Solid State Synthesis and Characterization of Mixed Rare Earth Hexaaluminates

Hexaaluminates are promising candidates for thermal barrier coatings in gas turbine technology operating at temperatures exceeding 1350°C. The magnetoplumbite structure of the rare earth magnesium hexaaluminates (REMgAl₁₁O₁₉) enables host element substitutions in both rare-earth (RE) and Mg sites of the lattice. The chosen compositions for the experimental studies were based on the estimation of the thermal conductivity variation upon single and multiple substitutions in the rare-earth sites. A mathematical equation was developed, for approximate estimation of the thermal linear expansion coefficient in solid crystal dielectrics above the Debye temperature. Thus materials having required values of thermal expansion coefficient may be made. Compositions corresponding to RE_1-x-yM_xM'_yMgAl₁₁O₁₉ (RЕ=La, Sm, Gd; M, M'=Gd, Yb; x, y=0; 0.15; 0.3; 0.5) were prepared in the form of ceramics by single step solid state synthesis. The microstructure, phase composition and distribution of elements in the materials were investigated by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray microanalysis. It is found that compositions involving double and triple substitutions by different rare-earth ions may contain traces of secondary phases in addition to the hexaaluminate phase. The hexaaluminate phase is sintered forming platelet-like grains of ~5μm, while the secondary phases are separated in the form of near-spherical granules and their conglomerates. The content of secondary phases is increasing with concentration of introduced substituent ions, especially in the case of Yb. Variation of the temperature and duration of the process, respectively in between 1600-1700°C and 10-30h, does not significantly affect the resulting phase composition. It is concluded that the melt preparation techniques may be more efficient for preparation of single phase products of compositions selected in this study.    

Financial support by «THEBARCODE - Development of multifunctional Thermal Barrier Coatings and modeling tools for high temperature power generation with improved efficiency» FP7-NMP-2012-SMALL-6, Collaborative project is acknowledged.