USE OF IN-SITU XRD TO DEVELOP CONDUCTING CERAMICS WITH THE AURIVILLIUS CRYSTAL STRUCTURE
In-situ X-ray diffraction was used to study structural transformations in several n=3 Aurivilliustype phases: Bi4Ti3O12, Bi2La2Ti3O12, Bi2Sr2Nb2TiO12, Bi2Sr2Nb2AlO11.5 and Bi2Sr2Nb2GaO11.5. Order-disorder transformations have been linked to transitions to fast ion conduction in certain oxygen-deficient phases, such as Bi 2Sr2Nb2AlO11.5 and Bi2Sr2Nb2GaO11.5. Bi2La2Ti3O12 and Bi2Sr2Nb2TiO12 were synthesized and indexed as body-centered tetragonal with 4/mmm Laue symmetry. At room temperature, Bi4Ti3O12 has an orthorhombic unit cell with space group symmetry B2ab. As previously documented, it transforms to tetragonal above its Curie temperature of 675 �C. Indexing of XRD data collected for this study indicates the high temperature phase is body-centered tetragonal with 4/mmm Laue symmetry, similar to the structures of Bi2La2Ti3O12 and Bi2Sr2Nb2TiO12. None of these phases undergo additional phase transformations up to 1100 �C. Both Bi 2Sr2Nb2AlO11.5 and Bi2Sr2Nb2GaO11.5 have oxygendeficient perovskite-layers, with either primitive tetragonal or orthorhombic unit cells at room temperature. Both materials undergo multiple phase transformations. Above 875 �C, the unit cells are similar to those of the non-oxygen deficient n=3 Aurivillius-type phases, being bodycentered tetragonal with 4/mmm Laue symmetry. This behavior agrees with the expectations for oxygen-vacancy order-disorder transformations. The intermediate phases are not yet fully understood: they may be associated with partial disordering of oxygen vacancies or with a ferroelectric transition.
Recent developments in oxide ion conductors: Aurivillius phases
Recent advances in the study of Aurivillius phases (Bi2An-1BnO3n+3, n = 1−5) as oxide ion conductors are presented. The structures of modified Aurivillius phases containing extrinsic oxygen vacancies as well as Aurivillius phases containing intrinsic oxygen vacancies are surveyed. A detailed discussion of the conductivity behavior of these Aurivillius phases is given.
Effects of the mineral phase and valence of vanadium on vanadium extraction from stone coal
The influence of roasting on the leaching rate and valence of vanadium was evaluated during vanadium extraction from stone coal. Vanadium in stone coal is hard to be leached and the leaching rate is less than 10% when the raw ore is leached by 4 mol/L H2SO4 at 90°C for 2 h. After the sample is roasted at 900°C for 2 h, the leaching rate of vanadium reaches the maximum, and more than 70% of vanadium can be leached. The crystal of vanadium-bearing mica minerals decomposes and the content of V(V) increases with the rise of roasting temperature from 600 to 900°C, therefore the leaching rate of vanadium increases significantly with the decomposition of the mica minerals. Some new phases, anorthite for example, form when the roasting temperature reaches 1000°C. A part of vanadium may be enwrapped in the sintered materials and newly formed phases, which may impede the oxidation of low valent vanadium and make the leaching rate of vanadium drop dramatically. The leaching rate of vanadium is not only determined by the valence state of vanadium but also controlled by the decomposition of vanadium-bearing minerals and the existence state of vanadium to a large extent.