Carbon nanofibers
Carbon nanofibers (CNFs), vapor grown carbon fibers (VGCFs), or vapor grown carbon nanofibers (VGCNFs) are cylindric nanostructures with graphene layers arranged as stacked cones, cups or plates. Carbon nanofibers with graphene layers wrapped into perfect cylinders are called carbon nanotubes.
Carbon is the building block of a myriad of organic and inorganic matter around us. It is a versatile atom capable of joining to other atoms in sp, sp2, and sp3 hybridized structures giving rise to millions of stable molecules. In its single element form, it has a number of allotropes (polymorphs) like diamond, graphite, and fullerenes with different properties ranging from extremely hard to very soft scope. Carbon can be made to form tubular microstructure called filament or fiber. The unique properties of carbon fibers have expanded the science and technology of composite materials in recent decades.
VGCFs and their smaller size variant, VGCNFs (Vapor Grown Carbon Nanofiber) are among short carbon fibers that have drawn lots of attention for their potential thermal, electrical, frequency shielding, and mechanical property enhancements. They are being more and more utilized in different material systems like composites thanks to their exceptional properties and low cost.
Catalytic chemical vapor deposition (CCVD) or simply CVD with variants like thermal and plasma-assisted is the dominant commercial technique for the fabrication of VGCF and VGCNF. Here, gas-phase molecules are decomposed at high temperatures and carbon is deposited in the presence of a transition metal catalyst on a substrate where subsequent growth of the fiber around the catalyst particles is realized. In general, this process involves separate stages such as gas decomposition, carbon deposition, fiber growth, fiber thickening, graphitization, and purification and results in hollow fibers. The nanofiber diameter depends on the catalyst size. The CVD process for the fabrication of VGCF generally falls into two categories: 1) fixed-catalyst process (batch), and 2) floating-catalyst process (continuous).
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