The electrical conductivity and Hall effect of glassy carbon

The electrical conductivity and Hall effect of glassy carbon heat treated for three hours between 1200 and 2700°C was measured at temperatures from 3 to 300 K in magnetic fields up to 5 tesla.The electrical conductivity, of the order of 200 (ohm cm)^?1 at room temperature, can be empirically written $\text{σ = A + B}\text{exp}\text{(?CT}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}\text{)-DT}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$, where the first term is a strongly scattering metallic component, the second term is attributed to variable range hopping, and the third and new term is negative correction to the metallic conductivity associated with one-dimensionality. All of the constants A, B and C were insensitive to heat-treatment temperature; the constant D decreased with increasing temperature until it disappeared at about 2200°C.The Hall coefficient was independent of magnetic field, insensitive to temperature, but was a strong function of heat-treatment temperature, crossing over from negative to positive at about 1700°C and ranging from ?0.048 to 0.126 cm³/coul.The idea of one-dimensional filaments in glassy carbon suggested by the electrical conductivity is compatible with the present consensus view of the microstructure constructed through such means as lattice imaging in transmission electron microscopy, and X-ray diffraction and small angle scattering.