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| Home > Publications database > B-site ordered double perovskite, La2(Al0.5MgTa0.5)O6 for thermal barrier applications and its high-temperature phase transition |
| Home > Publications database > B-site ordered double perovskite, La2(Al0.5MgTa0.5)O6 for thermal barrier applications and its high-temperature phase transition |
| Poster (Other) | FZJ-2018-04167 |
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2018
Abstract: An improvement of gas turbine engines can be obtained by increase of the inlet temperatures. The standard thermal barrier coating (TBC) material yttria partially stabilized zirconia (YSZ) decomposes at elevated temperatures into high-yttria and low-yttria phases. The latter transforms upon cooling into the monoclinic phase with an associated large volume increase, which may result in failure of the TBC [1]. Thus, new TBC materials are widely searched to further improve the gas turbine engine efficiency at long-term operation temperatures above 1200 °C. Over the last decades a large amount of candidates have been investigated to identify alternative TBC materials. Among them, the complex rare-earth perovskites gained interest due to their high melting point and possible tailoring properties with B-site cation ordering effect [2,3]. Recently, plasma sprayed La2(Al0.5MgTa0.5)O6 (LAMT) coatings showed significantly improved thermal cycling lifetime results, and were suggested to be a promising candidate [4,5]. Up to now, synthesized LAMT-powder was identified to be an orthorhombic phase with the space group symmetry, Pnma, which is isostructural to orthorhombic LaFeO3. The Pawley refinement on the measured powder X-ray diffraction (XRD) data revealed no oddity. However, a strong mismatch of the peak intensities was detected during the Rietveld analysis. A possible texture effect was excluded because of the powder morphology, as well as, by trying out the different sample preparation methods. To clarify the correct crystal structure of LAMT and to check the phase stability, in-situ high-temperature XRD was carried out in the temperature range of 25-1200 °C. Unlike reported earlier [4], a structural phase transition was observed at ~ 855 °C upon heating, and this phase transition was completely reversible. The crystal structure of LAMT was then refined by Rietveld analysis in the monoclinic space group symmetry, P21/n [6] at room temperature, with β = ~ 90°. In the monoclinic crystal structure, the B-site cations are ordering in a rock-salt type arrangement. The Mg2+-ions take the fully occupied 2c-Wyckoff position whereas the Al3+- and Ta5+-ions occupy each half of the 2d-Wyckoff position. The crystal structure of LAMT becomes rhombohedral with the space group, R-3 above ~ 855°C. The unit cell volume changes gradually as a function of temperature without any abrupt jump. [1] Vaßen R., Jarligo M.O., Steinke T., Mack D.E. and Stöver D. Surf. Coat. Technol., 2010, 205, 938. [2] Guo R., Bhalla A.S. and Cross L.E. J. Appl. Phys., 1994, 75, 4704. [3] Tarvin R. and Davies P.K. J. Am. Ceram. Soc., 2004, 87, 859. [4] Jarligo M.O., Mack D.E., Vaßen R. and Stöver D. J. Therm. Spray Technol., 2009, 18, 187.[5] Schlegel N., Sebold D., Sohn Y.J., Mauer G. and Vaßen R. J. Therm. Spray Technol., 2015, 24, 1205.[6] Kim Y.I. and Woodward P.M. Solid State Chem., 2007, 180, 2798.Keywords: thermal barrier coatings, ordered double perovskite, in-situ high-temperature XRD
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