Degradation of Electrical Conductivity in Lanthanum Silicate Oxyapatite

Lanthanum silicate oxyapatite (La_9.33+xSi₆O_26+1.5x) is known to exhibit higher oxygen ionic conductivity than yttria-stabilized zirconia at temperatures below 700^oC, making this material attractive for its potential use in intermediate-temperature fuel cells operated at 500~700 ^oC. Mineshige et. al. reported that the lanthanum content in oxyapatite phase is strongly related to its electrical conductivity, with the highest conductivity obtained when x=0.67 (La₁₀Si₆O₂₇) ^[1]. It was further reported that La₂SiO₅ existed in addition to the oxyapatite phase even in the as-sintered sample. Another important aspect that can influence electrical conductivity is the phase stability of lanthanum apatite, which remains unclear. In this study, electrical conductivity measurements of lanthanum silicate apatite were conducted for over 100h and its phase stability was evaluated.

 Lanthanum silicate apatite with nominal composition of La₁₀Si₆O₂₇ was made by conventional solid state reaction. The pellet was sintered at 1700^oC for 10h after calcining at 1400^oC for 10h. Electrical conductivities were determined by impedance spectroscopy measurements conducted at 600, 800 and 1000^oC for 106~201h in dry air. The phase stability was evaluated by annealing at 1500^oC for 20h.

 The degradation of electrical conductivity was observed at all temperatures. In particular, the conductivity decreased significantly from 5.57x10^-2 Scm^-1 to 3.57x10^-2 Scm^-1 after 201h at 1000^oC. Crystallites having small facets with the size of several hundred nanometers were observed on the oxyapatite phase after conductivity measurement. On the other hand, significantly larger particles were found segregated on the surface of the sample annealed at 1500^oC; these particles have a composition determined to be La₂SiO₅ by EDS analysis. This result suggests that the same phase segregation could have occurred at the temperature range of 600 ~ 1000^oC but at a diminished degree. Moreover, the degradation of electrical conductivity could be correlated to the observed phase segregation. In this presentation, we will discuss the causes of the degradation of electrical conductivity in consideration of the phase equilibrium of lanthanum silicate.

Ref. 1: A. Mineshige, Y. Ohnishi, R. Sakamoto, Y. Daiko, M. Kobune, T. Yazawa, H. Yoshioka, T. Nakao, T. Fukutsuka, Y. Uchimoto, Solid State Ionics 192 (2011) 195.