Synthesis of Al-Substituted Lanthanum Silicate with Oxyapatite Structure By Hydrothermal Method and Local Structural Analysis

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INTRODUCTION The solid oxide fuel cell (SOFC) can be regarded as one of the most promising power generation systems because of the high efficiency and low emission of pollutants. At present, however, SOFC must be operated at temperature as high as 1000 oC, and thus it is expected to decrease the operating temperature to the intermediate temperature range around 500 oC for the wide-spread use. Since La9.33Si6O26-based oxide-ion conductors have high oxide-ion conductivity and low activation energy even at the intermediate temperature, the materials are considered as a promising candidate for an electrolyte of SOFC. In addition, it was also reported that the conductive properties were improved by Al substitution1). However, it is well-known that both the synthetic and sintering processes of the lanthanum silicates need extremely high temperature, and this demerit obstruct the practical use. In this work, we synthesized nanopowders of the lanthanum silicates substituted by Al for Si with a hydrothermal process at 180 oC, and then investigated effects of the substitution on conductive properties and crystal structure. We also examine the substitution effect on the local structure in lanthanum silicates by means of the reverse Monte Carlo (RMC) simulation. EXPERIMENTAL La9.33Si6-xAlxO26- d(x=0~1) were synthesized by hydrothermal method. In the hydrothermal synthesis, LaCl3・6H2O and SiO2, Al(OH)3 were mixed in the 4.8mol/l NaOH aqueous solution. The mixture was stirred for 1 hour at room temperature and then transferred into a Teflon lined autoclave. Synthesis was carried out at 180 oC for 96h. The powder after washing with distilled water was dried at 100 oC for a day. The product with PVA addition was pressed into a pellet and then sintered at 1250 oC for 12 h. The sample was identified by X-ray diffraction. Particle morphologies of the synthesized power, cross section and surface morphologies of the bulk samples were observed by the scanning electron microscopy (SEM). The densities of the sintered pellets were evaluated by the Archimedes method. The conductivity measurements were carried out between 300-900oC by the AC impedance method. In order to analyze crystal structures of the samples, synchrotron X-ray diffraction measurements were performed. The crystal structures were refined with Rietveld technique (REITAN-FP) using the data. Atomic arrangements (local structures) were also investigated in detail by the RMC simulation (RMC Profile) using the results of synchrotron X-ray (BL02B2, SPring-8) and neutron total scatterings (NOVA, J-PARC). RESULTS AND DISCUSSION La9.33Si6-xAlxO26- d(x=0~0.3) prepared by hydrothermal method had a single phase of the oxyapatite structure. However, La9.33Si6-xAlxO26- d(x=0.35~1) contained a secondary phase just after the hydrothermal process. By sintering the samples at 1250 oC, a single phase was obtained for La9.33Si6-xAlxO26- d(x=0.35~0.75). Fig.1 shows an SEM image of the powder of La9.33Si5.25Al0.75O26- d after hydrothermal method. An average particle size of the powder was 150 nm. From the measurements results of Archimedes method, relative densities of the sintered sample were more than 90 % regardless of the Al substitution amount. As reported in literatures, the Al-substituted samples showed higher conductivity and lower activation energies compared to the sample without the Al substitution. Since the conductivity did not depend on oxygen partial pressure, it can be considered that the oxygen ion conduction is dominant in the samples. In the Rietveld analysis, four possible space groups (P63/m, P63 P-3 and P21) were examined. From the refined unit cell, an initial model of 3 × 3 × 4 super cell was constructed for RMC simulation. In the simulation, we used synchrotron X-ray diffraction for the average structure, neutron and synchrotron X-ray total scattering for the local structure, simultaneously. As a result, it was suggested that the local structure around La vacancies was different significantly from that around La. REFERENCES 1)S. Chefi, A. Madani, H. Boussetta, C. Roux, A. Hammou, J. Power Sources, 177, 464 (2008) Figure 1