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 an d 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 pres ence of oxygen was studied by fractional crystallisation in a temperature g radient tube furnace. It was observed that the oxidation of zinc vapour is a heterogeneous process. In the absence of nucleation sites, zinc vapour an d oxygen can coexist in a meta-stable state. The different stages and forms of nucleation, crystal growth and oxidation were deduced from the morpholo gy of the quenched products, as determined by scanning and high-resolution transmission electron microscopy. Zinc morphology depends on whether it cry stallised at above or below the melting point. (C) 1998 Elsevier Science Lt d. All rights reserved.