294 



tHE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



and approaches a limiting value of about — 1.8- 10~^ deg C~^ which is found 

 to be independent of the nature of the base. This value is, therefore, charac- 

 teristic of the carbon fikn itself. The data illustrated in Fig. 14 were obtained 

 over the temperature interval of 30 deg C to 60 deg C. Figure 15 shows the 

 relationship between a and temperature for a typical film, the slope of 

 which is common for all films deposited on the same base. As the coefficient 

 of thermal expansion of the base increases, the value of a for any given film 

 thickness less than 3 X 10~^ cm also increases, which serves to emphasize 



10,000 

 6000 

 6000 



1000 

 800 

 600 



400 



200 



100 

 80 

 60 



40 



20 



10 



- 0.005 



0.002 X 10-* 



0.01 



0.05 



- 0.1 ^ 



- 0.5 



1.0 



-175 -200 -225 -250 -275 -300 -325 -350 -375 -400 -425 -450 -475 



TEMPERATURE COEFFICIENT OF RESISTANCE IN PARTS PER MILLION 



PER DEGREE CENTIGRADE 



Fig. 14 — Dependence of the temperature coefficient of resistance of pyrolytic carbon 

 films on film thickness. Thickness expressed in terms of film resistance. 



the role of the intercrystal boundaries in determining the properties of 

 pyrolytic carbon, in suggesting that the resistances of these boundaries are 

 dependent on pressure. 



The thermal coefficient of expansion for graphite crystals along the c-axis^ 

 is 26 X 10-« deg C-^ and parallel to the base plane^^ is 6.6 X 10-« deg C-^; 

 that for films of pyrolytic carbon was estimated to be of the order of this 

 latter value. Carbon films which had stripped spontaneously from smooth 

 cylindrical fused silica bases were found to curl away from them, the radii 

 of curvature increasing with film thickness in the manner to be expected if 



