WO2018055227A1

WO2018055227A1 - Suspensions of carbon nanoparticles for use as inks and thin films and methods for preparing the same

Info

Publication number: WO2018055227A1
Application number: PCT/FI2016/050648
Authority: WO
Inventors: Jorma Jokiniemi; Anna LÄHDE; Mirella MIETTINEN
Original assignee: University Of Eastern Finland
Priority date: 2016-09-20
Filing date: 2016-09-20
Publication date: 2018-03-29

Abstract

The method of the invention for producing suspensions of carbon nanoparticles comprises the steps of providing an organosilane compound comprising a silicon-silicon bond as a precursor material, and vaporizing the precursor material, heating the vaporized precursor material for obtaining silicon-carbon nanoparticles, heating the obtained silicon-carbon nanoparticles by induction for obtaining multilayer graphene, MLG, sheets and carbon nanoflowers, CNFs, and mixing the obtained silicon-carbon nanoparticles with an organic solvent a sufficient time for obtaining a homogenous suspension as a result. The suspensions of the inventions are suspensions of carbon nanoparticles in an organic solvent, wherein the carbon nanoparticles comprises multilayer graphene, MLG, sheets and/or carbon nanoflowers, CNFs. Conductive inks, thin films and spray coatings that are prepared from the suspensions are also part of the invention.

Description

SUSPENSIONS OF CARBON NANOPARTICLES FOR USE AS INKS AND THIN FILMS AND METHODS FOR PREPARING THE SAME

TECHNICAL FIELD The inventions is concerned with suspensions of carbon nanoparticles for use as inks and thin films and methods for preparing such suspensions.

BACKGROUND

Carbon can exist in several different forms. The most common form of carbon is graphite, which consists of stacked sheets of carbon with a hexagonal structure. Under high pressure diamond is formed, which is a metastable form of carbon. Graphite contains two dimensional lattice bonds, while diamond contains three dimensional lattice bonds. Graphite is a commonly found mineral and is composed of many layers of graphene. Thus, graphene, simply, is one atomic layer of graphite - a layer of sp2 bonded carbon atoms arranged in a hexagonal or honeycomb lattice.

What exactly can be called graphene is, however, not commonly precisely defined since graphene is often referred to by number of layers, like single layer graphene, few layer graphene or even multi-layer graphene. The use of the term graphene is also often related to particle morphology and aspect ratio, Graphene might e.g. be said to exist in the form of related two-dimensional (2D) sheet-like or flake-like mono-, few- or multilayered carbon forms. Graphene nanoplatelet is a term used for disk shaped graphene sheet and is in fact a type of a graphene flake. The term graphene is nowadays so commonly used that authors, in order to avoid misunderstandings, may now write "monolayer" graphene or "single-layer" graphene just to emphasize that the idea being communicated is about the true single-layer 2D material, graphene reflecting its original meaning, and not one of these related few- or even multilayered materials. Regardless of arrangement, the structural unit in all of these materials is, however, the "graphene layer". An alternative and general method for describing the materials listed above (bilayer graphene, trilayer graphene, few-layer graphene, multi-layer graphene) is to identify the form (flake, film, coating) modified by a compound adjective based on the "graphene layer" concept as a structural component. For example, one could describe a "four-graphene-layer flake", a "bi-graphene-layer coating", or a "multi-graphene-layer shell". This is a logical usage and is flexible enough to describe a wide variety of carbon architectures

Thus, classification of nanoscale materials, however, are not necessarily based on crystallography (phase) alone, but can also include morphological descriptors for shape and size.

Carbon nanotubes constitute a related quasi-one-dimensional form of carbon, and there are single and multi-walled carbon nanotubes. These can be formed from graphene sheets which are rolled up to form tubes, and their ends are half spherical. The electronic and mechanical properties of metallic single walled nanotubes have many similarities with graphene, which has proved to be an extremely exciting new material which has many fascinating properties.

A graphene sheet can be cut into different shapes. For graphene materials, shape and size are most conveniently expressed by the number of layers (or thickness), lateral dimension, and in-plane shape. Graphene research involves the study of several different physical forms of the material: powders, flakes, ribbons, and sheets and others not yet named or imagined and they can consist of everything from atomic monolayers to thicker flakes. The differences between two layers and a few layers of graphene sheets are not obvious in Raman spectra.

The three basic type of graphene are, (a) an infinite graphene sheet or membrane, (GMBs, a 2-D graphene), (b) a graphene nanoribbon (GNRs, a 1 -D graphene), and (c) a graphene nanoflake or graphene nanodot (GNFs, a 0-D graphene).

Much of the research on graphene has been concerned with 2-D graphene and 1 -D graphene, because of their interesting and potentially very useful properties have been relatively simple to isolate and address. Less well studied is the 0-D form of graphene, which presents a greater degree of complexity, but offers a greater potential for flexibility and selectivity (both literally and figuratively). In a general meaning, a nanoflake is a flake with at least one nanometric dimension (1 - 100 nm). A flake is not necessarily perfectly flat but it is characterized by a plate-like morphology and is, an uneven piece of material with one dimension substantially smaller than the other two. A new carbon nanostructure has been reported by M. Miettinen et al e.g. in the article "Synthesis of novel carbon nanostructures by annealing of silicon-carbon nanoparticles at atmospheric pressure" in J. Nanopart Res (2014) 1 6:21 68. DOI 10.1007/s1 1051 -013-21 68- 2. Post-processing of previously synthesized Silicon-carbon (Si-C) nanoparticles by annealing in argon at atmospheric pressure produced, Si-C crystals, graphene sheets and novel carbon nanoflowers. The carbon nanoflowers are spherical carbon particles consisting of curved carbon layers growing from the SiC core forming a "nanoflower" with a diameter below 60nm. A nanoflower, in chemistry, refers to a compound of certain elements that results in formations which in microscopic view resemble flowers or, in some cases, trees that are called nanobouquets or nanotrees. These formations are nanometers long and thick so they can only be observed using electron microscopy.

In "Structure of a new rotationally faulted multi-layer graphene-carbon nanoflower composite (Carbon, Volume 84, April 2015, Pages 214-224), M. Miettinen et al reports examination of the structure of a new carbon-carbon nanocomposite that consists of thin (<15 layers) multi-layer graphene microsheets and carbon nanoflowers (CNF) by high- resolution transmission electron microscopy combined with selected area electron diffraction (SAED) analysis, and Raman spectroscopy. Both SAED and Raman analyses verified that the graphene layers in the sheets were rotated to each other. A typical rotation angle in SAED analysis was 30 ± 2° but also other rotation angles (e.g., 2 ± 1 °, 12 ± 2°, 19 ± 2° and 25 ± 2°) were detected. Raman analysis designated the rotation angle of 1 1— 12°. Both folded and free standing, unfolded edges were present in the sheets. The free standing edges were rough and no preferred chirality was found. Overlapping boundary interfaces were dominant between the graphene domains in the sheets. The interlayer distance in the sheets was increased by ca 12% compared to graphite.

Recently, the focus has been on graphene because of its enhanced properties and easier production and handling. Graphene, generally, has superb optical, electrical and thermal properties, is chemically and thermally stable and as presented above is structurally flexible. Sheets of graphite and graphene materials are hard to produce in part due to the fact that they tend to agglomerate in processing media, such as solvents. Consequently, also the production of stable suspensions of graphite materials, graphene materials, and sheets thereof is challenging. Graphene is practically transparent and thus, suspended graphene does not have any color. Graphene suspensions are therefore considered to be especially suitable for use as conductive inks, thin films, transparent films and flexible films.

A disadvantage of graphene inks is, however, that they are not stable in ordinary solvents and sedimentation of the ink suspensions occur. Therefore they usually need to be chemically modified in order to prolong the time of storage and use.

The problem has been attended to in prior art. Graphene is hydrophobic and is therefore difficult to solubilize or disperse in most liquids, which limits their easy processability by many traditional methods.

Chemists have spent considerable effort in functionalizing graphene to be more readily solubilized or dispersed, especially in water. Groups, such as carboxyl epoxy and hydroxyl groups, are commonly used to solubilize in water and long alkyl chains make graphene soluble in many organic solvents.

CN publication 1031 13786B discloses a conductive graphene ink having an anti- sedimentation property comprising resin, graphene additive, and solvent. The preparation involves adding and dispersing all components, stirring, mixing uniformly to obtain a pre- dispersed conductive ink, and finally grinding for 0.1 -120 hours, filtering, checking, and packing. The solvent can be e.g. water or alcohol, and the additive is typically a thickener or dispersant, a crosslinking agent, or a stabilizer.

CN publication 102515149A presents preparation of a high stability graphene dispersion liquid comprises agitating graphene and cyclodextrin; adding alcohol; grinding the mixture for 2-4 hours; and drying at 40-100° C to obtain a mixture; and carrying out ultrasonic dispersion for 30 minutes to 3 hours. The solvent is water, ethylene glycol, methanol, N, N'- dimethylformamide, or ethanol. WO publication 2010/083378 comprises a suspension of chemically functionalized graphene or graphite in an organic solvent, such as e.g. ethanol or N-methyl pyrrolidone, and more than 0,1 % by volume of water. The suspension is presented to be stable for at least one month.

SUMMARY

The object of this inventions is to provide a suspension of graphene or the like, with an improved stability for different applications, such as conductive inks and thin films, and improved methods for preparing such suspensions. The method of the invention for producing suspensions of carbon nanoparticles comprises the steps of providing an organosilane compound comprising a silicon-silicon bond as a precursor material, and vaporizing the precursor material, heating the vaporized precursor material for obtaining silicon-carbon nanoparticles, heating the obtained silicon-carbon nanoparticles by induction for obtaining multilayer graphene, MLG, sheets and carbon nanoflowers, CNFs, and mixing the obtained silicon-carbon nanoparticles with an organic solvent a sufficient time for obtaining a homogenous suspension as a result.

The precursor material is vapourized with by an aerosol method and the heating of the obtained silicon-carbon nanoparticles is performed by annealing in argon at high temperature and atmospheric pressure in an inductively heated furnace. The mixing is performed by sonication or ultrasonication. The obtained graphene sheets and carbon nanoflowers can be separated from eachother by (ultra)sonication and centrifugation.

The suspensions of the inventions are suspensions of carbon nanoparticles in an organic solvent, wherein the carbon nanoparticles comprises multilayer graphene, MLG, sheets and/or carbon nanoflowers, CNFs.

The carbon nanoparticles can consist of interconnected or separated MLG sheets and CNFs. The suspensions of the invention comprises 0,05 - 20 % of weight of the carbon nanoparticles in an organic solvent and are very stable having a sedimentation time of from at least 2 weeks and up to more than 10 months. The solvent is an organic alcohol, such as ethanol, or Ν,Ν-dimethylformamide or N-methyl-2- pyrrolidone or mixtures thereof, preferably N-methyl-2- pyrrolidone.

The suspensions can be doped by a metal or metal oxide by adding ultrafine particles of the metal or metal oxide within the carbon nanoflower structures and/or the graphene sheets, by forming a connective network of either particles or layers of doping material or by mixing the metal or metal oxide with the G/CNFs and subsequently heating to get conductive metals inside the G/CNFs.

Conductive inks, thin films and spray coatings that are prepared from the suspensions are also part of the invention.

The preferable embodiments of the inventions have the characteristics of the sub claims.

The inventors have found that the use of particular Carbon-Silicon (C-Si) compounds as precursors in the inventive methods results in carbon nanomaterials, which can be used to produce extremely stable suspensions for use as e.g. inks and thin films. These particular precursor C-Si compounds are Organo-Silicon compounds, which are organodisilanes containing at least a silicon-silicon bond and preferably also one or more methyl groups, such as hexamethyldisilane, Si2(CH3)e, and 1 ,1 ,3,3 - tetramethyl- 1 ,3-disilanocyclobutane, or alternatively, they contain a Si-N-Si bond, such as hexamethyldisilazane ((CH₃)3SiNHSi(CH₃)3).

The inventive method used to produce such compounds result in a mixture of carbon nanoflowers (CNFs) and multilayer graphene sheets (G), wherein the CNFs and the graphene layers are interconnected but can be separated. The interlayer distance in the graphene layers is expanded compared to graphite and the graphene layers are rotationally disordered. The graphene component primarily consists of multi-layer graphene (MLG) sheets, which can be rotated to each other. The invention is thus related to suspensions of a novel composite material comprising graphene-carbon nanof lower (G/CNF) structures, or synthetized via a two-stage process. The invention is also concerned with suspensions of the new Multilayer Graphene (MLG) sheets alone and suspensions of the new carbon nanoflowers (CNFs) alone. The invention is also concerned with the use of such suspensions in conductive inks for printing and in printed thin films as well as to methods of preparing such suspensions.

When prepared in the way of the invention, the carbon nanoflowers (CNFs) and multilayer graphene (MLG) sheets can be used to form very stable suspensions in organic solvents. These suspensions provide improved possibilities for storage. They remain unaltered for several months and no sedimentation is observed. In the suspensions, the carbon nanoflowers (CNFs) and MLGs are mixed in a solvent but not dissolved therein and sedimentation takes place very slowly. The suspensions can therefore be advantageously used in conductive inks, printed thin films, injection coating and thin layers, such as solar panels. Furthermore, the carbon nanoflowers (CNFs) made by the method of the invention have promising electrochemical properties in e.g. Li-ion batteries and supercapacitors.

The manufacturing and production of graphene materials in general is challenging, as the manufacturing of high-grade graphene continues to be extremely expensive and complicated. The method of the invention also has advantages relating to the manufacturing of the products of the invention, since the aerosol-assisted induction heating step of the method constitutes a cost-efficient way of manufacturing large amounts of the high-grade carbon nanoflowers (CNFs) and multilayer graphene (MLG) sheets.

The inventors have found that the suspensions of the invention show surprisingly stable properties. The graphene-carbon nanoflower (G/CNF) structures used in the suspensions, interconnected or separated, have in the invention been synthetized by inductively annealing silicon-carbon (Si-C) nanoparticle precursors. The Raman spectra of the carbon nanoflowers (CNFs) and multilayer graphene sheets (MLGs) display narrow bands, which are characteristic of high-quality graphene. Both the CNFs and the MLGs, interconnected or separately, have a high stability in organic solvents without the aid of surfactants.

The printability of the inks of the invention is very good. Their adhesive properties can be further improved by adding conductive polymers, such as PVDF, to the suspensions. The new materials used in the ink suspensions of the invention are optionally doped with a metal or metal oxide, such as with Gold (Au), Silver (Ag), Copper (Cu) or Zinc oxide (ZnO) or with Silicon (Si). Doping is to introduce impurities into a pure material. In the doping of the invention, some of the carbon atoms will be replaced by other atoms. The doping can be performed by adding ultrafine particles of the above mentioned metals or metal oxides or silicon within the carbon nanoflower structures and/or graphene sheets. The doping can also be performed to form a connective network of either particles or layers of doping material. Still further liquid metalorganic precursors can be mixed with the new carbon particles of the invention and then heated to get conductive metals inside them. "Nanoflower" refers to a compound, wherein the elements have formations which in microscopic view resemble flowers or, in some cases, trees that are called nanobouquets or nanotrees. The new structures developed by the inventors have such a structure, in which the petals are formed from graphene sheets. However, by referring to the background section, graphene has traditionally been produced in varying formats, but as nanoflowers reminding structures have not been reported, the new type of nanoflowers of the invention is referred to in this text as just carbon nanoflowers (CNFs). The term, however, covers in this text, all types of carbon nanoparticles that can be considered to have a nanoflower structure, and includes also the term graphene nanoflowers (GNFs), since the CNFs consist at least partly and usually completely of graphene sheets arranged in the form of a flower. The petals are hollow in the middle and the graphene sheets are curved in the shape of a flower petal.

These formations are nanometers long and thick so they can only be observed using electron microscopy. The number of layers is typically below 10 layers. Graphene research involves the study of several different physical forms of the material: powders, flakes, ribbons, and sheets and others not yet named or imagined. Within those forms, graphene can include a single layer, two layers, or <10 sheets of sp² carbon atoms.

The Multi-Layer Graphene sheets (MLGs) used in the invention are high-grade graphene, with a minimal number of crystal defects in comparison to other graphene materials that are currently commercially available. Terms used

Graphite is a material composed of many layers of graphene, wherein the given distance between the layers is 0,335nm.

Grapherse as material, generally, is one atomic layer of graphite - a layer of sp2 bonded carbon atoms arranged in a hexagonal or honeycomb lattice.

Graphene f ake is a G D form of graphene characterized by a plate-like (not necessarily flat) morphology and is an uneven piece of material with one dimension substantially smaller than the other two.

Graphene sheet is a 2-dimensionai (2-D) infinite form of a graphene layer and composed of carbon atoms linked in hexagonal shapes with each carbon atom covalentiy bonded to three other carbon atoms. Each sheet of graphene is only one atom thick, and each graphene sheet is considered a single molecule.

Single- layer Graphene (SLG) is a graphene material comprising one graphene sheet

Multi-layer Graphene (MLG) is a graphene material comprising more than one but typically less than fifteen graphene sheets, it differs from graphite in some respects. Graphite can comprise more than fifteen graphene layers and in addition they are distinguished by having some differences in their Raman spectra, i.e. different relations between the D, G and 2D peaks. Still further, the distance between the layers is increased compared to that of graphite, being ca 0,38nm. Carbon nanoflower (CNF) covers in this texts all types of carbon nanoparticles with a nanoflower structure.

Graphene nanoflower (GNF) is a type of carbon nanoflower, wherein the petals of the carbon consist of graphene sheets curved into the form of flower petals.

Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained and come to rest against a barrier.

Settling is the process of falling of suspended particles through the liquid, whereas sedimentation is the termination of the settling process. A dispersion is a system in which particles are dispersed in a continuous phase of a different composition (or state).

A suspension as the term is used in this text is an opaque type of dispersion being a mixture of solid particles in a liquid, solid or gas with the property that one does not rapidly settle out. A suspension is thus a heterogeneous mixture. The particles in a suspension settle down under the influence of gravity. The solid particles in the suspension are sufficiently large for sedimentation and they are not dissolved nor aggregated but dispersed in the liquid. The liquid can be a mixture of liquids. Suspensions are classified on the basis of the dispersed phase and the dispersion medium, where the former is essentially solid while the latter may either be a solid, a liquid, or a gas. The suspension of the invention is a mixture of solid particles in a liquid.

An aerosol or a particulate is a dispersion of liquid droplets or fine solid particles in a gas.

Sonication is the act of applying sound energy to agitate particles in a sample, for various purposes. Ultrasonic frequencies (>20 kHz) are usually used, leading to the process also being known as ultrasonication or ultra-sonication. In the laboratory, it is usually applied using an ultrasonic bath or an ultrasonic probe, colloquially known as a sonicator.

In the following the products and methods of the invention will be described by referring to some different embodiments by means of figures. The invention is not restricted to the details of these embodiments.

FIGURES

Figure 1 illustrates the first phase of the method of the invention

Figure 2 illustrates the second phase of the method of the invention

Figure 3 shows the structures of the MLG sheets and CNFs in a Transmission Electron Microscope (TEM) image of a suspension of the invention

Figure 4 presents some conductivity values of the suspensions of the invention

Figure 5 presents some resistance values of the suspensions of the invention Figure 6 shows the effect of the solvent to the sheet size of the MLG and CNFs in DMF and ethanol

Figure 7 shows the deagglomeration and structure of Graphene/Carbon nanoflower structures in ethanol and DMF

Figure 8 shows the Ultraviolet (UV) absorption spectra of conventional graphene, the MLG of the invention and the CNFs of the invention.

Figure 9 shows a photography of three different thin films made from GNF suspensions of the invention printed on a surface.

Figure 10 shows the film transparency of thin films made from suspensions of the invention Figure 1 1 shows Transmission Electron Microscope (TEM) images suspensions of G/CNF doped with Ag

DETAILED DESCRIPTION

The production of the suspensions of the invention are prepared in a method comprising three main phases.

First phase

Figure 1 illustrates the first phase of the method of the invention for forming the inventive suspensions. It is pointed out that the amounts and chemicals as well as reaction conditions can vary and are presented below as examples only. The first phase consists of the production of pre-ceramic Silicon-Carbon (Si-C) nanoparticles from e.g. hexamethyldisilane (C6HisSi2, HMDS) by atmospheric pressure chemical vapor synthesis.

Also 1 ,1 ,3,3-tetramethyl-1 ,3-disilacyklobutane (trade name CVD-742) or other corresponding orqanosilane compounds could be used instead of hexamethyldisilane. A gas mixture containing ca 10% of hydrogen (H2) in nitrogen (N2) is introduced as a combined reaction/carrier gas through an inlet 3 into a bubbler vessel 1 with a precursor liquid of hexamethyldisilane (C6H18S12, HMDS) to be mixed with the liquid precursor by bubbling it with a flow of e.g. 0,3 litres per minute (Ipm) through the same, thereby causing a transition of the liquid precursor into gaseous state. The amount of hydrogen in the reaction/carrier gas can be varied for influencing the properties of the particles to be formed and can be increased up to 25 volume-%. As a result, the HDMS is vapourised in this step to give an aerosol of HDMS particles in said carrier gas mixture. The bubbler vessel is to be held in a thermostatic water bath 2 of a temperature of ca 30°C in order to prevent the HDMS vapour from condensing before reaching the reactor 5.

In the vaporization, the precursor material undergoes a phase transition from liquid phase to gas phase. The vaporization is performed by evaporation rather than by boiling.

Evaporation is a phase transition from the liquid phase to gas phase that occurs at temperatures below the boiling temperature at a given pressure. Evaporation usually occurs on the surface. In boiling, a phase transition takes place from the liquid phase to gas phase that occurs at or above the boiling temperature. Boiling, as opposed to evaporation, occurs below the surface.

The reactor 5 is preferably vertical for upwards moving of the reactants and the reaction products forward in the reactor 5.

The resulting aerosol of HDMS particles in said carrier gas mixture mentioned above is carried with the carrier gas mixture through inlet 4 to the high temperature aerosol flow reactor at a flow rate of ca 0,3 Ipm (litres per minute) at ca 20°C and ca 01325Pa.

A flow of ca 1 ,5 Ipm of N2 is added to the gas flow of H2/N2 in order to get an optimal flow profile in the reactor.

A useful overall height of the reactor is ca 1000 mm (heated distance) and it has a ceramic reactor tube 6 with an inner diameter of about 22 mm.

The flow reactor is here divided into two heating zones, a first heating zone 5a and a second heating zone 5b, both being about 500 mm long, but alternatively, a single heating zone can also be used.

As a result of the heating, particles are formed in the heating zones 5a, 5b. The particle formation is based on the decomposition of the precursor and subsequent reaction of the precursor in the reactor in a temperature of 800 - 1400°C, preferably 1400°C, in atmospheric pressure. When heated to said temperature, decomposition of the precursor material occurs with said particle formation as a result. The decomposition leads to homogenous nucleation and formation of nanoparticles with a narrow size distribution. The geometric mean diameter range of the particle size distribution is within 1 60 - 200nm in a temperature of < 1000°C and < 10Onm in a temperature > 1000°C with a 1 .5 - 1 .6 standard deviation. The properties, such as crystallinity, amount of excess carbon, particle size etc. depends on the temperature used.

Thus, a reaction product consisting of nanoparticles is formed. Immediately after leaving the reactor 5, the nanoparticles particles are introduced in a porous tube diluter 7, wherein nitrogen N2 or some other inert gas is introduced as cooling and diluting gas from inlet 8 among the nanoparticles at a flow rate of ca 20 litres per minute at ca 20°C and 101325Pa.

The reaction product consisting of the nanoparticles is cooled and diluted by mixing the reaction product with the cooling gas, whereby the nanoparticles are cooled and diluted with the cooling gas in order to stop further collisions and avoid agglomeration and sintering. A typical dilution gas temperature is from -20°C to +20°C and a flow rate from 10 to 60 Ipm. The dilution gas can also be the same as the inert carrier gas and/or reaction gas.

After the diluter 7, the reaction product is fed through inlet 9 to a sampling collector 10 (such as a filter) and collected and sampled in via an outlet 1 1 .

The nanoparticles received by using a temperature of 1400°C have a crystal size of 5 nm. When produced in 1200°C, the crystal size is 1 ,3 nm, and in 1000°C, the crystal size is 0,7 nm, which still can be used as a starting material in the second phase of the method. With still lower temperatures, the nanoparticels are amorphous and preceramic Nanoparticles with a too big crystal size (such as 0,5 μηπ) do not produce any nanoflowers. Particle sizes of 1 10 nm can till be used as a starting material for the next phase, but a more preferable particle size is 77 nm (and the crystal size being 0,7nm), and a still more preferable particle size is 69 nm (and the crystal size being 1 ,3 nm). The most preferable particle size is < 69 nm (and the crystal size being 5 nm). The particles have a bigger amount of carbon (atom-%) and much less oxygen compared to particles produced in lower temperatures. Thus, the particle size decreases and the specific surface area of the particle increases with increasing temperatures in the following way:

800 ^QC -> =180 nm

900 ^QC -> =1 10 nm

1000 ^QC --> = 77 nm

1200 ^QC --> = 69 nm

1400 ^QC --> < 69 nm

The Specific surface area (SSA) is a property of solids defined as the total surface area ofmaterial per unit of mass.

Thus, by altering the temperature of the first phase, preceramic (T=1000 ^QC, crystal size 0.7 nm), amorphous or nanocrystalline (T=1400 ^QC, crystal size 5 nm) particles can be produced. Big crystals (> 0.5 μηΊ/οοηΊΓηβΓθί3ΐ SiC products ) do not produce nanoflowers.

The higher the crystallinity is and the lower particle size of the end product (Si-C nanopraticles) of the first phase is, the lower is the number of graphene layers in the end product of the second phase of the method of the invention (to be described in connection with figure 2). Furthermore, there are more graphene sheets compared to carbon nanoflowers in the end product of the second phase of the method of the invention, the higher the crystallinity is higher, and the lower the particle size is in the end product of the first phase.

By altering the reaction conditions of the first phase of the method of the invention, such as the temperature, the Si/C/O ratio of the nanoparticles of the first phase can be influenced on:

Reactor temperature (°C) Atomic % of element Atomic ratio of elements

Si C 0

800 31 .7 49.1 19.2 SiC1.54O0.6i

900 38.8 53.8 7.4 SiCl.38Oo.19

1000 40.7 58.1 1 .2 SiC1.42O0.03

1200 37.7 61 .4 0.9 SiC1.63O0.02 The Si/C/O ratio of the nanoparticles have an effect on the end products of the second phase and the suspensions of the invention. A useful range is 41 /58/1 atomic %, preferably the range should be about 32/49/90 atomic %, and most preferably 38/61 /1 atomic %. The properties of the nanoparticles produced in the first phase of the method and have an effect on the size of the graphene sheets and the carbon nanoflowers, the number of the atomic layers in those, and the distance between the layers. The characteristics (such as the size and number of atomic layers) of the multilayer graphene sheets and the carbon nanoflowers, in turn, have an influence on the suspensions to be prepared.

For example, a pre-ceramic particle size of 77 nm the first phase particle gives usually 1 1 atomic layers of carbon nanoflowers.

A nanocrystalline particle size of the first phase particle below 69 nm with crystal size of ca 5nm gives a graphene sheet size of up to some square micrometers, (a minimum graphene sheet size of 1 μηπ being, however, preferable), a carbon nanoflower (diameter) size of 20 nm, and in average less than 15 atomic layers of graphene sheets,, which is a useful a number. The layers of the graphene sheets are preferably less than 10, and most preferably abut 5. The layers of the carbon nanoflowers can be less than 15, preferably less than 10, and most preferably about 4 atomic layers of carbon nanoflowers.

A distance of 0.38±0.03 nm between the graphene sheet layers, and a distance of 0.36±0.02 nm between the layers of carbon nanoflowers are achieved with these first phase particles (particle size 69nm, crystal size 5 nm), which are the most preferable values. A useful distance between the graphene sheet layers can, however, be about 0,335 nm, but more preferably ca 0,38 nm and most preferably up to 40 nm.

Second phase

Figure 2 illustrates the second phase of the method of the invention in which the nanoparticles were annealed in argon at high temperature and atmospheric pressure in an inductively heated furnace 12 having an airflow inlet 13.

Induction heating is the process of heating an electrically conducting object by electromagnetic induction, through heat generated in the object by eddy currents (also called Foucault currents). An induction heater consists of an electromagnet, and an electronic oscillator that passes a high-frequency alternating current (AC) through the electromagnet. An important feature of the induction heating process is that the heat is generated inside the object itself, instead of by an external heat source via heat conduction. Thus objects can be very rapidly heated. In addition there need not be any external contact, which can be important where contamination is an issue.

In the second phase, the collected nanoparticles 14 of the first phase is first placed in a graphite crucible 15 with an inner diameter of 35 mm and with 9 mm thick walls and which is closed except from a 10 mm hole in the lid. The crucible 15 is encased in a graphite felt thermal insulation 1 6. The annealing is performed by raising the temperature from room temperature to most preferably 2600°C and kept there in 18 min before cooling back to room temperature.

Raising the temperature to 2600°C has given the best results, even of a temperature range of 1900°C - 2600°C is useful, more preferably 2200°C. A range of 1900°C - 2600°C gives diameter sizes of 20 - 60 nm for the carbon nanoflowers. In 2600°C, the most preferable diameter size of 20nm for the carbon nanoflower is achieved, whereas size of 60nm is still useful, which is obtained in the lower end of the temperature range. The most preferable void/cavity diameter with respect to the suspensions to be prepared is 10 nm, achieved in 2600°C. The temperature was monitored with a Kleiber 730-LO infrared pyrometer 19 through the hole. A pyrometer is a type of remote-sensing thermometer used to measure the temperature of a surface.

An alternating voltage is imposed in the induction coil (not shown) of the furnace 12, which induction coil generates an alternating current and a magnetic field by inducing eddy currents that result in heat release in the crucible 15 due to the Joule effect of production of heat as the result of a current flowing through a conductor. The nanoparticles inside the crucible is mainly heated by conduction and radiation.

Air is introduced through air flow inlet 13 into an alumina support tube 18 of the furnace 12. The air is lead out through an air flow outlet 20 at the top of the furnace 12. The alumina support tube 18 is surrounded by a quartz glass tube 17. An example of a useful furnace has a heating power of 50kW, an input current of up to 95A and a working frequency of ca 9 - 10 kHz.

A flow of 1 ,5 Ipm of Argon is lead through the sample space.

The step-by-step annealing gives first a mixture of SiC crystals and graphene sheets. At temperatures below 2200°C, there are still some Si-C cores but above 2200°C, they disappeared and carbon nanof lowers (CNF) are formed. Tests were also made in temperatures of 1900 - 2600°C. Preferably, the temperature should be 2200 - 2600°C, most preferably 2600°C. When using a temperature of 2600 °C, the content of SiC residues of nanoparticles in the end product of the second phase is less than 10 wt% on the basis of thermogravimetric analysis. The rest of the resulted product (over 90 wt%) is a mixture of carbon nanoflowers (CNF) and Multi-Layer Graphene (MLG) flakes.

The step-by step annealing causes probably a separation of the Si and C phases into a molten form (phase separated molten dissolution), from which the Si is vaporized and partly removes from the system. Thus the amount of Si decreases and the remaining "lonely" carbon forms CNF and MLG structures.

Since the melt point of crystalline SiC is 2700°C, Si can not discharge at temperatures < 2600°C otherwise than the preceramic and nanocrystalline particles.

Dilution and cooling with a 10 Ipm flow of nitrogen of the end product after it has left the reactor is performed.

Third phase

In the third phase of the method of the invention, suspensions of the carbon nanoflowers (CNF) and/or the Multi-Layer Graphene (MLG) sheets of the second phase were prepared. The product received from the second phase described in connection with figure 2 consisted of both carbon nanoflowers (CNF) and/or Multi-Layer Graphene (MLG) flakes. If desired, they can first be separated by a series of sonication or ultrasonification and centrifugation steps.

They are then mixed with a solvent, such as a total weight of 0,05 - 20 weight-% (such as 1 weight-%) of graphene, of sheets and/or carbon nanoflowers in N-methylpyrrolidine or some other organic solvent, and then sonicated or ultrasonicated in e.g. 22°C and 15 min in a bubble bag or a glass tube. The Bubble bag is a disposable plastic bag that improves the effect of the ultrasound. The surface of the bag has been modified with small pits that trap gas bubbles and lead them to the enhance bubble formation.

The sonication or ultrasonication time can suitably be within the range of 0 - 30 min, preferably 10 - 15 min, and most preferably 15 min in e.g. 6000rpm. The temperature can be in the range of ca 22 - 50°C.

The particle size of the carbon nanoflowers is most preferably 135 nm when using ethanol as solvent, 148 nm when using N-methyl- 2-pyrrolidine (NMP), and 74 nm when dimethylformamide (DMF).

The particle size of the graphene sheets is most preferably 180 nm when using ethanol as solvent, 206 nm when using N-methyl- 2-pyrrolidine (NMP), and 100 nm when dimethylformamide (DMF).

Optional fourth phase Doping can be performed as a fourth phase of the method of the invention in order to improve the suspensions for ink and thin film use.

Doping with Ag can be carried out in a water/DMF solution or an ethanol/ DMF solution. The size of the Ag particles can be e.g. 10 nm and can be adjusted depending on the requirements for the final application. E.g. optical properties can be adjusted in applications, wherein the reactions with light are important. Other properties that can be adjusted are magnetic properties and conductivity. Doping with Au can be carried out in organic solvents, e.g. in hexanol or ethanol. The precursor for Au nanoparticles can be for example an organometallic compound and the doping can be carried out at the same time with the suspension procedures. A suitable particle size is 2 - 50 nm.

Zinc Oxide can be used for improving the characteristics of printed thin films of carbon nanoflowers (CNFs) in an ethanol suspension, such as to ensure that a uniform "CNF network" is formed.

Another optional fourth phase

The properties of the suspensions of graphene and/or carbon nanoflowers can be modified by adding of polymers, like e.g. Poly Vinyl Fluoride, PVDF, in order to improve the adhesion when printing of the suspensions on surfaces. Also the viscosity and resistivity can be adjusted by means of polymers.

Desired properties Desired properties for the multilayer graphene sheets are:

- size of graphene sheets: >1 μηπ²

- number of layers: ca 5 -15, preferably ca 5

- distance between layers: 0, 30 nm - 0,40 nm

·· can be both folded or straight

Desired properties for the carbon nanoflowers are:

- diameter of an individual carbon nanoflower: approximately 20 nm

- diameter of the void in an individual carbon nanoflower: up to 10 nm

- number of layers: ca 4 - 15, preferably ca 4

- distance between layers: ca 0,36 nm, preferably 0,36 nm ± 0,02nm

A desired zeta potential for the ink suspensions of the invention is > ± 30mV, preferably > ± 40mV, and most preferably > ± 60mV. - CNFs are interconnected to sheets and can be separated using repeated sonication / centrifugation method.

- Raman

• MLG G peak -1576 cm-1583 cm-1 (for graphite the G position is typically at 1560 cm-1 )

Figure 3 shows different graphene/carbon nanoflower structures in a suspension of MLGs and CNFs of the invention in a Transmission Electron Microscope (TEM) image.

There are usually different structures at the same time if no separation has been performed. The interlayer distance in the graphene layers is expanded compared to graphite and the graphene layers in the sheets are rotationally disordered. The CNFs and the graphene layers can be interconnected or separated.

Reference number 21 shows interconnected CNFs and MLG sheets. Reference number 22 shows CNFs, which are separated from MLGs. Reference number 23 shows MLGs, which are separated from CNFs.

Figure 6 shows the effect of the solvent to the sheet size of the MLG and CNFs in DMF and ethanol. As can be seen, the sheet size of the suspended material can be effected by the selection of the solvent. Transmission electron microscopy (TEM) images of the suspended and deagglomerated graphene/carbon nanoflowers in ethanol and DMF are shown on figure 7. Image A shows the deagglomeration and structure of graphene/Carbon nanoflowers in ethanol and B shows the deagglomeration and structure of graphene/Carbon nanoflowers in DMF. It can be seen that a better agglomeration is achieved with DMF and the agglomerates are better dispersed in image B.

NMP and DMF are good solvents for exfoliation and dispersion of graphene, in part, because it has a surface tension similar to that of graphene. The Transmission Electron Microscope (TEM) images show that suspensions of DMF contributes to the de-agglomeration of the carbon nanoflowers and the graphene multilayers. A smaller particle size and a darker colour is observed with DMF compared to ethanol. CN F suspensions in DMF can be considered monodisperse. Dispersions of the MLGs and CNFs used in the invention for the suspensions in NMP, DMF or ethanol can be attained without chemical modification by sonication. Shaking is not enough.

Hansen solubility parameters, i.e. the sum of polarity cohesion ( δ_Ρ) and the hydrogen bonding cohesion ( 5h) parameters, should be in the around 1 3-29 (e.g. DMF, NMP) for graphene suspensions.

The sonication times for the preparation of graphene suspensions in organic solvents (e.g. DMF) can be up to 2h (temp, of the sonication bath below 30 ^QC).

Figure 8 shows the UV absorption spectra of conventional graphene, the MLG of the invention and the CNFs of the invention in DMF (first image) and ethanol (second image).

First image:

- The upmost curve in the first image is for conventional graphene nanopowder. This peak at 278 nm is consistent with what has been reported in literature for conventional graphene suspensions.

- The intermediate curve in the first image is for the MLG of the invention. This peak is also consistent with what has been reported in literature for conventional graphene suspensions.

The lowest curve in the first image is for the CNFs of the invention, here graphene nanoflowers. An absorption band is observed at around 248 nm in the spectrum of the carbon nanoflower. This peak has not been reported in literature for conventional graphene suspensions.

Second image:

- The upmost curve in the first image is for conventional graphene nanopowder. This peak at 278 nm is consistent with what has been reported in literature for conventional graphene suspensions. ^■ The intermediate curve in the first image is for the MLG of the invention. The lowest curve in the first image is for the CNFs of the invention, here graphene nanoflowers. An absorption band is observed at around 204 nm in the spectrum of the carbon nanoflower and MLGs. This peak has not been reported in literature for conventional graphene suspensions.

- There is a strong absorption peak at 204 nm and a weak peak at 241 nm in all

ethanol-based dispersions.

Figures 9a - 9c shows photographies of three different thin films made from GNF suspensions of the invention printed on a surface.

Figure 9a is a thin film made from a suspension of the invention comprising Graphene Nanoflowers (GNFs) in buthanol and polyvinylpyrrolidone (PVP) as stabilizer. Obvious stripes as pointed to with reference number 24 can be seen, which is an indication of that the printability is not the best possible giving an uneven result.

Figure 9b is a thin film made from a suspension of the invention comprising Graphene Nanoflowers (GNF) mixed with ZnO nanoparticles in ethanol, a stabilising agent. (The commercial ZnO particles from Nanograde). Spreading of the film suspension as is can be seen as indicated by reference number 25, which is an indication of that the printability is not the best possible by not giving a sharp result.

Figure 9c is a thin film made from a suspension of the invention comprising Graphene Nanoflowers (GNF) mixed with ZnO nanoparticles in isopropanol (IPA), a stabilising agent, (The commercial ZnO particles from Nanograde). As reference number 26 shows, the best printability was achieved with isopropanol as solvent, which gave the best film with no spreading or stripes.

All suspensions were stable and no visible sedimentation had occurred after several weeks. Nor were there any significant phase-separation or precipitation observed. Both the PVP addition and the ZnO addition seem to improve the results when compared to pure Graphene Nanoflowers GNF in ethanol or other solvents. All the suspensions were made by mixing the GNFs with the solvent and stabilizers in ultrasonic bath and the suspensions were printed on the surface with gravure on PET ST506. Figure 10 shows transmittance values of films of the invention prepared by using CNF and MLG suspension with varying deposition times. The deposition time affects directly the film thickness. The films were prepared from 0.1 - wt% MLG or CNF suspensions. The coverage times of 7 min and 14 min resulted in 93% and 100 & surface coverage, respectively.

Figure 11 shows the Transmission Electron Microscope images (TEM) of suspensions of G/CNF doped with Ag (silver) nanoparticles 27. The carbon nanof lowers are those with reference number 22 and the MLGs with reference number 23. The Ag doping was carried out in a water/DMF solution. The size and concentration of the Ag particles can be adjusted depending on the requirements of the final application. The images show an even distribution of small Ag particles, which is not achieved with conventional graphene.

An Au doping can be carried out in organic solvents, such as ethanol or hexane. The precursor for the Au (gold) nanoparticles, can for example be an organometallic compound and the doping can be done at the same time with the preparation of the suspensions. An even distribution is also achieved with small Au particles, which is not achieved with conventional graphene.

TEST EXAMPLE 1

The conductivity of ink suspensions of the invention are presented in figure 4. They are made by the methods of the invention (temperature in the first phase 1400^°C and 2600^°C in the second phase) The conductivity of the suspensions are improved compared to that of the pure solvents, which is ca -2.5 - 6.5 με/αη for DMF, < 0.02 με/αη for NMP, and ca 0.1 με/αη for ethanol.

The conductivity is improved by ca 88% compared to the pure solvents.

For the MLG/ethanol suspension, sedimentation occurred in a few days.

For the MLG/NMP and MLG/DMF suspensions, there were no sedimentation after 10 months from the preparation of the suspensions even if the tests were centrifuged at 500rpm in 90 min. The CNF/ethanol suspension was stable in more than three days. For the CNF/NMP suspension, there still is at the time of filing of this application no sedimentation after 1 1 months. The CNF/DMF suspension was stable after several weeks.

TEST EXAMPLE 2 - TRANSMITTANCE The transmittance of some thin films made of suspensions of the invention are presented below

Q= 90 mL/h, L= 3 cm, T_Piate= 170 °C, F= 12 L/min, Cone. = 0.004 wt% (approx.)

Q = Feeding speed of presurcor,

L= Nozzle distance from the substrate,

T= Temperature of substrate,

F= Flow rate from spraying nozzle,

Concentration of CNF or MLG They are made by the methods of the invention (temperature in the first phase 1400^°C and 2600^°C in the second phase)

30min deposition CNF/NMP:EtOH 82.7%

30min deposition MLG/NMP:EtOH 54.80%

30 min deposition CNF/DMF:EtOH 91 .95%

30 min deposition MLG/DMF:EtOH 48.92%

The transmittance of the thin films made from the suspensions of the invention have been measured with a deposition time of 30 min at a wavelength of 550nm. The deposition time has an influence on the surface coverage of the thin film.

In this example, a deposition time of 30 min was used since the concentration was so low. In another example, films prepared from 0.1 - wt% MLG suspensions with deposition times of 7 min and 14 min result in 93% and 100 & surface coverage, respectively.

The results show the best result for a suspension of CNF in a mixture of /DMF:EtOH (91 .95%) being almost transparent. TEST EXAMPLE 3 - RESISTANCE

The resistance of some thin films made of suspensions of the invention are presented below:

3.7x 1 0^"5 Ωι CNF/ glass substrate

2.6 x 1 0^"6 ΩΓΠ CNF/MA-glass substrate (MA = Methacrylic acid-methyl methacrylate) 9.3 x 1 0^"5 Ωι MLG on glass substrate

1 x 1 0^"4 Ωι MLG MA-glass substrate

MLG/PVDF 5.8 x 1 0^"5 Qm MLG glass substrate

MLG/PVDF/Au 1 .1 χ 1 0^"4 Ωηι MLG glass substrate

The best value was obtained for a thin film prepared from a suspension made from Multi- Layer Graphene sheets (MLGs) in PolyVinylidene Fluoride (PVDF) and Au doped.

TEST EXAMPLE 4 - ADHESION

The adhesion of some thin films made of suspensions of the invention have been tested:

1 . MLG/PVDF on Glass substrate

2. MLG/PVDF on MA-Glass substrate

3. CNF/PVDF on Glass substrate

4. CNF/PVDF on MA- Glass substrate

A scotch tape test was performed by testing how much material adheres to the tape when it is placed on the thin film and then peeled off.

The adhesion of the thin film to the substrate is better the less material is adhered to the tape when peeling it off. In the above test, the best results were obtained for test no 1 (MA- polymer coated glass substrate, MLG+PVDF deposition), then test no 4 (MA-polymer coated glass substrate, CNF+PVDF deposition). Thus, the best results were achieved best with the MA-polymer coated glass substrate, i.e. the polymer used on the surface of the glass improved significantly the adhesion. Compared to conventional thin films made of graphene, a typical resistance value is 1 x 1 0^"' ΩΓΠ at 20 °C (for a single uniform graphene sheet) .

TEST EXAMPLE 5 - RESISTIVITY

Figure 5 illustrates the resistivity of some thin films made from the suspensions of the invention and how the addition of the PVDF polymer influences on the resistivity. The products consists of suspensions of graphene sheets or carbon nanoflowers (CNFs) deposited on a glass or MA glass surface.

The products have been improved by adding polymer (PVDF) either directly on the surface of a glass substrate (MA-glass) or directly to the suspension (PVDF). The lowest and best resistivity was obtained for CNF/PVDF/glass (2.7 Ω).

COMPARISON EXAMPLE 1

Three sample suspensions of graphite, MLGs & carbon nanoflowers of the invention, and commercial Angstron graphene, respectively in N-methyl-2-Pyrrolidone were prepared for comparison on 3.1 1 .2015 by sonication in 15 min using ultrasound at 22^QC. The ultrasound device was Ultrasound VWR used in full power.

Sample 1 :

A suspension comprising 1 mg/ml graphite with a particle size of < 25 pm

Sample 2:

A suspension comprising 1 mg/ml MLGs & carbon nanoflowers of the invention,

The graphene sheets & carbon nanoflowers used were prepared by the method of the invention, the temperature of the first phase being 1400^°C, and the second phase 2600 ^°C.

Sample 3:

A suspension comprising 1 mg/ml commercial Angstron graphene with a particle size of <5 μηπ

N002-pdr X-Y dimension <5 μηηηι, thickness <1 μηπ Results: There is still no sedimentation at the day of filing of this patent application (more than ten months after preparation of the samples), whereas sedimentation of samples 1 and 3 occured within the first day. COMPARISON EXAMPLE 2

Suspension 1 (PRIOR ART SUSPENSION):

A suspension prepared 4.1 1 .2015 by sonication in a glass tube comprising 1 % of weight graphene in N-Methylpyrrolidone (1 mg/ml)

with a particle size of <5 μηιι , thickness <1 μηπ

Graphene type: Nano graphene Platelets (N002-PDR graphene) (Angstron)

Sedimentation in two days

Suspension 2 (PRIOR ART SUSPENSION):

A suspension prepared 4.1 1 .2015 by sonication in a Bubble bag comprising 1 % of weight graphene in N-Methylpyrrolidone

with a particle size of <5 μηιι , thickness <1 μηπ

Graphene type: Nano graphene Platelets (N002-PDR graphene) (Angstron) A slow sedimentation occurs. There is an apparent precipitate after a month

Bubble Bag is a disposable plastic bag that improves ultrasound. The inner bag surface has been modified with small pits that traps gas bubbles and lead to the enhanced bubble formation.

Suspension 3:

A suspension prepared 4.1 1 .2015 comprising 1 % of weight carbon nanoflower in N- Methylpyrrolidone

Graphene type: Nano flower

Particle size: mixture of graphene sheets and CNFs. The size of CNFs approximately 20 nm, the number of layers less approximately 4, interlayer distance 0.36±0.02 nm.

The MLG sheets in the sample both folded and free standing, the size of the can be up to several micrometers, number of layers less than 15, Interlayer distance 0.38±0.03 nm. Still no so sedimentation in September 201 6

COMPARISON EXAMPLE 3

The following table shows the properties of some example suspensions of the inventions.

Suspension 1 :

Suspension 1 contains 1 weight% of carbo nanoflowers in a solvent mixture of N,N- dimethylformamide and ethanol (1 :19 v/v).

Suspension 1 were prepared by using carbon nanoparticles prepared by the method of the invention, by bath ultrasonication (10 min), followed by a two step centrifugation (6000rpm, 1 h and 6000rpm, ¹/2h) and separation of the supernatant and sediment. The carbon nanoflowers were separated from the obtained mixture of graphene sheets and carbon nanoflowers by sonication and ultracentrifugation.

Suspension 2:

Suspension 2 contains 1 weight % of graphene sheets (particle size?) in a solvent mixture of Ν,Ν-dimethylformamide and ethanol (1 :19 v/v).

Suspension 2 were prepared by using carbon nanoparticles prepared by the same method parameters of the invention as above. The graphene sheets were separated from the obtained mixture of graphene sheets and carbon nanoflowers by sonication and ultracentrifugation.

Suspension 3:

Suspension 3 contains 1 weight % of carbo nanoflowers in a solvent mixture of N- methylpyrrolidine and ethanol (1 :19 v/v).

Suspension 3 were prepared by using carbon nanoparticles prepared by the same method parameters of the of the invention as above. The carbon nanoflowers were separated from the obtained mixture of graphene sheets and carbon nanoflowers by sonication and ultracentrifugation.

Suspension 4:

Suspension 4 contains 1 weight % of graphene sheets (particle size?) in a solvent mixture of N-methylpyrrolidine and ethanol (1 :19 v/v). Suspension 2 were prepared by using carbon nanoparticles prepared by the same method parameters of the invention as above. The graphene sheets were separated from the obtained mixture of graphene sheets and carbon nanoflowers by sonication and ultracentrifugation.

The zeta potential values of the suspensions indicate a good stability of the suspensions.

The general dividing line between stable and unstable suspensions is generally taken at either +30mV or -30mV. Particles with zeta potentials more positive than +30mV or more negative than -30mV are normally considered stable.

N-methylpyrrolidine gives the best results in this respect.

Claims

Method of producing suspensions of carbon nanoparticles comprising the steps of a) providing an organosilane compound comprising a silicon-silicon bond as a precursor material, and vaporizing the precursor material,

b) heating the vaporized precursor material for obtaining silicon-carbon nanoparticles, c) heating the obtained silicon-carbon nanoparticles by induction for obtaining multilayer graphene, MLG, sheets and carbon nanoflowers, CNFs,

d) mixing the obtained silicon-carbon nanoparticles with an organic solvent a sufficient time for obtaining a homogenous suspension as a result.

Method of claim 1 , characterized by vaporizing the precursor material by an aerosol method.

Method of claim 1 , characterized by the precursor material being an organodisilane comprising a silicon-silicon bond and one or more methyl groups, and selected from hexamethyldisilane, Si2(CH3)e, and 1 ,1 ,3,3 - tetramethyl- 1 ,3-disilanocyclobutane.

Method of claim 1 , characterized by the precursor material being an organodisilane comprising a Si-N-Si bond, such as hexamethyldisilazane ((CH3)3SiNHSi(CH3)3).

Method of any of claims 1 - 4, characterized by using in step b), a temperature of 900 °C - 1400°C, more preferably 1200°C and most preferably 1400°C, for obtaining a crystal size of have a crystal size of 7nm - 5 nm at 800 °C - 1400°C , preferably 1 ,3nm at 800 °C - 1400°C and most preferably 5nm at 1400°C, corresponding to a particle size of 1 10 nm at 900 °C 77nm at 1000°C, most preferably < 69nm at 1400°C.

Method of any of claims 1 - 5, characterized by using a heating temperature in step b) of 1900 °C - 2600 °C, preferably more than 2200°C, most preferably 2600°C.

Method of any of claims 1 - 6, characterized by performing the heating in step c) by annealing in argon at high temperature and atmospheric pressure in an inductively heated furnace.

8. Method of claim any of claims 1 - 7, characterized by using Carbon-Silicon, C-Si, nanoparticles of a particle size of 69 nm for obtaining a graphene sheet size of > 1 μηι, less than 15 atomic layers of graphene sheets, a distance between the layers 0.36 ± 0.02 nm, and a carbon nanoflower diameter size of 10 - 20 nm . 9. Method of any of claims 1 - 8, characterized by performing the mixing in step d) by ultrasonication.

10. Method of any of claims 1 - 9, characterized by mixing in step d) 0,004 - 20 % of weight, preferably 0,05 - 1 % of weight, of the obtained carbon nanoparticles into the organic solvent.

1 1 . Method of any of claims 1 - 10 further characterized by separating the graphene sheets and carbon nanoflowers from eachother after step d) by (ultra)sonication and centrifugation.

12. Method of any of claims 1 - 1 1 , characterized by using ethanol, N,N- dimethylformamide or N-methyl-2-pyrrolidone or mixtures thereof as the solvent, preferably N-methyl-2-pyrrolidone. 13. Method of any of claims 1 - 12, characterized by doping the suspension by a metal or metal oxide by adding ultrafine particles of the metal or metal oxide within the carbon nanoflower structures and/or the graphene sheets, by forming a connective network of either particles or layers of doping material or by mixing the metal or metal oxide with the G/CNFs and subsequently heating to get conductive metals inside the G/CNFs.

14. Suspensions of carbon nanoparticles in an organic solvent, wherein the carbon nanoparticles comprises multilayer graphene, MLG, sheets and/or carbon nanoflowers, CNFs. 15. Suspensions of claim 14, characterized in that the carbon nanoparticles consists of carbon nanoflowers, CNFs.

1 6. Suspensions of claim 14, characterized in that the carbon nanoparticles consist of multilayer graphene, MLG, sheets.

17. Suspensions of claim 14, characterized in that the carbon nanoparticles comprises interconnected or separated MLG sheets and CNFs.

18. Suspensions of any of claims 14 - 17, characterized by a sedimentation time of from at least 2 weeks and up to more than 10 months. 19. Suspensions of any of claims 14, 1 6, or 18, characterized in that the layers of the graphene sheets are 5 -15, preferably 5 - 10, and most preferably 5 and the distance between the layers are 0.30 - 0.40 nm.

20. Suspensions of any of claims 14, 15, or 18, characterized in that the diameter of an individual carbon nanoflower is approximately 20 nm, the diameter of the void in an individual carbon nanoflower is up to 10 nm, the number of layers ca 4 - 15, preferably ca 4, and the distance between layers: ca 0,36 nm, preferably 0,36 nm ± 0,02nm

21 . Suspensions of claim any of claims 14 -20, characterized by comprising 0,05 - 20 % of weight of the carbon nanoparticles in an organic solvent.

22. Suspensions of claim 21 , characterized by the solvent being an alcohol, such as ethanol, or Ν,Ν-dimethylformamide or N-methyl-2- pyrrolidone or mixtures thereof, preferably N- methyl-2- pyrrolidone.

23. Suspensions of claim any of claims 14- 22, characterized by an electrical conductivity

24. Suspensions of any of claims 14 - 23 with a zeta potential of >± 30mV, preferably >± 40mV, and more preferably >± 60mV.

25. Suspensions of claim any of claims 14 - 24, characterized in that the carbon nanoparticles are doped with Silver (Ag), Gold (Au), Copper (Cu), or Zinc oxide (ZnO).

26. Use of the suspensions of any of claims 13 - 25 as conductive inks.

27. Use of the suspensions of any of claims 13 - 25 as thin films and spray coatings.

28. Thin film of claim 27, characterized by comprising Polyvinylidene fluoride, PVDF, to improve the adhesion.

30. Thin film of claim 27 or 28 made from a suspension of CNF in a mixture of N,N- dimethylformamide and ethanol, the film having a transparency of > 90%.