Shelly L. P. Savin

Structural studies of silica- and alumina-pinned nanocrystalline SnO2

Sol–gel-prepared nanocrystalline tin (IV) oxide has been characterized by magic angle spinning nuclear magnetic resonance (MAS NMR) in a pure form and also with silica and alumina pinning phases present. The 119Sn MAS NMR lineshape was observed to broaden as the SnO2 nanocrystal sizes decreased, a reflection of the increasingly important contribution of the more disordered surface region. A second local Sn environment other than the cassiterite phase was observed, probably an orthorhombic phase that has previously been observed in nanoparticles of SnO2. The silica and alumina pinning particles were characterized by 29Si and 27Al MAS NMR. The silica pinning particles retained a high degree of disorder over the range of annealing temperatures, whereas alumina crystallized at ~1000 °C. A possible explanation for this difference is that since the alumina pinning particles are less successful at restricting nanocrystal growth than the silica particles, they may aggregate together and combine, hence they are larger, less constrained particles.

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.

Low X-Ray Energy Fluorescence Gas Electron Multiplier (GEM) for XAS Studies

X-ray absorption spectroscopy at sub-keV energies dictates the use of detectors that can attain a good signal-to-noise ratio. We report a recent study undertaken at Science and Technology Facilities Council, Daresbury Laboratory to evaluate the performance of a gas electron multiplier (GEM) for this purpose. The main impetus of this study was to investigate the relevant parameters such as the effective gain and energy resolution in the X-ray energy range of 270-930 eV. This study demonstrated that a single stage GEM can sustain effective gains up to 60,000 in a helium-isobutane counter gas mixture at atmospheric pressure. Consequently, high signal-to-noise ratios were achieved (electronic noise electrons r.m.s) thereby permitting adequate X-ray energy resolution.