ARCI had initiated the Graphite Activity (Exfoliated Graphite) in 1994 at laboratory scale. Synthetic Graphite is having a limitation of brittle failure due to binder. To overcome this problem, a unique process was developed at laboratory scale, where natural graphite flakes were chemical intercalated and subjected to thermal treatment to produce Exfoliated Graphite Powder. This powder is vermicular in shape and facilitates pressing or rolling etc, without any binder (Shown in figure). In collaboration with Institute of Gas, Kiev, Ukraine, facilities of pilot scale production of exfoliated graphite was set up.
Process parameters were established for achieving more than 300 times of Exfoliation. Various value added products were developed from this graphite powder which finds various engineering and industrial applications. Exfoliated Graphite plant with a capacity of 2.5 Kg/shift consisting of intercalation and exfoliation unit was set up.
Flexible Graphite & Tapes
Using the Exfoliated Graphite Powder developed at ARCI, Flexible graphite sheets and tapes of various thicknesses ranging from 0.2 mm - 0.8 mm have been developed for electrical contact applications.
Plain Graphite seals of various dimensions were developed for chemical and pharmaceutical industries. A special type of reinforcement was developed with metallic wires from the Metallic Knitting Unit, which is useful for the development of seals for high pressure applications.
By suitable metallic reinforcement, automobile gasket graphite sheets were developed. A rolling mill for continuous graphite sheet was set up at Carbon Center. Studies were conducted on effect of metallic reinforcement i.e. its perforation geometry, hardness etc. on the properties of gasket.
Various trial samples have been supplied to Gasket manufacturer for actual development of automobile cylinder head gaskets. For developing the Asbestos free Cylinder Head Gasket for Indigenous Car a joint work was carried out with the financial support of DST under their PATSER Scheme with M/s Sankar Sealing Systems, Chennai. Gasket developed with the ARCI material has been fitted in car and successfully completed many thousands kilometers of running.
Graphite Boards
Using the highly anisotropic properties of graphite, thermal insulating boards were developed. Improved boards with exfoliated graphite powder is sandwiched between the flexible graphite sheets were also developed. Graphite wave shaper was also developed for defence application.
Oil Spill Management through Exfoliated Graphite
Exfoliated Graphite Powder has a unique property of high adsorption of oil from the oil water mixture. Detailed studies were carried out for the adsorption behavior of various grades of oils on the different types of graphite powder (different density). Different types of set-ups were designed for the adsorption and recovery of the oil from the exfoliated graphite powder like different stack height of graphite etc. An interesting observation was made that adsorption of oil depends on the viscosity of oil.
Carbon Nanotubes (CNT’s)
Just two decades before, we knew of only three forms of carbon, namely diamond, graphite and amorphous carbon. Today we know there is a whole family of other forms of carbon. The first to be discovered was the hollow, cage like buckminsterfullerene molecule - also known as the buckyball, or the C60 fullerene. There are now thirty or more forms of fullerenes and also an extended family of linear molecules, carbon nanotubes. Possibly more important than fullerenes are Carbon Nanotubes (CNT’s), which are related to graphite. A nanotube may consist of one tube of graphite, a one-atom thick Single-Wall Nanotubes (SWNT) or a number of concentric tubes called Multiwalled Nanotubes (MWNT). There are different types of CNTs, because the graphitic sheets can be rolled in different ways.
There are many useful and unique properties of CNTs. The list includes High Electrical Conductivity, Very High Tensile Strength, Highly Flexible - can be bent considerably without damage, Very Elastic ~18% elongation to failure, High Thermal Conductivity, Low Thermal Expansion Coefficient, Good Field Emission of Electrons, Highly Absorbent and High Aspect Ratio (length = ~1000 x diameter).
Arc Discharge Technique
The carbon arc discharge method as shown in figure is the most common and perhaps most popular way to produce high temperature resistant CNTs (Multiwall, Single Wall Carbon Nanotubes and Fullerenes). However, it is a technique that produces a complex mixture of components and requires further purification - to separate the CNTs from the soot and the residual catalytic metals present in the crude product. This method creates CNTs through arc-vaporization of two carbon rods placed end to end, separated by few mm, in an enclosure that is usually filled with inert gas (Helium) at low pressure. A direct current of order of few hundreds Ampere (based on Graphite Current Density), driven by a potential difference of approximately 20 - 50 V, creates a high temperature discharge between the two electrodes as shown in figure 1. The discharge vaporizes the surface of one of the carbon electrodes and forms a small rod-shaped deposit on the other electrode. Producing CNTs in high yield depends on the uniformity of the plasma arc and the temperature of the deposit forming on the carbon electrode.
Carbon Nanotubes by CVD Technique
Chemical vapor deposition of hydrocarbons over a metal catalyst is a classical method that has been used to produce various carbon materials such as carbon fibers and filaments for last few decades. In the same line large amounts of Carbon nanotubes and Carbon nano-fibers can be synthesized by catalytic CVD of hydrocarbon gas such as acetylene over Cobalt, Nickel or Iron catalysts supported on ceramic support such as silica at the temperature range of around 900°C. A simple laboratory type set up of CVD installed at ARCI to synthesize Carbon nanotubes.
Synthesis of multi-walled carbon nanotubes through arc discharge from various grades of graphite.
