Biomedical Engineering Reference
In-Depth Information
silicon-alloyed pyrolytic carbon, a diffraction pattern
characteristic of the b form of silicon carbide also appears
in the diffraction pattern along with the carbon bands.
The carbon diffraction pattern indicates a turbostratic
structure (Kaae and Wall, 1996) in which there is order
within carbon layer planes, as in graphite; but, unlike
graphite, there is no order between planes. This type of
turbostratic structure is shown in Fig. 3.2.11-3 compared
to that of graphite. In the disordered crystalline struc-
ture, there may be lattice vacancies and the layer planes
are curved or kinked. The ability of the graphite layer
planes to slip is inhibited, which greatly increases the
strength and hardness of the pyrolytic carbon relative to
that of graphite. From the Bragg equation, the pyrolytic
carbon layer spacing is reported to be 3.48 ˚ , which is
larger than the 3.35 ˚ graphite layer spacing (Kaae and
Wall, 1996). The increase in layer spacing relative to
graphite is due to both the layer distortion and the small
crystallite size, and is common feature for turbostratic
carbons. From the Scherrer equation the crystallite size is
typically 25-40 ˚ (Kaae and Wall, 1996).
During the coating reaction, gas-phase nucleated
droplets of carbon/hydrogen form that condense and
deposit on the surfaces of the reactor wall and bed par-
ticles within the reactor. These droplets aggregate, grow,
and form the coating. When viewed with high-resolution
transmission electron microscopy, a multitude of near-
spherical polycrystalline growth features are evident as
shown in Fig. 3.2.11-4 (Kaae and Wall, 1996). These
growth features are considered to be the basic building
blocks of the material, and the shape and size are related
to the deposition mechanism. In the silicon-alloyed
carbon small silicon carbide particles are present within
the growth features as shown in Fig. 3.2.11-5 . Based on
a crystallite size of 33 ˚ , each growth feature contains
about 3 10 9 crystallites. Although the material is quasi-
crystalline on a fine level, the crystallites are very small
and randomly oriented in the fluidized bed pyrolytic
B
A
C
Fig. 3.2.11-3 Structures of (A) diamond, (B) graphite, and (C) turbostratic pyrolytic carbon.
Search WWH ::




Custom Search