Preparation of dense and high-purity SiC ceramics by pressureless solid-state-sintering

High-purity SiC materials have been used in semiconductor processes due to their excellent properties. However, they are difficult to densify without sintering aids. In this work, dense and high-purity SiC ceramics have been obtained by pressureless solid-state-sintering with ultra-low contents of sintering additives. The amount of residual B, C and O in the high-purity SiC ceramics was less than 0.15 wt%, respectively, and the total content of other impurity elements (such as aluminum, magnesium, calcium, iron, etc.) was less than 0.015 wt%. Finally, the purity of the as-prepared SiC ceramics was more than 99.5 wt%.     

Permeability and corrosion resistance of porous SiOC-bonded SiC ceramics prepared by the preceramic polymer

Porous SiC ceramics have been used in high temperature flue gas filtration fields because of their excellent properties such as high strength, high temperature resistance, corrosion resistance, and long service time. This work reports the porous SiOC-bonded SiC ceramics prepared at low temperature. The properties of porous SiC ceramics were first investigated with silicone resin content from 10 to 25 wt%, and then the effects of different pore-forming agent contents on the behaviors of porous SiC ceramics were discussed by adjusting poly (methyl methacrylate) PMMA microbeads from 5 to 20 wt%. The prepared porous SiC ceramics showed apparent porosity from 17.3% to 57.7%, compressive strength from 6 to 216 MPa, and Darcy permeability k₁ ranging from 7.02 × 10⁻¹⁴ to 1.45 × 10⁻¹² m². The corrosion behavior of porous SiC ceramics was investigated in acidic and alkaline media. The porous SiC ceramics showed better corrosion resistance in acidic solutions.     

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.     

Preparation and properties of SiC/SiC-SiYC with excellent water-oxygen corrosion resistance

In this study, the SiC/SiC-SiYC composites were fabricated via chemical vapor infiltration (CVI) combined with the reactive melt infiltration (RMI) process. The excellent infiltration of Si-Y alloy assisted in fabricating composites with a density of 2.94 g/cm³ and a porosity of only 2.0%. After 20 h of corrosion at 1300 °C in the water-oxygen environment, the generated oxide layer, consisting of a glass layer and a diffusion layer, effectively protected the composites, and the flexural strength retention is 114.2%. This study highlights the significant potential of Si-Y alloy as a modification phase that is resistant to water and oxygen. It also presents a novel approach for developing high-density ceramic matrix composites that are resistant to water-oxygen corrosion.     

In situ formation of mullite strengthened SiC porous ceramics via gelcasting with high strength and good alkali resistance

Mullite-bonded porous SiC ceramics sintered in air by gelcasting are still challenges due to the high porosity induced severe oxidation of SiC, which results in the formation of large amount of detrimental cristobalite phase. Here in this work, small amounts of Y₂O₃ and CaF₂ were added in SiC and Al(OH)₃ raw materials as sintering additives for the in situ growth of mullite reinforcement. This additive system promoted the reaction between oxidation-derived SiO₂ from SiC and Al₂O₃ decomposed from Al(OH)₃ to mullite phase. Almost no cristobalite phase was detected when sintered at 1450℃/2 h with CaF₂ addition of more than 2.0 wt%. Mullite whisker reinforcement was in situ formed due to the gas reaction mechanism caused by CaF₂ addition. Thus obtained porous SiC ceramics exhibited a flexural strength of 67.6 MPa at porosity of 41.3%, which maintained exceeding 36 MPa after 8 h corrosion in 10 wt% NaOH 80℃ solution, being the best performance up to now. This high performance of porous SiC was attributed to the additive induces proper phase control and in situ formation of whisker-like mullite reinforcement.     

Effect of Ti combined with Si and C on mechanical performance and oxidation resistance of SiC castables for plasma gasifier

Gaseous products released during the oxidation of SiC at 1700?°C lead to serious degradation of SiC castables. Ti combined with Si and carbon black are added to improve the mechanical behavior and oxidation resistance of SiC castables in this study. The mechanical behavior, isothermal oxidation, microstructure, and thermodynamic analysis are used to evaluate the properties of SiC castables. The result shows that SiC castables with more Ti exhibit better degradation resistance at high temperature oxidation atmosphere. The preferential oxidation of metal Ti to TiO₂ reduces the oxidizing gases and increases the content of SiO (g) in the matrix, which is beneficial for the generation of SiC fibers; in turn, this reinforces the mechanical behavior. In addition, a certain amount of TiO₂ dissolves into SiO₂ glass following the decrease in viscosity. TiO₂ is not only more difficult to volatilize than SiO₂, but also can decrease the viscosity of SiO₂ glass to improve the mobility of the liquid, which is good for healing the pores on the surface and protecting the inner SiC from being oxidized; this improves the mechanical properties and oxidation resistance.     

Dielectric properties of alumino-silicate ceramics synthesized by plasma sintering

The polycrystalline ceramic specimens of three different alumino-silicate solid solutions (Al_0.70Si_0.30O, Al_0.73Si_0.27O and Al_0.75Si_0.25O) consisting of different alumina and silica concentrations have been synthesized by thermal plasma sintering technique. From structural analysis carried out by X-ray diffraction, the ceramics are mostly found to consist of two different phases of mullite and sillimanite. SEM images of these ceramics reveal a high dense and less porous microstructure with homogeneous distribution of grains throughout their surface. These materials exhibit high dielectric constant value (>10³) with low dissipation factor. The AC conductivity analysis reveals that Al_0.70Si_0.30O and Al_0.75Si_0.25O ceramics possess room temperature conductivity values of the order of 10^?5, whereas Al_0.73Si_0.27O has conductivity of 10^?7 order that increases with rise in temperature. From the Nyquist plots, the grain and grain boundary conductivities are distinguished and negative temperature coefficient of resistance behavior is identified in these ceramics with small positive temperature coefficient of resistance effect.     

Optimized ablation resistance behavior and mechanism of C/SiC composites with high thermal conductive channels

Aimed to enhance the high-temperature service performance of C/SiC composites in high-speed aircraft thermal protection system, in this article, pitch-based carbon fibers were used to construct high thermal conductive channels to improve the heat transfer capability of C/SiC composites. The results revealed that the as-prepared composites equipped with 4.7 times higher thermal conductivity than that of conventional C/SiC composites. The oxyacetylene flame ablation test confirmed that the constructed high thermal conductive channels, which quickly conducted the heat flow from the ablation center area to other areas is the main reason of as-prepared composites exhibiting a very impressive ablation resistance property. Briefly, the ablation temperature of the as-prepared composite surfaces considerably dropped by about 300°C compared with conventional C/SiC composites, while the linear ablation rate and mass ablation rate of the composites are 1.27 μm/s and 0.61 mg/s respectively, which is superior to many recent reports, demonstrating that this article provides a simple but highly effective measure to improve the ablation resistance property of C/SiC composites.     

高温除尘碳化硅膜的制备及其抗腐蚀特性

采用喷涂法在碳化硅（SiC）支撑体上覆膜,根据碳化硅材料的氧化特性,设计了有氧烧结和氩气烧结随温度转换的组合烧结制度,并通过优化保温时间降低碳化硅陶瓷膜烧结成本。研究结果表明,新的烧结制度能有效地控制有氧烧结阶段产生的二氧化硅（SiO₂）量,并促进其与烧结助剂氧化锆（ZrO₂）等在气氛烧结阶段的反应,反应生成的锆英石相和添加莫来石相共同形成SiC颗粒连接颈部。制备的碳化硅陶瓷膜平均孔径为3.03 μm,气体通量为175 m³?m^-2?h^-1?kPa^-1。且在100℃的0.25 mol?L^-1的H₂SO₄溶液和0.25 mol?L^-1的NaOH溶液中腐蚀6 h后,膜层表面形貌无明显变化,具有较强的抗腐蚀性能。     

Preparation of dense La0.67Ca0.33MnO3 ceramics by plasma activated sintering and hot-pressing

Highly dense La_0.67Ca_0.33MnO₃ (LCMO) ceramics were prepared by plasma activated sintering (PAS) and hot-pressing (HP). The comparison of structural, magnetic, and electrical transport properties of the LCMO ceramics after PAS and HP were investigated. XRD and SEM analyses confirmed that both samples exhibited orthorhombic phase with highly dense microstructure. The differences of magnetic and electrical transport properties in PAS and HP LCMO ceramics could be attributed to the different grain sizes, which have an effect on grain boundaries, domain states, Mn–O–Mn bond angles and Mn–O bond lengths. On analyzing the fitting data with several empirical equations, the conduction mechanism of the samples was found to be electron-magnon scattering in the ferromagnetic low-temperature region and variable range hopping (VRH) in the paramagnetic high-temperature region.     

Rapid densification of SiC ceramic rollers by microwave sintering

Ultralarge silicon carbide (SiC) ceramic rollers are successfully prepared in a microwave multimode cavity at 2·45 GHz. The length of the rollers is 1220 mm with outer diameter of 62 mm and thickness of 7 mm. The optimised sintering temperature of microwave sintering is ～980°C, which is ～400°C lower than that of conventional sintering. Mullite was detected within microwave sintered samples, rather than conventional sintered ones, which preferentially appeared around SiC as well as inside the residual pores. This might be attributed to the unique features of microwave sintering of ‘hot spot effect’, plasma behaviour and high frequency electromagnetic fields.     

Pressureless sintering of SiC ceramics with improved specific stiffness

The effects of B₄C content on the specific stiffness and mechanical and thermal properties of pressureless-sintered SiC ceramics were investigated. SiC ceramics containing 2.5 wt% C and 0.7–20 wt% B₄C as sintering aids could be sintered to ≥ 99.4% of the theoretical density at 2150 °C for 1 h in Ar. The specific stiffness of SiC ceramics increased from 136.1 × 10⁶ to 144.4 × 10⁶ m²‧s⁻² when the B₄C content was increased from 0.7 to 20 wt%. The flexural strength and fracture toughness of the SiC ceramics were maximal with the incorporation of 10 wt% B₄C (558 MPa and 3.69 MPa‧m^1/2, respectively), while the thermal conductivity decreased from ∼154 to ∼83 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 20 wt% B₄C were ∼346 MPa and ∼105 W‧m⁻¹‧K⁻¹, respectively.     

Preparation of ZrC/SiC composites by using polymer-derived ceramics and spark plasma sintering

ZrC/SiC composites were fabricated using Spark Plasma Sintering (SPS) combined with the Polymer-Derived Ceramics (PDC) route. The originality of this work, consisted in the direct ceramization/sintering of a ZrC/polymer mixture without any previous crosslinking steps. The presence of a polymer in our mixtures required an optimization of the thermomechanical parameters of SPS in order to promote the polymer-to-ceramic conversion prior to the densification. The optimized SPS conditions were composed of a dwell temperature of 700 °C for 10 min followed by a soaking temperature of 1700 °C for 5 min under an applied load of 75 MPa. A fine and homogenous phase distribution was achieved at relatively low temperature (1700 °C) compared to the high temperatures required for the sintering of the refractory composite. The influence of the addition of SiC (up to 10 wt%) was also studied and related to the final relative densities and mechanical properties of the composites.     

SiC-based ceramics with remarkable electrical conductivity prepared by ultrafast high-temperature sintering

SiC-based ceramics with high electrical conductivity are applied widely as electrode materials and semiconductor materials. In this study, a SiC-based ceramic with relative density of 96% was prepared by ultrafast high-temperature sintering (UHS) at 2000 ℃ (with a heating rate of 1000 ℃/min) for 40 s. The resistivity of as UHS-ed SiC-based ceramic was 1/15 of that prepared by the pressureless sintering. We found that the components of as-sintered body (SiC, Si and Y₃Si₅) by UHS were different from those (SiC and YAG) prepared by the pressureless sintering. The reason for the remarkable increase of the electrical conductivity of UHS-ed body was that the Si with higher electrical conductivity than SiC had emerged. Besides, the reaction mechanism was proposed and the unusual composition of the SiC-based ceramic sintered by UHS may also provide new reference for the application of SiC in specific fields.     

A route for the pressureless liquid-phase sintering of SiC with low additive content for improved sliding-wear resistance

A colloidal processing route has been developed for the pressureless sintering of dense SiC with a low content of sintering additives. In this route, a sol-gel solution precursor of the sintering additives is deposited onto the surface of the SiC particles, achieving a uniform distribution of the sintering additives in the green compact. This in turn promotes complete densification at short sintering times, which is not otherwise achievable when the batch is prepared by the standard method of mechanically mixing powders. It is also shown that the resulting ceramic has improved sliding-wear resistance compared to its counterpart prepared by the classical method, with essentially the same rate of mild and severe wear but a notably delayed transition from the mild to the severe wear regimes. This improvement is attributed to the reduction in the microstructural defect size achieved by the colloidal processing. Implications for the fabrication of low-cost SiC ceramics for wear-resistance applications are discussed.     

ZCAS-assisting low-temperature hot-press sintering of SiC ceramic for immobilizing simulated radioactive graphite

The safe treatment and disposal of high-level radioactive graphite is an essential challenge in the governance of irradiated graphite. SiC has an insurmountable defect in densification sintering at low temperatures, although it is an ideal host material for immobilizing high-level radioactive graphite. To solve this issue, we employ ZCAS (short for ZnO–CaO–Al₂O₃–SiO₂ glass) as a sintering aid to prepare SiC-ZCAS composite ceramic with the relative density up to 98% by vacuum hot-press sintering. For investigating the optimum formula and technological conditions of (1-x) SiC-x ZCAS composite ceramic, the effects of synthesis and sintering process on preparation (1-x) SiC-x ZCAS (x = 25-40 wt%) composites were investigated in detail. The results show that the SiC-ZCAS composite powder with x = 25-40 wt% can be synthesized at 1350 °C for 2 h when the Si/C mole ratio is 1.05:1. The relative density, Vickers hardness, and thermal conductivity of SiC-ZCAS composites ceramic increase rapidly by increasing sintering temperature and pressure. However, these properties will display different effects from the increase in the content of ZCAS, such as continuous elevation of the relative density and dramatic depression of thermal conductivity. the Vickers hardness starts to decline when the content of ZCAS is 30 wt%. Considering the above mentioned, we can conclude that the 0.7SiC-0.3ZCAS composite ceramic has choiceness comprehensive properties when sintered at 1550 °C and 60 MPa for 1 h, with the relative density, Vickers hardness and thermal conductivity of 95.5%, 1084 (HV 10) and 7.104 W/m·k, respectively.     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Liquid phase sintering and characterization of SiC ceramics

The present study focuses on the sintering of silicon carbide-based ceramics (SiC) by liquid phase sintering (LPS) followed by characterization of the produced ceramics. AlN/Re₂O₃ mixtures were used as additives in the LPS process. In the first step, the LPS-SiC materials were produced in a graphite resistance furnace in the form of discs at different temperatures. The conditions with the best results regarding real density and relative density were taken as reference for sintering in the form of prismatic bars. In the second step, these samples were evaluated regarding fracture toughness (K_IC), by the Single Edge V Notch Beam – SEVNB – method, and flexural strength. K_IC behavior was evaluated according to the depth and curvature radius of the notches. Reliable K_IC values were presented when the ceramic displayed a small curvature radius at the notch tip. When the radius was large, it did not maintain the square root singularity of the notch tip. Tests were carried out to determine K_IC values in atmospheric air and water. K_IC results were lower in water than air, with a decrease ranging between 2.56% and 11.26%. The observations indicated a direct grain size correlation between K_IC values and fracture strength of the SiC ceramics.