Carbon fibers/filaments are forms of carbon which are known in the art and which have a diameter of, normally, from 5 to 15 micrometers. They are flexible, light in weight, thermos table, chemically inert and are good thermal and electrical conductors. They can be divided into two categories, low-modulus fibers having a Young’s modulus below about 140 g Pa and high-performance fibers having a Young’s modulus above about 170 g Pa and having a very high tensile strength.
Carbon nanotubes, carbon nanofibers, carbon microtubes, nanobeads etc which are allotropic forms of carbon are sought for many applications reflecting their novel mechanical, thermal and electrical properties including reinforcing, catalyst support, gas storage and electrochemical energy storage. Extensive research related to the unique mechanical and electrical behavior of carbon filaments thus has necessitated need for improved processes for preparation of carbon filaments.
The discovery of single walled carbon nanotubes (SWNT), multiwalled carbon nanotubes(MWNT), carbon fibers (CF) by Iijima has started extensive research in the field due to the promising physical properties of carbon nanotubes. Carbon nanotubes are categorized as single walled nanotubes (SWNT) and multiwalled nanotubes (MWNT). Single walled carbon nanotube (SWNT) is a single atom thick layer of graphite (graphene) rolled up such that the diameter is of the order of nanometers. The length to diameter ratio of these carbon filaments are of the orders of 1,000,000. Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphite. These cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of material science and technology.
Carbon Nanotubes have also been found in the soot of Industries and places which burn Methane, benzene and ethylene, but the irregularities in size and quality is enormous due to highly uncontrolled environment.
Multi-walled nanotubes (MWNT) consist of multiple layers of graphite rolled in on them to form a tube shape. There are two models which can be used to describe the structures of multi-walled nanotubes. In the Russian Doll model, sheets of graphite are arranged in concentric cylinders, e.g., a (0.8) single-walled nanotube (SWNT) within a larger (0.10) single-walled nanotube. In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled up newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.3 A (330 pm).
A number of synthetic method for production of vertically aligned carbon filaments are known from the prior art. The majority of methods of synthesis comprise formation of catalyst layer on which carbon filaments are developed followed by their purification. It is a two step process, first the production of carbon filaments and then their purification, which is not economical for industrial applications. Popular methods for obtaining such a catalyst layer are sputtering, deposition processes, such as electron beam deposition, thermal deposition and the like. Preferred process for growing carbon filaments thereon include arc discharge, laser vaporization, gas phase synthesis, CVD (Chemical Vapor Deposition) method, Plasma enhanced chemical vapor deposition vapor-phase method, Alcohol catalytic chemical vapor deposition, High Pressure CO-disproportionation process, Flame synthesis (E T. Thostenson, Z. Ren, and T. W Chou, Composites Science and Technology, Vol. 61, 2001, p. 1899-1912).
Currently, there are three principal techniques to produce high-quality CNTscarbon filaments:
(i) Laser Ablation
(ii) Electric Arc Discharge and
(iii) Chemical Vapor Deposition (CVD)
In the laser ablation process, a pulsed laser is used to vaporize a graphite target in a high-temperature reactor while an inert gas is bled into the chamber. Nanotubes are developed on the cooler surfaces of the reactor as the vaporized carbon condenses. A water-cooled surface may be included in the system for separation and collection of the nanotubes.
In the arc-method, current is passed between carbon anode and cathode in a suitable container filled with a gas. An arc is created between the electrodes, and carbon evaporates from anode and deposits on the cathode which is a mixture of different carbon nano structures. These can be subsequently separated and purified.
However, Bboth laser ablation and electric arc discharge methods suffer the problem that it is difficult to scale-up the production of carbon filaments CNTs to industrial level.
The flame synthesis method is based on the use of controlled flame environment, where carbon atoms are formed from hydrocarbon fuels along with aerosols of metal catalyst The SWNTs grow on the metal islands. A sub monolayer film of metal (cobalt) catalyst was applied to the stainless steel by Physical Vapor Deposition (PVD). In this manner, metal islands resembling droplets were formed upon the mesh support to serve as catalyst particles. These small islands become the aerosol when exposed to flame. The reaction is carried out at 800[deg.] C. and requires purification. The hydrocarbon decomposition produces also CO/CO2 contaminated hydrogen.
The CVD method is currently the only hope for large-scale production of carbon filaments CNTs. Traditionally, the catalyst for the CVD method is made either by impregnating a catalyst precursor into a powdered catalyst support, such as silica powder and alumina powder or by decomposing the precursor in the gas phase together with a carbon source at elevated temperature.
However, the production of carbon filaments CNTs by CVD method requires several conditions to be satisfied, as follows.
(i) Maintaining High Temperature-of about 860°C for the process.
(ii) Maintaining of pressure and inert atmosphere all through the chamber.
(iii) Continuous flow of air.
(iv) Maintaining the flow of exhaust gases containing the likes of C,H2,CH 4 etc.,
In view of the wide range of high technological applications of carbon filaments CNTs, and resultant heavy demand for carbon filamentsCNTs, especially in the steel making industry, the traditional method of the preparation of CNTs, the conventional materials employed and process conditions adopted thereof, have all been contributing to high cost of carbon filaments CNTs. Maintaining and satisfying all these conditions is not only costly but also difficult, thus resulting in a higher cost of carbon filaments CNTs.
Further, EP0198558 disclose a method of preparing carbon filaments comprising exposing a suitable thermostable support having substantially completely reduced monocrystalline metal particles to a carbon containing gas at a temperature of from about 250[deg.]C up to about 700[deg.]C to 800[deg.]C for a period of time sufficient to form carbon filaments of a desired dimension followed by removing the substrate and/or the metal particles. The metal particles disclosed in said patent is selected from ferromagnetic particles such as nickel, metallic iron or alloys thereof. The said patent further maintains that when ferromagnetic metal particles are used for the growth of the carbon filaments, these filaments can be oriented in a parallel relationship with each other by placing the substrate in an electric or magnetic field having the desired strength to promote growth of the fibers in a parallel manner. The substrate used is silica, alumina where a dense network of filaments is preferably grown using a flat substrate in a restricted volume, optionally in the presence of an electric or magnetic field. The metal tips are subsequently removed by treatment with an acid.
An article titled, Carbon nanotube Synthesis via the catalytic CVD method : A review on the effect of reaction parameters by Cmar Oncel and Yuda Yurium covers the results obtained in carbon nanotube synthesis by chemical vapor deposition. Various parameters such as catalysts, supports, carbon precursors, reaction time, temperature etc used in the production of carbon nanotubes are discussed in said article.
An article titled ‘Carbon synthesis of carbon Nanotubes and nanofibres’ by Kenneth teo, Cahranjeet Singh et. al in encyclopedia of Nanoscience and Nanotechnology, volume X, pages 1-22 disclose the catalytic chemical vapor deposition for the preparation of carbon nanotube and nanofibres. Further, the article disclose that branching of the carbon filament is promoted with the catalyst doped with silica or calcium.
There still remain a need to provide processes for the production of carbon filaments which is industrially feasible, cost effective and yields carbon filaments in high yield and purity. Accordingly, our invention has sought to provide an economical but superior alternative to redress the problems in the prior art. We have thus come up with the innovative process to produce carbon filaments CNTs at low cost by utilizing modifying the already existing conditions required for the production of carbon filaments CNTs using CVD which are found at exhaust level of the industrial plant. Commercially, this could solve not only the problem of high cost of producing carbon filaments CNTs but also bring down environmental pollution and carbon emission, leading to reduction of global warming and climate catastrophes.