A (27)Al MAS NMR study of a sol-gel produced alumina: Identification of the NMR parameters of the theta-Al(2)O(3) transition alumina phase.

(27)Al MAS NMR has been used to study a sol-gel prepared alumina annealed at various temperatures. Two-field simulation of the sample heated to 1200 degrees C confirmed the presence of corundum, as suggested by XRD, and also the presence of nanocrystalline theta-Al(2)O(3). (27)Al MAS NMR chemical shifts, quadrupolar coupling constants and asymmetry parameters are reported for the tetrahedral and octahedral aluminium sites within theta-Al(2)O(3).

A 27Al, 29Si, 25Mg and 17O NMR investigation of alumina and silica Zener pinned, sol-gel prepared nanocrystalline ZrO2 and MgO.

Alumina and silica Zener pinning particles in sol-gel prepared nanocrystalline ZrO2 and MgO have been characterised using 27Al and 29Si MAS NMR after annealing at various temperatures up to 1200 degrees C. The structures of the pinning phases were found to differ not just between the two metal oxide systems but also depending on the exact method of manufacture. Three distinct transitional alumina phases have been observed in different alumina-pinned samples annealed at 1200 degrees C, one in particular identified by a peak at a shift of 95 ppm in the 27Al NMR spectrum. Both the alumina and silica pinning phases reacted with the MgO nanocrystals, forming spinel in the case of alumina, and enstatite and forsterite in the case of silica. Despite reacting readily with the MgO, the silica pinning particles were effective at restricting grain growth, with 11 nm MgO nanocrystals remaining after annealing at 1000 degrees C.

Local atomic and electronic structure in nanocrystalline Sn-doped anatase TiO2.

Tin-doped anatase TiO(2) nanopowders and nanoceramics with particle sizes between 12 and 30 nm are investigated by X-ray absorption fine-structure (EXAFS) and Mössbauer spectroscopies. Furthermore, ab initio calculations based on the density functional theory are performed to analyze changes in the electronic structure due to Sn doping. The three approaches consistently show that Sn is dissolved on substitutional bulk sites with a slight increase of the bond lengths of the inner coordination shells. The Debye-Waller factors show that the nanocrystallites are highly ordered. There is no indication of defect states or bandgap changes with Sn doping.