Elsevier

Ceramics International

Volume 48, Issue 2, 15 January 2022, Pages 1512-1521
Ceramics International

Thermophysical performances of (La1/6Nd1/6Yb1/6Y1/6Sm1/6Lu1/6)2Ce2O7 high-entropy ceramics for thermal barrier coating applications

https://doi.org/10.1016/j.ceramint.2021.09.172Get rights and content

Abstract

In this study, a novel high-entropy oxide of (La1/6Nd1/6Yb1/6Y1/6Sm1/6Lu1/6)2Ce2O7was prepared using a sol–gel and high-temperature sintering technology. Additionally, its lattice structure, micro-morphology, elemental composition, and thermophysical and mechanical properties were evaluated. The results revealed that the obtained oxide powder has a typical fluorite-type lattice with particle sizes in the range of∼30–100 nm. The bulk sample has a dense microstructure and uniform elemental distribution. Owing to its low lattice order, the thermal expansion coefficient of (La1/6Nd1/6Yb1/6Y1/6Sm1/6Lu1/6)2Ce2O7 is greater than that of Sm2Ce2O7, which also exhibits excellent lattice stability up to 1200 °C. Further, owing to phonon scattering due to lattice distortion, oxygen vacancy, and cation substitution, the thermal conductivity of (La1/6Nd1/6Yb1/6Y1/6Sm1/6Lu1/6)2Ce2O7 is lower than that of Sm2Ce2O7, while its mechanical performance is inferior to that of 7YSZ.

Introduction

Over the past few decades, Y2O3-stabilised ZrO2 (YSZ) thermal barrier coatings prepared by air plasma spraying or electron beam physical vapour deposition have been used to protect the metallic components of advanced turbine engines to prevent their corrosion at high temperatures and in servicing environments [1,2]. However, the prevailing phase transformation, poor sintering resistance, and inferior thermal insulation properties of YSZ above 1200 °C result in short service lives of thermal barrier coatings [3]. The most feasible and economical method for overcoming the shortcomings of the current YSZ thermal barrier coatings is to develop novel candidates with a high thermal expansion coefficient, low thermal conductivity, and excellent lattice stability at high temperatures [[4], [5], [6], [7], [8]].

It has been suggested that the addition of multi-type metal cations into ceramic lattices can induce lattice distortions and point defects, which can help reduce the thermal conductivity of ceramics [9]. Furthermore, the thermal expansion coefficient of ceramics can be regulated via elemental doping [10,11]. Recently, high-entropy ceramics have attracted increased attentions as novel candidates for thermal barrier coatings [12]. High-entropy ceramics, which are solid solutions of inorganic compounds with multiple principal elements at equal or near-equal atomic ratios, exhibit an amorphous thermal conductivity, excellent lattice stability, and enhanced mechanical properties [[13], [14], [15]]. The thermophysical and mechanical performances of high-entropy ceramics are expected to improve the service life of thermal barrier coatings [7,8], and several novel high-entropy oxides have been reported as thermal barrier coatings. For example, high-entropy (5RE1/2)2Zr2O7 ceramics exhibit excellent phase stability and low thermal conductivity between 300 °C and 1200 °C (<1 W/m∙ K) [16]. The room-temperature thermal conductivity of (Y0.25Yb0.25Er0.25Lu0.25)2(Zr0.5Hf0.5)2O7 is ∼1.40 W/m∙ K, which is lower than that of Yb2Zr2O7 [17]. The thermal expansion coefficients of (Sm1/6Er1/6Y1/6Yb1/6Lu1/6Er1/6)3(Nb1/2Ta1/2)3O7, (Y1/3Yb1/3Er1/3)3NbO7, and (Y1/3Yb1/3Er1/3)3TaO7 are comparable with that of RE3TaO7, and their Vickers hardness values are in the range of 10.9–12.0 GPa [18]. Compared with TiP2O7, ZrP2O7, and HfP2O7, (TiZrHf)P2O7 has better thermal stability up to 1550 °C and lower thermal conductivity (0.78 W/m∙ K) [19].

The RE2Ce2O7 oxides have higher thermal expansion coefficients than other oxides studied as candidate materails for thermal barrier coatings [20,21]. However, the thermophysical characteristics of high-entropy RE2Ce2O7 ceramics have not been reported to date. In this study, a novel high-entropy oxide of (La1/6Nd1/6Yb1/6Y1/6Sm1/6Lu1/6)2Ce2O7 was prepared, and its lattice type and thermophysical and mechanical properties were examined.

Section snippets

Experimental

For the synthesis of the nano-scale powder of (La1/6Nd1/6Yb1/6Y1/6Sm1/6Lu1/6)2Ce2O7 using a modified sol–gel method, high-purity RE2O3 oxides (RE = La, Nd, Yb, Y, Sm, and Lu; ≥99.9% purity) were selected as the raw materials, and analytically pure Ce(NO3)·6H2O was selected as the Ce precursor. The weighted rare-earth oxides were first dissolved in diluted nitric acid to obtain a mixed Ln(NO3)3·nH2O solution. After continuous stirring for approximately half an hour, a solution of Ce(NO3)·6H2O in

XRD

The XRD pattern of the obtained (La1/6Nd1/6Yb1/6Y1/6Sm1/6Lu1/6)2Ce2O7powder is presented in Fig. 1, together with the XRD pattern of standard Sm2Ce2O7. The XRD pattern of the (La1/6Nd1/6Yb1/6Y1/6Sm1/6 Lu1/6)2Ce2O7 powder is nearly identical to that of standard Sm2Ce2O7, implying that it has a typical fluorite lattice. Further, Fig. 2 shows that the obtained ceramic powder also yields the same Raman spectrum as Sm2Ce2O7, confirming that a pure (La1/6Nd1/6 Yb1/6Y1/6Sm1/6Lu1/6)2Ce2O7 powder with

Conclusions

  • (1)

    A nano-scale powder of (La1/6Nd1/6Yb1/6Y1/6Sm1/6Lu1/6)2Ce2O7 with single fluorite-type lattice was prepared by a sol–gel method. Using this nano-scale powder, a sintered bulk sample that retained the fluorite lattice was formed. The bulk sample also exhibited a dense microstructure and clear grain boundaries.

  • (2)

    Owing to the relatively low structural order of the obtained high-entropy oxide, its thermal expansion coefficient is higher than that of Sm2Ce2O7. The obtained high-entropy oxide also

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the fiscal foundation of the Henan Province Science and Technology Research Plan Project (212102210181) and the National Natural Science Foundation of China of China (No. U2004182).

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