On the microstructures of equilibrated and quenched spinel-containing nickel titanates

Electron microscopy has been used to study the microstructures of equilibra ted and quenched NiO-TiO2 mixtures containing 5, 10, 15, 20 and 25wt% TiO2. All the specimens contained rocksalt- and spinel-structured phases on a na nometre scale. The microstructure suggests that a nonstoichiometric spinel decomposed during quenching. The 25wt% TiO2 specimen exhibited a lamellar m icrostructure composed of NiTiO3 and spinel-NiO. These phases had the usual orientation relationships, i.e., (111)(cubic)parallel to (0001)(hex) and ( 1 (1) over bar 0)(cubic)parallel to (1 (1) over bar 00](hex). Faceted NiO s olid-solution regions with coherent cube-on-cube interfaces with the spinel were found in the four specimens of lower TiO2 contents. When large (15 an d 20wt% TiO2), these regions were faceted cuboidal particles. With lower Ti O2 contents, these regions were no longer equiaxed and could be interconnec ted. The specimens were chemically homogeneous on a length scale of about 1 mu m. Morphological evidence is presented to explain which part of the mic rostructure most probably already existed in this form during equilibration . In particular, the large-scale NiTiO3 and the spinel lamella in the 25wt% TiO2 material form during long-term equilibration.