Aguas, Marco Diogo; (2001) Self-propagating high-temperature synthesis of metal oxides; Reactions in external magnetic fields. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The preparation of metal oxides by Self-Propagating High-Temperature Synthesis is reported. The reactions are started with a point source of ignition; typically a hot wire. A synthesis wave is observed moving out from the point source and reactions terminate in seconds. Products obtained can be classified into ferrites (magnetic applications) and stannates (gas sensing applications). Ferrites were synthesised under variable external magnetic fields. The synthesis wave is hotter in the presence of an external magnetic field for hard ferrite synthesis. For spinel ferrites the opposite was observed. Materials synthesised in the field show differences in their bulk magnetic properties (coercivity and saturation magnetisation), structures and microstructures. Combustion reactions in large fields revealed changes in unit cell volume (shrinkage was observed for hard ferrites while expansion was observed for spinel ferrites). SHS synthesised hard ferrites show two distinct components; one has large grain structure consisting of 50% acicular particles of 100 μm and another that has a finer microstructure. The ferrites studied were BaFe12O19, SrFe12O19, MgFe2O4, and Mg0.5Zn0.5Fe2O4. Formation of ferrites by SHS was also studied by time resolved X-ray diffraction. Patterns were successfully recorded at up to 0.8 s intervals. These showed that synthesis of ferrites is in some cases through intermediates. TRXRD has also helped form theories for the mechanistic pathways of the reactions. Work carried out has shown that magnetic fields act on SHS reactions in 3 stages; before, during and after reactions. BaSnO3 was prepared by SHS from various barium and tin reagents. Sintering of the SHS prepared powders at 800 °C for 2-72 h produced phase pure BaSnO3. Powders obtained showed sensitivity towards CO2. All products were analysed by X-ray powder diffraction, SEM/ED AX, electron microprobe, and FT-IR. The ferrites were also analysed by 57Fe Mossbauer spectroscopy, VSM, thermal imaging camera, optical pyrometry and optical microscopy.

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