FR2911333A1 - New, low-friction composition material, useful as electrical conductor, lubricant or mechanical damping agent, comprises core covered by layer of aligned, multilaminar carbon nanotubes - Google Patents

Abstract

A new material (I), in spherical, spheroidal or ellipsoidal form, comprises a core (II) completely covered by a continuous layer (III) of aligned, multilaminar carbon nanotubes, each tube being perpendicular to the tube surface. An independent claim is included for a method for preparing aligned, multilaminar carbon nanotubes on a support (specifically for the preparation of (I)), involving: (A) providing a solid catalyst (IV) consisting of material in spherical, spheroidal or ellipsoidal form, comprising a core covered by a layer of transition metal oxide (forming less than 50 wt. % of (IV)); and (B) growing carbon nanotubes on the support by chemical vapor deposition (CVD) using a carbon source in contact with the surface, in a fluidized bed at atmospheric pressure.

Description

CARBON NANOTUBES ALIGNED ON SPHERICAL SUPPORT, SPHERIOID OR

  ELLIPSOIDAL, PROCESS FOR THEIR PREPARATION AND USE THEREOF

  Field of the invention The present invention relates to carbon nanotubes aligned on a spherical, spheroidal or ellipsoidal support, their method of preparation and their use. Technological Background of the Invention

  At present, obtaining films of aligned nanotubes is mainly by catalytic chemical vapor deposition of a carbon source, either without substrate (deposition on the walls of the reactor) or on flat or nanoparticulate substrates . Thus, in the case of growth of carbon nanotubes aligned on nanoparticle support, these nanoparticles can be prepared in an independent first step, by various conventional methods such as lithography, sputtering magnetron, liquid impregnation or electroplating, before the growth of the nanotubes by catalytic chemical vapor deposition of a carbon source.

  Most often, most of the nanotube organizations obtained by these methods are entanglements of nanotubes or bundles or "bundles" of nanotubes which, in the majority of cases, are not systematically aligned. Nevertheless, the document WO 2004/043858 claims the growth of aligned nanotubes in the form of a film on each face of a divided support having facets that are almost flat and smooth in order to be able to control the length of the nanotubes and make them more easily dispersible after purification. by their non-entanglement. Moreover, the document WO 2006/008385 describes a process for the preparation of nanotubes on a spherical substrate, surrounded by an iron matrix, with an average particle size of between 25 μm and 2.5 mm in order to increase the productivity and the selectivity of carbon nanotubes. Nevertheless, the nanotubes obtained also grow entangled. Indeed, the notion of ordered nanotubes cited in this document is not confirmed by the photographs 7a and 7b which typically show CNT entanglements.

  It therefore appears that the growth of aligned carbon nanotubes is a non trivial task and a sharp control of many parameters is necessary. It appears that such structures preferentially grow on flat substrates from a catalyst film (iron, cobalt, etc.) of a few nanometers.

  There is therefore a need for new structures based on aligned carbon nanotubes and a method for producing such structures, in particular on a large scale. Summary of the invention. The Applicant has discovered that a catalyst consisting of a spherical, spheroidal or ellipsoidal support on which a metal has been deposited, could allow the growth of a continuous layer of carbon nanotubes (CNTs) aligned, covering the whole of the catalyst grain. Thus, the invention provides a spherical-shaped material whose diameter is less than 7 mm, or spheroidal or ellipsoidal whose length of the main axis is less than 7 mm, for example in the range of 10 pm to 5 mm , preferably between 10 μm and 1 mm and the length of the secondary axis is less than 6.99 mm, for example in the range from 9 μm to 4.99 mm and preferably between 9 μm and 0 μm. , 9 mm, comprising a core completely covered with a continuous layer formed of aligned multifilette carbon nanotubes, each of the tubes being perpendicular to the surface of the core. This material comprises a core made of a material selected from alumina, silica, activated carbon, magnesia, zirconia, titanium dioxide, silico-aluminates.

  The core of the material according to the invention comprises, on a part of its surface, at least one transition metal chosen from those of group VIIIB such as iron Fe, cobalt Co, nickel Ni, ruthenium Ru, rhodium Rh, Pd palladium, osmium Os, Ir irium, Pt platinum or their mixture, preferably the metal is selected from iron Fe, cobalt Co, or nickel Ni, or a mixture thereof.

  The metal of the material according to the invention represents at most 50% by weight of the surface of the core and extends in the form of a discontinuous film. The continuous layer of carbon nanotubes of the material according to the invention has a thickness ranging from 1 to 500 μm, for example ranging from 1 to 300 μm, preferably ranging from 3 to 50 μm. In this layer, the carbon nanotubes are densely aligned and cover 100% of the surface of the heart. The material according to the invention is of spherical shape whose diameter ranges from 2 microns to 7 mm.

  According to the invention, the expressions "ranging from ... to" or "between" also cover the terminals. The subject of the invention is also a process for the preparation of multilayered carbon nanotubes aligned on a support comprising: a) the provision of a solid catalyst consisting of a material of spherical shape, in particular of diameter less than 6.5 mm or of spheroidal or ellipsoidal shape in particular having a length of the principal axis less than 6.5 mm, and the length of the secondary axis is less than 6.49 mm and comprising a core covered with a layer of a transition metal oxide, the solid catalyst comprising less than 50% by weight of unoxidized transition metal relative to the total weight of the catalyst, b) the growth of the carbon nanotubes perpendicular to the catalyst surface by deposition in phase vapor (CVD) of a carbon source contacted with said surface, in a fluidized bed, and at atmospheric pressure. In the process according to the invention, the solid catalyst comprises less than 20%, preferably between 5.5% and 19.5% by weight of unoxidized transition metal relative to the total weight of the catalyst. This percentage corresponds to the amount of the metal in metallic or unoxidized form in the oxide layer of the transition metal which covers the catalyst support. Preferably, the core of the solid catalyst is a porous material and comprises at least part of the accessible pores for step b). According to one embodiment, the metal is iron and the support is gamma alumina or a pseudo boehmite or a mixture thereof. The method according to the invention allows the preparation of a material as described above in the text. The invention also relates to the use of a material according to the invention in the field of electric charge transfer or as a lubricant. Figures. Figure 1: Figure 1 shows a SEM micrograph of the material consists of a core and a continuous layer, which covers it, formed of carbon nanotubes multifilettes. Figure 2: Figure 2 shows a SEM micrograph of the section (obtained by grinding) of a grain coated with a layer of aligned multilayer carbon nanotubes.

  FIG. 3: FIG. 3 represents a SEM micrograph of the section (obtained by grinding) of a grain coated with a layer of aligned multi-walled carbon nanotubes and where perpendicular alignments are observed. Figure 4: Figure 4 shows a high resolution TEM micrograph of a multi-walled nanotube produced. Figure 5: Figure 5 shows, according to Figure 1, a reactor for the deposition of catalytic material by the process under reduced pressure in a fluidized bed and chemical vapor deposition. Figure 6: Figure 6 shows, according to Figure 2, a reactor for the growth of carbon nanotube films aligned by a process under atmospheric pressure in a fluidized bed and by chemical vapor deposition. DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION The object of the invention is to provide a material of spherical, spheroidal or ellipsoidal shape comprising a core or support grain entirely covered by a continuous layer formed of aligned multi-plane NTC carbon nanotubes, each of the tubes being perpendicular to the surface of the core. Each support grain has CNTs forming a homogeneous continuous surface layer extending along the entire closed surface around the core. By "continuous" layer of CNT is meant the fact that it is possible to travel continuously all the outer surface of this layer, without having to cross a portion of another nature (including a portion free of CNT). Thus the CNTs are not dispersed on the surface of each support grain, but instead form a continuous layer of visible area substantially homothetic to that of the support grains. This continuous layer is, moreover "homogeneous" in that it is formed of CNT, and has a nature and an identical geometry in all its volume. In fact, this layer consists of aligned NTC multifolds, each of the tubes being substantially perpendicular to the surface of the core. In the material according to the invention, the continuous layer of carbon nanotubes has a thickness ranging from 1 to 500 μm, preferably ranging from 3 to 50 μm. Moreover, the layer of carbon nanotubes, densely aligned, on the surface represents up to 100% of the geometric surface of the grains. The supported CNTs have a uniform length of 1 to 500 microns and a diameter of 4 to 30 nm.

  The expression "closed surface" is used in the topological sense of the term, that is to say a surface that delimits and surrounds an internal finite space which is the heart of the grain, and which can take various spherical, spheroidal or ellipsoidal forms.

  Thus when the material is spherical in shape, it has a mean diameter ranging from 2 microns to 7 mm. When the material is of spheroidal or ellipsoidal shape, it has a main axis less than 7 mm long, for example in the range from 10 pm to 5 mm and preferably between 10 microns and 1 mm and a secondary axis length less than 6.99 mm, for example in the range of 9 pm to 4.99 mm and preferably between 9 microns and 0.9 mm. The core of the material according to the invention consists of a material chosen from alumina, silica, activated carbon, magnesia, silicoaluminates, zirconia and titanium dioxide. According to a particular embodiment, the core may also comprise, deposited on a part of its surface, at least one transition metal chosen from those of group VIIIB such as iron Fe, cobalt Co, nickel Ni, ruthenium Ru Rh rhodium, Pd palladium, Os osmium, Ir irium, Pt platinum or a mixture of these metals.

  Alternatively, advantageously and according to the invention, the catalyst deposit is formed of iron and at least one metal selected from nickel and cobalt. Preferably, the ferrous metal deposit is mainly iron. According to one embodiment, each object of spherical, spheroidal or ellipsoidal shape has a metal deposit which can represent at most 50% by weight relative to the total weight of the core. This deposit may extend to the surface of the core as a continuous or discontinuous film as in the form of clusters or nodules. The invention proposes a process for preparing carbon nanotubes multifilets aligned on a support. This process, shown in Fig. 2 of Fig. 6, comprises the steps of: a) providing a solid supported catalyst in the form of grains of a spherical, spheroidal or ellipsoidal shaped material. b) growing the carbon nanotubes perpendicular to the catalyst surface by vapor deposition (CVD) of a carbon source contacted with said surface, in a fluidized bed reactor, and advantageously at atmospheric pressure . Indeed, the increase in pressure decreases the efficiency of the process and a pressure lower than the atmospheric pressure is to be avoided for reasons of safety and inadvertent reaction in the reactor at 650 C with the incoming air, in case of leakage . This catalyst, in the form of grains, comprises a core covered with a layer of a non-oxidized transition metal. Thus, when the catalyst is spherical in shape, it has an average diameter of less than 6.5 mm. When it is of spheroidal or ellipsoidal shape, it has a main axis less than 6.5 mm in length, and a secondary axis less than 6.49 mm long. The core of the catalyst consists of a material selected from alumina, silica, activated carbon, magnesia, zirconia, titanium dioxide, silicoaluminates. Preferably, the core is made of a porous material which has a specific surface area greater than 10 m 2 / g, and whose pores have a mean size of less than 50 nm. According to a particular embodiment, the oxide layer of the transition metal deposited on a part of the surface of the core covers more than 75% of the surface of the core. This deposit may extend to the surface of the core as a continuous or discontinuous film as in the form of clusters or nodules. Preferably, the metal oxide deposition makes it possible to keep at least a portion of the accessible pores in order to facilitate the growth of the nanotubes perpendicularly to the surface of the support. The catalyst comprises less than 50% by weight of unoxidized transition metal based on the total weight of the catalyst. Preferably, and to obtain good yields of CNT, the transition metal content in its metal form in the metal oxide covering the support is less than 20%, preferably between 2 and 20% or between 5.5 % and 19.5% of the total weight of the catalyst. The transition metal is chosen from those of group VIIIB such as iron Fe, cobalt Co, nickel Ni, ruthenium Ru, rhodium Rh, palladium Pd, osmium Os, iridium Ir, platinum Pt or a mixture of these metals. Alternatively according to the invention, the metal deposit is formed of iron and at least one metal selected from nickel and cobalt. Indeed, in particular it is possible to use a bimetallic catalyst Fe-Ni or Fe-Co with results similar to a catalyst of pure iron, all things being equal. Preferably, the ferrous metal deposition is mainly iron. Advantageously, the catalyst consists of iron supported on gamma-alumina or on a pseudo-boehmite or on their mixture.

  This process allows a fine control of the length of all carbon nanotubes produced, which was not possible until now on spherical, spheroidal or ellipsoidal supports and for large scale production of NTC with high returns.

  Thus, the CNTs obtained have lengths of up to 500 microns. The solid supported catalyst described above and used in the process according to the invention is prepared according to known methods for low-pressure chemical vapor deposition, in particular that described in patent WO 2006/008385 or WO2003 / 002456. In particular, the catalyst is prepared by deposition of catalytic material under reduced pressure, less than 100 torr (13 KPa) in a fluidized bed and chemical vapor deposition of an organometallic precursor, preferably ferrocene, in a reactor represented by the Scheme 1 of Figure 5. The supported metal oxide is subsequently reduced in situ (in the nanotube reactor of Scheme 2) prior to deposition, so as to become active for the synthesis of the nanotubes. The amounts of precursor and carrier introduced into the reactor are such that iron represents less than 50%, for example between 2 and 20%, and especially between 5.5% and 19.5% by weight of the catalyst after deposition. The invention also relates to the use of a material according to the invention as an electrical conductor, or as a lubricant (agent whose outer surface has a low coefficient of friction) or as a mechanical damping agent. Indeed, taking into account the fact that the material consists of a multitude of nanotube tips available on the surface of the material, various applications can be envisaged in the fields of electrical conduction, and the transfer of electrical charges. In addition, since the material can be considered as an agent whose external surface is of low coefficient of friction because the nanotubes are elastic and light, it is conceivable that the object according to the invention has a shock absorbing effect. mechanical constraints and thus serve as low friction product or lubricant. Examples. The following examples are intended to illustrate the invention without limiting its scope. Example 1: General description of the process for preparing the material according to the invention. 1) Deposition of iron on the support: The iron is deposited on the support by fluid vapor deposition (CV) in fluidized bed (LF) under reduced pressure (PR) from an organometallic precursor (OM), ferrocene . The reactor used is shown in diagram 1. It consists, in its lower part, of a sublimator (A) in stainless steel jacketed for the circulation of a heat transfer fluid (B) which allows to maintain the temperature of the sublimator to the desired set point T. The sublimator is connected to the stainless steel reactor (C) which has a perforated dispenser surmounted by a wire cloth (D) ensuring the maintenance of the carrier powder bed in the reaction zone and the good distribution of the ascending gas flow. The stainless steel reactor has an internal diameter of 8 cm and a height of 1 m, it is brought to the desired temperature TR for the deposition of the catalyst film by a system of heating elements controlled by a PID (Proportional Integral Derivative) (E) .

  The rest of the installation comprises a filter cartridge (F) for separating any sorted particles by elution, a liquid nitrogen trap (G) for condensing any decomposition products or unreacted precursor and a pump for vacuum (H) assisted by a vacuum regulator (I) to obtain the desired reduced pressure in the chamber PR.

  The vacuum pump is itself connected to a filter system (J) upstream, and, downstream, to an oven (K) to dilute and burn any residues before sending the gas to the vent. Mass flow controllers allow regulation and control of the carrier gas flow (QN2a) fed into the sublimator (L) to drive the precursor vapor and the dilution carrier gas flow (QN2b) fed directly into the reactor (M) ( just before the dispenser) to ensure sufficient flow to control fluidization. At this rate of dilution gas, can be added a reactive gas (QGR) (itself controlled by a mass flowmeter (N)) assisting the decomposition of the precursor in the bed (air, hydrogen, water, ...). 2) Growth of nanotubes: The growth of aligned carbon nanostructures is carried out by catalytic CVD in a fluidized bed at atmospheric pressure. It is carried out on the support-catalyst composite material whose synthesis is described above. The reactor used is shown in Figure 2: it has in its lower part, a windbox (A) for the entry and homogenization of the gas stream. The upper part, which represents the body of the reactor, consists of a stainless steel tube (C) containing a perforated distributor surmounted by a wire mesh (B) ensuring the maintenance of the catalytic powder bed in the reaction zone and a good distribution. rising gas flow. The stainless steel reactor has an internal diameter of 5 cm and a height of 1 m, it is brought to the desired temperature TR for deposition of nanostructures 5 by a system of heating elements controlled by a PID (D). The rest of the installation comprises a filter cartridge (E) for separating any fine particles from the gas leaving in the direction of the vent. A gas chromatographic system (F) allows on-line or "on-line" analysis at the outlet of the reactor from the gaseous atmosphere by quenching at the outlet of the reactor downstream of the cartridge. The flow rates of gas entering the reactor are adjusted and controlled by mass flowmeters (G, H, I). Example 2 Preparation of the Support / Catalyst Products All of the carrier / catalyst preparation examples were carried out in the 5 cm diameter fluidized bed reactor described in Scheme 2 from a catalyst made of an iron oxide film deposited according to the described process. in step 2 from ferrocene as an iron precursor in the presence of air. The preparation of the catalytic powder was carried out in the reactor described in scheme 1 according to the following procedure: The pressure prevailing in the enclosure is PR = 80 Torr, the sublimator is at Ts = 185 ° C. and the reactor TR = 575 C. The amounts of precursor and carrier introduced are such that iron represents 10% by weight of the product after deposition. The gas flow rates are as follows: QN2a = 0.75 slm (standard liter / minute), QN2b = Os1m, QGR = 1.75s1m air. The support material used in all the examples is a pseudo boehmite of average diameter 315 μm which, after the deposition of catalytic material, can undergo a partial or complete phase transition. As a result, the carrier of the supported catalyst is either gamma alumina, pseudo boehmite, or a pseudo boehmite and gamma alumina mixture. The mass of introduced ferrocene precursor is mpr = 400 g and the mass of support introduced is ms = 1000 g. The reactor is then brought back to ambient temperature and pressure to be emptied. The supported iron oxide is subsequently reduced in situ (in the nanotube reactor of Scheme 2) prior to deposition so as to become active for nanotube synthesis.

  The following example gives the conditions of preparation of products 1 to 6 with ethylene as a carbon source and at different growth times and temperatures. Example 3 Preparation of the Products According to the Invention

  Product 1: 60 g of support / catalyst - growth at 650 ° C for 30 min -QH2 = QN2 = 0.5 QC2H4 = 1.33s1m. The product consists of the initial support (pseudo boehmite) coated with a layer of multi-walled carbon nanotubes 25 μm thick (FIG. 2). The mass of deposited carbon is 25g. Product 2: 60 g of support / catalyst - 650 C - 60 min - QH2 = QN2 = 0.5 QC2H4 = 1.33s1m. The product consists of the initial support (pseudo boehmite) coated with a layer of multi-walled carbon nanotubes 50 μm thick. The mass of deposited carbon is 54g. Product 3: 60 g of support / catalyst - 550 C - 30 min - QH2 = QN2 = 0.5 QC2H4 = 1.33s1m. The product consists of the initial support (pseudo boehmite) coated with a layer of multifocal carbon nanotubes of 3-4 μm thick. The mass of deposited carbon is 4g. Product 4: 60 g of support / catalyst - 550 C - 90 min - QH2 = QN2 = 0.5 QC2H4 = 1.33s1m. The product consists of the initial support (pseudo boehmite) coated with a layer of multi-walled carbon nanotubes 10 μm thick. The deposited carbon mass is 11g. Product 5: 60 g of support / catalyst - 650 C - 90 min - QH2 = 1.33s1m QN2 = 2.66s1m QC2H4 = 1.33s1m. The product consists of the initial support (pseudo boehmite) coated with a layer of multi-walled carbon nanotubes 30 μm thick. The mass of deposited carbon is 60g.

  Product 6: 60 g of support / catalyst - 650 C - 90 min - QH2 = 1.33s1m QN2 = 3slm Qc2H4 = 1slm. The product consists of the initial support (pseudo boehmite) coated with a layer of multi-walled carbon nanotubes 35-40 μm thick. The mass of deposited carbon is 63g. Figures 1 and 2 show the spherical materials obtained with a continuous layer of carbon nanotubes covering the support. Figure 3 shows, more particularly, the distribution of nanotubes aligned and perpendicular to the surface of the support. In the above examples, in order to evaluate the production yields of CNTs, thermogravimetric analysis or ATG percentages of losses of mass corresponding to the carbon of the CNTs formed are measured. ATG makes it possible to measure the loss of mass of a sample in a given atmosphere. Thus, by subjecting a catalyst sample coated with nanotubes to a temperature ramp under air, the weight loss due to the combustion of the nanotubes, and therefore to the quantity of nanotubes produced on the inorganic catalyst, will be achieved. The results show that these percentages are greater than 50%, or even greater than 80%, and correspond to values of 1 g to more than 2 grams of carbon per gram of catalyst.

Claims (16)

  1. Spheroidal or ellipsoidal spherical material comprising a core completely covered with a continuous layer formed of aligned multilayered carbon nanotubes, each of the tubes being perpendicular to the surface of the core.   2. Material according to claim 1 of spherical shape whose diameter is less than 7 mm or of spheroidal or ellipsoidal shape whose length of the main axis is less than 7 mm, preferably in the range of 10 microns to 5 mm or still in the range of 10 microns to 1 mm and the length of the secondary axis is less than 6.99 mm, preferably in the range of 9 microns to 4.99 mm or in the range of 9 microns and 0.9 mm.   3. Material according to claim 1 wherein the core is made of a material selected from alumina, silica, activated carbon, magnesia, zirconia, titanium dioxide, silicoaluminates.   4. Material according to one of claims 1 to 3, wherein the core comprises on a portion of its surface at least one transition metal selected from those of group VIIIB such as iron Fe, cobalt Co, nickel Ni, ruthenium Ru, rhodium Rh, palladium Pd, osmium Os, Ir irium, platinum Pt or their mixture.   5. The material of claim 4, wherein the metal is selected from iron Fe, cobalt Co or nickel Ni or a mixture thereof.   6. The material of claim 4 or 5, wherein the metal is at most 50% by weight of the core surface and extends as a discontinuous film.   7. Material according to one of claims 1 to 6, wherein the continuous layer of carbon nanotubes has a thickness ranging from 1 to 500 pm, preferably from 3 to 50 pm.   8. Material according to one of claims 1 to 7, wherein the layer of densely aligned carbon nanotubes covers 100% of the surface of the core.   9. Material according to one of claims 1 to 8, of spherical shape whose diameter ranges from 2 microns to 7 mm.'13   A process for preparing multilayered carbon nanotubes aligned on a support comprising: a) providing a solid catalyst consisting of a spherical, spheroidal or ellipsoidal shaped material comprising a core coated with an oxide layer; a transition metal, the solid catalyst comprising less than 50% by weight of unoxidized transition metal relative to the total weight of the catalyst, b) the growth of the carbon nanotubes perpendicular to the surface of the catalyst by vapor deposition (CVD ) a carbon source contacted with said surface, in a fluidized bed, and at atmospheric pressure.   11. The method of claim 10, wherein the solid catalyst is made of a spherical material of diameter less than 6.5 mm or of spheroidal or ellipsoidal shape whose length of the main axis is less than 6.5 mm, and the length of the secondary axis is less than 6.49 mm.   The process according to claim 10 or 11, wherein the solid catalyst comprises less than 20%, preferably 5.5 to 19.5% by weight of unoxidized transition metal based on the total weight of the catalyst.   13. Method according to one of claims 10 to 12, wherein the core of the solid catalyst is a porous material and comprises at least a portion of accessible pores for step b).   14. Process according to one of Claims 10 to 13, in which the metal is iron and the support is gamma-alumina or a pseudo-boehmite or a mixture thereof.   15. Method according to one of claims 10 to 14, for the preparation of a material according to one of claims 1 to 9.   16. Use of a material according to one of claims 1 to 9, as an electrical conductor, as a lubricant or as mechanical damping agent.