Variable Absorptance and Emittance Devices for Thermal Control

Abstract

Variable absorptance (α) and variable emittance (ε) devices were developed and tested by electrochemical and optical methods for thermal control applications. A unique concept was investigated whereby a highly reflective metal was reversibly deposited on UV/vis/near-infrared (Mylar) and long-wavelength infrared (ZnSe) transparent substrates. Copper was ultimately chosen as a reflective metal due to its desirable optical characteristics, relatively low toxicity, and well-explored electrochemical deposition/stripping phenomena. Dramatically enhanced current densities were obtained by the presence of catalytic, inert metals (Pt, Au, Pd) on the Mylar and ZnSe substrates (working electrodes). Devices functioned by reversible electrochemical transfer of metal (Cu) between the working electrode and Cu/Kapton counter electrode that were sandwiched around 0.1 M Cu(BF4)2/0.1 M LiBF4/propylene carbonate, radiation-absorbing, redox-active electrolyte by application of <±1 V and <±30 mA/in2 (<±30 mW/in2) between the electrodes for ∼1−2 min at ambient temperature. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to confirm the presence of electrochemically deposited copper on the working electrode. Variable absorptivity spectra (300−2400 nm) were convolved with solar blackbody radiance at 5800 K. The resulting maximum absorptance modulation was ∼0.32 (0.83−0.51). Variable emissivity spectra (8−12 μm) were convolved with blackbody radiance at 300 K. The resulting maximum emittance modulation was ∼0.53 (0.73−0.20). The convolved optical modulations of each device had decreased by <15% after 500 potentiostatic square-wave cycles. Applications to thermal control were quantified by calculating the device surface temperature in each state.