PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 291 



conductivity of poly crystalline Acheson graphite was found to be 0.4 watt 

 cm-^ deg C~\ in agreement with published values^'*. 



The thermal conductivity of films of mesomorphic carbon is thus much 

 smaller than those for single crystal or grossly polycrystalline graphite, and 

 this is probably due to the definitive influence of intercrystal boundaries. 

 As noted above, the crystals of mesomorphic carbon are more anisotropic 

 than are those of graphite; but, despite this, the effect of the crystal boun- 

 daries is sufficient to suppress any influence of crystal orientation on the 

 thermal conductivity. 



5.4 The Specific Resistance 



For determination of the specific resistance of pyrolytic carbon, silver 

 electrodes were applied to the ends of films on rods or plates of fused silica 

 and measurements were made at currents so small that there was no de- 

 tectable joule heating. Comparative measurements made with and without 

 potential probes showed no detectable contact resistance between these 

 electrodes and the carbon film. Furthermore, the potential drop was linear 

 along the specimens, thus indicating their uniform thicknesses. 



Within the limits of experimental accuracy, the specific resistance of 

 pyrolytic carbon films is independent of film thickness: From the meas- 

 urements of the weights and film conductances of carbon films discussed in 

 Section 2.2, and using the value 2.07 gm cm~^ for the density, the data of 

 Fig. 12 relating the film resistance to its thickness were obtained. Over the 

 measured range from about 2.5 X 10"^ cm to about 2.5 X 10"^ cm thick- 

 ness there is no change in resistivity and a strictly linear dependence of 

 film resistance on thickness is obtained. While direct measurements of the 

 thickness of still thinner films have not been made, the actual thicknesses 

 have been approximated on the basis of the relationship between film thick- 

 ness and duration of deposition given by Fig. 3. On this basis, films having 

 resistances in excess of 2 X 10^ ohmfe for a square lie on an extrapolation of 

 the curve of Fig. 12. These films are but a few Angstroms in calculated 

 average thickness. 



The specific resistance does, however, depend on the conditions under 

 which the carbon is prepared, and it decreases with increase in the degree 

 of preferential crystal orientation for films greater than 3 X 10~^ cm in 

 thickness. As a probable result of the influence of crystal orientation, the 

 specific resistance of the carbon films measured parallel to the film surface 

 passes through a minimum value at 1025 deg C as a function of increasing 

 furnace temperature at constant methane concentration, while, with in- 

 creasing methane concentration at a constant pyrolyzing temperature, it 

 decreases monotonically. 



