(La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇: A novel high-entropy ceramic with low thermal conductivity and sluggish grain growth rate

1. Introduction

Thermal barrier coatings (TBCs) have widely been used in aircraft and industrial gas-turbine engines to insulate turbine and combustor engine components from hot gas stream, as well as to improve the durability of metallic components and energy efficiency at elevated operation temperatures [1,2]. The general requirements for the backbone of thermal barrier coatings include (1) high thermal stability, (2) low thermal conductivity, (3) matched thermal expansion coefficient with metallic substrate [3,4]. Because of phase transformation, yttria-stabilized zirconia (YSZ) is restricted to be used below 1200 °C [5]. At higher operation temperatures, new thermal barrier coating (TBC) materials such as La₂Zr₂O₇, LaPO₄ and Yb₃Al₅O₁₂ with higher thermal stability and lower thermal conductivity have been proposed as potential candidates [4,[6], [7], [8], [9], [10]]. However, thermal stress-induced cracking and increase in thermal conductivity are still serious problems for TBC materials, which are caused by the grain growth during long-term service at higher temperatures. Thus, reducing grain growth rate is effective in preventing the thermal stress-induced cracking and reducing the thermal conductivity of TBC materials.

Recently, high-entropy ceramics (HECs), which are solid solutions containing three or more principal constituting single component compounds in equal or near equal molar ratio, have attracted increasing attentions for their intriguing properties, such as lower thermal conductivity, higher hardness and better environmental resistance than those of the single component compounds [[11], [12], [13], [14], [15], [16], [17], [18]]. One of the core effects for high-entropy materials is the sluggish diffusion [19,20], wherein atomic movement and effective diffusion of the atoms are hindered due to the lattice distortion induced by solid solution and the cooperative diffusion of multiple elements [21]. Thus, fine grains can be maintained and slow grain growth rate is expected when high-entropy materials are used at high temperatures. This sluggish diffusion effect opens a new window to design new TBC materials, i.e., high-entropy solid solutions with fine grains and slow grain growth rate.

Inspired by the sluggish diffusion effect of high-entropy materials, a novel high-entropy rare-earth zirconate (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ is designed and successfully synthesized in this work. The microstructure of as-synthesized (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ solid solution is characterized and low thermal conductivity as well as the slow grain growth rate is demonstrated in the later section.

2. Experimental procedure

(La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ powders were synthesized by a co-precipitation method using LnNO₃·6H₂O (Ln = La, Ce, Nd, Sm, Eu) (99.9% purity, Aladdin Biochemical Technology Co., Ltd, Shanghai, China) and ZrOCl₂·8H₂O (99% purity; Sinopharm Chemical Reagent Co., Ltd, Beijing, China) as starting materials. Firstly, five rare-earth nitrates (LnNO₃·6H₂O, Ln = La, Ce, Nd, Sm, Eu) in equal molar ratio were dissolved in distilled water and mixed. Separately, ZrOCl₂·8H₂O was also dissolved in distilled water and added into the LnNO₃ solution and then stirred vigorously for 30 min until clarified solution was formed. The molar ratio of total Ln element and Zr is Ln_total: Zr = 1:1. Secondly, excess aqueous ammonia (NH₃·H₂O, pH = 12.5) was added slowly into the mix solution with stirring to obtain gel-like precipitants. The precipitants were filtered and washed with distilled water for several times to ensure that impurity ions (NH₄⁺ and NO₃^-) were eliminated before being dried in an oven at 110 °C for 12 h. Finally, (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ powders were synthesized by calcining at 1300 °C for 1 h in argon then annealing at 1500 °C for 1 h in air. For comparison, phase-pure La₂Zr₂O₇ powders were also synthesized by the same method. The obtained powders were ball-milled with agate balls for 20 h and screened with 300 mesh sieve.

The phase composition of as-synthesized powders was identified by an X-ray diffractometer (XRD, D8 Advanced, Bruker, Germany) using Cu K_α radiation (λ = 1.5406 Å) with a step size of 0.02° at a scanning rate of 2°/min. The lattice constant of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ were refined by the Rietveld method using total pattern analysis solutions software (TOPAS, Bruker Corp., Karlsruhe, Germany).

In order to investigate the thermal conductivity of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇, the preparation of dense bulk compact is essential. The green bodies of (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ were prepared by pressing the as-synthesized powders under a uniaxial pressure of 360 MPa in 13 mm diameter steel die and then sintered at 1600 °C for 1 h. The density of as-prepared bulk compacts was measured by Archimedes' method. The microstructure analysis and element distributions of bulk samples were performed by using a scanning electron microscope (SEM, Apollo300, CamScan, Cambridge, UK) with the attached energy dispersive X-ray spectroscopic system (EDS Inca X-Max 80 T, Oxford, UK).

The thermal conductivity of bulk HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ was obtained through the relationship between thermal conductivity κ, thermal diffusivity D_th, heat capacity C_p and density ρ:

κ=D_th⋅C_p⋅ρ (1)

The thermal diffusivity of bulk HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ was measured by laser flash method using a sample with the size of Ø 10 mm × 2 mm. The heat capacity was obtained by Neumann-Kopp rule using the data of their constituent oxides [22].

To investigate the grain growth rate of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ and La₂Zr₂O₇, the green bodies of (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ and La₂Zr₂O₇ were prepared by pressing the as-synthesized powders under a uniaxial pressure of 360 MPa in 8 mm diameter steel die and then annealed at 1500 °C for different time in air. The microstructure and grain size of annealed samples were observed by SEM.

3. Results and discussion

Fig. 1 shows the crystal structure of Ln₂Zr₂O₇ pyrochlores (Ln = La, Ce, Nd, Sm, Eu). Ln₂Zr₂O₇ pyrochlores crystallize in a cubic structure with the space group of Fd $\bar{3}$ m (Z = 8). Ln atoms and Zr atom occupy 16d site (1/8, 1/8, 3/8) and 16c site (1/8, 1/8, 1/8), respectively. There are two O atom sites: O1 atom locates at 8b site (0, 0, 0), which is stable and tetrahedrally coordinated by Ln atoms. O2 atom locates at 48f (x, 1/8, 1/8), and is bonded with two Zr atoms and two Ln atoms. The 8a site (1/8, 1/8, 1/8), however, is unoccupied. The equilibrium position of O2 is expressed by x, which ranges from 0.3125 to 0.375 [23,24]. The same crystal structure of Ln₂Zr₂O₇ enables them to form a solid solution.

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Fig. 1.   Crystal structure of Ln₂Zr₂O₇ (Ln = La, Ce, Nd, Sm, Eu) pyrochlores.

Fig. 2 compares the XRD pattern of (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ powders with those of La₂Zr₂O₇, Ce₂Zr₂O₇, Nd₂Zr₂O₇, Sm₂Zr₂O₇ and Eu₂Zr₂O₇ obtained from ICDD/JCPDS cards. The peak positions of the single component Ln₂Zr₂O₇ pyrochlore shrift to the higher angle direction with the decrease of the Ln³⁺ radius (La³⁺→Eu³⁺). Since (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ solid solution has the same crystal structure with the single component Ln₂Zr₂O₇, the peak positions of (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ are lying in the middle of those of five single components Ln₂Zr₂O₇ (Ln = La, Ce, Nd, Sm, Eu) and almost coincide with those of Nd₂Zr₂O₇. The broaden of peaks of (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ indicates that complete (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ solid solution is formed and the five Ln³⁺ cations (La, Ce, Nd, Sm, Eu) averagely occupy the Ln sites of the solid solution. In short HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ powders have been successfully synthesized.

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Fig. 2.   Comparison of XRD patterns of as-synthesized (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ powders with those of single component Ln₂Zr₂O₇ (Ln = La, Ce, Nd, Sm, Eu) obtained from ICDD/JCPDS cards.

The cell dimension of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ was calculated by Rietveld refinement using TOPAS software. Table 1 illustrates the refined cell dimension of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ together with those of single component Ln₂Zr₂O₇ pyrochlore reported in literature [[25], [26], [27], [28], [29]]. The cell dimension of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ (10.62 Å) is smaller than those of La₂Zr₂O₇ (10.80 Å), Ce₂Zr₂O₇ (10.75 Å), Nd₂Zr₂O₇ (10.67 Å) but larger than those of Sm₂Zr₂O₇ (10.59 Å), Eu₂Zr₂O₇ (10.53 Å) and close to the average cell dimension of five single components Ln₂Zr₂O₇ (10.67 Å). Based on the refined cell dimension, the theoretical density of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ is calculated to be 6.49 g/cm³.

Fig. 3 shows the SEM image and EDS elemental mappings of the surface of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ compact sintered at 1600 °C for 1 h. No pores and cracks can be seen from the observed region indicating that the sample has a high relative density (ca. 96%). Furthermore, the five rare earth elements La, Ce, Nd, Sm, Eu are homogeneously distributed in the HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ and no obvious element segregation can be observed.

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Fig. 3.   SEM image and EDS elemental mappings of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ compact.

To demonstrate the advantage of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ as a promising thermal barrier coating material, thermal conductivity vs thermal expansion coefficients for HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ together with some thermal barrier coatings and nickel-based alloys [[30], [31], [32], [33], [34]] are presented in Fig. 4. The thermal expansion coefficient of RE₂Zr₂O₇ is (7.6-11.6) × 10^-6/°C, which is close to that of nickel-based alloys ((14-16) × 10^-6/°C). The close TEC between RE₂Zr₂O₇ and nickel-based alloys insures that the coating has good thermal cycling resistance. Furthermore, HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ has much lower thermal conductivity (0.76 W m^-1 K^-1 at room temperature) than those of single component RE₂Zr₂O₇ (1.2-1.9 W m^-1 K^-1) and other thermal barrier coating materials like YSZ (1.7-1.9 W m^-1 K^-1), TMP₂O₇ (0.8-1.1 W m^-1 K^-1) etc., as shown in Fig. 4. Low thermal conductivity and close TEC with nickel-based alloys indicate that HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ is promising for thermal barrier coating application.

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Fig. 4.   Thermal conductivity versus thermal expansion coefficients for HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ together with some thermal barrier coatings and nickel-based alloys.

Annealing experiment was performed to study the grain growth rate during high temperature annealing. Fig. 5 displays the microstructure of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ and La₂Zr₂O₇ compacts after annealing at 1500 °C for 1-18 h in air ((a): 1 h, (b): 6 h, (c): 12 h, (d): 18 h for (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ and (e): 1 h, (f): 6 h, (g): 12 h, (h): 18 h for La₂Zr₂O₇). When annealed at 1500 °C for 1 h, (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ and La₂Zr₂O₇ have average grain size of 1.69 and 1.96 μm, respectively. However, the grain growth rate of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ is much lower than that of La₂Zr₂O₇ when the annealing time is prolonged, as plotted in Fig. 6. After annealed at 1500 °C for 18 h, the grains of La₂Zr₂O₇ are coarse, with an average size of 8.89 μm. On the contrary, HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ has a narrow grain size distribution and most grains are fine with an average size of 3.92 μm when annealed at the same condition. In addition, some abnormally grown grains with the size of ∼18 μm can also be observed on the surface of La₂Zr₂O₇ compact, which can not be detected in HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇. The above result provides clear evidences that HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ has a lower grain growth rate than that of La₂Zr₂O₇.

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Fig. 5.   Microstructures of (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ and La₂Zr₂O₇ compacts after annealing at 1500 °C for 1-18 h in air: ((a): 1 h, (b): 6 h, (c): 12 h, (d): 18 h for (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ and (e): 1 h, (f): 6 h, (g): 12 h, (h): 18 h for La₂Zr₂O₇).

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Fig. 6.   Average grain size of (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ and La₂Zr₂O₇ compacts after annealing at 1500 °C for 1-18 h in air.

The slow grain growth rate of HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ can be explained by the sluggish diffusion effect of high-entropy materials, which results in HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ retaining fine grains when being heated at high temperatures. Slow grain growth rate is significant for thermal barrier coating materials, which is beneficial to improving the thermal stress-induced cracking resistance and prevent increasing of thermal conductivity. From this point, HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ is more suitable than single component Ln₂Zr₂O₇ as a promising thermal barrier coating material. More broadly, slowing down the grain growth rate by forming high-entropy solid solutions is not limited to rare-earth zirconate but is feasible for other TBC materials. Thus, forming high-entropy solid solution may open a new window to improve the thermal stress-cracking resistance and the thermal insulation performance of TBC materials.

4. Conclusion

In this work, a novel high-entropy rare-earth zirconate (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ was designed and successfully synthesized for the first time. The HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ has the same crystal structure as that of La₂Zr₂O₇. XRD and SEM analyses reveal that the as-synthesized HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ powders are phase-pure with homogeneous rare-earth elements distribution. The thermal conductivity of as-synthesized (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ at room temperature is as low as 0.76 W m^-1 K^-1. Due to sluggish diffusion effect of high-entropy solid solutions, HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ exhibits slower grain growth rate than that of La₂Zr₂O₇. After annealing at 1500 °C for 1-18 h, the average grain size of (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ increases from 1.69 to 3.92 μm, while the average grain size of La₂Zr₂O₇ increases from 1.96 μm to 8.89 μm. Low thermal conductivity and sluggish grain growth at high temperatures renders HE (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ promising for applications as a thermal barrier coating material. Furthermore, forming high-entropy solid solution may open a new window to improve the properties of TBC materials especially those related to grain size and stability.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos. 51672064 and U1435206).