It is of great significance to search oxide thermal/environmental barrier coatings (T/EBCs) with high working temperatures and thermal expansion coefficients (TECs) matching to different substrates. ABO₄-type oxides have been widely studied due to their high working temperatures, adjustable TECs, and low thermal conductivity. In this work, ABO₄-type (A=Ga, In, Cr; B=Nb, Ta) oxides are studied as EBC candidates based on their relatively low TECs. The influences of crystal structures, distortion degree, types of polyhedrons, as well as the A- and B-site ionic radii and atomic weights on TECs are discussed. It is found out that the TECs of ABO₄-type oxides are not depended on one single factor, and reducing A-site ionic radius may be a good way to decrease their TECs. Based on the TECs, AlNbO₄, InNbO₄, and GaTaO₄ are chosen as EBCs for C-, SiC-, and Al₂O₃-based substrates, respectively. The similar TECs between ABO₄-type oxide EBCs and substrates are beneficial for reducing interfacial thermal stress, which is good for their long-term applications. This work shows that the applications of ABO₄-type oxides can be expanded by effectively regulating TECs.

Rare-earth tantalates RETaO₄ have been extensively investigated as thermal protective materials, but their applications as environmental barrier coatings (EBCs) for ceramic matrix composites (CMCs) are limited by their high thermal expansion coefficients (TECs≥9.0×10^-6 K^-1). The high-entropy design provides a feasible route to tailor the thermal properties of RETaO₄; however, most reports focus on equimolar RETaO₄ HECs. In this study, a series of non-equimolar monoclinic-prime (m´) RETaO₄ HECs are designed and synthesized to clarify the composition-structure-property relationships. The non-equimolar design is used to regulate configurational complexity, polyhedral distortion, and lattice strain in m´-RETaO4 within the same m´ phase. Atomic-scale TEM and GPA characterizations reveal the associated local distortion and lattice strain. These observations link non-equimolar composition with local structural distortion and macroscopic thermal transport behavior. The lowest thermal conductivity reaches 1.52-2.68 W·m^-1·K^-1 at 25-900 °C, and is mainly associated with RE-site disorder, lattice strain, and polyhedral distortion. Polyhedral distortion also suppresses thermal expansion by restricting atomic anharmonic vibrations. Among the designed compositions, the optimized non-equimolar m´-RETaO₄ HECs (Sc_0.2Y_0.2Tm_0.2Ho_0.2Dy_0.1Gd_0.1)TaO₄ exhibits the lowest TECs of 6.2×10^-6 K^-1 at 1500 °C, closer to SiC-based CMCs than the other compositions, indicating its potential as candidate EBCs.  This work proposes that polyhedral distortion and lattice strain are important structural factors for tailoring the thermal properties of non-equimolar RETaO₄ HECs, providing guidance for the design of complex oxides.

Tailoring crystals and microstructures of multicomponent rare-earth tantalate RETaO₄ nano powders can promote their application as thermal protective coating materials because their properties are dominated by the structures. In this work, multicomponent RETaO₄ nano powders are synthesized via the chemical coprecipitation method, and their structures are tailored by changing the annealing temperature. After annealed at 800–1500 °C, the multicomponent RETaO₄ nano powders can be crystallized into metastable tetragonal (t′), monoclinic-prime (m′), and monoclinic (m) phases, and their particle sizes (8–652 nm) gradually increase with increasing temperature. The optimal annealing temperature of RETaO₄ powders is determined to be 1000 °C based on the crystallinity degree and particle sizes, which are 11.3 nm and 89.5%, respectively. High-resolution transmission electron microscopy (HR-TEM) and corresponding energy dispersive X-ray spectroscopy (EDS) mapping have validated the compositional uniformity of each element at the nanoscale, and the interplanar spacing of different phases corresponds to the X-ray diffraction (XRD) Rietveld refinements. This work demonstrates that high-temperature heat treatment can act as an effective mean to tailor the particle sizes and crystal structures of RETaO₄ ceramic, which can be further applied to synthesize nano spherical powders and coatings in future studies.

Cu₃SbSe₄ compounds with a diamond-like structure are promising eco-friendly p-type thermoelectric materials. However, their thermoelectric performance is limited by low intrinsic carrier concentration and mobility. This study employed GeMnTe₂ doping to simultaneously regulate the electronic band structure and carrier transport properties of Cu₃SbSe₄. First-principles calculations show that Mn ions introduce a resonant state (3d orbital) and promote band convergence, enhancing the density-of-states effective mass near the Fermi level. Concurrently, GeMnTe₂ doping induced the lattice relaxation effect and promoted the transformation of the matrix structure to a pseudocubic structure with higher symmetry, which reduced the deformation potential and inhibited the decreasing in carrier mobility under heavy doping. Multi-scale defects, including point defects and Ge/MnO_x nanoprecipitates, strongly suppressed the lattice thermal conductivity. Therefore, the power factor of 11.5 μW·cm⁻¹·K⁻² was reached by the 1.5% (in mole) GeMnTe₂ doped sample, and the peak ZT of ~1.2 was obtained at 723 K. Meanwhile, the hardness and modulus of the optimized sample were increased by 10%. These findings indicate that the electrical transport properties of Cu₃SbSe₄ can be synergistically optimized by band engineering and lattice symmetry tuning. This study provides a valuable method for the design of high-performance diamond-like thermoelectric materials.

Bismuth sulfide (Bi₂S₃) has garnered extensive attention for thermoelectric (TE) applications due to its earth abundance, nontoxicity, and ultralow lattice thermal conductivity. However, simultaneously improving its electrical and thermal properties remains challenging due to the strong coupling between these parameters. Herein, we propose a structural evolution strategy to synergistically optimize the electrical and thermal transport properties of Bi₂S₃-based TE compounds. By leveraging the distinct crystal structures of Bi₂S₃ and Bi₂Se₃, alloying Se at S sites successfully reconstructs the carrier transport channels within the Bi₂S₃ matrix, thereby facilitating carrier mobility. Additionally, the weakened chemical bonding leads to a significant increase in carrier concentration, approaching the optimal range. Furthermore, the structural evolution from particle-like to lamellar grains, coupled with the large mass difference between Se and S, induces lattice distortion that effectively reduces the lattice thermal conductivity. Combining the dramatically enhanced power factor and ultralow thermal conductivity, an optimized figure of merit (ZT) of 0.38 at 623 K is achieved in the Bi₂SSe₂ solid solution. This structural evolution strategy offers a promising avenue for enhancing the TE performance of binary compounds.

As the operating temperatures of aero-engine and gas turbine combustion chambers continue to rise, the high-temperature components face increasingly harsh service environments. The designed inlet temperatures of gas-turbine engines of advanced fighter jets have reached 1900 ℃, when the working temperatures of combustion chamber of H-level heavy-duty gas turbines have approached 1600 ℃, which are far higher than the limited application temperatures of Ni-based alloys (1150 ℃). Besides, the high-temperature components, such as rotating blades in the combustion chambers, are subjected to high-speed rotations, erosions of high-speed particles, thermal corrosions, thermal cycling stress, and corrosions of melted oxides, which will further lead to premature failures of these components. To overcome these challenges, ceramic thermal/environmental barrier coatings (T/EBCs) are widely used on surfaces of high-temperature components of gas turbines and aircraft engines to provide specific protections. The T/EBC systems are consist of substrates (high-temperature alloys or ceramic matrix composites), bond coat (BC) and surficial ceramic thermal/environmental barrier coatings. The main functions of T/EBCs are to provide thermal insulation performance, resisting particle impact, anti-oxidation, anti-corrosion, and so on. In order to maintain the excellent thermal insulation performance and exhibit a long-term service life, ceramic T/EBCs must display a low thermal conductivity, thermal expansion coefficients matching to those of different substrates, extraordinary high-temperature stability, high toughness and hardness, as well as relatively low modulus. It is of great significance to effectively measure and calculate the key mechanical and thermal properties of various potential T/EBC materials, which can promote the investigation and application of high-performance T/EBCs. In this work, we analyze and discuss the measurements and calculation methods of various mechanical and thermal properties of ceramic T/EBCs in detail, including hardness, elastic modulus, fracture toughness, thermal expansion coefficients, and thermal conductivity. For mechanical properties, the Vickers hardness device, nano indentation, and the ultrasonic reflection method are used to measure hardness, elastic modulus, and fracture toughness. For thermal properties, the thermal expansion device and high-temperature X-ray diffraction are used to calculate the thermal expansion coefficients, and their differences are discussed. Specifically, four different models are discussed in detail to elucidate the theories and calculations of thermal diffusivity, and the choosing rules of different models are proposed based on their calculation principle, which are good for obtaining reliable thermal conductivity. The object of this work is to provide scholars with suitable measurements and calculation methods of various mechanical and thermal properties for T/EBC materials, which can advance the quick screening, investigations, and designs of high-performance TBCs.

High entropy engineering has been widely used to optimize properties of various materials, and we improve comprehensive performance of rare-earth tantalates RETaO₄ (RE is rare earth) by changing configurational entropy in this work. Four medium/high entropy RETaO₄ (M/HERT) are successfully prepared, and the variations of disorders and distortion degree of lattices with the increasing configurational entropy are described in detail. It is revealed that M/HERT with the highest configurational entropy does not correspond to the best comprehensive properties. Unexpected variations in properties of M/HERT compared to RETaO₄ are observed. By comparing with values obtained from rule of mixture (ROM), it is believed that the cocktail effect exists in M/HERT. The synergistic optimizations of thermo-mechanical properties are realized, including reducing thermal conductivity, increasing thermal expansion coefficients (TECs), and enhancing mechanical properties. M/HERT exhibit excellent high temperature stability and provide a good thermal insulation gradient, which is significant for high-temperature applications of RETaO₄. This work serves as an important part for thermal barrier coatings materials with high working temperatures and low thermal conductivity.

High fracture toughness, low thermal conductivity, and thermal expansion coefficient (TEC) matching substrate are essential for thermal barrier coatings (TBCs) and abradable seal coatings (ASCs). In this work, TmNbO₄/Tm₃NbO₇ composites are designed and synthesized to increase their fracture toughness (K_IC) and thermal insulation performance. Compared with those of TmNbO₄ (K_IC = 2.2±0.1 MPa·m^1/2) and Tm₃NbO₇ (K_IC = 1.7±0.2 MPa·m^1/2), the increments in fracture toughness are as high as 50.0% and 91.1%, respectively. The highest toughness reaches 3.3±0.4 MPa·m^1/2, which is attributed to the superior combination of grains between TmNbO₄ and Tm₃NbO₇, as well as the simultaneous effects of microcracks and crack bridging and bifurcation. Accurate estimation of the effect of the interfacial thermal resistance on the thermal conductivity at low temperatures was achieved using the minimum interfacial thermal resistance model. A novel method is proposed to inhibit radiative heat transfer by utilizing oxides with glass-like thermal conductivity to suppress thermal radiation. Consequently, the TmNbO₄/Tm₃NbO₇ composite maintains a low thermal conductivity (1.19–2.02 W·m⁻¹·K⁻¹) at 1000 °C. The high TECs (10.4×10⁻⁶–11.8×10⁻⁶·K⁻¹ at 1500 °C) and excellent high-temperature stability ensure that the designed TmNbO₄/Tm₃NbO₇ composites can be used at temperatures reaching 1500 °C. Accordingly, simultaneous enhancement of fracture toughness and thermal insulation in TmNbO₄/Tm₃NbO₇ composites is effective, and the revealed mechanisms are useful for various materials.

In situ phase separation precipitates play an important role in enhancing the thermoelectric properties of copper sulfides by suppressing phonon transmission. In this study, Cu_1.8S composites were fabricated by melting reactions and spark plasma sintering. The complex structures, namely, micron-PbS, Sb₂S₃, nano-FeS, and multiscale pores, originate from the introduction of FePb₄Sb₆S₁₄ into the Cu_1.8S matrix. Using effective element (Fe) doping and multiscale precipitates, the Cu_1.8S+0.5 wt% FePb₄Sb₆S₁₄ bulk composite reached a high dimensionless figure of merit (ZT) value of 1.1 at 773 K. Furthermore, the modulus obtained for this sample was approximately 40.27 GPa, which was higher than that of the pristine sample. This study provides a novel strategy for realizing heterovalent doping while forming various precipitates via in situ phase separation by natural minerals, which has been proven to be effective in improving the thermoelectric and mechanical performance of copper sulfides and is worth promoting in other thermoelectric systems.

Four high-entropy perovskite (HEP) RETa₃O₉ samples were fabricated via a spark plasma sintering (SPS) method, and the corresponding thermophysical properties and underlying mechanisms were investigated for environmental/thermal barrier coating (E/TBC) applications. The prepared samples maintained low thermal conductivity (1.50 W·m^-1·K^-1), high hardness (10 GPa), and an appropriate Young’s modulus (180 GPa), while the fracture toughness increased to 2.5 MPa·m^1/2. Nanoindentation results showed the HEP ceramics had excellent mechanical properties and good component homogeneity. We analysed the influence of different parameters (the disorder parameters of the electronegativity, ionic radius, and atomic mass, as well as the tolerance factor) of A-site atoms on the thermal conductivity. Enhanced thermal expansion coefficients, combined with a high melting point and extraordinary phase stability, expanded the applications of the HEP RETa₃O₉. The results of this study had motivated a follow-up study on tantalate high-entropy ceramics with desirable properties.