274 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



thickness. The resistors can be produced in values ranging from a fraction of 

 an ohm to tens of megohms, and, in certain versions, they exhibit exception- 

 ally high order of stability.^ Because of the relatively high specific resistance 

 of carbon, the "skin depth" at high frequencies exceeds that of the thickest 

 films employed, so that there is no increase in resistance due to skin effect. 

 This, coupled with the fact that film configurations of inherently low react- 

 ance can be employed, permits advantageous use of these resistors at very 

 high frequencies. Further, when suitably protected from oxidation, pyrolytic 

 carbon films can dissipate very large amounts of power per unit area 

 without permanent change, which is of particular advantage for certain 

 high frequency uses.^ 



Despite their otherwise favorable characteristics and growing acceptance, 

 pyrolytic carbon resistors have been inferior to wire-wound varieties be- 

 cause of the relatively large magnitudes of their temperature coefficients of 

 resistance. This relatively large absolute magnitude of the temperature co- 

 efficient of resistance, which is always negative, has served as a deterrent to 

 the use of pyrolytic carbon resistors where their characteristics would be 

 otherwise suitable. It has now been found, however, that the incorporation 

 of a few percent of boron in pyrolytic carbon films reduces the temperature 

 coefficients of resistance to values smaller than are available, on the aver- 

 age, in wire-wound types. Further, this improvement is accompanied by an 

 increased time stability of resistance value. As a result of these improvements 

 it now appears that the borocarbon resistor, which will undoubtedly find 

 widespread use, possesses the stability of resistance value of the wire-wound 

 type for most applications and surpasses it for some. 



2. The Production of Pyrolytic Carbon Films 



2.1 The Technique of Pyrolysis 



Pyrolytic carbon films are produced over the surfaces of suitably refrac- 

 tory and chemically stable objects inserted into a heated enclosure in the 

 presence of a hydrocarbon gas or vapor. These films, which result from the 

 pyrolysis or "cracking" or thermal decomposition of the hydrocarbons, de- 

 pend in their nature and properties both on the pyrolyzing conditions and 

 on the characteristics of the supporting surface. The pyrolysis may be car- 

 ried out by maintaining a suitable vapor pressure of the hydrocarbon in an 

 otherwise evacuated furnace or by employing a suitable carrier gas to dilute 

 the hydrocarbon and to transport it through the furnace at atmospheric 

 pressure. Both techniques have been studied, and no observable differences 

 in film properties were found; but, because of their greater simplicity, carrier- 

 gas systems operatmg at atmospheric pressure were employed in the work to 

 be described. The principal reasons for this choice are that continuous coat- 



