A Model for Extraterrestrial Solar Degradation of Zinc Oxide

Discoloration of ZnO from ultraviolet absorption can be produced in the laboratory, but is most severe in extraterrestrial environments, when microcrystalline ZnO is used as a pigment for spacecraft thermal control surfaces. Understanding of this degradation process should lead to development of pigments with improved thermal stability under prolonged ultraviolet exposure. It is noted that movement of photoexcited holes to the surfaces of crystallites, followed by oxygen evolution from the lattice and the creation of interstitial zinc (which causes the optical degradation), can continue indefinitely if the interstitial zinc is able to migrate inwards. The evidence supports a finite mobility for interstitial zinc at temperatures as low as 300°K. Doping techniques aimed at minimizing the tendency towards continued degradation are discussed.