No abstract

No abstract

No abstract

The electrical resistivity of sheet mica, a ground coat porcelain enamel, two alkali-free ceramic coatings, a silicone varnish, and a coating consisting of a silicone varnish mixed with ground mica was measured in the temperature range 100 to 500 C and at an applied direct current potential of 200 v. A plot of the logarithm of the resistivity versus the reciprocal of the absolute temperature for each of these materials showed that marked deviations from the Rasch-Hinrichsen law occurred only with silicone and silicone-mica coatings. The electrical resistivity of all coatings tested was found to be independent of coating thickness and of duration of aging at 200 C. The measured resistivities in ohm-cm of the materials when tested at 400 C were (a) silicone-mica coating, 1400 × 108, (b) mica, 320 × 108, (c) silicone varnish coating, 280 × 108, (d) alkali-free ceramic coating, 1.6 × 108, and (e) ground coat enamel, 0.0025 × 108.

The resistivities of selected vitreous t ernary lead silicates containing ions of g roups I , II, III, IV, and a lso the ions chromium, mangan ese, iron, cobalt, and nickel were meas ured in the range 200 0 to 500 0 C and at an applied direct-current fi eld of 525 volts per centimete r. In addition, the resis tivities of vitreous silica and of quartz, wit h op t ic ax is parallel and perpendicular to t he applied fi eld, were m eas ured over t he same te mperatu re range. The resu lting data a re given as log resistivity-composi tion isotherms for the alkalies and alkaline earths and as log resisti vity-temperat ure curves for the other ions. Correlations between resistivity and bonding energy and between frequency factor and heat of activation are presented. The resistivit ies of the al kali lead silicates were interpreted from the stan dpoint of t he activation energy prin ciple.