Arkema France (Colombes, FR) reveals in U.S. Patent 7,622,059 a method for synthesis of carbon nanotubes of the highest carbon purity by combining the process of vapor phase chemical deposition. with ball milling. The nanotubes produced can be used to advantage in all known applications of carbon nanotubes. Inventors Serge Bordere, Patrice Gaillard and Carole Baddour say the process produces 15 grams of carbon nanotubes for each gram of catalyst used.
FIG. 1 is a scanning electron micrograph of the CNT agglomerates obtained according to the prior manufacturing methods.
FIG. 2 is a scanning electron micrograph of the milled CNTs obtained from step b) according to Arkema's invention; and by comparing FIG. 2 with FIG. 1, it may be clearly seen that the process according to the invention results in a very small number of CNT agglomerates with a diameter greater than 200 .mu.m. The final product thus formed is therefore more easily dispersed within a material, in particular a polymer.
FIG. 3 illustrates a milling device according to Arkema's invention which may be installed either within an actual synthesis reactor (6) for synthesizing CNTs by CVD (in situ milling) or in an external loop allowing possible recycling of all or part of the CNTs milled within the reactor (ex situ milling).
The milling device shown in FIG. 3 comprises a system of high-velocity gas jets generated through injectors (2) which entrain the CNT powder onto one or more targets (5) held by a support (4), that has to be subjected to the bombardment of the CNT agglomerates thus reducing the particle size by impact. The fluidization may be carried out by just these injectors (2) and/or in combination with a gas stream diffused by the distributor (3) around these injectors (2). The dimensions of the milling system and the flow rates of incoming gas (1) and (2a) used are suitable for obtaining good fluidization and the desired particle size, depending on the hardness and the density of the catalyst substrate. The distributor (3) is designed to support the catalyst, which is in powder form, at the time T0 of the synthesis.
The form of the milling device will advantageously be adapted according to the materials used and/or the behavior of the fluidized bed. The process may be carried out semi-continuously or in batch mode, but preferably continuously. At least part of the entangled CNT/catalyst network resulting from step a) may be extracted from the synthesis reactor to a milling device operating continuously, semi-continuously or in batch mode, then injected (step c)) either into the same synthesis reactor of step a) or into a second CNT synthesis reactor by fluidized-bed CVD (finishing reactor).
It is also possible to carry out the milling (step b) in the synthesis reactor of step a), provided with milling means as shown by the device in FIG. 3, which avoids having to extract the powder from the reactor and therefore reduces the head losses and the risk of powder fly-off.
Step b) is carried out inside the CNT synthesis reactor (6) by injecting some of the reactive gas or gases and/or an additional gas through injection nozzles (2) distributed over the surface of the distributor (3), the vertical gas jet or jets (1) entraining the particles toward a target (5). The particles consist of CNT agglomerates and/or catalyst. The target (5) is in the form of a cone, made of stainless steel, preventing deposition of particles at the top of the target (5).
This milling makes catalytic CNT growth sites accessible, thereby making it possible, during step c), to grow further CNTs on these now accessible sites, but also on the CNT agglomerates formed during step a), the size and/or the number of which have been reduced thanks to the milling. Growth of the CNTs during step a) and step c) may take place using identical gas sources (which is the case during a process involving in situ milling) or sources that differ both in terms of nature and flow rate (which is especially the case during a process involving ex situ milling). The CNTs synthesized during introduction of synthesis gas and fresh catalyst, during step c), may be subjected to a further milling step d) under the conditions described above. The CNTs thus obtained after step c) or d) are finally recovered.
These CNTs have improved properties, especially their dispersion in a material, in particular a polymer. It is thus possible to introduce a higher quantity of CNTs compared with the prior art, with better distribution and/or homogeneity, thereby improving the final properties of the material containing the CNTs.
These CNTs can be used in all applications in which CNTs are employed, especially in fields in which their electrical properties are desired (depending on the temperature and their structure, they may be conductors, semiconductors or insulators), and/or in fields in which their mechanical properties are desired, for example for the reinforcement of composites (the CNTs are one hundred times stronger and six times lighter than steel) and in electromechanical applications (they can elongate or contract by charge injection). For example, mention may be made of the use of CNTs in macromolecular compositions intended for example for the packaging of electronic components, for the manufacture of fuel lines (gasoline or diesel), antistatic coatings, in thermistors, electrodes, especially in the energy sector, for supercapacitors, etc
