PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 



287 



5. The Physical Properties of Pyrolytic Carbon Films 



The physical and chemical properties of graphite are different in its basal 

 plane and along its c-axis.^^ As one common example, it is the ease with 

 which shear occurs perpendicular to the c-axis which is fundamental to its 

 value as a lubricant, even though in the base plane its hardness is great 

 enough to warrant, in principle, its use as an abrasive. This pronounced 

 anisotropy extends to other properties of graphite as well; and, since the 

 crystals of pyrolytic carbon are very probably even more anisotropic because 

 of the structural differences noted above, it is reasonable to expect that the 

 properties of pyrolytic carbon will depend on the relative orientations of 

 its constituent crystals. Though in some instances this expectation is con- 



10"® CM 



3xtO"^CM 



THE APPROXIMATE SIZE 

 OF A SINGLE CRYSTALLITE, 

 SHOWING THE ORIt.NTATION 

 OF CRYSTAL AXES 



RANDOM ORIENTATION 



PREFERENTIAL ORIEN- 

 TATION WITH BASE 

 PLANE PARALLEL TO 

 SURFACE 



Fig. 10— Crystallite size and orientation in pyrolytic carbon. 



firmed, as measurements to be described show, the influence of the inter- 

 crystal boundaries is always present and, in many cases, it is controlling. 



5.1 Density 



The density of pyrolytic carbon fihns composed of crystals about 25 A 

 in diameter was determined, by flotation in a mixture of bromoform and 

 carbon tetrachloride, to be 2.07 d= 0.04 gm^ cm-^ Since the interplanar 

 spacing in the crystal packets averages 3.59 A and the interatom distance 

 within the base plane is 1.40 A, the computed density, accepting that of 

 graphite as 2.26, is 2.15 d= 0.04 gm cm"'. The value 2.07 gm cm-^ was 

 employed in determinations of the thicknesses and specific resistances of all 

 carbon fihns, since there was no observable systematic variation of density 

 with change in the conditions under which the carbon was prepared, despite 



