Influence of sintering temperature and various atmospheres on structural and electrical properties of LaNi1-xNbxO3 (x = 0.005, 0.05)

Abstract

Ternary lanthanum nickelates, La2NiO4, La3Ni2O7, La4Ni3O10 and LaNiO3 belong to Ruddlesden-Popper (RP) homologous series with the general formula Lan+1NinO3n+1, where n = 1, 2, 3 and ∞, respectively.The crystal structure of lanthanum nickelates consist of n perovskite layers (LaNiO3) separated by rocksalt layeres (LaO) along the crystallographic c direction. These compounds exhibit interesting electrical, magnetic, optical and catalytic properties. Owing to these properties, lanthanum nickelates find application such as catalysts in batteries and fuel cells, cathode materials for solid oxide fuel cells (SOFC), oxygen separation membranes, capacitors, superconductors, electrical contacts and resistors. LaNiO3 (LNO) has a rhombohedrally distorted perovskite structure. The conduction band in LNO is formed by the hybridization of the egorbitals of Ni3+ and the p-orbitals of O2-. As a result, LNO has a metallic behaviour, which exibits high electrical conductivity in wide range of temperature. The main drawbacks of this material are poor density and thermal unstability at temepratures above 850°C. Niobium, Nb5+, as a dopant could improve density and thermal stability of the ceramic samples. Also, it could enhance electron concentration and mobility, leading to improved electrical conductivity of ceramic. The type of atmosphere during thermal treatment has an important influence on the stoichiometry of this material and preventing decomposition. The aim of this work was to prepare ceramic materials of LaNi1-xNbxO3 (x = 0.005, 0.05) by mechanochemically assisted solid state method. Commercial powders, La2O3, NiO and Nb2O5, were mechanochemically activated in the planetary ball mill for 5 h and calcined at 700 °C for 3 h in air. Obtained powders were sintered at 900 °C and 1200 °C for 2 h and 10 h in various atmospheres (air and oxygen). LaNi1-xNbxO3 ceramics was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The XRD analysis confirmed the existence of rhombohedral LaNiO3 and secondary phases, Lan+1NinO3n+1 and NiO. Sintering in air atmosphere resulted in LaNiO3 ceramics with lower content of secondary phases. However, sintering in oxygen atmosphere provided LaNi1-xNbxO3 ceramics with improved density, thermal stability and electrical conductivity.