Morphology of ZnO Crystals formed on the Surface of Brass or Zinc at High Temperatures

It was electron-microscopically observed that thin needles are projecting on the surface of brass or zinc heated in the air. Electron diffraction diagrams proved that they are needle-shaped crystals of hexagonal ZnO with axes parallel to [110] direction; which presents an interesting contrast to the ZnO smoke needles having axes parallel to [001]. For brass, needle-shaped crystals begin to form at about 300°C, and the largest crystals grow at 550–600°C. At temperatures higher than 700°C the growth of the needles is suppressed and no oxide is found at all near the melting point. In the case of zinc, the largest crystals are obtained at 450–550°C (melting point 419°C), where dendritic crystals having 60°C branching are observed together with flat sword-like crystals. When the substrate brass or zinc is mechanically distorted heavily, plate-shaped ZnO crystals showing rotational slips are obtained.