(Ln_0.9Gd_0.05Yb_0.05)₂Zr₂O₇ Ceramics with Pyrochlore Structure as Thermal Barrier Oxides

Abstract

Ceramic thermal barrier coatings (TBCs) used for thermal insulation against hot combustion gases greatly enhance operating temperatures and thermal efficiency of hot-section components in gas-turbine engines, and reduce fuel consumption and gas emissions at elevated temperatures. [1][2][3] Up to now, the most successful TBC materials in use are 7 ± 1 wt.%Y 2 O 3 stabilized ZrO 2 (7YSZ). However, 7YSZ is limited to operate below 1200°C. Above 1200°C, the t'-phase zirconia transforms into tetragonal and cubic phases. During cooling the tetragonal phase will further transform into monoclinic phase, which is accompanied with a volume change of 3-5 % and invariably causes cracking and failure. The relatively porous 7YSZ plasma-sprayed coatings are prone to sintering above 1200°C, which increases thermal conductivity and makes them less effective. To further improve the thermal insulation properties, it is urgently needed to develop new thermal barrier oxide materials with a lower thermal conductivity than 7YSZ. Some important requirements for excellent TBC materials are high melting point, high phase stability, low thermal conductivity, chemical inertness, high thermal expansion coefficient, high strain tolerance and low sintering rate at elevated temperatures. [4] Among high-melting temperature ceramic materials, rareearth zirconates, with the general formula Ln 2 Zr 2 O 7 (Ln = La, Nd, Sm, Eu, Gd, etc), have a distinctly lower thermal conductivity than 7YSZ. [5,6] Pyrochlore-structured La 2 Zr 2 O 7 -based TBCs were successfully prepared with electron beam physical vapor deposition (EB-PVD) method by Saruhan et al. [7] The hot corrosion resistance of La 2 Zr 2 O 7 coatings was better than that of 8 wt.% yttria-stabilized zirconia coatings produced by thermal spraying, and La 2 Zr 2 O 7 coatings remained well bonded to the substrate following exposure to vanadium compound at 1000°C. [8] La 2 Zr 2 O 7 TBCs exhibited a better thermal cycling performance of 1865 cycles at 1050°C than EB-PVD 7YSZ (1380 cycles) TBCs. [9] The rare-earth zirconate pyrochlore/YSZ double-layer material systems produced by atmospheric plasma spraying had a longer thermal cycling life than the single layer YSZ coatings. This indicates that the double-layer structure was an efficient way to use the advantages and overcome the disadvantages of different coating materials, giving promise to the application of the TBCs at temperatures higher than 1250°C. [10,11] It was reported that the substitution of some Ln sites in Ln 2 Zr 2 O 7 by other rareearth cations led to a low thermal conductivity as contrasted with pure rare-earth zirconates. [12][13][14] In the present study, Ln 2 Zr 2 O 7 (Ln = Sm, Nd) ceramics codoped with both 5 mol.%Gd and 5 mol.%Yb were prepared in order to further reduce thermal conductivity, and microstructure and thermophysical properties of (Ln 0.9 Gd 0.05 Yb 0.05 ) 2 Zr 2 O 7 (Ln = Sm, Nd) ceramics were examined. Experimental ProcedureIn the present study, rare earth oxide powders (Nd ; purity ≥ 99.99 %) and zirconium oxychl...

Effects of Y2O3 addition on the phase evolution and thermophysical properties of lanthanum zirconate

“…It is important to note that there are some structural features (e.g., defects such as oxygen vacancy V ö , substitution cation Y 3+ and other imperfections in the lattice such as pores or varying properties in thermal expansion along with different orientations) that have a significant effect on the overall TECs of different La 2 O 3 -ZrO 2 -Y 2 O 3 ceramics [23,24].…”

Synthesis and Characterization of a Neodymium Zirconate Pyrochlore Doped with Ytterbium: (YbxNd1-x)2Zr2O7 (x = 0.1, 0.25 and 0.4)

“…This interval is included in that of the formation of pyrochlore crystal, which is in the range of 1.46 to 1.78, at the atmospheric pressure [8]. The ionic radius of Zr 4+ is 0.72 Å in six-coordinated; however, the ionic radius of Nd 3+ and Yb 3+ are 1.109 and 0.985 Å in an eight-fold coordination, respectively [59].…”

Optimized functionally graded La2Zr2O7/8YSZ thermal barrier coatings fabricated by suspension plasma spraying

“…Many methods were proposed and used to improve the anti-spallation ability of the LZ coatings, such as nano-structured coatings or modified LZ compound with doped rare earth 50 element [10,11], double-ceramic-layer coatings [12], graded coatings [13]. Typically, current state-of-the-art TBCs are double ceramic layers system, consisting of outer LZ ceramic coatings and YSZ ceramic coating [8,12,14].…”