High temperature resistivity of pyrolytic graphite bromine residual compounds

The electrical resistivity of pyrolytic graphite (PG) deposited at 2300°C decreases in both the a- and c-axis directions as the temperature is increased up to 1000°C, but the high temperature resistivity of PG-bromine residual compounds differ somewhat from that of the original PG. When the temperature is increased, the a-axis resistivity remains nearly constant at the lower temperature range but increases rapidly between 300 and 500°C. Above 500°C the value of the resistivity approaches that of the original PG. On cooling, the change of the electrical resistivity of PG-bromine residual compounds is the same as that on heating, but its value is larger between 500 and 100°C. At room temperature, the a-axis resistivity of the compound before and after the cyclic heat-treatment is the same. This behaviour is closely related to the c-axis thermal expansion and is well explained by the same bromine vaporisation model. As to the c-axis direction, the electrical resistivity of PG-bromine residual compounds show a different behaviour. However, the factors affecting the electrical conduction in this case are complicated and cannot be resolved.