Effective thermal conductivity of MgH2 compacts containing expanded natural graphite under a hydrogen atmosphere

• The thermal conductivity of MgH2–graphite compacts is measured under H2 pressure.
• H2 pressure up to 20 bar increases the thermal conductivity.
• The thermal conductivity reaches the maximum at a graphite flake size of 200 μm.

Expanded natural graphite (ENG) was added to enhance the effective thermal conductivity of MgH2, which is one of the important parameters in the design of MgH2-based hydrogen storage tanks. Cylindrical MgH2 compacts containing up to 20 wt% ENG flakes with various average sizes (20, 50, 200, 350 and 1200 μm) were fabricated to measure the effective thermal conductivity of MgH2–ENG mixtures. The radial direction effective thermal conductivity of the compacts was measured under a hydrogen atmosphere up to 70 bar. The conductivity was significantly enhanced by the addition of ENG flakes, reaching 9.3 W m−1 K−1 at 20 wt% ENG at 1 bar of hydrogen. It was observed that hydrogen pressure and the size of ENG flakes influenced the conductivity together with the amount of ENG. As hydrogen pressure increased up to 20 bar, the conductivity continued to increase. On the other hand, the conductivity very slowly increased above 20 bar, exhibiting a saturation tendency. It relatively rapidly increased with increasing average flake size up to 200 μm and then gradually decreased with further increasing size up to 1200 μm, exhibiting the maximum value at an average flake size of 200 μm. This trend might be determined by the competition between the thermal resistance at ENG/MgH2 interfaces and the formation of conductive networks of ENG flakes.