High porosity and low thermal conductivity high entropy (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C

1. Introduction

Porous ultrahigh temperature ceramics (UHTCs) exhibit a number of unique properties, including low bulk density and high specific surface area [1,2], high-melting point (>3000 °C), tailorable thermo-mechanical performance [[3], [4], [5], [6], [7]], good chemical and high-temperature stability [[8], [9], [10], [11], [12], [13], [14]]. The combination of these properties makes them promising in harsh environment applications, including ultrahigh temperature thermal insulation, scramjet engine component, filtration of high-temperature molten metals and corrosive gases, catalyst supports, high-temperature solar absorbers [[15], [16], [17], [18], [19], [20], [21]]. However, the application of porous UHTCs in ultrahigh-temperature thermal insulation is encumbered by their high thermal conductivity. It has been taken for granted that the thermal conductivity of porous UHTCs depends on both porosity and the backbone material. Therefore, besides controlling grain size and porosity, decreasing the thermal conductivity of backbone materials is significant in reducing the thermal conductivity of porous UHTCs.

In order to further reduce the thermal conductivity of porous UHTCs, high-entropy (HE) (Zr_0.2Hf_0.2Ti_0.2 Nb_0.2Ta_0.2)C is chosen as the backbone material. The idea comes from the fact that high entropy ceramics have low thermal conductivity, good high-temperature stability, sluggish diffusivity and severe lattice distortion [[21], [22], [23], [24]]. For example, the dense HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C exhibits lower thermal diffusivity (3.6 mm²/s, 29.5 °C) and lower thermal conductivity (6.45 W/m·K, 29.5 °C) than those of the monophase carbides (HfC, TaC, ZrC and TiC) [21].

In our previous works, a novel and simple method combining in-situ reaction and partial sintering has been successfully used to prepare high porosity, high strengths and low thermal conductivity borides, carbides and boridecarbides [[25], [26], [27], [28]]. Through precisely controlling the shrinkage and reaction during high-temperature process, the thermal conductivity of porous UHTCs can be reduced, which is only ca 1/20 of the corresponding dense materials [25].

In this work, we aim to use low thermal conductive (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C as the backbone material to prepare low thermal conductivity porous UHTCs. To achieve such a goal, the following in-situ reaction (1) is designed to prepare porous HE (Zr_0.2Hf_0.2Ti_0.2 Nb_0.2Ta_0.2)C. This process is based on the fact that transition metal carbides can be synthesized according to carbothermal reduction reactions (2), (3), (4), (5), (6) [29] and HE (Zr_0.2Hf_0.2 Ti_0.2Nb_0.2Ta_0.2)C can be synthesized following reaction (7) [21,22]. The reaction released CO in reaction (1) can be used for the formation of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C. The phase composition, microstructure, properties of porous (Zr_0.2Hf_0.2Ti_0.2Nb_0.2 Ta_0.2)C are investigated, and low thermal conductivity and good high-temperature stability are demonstrated.

ZrO₂(s) + 2HfO₂(s) + 2TiO₂(s) + Nb₂O₅(s) + Ta₂O₅(s) + 32C(s) = 10(Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C(s) + 22CO(g) (1)

ZrO₂(s) + 3C(s) = ZrC(s) + 2CO(g) (2)

HfO₂(s) + 3C(s) = HfC(s) + 2CO(g) (3)

Nb₂O₅(s) + 7C(s) = 2NbC(s) + 5CO(g) (4)

Ta₂O₅(s) + 7C(s) = 2TaC(s) + 5CO(g) (5)

TiO₂(s) + 3C(s) = TiC(s) + 2CO(g) (6)

ZrC(s) + HfC(s) + TiC(s) + NbC(s) + TaC(s) = 5(Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C(s) (7)

2. Experimental

2.1. Materials

The raw materials used in the present study were TiO₂, ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅ powders (99.9%, 1 μm, China New Metal Materials Technology Co., Ltd, Beijing, China) and carbon black (99%, -100 mesh, Mudanjiang Qianjin Reagent Co., Ltd, Mudanjiang, China). According to Eq. (1), reactant powders were mixed using ball-milling in polyurethane jars with absolute ethyl alcohol and zirconia grinding balls for 20 h. After ball-milling, the slurries were dried in an oven at 60 °C until the alcohol evaporated completely. Then the powders were sieved through a 120-mesh screen to obtain homogeneous reactants. In order to determine the synthesis temperature of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C and control the gas releasing process, the reactant powders were heated to 1750 °C using a thermal analyzer (NETZSCH STA 409 CD/7/403/5/G, Germany) in flowing argon at a heating rate of 5 °C /min. Meanwhile, shrinkage behavior was monitored using a thermal dilatometer (NETZSCH DIL 402 E, Germany) through heating the columnar shaped green bodies (7 mm in diameter and 15 mm in height) up to 1850 °C in flowing argon atmosphere at a heating rate of 5 °C/min to investigate the shrinkage behavior during reaction and sintering process.

Porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C was prepared through gas-released in-situ reaction (1) followed by high-temperature sintering. The reactant powders were uniaxially pressed into columnar shaped green bodies of 30 mm in diameter. According to the thermal analysis results, the porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C with excellent shape stability was successfully prepared by in-situ reaction /partial sintering.

2.2. Characterization

The density of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C was determined geometrically, through measuring the volume and weight of five identical samples and averaging the data to ensure the accuracy. The theoretical density of (Zr_0.2Hf_0.2Ti_0.2 Nb_0.2Ta_0.2)C was calculated from the mass of the atoms in a unit cell and the lattice parameters measured from the XRD data. The porosity (P) was calculated according to Eq. (8):

P=(1-$\frac{\rho}{\rho'}$)×100% (8)

where ρ is the density of as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C, ρ′ is the theoretical density of (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C.

The radial shrinkage (S) was evaluated by Eq. (9):

S=(1-$\frac{r}{r'}$)×100% (9)

where r is the radius of as-prepared porous sample, r′ is the radius of green body.

The as-prepared samples were analyzed by X-ray diffraction (XRD, D8 advanced, Bruker, Karlsruhe, Germany) using CuK_α radiation (λ = 1.54178 Å), scanning electron microscopy (SEM, Apollo 300, CamScan, Cambridge, UK) and transmission electron microscopy (TEM, Philips EM420) equipped with an energy dispersive spectroscopy (EDS) system.

Thermal conductivity (κ) was calculated according to Eq. (10):

κ=α⋅ρ⋅C_p (10)

where α is the thermal diffusivity, ρ is the density of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2 Ta_0.2)C, C_p is the specific heat capacity. Thermal diffusivity was measured using a laser thermal conductivity testing instrument (ANTER FLASHLINE 5000, USA) with a sample size of Φ10 mm × 2.8 mm. The specific heat capacity of (Zr_0.2Hf_0.2Ti_0.2Nb_0.2 Ta_0.2)C was measured on a differential scanning calorimeter (DSC 250, TA Instruments, USA). The compressive strength was characterized by using a universal testing machine (DZS-III, China Building Material Test and Certification Center, Beijing, China). The tested samples were rectangular bars with a dimension of 5 mm × 5 mm × 10 mm.

3. Results and discussion

3.1. Reaction and shrinkage process of HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C

As we have demonstrated in our previous works, for preparing porous samples with excellent shape stability, the in-situ synthesis/shrinkage process must be precisely controlled to avoid cracking and collapse. To achieve this goal, the reaction and shrinkage behavior need to be studied to determine the processing parameters. The linear shrinkage (dL/L₀, with L₀ being the height of columnar shaped green bodies at room temperature) and linear shrinkage rate ((1/L₀)dL/dt) curves together with the corresponding TG and DTG curves are shown in Fig. 1.

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Fig. 1.   The linear shrinkage, linear shrinkage rate, TG and DTG curves recorded during the heating of green body of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C.

From Fig. 1, the shrinkage curve is obviously divided into several stages with the increase of temperature, namely, reaction shrinkage stage and sintering shrinkage stage. Intriguingly, the shape of TG curve is consistent with the reaction stages of linear shrinkage curve, which indicates that the reactions are accompanied by mass loss and shrinkage. The TG curve is divided into five parts by apparent rate change in the range of 1200-1600 °C. The temperatures with the fastest mass loss rate are 1240 °C, 1324 °C, 1358 °C, 1436 °C and 1447 °C, which are corresponding to carbothermal reduction reactions for NbC, TaC, TiC, ZrC and HfC formations as reported by Feng et al. [23]. The in-situ reaction (1) is consisted of carbothermal reduction reaction and solid solution formation. The result of TG-DTG indicates that the reactions for synthesizing HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C are completed at 1600 °C. In order to avoid cracks caused by rapid shrinkage and gas-releasing, the heating rate must be controlled under 5 °C/min during 1200-1600 °C. As the temperature continues to increase, the solid solution formation starts at 1700 °C [23]. Following the completion of reduction reactions, shrinkage is caused by partial sintering. Partial sintering temperature affects both microstructure and property. Like porous ZrC and HfC [25], the partial sintering temperature for preparing porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2 Ta_0.2)C is determined as the temperature with similar linear shrinkage rate (-0.1 ˜ -0.15%/min). Therefore, the partial sintering temperature is selected as 1850 °C for preparing porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2 Ta_0.2)C.

3.2. Composition and microstructure of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C

Based on the thermal analysis results, porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C and TiC, NbC, TaC, ZrC, HfC without cracks and other defects have been prepared by in-situ reaction/partial sintering process. The sintered density, porosity, radial shrinkage of as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C together with those of TiC, NbC, TaC, ZrC [25], HfC [25] are listed in Table 1. It can be seen that the HE (Zr_0.2Hf_0.2 Ti_0.2Nb_0.2Ta_0.2)C prepared by in-situ reaction/partial sintering possesses high porosity (80.99%). During in-situ reaction/partial sintering process, gas product CO can prevent the densification and consequently develop a great number of pores in microstructure.

Fig. 2(a) shows the XRD patterns of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C together with those of porous TiC, NbC, TaC, ZrC [25] and HfC [25]. The peaks of monophase carbides coincide with rock-salt structured TiC, NbC, TaC, ZrC and HfC, respectively, indicating that these porous UHTCs prepared by in-situ reaction/partial sintering are TiC (according to JCPDF card# 32-1383), NbC (according to JCPDF card# 38-1364) and TaC (according to JCPDF card# 35-0801), ZrC (according to JCPDF card# 35-0784) and HfC (according to JCPDF card# 39-1491), respectively. From the broadening of peaks, a single-phase solid solution (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C is formed with rock-salt structure. The lattice parameter a of (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C is calculated to be 4.5118 Å, which is close to the average value of ZrC, HfC, TiC, NbC and TaC [[21], [22], [23], [24]].

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Fig. 2.   (a) XRD patterns of as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2 Ta_0.2)C and TiC, NbC, TaC, ZrC [25] and HfC [25], (b) SEM micrograph of as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C with porosity of 80.99%.

Fig. 2(b) presents the SEM micrograph of the fractured surface of as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C. It shows that the porous HE (Zr_0.2Hf_0.2Ti_0.2 Nb_0.2Ta_0.2)C prepared by in-situ reaction/partial sintering possesses homogeneous microstructure. The porous structure is formed during high-temperature sintering process, which is due to that the carbothermal reduction liberated CO gas is thought to promote the bridging of grains by neck growth in the initial stage of high-temperature sintering [32,33]. Since sintering is carried out immediately after reaction, microstructural coarsening can be avoided. As shown in Fig. 2(b), the grain size of porous HE (Zr_0.2 Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C is in the range of 100-500 nm, and the pore size is in the range of 0.2-1 μm. Furthermore, TEM micrograph of HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C along with the corresponding elemental distributions analyzed by EDS is demonstrated in Fig. 3. It can be seen that the as-prepared HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C is chemically and structurally homogeneous.

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Fig. 3.   TEM-EDS analysis of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C, (a) TEM micrograph, (b)-(f) the corresponding EDS mapping of Zr, Hf, Ti, Nb and Ta elements.

Based on the results of XRD, SEM and TEM-EDS analyses, it can be concluded that high porosity (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C with a homogeneous porous microstructure has been successfully prepared by in-situ reaction combined with partial sintering process.

3.3. Compressive strength, thermal conductivity and stability of porous (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C

The compressive strengths of as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2 Ta_0.2)C together with those of TiC, NbC, TaC, ZrC [25], HfC [25] are listed in Table 1. It can be seen that the porous UHTCs prepared by in-situ reaction/partial sintering possess high compressive strength, which is due to that the necks between particles can result in substantially high strength [34,35].

Previous work demonstrated that porous ZrC and HfC prepared by in-situ reaction/partial sintering method possess low thermal conductivity which is only ca 1/20 of the dense counterparts [25]. The thermal diffusivity, heat capacity and thermal conductivity of dense HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C [21] and porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C are listed in Table 2. It has been well accepted that the thermal diffusivity and thermal conductivity of HE ceramics are significantly lower than that of the single components, which is caused by the presence of the enhanced point-defect scattering due to the strain field fluctuations of the anion sublattice distortion during the formation of solid solution [[21], [22], [23], [24]]. For example, the HECs prepared by Yan et al. [21] has a grain size of 16.4 ± 4.5 μm, thermal conductivity of 6.45 W/m·K (29.5 °C), which is only 1/4-1/2 of that of dense ZrC, HfC, TiC, NbC and TaC. From Table 2, the thermal conductivity at 29.5 °C of porous HE (Zr_0.2Hf_0.2Ti_0.2 Nb_0.2Ta_0.2)C is 0.40 W/m·K, which is only ca 1/20 of the dense material.

The other advantage of HECs is their excellent high temperature stability. In order to characterize the stability of as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C, the sample dimension, phase composition and microstructure are studied after heating porous sample to 1850 °C. Fig. 4(a) compares the linear shrinkage curve of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C during its synthesis process and during the second round heat treatment. It can be seen that the sample dimension did not change during the second round heating up to 1850 °C. The microstructures of as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C (Fig. 2(b)) and that after the second round heat treatment (Fig. 4(b)) are also identical, revealing good stability of porous HE (Zr_0.2Hf_0.2Ti_0.2 Nb_0.2Ta_0.2)C. Fig. 4(c) compares the XRD pattern of as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C and that after second round heat treatment. There is no evidence of phase decomposition or transformation, which indicates that the porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C is thermally stable at least up to 1850 °C in argon atmosphere. Based on these results, porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C possesses excellent thermodynamic stability at high-temperatures, which can be attributed to the sluggish diffusivity [36,37].

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Fig. 4.   (a) Linear shrinkage curve of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C during its synthesis process and during the second round heat treatment, (b) SEM micrograph of porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C after second round heat treatment, (c) XRD pattern of as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C and that after second round heat treatment.

4. Conclusion

Porous high entropy (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C with low thermal conductivity and high porosity has been prepared using in-situ reaction and partial sintering method. The as-prepared porous HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C has homogeneous porous structure with grain size in the range of 100-500 nm and pore size in the range of 0.2-1 μm, and possesses low density of 0.97 g/cm³, high porosity of 80.99%, high compressive strength of 3.45 MPa, low room temperature thermal conductivity of 0.39 W·m^-1 K^-1, low thermal diffusivity of 0.74 mm²·s^-1. Low thermal conductivity, which is only ca 1/20 of the dense material, is due to low thermal conductivity of HE (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C backbone, small grain size and high porosity. The porous high entropy (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C ceramic exhibits excellent stability at least up to 1850 °C in argon atmosphere due to the sluggish diffusivity. The unique combination of these properties makes porous high entropy (Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)C promising for ultra-high temperature thermal insulation applications.

Acknowledgments

This work was supported by the National Natural Science Foundation of China under Grant Nos. U1435206 and 51672064, and by Beijing Municipal Science & Technology Commission under Grant No. D161100002416001.

The authors have declared that no competing interests exist.