Electron spin resonance in turbocrystalline carbons—I

Abstract

In continuation of the work on ESR in polycrystalline graphites, experiments on temperature dependence and on neutron irradiation of the ESR for two carbon blacks, P33 and Thermax, were performed in the range of heat treatments from 1400° to 3000°C. Intensity, g-value, and line width data are reported. The results show that throughout the whole range the ESR absorption can be analyzed in terms of two components, localized spins and carrier absorption, producing a single ESR line as a result of exchange mixing of the g-values. The presence of the intensity minimum, observed by castle and wobschall, is confirmed and explained as being the result of a variation in band overlap. Neutron irradiation leads to an increase in the ESR intensity, mainly through an increase in the concentration of localized spins. The stability of the so-created localized spin centers against anneal is higher in the carbons of lower heat treatment (that is for lesser organized structures). It is shown that the dependence of the room temperature g-factor for the carriers (after correcting the measured values for the M-effect) on the heat-treatment temperature parallels closely the variation of the diamagnetic susceptibility and also agrees well with the dependence of the g-value on crystallite size L a found in previous work for a variety of carbon blacks (heat treated to 2900°C).