nanofibers, has enthralled us with the thrilling research pertaining to various applications. I would like to pay my obeisance and gratitude to all those who have strived very hard to contribute to this book.
Dr. Madhuri Sharon
Director, Walchand Center for Research in Nanotechnology and Bionanotechnology, Solapur, India
December 2020
1
An Introduction to Carbon Nanofiber
Maheshwar Sharon
Walchand Center for Research in Nanotechnology and Bionanotechnology, WCAS, Solapur, Maharashtra, India
The goal of education is not to increase the amount of knowledge but to create the possibilities for a child to invent and discover, to create men who are capable of doing new things.
Jean Piaget
1.1 Introduction
Carbon (Latin word: carbo “coal”) is a chemical element with symbol C belongs to group 14 of the periodic table. Its atomic number is 6 and has the electronic structure of 1s22s22p2. The valence of carbon will depend upon the arrangement of electrons in 2s and 2p orbitals. 2s orbitals are supposed to house maximum of 2 electrons and 2p can possess a maximum of 6 electrons. The energy of electrons in 2s and 2p orbitals is not much different from each other. Therefore, depending upon the condition of the electronic arrangement, carbon atom can show valence of 2 due to hybridization of 2s1 2p1 orbitals. The 2s2p orbital can also be hybridized as 2s12p2 showing a valence of 3. Likewise, 2s2p orbital can be hybridized to show 2s12p3 configuration making a carbon valence of 4. Carbon compound with 2s12p1, i.e., with valence of 2, is rarely seen, and if some carbon compounds are made with such configuration, they are not stable. Most common compounds with configuration of 2s12p2 (like graphite) and 2s12p3 (like diamond) are found in nature. Angles between each carbon for 2s12p1, 2s12p2 and 2s12p3 are shown in Figure 1.1.
Figure 1.1 The formation angle between the carbon for sp, sp2 and sp3 configuration.
In this chapter we shall restrict our discussions to carbon with configuration of sp2 and sp3. In addition, we shall also restrict our discussions especially regarding carbon fiber (discussed in detail in later on), though we may touch upon carbon nanotube.
It will not be the scope of this chapter to discuss diamond or graphite, mainly because information about these materials are available in almost all textbooks dealing with carbon. Our discussion will be confined to materials like carbon fiber, carbon nanofibers and to some extent carbon nanotubes for making a clear distinction between these materials.
1.1.1 History of Carbon Fiber
The development of carbon fibers dates back as early as the 18th century but its applications were only realized after 1960, making it possible to develop carbon fibers with high strength. Some landmarks in the development of carbon fibers are given here. In 1860, Joseph Swan first produced carbon fibers. In 1879, Thomas Edison carbonized cotton threads or bamboo slivers to produce carbon fiber. He used it as wire filament which glows under electric current. Roger Bacon prepared carbon fiber containing about 20% carbon in 1958 by carbonizing rayon. The process of carbonizing rayon was later improved in 1960 by Richard Millington (US Patent No 3,294,489) [1] to get carbon fiber with almost 99% carbon. During the 1960s, a Japanese firm produced carbon fibers from a petroleum pitch which contained 85% carbon and had excellent flexural strength. After 1960, it seems various efforts were made to prepare carbon fibers (Figure 1.2) from different types of precursors to improve the quality of carbon fibers; for example, giving tensile strength 4,000 MPa and a modulus of 400 GPa to be used in aircraft.
Figure 1.2 SEM image of carbon fiber.
1.1.2 What Is a Carbon Fiber?
Carbon fibers are classified symbolically as CF. These fibers are of size about 5–50 μm in diameter and composed mostly of carbon atoms. They are also known as graphite fiber. Graphite is made of pure carbon arranged into big sheets of hexagonal aromatic rings (Figure 1.3X). The structure of carbon fibers is like the structure of graphite arranged in layers of long graphite sheets of length about 5–50 μm and each layer of graphite is stacked together forming a ribbon-like structure. Depending upon the precursors used to synthesize carbon fibers and the manufacturing processes, each layer planes may be either turbostratic graphitic, or a hybrid structure. In graphitic crystalline regions, the layer planes are stacked parallel to each other in a regular fashion. The atoms in each layer are covalently bonded through sp2 bonding while the interaction between each layer is relatively weak van der Waals forces. In a single graphitic crystal, d-spacing between two graphene layers (d002) is about 0.335 nm (Figure 1.3Y).
Figure 1.3 (X) A sheet of graphite showing free edges A, which are reactive and can be functionalized, (Y) shows four layers of graphene sheet separated by van der Waals forces. In graphite this separation distance between two consecutive layers is 0.335 nm.
If carbon content in the fiber is 92 wt% then it is considered as carbon fiber. However, if carbon content is more than 99 wt% then the fiber is known as graphite fiber. This classification depends on the percentage content of carbon, process of synthesis and precursors; Carbon fibers can possess excellent tensile strength, low densities, high electrical conductivities, high temperature tolerance, low thermal expansion and chemical stabilities. High tensile strength and lightweight carbon fiber can be developed by making its composite with suitable polymers. Because of these properties, carbon fiber has found its place in aerospace, civil engineering, the military, and motorsports, along with other areas like sports.
Classification of carbon fibers based on precursor used to make them:
PAN-based carbon fibers (90% of fibers made from this precursor)
Pitch-based
