High-entropy (Ho0.2Y0.2Dy0.2Gd0.2Eu0.2)2Ti2O7/TiO2 composites with excellent mechanical and thermal properties

High-performance ceramics with low thermal conductivity, high mechanical properties, and idea thermal expansion coefficients have important applications in fields such as turbine blades and automotive engines. Currently, the thermal conductivity of ceramics has been significantly reduced by local doping/substitution or further high-entropy reconfiguration of the composition, but the mechanical properties, especially the fracture toughness, are insufficient and still need to be improved. In this work, based on the high-entropy titanate pyrochlore, TiO₂ was introduced for composite toughening and the high-entropy (Ho_0.2Y_0.2Dy_0.2Gd_0.2Eu_0.2)₂Ti₂O₇-xTiO₂ (x = 0, 0.2, 0.4, 1.0 and 2.0) composites with high hardness (16.17 GPa), Young's modulus (289.3 GPa) and fracture toughness (3.612 MPa·m^0.5), low thermal conductivity (1.22 W·m⁻¹·K⁻¹), and thermal expansion coefficients close to the substrate material (9.5 ×10⁻⁶/K) were successfully prepared by the solidification method. The fracture toughness of the composite toughened sample is 2.25 times higher than that before toughening, which exceeds most of the current low-thermal conductivity ceramics.     

Single-phase rare-earth high-entropy zirconates with superior thermal and mechanical properties

Emerging of high-entropy ceramics has brought new opportunities for designing and optimizing materials with desired properties. In the present work, high-entropy rare-earth zirconates (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ and (Yb_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ are designed and synthesized. Both high-entropy ceramics exhibit a single pyrochlore structure with excellent phase stability at 1600 °C. In addition, the Yb-containing system possesses a high coefficient of thermal expansion (10.52 × 10^?6 K^-1, RT～1500 °C) and low thermal conductivity (1.003 W·m^-1 K^-1, 1500 °C), as well as excellent sintering resistance. Particularly, the Yb-containing system has significantly improved fracture toughness (1.80 MPa·mm^1/2) when compared to that of lanthanum zirconate (1.38 MPa·mm^1/2), making it a promising material for thermal barrier coatings (TBCs) applications. The present work indicates that the high-entropy design can be applied for further optimization of the comprehensive properties of the TBCs materials.     

Marked reduction in the thermal conductivity of (La0.2Gd0.2Y0.2Yb0.2Er0.2)2Zr2O7 high-entropy ceramics by substituting Zr4+ with Ti4+

The (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ (x = 0–0.5) high-entropy ceramics were successfully prepared by a solid state reaction method and their structures and thermo-physical properties were investigated. It was found that the high-entropy ceramics demonstrate pure pyrochlore phase with the composition of x = 0.1–0.5, while (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂Zr₂O₇ shows the defective fluorite structure. The sintered high-entropy ceramics are dense and the grain boundaries are clean. The grain size of high-entropy ceramics increases with the Ti⁴⁺ content. The average thermal expansion coefficients of the (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ high-entropy ceramics range from 10.65 × 10^?6 K^?1 to 10.84 × 10^?6 K^?1. Importantly, the substitution of Zr⁴⁺ with Ti⁴⁺ resulted in a remarkable decrease in thermal conductivity of (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ high-entropy ceramics. It reduced from 1.66 W m^?1 K^?1 to 1.20 W m^?1 K^?1, which should be ascribed to the synergistic effects of mass disorder, size disorder, mixed configuration entropy value and rattlers.     

Rare-earth-tantalate high-entropy ceramics with sluggish grain growth and low thermal conductivity

A series of rare-earth-tantalate high-entropy ceramics ((5RE_0.2)Ta₃O₉, where RE = five elements chosen from La, Ce, Nd, Sm, Eu and Gd) were prepared by conventional sintering in air at 1500 ^°C for 10 h. The (5RE_0.2)Ta₃O₉ high-entropy ceramics exhibit an orthogonal structure and sluggish grain growth. No phase transition occurs in the test temperature of 25–1200 ^°C. The thermal conductivities of all (5RE_0.2)Ta₃O₉ ceramics are in the range of 1.14–1.98 W m^?1 K^?1 at a test temperature of 25–500 ^°C, approximately half of that of YSZ. The sample of (Gd_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)Ta₃O₉ exhibits a low glass-like thermal conductivity with a value of 1.14 W m^?1 K^?1 at 25 ^°C. The thermal expansion coefficient of (5RE_0.2)Ta₃O₉ ceramics ranges from 5.6 × 10^?6 to 7.8 × 10^?6 K^?1 at 25–800 ^°C, and their fracture toughness is high (3.09–6.78 MPa·m^1/2). The results above show that (5RE_0.2)Ta₃O₉ ceramics could be a promising candidate for thermal barrier coatings.     

Realizing load bearing-heat insulation integration of lightweight porosity-controlled SiC(rGO) bulk ceramics via precursor route for hypersonic vehicles

Realizing integration of admirable load bearing and outstanding heat insulation characteristics of porosity-controlled ceramics for hypersonic vehicles is a great challenge. Herein, an ingenious strategy based on re-pyrolysis process of ball-milling-induced fillers/precursors(pore-forming agents) blends is proposed to prepare porous SiC(rGO) bulk polymer-derived ceramics (PDCs) for thermal protection. During re-pyrolysis, dense integrated β-SiC/SiOxCy/C_free(rGO) framework, belonging to SiC(rGO)_p tightly tied by SiC(rGO) from polycarbosilane-vinyltriethoxysilane-graphene oxide (PCS-VTES-GO, PVG), can be formed to maintain brilliant mechanical properties of products. Meanwhile, good interfacial compatibility of nanocomposite structure within the framework also contributes to load capacity. A uniquely uniform distribution of dentinal tubules-like pores, originated from polystyrene (PS) in SiC(rGO) region, could relax stress at crack tips and ensure good thermal insulation. Particularly, lightweight porous SiC(rGO) bulk PDCs with 10 wt% PS content possess low thermal conductivity (0.25 W·m^?1·K^?1), excellent fracture toughness (1.96 MPa·m^1/2), outstanding hardness (3.58 GPa), optimal compressive strength (51.80 MPa) and good flexural strength (33.86 MPa). Their large-sized molding ability and good high-temperature oxidation resistance were significantly demonstrated by further exploration. Such well-balanced high load bearing and good heat insulation integration nature can be used to make thermal insulators with complex shapes in a facile and economical manner.     

Carbothermal reduction synthesis of high porosity and low thermal conductivity ZrC-SiC ceramics via an one-step sintering technique

In this work, porous ZrC-SiC ceramics with high porosity and low thermal conductivity were successfully prepared using zircon (ZrSiO₄) and carbon black as material precursors via a facile one-step sintering approach combining in-situ carbothermal reduction reaction (at 1600 °C for 2 h) and partial hot-pressing sintering technique (at 1900 °C for 1 h). Carbon black not only served as a reducing agent, but also performed as a pore-foaming agent for synthesizing porous ZrC-SiC ceramics. The prepared porous ZrC-SiC ceramics with homogeneous microstructure (with grain size in the 50–1000 nm range and pore size in the 0.2–4 µm range) possessed high porosity of 61.37–70.78%, relatively high compressive strength of 1.31–7.48 MPa, and low room temperature thermal conductivity of 1.48–4.90 W·m^?1K^?1. The fabricated porous ZrC-SiC ceramics with higher strength and lower thermal conductivity can be used as a promising light-weight thermal insulation material.     

Synthesis,microstructure and property characterization of Mo4Y2Al3B6 ceramic fabricated by spark plasma sintering

Polycrystalline Mo₄Y₂Al₃B₆ ceramic (92.84 wt% Mo₄Y₂Al₃B₆ and 7.16 wt% MoB) was prepared by spark plasma sintering at 1250 ℃ under 30 MPa using Mo, Y, Al, and B as starting materials. The dense sample obtained has a high relative density of 96.6 %. The average thermal expansion coefficient is 8.38 × 10^?6 K^?1 in the range of 25–1000 ℃. The thermal diffusivity decreases from 6.50 mm²/s at 25 °C to 4.33 mm²/s at 800 °C. The heat capacity, thermal conductivity, and electrical conductivity are 0.30 J·g^?1·K^?1, 11.73 W·m^?1·K^?1, and 0.66 × 10⁶ Ω^?1·m^?1 at 25 °C, respectively. Vickers hardness with increasing load in the range of 10–300 N at room temperature decreases from 10.82 to 9.49 GPa, and the fracture toughness, compressive strength, and flexural strength are 5.14 MPa·m^1/2, 1255.14 MPa, and 384.82 MPa, respectively, showing the promising applications as structural-functional ceramics.     

Physical Properties of La1−xEuxPO4,0 ≤ x ≤ 1, Monazite‐Type Ceramics

Synthetic La_1?xEu_xPO₄ monazite‐type ceramics with 0 ≤ x ≤ 1 have been characterized by ultrasound techniques, dilatometry, and micro‐calorimetry. The coefficients of thermal expansion and the elastic properties are, to a good approximation, linearly dependent on the europium concentration. Elastic stiffness coefficients range from 182(1) to 202(1) GPa for c₁₁ and from 53.8(7) to 61.1(4) GPa for c₄₄. They are strongly dependent on the density of the sample. The coefficient of thermal expansion at 673 K is 8.4(3)  × 10^?6 K^?1 for LaPO₄ and 9.9(3)  × 10^?6 K^?1 for EuPO₄, respectively. The heat capacities at ambient temperature are between 101.6(8) J·(mol·K)^?1 for LaPO₄ and 110.1(8) J·(mol·K)^?1 for EuPO₄. The difference between the heat capacity of LaPO₄ and the Eu‐containing solid solutions is dominated by electronic transitions of the 4f‐electrons at temperatures above 75 K.     

Structural and Thermo‐Mechanical Properties of Nd: Y2O3 Transparent Ceramics

Various content of neodymia Nd: Y₂O₃ (Nd: 0.5–5.0 at.%) transparent ceramics were fabricated by vacuum sintering. The prepared Nd: Y₂O₃ ceramics exhibit high transmittance (~80%) at the wavelength of 1100 nm. It is found that the increase in Nd concentration enhances the grain size growth, while decreases the phonon energy, which is benefit for improving both the luminescence quantum and up‐conversion efficiency. The thermal conductivity and thermal expansion coefficient of the transparent 1.0 at.% Nd: Y₂O₃ ceramic is 5.51 W·(m·K)^?1 and 8.11 × 10^?6 K^?1, respectively. The hardness and the fracture toughness of the transparent ceramic is 9.18 GPa and 1.03 Mpa·m^1/2, respectively. The results indicate that the Nd: Y₂O₃ transparent ceramic is a potential candidate material for laser.     

Tailoring thermal and mechanical properties of rare earth niobates by coupling entropy and composite engineering

In this paper, a series of high-entropy rare earth niobates, including fluorite RE₃NbO₇ (HE317), monoclinic RENbO₄ (HE114) and RENbO₄/RE₃NbO₇ composite (HE-composite), are prepared via solid state reaction, following by a study about their thermal and mechanical properties. The high-entropy rare earth niobates exhibit excellent phase stability after thermally exposed to 1300 °C for 100 h, indicating entropy can stabilized high-entropy rare earth niobates. Compared with the single element rare earth niobates, high-entropy rare earth niobates have higher fracture toughness and hardness. The high-entropy RENbO₄/RE₃NbO₇ composite has the best mechanical properties, with a fracture toughness of 2.71 ± 0.17 MPa·m^1/2 and hardness of 9.46 ± 0.24 GPa, respectively. The high-entropy niobates exhibit high coefficients of thermal expansion which is close to 7 wt% Y₂O₃ stabilized ZrO₂. It is also proved that the configurational entropy has little effect on the critical temperature from monoclinic to tetragonal phase transition. The thermal conductivity of HE-composite is lower than HE114, indicating the combination of HE114 and H317 is a more efficient strategy to decrease the thermal conductivity of HE114 than entropy engineering.     

High-entropy (HfTaTiNbZr)C and (HfTaTiNbMo)C carbides fabricated through reactive high-energy ball milling and spark plasma sintering

Powders of high-entropy Hf_0.2Ta_0.2Ti_0.2Nb_0.2Zr_0.2C (HEC_Zr) and Hf_0.2Ta_0.2Ti_0.2Nb_0.2Mo_0.2C (HEC_Mo) carbides were fabricated through the reactive high-energy ball milling (R-HEBM) of metal and graphite particles. It was found that 60 min of R-HEBM is adequate to achieve a full conversion of the initial precursors into a FCC solid solution for both compositions. The HEC_Zr powder possesses a unimodal particle size distribution (40% d ≤ 1 μm, 95% d ≤ 10 μm), and the HEC_Mo powder features a bimodal distribution with a slightly larger particle size overall (30% d ≤ 1 μm, 80% d ≤ 10 μm). Bulk high-entropy ceramics with a minor presence of an oxide phase were fabricated through the spark plasma sintering of these high-entropy powders at 2000 °C with a 10 min dwelling time. The HEC_Zr ceramics possess a relative density of up to 94.8%, hardness of 25.7 ± 3.5 GPa, Young's modulus of 473 ± 37 GPa, and thermal conductivity of 5.6 ± 0.1 W/m·K. HEC_Mo ceramics with a relative density of up to 93.8%, hardness of 23.8 ± 2.7 GPa, Young's modulus of 544 ± 48 GPa, and thermal conductivity of 5.9 ± 0.2 W/m·K were also fabricated. A comparison of the properties of the HECs produced in this study and those previously reported is also provided.     

Determinations of Ce solid solution mechanism and limit thermal conductivity of YTaO4 ceramics

x mol% CeO₂-YTaO₄ (x = 0, 3, 6, 9, 12) ceramics have been synthesized by the spark plasma sintering (SPS) technique. We focus on the changes in lattice distortion, bonding length, thermal conductivity, thermal expansion, and phase stability of the prepared samples. XRD, Raman, and XPS are used to determine the chemical valence and solid solution mechanism of Ce in the lattice of YTaO₄, while its effects on thermal/mechanical properties are elucidated from microstructures. Y³⁺ is substituted via Ce³⁺, and all samples maintain a monoclinic phase. The limit thermal conductivity (1.2 W?m^?1?K^?1, 900 °C) is realized in 9 mol% CeO₂-YTaO₄, and the thermal expansion coefficients are increased to 10.2 × 10^?6 K^?1 at 1200 °C. Furthermore, the exceptional phase stability and mechanical properties of all samples indicate that they can provide good thermal insulation at high temperatures, and have higher working temperatures than the current YSZ thermal barrier coatings.     

Novel (Sm0.2Lu0.2Yb0.2Y0.2Dy0.2)3TaO7 high-entropy ceramic for thermal barrier coatings

A novel (Sm_0.2Lu_0.2Dy_0.2Yb_0.2Y_0.2)₃TaO₇ (SLT-5RE_0.2) oxide with a single-fluorite structure was synthesized via an optimized sol-gel and sintering method, and its crystal structure, mechanical and thermophysical properties were investigated. The results indicate that the calcined nanoscale powder is of high crystallinity, and bulk sample is of a uniform elemental distribution. Compared to YSZ (6–8 wt.% Y₂O₃ partially stabilized by ZrO₂), SLT-5RE_0.2 exhibits lower Young's modulus, less mean acoustic velocity, and higher Vickers microhardness. Owing to the strengthened anharmonic vibration and phonon scattering, SLT-5RE_0.2 exhibits low thermal conductivity (1.107 W K^?1·m^?1, 900 °C). The high thermal expansion coefficient (11.3 × 10^?6 K^?1, 1200 °C) of SLT-5RE_0.2 ceramic can be ascribed to the reduced lattice energy and ionic spacing as well as the cocktail effect of high-entropy ceramics. The excellent mechanical and thermophysical properties, and excellent phase steadiness during the whole testing temperature cope, indicate that SLT-5RE_0.2 high-entropy ceramic can be a candidate material for thermal barrier coatings.     

Phase evolution and thermal stability of novel high-entropy (Mo0.2Nb0.2Ta0.2V0.2W0.2)Si2 ceramics

Owing to the high melting points and high-temperature stability, transition-metal disilicides are potential components for aerospace, automotive, and industrial engineering applications. However, unwanted oxidation known as PEST oxidation severely limits their application owing to the formation of volatile transition metal oxides, especially in the temperature range of 500–1000 °C. To overcome this problem, a new class of high-entropy disilicides, (Mo_0.2Nb_0.2Ta_0.2V_0.2W_0.2)Si₂, was selected by first-principles calculations and then successfully fabricated using a hot-pressing sintering technique. Furthermore, the phase evolution, thermal expansion behavior, thermal conductivity, and oxidation behavior were systematically investigated. Compared with MoSi₂, (Mo_0.2Nb_0.2Ta_0.2V_0.2W_0.2)Si₂ possessed a lower thermal conductivity (10.9–14.7 W·m^?1·K^?1) at 25–1000 °C, higher thermal expansion coefficients (8.6 ± 1.3^–6 K^–1) at 50–1200 °C, and especially an excellent thermal stability at 500–1000 °C owing to slow diffusion and selective oxidation. This work provides a strong foundation for the synthesis and application of high-entropy disilicides.     

Improving specific stiffness of silicon carbide ceramics by adding boron carbide

A strategy for improving the specific stiffness of silicon carbide (SiC) ceramics by adding B₄C was developed. The addition of B₄C is effective because (1) the mass density of B₄C is lower than that of SiC, (2) its Young’s modulus is higher than that of SiC, and (3) B₄C is an effective additive for sintering SiC ceramics. Specifically, the specific stiffness of SiC ceramics increased from ~142 × 10⁶ m²?s^?2 to ~153 × 10⁶ m²?s^?2 when the B₄C content was increased from 0.7 wt% to 25 wt%. The strength of the SiC ceramics was maximal with the incorporation of 10 wt% B₄C (755 MPa), and the thermal conductivity decreased linearly from ~183 to ~81 W?m^?1?K^?1 when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 25 wt% B₄C were ~690 MPa and ~95 W?m^?1?K^?1, respectively.     

Investigation of the thermal shrink mechanism,thermal conductivity and compressive resistance of TaTiP3O12 ceramics

Dense TaTiP₃O₁₂ ceramics were synthesized by the solid-state method and spark plasma sintering (SPS) with 6 wt% V₂O₅ as a sintering aid, and their phase, microstructure, thermal conductivity, hardness, compressive strength, and expansion property and mechanism were investigated. Results show that the pure phase can be achieved by the two methods. In particular, the sample prepared by SPS possesses a relative density of 97.62% and a porosity of 3.07%, and has better properties than that prepared by the solid-state method. The SPS sample has a thermal conductivity at room temperature of 2.03 w/(m· °C), a Vickers hardness of 4.34 GPa and a compressive strength of 175.98 MPa, which are 0.95, 1.49 and 1.59 times greater than those of the sample prepared by the solid-state method, respectively. In addition, the TaTiP₃O₁₂ ceramic prepared by SPS exhibits a linear ultralow negative thermal expansion property with a coefficient of thermal expansion of ?0.74 × 10^?6 °C ^?1 (-100–400 °C). The negative thermal expansion in TaTiP₃O₁₂ is induced by the coupling effect of [Ta(Ti)O₆] octahedron and [PO₄] tetrahedron caused by the transverse vibration of bridging oxygen atoms.     

Lightweight porous silica-alumina ceramics with ultra-low thermal conductivity

Thermal protection has always been an important issue in the energy, environment and aerospace fields. Porous ceramics produced by the particle-stabilized foaming method have become a competitive material for thermal protection because of their low density and low thermal conductivity. However, the study of porous ceramics for composite systems using particle-stabilized foaming method was relatively rare. Here, silica-alumina composite porous ceramics were prepared by particle-stabilized foaming method, which was achieved by tailoring the surface charges of silica and alumina through adjustment of the pH. Porous ceramics exhibited porosity as high as 97.49% and thermal conductivity (25 °C) as low as 0.063 W m^?1 K^?1. The compressive strength of porous ceramics sintered at 1500 °C with a solid content of 30 wt% could reach 0.765 MPa. Based on the light weight and excellent thermal insulation properties, the composite porous ceramic could be used as a potential thermal insulation material in the spacecraft industry.     

Highly porous (La1/5Nd1/5Sm1/5Gd1/5Yb1/5)2Zr2O7 ceramics with ultra-low thermal conductivity

The high-entropy rare earth zirconate (La_1/5Nd_1/5Sm_1/5Gd_1/5Yb_1/5)₂Zr₂O₇ porous ceramics ((5RE_1/5)₂Zr₂O₇ PCs) were prepared using a foam-gel casting-freeze drying method combined with segmented calcination process. The results of SEM, TEM, and XRD analyses of the (5RE_1/5)₂Zr₂O₇ PCs indicated the formation of a defective fluorite crystal structure, with the rare earth elements homogeneously distributed. Meanwhile, the as-prepared (5RE_1/5)₂Zr₂O₇ PCs exhibited high porosity, low bulk density, low thermal conductivity, and relatively high compressive strength. Moreover, the high-temperature thermal conductivity of the samples was evaluated, and the results showed that the (5RE_1/5)₂Zr₂O₇ PCs maintain a thermal conductivity of 0.150 ± 0.002 W m^?1 K^?1 even at 1000 °C. The strategy used in this paper can be extended to the synthesis of other high-entropy porous ceramics with high porosity and low thermal conductivity, which is suitable for applications as thermal insulation materials.     

Effect of carbon content on electrical,thermal, and mechanical properties of porous SiC ceramics with B4C and C additives

The electrical, thermal, and mechanical properties of porous SiC ceramics with B₄C-C additives were investigated as functions of C content and sintering temperature. The electrical resistivity of porous SiC ceramics decreased with increases in C content and sintering temperature. A minimal electrical resistivity of 4.6 × 10^?2 Ω·cm was obtained in porous SiC ceramics with 1 wt% B₄C and 10 wt% C. The thermal conductivity and flexural strength increased with increasing sintering temperature and showed maxima at 4 wt% C addition when sintered at 2000 °C and 2100 °C. The thermal conductivity and flexural strength of porous SiC ceramics can be tuned independently from the porosity by controlling C content and sintering temperature. Typical electrical resistivity, thermal conductivity, and flexural strength of porous SiC ceramics with 1 wt% B₄C-4 wt% C sintered at 2100 °C were 1.3 × 10^?1 Ω·cm, 76.0 W/(m·K), and 110.3 MPa, respectively.     

High-entropy ferroelastic rare-earth tantalite ceramic: (Y0.2Ce0.2Sm0.2Gd0.2Dy0.2)TaO4

In this study, high-entropy rare-earth tantalate ceramics (Y_0.2Ce_0.2Sm_0.2Gd_0.2Dy_0.2)TaO₄ ((5RE_0.2)TaO₄) have been successfully fabricated. The possibility of formation of (5RE_0.2)TaO₄ was verified via first-principles calculations. In addition, the phase structure, ferroelastic toughening mechanism, thermophysical, and mechanical properties were systematically investigated. The (5RE_0.2)TaO₄ ceramics have lower phonon thermal conductivity (1.2–2.6 W·m^–1·K^–1) in the entire temperature range than that of RETaO₄ and YSZ. (5RE_0.2)TaO₄ has a higher fracture toughness and lower brittleness index than YSZ. The thermal expansion coefficients of (5RE_0.2)TaO₄ are as high as 10.3 × 10^-6 K^–1 at 1200°C and Young's modulus is 66–189 GPa, and thus, (5RE_0.2)TaO₄ possesses great potential for application in thermal barrier coatings (TBCs).