1.3.3 Use of Catalyst in the Synthesis of CNF
There are variations in the type of catalyst used for the synthesis of CNF. There are some catalysts which are solid like ferrocene, cobaltocene, etc. These catalysts vaporize in the temperature range of 300 to 400 °C. Such catalysts are kept in a boat (Figure 1.8B1) in the furnace (Figure 1.8A1) along with the precursor like oil. The vapor of oil and the vapor of catalyst like ferrocene are carried with carrier gas to the furnace (Figure 1.8A2). Catalyst decomposes into its metal oxide and in-situ reaction takes place with the vapor of oil to produce CNF. These CNFs are collected in the boat (Figure 1.8B2).
Catalyst can be used either in the form of powder or as a thin film deposited on a substrate like quartz glass, alumina plate, or as a metal plate like nickel, iron metal, etc. Normally, after scratching a metal plate with diamond powder it is cleaned with water and dried with acetone, which is also used as a catalyst. Scratching the plate creates more roughness of the metal, which produces more surface states and surface area over which the reaction of vapor of precursor reacts more favorably with catalyst. The preferred size of catalyst particles should be in the range of 5–25 nm in diameter. Some examples of the role of different types of catalysts and their effect on the carbon nanomaterials produced are shown in Figure 1.4.
There is no hard and fast rule to suggest which type of catalyst will produce better CNF. Better CNF means the value of its diameter, length, surface area and surface states. Selection of catalyst and its type depends upon the type of precursor and size of catalyst powder, which can only be decided by carrying out some specific experiments. Powder or thin films of Fe/Ni, Ni, Co, Mn, Cu, V, Cr, Mo, Pd, MgO, and Al2O3 are used as catalyst. But most popular catalysts are powder or thin film of organometallic compounds of iron or nickel or cobalt. Details of the use of different catalysts are presented in Chapter 3 of this book.
1.3.4 Selection of Variable Parameters for Growth of CNF
Variable parameters of pyrolysis under the CVD units are: precursor (oil, solid materials like plant materials, camphor, etc.), temperature (Figure 1.8A1 and A2), type and size of catalyst (usually nickel, iron and cobalt), carrier gas (argon, nitrogen, etc.), reactive gas (hydrogen), and duration of pyrolysis. Due to the large number of variables, it becomes very time-consuming to reach a suitable condition to obtain the desired type of CNF.
It is advisable that one should adopt the Taguchi method, which can provide information of effective variables to produce the required type of CNF [1–3]. For example, with 4 variables (requires 24 experiments) one needs to carry out only 9 sets of experiments; whereas, with 6 variables (requires 720 experiments) one needs to carry out only 16 sets of experiments.
1.3.5 Epitaxial Growth of Aligned CNF
Afre et al. [8] have developed a CVD with spray techniques (Figure 1.9A) to get epitaxial growth of aligned, continuous (Figure 1.9B), catalyst-free carbon nanofiber from vapor of turpentine oil. Aligned CNF possesses properties like high mechanical strength, high directional electrical conductivity, and high thermal conductivity. The main difference between the CVD unit described in Figure 1.8 and 1.9 is the inclusion of a spray facility so that solution containing precursor and the catalyst sprays its liquid in the furnace like a cloud of material.
1.3.6 Mechanism of CNF Synthesis
There are different mechanisms suggested to explain the role of catalyst but none of them can explain why CNFs are formed over the entire reaction tube in a larger amount than the amount of catalyst used for the purpose. The author of this chapter suggests that vapor of precursor at high temperature breaks down into lower very reactive carbon fragments and this decomposition is catalyzed by the catalyst. These reactive fragments move around the furnace (Figure 1.8A2) with the help of the carrier gas. Since these fragments are highly reactive, they recombine to give CNF. The process of recombination giving the product depends upon the variable conditions used for the synthesis of CNF. The nature of CNF formed thus depends upon variables like temperature, the flow rate of carrier gas, etc. It is for this reason one finds deposition of CNF not only in the boat (Figure 1.8B2) but throughout the quartz tube (Figure 1.8C). Moreover, the amount of CNF formed are far in excess of the amount of catalyst used for the purpose.
Figure 1.9 (a) typical sketch of CVD unit with a spray system and (b) the aligned carbon nanofibers produced from this process using turpentine oil.
1.4 Properties of CNF and Its Composites
Electrical conductivity in CNF/polymer composites depends upon the nature of the network and their alignment in the matrix. Therefore, for good electrical conductivity a good fiber dispersion and the continuation of the network of carbon nanofibers is necessary. Also, by controlling the loading amount of CNF in the composite different electrical resistivity values can be achieved. CNF/polymer composite can increase the tensile strength (gain of 50 to around 300%), compression strength (50–100%), Young’s modulus (almost 100%), inter-laminar shear strength (with 1% CNF increases in the range of 20–50%), fracture toughness (with 1% CNF increases in the range of 30–50%), and vibration damping of the base polymer. These improvements depend upon the nature of polymer, extent of dispersion, etc. [9–15]. Thermal conductivity of CNF with 20 wt% epoxy resin has been observed to increase from 0.2 W/m-K to 2.8 W/m-K [15]. CNF composite with thermoplastic material is found to retard fire properties like delay in burning when exposed to flame and slow heat transmission [16, 17]. Polymeric composites with carbon nanofibers have shown substantially lower coefficients of thermal expansion as compared to graphite. Details of CNF and its composites are presented in Chapter 4 of this book.
1.5 Applications of CNF
Some of the applications of CNF are enumerated here:
Scientists, after understanding the properties of carbon fibers, have started using this material in various engineering applications to reinforce concrete, replace steel with carbon fibers, as material for bridge work, etc. CNFs are being applied or are thought to apply for strengthening of structures with carbon fiber composites in precast concrete and plastic, and fiber-cement. The traditional method of strengthening the reinforcement of brick is by utilizing steel clamps, which provide strength but