Atomic layer deposition of semiconductor oxides on electric sail tethers

Electric Solar sail (ESAIL) tethers may be subjected to barrage of incident charged particles, high energy photons and strong vibrations, depending on their working mechanism. Atomic Layer Deposition (ALD) technique can produce light-weight, conformal and defect-free coatings that can protect the metallic tethers from many anticipated harsh space effects. In addition to this, the coatings may be used to steer the equilibrium temperature of the tethers, especially when used in the deep solar system. We have studied the effectiveness of ALD coatings of ZnO, TiZnO and TiO2 with nanometric thickness to protect tethers from UV attack and physical and optical degradation due to exposure to ultraviolet radiation. Numerical calculations for the different oxide layers and different thicknesses showed that films thicknesses in the order of nanometres act as anti-reflecting coatings and maximize the absorbance in the case under study. The reflectance spectra of the coatings was obtained from a standard spectrophotometric device, showing that the absorbance changes significantly depending on the coating material, thus affecting the equilibrium temperature of the ESAIL tether in space. From the results of the optical measurements, we may conclude that a TiO2, ZnO or TiZnO thin coating could absorb almost twice as much power from the Sun than Al2O3, provided that the value of the emissivity is unchanged. Focus Ion Beam technique provided a precise measure of the thickness of the coated samples and information of the morphology of the coating. Additionally, the coating thickness was measured indirectly using the reflectance measurement, providing results in agreement with the FIB technique. To quantify electrochemically the protection capability of the coatings, Potentiodynamic Polarizartion technique was employed both in dark and under UV conditions. Shift in corrosion potential of TiO2 coating is nearly double the positive shift of Ti-Zn-O compared to bare tether. ZnO coating failed to prove its efficacy in this regard. TiO2 coating outperforms all other coatings both in dark and UV both from a morphological and electrochemical point of view.