PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 



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randomly stacked, the only crystallographic order along the c-axis being 

 the uniform separation of the layers. ^^•^'* 



The carbon atom has four valence bonds and, in graphite, these valences 

 are completely satisfied within the plane hexagonal network. There is no 

 valence bonding between successive atom layers, these being held together 

 only by relatively weak van der Waals forces. The valence bonding between 

 carbon atoms within any one plane is of the resonance-stabilized type, with 

 the result that there is effectively one electron from each atom left over. 

 Some such electrons are free to move over the entire extent of the atom 

 plane, and these provide metaUic conductivity. With the larger interatomic 

 spacing along the c-axis, many fewer electrons move from one plane to the 

 next and along the c-axis, accordingly, the conductivity of graphite is 

 much smaller. 



H H^^.^^H i^^y 



CxHy 



CxHy 



CxHy 



Fig. 8 — Two resonance forms of the valence structure in the carbon atom layer, showing 

 the free valences at the crystal periphery with possible bonding of hydrogen and a hydro- 

 carbon. 



Any single plane of carbon atoms in graphite may be considered to be a 

 single giant molecule. Examination of such a plane of carbon atoms will 

 show, however, as in Fig. 8, that it could better be considered as a free 

 radical since there are free valences at its periphery; and these valences are 

 quite probably satisfied by hydrogen or hydrocarbon fragments, as shown. 

 Since the number of free valences in a graphite crystal is small relative to 

 the total number of carbon atoms, the actual percentage of hydrogen is 

 very small. Nevertheless, each plane of carbon atoms may be considered 

 to be surrounded by a ''hydrocarbon skin." 



In pyrolytic carbon, the atom planes may likewise be considered to be 

 surrounded by hydrocarbon skins. However, with an average diameter for 

 these planes of approximately 25 A, the number of free valences is appreci- 

 able relative to the total number of carbon atoms, so that the hydrogen 

 content of pyrolytic carbon may be greater than that of graphite. This 

 hydrogen content is primarily dependent on the temperature at which the 



