Formation of Spin Centers in Carbons by Oxidation

Abstract

It was found that a high concentration of electron spin centers is obtained in carbons originally heat treated to a temperature of 1000°—2400°C by heating such carbons to 700°—1100°C in an oxidizing atmosphere. The conditions for the most efficient formation of spin centers by oxidation are: a temperature of about 800°C or somewhat higher, gas pressure above atmospheric, and a proper duration of heating. The presence of spins was detected using the usual experimental techniques of paramagnetic spin resonance absorption. Spin centers were created in various types of carbons (hard, soft, and carbon blacks). The shape and width of the paramagnetic resonance line vary greatly depending on spin concentration and on the original heat-treatment temperature of the carbon. In general the g value is close to or slightly higher than that for the free electronic spins. The experiments show that the presence of spin centers in equilibrium with an oxidizing gas is the result of the operation of two opposing mechanisms, one creating and the other destroying the centers. The creation occurs as a result of a direct chemical attack on the crystal surface. The destruction at high temperature is a result of the improvement of the crystal surface by atomic diffusion. Studies of the dynamics of formation and destruction of spin centers were carried out. The temperature dependence of spin absorption indicates a continuous change in character of spin centers with change in heat-treatment temperature. For low heat treatments the spin centers show a localized broken edge-bond character. With increasing heat-treatment temperature of a carbon the spin centers gradually change in their character into ``metallic'' conduction band electrons.

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