Scalable Spray-On Fabrication Approach for Voltage-Tunable Thermal Emissivity Surfaces

We report a new approach for fabricating gate-tunable thermal emissivity surfaces by spraying them on graphene ink. The devices consist of a multilayer graphene (MLG)/porous alumina membrane/gold stack, in which the MLG is deposited by spraying the graphene ink onto the porous membrane using an airbrush. The graphene ink consists of μm-sized flakes of MLG suspended in a solution of polyvinylpyrrolidone and ethylene glycol. The alumina porous membrane is filled with the ionic liquid [DEME][TFSI], forming an electrochemical cell. When a positive voltage is applied to the device, the intercalation of [TFSI]^- anions causes large changes in the thermal emissivity of the MLG. This, in turn, gives rise to a substantial change in the apparent temperature (i.e., thermal camouflage) as measured by a thermal imaging (i.e., FLIR) camera sensitive over the 7.5-14 μm wavelength range. In this work, the apparent temperature change reaches ΔT = 14 °C. This method of spray coating offers a scalable solution that is compatible with roll-to-roll printing and manufacturing techniques.