Electron spin resonance in turbocrystalline carbons—II

Abstract

In continuation of the work on ESR in turbostratic carbons, experiments on introduction of boron acceptors and sodium donors were performed using P33 carbon black samples heat treated to various temperatures mainly in the range 1600–2400°C. The experiments with boronation show a complicated behavior and lead to the conclusion that boron atoms can act not only as acceptors, but neutralize some of the existing localized spin centers and also can under special conditions become spin centers themselves. Experiments with sodium doping have shown that the stability of the sodium against oxidation increases with decrease in HTT. With increasing doping the “Curie-like” behavior (the factor R = I LN I RT ) goes through a maximum and the intensity of the ESR line through a minimum, as expected from the simple band model with the Fermi level originally located below, and moving across and then above the band overlap with increase in doping. Using two spectrometers 9.2 Gc and 35 Gc and a variety of samples it was found that while for low heat treatments the widths are the same (in Gauss), for well graphitized materials the use of the higher frequency results in a considerably broader line. Considerations concerning the temperature and heat treatment dependence of the Δg cond and its relation to the diamagnetic susceptibility are presented. The nature of the band structure in turbostratic carbons and of the localized spin centers are discussed.