Thermal and electrical conduction in the compaction direction of exfoliated graphite and their relation to the structure

The effects of the compaction and graphite layer preferred orientation on the thermal and electrical conductions in the compaction direction of graphite-flake-based exfoliated graphite have been decoupled. The compact's electrical and thermal conductivities decrease with increasing compaction (density increasing from 0.047 to 0.67 g/cm³, solid content increasing from 2.1 to 30 vol.%) and preferred orientation. The essentially linear correlation between electrical and thermal conductivities (Wiedemann-Franz Law) is because both conductions are governed by the preferred orientation. With increasing compaction, the fraction (f) of conduction path that is the graphite a-axis decreases from 0.997 to 0.937 and from 0.994 to 0.798 for thermal and electrical conductions respectively. For the solid-part thermal and electrical conductivities to exceed 140 W/(m K) and 60 kS/m respectively, f must exceed 0.95; the highest solid-part conductivities are 550 W/(m K) and 230 kS/m. The compaction-related variation in the solid-part conductivities is large [21-550 W/(m K) and 10-230 kS/m], due to the preferred orientation variation. The through-thickness Lorentz number (7.3 × 10 ^-6 W Ω/K²) is similar to the in-plane value, being independent of the preferred orientation. At 2-7 vol.% solid, conductivities of 7 W/(m K) and 3 kS/m are obtained for the compact - toward the targets for fuel cell biopolar plates.