39 research outputs found

    Thermische ontleding van calciumnitraat

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    Document(en) uit de collectie Chemische ProcestechnologieDelftChemTechApplied Science

    Elucidating structural mechanisms for cordierite ceramic formation using synchrotron radiation

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    Cr₂O₃ is a common nucleating agent useful for forming ceramics from oxide glasses. In this study we have used a variety of synchrotron radiation techniques to examine the atomic structure, crystallinity and microstructure of a magnesium alumino-silicate glass ceramic whose composition is close to that of the mineral cordierite, Mg2Al4Si5O18. X-ray Absorption spectra on the Cr K-edge have been performed with samples that were heat treated at different temperatures and times to examine the metamorphosis of the nucleating site. This study reveals that Cr³⁺ is always octahedrally coordinated with oxygen. In addition, the second nearest neighbour environment changes from an amorphous, single shell of Al/Si, but upon crystallisation, develops into a well ordered Al/Mg shell indicative of the formation of a dilute Cr spinel phase. Powder X-ray Diffraction (XRD) patterns, however, reveal that the major phase initially precipitated (approximately 900°C) is a stuffed quartz and from the lattice parameters that the composition is SiO2-rich. With prolonged heat treatment a small quantity of a spinel phase emerges whose composition from its lattice parameter is close to MgAl2O4. Combined Small Angle X-ray Scattering (SAXS) and XRD establish that devitrification at these temperatures is associated with particles about 250A in size, on average. Energy dispersive powder diffraction patterns were collected in situ to observe changes in crystallinity with temperature and time. From these measurements the stuffed quartz phase identified at 900°C is found to be intermediate, being eventually replaced at higher temperatures by cordierite with a small quantity of spinel.6 page(s
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