**3. Vapor-grown carbon fibers**

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

structure renders them to show semi- conducting behavior [16] and to have synthetically dynamic end planes on both the inner and outer surfaces of the nanofibers, in this way making them helpful as supporting materials for catalysts [17], reinforcing fillers in polymeric composites [18], hybrid type filler in carbon fiber reinforced plastics [19–21], and photocurrent generators in photochemical cells [22, 23].

*Schematic comparison of the diameter dimensions for various types of fibrous carbons [4].*

Since the temperature and pressure essential to synthesize a carbon fiber from the fluid stage is at the triple point (Temperature—4100 K, Pressure—123 kbar), it would be practically difficult to get ready carbon fibers from the liquefy under modern preparing conditions. As a result, carbon fibers are arranged from organic precursors. This preparation is commonly done in three stages, including stabilization of a precursor fiber in air (at 300°C), carbonization at 1100°C, and subsequent graphitization (*>* 2500°C). Fibers undergoing only the first two steps are commonly called carbon fibers, while fibers undergoing all three steps are called graphite fibers. Carbon fibers are commonly utilized for their high strength, while graphite fibers are utilized for their high modulus. Graphitic whiskers were developed under conditions close to the triple point of graphite. At that point, the structural model was proposed, in which the layers comprising of graphene sheets are twisted around the axis like as in rolling up a carpet. These whiskers were utilized as the presentation focus in the beginning times of carbon fiber technology, despite the fact that


**2. Synthesis and properties of carbon fibers**

they have never been manufactured on a large scale.

2.A high-modulus PAN-based fiber

1.High-strength polyacrylonitrile (PAN)-based fiber

3.A mesophase pitch-based carbon fiber (MPCF).

lined up with the carbon hexagonal portions corresponding to the fiber axis. This orientation is liable for the tensile strength of PAN-based carbon fibers [28].

The PAN-based fibers comprise of little sp2

Carbon fibers are

**Figure 1.**

**52**

PAN- based fibers have high strength and MPCFs have high modulus, while VGCFs afford fundamentally ultra-high modulus materials. VGCFs have an extremely unique structure like annular-rings and are prepared by something diverse arrangement process than that used to prepare PAN-based and MPCFs. Specifically, VGCFs are not prepared from a fibrous precursor, but instead from hydrocarbon gas, utilizing a catalytic growth process [5, 11–15]. Ultrafine transition metal particles, for example, iron particles with diameter under 10 nm, are scattered on a ceramic substrate, and a hydrocarbon, for example, benzene diluted with hydrogen gas is introduced at temperatures of around 1100°C. Hydrocarbon decomposition happens on the catalytic particle, showing a ceaseless carbon take-up by the catalytic particle and a persistent output by the particle of efficient tubular filaments of hexagonal sp2 carbon. The swift development rate which is 106 times quicker than that analyzed for the development of normal metal whiskers [30], permits the production of commercially suitable amounts of VGCFs.

## **4. Vapor grown carbon Fiber growth**

While portraying the production of VGCF, two distinct strategies can be discriminated. In the primary strategy, the fibers are manufactured on catalyst-seeded substrates, in two independent consecutive stages. In the first stage, the impetus molecule, for the most part with iron being the prevalent constituent, starts a long, thin, somewhat graphitic fiber when presented to a hydrocarbon gas close or more 1000°C.

A petite fraction of these fibers grows to macroscopic lengths when exhibited to a low carburizing potential gas, while keeping up the outside diameter of the initial catalytic particle. The sizes of these particles, and subsequently the filament diameter, have been considered from 10 to 200 nm [31–33]. The development of the filament may extend as quickly as 1 mm/min and persevering for a few minutes until the catalytic particle is deactivated [33]. At this stage filament lengths of a few centimeters might be acquired.

In the second stage, when the gas potential is increased, the filament thickens because of the deposition of pyrolytic carbon. Fibers with diameters up to 100 nm have been produced in batch conditions by this method. Pyrolytic carbon is deposited with the basal planes specially arranged corresponding to the surface, the properties of the fiber are moderately graphitic [34].

Methane, hydrogen and an inert gas are utilized to develop the fibers at atmospheric pressures on nesting cylindrical substrates inside a growth tube. Lower centralizations of the methane-hydrogen blends are utilized for fiber extending, though higher concentrations are utilized for thickening. As methane is an economical feedstock in certain parts of the world, the production of VGCFs can turn out to be exceptionally financial contrasted with other carbon filaments.
