Pyrolytic Graphite (PG) is a unique form of graphite manufactured by decomposition of a hydrocarbon gas at very high temperature in a chemical vapor deposition furnace. The result is an ultra-pure product which is near theoretical density and extremely anisotropic. This anisotropy results from the layered structure as indicated in the drawing. As an example, PG exhibits a thermal conductivity comparable with the best conductors in the "ab" plane and lower than alumina brick in the "c" direction. Mechanical, thermal, and electrical properties are generally far superior to conventional graphites. PG is available as plate or machined shapes and as a coating on graphite and other substrates. Typical properties are listed below
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Typical Properties
|
Density
|
2.18-2.22 g/cc
|
Flexural Strength
|
(a b)
|
18,000 psi (120 MPa)
|
Tensile Strength
|
(a b)
|
12,000 psi (80 MPa)
|
Compressive Strength
|
(a b)
|
15,000 psi (100 MPa)
|
Young’s Modulus
|
(a b)
|
3x106 psi (20,000 MPa)
|
Coefficients
|
Thermal Expansion
|
(a b)
|
0.5x10-6 cm/cm/°C
|
|
(c)
|
2-x10-6 cm/cm/°C
|
Thermal Conductivity
|
(a b)
|
300 watts/meter °C
|
|
(c)
|
3.5 watts/meter °C
|
Electrical Resistance
|
(a b)
|
0.5x10-3 ohm-cm
|
|
(c)
|
0.5 ohm-cm
|
Crystal Structure
|
Hexagonal
|
(C/2 Spacing)
|
|
(3.4Å)
|
Melting Pt. (Atmosphere)
|
Sublimes @ 3650°C
|
Purity
|
Total
|
0.01% Maximum
|
| Total Metallic |
10 ppm |
| Outgassing |
Negligible |
PG Coatings
PG is a popular coating for sealing machined graphite hardware which does not require oxidation protection. It is the most anisotropic commercially available coating and is therefore useful where thermal management is important. PG coatings can be applied to PBN crucibles and plates as in Momentive's patented "Hot-Lip" MBE crucibles and PBN/PG heaters.
Performance Features
- Chemically Inert
- High Purity
- Stable to 3000°C
- Impermeable
- Directional Electrical & Thermal Characteristics
- Self-Lubricating
- Nondusting
- Low Etch Rate
A Unique Combination
PG, a very refractory electrical and thermal conductor, can be combined with PBN, a superb dielectric, to form heating elements for high temperature UHV applications. These nonbrittle refractory materials owe their exceptional purity to the high temperature vapor deposition production method. The high thermal conductivity and anisotropy of both materials result in excellent thermal leveling characteristics.