Low-temperature Pr3Si2C2-assisted liquid-phase sintering of SiC with improved thermal conductivity

A novel Pr₃Si₂C₂ additive was uniformly coated on SiC particles using a molten-salt method to fabricate a high-density SiC ceramics via liquid-phase spark plasma sintering at a relatively low temperature (1400°C). According to the calculated Pr–Si–C-phase diagram, the liquid phase was formed at ∼1217°C, which effectively improved the sintering rate of SiC by the solution–reprecipitation process. When the sintering temperature increased from 1400 to 1600°C, the thermal conductivity of SiC increased from 84 to 126 W/(m K), as a consequence of the grain growth. However, an increasing amount of the sintering additive increased the interfacial thermal resistance, resulting in a decrease of thermal conductivity of the materials. The highest thermal conductivity of 141 W/(m K) was obtained for the material having the largest SiC grains and an optimized amount of the additive at the grain boundaries and triple junctions. The proposed Pr₃Si₂C₂-assisted liquid-phase sintering of SiC can be potentially used for the fabrication of SiC-based ceramic composites, where a low sintering temperature would inhibit the grain growth of SiC fibers.     

Almost seamless joining of SiC using an in-situ reaction transition phase of Y3Si2C2

A new design of seamless joining was proposed to join SiC using electric field-assisted sintering technology. A 500 nm Y coating on SiC was used as the initial joining filler to obtain a desired transition phase of Y₃Si₂C₂ layer via the appropriate interface reactions with the SiC matrix. The phase transformation and decomposition of the transition phase of Y₃Si₂C₂ was designed to achieve almost seamless joining of SiC. The decomposition of the joining layer to SiC, followed up by the inter-diffusion and complete densification with the initial SiC matrix, resulted in the formation of an almost seamless joint at the temperature of 1900 °C. The bending strength of the seamless joint was 134.8 ± 2.1 MPa, which was comparable to the strength of the SiC matrix. The proposed design of seamless joining could potentially be applied for joining of SiC-based ceramic matrix composites with RE₃Si₂C₂ as the joining layer.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Effect of a new nonoxide additive,Y3Si2C2, on the thermal conductivity and mechanical properties of Si3N4 ceramics

Enhancement of the thermal conductivity of silicon nitride is usually achieved by sacrificing its mechanical properties (bending strength). In this study, β-Si₃N₄ ceramics were prepared using self-synthesized Y₃Si₂C₂ and MgO as sintering additives. It was found that the thermal conductivity of the Si₃N₄ ceramics was remarkably improved without sacrificing their mechanical properties. The microstructure and properties of the Si₃N₄ ceramics were analyzed and compared with those of the Y₂O₃-MgO additives. The addition of Y₃Si₂C₂ eliminated the inherent SiO₂ and introduced nitrogen to increase the N/O ratio of the grain-boundary phase, inducing Si₃N₄ grain growth, increasing Si₃N₄ grain contiguity, and reducing lattice oxygen content in Si₃N₄. Therefore, by replacing Y₂O₃ with Y₃Si₂C₂, the thermal conductivity of the Si₃N₄ ceramics was significantly increased by 31.5% from 85 to 111.8Wm⁻¹K⁻¹, but the bending strength only slightly decreased from 704 ± 63MPa to 669 ± 33MPa.     

Fabrication of high thermal conductivity silicon nitride ceramics by pressureless sintering with MgO and Y2O3 as sintering additives

The fabrication of silicon nitride (Si₃N₄) ceramics with a high thermal conductivity was investigated by pressureless sintering at 1800 °C for 4 h in a nitrogen atmosphere with MgO and Y₂O₃ as sintering additives. The phase compositions, relative densities, microstructures, and thermal conductivities of the obtained Si₃N₄ ceramics were investigated systemically. It was found that at the optimal MgO/Y₂O₃ ratio of 3/6, the relative density and thermal conductivity of the obtained Si₃N₄ ceramic doped with 9 wt% sintering aids reached 98.2% and 71.51 W/(m·K), respectively. EDS element mapping showed the distributions of yttrium, magnesium and oxygen elements. The Si₃N₄ ceramics containing rod-like grains and grain boundaries were fabricated by focused ion beam technique. TEM observations revealed that magnesium existed as an amorphous phase and that yttrium produced a new secondary phase.     

The effect of rare earth oxides on the pressureless liquid phase sintering of α-SiC

Silicon carbide (SiC) ceramics have been fabricated by pressureless liquid phase sintering with Al₂O₃ and rare-earth oxides (Lu₂O₃, Er₂O₃ and CeO₂) as sintering additives. The effect was investigated of the different types of rare earth oxides on the mechanical property, thermal conductivity and microstructure of pressureless liquid phase sintered SiC ceramics. The room temperature mechanical properties of the ceramics were affected by the type of rare earth oxides. The high temperature performances of the ceramics were influenced by the triple junction grain boundary phases. With well crystallized triple junction grain boundary phase, the SiC ceramic with Al₂O₃–Lu₂O₃ as sintering additive showed good high temperature (1300 °C) performance. With clean SiC grain boundary, the SiC ceramic with Al₂O₃–CeO₂ as sintering additive showed good room temperature thermal conductivity. By using appropriate rare earth oxide, targeted tailoring of the demanding properties of pressureless liquid phase sintered SiC ceramics can be achieved.     

Thermal conductivity of spark plasma sintered SiC ceramics with Alumina and Yttria

In this study, dense SiC ceramics were fabricated at 1650?1750 °C for 10?60 min by spark plasma sintering (SPS) using 3?10 wt.% Al₂O₃-Y₂O₃ as sintering additives. Effects of sintering temperature, sintering additive content and holding time on microstructure as well as correlations between microstructure and thermal conductivity were investigated. An increase in the sintering temperature promotes grain growth. Extending holding time has little influence on grain size but results in formation of continuous network of sintering additive, which increases interfacial thermal resistance and thus decreases thermal conductivity. For SiC ceramics composed of continuous SiC matrix and discrete secondary phase (yttrium aluminum garnet, YAG), an increase in the sintering additive content results in smaller grain size and lower thermal conductivity. The lower thermal conductivity of the SiC ceramic with higher sintering additive content is mainly due to the smaller grain size rather than the low intrinsic thermal conductivity of YAG.     

Ultrafast low-temperature near-seamless joining of Cf/SiC using a sacrificial Pr3Si2C2 filler via electric current field-assisted sintering technique

A novel layered structure material, Pr₃Si₂C₂, was synthesized at a low temperature of 850 °C using a molten salt approach for the first time, and subsequently used as the joining filler for carbon fibers reinforced SiC composites (C_f/SiC). A robust near-seamless C_f/SiC joint was successfully obtained at 1509 °C (T_i) for 30 s, while an ultrafast heating rate of 6000 °C/min was applied via electric field-assisted sintering technology. The near-seamless joining process was attributed to the newly precipitated SiC grains, which were densified well with the C_f/SiC matrix by liquid-assisted sintering. The liquid phase was in-situ formed by the eutectic reaction between Pr₃Si₂C₂ and SiC. The shear strength of the near-seamless joint obtained at 1509 °C for 30 s was 17.6 ± 3.0 MPa. The failure occurred in the C_f/SiC matrix. The formation of near-seamless C_f/SiC joints dismisses the issues related to thermal mismatch between C_f/SiC matrices and traditional joining fillers.     

Seamless joining of silicon carbide ceramics through an sacrificial interlayer of Dy3Si2C2

A ternary carbide Dy₃Si₂C₂ coating was fabricated on the surface of SiC through a molten salt technique. Using the Dy₃Si₂C₂ coating as the joining interlayer, seamless joining of SiC ceramic was achieved at temperature as low as 1500 °C. Phase diagram calculation indicates that seamless joining was achieved by the formation of liquid phase at the interface between Dy₃Si₂C₂ and SiC, which was squeezed out under pressure and continuously consumed by the joining interlayer. This work implies the great potential of the family of ternary rare-earth metal carbide Re₃Si₂C₂ (Re = Y, La-Nd) as the sacrificial interlayer for high-quality SiC joining.     

Porous Si3N4/SiC Ceramics Prepared via Nitridation of Si Powder with SiC Addition

Porous Si₃N₄/SiC ceramics with high porosity were prepared via nitridation of Si powder, using SiC as the second phase and Y₂O₃ as sintering additive. With increasing SiC addition, porous Si₃N₄/SiC ceramics showed high porosity, low flexural strength, and decreased grain size. However, the sample with 20wt% SiC addition showed highest flexural strength and lowest porosity. Porous Si₃N₄/SiC ceramics with a porosity of 36–45% and a flexural strength of 107‐46MPa were obtained. The linear shrinkage of all porous Si₃N₄/SiC ceramics is below 0.42%. This study reveals that the nitridation route is a promising way to prepare porous Si₃N₄/SiC ceramics with favorable flexural strength, high porosity, and low linear shrinkage.     

Effect of MgF2 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Dense silicon nitride (Si₃N₄) ceramics were prepared using Y₂O₃ and MgF₂ as sintering aids by spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering annealing at 1900 °C for 4 h. Effects of MgF₂ contents on densification, phase transformation, microstructure, mechanical properties, and thermal conductivity of the Si₃N₄ ceramics before and after heat treatment were investigated. Results indicated that the initial temperature of liquid phase was effectively decreased, whereas phase transformation was improved as increasing the content of MgF₂. For optimized mechanical properties and thermal conductivity of Si₃N₄, optimum value for MgF₂ content existed. Sample with 3 mol.% Y₂O₃ and 2 mol.% MgF₂ obtained optimum flexural strength, fracture toughness and thermal conductivity (857 MPa, 7.4 MPa m^1/2 and 76 W m⁻¹ K⁻¹, respectively). It was observed that excessive MgF₂ reduced the performance of the ceramic, which was caused by the presence of excessive volatiles.     

Thermal conductivity and mechanical properties of Si3N4 ceramics with binary fluoride sintering additives

Silicon nitride (Si₃N₄) ceramics were fabricated by gas pressure sintering (GPS) using four sintering additives: Y₂O₃–MgO, Y₂O₃–MgF₂, YF₃–MgO, and YF₃–MgF₂. The phase composition, grain growth kinetics, mechanical properties, and thermal conductivities of the Si₃N₄ ceramics were compared. The results indicated that the reduction of YF₃ on SiO₂, induced a high Y₂O₃/SiO₂ secondary phase ratio, which improved the thermal conductivity of the Si₃N₄ ceramics. The depolymerization of F atom reduces the diffusion energy barrier of solute atom and weakens the viscous resistance of anion group, which was beneficial to grain boundary migration. Besides exhibiting a lower grain growth exponent(n = 2.5)and growth activation energy (Q = 587.94 ± 15.35 kJ/mol), samples doped with binary fluorides showed excellent properties, including appreciable thermal conductivity (69 W m⁻¹ K⁻¹), hardness (14.63 ± 0.12 GPa), and fracture toughness (8.75 ± 0.18 MPa m^1/2), as well as desirable bending strength (751 ± 14 MPa).     

Low temperature seamless joining of SiC using a Ytterbium film

Monolithic SiC, for the first time, was seamless joined at a low temperature of 1200 °C using electric field-assisted sintering technology. A 300 nm Yb coating on SiC was used as the joining filler to form Yb₃Si₂C₂ via an in-situ reaction with the SiC. A liquid phase was formed by an eutectic reaction between Yb₃Si₂C₂ and SiC. Almost completely seamless joints were formed by the precipitated SiC grains, which were fully consolidated with the SiC matrix with the help of in-situ formed liquid phase, followed by its elimination under the uniaxial pressure. The bending strength of the seamless joint joined at 1500 °C for 15 min was as high as 257.2 ± 31.1 MPa, which was comparable to the strength of the SiC matrix. As a result, the failure occurred in the matrix indicated a sound joint was obtained. The proposed low temperature seamless joining could potentially be used for joining of SiC-based composite.     

Mechanical and Thermal Properties of Pressureless Sintered Silicon Carbide Ceramics with Alumina–Yttria–Calcia

The effect of sintering temperature on the mechanical and thermal properties of SiC ceramics sintered with Al₂O₃–Y₂O₃–CaO without applied pressure was investigated. SiC ceramics containing A₂O₃–Y₂O₃–CaO as sintering additives can be sintered to >97% theoretical density at temperatures between 1750°C and 1900°C without applied pressure. A toughened microstructure, consisting of relatively large elongated grains and relatively small equiaxed grains, has been obtained when sintered at temperatures as low as 1800°C for 2 h in an argon atmosphere without applied pressure. The achievement of toughened microstructures under such mild conditions is the result of the additive composition. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature because of the decrease in the lattice oxygen content of the SiC grains. Typical sintered density, flexural strength, fracture toughness, hardness, and thermal conductivity of the 1850°C‐sintered SiC, which consisted of 62.2% 4H, 35.7% 6H, and 2.1% 3C, were 99.0%, 628 MPa, 5.3 MPa·m^1/2, 29.1 GPa, and 80 W·(m·K)^?1, respectively.     

Microstructure and corrosion behavior of in-situ grown Y3Si2C2 coated SiC fibers exposed to air and wet-oxygen at 1400 ℃

Y₃Si₂C₂ ternary ceramics were in-situ grown on the third-generation Chinese commercial SiC fiber (KD-SA SiC fiber) surface via molten salt method. Microstructures and oxidation/corrosion behavior of in-situ grown Y₃Si₂C₂ coated SiC fibers exposed to air and wet-oxygen at 1400 ℃ were investigated. Results indicated that the layered Y₃Si₂C₂ slices with thickness of approximately 15 nm can be successfully in-situ grown on SiC fibers. The product on the fibers surface after oxidation/corrosion at 1400 ℃ for 1 h in both ambient air and wet-oxygen are Y₂Si₂O₇ and SiO₂. Moreover, microstructural characterization indicates that the immigration and expansion of gaseous bubbles induced by oxidation product, mainly CO, result in microstructural differences of SiC fiber specimens, and finally oxidation mechanism based on the microstructural difference were proposed.     

Enhanced thermal conductivity in Si3N4 ceramics by carbonizing polydopamine coatings

To enhance the thermal conductivity of Si₃N₄, a polydopamine (PDA) coating was creatively introduced into ceramics sintered with different additive contents through a two-step sintering process consisting of a first treatment at 1500 °C for 8 h followed by 12 h at 1900 °C under 1 MPa nitrogen pressure. After the first-step sintering, the PDA-coated sample exhibited a higher elimination effect of the liquid phase and an increase in the N/O ratio, which allowed the additives to directly interact with the Si₃N₄ grains, resulting in a microstructure with more and larger rod-shaped grains. After the second-step sintering, the densified samples of the PDA-coating attained slightly coarser rod-shaped grains, lower O content, higher N/O ratio and peak intensity (XRD) of the Y₂Si₃O₃N₄ phase, thicker grain boundary film, and secondary phases mainly residing in multigrain junctions. Consequently, the thermal conductivity of all the PDA-coated samples typically showed a 10–12% increment in comparison to the PDA-free samples for each additive content.     

Direct bonding of silicon carbide ceramics sintered with yttria

SiC ceramics sintered with yttria were successfully joined without an interlayer by conventional hot pressing at lower temperatures (2000–2050 °C) compared to those of the sintering temperatures (2050–2200 °C). The joined SiC ceramics sintered with 2000 ppm Y₂O₃ showed almost the same thermal conductivity (˜198 Wm⁻¹ K⁻¹), fracture toughness (3.7 ± 0.2 MPa m^1/2), and hardness (23.4 ± 0.8 GPa) as those of the base material, as well as excellent flexural strength (449 MPa). In contrast, the joined SiC ceramics sintered with 4 wt% Y₂O₃ showed very high thermal conductivity (˜204 Wm⁻¹ K⁻¹) and excellent flexural strength (˜505 MPa). Approximately 16–22% decreases in strength compared to those of the base SC materials were observed in both joined ceramics, due to the segregation of liquid phase at the interface. This issue might be overcome by preparing well-polished and highly flat surfaces before joining.     

Fabrication of fine-grained undoped Y2O3 transparent ceramic using nitrate pyrogenation synthesized nanopowders

Y₂O₃ ceramic is a promising optical material for mid-infrared (IR) windows and domes. Improvements in the mechanical and thermal performance of this material have become urgent if it is to perform adequately under extreme conditions. Herein, Y₂O₃ nanopowders were produced through the nitrate pyrogenation method. The final Y₂O₃ transparent ceramics were fabricated with a hybrid sintering method combining low temperature presintering and a subsequent hot isostatic pressing (HIP) treatment. The synthesis of nanopowders and the fabrication of the final ceramic products were investigated in detail. The Y₂O₃ ceramic sample that was presintered at 1350?°C provided the optimum microstructure for HIP treatment and resulted in an average grain size of 0.5?µm. Owing to the reduced grain size, the flexure strength and Vickers hardness of the sample were improved to 180?MPa and 8.4?GPa, respectively. Furthermore, the achieved pure Y₂O₃ ceramic demonstrated an excellent thermal conductivity at high temperature.