Thermal expansion performance and intrinsic lattice thermal conductivity of ferroelastic RETaO4 ceramics

Ferroelastic RETaO₄ ceramics are promising thermal barrier coatings (TBCs) because of their attractive thermomechanical properties. The influence of crystal structure distortion degree on thermomechanical properties of RETaO₄ is estimated in this work. The relationship between Young's modulus and TECs is determined. The highest TECs (10.7 × 10⁻⁶ K⁻¹, 1200°C) of RETaO₄ are detected in ErTaO₄ ceramics and are ascribed to its small Young's modulus and low Debye temperature. The intrinsic lattice thermal conductivity (3.94-1.26 W m⁻¹ K⁻¹, 100-900°C) of RETaO₄ deceases with increasing of temperature due to an elimination in thermal radiation effects. The theoretical minimum thermal conductivity (1.00 W m⁻¹ K⁻¹) of RETaO₄ indicates that the experimental value is able to be reduced further. We have delved deeply into the thermomechanical properties of ferroelastic RETaO₄ ceramics and have emphasized their high-temperature applications as TBCs.     

Preparation of ultralow thermal conductivity (Mg0.5Ca0.3Ba0.2) (AlSi)2O8 ceramics

A type of nonequimolar multicomponent ceramic solid solution (Mg_0.5Ca_0.3Ba_0.2) (AlSi)₂O₈ with a low thermal conductivity was prepared through solid-state synthesis. Results show that the (Mg_0.5Ca_0.3Ba_0.2) (AlSi)₂O₈ solid solution exhibits excellent high-temperature stability and an ultralow thermal conductivity (.3676 W m⁻¹ K⁻¹), far lower than widely used 3YSZ (2.9 W m⁻¹ K⁻¹), La₃NbO₇ (1.5 W m⁻¹ K⁻¹), and Gd₂Zr₂O₇ (1.28 W m⁻¹ K⁻¹). Furthermore, the Young modulus of the final product is 64.56 GPa. Therefore, the proposed ceramic solid solution provides a new research direction for ultralow thermal conductivity materials and has a practical application value for the field of wall thermal insulation.     

Thermophysical properties and cyclic lifetime of plasma sprayed SrAl12O19 for thermal barrier coating applications

About 6-8 wt% yttria-stabilized zirconia (YSZ) is the industry standard material for thermal barrier coatings (TBC). However, it cannot meet the long-term requirements for advanced engines due to the phase transformation and sintering issues above 1200°C. In this study, we have developed a magnetoplumbite-type SrAl₁₂O₁₉ coating fabricated by atmospheric plasma spray, which shows potential capability to be operated above 1200°C. SrAl₁₂O₁₉ coating exhibits large concentrations of cracks and pores (~26% porosity) after 1000 hours heat treatment at 1300°C, while the total porosity of YSZ coatings progressively decreases from the initial value of ~18% to ~5%. Due to the contribution of porous microstructure, an ultralow thermal conductivity (~1.36 W m⁻¹ K⁻¹) can be maintained for SrAl₁₂O₁₉ coating even after 1000 hours aging at 1300°C, which is far lower than that of the YSZ coating (~1.98 W m⁻¹ K⁻¹). In thermal cyclic fatigue test, the SrAl₁₂O₁₉/YSZ double-ceramic-layer coating undertakes a thermal cycling lifetime of ~512 cycles, which is not only much longer than its single-layer counterpart (~163 cycles), but also superior to that of YSZ coating (~392 cycles). These preliminary results suggest that SrAl₁₂O₁₉ might be a promising alternative TBC material to YSZ for applications above 1200°C.     

Functionalized few-layered graphene nanoplatelets for superior thermal management in heat transfer nanofluids

The superior thermal conductivity and lightweight of graphene flakes make them materials of choice for advanced heat transfer applications, especially for transport of electricity from sustainable power stations such as concentrating solar power plants. In view of the excellent thermal conductivity of graphene or graphene-like nanomaterials (3000–5000 W m⁻¹ K⁻¹), their dispersion into conventional host fluids such as water (0.613 W m⁻¹ K⁻¹) or ethylene glycol (0.25 W m⁻¹ K⁻¹) can significantly improve fluid heat transfer characteristics. The two-dimensional structure and high surface area as well as the cost-efficient carbon-based material make graphene nanoplatelets (GNPs) suitable for large-scale applications in colloidal thermal conductive fluids. For an efficient dispersion of GNPs in base fluids, intrinsically hydrophobic GNPs were acid treated to obtain highly concentrated (4 wt.%) graphene-based nanofluids. Investigations on various GNP sizes and reaction parameters showed significant influences on the resulting thermal conductivity values of the nanofluid. After 14 h measurements in a dormant system, the most efficient nanofluid reached a thermal conductivity of 0.586 W m⁻¹ K⁻¹ (the base fluid of 0.391 W m⁻¹ K⁻¹) and a low viscosity of 6.39 cP resulting in an overall efficiency improvement of 77%, when compared to the base fluid without particles.     

SPS-assisted synthesis of InGaO3(ZnO)m ceramics,and influence of m on the band gap and the thermal conductivity

In this study, we report on the use of a two-stage annealing treatment at 1100°C coupled to reactive Spark Plasma Sintering to reduce the synthesis temperature of InGaO₃(ZnO)_m (m = 1 to 9) dense polycrystalline pellets below 1200°C, in order to suppress the volatilization of ZnO and get a better control of the crystalline quality of the pellets. We show that using this treatment, dense single-phase pellets can be prepared with randomly oriented grains. Besides, we evidence a monotonic evolution of the band gap in the series from 3.27 eV in InGaO₃(ZnO) to 3.02 eV in InGaO₃(ZnO)₉, as well as a non-monotonic evolution of the lattice thermal conductivity that reaches a minimum for InGaO₃(ZnO)₃, lower than 2 W m⁻¹ K⁻¹ above 350°C. Last, we propose a procedure for the high-temperature measurement of the thermal diffusivity of oxides by the laser flash method to avoid possible reactions between the measured material and the graphite spray.     

Spark plasma sintering of (5RE.2)Ta3O9 high-entropy ceramics and their thermal properties

Six rare-earth tantalate high-entropy ceramics of (5RE_.2)Ta₃O₉ (RE represents any five elements selected from La, Ce, Nd, Sm, Eu, Gd) were designed and prepared by spark plasma sintering process at 1400°C in this study. The (5RE_.2)Ta₃O₉ ceramics only consist of a single-phase solid solution with perovskite structure. Their relative densities are all above 90%, and the average grain size is in the range of 1.47–2.92 μm. The thermal conductivity of (5RE_.2)Ta₃O₉ ceramics is in 2.24–1.90 W m⁻¹ K⁻¹ (25°C–500°C), which is much lower than that of yttria-stabilized zirconia. In six samples, (La_.2Nd_.2Sm_.2Gd_.2Eu_.2)Ta₃O₉ possesses a thermal conductivity of 1.90 W m⁻¹ K⁻¹, a thermal expansion coefficient of 3.47 × 10⁻⁶ K⁻¹ (500°C), a Vickers hardness of about 7.33 GPa, and a fracture toughness of about 5.20 MPa m^1/2, which are suitable for its application as thermal barrier coatings.     

Reticulated open-cellular aluminum nitride ceramic foams: Effect of sintering aids on microstructural,thermal, and mechanical properties

Open-celled aluminum nitride ceramic foams were prepared by the polymer sponge replication technique involving aqueous dispersions of passivated AlN. The amount of the Y₂O₃ and Dy₂O₃ as sintering aid was varied, and the effects on the densification, microstructure formation, phase composition, and finally, the thermal conductivity were investigated. A typical thermal conductivity of 1.1 W m⁻¹ K⁻¹ was determined for foams at a porosity level of 94.3 vol.%, on average. This measured foam thermal conductivity was subsequently modeled using different porosity ↔ thermal conductivity relations considering the different hierarchical levels of porosity in these foams. From these models, the thermal conductivity of the bulk AlN strut material was determined, correlated with the strut microstructure and the phase composition, and compared to literature data.     

Processing and thermal properties of mayenite electride

A mayenite-based electride is being considered for field emitter applications where it will be critical to have a good understanding of processing repeatability and thermal properties. This paper focuses on the details of processing of mayenite compounds in a strongly reducing graphite environment to form an electride solid-solution phase as Ca₁₂Al₁₄O₃₂(C₂,O₂)_1-x(e₂)_x. The work reports, in addition to processing details, several thermal properties. The electride phase was found to have coefficient of thermal expansion (CTE) of 7e-6 K⁻¹, thermal diffusivity between 0.0063 and 0.0102 cm²/s, and thermal conductivity between 1.9 and 3.1 W m⁻¹ K⁻¹. Rattling motion of O²⁻ or C₂²⁻ ions in cage structure has been suggested to explain low thermal conductivity of the electride phase.     

Magneto-optical,thermal, and mechanical properties of polycrystalline Tb3Al5O12 transparent ceramics

Terbium aluminum garnet (Tb₃Al₅O₁₂, TAG) ceramics have become a promising magneto-optical material owing to the outstanding comprehensive performance, including the magneto-optical, thermal, and mechanical properties. Fine-grained TAG ceramics with high optical quality and mechanical properties have attracted much attention. In this study, TAG ceramics with fine grains and high optical quality are fabricated successfully by a two-step sintering method from co-precipitated nano-powders. After pre-sintered at 1525°C in vacuum and hot isostatic pressed at 1600°C, the in-line transmittance of TAG ceramics reaches 81.8% at 1064 nm, and the average grain size is 7.1 μm. The Verdet constant of TAG ceramics is −179.6 ± 4.8 rad T⁻¹ m⁻¹ at 633 nm and −52.1 ± 1.9 rad T⁻¹ m⁻¹ at 1064 nm, higher than that of commercial Tb₃Ga₅O₁₂ crystals. The thermal conductivity of TAG ceramics is determined from 25 to 450°C, and the result is 5.12 W m⁻¹ K⁻¹ at 25°C and 3.61 W m⁻¹ K⁻¹ at 450°C. A comparison of mechanical properties between large- and fine-grained TAG ceramics fabricated under different conditions is conducted. The fine-grained TAG ceramics possess a bending strength of 226.3 ± 16.4 MPa, which is 9.7% higher than that of the large-grained ceramics. These results indicate that reducing the grain size on the premise of high optical quality helps improve the comprehensive performance of TAG ceramics.     

Determining the thermal conductivity of ceramic coatings by relative method

Thermal conductivity is a crucial parameter for evaluating the quality and thermal effects of ceramic coatings, especially for thermal barrier coatings. However, measurement by conventional method involves two problems: (a) it is difficult to peel off a ceramic coating from a substrate; (b) even if the coating can be peeled off, it is still hardly used as standard specimen in test. Therefore, the relative method was proposed to evaluate the thermal conductivity of ceramic coating. An analytical relationship among the thermal conductivities of the coating, the substrate, and the coating/substrate composite was established. Experiments on TA4 coated with YSZ coatings were carried out to demonstrate the feasibility of this novel method and to investigate the impact of temperature on the thermal conductivity of YSZ coatings. The experimental results demonstrated the validity and convenience of the relative method. With the increasing testing temperature, the thermal conductivity value of YSZ coatings displayed nonlinearity feature, that is, decreased from 1.4 to 1.3 (W m⁻¹ K⁻¹) in the temperature range of 32-300°C and then increased up to 1.58 W m⁻¹ K⁻¹ at 1000°C.     

High-entropy La(Fe0.2Co0.2Ni0.2Cr0.2Mn0.2)O3 ceramic exhibiting high emissivity and low thermal conductivity

A novel, high-entropy, perovskite-structured, solid solution La(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)O₃ ceramic was successfully synthesized via high-temperature solid-state reaction. The crystal structure, microstructure, infrared emissivity, and thermophysical properties were investigated. The experimental results indicated that La(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)O₃ exhibited an infrared emissivity as high as .92 in the near-infrared region of .76–2.50 μm. The thermal conductivity was 1.38–1.72 W m⁻¹ K⁻¹ in the temperature range of 25–1200°C.     

Synergistic modification of properties in Ba[(Zn0.8Mg0.2)1/3Nb2/3]O3 ceramics through ordered domain engineering

The ordered domain engineering was investigated for Ba[(Zn_0.8Mg_0.2)_1/3Nb_2/3]O₃ microwave dielectric ceramics to synergistically modify the physical properties especially the temperature coefficient of resonant frequency τ_f and quality factor Q value together with the thermal conductivity. The ordered domain structure could be tailored and controlled by the post-densification annealing, and the fine ordered domain structures with high ordering degree and low-energy domain boundary were obtained in the present ceramics annealed around 1400°C for 24 h, where the Qf value was improved from 51 000 to 118 000 GHz, τ_f was suppressed from 30 to 25.5 ppm/°C. Moreover, the thermal conductivity at room temperature was increased from 3.79 to 4.30 W m⁻¹ K⁻¹, and the Young's modulus was improved from 98 to 214 GPa. The present work provided a promising approach for synergistic modification of physical properties in Ba-based complex perovskite microwave dielectric ceramics.     

Potential thermal barrier coating materials: RE3NbO7 (RE=La, Nd,Sm, Eu,Gd, Dy) ceramics

In this work, RE₃NbO₇ ceramics are synthesized via solid‐state reaction and the phase structure is characterized by X‐ray diffraction and Raman spectroscopy. The relationship between crystal structure and thermophysical properties is determined. Except Sm₃NbO₇, each RE₃NbO₇ exhibits excellent high‐temperature phase stability. The thermal expansion coefficients increase with the decreasing RE³⁺ ionic radius, which depends on the decreasing crystal lattice energy and the maximum value reaches 11.0 × 10^?6 K^?1 at 1200°C. The minimum thermal conductivity of RE₃NbO₇ reaches 1.0 W m^?1 K^?1 and the glass‐like thermal conductivity of Dy₃NbO₇ is dominant by the high concentration of oxygen vacancy and the local structural order. The outstanding thermophysical properties pronounce that RE₃NbO₇ ceramics are potential thermal barrier coating materials.     

Enhanced thermal stability of the lepidocrocite-type titanates by intercalation of large alkaline ions

Layered materials are promising candidates to serve for thermal insulation coatings because of their low out-of-plane thermal conductivity. The thin film preparation of the layered materials in layer-by-layer fashion can be realized by several solution processes. However, the layered materials suffer low thermal stability after the solution processes. Herein, we report a strategy to improve the thermal stability of the layered materials processed in solution. In this work, we studied the thermal stability of the lepidocrocite-type titanate with various interlayer ions (Li⁺, Na⁺, K⁺, and Cs⁺) at elevated temperatures. We proved that the thermal stability of the titanate increased with the increase of the ionic radius. The Cs⁺ intercalated titanate can remain in its layered structure up to 1000°C, while the Li⁺ interacted titanate loses its structural stability at ~280°C. Our work suggests that increasing the size of the interlayer ions is an effective strategy to enhance the structural stability of layered titanates.     

Balancing thermal conductivity and strength of hot-pressed AlN ceramics via pre-sintering and annealing process

The heat dissipation requirements of the new generation of semiconductors are placing higher demands on the overall performance of aluminum nitride (AlN) ceramics. This has led to an increasing emphasis on AlN ceramics that combine thermal conductivity and strength. AlN ceramics are often strengthened by hot-press sintering, but their thermal conductivity is typically modest. In this study, pre-sintering and annealing processes are introduced to optimize the thermal conductivity of hot-pressed AlN ceramics to avoid the detrimental effects of oxygen impurities in the AlN lattice. The effect of the oxide layer on the surface of commercial AlN particles is investigated, including density, phase composition, microstructure, and fracture behavior. The effect of annealing on the improvement of thermal conductivity and flexural strength is also verified. Electron paramagnetic resonance (EPR) analysis is performed to examine the concentration of intercrystalline defects under various conditions. Finally, AlN ceramics with a thermal conductivity of 204 W m⁻¹ K⁻¹ and a flexural strength of 376 MPa are obtained.     

Screening rare-earth aluminates as promising thermal barrier coatings by high-throughput first-principles calculations

Thermal barrier coatings (TBCs) play an important role in gas turbines to protect the turbine blades from the high-temperature airflow damage. In this work, we use first-principles calculations to investigate a specific class of rare-earth (RE) aluminates, including cubic-REAlO₃ (c-REAlO₃), orthorhombic-REAlO₃ (o-REAlO₃), RE₃Al₅O₁₂, and RE₄Al₂O₉, to predict their structural stability, bonding characteristics, and mechanical and thermal properties. The polyhedron structures formed by the Al–O bonds are stronger and exhibit rigid characteristics, whereas the polyhedra formed by the RE–O bonds are relatively weak and soft. The alternating stacking of AlO₄ tetrahedra, AlO₆ octahedra, and RE–O polyhedra, as well as the selection of RE elements, shows intensive influences on the expected mechanical and thermal properties. The B, G, and E of these four types of aluminates decrease in the order of c-REAlO₃ > o-REAlO₃ > RE₃Al₅O₁₂ > RE₄Al₂O₉. REAlO₃ and RE₄Al₂O₉ are brittle and quasi-ductile ceramics, respectively, whereas RE₃Al₅O₁₂ is tailorable. The minimum thermal conductivity is in the range of 1.4–1.5 W m⁻¹ K⁻¹ for c-REAlO₃, 1.3–1.4 W m⁻¹ K⁻¹ for o-REAlO₃, 1.25–1.35 W m⁻¹ K⁻¹ for RE₃Al₅O₁₂, and 0.8–0.9 W m⁻¹ K⁻¹ for RE₄Al₂O₉. RE₄Al₂O₉ with low thermal conductivity and damage tolerance is predicted to be the potential candidates for next-generation TBC materials.     

New class of high-entropy rare-earth niobates with high thermal expansion and oxygen insulation

Tailoring the structure and properties of materials using the high-entropy (HE) effect is of significant interest in the fields of environmental and thermal barrier coatings (TBCs). In this work, a new class of dense HE rare-earth niobates was successfully prepared by a solid-phase reaction method, including (Sm_1/5Dy_1/5Ho_1/5Er_1/5Yb_1/5)NbO₄ (5HERN), (Sm_1/6Dy_1/6Ho_1/6Er_1/6Yb_1/6Lu_1/6)NbO₄ (6HERN), (Sm_1/7Dy_1/7Ho_1/7Er_1/7Yb_1/7Lu_1/7Gd_1/7)NbO₄ (7HERN), and (Sm_1/8Dy_1/8Ho_1/8Er_1/8Yb_1/8Lu_1/8Gd_1/8Tm_1/8)NbO₄ (8HERN), along with eight single rare-earth niobates (RENbO₄, RE = Sm, Dy, Ho, Er, Yb, Lu, Gd, and Tm). X-ray diffraction analysis showed that 5–8HERN are single-phase solid solutions with a monoclinic structure (space group C12/c1). The thermal expansion coefficients of 7HERN and 8HERN exceed 11 × 10⁻⁶ K⁻¹ at 1200°C and are much higher than those of the RENbO₄ compositions (10.13–10.74 × 10⁻⁶ K⁻¹) and other some HE rare-earth oxides (10.27–10.87 × 10⁻⁶ K⁻¹). Importantly, 5–8HERN have lower oxygen-ion conductivity and higher activation energy than yttrium-stabilized zirconia (YSZ) and the RENbO₄ compositions. The oxygen-ion conductivity of 5HERN (7.52 × 10⁻⁷ S cm⁻¹, 900°C) was 10⁵ times lower than that of YSZ (0.01 S cm⁻¹, 750°C). The hardness of 5–8HERN is ∼7.81–8.46 GPa and these compositions have low intrinsic lattice thermal conductivity at high temperature (1.28–1.69 W m⁻¹ K⁻¹ at 900°C). The mechanism by which the HE effect improved the material properties was elucidated. Young's modulus, hardness, thermal expansion coefficient, and intrinsic lattice thermal conductivity are linearly related to the mass, size, and distortion degree of samples. In contrast, the oxygen-ion conductivity depends on both the degrees of disorder and distortion and the oxygen-ion vacancy concentration. Based on their overall performance, especially their high thermal expansion coefficients and excellent oxygen-barrier performance, HE rare-earth niobates show potential for further development as TBC materials.     

Fabrication of Si3N4 ceramics substrates with high thermal conductivity by tape casting and gas pressure sintering

In this paper, high thermal conductivity Si₃N₄ ceramics were successfully fabricated through exploring and optimizing the tape casting process. The impact of various organic additives on the rheological characteristics of Si₃N₄ slurry was explored, and the pore size distribution and microstructure of the green tapes at different solid loadings were investigated, as well as the microstructure of Si₃N₄ ceramics. Green tapes with a narrow pore size distribution, a small average pore size, and a high density of 1.88 g cm⁻³ were prepared by the investigation and optimization of the Si₃N₄ slurry formulation. After gas pressure sintering, Si₃N₄ ceramics with a density of 3.23 g cm⁻³, dimensions of 78 mm × 78 mm, and a thickness of 0.55 mm were obtained. The microstructure of the Si₃N₄ ceramics showed a bimodal distribution and a low content of glassy phases. The thermal conductivity of the Si₃N₄ ceramics was 100.5 W m⁻¹ K⁻¹, the flexural strength was 735 ± 24 MPa, and the fracture toughness was 7.17 MPa m^1/2.     

Evolution mechanism of the microstructure and mechanical properties of plasma-sprayed yttria-stabilized hafnia thermal barrier coating at 1400 °C

Yttria stabilized hafnia (Hf_0.84Y_0.16O_1.92, YSH16) coatings were sprayed by atmospheric plasma spraying (APS). The effects of thermal aging at 1400 °C on the microstructures, mechanical properties and thermal conductivity of the coatings were studied. The results show that the as-sprayed coating was composed of the cubic phase, and the nano-sized monoclinic (M) phase was precipitated in the annealed coating. The presence of M phase effectively constrained the sintering of the coating due to its superior sintering-resistance. The Young's modulus kept at a nearly same level of ~78 GPa even after annealing, and the coating annealed for 6 h yielded a maximum value of hardness but revealed a declining tendency in the Vicker's hardness with prolonged sintering time. The thermal conductivity increased from 0.8-0.95 W m^-1 K^-1 at as-sprayed state to 1.6 W m^-1 K^-1 after annealing at 1400 °C for 96 h. The dual-phase coating is promising to serve at temperatures above 1400 °C due to its excellent thermal stability and mechanical properties.