Stretchable Selective Emitters based on Carbon Nanotube Films for Adaptive Thermal Control

Selective emitters based on nanostructured materials have received increasing attention around the world as a novel branch in the development of thermal control systems. However, conventional selective emitter designs do not offer a dynamic control mechanism that is highly desired in many applications such as surface cooling. The self-adaptation properties of selective emitters are essential to address large-scale surface cooling and heating demands for buildings and civil infrastructures. In this work, we demonstrate the tunable emissivity of the thermal emitter consisting of carbon nanotubes (CNTs) film on top of a stretchable silicone gel substrate. The broad wavelength-selective spectral emissivity in the Near-Infrared (NIR) and Middle-Infrared (mid-IR) ranges for this thermal emitter is illustrated and it can be continuously tuned when the film is under different strains. For instance, the average emissivity decreases by 0.2 in the UV-Vis-NIR range and 0.1 in the mid-IR range when the film goes through a strain of 80%. The hypothesis is that the emissivity changes with porosity inside the CNTs film when it suffers certain amounts of strains. We characterize the switchable emissivity by Ultraviolet-Visible-Near IR Spectroscopy (UV-Vis-NIR) and Fourier-Transform Infrared Spectroscopy (FTIR). Our recycling measurement also confirms the repeatability of this emissivity change. The temperature prediction based on emissivity data suggests that the temperature difference can be up to 8 K between strained and unstrained samples assuming a steady ambient environment, typical weather condition and solar irradiance in California. Through this successful control of thermal emissivity, the way is paved towards some potential applications in thermal areas like heat management, thermal energy harvesting and passive radiative cooling.