Effect of reaction routes on the porosity and permeability of porous high entropy (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ for transpiration cooling

Thermal protection systems (TPS) play a vital role in protecting hypersonic vehicles from the damage in re-entry environment [[1], [2], [3]]. Among the candidates for reusable and high-efficiency TPS, transpiration cooling technique is potential to improve the reusability and security of re-entry space vehicles [[4], [5], [6]]. In transpiration cooling systems, a coolant gas flowing through the porous structure absorbs the heat by convection, then forms a coolant layer on the hot surface to reduce heat transfer [[7], [8], [9]]. However, traditional porous metals will thermally and structurally fail if local hot spots occur [8]. Therefore, developing novel materials as potential porous media for transpiration cooling is urgently awaited. Porous ultra-high temperature ceramics (UHTCs) are candidate materials for transpiration cooling technique due to their high melting point, good thermal stability and corrosion resistance, high modulus and strength [[10], [11], [12], [13], [14], [15], [16], [17]]. Recently, investigations on porous UHTCs are mainly focused on porous ZrB₂. For example, Ifti et al. [9] prepared porous ZrB₂ with a porosity of 40%, permeability of 2.99 × 10^-14 m² and strength of 50 MPa, and investigated its external flow characteristics. However, the relatively high density, low permeability and low porosity are limitations for such an application. Therefore, exploring novel porous UHTCs with low density, high porosity and high permeability for transpiration cooling is necessary.

There are two ways to decrease the density of porous UHTCs. The first one is to use low density UHTCs, such as metal hexaborides [[18], [19], [20]], as the backbone materials. The second one is to increase the porosity of porous UHTCs. Our previous works have demonstrated that using the novel in-situ reaction/partial sintering method can effectively increase the porosity of porous UHTCs and form homogeneous microstructure without coarsening [[13], [14], [15]]. This method has been used to prepare porous YbB₆ ceramics with high porosity (∼72%) and high compressive strength (∼21.34 MPa) [13].

In order to improve the thermal stability of porous metal hexaborides, high entropy (HE) (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ is chosen as the backbone material. The idea comes from the fact that high entropy ceramics (HECs) possess excellent properties compared with single components, such as better high temperature stability, good corrosion resistance, lower thermal conductivity, higher strength and hardness [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]]. Our previous works have demonstrated that porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C [38] and (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ [39] exhibit high porosity and excellent high-temperature stability (up to 1850 °C).

In this work, we aim to prepare low density and high permeability porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ by the in-situ reaction/partial sintering method. To achieve such a goal, we have designed the boron thermal reduction reaction (1) and the borocarbon thermal reduction reaction (2) as following.

Y₂O₃ + Yb₂O₃ + Sm₂O₃ + Nd₂O₃ + Eu₂O₃ + 70B=10(Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ + 5B₂O₃ (1)

Y₂O₃ + Yb₂O₃ + Sm₂O₃ + Nd₂O₃ + Eu₂O₃ + 15B₄C=10(Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ + 15CO (2)

To gain insight inon the effect of reaction routes on the phase composition, porosity, strength and permeability, the properties of as-prepared porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ from both of the aforementioned reaction routes are investigated.

The raw materials used in the present study were Yb₂O₃, Y₂O₃, Sm₂O₃, Nd₂O₃, Eu₂O₃ (99.99%, 1 μm, Rare Chemical Technology Co., Ltd, Huizhou, China) powders, and the reducing agents were B₄C (99%, 2 μm, Mudanjiang Qianjin Boron Carbide Co., Ltd, Mudanjiang, China) and B (99%, 1 μm, HWRK Chem. co. Ltd, Beijing, China) powders. According to Eqs. (1) and (2), reactant powders were mixed with ethyl alcohol using a polyurethane jar and zirconia grinding balls. After ball-milling for 8 h, the slurry was dried in an oven at 50 °C until the ethanol evaporated completely. Then, the mixed powders and zirconia balls were separated by using a 120-mesh screen. In order to determine the synthesis temperature of HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆, the reaction processes were monitored using a thermal analyzer (NETZSCH STA 409 CD/7/403/5/G, Germany) through heating reactant powders to 1700 °C in flowing helium atmosphere at a heating rate of 5 °C /min.

To investigate the effect of reaction routes (reaction (1) and (2)) on the microstructure and properties, the reactant powders were uniaxially pressed into columnar shaped green bodies with 12 mm in diameter. The green bodies were heated to 1850 °C for 1 h in vacuum at a heating rate of 5 °C/min. Porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ ceramics with excellent shape stability were successfully prepared by the in-situ reaction/partial sintering process.

The density of porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ was determined geometrically, wherein the theoretical density of (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ was calculated from the mass of the atoms in a unit cell and the lattice parameter measured from the XRD pattern. The lattice parameter of (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ was refined by Rietveld method (TOPAS, Bruker Corp., Karlsruhe, Germany). The porosity (P) was calculated according to Eq. (3):

p=(1-$\frac{p}{p'}$)×100% (3)

where ρ is the density of porous samples, ρ′ is the theoretical density.

The formation of (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ solid solution was identified by X-ray diffraction analysis (XRD, D8 advanced, Bruker, Karlsruhe, Germany) using CuKα radiation (λ = 1.54178 Å). The residual oxygen content of samples was detected by an oxygen-nitrogen analyzer (TC-600, LECO Corporation, USA). The microstructure and rare earth metal element distribution were investigated by using a scanning electron microscope (SEM, Apollo 300, CamScan, Cambridge, UK) equipped with an energy dispersive spectroscope (EDS, Inca X-Max 80 T, Oxford, UK) system. And the internal structure was demonstrated by X-ray tomography (XRT, Xradia Versa XRM-500, Carl Zeiss X-ray Microscopy Inc., Pleasanton, California, United States). The image signal magnified by optical lens was recorded by a 2000 × 2000 CCD camera (Andor Technology Ltd., Belfast, Northern Ireland, UK). The compressive strength was measured by a universal testing machine (DZS-III, China Building Material Test and Certification Center, Beijing, China) using rectangular bars with the dimension of 5 mm × 5 mm × 10 mm. To calculate gas permeability, the pressure drops of the disc samples (Φ10 × 5 mm) were measured with increasing volumetric flow rate of air.

In order to synthesize high purity HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ and determine the reaction process, the boron thermal reduction reaction (1) and borocarbon thermal reduction reaction (2) are investigated. Since a large amount of B₂O₃ or CO gases are produced, the change of mass during heating process should be monitored. Fig. 1(a) and (b) show the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves, corresponding to reaction (1) and (2). Intriguingly, it can be seen that the boron thermal reduction (1) and borocarbon thermal reduction (2) have similar mass-loss process. In the low temperature range (∼ 400 °C), a slight mass loss can be observed. The possible reason is that the RE₂O₃ can transform to RE(OH)₃ during ball-milling process, and will decompose to RE₂O₃ and H₂O when heated to 260 °C [13]. As the temperature increases, a significant mass loss can be observed in the range of 1100-1600 °C. The starting temperature for boron thermal reduction (1) can be recognized as 1150 °C when the mass loss starts. Similarly, the borocarbon thermal reduction (2) starts at 1190 °C. According to the corresponding DTG curves, the significant mass-loss can be divided into two stages. In order to investigate the phase evolution during the two stages, the phase compositions of samples are investigated, as shown in Fig. 1(c) and (d). It can be seen that the phases are mainly composed of (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ and (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)BO₃ in two stages. Previous works have proven that REBO₃ is an intermediate product in the synthesis of REB₆ and can be removed through decomposing or reacting with reducing agents when increasing temperature [13,40]. In addition, a small amount of ZrB₂ can also be observed due to the use of ZrO₂ as ball-milling media which could react with B₄C or B to form ZrB₂. In the boron thermal reduction process, there are two continuous reactions (1) and (4) in the first stage, and the (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)BO₃ can be removed through reacting with B in light of reaction (5) in the second stage. Similarly, the borocarbon thermal reduction process consists of synthesis of (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ (reactions (2)) and (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)BO₃ (reaction (4)) and then the reduction of (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)BO₆ (reaction (6)).

Y₂O₃ + Yb₂O₃ + Sm₂O₃ + Nd₂O₃ + Eu₂O₃ + 5B₂O₃=10(Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)BO₃ (4)

(Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)BO₃ + 7B= (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ +B₂O₃ (5)

2(Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)BO₃ + 3B₄C=2(Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ + B₂O₃ + 3CO (6)

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Fig. 1.   (a) and (b) TG and DTG curves recorded during boron and borocarbon thermal reduction, (c) and (d) Phase evolution during boron and borocarbon thermal reduction.

In the following part of this letter, the porous samples synthesized through boron and borocarbon thermal reductions are named as HEREB₆-1 and HEREB₆-2, respectively. Fig. 2 shows the X-ray diffraction (XRD) patterns of HEREB₆-1 and HEREB₆-2, together with those of YB₆ (JCPDF card# 16-0732), YbB₆ (JCPDF card# 25-1343), EuB₆ (JCPDF card# 40-1308), SmB₆ (JCPDF card# 36-1326), NdB₆ (JCPDF card# 11-0087). It is obvious that a single-phase solid solution (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ is formed, which possesses the same crystal structure as the single component phases. Rietveld refinement is conducted by TOPAS software and the lattice parameter of the (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ solid solution is calculated to be a = 4.135 Å. Using the refined cell dimension and atomic mass as input, the theoretical density is calculated to be 4.85 g/cm³, which is lower than that of ZrB₂ (6.09 g/cm³) [41]. As listed in Table 1, the densities of HEREB₆-1 and HEREB₆-2 are 1.42 and 1.73 g/cm³, respectively, which is only 1/3-1/2 of that of previously reported porous ZrB₂ (3.65 g/cm³) [9]. According to Eq. (3), the porosities of HEREB₆-1 and HEREB₆-2 are determined as 70.92% and 64.35%, respectively. Similar to carbothermal reduction process, residual oxygen will remain in the samples after boron and borocarbon thermal reduction. The oxygen contents of HEREB₆-1 and HEREB₆-2 are 0.395 wt% and 0.405 wt%, respectively.

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Fig. 2.   X-ray diffraction (XRD) patterns of as-synthesized HEREB₆-1 and HEREB₆-2, together with those of YB₆, YbB₆, EuB₆, SmB₆, NdB₆.

Fig. 3(a) and (b) show the SEM images of HEREB₆-1 and HEREB₆-2, respectively. It can be seen that the as-prepared porous samples possess homogeneous microstructure. Similar to the as-reported HE (Zr_0.25Ta_0.25Nb_0.25Ti_0.25)C (33), the grain growth is in a characteristics of ledge growth, and grains exhibit a coral-like morphology. The porous structure is formed by necking between grains during partial sintering process, which can lead to increased strength of porous ceramics [42,43]. As listed in Table 1, HEREB₆-1 and HEREB₆-2 exhibit high compressive strength of 12.71 and 31.59 MPa, respectively. Fig. 3(c) and (d) show the pore diameter distributions of HEREB₆-1 and HEREB₆-2. The average pore diameter of HEREB₆-1 is 4.76 μm, and that of HEREB₆-2 is 9.70 μm. Furthermore, SEM-EDS elemental mappings of porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ at micrometer scale are shown in Fig. 4. It can be seen that the distributions of metal elements are homogeneous without evident localization of any elements.

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Fig. 3.   (a) and (b) SEM images of HEREB6-1 and HEREB6-2, (c) and (d) Pore diameter distributions of HEREB6-1 and HEREB6-2.

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Fig. 4.   (a) SEM image of as-prepared porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆, (b)-(f) Corresponding element mappings of Eu, Nd, Sm, Y and Yb.

Based on the results of XRD and SEM-EDS analysis, porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ is homogeneous in composition. Moreover, high porosity demonstrates the advantages of porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ for applying in transpiration cooling.

In addition to high strength, the permeability of porous media is considered as an important parameter for transpiration cooling technique. In order to improve the cooling capacity, it is necessary to increase the permeability of porous media [[7], [8], [9]]. The relationship between pressure drop, flow rate and permeability is described by Darcy equation [44]:

$\frac{ΔP}{L}=\frac{ηQ}{kA}$ (7)

where ΔP is the pressure drop, L is the length of a porous sample, η is the dynamic viscosity of air at ambient temperature (1.84 × 10^-5 Pa·s), Q is the volumetric flow rate, k is the Darcy permeability coefficient and A is the flow area. By linear fitting the data in the plot of ΔP/L and Q/A, the Darcy permeability of HEREB₆-1 and HEREB₆-2 are determined to be 2.73 × 10^-11 m² and 2.65 × 10^-11 m², respectively, as shown in Fig. 5(a) and (b). In short, porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ possesses higher permeability than that of porous disc of sintered bronze (2 × 10^-12 m²) and porous ZrB₂ (2.99 × 10^-14 m²) [9]. During in-situ reaction/partial sintering process, gas products act as a pore-forming agent and lead to a large number of interconnected pores [[11], [12], [13],38,39]. As shown in Fig. 5(c), it can be observed that the as-prepared porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ possesses a large number of interconnected pores with large pore diameter, which results in high permeability. In addition, our previous works have demonstrated that porous UHTCs prepared by in-situ reaction/partial sintering process possess controlled porosity [[11], [12], [13]]. Since porosity and permeability are directly related [45], the permeability is also controllable. This is beneficial to design various transpiration cooling systems for different application environments.

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Fig. 5.   Normalized differential pressure versus superficial input velocity of (a) HEREB6-1 and (b) HEREB6-2, (c) XRT image of HE porous (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆.

In conclusion, a novel porous high entropy (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ is prepared by in-situ reaction/partial sintering process in this work. Using boron and borocarbon thermal reduction processes, a single-phase solid solution (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ is formed. The porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ ceramics synthesized through boron and borocarbon thermal reductions possess low density of 1.42 g/cm³ and 1.73 g/cm³, high porosity of 70.92% and 64.35%, high permeability of 2.73 × 10^-11 m² and 2.65 × 10^-11 m², high compressive strength of 12.71 MPa and 31.59 MPa, respectively. The combination of these properties makes porous HE (Y_0.2Yb_0.2Sm_0.2Nd_0.2Eu_0.2)B₆ promising as a candidate porous media for various transpiration cooling applications.

Acknowledgements

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