Effect of substrate surface temperature on the microstructure and ionic conductivity of lanthanum silicate coatings deposited by plasma spraying

Apatite-type lanthanum silicate shows a high ionic conductivity and low activation energy at intermediate temperature (600-800 °C) compared with Yttria stabilized Zirconia (YSZ). In addition, its excellent stability in a wide oxygen partial pressure range meets the requirement for the electrolyte. Thus it is a potential candidate for intermediate temperature solid oxide fuel cell (IT-SOFC) electrolytes. Moreover, atmospheric plasma spraying is expected to be a promising alternative to other costly or low-deposition rate electrolyte processing methods. However, the amorphous phase and pores existing in the plasma sprayed coatings might impair the stability and the higher ionic conductivity of lanthanum silicate. The present work investigated the effect of substrate surface temperature from 545 °C to 900 °C on the microstructure and ionic conductivity of the lanthanum silicate coating. The crystallinity of as-deposited coatings increased with the increase of substrate surface temperature, whereas the porosity showed a contrary tendency. Ionic conductivity increased about four times with increasing substrate surface temperature, which is related to the low porosity and high crystallinity of coatings. Increasing the substrate surface temperature during plasma spraying is a feasible method to increase ionic conductivity of the lanthanum silicate coatings by reducing the porosity and enhancing the crystallinity.