PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 



283 



the more rapid formation of aromatic radicals, should, as is observed, pro- 

 vide even more rapid deposition than does the use of benzene. 



Rapid generation of free radicals, whether by catalytic surface reactions 

 or through use of easily "ionized" hydrocarbons, is necessary for rapid dep- 

 osition of pyrolytic carbon films. However; an excessive rate of generation, 

 as from large concentrations of acetylene, leads to so rapid a gas phase 

 polymerization that coherent surface films can be formed only with difficulty, 

 the principal product being an "aerosol" of soot. Methane is employed in 

 most instances because, being the most thermally stable hydrocarbon, the 

 deposition from it can be so controlled as to yield thin and coherent films 



INTERPLANAR 

 DISTANCE 



Fig. 6 — Structure of the most abundant form of graphite. 



4. The Structure of Pyrolytic Carbon Films 



X-ray and electron diffraction analysis of pyrolytic carbon has shown 

 clearly that its fundamental structure is similar to that of graphite, although 

 it differs in two respects: The lattice constants are not quite the same, and 

 the structure possesses a greater randomness, in a sense which will presently 

 be specified. 



The hexagonal structure of the most abundant form of graphite" is shown 

 in Fig. 6. The carbon atoms are arranged in parallel plane sheets, being 

 located at vertices of hexagons in these sheets. The interatom separation in 

 the sheets is 1.415 A and the separation between neighboring sheets is 

 3.345 A. Alternate sheets of atoms are so displaced that the repeating dis- 

 tance perpendicular to the layers, or along the c-axis of the crystal, is twice 

 the interplanar spacing, or 6.690 A. Other relatively rare forms of graphite 



