Preparation, structures and electrical properties of alkaline earth and transition metal-doped NdAlO3 conducting ceramics

Abstract

Novel Nd0.9Sr0.1Al1-xMxO3-δ (M = Co, Fe, Mn; x =0, 0.15, 0.3, 0.5) conducting ceramics each with a hexagonal perovskite structure are prepared using organic-gel method combined with subsequent high temperature sintering. The influences of transition elements (Co, Fe, and Mn) and their dosages on the structure characteristics and electrical properties are investigated in detail. The experimental results reveal that well-crystallized Nd0.9Sr0.1Al1-xMxO3-δ perovskite oxide ultrafine powders can be obtained by calcining the gel precursors at 900 ℃ for 5 h. The lattice parameters of sintered ceramics increase with the increase of transition metal content (x), and they increase according to the order of Co, Mn, and Fe. All the samples are mixed conductors of oxygen ions and holes in air, and the oxygen ion transport number is enhanced monotonically from 0.32 at 500 ℃ to 0.63 at 850 ℃ for NdAlO3-δ ceramic single-doped with alkaline earth metal Sr, indicating that this material has an electronic-to-ionic dominant transition in electrical conductivity with measurement temperature increasing. Whereas the oxygen ion transport numbers are all below 0.001 for the samples co-doped with Sr and transition metals (Co, Fe, and Mn), and their electrical conductivities are absolutely dominated by p-type conduction. It is found that the conductivity values increase with the increase of x value, and they increase according to the order of Mn, Fe, and Co, while the change of corresponding apparent activation energies is just the opposite. Nd0.9Sr0.1Al0.5Co0.5O3-δ ceramic has the highest electrical conductivity, ～100.8 S/cm at 800 ℃, and the lowest apparent activation energy (0.135 eV) in all the synthesized samples. The observed changes in structure and electrical property in this study can be explained on the basis of the difference in ionic radii among the doped transition metals as well as the differences in bond energies and covalencies among the M-O bonds.