Papers by Keyword: Nanocrystallinity

Abstract: Positron annihilation techniques (PAT) have recently been successfully employed for the characterization of phase transitions in metals and compounds. In the present study, positron annihilation lifetime (PAL) measurements have been carried out on a nanocrystalline titania (TiO2) in the form of powders that had been heat-treated at temperatures ranging from 300 to 1273K. The PAL spectra were analyzed into two lifetime components. The shorter lifetime τ1 (185-300 ps) is attributed to positron annihilation in vacancies and the longer lifetime τ1 (400-580 ps) to positrons in microviods at interfaces. The rutile phase of TiO2 powders was utilized as a reference in order to compare their behavior with the commercially supplied and widely available anatase phase (Degussa P25). The influence of the heat-treatment upon the nanostructure during the transition of the anatase to rutile phase were also investigated by X-ray diffraction (XRD), TEM and BET surface area methods. Understanding of this effect is expected to enhance our knowledge of the morphology and nanocrystallite size of TiO2 powders and their T-dependence, and hence their physical properties.

Abstract: Nanocrystalline tetragonal zirconia was obtained from ZrOCl2 via the modified forced hydrolysis method combined with aging of the hydrous amorphous precipitate in the mother liquor at 100 °C for 48 h (pH = 9.3). The role of the precipitation and aging temperatures in the metastabilization of the tetragonal ZrO2 polymorph is discussed in terms of the structural and textural data of the resultant oxide. The influence of low concentrations of silica (0.01 – 0.35 wt. % Si), spontaneously leached from the glass vessel or intentionally introduced to the parent solution, was shown to be a vital factor, controlling the phase composition of the final prepared zirconia. Using the concepts of zirconium aquatic chemistry, this effect was explained by incorporation of silicates into hydrous zirconia protostructures.

Abstract: Nanocrystalline calcium bismuth titanate (CaBi4Ti4O15), which exhibits a layer structure, has been successfully synthesized by mechanical activation of constituent oxides of CaO, Bi2O3 and TiO2 in a nitrogen atmosphere at room temperature. The phase-forming calcination at elevated temperatures that is always required is skipped. CaBi4Ti4O15 derived from mechanical activation consists of nanocrystallites, which occur as aggregates of ~50 nm in sizes. It demonstrates an improved sinterability and was sintered to a density of 93.4% theoretical density at 1175oC for 2 hours. Ferroelectric properties of sintered CaBi4Ti4O15 derived from mechanical activation have been studied. A peak dielectric constant of 1049 at the Curie temperature of 774oC was measured at 1MHz for CaBi4Ti4O15 sintered at 1175oC.