PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 275 



ing systems can be employed and that accurate and readily adjustable con- 

 trol of the composition of the furnace atmosphere can be maintained. 



The furnace employed in producing pyrolytic carbon films over the surface 

 of cyUndrical ceramic rods may, for example, be either of the batch type,®- ^ 

 or the continuous type,^- ^ through which the rods are passed in sequence. 

 In each case, suitable precautions are necessary in order that tightly ad- 

 herent, clean films be produced on the rod surfaces. 



Figure 1 shows a furnace of the batch type suitable for small scale pro- 

 duction. In the batch process, a rotating or oscillating gas-tight refractory 

 core is employed as the pyrolyzing chamber. Into this core is loaded a quan- 

 tity of cylindrical ceramic rods, or ceramics of other shapes, together with 

 a quantity of silica sand or other granular material which serves to support 

 the rods and to prevent damage to them or to the core as they are tumbled. 

 This "floating" material also has an influence on the quality of the film, and 

 on the rate of carbon deposition. In a static process, where the rods main- 

 tain fixed positions in the furnace during coating, it would be virtually im- 

 possible to achieve the requisite uniformity in coating conditions. This is, 

 in part, due to the fact that the composition of the atmosphere is dependent 

 on the length of time it remains in the furnace, or, in other words, on its 

 velocity and the actual path it transverses through the coating chamber. 

 Rotation or oscillation of the core with the resultant random tumbling of 

 the rods ensures that each of these is, on a statistical basis, exposed to the 

 same deposition conditions as any other. 



With the furnace core loaded with ceramic rods and sand, it is brought 

 to the desired temperature — usually from about 975 deg C to 1300 deg C — 

 while being flushed with an inert gas such as nitrogen before admission of 

 the hydrocarbon. When temperature equilibrium is attained, the coating 

 gas is admitted and permitted to flow for the requisite time. The hydro- 

 carbon supply is then shut off and the furnace core cooled while a flow of 

 oxygen-free nitrogen is maintained. It has been found that, if this procedure 

 is followed, clean, uniform and strongly adherent carbon fihns can be pro- 

 duced successfully on a commercial basis. 



In Fig. 2 is illustrated a typical continuous furnace,^ which consists of 

 three zones through which the ceramic rods to be coated are passed auto- 

 matically in sequence. The first of these has been termed a "preheating" 

 zone and its purpose is to raise the rods to a temperature at least as high 

 as that of the next following "coating" zone before they come into contact 

 with the coating gases. Through this preheating zone there is maintained a 

 flow of an inert gas, such as nitrogen, moving in the same direction as the 

 rods and emptymg into the coating zone. At the junction of the preheating 

 and coating zones the coating gas is admitted at high velocity through 

 smaU jets so disposed as to facilitate thorough mixing in the furnace atmos- 



