Preparation and characterization of Ln9.33+x/3Si6-xAlxO26 (Ln = La, Nd and Sm) with apatite-type structure and its application to a potentiometric O2 gas sensor

The compacted sinters of Ln(9.33+x/3)Si(6-x)Al(x)O(26) (0 <= x <= 2.0), (Ln=La, Nd and Sin) are composed of an apatite-like phase with a hexagonal structure. The existence of Ln(2)SiO(5) was confirmed for x <= 1.0 as a minor product, but its formation was suppressed by the increasing Al-content. The densification of the sinters was also improved by partially replacing Si4+ with Al3+. For all the Ln systems, the highest ionic conductivity was observed at x=1.5 in ambient air. The compacted sinters were used as a potentiometric oxygen sensor. The concentration dependence of EMF was well expressed by the Nernst equation, Delta E-obs=(RT/nF) ln (P-O2(I)/P-O2(II)). Furthermore, the electron number n is comparable to the theoretical value of 4. The sensing characteristics of the sinters are comparable to those of the sensors with 3 and 8 mol% YSZ. (c) 2005 Published by Elsevier B.V.