PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 273 



transmitter, are still difficult to control and reproduce. The film resistance 

 depends principally on the contact areas and on the elastic properties of the 

 contact assemblage. These areas, in turn, depend on the shrinkage of the 

 matrix during curing; on differences in the thermal expansion coefficients of 

 the matrix, the particles, and the base; on cold flow of the matrix after cur- 

 ing; on its swelling through Hquid absorption; and on other factors, all of 

 which affect the stability and temperature coefficient of resistance. 



In view of the complex nature of such composite conductors it is a note- 

 worthy achievement to have attained the reproducibility and general re- 

 liability characterizing present-day carbon composition resistors of the film 

 type. Nevertheless, these resistors suffer serious shortcomings, inherent in 

 their structures, among which are numerically large and variable tempera- 

 ture coefficients of resistance, lack of availabihty to close and constant toler- 

 ances, relatively low power handling capacity, and poor stabiUty with time, 

 temperature and humidity. In addition, such resistors exhibit noise, many 

 times greater than thermal noise, which is characteristic of contact assem- 

 blages; and they also frequently exhibit appreciable voltage coefficients of 

 resistance. 



In order to provide a carbon film free from most of these undesirable 

 properties, the production of homogeneous carbon films by the pyrolysis of 

 hydrocarbon vapors has been widely studied,^ and it is with these that the 

 present work is concerned. The production of carbon films by the thermal 

 decomposition of hydrocarbon materials, even though unintentional, was 

 probably effected in antiquity; and, before the present century, the use of 

 such pyrolytic carbon films as resistors was suggested. It is only in roughly 

 the past two decades, however, that pyrolytic carbon resistors have been 

 commercially available, principally in Europe; but their production has been 

 based largely on "art" without quantitative information concerning the films 

 themselves. The purpose of this paper is to present more specific information 

 on the structures and properties of these films and to describe the bearing 

 of this information on the production and properties of pyrolytic carbon 

 resistors of improved stabiHty and enlarged fields of application. A particular 

 object of the present paper is to describe the recent development of the boro- 

 carbon resistor wherein modification of a pyrolytic carbon film by the incor- 

 poration of boron permits the production of non-metallic resistors which are 

 equivalent or superior to the wire-wound type. 



Pyrolytic carbon resistors^- ^' ^ comprise homogeneous carbon films, of 

 specific resistance appreciably greater than for metallic resistance alloys, 

 which are produced or deposited on suitable refractory bases in continuous 

 films of thicknesses controllable over a wide range. The specific resistance of 

 the carbon even in the thinnest films is essentially the same as in bulk, so 

 that purely geometrical or mechanical factors determine the minimum usable 



