Preparation and Electrical Properties of Lnx(SiO4)6O(1.5x?12) (Ln: Nd, La) with Apatite Structure

Abstract

In this study, Ln x (SiO4)6O(1.5x−12) (Ln: Nd, La) materials as electrolytes for solid oxide fuel cells (SOFC) were prepared by the sol-gel process. It has been reported that the apatite structure of Ln x (SiO4)6O(1.5x−12) shows higher ionic conductivity than yttrium-stabilized zirconium oxide at the working temperature of the SOFC. Ln10(SiO4)6O3 is a major component of the Ln x (SiO4)6O(1.5x−12) system. Ln10(SiO4)6O3 consists of Ln9.33(SiO4)6O2 and a small amount of Ln2SiO5. It has been proposed that the ionic conductivity of Ln x (SiO4)6O(1.5x−12) decreases with increasing Ln2SiO5 with non-apatite structure. The object of the present study was to bring about this decrease by generating Ln2SiO5 in the system. Precursor solutions for synthesis of the powder were prepared using tetraethoxysilane (Si(OC2H5)4) and neodymium acetate monohydrate (Nd(CH3COO)3·H2O) or lanthanum acetate monohydrate (La (CH3COO)3·H2O) as raw materials and acetic acid (CH3COOH), 2-methoxyethanol (C2H5OCH2CH2OH), and triethanolamine (N(CH2CH2OH)3) as solvents. To obtain the powder, the solution was dried and heat-treated at 600 °C for 2 h. Disks made from the powder were heat-treated at temperatures between 1100 and 1500 °C for 10 h. The results of an XRD investigation indicate that almost all diffraction peaks of these samples could be assigned to Ln9.33(SiO4)6O2. The sample with x = 10.00 included a small amount of Ln2SiO5. The ionic conductivity of this latter sample was higher than that of other samples with similar values of x (x = 9.33 and 10.67).