Journal of Materials Science & Technology, 2020, 47(0): 45-51 DOI: 10.1016/j.jmst.2020.02.011

Research Article

High-entropy (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3: A promising thermal/environmental barrier material for oxide/oxide composites

Zifan Zhaoa,b, Heng Chena, Huimin Xianga, Fu-Zhi Daia, Xiaohui Wangc, Wei Xud, Kuang Sund, Zhijian Peng,b,*, Yanchun Zhou,a,*

aScience and Technology on Advanced Functional Composite Laboratory, Aerospace Research Institute of Materials & Processing Technology, Beijing, 100076, China

bSchool of Engineering and Technology, China University of Geosciences, Beijing 100083, China

cShenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China

dShanghai Chenhua Science and Technology Corporation Limted, Shanghai, 201804, China

Corresponding authors: * E-mail addresses:pengzhijian@cugb.edu.cn(Z. Peng),yczhou@alum.imr.ac.cn(Y. Zhou).

Received: 2019-11-22   Accepted: 2020-01-28   Online: 2020-06-15

Abstract

Yttrium aluminum perovskite (YAlO3) is a promising candidate material for environmental barrier coatings (EBCs) to protect Al2O3f/Al2O3 ceramic matrix composites (CMCs) from the corrosion of high-temperature water vapor in combustion environments. Nevertheless, the relatively high thermal conductivity is a notable drawback of YAlO3 for environmental barrier coating application. Herein, in order to make REAlO3 more thermal insulating, a novel high-entropy rare-earth aluminate ceramic (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 was designed and synthesized. The as-prepared (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 ceramic possesses close thermal expansion coefficient (9.02 × 10―6 /oC measured from room temperature to 1200 °C) to that of Al2O3. The thermal conductivity of (Y0.2Nd0.2Sm0.2Eu0.2Er0.2) AlO3 at room temperature is 4.1 W·m-1 K-1, which is almost one third of the value of YAlO3. Furthermore, to effectively prevent the penetration of water vapor from possible pores/cracks of coating layer, which are often observed in T/EBCs, a tri-layer EBC system REAlO3/RE3Al5O12/(Al2O3f/Al2O3 CMCs) is designed. Close thermal expansion coefficient to Al2O3 and low thermal conductivity of (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3, as well as the formation of dense garnet layer at (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3/Al2O3 interface, indicate that this new type of high-entropy ceramic is suitable as a candidate environmental barrier coating material for Al2O3f/Al2O3 CMCs.

Keywords: High-entropy ceramics ; (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 ; Tri-layer EBC system ; Low thermal conductivity ; Al2O3f/Al2O3 CMCs

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Zifan Zhao, Heng Chen, Huimin Xiang, Fu-Zhi Dai, Xiaohui Wang, Wei Xu, Kuang Sun, Zhijian Peng, Yanchun Zhou. High-entropy (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3: A promising thermal/environmental barrier material for oxide/oxide composites. Journal of Materials Science & Technology[J], 2020, 47(0): 45-51 DOI:10.1016/j.jmst.2020.02.011

1. Introduction

Oxide/oxide ceramic matrix composites (CMCs) have potential applications in gas turbine engines for their heat resistance, good chemical stability and oxidizing-free capacity [[1], [2], [3], [4], [5]]. Among them, the state-of-the-art one is Al2O3f/Al2O3 ceramic matrix composite (CMC) [6]. However, the major stumbling blocks to realizing Al2O3f/Al2O3 CMCs turbine hot section components are their insufficient creep rupture strengths at high temperatures and the recession of Al2O3 in combustion environments due to the formation of volatile aluminum hydroxide products by water vapor corrosion [[6], [7], [8]]. Thus, the application of environmental barrier coatings (EBCs) is the key for Al2O3f/Al2O3 CMCs that will be used as the hot section components of gas turbine engines in combustion environments.

In general, there are several key requirements must be taken into account when selecting EBC materials [6,9]. Fig. 1 schematically illustrates the critical issues in selecting the EBC materials of Al2O3f/Al2O3 CMCs. Firstly, good phase stability and absence of decomposition and phase transformation from room temperature to the service temperature is essential for EBC materials. Secondly, EBC materials should possess good compatibility with the Al2O3f/Al2O3 CMCs, i.e., (1) close thermal expansion coefficient with the substrates to prevent cracking or delamination due to CTE mismatch stress; (2) good chemical compatibility with the substrates. Thirdly, EBC materials must have good durability to extreme environments, such as high temperature heat flux and H2O/O2 corrosion. Thus, low thermal conductivity and good water vapor resistance are also required for EBC materials.

Fig. 1.

Fig. 1.   Schematic illustration of the critical issues in selecting the EBC materials of Al2O3f/Al2O3 CMCs.


Over the past years, many investigations have demonstrated that rare-earth aluminates are promising candidates for EBC application due to their superior high temperature stability and outstanding durability in water vapor [[10], [11], [12], [13], [14], [15], [16], [17]]. Among these compounds, yttrium aluminum perovskite (i.e. YAlO3) attracted specific attention due to its high melting point (1875 °C), close thermal expansion coefficient (α = 8.8 × 10―6 /oC) to Al2O3 (9.0 × 10―6 /oC), and good stability in combustion environments [[18], [19], [20], [21]]. However, relatively high thermal conductivity of YAlO3 (11 W·m-1 K-1 at room temperature) is the bottleneck that limiting its applications for EBCs [19]. Thus, it is significant to reduce the thermal conductivity of YAlO3.

In recent years, high-entropy ceramics (HECs) have attracted enormous attention because they exhibit some superior properties such as high hardness, good water-vapor corrosion resistance, good thermal stability, low thermal conductivity, low grain growth rate, and tunable thermal expansion coefficient [[22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]]. Especially, because the enhanced phonon scattering resulted from the high entropy effect and lattice distortion effect, high-entropy ceramics usually possess lower thermal conductivities than those of the single-component compounds [[27], [28], [29], [30], [31], [32]]. Therefore, in order to make REAlO3 more thermal insulating, a novel high-entropy rare-earth aluminate ceramic (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 was designed and synthesized in this work. The chosen of this composition is due to the following reasons: firstly, the five compounds possess similar crystal structures; secondly, the difference of ion radius of five rare-earth elements is small (less than 11%). Based on these facts, the single-phase solid solution containing five rare-earth elements is proven to form. In order to demonstrate the suitability of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 as T/EBC material, the phase composition, microstructure, thermal expansion coefficient, thermal conductivity and chemical compatibility of this new HE rare-earth aluminate were investigated.

2. Experimental procedure

A co-precipitation method was performed to synthesize HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2) AlO3 powders. The synthesizing process was introduced as following: firstly, Y2O3 and Er2O3 powders (99.9% purity; HWRK Chem. Co., Ltd, Beijing, China) with molar ratio of 1:1 were dissolved in HNO3 solution (Beihua Fine Chem. Co., Ltd. Beijing, China) to form clear Y(NO3)3 and Er(NO3)3 solutions by heating at 40 °C with stirring. Respectively, RE(NO3)3·6H2O (RE= Nd, Sm, and Eu) (99.9% purity, Aladdin Biochemical Technology Co., Ltd, Shanghai, China) were also dissolved in distilled water. Then five rare-earth nitrate solutions were mixed in equal molar ratio. After that, Al(NO3)3·9H2O (99.9% purity, Aladdin Biochemical Technology Co., Ltd, Shanghai, China) was added into the RE(NO3)3 solution and then stirred vigorously until clarified solution was formed. The molar ratio of total RE elements and Al is REtotal: Al = 1:1. Secondly, excess aqueous ammonia (NH3·H2O, pH = 12.5, Beihua Fine Chem. Co., Ltd. Beijing, China) was added slowly into the mixed solution with stirring to obtain gel-like precipitants. Afterwards, the obtained precipitants were filtered and washed with distilled water for several times to ensure impurity ions (NH4+ and NO3-) were eliminated before being dried in an oven at 110 °C for 12 h. Finally, (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 powders were synthesized by calcining at 1500 °C for 1 h. The synthesized powders were ball milled in ethyl alcohol with agate balls for 8 h and then dried and filtered by a 300 mesh sieve to obtain fine particles.

The phase composition of as-synthesized powders was determined by an X-ray diffractometer (XRD, D8 Advanced, Bruker, Germany) with Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 2°/min. The lattice parameters were refined by Rietveld method using total pattern analysis solutions software (TOPAS, Bruker Corp., Karlsruhe, Germany). The theoretical density of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 was calculated by the refined lattice parameters.

To investigate the thermal properties of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3, the dense bulk compacts need to be prepared first. The densification of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 powders was performed by a spark plasma sintering apparatus (SPS-20T-6-IV, Shanghai Chenhua Science and Technology Co., Ltd, China) at 1650 °C for 4 min under a pressure of 30 MPa. The details of the sintering process have been described in our previous papers [32,36].

Bulk density of the as-prepared compacts was measured by Archimede’s method. The relative density of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 compacts was determined as the ratio of bulk density to theoretical density. Phase composition of the sintered samples was identified by a powder X-ray diffractometer. The microstructure and element distributions of the sintered samples were investigated by a scanning electron microscope (SEM, Apollo300, CamScan, Cambridge, UK) equipped with an energy dispersive spectroscopy (EDS Inca X-Max 80 T, Oxford, UK) system. Before SEM observation, the surface of the samples was polished to a 5-μm finish and then thermally etched at 1400 °C for 1 h in a muffle furnace.

Thermal diffusivity (Dth) of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 was measured by laser flash method using a sample with the size of Ø 10 mm × 2 mm. Before test, a layer of graphite was coated on the surface of the sample to prevent heat radiation from penetrating through it. The heat capacity (Cp) was calculated by Neumann-Kopp rule using the data of its constituent oxides (Nd2O3, Sm2O3, Eu2O3, Er2O3, Y2O3, and Al2O3). Thermal conductivity (κ) of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 was calculated from its thermal diffusivity (Dth), heat capacity (Cp) and bulk density (d) according to the following relationship:

κ=Dth⋅Cp⋅d

The linear thermal expansion coefficient of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 was examined by an optical dilatometer (Misura ODHT 1600-50, Expert System Solutions, Italy) using a polished specimen with the size of 3 mm × 4 mm × 15 mm. A 45° chamfer was cut at one end of the sample. In order to investigate the anisotropy of the thermal expansion of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3, high temperature XRD patterns of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 powders were conducted on a powder X-ray diffractometer (XRD, X’Pert MPD Pro Panalytical, Holland). The sample was heated from 27 °C to 1200 °C at a heating rate of 5 °C/min, and held for 5 min to achieve temperature equilibration at 100 °C intervals, and then the XRD pattern of the sample at the desired temperature was recorded with a scanning rate of 1°/min. The lattice parameters of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 at different temperatures were refined by the Rietveld method (Topas software, Bruker, Germany). The linear thermal expansion coefficient along principal axes (αa, αb and αc) and the volumetric thermal expansion coefficient (αV) were obtained by fitting the lattice parameters versus temperature curves of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3.

Good chemical compatibility with the substrates is also essential for environmental barrier coating materials. In order to investigate the chemical compatibility between HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 and the Al2O3 substrate, the as-synthesized (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 powders and Al2O3 powders (99.9% purity, Aladdin Biochemical Technology Co., Ltd, Shanghai, China) were ball-milling mixed in equal volumes and molded under an uniaxial pressure of 40 MPa then annealed at 1500 °C for 1 h in air. The obtained products were identified by XRD. Furthermore, the interface reaction between HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2) AlO3 and Al2O3 at high temperature was also investigated in detail. Firstly, Al2O3 powders were put into a graphite die with an inside diameter of 25 mm and flatten. Then HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2) AlO3 powders in equal volumes were covered on Al2O3 powders. The double-layered green body was sintered by SPS apparatus at 1650 °C for 4 min under a pressure of 30 MPa. After that, the sintered compact was cut by using a low-speed diamond saw and then the cross-sections were polished to a 2-μm finish. Finally, the polished compacts were annealed at 1500 °C for different hours and the cross-sections were observed by SEM.

3. Results and discussion

3.1. Phase composition and microstructure

The XRD pattern of as-synthesized HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 powders is shown in Fig. 2. The data of REAlO3 (RE=Y, Nd, Sm, Eu, and Er) obtained from ICDD/JCPDS cards are also exhibited for comparison. As previous works reported, REAlO3 (RE=Y, Sm, Eu, Er) show orthorhombic structures, while NdAlO3 possesses a trigonal structure. As shown in Fig. 2, HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 exhibits orthorhombic structure, which is similar to those of REAlO3 (RE=Y, Sm, Eu, Er). It indicates that as-synthesized HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 forms an single orthorhombic phase despite that the solid solution contains multiple elements with different crystal structures. This result is significant because it indicates that high entropy oxide can be formed from components of different structures. Some weak peaks of RE3Al5O12 phase were also identified in the XRD pattern. Based on the XRD pattern in Fig. 2, the lattice parameters of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 were refined as a = 5.253 Å, b = 5.296 Å, c = 7.444 Å. The theoretical density of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 is calculated to be 6.92 g/cm3 when assuming that all rare-earth elements are evenly distributed at lattice sites.

Fig. 2.

Fig. 2.   XRD pattern of as-synthesized HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 powders and the data of REAlO3 (RE=Y, Nd, Sm, Eu, and Er) obtained from ICDD/JCPDS cards are also exhibited for comparison.


Fig. 3 shows the XRD pattern of the bulk (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 compact sintered by SPS method. It can be seen that no new impurity phase was identified, which indicates that HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 has good high-temperature capability. Fig. 4 exhibits the surface morphology of bulk (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 compact thermally etched at 1400 °C for 1 h together with the EDS mappings of the containing rare-earth elements. The microstructure reveals equiaxial grains and the average grain size is about 0.95 μm. No micro-pores and micro-cracks are observed, which indicates that the sintered compact possesses high relative density. The density of the bulk compact measured by the Archimede’s method is 6.71 g/cm3, corresponding to 97% of the theoretical value. As exhibited in the EDS mappings, all the containing rare-earth elements are distributed in the grains evenly and no elements segregation is observed, proving that the single phase solid solution is formed.

Fig. 3.

Fig. 3.   XRD pattern of the bulk (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 compact sintered by SPS method.


Fig. 4.

Fig. 4.   Surface morphology of bulk (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 compact thermally etched at 1400 °C for 1 h together with the EDS mappings of the containing rare-earth elements.


3.2. Thermal properties and phase stability

The thermal conductivity of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 at room temperature together with those of REAlO3, Y2O3 and Al2O3 are listed Table 1. Clearly, the thermal conductivity of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 (4.1 W·m-1 K-1) is much lower than those of the single component compounds, YAlO3 (11 W·m-1 K-1) [19] and NdAlO3 (7.5 W·m-1 K-1) [37], the substrate material, Al2O3 (8.0 W·m-1 K-1) [18] and Y2O3 (27 W·m-1 K-1) [38], the reported candidate coating material for Al2O3 CMCs.

Table 1   Thermal conductivity of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 at room temperature together with those of REAlO3 (RE= Y, Nd), Y2O3 and Al2O3.

Compoundsκ (W·m-1·K-1)
(Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO34.1
YAlO311.7
NdAlO37.5
Al2O38.0
Y2O327.0

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The linear thermal expansion curve of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 measured from room temperature to 1200 °C is illustrated in Fig. 5. As shown in Fig. 5, the expansion of the test specimen increases almost linearly with the increase of temperature. The linear thermal expansion coefficient is determined in terms of the slope of thermal expansion versus temperature during heating process, and the linear CTE of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 is 9.02 × 10―6 /°C, which is close to those of Al2O3 and Al2O3/Al2O3 CMCs (∼9 × 10―6 /°C) [18,39]. Furthermore, according to previous reports, SmAlO3 shows a reversible phase transition (orthorhombic to rhombohedral structure) at 785 °C, which causes a sudden change of its crystal volume at this temperature [40,41]. However, it can be seen from Fig. 5 that HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 compact expands almost linearly when the temperature increases from room temperature to 1200 °C and no sudden change in length occurs. This result indicates that HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 exhibits good phase stability at high temperatures.

Fig. 5.

Fig. 5.   Linear thermal expansion curve of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 measured from room temperature to 1200 °C.


The anisotropy of the thermal expansion of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 is obtained from the high temperature XRD patterns of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 powders. Fig. 6 exhibits the XRD patterns of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 powders at different temperatures. When the temperature increases from 27 °C to 1200 °C, there is no new phase detected, indicating that HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 is stable at high temperatures. The normalized lattice parameters of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 at different temperatures were calculated and shown in Fig. 7. Clearly, the lattice parameter a, b, c and the unit cell volume V increase steadily with the temperature up to 1200 °C. Based on the data in Fig. 7, the anisotropic thermal expansion coefficients and the volumetric thermal expansion coefficient of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 are calculated, which are listed in Table 2 and compared with the data of REAlO3 (RE= Y, Eu, and Er) [42]. As shown in Table 2, the linear thermal expansion coefficients along three crystallographic directions (a, b, and c) are αa = 12.04 × 10-6 /°C, αb = 7.46 × 10-6 /°C, αc = 11.28 × 10-6 /°C, and the volumetric thermal expansion coefficient is αV = 30.78 × 10-6 /°C, which are higher than those of REAlO3 (RE= Y, Eu, and Er).

Fig. 6.

Fig. 6.   XRD patterns of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 powders at different temperatures.


Fig. 7.

Fig. 7.   The normalized lattice parameters of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 at different temperatures.


Table 2   Anisotropic thermal expansion coefficients of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 and REAlO3.

αa (×10―6 /oC)αb (×10―6 /oC)αc (×10―6 /oC)αV (×10―6 /oC)
(Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 (27-1200 °C)12.047.4611.2830.78
YAlO3 (27-973 °C)11.224.5510.6726.57
EuAlO3 (27-973 °C)10.485.979.6326.17
ErAlO3 (27-973 °C)10.634.8010.3325.91

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Fig. 8 presents the thermal conductivities versus thermal expansion coefficients for HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 together with selected EBC materials and Al2O3 [36]. Clearly, the thermal expansion coefficient of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 is very close to that of Al2O3, but its thermal conductivity is much lower than those of Al2O3, YAlO3, and other EBC materials. In short, low thermal conductivity, close CTE and good phase stability at high temperatures indicate that HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 is promising as a T/EBC material of Al2O3f/Al2O3 CMCs.

Fig. 8.

Fig. 8.   Thermal conductivities versus thermal expansion coefficients for HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 together with selected EBC materials and Al2O3.


3.3. Design of tri-layer coating system

Fig. 9 shows the XRD pattern of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3/Al2O3 mixed powders after annealing at 1500 °C for 1 h in air. Well-defined peaks from (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)3Al5O12, (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 and Al2O3 phases were observed in the XRD pattern, indicating that HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 reacted with Al2O3 and (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)3Al5O12 garnet phase was formed. For T/EBC applications, a dense thermally grown oxide (TGO) layer is needed to prevent the penetration of oxidant species [43]. The formation of garnet implies the possible design of a tri-layer system similar to the YSZ/TG Al2O3/Ni-based super alloy (as illustrated in Fig. 10(a)) and the RE-silicates/TG SiO2/(SiCf/SiC composites) (as illustrated in Fig. 10(b)) [1,41]. To demonstrate the possibility of this tri-layer system, the interfacial microstructure of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3/Al2O3 is investigated. The SEM images of the cross-sectional microstructure of the double-layer compacts (upper layer: Al2O3; under layer: HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3) after annealing at 1500 °C for different time are shown in Fig. 11. Before annealing treatment, the interface of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 and Al2O3 is clear and no interaction occurs between them, as shown in Fig. 11(a). When annealled at 1500 °C for 1 h, it can be seen that reaction occurs at the HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3/Al2O3 interface and a new reaction layer with ∼25 μm thick is formed (as shown in Fig. 11(b)). Fig. 11(d) presents a high-magnification micrograph of the reaction layer. It is found that the reaction layer is dense and the average grain size is ∼1.7 μm. Small amount of pores are observed between grains, which is formed due to the volume shrinkage after interface reaction. However, when the annealing time is prolonged to 5 h, the thickness of the reaction layer is almost unchanged (as shown in Fig. 11(c)). The grains of the reaction layer grow up and have an average size of ∼2.2 μm, but no micro-cracks are found at the reaction layer. Thus, the dense garnet barrier formed at the interface can seal HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 away from Al2O3 for further reaction. Furthermore, analogous to the YSZ/TG Al2O3/Ni-based super alloy and the RE-silicates/TG SiO2/(SiCf/SiC composites) EBC structures, a tri-layer EBC system REAlO3/RE3Al5O12/(Al2O3f/Al2O3 CMCs) can be established, as illustrated in Fig. 10(c). In this way, the reacting-formed garnet layer can act as a barrier for penetrating the water vapor from the top REAlO3 layer.

Fig. 9.

Fig. 9.   XRD pattern of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3/Al2O3 mixed powders after annealing at 1500 °C for 1 h in air.


Fig. 10.

Fig. 10.   Schematic illustration of (a) YSZ/TG Al2O3/Ni-based super alloy, (b) RE-silicates/TG SiO2/(SiCf/SiC composites), and (c) REAlO3/RE3Al5O12/(Al2O3f/Al2O3 CMCs) tri-layer EBC systems.


Fig. 11.

Fig. 11.   SEM images of the cross-sectional microstructure of the double-layer compacts (upper layer: Al2O3, under layer: HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3) after annealing at 1500 °C for different time: (a) 0 h, (b) 1 h, (c) 5 h. (d), (e) high-magnification micrographs of the reaction layer after annealed for 1 and 5 h.


4. Conclusion

A novel high-entropy rare-earth aluminate ceramic (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 is designed and synthesized in this work. HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 crystallizes in an orthorhombic structure and the refined lattice constant is a = 5.253 Å, b = 5.296 Å, and c = 7.444 Å. The linear thermal expansion coefficient of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 from room temperature to 1200 °C is 9.02 × 10―6 /oC, which is close to that of Al2O3, the promising candidate material of blades and combustor liners of future gas turbines. The anisotropy of thermal expansion coefficients and volumetric thermal expansion coefficient of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 are αa = 12.04 × 10-6 /oC, αb = 7.46 × 10-6 /oC, αc = 11.28 × 10-6 /oC, and αV = 30.78 × 10-6 /oC. The thermal conductivity of HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 at room temperature is 4.1 W·m-1 K-1, which is much lower than those of YAlO3, NdAlO3, Y2O3, and Al2O3. The interaction experiment indicates that HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 reacts with Al2O3 at 1500 °C, but a dense garnet reaction layer with a thickness of ∼25 μm is formed at interface, which separates HE (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3 layer from Al2O3 layer and prevents the interaction from further reaction. Based on this result, a tri-layer EBC system REAlO3/RE3Al5O12/(Al2O3f/Al2O3 CMCs) is designed and the RE3Al5O12 layer can act as a barrier to prevent penetration of the water vapor from the top REAlO3 layer.

Acknowledgement

This work was financial supported by the National Natural Science Foundation of China (Nos. 51672064 and 51972089).

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Predicting 3D structure of protein from its amino acid sequence is one of the most important unsolved problems in biophysics and computational biology. This paper attempts to give a comprehensive introduction of the most recent effort and progress on protein structure prediction. Following the general flowchart of structure prediction, related concepts and methods are presented and discussed. Moreover, brief introductions are made to several widely-used prediction methods and the community-wide critical assessment of protein structure prediction (CASP) experiments.

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Hundreds of different types of coatings are used to protect a variety of structural engineering materials from corrosion, wear, and erosion, and to provide lubrication and thermal insulation. Of all these, thermal barrier coatings (TBCs) have the most complex structure and must operate in the most demanding high-temperature environment of aircraft and industrial gas-turbine engines. TBCs, which comprise metal and ceramic multilayers, insulate turbine and combustor engine components from the hot gas stream, and improve the durability and energy efficiency of these engines. Improvements in TBCs will require a better understanding of the complex changes in their structure and properties that occur under operating conditions that lead to their failure. The structure, properties, and failure mechanisms of TBCs are herein reviewed, together with a discussion of current limitations and future opportunities.

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