PYROLYTIC FILM RESISTORS! CARBON AND BOROCARBON 313 



sistance. Thicker films of materials with higher specific resistances but with 

 the same resistance for a square might, therefore, have lower temperature 

 coefficients of resistance. 



\\ hile such considerations may be generally applicable to thin films, the 

 low temperature coeflftcients of thick borocarbon films are probably due to 

 another cause. It appears that boron decreases the temperature coefficient 

 of resistance of carbon films because it acts as a dehydrogenating or "graphi- 

 tizing" agent. As noted earlier, the crystal packets of which pyrolytic carbon 

 is composed are surrounded by complex hydrocarbons which have a pro- 

 found influence on the resistivity of the films. Boron is believed to act 

 primarily to decrease the thickness and total volume of these peripheral 

 layers, since it is known that the average diameter of the packets in boro- 

 carbon films is at least twice as great as in carbon films and that boron 

 strongly catalyzes the pyrolysis of hydrocarbons. 



Although the principal result of adding boron to the carbon fihns is thus 

 believed to be a type of graphitization, it is possible that its effect may be 

 due to other causes: Boron may act to produce "cross links" between the 

 atom layers in adjacent packets; or, in view of its favorable atomic diameter, 

 boron may enter substitutionally into the graphite lattice itself. 



It will be recalled that the resistance stability of the pyrolytic carbon 

 resistor is largely dependent on the characteristics of the boundaries between 

 crystal packets, and that adsorption of contaminants at these boundaries 

 produces changes in resistance. With the relatively decreased importance 

 of interpacket boundaries in borocarbon fihns it is reasonable to expect that 

 the films might also be more stable with time. While data as extensive as 

 those for pyrolytic carbon films are not yet available, it now appears that 

 this expectation is fuUy confirmed. 



Borocarbon films thus make possible the production of film type resistors 

 of improved stability with temperature coefficients of resistance as low as 

 and in many cases lower than can be obtained in the wire-wound type. 

 Further, borocarbon films make accessible the very high resistance ranges 

 hitherto inaccessible to stable film t)^e resistors. 



Acknowledgment 



It is a pleasure to acknowledge the assistance of many of our colleagues 

 in the Bell Telephone Laboratories. Messrs. K. H. Storks, and A. H. White 

 performed most of the x-ray diffraction and electron diffraction analyses. 

 Mr. M. D. Rigterink developed ceramic materials and processing techniques. 

 Mr. G. K. Teal made the first study of borocarbon fihns in these Labora- 

 tories and has been of continued help in their development. For preparation 

 of the films and for the many detailed measurements of fihn and resistor 

 characteristics we are indebted to Miss S. E. Koonce and to Messrs. R. F. 



