WO2006012921A1

WO2006012921A1 - Process of production of carbon nanotube rings

Info

Publication number: WO2006012921A1
Application number: PCT/EP2004/051696
Authority: WO
Inventors: Maurizio Prato; Francesco Stellacci
Original assignee: Universita' Degli Studi Di Trieste
Priority date: 2004-08-03
Filing date: 2004-08-03
Publication date: 2006-02-09
Also published as: EP1786727A1

Abstract

The present invention describes a method for the preparation of carbon nanotube rings starting from commercial carbon nanotubes by means of organic chemical reactions. The method provides formation of rings from any kind of carbon nanotubes. The nanotube rings obtained possess unique magnetic properties and can be used in a variety of ways in electronic circuits.

Description

Title

Process of production of carbon nanotube rings Field of the invention

The present invention relates to production of carbon nanotube rings through an organic functionalization, magnetic carbon nanotube rings and their use in electronic circuits for their electronic and magnetic properties. Prior art

Carbon nanotubes (hereinafter the terms carbon nanotubes or nanotubes have, if not otherwise indicated, the meaning of straight or slightly curved carbon nanotubes) are carbon materials that are currently fabricated either from graphite or other starting materials, through procedures generally known as electric arc discharge, HIPCO (High-pressure CO), laser-desorption, laser-ablation, plasma (PVD) or chemical vapour deposition (CVD). Following these treatments, the initial material adopts a highly orderly straight structure constituted either by one wall (single-walled nanotubes SWNTs), by two walls (double-walled nanotubes DWNTs) or several walls (multi-walled nanotubes MWNTs) with miniaturized cylindrical shape varying in diameter and length according to the type of treatment, in which the carbon atoms ' combined together form a prevalently, but not exclusively, hexagonal honeycomb pattern. This particular arrangement provides these carbon materials with unexpected and interesting chemical-physical and mechanical properties, making them extremely important materials for a variety of applications. Carbon nanotubes are in fact highly resistant to high current densities, which determines important electronic, optical and mechanical properties for the use of these materials in many fields of application as metal conductors or semi-conductors, insulating materials or materials with high mechanical strengths. They can thus be used in electronic and opto-electronic equipment (electrical and electronic microcircuits, diodes, transistors, sensors, field emission displays, vacuum fluorescent displays or sources of white light), and also polymeric compound materials with high electrical, thermal and mechanical strength.

The electronic properties of carbon molecular structures have been extensively studied since Smalley and co-workers suggested a large diamagnetic response for C_6O fullerenes for their peculiar electronic structure (H.W. Kroto et a!., Nature, 1985, 318, 162-163). In a comparison study on magnetic susceptibility of various forms of molecular carbon, including diamond, graphite, fullerenes and nanotubes, R. E. Smalley and co-workers observed a large susceptibility in the straight carbon nanotubes, implying an average band structure similar to that of graphite, and suggested that the diamagnetism of said carbon nanotubes would be greater than that of graphite (A. P. Ramirez et al., Science, 1994, 265, 84-86). Similar conclusions have been achieved in a study on magnetic susceptibility of carbon structures by J. Heremans, C. H. Oik and D. T. Morelli (Phys. Rev. B, 1994, 49, 15122-15125), where for carbon nanotubes a mostly diamagnetic behaviour with a magnetic field and temperature dependence different from graphite has been found. Afterwards in a theoretical study it has been suggested that carbon nanotubes can exhibit both diamagnetic and paramagnetic susceptibility and that these responses can depend on features such as helicity and radius of carbon nanotube, field direction and Fermi energy (J. P. Lu, Phys. Rev. Lett. 1995, 74, 1123-1126). Also, because of their cylindrical shape and symmetry and their closed shell electronic structure (that leads to the absence of free spin), straight carbon nanotubes are not ferromagnetic, nor present magnetism hysteresis under an applied magnetic field. In some cases carbon nanotube rings or tori (a torus refers to a single carbon nanotube closed on itself) have been observed and their properties studied. Single-walled carbon nanotube circular formations, many of them assumed as to be perfect tori or toroids and not as coiled carbon nanotubes, in laser-grown single-walled carbon nanotube deposits have been reported by J. Liu et al. (Nature, 1997, 385, 780-781). Carbon nanotube rings of about 0.5 μm diameter have been observed also in carbon nanotube deposits catalytically produced by thermal decomposition of acetylene. In this case the carbon nanotube rings found have been interpreted as single turn coil - multi-walled carbon nanotubes with overlapping ends and not as toroids (M. Ahlskog et al. Chem. Phys. Lett., 1999, 300, 202-206).

Subsequently, a method to prepare single-walled nanotube rings from SWNTs under ultrasonic irradiation has been reported. The single-walled nanotubes, produced by laser vaporization, were oxidized with a mixture of concentrated H₂SOVH₂O₂ in order to reduce the length of the nanotubes themselves and then irradiated with ultrasonication (40 kHz, 190W) for 1-3 hours at 40-50⁰C for ring formation. The carbon nanotube rings formed were assumed to be in form of coils, due to ultrasonication self-folding of SWNTs, with a distribution of radii of 300-400 nm (R. Martel et al. J. Phys. Chem. B, 1999, 103, 7551-7556). A similar method for production of carbon nanotube rings has been disclosed in US Patent 6590231. Single-walled nanotube rings were prepared from SWNTs, produced by the arc discharge technique, dispersed in a methanol solution of benzalkonium chloride as surfactant and then ultrasonicated at frequency of 2OkHz, output 5OW for about 1 hour.

More recently a method of preparation of carbon nanotube rings fully closed of average diameter of 540nm has been described by M. Sano et al. (Science, 2001 , 293, 1299-1301). The method is based on organic chemical reaction of SWNTs, lightly etched in H₂SO₄ZH₂O₂ at the purpose not only to reduce the length of the nanotubes but also to functionalize the ends with oxygen-containing groups, dispersed in dry dimethylformamide with an excess dicyclohexylcarbodiimmide (DCC). The ring formation in this case has been obtained for covalent binding between the oxygen-containing groups at the ends introduced with oxidation. Furthermore, with reference to the properties of these molecular carbon circular structures, in a theoretical study L. Liu et al. (Phys. Rev. Lett., 2002, 88, 21722061-2172064 ascribes to the carbon nanotube tori having either semiconducting or metallic behaviour respectively either a diamagnetic or a paramagnetic response. However, all the procedures generally used for fabrication of carbon nanotubes provide raw carbon nanotubes with a great number of impurities, differing according to the production method used, which include inert carbon particles, amorphous carbon, fullerenes and catalysis metals, so that these contaminants and, in particular the catalysis metals, can have a great influence on the formation of carbon nanotube rings fully closed. Furthermore even the aggregation process of the carbon nanotubes themselves can prevent the formation of rings. The oxidation step used both by R. Martel et al. {ref. cit.) and M. Sano et al. (ref. cit.) is therefore essential, not only to cut the SWNTs, but also to disperse the same and reduce the residual metal catalysts and carbon impurities (R. Martel et al. ref. cit). However the oxidation and the ultrasonication, both widely used for purification of carbon nanotubes, can be also the major limits of these ring preparation methods. In fact, the carbon nanotubes can be impaired in consequence of the processes above mentioned with the introduction of structural defects, influencing their chemical-physical and mechanical properties. Consequently, even the carbon nanotube rings formed can be impaired and then useless for industrial application in spite of their remarkable properties. Accordingly, one purpose of the present invention is to provide a simple method for the preparation of carbon nanotube rings from industrial straight carbon nanotubes and another purpose is to prepare said carbon nanotube rings pure and also without impairing their molecular structure. Summary A new process for preparing carbon nanotube rings from industrially produced carbon nanotubes (pristine nanotubes p-NTs), either as single-walled nanotubes (SWNTs) or double-walled nanotubes (DWNTs) or multi-walled nanotubes (MWNTs), has now been found. Furthermore, the carbon nanotube rings obtained with the process of the invention, hereinafter disclosed, have shown new and surprising dia- and para¬ magnetic properties.

Therefore an object of the invention is a process for preparation of carbon nanotube rings wherein said carbon nanotube rings are obtained by means of at least the following step: - functionalization of carbon nanotubes through an organic reaction using 1 ,1 ,2,2,-tetrachloroethane as solvent obtaining that said carbon nanotubes form rings.

It is still further object of the invention carbon nanotube rings having magnetic properties, hereinafter by the use of the term "magnetic" meaning the generation of a magnetic dipole within a nanotube ring via the formation of ring currents along the nanotube ring induced by an external field. It is yet another object of the invention a process for preparation of carbon nanotube rings comprising a further step of purification and preferably comprising at least the following steps:

- dispersion of said carbon nanotubes rings in a non-solvent or poor solvent medium or a mixture of such solvents, or a mixture of a good and a bad solvent,

- separation of carbon nanotubes rings having magnetic properties by attraction thereof to means generating a magnetic field.

It is a further object of the invention the use of the carbon nanotube rings in an electronic circuit and in particular as inductors.

Brief description of the drawings

Figure 1. Illustration of the carbon nanotube rings obtained. The image shown is an Atomic Force Microscopy (AFM) picture.

Figure 2. Illustration of the attraction of carbon nanotube rings to magnets. The image shown is a photographic picture taken with a digital camera.

Figure 3. Magnetic Force Microscopy images of carbon nanotube rings: (left) height image of the nanotube rings used a reference, (right) magnetic image displayed as a map of the phase shift of the magnetic cantilever.

Figure 4. Magnetization curves of various carbon nanotube ring samples showing an almost perfectly linear magnetic response to the applied field.

Figure 5. Atomic Force Microscopy images of (left) non magnetic material collected at the bottom of the cuvette as shown in fig. 2 and (right) the magnetic material collected at its walls as shown in fig. 2.

Detailed description of the invention The objects above mentioned and other objects, features and advantages of the invention will become apparent in the course of the description set forth below and from the preferred embodiments which are given for illustration and not limiting purposes.

The present invention provides a process for the preparation of carbon nanotube rings by means of at least a step of functionalization of carbon nanotubes, either as single-walled nanotubes (SWNTs) or double-walled nanotubes (DWNTs) or multi-walled nanotubes (MWNTs), obtained with an organic reaction using 1 ,1 ,2,2,-tetrachloroethane as solvent, and obtaining that said carbon nanotubes form rings. The carbon nanotube rings (hereinafter indicated also as nanotube rings or nanorings) obtained can be then SWNT rings or DWNT rings or MWNT rings, the number of walls of rings obtained depending exclusively from the pristine carbon nanotubes used at the beginning.

The functionalization can be carried out through different organic reactions. In one case the step of functionalization is performed adding to carbon nanotubes an aziridine represented by the formula (I)

1 (I) where Ri, R₂ and R₃, equal or different from each other, are organic residues or a hydrogen and preferably Ri can be selected in the group comprising any alkyl or aryl groups, R₂ and R₃ can be selected in the group comprising carboxy-alkyl, carboxy-aryl groups, hydrogen, or any alkyl or aryl groups and most preferably Ri can be selected in the group comprising any alkyl group, R₂ and R₃ can be selected in the group comprising any carboxy-alkyl in general and in particular carboxy-methyl, carboxy-ethyl.

The addition of an aziridine 1 to carbon nanotubes is performed in a time comprised between 1 to 10 days, adding an excess of aziridine (at least 1 equivalent in weight) every day at the temperature of reflux of the solvent 1 ,1 ,2,2,- tetrachloroethane. After cooling, the organic solution is separated from the solid residue, which is washed several times with solvents, such as for example dichloromethane. The combined organic solutions are evaporated under vacuum and the remaining oily residue is extracted with a mixture of solvent and water as for example CH₂CI₂/H₂O and dried. The organic residue is washed many times with a solvent such as hexane or diethyl ether, then becoming a solid. The solvents for dissolving and precipitating can be selected respectively between solvents where the carbon nanorings obtained are soluble or insoluble respectively. Alternatively, instead of aziridine 1 , a mixture of a reagent 2 represented by the general formula (II) and a reagent 3 represented by the general formula (III) can be used in the functionalization of carbon nanotubes to obtain rings:

2 ³

(II) (III) where

R₄, R₅, Re, R₇ and Rs, equal or different from each other, are organic residues or a hydrogen and preferably R₄ or R₅ or R₆ or R₇ or R₈ can be selected in the group comprising any alkyl or aryl groups.

• The addition of a reagent 2 and an aldehyde 3 to carbon nanotubes is performed in a time comprised between 1 to 10 days, adding an excess of a reagent 2 and aldehyde 3 (at least 1 equivalent in weight) every day at the temperature of reflux of the solvent 1 ,1,2,2,-tetrachloroethane. After cooling, the organic solution is separated from the solid residue, which is washed several times with solvents, such as for example dichloromethane. The combined organic solutions are evaporated under vacuum and the remaining oily residue is extracted with a mixture of solvent and water as for example CH₂CI₂/H2O and dried. The organic residue is washed many times with a solvent such as hexane or diethyl ether, then becoming a solid. The solvents for dissolving and precipitating can be selected respectively between solvents where the carbon nanorings obtained are soluble or insoluble respectively.

The reaction can be performed with different types of commercial carbon nanotubes. The ones tried within this invention include SWNT, DWNT and MWNT, but can be easily presumed that all types of carbon nanotubes will give similar results. The carbon nanotube rings obtained are soluble in most organic solvents, e.g., chloroform, dichloromethane, tetrahydrofuran (THF), toluene, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The organic reaction with 1 ,3-dipolar cycloaddition of azomethine ylides, generated by condensation of an α amino acid and an aldehyde, is widely applied for organic modification of C₆o fullerenes and the same functionalization process in dimetylformamide (DMF) for carbon nanotubes has been first previously described at the purpose to obtain soluble carbon nanotubes (Georgakilas V. et al. J. Am. Chem. Soα, 2002, 124, 760-761 (a)). The functionaiized carbon nanotubes obtained have been proved to be soluble in the most common organic solvents, such as chloroform, methylene chloride, acetone, methanol, ethanol, and also in water, less soluble in toluene and tetrahydrofuran (THF), insoluble in diethylether and hexane. Later on this functionalization process has been also exploited in combination with other treatments for a purification process of industrial carbon nanotubes from metal and carbon contaminants (Georgakilas V. et al. J. Am. Chem. Soc, 2002, 124,14318-14319 (b)). However in both these cases, formation of carbon nanotube rings after 1 ,3-dipolar cycloaddition of azomethine ylides has not been observed, so that the formation of carbon nanotube rings through this organic reaction in a medium formed by a particular solvent, e.g. 1,1 ,2,2,-tetrachloroethane, is new and surprising, being the reaction conditions almost the same as previously reported in the papers above mentioned and different only for the solvent used. The found nanorings represent the majority of the forms observed through the microscopic analysis, some other possible forms present being not complete nanorings or carbon nanotubes. The diameters of the nanorings range between few nanometers and few microns and in particular between 10 nanometers to 950 microns. The functionaiized carbon nanotubes rings obtained show similar solubility properties in most of the organic solvents such as for example chloroform, dichloromethane, THF, toluene, DMF, DMSO and even in water as reported previously for functionaiized carbon nanotubes (Georgakilas V. et al. ref. cit. (a)). Therefore being soluble in organic solvents, they can be further processed as described for functionaiized carbon nanotubes in order to eliminate contaminants, such as metal contaminants from catalysis process of production and especially carbon materials, without impairing their structure as previously described for carbon nanotubes (Georgakilas V. et al. 2002 ref. cit. (b)). They can also be purified by column chromatography or size exclusion HPLC. Different from the known art, the carbon nanotube rings prepared have strong magnetic properties, so that they can be purified from all types of carbon contaminants after functionalization with a process further comprising:

- a dispersion of the carbon material obtained after functionalization in a non-solvent or bad-solvent medium, or a mixture of those or a mixture of a good and a bad or non-solvent,

- an attraction thereof to a region in space with means generating a magnetic field, thus obtaining a separation of pure magnetic carbon nanotube rings from other carbon contaminants.

In this disclosure a "non solvent" medium has the meaning of a fluid in which precipitation of more that 90% of carbon nanotube rings is observed when dispersing 20 mg of such nanotube rings (pure or impure) in 20 ml of such solvent. If in the same conditions the precipitate consists of a percentage of more than 20 and less that 90 then the "solvent medium" is called hereafter bad solvent. If in the same conditions the precipitate consists of a percentage of more than 10 and less that 20 then the solvent is called hereafter good solvent. If in the same condition nothing precipitates then the solvent is called simply solvent. The quantitative assessment of the quantity of nanotube rings precipitated is done by filtering the solution (one day after the dispersion is made), drying the filtered part and weighting it. In particular, the purification process of carbon nanotubes rings comprises the following steps:

- a dispersion of the carbon material obtained after the functionalization in a non-solvent or poor-solvent medium (such as a fluid or a gas), or in a good solvent or a solvent or a mixture of any of these types of solvents, - an attraction of said carbon material to a region in space by a magnetic field, - a collection of the carbon nanotube rings attracted to the magnetic field

- a collection of the carbon contaminants non attracted to the magnetic field. The same process can be applied to purify carbon nanotubes rings contained in carbon material obtained with known procedures for preparation of carbon nanotubes in the case they posses magnetic properties.

The non-solvent medium, when is a gas, can be selected in the group comprising but not limited to air, oxygen, nitrogen, compressed air, methane, argon, acetylene, tetrafluoromethane, tetrachloromethane, carbon dioxide, surpercritical carbon dioxide, butane, propane, ethane, benzene vapours. The non-solvent medium, when is a fluid, can be selected in the group comprised of but not limited to apolar organic solvents such as hexane, heptane, pentane, mildly polar solvents as chloroform, dichloromethane, 1 ,2 dichlorobenzene, acetonitrile, dimethyl formamide, acetone, or polar solvents as water, ethanol, methanol, propanol.

The magnetic field can vary from 0.02 to 50 Tesla and can be generated by conventional generating a magnetic field means known to an artisan expert in the field, such as for example magnet having from 0.02 to 20 Tesla surface field or a solenoid having from 0.2 to 100 Tesla.

With this method the carbon nanotube rings can be separated from amorphous carbon materials and non-ring material.

After purification, carbon nanotube rings can go back to carbon nanotubes, so that the process for preparation and purification of carbon nanotube rings herein described can be useful also as purification process of pristine nanotubes. That is once the rings have been separated from all forms of impurities the same can be dissolved in solvent such as dichlorometane and exposed to a disrupting shock suitable to break the carbon nanotube rings such as for example a sonication with a standard commercially available sonicator, this has been observed to make them go back to straight or almost straight nanotubes. As a consequence this purification method can also be used to produce pure carbon nanotubes. Responding to a magnetic field with a ring current-like response as disclosed hereunder, the carbon nanotube rings can be used as inductors, in particular in electronic circuits.

The processes herein disclosed are extremely simple and from an industrial point of view can be scaled up very easily in different ways. Further implementation or adaptations as well as embodiments readily apparent to those skilled in the art are to be considered within the scope of the present invention. Description of the preferred embodiments Example 1. Preparation of carbon nanotube rings. 50 mg of commercial SW carbon nanotubes was suspended in 40 ml of 1 ,1 ,2,2- tetrachloroethane. Aziridine 1 (R₁ = - (CH₂)TCH₃, R₂ = COOEt, R₃ = H) was added in 4 portions of 50 mgs each, once every day and the reaction mixture was refluxed for 96 hours. After cooling, the organic phase was separated by paper filtration and the solid was washed several times with dichloromethane. The combined filtrates were evaporated under vacuum and the remaining oily residue was extracted with CH₂CI₂ZH₂O, the organic phase separated and dried over Na₂SO₄. Subsequently, the filtrated organic solution was evaporated to dryness and the residue was washed many times with hexane and diethyl ether. After taken to dryness, 10 mg of brown-colored residue was obtained. The functionalized carbon nanotube obtained are in form of rings as shown in figure 1 where a typical AFM image is reported. Example 2. Preparation of carbon nanotube rings.

50 mg of commercial SW carbon nanotubes was suspended in 40 ml of 1 ,1 ,2,2- tetrachloroethane. Amino ester 2 (R₄ = - (CH₂)₇CH₃, R₅ = R₆ = H, R₇ = CH₂CH₃) and aldehyde 3 (Rs = H) were added in 4 portions of 50 mgs each, once every day and the reaction mixture was refluxed for 96 hours. After cooling, the organic phase was separated by paper filtration and the solid was washed several times with dichloromethane. The combined filtrates were evaporated under vacuum and the remaining oily residue was extracted with CH₂CI₂/H₂O, the organic phase separated and dried over Na₂SO₄. Subsequently, the filtrated organic solution was evaporated to dryness and the residue was washed many times with hexane and diethyl ether. After taken to dryness, 10 mg of brown-coloured residue was obtained.

The functionalized carbon nanotube rings obtained are identical to those obtained in example 1. The carbon nanotube ring material obtained has proven to posses very interesting magnetic properties as demonstrated hereunder, so that these properties can be used for a very simple and easy to implement in a large scale purification process.

Example 3: Purification of carbon nanotube rings with a magnet.

2 mg of carbon nanotube ring material prepared as in example 1 were dispersed in 2 ml of toluene in a 5 cm³ quartz cuvette. The solution was sonicated (exposed to sonic waves using a standard commercially available sonicator from VWR of the ones used in chemistry to induce solubility) for 2 minutes. A blackish solution was obtained. 2 magnets (5 Tesla surface field) were placed on two faces (1 cm apart) of a cuvette. After about 1 minute most of the material suspended in solution was collected either on the cuvette walls at the height correspondent to the lower edges of the magnets or at the bottom of the cuvette. The material at the bottom was collected with a pipette and so was the material at the walls. The two samples were placed in two different cuvettes and a cycle of purification was run again.

After three cycles all of the magnetic material was combined and so was the non magnetic.

About 70% of the starting materials was found to be magnetic.

Imaging of the non magnetic material showed no presence of rings while rings were found only in the magnetic part as illustrated in fig. 5.

The magnetic properties of the carbon nanotubes obtained as previously disclosed were analyzed in 3 different ways in order to establish and quantify the nature and strength of their induced magnetic field in response to a magnetic or electric stimulus. First, it was proven that such material would be attracted to a magnet. Second, Magnetic Force Microscopy was used to image such rings. Third, an Alternating Force Gradient Magnetometer (AFGM) was used to measure the response of these materials to a magnetic field. All these techniques showed a magnetic response that is consistent with the presence of a ring current. Test example A: attraction to a magnet

5 mg of carbon nanotube ring material prepared as in example 1 were dispersed in 2 ml of hexane in a 5 cm³ quartz cuvette The solution was sonicated (exposed to sonic waves using a standard commercially available sonicator from VWR of the ones used in chemistry to induce solubility) for 2 minutes. A blackish suspension was obtained. 2 magnets (0.5 Tesla surface field) were placed on two faces of the cuvette as in the figure 2. After about 1 minute most of the material suspended in solution was collected either on the cuvette walls at the height correspondent to the lower edges of the magnets or at the bottom of the cuvette. This result is a clear indication that part of the carbon nanotube material synthesized is magnetic.

Test example B: Magnetic Force Microscopy (MFM)

MFM is an imaging technique that is able to image object that have a magnetic moment. A Digital Instrument Multimode Nanoscope Ilia with cobalt coated tips was used. Before each measurement the tips were magnetized by passing a magnet (0.5 tesla surface field) 1 cm over them. Repeated measurements showed that no magnetic imaging was possible for standard carbon nanotubes of the same type used as starting materials for the preparation of the carbon nanotube rings. Conversely, single walled, double walled and multiwalled carbon nanotube rings showed up in magnetic imaging as shown in figure 3.

Test example C: Alternating Force Gradient Magnetometer (AFGM) AFGMs are commonly used to measure hysterisis curves of magnetic materials. At the purpose of measuring the response of the carbon nanotube rings obtained in example 1 the aforementioned measurements were carried out. Figure 4 illustrates some indicative curves. With these measurements it was found that the carbon nanotube rings respond to a magnetic field exactly as inductors. Alternating Force Gradient Magnetometer (AFGM) experiments described above fully characterize the carbon nanotube rings prepared as inductors. In fact they show a clear presence of an induced magnetic field in response to an external magnetic field, thus showing the presence of ring currents. In the case of an inductor these ring currents would be induced on the ring by electrical contact and the generated magnetic field would respond with a time retardation, exactly as needed in an inductor.

Claims

1. A process for preparation of carbon nanotube rings wherein said carbon nanotube rings are obtained by means of at least the step of :

- functionalization of carbon nanotubes through an organic reaction using 1 ,1 ,2,2,-tetrachloroethane as solvent obtaining that said carbon nanotubes form rings.

2. The process according to claim 1 wherein the functionalization is performed by reacting carbon nanotubes with an aziridine represented by the general formula

(I)

(I) where:

Ri, Ra and R₃, equal or different from each other, are organic residues or a hydrogen.

3. The process according to claim 2 wherein R-i, R₂ and R₃ are selected in the groups comprising:

- Ri any alkyl or any aryl groups,

- R₂ and R₃ any carboxy-alkyl, any carboxy-aryl groups, hydrogen, or any alkyl or aryl groups.

4. The process according to claim 3 wherein R₁ is any alkyl groups, R2 hydrogen and R₃ are carboxyalkyl groups.

5. The process according to claim 2 wherein the functionalization is performed in a time comprised between 1 to 10 days, adding an excess of aziridine represented by the general formula (I) and at least 1 equivalent in weight every day of reaction at the temperature of reflux of the solvent 1 ,1 ,2,2,- tetrachloroethane.

6. The process according to claim 1 wherein the functionalization is performed by reacting carbon nanotubes with a mixture of the two reagents represented by the general formula (II) and (III)

(M) (Hi) where

R₄, R₅, RΘ, R₇ and Re, equal or different from each other, are organic residues or a hydrogen.

7. The process according to claim 6 wherein R₄, R5, R ₆, R7 and Rs are selected in the groups comprising any alkyl or any aryl groups.

8. The process according to claim 6 wherein the functionalization is performed in a time comprised between 1 to 10 days, adding an excess of reagents represented by general formula (II) and (III) and at least 1 equivalent in weight every day of reaction at the temperature of reflux of the solvent 1 ,1 ,2,2,- tetrachloroethane.

9. The process according to claim 1 wherein a further step of purification is added.

10. The process according to claim 9 wherein said purification is obtained by means of at least the steps of:

- a dispersion of the carbon material obtained after functionalization in a non-solvent or bad-solvent medium, or a mixture of those, or a mixture of a good-solvent with a bad or a non-solvent or a mixture of a solvent with a bad or a non-solvent or a mixture of a solvent with a good-solvent, or a good solvent or a solvent,

- an attraction thereof to a region in space with means generating a magnetic field.

11. The process according to claim 10 wherein the non-solvent or bad-solvent medium is a gas or a fluid.

12. The process according to claim 11 wherein the gas is selected in the group comprising air, oxygen, nitrogen, compressed air, methane, argon, acetylene, tetrafluoromethane, tetrachloromethane, carbon dioxide, surpercritical carbon dioxide, butane, propane, ethane, benzene vapours.

13. The process according to claim 11 wherein the fluid is selected in the group comprising apolar organic solvents or mildly polar solvents or polar solvents.

14. The process according to claim 9 wherein the magnetic field is comprised in a range from 0.02 to 50 Tesla.

15. The process according to claim 14 wherein the magnetic field is obtained by means of a magnet.

16. The process according to claim 14 wherein the magnetic field is obtained by means of a solenoid.

17. The process according to claim 9 wherein said purification comprises the following steps: - ^■ _

- a dispersion of the carbon material in a non-solvent or bad-solvent medium, or a mixture of those, or a mixture of a good-solvent with a bad or a non-solvent or a mixture of a solvent with a bad or a non- solvent or a mixture of a solvent with a good-solvent, or a good solvent or a solvent

- an attraction of said carbon material to a region in space by a magnetic field, - a collection of the carbon nanotube rings attracted to the magnetic field

- a collection of the carbon contaminants non attracted to the magnetic field.

18. A process for purification of carbon nanotube rings having magnetic properties contained in a carbon material wherein said purification comprises at least the following steps:

- a dispersion of the carbon material in a non-solvent or bad-solvent medium, or a mixture of those, or a mixture of a good-solvent with a bad or a non-solvent or a mixture of a solvent with a bad or a non- solvent or a mixture of a solvent with a good-solvent, or a good solvent or a solvent - an attraction of said carbon material to a region in space by a magnetic field.

19. A process for purification of carbon nanotube rings having magnetic properties contained in a carbon material wherein said purification comprises the following steps:

- an attraction of said carbon material to a region in space by a magnetic field,

- a collection of the carbon nanotube rings attracted to the magnetic field - a collection of the carbon contaminants non attracted to the magnetic field.

20. The process for purification according to claims 18 and 19 wherein the non- solvent or poor-solvent medium is a gas or a fluid.

21. The process according to claim 20 wherein the gas is selected in the group comprising air, oxygen, nitrogen, compressed air, methane, argon, acetylene, tetrafluoromethane, tetrachloromethane, carbon dioxide, surpercritical carbon dioxide, butane, propane, ethane, benzene vapours.

22. The process according to claim 20 wherein the fluid is selected in the group comprising apolar organic solvents or mildly polar solvents or polar solvents.

23. The process for purification according to claims 18 and 19 wherein the magnetic field is comprised in a range from 0.02 to 50 Tesla.

24. The process according to claim 23 wherein the magnetic field is obtained by means of a magnet.

25. The process according to claim 23 wherein the magnetic field is obtained by means of a solenoid.

26. A process for purification of carbon nanotubes wherein said purification comprises at least the following steps: - preparation of carbon nanotube rings having magnetic properties;

- a dispersion of the carbon material obtained in the previous step in a non-solvent or bad-solvent medium, or a mixture of those, or a mixture of a good-solvent with a bad or a non-solvent or a mixture of a solvent with a bad or a non-solvent or a mixture of a solvent with a good-soivent, or a good solvent or a solvent;

- an attraction of said carbon material to a region in space by a magnetic field;

- the collection of said carbon material attracted to said region in space,

27. A process of purification according to claim 26 wherein after collection said carbon material undergoes one or more steps of which at least one is:

- the production of a solution of the carbon nanotube rings attracted to a region in space by a magnetic field in a good-solvent or in a solvent and a disruption of the carbon nanotube rings.

28. The process for purification of carbon nanotubes according the claim 26 wherein said preparation of carbon nanotube rings having magnetic properties is obtained by means of at least the step of :

- functionalization of carbon nanotubes through an organic reaction using 1 ,1 ,2,2,-tetrachloroethane as solvent.

29. Carbon nanotube rings wherein said carbon nanotube rings are obtained by means of at least the step of :

- functionalization of carbon nanotubes through an organic reaction using 1,1 ,2,2,-tetrachloroethane as solvent.

30. Carbon nanotube rings according to claim 29 wherein the functionalization is performed by reacting carbon nanotubes with an aziridine represented by the formula (I)

R₁

(I) where: Ri, R2 and R₃, equal or different each other, are organic residues or a hydrogen.

31. Carbon nanotube rings according to claim 30 wherein R-i, R₂ and R₃ are selected in the groups comprising: - Ri any alkyl or any aryl groups,

32. Carbon nanotube rings according to claim 31 wherein Ri is any alkyl groups, R₂ hydrogen and R₃are carboxyalkyl groups.

33. Carbon nanotube rings according to claim 30 wherein the functional ization is performed in a time comprised between 1 to 10 days, adding an excess of aziridine represented by the general formula (I) and at least 1 equivalent in weight every day of reaction at the temperature of reflux of the solvent 1 ,1 ,2,2,- tetrachloroethane.

34. Carbon nanotube rings according to claim 29 wherein the functionalization is performed by reacting carbon nanotubes with a mixture of the two reagents represented by the general formula (II) and (III)

(N) <'"> where R₄, R₅, R ₆, R7 and Re, equal or different from each other, are organic residues or a hydrogen.

35. Carbon nanotube rings according to claim 34 wherein R₄, R₅, R₆, R₇ and R₈, are selected in the groups comprising any alkyl or any aryl groups.

36. Carbon nanotube rings according to claim 34 wherein the functionalization is performed in a time comprised between 1 to 10 days, adding an excess of reagents represented by general formula (II) and (III) and at least 1 equivalent in weight every day of reaction at the temperature of reflux of the solvent 1 ,1 ,2,2,- tetrachloroethane.

37. Carbon nanotube rings wherein said carbon nanotube rings have magnetic properties.

38. A use of carbon nanotube rings according to claim 37 in an electronic circuit.

39. The use according to claim 38 as inductors.