Development of Thin Film Production Process for Lanthanum Silicate Oxyapatite Electrolyte By Tape Casting Method
Authors: Nishimoto Yuzo , Kobayashi Kiyoshi , Akashi Takaya , Suzuki Tohru S. 
Published: December 2020

Lanthanum silicate oxyapatite (LSO) has attracted attentions for applications as solid electrolyte of solid oxide fuel cells (SOFCs) and chemical sensors due to its high oxide ion conductivity along to the c-axis [1]. For applications as solid electrolyte of SOFC and chemical sensors, thin and dense electrolyte is preferable because of lower resistance of the electrolyte. Among thin film production processes, tape casting method has an advantage to produce large area thin film ceramics with low cost. In this research, we developed the tape casting process for LSO thin film electrolyte production. The conditions for preparation of slurry and sintering temperature were optimized in order to produce a dense thin film electrolyte without cracks.

LSO was synthesized from La(OH)3 and SiO2 raw powders using solid state reaction method [2]. After the synthesis, the LSO was grinded by planetary ball mill in ethanol with polyethyleneimine as dispersant. Then, methyl ethyl ketone (MEK) was added in the slurry. It was known that mixture of ethanol and MEK at volume ratio of 4 : 6 becomes an azeotropic state [3]. Solid content of LSO powder in the prepared slurries was 5, 10, and 15 vol.%. A binder was added to the slurry at 5, 10, and 15 wt.%. Also, 0, 5, 10, and 15 wt.% of plasticizers were added to improve flexibility without crack formation in a green tape. After stirring the slurry, residual gas was removed in vacuum desiccator. Green film was cast at 1 mm thickness on PET (polyethyleneterephthalate) film using metal applicator. The cast film was dried in air at room tempareture. Frist, we focused on finding concentration in which the cast green tape can be peeled off from PET film without macroscopic crack. The films were heated at 773 K to remove organic additives. Optimized concentration of slurry was determined by the results that macroscopic crack and warp were not formed after the heating. This heated film produced from the optimized slurry was represented by preheated film. Final sintering tempreture was 1873 K. Warp was observed by the sintering even use of the preheated film when the film was set at free space. In order to avoid the warp, the preheated film was sandwitched by LSO ceramic disks during sintering. The disks were produced by conventional sintering using LSO powder.

A sintered thin film was obtained under the conditions ; solid content at 15 vol.% ; binder concentration at 5 wt.% ; plasticizer concentration at 0 wt.% at this stage. The prepared electrolyte thin film was characterized by scanning electron microscope (SEM) to observe microstructure and film thickness. Electrical conductivity was measured by impedance spectroscopy. In addition, we produced a SOFC single cell by obtained film and platinum electrode. The results of SOFC cell performance test from 1273 K to 1073 K are shown in the figure. Thickness of the LSO thin film was 125 µm. From the cell performance test, mechanical gas leak was negligible because open circuit voltage at 1273 K was 1.04 V.

Due to the use of platinum electrode, cell performance was still low at 13.1 mW cm-2 at 1273 K. To improve the cell performance, more suitable electrode materials for SOFC cell is necessary. Moreover, alignment of crystal orientation is effective to improve the cell performance by decreasing of electrolyte resistance.

Reference

[1] S. Nakayama, et al, J. Mater. Chem.
5 (1995), 1801-1805

[2] K. Kobayashi, et al., J. Ceram. Soc. Japan.
123 (2015), 274-279

[3] S. Beaudet Savignat et al., J. Eur. Ceram. Soc.
27 (2007), 673-678

Figure 1