Copper nanoparticle–expanded graphite (CuNPs/EG) composites were developed to improve through-plane heat transport 

in graphite-based thermal interface materials (TIMs). Copper nanoparticles (CuNPs) were synthesized by chemical reduction

using polyethylene glycol and ascorbic acid to suppress aggregation and oxidation, and were then incorporated into 

expanded graphite (EG) by a supercritical carbon dioxide (scCO₂)-assisted intercalation process. X-ray diffraction (XRD) 

confirmed the formation of metallic CuNPs, and dynamic light scattering (DLS) showed a particle size distribution of 

approximately 150–650 nm with a main peak near 350 nm. Scanning electron microscopy (SEM) and energy-dispersive 

X-ray spectroscopy (EDS) indicated that ethanol used as a co-solvent produced a more homogeneous distribution of Cu 

species within the expanded interlayer voids and porous regions of EG than isopropyl alcohol. Cross-sectional SEM/EDS 

analysis further showed Cu-containing particles distributed inside the composite film. After 14 days of air exposure at 

25 °C and 40% relative humidity, only weak oxygen signals were detected from CuNPs embedded within EG, indicating 

improved oxidation stability. Laser flash analysis showed that the CuNPs/EG composite prepared using ethanol exhibited 

a through-plane thermal conductivity of 8.402 W/m·K, a 124.5% increase compared with pristine EG, while retaining an 

in-plane thermal conductivity of 90.854 W/m·K. These results demonstrate that ethanol-assisted scCO₂ intercalation is an 

effective strategy for developing oxidation-stable CuNPs/EG composites with enhanced through-plane thermal transport.