Process for synthesizing carbon nanotubes on multiple supports

Abstract

The present invention relates to a process for synthesizing carbon nanotubes by continuous chemical vapor deposition at the surface of reinforcements, said reinforcements constituting a mixture A (i) of particles and/or fibers of a material comprising at least one oxygen atom and (ii) of particles and/or fibers of a material chosen from carbides and/or of a material comprising at least one silicon atom, said process comprising the following steps, carried out under a stream of inert gas(es) optionally as a mixture with hydrogen: (i) heating of said mixture of reinforcements A in a reaction chamber at a temperature ranging from 400° C. to 900° C.; (ii) introducing into said chamber a source of carbon consisting of acetylene and/or xylene, and a catalyst comprising ferrocene; (iii) exposing said heated mixture A to the source of carbon and to the catalyst comprising ferrocene for a sufficient time to obtain carbon nanotubes at the surface of the reinforcements constituting said mixture A; (iv) recovering a mixture B at the end of step (iii), optionally after a cooling step, said mixture B consisting of the mixture (A) of reinforcements comprising carbon nanotubes at their surface; (v) optionally, separation (a) of the particles and/or fibers of a material comprising at least one oxygen atom, (b) of the particles and/or fibers of a material chosen from carbides and/or of a material comprising at least one silicon atom.

Claims

1. A process for the synthesis of carbon nanotubes (abbreviation CNTs) by chemical vapor deposition (abbreviation CVD) at the surface of articles, said articles being provided in the form of a mixture A (i) of particles and/or fibers of a material comprising at least one oxygen atom and (ii) of particles and/or fibers of a material chosen from carbides and/or of a material comprising at least one silicon atom, said process comprising the following steps, carried out under a stream of inert gas(es), optionally in mixture with hydrogen: (i) heating, in a reaction chamber, said mixture A of articles at a temperature of between 400° C. and 900° C.; (ii) introducing, into said chamber, a carbon source comprising acetylene and/or xylene and a ferrocene-comprising catalyst; (iii) exposing said heated mixture A to said carbon source and to said ferrocene-comprising catalyst for a period of time sufficient to obtain CNTs at the surface of the articles forming said mixture A; (iv) recovering a mixture B at the end of step (iii), optionally after a cooling step, said mixture B being formed of the mixture A of articles comprising CNTs at their surface; (v) optionally separating the particles and/or fibers of a material comprising at least one oxygen atom, said particles and/or fibers comprising CNTs at their surface, from the particles and/or fibers of a material chosen from carbides and/or comprising at least one silicon atom, said particles and/or fibers comprising CNTs at their surface.

2. The process according to claim 1, in which the material chosen from carbides and/or comprising at least one silicon atom is chosen from silicon nitride (Si.sub.3N.sub.4), silicon carbide (SiC), silica (SiO.sub.2), TiC and B.sub.4C and/or in which the material comprising at least one oxygen atom is Al.sub.2O.sub.3.

3. The process according to claim 1, in which the ratio by weight of the mixture A of (particles and/or fibers of a material comprising at least one oxygen atom)/(particles and/or fibers of a material chosen from carbides and/or comprising at least one silicon atom) is between 10/90 and 90/10.

4. The process according to claim 1, in which the heating temperature of step (i) is between 650 and 900° C. or between 400 and 550° C.

5. The process according to claim 1, in which the material in step (i) is provided in the form of fibers having a diameter of 1 to 100 μm or of particles having a diameter of 0.1 to 100 μm.

6. The process according to claim 1, in which, in step (ii), the acetylene is introduced into the reaction chamber in the gas form in an amount of greater than 0% and ranging up to 20% by volume of the total gas with a linear velocity of 5.0×10.sup.−6 to 1.0×10.sup.−1 m/s.

7. The process according to claim 1, in which, in step (ii), the xylene is introduced into the reaction chamber in the form of microdroplets via a spray, optionally mixed with the ferrocene, the flow rate of xylene being controlled from 0.1 to 0.7 ml/min.

8. The process according to claim 7, in which the ferrocene content of the xylene and ferrocene mixture is between 0.001 and 0.3 g of ferrocene/ml of xylene.

9. The process according to claim 1, in which, in step (iii), the mixture A is exposed to the carbon source and to the catalyst for a period of time of 1 to 120 minutes.

10. The process according to claim 1, in which steps (i) to (iv) are carried out under a stream of inert gas(es), optionally in mixture with hydrogen, with a hydrogen/inert gas(es) ratio of 0/100 to 50/50.

11. A mixture (i) of particles and/or fibers of a material comprising at least one oxygen atom, advantageously Al.sub.2O.sub.3, and (ii) of particles and/or fibers of a material chosen from carbides and/or comprising at least one silicon atom, said particles and/or fibers comprising CNTs at their surface, said mixture being obtained or obtainable by a process according to claim 1.

12. The mixture according to claim 11, having a rise in weight of between 0.2% and 80%, with respect to the weight of the starting material.

13. The mixture according to claim 11, in which the number of CNTs at the surface of the material is between 5 and 200 per microm.sup.2.

14. The mixture according to claim 11, exhibiting a specific surface of between 150 and 2000 m.sup.2/g.

15. The mixture according to claim 11, additionally comprising a polymer, a metal or a ceramic material.

16. An object comprising a mixture according to claim 11 or comprising a mixture obtainable at the end of the process according to claim 1.

17. A process for preparing reinforced structural and functional composite materials comprising the step of adding (i) a mixture according to claim 11, or (ii) particles and/or fibers of a material comprising at least one oxygen atom, advantageously Al.sub.2O.sub.3, said particles and/or fibers comprising CNTs at their surface, or (iii) particles and/or fibers of a material chosen from carbides, said particles and/or fibers comprising CNTs at their surface, or (iv) particles and/or fibers of a material comprising at least one silicon atom, said particles and/or fibers comprising CNTs at their surface, and said fibers and/or particles of (ii)-(iv) being obtainable at the end of the process according to claim 1, during the process for preparing reinforced structural and functional composite materials.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 represents the overall weight and chemical yields of syntheses of CNTs carried out under the same conditions at 600° C. for different mixtures of Al.sub.2O.sub.3 (purity 99.8%, comprising 800 ppm of SiO.sub.2 and 600 ppm of Na.sub.2O) microparticles and of SiC microparticles, said microparticles having a mean diameter between 3 and 7 micrometers.

(2) FIG. 2 represents the weight percentage of CNTs in the mixtures of Al.sub.2O.sub.3 (purity 99.8%, comprising 800 ppm of SiO.sub.2 and 600 ppm of Na.sub.2O) microparticles and of SiC microparticles, said microparticles having a mean diameter between 3 and 7 micrometers, as a function of the percentage of SiC/Al.sub.2O.sub.3.

(3) FIG. 3 represents the length of the CNTs as a function of the temperature for substrates not in accordance with the invention (substrate SiC alone or Al.sub.2O.sub.3 alone) and for substrates in accordance with the invention.

(4) FIGS. 4 and 5 represent photographs taken with a scanning electron microscope respectively (i) of a mixture of microparticles, with mean diameters of between 3 and 7 micrometers, of Al.sub.2O.sub.3 and SiC (50/50) and (i) of Al.sub.2O.sub.3 microparticles alone after having carried out the process of the synthesis of the CNTs according to the invention at a temperature of 700° C.

(5) FIG. 6 represents the diameter of the CNTs as a function of the temperature for substrates not in accordance with the invention (substrate SiC alone or Al.sub.2O.sub.3 alone) and for substrates in accordance with the invention.

(6) FIGS. 7 and 8 represent photographs taken with a scanning electron microscope respectively (i) of a mixture of microparticles, with mean diameters of between 3 and 7 micrometers, of Al.sub.2O.sub.3 and SiC (50/50) and (i) of Al.sub.2O.sub.3 microparticles alone after having carried out the process for the synthesis of the CNTs according to the invention at a temperature of 650° C.

(7) FIG. 9 represents the density of the CNTs as a function of the temperature for substrates not in accordance with the invention (substrate SiC alone or Al.sub.2O.sub.2 alone) and for substrates in accordance with the invention.

(8) FIGS. 10 and 11 represent photographs taken with a scanning electron microscope respectively (i) of a mixture of microparticles, with mean diameters of between 3 and 7 micrometers, of Al.sub.2O.sub.2 and SiC (50/50) and (i) of SiC microparticles alone after having carried out the process for the synthesis of the CNTs according to the invention at a temperature of 650° C.

(9) FIG. 12 represents a device for the synthesis of the CNTs in accordance with the invention.

(10) FIG. 13 represents the dielectric permittivity of a composite having a PVDF matrix reinforced by hybrid fillers (Al.sub.2O.sub.2+SiC) as a function of the Al.sub.2O.sub.2/SiC ratio of the mixtures of Al.sub.2O.sub.2 (purity 99.8%, comprising 800 ppm of SiO.sub.2 and 600 ppm of Na.sub.2O) microparticles and of SiC microparticles, said microparticles having a mean diameter between 3 and 7 micrometers.

(11) FIG. 14 represents the AC conductivity of a composite having a PVDF matrix reinforced by hybrid fillers (Al.sub.2O.sub.2+SiC) as a function of the Al.sub.2O.sub.2/SiC ratio of the mixtures of Al.sub.2O.sub.2 (purity 99.8%, comprising 800 ppm of SiO.sub.2 and 600 ppm of Na.sub.2O) microparticles and of SiC microparticles, said microparticles having a mean diameter between 3 and 7 micrometers.

(12) FIG. 15 represents the tangential loss as a function of a composite having a PVDF matrix reinforced by hybrid fillers (Al.sub.2O.sub.2+SiC) related to the Al.sub.2O.sub.2/SiC ratio of the mixtures of Al.sub.2O.sub.2 (purity 99.8%, comprising 800 ppm of SiO.sub.2 and 600 ppm of Na.sub.2O) microparticles and of SiC microparticles, said microparticles having a mean diameter between 3 and 7 micrometers.

DETAILED DESCRIPTION AND EXAMPLES

(13) It should be remembered that the conversion of degrees Celsius into degrees Kelvin is K=° C.+273.15 and of degrees Kelvin into degrees Celsius is ° C.=K−273.15.

Example 1: Preparation of Mixture of Materials Covered with CNTs

(14) 1/Assembly Used

(15) The assembly (FIG. 12) is made up so as to control the simultaneous injections of the chemical precursors and the flow rates of gases into a reactor of the quartz tube type, the heating of which is provided by a resistance thermal furnace sold by Carbolite equipped with a temperature programmer.

(16) The flow rate of gases (acetylene (C.sub.2H.sub.2), argon (Ar), hydrogen (H.sub.2)) are measured and controlled by digital mass flow meters sold by Bronkhorst France and Serv Instrumentation.

(17) The flow rates of liquid precursors (xylene, xylene/ferrocene mixture) are controlled with a mechanism of medical syringe driver type (sold by Razel or by Fisher Bioblock Scientific) or mixer equipped with a liquid flow meter (sold be Bronkhorst France and Serv Instrumentation).

(18) The ferrocene is injected dissolved in the xylene or else directly vaporized and injected by convection by means of a neutral carrier gas, such as, for example, argon, by virtue of an appropriate device.

(19) In the examples, when the ferrocene is directly vaporized, the vaporization is carried out in a glass vaporization chamber (heated 100 ml three-necked round-bottomed flask sold by Fisher Bioblock); the vaporization temperature is between 200 and 400° C.; the carrier gas is argon with a flow rate of 0.1 to 0.4 l/min. More generally, for the vaporization of the ferrocene, a device external to the reactor or reaction chamber makes it possible to heat the ferrocene in order to vaporize it. The vapor is then injected by convection: a stream of neutral gas sweeps across the vaporization chamber.

(20) For a given temperature, the amount of ferrocene vaporized is proportional to the flow rate of the neutral gas. By taking into account the vapor pressure of the ferrocene in the vaporization chamber (P expressed in mmHg), the amount of ferrocene can be calculated by the relationship:
Log P(mmHg)=7.615-2470/T(° K).

(21) The mixtures A used are mixtures of spherical alumina (μ-Al.sub.2O.sub.3, with a purity of 99.8%, comprising 800 ppm of SiO.sub.2 and 600 ppm of Na.sub.2O) microparticles and of silicon carbide microparticles having a mean size of between 3 and 7 μm. These particles are sold by Performance Ceramics.

(22) 2/Synthesis of CNTs by Aerosol CVD on Alumina (Al.sub.2O.sub.3) and/or SiC Particles

(23) The assembly used is that of FIG. 12. The synthesis of the CNTs was carried out on mixtures in accordance with the invention of alumina and SiC particles defined above and on alumina particles alone defined above or SiC particles alone defined above, by way of comparatives.

(24) The operating conditions are as follows: flow rate of gases=H.sub.2 0.3 l/min, Ar 0.7 l/min, C.sub.2H.sub.2 0.04 l/min, concentration of ferrocene in xylene: 0.05 g/ml and liquid flow rate at 12 ml/h, time=10 min, temperature=575° C.

(25) The device employed for carrying out this synthesis is composed of a cylindrical quartz chamber, with a length of 110 cm and a diameter of 45 mm, heated between 500 and 900° C. by a horizontal furnace with a length of 60 cm. The microparticles, which have not been subjected to any pretreatment, are mixed and then homogeneously deposited for a given weight on a quartz plate having a length of approximately fifteen cm. The combination is then placed in the tube at the center of the furnace and brought to temperature under an inert atmosphere (mixture of argon and hydrogen). The total flow rate of the gases present in the reactor is kept constant at 1 l/min using electronic flow meters of Brook Smart type. A solution of ferrocene (Fe(C.sub.5H.sub.5).sub.2) diluted in xylene (C.sub.8H.sub.10), the concentration of which is variable from 0.01 to 0.3 g/ml, will act both as catalytic precursors (iron) and carbon source. This solution is subsequently injected in the spray form into the tube using an electronic syringe driver, the flow rate of which can be adjusted manually. Finally, another hydrocarbon, the acetylene (C.sub.2H.sub.2), is also injected into the system at controlled flow rates from 0.01 to 0.1 l/min. The growth of nanotubes lasts between 5 and 50 min for each sample. In order to finish, the system is cooled to ambient temperature under an inert atmosphere (argon) in order to collect the samples.

(26) 3/Results

(27) 3.1. Yields

(28) The overall weight and chemical yields were calculated. The calculation is carried out according to the equations below. The overall weight yield refers to the ratio of the weight of nanotubes synthesized to the total weight of the hybrids produced (nanotubes+microparticles), such that:

(29) $Y_{\mathrm{weight}}=\frac{\mathrm{weight}\mathrm{nanotubes}}{\mathrm{total}\mathrm{weight}\mathrm{hybrids}\left(\mathrm{nanotubes}+\mathrm{microparticles}\right)}$ The overall chemical yield is the degree of conversion of the reactants introduced into the device (acetylene, ferrocene and xylene) to give synthesized products (carbon-based and metal products). The formula for this degree of conversion is as follows:

(30) $Y_{\mathrm{chem}}=\frac{\mathrm{total}\mathrm{weight}\mathrm{products}}{\mathrm{total}\mathrm{weight}\mathrm{reactants}}\times 100$ Although the phenomenon observed is reproducible over a broad temperature range (from 400 to 900° C.), the weight and chemical yields were calculated for different mixtures of microparticles having an SiC/Al.sub.2O.sub.3 weight ratio of 1/0 (comparative), 7/3, 5/5, 3/7 and 0/1 (comparative) under the same conditions of synthesis at 600° C. The results, set out in FIG. 1, have formed the subject of a mean carried out on two series of syntheses which are identical, for the sake of reproducibility.

(31) It is noticed first of all that the values for overall chemical yield are relatively low, in view of the fact that not all the aerosol is completely consumed during the reaction and that a portion is re-encountered in the traps of the reactor outlet. Nevertheless, these values have a tendency to increase with the temperature since more aerosol may then be consumed. Thus, if these data are compared with literature results obtained under similar aerosol CVD conditions but on other substrates, it is noted that, for processes optimized at higher temperatures, the overall chemical yields do not exceed 8% (2% at 800° C., 7.8% at 850° C. and 4.6% at 900° C.).

(32) In addition, a significant increase in the overall weight yield is noticed when the alumina and silicon carbide microparticulate substrates are mixed, with respect to the comparatives, the SiC/Al.sub.2O.sub.3 weight ratio of which is 1/0 or 0/1.

(33) Specifically, the weight yield has virtually doubled, with respect to the comparatives, whereas the chemical yield has for its part virtually tripled!

(34) Generally, a main increase in the weight yield of greater than 25% is obtained, with respect to the substrates for which the SiC/Al.sub.2O.sub.3 weight ratio is 1/0 or 0/1, whatever the CVD conditions employed.

(35) 3.2. Length of the CNTs

(36) If the growth of CNTs on a substrate for which the SiC/Al.sub.2O.sub.3 weight ratio is 0/1 is compared with the growth of the CNTs on substrates comprising a mixture of alumina and silicon carbide particles, it is found that the nanotubes synthesized on a substrate comprising a mixture of alumina and silicon carbide particles are overall longer than those synthesized on alumina alone. The nanotubes having grown on the alumina portion of the alumina/silicon carbide mixture are on average 20 μm longer than those having grown on a substrate composed solely of alumina microparticles, under identical CVD synthesis conditions. The addition of SiC particles thus has a synergistic effect favorable to the growth of CNTs on alumina since it makes it possible to increase the rate of growth of the CNTs on alumina by approximately 72% under the conditions of synthesis according to the invention.

(37) FIG. 3 below shows us the change in the length of the nanotubes as a function of the temperature for different substrates under consideration with the comparatives (substrate composed solely of alumina microparticles and substrate composed solely of silicon carbide microparticles) in solid lines and the mixtures B according to the invention in dotted lines.

(38) The SEM images of FIGS. 4 and 5, on the same scale and taken at the end of a process for the synthesis of CNTs under the same operating conditions (700° C.), make it possible to display the difference in growth of the nanotubes for a 50/50 mixture of alumina/SiC particles in accordance with the invention, in the case of FIG. 4, and for a substrate of alumina microparticles not in accordance with the invention, in the case of FIG. 5.

(39) 3.3. Diameter of the CNTs

(40) An increase in the diameter of the nanotubes is observed when the substrates are substrates comprising a mixture of alumina and silicon carbide particles, with respect to the mean diameter of the CNTs when the substrate is composed of alumina particles or of SiC particles.

(41) The diameter is thus greater on average by 18% with regard to the alumina particles and by 21% with regard to the silicon carbide particles in a mixture B according to the invention, compared respectively with the mean diameter of CNTs on a substrate composed solely of alumina particles and on a substrate composed solely of SiC particles. Furthermore, FIG. 6 shows that this increase is significant over the whole of the temperature range considered.

(42) The SEM images of FIGS. 7 and 8, on the same scale and taken at the end of a process for the synthesis of CNTs under the same operating conditions (650° C.), make it possible to display the difference in diameters of the nanotubes for a 50/50 mixture of alumina/SiC particles in accordance with the invention, in the case of FIG. 7, and for a substrate of alumina microparticles not in accordance with the invention, in the case of FIG. 8.

(43) 3.4. Density of the CNTs

(44) In order to calculate the surface density of nanotubes for the samples, the mean density per unit length over 1 μm of length of CNTs was first determined and then this density per unit length was subsequently squared. Thus, it is noticed that the mean density of CNTs on mixed SiC and alumina particles respectively increases by 43% and by 18% when the substrates are mixed and in accordance with the invention, in comparison with respectively (i) the mean density of CNTs on a substrate composed solely of SiC particles and (ii) the mean density of CNTs on a substrate composed solely of alumina particles.

(45) Furthermore, FIG. 9 shows that this increase in density is generally greater for SiC than for alumina, this being the case over the whole of the temperature range considered.

(46) The SEM images of FIGS. 10 and 11, on the same scale and taken at the end of a process for the synthesis of CNTs under the same operating conditions (650° C.), make it possible to display the difference in density of the nanotubes for a 50/50 mixture of alumina/SiC particles in accordance with the invention, in the case of FIG. 10, and for a substrate of silicon carbide microparticles not in accordance with the invention, in the case of FIG. 11.

(47) 4/Conclusion of Example 1

(48) The simultaneous growth of CNTs on two types of different and mixed substrates chosen from alumina fibers, alumina particles, the fibers of material comprising at least one silicon atom and the particles of a material comprising at least one silicon atom (for example, a mixture of alumina and silicon carbide microparticles) exhibits significant beneficial effects on: (i) the yield of the process for growth of CNTs by aerosol CVD (Mean increase in the weight yield, whatever the conditions of synthesis, of greater than 25%); (ii) the diameter of the CNTs (Mean increase of approximately 20%); (iii) the length of the CNTs (CNTs approximately 20 μm longer on average on alumina and 5 μm shorter on average on SiC); (iv) the rate of growth (Increase of 72% on average in the rate of growth of the CNTs on alumina) and; (v) the density of the CNTs (Increase of 43% on average in the density of the CNTs on silicon carbide and of 18% on average on alumina).

(49) There is thus a true synergistic effect of the two substrates which makes it possible to obtain all these advantages. In addition to its undeniable advantage over the processes for the syntheses of CNTs in a large amount, the present invention also confers a significant benefit on the preparation of novel composite materials which may be used for various types of applications.

Example 2: Preparation of Dielectric Composite Materials

(50) Compositions comprising (i) a mixture of particles covered with CNTs (5 different mixtures are tested) according to the invention and a PVDF polymer were prepared by employing an extrusion method which makes it possible to provide a uniform dispersion of the mixture of particles covered with CNTs in said polymer. First of all, the mixture of particles covered with CNTs, prepared as defined above, and the PVDF powder are dispersed in N,N-dimethylformamide (abbreviation DMF). The resulting mixture is subsequently treated by magnetic stirring overnight so as to obtain a precursor composite solution.

(51) The precursor composite solution obtained above is subsequently applied to ceramic particles which are subsequently heat treated at 150° C. for 2 h.

(52) Subsequently, the resulting composite particles are again mixed in a corotating, conical, twin-screw microcompounder (Micro 5 cm.sup.3, twin-screw compounder, DSM) at 200° C. for more than 10 minutes and at a stirring rate of 20 rev/min under an argon atmosphere. 5 samples of composite particles are thus obtained, the initial composition of the mixture of particles covered with CNTs of which is different and defined in table 1 below. The dielectric permittivity, AC conductivity and tangential loss properties are expressed respectively in FIGS. 13, 14 and 15 as a function of the silicon carbide particles/alumina particles ratio of the starting mixture, said particles being covered with CNTs.

(53) Blocks with a thickness of 1.5 mm were prepared by injection molding of composites (Micro 5 cm.sup.3 Injection Molder, DSM) using a pressure of 1.6 MPa for 1 minute, while the temperature of the mold was maintained at 60° C. The temperature is then decreased from 60° C. down to ambient temperature.

(54) The characteristics of each sample 1-5 produced are given in table 1 below. The results are illustrated in FIGS. 13-15.

(55) TABLE-US-00001 TABLE 1 Amount of SiC/ the mixture Amount of Amount of Amount of Sam- Al.sub.2O.sub.3 of SiC/Al.sub.2O.sub.3 ceramic CNTs PVDF ple ratio particles (g) (g) (g) 1 10/0  0.080 0.05 0.030 5.00 2 7/3 0.130 0.05 0.080 5.00 3 5/5 0.185 0.05 0.135 5.00 4 3/7 0.140 0.05 0.090 5.00 5  0/10 0.105 0.05 0.055 5.00