Fabrication of Open-Cell, Microcellular Silicon Carbide Ceramics by Carbothermal Reduction

A novel processing route for developing open-cell, microcellular SiC ceramics has been developed. The strategy adopted for making microcellular SiC ceramics involved the following: (i) fabricating a formed body from a mixture of polysiloxane, phenol resin (used as a carbon source), polymer microbeads (used as sacrificial templates), and Al₂O₃–Y₂O₃ (an optional sintering additive); (ii) cross-linking the polysiloxane in the formed body; (iii) transforming the polysiloxane and phenol resin by pyrolysis into silicon oxycarbide and C, respectively; and (iv) synthesizing SiC by carbothermal reduction. By controlling the microbead and additive contents, it was possible to adjust the porosity so that it ranged from 60% to 95%.     

Porous 2H-Silicon Carbide Ceramics Fabricated by Carbothermal Reaction between Silicon Nitride and Carbon

Porous SiC ceramics were synthesized by sintering pressed and pressed/CIPed powder compacts of α-Si₃N₄, carbon (Si₃N₄:C = 1:3 mol as ratio), and sintering aids, at 1600°C for few hours to achieve a reaction, and subsequently sintering at a temperature range of 1750°–1900°C, in an argon atmosphere. High porosities from 45%–65% were achieved by low shrinkage with large weight loss. Formation of pure 2H-SiC phase via a reaction between Si₃N₄ and carbon can be demonstrated by X-ray diffractometry. The resultant porous SiC samples were characterized by SiC grain microstructures, pore-size distribution, and flexural strength. This method has the advantage of fabricating high-porous SiC ceramics with fine microstructure and good properties at a relatively low temperature.     

碳热还原合成碳化硅过程的数值模拟

为揭示碳化硅合成过程中能量及物质扩散机理,从而为碳化硅的提质增产奠定理论基础,采用数值模拟的方法对碳化硅合成过程中的温度场、压力场、气体流动规律进行模拟研究。结果表明,随着合成时间的延长,炉内热量呈辐射状向外扩散,合成炉内气体呈现三维多向流动特性,反应进行到24 h时CO气体流量达到最大,而此时由于炉底透气性差的原因,致使炉底部压力高于其余位置,最大可达1.525×101 k Pa,此时可加入少量木屑以增加炉底透气性来改善因压力过高所造成的喷炉事故。模拟结果得到了生产实践验证。     

Preparation of Silicon Carbide and Aluminum Silicon Carbide from a Montmorillonite–Polyacrylonitrile Intercalation Compound by Carbothermal Reduction

Both silicon carbide and aluminum silicon carbide have simultaneously been obtained directly from naturally occurring aluminosilicate by carbothermal reduction for the first time. A precursor of a montmorillonite–polyacrylonitrile (PAN) intercalation compound was heated at 1700°C in Ar. For comparison, montmorillonite–carbon mixtures were similarly heated. α-SiC, β-SiC, and Al₄Si₂C₅ formed from the montmorillonite–PAN intercalation compound. Mainly α-Al₄SiC₄ was obtained with ternary carbides from the montmorillonite–carbon mixtures in addition to a large amount of β-SiC. Hence, aluminum silicon carbide formation was affected by the mixing condition of the starting materials.     

Microstructure of Silicon Carbide Whiskers Synthesized by Carbothermal Reduction of Silicon Nitride

The microstructure of silicon carbide whiskers synthesized by carbothermal reduction of silicon nitride has been studied using transmission electron microscopy. All of the whiskers examined are single crystals, and grow in the (111) crystallographic direction. Two different forms of stacking faults and microtwins were observed; in one the planar defects are normal to the whisker growth direction, and the other has the defect planes at an angle of about 70° to the growth axis, while both forms of the defects are on the [111] closed-packed planes. Without the addition of catalyst, droplets containing metallic impurities were not found at the tips of the whiskers synthesized by the present process. A core and outer regions were observed in the single-crystal whiskers, which may be evidence that the whiskers were formed by a two-stage mechanism.     

Wood-Derived Porous SiC Ceramics by Sol Infiltration and Carbothermal Reduction

A new method for transforming wood structures into ceramic by carbothermal reduction of silica and carbonized wood has been developed. Application of this technique allows the preservation of wood microstructures in the ceramic state and the conversion of wood components without constraints in component size. The chosen infiltration technique of silica sol incorporation into carbonized wood structures is examined in terms of sol and wood type, carbonization conditions of the wood, and thickness of the infiltrated carbon body. Ceramization conditions were optimized, and the reaction mechanism is discussed.     

High-Strength Porous Silicon Carbide Ceramics by an Oxidation-Bonding Technique

Porous silicon carbide (SiC) ceramics were fabricated by an oxidation-bonding process in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO₂ glass. Because of the crystallization of amorphous SiO₂ glass into cristobalite during sintering, the fracture strength of oxidation-bonded SiC ceramics can be retained to a relatively high level at elevated temperatures. It has been shown that the mechanical strength is strongly affected by particle size. When 0.6 μm SiC powders were used, a high strength of 185 MPa was achieved at a porosity of ∼31%. Moreover, oxidation-bonded SiC ceramics were observed to exhibit an excellent oxidation resistance.     

Thermal Shock Behavior of Porous Silicon Carbide Ceramics

Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications.     

Structural and Optical Characteristics of Crystalline Silicon Carbide Nanoparticles Synthesized by Carbothermal Reduction

Silicon carbide (SiC) nanoparticles were successfully synthesized by using carbothermal reduction method. Nanoparticles with zincblende structure (3C–SiC) could be prepared using polysiloxane as silicon source and phenol resin (MH) or xylene resin (YN) as carbon source. The sample YN has an average grain size of 22 nm, larger than that of the sample MH (8 nm). Raman spectroscopy revealed that all nanoparticle samples contain graphitic surface carbon layers. Oxygen contamination on the nanoparticle surface could be reduced by postfluorine treatment (MH-F). But the sample MH-F showed reduced SiC crystallinity compared with the sample MH. The nanoparticle samples exhibited an intensive emission band in the blue region observed by photoluminescence (PL) spectroscopy. The band-gap energy of the nanoparticle samples is estimated to be ∼3 eV from the PL spectra, blueshifted by ∼0.6 eV from that of bulk 3C–SiC due to the quantum confinement effect.     

Intrinsic Reaction and Self-Diffusion Kinetics for Silicon Carbide Synthesis by Rapid Carbothermal Reduction

A one-dimensional nonisothermal model has been developed for the "rapid carbothermal reduction" synthesis of fine silicon carbide powders. Intrinsic reaction and self-diffusion kinetics are identified through simulation of the model and comparison to experimental results. The reaction rate follows a shrinking-core mechanism and is described by the relation [formula omitted] The self-diffusion coefficient for SiC in the aerosol flow reactor is described by the relation The self-diffusion coefficient for SiC in the aerosol flow reactor is described by the relation      

Characterization of β-Silicon Carbide Powders Synthesized by the Carbothermal Reduction of Silicon Carbide Precursors

Boron-doped and nondoped ultrafine β-silicon carbide (β-SiC) powders were synthesized via the carbothermal reduction of SiC precursors at temperatures of 1773–1973 K. Although the reaction rate of carbothermal reduction was generally higher when a boron-doped precursor was used, the reaction rate for the boron-doped precursor was reduced considerably at 1873 K. For boron-doped and nondoped precursors, the reaction rates were almost the same. Powder characterization via transmission electron microscopy indicated that the suppression of the reaction rate for boron-doped precursor at 1873 K was due to the formation of a special coexistent system with two types of particle agglomerates. As expected, boron doping inhibited the particle growth in the synthesis of SiC powder.     

Analysis of the Rapid Carbothermal Reduction Synthesis of Ultra-Fine Silicon Carbide Powders

A nondimensionalized and scaled nonisothermal model is developed for the "rapid carbothermal reduction" synthesis of sub-micron silicon carbide particles in an aerosol flow reactor to determine the minimum parametric representation of the system. Seven dimensionless groups are needed to completely describe the system, and these dimensionless groups are varied to determine the effects of the furnace wall temperature, inlet carbon particle size, carrier gas flow rate, and solids feed rate on final product quality. Analysis shows that radiation dominates the heating process, sintering dominates the primary particle growth, and conversion is controlled with precursor carbon particle size, wall temperature, and carrier gas flow rate.     

碳化硅多孔陶瓷的制备工艺比较

多孔碳化硅陶瓷由于具有优良的高温强度、耐磨性、耐腐蚀性以及抗热震性而得到越来越广泛的关注.随着科技的发展,其已在环境保护、过滤分离、尾气吸收、吸声降噪、生物医学、航空航天和能源化工等方面发挥着重要的作用.本文着重分析了多孔碳化硅陶瓷的传统制备工艺与先进制备工艺的优缺点,并对其未来的制备工艺作出展望.     

Highly Aligned Porous Silicon Carbide Ceramics by Freezing Polycarbosilane/Camphene Solution

We fabricated highly aligned porous silicon carbide (SiC) ceramics with well-defined pore structures by freezing a polycarbosilane (PCS)/camphene solution. In this method, the solution prepared at 60°C was cast into a mold at temperatures ranging from 20° to −196°C, which resulted in a bicontinuous structure, in which each phase (camphene or PCS) was interconnected in a regular pattern. After the removal of the frozen camphene network, the samples showed highly porous structures, in which long straight and short elongated pore channels were formed parallel and normal to the direction of freezing, respectively. Thereafter, porous SiC ceramics were produced by the pyrolysis of the porous PCS objects at 1400°C for 1 h in a flowing Ar atmosphere, while preserving their mother pore structures having aligned pore channels.     

碳化硅/堇青石复相多孔陶瓷的制备及性能研究

采用碳化硅、烧高岭土、氢氧化铝、滑石为主要原料,石墨为造孔剂制备了碳化硅/堇青石复相多孔陶瓷.研究了烧结温度和烧结助剂二氧化铈对碳化硅/堇青石复相多孔陶瓷气孔率和强度的影响,并分别用XRD和SEM分析晶相组成和断面显微结构表明:制备出的SiC多孔陶瓷的主相是SiC,结合相是堇青石与方石英,多孔陶瓷具有相互连通的开孔结构;在1350℃烧结,并保温3h,当造孔剂含量为15％时,碳化硅/堇青石复合多孔陶瓷性能最佳,其气孔率31.80％,相应的弯曲强度为63.74 MPa.在1200℃下,添加不同含量的CeO2,对烧结样品的相组成有影响,能够降低生成堇青石的温度,在CeO2含量为3％的样品中,堇青石的峰最明显,但是过量的氧化铈会抑制了堇青石的生成;随着CeO2加入量的增加,其气孔率和弯曲强度也会随之变化,1200℃下,在CeO2加入量为4％时其弯曲强度最优.但随着CeO2的含量的增加,其气孔率逐渐下降.     

Effects of Pore-forming Agents on Properties of Silicon Carbide Porous Ceramics Prepared from Crystalline Silicon Cutting Waste

SiC powder was rapidly synthesized in an induction furnace with crystalline silicon cutting waste and active carbon as raw materials, and then SiC porous ceramics were prepared at 1 600 ℃ for 4 h with carbon embedded using the powder as raw material, the starch and the graphite as pore-forming agents. Effects of additions of different pore-forming agents on the phase composition, microstructures, physical properties, and cold crushing strength of the porous ceramics were investigated. The results show that the main crystalline phases of the synthetic powder are α-SiC(6 H-SiC) and β-SiC(3 C-SiC). The phase composition of the porous ceramics includes α-SiC(6 H-SiC), β-SiC(3 C-SiC), Fe Si, quartz and Si_2N_2O. The apparent porosity and closed porosity of the porous ceramics prepared by adding starch are higher, and the cold compressive strength of the porous ceramics added with graphite is higher. As increasing the additions of the starch, the apparent porosity, closed porosity and linear shrinkage ratio of the porous ceramics increase, and the bulk density decreases correspondingly. When 20 mass% starch is added, the apparent porosity, closed porosity, linear shrinkage ratio and cold compressive strength are 57.05%, 2.03%, 5.10% and 10.20 MPa, respectively.     

Preparation and Properties of Porous Aluminum Nitride–Silicon Carbide Composite Ceramics

Microporous two-phase AlN–SiC composites were prepared using Al₄C₃ and either Si (N₂ atmosphere) or Si₃N₄ (Ar atmosphere) as precursors. The reaction mechanisms of the two synthesis routes and the effect of processing conditions on reaction rate and the material microstructures were demonstrated. The exothermic reaction between Si and Al₄C₃ under N₂ atmosphere was shown to be a simple processing route for the preparation of porous two-phase AlN–SiC materials. The homogeneous two-phase AlN–SiC composites had a grain size in the range of 1–5 μm, and the porosity varied in the range of 36%–45%. The bending strength was 50–60 MPa, in accordance with the high porosity.     

Porous Silicon Carbide Ceramics Produced by a Carbon Foam Derived from Mixtures of Mesophase Pitch and Si Particles

Carbon foam templates were prepared from a mixture of mesophase pitch (MP) and Si particles, followed by foaming and carbonization. Subsequent molten Si infiltrated into the carbon foam at 1500°C for 4 h in an inert atmosphere resulted in the formation of porous SiC ceramics. Micrographs were investigated by a scanning electron microscope (SEM), and phase identification of porous SiC ceramics was performed by X-ray diffraction (XRD). The flexural strength and bulk density of porous SiC ceramics were also measured and calculated. The results revealed that the flexural strength of porous SiC ceramics increases with increasing Si content and decreasing porosity. The addition of Si in MP results in an increased densification of porous SiC struts. With 50 wt% Si, porous SiC ceramics with a high flexural strength of 23.9 MPa and a porosity of 55% were obtained.     

碳化硅陶瓷的研究进展

碳化硅陶瓷以优异的高温力学性能以及优良的耐化学腐蚀性能得到了越来越广泛的应用,本文重点介绍了碳化硅陶瓷的烧结工艺以及性能特点等。     

热浇注成型烛状碳化硅多孔陶瓷过滤元件的研制

本文研究了热浇注成型烛状碳化硅多孔陶瓷过滤元件的制备工艺技术,及不同添加剂和含量对过滤元件物理性能的影响.结果表明:采用粘土作为结合剂,加入量为10～15%(质量含量,下同)时,碳化硅过滤元件的显气孔率可达到33～40%,抗压强度达到26～29 MPa,可以满足PFBC、IGCC和煤炭气化系统装置对过滤器气孔率和强度的要求.