FR2861089A1 - Production of composite carbon-based electrodes for synthesis of carbon nanotubes by electric arc process, using carbon and carbon nanotube catalyst powders - Google Patents

Abstract

The production of composite carbon-based electrodes comprises mixing compounds rich in carbon atoms with a binder defined by at least one thermo-hardening resin to prepare a paste, giving the paste a specific shape by the application of pressure and subjecting the shaped paste to heat treatment. The paste is prepared from graphite powder, powdered catalyst elements for carbon nanotube synthesis and at least one liquid phenol resin thermo-hardening at ambient temperature and homogenizing them at ambient temperature. An independent claim is also included for an electrode produced by the above process.

Description

The present invention relates to a method for manufacturing electrodes

  carbon-based composites of the type used for the synthesis of carbon nanotubes by the so-called electric arc technique, said process consisting in mixing compounds

  carbonaceous material with a binder, defined by at least one thermosetting resin, so as to prepare a paste sufficiently compact to be shaped under the effect of a pressure, and to subject the resulting shape to heat treatments intended to adjust the properties electrical, mechanical and thermal of said electrodes.

  In known manner, the synthesis of carbon nanotubes by the electric arc technique consists in placing two electrodes, one of which is generally made of pure graphite and the other of a mixture of graphite and catalytic elements, a few millimeters apart from each other and apply a strong intensity to them.

  This has the effect of causing between the two electrodes a spark whose high heat is capable of inducing a sublimation of the reactive electrode, that is to say that containing the elements of catalysis, which is consumed as a result. and as a result of the reaction, and then recondense, in the form of a collar rich in carbon nanotubes, on the pure graphite electrode.

  If many parameters are involved in the success and control of this reaction, the homogeneity, the crystallographic and mechanical characteristics as well as the resistivity of the reactive electrode, play a determining role in the production yield and the quality of the nanotubes as well as synthesized.

  Therefore, the possibility of having available and industrial manufacturing suitable electrodes is one of the key factors, likely to allow the development of technology related to carbon nanotubes, through the improvement and especially the industrialization of this method of synthesis by electric arc.

  A unique method of manufacturing such electrodes, able to allow the synthesis of carbon nanotubes by the technique of the electric arc, is known at present.

  It consists mainly of using bars of pure graphite, typically 152 mm long, 6.15 mm in diameter and about 6 grams, in which each time a blind hole of 3 millimeters in diameter is drilled for a distance of 110 mm, which is then filled with a powdery reactive mixture consisting of graphite and catalytic elements of the synthesis reaction of carbon nanotubes.

  This method, however, has many disadvantages, both as to the final structure of the electrode obtained, as to its implementation, exclusively manual, and therefore particularly hazardous and time-consuming.

  Indeed, the graphite, which composes the filling powder to more than 90% presents, due to its very small particle size, a very low bulk density, of the order of 0.2 to 0.3 grams per cubic centimeter. which makes that when the powder is packed in the hole, its volume is divided by four, forcing the manipulator to fill each bar little by little, with little powder at a time, by packing it gradually, as and when filling .

  The fact of filling such bars in an automated way by tamping the powder in a single operation is not feasible in practice because it would require the use of a piston driven by an extremely large stroke relative to the section of the hole , ie more than 50 centimeters for a section of 3 millimeters.

  On the other hand, it is extremely difficult to obtain, by this method, electrodes having a homogeneous distribution of the catalysts and the compacted powder in its entirety, and one often finds a concentration of elements of catalysis in the center of bars, because of a compaction of the powder made manually, irregularly, inaccurately or poorly.

  The electric arc generated by such an electrode is then located only on a very small surface, and the amount of nanotubes synthesized is therefore much lower than with an electrode having a homogeneous distribution of catalysts over the entire its surface.

  On the other hand, this technique still has the disadvantage of being limiting in the geometry of the electrodes used. It is indeed difficult to perform drilling on pure graphite elements on diameters of 3 mm for stroke lengths greater than 13 or 14 cm.

  Thus, this method is both laborious, time-consuming, and unsatisfactory from the point of view of the final properties of the electrodes manufactured.

  It has therefore been sought to develop a new manufacturing process capable of obtaining reactive electrodes having the characteristics required for the synthesis of carbon nanotubes by the electric arc technique, said process being able to be carried out with industrial way.

  It has thus been necessary to check whether the numerous documents describing processes for manufacturing carbon-based composite electrodes intended for other applications, in particular relating to the field of metallurgy, could describe suitable solutions for the synthesis of electrodes adapted to the production of carbon nanotubes.

  As is apparent from the state of the art, the industrial manufacture of carbonaceous electrodes is conventionally carried out by extrusion or pressing of a paste, obtained by mixing carbon compounds with a binder which, after specific heat treatments are carried out. carbonizes, releasing a fraction of volatile gases while another fraction remains in the electrode in the form of more or less organized carbon.

  A first series of documents concerning the industrial manufacture of carbonaceous electrodes has been ruled out from the outset because of the use, as a binder, of petroleum derivatives which, in a known manner, not only give off toxic vapors during certain stages of the described processes, but which in QQtre are not adapted to the application of synthesis of carbon nanotubes considered, due to the fluctuation of their composition and the strong presence of impurities unacceptable to the process, such as the oxygen and heavy organic elements, which does not guarantee the reproducibility of the final structure of the electrodes manufactured.

  However, as already mentioned above, the success of the carbon nanotube synthesis reaction depends directly on this latter characteristic.

  Other documents showing the use of synthetic binders which make it possible to avoid these various disadvantages therefore seem more interesting.

  Thus, document US 4,431,503, for example, relates to a process for manufacturing carbonaceous electrodes intended for use in particular in the field of electrolytic production of aluminum and which must therefore have properties enabling them to withstand strong constraints inherent to this technology.

  This document recommends, among other things, mixing carbon powder, preferably coke, containing aluminum oxides, with a thermosetting liquid phenolic resin, to put the product obtained in shape by vulcanization under pressure at low temperature, then to anneal the product.

  Although this process has a certain number of advantages, it is not however suitable for the problem of the synthesis of electrodes intended for the production of carbon nanotubes in which the metals must necessarily, at the end of manufacture, be found in their metallic form, and not oxidized, to be able to play their role of catalysts.

  On the contrary, the document provides for the use, from the outset, of the oxidized aluminum in the starting mixture; in addition, the various treatments recommended, in particular thermal, are likely to induce inevitably oxidation of any particles in the form of metal initially.

  On the other hand, it has been found that with such a method, it is practically impossible to use graphite because its wetting by the recommended resin is a particularly delicate operation and difficult to achieve properly.

  US 4,775,455 also relates to a method of manufacturing electrodes in which carbon compounds are mixed with phenolic resins to obtain a product shaped by pressure and then subjected to a series of heat treatments.

  This document also does not provide for the introduction of metals into the initial carbonaceous mixture and therefore does not provide a solution for keeping these in their metallic form during the process.

  In addition, it advises against the use of pure graphite as part of electrode fabrication through the process in question, while the use of very high carbon graphitic structure is quite fundamental in the part of a production of electrodes for the synthesis of carbon nanotubes, this raw material having the most suitable crystallographic structure for this application.

  On the other hand, and contrary to what is taught in the aforementioned document, the document US Pat. No. 4,775,455 advises against the use of a thermosetting liquid resin, on the pretext that the latter does not make it possible to reach the density characteristics. required for a quality electrode.

  Finally, another major disadvantage encountered in each of the two documents cited, relates to the structure of the electrodes, in which it is found that the carbon that composes them is defined by vitreous carbon, unordered, and not conducive to optimal synthesis of nanotubes. carbon.

  The documents of the prior art therefore do not propose a process capable of allowing the industrial manufacture of electrodes having a structure and a composition meeting the strict requirements required in the context of the synthesis of carbon nanotubes by the technique of the electric arc.

  The object of the present invention is to provide such a method.

  Thus, the present invention relates to a process for manufacturing carbon-based composite electrodes, of the type used for the synthesis of carbon nanotubes by electric arc, in which compounds rich in carbon atoms are mixed with a binder defined by FIG. less than a thermosetting resin for preparing a paste, a specific shape is given to said paste by applying a pressure to it, the form obtained is subjected to heat treatments, characterized in that the powder used to prepare said paste is graphite, the powder of catalyst elements of the synthesis of nanotubes, at least one thermosetting phenolic resin liquid at room temperature, and which is homogenized at room temperature.

  Thus, the first step of the present process is, firstly, to intimately mix, at room temperature, graphite powder, preferably synthetic and controlled particle size, with catalyst elements.

  According to a first characteristic of the process according to the invention, use is made of graphite powders and catalyst elements whose particles have a size of between lpm and 80 μm, preferably of the order of 40 μm.

  This characteristic may, if necessary, be adjusted by simple grinding of the powders before homogenization with the resin, carried out, as the case may be, manually or mechanically.

  This powder mixture is kneaded using a powder mixer until a homogeneous mixture is obtained.

  According to another characteristic of the present process, a mixture of at least two catalyst elements of the nanotube synthesis reaction, preferably a mixture of at least two metals in a total concentration of less than 6% of the mass, is introduced. total of graphite powder, preferably less than 2%, more preferably less than 1%. Due to the small amount of catalyst elements introduced, it is very difficult to obtain an ideal mixture of the perfect powders, it is It is therefore essential to attach particular importance and care to their homogenisation.

  According to another advantageous characteristic of said process, it is also planned to mix the powders dry to prevent any oxidation of the chemical elements in contact with the binder used.

  As already mentioned above, the type and the quantity of metals used for the manufacture of said electrodes are determining parameters for the optimal course of carbon nanotube synthesis, in which these elements act as catalysts capable of inducing the formation of carbon nanotubes. the electric arc necessary for the reaction.

  Thus, according to an additional feature of the present process, a mixture of at least two metals, one of which is selected from transition metals, and the other of rare earths, chalcogenides or heavy metals, is used. .

  Such metals will thus be defined for example by nickel, yttrium, cobalt, iron or calcium carbide.

  According to another characteristic of the present process, an atomic% of each metal constituting the mixture is introduced, the ratio between the majority element and the minority element to be between 0.15 and 0.25.

  To the mixture of graphite powder and metal powder is then added a binder based on one or more phenolic resins, the respective proportions of which are adjusted according to the properties that are sought for the bars, such as density, or conductivity, in particular.

  The viscosity of the binder, and therefore its ability to wet the graphite, is adjusted according to three parameters, on the one hand the nature of the resin (s) used, and on the other hand the dilution ratio of each resin and finally the nature of the solvent used. .

  The binder is preferably introduced in the form of droplets, by means of a spray, into the mixture of powders.

  The viscosity of the binder is adjusted in adequacy with the spray means and with the wettability of the graphite, between 120 and 290 centipoise and preferably between 200 and 250 centipoise, by means of an organic solvent, such as, for example, acetone.

  This solvent is very volatile and allows to adjust the viscosity of the mixture without remaining there. It is necessary to use a very pure acetone to avoid polluting the mixture.

  The present process further provides for adding the binder preferably at the last moment to the mixture of graphite powder and catalyst elements, to wet it optimally.

  In fact, the addition of the binder is possible when the mixture of powders is considered homogeneous.

  Moreover, the proportion of resin introduced into the powder mixture is previously calculated relative to the total mass of the medium and varies according to the desired bar grade.

  According to the present method, the entire step of mixing the constituent elements of the electrode, which has just been described, can be automated and therefore alone represents a major time saving compared to the methods used previously.

  When the powder composed of metals and graphite is suitably homogenized with the resin, a tacky powder is obtained which exhibits a strong tendency to agglomeration, and which retains its properties for at least one hour, if it is not subjected to excessive evaporation.

  We can then proceed to the next step of the present process which consists of shaping the dough by applying pressure to it, by means of a shaping apparatus such as an extruder or an injection machine. in a relatively wide pressure range of 50 to 300 bar continuously, for example between 150 and 300 bar, preferably between 250 and 300 bar This extrusion is thus carried out under a temperature of between 50 C and 120 C, obtained by heating of the powder in rotation by means of the extrusion screw inside the extrusion nozzle, which makes it possible to give a first cross-linking of the upper crust of the bar, and to facilitate subsequent training thereof on a conveyor belt.

  The shaping makes it possible to obtain bars of good quality, having a state of surface perfect with the naked eye, and whose length and diameter can advantageously vary and be adapted according to the subsequent needs.

  Of course, the present method also provides the possibility of putting the material into shape in a mold by applying the same type of pressure.

  This generally leads to bars smaller than those obtained by extrusion, but which nevertheless have the same properties and characteristics from the point of view of their composition and structure.

  According to another particularity of the present process, an evacuation of the vaporized solvents is also carried out during the step of forming the tacky material, which advantageously makes it possible to improve the density of the bars, and to evacuate most of the oxygenated compounds, thanks to a depletion in solvent.

  In addition, this suction makes it possible to eliminate the air that could be trapped in the viscous paste obtained after mixing the binder with the powders.

  The presence of air bubbles is unacceptable in the context of the production of electrodes for the synthesis of carbon nanotubes. Indeed if an air bubble was to be blocked in a bar, the volume expansion of the air caused by the heat treatment, would lead to the pressurization of the bar and its explosion.

  The bars obtained after the shaping step can then optionally be subjected to a first heat treatment, carried out between 110 ° C. and 200 ° C., preferably between 150 ° C. and 180 ° C., preferably between 160 ° C. and 170 ° C. for a duration proportional to their temperature. size, and by means of heater used.

  This corresponds to a vulcanization whose object is to obtain the hardening of the bars by crosslinking the resin.

  This vulcanization is performed by means of a system capable of delivering a homogeneous heat over the entire distance traveled by the bars on a carpet, driven by a conventional advance system.

  Because of the density of the rods sought, and the obligation to cook them in their entirety, the present method provides for the use of a vulcanization system which relies on an infra-red heating to cook the parts and to heat the parts. have a homogeneous heating.

  According to another possibility of said process, which makes it possible to improve the progress of the reaction, the end of the vulcanization step is checked with a volatile compound trap, defined for example by a basic solution capable of changing color. as the reaction progresses and indicates the end.

  Although this vulcanization step is optional, it has the advantage of facilitating subsequent reactions of the process; in addition, after this first heat treatment, the bars can be stored for a long time, have a high internal cohesion and are easily manipulated, both by an operator than by an automaton.

  According to another characteristic, said bars are subjected to carbonization carried out in stages between the vulcanization temperature and a temperature of between 900 and 1100 ° C., preferably around 1000 ° C., depending on the binder used under a controlled atmosphere, preferably inert mainly, but may consist of a mixture of inert gas and a reducing gas such as hydrogen.

  The bearings are a function of the characteristics of the binder used. In fact, most of the volatile groups go from 90 to 450 C and then from 600 to 800 C in the form of recombination.

  It is therefore necessary to adapt the heating ramps according to the quantities of volatiles to be released. On phases where 50% of the volatiles must leave, the temperature ramp is very low and vice versa. This step is done under a controlled and reducing atmosphere because the lesser presence of oxygen destroys the structure of the bars and renders them unusable.

  This treatment makes it possible to give the bars their final structure, not only thanks to the evacuation of all the nitrogen and oxygen atoms present in different forms within the material, but also thanks to an improvement of the carbon structure. , which evolves from a destructured state to a more structured state and therefore has properties closer to those of graphite than those of amorphous carbon.

  The high temperatures used therefore confer on the carbon the specific crystallographic structure in which the synthesis of carbon nanotubes can take place optimally.

  Thus, the present invention also relates to an electrode obtained by means of the manufacturing method as previously detailed, such an electrode being characterized in that it comprises graphite and catalyst elements of the synthesis of carbon nanotubes, said graphite being arranged, from the standpoint of its crystallographic structure, in a manner to impart to said electrode a density of at least 1.3, and a resistivity of less than 0.026 ohm.cm.

  Moreover, according to a preferred embodiment, such an electrode is further characterized in that said catalyst elements for the synthesis of carbon nanotubes it contains are distributed homogeneously over its entire volume.

  The various examples described below make it possible, inter alia, to illustrate that the method according to the invention is perfectly suited to an industrial implementation which can also be carried out using conventional and inexpensive machines.

Example 1

  4 kg of graphite, with a particle size distribution centered at 40 μm, are charged with nickel and yttrium powder metal catalysts of controlled particle size in proportions of between 1 and 0.3 at%, and 1500 ml of a resole resin of viscosity. equal to 125 centipoise.

  The machine is rotated until the powder is tacky, agglomerates with a simple pressure of a spatula, and has a homogeneous appearance.

  It is then introduced into the hopper of an extruder, having at least one worm to ensure its pressurization, or having two worms placed in series, the first to ensure the conveying and a complementary stirring of matter, while the second performs the actual extrusion.

  During all the manipulation, an evacuation of the evacuated solvents is carried out, for example by means of a paddle pump, coupled to a nitrogen trap in order to avoid the formation of emulsions, placed at the rear of the screw of extrusion, so as to minimize the porosity of the material.

  The material that leaves the extruder after a few minutes then has a smooth surface, a homogeneous and solid appearance.

  It is then subjected to vulcanization, by heating at a temperature above 160 ° C., preferably at 165 ° C. on a drying belt, the speed of which can be programmed as a function of the volume of the bars.

  At the exit of this carpet, the bars are strong enough to be handled without risk of deterioration.

  Charring is then carried out under nitrogen in a scanning oven.

  The flow of gas, set at 0.51 / min, passes into a shovel with a working volume of 3 l.

  The first ramp, set at 1.5 C / min, starts at 150 C and ends at 835 C.

  The cooling of the bars is performed with the same flow rate and cut at 150 C.

  After analysis of the bars, it is found that they have a conductivity of 0.020 Ohm.cm, a density of 1.4, they are resistant to the electric arc and allow the synthesis of carbon nanotubes mono-wall type.

Example 2

  The same operations as in Example 1 are carried out, but only after the end of the extrusion has been cured 30h, by placing the bars in a preheated tubular furnace at a temperature greater than 160.degree. C., preferably 165.degree. .

  The properties of the bars obtained after the second heat treatment are identical to those of the bars obtained in Example 1.

Example 3

  The same operations as in Example 1 are carried out, but the two heat treatments are eliminated.

  Although the evacuation of the solvents takes place little by little, over time, and that there is a slight solidification of the bars, the resin does not harden completely, and the electrodes do not have the mechanical, electrical and chemically treated bars.

  Such bars are not usable in an electric arc furnace.

Example 4

  2 kg of graphite are introduced into a mixer which is started before adding, with a funnel, nickel, yttrium, and a resole resin of suitable viscosity.

  This procedure advantageously makes it possible to guarantee a constant flow rate of the resin in the mixture.

  The mixer is pushed to its maximum speed for 10 minutes and then the extrusion of the powder is carried out with an extruder whose speed is set to a minimum.

  The material leaves after 10 minutes with a satisfactory surface finish.

  It is furthermore noted that the friction with the wear liners of the extruder advantageously heats the material, which facilitates the evaporation of the solvent, and causes a partial vulcanization of the resin on the surface of the bars.

  After the extrusion, the material is placed on a drying mat, preheated to 200 C, on which it is conveyed for more than 20 minutes.

  Convection of the gaseous effluents is carried out jointly by a fan which sweeps the carpet.

  The bars are then machined so that they can be inserted into the supports of an electric arc reactor. This machining is performed before the second heat treatment.

  The latter is carried out at once, from 300 C to 985 C, with a ramp of 2 C / min up to 700 C, then 1.5 C / min up to 985 C.

  A plateau of 2h is observed at this temperature.

  The oven used has a sealed box to ensure the circulation of neutral gas in the enclosure.

  The gas flow rates and gas type are the same as for Example 1.

  The density of the bars obtained is 1.3, which corresponds, here again, to a value quite satisfactory for the intended application.

EXAMPLE 5 The mixture is made between the pulverulent materials and the resin at

  hand on a few grams, for 5 minutes, with a stainless steel spatula until obtaining a wet powder in the form of small flakes.

  The powder is then pressurized with a pestle to form pellets having a material density of about 2 g / cm 3.

  In a first experiment, the first heat treatment of these pellets is carried out in the tubular furnace, without particular convection, while the second heat treatment is carried out in one go using quartz crucibles.

  The nitrogen flow rate used is set at 0.5L / min after purging the medium for 5 minutes. The flow of nitrogen is cut after a plateau of 2 hours at 950 C. The cooling of the bars takes 10 hours.

  There is no tangible result here. All the graphite is found in powder form, as well as the crucibles. The carbonaceous mass is lower than that introduced.

  The pellets are then subjected to the conditions of Example 2, including the heat treatment up to 635 C.

  A solid pellet is then obtained, but however more fragile than the bars of the preceding examples.

  Thus, in this example, the conditions and the quantity of carbonized resins do not make it possible to lead to electrodes having, after the second heat treatment, a sufficient mechanical cohesion, even if the density observed is greater than 1.6.

  This example of implementation of the method according to the invention therefore makes it possible to show that a manual implementation leads to hazardous, poorly reproducible results, and that the electrodes manufactured in this way do not have the properties required in the context of the invention. of use as a fuel for the synthesis of carbon nanotubes.

  As is clear from the foregoing, the present process is capable of allowing an industrial and reproducible fabrication of electrodes having the structural, physical, electrical and thermal characteristics required by this specific application, while guaranteeing conditions of implementation with the lowest cost for the minimum of risk, both from the point of view of the safety of the manipulators as of that of the degradation of the environment.

  Although the invention has been described with respect to some particular embodiments, it is understood that it is in no way limited and that various modifications of shape, materials and combinations thereof can be made. without departing from the framework and spirit of the invention.

Claims (13)

  1) Process for manufacturing carbon-based composite electrodes, of the type used for the synthesis of carbon nanotubes by electric arc, in which compounds rich in carbon atoms are mixed with a binder defined by at least one thermosetting resin for preparing a paste, giving a specific shape to said paste by applying a pressure to it, the resulting form is subjected to heat treatments, characterized in that, to prepare said paste, graphite powder, powder of catalyst elements of the synthesis of carbon nanotubes at least one thermosetting phenolic resin liquid at room temperature, and that these are homogenized at room temperature.   2) Process according to claim 1, characterized in that one uses graphite powders and metals whose particles have a size between lpm and 80pm, preferably of the order of 40 pm.   3) A method according to claim 1 or claim 2, characterized in that one introduces a mixture of at least two catalyst elements of the carbon nanotube synthesis reaction, preferably a mixture of at least two metals in a total concentration of less than 6% of the total mass of graphite powder, preferably less than 2% of this mass, more preferably less than 1%, the ratio between the majority element and the minority element being between 0 , 15 and 0.25.   4) Process according to any one of the preceding claims, characterized in that one uses a mixture of powders of at least two metals, one of which is selected from transition metals, and the other of rare earths heavy metals or chalcogenides.   5) Process according to any one of the preceding claims, characterized in that the shaping of the dough is carried out by applying a pressure of between 50 bar and 300 bar continuously, preferably between 250 bar and 300. bars, through, for example, an extruder machine, to obtain bars.   6) Process according to any one of claims 1 to 4, characterized in that one carries out the shaping of the dough by pressing it into a mold.   7) Process according to any one of the preceding claims, characterized in that the shaping is carried out at a temperature between 50 C and 120 C.   8) Process according to any one of the preceding claims, characterized in that it carries out an aspiration of solvents and volatile compounds discharged during the shaping step of said paste.   9) Process according to any one of the preceding claims, characterized in that the forms obtained are subjected to vulcanization, carried out between 110 C and 200 C, preferably between 160 c and 170 C for a duration proportional to their size and by means of heater used.   10) Process according to claim 9, characterized in that the end of the vulcanization step is checked with a volatile compound trap.   11) Process according to any one of the preceding claims, characterized in that said rods are subjected to carbonization carried out in stages between the vulcanization temperature and a temperature between 900 and 1100 C, preferably around 1000 C, under controlled atmosphere.   12) Electrode obtained by means of the manufacturing method according to claims 1 to 11, characterized in that it comprises graphite and catalyst elements of the synthesis of carbon nanotubes, said graphite being organized, from the point of view of its crystallographic structure, in a manner adapted to impart to said electrode a density of at least 1.3, and a resistivity of less than 0.026 ohm.cm.   13) Electrode according to claim 12, characterized in that the catalyst elements of the synthesis of carbon nanotubes are distributed homogeneously over its entire volume.