WO2000066813A1

WO2000066813A1 - Process for producing a superconductive layered material and product obtainable therefrom

Info

Publication number: WO2000066813A1
Application number: PCT/EP2000/003871
Authority: WO
Inventors: N. Oleg Efimov; P. Aleksander Moravskii; A. Yuri Dubitsky; Antonio Zaopo
Original assignee: Pirelli Cavi E Sistemi S.P.A.; Russian Academy Of Science Institute Of Problems Of Chemical Physics
Priority date: 1999-05-04
Filing date: 2000-04-28
Publication date: 2000-11-09
Also published as: RU2243298C2; AU4753200A

Abstract

The present invention relates to a process for producing a superconductive layered material which comprises at least a layer of a graphite-like material and at least a layer of a fulleride. Said process comprises the following steps: a) preparing an organic electrolytic solution comprising a neutral fullerene and at least one metal salt using as solvent a mixture of at least a first solvent and at least a second solvent, said first solvent dissolving said neutral fullerene and said second solvent dissolving said metal salt; b) depositing said fulleride layer on said graphite-like material layer by electrochemical deposition. Besides, the present invention relates to a superconductive layered element comprising at least a layer of a graphite-like material and at least a layer of a fulleride.

Description

"PROCESS FOR PRODUCING A SUPERCONDUCTIVE LAYERED MATERIAL AND PRODUCT OBTAINABLE THEREFROM"

The present invention relates to a process for producing a superconductive layered material which comprises at least a graphite-like material layer and a fulleride layer. More particularly, said process comprises a cathodic deposition phase in which a fulleride layer is deposited on a graphite-like material layer.

Furthermore, the present invention relates to a superconductive layered material obtainable by said process. Recently there has been a remarkable interest in organic materials which possess conductive properties or which can be suitably doped so as to show conductive properties. For example, organic materials are generally easily formed in thin films in order to be used as conductive components in devices such as switches, antistatic devices or magnetic shielding. Among said organic materials, typically used carbon-based conductors are graphite and polyacetylene. Graphitic materials, characterized in having an infinite sheet-like structure of carbon atoms, present conductivity values in the range from 10³ to 10⁵ Siemens/cm; however they are rather intractable and can not be used in a plurality of applications. Polyacetylene, instead, is known to possess conductivity values higher than 10^ Siemens/cm only if it is suitably doped. Other organic conductors, such as those based on tetrathiafulvalene, are known to have high conductivity properties (10³ Siemens/cm); however they are difficult to be shaped into desired geometries. Among the organic materials, those based on fullerenes are generally known as insulators and many attempts have been made to modify their structure in order to improve their conductivity. Fullerenes represent a particular allotropic form of carbon, and are typically represented by 12 pentagons combined with a plurality of hexagons to form a cage structure. The pentagons are required in order to allow the curvature and the closure of the surface upon itself. Nowadays, the most abundant species of identified fullerenes is the C o molecule or

"Buckminsterfullerene" or "buckyball". CβQ fullerene is a hollow molecule the carbon atoms of which are located at the vertices of said 12 pentagons and possessing 20 hexagons arranged to form an icosahedron. More particularly, the C o fullerene molecule consists of 60 carbon atoms joined together to form a cage structure with 20 hexagonal and 12 pentagonal faces symmetrically arrayed in a soccer ball-like structure. Cβo fullerene molecules form a close-packed solid material having a face-centered cubic structure. The inner hollow space of the fullerene can be used to accomodate any metal ion inside (i.e., endohedral) or outside (i.e., exohedral) the fullerene cage. The second most abundant species of identified fullerenes is C70 molecule, said molecule containing 12 pentagons combined with 25 hexagons. The C70 fullerene molecule presents a shape which is reminiscent of a rugby ball. Furthermore, fullerene molecules containing from 30 to many hundreds carbon atoms have been detected by mass spectrometry.

Fullerene films can be applied to a suitable support by using different techniques, such as impregnation or spin-coating (i.e., obtaining a coating from a solution by using a centrifuge) from fullerene solutions as well as deposition from a gas (vapour) phase. For instance, US-5,698,140 discloses a combination of fullerenes with extremely porous materials, said fullerenes being deposited by means of a vapour deposition chamber. However, a deposition method from a gas phase does not lead to satisfactory results since an undesired inhomogeneity of the film generally occurs. Furthermore, the obtained film exhibits very low conductivity properties since fullerenes are in their zero oxidation state.

Therefore, in order to enhance conductivity of a fullerene coating, it was proposed to intercalate the latter by one or more alkali metals. US-5,294,600 discloses a superconducting material which comprises a fullerene doped with rubidium and caesium, as well as the production process thereof. In particular, said production method comprises at least one of the following processes: a) ultrasonically solid-phase mixing of an alkali metal or metals and a fullerene before heat treatment; b) finely pulverizing a solid- phase fullerene before mixing with an alkali metal or metals; c) annealing a sintered body of an alkali metal or metals and a fullerene while heating and then gradually cooling. US-5,324,495 discloses a method for obtaining a metal fulleride composition by contacting a metal and a fullerene in a solvent or a mixture of solvents in which the fullerene is at least partly soluble. Typically the fullerene is dissolved or slurried in an appropriate solvent and the metal is added thereto. According to said process the obtained metal fulleride compositions are neutral molecules and show insulating properties. US-5,391,323 relates to highly conducting tractable materials (conductivity greater than 500 Siemens/cm at room temperature) which are obtained by electronic structure modifications of fullerenes. According to a form of embodiment described in said document, electrons are added to the fullerene structure by charge transfer from a species more electropositive than the fullerene, e.g. it is possible to electronically modulate a fullerene by subjecting it to an alkali metal vapour. Alkali materials, such as potassium, rubidium and caesium, are significantly more electropositive than fullerene and therefore donate electrons to the fullerene structure. US-5,223,479 relates to metal-doped fullerenes and, in particular, to a stoichiometrically-controlled method for the preparation thereof. Said method consists in preparing a more fully doped fullerene, preferably a metal-saturated fullerene, and then diluting the more fully doped moiety by contacting it, inside of a dry box, with a previously weighed amount of non-doped fullerene to give a desired stoichiometry. This accurate and difficult stoichiometric control is extremely important to be achieved since the superconductivity of a metal doped fullerene is sensitive to the exact degree of doping.

Furthermore, in order to enhance the conductivity of a fullerene coating, it is known in the art to preliminarily deposit a fullerene thin film on a suitable support and successively doping said film by carrying out its electrochemical reduction in an organic electrolytic solution containing a suitable cation, such as an alkali metal cation, to obtain the corresponding fulleride, i.e. the fullerene salt (see, for instance, Trans. Mat. Res. Soc. Jpn. / 1994 Elsevier Science B.V. / Vol. 14B / pages 1107-1112). WO 93/11067 discloses a method for producing solutions or precipitates of fullerene by electrochemical reduction in an organic electrolyte, said organic electrolyte containing a solution of an alkali metal salt and fullerene CβO or C70. According to said document a suitable electric potential is applied to a non- aqueous solution of fullerenes in the presence of a soluble salt containing a cation for a time sufficient to generate fullerene anions in the solution. According to a further embodiment, the fulleride compounds are prepared in the solid state. A fulleride coating can be obtained on a gold or a platinum electrode by cathodic deposition from an electrolyte consisting of an acetonitrile solution of CβO^{2" ar}>d Cβo^3". said solution being respectively prepared by bulk controlled-potential electroreduction (at -1,2 V) of a CβO suspension in solutions of CsAsFβ, KPFβ or Ca(PFβ)2 and by bulk controlled-potential electroreduction (at -1,6 V) of a CβO suspension in solutions of

(TBA)CIO4 (see "Synthesis and Electrodoping of C60^n" (n = 0. */ 2, 3) Films: Electrochemical Quartz Microbalance Study in Acetonitrile Solutions of Alkali Metal, Alkaline-Earth Metal, and Tetra-n-butylammonium Cations" by W. Koh, D. Dubois, W. Kutner, M. T. Jones, K. Kadish; J. Phys. Chem. 1993, 97, 6871- 6879).

According to the Applicant, the processes of the above described prior art show a plurality of drawbacks and disadvantages which are mainly related to complexity and duration of the different steps involved in said processes, as well as to poor control of the intercalation degree of said alkali metals.

For instance, referring to the process according to which a preliminary deposition of a fullerene film is followed by a doping phase of said film by means of its electrochemical reduction so that a fulleride coating is obtained, it can be pointed out that said electrochemical reduction is accompanied by a transfer of fullerene anions into the electrolyte solution, thus causing degradation and dissolution of the obtained fulleride coating. The fulleride coating dissolution occurs also in the process according to document WO 93/11067.

Furthermore, with reference to the fulleride coating deposition carried out by Koh et al., it has to be noted that, since neutral Cβø fullerene is insoluble in acetonitrile, the suspension of neutral C o fullerene was firstly reduced to obtain the desired

Cβθ^n" solution and then the obtained Cβo^n" was oxydized to obtain the deposition of the fulleride coating on the electrode surface, the fulleride being insoluble in acetonitrile. However this method is rather complex and requires at least two separate steps, i.e. the reduction of neutral Cβo fullerene followed by the oxydation of reduced CβQ^n" species thus obtained. Moreover, this method reveals very high sensitivity of fullerene anions to oxygen and water traces due to C o epoxidation and other subsequent reactions. Besides, according to the described method, the fulleride coating is obtained on a gold or platinum cathode. This implies that, in order to use the fulleride coating as a conductive element, the fulleride coating has to be separated from the cathode on which it is obtained. The Applicant has now found a process for producing a superconductive layered material which involves few and simple steps which remarkably decrease the complexity and the duration of the processes known in the art. Furthermore, thanks to the particular choice of the solvents mixture, as defined above, no degradation or dissolution of the fulleride coating, obtained on the electrode at the end of the electrochemical deposition, occurs. Therefore, all the fulleride coating which is produced can be advantageously utilized.

Besides, the present process allows to carefully control the intercalation degree of the alkali metals and thus the correct stoichiometry of the process can be carried out. This fact mainly results in depositing a fulleride coating on the graphite-like material electrode which possesses all the desired properties which are requested to the final product, above all its electrical properties.

Moreover, the process of the present invention allows to deposit a fulleride coating, i.e. a superconductive layer, on a graphite-like material, i.e. a conductive layer. Therefore the fulleride coating coupled to the graphite-like material can be readily used as a superconductive element with no need to separate the fulleride coating from the supporting electrode on which it has been deposited. Besides, the present process allows to obtain a fulleride layer which possesses a macroscopically and microscopically ordered structure that enhances the superconductive properties of the product of the present invention. According to a first aspect, the present invention relates to a process for producing a superconductive layered material which comprises at least a layer of a graphite-like material and at least a layer of a fulleride, said process comprising the following steps: a) preparing an organic electrolytic solution comprising a neutral fullerene and at least one metal salt using as solvent a mixture of at least a first solvent and at least a second solvent, said at least a first solvent dissolving said neutral fullerene and said at least a second solvent dissolving said at least one metal salt; b) depositing said fulleride layer on said graphite-like material layer by electrochemical deposition. In the following, the wording "graphite-like material" is referred to carbon substances which are composed of graphene layers, i.e. monolayers of hexagonal flat arrangement of carbon atoms. According to a preferred embodiment of the present invention the term "graphite-like material" is graphite, preferably in the form of a carbon fiber. Other forms which can be advantageously employed are, for instance, rods, films, fabrics. In the following, the wording "fullerenes" indicates any hollow, all carbon-containing molecule having carbon atoms located at the vertices of 12 pentagons (five membered carbon rings) combined with a plurality of hexagons (six membered carbon rings), said fullerenes corresponding to the general formula C2n wherein n>12.

According to a preferred embodiment of the present invention the used fullerene molecule is Cβo fullerene.

In the following, the wording "fulleride" indicates a salt of a fullerene, namely a neutral molecule that can be represented by the formula A_n(C_x)_m wherein C_x is a fullerene anion (preferably an anion of Cβo), m is the number of fullerene anions in the metal fulleride composition and is equal to the absolute value of the valence of the metal, wherein A is a metal cation and n is a number that renders the composition neutral in charge. In particular, in the case a Cβo fullerene is used, n is from +1 to +3 depending on the negative charge of the CβQ fullerene which ranges from -1 to -3.

Furthermore, the superconductive layered material according to the invention has a non-polymeric structure, since it is known that fullerene polymeric materials generally are devoid of any superconducting properties, usually showing an insulating or semiconductive behaviour. According to a second aspect, the present invention relates to a superconductive layered material which comprises at least a graphite-like material layer on which at least a fulleride layer is electrochemically deposited. Thanks to the fact that said fulleride layer possesses superconductive features and is supported on a graphite-like material, i.e. a conductive material which can be shaped in advantageous elongated structures, said superconductive layered material does not need to be successively processed in order to separate the fulleride layer from the supporting layer, i.e. said superconductive layered material can be readily used to produce superconducting elements. Particularly, since the graphite-like layer material can assume any suitable elongated structure, e.g. a fiber or a rod, said superconductive layered material can be advantageously used for producing superconducting elongated elements. Besides, the so obtained fulleride layer possesses a macroscopically and microscopically ordered structure which, according to the Applicant, is mainly due to the presence of the particular employed supporting layer, i.e. a graphite-like material, the ordered structure of which enhances the superconductive properties of the layered material of the present invention.

The electrochemical cathodic deposition of the present process is carried out by using an organic electrolytic solution which comprises, as mentioned above, one or more metal salts to which a suitable amount of neutral fullerenes is added. Preferably said metal elements are selected from alkali, alkaline- earth and rare-earth metals, or mixtures thereof. In particular, potassium (K), rubidium (Rb) and caesium (Cs) are particularly preferred.

Metal salts which are particularly preferred are perchlorates, tetrafluoroborates, hexafluorophosphates, acetates and tetraphenylborates of potassium, rubidium and caesium, or mixtures thereof As mentioned above, the organic electrolytic solution of the process according to the present invention comprises a mixture of at least a first solvent, which is able to dissolve the neutral fullerenes, and at least a second solvent, which is able to dissolve the metal salt or mixture of metal salts. Said first solvent can be selected from benzene, toluene, xylene and the like (1st group), while said second solvent can be selected from acetonitrile, dimethylformamide, dimethylacetamide, dimethylsulfoxide, propylene carbonate and the like (2nd group). In general, the ratio between the amount of the first solvent and the amount of the second solvent is predetermined mainly on the basis of the fullerene/metal salt ratio present inside the organic electrolytic solution, therefore it can widely vary from case to case. Generally, the ratio between the first solvent and the second solvent ranges from 1/1 to 1/10 v/v, preferably from 1/1 to 1/4 v/v.

According to a further embodiment of the present invention the process for producing a superconductive layered material, which comprises at least a layer of graphite-like material and at least a layer of fulleride, comprises the following steps: a) preparing an organic electrolytic solution by introducing at least one metal salt, at least a second solvent which dissolves said at least one metal salt and a neutral fullerene previously dissolved in a solvent of said 1st group, in particular toluene; b) depositing said fulleride layer on said graphite-like material layer by electrochemical deposition.

Moreover, the Applicant has found that said metal salt or salts can be advantageously added in solution with one or more crown ethers, said ethers positively enhancing the metal salt solubility and alkali cation intercalation rate. Among said crown ethers, dibenzo-18-crown-6 ether is particularly preferred. According to a particular embodiment of the present invention, the process is carried out by applying an electric potential with a potentiodynamic regime to the organic electrolytic solution wherein the fullerene is dissolved. This means that the applied potential is varied within a predetermined range, e.g. from +0.5 V to -2.5 V (versus Ag/AgCI reference electrode), at a predetermined scanning rate and the cycle is repeated for a predetermined number of times.

In a further embodiment, the electrochemical cathodic deposition is carried out at a potentiostatic regime, i.e. by applying a predetermined electric potential so as to obtain a desired fullerene reduced species which consequently results in a desired corresponding fulleride coating on the graphite-like material layer.

According to a further embodiment of the present invention, the electrochemical deposition process is divided into two different steps. A first step wherein the electrolyte is subjected to a potentiodynamic regime in order to carry out the intercalation of said metal cations onto the cathode surface, and thus to carry out the doping of the cathode. A second step wherein, after the neutral fullerene amount has been added to the organic electrolytic solution, a potentiodynamic regime is applied to the electrolytic solution to obtain the deposition of the desired fulleride. According to a particular embodiment of the present invention, the cathodic potential range of the potentiodynamic regime applied during the first step is substantially identical to that of the potentiodynamic regime applied during the second step. Besides, according to a further embodiment, a suitable potentiostatic regime is applied to the organic electrolytic solution after said second step so as to better consolidate the formation of the desired fulleride. The Applicant has found that the above mentioned intercalation of said metal cations onto the graphite-like material layer, i.e. the doping of the cathode surface, is particularly facilitated by using one or more crown ethers. Furthermore, the Applicant has found that dibenzo-18-crown-6 ether is particularly convenient for potassium cations intercalation.

According to the Applicant, the doping of the cathode surface is believed to cause an electrochemical intercalation of a predetermined amount of metal cations present inside the organic electrolytic solution in order to ensure a preliminary intercalation of the graphite-like material surface structure as well as to ensure the concomitant storage of a specific amount of metal cations inside this structure. The Applicant believes that said stored amount of metal cations in the cathode structure enhances its conductivity and can be useful during the successive formation of the fulleride coating on the graphite-like material layer.

The process according to present invention is generally carried out in an electrochemical cell which is advantageously operated under oxygen-free conditions. In fact, the presence of even little oxygen amounts inside the cell can alter the fulleride coating composition by introducing bound oxygen groups into the coating structure. To achieve said oxygen-free conditions, an inert gas flow, e.g. a pure Argon flow containing less than 1 ppm of oxygen, is generally bubbled inside the electrolyte solution and successively maintained through the cell during the whole electrochemical process.

The present invention is now described in more details in the following examples with reference to the attached Figure 1 which represents a fulleride coating photograph obtained according to a diffraction technique.

EXAMPLE 1

A conventional electrochemical three-electrode cell with anodic and cathodic compartments, separated with a medium pore sintered glass frit, was used for the preparation of a fulleride coating. The cell was operated under oxygen-free conditions since the presence of even little oxygen amounts alters the fulleride coating composition by introducing bound oxygen groups into the coating structure. To achieve said oxygen-free conditions, an inert gas flow, e.g. a very pure Argon flow containing less than 1 ppm of oxygen, was bubbled for 30 minutes inside the electrolytic solution. Said Argon flow was successively maintained through the cell during the whole electrochemical process. The three electrodes of the electrochemical cell were:

1) a platinum grid (having a surface area of about 3x8 cm²) was used as counter electrode (anode), the surface area of which was comparable with the surface area of the working electrode (cathode); 2) a graphite fiber (which was 3 cm in length and 16 mg in weight) was used as working electrode; the graphite fiber was a YS-80-60S graphite fiber by Nippon Graphite Fiber Co., said fiber being a bunch of several hundred filaments, each one of about 7 mm in diameter and 2.17 g/cm³ in density;

3) a reference electrode of the type silver/silver chloride (Ag/AgCI), wherein a silver wire was immersed into a 4M aqueous solution of Lithium Chloride (LiCI); the reference electrode was separated from the electrolytic solution contained inside the cell by a salt bridge filled with a solution of the supporting electrolyte. All the potentials determined in the following description are relative to said Ag/AgCI reference electrode. The potential of a reference electrode was standardized by measuring the potential of the redox transition in the ferrocene/ferrocenium cation couple (Fc/Fc+). A deoxygenated electrolytic solution was introduced inside of the electrochemical cell, said electrolytic solution consisting of 1.3xl0^-2 M KBF4, 1.3x10-2 M dibenzo-18-crown-6 ether and a mixture of acetonitrile/toluene the ratio of which being of about 1/3 v/v (1 is referred to acetonitrile and 3 to toluene). The deoxygenated electrolytic solution was obtained by using an Argon flow as mentioned above. All the solvents and reagents were purchased from Aldrich and dried.

Successively to the introduction of the electrolytic solution, i.e. the fullerene-free electrolytic system, inside the electrochemical cell, a potentiodynamic regime was applied to the cell by operating a potential scanning within the range of from +0.5 V to -2.5 V. Said potential scanning was repeated for twenty cycles at a 10 mV/s scan rate by means of a programmer (PR-8 model made in Russia) and a potentiostat (PI-50-1 model made in Russia). As a result of the application of said potentiodynamic regime, the doping of the carbon fiber by potassium cations (K+) was obtained, i.e. the intercalation into the carbon fiber of potassium cations coming from the metal salt KBF4 of the electrolyte in use was occured.

Successively, a neutral fullerene C o (purity 99.9%) amount was added to the electrolytic solution. Said neutral fullerene (deep violet in colour) was introduced in solution with toluene so that the CβQ concentration in the electrochemical cell was of about

3xl0^-4 M. The concentration of CβQ in the toluene solution was of about 1.13 g/L (1.57xl0^-3 M). Once said addition was performed, the electrolytic process was carried out under the same potentiodynamic regime mentioned above (from +0.5 V to

-2.5 V) and a Cβo^3" fullerene coating was deposited onto the carbon fiber electrode, said deposition being due to the reduction of neutral Cβo fullerene present in the electrolytic solution. After a deposition period of about 5-10 hours, a fullerene coating of about 1 mm in thickness was obtained as revealed by a scanning electron microscopy analysis of the fiber electrode of the electrochemical cell.

While applying the potentiodynamic regime mentioned above, cyclic voltammetry measurements of the occurred chemical reaction were performed to obtain a voltammetric profile of Cβo solution in the described acetonitrile/toluene solution containing KBF4/dibenzo-18-crown-6 ether. Said voltammetric profile revealed that a first reversible reduction wave, corresponding to the Cβo/ Cβo^" redox couple, occured at -0.75 V, while a second reversible reduction wave, corresponding to the Cβo^"/ Cβo^2" redox couple, occured at -1.4 V, and a third reversible reduction wave, corresponding to the Cβo ^"/Cβo^3" redox couple, occured at -1.8 V. Carrying out a voltammetric profile of the metal doped graphite-like material it was possible to verify the complete saturation of the surface layer of the working electrode by the metal cations present in the organic electrolytic solution. In particular, the saturation was complete when the current through the cell, in which the organic electrolytic solution was contained, did not grow any more.

Successively to the addition of the neutral fullerene solution and the application of said potentiodynamic regime, the electrolytic process was carried out at a potentiostatic regime, in particular at a cathode potential equal to -1.8 V, until the current through the cell vanished to zero. The use of said potentiostatic regime was applied in order to achieve a complete charging of the CβQ^3" fullerene coating and thus a corresponding full completion of potassium cations intercalation in the fullerene coating up to the desired stoichiometric composition K3CβQ.

Therefore, potassium counterions of fulleride were obtained on the carbon fiber at the end of the electrolytic process.

The obtained fulleride was insoluble in the solvent mixture and thus no losses of fulleride had occured.

In order to determine the structure of the obtained fulleride coating, two different kinds of tests have been carried out. Since a K3Cβø coating is not stable under normal environmental conditions, to carry out the structural analysis of the obtained ^K3^C60 coating, the latter was oxydized, at room temperature and at a potential value equal to -0.75 V, so that the corresponding KCβo coating was obtained. In fact, the corresponding KCβo coating is stable in a dry oxygen-free atmosphere and can be analyzed after washing with ethanol to remove soluble impurities, such as solvents.

It has to be pointed out that oxidation of the K3Cβo coating to a

KCβo coating, the latter being obtained in order to be structurally analyzed, does not cause any changes in the K3Cβo coating structure. This means that, from a structural point of view, it is equivalent to analyze the stable KCβo coating instead of the unstable K3Cβo coating.

Therefore, a Scanning Electron Microscopy Analysis was carried out on the 1 mm thick KCβo coating as well as an Electron Beam

Diffraction Analysis by using a Jeol model EM 1200 CX instrument. Said analysis have been carried by spots on different zones of the KCβo coating; in particular 5 zones have been inspected. For each of said zones the diffraction analysis revealed that the KCβo coating, and thus the obtained K3Cβo coating, possessed a very ordered structure both from a macroscopic and a microscopic point of view. The coating revealed an advantageous surface uniformity and all the analyzed crystals, even though belonging to different zones of the coating, showed always the same pattern. For instance, Figure 1, which is a photography of a coating zone as resulted from the diffraction technique, clearly shows a pattern that is typical of a cubic type symmetry lattice.

Furthermore, elemental analysis of coatings of different composition revealed the presence of potassium in all the analyzed samples, the potassium content being about three times higher in K3CβQ coatings compared to that in KCβo coatings.

The obtained fulleride K3CβQ coating was also subjected to magnetic susceptibility measurements in order to reveal the presence of a superconducting transition temperature. Thus, the cell was cooled to -20°C, the electrolyte was removed by a syringe, the cathode fiber was rinsed with cold dry acetonitrile and transferred under helium atmosphere in an ampule which was sealed and then stored in liquid nitrogen. The temperature dependence of the magnetization of the obtained K3Cβo coating revealed a distinct jump at 19K showing the presence of a superconducting phase in the coating having a transition temperature (Tc) equal to 19K, a value typical of a superconductive K3Cβo material.

A differential AC magnetometer was used to perform magnetic susceptibility measurements in the range from liquid helium to room temperature. The magnetometer setup consisted of three coaxial coils: a middle transmitting coil and two external receiving coils. The middle transmitting coil was connected to a frequency generator and it induced the AC magnetic field in both the receiving external coils which were connected to a differential preamplifier in order to register the disbalance of the induction voltages. The registered signal was set close to zero if the sample inserted into an external coil was in a normal state or if the cell was empty. The occurence of the superconducting transition in the sample produced an extra magnetic flux which resulted in a disbalance of voltages in the receiving coils, said disbalance being registered by a computer. The setup was placed in a helium Dewar vessel and cooled down to liquid helium temperature. An electric heater caused the temperature increase controlled by a thermocouple and thus the temperature dependence of the susceptibility was obtained.

EXAMPLE 2

Example 2 was carried out under the same conditions of Example 1, the only differences being: a) the use of 1.25xl0^-3 M KPFβ as alkali metal salt component of the electrolytic solution; b) no crown ether was used; c) a different kind of graphite fiber as cathode was used, i.e. YS-15-60S graphite fiber (Nippon Graphite

Fiber Co.) which consists of several hundred 7 to 10 mm in diameter filaments, the filament material density being 1.85 g/crm³. Though the concentration of the electrolyte was an order of magnitude lower than that used in Example 1, a 1 mm thick fulleride coating was deposited in a reasonable time (10 - 15 h) despite much lower conductivity of the cell and concomitant high ohmic losses which should not favour the growth of the coating.

EXAMPLE 3

Example 3 was carried out under the same conditions of Example 1, the only differences being : a) the use of 0.1 M KCIO4 as alkali metal salt component of the electrolytic solution comprising dimethylformamide/toluene in the ratio of 2/3; b) the introduction of 1.5xl0^-4 M of Cβo fullerene.

Furthermore, the electrolytic process of Example 3 was carried out under potentiostatic conditions. In particular: a) a first constant cathode potential equal to -0.9 V vs the Ag/AgCI reference electrode, value at which a two electron reduction of the fullerene moiety occurred; b) successively a second constant cathode potential equal to -0.4 V. After a period of about 20 hours, a 1 to 2 mm thick K3Cβø coating was deposited on the carbon fiber surface.

EXAMPLE 4

Example 4 was carried out under the same conditions of Example 1, the only differences being : a) the cathode was a polyacrilonitrile fiber pyrolysed at 1.100°C, 2-3 cm long and consisting of several hundreds of separate filaments, each one of about 10 mm in diameter; b) the organic electrolytic solution was a saturated solution of potassium perchlorate in a mixture of acetonitrile/toluene (in the ratio 1/4) containing

The fullerene was introduced at the beginning of the electrolytic process and no previous doping of the cathode occurred. The electrolytic deposition of a K3C o coating on the surface of the filaments was carried out potentiodynamically at a 50 mV/s scanning rate in the potential scanning range from +0.5 V to - 3.0 V and a coating of about 1 mm in thickness was deposited after a period of about 10 hours.

Electron Beam Diffraction Analysis and magnetic susceptibility measurements have been carried out as for Example 1 : the diffraction analysis showed an ordered structure of the obtained fulleride coating and the magnetic susceptibility measurements revealed the superconductivity of the graphite-like layered material, with results fully in accordance to that obtained in Example 1.

Claims

1. Process for producing a superconductive layered material, said layered material comprising at least a layer of a graphite- like material and at least a layer of a fulleride, characterized in that said process comprises: a) preparing an organic electrolytic solution comprising a neutral fullerene and at least one metal salt using as solvent a mixture of at least a first solvent and at least a second solvent, said first solvent dissolving said neutral fullerene and said second solvent dissolving said metal salt; b) depositing said fulleride layer on said graphite-like material layer by electrochemical deposition.

2. Process according to claim 1, characterized in that said step a) comprises the phase of preparing an organic electrolytic solution by dissolving the metal salt in said second solvent and then mixing the so obtained solution with a neutral fullerene previously dissolved in said first solvent.

3. Process according to claim 1 or 2, characterized in that said electrochemical deposition is carried out at a potentiodynamic regime.

4. Process according to claim 3, characterized in that said potentiodynamic regime is carried out in the range of from +0.5 V to -2.5 V.

5. Process according to claim 1 or 2, characterized in that said electrochemical deposition is carried out at a potentiostatic regime.

6. Process according to anyone of the preceding claims, characterized in that an intercalation of the metal cations of said metal salt is carried out onto said graphite-like material layer from said organic electrolytic solution before introducing said neutral fullerene into said organic electrolytic solution, said intercalation being carried out at a potentiodynamic regime.

7. Process according to claims 1 to 3 or 6, characterized in that said electrochemical deposition is followed by application of a potential at a potentiostatic regime.

8. Process according to anyone of the preceding claims, characterized in that said graphite-like material is graphite.

9. Process according to claim 8, characterized in that said graphite is in the form of a carbon fiber.

10. Process according to anyone of the preceding claims, characterized in that said fullerene is C60 fullerene.

11. Process according to anyone of the preceding claims, characterized in that said metal cations are selected from the group consisting of alkali, alkaline-earth and rare-earth metal cations, or mixtures thereof.

12. Process according to claim 11, characterized in that said metal cations are potassium, rubidium or caesium cations, or mixtures thereof.

13. Process according to claim 11 or 12, characterized in that said metal salts are selected from the group consisting of perchlorates, tetrafluoroborates, hexafluorophosphates, acetates and tetraphenylborates of potassium, rubidium and caesium, or mixtures thereof.

14. Process according to anyone of the preceding claims, characterized in that said first solvent is selected from the group consisting of benzene, toluene and xylene, or mixtures thereof.

15. Process according to claims 1 to 13, characterized in that said second solvent is selected from the group consisting of acetonitrile, dimethylformamide, dimethylacetamide, dimethylsulfoxide and propylene carbonate, or mixtures thereof.

16. Process according to anyone of the preceding claims, characterized in that said organic electrolytic solution comprises at least one crown ether.

17. Process according to claim 15, characterized in that said crown ether is dibenzo-18-crown-6 ether.

18. Process according to anyone of the preceding claims, characterized in that the ratio between the first solvent and the second solvent is of from 1/1 to 1/10 v/v.

19. Process according to claim 18, characterized in that said ratio is of from 1/1 to 1/4 v/v.

20. Superconductive layered element comprising at least a layer of a graphite-like material and at least a layer of a fulleride.

21. Superconductive layered element according to claim 20, characterized in that said fulleride layer is

22. Superconductive layered element obtainable by the process according to anyone of claims from 1 to 19.