282 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



While the deposition of pyrolytic carbon fibns is not a surface reaction in 

 the usual sense, the nature of the substrate surface can profoundly affect 

 the reaction through its catalytic influence. For a ceramic surface contami- 

 nated with iron or other heavy metals or their oxides this influence is evi- 

 denced by the production of soft, sooty, easily removed deposits which can 

 be formed at temperatures considerably below those normally required. 

 There is evidence that these loosely adherent films may result through the 

 formation of the metal carbides as intermediates. 



A great variety of catalytic influences on the deposition of pyrolytic carbon 

 films has been observed: For instance, fingerprints are very clearly ''devel- 

 oped" by deposition of thin fihns, the salts in them appearing to inhibit 

 carbon deposition. If there is back diffusion of gases from the coating zone 

 into the preheating zone and end chambers of a continuous furnace, then 

 several phenomena may be observed: Colloidally dispersed complex hydro- 

 carbons may deposit on the cooler ceramic rod surfaces from the gas phase 

 or they may be mechanically transferred to the rods by contact with already 

 contaminated portions of the furnace mechanism. In either event, their dis- 

 tribution is nonuniform and the contaminated areas provide catalytic nuclei 

 which accelerate carbon deposition in their immediate vicinities, resulting 

 in a pyrolytic film with locally thicker areas. On the other hand, if these 

 complex materials come into contact with certain metallic portions of the 

 mechanism, complex organo-metallic compounds are occasionally formed, 

 and transfer of these to the rod surface generally results in a local inhibition 

 of deposition and hence in films with locally thin areas. 



For the production of uniform films of pyrolytic carbon it is generally 

 necessary to employ a substrate which is uniformly clean. Chemical methods 

 of cleaning contaminated surfaces have not proved generally feasible, and 

 to achieve the requisite cleanliness firing of the ceramics at high tempera- 

 tures in air is usually required. Even this may not be adequate, however, 

 and it is occasionally necessary to reject ceramics with badly contaminated 

 surfaces. 



Since the production of pyrolytic carbon involves the synthesis of progres- 

 sively more complex hydrocarbons, it is natural to expect that the nature 

 of the hydrocarbon employed would be of considerable significance. As 

 discussed in a later section, pyrolytic carbon is graphitic in nature and thus 

 can be considered as originating from aromatic hydrocarbons which possess 

 similar hexagonal carbon ring structures. Isolation of benzene, napthalene, 

 anthracene and other more complex aromatic compounds from the pyrolysis 

 of methane is evidence that the aromatization of methane is probably an 

 intermediate step in the production of carbon. It is therefore to be expected 

 that the use of benzene should increase the rate of carbon deposition and 

 this increase is observed. Similarly, the use of toluene or xylene, leading to 



