PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 281 



the object being coated, and particularly so if this distance is less than the 

 thickness of the conduction zone. 



While the rates of deposition associated with the film resistances of Fig. 

 4 and Fig. 5 are related to the corresponding absolute rates of hydrocarbon 

 pyrolysis in the furnace atmosphere, they cannot be identified with them 

 because of the important influence of the conduction zone, and because the 

 rate of flow of the gases through the furnace is not specified. In fact, the 

 rates of carbon deposition in individual furnaces are virtually independent 

 of the rates of flow of the furnace atmospheres over wide limits, but they 

 may differ appreciably from one furnace to the next. The viscosity in the 

 conduction zone is so great that the rotation of an object in the furnace can 

 be seen to drag with it the immediately surrounding gas; and it appears 

 unlikely that this viscous gas layer is greatly altered in its thickness or other 

 properties by any reasonable change in flow conditions of the furnace at- 

 mosphere. 



The rates of carbon deposition were determined by weighing ceramic 

 blanks before and after deposition of carbon films and are expressed in terms 

 of weight deposited per unit area and unit time. This procedure is best 

 suited to the thicker fikns, and for thin films or low rates of deposition large 

 errors may obtain. For this reason, it has proved desirable to determine the 

 rate of deposition from the film resistance, for which is required knowledge 

 of the relationship between the fihn resistance and its thickness, and between 

 its thickness and its mass, which involves a knowledge of the density of the 

 carbon fihn. The determination of the density is discussed in a later section. 



3. The Mechanism by Which Pyrolytic Carbon is Produced 



It seems reasonably weU estabhshed that the mechanism by which pyro- 

 lytic carbon is produced is not simply a surface reaction, but is related to 

 that of the gas phase dehydrogenation and polymerization of hydrocarbons. 

 Thus, in the case of methane, the simplest hydrocarbon, it is found that, 

 among others, free radicals such as methyl and methylene are present in 

 the gas phase. These combine or polymerize and the resultant products 

 lose hydrogen to yield radicals and molecules of increasing size and com- 

 plexity. Analysis of the furnace gases from the pyrolysis of methane has 

 shown the presence of acetylene, ethane, ethylene, benzene, napthalene, 

 anthracene and a long series of more C9mplex materials of decreasing hydro- 

 gen content up to pure "carbon" soot itself. It thus appears that pyrolysis 

 of a gaseous hydrocarbon involves the formation of an entire series of molec- 

 ular species of progressively decreasing hydrogen contents, which are inter- 

 mediates in the formation of carbon. Chemical, structural, and physical 

 tests are, ui fact, incapable of distinguishing between some of the higher 

 members of this series and pyrolytic carbon.^^ 



