Magnetic Susceptibility of Carbons and Polycrystalline Graphites. I

In this investigation, the relation of susceptibility to the $a$ dimension of crystallites was studied over the range from turbostratic carbons with diameters of 50 angstroms, to graphitic crystals with diameters of at least several thousand angstroms. Two series of carbons, one Thermax carbon black in powder form, the other National Carbon baked rods, were prepared in which the crystallite size was varied by heat treatment to different temperatures. A third series consisting of carbon blacks with different particle sizes from 50A to 3000A was prepared by heating to 3000°C, the particle size limiting the crystallite growth. The crystallite diameter was determined from the broadening of the x-ray diffraction lines, 10 (two-dimensional asymmetric diffraction line), and 110. The magnetic susceptibility was measured at room temperature by the Gouy method, and the measurements were made with fields ranging from 2000 to 11 500 oersteds to correct for ferromagnetic impurities. The mass susceptibilities of all carbons plotted against crystallite diameter fall within a narrow band showing that the susceptibility is primarily a function of crystallite size (Fig. 5). Almost all the increase in susceptibility from -0.5×${10}^{-6\mathrm{}}$/g for very small crystallites (aromatic molecules) to -7.8×${10}^{-6\mathrm{}}$/g for large crystals takes place in the range of diameters from 50A to 150A. The sharp increase in susceptibility is discussed in relation to the changes in the Hall effect and thermoelectric power as a function of heat treatment. It is concluded that this increase is due to an increase in the filling of the $\pi$ band which brings the Fermi level into a region of higher curvature of the constant energy surfaces.