Multiphase carbon and its properties in complex mixtures Page: 12 of 19
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van Thiel, paper No. 6.30
molecules allowed in the mixture always dominate at these high pressures.
Even smaller molecules like benzene appear to form extended diamond
networks with formation of hydrogen. Dissociation is not important in
shock compression of hydrocarbons to the conditions in the table. The
carbon atoms simply crosslink with formation of interstitial hydrogen.
The same process may occur in polyacrylpnitrile which has not been
studied at such high pressures and densities. Some foams have been
studied. These foams shock compress to high T low p states in which
molecules with carbon chains of less than 2 carbon atoms dominate. In
Figure 5 we show computed and experimental (Marsh 1980) shock states of
polystyrene foam with an initial sample density of .055 g/cc. The original
uncompressed samples contained air at the ambient pressure and perhaps one
monolayer of adsorbed air. Adding this amount of air to the hydrocarbon
mixture makes a negligible difference in the calculated compression
compared to the deviation between theory and experiment. The principal
difference between the calculation and the experiment, are the degree of
mixing between the gas in the voids and the carbon rich polymer products.
In the theory the mixture is uniform. The experiment if not mixed would
also have very hot compressed air pockets that complicate the
interpretation of the data.
4.0
3,- CHEQ
+ LASL o
3.0 +
+
m 2.5 4-
0L + +Figure 5. Hugoniot of polystyrene: (0) CHEQ
C .+ calculation; (+) Experiment, (Marsh 1980).
2.0
0
1.5
4- +
1.0 0
4 +
4 6 8 10 12
P / Po
Removing the air by shock compression in vacuum, indeed achieves higher
compressions. A (CH2)n polymer with a much reduced void size and initial.
density of .04 g/cc shock-compresses to p/p0 - 7.5 at 2 GPa (Holmes 1990)
while the theory still indicates a 10 fold compression. The smallest
deviation between theory and experiment was observed with a sample of
polyacrylonitrile (C3H3N) with a higher initial density of
about .08 g/cc. Here experiment and theory achieved compressions of 7.3
and 8.3, respectively (Holmes, Yoo and Ree 1990). This seems to show that
shock compressed foams can achieve reasonably uniform conditions if
entrained hot air is kept to a minimum. The compressional heating is still
concentrated in the material on the walls of the voids. But compression
under vacuum replaces that with carbon-rich material. The relatively high.
thermal conductivity and heat of formation of the carbon products aids the
equilibration rate of the hot spots.page -10-
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van Thiel, M. & Ree, F. H. Multiphase carbon and its properties in complex mixtures, article, September 1, 1990; California. (https://digital.library.unt.edu/ark:/67531/metadc1212167/m1/12/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.