Limitation-aware thermal transport in powder metallurgy silicon–graphene nanocomposites: Effects of dispersion, porosity and structural defects

Silicon–graphene nanocomposites containing 0, 5, 10 and 15 wt% graphene were fabricated by powder metallurgy to investigate why the high intrinsic thermal conductivity of graphene does not necessarily translate into enhanced bulk heat transport. The composites were prepared by high-speed dry blending (8000 rpm, 10 min), uniaxial compaction at 15 kN, and solid-state sintering in a continuous belt furnace under an N₂–H₂ atmosphere at a maximum temperature of 870 °C, with an overall furnace residence time of approximately 2 h. The resulting composites were characterized using field-emission scanning electron microscopy–energy-dispersive spectroscopy, X-ray diffraction, Raman spectroscopy, Archimedes density measurement, thermogravimetric analysis and laser flash analysis. The dispersion standard deviation increased from 3.31 for S5 to 5.10 for S10 and 6.08 for S15, indicating increasing spatial heterogeneity of the graphene-rich domains with graphene loading. The experimental density ranged from 1.980 to 2.167 g cm⁻³, corresponding to relative densities of 85.23–93.83% and porosities of 6.17–14.77%. X-ray diffraction confirmed retention of crystalline silicon, with an apparent crystallite size of 27.05 nm for S15. Raman spectroscopy revealed non-monotonic defect evolution, with ID/IG values of 0.176, 0.325 and 0.189 for S5, S10 and S15, respectively, while S15 exhibited the highest I2D/IG ratio of 0.322. For the graphene-containing samples, the measured thermal conductivity remained low, ranging from 0.362 to 0.568 W m⁻¹ K⁻¹, with S5 exhibiting the highest room-temperature value of 0.538 W m⁻¹ K⁻¹. The combined results show that graphene loading alone does not determine bulk thermal transport; instead, spatial heterogeneity, incomplete densification, structural disorder and associated interfacial transport barriers collectively restrict effective heat-transfer pathways. These findings establish a limitation-aware microstructure–thermal transport framework for the design of Si–graphene composites for thermal-management applications.