Yttria-zirconia coatings studied by grazing-incidence small-angle X-ray scattering during in situ heating.

The morphology of sol-gel derived dip-coated yttria-doped zirconia films containing variable amounts of yttria has been studied using in situ grazing-incidence small-angle X-ray scattering (GISAXS) whilst heated incrementally to 1000 °C. A procedure to analyse in situ GISAXS data has been devised which allows a quantitative analysis of time-dependent GISAXS data tracing processes such as chemical reactions or manufacturing procedures. To achieve this, the relative positions of the Yoneda peak and the through beam are used to fix the vertical q scale when the sample thickness is subject to fluctuations due to chemical reactions or deposition processes. A version of Beaucage's unified model with a structure factor from Hosemann's model for paracrystals describes the yttria-zirconia film data best. It is interpreted in terms of particles forming from a polymeric gel network and subsequently agglomerating into larger units subject to Ostwald ripening as both size and average separation distance of the scattering objects increase. The sample with the highest yttria content shows progressive surface roughening from 850 °C which may indicate the onset of chemical segregation.

23Na, 29Si, and 13C MAS NMR investigation of glass-forming reactions between Na2CO3 and SiO2.

The glass-forming reactions between sodium carbonate (Na2CO3) and silica (SiO2) have been investigated by 23Na, 29Si, and 13C magic-angle spinning (MAS) NMR spectroscopy. The multinuclear MAS NMR approach identifies and quantifies reaction products and intermediates, both glassy and crystalline. A series of powdered batches of initial composition Na2CO3.xSiO2 (x = 1, 2) corresponding to a sodium metasilicate (Na2SiO3) and sodium disilicate (Na2Si2O5) stoichiometry were investigated after periods of isothermal and nonisothermal heat treatments at different temperatures. Analysis of the 23Na quadrupolar coupling parameters has identified the early reaction product in all cases as crystalline Na2SiO3. In the nonisothermal experiment, this reaction is preceded by an early silica-rich melt phase formed around 850 degrees C. The early reactions are controlled by solid-state Na+ diffusion across the reaction zone in the grain interface layer. Crystalline Na2SiO3 precipitates in the interface layer, increasing its thickness between the Na2CO3 and the SiO2 grains and slowing down the rate of Na+ migration. This creates a secondary phase, which is temperature dependent. At low temperatures, where Na+ migration is impaired, the production of Na2SiO3 ceases and silica-richer phases are precipitated. In the case of the sodium disilicate batch, where excess SiO2 is present, a secondary reaction of Na2SiO3 with SiO2 forming a glassy phase is observed. A transient carbon-bearing phase has been identified by 13C NMR as a NaCO3- complex loosely bound to bridging oxygens in the silicate network at the SiO2 grain surface.

Tracing the reactive melting of glass-forming silicate batches by in situ 23Na NMR.

The kinetics of the reaction of batches of powdered quartz and sodium carbonate was studied by in situ (23)Na nuclear magnetic resonance (NMR) spectroscopy using a laser-heated probe. We show for the first time that the technique allows one to study solid-state reactions at high temperatures with good time resolution and without the risk of quenching artifacts. The reaction is controlled by solid-state Na(+) diffusion across the grain interface. Independent of the batch composition, the first reaction product is crystalline sodium metasilicate, Na(2)SiO(3), even if the temperature is high enough for much of the composition space between silica and metasilicate to be above the equilibrium liquidus. Fast Na(+) diffusion allows the reaction front to cross the grain interface and form the solid product before liquid intermediate equilibrium products can be formed. This purely solid-state reaction slows down as the thickness of the interface increases; the reaction is more deceleratory than published models suggest. If excess quartz is present, it reacts in a second step involving a liquid film wetting the excess grains. Once this reaction has started, it pulls the reaction into the thermodynamic regime, which leads to an increase even in the rate of the first step leading to intermediate solid metasilicate.

Following the formation of nanometer-sized clusters by time-resolved SAXS and EXAFS techniques.

Time-resolved in situ SAXS and XAS measurements were carried out to monitor the formation of nanoparticles of the sulfides of cadmium and zinc, from solutions containing he corresponding acetate, and thioacetamide under solvothermal conditions. Analysis of the SAXS data shows that particles of ca 5 nm in radius form within the first few minutes of the reaction and then grow uniformly to ca 20 nm over a period of two hours resulting in a highly mono-dispersed particle distribution. EXAFS data of the CdS particles also prepared by solvothermal methods and recorded at 20 K, support the formation of nano-meter sized particles.