Electronic Properties of Carbons and of Their Interstitial Bisulfate Compounds

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Abstract
The conductivity of graphite increases when it is oxidized into graphite bisulfate, this being the result of the expulsion of resonance electrons by the negative ions and the resulting creation of excess holes in the π‐band. This study concerns the electronic properties of the bisulfate compounds formed by the oxidation of carbons in sulfuric acid, the range of heat‐treatment of carbons extending from 1400°C (baked carbons) to 3000°C (polycrystalline graphite). The resistivities of all samples thus treated decrease with increase in the degree of oxidation. The rate of decrease in resistance depends upon the heat‐treatment, the rate being increasingly greater for samples treated to higher temperatures. The curve of the Hall constant of unoxidized carbons versus the heat‐treatment temperature H has a sharp positive peak at about H=2000°C. The curve rapidly drops on either side of the peak, the coefficient becoming negative and exhibiting a minimum around H=1400°C. The thermoelectric power of all samples studied becomes more positive with oxidation, with maxima appearing for lamellar compounds. The rate of the initial rise is greater for higher heat‐treatment temperatures. The temperature coefficient of resistance depends strongly upon the ion concentration, changing from negative to positive for carbons heat‐treated above 2000°C. On the other hand, the coefficients remain negative and are remarkably independent of ion concentration for carbons treated to temperatures below 2000°C. The results are in general consistent with the electronic model for carbons proposed by Mrozowski whereby the Fermi level is initially depressed during devolatilization of the hydrocarbon material by the formation of a large number of excess holes (heat‐treatments below 1400°C) and later steadily rises with the growth of the crystallites towards the top of the π band, the growth of the crystal planes resulting also in a steady decrease of the energy gap between the π and conduction bands. The negative Hall coefficient for baked carbons indicates the π band to be so depleted that the Fermi level drops at least partially below the inflection curve. At room temperature the conduction is therefore semi‐metallic in nature for carbons treated to H2000°C where the number of excess holes is relatively small and the current is carried mainly by the holes and electrons thermally activated into the conduction band.