Crystallographic Correlations with Anisotropic Oxide Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes
To better understand the oxide ion conduction mechanism of rare earth silicate apatites as intermediate temperature electrolytes for solid oxide fuel cells (SOFC), the effect of lower valent metal doping on the performance of Nd(28+x)/3AlxSi6-xO26 (0 ≤ x ≤ 2) single crystals has been examined. The m...
| Main Authors: | , , , , , , , , , |
|---|---|
| Other Authors: | |
| Format: | Journal Article |
| Language: | English |
| Published: |
2017
|
| Subjects: | |
| Online Access: | https://hdl.handle.net/10356/85584 http://hdl.handle.net/10220/43744 |
| _version_ | 1824454168482414592 |
|---|---|
| author | An, Tao Baikie, Tom Wei, Fengxia Pramana, Stevin S. Schreyer, Martin K. Piltz, Ross O. Shin, J. Felix Wei, Jun Slater, Peter R. White, Timothy John |
| author2 | School of Materials Science & Engineering |
| author_facet | School of Materials Science & Engineering An, Tao Baikie, Tom Wei, Fengxia Pramana, Stevin S. Schreyer, Martin K. Piltz, Ross O. Shin, J. Felix Wei, Jun Slater, Peter R. White, Timothy John |
| author_sort | An, Tao |
| collection | NTU |
| description | To better understand the oxide ion conduction mechanism of rare earth silicate apatites as intermediate temperature electrolytes for solid oxide fuel cells (SOFC), the effect of lower valent metal doping on the performance of Nd(28+x)/3AlxSi6-xO26 (0 ≤ x ≤ 2) single crystals has been examined. The measurement of ionic conductivity via AC impedance spectroscopy showed that the conductivities were anisotropic and superior along the c direction. An interesting aspect from the impedance studies was the identification of a second semicircle with capacitance similar to that of a grain boundary component, despite the fact that polarized optical microscopy and electron backscattered diffraction showed that the single crystals consisted of a single grain. This semicircle disappeared after long-term (up to 3 months) annealing of the single crystals at 950 °C, also leading to a reduction in the bulk conductivity. In order to explain these observations, single-crystal X-ray diffraction studies were performed both before and after annealing. These studies found the undoped crystal conformed to P63/m, but with the O(3) oxygen positions, that participate in conduction, split nonstatistically across two sites with a shortened Si–O(3) bond. Consequently, the bond valence sum (BVS) of the Si (4.20) is larger than the formal valence. Fourier difference maps of the Al-doped crystals contain regions of excess scattering, suggesting the possible lowering of symmetry or creation of superstructures. After long-term annealing, the single crystal structure determinations were of higher quality and the experimental and nominal compositions were in better agreement. From these observations, we propose that in the as-prepared single crystals there are regions of high and low interstitial content (e.g., Nd9.67Si6O26.5 and Nd9.33Si6O26), and the second semicircle relates to the interface between such regions. On annealing, Nd redistribution and homogenization removes these interfaces and also reduces the number of interstitial oxide ions, hence eliminating this second semicircle while reducing the bulk conductivity. The results therefore show for the first time that the conductivity of apatite materials containing cation vacancies is affected by the thermal history. |
| first_indexed | 2025-02-19T03:18:01Z |
| format | Journal Article |
| id | ntu-10356/85584 |
| institution | Nanyang Technological University |
| language | English |
| last_indexed | 2025-02-19T03:18:01Z |
| publishDate | 2017 |
| record_format | dspace |
| spelling | ntu-10356/855842020-09-26T22:11:32Z Crystallographic Correlations with Anisotropic Oxide Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes An, Tao Baikie, Tom Wei, Fengxia Pramana, Stevin S. Schreyer, Martin K. Piltz, Ross O. Shin, J. Felix Wei, Jun Slater, Peter R. White, Timothy John School of Materials Science & Engineering A*STAR SIMTech Solid electrolyte Apatite To better understand the oxide ion conduction mechanism of rare earth silicate apatites as intermediate temperature electrolytes for solid oxide fuel cells (SOFC), the effect of lower valent metal doping on the performance of Nd(28+x)/3AlxSi6-xO26 (0 ≤ x ≤ 2) single crystals has been examined. The measurement of ionic conductivity via AC impedance spectroscopy showed that the conductivities were anisotropic and superior along the c direction. An interesting aspect from the impedance studies was the identification of a second semicircle with capacitance similar to that of a grain boundary component, despite the fact that polarized optical microscopy and electron backscattered diffraction showed that the single crystals consisted of a single grain. This semicircle disappeared after long-term (up to 3 months) annealing of the single crystals at 950 °C, also leading to a reduction in the bulk conductivity. In order to explain these observations, single-crystal X-ray diffraction studies were performed both before and after annealing. These studies found the undoped crystal conformed to P63/m, but with the O(3) oxygen positions, that participate in conduction, split nonstatistically across two sites with a shortened Si–O(3) bond. Consequently, the bond valence sum (BVS) of the Si (4.20) is larger than the formal valence. Fourier difference maps of the Al-doped crystals contain regions of excess scattering, suggesting the possible lowering of symmetry or creation of superstructures. After long-term annealing, the single crystal structure determinations were of higher quality and the experimental and nominal compositions were in better agreement. From these observations, we propose that in the as-prepared single crystals there are regions of high and low interstitial content (e.g., Nd9.67Si6O26.5 and Nd9.33Si6O26), and the second semicircle relates to the interface between such regions. On annealing, Nd redistribution and homogenization removes these interfaces and also reduces the number of interstitial oxide ions, hence eliminating this second semicircle while reducing the bulk conductivity. The results therefore show for the first time that the conductivity of apatite materials containing cation vacancies is affected by the thermal history. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version 2017-09-15T07:27:03Z 2019-12-06T16:06:35Z 2017-09-15T07:27:03Z 2019-12-06T16:06:35Z 2013 Journal Article An, T., Baikie, T., Wei, F., Pramana, S. S., Schreyer, M. K., Piltz, R. O., et al. (2013). Crystallographic Correlations with Anisotropic Oxide Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes. Chemistry of Materials, 25(7), 1109-1120. 0897-4756 https://hdl.handle.net/10356/85584 http://hdl.handle.net/10220/43744 10.1021/cm4000685 en Chemistry of Materials © 2013 American Chemical Society. This is the author created version of a work that has been peer reviewed and accepted for publication by Chemistry of Materials, American Chemical Society. It incorporates referee’s comments but changes resulting from the publishing process, such as copyediting, structural formatting, may not be reflected in this document. The published version is available at: [http://dx.doi.org/10.1021/cm4000685]. 19 p. application/pdf |
| spellingShingle | Solid electrolyte Apatite An, Tao Baikie, Tom Wei, Fengxia Pramana, Stevin S. Schreyer, Martin K. Piltz, Ross O. Shin, J. Felix Wei, Jun Slater, Peter R. White, Timothy John Crystallographic Correlations with Anisotropic Oxide Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes |
| title | Crystallographic Correlations with Anisotropic Oxide Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes |
| title_full | Crystallographic Correlations with Anisotropic Oxide Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes |
| title_fullStr | Crystallographic Correlations with Anisotropic Oxide Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes |
| title_full_unstemmed | Crystallographic Correlations with Anisotropic Oxide Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes |
| title_short | Crystallographic Correlations with Anisotropic Oxide Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes |
| title_sort | crystallographic correlations with anisotropic oxide ion conduction in aluminum doped neodymium silicate apatite electrolytes |
| topic | Solid electrolyte Apatite |
| url | https://hdl.handle.net/10356/85584 http://hdl.handle.net/10220/43744 |
| work_keys_str_mv | AT antao crystallographiccorrelationswithanisotropicoxideionconductioninaluminumdopedneodymiumsilicateapatiteelectrolytes AT baikietom crystallographiccorrelationswithanisotropicoxideionconductioninaluminumdopedneodymiumsilicateapatiteelectrolytes AT weifengxia crystallographiccorrelationswithanisotropicoxideionconductioninaluminumdopedneodymiumsilicateapatiteelectrolytes AT pramanastevins crystallographiccorrelationswithanisotropicoxideionconductioninaluminumdopedneodymiumsilicateapatiteelectrolytes AT schreyermartink crystallographiccorrelationswithanisotropicoxideionconductioninaluminumdopedneodymiumsilicateapatiteelectrolytes AT piltzrosso crystallographiccorrelationswithanisotropicoxideionconductioninaluminumdopedneodymiumsilicateapatiteelectrolytes AT shinjfelix crystallographiccorrelationswithanisotropicoxideionconductioninaluminumdopedneodymiumsilicateapatiteelectrolytes AT weijun crystallographiccorrelationswithanisotropicoxideionconductioninaluminumdopedneodymiumsilicateapatiteelectrolytes AT slaterpeterr crystallographiccorrelationswithanisotropicoxideionconductioninaluminumdopedneodymiumsilicateapatiteelectrolytes AT whitetimothyjohn crystallographiccorrelationswithanisotropicoxideionconductioninaluminumdopedneodymiumsilicateapatiteelectrolytes |