preferably be 6h or longer.
In step (iv), the heat treatment can be performed under an inert atmosphere (such as Argon). The heat treatment can be performed using a tube furnace, e.g. by placing the precursor into a second vessel (e.g. in the form of a crucible) which can be for instance made of graphite, preferably enveloping the second vessel with carbon paper, and then keeping the second vessel in the tube furnace heated to a temperature within the range between 1450°C and 1550°C. The duration of heat treatment can preferably be 6 hours or longer. Preferably, the initial mixture can be prepared by adding a first mixture comprising boric acid, elementary boron and a first carboxylic acid, with a second mixture comprising glycerin and a second carboxylic acid which is stronger than the first carboxylic acid.
Throughout the method, the molar ratio of carbon to boron used (C/B molar ratio) can be arranged to be within the range between 3: 1 and 5: 1 (for the variation above, in the initial mixture). This molar ratio range eliminates the formation of residual C and B in the product, thereby achieving an enhanced purity and crystallinity.
In a variation of the method proposed in the present application, the initial mixture of step (i) comprises one or more carboxylic acids, glycerin (as solvent for the carboxylic acids) and (nano-) elementary boron. Preferably, boric acid is introduced into the precursor upon the pyrolysis of step (iii), along with boron oxide (B2O3) (preferably, in the form of boron oxide nanoparticles). Thus, in this variation, the boric acid can preferably be excluded/absent in the initial mixture prepared in step (i). The initial mixture of step (i) can include a single carboxylic acid instead of a plurality/variety of carboxylic acids.
In both of the variations described above, components of the initial mixture(s) of step (i) enable the formation of borat-ester and carbon-ester bonds upon gelling of step (ii).
The use of an ultrasonic probe in the gelling of step (ii) provides a nanometer-level homogeneity (in the matrix/network/lattice), thereby enhances/maximizes the efficiency in pyrolysis of step (iii) and heat treatment of step (iv).
Composition of the initial mixture in step (i) affects the geometric characteristics (i.e. length, diameter and L/D ratio) of the boron carbide nanofibers in the product.
The one or more carboxylic acids to be employed in the method according to the present invention can preferably be selected from the list consisting of citric acid, tartaric acid, tannic acid, glycolic acid, malic acid, mandelic acid, oxalic acid, and mixtures thereof.
EXAMPLES
The method as exemplified below can be considered to include a modified sol-gel process, in which the formation of free boron oxide, thereby enabling the synthesis of boron carbide fibers with arrangeable geometric characteristics (in particular, L/D ratio). None of the variations of the method necessitates the use of catalytic particles. The variations of the method enable the obtention of high purity and high crystallinity boron carbide nanofibers.
In both of the variations described in the Example 1 and Example 2 below, components of the initial mixture(s) of step (i) enabled the formation of borat-ester and carbon-ester bonds upon gelling of step (ii). The use of an ultrasonic probe in the gelling of step (ii) provided a nanometer-level homogeneity (in the matrix/network/lattice), thereby enhancing/maximizing the efficiency in pyrolysis of step (iii) and heat treatment of step (iv). It is considered that the composition of the initial mixture in step (i) affected the geometric characteristics (i.e. length, diameter and L/D ratio) of the boron carbide nanofibers obtained with each variation of the method. The one or more carboxylic acids to be employed in the method variations described below were (as preferred) selected from the list consisting of citric acid, tartaric acid, tannic acid, glycolic acid, malic acid, mandelic acid, oxalic acid, and mixtures thereof.
Example 1: i. An "initial mixture" was prepared such that the C/B molar ratio was arranged to be within the range between 3: 1 and 5: 1. With this molar ratio range, the formation of residual carbon and boron (in particular, residual carbon) in the product was eliminated for achieving high purity and crystallinity. The initial mixture comprised one or more carboxylic acids, boric acid (H3BO3), glycerin as common solvent for carboxylic acids and boric acid and (nano-) elementary boron. Preferably, the initial mixture can be prepared by combining a first mixture comprising boric acid, elementary boron and a first carboxylic acid, with a second mixture comprising glycerin and a second carboxylic acid which is stronger than the first carboxylic acid. ii. The initial mixture was subjected to gelling, preferably under heating to a temperature below the degradation temperature of carboxylic acid(s) in the initial mixture. Degradation temperatures information for various carboxylic acids are available to a person skilled in chemistry, and can be exemplified as 230°C for citric acid, 257°C for tartaric acid, 190°C for tannic acid, 100°C for glycolic acid, 225°C for malic acid, 322°C for mandelic acid and 190°C for oxalic acid. The selection of (one or more) carboxylic acid(s) is considered to have an effect in modifying the network structure of the gel obtained upon the gelling step. The gelling was performed by mixing, using an ultrasonic probe; thereby a high extent of homogeneity can be availed. The mixing was continued for a time period of 2 hours. The extent of homogeneity corresponds to a high yield in obtainment of boron carbide as product of the method. Borate ester and carbon ester bonds formed at the gelling step; thus the gelling step resulted in formation of a "gel". iii. The gel was subjected to a pyrolysis step, and thereby the gel was converted into a "precursor". The pyrolysis was performed by subjecting the gel to a temperature within the range between 450°C and 750°C. The temperature used in the pyrolysis step can be selected based on the carboxylic acid(s) employed in the initial mixture: for instance, a relatively high temperature value within said range can be selected for achieving a relatively quick conversion (into precursor) of a gel formed from an initial mixture comprising one or more acids with relatively high degradation temperature(s). An exemplary period of 6 hours for the pyrolysis step corresponds to a quick conversion within the context of the present application. The pyrolysis was performed by placing the gel into a first vessel which was an alumina crucible and by keeping the first vessel for 6 hours in a shuttle kiln heated to the temperature within the range between 450°C and 750°C. iv. The precursor was then subjected to a heat treatment at a temperature within the range between 1450°C and 1550°C, and thereby a "product" is obtained which includes boron carbide nanofibers. The temperature used in the heat treatment step can be selected based on the carboxylic acid(s) employed in the initial mixture; such as in the gelling step (iii) (mutatis mutandis). The heat treatment was performed under an inert atmosphere (here: Argon, fed at a flow rate of e.g. 200 mL/min). Within the context of the present application, the atmosphere being "inert" corresponds to that the precursor is protected from contacting with any reactive gases (such as air oxygen), to avoid the oxidation of the precursor. The heat treatment was performed by placing the precursor into a second vessel which was a graphite crucible enveloped with carbon paper, and then keeping the second vessel in a tube furnace heated to the temperature within the range between 1450°C and 1550°C. The duration of heat treatment was 6h. v. The product was then subjected to physical separation, for separating boron carbide nanofibers out of the product. The physical separation included the following steps: dry vibration of the product in an ultrasonic bath, followed by preparation of an aqueous mixture which includes the product, ionized water, one or more alcohols and one or more surfactants, followed by subjecting the aqueous mixture to an ultrasonic probe, followed by vacuum filtration (preferably using filter papers, more preferably a progressive vacuum filtration using a series of filter papers with gradually decreasing pore sizes), followed by drying.
Example 2:
The method variation of Example 2 was different from the variation described in Example 1 in that:
- The initial mixture of step (i) comprised glycerin as solvent for the carboxylic acid(s) and (nano-) elementary boron. The initial mixture of step (i) further comprised a single carboxylic acid (here: citric acid as an exemplary carboxylic acid) (instead of a plurality/variety of carboxylic acids).
- boric acid (as preferred, in the form of boric acid particles) was introduced into the precursor upon the pyrolysis of step (iii), along with boron oxide (B2O3) (as preferred, in the form of boron oxide nanoparticles). Thus, in this variation, the boric acid was (as preferred) excluded/absent in the initial mixture prepared in step (i). The introduction of boric acid particles and boron oxide (preferably, nanoparticles) was performed by grinding/mixing into the precursor, resulting in a "modified precursor" which equals to a "precursor which contains boric acid particles and boron oxide (preferably, nanoparticles)".
The modified precursor is then subjected to heat treatment as described in step (iv) of Example 1. 1500°C was selected as an exemplary heat treatment temperature. Then, the product of the heat treatment step was subjected to physical separation as described in step (v) of Example 1.
Fig.l shows SEM image of boron carbide nanofibers obtained with the method as described in the Example 2. The method variation described in the Example 1 also results in boron carbide nanofibers which are highly comparable (or equivalent) with those shown in Fig.l. In other words, the present invention proposes a method for obtaining boron carbide nanofibers, including the following steps: i. preparation of an initial mixture comprising one or more carboxylic acids, , glycerin and elementary boron nanoparticles; ii. gelling of the initial mixture to obtain a gel; iii. subjecting the gel to a pyrolysis step, to obtain a precursor; performed at a temperature within the range between 450°C and 750°C; iv. subjecting the precursor to a heat treatment at a temperature within the range between 1450°C and 1550°C, thereby obtaining a product which includes boron carbide nanofibers.
In the method, the initial mixture of step (i) further comprises boric acid particles, or alternatively, the method includes an introduction of boric acid particles along with boron oxide into the precursor obtained in step (iii) prior to the step (iv).
The heat treatment described in step (iv) can be followed by physical separation of boron carbide nanofibers out of the product obtained upon said heat treatment step (iv). The physical separation can include dry vibration of the product in an ultrasonic bath, followed by preparation of an aqueous mixture which includes the product, ionized water, one or more alcohols and one or more surfactants, followed by subjecting the aqueous mixture to an ultrasonic probe, followed by centrifugation, followed by vacuum filtration, followed by drying.
In the variation where the initial mixture of step (i) comprises boric acid particles, in the step (i), the molar ratio of carbon to boron in the initial mixture can be preferably arranged to be within the range between 3: 1 and 5: 1. The initial mixture can preferably be prepared by combining a first mixture comprising boric acid particles, elementary boron nanoparticles and a first carboxylic acid, with a second mixture comprising glycerin and a second carboxylic acid which is stronger than the first carboxylic acid.
In the alternative variation where prior to the step (iv), the method includes an introduction of boric acid particles and boron oxide are into the precursor obtained in step (iii); the molar ratio of carbon to boron obtained upon the introduction of boric acid particles and boron oxide into the precursor, can preferably be arranged to be within the range between 3: 1 and 5: 1. Said introduction can preferably be performed by grinding/mixing of the boron oxide nanoparticles and boric acid into the precursor obtained in step (iii). In this alternative variation, the initial mixture of step (i) can be arranged to comprise a single carboxylic acid.
In any variation of the method according to the present invention, the following options/preferences apply:
- the one or more carboxylic acid can be for instance (or preferably) selected from the list consisting of citric acid, tartaric acid, tannic acid, glycolic acid, malic acid, mandelic acid, oxalic acid, and mixtures thereof;
- the gelling in step (ii) can be performed by subjecting the initial mixture to a mixing by an ultrasonic probe, preferably for 2 hours or longer; the gelling in step (ii) can be performed at a gelling temperature below a degradation temperature of the one or more carboxylic acids; the one or more carboxylic acids can include (or be) citric acid and the gelling temperature can be 150°C
- the heat treatment of step (iv) can be performed at a temperature of e.g. 1500°C.
With the method variations described in the present specification, pure boron carbide (B4C) nano-fibers were produced using a non-catalytic sol-gel process. The characterization of the boron carbide nano-fibers were also investigated in terms of their phase content, and their potential in application of supercapacitors as an electrode material was also assessed. The effects of the chemical substances (i.e. starting materials or reactants) used in the method, temperature and time/periods for jelling, pyrolysis and heat treatment steps/operations for the final formation of B4C nanofiber on the purity, diameter and the length of the nanofiber are optimised. In the present application, a novel method is developed, which has the ability of composition control in molecular scale and preferential crystal growth without necessitating the assistance of any catalytic metal particles. The method provides homogeneous mixing of reactants at molecular scale, therefore final nano-fibers are fully crystalline boron carbide free from residual carbon or boron oxides.
It is observed that the method enables the formation of boron carbide nanofibers with a diameter within the range between 20 nanometers and 400 nanometers, and with a length within the range between 10.000 nanometers and 300.000 nanometers (i.e. within the range between 10 micrometers and 300 micrometers). Accordingly, the present invention proposes boron carbide nanofibers having a diameter within the range between 20 nanometers and 400 nanometers and a length within the range between 10.000 nanometers and 300.000 nanometers.
By optimizing the polymeric gel structure, boron carbide nano-fibers with tunable cross section/length are synthesized. The obtained products can be used in supercapacitors (e.g. for being employed in electrical vehicles with high efficiency), in defence, electronics as well as medicine industries. Accordingly, the present invention further proposes the use of boron carbide nanofibers having a diameter within the range between 20 nanometers and 400 nanometers and a length within the range between 10.000 nanometers and 300.000 nanometers, in supercapacitors.

Claims

CLAIMS Boron carbide nanofibers having a diameter within the range between 20 nanometers and 400 nanometers and a length within the range between 10.000 nanometers and 300.000 nanometers. A method for obtaining boron carbide nanofibers, including the following steps: i. preparation of an initial mixture comprising one or more carboxylic acids, glycerin and elementary boron nanoparticles; ii. gelling of the initial mixture to obtain a gel; iii. subjecting the gel to a pyrolysis step, to obtain a precursor; performed at a temperature within the range between 450°C and 750°C; iv. subjecting the precursor to a heat treatment at a temperature within the range between 1450°C and 1550°C, thereby obtaining a product which includes boron carbide nanofibers; and either the initial mixture of step (i) further comprises boric acid particles, or the method includes an introduction of boric acid particles and nano-boron oxide into the precursor obtained in step (iii) prior to the step (iv). The method according to the claim 2, wherein the heat treatment described in step (iv) is followed by physical separation of boron carbide nanofibers out of the product obtained upon said heat treatment step (iv). The method according to the claim 3, wherein the physical separation includes dry vibration of the product in an ultrasonic bath, followed by preparation of an aqueous mixture which includes the product, ionized water, one or more alcohols and one or more surfactants, followed by subjecting the aqueous mixture to an ultrasonic probe, followed by centrifugation, followed by vacuum filtration, followed by drying. 5. The method according to any of the claims 2 to 4, wherein the initial mixture of step (i) comprises boric acid particles, and the molar ratio of carbon to boron in the initial mixture is arranged to be within the range between 3: 1 and 5: 1.
6. The method according to the claim 5, wherein in the step (i), the initial mixture is prepared by combining a first mixture comprising boric acid particles, elementary boron nanoparticles and a first carboxylic acid, with a second mixture comprising glycerin and a second carboxylic acid which is stronger than the first carboxylic acid.
7. The method according to any of the claims 2 to 4, wherein prior to the step (iv), the method includes an introduction of boric acid particles and boron oxide are into the precursor obtained in step (iii); the molar ratio of carbon to boron obtained upon the introduction of boric acid particles and boron oxide into the precursor, is arranged to be within the range between 3: 1 and 5: 1.
8. The method according to the claim 7, wherein said introduction is performed by grinding/mixing of the boron oxide nanoparticles and boric acid into the precursor obtained in step (iii).
9. The method according to any of the claims 7 or 8, wherein the initial mixture of step (i) is arranged to comprise a single carboxylic acid.
10. The method according to any of the claims 2 to 9, wherein the one or more carboxylic acid is selected from the list consisting of citric acid, tartaric acid, tannic acid, glycolic acid, malic acid, mandelic acid, oxalic acid, and mixtures thereof. -14-
11. The method according to any of the claims 2 to 10, wherein the gelling in step (ii) is performed by subjecting the initial mixture to a mixing by an ultrasonic probe, preferably for 2 hours or longer. 12. The method according to the claim 11, wherein the gelling in step (ii) is performed at a gelling temperature below a degradation temperature of the one or more carboxylic acids.
13. The method according to the claim 12, wherein the one or more carboxylic acids include citric acid and the gelling temperature is 150°C.
14. The method according to any of the claims 2 to 13, wherein the heat treatment of step (iv) is performed at a temperature of 1500°C. 15. Use of boron carbide nanofibers having a diameter within the range between 20 nanometers and 400 nanometers and a length within the range between 10.000 nanometers and 300.000 nanometers, in a supercapacitor.
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