Toshio Okabe

Growth of large tetrapod-like ZnO crystals: I. Experimental considerations on kinetics of growth

Abstract A new method to grow large and uniform tetrapod-like crystals has been developed and the kinetics of the growth has been analyzed based on the reaction curves obtained experimentally. The formation of such crystals, having a leg length of 30 to 200 μm and an edge size of centering nucleus of 1 to 10 μm, is achieved by using ZnO/Zn particles prepared by oxidation of the surface of Zn powder, by making use of a smouldering reaction controlling the reaction rate, and by introducing zeolite as a reaction catalyst. The plot of the amount of O 2 absorbed during the reaction showed that the oxidation process proceeded parabolically and the reaction could be divided into two elementary reactions: the growth reaction of the nucleus and that of the succeeding legs, which can be controlled independently by the amount of zeolite added or the air-flow rate. The condition for reducing the secondary growth crystals like platelets between legs was elucidated by connecting the amounts of Zn reacted with the reaction rates.

Growth of large tetrapod-like ZnO crystals II. Morphological considerations on growth mechanism

Abstract A newly developed method to grow large tetrapod-like ZnO crystals, described in part I, has made possible morphological observations of the products formed at each stage of the growth process. The octahedral nuclei are formed and grown in the oxide scales accompanying the generation of stresses and these crystals are expelled out of the grain boundary when they become unable to resist the internal stress. The emerged octahedral crystals begin to develop their legs on their alternative faces to form large and uniform tetrapod-like crystals in the vapor of zinc and oxygen. The octahedral nucleus as well as the accurate shape of the fourlings can be understopd by supposing that at the initial stages of growth a nucleus was formed with cubic symmetry, corresponding to zinc-blende type ZnO.

Decagonal phase and pseudo-decagonal phase in the Al-Cu-Cr system

Abstract A decagonal phase with a periodicity of 3·78 nm (= 3 × 1·26nm) along the periodic axis has been found for the first time in Al–Cu–Cr alloys in the as-cast state as well as in the annealed state. The temperature range of stability has been found to be very narrow around 1000°C by investigation of differently annealed specimens in a transmission electron microscope. The broken bulk specimens exhibited a few particles with a tenfold-facetted decagonal prismatic shape. The chemical composition of these particles was determined to correspond to Al72Cu12Cr16 by energy-dispersive X-ray spectrometry. High-resolution electron microscopy image of the decagonal phase taken along the periodic axis (tenfold axis) has shown a ring contrast with ten bright spots situated symmetrically and located at the vertices of a random tiling with four tiles with the same edge length of about 2·0 nm. The decagonal phase has been examined by comparing with the pseudo-decagonal phase obtained at a lower annealing temperature.

Kinetics and mechanism of hydrogen-induced disproportionation of ZrCo

Abstract The kinetics and mechnism of the hydrogen-induced disproportionation of ZrCo was studied experimentally. In general, the time dependence of the disproportionation has a sigmoidal shape. The disproportionation rate increased with increasing initial hydrogen pressure and showed a 3/2-power dependence on the pressure. As for the temperature dependence, the rate increased with temperature only up to 815 K and then decreased at higher temperatures. Namely, a volcano-type pattern was observed in the Arrhenius plot of the rate constant. In addition, a fairly large isotope effect was observed for the rate of disproportionation between hydrogen and deuterium. These results were explained well by a nucleation and nuclei growth model, in which hydrogen solubility in the ZrCo phase was assumed to play an important role.

Morphology and growth mechanism of new-shaped ZnO crystals

We report new-shaped ZnO crystals grown by using the similar method reported previously for growing four-legs-form tetrapod-like crystals with adding Sn Zn alloy powder to ZnO/Zn powder. The crystal has four long and four short legs which are united at a same junction and make an equal angle of 70.5° to each other. From the determination of the crystallographic polarity of the faces of octahedral nuclei, we consider that the additional short legs grow on the four oxygen faces of an octahedral nucleus by VLS mechanism where liquid Sn ZnO alloy acts as an agent for the formation of the stable liquid-solid interface.

Crystallization behavior and local order of amorphous GexTe1−x films

Amorphous Ge x Te 1 − x (0.1 x x = 1/3 and x = 2/3, which has been interpreted to predict the chemically ordered 4–2 coordination to be favored for the amorphous Ge-Te alloys. Also, resistance measurements have been performed on the films protected from impurity contamination and tellurium sublimation and they revealed remarkable annealing effects accompanying no structural changes detectable by electron microscopy, which sometimes mask the change corresponding to the crystallization taking place in the films.

High resolution electron microscopic observation on a pentagonal nucleus formed in amorphous germanium films

Abstract A structure analysis has been carried out on the nucleus formed in amorphous Ge films at the initial stage of crystallization by means of high resolution electron microscopy. Small crystallites displaying fivefold symmetries were formed in the amorphous matrix with sizes down to about 10 nm. A columnar structure model for the nuclei which consists of five-membered rings, stacking with a common axis, has been proposed and the computer-simulated electron microscope image produced, based on this model, is in good agreement with the observed images. The results suggest the existence of pentagonal structural units in amorphous germanium which play a salient role as preferential nucleation sites at the stage of incipient crystallization.

Simultaneous crystallization of both elements in amorphous GeSb and GeAl eutectic alloys

Abstract Electron microscopic investigations have been carried out on the early stages of crystallization of amorphous eutectic GeSb and GeAl alloys with no intermediate compound. The GeSb and GeAl films, prepared by vacuum coevaporation on the substrates at room temperature, are amorphous for Ge concentrations larger than 0.1 and 0.5, respectively. The crystallization of these films are characterized by simultaneous formation of both constituents in their crystalline forms, adjacent each other with specific orientation relationships: (246)Ge  (003)Sb and [03 2] Ge  [110]Sb for GeSb, and (111)Ge  (224)Al and [1 2 1] Ge  [1 1 0] Al for GeAl. The crystallization temperature decreases monotoneously with increasing Sb or Al content, and is well fitted to 2 3 values of the liquids temperature in Kelvin in the case of GeSb.

Growth of electrocrystallized ZnO particles by reaction of vacuum-deposited Zn films with distilled water

Products by electrochemical reaction of vacuum-deposited Zn films with distilled water at 20°C are investigated by scanning electron microscopy (SEM) and an X-ray diffraction method. The time dependence of the pH of the water indicates three stages of the reaction; the initial stage I at pH = 7 for about 20 h, the middle stage II at pH > 7 (Max = 7.7) for about 100 h and the final stage III at pH = 7. In stage I etching pits occur as a result of the local cell formation. Each pit is enclosed by a ZnO bank on the ring margin of 10–100 |gmm in diameter. Stage II is characterized by the growth of many spindle-like ZnO particles on the speckles which are traces of the pits of stage I. The ZnO banks dissolve and dendritic |Gb-Zn(OH)|2 crystals also appear near the margins. The spindle crystals disappear in stage III. The formation and morphology of the spindle crystals are described.

Growth of α-Ag2S whiskers in a VLS system

Abstract Whiskers of α-Ag2S have been grown by the direct reaction of vapour sulphur with diffusing silver atoms through its own solid crystals, and examined by scanning electron microscopy, X-ray microprobe analysis and X-ray diffraction. The whiskers, which have grown in a 〈111〉 direction with six {110} lateral surfaces, have irregular tops and sometimes have spheres at the tips whose constitution is nearly the same as Ag2S. A mechanism of growth for the whiskers is proposed. Molten droplets of metastable sulphides, formed on the balance of fluxes of sulphur vapour and solid silver, proceed the whisker growth. Also the polymorphic transformations of the whiskers to β-phase are described.