Page 188 - Fiber Fracture

FRACTURE OF CARBON FIBERS 173

Fig. 21. High-resolution scanning electron micrograph of pitch-based carbon fiber surface.

VAPOR-GROWN CARBON FIBERS

Pure carbon fibers may be grown by a catalytic process from carbon-containing gases.
The catalysts are typically transition or noble metals, and the gases are CO or virtually
any hydrocarbon. The fibers were first identified by Schutzenberger and Schutzenberger
(1890), and they were the subject of study within the oil industry more recently, with
the objective of preventing their growth in petrochemical processes. The fibers may
take a variety of forms, depending upon the catalyst system and the constituents of the
feed gas. The interested reader is referred to an excellent review article by Rodriguez
(1993).
A generic process for catalytic formation of carbon fibers is described by Rodriguez
(1993). Typically, about 100 mg of powdered catalyst is placed in a ceramic boat
which is positioned in a quartz tube, located in a horizontal tube furnace. The catalyst
is reduced in a dilute hydrogen/helium stream at 600°C, and quickly brought to the
desired reaction temperature. Following this step, a mixture of hydrocarbon, hydrogen
and inert gas is introduced into the system, and the reaction is allowed to proceed for
about 2 h. This approach will produce about 20 g of carbon fibers from the more active
catalyst systems. In this process, the fiber diameter is typically related to the catalyst
particle size. The process proposed for fiber formation by Oberlin et al. (1976) involves
adsorption and decomposition of a hydrocarbon on a metal surface to produce carbon
species which dissolve in the metal, diffuse through the bulk, and ultimately precipitate
at the rear of the particle to produce the fiber. This process is described as tip growth.
There is an analogous process in which the catalyst particle remains attached to the
support.