handbook of carbon, graphite, diamond and fullerenes

Pyrolytic Graphite 161
Table 7.3. Properties of Isotropic Pyrolytic Carbon at 25°C (as deposited)
Density, g/cm 3
Vickers DPH* hardness,
kg/mm 2
Flexural strength, MPa
Young's modulus, GPa
Strain to failure, %
2.1
240 - 370
350
28
1.2
* DPH: diamond pyramid hardness
Gas Permeability. Isotropic pyrolytic carbon provides a better barrier
to gases than the more crystalline pyrolytic graphites.! 261 The permeability,
K, is as follows:
Isotropic pyrolytic carbon: 10 6 to 10 15 cm 2 s
Pyrolytic graphite: 10 2 to about 10 cm 2 s
5.0 APPLICATIONS OF PYROLYTIC GRAPHITE AND CARBON
5.1 High-temperature Containers and Other Free-Standing Products
Containers and otherfree-standing (monolithic) parts are produced by
depositing the pyrolytic carbon or graphite onto a graphite mandrel. After
the deposition is completed, the mandrel is removed, usually by machining.
It is difficult to deposit pyrolytic graphite in shapes having sharp radii
without interlayer delaminations. These delaminations are caused by the
stresses generated by the considerable differences in the thermal expansion
in the ab and c directions which can overcome the low interlaminar
strength. These delaminations do not occur with isotropic pyrolytic carbon.
Common applications of pyrolytic graphite are:
• Boats, and crucibles for liquid-phase epitaxy
• Crucibles for molecular-beam epitaxy
• Reaction vessels for the gas-phase epitaxy of Ill-V
semiconductor materials such as gallium arsenide
• Trays for silicon-wafer handling