284 



THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



differ from this form only in the way successive planes are displaced or in 

 the repeating distance.^^ 



Pyrolytic carbon consists of minute crystal packets composed of parallel 

 plane sheets of carbon atoms in hexagonal arrays as in graphite.^^ The areas 

 of these planes are, however, very small, their diameters generally being 

 less than 50 A. Associated with their small size, there are differences in 

 lattice constants, the interatom distance within the planes being less than 

 in graphite and the interplanar spacing being greater. The extent of these 

 differences is dependent on the size of the crystal packet. The interplanar 



120 

 110 

 100 

 90 

 80 

 70 

 60 

 50 



Ui 



}C 40 



o 



t 



30 



10 



3.30 



3.55 3.60 



IN ANGSTROMS 



3.65 



3.35 3.40 3.45 3.50 



INTERPLANAR DISTANCE 



Fig. 7 — Dependence of the interplanar separation in crystallites of pyrolytic carbon 

 on crystallite size. 



separation as determined in the present work and by other investi- 

 gators^^ . 14, 15, 16 js given as a function of the packet size in Fig. 7. 



The average crystal packet size in pyrolytic carbon appears, for a given 

 parent hydrocarbon, to depend principally on the rate of carbon deposition 

 whether this rate is altered by change in pyrolyzing temperature or in 

 hydrocarbon concentration. When the rate of deposition is changed through 

 use of other hydrocarbons there appears also to be a correlation with packet 

 size. 



Pyrolytic carbon differs from graphite in another important respect: 

 Whereas in graphite the atom layers lie one above the other with the atoms 

 in successive layers in a definite relationship, those in pyrolytic carbon are 



