TITANOMAGNETITE PROPERTIES AND MICROSTRUCTURES

摘要

Ulvospinel (Fe_2TiO_4) and titanomagnetites (TM) (pseudo-binary (1-x)Fe_3O_4·xFe_2TiO_4 system, where, e. g. TM95 corresponds to x = 0.95) were made using a solid-state synthesis technique. For single phase titanomagnetites (x＞0.75), our measured lattice constants extends the previous work of Wechesler to Ti-rich solid solutions, and extrapolates to a value of 8.531 A for single phase ulvospinel, agreeing closely to the literature value of 8.534 A. The lattice constants as a function of composition (x＞0.75) were fit to a straight line with the equation y = 0.09x + 8.441. Grain sizes of single phase TM pellets of as-quenched and annealed samples were determined, showing that the grain size of the annealed samples grew noticeably after 100 hrs in Ar flow at 450 ℃. For TM75, the grains grew from 11.7 μm to 15.4 μm. The ratios of grain sizes vary over the composition range, from 4% to 31%, suggesting coarsening driven by diffusional growth. In the samples that initially contained wustite as a second phase, there are grains with a lath microstructure with well-defined angles, indicating preferred orientations. Small amounts of Fe-or Ti-cation rich phases were observed to decorate the grain boundaries after long annealing. TiO_2 formed on grain boundaries of TM75 annealed sample.