Dynamic radiative thermal management by crumpled graphene

Systems and surfaces used for aerospace applications require dynamic temperature control for optimal system performance, simultaneously fulfilling the thermal needs for personal human comfort and maintaining equipment functionality, and at the same time avoiding overheating. Radiative cooling is becoming an increasingly attractive method of passive thermal management that utilizes spectral radiation characteristics in the ambient environment. This is achieved by making use of the solar spectrum to heat up surface areas with incident solar radiation (from 200 nm to 2.5 um, visible to near-infrared wavelengths), and utilizing the atmospheric transmission window (from 8 um to 14 um, mid-infrared wavelengths) to cool down the surfaces via reemission of the heat to the outer space. The distinct spectral ranges enabling heating and cooling negate the use of conventional materials with fairly uniform high or low emissivity values, and the need for tunable thermoregulation rules out most existing rigid cooling surfaces due to lack of dynamic modulation over emissivity. Here we show selective mid-infrared emissivity control by mechanically reconfigurable graphene, in which mechanical stretching and releasing induces controlled morphology changes of graphene. By integrating graphene with stretchable, elastomeric substrates, we fabricate crumpled graphene with controlled pitch size optimized for radiative cooling at 10 um. Our emissivity measurements based on reflectance and Fourier transform infrared spectroscopy, validated with computations based on rigorous coupled wave analysis (RCWA) and finite-difference time-domain (FDTD) methods, demonstrate that the optimized crumpled graphene attains topography-driven high emissivity values at 9.9 um and 13 um wavelengths. These emissivity variations are attributed to interference between adjacent crumpled features, diffraction at the graphene/air interface, and incoherence of light. The tunability of the crumpled graphene for thermoregulation is demonstrated by mechanical stretching and releasing for controlled modulation of the surface morphology. The results show reversible changes of emissivity over 30 cycles at the wavelength of 9.9 um. Our thermal analysis shows that the optimally crumped graphene will achieve a net radiative cooling power of 77 Wnr ² and a surface temperature reduction 7 K below the ambient air. The spectral-selective control of emissivity governs the thermal energy exchange in the ambient environment and enables optimal control of the surface temperature without running electricity or any active components such as bulky heat exchangers. The proposed mechanically tunable crumpled graphene could potentially lead to breakthroughs in aerospace thermal management, especially for surface systems where radiative heat transfer is critical to performance and reliability.