The phonon-drag thermoelectricity of graphite in relation to structural imperfection

The low-temperature thermoelectricity has been studied extensively for single-crystal graphite and highly oriented pyrolytic graphites (PG) annealed at 3600°c, 3200°c and 2800°c respectively. In the thermoelectric power versus temperature diagram for these specimens, the characteristic phonondrag dip is found to deepen stepwise in the order of structural imperfection as follows : - 11.5 μv/°k for single crystal, - 29.5 μv/°k for 3600°c-annealed PG and - 39 μv/°k for 3200°c-annealed PG. For the 2800°c-annealed, however, the dip becomes as shallow as - 8.5 μv/°k again. The deepening is appropriately ascribed to the perturbation of band structure which takes place so as to bring about relative increase of conduction electrons with the increase of imperfection, whereas it is ultimately cancelled by the decline of the phonon-drag itself. A phenomenological analysis made of galvano-magnetic properties of the same specimens verifies such an interpretation.