Carbon fibers are extremely strong thin fibers made by pyrolyzing synthetic fibers, such as rayon, until charred. It is used to make high-strength composites.
Use of Carbon Fibers
Carbon fibers are used in nearly every type of high-performance vehicle like boats, cars, motorcycles etc. Example of practical civilian uses includes Feather Carbon. Expensive civilian and military uses of carbon fibers includes some planes, jets and portable floating cubes that are used for bridges. An area where carbon fiber has found good use is in the manufacture of bicycles. The vibrations absorbing properties of carbon smooth out the road and offers improved weight over an aluminium design. The choice of weave can be chosen so as to maximize stiffness. A variety of shapes of carbon can be built. This has further increased stiffness and also allows aerodynamic considerations into tube profiles.
The importance of strong fibres was realised way back in the 1950s. Technologists realised the magnitude of the importance of such fibres and started exploring newer and better fibres. Soon it was realised that ceramics and carbon/graphite fibres can provide a solution for this challenge.
Two different methods were used for production of the same:
- Using shape of the existent filament
- Growing it as whiskers or crystals
Continuous filaments of carbon were successfully produced by means of the first method.
Consequently, many modifications were made on this basic finding. These were followed by the composite structures. The attempts to correlate Youngs modulus of elements with their position in the Mandeleefs periodic table have not met with much success.
However, it is a proven fact that the bonds that exist between the lighter elements are as stiff as those between the heavier elements. Elements of high modulus or high specific modulus have been placed in the first two rows of the periodic table. A further analysis reveal that the first row elements dominate more than the second row elements, in this respect. The third row too contributes a little here. To be more precise and clear, the list goes as shown below:
- First row: lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine
- Second row: magnesium, aluminium, silicon
- Third row: titanium
It is always important to bear in mind, while viewing the elements as the contributors of modulus, there are different factors that make up this property. However, the available methods for the measurement as well as determination of properties are too difficult to be accurate or error-proof. There exists a great degree of possibility that many materials might not have been inferred with dependable accuracy with regards to their values of modulus. Many a time, the resultant derivations of even the established and reputed methods do not give the same results. Requirements of thermal stability, the chemical stability, strength, and even the isotropy differ greatly in substances. It is this difference that has made the measurement of modulus of substances in the periodic table rather important. Interest in fibres possessing high modulus is centred on few elements like carbon and boron, and some other common abrasives like silicon carbide and aluminium trioxide. Unless pure, these materials tend to show large, unexpected and unpredictable changes in modulus. Of these, the properties of carbon fibres deviate more than any other element or compound when impurities are added.
Fibres for reinforcements should be chosen only after understanding their properties and methods of manufacturing them. Few major factors that comprise this arena are:
- Chemical compatibility
- Physical packing
- Energy absorbing capability
- Elongation of fracture
Manufacturing carbon fibres
The manufacturer of carbon fibres makes use of decomposing bamboo. Degrading cotton wool paves the path to manufacture carbon fibre insulation material. However, a useful type of carbon fibre is made by means of stoving of viscose rayon fibres, followed by the removal of water and gases like carbon-di-oxide and hydrogen under controlled conditions. This produces a stiff fibre, which is also strong. Poly-acrylonitrile is also used for manufacturing another type of carbon fibre. It is possible to derive a partly oriented structure during the progressive decomposition of acrylonitrile. The process is open to accommodate improved orientation by means of highly oriented polymer and careful oxidation. Lesser temperature of operation facilitates tensioning of acrylonitrile. It is rather customary to have slow preliminary oxidation at a low temperature to stabilise the fibre.
In general, the three vital stages in the production of carbon fibres are:
- Metal spinning or wet spinning
Each step differs for each of the known production processes.
In spite of the many contemporary aids of science, the exact molecular processes in the conversion of polymeric fibres to high modulus carbon fibre is still subject to some degree of speculation. As per the recorded derivations available in the present time, carbon stands tall in the value of modulus. Carbon fibres are thinner than even hair. In spite of their diameter, which range from 6 to 9 micron, the family of carbon fibres covers a range of Youngs modulus values ranging from 220 to 1000 GN/m2. The highest values are achieved when graphite is used and when it is measured along the plane of the layer structure, in which it is supposed to have higher concentration of covalent bonds. As diamond, the values range from 600 to 1000 GN/m2. Carbon fibres are black in colour, and thin in shape. Generally, 99.9 per cent of the composition is carbon, in its purest form. The strength is in a range of 2000 to 4500 MN/m2. The one material that shows a comparable specific stiffness to that of the carbon fibre as graphite is the carbon chain that runs through the polyethene.
For carbon fibre oriented matrixes, strength properties exhibited at room temperatures remain unchanged even at very low temperatures. Even the inter-molecular shear strengths improve. But, its otherwise appreciable properties of good thermal and electrical conductivity do not seem to continue at lower temperatures. Advanced composites commonly consist of 60 per cent carbon fibres. Glass fibres are the most liberally and generally used reinforcement. It renders more stiffness and the composite is lighter in weight than carbon fibres. But, it may be rather difficult to accept that the two perform equally when the strength is measured.
Carbon fibres are of two categories:
- Poly-acrylonitrile (PAN) based
- Pitch based
The latter category finds wide application in reinforcements. The former category is selected for insulation purposes, fillers etc, as these exhibit only medium high modulus of the range 200-400 GN/m2, clubbed with higher tensile strengths ranging around 2000 MN/m2. The existence of very strong chemical bonds between the carbon atoms, which form the major part of the graphite layers, form the foundation for the high strength and the high Youngs modulus. It is this bond strength that controls the lattice elasticity too. The homo polar nature plays a vital role in getting the bond strength between the carbon atoms. The lower strength in the lattice direction is
due to the Van Der Waals bonds between the atoms. Thermal and electrical conductivity of graphite, in a direction that is parallel to the layers, tend to be high due to the electrons between the layers. Only highest preferred orientations can exert highest modules and strengths in carbon fibres.
Coated or composite fibres and their cheaper precursors are also popular and continuous research takes place in this field. Deposition of foreign materials such as silicon or silicon carbide or alumina is one fruitful derivation of such experiments. The fibres thus modified by such treatments show a great stability towards chemicals than the original or parent fibre. They withstand higher temperatures, exhibit good bonding characteristics between the fibre and the matrix and reveal high toughness to fractures. In some cases, the material imposes good resistance to corrosion of the base material with which this composite is used. The growth and improved orientation of the layers will increase Youngs modulus in the direction of the fibre. Usage of highly pure forms of the polymer precursors helps the carbon fibres maintain strength even at higher temperatures. The structure of the polymer plays a vital role in the control of the lattice defects in the fibres.