Carbon and Graphite
Carbon and graphite are excellent refractory materials but might readily oxidise and hence are utilized only either in reducing or neutral conditions. Graphite is more stable form of carbon. The amorphous carbon is converted into crystalline graphite on prolonged heating at about 2500oC. Graphite does not melt however sublimes at a temperature of 4200oC. Carbon and graphites are highly resistant to acids, alkalies & all of solvents except oxidising ones.
Graphite is utilized at high temperature as rocket nozzle or nozzle throat insert. The graphite nozzles are coated with molybdenum silicate, silicon carbide, or impregrated along Zr to improve oxidation resistance. The molten Zr along which graphite surface is impregrated forms zirconium carbide with good chemical bond with graphite surface. The outer surface is oxidised to zirconia whereby further oxidation is stopped.
Carbon and graphite are utilized in industry in fabricated and non-fabricated forms. In fabricated forms carbon and graphites are utilized as materials of construction for electrodes, refractories, chemically resistant equipment and nuclear reactors. Carbon and graphite shapes produced by common techniques are porous. They might be made impervious to fluids by impregnating with synthetic resin, typically of phenolic type and then curing resin at high temperature.
Now fabricated carbon includes carbon black, lamp black, activated carbon, graphite and diamond. Lamp black and carbon black are utilized as fillers and pigments while activated carbon is utilized as a crystal in chemical processes. Graphite is utilized as a lubricant, in batteries and for crucible manufacture. Industrial diamond is utilized as cutting tool for very hard materials. Bort & Carbonado are general industrial diamonds.
The tensile strength of carbon materials varies with type grade and method of manufacturer between 80 and 250 MPa, at room temperature. The strengths are about doubles at temperature above 2450oC. The spell resistance of carbon is excellent. Carbon as graphite also possesses low thermal expansion and high thermal conductivity. Both these properties shall enhance resistance to thermal shock. Whereas other forms of carbon are not easily machinable but graphite is often preferred as an electrical conductor instead of its low specific resistance. The largest tonnage of carbon & graphite is utilized in metallurgical and electrochemical industries as anode in arc melting furnaces.