PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 299 



or over- vitrified porcelain, many steatites, and fused silica provide rela- 

 tively inferior adherence, which can, however, be improved by chemical 

 etching or by thermal oxidation of a previously deposited carbon film. 



Even though free from mechanical imperfections resulting from corre- 

 sponding faults in the substrate surface, the carbon film may still exhibit 

 local variations in thickness as a result of catalytic influences, as described 

 earlier. The increase in resistance of a uniformly coated core or blank due 

 to cutting a heUcal groove through the fihn can accurately be calculated 

 when the helix angle and groove width are known, but if there are variations 

 in fihn thickness, then the observed helixing increase is greater than that 

 calculated, because the high resistance areas become series elements in the 

 helix. Aside from the fact that such a variation would present production 

 problems, an increase in temperature coefficient of resistance also results 

 since, as shown in Fig. 14, this coefficient is larger the thinner is the film. 

 This increase is particularly undesirable if resistors with closely reproduci- 

 ble temperature coefficients are required. 



The core of a pyrolytic carbon resistor must, obviously, be a good in- 

 sulator, particularly where very high values of resistance are obtained by 

 helixing; and when extreme stabiUty of resistors under severe operating 

 conditions is required, great care is necessary in the choice of the substrate 

 material. Thus, the usual wet process electrical porcelain, when properly 

 compounded from purified raw materials, can be made into resistor cores 

 with very high surface perfection and good adherence for carbon films. 

 However, it cannot be employed in resistor cores because at elevated oper- 

 ating temperatures the mobihties of the alkafi ions in the glass matrix of 

 this material are too great. The result of this mobility is that, under the 

 influence of the fields between successive turns of a helixed resistor, electro- 

 chemical migration sufficient to alter the shunting resistance between turns 

 occurs even with the resistive element sealed in a thoroughly dry and evac- 

 uated enclosure. 



To obviate these electrochemical effects, which are quaUtatively correlated 

 with the analytically determined alkali concentrations, new alkali-free 

 ceramic materials^*^ have been developed for use in fabrication of cores for 

 pyrolytic carbon resistors. These materials are essentially porcelains in 

 which all but small residual traces of sodium and potassium have been re- 

 placed with alkaline earths such as magnesium, calcium, barium, and stron- 

 tium. The ionic radii of these alkaUne earth metals are sufficiently larger 

 than those of the alkaU metals that field migration is largely prevented. 

 These alkahne earth porcelains show no evidence of electrochemical polari- 

 zation when employed as resistor cores; and they have, in addition, high 

 specific resistances and relatively low dielectric losses. 



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