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The microwave (MW)-assisted formation of ZnO micro-javelins from zinc nitrate and urea in aqueous solution is described. The particles (named as ‘micro-javelins’ because of their high aspect ratio and needle-like tips) grow hexagonally with well-defined facets in the 〈0110〉 direction and pointed tips in (0001) direction. Powder X-ray diffraction patterns show the appearance of a strikingly dominant (1000) orientation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations reveal the morphological evolution of these hexagonal ZnO particles with time. The effect of precursor concentrations, counterion type and MW irradiation power and their consequent influence on pH and Zn2+ion concentration are investigated. A mechanism for the formation of the micro-javelins is postulated. The microwave induced supersaturation of Zn(OH)+species under the weakly basic pH condition and the initial growth through the (0001) direction (oxygen-rich face) are proposed to be the key factors that dictate the formation of these ZnO micro-javelins. The present one-step microwave process is a straightforward and a reproducible method for the bulk synthesis of defect-free ZnO micro-javelins, which would find potential applications in microelectronic devices (e.g.lasers, cantilevers in surface probing equipment, etc.). [ABSTRACT FROM AUTHOR]

An industrially viable solution-based processing route using minimal amounts of solvent has been used to prepare bulk quantity nanopowders (average particle size 15 ± 3 nm) for the fabrication of ZnO varistors. The xerogels, calcined powders and sintered materials were fully characterised. The preparation of varistors from nanopowders has been optimised by studying the effect of temperature on grain growth, densification and breakdown voltage. The varistors are prepared by sintering at 1050 °C for 2 hours, a temperature that is significantly lower than that used in the current industrial process. Highly dense varistor discs prepared from the sintered material produce devices, with a breakdown voltage 85% higher than that of commercial varistors, making them suitable for use in miniaturised electronic circuitry. Improved performance of these materials has been attributed to the small grain size and better dispersion of additives on ZnO grains. [ABSTRACT FROM AUTHOR]

A rapid, straight-forward method for the preparation of silver colloidal solutions of various colours has been developed. This is based on the reduction of silver ions by ascorbic acid in the presence of citrate-stabilized silver seeds, additional trisodium citrate and a polymer such as polyvinylpyrrolidone. The colour of the sol is controlled by varying the concentration of only one reagent, namely the trisodium citrate. TEM studies show that this effect is due to changes in morphology of the particles. Working at temperatures above 50 °C yields well-defined triangular plates, which are stable in the reaction medium. Samples prepared at lower temperatures and left in the reaction medium experience etching and a consequent change in particle shape. The mechanisms of the initial particle growth and the etching process are discussed. [ABSTRACT FROM AUTHOR]