Oxygen Transport Ceramic Membranes
The present quarterly report describes some of the investigations on the structural properties of dense OTM bars provided by Praxair and studies on newer composition of Ti doped Ti-substituted perovskites, La 0.7 Sr 0.3 Mn 1-x Ti x O 3 , with 0≤x≤0.20, were investigated by neutron diffraction, magnetization, electric resistivity, and magnetoresistance (MR) measurements. All samples show a rhombohedral structure (space group R3C) from 10 K to room temperature. At room temperature, the cell
... ure, the cell parameters a, c and the unit cell volume increase with increasing Ti content. However, at 10 K, the cell parameter a has a maximum value for x=0.10, and decreases for x>0.10, while the unit cell volume remains nearly constant for x>0.10. The average (Mn,Ti)-O bond length increases up to x=0.15, and the (Mn,Ti)-O-(Mn,Ti) bond angle decreases with increasing Ti content to its minimum value at x=0.15 at room temperature. Below the Curie temperature TC, the resistance exhibits metallic behavior for the x ≤ 0.05 samples. A metal (semiconductor) to insulator transition is observed for the x ≥0.10 samples. A peak in resistivity appears below TC for all samples, and shifts to a lower temperature as x increases. The substitution of Mn by Ti decreases the 2p -3d hybridization between O and Mn ions, reduces the bandwidth W, and increases the electron-phonon coupling. Therefore, the TC shifts to a lower temperature and the resistivity increases with increasing Ti content. A field-induced shift of the resistivity maximum occurs at x≤0.10 compounds. The maximum MR effect is about 70% for La 0.7 Sr 0.3 Mn 0.8 Ti 0.2 O 3 . The separation of TC and the resistivity maximum temperature T ρ,max enhances the MR effect in these compounds due to the weak coupling between the magnetic ordering and the resistivity as compared with La 0.7 Sr 0.3 MnO 3 .