Ionic Conductivity of Anisotropically Sintered Apatite-Type Lanthanum Silicon Oxide
Authors: Hidaka Shigekazu , Yamamoto Takahisa 
Published: December 2024

Apatite-type lanthanum silicon oxide (LSO) has attracted keen attention as one of the new solid electrolytes for its high oxygen ionic conductivity. LSO has a hexagonal apatite structure with oxygen sites coordinated along the c-axis direction and exhibits higher ionic conductivity in the c-axis direction than that of yttria stabilized zirconia, which has been put to practical use as a solid electrolyte. However, to fabricate a dense sintered body with LSO single phase, high sintering temperature of around 1700 ℃ is required due to its very small diffusivity of La and Si. From a practical standpoint, it is necessary to develop a process to lower the sintering temperatures and foster orientation along the c-axis direction to realize practice use of LSO as solid electrolytes.

In this study, a powder coated with silicon oxide (SiO2) on the surface of lanthanum oxide (La2O3) powder was prepared for the purpose of completing the diffusion of La and Si at an early stage in sintering. This coated powder is noted as additive-free LSO. Further, a powder doped with tetrafluoroboric acid was also prepared for the purpose of lowering the sintering temperature by enhancing the diffusion of La and Si. This doped powder is noted as added LSO. Sintering behaviors of each powder, crystal phases and ionic conductivities of the obtained sintered bodies were measured.

The experimental procedure is as follows. La2O3 powder and tetraethoxysilane (TEOS) were weighed to be La9.33Si6.00O26 and mixed in ethanol followed by dripping 10% ammonia water up to the pH of 10 in the mixed solution. Additive-free LSO was obtained by drying the mixed solution in an oven at 120 °C and then calcining at 400 °C. For the added LSO, the additive-free LSO was stirred in pure water in which a 42% tetrafluoroboric acid aqueous solution was added 10 wt.% to the additive-free LSO powder, and dried in an oven at 90 °C. Two types of prepared powders weighing 0.3 g were pelletized to a size of about 1 mm thick with a diameter of 10 mm by a uniaxial press of 10 MPa and an isostatic pressure of 100 MPa and sintered in a furnace. For the sintered bodies, sintered densities were measured, crystal phases were confirmed by X-ray diffraction (XRD), surface structures were observed by a scanning electron microscope (SEM), and the ionic conductivities were measured by an impedance analyzer.

As a result, dense sintered bodies with a relative density exceeding 90 % were achieved by holding the pellet of additive-free LSO at 1700 °C and the added LSO at 1450 °C for 10 hours. From the XRD, it was confirmed that each sintered body consisted of LSO single-phase. The sintered body of the additive-free LSO exhibited high peak intensities associated with (100), which means the c-axis of LSO is oriented in the in-plane direction. SEM observation revealed that the sintered body of additive-free LSO is composed of many columnar grains grown in the in-plane direction. On the other hand, the sintered body of added LSO exhibited an equiaxed-grained structure. Further, by measuring the ionic conductivity of each sintered body in the in-plane and out-of-plane directions, it is revealed that the ionic conductivity of the additive-free LSO showed a higher ionic conductivity in the in-plane direction than in the out-of-plane direction, while the added LSO showed lower ionic conductivity regardless of along in-plane / out-of-plane directions.

These results indicate that the difficult-to-sinter properties of LSOs is not only due to the low diffusivity of La and Si, as previously reported, but also preferential grain growth in the c-axis direction impinging on and blocking each other to lower shrinkage speed. Although the sintering temperature could be lowered by enhancing equiaxial grain growth by adding tetrafluoroboric acid, the ionic conductivity became lower, which is probably due to too many grains with a, b-axis orientation of poor ionic conductivity and grain boundaries between them. It could have better prospect to support high ionic conductivity and low sintering temperature at the same time that exploiting the preferential grain growth characteristics of LSO in the c-axis direction.

Figure 1