Nanostructured Materials最新文献

Nanoparticles of ZnO and Al₂O₃ were prepared by chemical arrested precipitation. The average particle size was determined from x-ray line broadening and TEM. The ac electrical response of the consolidated nanoparticles of ZnO and ZnO-Al₂O₃ nanocomposites was studied using impedance spectroscopic technique. The equivalent circuit parameters were determined by computer simulation. The grain and grain boundary contributions to the conductivity were determined and the difference in the conductivity was attributed to the grain boundary properties of nanomaterials. The variation in conductivity of nanocomposites with weight percentage of Al₂O₃ was attributed to the change in interfacial defect structure, band structure modifications and compensation of oxygen vacancies.

Annealing products of mechanically treated Al₇₅Mo₂₅ powders have been studied by X-ray powder diffraction with the Rietveld method. The differential scanning calorimetry traces of as-milled powders showed transformation features whose temperature decreased as a function of mechanical treatment time. Different phases were identified across the calorimetric events, which were also reported in the high-temperature phase diagram of the Al-Mo system, such as Al₁₂Mo, Al₅Mo, Al₄Mo, Al₃Mo and Al₂₂Mo₅. Nevertheless, the solid state transformation processes activated in the calorimeter at ambient pressure are of different nature with respect to those observed by means of neutron diffraction during in situ annealing under vacuum. In the specimen mechanically alloyed for 32 hours the Al₁₂Mo phase was observed after annealing with both processes. In the crystal geometry of Al₁₂Mo, which is regarded as a first order approximation of the quasi-crystalline phase, the molybdenum atoms are arranged in a BCC fashion and are surrounded icosahedrally by twelve aluminum atoms.

Inorganic and organic approaches for the synthesis of nanocorundum powders and products are described on a scale of 50 g batches. The median particle size of the volume distribution of redispersed powders is about 50 nm after calcination at T < 950 °C. A key issue of advanced processing is the preparation of dense sintered bodies at a sintering temperature < 1300 °C with a submicrometer grain size; the bodies are large enough to enable the measurement of a Vickers macrohardness > 21 GPa at a testing load of 10 kg. Porous sintered corundum bodies with an average pore size between 15 and 60 nm and a porosity of about 40–55% have been produced within the framework of the described synthesis by both powder or sol/gel technologies for the use as chemically highly stable filtration membranes or as catalyst supports. Other applications are amorphous or crystalline nanoporous products with average pore sizes of about 1 nm.

A solution-based processing method has been used to synthesize nanocrystalline TiO₂ powders by controlling the hydrolysis of TiCl₄ in aqueous solution. As-prepared powder was characterized by TEM, HREM, XRD, BET and ED techniques. In the presence of a small amount sulphate ions, the powder was pure anatase and its primary particle size is finer than that of alkoxide-derived powder, moreover the transformation of anatase to rutile was retarded. Both hydrolysis temperature and amount of sulfate ions had effects on the morphology and crystallization of the powder. In an optimum process, TiO₂ powder in the anatase phase was fabricated without calcination and its primary size was 4nm, BET surface area was 290m²/g.