STB Model and Transport Properties of Pyrolytic Graphites

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Abstract
We propose to demonstrate that a coherent description of pyrolytic graphite (PG) layer‐plane phenomena can be based on a parabolic two‐band system with cylindrical equal‐energy surfaces. In this simple two‐band model (STB model), band overlap and effective mass must be interpreted as phenomenological parameters to be derived from experiments on highly heat‐treated pure PG. With p‐type (boron‐doped) specimens, the objective is to describe the situation from the shift of the Fermi level, on the assumption that the presence of trapping centers would not inject major perturbations in the band structure. (1) Galvanomagnetic effects: As derived from zero‐field resistivity and magnetoresistance mobility, the intrinsic carrier concentration and its variation with temperature in the range 0° to 1500°K implies that past a given graphitization stage band‐structural features may no longer be seriously affected by the size of the carbon networks. Above room temperature the carrier concentration increases almost linearly and in accordance with a constant mean effective mass of about 0.0125 m0; low‐temperature data suggest an energy‐band overlap of 0.01 eV, which may be temperature‐sensitive through thermal expansion. (2) Thermoelectric effects: Along the layer planes the carrier‐mobility behavior is strong evidence for a relaxation time that may be taken of the form E−½. On this basis it can be shown that the STB model accounts for the Seebeck coefficient, provided that exact Fermi‐Dirac statistics are used in conjunction with standard equations for semimetallic systems. (3) Magnetothermal effects: The discovery of a substantial electronic component in the low‐temperature thermal conductivity of PG layer planes makes it desirable to develop methods of separating lattice‐phonon and charge‐carrier contributions to heat transport in graphite. It is shown that saturation magnetothermal conductivity measurements can be successfully utilized for this purpose.