Production of Crystal Oriented Neodymium Silicate Oxyapatite Ceramics and Their Conductivity
Authors: Tsunoda Yuichi , Kobayashi Kiyoshi , Higuchi Tohru , Suzuki Tohru S. 
Published: December 2020

Production of oxide ion conductor ceramics having high conductivity is important for applications of solid oxide fuel cells (SOFC) and chemical sensors. Particularly, the ceramics with high ionic conductivity around 873 K have attracted attention for development of SOFC and sensors operated at middle-temperature.
Neodymium silicate oxyapatite, denoted by NSO hereafter, was known to show very high oxide ion conductivity along to the c-axis below 873 K [1]. To utilize this property, production of the crystal-oriented ceramics is necessary. In this research, we produced crystal-oriented NSO green bodies by a slip casting under a magnetic field less than 1 T. Dense and crystal-oriented NSO ceramics were obtained by subsequent conventional sintering under an ambient atmosphere. In addition, NSO ceramics having different degree of crystalline orientation were successfully produced. Using these ceramics, the degree of the crystal orientation and its dependence on conductivity were discussed on the basis of experimental data.
Composition of NSO was Nd9.43(SiO4)6O2, which was confirmed to be close to the oxyapatite single phase (NSO has a wide composition range [2], [3]). The raw materials were Nd2O3 and SiO2. NSO was synthesized by solid state reaction method at 1473 K or 1773 K.
For production of crystal-oriented NSO green body, controlling the distribution of particle size was important to prepare a slurry used for slip casting. Therefore, the synthesized NSO powder was grinded by planetary ball mill. The slurry was prepared by mixing of the grinded NSO powder and polyethyleneimine in distilled water. The c-axis oriented NSO green body was prepared by slip casting under a magnetic field of 1 T and 0.5 T. These green bodies were denoted by NSO-1T and NSO-0.5T, hereafter. The green body produced without magnetic field was denoted as NSO-0T. The green bodies were sintered at 1873 K for 6 h in air in order to obtain the dense ceramics.
X-ray diffraction (XRD) patterns on surfaces of (a) NSO-0T, (b) NSO-0.5T and (c) NSO-1T are shown in the Figure. XRD profile of (a) NSO-0T is good agreement with that of the oxyapatite single phase (standard data (d), ICSD No. 01-075-7767). Regarding to the patterns of (b) NSO-0.5T and (c) NSO-1T, intensities of (00l) peaks were very high, compared to those of (a) NSO-0T and standard data. The degree of crystal orientation was evaluated by Lotgering factor [4] calculated from these XRD patterns. Lotgering factors of the densified NSO-1T and NSO-0.5T were 0.93 and 0.95, respectively. In the case of perfect crystal-orientation, Lotgering factor is unity. Contrary, the factor is zero for random crystalline orientation. The factor values of 0.93 and 0.95 indicate that the degrees of the c-axis orientation are very high.
Conductivity was measured by electrochemical impedance method between 773 K and 1273 K under air. The conductivity was found to be depended on the Lotgering factor values. Conductivities at 773 K of the samples sintered from NSO-1T, NSO-0.5T, and NSO-0T were 3.97×10-3, 3.04×10-3, and 3.88×10-4 S cm-1, respectively. The conductivities of the samples with highly c-axis orientation was about 10 times higher compared to that of randomly oriented samples. The conductivities of sintered NSO-0.5T and NSO-1T along to the c-axis at 773 K are confirmed to be close to that of a single crystal (6.4×10-3 S cm-1) [5].
Considering into high magnetic susceptibility of neodymium, the crystal-oriented NSO ceramics can be fabricated under magnetic fields less than 1 T and high conductivity was achieved comparable to that of single crystal.
Reference
[1] S. Nakayama, et al, Journal of the European Ceramic Society, 19, 507-510, (1999).
[2] R. O. Miller, et al, J. Am. Ceram. Soc., 47, 653-654, (1964).
[3] G. J. McCathy, et al, J. Inorg. Nucl. Chem., 29, 253-254, (1967).
[4] F. K. Lotgering, J Inorg. Nucl. Chem., 9, 113-123 (1959).
[5] S. Nakayama, et al, Journal of materials science letters, 20, 913-915, (2001).

Figure 1