Fabrication of Dense Ceramics and the Electrical Conductivity Anisotropy of the Textured MgO-Doped Lanthanum Silicate Oxyapatite
Authors: Shimura Yuki , Kobayashi Kiyoshi , Suzuki Tohru S. , Uchikoshi Tetsuo , Sakka Yoshio , Akashi Takaya 
Published: February 2020

【Introduction】
Lanthanum silicate oxyapatite (LSO) has attracted attention as a new solid electrolyte for solid oxide fuel cell due to its high oxygen-ion conductivity and chemical stability[1]. Magnesium oxide has been reported as the effective dopant to improve the oxygen-ion conductivity of the LSO [2]. It has been revealed that non-doped LSO shows the anisotropic oxygen-ion conductivity depending on the crystal structure direction. It is already known that the oxygen-ion conductivity parallel to the c-axis of the non-doped LSO is much higher than that perpendicular to the c-axis. On the other hand, the anisotropic oxygen-ion conductivity of MgO-doped LSO has not been reported yet. In this study, influence of the crystal orientation on the total conductivity of the MgO-doped LSO was investigated by using texture ceramics which were fabricated by colloidal processing under a strong magnetic field.
【Experimental】
Lanthanum hydroxides La(OH)3, amorphous silica SiO2, Magnesium oxide MgO were used as the raw materials. The raw powders, YSZ balls with a diameter of 5mm, and ethanol were mixed by a pod mill, and the resulting slurry was sieved and dried at 373K. The dried powder was pulverized and the resulting powder was pressed at 20MPa into pellets. The pellets were calcinated at 1673K for 10h in air. The calcinated pellets were pulverized into powder. The resulting powder was dispersed into a water dissolving of ammonium polycarboxylic acid. After the YSZ balls with a diameter of 1mm was added into the mixture, the dispersed MgO-doped LSO powder was ground into small particle using planetary ball mill. This slurry was consolidated by slip casting in a strong magnetic field (12T). A magnetic field was applied parallel or perpendicular to the casting direction. In the case of the latter, the green compacts were prepared in two ways that slurry was rotated or not. The green compacts were pressed by cold isostatic pressing (CIP) at 392MPa, and were then calcinated at 1073K for 5h to remove dispersant. Texture ceramics were obtained by sintering at 1873K for 10h in air. The degree of crystalline orientation was determined by X-ray diffraction (XRD) analysis. The total conductivity of the samples were measured using the two-probe impedance method.
【Results and discussion】
The relative intensities of the (h00) peaks in the surface perpendicular to the static magnetic field were higher than those of the non-oriented one. In contrast, on the surface parallel to the rotating plane of strong magnetic field, the intensities of the (00l) peaks are relatively strong compared to the other peaks. These XRD data show that the crystalline orientation of MgO-doped LSO was controlled by slip casting in a strong magnetic field, and c-axis was aligned by the rotating magnetic field. It is different from non-doped LSO whose the c-axis was aligned parallel to the magnetic field. By comparing to the total conductivity of the above ceramics, the order of the magnitude was found to be the c-axis oriented ceramics > non-textured > a,b-axes oriented one.
【References】
[1] S. Nakayama et al., J. Eur. Ceram. Soc., 18, 1413-1418 (1998)
[2] H. Yoshioka et al., Solid State Ionics, 179, 2165-2169 (2008)

Figure 1