Preparation and electrical properties of Ln _x (SiO ₄)₆O_(1.5x-12) (Ln: Nd, La) with apatite structure

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(SiO ₄)₆O₃ is a major component of the Ln _x (SiO₄)₆O_(1.5x-12) system. Ln ₁₀(SiO₄)₆O₃ consists of Ln _9.33(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(OC₂H ₅)₄) and neodymium acetate monohydrate (Nd(CH ₃COO)₃•H₂O) or lanthanum acetate monohydrate (La (CH₃COO)₃•H₂O) as raw materials and acetic acid (CH₃COOH) 2-methoxyethanol (C ₂H₅OCH₂CH₂OH) 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 Ln _9.33(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).