278 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



zone, prevents entry of the coating gases into the preheating zone either by 

 flow or by diffusion. 



From the coating zone, the coated rods enter a ''coohng" or ''after-heat- 

 ing" zone, during passage through which they are brought very nearly to 

 room temperature. Through this zone, in a direction countercurrent to that 

 of the rods, there is maintained a flow of oxygen-free nitrogen, or of this 

 with small additions of hydrogen, since it has been found virtually impossible 

 adequately to deoxidize commercial nitrogen except by the addition of 

 a small amount of hydrogen and passage over a catalyst such as palladinized 

 alumina prior to thorough drying and admission to the cooling zone. To 

 prevent entry of coating-zone gases into the coohng zone, the counterflow of 

 gas through this zone is maintained at a higher linear velocity. All gases 

 admitted to the furnace are exhausted at the junction between the coating 

 and cooling zones. To produce circumferentially uniform films on the rods 

 they are rotated about their axes as they advance through the furnace. 



At reasonably high hydrocarbon concentrations in the atmosphere, an 

 opaque fog is formed over most of the coating zone cross-section in both the 

 batch and continuous furnaces. Immediately surrounding the rod surfaces 

 or other surfaces on which deposition takes place, however, there is a fog- 

 free region, called the conduction zone, which is the zone in which the trans- 

 fer of heat from the surfaces to the gas occurs by conduction rather than by 

 convection. The well-defined outer edge of this conduction zone is thus con- 

 sidered to be the boundary of the region of generally streamline flow in the 

 body of the furnace atmosphere, with the conduction zone, contiguous to the 

 hot surfaces, being a more viscous, stationary region in which diffusion proc- 

 esses are operative. The fog consists of minute particles of sooty and tarry 

 substances which do not penetrate the conduction zone appreciably and 

 which, therefore, do not deposit appreciably on the rod surfaces. The cause 

 of this behavior is that the surface temperature of the rod exceeds that of 

 the body of the gas; and, under the influence of this temperature gradient 

 and the associated viscosity gradient, the heavier particles of soot and tar 

 tend to diffuse away from the surface. If this temperature gradient is re- 

 versed, as by the introduction of a cool rod into a hotter gas, then there is 

 produced on its surface a soft and easily removed sooty coating. It is for 

 this reason that, in the continuous process, the ceramic rods are preheated 

 before entry into the coating zone and that the carbon-coated rods are pro- 

 tected in the cooUng zone from contact with furnace effluents of higher 

 temperature. 



2.2 Process Variables Controlling the Rate of Carbon Deposition 



The thickness of pyrolytic carbon films is dependent not only on the na- 

 ture of the hydrocarbon employed, but also on its concentration in the 



