PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 



295 



the surfaces of the fibns originally contiguous to the bases had been de- 

 formed largely in conformity with them according to the differential con- 

 tractions during cooling. The thermal expansion coefficient of the pyrolytic 

 carbon films was thus determined from measurements of their radii of 

 curvature and of those of the bases from which they had stripped. This coef- 

 ficient might be expected to depend on the nature of the intercrystal boun- 

 daries. 



The anisotropy in the temperature coefficient of resistance of the con- 

 stituent crystals of graphite might be expected to exist also in pyrolytic 

 carbon, giving a dependence of a on the orientation of the crystallites. This 

 dependence has not been observed, however, and the failure to observe it 



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lij 

 ujq: 



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ujO 



UJ< 

 l-CL 



-50 -40 -30 -20 -10 10 20 30 40 50 60 



TEMPERATURE IN DEGREES CENTIGRADE 



Fig. 15 — Dependence of the temperature coefl&cient of resistance of a typical pyrolytic 

 carbon film on temperature. 



may be due to the primary influence of the intercrystal boundaries in con- 

 junction with the usual fluctuations in the value of a for a given set of coating 

 conditions. 



5.6 Summary 



Pyrolytic carbon, graphitic in structure as are most black carbons, has 

 physical properties which can be correlated in part with the size and prop- 

 erties of its constituent crystals and the way in which these crystals are 

 arranged. While the lattice of these minute crystals differs in certain respects 

 from that of graphite, it is probable that the metaUic character of the layer 

 planes is retained, that the anisotropy in properties of the crystal packets is 

 somewhat greater than for graphite, and, hence, that conclusions as to the 

 properties of pyrolytic carbon based on this anisotropy are valid to good 



