| In this project, amorphous tungsten oxide powders were synthesized by a low temperature reduction technique and then heat-treated in a gaseous reduction atmosphere to obtain nanocrystalline tungsten powders. Synthesis was carried out in ethanol and aqueous media using NaBH4 as a reducing agent. The x-ray diffraction patterns of as-synthesized powders from both synthesis techniques were amorphous. In order to promote crystallization, the powders were heat-treated at 1000°C for 3 hours in air. The x-ray diffraction patterns of the heat-treated powders prepared in ethanol showed peaks of sodium tungsten bronzes, whereas the powders synthesized in aqueous media showed peaks of tungsten oxides, showing that the powders prepared in aqueous media are better for obtaining pure tungsten, since they only require a simple reduction treatment. In addition, after washing these powders, energy dispersive spectroscopy showed that the powders had peaks for oxygen and tungsten for powders synthesized in aqueous medium, whereas it showed peaks of sodium, chlorine, tungsten, and oxygen for powders synthesized in ethanol medium. The powders, which were synthesized in aqueous medium, were heat-treated at 800°C, 650°C and 500°C in a H2/CH4 reducing atmosphere for 2 hours and characterized using x-ray diffraction, scanning electron microscopy, and dynamic light scattering. X-ray diffraction results after reduction treatment showed alpha-W peaks for the powders heat-treated at 800°C and 650°C and a mixture of beta-W and alpha-W for the powders heat-treated at 500°C. The crystallite sizes determined from x-ray peak broadening were 20 nm, 15.8 nm and 11.5 nm, respectively. Dynamic light scattering results showed average particle sizes of 260 nm, 450 nm and 750 nm, respectively. Scanning electron micrographs for all the heat-treated powders that form larger agglomerates of irregular shape. |
