Preparation and Study of Rare-Earth Hexaaluminates with Double Substitutions

During the last decade there is a continuous search for new thermal barrier coatings stable at temperatures above 1200 C for applications in turbogenerators and jet engines operating at elevated temperatures. Hexaaluminates (LnMgAl₁₁O₁₉) are in the list of actively studied compounds; they exhibit phase stability at high temperatures and low thermal conductivity. The goal of this work was to perform double substitutions in lanthanide sites and to study their effect upon the thermal conductivity.

The following compositions were prepared for the first time applying solid state reactions: lanthanum based La_1-х-yM_xM`<sub>y</sub>MgAl<sub>11</sub>O<sub>19</sub> (х=y=0.15) with pair impurities M<sub>x</sub>M`_y(Gd–Yb, Gd–Y, Y–Yb, Y–Lu, Y–Sc) and samarium-based Sm_1-х-yM_xM`_yMgAl₁₁O₁₉ (х=y=0.15) with pair impurities of Gd–Yb and Y–Yb. The phase composition of obtained products and unit cell parameter of the hexaaluminate phase were determined. Optimization of conditions (including the temperature, duration, atmosphere, mixing procedure before tableting) resulted in improvement of homogeneity and decrease of percentage of secondary phases (α-Al₂O_3, LaAlO₃, MgAl₂O₄).  Basing on the studies of micro-structure and elemental composition on the surface and in transverse sections it is shown that the evaporation rate of magnesium oxide is higher, in comparison to other oxides, resulting in deviation from stoichiometry on surfaces. It is shown that the thermal conductivity of both lanthanum- and samarium- based compounds decreases upon introduction of Gd-Yb pairs; it is however increasing to some extent in the case of pairs involving Y. The hexaaluminates with double substitutions may thus present interest as thermal isolation coatings only in the case of properly selected pair impurities, in terms of their ionic radii ratio and atomic weights. 

 The work is performed in the frame of grant №310750 «THEBARCODE - Development of multifunctional Thermal Barrier Coatings and modeling tools for high temperature power generation with improved efficiency» FP7-NMP-2012-SMALL-6.