PYROLYTIC FILI* RESISTORS: CARBON AND BOROCARBON 



293 



the role of intercrystal boundaries in determining the specific resistance of 

 mesomorphic carbon. 



This influence is also evident from the dependence of the resistance on 

 ambient gas pressure. After a carbon specimen is heated in vacuo to about 

 500 deg C in order to remove adsorbed gases, it is found on subsequent 

 exposure to air that the resistance exhibits a gradual, relatively small in- 

 crease with time. If, at any later time it is reheated in vacuo, the resistance 

 returns to its original value. These changes in resistance are completely 

 reversible and are presumably associated with the adsorption of atmospheric 

 constituents at intercrystal boundaries. 



The influence of the intercrystal boundaries is further illustrated by the 

 permanent decrease in film resistance by as much as 20 per cent, accompa- 



0.0040 



§ 0.0035 



g 0.0030 



0.0025 



0.0020 





0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 

 DEGREE OF PREFERRED ORIENTATION 



Fig. 13 — Dependence of the specific resistivity of pyrolytic carbon films on the degree 

 to which the crystallites are oriented parallel to the direction of flow of the measuring cur- 

 rent. Scale numeral "0" represents random orientation, while "10" represents perfect 

 orientation. 



nied by a decrease in the temperature coefficient of resistance, which results 

 from heating a pyrolytic carbon film in vacuo or in a neutral atmosphere to 

 a temperature appreciably in excess of that at which it was deposited. These 

 changes are presumably due to partial dehydrogenation at the boundaries 

 with a partial intergrowth of adjoining crystals, an effect which has been 

 confirmed by X-ray and electron diffraction examination. 



5.5 The Temperature Coefficient of Resistance 



The temperature coefficient of resistance, a, for carbon films deposited 

 on a suitable base depends on the thickness of the film, on the temperature 

 of the fihn, and on the coefficient of thermal expansion of the base. As 



Fig. 14 shows, the value of a, defined as — dR/dT, where R is the resistance 



K. 



and T the temperature, decreases in magnitude with increasing film thickness 



