Direct solar thermal dissociation of zinc oxide: Condensation and crystallisation of zinc in the presence of oxygen

The solar thermal production of zinc from zinc oxide is part of a two-step water splitting cycle. Zinc oxide thermally dissociates into zinc vapour and oxygen at elevated temperatures; ZnO(s)= Zn(g)+ 0.50(2) In practice, the yield of zinc depends on the kinetics of dissociation and the technical feasibility of quenching its gaseous products fast enough to avoid reoxidation. As the gaseous products cool, the rate of zinc oxidation varies as the zinc changes its phase. The condensation of zinc in the presence of oxygen was studied by fractional crystallisation in a temperature gradient tube furnace. It was observed that the oxidation of zinc vapour is a heterogeneous process. In the absence of nucleation sites, zinc vapour and oxygen can coexist in a meta-stable state. The different stages and forms of nucleation, crystal growth and oxidation were deduced from the morphology of the quenched products, as determined by scanning and high-resolution transmission electron microscopy. Zinc morphology depends on whether it crystallised at above or below the melting point. (C) 1998 Elsevier Science Ltd. All rights reserved.