Synthesis of Pure La10-XSrxSi6O27-0.5x Apatites Using an Optimized Sol-Gel Process

Apatite-type lanthanum silicates draw researcher’s attention due to their good performances as electrolyte materials for IT-SOFC (intermediate temperature solid oxide fuel cells) ^[1-5]. In this paper we present the synthesis and characterisation of Sr-doped La₁₀Si₆O₂₇ prepared through an optimized, water-based sol-gel ^[6] process. The parameters involved in this process have been investigated to obtain high purity, high crystallinity powders. The mechanisms of the sol-gel reactions that underlie the quality of the sol are discussed. The optimized sol gel process enables obtaining pure apatite structures through a calcination step for 2 hours at a temperature as low as 800℃. They were characterized by scanning electron microscopy, X-ray diffraction and Energy-dispersive X-ray spectroscopy. Ionic conductivities have been measured after sintering at 1500℃ in order to investigate the effect of the Sr-doping. 

The electrical properties were measured as a function of temperature by complex impedance spectroscopy, using a Solartron 1260A frequency response analyzer (FRA) in the frequency range 5 Hz - 15 MHz, under static air. The measurements were performed at the open circuit voltage as well as with an applied AC voltage of 100 mV. The impedance diagrams were recorded from 673 K to 1273 K with thermal steps and stabilization times of about 25 K and 20 minutes respectively. The results show that the ionic conductivity is thermally activated and its value lies between 3×10^-5 and 1×10^-6 S.cm^-1 at 873 K as function of the composition and powder preparation conditions.

Keywords: Sol-gel processes; SOFC; apatite type lanthanum silicates; Sr doping

References:

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