Electron spin resonance in neutron irradiated and in doped polycrystalline graphite—Part II

Abstract

In continuation of the study of influence of neutron irradiation and of doping on the electron spin resonance in incompletely graphitized P33 carbon black (HTT 2600°C), variable amounts of boron were introduced by heating grains of solid P33 mixed with boric acid powder to a temperature 1600°–1800°C. With increasing boron content the ESR line shifts gradually from the undoped position g p towards the free electron value gƒ, the temperature dependence of the g-value reversing its direction at about g = gƒ + 0.7 (g p−gƒ) , all in rough agreement with observations of Wagoner on a single graphite crystal. The study of the intensity of the ESR and of its temperature dependence shows that in the initial stages of boron doping some boron atoms are used up to neutralize the localized spin centers originally present in the undoped P33. Interlayer compound doping of the boronated samples shows additivity of the effects caused by the combined presence of the two types of doping agents. However, the processes of driving out the doping agents are not the inverse of the processes of doping. The passage of the Fermi level back and forth across the band overlap is nicely observed for Na or Li doping as a minimum in ESR absorption intensity. Analysis of the results leads to interesting conclusions concerning the band structure of the investigated carbon black and the rate of formation of holes and localized spin centers in neutron irradiation.