Ilmenite is the most common accessory mineral in many rocks. During weathering, the primary rocks disintegrate and the ilmenite grains accumulate to heavy mineral fractions also known as heavy mineral sands. These sands today represent the main source of titanium. The aim of my master thesis was to show that natural ilmenite-rich heavy sands can also be used for advanced applications such as preparation of functional ceramics with thermoelectric properties.
In my experimental work, I used ilmenite-rich sand provided by Cinkarna Celje as the starting powder. The starting (as received) powder was pre-oxidized at 600, 700 and up to 800 °C to trigger exsolution of rutile lamellae in ilmenite grains. The as-received powder and the pre-oxidized powders were sintered in an inert atmosphere using two procedures: pressureless conventional sintering (at 1200 °C) and pressure-assisted pulsed flow sintering (SPS; at 1100°C). The powders were analyzed by laser granulometry, TG, XRD, SEM/EDXS and MS, we also determined absolute density of the sintered samples. In the last step, thermoelectric properties of the sintered samples were measured.
The results have shown that all sintered samples consist of a matrix with the composition on the pseudobrookite–ferropseudobrookite tie-line, whereas the samples prepared from pre-oxidized powders contained rutile inclusions. The conventionally sintered samples were slightly more porous and had a lower density than the samples prepared by SPS. The average grain size was significantly smaller for the SPS sintered samples than for the conventionally sintered samples. Measurements of thermoelectric properties showed that both the conventionally sintered and SPS sintered samples have promising thermoelectric properties and show low thermal conductivity, relatively high Seebeck coefficient, but low electrical conductivity, which should be improved in the future. In my thesis, I have shown that natural ilmenite powder has great potential for the production of advanced functional ceramics.
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