Thermal conductivity of exfoliated graphite nanoplatelet paper

Exfoliated graphite nanoplatelet paper prepared by vacuum assisted self-assembly exhibits very high anisotropy in heat conduction with in-plane thermal conductivity as high as 190 W/mK for nanoplatelets of 15 μm in size. [Display omitted] ► Polyethyleneimine (PEI) functionalized xGnP form good suspension in water. ► The as-made paper prepared by vacuum filtration is porous. ► Decomposition of PEI improves thermal conductivity of the paper. ► Cold pressing reduces paper porosity and improves thermal contact. ► Paper exhibits strong anisotropy in heat conduction after annealing and pressing. Exfoliated graphite nanoplatelets (xGnP) were produced by acid intercalation followed by thermal exfoliation and a controlled size reduction to produce graphite nanoplatelets of 1–15 μm in lateral dimension and approximately 10 nm in thickness. These highly hydrophobic nanoparticles were dispersed and stabilized in a DI water/polyethyleneimine (PEI, a cationic polyelectrolyte) solution. A free standing, mechanically robust paper of xGnP was prepared by vacuum filtration. The effect of xGnP size, polyelectrolyte coating and paper porosity on thermal transport properties was investigated. It was found that the annealing process improves the thermal conductivity by decomposing the PEI molecule that is adsorbed on the xGnP particles while still maintaining the porosity of the paper. Mechanically compressing the sample effectively reduces the pore volumes within the paper and increases the contact area among individual platelet. The strong alignment effect and larger contact area was evidenced by a 80% increase in in-plane thermal conductivity (178 ± 12 W/mK) and a 10% reduction in through-plane conductivity (1.28 ± 0.12 W/mK). This flexible, lightweight, low-cost, paper material made of xGnP particles is a promising candidate for applications requiring 2D heat conduction.