materials, as well as to nanostructured carbon

material which is purified in such a manner. Nanostructured carbon materials such as carbon

nanotubes and carbon nanofibres have an increasing significance in different fields of application. For example, such materials are contemporarily used for fibre composite materials. With these, they lead to a significant improvement of the electrical, thermal, and/or mechanical properties. Moreover, one is working on a multitude of possible applications are under development in semiconductor technology and medical technology. For the synthesis of nanostructured carbon material, usually metallic catalysts are used. The resulting nanostructured material therefore contains impurities due to these catalyst metals, which may change the characteristics of the nanostructured carbon material in an undesired manner. For example, these impurities may influence the conductivity, trigger catalytic decomposition of polymers composite materials or interfere with functionalisation processes of the nanostructured carbon material. More importantly, catalyst metals used are often toxic or suspected of being carcinogenic. This greatly reduces the

applications of such materials . The state of the art describes different possibilities for removing metallic impurities from nanostructured carbon material .

At present, the method most frequently used is based on purifying nanostructured carbon material with mineral acids and/or acid mixtures. These oxidise the catalyst metals to water-soluble metal compounds, which must be removed in a subsequent washing step. Subsequently, the nanostructured carbon material, which was purified in such a manner, must be dried. A disadvantage of this method is the great expenditure with regard to material, energy and time, arising on carrying out the method. Considerable quantities of acid and washing water are used, so that not only material costs, but also significant disposal costs arise. The drying step too causes significant costs, in particular since, depending on the nanostructured material, a significant quantity of water may be located in the material, which is why high energy costs arise with the drying. Moreover, it is

disadvantageous that this method leads to material loss by means of wear and oxidation, as well as to structural damage. This is particularly- disadvantageous on purifying nanostructures with a high aspect ratio, since their aspect ratio is reduced due to the damage.

Moreover, according to the state of the art, it is known to subject nanostructured carbon material to high temperature treatment, analogously to the

purification of graphite. For this, the nanostructured carbon material in form of loose material is

introduced into an oven and heated in inert gas atmosphere to about 2000 to 3000 K. The duration of the purifying procedure is dependent on the dumping height and lies between several minutes and several hours. High energy costs arise due to the combination of long treatment duration and high treatment

temperature. Moreover, the long treatment duration is disadvantageous with regard to an achievable

throughput rate.

It is known to treat a pellet of nanostructured carbon material in the afterglow of plasma, which has already cooled down compared to the plasma in the excitation zone of 1100 K rotation temperature, from the

publication "Raman study of carbon nanotube

purification using atmospheric pressure plasma" . A direct treatment inside a thermal plasma is not disclosed. Thereby, the pellet is furthermore only partly immersed by the plasma afterglow. The treatment is specified as suitable for removing impurities, and in particular of amorphous carbon. A removal of metals is not disclosed and would not be possible by means of this method, since already the rotation temperature of the plasma is significantly too low for this. Also the input electrical energy is not at all sufficient to compensate heat radiation losses resulting from heating the pellet to temperatures which would allow evaporation of metallic constituents. Moreover, the oxygen-containing species generated by the plasma are capable of oxidising the pellet into volatile

compounds by which a relative enrichment of metallic impurities results. Thermo-gravimetric measurements explicitly show that the process does not achieve a depletion of impurities like for example metals that are not volatizable by thermal oxidation.

A method for treating carbon nanotube powder for removing amorphous carbon may be deduced from the publication "Surface modification of multi-walled carbon nanotubes by O ₂ plasma". For this, the powder is arranged in a rotating drum, which in turn is arranged in a vacuum chamber. A non-thermal O ₂ -plasma is subsequently generated in the vacuum chamber. This plasma acts on powder only in an indirect manner. The temperature of the powder is about 100° C during the treatment. A removal of metals is not described and would also not be possible due to the low

temperatures . On the contrary, enclosed metals are decapped by the treatment, i.e. the encapsulating carbon material is removed by means of oxidative attack of the plasma. After the treatment, XPS peaks assignable to the metal impurities become evident, which were not previously observed. Since the surrounding or encapsulating carbon material is decomposed to volatile species, the metallic impurities remain in the powder, thus the percentage of metallic impurities even increases during the treatment .

Moreover, both methods known according to the state of the art have the disadvantage that a continuous operation is not possible or appears to be difficult.

It is thus the object of the invention to develop a method for purifying nanostructured carbon material, which permits a more economic purification, wherein the structure of the nanostructured carbon material should remain intact as completely as possible.

Moreover, it is the object of the invention to suggest nanostructured carbon material, which on account of the favourable purification may at the same time be provided in an inexpensive manner with a high purity compared to the state of the art, and which moreover has an advantageously well-retained structure.

This object is achieved by a method according to claim 1. Advantageous further formations are mentioned in the dependent claims . The method according to the invention for removal of mechanical impurities from a nanostructured carbon material, which has a graphitic, partly graphitic or graphitisable structure comprises the immersion of a nanostructured carbon material which exhibits metal- containing impurities, and which may be of

individualized or agglomerated form, into a thermal plasma.

"Nanostructured" here is to be understood in that a respective material, on account of a structuring which has structure sizes of below one micrometer at least in one spatial direction, has significantly different properties such as for example tensile strength, elasticity, conductivity, or similar, than carbon in amorphous form or as macroscopic crystal. Preferably, the nanostructured carbon material is immersed directly into the thermal plasma, i.e. not only into the afterglow of the thermal plasma arising due to a process gas flow.

The temperature of the process gas in the thermal plasma is preferably at least 3350 K. After passing the plasma, a nanostructured carbon material with reduced metal content is found.

Finally, the nanostructured carbon material is separated from the process gas, wherein the removed metal-containing impurities advantageously are at least partly transported away with the process gas after separation. This in particular is possible by means of a separation being carried out whilst the process gas advantageously is still hot and in

particular still hotter than the boiling point of a prevailing metallic impurity.

Due to the use of a thermal plasma, utile high heating rates are possible. Due to the nanostructured

character of the material, impurities may escape significantly more rapidly, which allows for shorter residence times of the nanostructured material inside the hot process gas. Thereby, impurities accumulated inside the structure as well as located at its surface are removed. This effect is surprisingly rendered possible due to the fact that agglomerates of

nanostructured carbon material are sufficiently porous. Moreover, the evaporation is accelerated due to the fact that one is allowed to select high

temperatures, since even very high temperatures lead unexpectedly to no significant structural damage of the nanostructured carbon material. Advantageously, it is thus possible to remove at least 1% of the metal- containing impurities within less than 100 ms, in particular within 1 to 10 ms .

Due to achievable short processing duration, allowing for large throughput rates, this purification method can be carried out in a significantly more energy- saving way than methods according to the state of the art. Furthermore, no acids or similar media are required except a process gas, which may be recovered. A further advantage compared to methods according to the state of the art, is the fact that the structure of the nanostructured material is retained to an improved extent and less material loss occurs due to wear or oxidation.

It is advantageous if a structure improvement of the material is achieved at the same time as the removal of the impurities of the nanostructured material . Such an improvement may in particular consist in an

increase in the oxidation resistance or the degree of graphitisation.

In order to achieve a particularly good retention of the structure, the process gas may contain nitrogen or an inert gas such as helium, neon, argon, krypton, or xenon. In order to improve the purification effect, it may be advantageous to use a reactive gas or a mixture of a reactive gas and an inert gas, as process gas. The reactive gas may thereby advantageously be

selected depending on the impurity to be removed, wherein an impurity to be removed is advantageously determined before removal or is known from the

synthesis method of the nanostructured carbon

material. In particular, a hydrogen-containing gas may be used as a reactive gas, said hydrogen-containing gas being able to reduce metal oxides or to react with the metal-containing impurities to metal hydrides. This in particular may be exceedingly advantageous if the metal-containing impurity at least partly consists of a metal oxide, wherein the metal oxide has a

significantly higher boiling point than the

corresponding metal .

Likewise, it may be helpful to use a reactive gas, which is suitable for gas phase oxidation of the impurities. In particular, an oxygen-containing gas such as O ₂ , CO or CO ₂ and/or a hydrogen- and oxygen- containing gas such as H ₂ O or H ₂ O ₂ may be used as a process gas. The purification performance may be increased by way of this, depending on the prevailing metal-containing impurity. A further group of reactive gases which advantageously may be used are halogen-containing gases. Examples of iodine-containing gases are I ₂ , HI and HCI ₃ . Bromine- containing gases such as Br ₂ , HBr, and HCBr ₃ may likewise be used. Moreover, the use of chlorine- containing gases such as Cl ₂ , HCl or HCCl ₃ or of fluorine-containing gases such as F ₂ , CF ₄ , SF ₆ , CHF ₃ , and C _w H _x Cl _y F _z may be advantageous . Metal halogenides produced by such reactive gases may likewise exhibit a lower boiling point than the originally present impurity.

Advantageously, catalytically manufactured

nanostructured carbon materials may be purified by way of this method. In particular, partly-closed graphene structures such as single-walled, double-walled, or multi-walled carbon nanotubes, nanobuds, nanocones, or nanohorns may be purified. Contained metallic

impurities may escape particularly well due to the fact that these structures are only partly closed, whereas an acid treatment with a liquid acid would mostly be exceedingly time-consuming on account of the often high aspect ratio (length/diameter) of the structures .

Moreover, nanofibres, in particular carbon nanofibres and graphitic carbon nanofibres, may advantageously be purified. Moreover, impurity-doped or impurity- containing nanostructured carbon material such as for example impurity-doped carbon nanotubes may be

purified.

Of course, this is no conclusive list of the materials which may be treated. Other nanostructured materials, which are synthesised by means of a catalytic process, or nanoscaled carbon materials, may also be purified. What is essential is merely that they are sufficiently temperature resistant and that the impurities are metal-containing .

It is not even necessary for the nanostructured material to be a material synthesised by a catalytic process . Other materials such as for example

nanoscaled graphites which are manufactured from natural or synthetic raw materials, or graphenes which have metal-containing impurities may be purified.

Advantageously, the nanostructured carbon material may be present as a granulate or powder. In particular, it is advantageous if the nanostructured material is porous, wherein a bulk density and/or agglomerate density of the nanostructured carbon material is smaller than 2.5 g/cm ³ , advantageously smaller than 0.5 g/ctn ³ , particularly advantageously smaller than 0.2 g/cm ³ . By way of this, on the one hand, a

particularly optimal heat transfer from the process gas to the material is possible and, on the other hand, a particularly rapid escape of the impurities becomes possible.

Moreover, it is advantageous if the nanostructured carbon material is fluidised by the process gas prior to introduction into the plasma. This way, an optimal heat transfer from the process gas onto the

nanostructured carbon material is possible. An optimal fluidisation of the nanostructured carbon material is achieved if a flow speed of the process gas is more than twice of the minimum fluidisation speed of the largest occurring agglomerates of the nanostructured carbon material .

The thermal plasma may advantageously be operated at atmospheric pressure. This way, the implementation of the method is advantageously simple. However, it is also possible to optimise the purification performance by means of changing the process gas pressure. In dependence on the remaining parameters, in particular on the process gas, temperature, type and shape of the nanostructured material, reduced or enhanced pressure may be advantageous .

The plasma may advantageously be formed by a high- frequency plasma, which in particular may be excited by a frequency between 100 kHz and 10 MHz, between 10 MHz and 300 MHz or between 300 MHz and 300 GHz.

Alternatively, it is possible to produce the plasma by an electric arc, by means of direct current plasma or by means of alternating current plasma, with a frequency between 0 and 100 kHz. The use of a laser- induced plasma is also possible.

A temperature of the process gas in the plasma is advantageously above 3350 K, particular advantageously above 6000 K, in particular 10000 K. Due to the advantageous high temperature, the metal-containing impurities are transitioned into the gaseous phase in a particularly efficient manner wherein a

recondensation of the impurities on the nanostructured carbon material is particularly efficiently prevented.

The power density of the plasma is advantageously at least 5 W/cm ³ , particularly advantageously at least 10 W/cm ³ , in particular 30 W/cm ³ . This way, advantageous high heating rates are possible.

In a preferred embodiment of the method, the

nanostructured carbon material may be introduced into the plasma in a continuous manner and continuously separated from the process gas. This way, the method may be preferably carried out directly subsequent to a synthesis of the nanostructured carbon material, which advantageously may be likewise carried out in a continuous manner or has at least a short processing cycle time. This way, the necessity of an intermediate storage of the nanostructured carbon material between the synthesis and the purification is omittable. One advantageous possibility for the continuous separation of the nanostructured carbon material from the process gas is to change a flow direction of the process gas, wherein the nanostructured carbon

material precipitates out of the process gas flow.

This for example is possible in a particularly

efficient manner in a cyclone. Moreover, it is advantageous to provide a subsequent washing step, with which soluble impurities formed by a chemical reaction of the process gas with the metal- containing impurities, and which may be formed in particular as metal halogens, metal hydroxides, metal hydrates and/or metal oxides, or metal mixed oxides, may be eluted from the nanostructured material . This washing step may be supported by the effect of

ultrasound.

In one advantageous embodiment of the method, the treatment duration required for a significant metal removal is very short. For example, the treatment duration may be less than 100 ms, i.e. a residence time of the nanostructured carbon material in the thermal plasma is preferably shorter than 100 ms and particularly preferably shorter than 10 ms . This is preferably possible by means of very high heating rates of the nanostructured carbon material.

A heating rate of the nanostructured carbon material in the thermal plasma is for example preferably more than 50000 K/s and particularly preferably more than 200000 K/s and in particular results from a good heat transfer onto the material which is preferably

completely immersed by a process gas of a high

entropy, thus dispersed in gas. In this manner, one may achieve a significant purification effect, i.e. a reduction of the metal content percentage of the nanostructured carbon material by at least 20% or preferably at least 50% with respect to the original metal content percentage, already below 100 ms

treatment duration. During the plasma treatment, the nanostructured carbon material is preferably heated at least for a short time to above 2000 K, particularly preferably to above 3000 K or 3500 K.

The advantageous heating rates may be made possible by means of an sufficiently high plasma power. The plasma power required for a given heating rate depends functionally, among other parameters, on the process gas flow, the volume of the plasma excitation zone, the radiated power as well as the mass flow and the metal content of the nanostructured carbon material which is to be treated. The in practice processible mass flow therefore essentially depends on the

providable plasma power.

Advantageously, the nanostructured carbon material is completely immersed by a gas excited in thermal plasma. This for example may be achieved by means of dispersing the material into a plasma-thermalIy excited process gas or by means of dispersing into a process gas and subsequent passage of the plasma excitation zone. By means of fluidization of the material with a gas flow, the material may be conveyed independent of the effect of gravity. The angle at which the material encounters the plasma-excited gas or at which it is injected into the plasma excitation zone is of no significance with regard to the success of the method.

The separation of metal vapours arising in the method and the carbon material may be achieved by the

condensation of the metal vapours on surfaces which are in contact with the process gas. For this, their temperature must be below the boiling temperature of the metals. Alternatively or supplementarily, the separation may be achieved by a direction change of metal vapour containing process gas or carbon

material . In comparison to a metal removal by means of acid treatment according to the state of the art, the method according to the invention permits the

structure of the nanostructured carbon material to remain intact or even improves it. The graphitisation degree and/or the oxidation resistance are developed, i.e. enhanced. The characteristic structure of the multi-walled carbon nanotubes is retained with the plasma-thermal treatment. Preferably, an enhancement of the oxidation resistance of multi-walled carbon nanotubes by more than 10 K and preferably by more than 100 K is achieved by means of the plasma-thermal treatment, i.e. a temperature, at which half of the mass of the nanostructured carbon material is oxidised in air by heating, increases by more than 100 K by means of the plasma-thermal treatment. Preferably, a reduction of defect structures and an improvement in the graphitic structure of the nanostructured carbon material (for example multi-walled carbon nanotubes) is achieved with the plasma-thermal treatment, in particular in a manner such that a ratio of the G-peak to the D-peak in a Raman spectrum of the

nanostructured carbon material increases. Particularly preferably, the ratio of the intensity of the G-peak to the D-peak increases by at least 10%.

Preferably, the method is carried out in a non- oxidative manner. This procedural manner with the exclusion of oxidising process gases, i.e. without the use of oxidative media, permits a removal of metals and metal oxides from nanostructured carbon materials without substantially oxidatively damaging ordered or unordered carbon structures .

The presence of small quantities of oxidising gases may however be permissible and advantageous for the successful implementation of the method according to the invention, as long as the thermally-induced removal of metals or metal oxides occurs more rapidly than the oxidative decomposition of the nanostructured carbon, i.e. if nonetheless is assured, as with embodiment examples without oxidising gases, that a metal content percentage of a residual mass of the nanostructured carbon material remaining after the method, is reduced compared to the metal content percentage of the starting material .

If on the other hand the temperature required for a metal removal is not reached, then the oxidative carbon decomposition leads to an undesired relative enrichment of the material with metals or metal oxides, as is evident for example from the XPS spectra which are contained in the publication "Surface modification of multi-walled carbon nanotubes by O ₂ plasma" by Tao XU et al., and as is avoided with the method according to the invention.

As a rule, under oxidative conditions, non-graphitic carbon structures are preferentially decomposed compared to graphitic carbon structures. For this reason, with the presence of low quantities of

oxidising gases, the method may lead to a depletion of metallic or metal-oxide as well as non-graphitic impurities of a graphitic nanostructured carbon material. For this reason, a successful implementation of the method is possible even in presence of low quantities of oxidising gases, and is advantageous in particular for the simultaneous reduction of the percentage of metals and of non-graphitic impurities, with regard to the total mass. Such a presence of oxidising gasses may be possible in technical processes or may be caused by the reduction of metal oxides in the thermal plasma.

The invention is hereinafter explained in a more precise manner by way of several embodiment examples. There are shown in:

Fig. 1 an installation for carrying out the method according to the invention, during operation,

Fig. 2 a further installation for carrying out the method according to the invention, during operation,

Fig. 3 a transmission microscopy image of multi- walled carbon nanotubes that were purified by means of a plasma-thermal treatment according to a further embodiment of the invention.

Fig. 4 results of thermo-gravimetric measurement of the residual mass percentage in dependence on temperature for a nanostructured carbon material which was purified according to another embodiment of the invention, as well as for the starting material and

Fig. 5 a detail of a Raman spectrum of a

nanostructured carbon purified according to a further embodiment of the invention, in comparison to a Raman spectrum of the starting material. A conveyer system 1 for fluidisable solid matter is shown in Figure 1 into which injection port 78 litres of argon per minute are introduced at normal pressure through a filter 3 at an entry side 2. Porous

agglomerates of multi-walled carbon nanotubes, which are present in form of a granulate 5 in a supply container 6 and are contaminated with catalyst metals, are introduced by means of a screw conveyor 4 at a conveyer speed of 100 g per hour. Hereby, the filter 3 prevents a penetration of the carbon nanotubes into the gas supply. The bulk density of the granulate 5 is between 0.13 and 0.15 g/cm ³ . The size of the

individual agglomerates lies between 0.1 and 1 mm. The introduced granulate is fluidised by the gas flow.

The thus fluidised granulate is guided through a plasma zone 8 which is excited by a plasma source 7. Thereby, it is of no significance as to whether the gas flow is directed against gravity, with gravity, or inclined thereto. The plasma source 7 is designed as a high-frequency plasma source. The plasma 8 is excited with a frequency of 13.56 MHz, wherein a total power of 5 kW is applied. Since argon has a thermal capacity of 0.52 J/g/K and a throughput rate of 78 litres per minute corresponds to about 2.3 grams per second, the process gas heats by about 4200 K.

Metallic impurities evaporate out of the carbon nanotubes or from their surface due to the contact with the hot process gas. Due to the hot process gas, these may not attain to recondense on the carbon nanotubes, which are heated above the evaporation temperature. The agglomerates of carbon nanotubes fall out of the process gas flow with its direction change and are collected in a collection container 10. A further filter 9 is used, in order to prevent smaller agglomerates from reaching the exhaust gas system. The thus purified agglomerates of carbon nanotubes have an about 5% lower metal content than non-purified carbon nanotubes .

A second example is shown in Figure 2. Here too, the introduction takes place via a screw conveyor.

However, the carbon nanotubes, which are identical to those of the first embodiment example, are conveyed out of a tube 11 which is directly connected to a synthesis installation which is not represented. In this embodiment example, a mixture of 78 litres per minute of argon and 2.5 litres per minute of nitrogen is used as a process gas. The plasma is excited with a frequency of 4 MHz and a power of 12 kW. An improved purification is achieved by the higher power of the plasma and the thus higher temperature, in combination with the admixture of nitrogen, so that the metal content of the thus purified carbon

nanotubes is reduced by about 20% compared to

untreated carbon nanotubes. A separation of the nanostructured material takes place by means of a cyclone 12. The thus separated material is guided further through a further tube 13, for example to a filling installation, whilst the process gas is guided through a further filter 14 and is subsequently recovered and purified.

The installation shown in Figure 2 is likewise used in a third embodiment example. The single difference to the second embodiment example lies in the fact that instead of nitrogen a suitable quantity of hydrogen is contained in the process gas mixture. By way of this, the cleaning effect is significantly improved, so that only 47% of the metal content of non-purified carbon nanotubes is still contained in the purified carbon nanotubes . In a fourth embodiment example, the plasma is operated at a frequency of 4 MHz and a power of 11 kW. A mixture of 78 litres per minute of argon with 1.8 litres per minute of hydrogen chloride is used as a process gas. The purification effect is further improved by way of this, so that only 30% of the original metal content is still detectable. In a fifth embodiment example of the invention, a mixture dispersed in gas and of different

nanostructured carbon materials, which amongst other things contains single-walled and multi-walled carbon nanotubes as well as metallic and metal-oxide

impurities, is guided through a plasma zone excited by a plasma source. A variation of the plasma excitation frequency in a range of 2.5 to 5.0 MHz in this

embodiment example, at a constantly fed power, has no significant influence on the purification effect. The purification effect increases slightly with the use of frequencies larger than 5.0 MHz, such as e.g. 13.56 MHz

In a sixth embodiment example of the invention, a mixture of different nanostructured carbon materials, which amongst other things contains single-walled and multi-walled carbon nanotubes as well as metallic and metal-oxide impurities and which is dispersed in a gas, is guided through a high-frequency excited plasma. In this embodiment example, the delivery quantity of the material may be increased from 100 g/h to 2000 g/h with a comparable purification effect, by means of increasing the supplied plasma excitation power, from 3 to 40 kW.

In a seventh embodiment example of the invention, agglomerates dispersed in argon and of multi-walled carbon nanotubes with a content of metals and metal oxides of 2.5% with respect to the total weight, which were created from supported catalysts via CVD

synthesis, were guided at 100 g/h through a plasma zone excited at a frequency of 4 MHz and 40 kW. The treatment may preferably be carried out several times with the same material. The content with regard to metals and metal oxides with respect to the total weight thereby reduces with the number of treatments. For example, a metal content after a plasma-thermal treatment may be 1.9%, after two treatments 1.5% and after three treatments 1.1%. The relative reduction with respect to the content of metals and metal oxides in this embodiment example is about 25% per treatment.

For example, one may verify by means of elementary analyses with ICP-MS, that the content of

catalytically effective transition metals such as Fe, Co, Ni, Mn as well as the content of metals from catalyst supports such as MgO and Al ₂ O ₃ are reduced by the method.

In an eight embodiment example of the invention, agglomerates dispersed in argon and of multi-walled carbon nanotubes with a content of metals and metal oxides of 21% with respect to the total weight which were manufactured from supported catalysts via CVD synthesis, are guided at 100 g/h through a plasma zone excited at 4 MHz and 30 kW excitation power. The content of metals and metal oxides with respect to the total weight decreases to 15% by means of the plasma- thermal treatment . In a ninth embodiment example of the invention, agglomerates dispersed in argon and of single walled carbon nanotubes with a content of metals and metal oxides of 18% with respect to the total weight and which were manufactured from unsupported catalysts by- means of electric arc synthesis, were guided at 100 g/h through a plasma zone excited with a frequency of 4 MHz and 30 kw excitation power. The content of metals and metal oxides with respect to the total weight reduces to about 10% by means of the plasma- thermal treatment .

In a tenth embodiment example of the invention, graphitic nanofibres which are dispersed in argon and which are of a platelet structure, with a content of metals and metal oxides of 29% with respect to the total weight, which were manufactured from supported catalysts via CVD synthesis, were guided at 100 g/h through a plasma zone excited with a frequency of 4 MHz and 30 kW excitation power. The content of metals and metal oxides with respect to the total weight reduces to 21% by means of the plasma-thermal

treatment .

A transmission microscope image of multi-walled carbon nanotubes 15 after plasma-thermal treatment according to a further embodiment of the invention is presented in Figure 3. As may be noticed, the characteristic structure of the carbon nanotubes 15 remains intact with the treatment. An aspect ratio, i.e. a quotient of length and diameter of the carbon nanotubes 15 , was reduced at most by 20% of the aspect ratio of the starting material (i.e. of the still non-purified carbon nanotubes) and particularly preferably by 5% at the most or not at all, by way the plasma-thermal treatment. Such a low reduction of the aspect ratio is not only advantageous with carbon nanotubes 15, but also with other elongately formed nanostructured carbon materials. A mass loss percentage of carbon with graphitic structure compared to the starting mass of carbon with graphitic structure is likewise smaller than 10% and particularly preferably smaller than 1%.

The result of a thermo-gravimetric measurement on a nanostructured carbon material before and after treatment by means of a plasma-thermal method

according to a further embodiment form of the

invention is shown in Figure 4. With such a

measurement, a sample of nanostructured carbon

material is heated slowly in an oxygen-containing atmosphere, wherein the remaining mass percentage with respect to the starting mass is determined depending on the temperature.

The graph 16 shows the temperature dependency of the residual mass for a plasma-thermalIy treated,

nanostructured carbon material, whilst the graph 17 shows the temperature dependency for the corresponding starting material which is not plasma-thermally treated. The derivative of these graphs with respect to the temperature is represented by the graphs 18 (for the plasma-thermally treated material) and 19 (for the non-treated material) .

As may be noticed, a temperature, at which half the material has been decomposed, is more than 100 K larger for the plasma-thermally treated material than for the untreated material. Moreover, one may notice that the ash content of the material treated in the thermal plasma is significantly reduced compared to the untreated material . A Raman spectrum 20 of a plasma-thermally treated nanostructured carbon material in comparison to a Raman spectrum 21 of the untreated starting material is shown in Figure 5. As may be noticed, a ratio of the intensity of the G-peak to the intensity of the D- peak is significantly larger with the plasma-thermal material. In this embodiment example, the ratio for the untreated material is 0.95, whist for the plasma- thermally treated material it is 1.15. It is evident from this that the share of non sp ² -hybridised, i.e. non graphitic carbon structures, has reduced by the plasma-thermal treatment.