Natural graphite sheet (NGS) is compressible, porous, electrically and thermally conductive material that shows a potential to be used in fuel cells, flow batteries, electronics cooling systems, supercapacitors, adsorption air conditioning, and heat exchangers. We report the results of an extensive material characterization study that focuses on thermal conductivity, thermal diffusivity, electrical conductivity, coefficient of thermal expansion (CTE), compression strain, and emissivity. All the properties are density-dependent and highly anisotropic. Increasing the compression from 100 to 1080 kPa causes the through-plane thermal and electrical conductivities to increase by up to 116% and 263%, respectively. The properties are independent of the sheet thickness. Thermal and electrical contact resistance between stacked NGS is negligible at pressures 100 to 1080 kPa. In the in-plane direction, NGS follows the Wiedemann-Franz law with Lorenz number 6.6 $$\times $$ × 10$$^{-6}$$ - 6 W $$\Omega $$ Ω K$$^{-2}$$ - 2 . The in-plane CTE is low and negative (shrinkage with increasing temperature), while the through-plane CTE is high, increases with density, and reaches 33 $$\times $$ × 10$$^{-6}$$ - 6 K$$^{-1}$$ - 1 . Microscope images are used to study the structure and relate it to material properties. An easy-to-use graphical summary of the forming process and NGS properties are provided in Appendices A and B.

The compression behavior of natural graphite sheet (NGS) is studied during the forming process and during the compression of finished sheets. The forming process, in which the low-density semi-finished NGS is compressed to higher densities, is quantified by the proposed relationship p f = 0.35e 2.6d where d is the free-standing density in g cm −3 and p f is the forming pressure in MPa. The maximum achievable free-standing density is in the range 1.73-1.89 g cm −3. The compression behavior of finished NGS is non-linear, hysteretic, and density-dependent with the maximum strain of 6% at 0.55 g cm −3. The tangent compression modulus increases with the density and ranges from 20 to 220 MPa. A compact relationship for evaluating the strain at a given pressure and density is provided. NGS deforms viscously during the periods of constant pressure; the deformation is significant during the forming process but small during the compression of finished sheets. Keywords Natural graphite sheet • Flexible graphite • Compressed exfoliated natural flake graphite • Forming • Uniaxial compression • Tangent modulus • Viscoelastic behavior • Barreling List of symbols v Viscous strain d Density p Pressure p f Forming pressure t f Thickness after the forming cycle t n Thickness after the n-th cycle t v,0 Thickness at the beginning of the constant pressure period t v,end Thickness at the end of the constant pressure period * Majid Bahrami,

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Natural graphite sheet (NGS) is a candidate material for lightweight, high-performance heat sinks. We show that the low through-plane thermal conductivity can be mitigated by using heat pipes. In the measured configuration, the thermal resistance of an NGS heat sink with embedded heat pipes is comparable to that of a geometrically-identical aluminum one. The achieved weight reduction is 37 %. When electrical insulation of a heat sink is not required, soft and conforming NGS does not require thermal grease at the interface between the heat source and the heat sink. The low electrical conductivity of NGS does not lead to a decrease in common mode conducted emissions, but the potential to reduce the radiated emissions was quantified to be 12 to 97 % based on an analogy with antennas. In practical applications, replacing an existing heat sink with a geometrically identical NGS one is not recommended because it limits the achievable improvements in thermal performance, weight, and cost. Instead, we suggest using an optimization algorithm to determine the optimal heat sink geometry.

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