Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
  • C23C16/442—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating using fluidised bed process
• • 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
  • C01B32/158—Carbon nanotubes
  • C01B32/168—After-treatment
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
  • C23C16/26—Deposition of carbon only
• • 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
  • C01B2202/00—Structure or properties of carbon nanotubes
  • C01B2202/06—Multi-walled nanotubes
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2202/00—Structure or properties of carbon nanotubes
  • C01B2202/20—Nanotubes characterized by their properties
  • C01B2202/34—Length
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2202/00—Structure or properties of carbon nanotubes
  • C01B2202/20—Nanotubes characterized by their properties
  • C01B2202/36—Diameter
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2006/00—Physical properties of inorganic compounds
  • C01P2006/11—Powder tap density

Definitions

• the invention relates to a method for producing multi-walled carbon nanotubes. Furthermore, the invention relates to multi-walled carbon nanotubes and one of these
• Carbon nanotube carbon nanotube powder Carbon nanotube carbon nanotube powder.
• Carbon nanotubes in the prior art are understood to mean mainly cylindrical carbon tubes with a diameter between 3 and 100 nm and a length which is a multiple of the diameter. These tubes consist of one or more layers of ordered carbon atoms and have a different nucleus in morphology. These carbon nanotubes are for example also referred to as “carbon fibrils” or “hollow carbon fibers”. Carbon nanotubes have long been known in the literature. Although Iijima
• CCVD Deposition of carbon from gaseous hydrocarbons at reaction conditions
• Carbon donors called acetylene, methane, ethane, ethylene, butane, butene, butadiene, benzene and other carbon-containing reactants.
• the catalysts usually include metals, metal oxides or decomposable or reducible metal components.
• metals metal oxides or decomposable or reducible metal components.
• candidate metals for metals Fe, Mo, Ni, V, Mn, Sn, Co, Cu and others are mentioned.
• the individual metals usually alone have a tendency to catalyze the formation of nanotubes.
• high yields of nanotubes and small amounts of amorphous carbons are advantageously achieved with metal catalysts containing a combination of the above-mentioned metals.
• catalyst systems based on the prior art are based on combinations containing Fe or Ni.
• the formation of carbon nanotubes and the properties of the formed tubes depend in complex ways on the metal component used as catalyst or a combination of several metal components, the carrier material used and the catalyst-carrier interaction, the reactant gas and partial pressure, an admixture of hydrogen or other gases , the reaction temperature and the residence time or the reactor used. Optimization represents a special challenge for a technical process.
• the metal component used in the CCVD and referred to as a catalyst is consumed in the course of the synthesis process. This consumption is due to a deactivation of the metal component, for. B. due to deposition of carbon on the entire particle, which leads to the complete coverage of the particle (this is known to the skilled person as "Encapping".) A reactivation is usually not possible or economically not useful
• the catalyst here comprises the whole of the support and the catalyst used, and owing to the described consumption of catalyst, a high yield of carbon nanotubes, based on the catalyst used, is an essential requirement for catalyst and process.
• Typical structures of carbon nanotubes are those of the cylinder type (tubular structure).
• the cylindrical structures are differentiated between single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNTs). Common procedures to their
• Production are eg arc discharge, laser ablation, chemical vapor deposition (CVD process) and catalytic vapor deposition (CCVD process).
• CVD process chemical vapor deposition
• CCVD process catalytic vapor deposition
• Such cylindrical carbon tubes can also be produced.
• Iijima (Nature 354, 1991, 56-8) reports the formation of carbon tubes in the arc process which consist of two or more graphene layers rolled up into a seamlessly closed cylinder and nested together. Depending on the rolling vector, chiral and achiral arrangements of the carbon atoms along the longitudinal axis of the carbon fiber are possible.
• doped carbon nanotubes in which the properties of the tubes are modified by the doping of the carbon layers with foreign atoms.
• Doped carbon nanotubes are promising candidates for further miniaturized nano-scale electronic circuits.
• suitable and efficient production processes are required.
• Oberlin, Endo and Koyama have described in the prior art a way of producing cylindrical carbon nanotubes (Carbon 14, 1976, 133) wherein aromatic hydrocarbons, such as benzene, are reacted on a metal catalyst at about 1100 ° C. in a fly-back reactor. This forms carbon nanotubes with a graphitic core, which is however covered with a coating of amorphous carbon.
• a method for producing doped carbon nanotubes is known from WO 2009/080204 AI.
• catalytic growth of nitrogen-doped carbon nanotubes takes place in a fluidized bed.
• a targeted doping of the carbon nanotubes, in particular a different doping of the respective carbon layers, is not possible with the method.
• an object of the present invention to provide a method for the production of carbon nanotubes, with which the number of graphene-like layers of carbon nanotubes can be directly increased.
• the invention is based on the object to provide a method with which the thickness growth of carbon nanotubes can be adjusted specifically.
• One of the objects of the invention is to provide a process by means of which carbon nanotubes can be produced with targeted doping in specific graphene-like layers.
• the invention is based on the object to provide corresponding carbon nanotubes and a corresponding carbon nanotube powder available.
• carbon nanotubes can be used as the substrate.
• examples of carbon nanotube types are: single-wall nanotubes with a single graphene-like layer, multi-wall nanotubes with multiple graphene-like layers; Carbon nanotubes with a tubular structure, bamboo, winding or scroll structure; so-called capped carbon nanotubes, in which at least one tubular graphene-like layer is closed at their ends by fullerene hemispheres; or any combination of the aforementioned species and carbon nanofibers.
• the substrate is placed in a moving bed of a reactor.
• a reactor with a moving bed is procedurally delimited in particular by a fixed bed reactor and by a reactor without a bed, such as, for example, an entrained flow reactor.
• the substrate In a reactor with a bed, the substrate is spatially located above a support.
• the substrate In the case of a fixed bed reactor, for example, the substrate can be contained in an upwardly opened boat, the boat serving as a carrier here. The substrate is therefore in the process during the process
• the substrate is mixed with a moving bed while performing the process.
• the substrate is preferably first applied to the surface of a carrier, by which the substrate is held in the reactor and spatially localized.
• the substrate is then mixed, for example by a movement of the carrier or by the passage of the substrate through a gas stream.
• the mixing of the substrate leads to improved heat and mass transport within the substrate and to a more efficient reaction sequence.
• the substrate is not spatially located above a support, but is moved through the reactor, for example along with a gas flow.
• a moving bed reactor graphitic carbon can be deposited on the carbon nanotube.
• a soot layer especially in the form of amorphous carbon, on the nanotubes was found.
• Oberlin, Endo and Koyama have been unable to observe any graphitic deposition of carbon even in the case of a flow reactor, ie a bedless reactor.
• the substrate is preferably presented as a powder, in particular as a free-flowing powder.
• the flowability of the powder according to DIN EN ISO 6186 is preferably 5 mL / s to 100 mL / s, in particular 10 mL / s to 70 mL / s. As a result, a good mixing of the substrate in the moving bed is achieved during the process.
• the flowability can be determined, for example, with the free-flowing device from the company Karg-Industrietechnik (Code No. 1012,000) Model PM and a 15 mm nozzle according to Standard ISO 6186.
• the substrate can first with a bulk density according to DIN EN ISO 60 of preferably 20 to 500 kg / m 3 , more preferably from 20 to 450 kg / m 3 , in particular from 80 to 350 kg / m 3 , be placed on the support of the reactor.
• the surface of the substrate used is preferably greater than 20 m 2 / g, preferably greater than 100 m 2 / g, in particular greater than 200 m 2 / g. As a result, a larger surface for graphitic deposition is available, so that the yield can be increased.
• the carbonaceous precursor preferably contains or consists of an optionally substituted aliphatic, cyclic, heterocyclic, aromatic or heteroaromatic compound or a mixture thereof.
• Aliphatic means unbranched, branched and / or cyclic alkane, alkene or alkyne.
• the aliphatic molecules have from about 1 to about 20, more preferably from about 1 to about 12, and most preferably from about 2 to about 6 carbon atoms.
• the carbon-containing precursor is an at least partially unsaturated or aromatic compound or the precursor contains such a compound or a mixture thereof.
• partially unsaturated compounds are unbranched, branched and / or cyclic alkenes or alkynes, which may be optionally substituted.
• Alkene as used herein means a hydrocarbon base element which contains at least one carbon-carbon double bond.
• Carbon-containing precursors which can be used according to the invention are, for example, ethylene, propene, butene, butadiene, pentene, isoprene, hexene, 1-, 2- or 3- Heptene, 1-, 2-, 3- or 4-octene, 1-nonene or 1-decene in question, which may be optionally substituted, such as acrylonitrile.
• Alkyne as used herein means a hydrocarbon base element containing at least one carbon-carbon triple bond.
• Carbon-containing precursors which can be used in accordance with the invention include, for example, ethyne, propyne, butyne, pentyne, hexyne, 1-, 2- or 3-heptin, 1 - 2-, 3-, or 4-octyne, nonyne or decyne, which may be optionally substituted.
• Suitable cyclic alkenes or alkynes are nonaromatic, mono- or multicyclic ring systems of, for example, from about 3 to about 10, preferably from about 5 to about 10, carbon atoms which, in the case of the cycloalkenes, have at least one carbon atom.
• Carbon double bond in the case of cycloalkynes contain at least one carbon-carbon triple bond.
• monocyclic cycloalkenes are cyclopentene, cyclohexene, cycloheptene and the like.
• An example of a multicyclic alkene is norbornene.
• the carbonaceous precursor may also contain or consist of an optionally substituted heterocyclic molecule.
• heterocyclic means a mono- or multicyclic ring system of from about 3 to about 10, preferably from about 5 to about 10, more preferably from about 5 to about 6 carbon atoms wherein one or more carbon atoms in the ring system are replaced by heteroatoms.
• Heteroatom as used herein means one or more atoms of oxygen, sulfur, nitrogen, boron, phosphorus or silicon, with the respective oxidized forms being included in a preferred embodiment of the invention, those containing as carbon
• Precursors used heterocyclic compounds at least one carbon-carbon or carbon-heteroatom double bond.
• Aromatic molecule or “aromatic compound” as used herein includes optionally substituted carbocyclic and heterocyclic compounds containing a conjugated double bond system. Heterocyclic aromatics are also referred to as “heteroaromatics.” Examples of aromatic molecules according to the invention are optionally substituted monocyclic aromatic rings having 0 to 3 heteroatoms independently selected from O, N and S, or 8 to 12 membered aromatic bicyclic ring systems 0 to 5 heteroatoms which are selected independently of one another from O, N and S.
• Suitable carbonaceous precursors which can be used according to the invention are, for example, optionally substituted benzene, naphthalene, anthracene, pyridine, quinoline, isoquinoline, pyrazine, quinoxaline, acridine, pyrimidine, Quinazoline, pyridazine, cinnoline, furan, benzofuran, isobenzofuran, pyrrole, indole, isoindole, thiophene, benzothiophene, imidazole, benzimidazole, purine, pyrazole, indazole, oxazole, benzoxazole, isoxazole, benzisoxazole, thiazole and / or benzothiazole in question.
• substituents may be purely aliphatic or one or more heteroatoms According to a preferred embodiment, the substituents are selected from the group consisting of C 1 -C 10 aliphatic, C 3 -C 10 cycloaliphatic, C 10 -C 10 aryl, 5-10-membered heteroaryl or 3-10-membered Heterocyclyl Ci- to Cö-haloalkyl, ci- bis
• carbonaceous precursors which have achieved good to very good results in practice are unsaturated hydrocarbons such as ethylene or acrylonitrile and aromatic molecules such as benzene or pyridine.
• Carbon nanotubes for the formation of further graphene-like layers may, for example, have a tubular structure or else a scroll structure.
• Process parameters are given below examples of process conditions in which graphitic deposition could be detected.
• the process conditions include in particular the important parameters of the process temperature and the movement of the moving bed.
• the movement of the bed is determined, for example, by the gas flow through the fluidized bed.
• the duration of the process as well as the type of precursor used can be important for the production of the carbon nanotubes.
• the carbon nanotubes can be removed from the reactor. Due to the graphene-like layers deposited on the carbon nanotubes during the process, these now have, on average, a larger outer diameter.
• a process temperature in the range of 850 ° C to 1300 ° C preferably in the range of 900 ° C to 1300 ° C, in particular from 950 ° C to 1300 ° C are set. Measurements have shown that below a temperature of 850 ° C no significant deposition of graphitic carbon occurs. Above 1300 ° C, the thermal loads on the reactor become so great that only special reactor materials can be used, making the process expensive and uneconomical.
• the proportion of active catalyst material which could cause carbon nanotube elongation in the moving bed during the process is less than 5000 ppm, preferably less than 1000 ppm, especially less than 500 ppm. In this way, the growth in thickness can be increased, ie the proportion of precursor carbon deposited graphitically on the carbon nanotube.
• Catalysts which cause carbon nanotube elongation are used in particular in the production of carbon nanotubes in known processes.
• iron, cobalt or nickel-containing catalysts are often used for this purpose.
• the amount of catalyst in parts per million refers to the weight fraction of the catalyst metal in the total weight of the substrate. Furthermore, this specification is limited to the proportion of catalyst particles that are actively available for catalysis in the substrate. Passivated catalyst particles, in particular by encapsulation, such as, for example, catalyst particles enclosed in the interior of carbon nanotubes, are irrelevant in the present case and are therefore not taken into account in the quantitative specification.
• the low concentrations of the catalysts which are advantageous for the process can be achieved by using purified, in particular acid-cleaned, carboniianotubes for the substrate. In the catalytic production of Carbon ion tubes regularly remain residues of the catalyst used in the produced carbon nanotube powder.
• the process conditions in particular temperature, pressure and / or gas composition in the reactor, are chosen such that the kinetic constant for the thickness growth of the carbon nanotubes, i. for the graphitic deposition of carbon on the outer graphene layers of the carbon nanotubes is greater than the kinetic constant for the elongation of the carbon nanotubes caused by catalyst components.
• the kinetic constants are adjusted via the process conditions such that the ratio of the carbon which is consumed for the thickness increase to the carbon consumed for the growth in length is greater than 1, preferably greater than 5, in particular greater than 10.
• catalytic processes such as carbon nanotube elongation, typically show higher turnover than noncatalytic processes such as carbon nanotube thickness growth.
• the abovementioned ratios can therefore be achieved in particular at the highest possible process temperatures, preferably at process temperatures of more than 900 ° C., in particular of at least 950 ° C.
• a fluidized bed of a fluidized bed reactor is used as the moving bed.
• the substrate is deposited on a carrier, in particular a carrier plate.
• a gas stream is introduced into the substrate, so that the substrate and the gas stream form a so-called fluidized bed.
• the fluidized bed is characterized by a liquid-like behavior in which the individual particles of the substrate are mixed in the gas stream.
• a good heat and mass transfer is achieved in the fluidized bed, so that there are substantially homogeneous process conditions in the fluidized bed. This promotes uniform graphitic deposition of carbon on the carbon nanotubes.
• a fluidized bed reactor for example, a quartz fluidized-bed reactor can be used, in which the reactor is essentially formed by a quartz glass housing, for example a quartz glass tube.
• the moving bed may also be provided by a rotary tube reactor.
• a rotary tube reactor has a reactor tube, the longitudinal axis of which is aligned at a small angle of, for example, 1-5 ° to the horizontal.
• the reactor tube is rotatably supported about its longitudinal axis and driven for rotation about this axis.
• the substrate is first applied to the inner surface of the reactor tube.
• the reactor tube is rotated about its longitudinal axis, while a carbon-containing precursor is introduced into the reactor tube.
• a gas flow through the fluidized bed is adjusted for the process conditions causing the graphitic deposition to give a stable fluidization. With this gas flow range, experimentally good yields were found in the graphitic deposition of carbon.
• gas mixtures for example a mixture of an inert carrier gas with the carbonaceous precursor.
• Stable fluidization means that the gas flow has a velocity that is greater than or equal to the minimum fluidization velocity.
• WO 2007/118668 A2 the content of which should be incorporated herein by reference.
• formula (1) on page 7 of WO 2007/118668 A2 for determining the minimum fluidization rate.
• a precursor entry into the reactor of 0.0001 to 1 g, preferably 0.001 to 0.2 g, in particular 0.005 to 0.1 g, per gram of substrate and set per minute. This precursor entry has proven to be advantageous for a high yield of the method. With a lower precursor entry, too little carbon is available for optimal graphitic deposition. With a higher precursor entry, a part of the precursor is not reacted or even deposited in non-graphitic form, so that the results of the method are impaired.
• the process can be carried out continuously, quasi-continuously or batchwise.
• carbon nanotubes are continuously fed to the fluidized-bed reactor as a substrate and / or processed carbon nanotubes are removed.
• a discontinuous procedure the Process carried out in successive batches (batches).
• a substrate is introduced, and the substrate, which has been converted as completely as possible to the product, is substantially completely removed from the fluidized-bed reactor at the end of the process.
• a quasi-continuous procedure only a certain portion of the product is removed from the fluidized-bed reactor at the end of a procedure and the substrate is filled up again accordingly.
• the process time is adjusted so that the diameter distribution of the carbon nanotubes produced after the end of the process, a diameter ratio D90 / D10 of less than 4, preferably less than 3. More preferably, the process time is set so that the diameter ratio D90 / D10 of the produced carbon nanotubes is compared with the corresponding diameter ratio of the starting material, i. the carbon nanotube submitted as substrate is reduced by at least 20%, preferably by at least 30%, in particular by at least 40%.
• the graphitic deposition of the carbon is preferably on carbon nanotubes which have a below average diameter with respect to the substrate since they have a larger surface area to weight ratio and therefore in proportion a larger reaction area for the deposition of the carbon.
• the diameter value D90 or D10 means that 90% and 10%, respectively, of the carbon nanotubes are smaller than this diameter.
• the diameter ratio D90 / D10 corresponds to the quotient of D90 and D10.
• the carbonaceous precursor according to another embodiment of the process contains or consists of an at least partially unsaturated or aromatic compound.
• such molecules such as benzene or ethylene
• a carbon-carbon or carbon-heteroatom double bond or in particular an aromatic ring thus favor graphitic deposition.
• the production of carbon nanotubes with individual doped graphene-like layers can be achieved in a further preferred embodiment of the method in that the carbon-containing precursor contains or consists of a compound which comprises carbon and at least one heteroatom from the group consisting of nitrogen, boron, phosphorus or silicon, is used.
• the carbonaceous precursor may also contain at least two compounds, at least one of which comprises carbon and at least one other thereof comprises an element selected from nitrogen, boron, phosphorus or silicon.
• the carbonaceous precursor may also contain at least two compounds, at least one of which comprises carbon and at least one other thereof comprises an element selected from nitrogen, boron, phosphorus or silicon.
• oxygen, nitrogen, boron, phosphorus or silicon other impurities suitable for doping are also suitable in the embodiment described above.
• graphene-like layers can be deposited on the carbon nanotubes in the substrate in a targeted manner, which have a doping corresponding to the precursor.
• carbon nanotubes can be produced which have different layers with different doping or both doped and undoped layers.
• a first precursor may be introduced in a first step and a second precursor may be introduced into the moving bed in a second, temporally downstream step.
• a second precursor may be introduced into the moving bed in a second, temporally downstream step.
• successive different graphene-like layers in particular doped and undoped or differently doped layers, can be deposited on the carbon nanotube.
• the alternating introduction of different precursors can produce carbon nanotubes with layers that alternate with respect to their doping.
• the object underlying the invention is achieved in a multi-walled carbon nanotube comprising at least a first and a second graphene-like layer, wherein the second layer is arranged in the cross section of the carbon nanotube outside the first layer, according to the invention, that one of the two layers, a first doping and the other of the two layers has a second, different type of doping or is undoped.
• the second layer ie the outer layer, may have a first doping, while the first layer, ie the inner layer, is undoped or has a second, different type of doping.
• the first layer ie the inner layer
• the second layer ie the outer layer is undoped or has a second, different type of doping.
• the specific doping of individual graphene-like layers allows the properties of the carbon nanotubes to be adjusted as required. This results in new fields of application for the carbon nanotubes.
• the targeted adjustment of the electrical or electronic properties of the carbon nanotubes can be used as components in electronic circuits.
• the carbon nanotubes can be used as catalysts.
• the carbon nanotubes can be specifically functionalized by the doping.
• doping can be used to adjust the compatibility of the carbon nanotubes with other materials, for example for use in composite materials. Another possible potential of the selectively doped carbon nanotubes results in the range of electrode materials and lithium-ion batteries.
• use of the carbon nanotubes as a conductivity additive or anode material comes into question.
• the carbon nanotubes described above are preferably prepared by one of the previously described methods. Conversely, the methods described above are preferably used to prepare such carbon nanotubes.
• the second layer of the carbon nanotube is arranged in cross-section outside the first layer.
• Such an arrangement is understood to mean that the first layer is a layer further inward with respect to the cross section of the carbon nanotube and the second layer is farther out, i.e., more so. a further away from the center of the cross section location.
• the first and the second layer can lie directly above one another. Alternatively, further layers may be arranged between the first and the second layer.
• the first and second layers may each have a tubular structure such that each of the two layers has a tubular shape, with the second layer surrounding the first layer.
• the layers can also be present in a scroll structure, in which a structure of several superimposed, graphene-like layers is wound up. An outer graphene-like layer of this winding can then be regarded as the second layer and an inner graphene-like layer as the first layer.
• combinations are possible, for example, with tubular inner layers and outer layers of the scroll type or vice versa.
• a doping is understood to mean that the otherwise graphene-like structure of a layer in addition to the carbon atoms additionally has foreign atoms, preferably at least 1.5 at.%, Preferably at least 2 at.%, More preferably at least 5 at.%, In particular at least 10 at.%. These can be arranged, for example, instead of carbon atoms on lattice sites or impurities of the graphene lattice.
• An undoped layer is understood to be a graphene-like layer which has not been deliberately doped by foreign atoms, so that the defects within this layer lie in the natural impurity region, ie in particular in the region ⁇ 1 at.%, In particular ⁇ 0.5 at.%.
• one of the layers is doped with nitrogen, boron, phosphorus or silicon or a combination thereof.
• the properties, in particular the electrical properties, of the layers can be changed in a targeted manner.
• the other of the two layers is preferably undoped in this case or has a doping with a different type of foreign atom from the group nitrogen, boron, phosphorus or silicon.
• a next preferred embodiment of the carbon nanotube is characterized in that the carbon nanotube has a third graphene-like layer, that the second layer is arranged in the cross section of the carbon nanotube within the third layer, and that the first and third layers are undoped.
• a carbon nanotube with alternating layers is provided in which a doped layer is surrounded by two undoped layers.
• Such a carbon nanotube can be produced, for example, by one of the methods described above, in which various precursors are introduced into the moving bed at a time interval.
• the object underlying the invention is further solved by a carbon nanotube powder containing the above-described carbon nanotubes.
• the carbon nanotubes of the carbon nanotube powder preferably have an average diameter of 3 to 100 nm, preferably 5 to 50 nm, in particular 10 to 25 nm. This diameter range corresponds to frequent technical specifications and can be easily achieved with the invention.
• the diameter distribution of the carbon nanotubes after carrying out the method has a diameter ratio D90 / D10 of less than 4, preferably less than 3. In a further embodiment, the diameter distribution of the carbon nanotubes after performing the method
• Diameter ratio which is at least 20%>, preferably at least 30%>, in particular at least 40% lower than the diameter ratio of the starting material introduced as a substrate.
• the production can be carried out, for example, as described above for the process by appropriate adjustment of the process duration.
• the carbon nanotube powder preferably has a purity of at least 90%, preferably of at least 95%, in particular of at least 97%.
• the purity in the present case is understood to mean the proportion in% by weight of carbon nanotubes in the powder compared with other constituents, in particular amorphous carbon and inorganic metal oxides. It has been found that carbon nanotube powders of high purity can be produced by the present invention.
• the area ratio D / G of the D band to the G band in the Raman spectrum can be used.
• the D band is about 1300 cm -1 and the G band (graphite band) is about 1588 cm -1 .
• the integrals of the Raman spectrum are calculated via the D band and the G band and then put into proportion.
• the carbon nanotube powder preferably has a D / G ratio of less than 1.5, preferably less than 1, in the Raman spectrum.
• Fig. 1 shows an embodiment of the method according to the invention
• Fig. 4 is a schematic representation of a graph structure with possible
• Fig. 5 shows another embodiment of the method according to the invention.
• Fig. 1 shows an embodiment of the method according to the invention using a fluidized bed reactor.
• the fluidized-bed reactor 2 has a reactor housing 4 which surrounds a reactor space 6.
• the reactor housing 4 is presently designed as a closed quartz glass tube on both sides.
• the reactor chamber 6 is bounded below by a support plate 8, which has a plurality of nozzle openings 10.
• the reactor 2 also has a gas inlet 12 and a gas outlet 14, which are arranged so that a gas can flow via the gas inlet 12 through the nozzle openings 10 into the reactor chamber 6 and leave it again through the gas outlet 14.
• a condensation trap 15 may be provided to determine the amount of unreacted during the process precursor material can.
• condensation trap 15 instead of or in addition to a condensation trap 15, appropriate devices for exhaust aftertreatment, such as, for example, exhaust gas burners, filters, scrubbers and the like, may also be provided at this location.
• a pulverulent substrate 18 made of carbon nanotubes as starting material is introduced into the reactor chamber 6 via an access 16 provided for this purpose (see arrow 20) and applied to the carrier plate 8 with a bulk density of, for example, 20 to 450 kg / m 3 .
• a process gas such as nitrogen is introduced via the gas inlet 12 into the reactor 2 (see arrow 22), which is guided through the nozzle openings 10 (see arrow 24) into the substrate 18.
• the process gas flowing through the nozzle openings 10 forms with the substrate 18 a fluidized bed 26, in which the mixture of the process gas and the substrate is in a fluidizing, ie a liquid-like, state. In the fluidized bed 26, there is a strong mixing of the substrate and a good heat balance.
• the substrate is in the reactor 2 in a moving bed 27 before.
• a process temperature in the range of 950 ° C to 1300 ° C, in particular 1000 ° C is set by heating means provided for this purpose (not shown).
• the process gas can be heated by the heating means to the desired temperature before being introduced into the fluidized bed.
• the heat energy then stored in the process gas is transferred to the substrate in the fluidized bed.
• a carbonaceous precursor, in particular ethylene or benzene, is now introduced into the fluidized bed 26. This can be done via the inlet 12 together with the process gas or by a separate input. When using a gaseous precursor this can also be used as a process gas.
• the precursor reacts in the fluidized bed 26 and, under suitably adjusted process conditions, leads to the deposition of graphitic carbon onto the carbon nanotube.
• the process product i. the modified by the deposition of graphitic carbon Kohlenstoffiianorschreibchen the substrate 18 are removed through the access 16 again from the fluidized bed reactor 2 (arrow 28).
• the carbon nanotubes in the substrate 18 have, on average, an increased diameter and an increased bulk density at the end of the process, since graphite-like layers have formed around the individual carbon nanotubes due to the graphitic deposition.
• the access 16 may have a lock through which the starting material or at the end of the process, the product can be performed at the beginning of the process. In this case, a gas exchange of the fluidized bed reactor take place.
• a lock is provided in particular for discontinuous operation. However, a continuous or quasi-continuous procedure is also possible.
• Fig. 2 shows a second embodiment of the method using a rotary tube reactor.
• the rotary tube reactor 32 has a tube 34, the longitudinal axis 38 is inclined ⁇ by a small angle of, for example, 1 ° to 5 ° to the horizontal surface 36.
• the tube 34 is rotatable about its longitudinal axis 38 by means of a drive (arrow 40) provided for this purpose.
• a substrate 42 made of carbon nanotubes as educt is introduced into the tube 34 (arrow 44) and applied to the inner tube wall with a bulk density of, for example, 20 to 450 kg / m 3 .
• the tube 34 is rotated by means of the drive, so that the substrate 42 in the tube 34 is now in a moving bed 45.
• the substrate 42 is thoroughly mixed during the process in this way. Furthermore, a good heat balance results within the substrate.
• a process temperature in the range of 950 ° C to 1300 ° C, in particular 1000 ° C is set via provided heating means (not shown).
• a carbonaceous precursor such as ethylene or benzene is introduced into the tube 34 (arrow 46).
• the precursor can be introduced into the tube, for example, alone or with a process gas such as nitrogen.
• the precursor reacts under suitably set process conditions so that carbon is deposited graphitically on the carbon nanotube of the substrate 42.
• Excess precursor material or a process gas transporting the precursor material can emerge from the tube 34 on the other side (arrow 48).
• the process gas with the carbonaceous precursor material can also be guided in the same direction as the substrate.
• the product of the process ie the carbon nanotubes modified by the deposition of graphitic carbon in the substrate 42, can be removed from the tube 34 (arrow 50).
• the carbon nanotubes contained in the substrate have an increased diameter at the end of the process because the graphitic deposition of carbon has led to the formation of further graphene-like layers around the individual carbon nanotubes.
• FIGS. 3a-3e show exemplary embodiments of carbon nanotubes according to the invention, which can be produced, in particular, by the method according to the invention.
• Fig. 3a shows a first carbon nanotube 60 in a schematic isometric view.
• the carbon nanotube 60 has a first inner graphene-like layer 64 and a second outer graphene-like layer 62.
• the first and second graphene-like layers 62, 64 each have a tubular structure. While the first graphene-like layer 64 is undoped, the second graphene-like layer 62 has nitrogen doping. In the graphene structure of the second layer 62, therefore, nitrogen atoms are embedded in carbon lattice sites.
• FIG. 3b shows another carbon nanotube 70 in cross-section with a first layer 72, a second layer 74 and a third layer 76.
• the first and third layers 72 and 76 are undoped while the second layer 74 has nitrogen doping.
• Carbon nanotubes as shown in Figures 3a and 3b can be prepared by the method of the present invention.
• For the carbon nanotube 60 of FIG. 3 a This is done by depositing a carbon nanotube containing carbon and nitrogen, such as pyridine, into the moving bed during the process, and then placing the nitrogen-doped second layer 62 on a carbon nanotube initially introduced only from the first layer 64.
• nitrogen-doped second layer 62 By introducing only one carbon-containing precursor into the moving bed in an optional further step, finally a third undoped graphene-like layer can be deposited so that the structure of FIG. 3b results.
• FIG. 3 c shows another carbon nanotube 80 in cross section with a wound structure (winding structure) in which a single layer of graphene is wound up, resulting in a first inner layer 82 and an outer second layer 84.
• the inner layer 82 is doped with silicon while the outer layer 84 is undoped.
• the carbon nanotube 80 can be produced by the method according to the invention by applying a second undoped graphene-like layer 84 of carbon to a carbon nanotube with a wound structure consisting of a silicon-doped, graphene-like layer by introducing a carbon-containing precursor into the moving bed is applied.
• Fig. 3d shows another carbon nanotube 90 in cross-section, which has a first inner layer 92 in the form of a wound structure and an outer layer 94 with a tubular structure.
• the inner layer 92 is undoped while the outer layer 94 is doped with nitrogen.
• This carbon nanotube can be prepared by the described method by applying the tubular, nitrogen-doped graphene-like second layer 94 to a carbon nanotube having a wound structure from the substrate by introducing a carbon- and nitrogen-containing precursor into the moving bed.
• FIG. 3e shows a cross-section of a further carbon nanotube 100, which has a scroll structure with three graphene-like layers 102a-102c as well as an outer wound structure 104 inside.
• the layers of the scroll structure 102a-102c are undoped while the outer layer 104 is doped with nitrogen.
• the carbon nanotube 100 can be made by the described method by applying the further wound doped layer 104 to a carbon nanotube having a scroll structure from the substrate by introducing a carbon- and nitrogen-containing precursor into the moving bed of the reactor.
• Fig. 4 shows a schematic representation of a graphene-like layer.
• the carbon atoms 122 are in a characteristic hexagonal crystal structure arranged diatomic base, which gives a honeycomb-shaped arrangement of the carbon atoms 122.
• a proportion of foreign atoms 124 is deliberately introduced into the graphene layer 120.
• the foreign atoms can sit at carbon lattice sites (124a), replacing one carbon atom at a time.
• Fig. 5 shows a schematic representation of another embodiment of the method according to the invention.
• a carbon nanotube substrate is placed in a moving bed of a reactor.
• a carbonaceous precursor is introduced into the moving bed.
• the precursor in the moving bed reacts under suitable process conditions that cause graphitic deposition of carbon on the carbon nanotubes of the substrate.
• the carbon nanotubes are discharged from the reactor. These carbon nanotubes discharged as a process product differ from the carbon nanotubes presented as process in that additional graphene-like layers were deposited on at least part of the carbon nanotubes.
• a quartz glass fluidized bed reactor was used with a quartz glass tube with 5 cm inner diameter.
• carbon nanotubes of the type Baytubes (R) C 150 P were used (unless stated otherwise). These carbon nanotubes were (unless stated otherwise) acid-purified to remove catalyst residues to a large extent.
• the amount of deposited substance was determined by weight. From the ratio of weight to weight, the yield, i. calculated the relative weight gain of the substrate.
• the quality of the carbon nanotube powder was examined after conducting the procedure by transmission electron microscopy (TEM). The TEM also determined the diameter distribution of the carbon nanotubes.
• the quality of the deposited layers was examined by optical Raman measurements on the carbon nanotube powder.
• the relative signal strengths for the sp 3 -covalent and for the sp 2 -covalent carbon-carbon bonds were determined from the Raman spectrum and compared with each other.
• the sp 2 -covalent bonds correspond to the bonding type in graphite or graphene and therefore show a graphene-like structure of the layers.
• the sp 3 -covalent bonds correspond to the type of bond in the diamond and indicate the presence of amorphous carbon in the carbon nanotubes. A small ratio sp 3 / sp 2 is therefore an indication that the carbon was deposited graphitically.
• the impurity concentration in the product was furthermore determined by means of X-ray photoelectron spectroscopy.
• Tables 1 and 2 give the following data:
• Precursor The precursor used in each experiment.
• Process gas flow N2 Nitrogen gas flow through the fluidized bed in 1 / min.
• Process gas flow F hydrogen gas flow through the fluidized bed in 1 / min.
• Precursor inlet precursor gas stream through the fluidized bed in 1 / min below
• Yield Weight increase of the substrate in percent, calculated from: 100% * (weighted weight) / initial weight.
• TEM D90 / D10 ratio of diameter values D90 and D10.
• Raman D / G Ratio of the Raman signals for the D-band and the G-band.
• Impurity content percentage of impurities in at.% (Element given in each case), determined by X-ray photoelectron spectroscopy (XPS).
• the experiments L and M shown in Table 2 are comparative experiments in which carbon-free precursors were used. Therefore, in these experiments, no deposition of carbon on the carbon nanotube, so that the substrate also experienced no weight gain and the yield is zero.
• a ratio of the Raman signals D / G of about 1 was measured for these carbon nanotubes as in the output signal. From this comparison, it can be seen that the predominantly graphitic growth of carbon in the Raman measurement is indicated by an sp 3 / sp 2 signal ratio of 1 or less.
• the experiments C to F are a series of experiments with pyridine as a precursor, in which the process temperature was changed at otherwise substantially constant experimental conditions. At 1000 ° C in experiment C, a good yield of 41% is achieved. The ratio of the Raman signals at 0.78 indicates a predominant graphitic deposition of the carbon. With decreasing process temperature, a decrease in the yield was observed until at a process temperature of 850 ° C only a yield of 9% was reached. Since the precursor pyridine is a carbon- and nitrogen-containing precursor, the deposited graphene-like layers are nitrogen-doped in the experiments C to F. Exposure values of 1.5 at.% To 2.8 at.% Were detected by XPS studies.
• experiment G acid-purified carbon nanotubes from another manufacturer, namely the type Nanocyl (TM) NC 7000, were used as the substrate. Even with these carbon nanotubes, a very good yield of 70% could be achieved.
• carbon nanotubes of the type Baytubes (R) C 150 P were used as a substrate, which, however, were not acid washed, so that the substrate contained a certain proportion of catalyst residues. Nevertheless, a yield of 53% could be achieved at a temperature of 1000 ° C.
• experiment H shows that targeted thickness growth of carbon nanotubes can be achieved even in the presence of catalyst residues, for example, if the process parameters are chosen so that in particular the kinetic constant for the thickness growth is greater than the kinetic constant for the growth of carbon nanotubes caused by catalyst components ,
• Tables 1 and 2 show the D90 / DIO ratio of the TEM-determined diameter distribution of the carbon nanotubes. These values provide information about the diameter distribution of the carbon nanotubes. Since the graphitic deposition is preferentially on the smaller diameter carbon nanotubes, the diameter distribution becomes narrower in the course of the process. In the experiments carried out, the process duration in the range of 6 and 20 minutes was still relatively short. In all deposition experiments, this resulted in a D90 / D10 diameter ratio of significantly less than 4, while the starting material had a ratio significantly greater than 4. By increasing the process time to at least 20 or at least 30 minutes can be achieved that the diameter distribution becomes even narrower and the D90 / D10 ratio is correspondingly smaller.

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