Three-Tier Hierarchical Porous Structure with Ultrafast Capillary Transport for Flexible Electronics Cooling

The development of flexible electronics needs efficient cooling devices. The porous wick, the key component in a heat pipe (HP) and vapor chamber (VC), is generally fabricated by sintering copper particles at high temperatures (>1000 °C), which makes it only formed on an inflexible substrate. In this work, one three-tier hierarchical porous structure (mesocrack, micropore, and nanopapillae) was fabricated via a low-temperature sintering method based on the utilization of self-reducing metal precursors (∼300 °C), which can be used as a flexible porous wick. The mesocrack, acting as the main water flow channel, efficiently decreases the flow resistance. The micropore, covered with densely distributed spore-like nanopapillae, creates a heterogeneous wetting surface. By harnessing the synergistic effect of hydrophobic drag reduction and hydrophilic driving force enhancement, the capillary performance is significantly improved. The obtained wick on the flexible substrate can overcome the dilemma between diminishing viscous resistance and strengthening capillary force at different length scales. It can achieve an ultimate wicking coefficient of 7.132 mm/s^0.5, representing an enhancement of 9.1% compared to the best micro/nano wick structure in the previous works. Moreover, for the flexible light-emitting diode, the passive cooling approach utilizing the fluid transport and evaporation within the porous structure fabricated in this study, in comparison to the natural cooling, achieved a temperature decrease of 35.9 °C, resulting in a cooling effect of up to 35.1%. The proposed method resolves the challenge of fabricating a porous wick for flexible HP and VC, and it will open up a way for the cooling technique of flexible electronics.