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- Rietveld analysis (2)
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The transformation of doped or seeded pseudoboehmite to corundum was studied by combining thermal analysis, X-ray diffraction, transmission electron microscopy, and electron paramagnetic resonance spectroscopy. The temperature of phase transformation to corundum was lowered by about 130 °C when Fe3+ or corundum seeds were added to the sols. Action of Fe3+ ions depends on the actual degree of thermally induced transformation of pseudoboehmite via transition aluminas to corundum and the ability of these alumina phases to incorporate Fe3+ ions. These ions tend to aggregate with increasing iron concentration of the alumina phases and can work as nucleation centers. Small (not, vert, similar20 nm) corundum particles act as active nucleation sites whereas larger grains (200400 nm) also present in the samples are less effective. For the first time trapping and stabilization of NO2 molecules in transition aluminas formed by a solgel route was shown.
Binary organic-inorganic gels have been prepared by mixing a carbonaceous hydrosol and a Nb2O5 or Ta2O5 sol derived by hydrolysis of the alkoxides. The gels are pyrolyzed under an inert atmosphere into precursors in which carbon and the metal oxides are mixed very intimately. High temperature treatment converts the precursors into the cubic face centered carbides. The precursors as well as the carbides have been shown to be micro- and mesoporous materials. Measurements of nitrogen adsorption reveal a characteristic change of the shapes of the isotherms (Type I → Type IV) and of the hysteresis loops (H4 → H2 → H1) during the thermal processes. Pore widening has been observed with rising temperature. The phenomena of crystallization, carbothermal reduction and sintering were found to control the pore shape and size. The results of the adsorption measurement correlate well with those of the thermoanalytical and X-ray diffraction studies.
Binary hydrogels,. in which a niobium or tantalum oxide gel and a pyrolysable organic compound are combined, were prepared as preceramic materials. Carbonaceous gel or saccharose is used as the organic gel constituent, and alkoxides or peroxo acids are the starting materials for the transition metal component. Under pyrolysis at 600-700°C, the gels are transformed into carbide precursors in which a reactive carbon and finely dispersed oxide particles are mixed intimately. The subsequent carbothermal reduction proceeds at lower temperatures compared to physical mixtures of oxides and carbon black. The high-temperature treatment was monitored by TG/DTA, X-ray diffraction and nitrogen adsorption. Microporosity is generated in the temperature range 700-800°C, and mesoporosity is additionally formed at higher temperatures. The changes in porosity are controlled by the processes of crystallization, carbothermal reduction and sintering. The final products at 1400-1500°C are assemblages of fine particles of the face-centred cubic carbides with grain sizes of 1 µm and less.