A novel high-entropy (Sm_0.2Eu_0.2Tb_0.2Dy_0.2Lu_0.2)₂Zr₂O₇ ceramic aerogel with ultralow thermal conductivity

Abstract

Here, we report a novel high-entropy rare earth zirconate (HE-REZ) (Sm_0.2Eu_0.2Tb_0.2Dy_0.2Lu_0.2)₂Zr₂O₇ ceramic aerogel prepared through a sol-gel template method and high-temperature calcination followed by 3-D-structure reconstruction. The structural evolution and crystallisation behaviour of the prepared aerogel were characterised through scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The results indicated that the as-prepared HE-REZ ceramic aerogels had a typical nanoporous structure. The HE-REZ ceramic aerogels thermally treated at 900 °C presented an ultralow room temperature thermal conductivity of 0.031 W/(m·K), high specific surface areas of 443.26 m²/g and a relatively high strength of 12.95 MPa. The effects of different calcination temperatures on the microstructure of the samples were also investigated. Therefore, the excellent insulation performance of these unique HE-REZ ceramic aerogels indicate that they can be used as high-temperature insulators for hypersonic vehicles in the future.

Introduction

With the rapid development of aerospace industries, demands on hypersonic vehicle thermal protection systems with an enhanced performance have increased and intrigued great research interest. As is known to us, the hypersonic vehicles will face extremely harsh aerodynamic heating environment when serviced at high speed in hypersonic regimes and their surface temperature can reach up to 1300 °C or higher values [1,2]. Therefore, in order to make sure the safe operation of the internal components of hypersonic vehicle systems, a new type of lightweight thermal protection material that can withstand higher operation temperatures should be developed [3].

Aerogel is a new type of nanoporous material with a controllable structure. The porosity and specific surface areas of aerogels can reach up to 80.0%–99.8% and 800–1000 m²/g, respectively. The typical pore size and solid-state network structure unit size are 1–100 and 1–20 nm, respectively [4,5]. Aerogel performs ultralow solid and gaseous heat conduction because of its 3-D nanoporous network structure. Its total thermal conductivity under normal temperature and pressure is as low as 0.01 W/(m·K). Consequently, aerogel has been considered an excellent material with the lowest thermal conductivity and density amongst solid materials [6,7]. The aerogel is a kind of light and efficient insulation material, which has a wide applications in aerospace, chemical industry, metallurgy and energy-saving building fields.

One of the advanced insulating materials of single oxide aerogels is SiO₂ aerogel which obtains a very low thermal conductivity (≤0.015 W/(m·K)) at ambient temperature and pressure. However, its structure is unstable as the working temperature increases up to 650 °C [[8], [9], [10]]. In addition, traditional methods of SiO₂ aerogel preparation require high temperature and pressure which greatly limit the commercial applications of SiO₂ aerogel. The hydrophilicity of SiO₂ aerogel is prone to cause the collapse of its 3-D nanoporous network structure. SiO₂ aerogel easily cracks during the drying process, thereby reducing the thermal insulation performance of SiO₂ aerogel. Hence, other composite aerogel with more thermal stable properties has been explored. For example, Guo et al. [11] produced ZrO₂–SiO₂ composite aerogel with a high specific area (up to 172 m²/g) and large pore volume (0.97 m³/g). Wu et al. [12] synthesised an Al₂O₃–SiO₂ composite aerogel with ultra-low thermal conductivity (0.023 W/(m·K)) and a specific surface area of 166 m²/g. Zhang et al. [13] reported a high-strength ZrO₂–SiO₂ composite aerogel prepared with a newly modified method and obtained a good compressive strength property. However, the pore structures of these reported aerogels easily collapse and shrink when used at high temperature (≥1000 °C). Therefore, a new kind of aerogel with structural stability at a working temperature of above 1000 °C should be prepared.

High-entropy ceramics (HECs) are single-phase solid solution such as carbides [14], oxides [15], borides [16], and other systems. These systems were demonstrated to have many excellent properties, including low thermal conductivity colossal dielectric constants, super ionic conductivity and enhanced mechanical properties, which open up new possibilities to improve the performance of established ceramic systems. Resently, researchers have found that the high-entropy rare earth zirconate ceramics (HE-REZs) have been widely explored because of their super mechanical properties, excellent high-temperature stability, improved chemical stability, oxidation resistance, lower thermal conductivity and other excellent characteristics, which showed potential applications in high-temperature insulation environments [[17], [18], [19]].

Zhang et al. [20] prepared (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ HE-REZ powders through a ball milling method combining a high-temperature solid-phase reaction method. Obviously, the as-prepared coatings demonstrated improved durability during thermal cycling test at 1100 °C in air, and its lifetime was 53 times, compared to the lifetime with 10 times for the LZ system. However, the solid-phase reaction process required a very high sintering temperature. Wang et al. [21] fabricated (Sm_0.2Eu_0.2Tb_0.2Dy_0.2Lu_0.2)₂Zr₂O₇ HE-REZ powders which exhibited ultralow thermal conductivity (0.75 W/(m·K)) at high temperature. Zhang et al. [22] synthesised pyrochlore HE-REZ powders using as coating materials for thermal barriers and the thermal conductivity was below 1 W/(m·K) in the temperature range of 300–1200 °C. However, the average grain sizes are in a micrometer scale (4.0 μm). Zhou et al. [23] produced (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)₂Zr₂O₇ high-entropy ceramic with low thermal conductivity (0.76 W/(m·K)) at room temperature, which also exhibits slow grain growth rate.

Based on the previous study [24], the final microstructure and mechanical properties strongly depend on the particle size. Small grain size generally result in fine grain size and enhanced mechanical properties. When the size of HE-REZs decreased to a nanometer level, the increasing grain boundaries between particles will enhance the phonon scattering effect and reduce the thermal conductivity [25,26]. Compared with rare earth oxides, the decomposition of rare earth nitrates and the rearrangement of atoms occur simultaneously at high temperature. Consequently, the required activation energy will reduce and reactions can be easily carried out. However, the HE-REZs prepared with currently available technologies have many problems such as high preparation temperature and pressure, tedious process, large material size and poor mechanical properties, etc. The combination can form a new type of material with potential applications in the field of high-temperature insulation. However, rare earth materials for the preparation of HE-REZ ceramic aerogels are usually nitrates or chlorides and the formed structure is prone to collapse; as such, the development of HE-REZ ceramic aerogel materials is limited.

In this study, we aim to synthesize the HE-REZ ((Sm_0.2Eu_0.2Tb_0.2Dy_0.2Lu_0.2)₂Zr₂O₇) nanopowders with special 3-D porous structures. Therefore, a novel HE-REZ ceramic aerogel with good thermal insulation property was prepared and characterised through the sol-gel template method and high-temperature calcination followed by 3-D-structure reconstruction. Since the highly difference of ion radius can enhance the high mixing configuration and lattice distortion. The criteria for selecting the five rare earth elements are based on the difference of ion radius and types of crystal structure. The HE-REZ ceramic with defective fluorite structures have plenty of oxygen vacancies and the disordered distribution of rare earth elements will increase the configuration entropy, causing lattice distortion, which would affect both the mechanical and thermophysical properties [21]. The particle size of the material prepared by this method is very small (30–50 nm). The micro-structural, thermal and mechanical properties of HE-REZ ceramic aerogel were studied. This work was performed to develop a new preparation method and materials aims to using in the field of high-temperature thermal insulators applications.