Achieving excellent comprehensive thermal performances in (GF+CF)/(Cu+Ti) composites via hybrid reinforcement strategy and highly oriented structure

Although graphite flakes (GF)/Cu composites have high thermal conductivity (TC) in the basal plane direction of GF, the problem of high coefficient of thermal expansion (CTE) makes it unable to meet the requirements of thermal management. Herein, a hybrid reinforcement strategy combining graphite flakes and carbon fibers (CF) was employed, and the (GF+CF)/(Cu+0.7 wt%Ti) composites were fabricated by spark plasma sintering and matrix alloying. Structural characterization revealed that the composites have a highly oriented structure and a nanoscale thickness TiC interlayer is formed at the interface. The effect of the relative content of reinforcements on the (GF+CF)/(Cu+Ti) composites' microstructure and in-plane thermal properties was systematically investigated. As the ratio of GF to CF changed from 60 vol%:0 vol% to 30 vol%:30 vol%, although the in-plane TC of the composites decreased from 658.2 W/m·K to 495.1 W/m·K, the in-plane CTE significantly reduced from 12.90 ppm/K to 6.62 ppm/K, thereby meeting the low CTE requirement of thermal management materials (CTE<10 ppm/K). Notably, the CTE significantly decreased by sacrificing a small amount of thermal conduction performance, resulting in excellent comprehensive thermal properties. Improvement of CTE was attributed to the polycrystalline orientation characteristics of the graphene layers in CF, which effectively constrained the thermal expansion of the copper matrix. The maintenance of high thermal conductivity is related to the consistently high orientation distributions of GF and CF and interface modification. According to the acoustic mismatch model, it can be found that the formation of a nanoscale thick TiC interlayer is beneficial for reducing interfacial thermal resistance and improving the TC of composites. In summary, the hybrid reinforcement strategy, highly oriented structure, and interface modification effectively solve the challenges faced by graphite flakes/Cu composites as thermal management materials.