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WO2013107840A1 - Method of manufacturing microscopic graphene-containing grains and material obtainable thereby - Google Patents

Method of manufacturing microscopic graphene-containing grains and material obtainable thereby Download PDF

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Publication number
WO2013107840A1
WO2013107840A1 PCT/EP2013/050889 EP2013050889W WO2013107840A1 WO 2013107840 A1 WO2013107840 A1 WO 2013107840A1 EP 2013050889 W EP2013050889 W EP 2013050889W WO 2013107840 A1 WO2013107840 A1 WO 2013107840A1
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Prior art keywords
graphene
granular
vol
metal
copper
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PCT/EP2013/050889
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French (fr)
Inventor
Wlodzimierz Strupinski
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Isos Technologies Sarl
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Priority claimed from PL397832A external-priority patent/PL397832A1/en
Priority claimed from PL397831A external-priority patent/PL397831A1/en
Application filed by Isos Technologies Sarl filed Critical Isos Technologies Sarl
Publication of WO2013107840A1 publication Critical patent/WO2013107840A1/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
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
    • 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
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C01B32/184—Preparation
    • C01B32/186—Preparation by chemical vapour deposition [CVD]
    • 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
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
    • C01B32/194—After-treatment

Definitions

  • the present invention relates to a method of manufacturing microscopic graphene-containing grains.
  • the method is based on metal or metallic alloy granulate processing by Chemical Vapor Deposition (CVD) .
  • the invention relates also to material obtainable by this method, in particular graphene powder or graphene-covered metal or metallic alloy granules, especially granules of copper, nickel or aluminum (CuGP, NiGP or AIGP) .
  • Gra phene is a flat two-dimensional sheet of hexagonally arranged carbon atoms [K.S. Novoselov, et al. Science 306,666 (2004), A.K. Geim, K.S. Novoselov, Nat. Mat. 6 (2007) 183, Y.B.Zhang, Y. W.Jan, H.L.Stormer, and P.Kim, Nature 438, 20 1 (2005)] having characteristics of a semi-metal.
  • the exceptional electron properties of graphene and its high chemical stability make it a particularly attractive candidate for future electronics [Novoselov K.S., Geim A.K., Nature Materials 6, 183 (2007)] .
  • Graphene can be obtained by several methods. First of them, which was developed by K.S. Novoselov and A.K. Geim, entails detaching small flakes of graphite from a graphite block with a Scotch tape. Disadvantages of the method include the following: a substantially small size of obtained fla kes, which varies between a few hu ndred a nd one thousa nd sq uare micrometers, a significantly low efficiency of the flakes selection process, which is performed by hand and, consequently, requires high cost. Therefore, the method is inapplicable to electronics industry.
  • WO 2012021677 discloses a CVD process conducted on a copper foil cut into small pieces.
  • a "small" piece according to WO 2012021677 means a piece having the surface area of several cm 2 . It has neither been disclosed nor suggested to use metallic microgranulate as the substrate for CVD process for producing graphene.
  • the authors of the present invention have unexpectedly discovered that conducting a CVD process on metal or metallic alloy microgranulate (i.e. granular metal, in particular copper, nickel or aluminum, having the grain size of the order of single micrometers or hundreds of nanometers) leads to a useful product (graphene-covered metal micro-grains), which can be used as a raw material in some applications.
  • metal or metallic alloy microgranulate i.e. granular metal, in particular copper, nickel or aluminum, having the grain size of the order of single micrometers or hundreds of nanometers
  • graphene-covered metal micro-grains which can be used as a raw material in some applications.
  • graphene-covered metal micro-grains graphene-covered metal micro-grains
  • Graphene powder is usually manufactured using modified Hummer's method by graphite oxidation, fragmentation of the oxidized graphite into small flakes and reduction of the oxidized graphene to the form of graphene powder [Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958:80: 1339]. Obtaining platelets of the thickness as low as a single carbon atoms layer poses a problem. It has neither been disclosed nor suggested to produce graphene powder using the CVD technique.
  • the aim of the present invention is to provide a method of manufacturing microscopic graphene-containing grains.
  • the material obtainable by this method comprises graphene-covered metal or metallic alloy granules, especially granules of copper, nickel or aluminum (CuGP, NiGP or AIGP).
  • the inventive method utilizes the CVD process for deposition of carbon atoms on metallic substrate.
  • granular copper, granular nickel or granular aluminum of the grain size of the order of single micrometers or submicrometers (hundreds of nanometers) were used as the substrate, instead of a foil or other form of metal.
  • other metal granulates or alloy granulates, particularly Cu-Ni granulate, can be used, as long as the dimensions of single grains are of the order of microns.
  • the method of manufacturing microscopic graphene-containing grains is characterized in that it comprises the following steps: a) metal or metallic alloy granulate, preferably granular copper, granular nickel or granular aluminum, the grain size of which is between 0.1 m and ⁇ ⁇ , preferably between ⁇ ⁇ and ⁇ , is heated up to the temperature between 600°C and 1040°C, preferably between 920°C and 1040°C, and is held at this temperature for 2 to 30 minutes, preferably for 2 to 20 minutes, under an atmosphere comprising argon and propane, optionally with an addition of hydrogen and/or nitrogen, b) material obtained as a result of the stage a) is cooled to room temperature, preferably in argon atmosphere, preferably with optimized cooling rate.
  • the gas flow and pressure, the precursor ratio, temperature, and other process parameters of the step a) must be selected and optimized.
  • the time of carbonization has to be optimized according to the reactor (owen) construction a nd elongated in the case of bigger loadings req uiring granulate displacement, as a result of which it stays in better contact with propane.
  • the metal granulate is pre-processed before the step a) by annealing in an Ar/h mixture. If copper or nickel granulate is used, the preprocessing is preferably conducted in the temperature range between 900 and 1070C°C for 10-60min or longer, in the atmosphere of hydrogen, argon or nitrogen mixture.
  • Additional plasma assist and/or hydrocarbon sources activated in microwave plasma may be applied during step a), preferably with a plasma generator having a defined max voltage of 800V, current of 2.5A, power of 1 kW and frequency variable between 1 kHz and 100kHz.
  • Plasma generator with defined max voltage (preferably 800V), current (preferably 2.5A), power (preferably I kW), frequency (preferably variable 1 - 100kHz), with simultaneous control of voltage, current, power and frequency, is helpful to enable decomposition of hydrocarbon being a carbon source in the process of aluminum grains surface carbonization in the temperature below aluminum melting point.
  • max voltage preferably 800V
  • current preferably 2.5A
  • power preferably I kW
  • frequency preferably variable 1 - 100kHz
  • hydrocarbon sources activated in microwave plasma enable the carbonization process in the temperature below the thermal pyrolysis of the given hydrocarbon.
  • Plasma enhancement is utilized due to the impossibility of obtaining temperature high enough for thermal decomposition of the source.
  • granular aluminum is used as the metal granulate and said granular aluminum is heated up to the temperature between 600 and 660°C during step a).
  • the temperature range in the step a) must be held below aluminum melting point, i.e. between 600°C and 660°C. Therefore, if gra n u lar a lu min u m is used as the m eta l gra n u late, the aforementioned additional plasma assist and/or hydrocarbon sources activated in microwave plasma are necessary during step a), to enable decomposition of hydrocarbon being a carbon source in the process of aluminum grains surface carbonization in the temperature below aluminum melting point.
  • granular copper or granular nickel is used as the metal granulate and said metal granulate is heated up to the temperature between 920°C and 1040°C during step a).
  • the inventive method additionally comprises an additional step: c) granular copper or granular nickel is etched by chemical means, in particular comprising an aq ueous solution of a ny of the following: nitric acid, hydrochloric acid, ferric chloride, ammonium persulphate, preferably on a sieve, and the thus obtained material is strained or filtered.
  • chemical means in particular comprising an aq ueous solution of a ny of the following: nitric acid, hydrochloric acid, ferric chloride, ammonium persulphate, preferably on a sieve, and the thus obtained material is strained or filtered.
  • the etching means listed here explicitly are only given as examples. Actually, there are numerous etchants of copper or nickel well known ⁇ o those skilled in art. Any of such etchants is suitable to be used in the present method.
  • the inventive method preferably additionally comprises an additional step: d) oxidation and/or reduction of the material obtained as the result of step c).
  • oxidants and reductors well known to those skilled in art. Any of such oxidants and reductors is suitable to be used in the present method.
  • the step c) of etching is carried out with an aqueous solution of nitric acid having the concentration from 1 % vol. to 50% vol., preferably 1 ⁇ % vol. or 32% vol., or with an aqueous solution of hydrochloric acid having the concentration from 1 % vol. to 50% vol., preferably 4% vol., 9% vol. or 18% vol., or with an aqueous solution of ferric chloride having the concentration from 0.1 M to 5M, preferably 1 M, or with an aqueous solution of ammonium persulphate having the concentration from 0.05M to 1 M, preferably 0.1 M.
  • the invention relates also to microscopic graphene-containing grains obtainable according to the inventive method when no etching step is applied.
  • These grains comprise granules of metal or metallic alloy, preferably granules of copper, nickel or aluminum, with one or several layers of graphene on the surface of the granules, wherein the size of the grains is from 0.1 ⁇ ⁇ to ⁇ ⁇ , preferably from 0.1 ⁇ to ⁇ ⁇ .
  • the invention covers also microscopic graphene-containing grains obtainable according to the inventive method when etching step is applied.
  • These grains comprise graphene powder, wherein the size of the grains is from 0.1 to ⁇ ⁇ , preferably from 1 to 5 ⁇ , more preferably 1 to 2 ⁇ .
  • granular metal e.g. copper, nickel or granular metallic alloy ("granulate"), of the grain size of the order of single micrometers or submicrometers (hundreds of nanometers) is used with a view to manufacturing graphene.
  • the granulate is heated up to the temperature of at least 600°C (in case of aluminum), or at least 920°C, even more preferably at least 1000°C (in case of copper or other metals/alloys) and then is held at an argon and propane atmosphere for a given period of time, e.g.
  • Plasma assist in graphene growth enables application of granular aluminum instead of granular copper for similar purposes.
  • granular aluminum In the case of granular aluminum, it is heated up to the temperature not lower than 600°C, however - at the same time - not higher than 660°C, which is the aluminum melting temperature.
  • the granular aluminum is held at this temperature for a given period of time, e.g. 1 0 minutes (the time may vary from several to several dozens of minutes; the time is elongated in the case of bigger loadings requiring granulate displacement, as a result of which it stays in better contact with propane) .
  • Hydrocarbon sources activated in microwave plasma enable the carbonization process in the temperature below the thermal pyrolysis of the given hydrocarbon.
  • Plasma enhancement is utilized due to the impossibility of obtaining temperature high enough for thermal decomposition of the sou rce .
  • the tem peratu re sha l l not exceed the aluminum melting temperature (660°C) .
  • aluminum one may also modify the properties of the material metallurgically manufactured from granular Al- graphene.
  • other metal granulates or alloy granulates, particularly Ni or Cu-Ni granulate, can be used and plasma enhancement can be utilized, as well.
  • the metal-graphene grains obtained by the above-described method are subjected to additional step of etching.
  • the grains are preferably placed on a sieve and are subsequently etched in an aqueous solution of nitric acid, hydrochloric acid or ferric chloride. After performing a complete metal (in particular: copper) etching, graphene powder remains on the sieve.
  • Other known methods of straining and filtration of the suspension etc. can also be applied.
  • copper granulate In em bodiments of the inventive method of gra phene powder manufacturing, typically copper granulate was used. Copper is the most preferred metal to be used in the process of obtaining graphene powder, because it is the cheapest and the etching process is the easiest in case of copper. Therefore, copper microgranules on which carbon growth at 950 - 1000°C is performed are used for the most part. More specifically, at such temperature dissolution of carbon in Cu takes place, and subsequently as a result of cooling a thin layer of carbon appears on the surface of the copper. Due to low solubility of carbon in copper it is a process enabling self-control of the thickness of the obtained carbon layer, the value of which is close to that of a single atomic layer.
  • the thus obtained graphene powder comprises graphene flakes of the thickness of one monolayer.
  • a SEM image of such a flake is presented in Fig. 1 .
  • the thus obtained graphene powder can be oxidized and/or reduced applying different methods.
  • the graphene flake size is dependent on the grain size of metal-graphene grains, in particular - CuGP grains.
  • Epitaxial CVD graphene growth on metals substrates uses the atomic structure of a metal surface to seed the growth of the graphene. Copper appears to be the most promising for CVD because the resulting graphene is primarily monolayer.
  • the graphene growth on metal (Cu) grains is performed in CVD reactor (owen).
  • a typical CVD process of graphene synthesis on Cu starts with the annealing of metal granulates in order to reduce oxides on the surface in the temperature range between 900 and 1070C°C during 10-60min or longer in the atmosphere of hydrogen, argon or nitrogen mixture.
  • the nucleation and growth of graphene occurs by exposure of the metal surface to a hydrocarbon gas under low pressure, in the range 20-200mbar.
  • the carbonization of individual metal (copper) grains starts in the temperature between 920 and 1040°C. Reactive carbon species are being produced by thermal or/and plasma assist decomposition of hydrocarbon gas resulting in carbon atoms diffusion into the metal.
  • Plasma generator with defined max voltage (preferably 800V), current (preferably 2.5A), power (preferably I kW), frequency (preferably variable 1 - 100kHz), with simultaneous control of voltage, current, power and frequency, is used to enable decomposition of hydrocarbon being a carbon source in the process of aluminum grains surface carbonization in the temperature below aluminum melting point.
  • max voltage preferably 800V
  • current preferably 2.5A
  • power preferably I kW
  • frequency preferably variable 1 - 100kHz
  • the flow of hydrocarbon depends on reactor capacity, for the reactor used here - in the range of 0.5-20ml/min.
  • the carrier gas is argon or the mixture of argon or/and hydrogen or/and nitrogen.
  • the solubility of carbon in a metal increases with temperature. Subsequently, the metal grains are cool down to the room temperature. Some of carbon atoms dissolved in a metal at high temperature ca n precipitate as a graphitic film u pon cooling therefore the time of carbonization process has to optimized, usually in the range of 2-20min. Cooling down rate should be optimized, as well.
  • graphene as a two-dimensional one-atom-thick sheet of carbon covers the whole surface of each metal (copper) grain.
  • the grains of applied metal (copper) depends on final application and may vary from 0.1 tol OO micrometers or higher.
  • Example 1 The procedures described in Example 1 are essentially repeated, however, an aluminum microgranulate, having the grain size from 0.1 to ⁇ ⁇ is used instead of copper. Consequently, the annealing of aluminum granulates in order to reduce oxides on the surface is carried out in the temperature range between 600 and 660°C for 10-60min or longer in the atmosphere of hydrogen, argon or nitrogen mixture.
  • the graphene growth on metal (Al) grains is performed in CVD reactor (owen) .
  • the nucleation and growth of graphene occurs by exposure of the metal surface to a hydrocarbon gas under low pressure, in the range 20-200mbar.
  • the carbonization of individual metal (aluminum) grains occurs in the temperature between 600 and 660°C.
  • reactive carbon species have to be produced by thermal or/and plasma assist decomposition of hydrocarbon gas resulting in carbon atoms diffusion into the metal.
  • Plasma generator with defined max voltage (preferably 800V), current (preferably 2.5A), power (preferably I kW), frequency (preferably variable 1 - 100kHz), with simultaneous control of voltage, current, power and frequency, is used to enable decomposition of hydrocarbon being a carbon source in the process of aluminum grains surface carbonization in the temperature below aluminum melting point.
  • Such generators are well known in the art.
  • the flow of hydrocarbon depends on reactor capacity, for the reactor used here - in the range of 0.5-20ml/min.
  • the carrier gas is argon or the mixture of argon or/and hydrogen or/and nitrogen.
  • the solubility of carbon in a metal increases with temperature. Subsequently, fhe metal grains are cool down to fhe room temperature. Some of carbon atoms dissolved in a metal af high temperature can precipitate as a graphitic film upon cooling therefore the time of carbonization process has to optimized, usually in the range of 2-20min. Cooling down rate should be optimized, as well.
  • graphene as a two-dimensional one-afom-thick sheet of carbon covers fhe whole surface of each metal (aluminum) grain.
  • the grains of applied metal (aluminum) depends on final application and may vary from 0.1 fol OO micrometers or higher.
  • Example 1 The product obtained according to Example 1 (i.e. graphene-covered copper granules), after cooling down fo room temperature, was subjected fo chemical efching.
  • efching means, an aqueous solution of nitric acid, hydrochloric acid, ferric chloride or ammonium persulphate, have been tested, each of fhem providing very good results.
  • nitric acid, hydrochloric acid, ferric chloride or ammonium persulphate have been tested, each of fhem providing very good results.
  • nitric acid, hydrochloric acid, ferric chloride or ammonium persulphate have been tested, each of fhem providing very good results.
  • concentrations of fhe substances/solutions used for etching are not particularly limited insofar fhey provide the effect of efching copper. Therefore, the concentrations may vary in a very broad range, well known to those skilled in art for each of typical etchanfs of copper.
  • concentration of efchant influences the efching rafe, i.e. fhe efching rafe if higher for higher concentrations of efchant. Only by way of example, it may be mentioned that the inventors have successfully tested etching of graphene-covered copper granules with an aqueous solution of nitric acid having the concentration of 1 6% vol .
  • the etching step was performed on a sieve, which is preferred although not necessary, and the thus obtained material was strained or filtered.
  • the etching step was ended with rinsing the sieve with water, which is preferred although not necessary.
  • Copper is the most preferred metal to be used in the process of obtaining graphene powder, because it is the cheapest and the etching process is the easiest in case of copper.
  • similar procedure can be carried out with other metal or metallic alloy microgranulates, in particular with nickel.
  • the graphene flake presented in the figure 1 was obtained by the growth of graphene on copper grain with the average lateral size of 3-5 ⁇ a nd by etc hing ou t th e co p per in a q u eou s so l utio n of a m m oniu m persulphate having the concentration of 0.1 M.
  • the carbon flake was characterized by Scanning Electron Microscopy which revealed the graphene nature of the platelet - two-dimension form. The estimated thickness is 1 -2 carbon atomic layers and the size approximately 2 ⁇ 2 ⁇ .
  • the thus obtained graphene powder may be further subjected to an additional step of oxidation and/or reduction of the material obtained as the result of etching.
  • oxidants and reductors well known to those skilled in art. Any of such oxidants and reductors is suitable to be used in the present method.
  • metallic granules in particular copper granules, covered with graphene may be obtained.
  • Such material is a raw product suitable for further processing. It may be applied in different technical fields. For example, it has metallurgic applications - for manufacturing copper-graphene electrical cables, electric clutch plates, composite materials, sinters etc. Substantial quantity of such material can be obtained by the inventive method.
  • graphene powder is obtained.
  • Such graphene powder may be used e.g . in supercapacitors, graphene ink, printed conductive paths, nano- composites, electrodes, absorbers, anticorrosion covers and others fields.
  • the method according to the present invention enables manufacturing a large amount of graphene powder in relatively simple and inexpensive devices, the loading of which is even up to many kilograms of granulate. Copper (or other metal) can be electrolytically recovered from the solution and processed into granulate for reuse.
  • the quality of graphene platelets is very high, i.e. the thickness is 1 -2 monoatomic layers which is difficult to obtain by chemical methods.

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Abstract

The present invention relates to a method of manufacturing microscopic graphene-containing grains, characterized in that it comprises the following steps: a) metal or metallic alloy granulate, preferably granular copper, granular nickel or granular aluminum, the grain size of which is between 0.1 pm and 1000μm, preferably between 1μm and 6μm, is heated up to the temperature between 600°C and 1040°C, preferably between 920°C and 1040°C, and is held at this temperature for 2 to 30 minutes, preferably for 2 to 20 minutes, under an atmosphere comprising argon and propane, optionally with an addition of hydrogen and/or nitrogen, b) material obtained as a result of the stage a) is cooled to room temperature, preferably in argon atmosphere, preferably with optimized cooling rate. The invention covers also microscopic graphene-containing grains obtainable according to the inventive process, in particular - graphene powder or metal granules covered with graphene.

Description

Method of manufacturing microscopic graphene-containing grains and material obtainable thereby
The present invention relates to a method of manufacturing microscopic graphene-containing grains. The method is based on metal or metallic alloy granulate processing by Chemical Vapor Deposition (CVD) . The invention relates also to material obtainable by this method, in particular graphene powder or graphene-covered metal or metallic alloy granules, especially granules of copper, nickel or aluminum (CuGP, NiGP or AIGP) .
Gra phene is a flat two-dimensional sheet of hexagonally arranged carbon atoms [K.S. Novoselov, et al. Science 306,666 (2004), A.K. Geim, K.S. Novoselov, Nat. Mat. 6 (2007) 183, Y.B.Zhang, Y. W.Jan, H.L.Stormer, and P.Kim, Nature 438, 20 1 (2005)] having characteristics of a semi-metal. The exceptional electron properties of graphene and its high chemical stability make it a particularly attractive candidate for future electronics [Novoselov K.S., Geim A.K., Nature Materials 6, 183 (2007)] . Carriers mobility in graphene is significantly high, reaching up to 200000cm2/Vs, which is much higher than in the case of silicon tra nsistors [Lin Y.M. et al, Science 327, 662 (20 1 0)] . In addition, current density in graphene stays over 100 times higher than in copper (108 A/cm2) [M. Wilson, Phys.Today, p.21 (Jan.2006)] . Moreover, graphene has particula rly g ood th erm a l properties a n d exce ption a l mechanical strength.
Graphene can be obtained by several methods. First of them, which was developed by K.S. Novoselov and A.K. Geim, entails detaching small flakes of graphite from a graphite block with a Scotch tape. Disadvantages of the method include the following: a substantially small size of obtained fla kes, which varies between a few hu ndred a nd one thousa nd sq uare micrometers, a significantly low efficiency of the flakes selection process, which is performed by hand and, consequently, requires high cost. Therefore, the method is inapplicable to electronics industry.
Other methods enable obtaining graphene in the form of one or two carbon atoms layers on SiC substrates or on Cu, Ni, Ru, Ir, Co, Pt, Pd and W metallic substrates. This is mainly carried out by the process of Chemical Vapor Deposition (CVD) on a metallic substrate. For the known CVD techniques, the term "metallic substrate" covers many different forms of metal, including continuous layers, optionally deposited on a semiconductor wafer, foils, or even melted metal. A typical example of such a CVD process, conducted on a semiconductor wafer with a copper layer on its surface is disclosed in the Chinese patent application no. CN 102229420 (A).
The international patent application no. WO 2012021677 discloses a CVD process conducted on a copper foil cut into small pieces. However, a "small" piece according to WO 2012021677 means a piece having the surface area of several cm2. It has neither been disclosed nor suggested to use metallic microgranulate as the substrate for CVD process for producing graphene.
The authors of the present invention have unexpectedly discovered that conducting a CVD process on metal or metallic alloy microgranulate (i.e. granular metal, in particular copper, nickel or aluminum, having the grain size of the order of single micrometers or hundreds of nanometers) leads to a useful product (graphene-covered metal micro-grains), which can be used as a raw material in some applications. In particular, it has been discovered that by etching this metallic material, a graphene powder can be obtained.
As far as obtaining graphene powder is concerned, apart from the methods of obtaining graphene in the form of layers, chemical methods of manufacturing small carbon flakes of atomic thickness referred to as NGP, i.e. nano-g ra p h e n e p l a te l ets i n t h e f o rm of g ra p h e n e p owd e r we re independently established. The area of application of such powder predominantly includes supercapacitors, graphene ink, printed conductive paths, nano-composites components, electrodes on polymer foils and many others. Graphene powder is usually manufactured using modified Hummer's method by graphite oxidation, fragmentation of the oxidized graphite into small flakes and reduction of the oxidized graphene to the form of graphene powder [Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958:80: 1339]. Obtaining platelets of the thickness as low as a single carbon atoms layer poses a problem. It has neither been disclosed nor suggested to produce graphene powder using the CVD technique.
Hence, the aim of the present invention is to provide a method of manufacturing microscopic graphene-containing grains. In particular, in one of the preferred embodiments of the invention, the material obtainable by this method comprises graphene-covered metal or metallic alloy granules, especially granules of copper, nickel or aluminum (CuGP, NiGP or AIGP).
It is another aim of the present invention to provide a method of manufacturing of graphene powder (also called graphene nano-platelets) .
The inventive method utilizes the CVD process for deposition of carbon atoms on metallic substrate. However, in the case of the invention, granular copper, granular nickel or granular aluminum of the grain size of the order of single micrometers or submicrometers (hundreds of nanometers) were used as the substrate, instead of a foil or other form of metal. Alternatively, other metal granulates or alloy granulates, particularly Cu-Ni granulate, can be used, as long as the dimensions of single grains are of the order of microns.
According to the present invention, the method of manufacturing microscopic graphene-containing grains is characterized in that it comprises the following steps: a) metal or metallic alloy granulate, preferably granular copper, granular nickel or granular aluminum, the grain size of which is between 0.1 m and Ι ΟΟΟμιτι, preferably between Ι μπτι and όμιτι, is heated up to the temperature between 600°C and 1040°C, preferably between 920°C and 1040°C, and is held at this temperature for 2 to 30 minutes, preferably for 2 to 20 minutes, under an atmosphere comprising argon and propane, optionally with an addition of hydrogen and/or nitrogen, b) material obtained as a result of the stage a) is cooled to room temperature, preferably in argon atmosphere, preferably with optimized cooling rate.
The gas flow and pressure, the precursor ratio, temperature, and other process parameters of the step a) must be selected and optimized. The time of carbonization has to be optimized according to the reactor (owen) construction a nd elongated in the case of bigger loadings req uiring granulate displacement, as a result of which it stays in better contact with propane.
Preferably, the metal granulate is pre-processed before the step a) by annealing in an Ar/h mixture. If copper or nickel granulate is used, the preprocessing is preferably conducted in the temperature range between 900 and 1070C°C for 10-60min or longer, in the atmosphere of hydrogen, argon or nitrogen mixture.
Additional plasma assist and/or hydrocarbon sources activated in microwave plasma may be applied during step a), preferably with a plasma generator having a defined max voltage of 800V, current of 2.5A, power of 1 kW and frequency variable between 1 kHz and 100kHz.
Plasma generator with defined max voltage (preferably 800V), current (preferably 2.5A), power (preferably I kW), frequency (preferably variable 1 - 100kHz), with simultaneous control of voltage, current, power and frequency, is helpful to enable decomposition of hydrocarbon being a carbon source in the process of aluminum grains surface carbonization in the temperature below aluminum melting point. Such generators are well known in the art.
Similarly, hydrocarbon sources activated in microwave plasma enable the carbonization process in the temperature below the thermal pyrolysis of the given hydrocarbon. Plasma enhancement is utilized due to the impossibility of obtaining temperature high enough for thermal decomposition of the source.
In one of the preferred embodiments of the inventive method, granular aluminum is used as the metal granulate and said granular aluminum is heated up to the temperature between 600 and 660°C during step a).
In case of granular aluminum the temperature range in the step a) must be held below aluminum melting point, i.e. between 600°C and 660°C. Therefore, if gra n u lar a lu min u m is used as the m eta l gra n u late, the aforementioned additional plasma assist and/or hydrocarbon sources activated in microwave plasma are necessary during step a), to enable decomposition of hydrocarbon being a carbon source in the process of aluminum grains surface carbonization in the temperature below aluminum melting point.
In another preferred embodiment of the inventive method, granular copper or granular nickel is used as the metal granulate and said metal granulate is heated up to the temperature between 920°C and 1040°C during step a).
In such case, the inventive method additionally comprises an additional step: c) granular copper or granular nickel is etched by chemical means, in particular comprising an aq ueous solution of a ny of the following: nitric acid, hydrochloric acid, ferric chloride, ammonium persulphate, preferably on a sieve, and the thus obtained material is strained or filtered. The etching means listed here explicitly are only given as examples. Actually, there are numerous etchants of copper or nickel well known †o those skilled in art. Any of such etchants is suitable to be used in the present method.
Moreover, in case that the step c) of etching is applied, the inventive method preferably additionally comprises an additional step: d) oxidation and/or reduction of the material obtained as the result of step c). There are numerous oxidants and reductors, well known to those skilled in art. Any of such oxidants and reductors is suitable to be used in the present method.
Preferably, the step c) of etching is carried out with an aqueous solution of nitric acid having the concentration from 1 % vol. to 50% vol., preferably 1 ό% vol. or 32% vol., or with an aqueous solution of hydrochloric acid having the concentration from 1 % vol. to 50% vol., preferably 4% vol., 9% vol. or 18% vol., or with an aqueous solution of ferric chloride having the concentration from 0.1 M to 5M, preferably 1 M, or with an aqueous solution of ammonium persulphate having the concentration from 0.05M to 1 M, preferably 0.1 M.
The invention relates also to microscopic graphene-containing grains obtainable according to the inventive method when no etching step is applied. These grains comprise granules of metal or metallic alloy, preferably granules of copper, nickel or aluminum, with one or several layers of graphene on the surface of the granules, wherein the size of the grains is from 0.1 μ ιτΊ to Ι ΟΟΟμιτΊ, preferably from 0.1 μιτι to Ι ΟΟμιτι.
The invention covers also microscopic graphene-containing grains obtainable according to the inventive method when etching step is applied. These grains comprise graphene powder, wherein the size of the grains is from 0.1 to Ι ΟμιτΊ, preferably from 1 to 5μιτι, more preferably 1 to 2μιτι. Detailed description of the invention
According †o the invention, granular metal e.g. copper, nickel or granular metallic alloy ("granulate"), of the grain size of the order of single micrometers or submicrometers (hundreds of nanometers) is used with a view to manufacturing graphene. The granulate is heated up to the temperature of at least 600°C (in case of aluminum), or at least 920°C, even more preferably at least 1000°C (in case of copper or other metals/alloys) and then is held at an argon and propane atmosphere for a given period of time, e.g. 10 minutes (the time may vary from several to several dozens of minutes; the time is elongated in the case of bigger loadings req uiring gra n u late displacement, as a result of which it stays in better contact with propane) . In consequence, carbon dissolves at the surface area of grains and after cooling appears in the form of a mono-atomic layer of graphene. The size of grains depends on the size of metallic (in particular: copper) grains.
Plasma assist in graphene growth enables application of granular aluminum instead of granular copper for similar purposes. In the case of granular aluminum, it is heated up to the temperature not lower than 600°C, however - at the same time - not higher than 660°C, which is the aluminum melting temperature. The granular aluminum is held at this temperature for a given period of time, e.g. 1 0 minutes (the time may vary from several to several dozens of minutes; the time is elongated in the case of bigger loadings requiring granulate displacement, as a result of which it stays in better contact with propane) . Hydrocarbon sources activated in microwave plasma enable the carbonization process in the temperature below the thermal pyrolysis of the given hydrocarbon. Plasma enhancement is utilized due to the impossibility of obtaining temperature high enough for thermal decomposition of the sou rce . The tem peratu re sha l l not exceed the aluminum melting temperature (660°C) . For aluminum one may also modify the properties of the material metallurgically manufactured from granular Al- graphene. Alternatively, other metal granulates or alloy granulates, particularly Ni or Cu-Ni granulate, can be used and plasma enhancement can be utilized, as well.
In order to obtain graphene powder, the metal-graphene grains obtained by the above-described method, are subjected to additional step of etching. The grains are preferably placed on a sieve and are subsequently etched in an aqueous solution of nitric acid, hydrochloric acid or ferric chloride. After performing a complete metal (in particular: copper) etching, graphene powder remains on the sieve. Other known methods of straining and filtration of the suspension etc. can also be applied.
In em bodiments of the inventive method of gra phene powder manufacturing, typically copper granulate was used. Copper is the most preferred metal to be used in the process of obtaining graphene powder, because it is the cheapest and the etching process is the easiest in case of copper. Therefore, copper microgranules on which carbon growth at 950 - 1000°C is performed are used for the most part. More specifically, at such temperature dissolution of carbon in Cu takes place, and subsequently as a result of cooling a thin layer of carbon appears on the surface of the copper. Due to low solubility of carbon in copper it is a process enabling self-control of the thickness of the obtained carbon layer, the value of which is close to that of a single atomic layer.
The thus obtained graphene powder comprises graphene flakes of the thickness of one monolayer. A SEM image of such a flake is presented in Fig. 1 .
The thus obtained graphene powder can be oxidized and/or reduced applying different methods. The graphene flake size is dependent on the grain size of metal-graphene grains, in particular - CuGP grains.
Preferred embodiments of the invention
In the following, preferred embodiments of the present inventions are described in details, by way of non-limiting examples. Example 1
Epitaxial CVD graphene growth on metals substrates uses the atomic structure of a metal surface to seed the growth of the graphene. Copper appears to be the most promising for CVD because the resulting graphene is primarily monolayer. The graphene growth on metal (Cu) grains is performed in CVD reactor (owen). A typical CVD process of graphene synthesis on Cu starts with the annealing of metal granulates in order to reduce oxides on the surface in the temperature range between 900 and 1070C°C during 10-60min or longer in the atmosphere of hydrogen, argon or nitrogen mixture. The nucleation and growth of graphene occurs by exposure of the metal surface to a hydrocarbon gas under low pressure, in the range 20-200mbar. The carbonization of individual metal (copper) grains starts in the temperature between 920 and 1040°C. Reactive carbon species are being produced by thermal or/and plasma assist decomposition of hydrocarbon gas resulting in carbon atoms diffusion into the metal.
Plasma generator with defined max voltage (preferably 800V), current (preferably 2.5A), power (preferably I kW), frequency (preferably variable 1 - 100kHz), with simultaneous control of voltage, current, power and frequency, is used to enable decomposition of hydrocarbon being a carbon source in the process of aluminum grains surface carbonization in the temperature below aluminum melting point. Such generators are well known in the art.
The flow of hydrocarbon depends on reactor capacity, for the reactor used here - in the range of 0.5-20ml/min. The carrier gas is argon or the mixture of argon or/and hydrogen or/and nitrogen. The solubility of carbon in a metal increases with temperature. Subsequently, the metal grains are cool down to the room temperature. Some of carbon atoms dissolved in a metal at high temperature ca n precipitate as a graphitic film u pon cooling therefore the time of carbonization process has to optimized, usually in the range of 2-20min. Cooling down rate should be optimized, as well. Finally, graphene as a two-dimensional one-atom-thick sheet of carbon covers the whole surface of each metal (copper) grain. The grains of applied metal (copper) depends on final application and may vary from 0.1 tol OO micrometers or higher.
Although the embodiment described here relates to pure copper granulate with given grain size, it is clear for a skilled person that other metal or metallic alloy granulates, such as in particular Cu-Ni or Ni granulate, may be used in a method according to the invention.
Example 2
The procedures described in Example 1 are essentially repeated, however, an aluminum microgranulate, having the grain size from 0.1 to Ι ΟΟμΐτι is used instead of copper. Consequently, the annealing of aluminum granulates in order to reduce oxides on the surface is carried out in the temperature range between 600 and 660°C for 10-60min or longer in the atmosphere of hydrogen, argon or nitrogen mixture.
Subsequently, the graphene growth on metal (Al) grains is performed in CVD reactor (owen) . The nucleation and growth of graphene occurs by exposure of the metal surface to a hydrocarbon gas under low pressure, in the range 20-200mbar. The carbonization of individual metal (aluminum) grains occurs in the temperature between 600 and 660°C. In case of aluminum granulate, reactive carbon species have to be produced by thermal or/and plasma assist decomposition of hydrocarbon gas resulting in carbon atoms diffusion into the metal.
Plasma generator with defined max voltage (preferably 800V), current (preferably 2.5A), power (preferably I kW), frequency (preferably variable 1 - 100kHz), with simultaneous control of voltage, current, power and frequency, is used to enable decomposition of hydrocarbon being a carbon source in the process of aluminum grains surface carbonization in the temperature below aluminum melting point. Such generators are well known in the art. The flow of hydrocarbon depends on reactor capacity, for the reactor used here - in the range of 0.5-20ml/min. The carrier gas is argon or the mixture of argon or/and hydrogen or/and nitrogen. The solubility of carbon in a metal increases with temperature. Subsequently, fhe metal grains are cool down to fhe room temperature. Some of carbon atoms dissolved in a metal af high temperature can precipitate as a graphitic film upon cooling therefore the time of carbonization process has to optimized, usually in the range of 2-20min. Cooling down rate should be optimized, as well.
Finally, graphene as a two-dimensional one-afom-thick sheet of carbon covers fhe whole surface of each metal (aluminum) grain. The grains of applied metal (aluminum) depends on final application and may vary from 0.1 fol OO micrometers or higher.
Example 3
The product obtained according to Example 1 (i.e. graphene-covered copper granules), after cooling down fo room temperature, was subjected fo chemical efching. As efching means, an aqueous solution of nitric acid, hydrochloric acid, ferric chloride or ammonium persulphate, have been tested, each of fhem providing very good results. Actually, there are numerous substances and mixtures for etching copper or nickel, well known fo those skilled in art. Any of such substances or mixtures is suitable fo be used in fhe present method.
The concentrations of fhe substances/solutions used for etching are not particularly limited insofar fhey provide the effect of efching copper. Therefore, the concentrations may vary in a very broad range, well known to those skilled in art for each of typical etchanfs of copper. The concentration of efchant influences the efching rafe, i.e. fhe efching rafe if higher for higher concentrations of efchant. Only by way of example, it may be mentioned that the inventors have successfully tested etching of graphene-covered copper granules with an aqueous solution of nitric acid having the concentration of 1 6% vol . or 32% vol ., or with a n aq ueous solution of hydrochloric acid having the concentration of 4% vol., 9% vol. or 18% vol., or with an aqueous solution of ferric chloride having the concentration of 1 M, or with a n a q u e o u s s o l u tio n o f ammonium persulphate having the concentration of 0.1 M, each time obtaining high quality graphene powder as the result of etching.
The etching step was performed on a sieve, which is preferred although not necessary, and the thus obtained material was strained or filtered.
The etching step was ended with rinsing the sieve with water, which is preferred although not necessary.
Copper is the most preferred metal to be used in the process of obtaining graphene powder, because it is the cheapest and the etching process is the easiest in case of copper. However, similar procedure can be carried out with other metal or metallic alloy microgranulates, in particular with nickel.
The graphene flake presented in the figure 1 was obtained by the growth of graphene on copper grain with the average lateral size of 3-5 μΐτι a nd by etc hing ou t th e co p per in a q u eou s so l utio n of a m m oniu m persulphate having the concentration of 0.1 M. The carbon flake was characterized by Scanning Electron Microscopy which revealed the graphene nature of the platelet - two-dimension form. The estimated thickness is 1 -2 carbon atomic layers and the size approximately 2μΐτιχ2μΐΎΊ .
The thus obtained graphene powder may be further subjected to an additional step of oxidation and/or reduction of the material obtained as the result of etching. There are numerous oxidants and reductors, well known to those skilled in art. Any of such oxidants and reductors is suitable to be used in the present method.
As a result of the inventive method, as illustrated in Examples 1 and 2 above, metallic granules, in particular copper granules, covered with graphene may be obtained. Such material is a raw product suitable for further processing. It may be applied in different technical fields. For example, it has metallurgic applications - for manufacturing copper-graphene electrical cables, electric clutch plates, composite materials, sinters etc. Substantial quantity of such material can be obtained by the inventive method.
If an additional step of etching is applied, as illustrated in Example 3 above, graphene powder is obtained. Such graphene powder may be used e.g . in supercapacitors, graphene ink, printed conductive paths, nano- composites, electrodes, absorbers, anticorrosion covers and others fields. The method according to the present invention enables manufacturing a large amount of graphene powder in relatively simple and inexpensive devices, the loading of which is even up to many kilograms of granulate. Copper (or other metal) can be electrolytically recovered from the solution and processed into granulate for reuse. The quality of graphene platelets is very high, i.e. the thickness is 1 -2 monoatomic layers which is difficult to obtain by chemical methods.

Claims

Claims
1 . A method of manufacturing microscopic graphene-containing grains, characterized in that it comprises the following steps: a) metal or metallic alloy granulate, preferably granular copper, granular nickel or granular aluminum, the grain size of which is between 0.1 m and Ι ΟΟΟμιτι, preferably between Ι μιτι and όμιτι, is heated up to the temperature between 600°C and 1040°C, preferably between 920°C and 1040°C, and is held at this temperature for 2 to 30 minutes, preferably for 2 to 20 minutes, under an atmosphere comprising argon and propane, optionally with an addition of hydrogen and/or nitrogen, b) material obtained as a result of the stage a) is cooled to room temperature, preferably in argon atmosphere, preferably with optimized cooling rate.
2. The method according to claim 1 , characterized in that the metal granulate is pre-processed before the step a) by annealing in an Ar/h mixture.
3. The method according to claim 1 or 2, characterized in that an additional plasma assist is used during step a), preferably with a plasma generator having a defined max voltage of 800V, current of 2.5A, power of 1 kW and frequency variable between 1 kHz and 100kHz.
4. The method a ccording to c laim 1 , 2 or 3, characterized in that hydrocarbon sources activated in microwave plasma are used during step a).
5. The method according claim 3 or 4, characterized in that granular aluminum is used as the metal granulate and said granular aluminum is heated up to the temperature between 600 and 660°C during step a). ό. The method according a ny of the preceding claims from 1 to 4, characterized in that granular copper or granular nickel is used as the metal granulate and said metal granulate is heated up to the temperature between 920°C and 1040°C during step a).
7. The method according to claim 6, characterized in that it comprises an additional step: c) granular copper or granular nickel is etched by chemical means, in particular comprising an aq ueous solution of a ny of the following: nitric acid, hyd roch loric acid, ferric ch loride, ammonium persulphate, preferably on a sieve, and the thus obtained material is strained or filtered.
8. The method according to claim 7, characterized in that it comprises an additional step of d) oxidation and/or reduction of the material obtained as the result of step c).
9. The method according to claim 7 or 8, characterized in that etching is carried out with a n a q u eous sol utio n of nitric acid having the concentration from 1 % vol. to 50% vol., preferably 16% vol. or 32% vol., or with a n a q u eo us so l u tio n of hydrochloric acid having the concentration from 1 % vol. to 50% vol., preferably 4% vol., 9% vol. or 18% vol., or with an aqueous solution of ferric chloride having the concentration from 0.1 M to 5M, preferably 1 M, or with an aqueous solution of ammonium persulphate having the concentration from 0.05M to 1 M, preferably 0.1 M.
10. Microscopic graphene-containing grains obtainable according to any one of the claims from 1 to 6, characterized in that they comprise granules of metal or metallic alloy, preferably granules of copper, nickel or aluminum, with one or several layers of graphene on the surface of the granules, wherein the size of the grains is from 0.1 m to Ι ΟΟΟμιτΊ, preferably from 0.1 m to Ι ΟΟμιτι. Microscopic grophene-contoining grains obtainable according to any o ne of the c laims 7, 8 or 9, characterized in that they comprise graphene powder, wherein the size of the grains is from 0.1 to Ι Ομιτι, preferably from 1 to δμιτι, more preferably 1 to 2μιτι.
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