handbook of carbon, graphite, diamond and fullerenes

98 Carbon, Graphite, Diamond, and Fullerenes
3.2 Density and Porosity
The real density of molded graphite, that is the density exclusive of
open porosity (measured with a pycnometer) but including closed-internal
porosity, is 2.20 - 2.23 g/cm 3 . This value is near the theoretical density of
the graphite crystal (2.25 g/cm 3 ), indicating that the amount of internal
porosity is small and the structure of the material is well ordered.
The apparent density of molded graphite, that is the density inclusive
of porosity, varies considerably, depending on the process, the impregnation
cycle and other factors. Values generally quoted in the industry are 1.4
- 2.0 g/cm 3 , corresponding to an open porosity of 10 - 35%. A common
method of measuring apparent density is the Avogadro (or water-weight)
method. To avoid overstating the result, air bubbles must be eliminated and
the part must be perfectly dry before testing.
Typical pore-size distribution of several molded graphite materials is
shown in Fig. 5.5. I12]
9
7
-
-
Micro Grain
6 -
5
4
-
-
Fine Grain
3 -
Medium Grain
2 -
1m.,
A
12 3 4 30 100
Pore Size |xm
Figure 5.5. Pore-size distribution of various grades of molded graphite.' 12 '