Process-tolerant pressureless-sintered silicon carbide ceramics with alumina-yttria-calcia-strontia

Process-tolerant SiC ceramics were prepared by pressureless sintering at 1850–1950 °C for 2 h in an argon atmosphere with a new quaternary additive (Al₂O₃-Y₂O₃-CaO-SrO). The SiC ceramics can be sintered to a > 94% theoretical density at 1800–1950 °C by pressureless sintering. Toughened microstructures consisting of relatively large platelet grains and small equiaxed grains were obtained when SiC ceramics were sintered at 1850–1950 °C. The presently fabricated SiC ceramics showed little variability of the microstructure and mechanical properties with sintering within the temperature range of 1850–1950 °C, demonstrating process-tolerant behavior. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature from 1800 °C to 1900 °C due to decreases of the lattice oxygen content of the SiC grains and residual porosity. The flexural strength, fracture toughness, and thermal conductivity of the SiC ceramics sintered at 1850–1950 °C were in the ranges of 444–457 MPa, 4.9–5.0 MPa m^1/2, and 76–82 Wm^?1 K^?1, respectively.     

High-entropy titanate pyrochlore as newly low-thermal conductivity ceramics

Low-thermal conductivity ceramics play an indispensable role in maximizing the efficiency and durability of hot end components. Pyrochlore, particularly zirconate pyrochlore, is currently a highly promising and widely studied candidate for its extremely low thermal conductivity. However, there are still few pyrochlores that offer both stiffness, insulation, and good thermal expansion properties. In this work, the solidification method was innovatively introduced into the preparation of titanate pyrochlore, and combined it with the compositional design of high-entropy. Through careful composition design and solidification control, the high-density and uniform elements distributed high-entropy titanate pyrochlore ceramics were successfully prepared. These samples possess high hardness (15.88 GPa) and Young’s modulus (295.5 GPa), low thermal conductivity (0.947 W·m^?1·K^?1), excellent thermal expansion coefficient (11.6 ×10^?6/K) and an exquisite balance between stiffness and insulation (E/κ, 312.1 GPa·W^?1·m·K), in which the E/κ exhibits the highest value among the current reported works.     

Thermal conductivity and microstructure of Ti-doped graphite

Ti doped and Si-Ti doped graphites have been developed. The influence of the dopants on the properties and microstructure of doped graphites was analyzed. Test results reveal that Ti doped graphite has excellent bending strength and high thermal conductivity, with highest values reaching 50.2 MPa and 424 W m⁻¹ K⁻¹ for a Ti concentration of 15 wt% in the raw materials. Si added simultaneously with Ti promotes the growth of graphite crystals resulting in an increased thermal conductivity. A kind of Si-Ti doped graphite has been developed with thermal conductivity as high as 494 W m⁻¹ K⁻¹ by optimizing the compositions. Correlation between the content of dopant and the properties and microstructure of doped graphites was studied, and catalytic graphitization mechanism of dopants is also discussed.     

Effect of grain growth on the thermal conductivity of liquid-phase sintered silicon carbide ceramics

The effect of grain growth on the thermal conductivity of SiC ceramics sintered with 3 vol% equimolar Gd₂O₃-Y₂O₃ was investigated. During prolonged sintering at 2000 °C in an argon or nitrogen atmosphere, the β → α phase transformation, grain growth, and reduction in lattice oxygen content occurs in the ceramics. The effects of these parameters on the thermal conductivity of liquid-phase sintered SiC ceramics were investigated. The results suggest that (1) grain growth achieved by prolonged sintering at 2000 °C accompanies the decrease of lattice oxygen content and the occurrence of the β → α phase transformation; (2) the reduction of lattice oxygen content plays the most important role in enhancing the thermal conductivity; and (3) the thermal conductivity of the SiC ceramic was insensitive to the occurrence of the β → α phase transformation. The highest thermal conductivity obtained was 225 W(m K)⁻¹ after 12 h sintering at 2000 °C under an applied pressure of 40 MPa in argon.     

Lightweight porous silica ceramics with ultra-low thermal conductivity and enhanced compressive strength

In recent years, the need for robust thermal protection for reusable spacecraft and vehicles has spurred strong demand for high-performance lightweight thermal insulation materials that exhibit high strength. Herein, we report silica porous ceramics prepared via the direct foaming technique with lightweight, ultra-low thermal conductivity and enhanced compressive strength. Silica particles (particle size: 500 nm and 2 μm) were used as the raw materials. The nano-sized silica particles were easily sintered, thereby improving the compressive strength of the ceramics, whereas the micro-sized silica particles maintained the pore structure integrity without deformation. The addition of nano-silica enhanced the compressive strength by 764% (from 0.039 to 0.337 MPa). In addition, the thermal conductivity of the ceramics was as low as 0.039 W m^?1 K^?1. Owing to these outstanding characteristics, these porous silica ceramics are expected to be employed as thermal insulation material in diverse fields, especially aerospace and space where weight is an important constraint.     

Improved thermal conductivity of graphite though infiltration with SiC and Si3N4 inclusions

High thermal conductivity, wear resistant graphite is desirable for a range of applications, for example as an electrode to minimize energy consumption in aluminium smelting. This paper demonstrates that by infiltrating the porosity by reactive infiltration with SiC at levels ranging from 4 to 12 vol.% or Si₃N₄ ranging from 7 to 19 vol.% the thermal conductivity can be raised progressively from ～41 W/m K to ～53 and 64 W/m K respectively. A simple analytical model predicts thermal conductivity rises with increasing reinforcement fraction only 7–9 % higher than that observed. This suggests that heat transfer between the graphite and the reinforcing particles is good making these materials good options where improved thermal conductivity and wear resistance over graphite is required.     

Modified graphite filled natural rubber composites with good thermal conductivity

The rubber composites with good thermal conductivity contribute to heat dissipation of tires. Graphite filled natural rubber composites were developed in this study to provide good thermal conductivity. Graphite was coated with polyacrylate polymerized by monomers including methyl methacrylate, n-butyl acrylate and acrylic acid. The ratios between a filler and acrylate polymerization emulsion and those between monomers were varied. Eight types of surface modification formulas were experimentally investigated. Modification formula can affect coating results and composite properties greatly. The best coating type was achieved by a ratio of 1:1 between methyl methacrylate and n-butyl acrylate. The coating of graphite was thermal y stable in a running tire. Filled with modified graphite, the tire thermal conductivity reached up to 0.517–0.569 W·m-1·K-1. In addition, the mechanical performance was improved with increased crosslink density, extended scorch time and short vulcanization time.     

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.     

High thermal conductivity of spark plasma sintered silicon carbide ceramics with yttria and scandia

A fully dense SiC ceramic with a room‐temperature thermal conductivity of 262 W·(m·K)^?1 was obtained via spark plasma sintering β‐SiC powder containing 0.79 vol% Y₂O₃‐Sc₂O₃. High‐resolution transmission electron microscopy revealed two different SiC‐SiC boundaries, that is, amorphous and clean boundaries, in addition to a fully crystallized junction phase. A high thermal conductivity was attributed to a low lattice oxygen content and the presence of clean SiC‐SiC boundaries.     

Gradient microstructure development and grain growth inhibition in high-entropy carbide ceramics prepared by reactive spark plasma sintering

High-entropy carbide ceramics (Ti_0.2Hf_0.2Nb_0.2Ta_0.2W_0.2)C is prepared from five transition metal oxides and graphite by reactive spark plasma sintering. X-ray diffraction indicates the synthesized ceramics with the single-phase face-centered cubic structure. The elemental distribution maps by energy dispersive spectroscopy demonstrate homogeneous distribution of the five metal elements in both central and circumferential regions of the sample. SEM and corresponding back scattered electron observations show the residual graphite particles locating at the grain boundaries of high-entropy carbide ceramics. Moreover, the content of the residual graphite decreases and the grain size of the high-entropy carbide phase increases from central to circumferential region of the sample. Thermodynamic calculation results indicate that gradient gas pressure inside the sample affects the carbothermal reduction reactions during sintering and consequently results in the existence of residual graphite with gradient distribution feature. This study points out an effective way to inhibit the grain growth of high-entropy carbide phase during sintering process by the incorporation of graphite as the second phase particles acting as grain growth inhibitor.     

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.     

Thermal conductivity of liquid-phase sintered silicon carbide ceramics: A review

Silicon carbide (SiC) exhibits excellent thermal conductivity. Recently, thermal conductivity that amounts to 261.5 W/m-K has been obtained in polycrystalline SiC ceramic liquid-phase sintered (LPS) with Y₂O₃-Sc₂O₃ additives at 2050 °C under a nitrogen atmosphere. From the additive used to the sintering atmosphere selected, many factors affect the thermal conductivity of the SiC. In this review, important factors that are known to determine the thermal conductivity of LPS-SiC (lattice oxygen/nitrogen content, porosity, grain size, grain boundary structure, phase transformation, and additive composition) have been evaluated. While reviewing the impact of each factor on thermal conductivity, hidden correlations among different factors are also discussed. Among the factors that are claimed to be important, we suggest a few factors that are more critical to thermal conductivity than others. Based on the most critical factors on the thermal conductivity of LPS-SiC, a complete engineers’ guide for high thermal conductivity LPS-SiC is proposed.     

A novel aerogels/porous Si3N4 ceramics composite with high strength and improved thermal insulation property

A novel ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite with high strength, low density, good dielectric properties and low thermal conductivity was synthesized by filling ZrO₂-SiO₂ aerogels into the porous Si₃N₄ ceramics through vacuum sol-impregnating. The effects of aerogels on the microstructure and properties of composite were discussed. The results show that aerogels could form a mesoporous structure and significantly decrease the thermal conductivity from 9.8 to 7.3 W m^?1 K^?1. Meanwhile, the density, mechanical and dielectric properties of the porous Si₃N₄ ceramics could not be affected after introducing ZrO₂-SiO₂ aerogels. The composite exhibits high porosity (62.6%), high flexural strength (53.86 MPa) and low dielectric constant (2.86). The ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite shows great potential as radome materials applied in the fields of aerospace.     

Effects of (Ta0.2W0.2Nb0.2Mo0.2V0.2)C high-entropy carbide content on the microstructure and properties of Ti(C0.7N0.3)-based cermet

Using pre-synthesized high-entropy (Ta_0.2W_0.2Nb_0.2Mo_0.2V_0.2)C carbide as the reinforcing phase, Ti(C_0.7N_0.3)-based cermets were prepared by pressureless sintering at 1600 °C. The results revealed that due to the solid solution reaction between the mono-carbide and (Ta_0.2W_0.2Nb_0.2Mo_0.2V_0.2)C, only one set of face-centered-cubic diffraction peaks in XRD was detected in the as-sintered cermets, alongside the typical core-rim structure. Compared to the Ti(C_0.7N_0.3)-based cermets without high-entropy reinforcing phase, the Vickers hardness was increased from 17.06 ± 0.09 GPa to 18.42 ± 0.33 GPa and the fracture toughness was increased from 9.21 ± 0.31 MPa m^1/2 to 12.56 ± 0.23 MPa m^1/2 by adding 10 wt% (Ta_0.2W_0.2Nb_0.2Mo_0.2V_0.2)C. The wear resistance of the cermet was enhanced significantly with increasing (Ta_0.2W_0.2Nb_0.2Mo_0.2V_0.2)C content. This work provided a potential that the high-entropy carbide can be applied as an effective reinforcing phase in the preparation of high-performance Ti(C_0.7N_0.3)-based cermets.     

Isocyanate-crosslinked silica aerogel monolith with low thermal conductivity and much enhanced mechanical properties: Fabrication and analysis of forming mechanisms

The applications of silica aerogels are restricted due to their intrinsic fragile property. Polymerization of di-isocyanates can be templated onto the mesoporous surface of the –NH₂ group modified silica clusters, resulting in the conformal crosslinked coating on surface of silica clusters. Aminopropyltriethoxysilane (APTES), as the silica co-precursor and amine group modification agent, is involved containing tetramethyl orthosilicate (TMOS) silica precursor, while hexamethylene diisocyanate (HDI) is incorporated as the polymer crosslinking agent. The effects of different amounts of APTES on the physicochemical properties of the resulting crosslinked aerogels are investigated. The results show that the optimized APTES/TMOS volume ratio can be determined at 0.5:1. The resulting optimal crosslinked silica aerogel possesses large BET specific surface area of 150.9 m²/g, low thermal conductivity of 0.037 W/(m·K), and the Young's modulus is as large as 18 MPa under strain of 4.2%, much higher than that in the previously published works. The polymerization reaction mechanism forming the polyurethane chains has also been proposed. In addition, the interactions between silica clusters and polymer chains are studied by molecular mechanics and molecular dynamics. The interactions are mainly dependent on non-bonding energy, and the electrostatic energy has decisive impact on the combination of silica clusters and polymer chains. The density field of C, H, N, O, and Si elements overlaps with each other, indicating that the polymer crosslinked silica aerogel maintains typical three-dimensional porous structure. The N element enriches in the region between silica clusters, further verifying the formation –CONH–(CH₂)₆–CONH- polyurethane chains, which is actually responsible for the much enhanced mechanical property.     

High-entropy yttrium pyrochlore ceramics with glass-like thermal conductivity for thermal barrier coating application

A₂B₂O₇-type oxides with low thermal conductivities are potential candidates for next-generation thermal barrier coatings. The formation of high-entropy ceramics is considered as a newly effective way to further lower their thermal conductivities. High-entropy Y₂(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)₂O₇ (5HEO) and Y₂(Ti_0.25Zr _0.25Hf_0.25Ta_0.25)₂O₇ (4HEO) ceramics were prepared by in situ solid reaction sintering, considering the important roles of B-site cations on thermal conductivities of the A₂B₂O₇-type oxides. Reaction process, phase structures, microstructures, and thermal conductivities of the as-sintered ceramics were investigated. Lattice distortion effects on their thermal conductivities were also discussed by using the proposed criterion based on the supercell volume difference of the individual compounds. Near fully-dense 5HEO and 4HEO ceramics were obtained after being sintered at 1600°C. The former one had a dual-phase structure containing high-entropy Y₂(Ti_0.227Zr_0.227Hf_0.227Nb_0.136Ta_0.182)₂O_7.318 pyrochlore oxide (5HEO-P) and Y(Nb, Ta)O₄ solid solution, while the latter one was a single-phase pyrochlore oxide (4HEO-P) with homogeneous element distribution. The formed 5HEO-P oxide has larger lattice distortion than 4HEO-P oxide due to the larger total amounts of Nb and Ta cations at B sites in the 5HEO-P oxide. It results in lower thermal conductivity of 5HEO ceramics (keeping at 1.8 W·m^–1·K^–1) than those of 4HEO ceramics (ranging from 1.8 to 2.5 W·m^–1·K^–1) at temperatures from 25°C to 1400°C. Their glass-like thermal conductivities were determined by the selection of B site cations and high-entropy effects. These results provide some useful information for the material design of novel thermal barrier coating materials.     

Effect of Sm2O3 on microstructure,thermal shock resistance and thermal conductivity of cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline

Cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline were fabricated via pressureless sintering at a low sintering temperature with added Sm₂O₃. The effects of Sm₂O₃ on sintering behaviors, mechanical property, phase transformation, microstructure, thermal shock resistance and thermal conductivity of the composite ceramics were investigated. TEM analysis results demonstrated that Sm³⁺ located in glass and grain boundaries to facilitate the densification via the liquid-phase sintering mechanism and improve bending strength by grain refinement, respectively. Proper addition (3 wt%) of Sm₂O₃ could promote the crystallization of cordierite, and improve thermal shock resistance of the composite ceramics with an increasing rate of 16.70% for bending strength after 30 thermal shock cycles (air cooling from 1100 °C to RT). The composite ceramics possessed a superior thermal shock resistance, where a large amount of particles were formed to suppress crack initiation and propagation during thermal shock. Cordierite-mullite-corundum composite ceramics with proper Sm₂O₃ addition (3 wt%) had a lower thermal conductivity than that of composite ceramics without Sm₂O₃ addition by strengthening the scattering of phonon, which could reduce the heat loss during solar heat transmission process.     

Enhanced thermal conductivity in Si3N4 ceramic by addition of a small amount of carbon

In order to fabricate Si₃N₄ ceramic with enhanced thermal conductivity, 93 mol%α-Si₃N₄-2 mol%Yb₂O₃-5 mol%MgO powder mixture was doped with 5 mol% carbon, and sintered firstly at 1500 °C for 8 h and subsequently at 1900 °C for 12 h under 1 MPa nitrogen pressure. During the first-step sintering, the carbothermal reduction process significantly reduced the oxygen content and increased the N/O ratio of intergranular secondary phase, resulting in the precipitation of Yb₂Si₄O₇N₂ crystalline phase, higher β-Si₃N₄ content and larger rod-like β-Si₃N₄ grains in the semi-finished Si₃N₄ sample. After the second-step sintering, the final dense Si₃N₄ product acquired coarser elongated grains, lower lattice oxygen content, tighter Si₃N₄-Si₃N₄ interfaces and more devitrified intergranular phase due to the further carbothermal reduction of oxynitride secondary phase. Consequently, the addition of carbon enabled Si₃N₄ ceramic to gain a significant increase of ∼25.5% in thermal conductivity from 102 to 128 W∙m⁻¹ K⁻¹.     

Microstructure evolution of Si3N4 ceramics with high thermal conductivity by using Y2O3 and MgSiN2 as sintering additives

Silicon nitride (Si₃N₄) ceramics were prepared by gas-pressure sintering using Y₂O₃–MgSiN₂ as a sintering additive. The densification behavior, phase transition, and microstructure evolution were investigated in detail, and the relevance between the microstructure and the performance (including thermal conductivity and mechanical properties) was further discussed. A significant change from a bimodal to a homogeneous microstructure and a decreased grain size occurred with increasing Y₂O₃–MgSiN₂ content. When the small quantity of preformed β-Si₃N₄ nuclei grew preferentially and rapidly in a short time, an obvious bimodal microstructure was obtained in the sample with 4 mol% and 6 mol% Y₂O₃–MgSiN₂. When more β-Si₃N₄ nuclei grew at a relatively rapid rate, the sample with 8 mol% Y₂O₃–MgSiN₂ showed a microstructure consisting of numerous abnormally grown β-Si₃N₄ grains and small grains. When more β-Si₃N₄ nuclei grew simultaneously and slowly, there was a homogeneous microstructure and smaller grains in the sample containing 10 mol% Y₂O₃–MgSiN₂. Benefitting from the completely dense, significant bimodal microstructure, low grain boundary phase, and excellent Si₃N₄–Si₃N₄ contiguity, the sample containing 6 mol% Y₂O₃–MgSiN₂ exhibited great comprehensive performance, with a maximum thermal conductivity and fracture toughness of 84.1 W/(m⋅K) and 8.97 MPa m^1/2, as well as a flexural strength of 880.2 MPa.     

Effect of carbon content on the microstructure and mechanical properties of high-entropy (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)Cx ceramics

High-entropy (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics, with different carbon contents (x=0.55?1), were prepared by spark plasma sintering using powders synthesized via a carbothermal reduction approach. Single-phase, high-entropy (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics could be obtained when using a carbon content of x=0.70?0.85. Combined ZrO₂ and Mo-rich carbide phases, or residual graphite, existed in the ceramics due to either a carbon deficiency or excess at x=0.55 and 1, respectively. With the carbon content increased from x=0.70 to x=0.85, the grain size decreased from 4.36 ± 1.55 μm to 2.00 ± 0.91 μm, while the hardness and toughness increased from 23.72 ± 0.26 GPa and 1.69 ± 0.21 MPa·m^1/2 to 25.45 ± 0.59 GPa and 2.37 ± 0.17 MPa·m^1/2, respectively. This study showed that the microstructure and mechanical properties of high-entropy carbide ceramics could be adjusted by the carbon content. High carbon content is conducive to improving hardness and toughness, as well as reducing grain size.