Influence of mechanical activation on combustion synthesis of fine silicon carbide (SiC) powder

The influence of mechanical activation of powder mixtures of Si and C, via high energy attrition milling (up to 12 h), on combustion synthesis of SiC was experimentally investigated. β-SiC fine powder was successfully fabricated in 1.0 MPa N₂ atmosphere without other additional treatments, such as preheating, electric action, or chemical activation. Relatively weak peaks of α-SiC, α-Si₃N₄ and Si₂ON₂ were also found in the final products. The experimental results and their theoretical treatment showed that mechanical activation via high energy ball-milling provides to the initial Si/C powder mixture extra energy, which is needed to increase the reactivity of powder mixture and to make possible the ignition and the sustaining of combustion reaction to form SiC.     

以水玻璃为硅源喷雾干燥制备SiC前驱体

以廉价的水玻璃和炭黑为原料,通过制备均匀混合的前驱体,利用碳热还原反应合成出超细SiC粉体。比较研究了前驱体喷雾干燥与搅拌干燥两种制备方法对体系碳热还原反应的影响,重点考察了喷雾干燥制备过程中主要工艺参数对前驱粉体密度及收率的影响。结果表明:喷雾制备的前驱体具有更高的反应活性,在1550℃下反应2h就可使SiO2转化率达到89.4%,搅拌干燥制备的前驱体转化率只有65.2%。喷雾干燥过程中,适当提高进口温度、喷雾头转速并降低料液的固体含量,可获得高收率、高产率的前驱粉体,利于整个制备工艺生产效率及产率的提高。     

酚醛树脂对反应烧结碳化硅显微结构与性能的影响

以碳化硅(w(SiC)=99%,d50=5μm)、炭黑(w(C)=99.8%,d50=0.38μm)和单质硅(w(Si)=98.6%)为原料,无水乙醇(w(乙醇)=99.6%)、热塑性酚醛树脂(0.074μm、工业级)为结合剂,乌洛脱品为固化剂,以50 MPa的单向压力,分别将采用干混工艺、湿混工艺混合、干燥后的物料压制成型为50 mm×5 mm的生坯,在N2保护下经800℃焙烧、炭化处理,有机物热降解后得到陶瓷素坯。研究了酚醛树脂在不同加入量(其质量分数分别为4%、8%、10%、12%、16%)及混练工艺(干混、湿混)对反应烧结碳化硅素坯强度和烧结体显微结构的影响,并采用SEM和光学显微镜分析了试样的显微结构和断面形貌。结果表明:当酚醛树脂加入量为12%时,采用湿混工艺可以制备出具有良好可浸渗性且抗折强度高达45 MPa的素坯,完全可以满足复杂异型件在烧成以前进行机械加工的要求,烧结体的抗折强度最高可达455 MPa。与干混工艺相比,用湿混工艺制成烧结体的显微结构更加均匀,晶粒更加细小,裂纹扩展更加曲折。     

Effect of silicon carbide additive on microstructure and properties of porcelain ceramics

Porcelain green bodies with various silicon carbide contents (0-3 wt.%) were prepared from a porcelain tile powder as a major raw material and SiC particle as an additive, and were sintered at 1000-1240 °C. The samples were systematically characterized by the X-ray diffraction (XRD), scanning electron microscope (SEM) and metallurgical microscope. Effects of the SiC content and sintering temperature on the pore size, SiC particle size and sintered density were investigated in detail, and the correlative mechanism was also discussed. The SiC particle size decreased and the pore size augmented with increasing the sintering temperature. The sintered density decreased and the pore size enlarged with increasing the SiC content. The experimental results indicate that a small amount of SiC can cause porcelain ceramics to foam during sintering, and a foaming origin of the polishing porcelain waste during sintering could be attributed to the oxidation reaction of SiC particles under high temperature and alkaline molten salt conditions.     

热处理温度对反应烧结碳化硅材料组织与性能的影响

研究了真空热处理温度对反应烧结碳化硅材料显微组织和断裂强度的影响.结果表明反应烧结碳化硅中的游离硅在1600℃、1800℃真空热处理过程中已全部去除;经过1800℃真空热处理材料的强度均高于1600℃真空热处理材料的强度.在1800℃真空热处理过程中发生的碳化硅再结晶以及气孔形状的变化,是其强度较高的主要原因.     

Fabrication of B4C from Na2B4O7 + Mg + C by SHS method

This paper deals with the formation of boron carbide (B₄C) powders from Na₂B₄O₇ + Mg + C system by self-propagating high-temperature synthesis (SHS) method. B₄C without impurities could be obtained after the acid enrichment and distilled water washing. The reaction mechanism of SHS of B₄C was proposed: the synthesis of B₄C is a process involving the decomposition of Na₂B₄O₇ into the intermediate phase B₂O₃, which reacts with Mg and carbon to form B₄C.     

Zirconia ultrafiltration membranes on silicon carbide substrate: microstructure and water flux

This study reported the preparation of ZrO₂/SiC ceramic membrane with silicon carbide as the substrate and intermediate layers and zirconia as the selective layer. The substrate and intermediate layers were sintered by evaporation-condensation process at 2200 and 1900 ℃, respectively. After sintering, the intermediate layer presented layer thickness of 50 μm, pore size of 0.87 μm and pure water permeability of 2140 L/(m²·h). The selective lay was deposited on the silicon carbide substrate by dip-coating method and then sintered in the temperature range from 800 to 1000 ℃. For the membrane coated by one dip-coating cycle and sintered at 800 ℃, it presented average pores of 82 nm and water flux of 850 L/(m²·h). Due to the exclusion of low-melting oxides during sintering, the ZrO₂/SiC ceramic membrane can satisfy the separation and purification of chemical corrosion and high temperature wastewater.     

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.     

A perspective on non-stoichiometry in silicon carbide

The non-stoichiometric ceramics are amazing materials with potential to offer applications that are unachievable by using otherwise ideal stoichiometric counterparts. These materials have contributed in wide areas including superconductivity, optical, magnetic, electronic, structural, mechanical and transport applications. The deviation form stoichiometry in a large number of compounds, though usually avoided, has numerous benefits; by increasing ionic conductivity, offering band structure modifications, causing paramagnetic to ferromagnetic transitions, reducing magnetoresistance, increasing mechanical strength, enhancing electrochemical efficiency etc. Keeping in mind the promising contributions of silicon carbide among family of ceramic materials, this review highlights the implications of non-stoichiometry and its properties. The non-stoichiometry produced unintentionally or purposefully is strongly influenced by synthesis conditions and varies for silicon carbide grown in amorphous, crystalline, polycrystalline polytypes in the form of bulk, surfaces and low dimensional structures. The prospects of tuning the properties of silicon carbide on the basis of fabrication of silicon rich and carbon rich by monitoring silicon to carbon ratio are discussed in detail.     

Effect of microstructure on the mechanical properties of liquid-phase-sintered silicon carbide at pre-creep temperatures

The effect of the microstructure on the mechanical properties of pressureless, liquid-phase-sintered (LPS) α-SiC ceramics above room-temperature was studied. LPS-SiC ceramics were fabricated with different microstructural features (grain size and morphology, and content of the intergranular phase), and their mechanical behaviour under contact stresses was evaluated by high temperature Hertzian testing (HTHT) from room temperature up to the creep temperature (1000 °C). The amount of intergranular phase was found to control the elasto-plastic properties of LPS-SiC at intermediate temperatures. Grain size and morphology had a significant influence only on toughness, since the crack bridging mechanism was enhanced by elongated grains, the more so the larger their size. Implications of these results for the design and fabrication of LPS-SiC ceramics with tailored contact-mechanical properties are discussed.     

Silicon carbide ceramics: Mechanical activation,combustion and spark plasma sintering

Single-stage fabrication of SiC ceramics by a combination of self-propagating high temperature synthesis (SHS) and spark plasma sintering (SPS) is reported. SHS+SPS is demonstrated to be an efficient method for production of SiC ceramics with density 3.1 g/cm³, hardness of 24 GPa and toughness of 5 MPa m^1/2. The starting material for the process is fine (50–300 nm in size) highly reactive powder, which involves composite particles of elemental carbon and silicon. This powder was prepared using a high-energy ball milling (HEBM). To optimize precursor preparation conditions, the structure transformation in nano-composite Si/C particles at different HEBM stages is also investigated.     

Influence of silicon carbide interlayer evolution on diamond heteroepitaxy during bias enhanced nucleation on silicon substrates

In order to improve the crystalline quality of diamond films produced by microwave plasma assisted chemical vapour deposition (MPCVD), the structural evolution of the silicon carbide interlayer during the bias nucleation step has been investigated by reflection high energy electron diffraction (RHEED). Here we highlight the fact that the carbonisation pre-treatment induces a strong extension of the silicon carbide lattice in the direction perpendicular to the surface. This extension gives a lattice constant close to that of silicon. Then, during bias enhanced nucleation, the carbide lattice relaxes. At the same time, this modification is accompanied by an increase of the surface roughness and by a progressive polar misorientation of the silicon carbide. All these transformations could be responsible for the observed drop of the diamond epitaxial ratio when the duration of the bias step is extended. Finally, we found that a lower methane concentration in the plasma slows down this carbide transformation, allowing us to obtain a promising 37% epitaxial ratio.     

Influence of cover thickness on the ballistic performance of silicon carbide subjected to large-scale tungsten projectiles

Al alloy was the optimal metal cover material for confined silicon carbide (SiC) against blunt tungsten-heavy-alloy projectiles based on a previous research. In this study, the influence of the aluminium (Al) alloy plate thickness on the ballistic performance of SiC was investigated to determine the optimal plate thickness. In addition, the ballistic responses of covered ceramic composite structures under various impact velocities were examined. The failure features of the cover plates and ceramic were elaborated based on tests and simulation analyses. The penetration weight calculated using the measured residual penetration was applied to characterise the ballistic performance of ceramic armour modules. The results showed that thin and thick cover plates could increase the loading rate of the ceramic pressure, thus attenuating their buffering effects on the ceramic. Therefore, the determination of the optimal cover plate thickness must simultaneously meet the requirements of stiffness and the reduced ceramic pressure loading rate. The optimal thickness for the Al alloy cover plate was 4 mm for lateral and back confined ceramic against blunt projectiles in this paper, which was obtained via a numerical simulation. Finally, the buffering effect of the cover plate on enhancing the structural ballistic performance varied with the impact velocities of the projectiles. This study provides insights for covered ceramic armour applications.     

Effect of Dilution and Porosity on Self-Propagating High-Temperature Synthesis of Silicon Nitride

Self-propagating high-temperature synthesis is a very easy and low-cost method to synthesize Si₃N₄. The nitriding of silicon powder takes place in a self-sustained regime under high pressures of nitrogen with dilution of silicon by Si₃N₄. In this work effects of dilution and green-mixture porosity on combustion velocity and phase content of reaction products are studied. Results are compared with previous work of other authors and different behaviors are found. An explanation of these behaviors is given.     

氮化硅结合碳化硅耐火材料在钢水中的腐蚀行为

运用扫描电镜（ＳＥＭ）技术，研究了氮化硅结合碳化硅耐火材料在钢水中的腐蚀行为。结果表明，金属与氮化硅结合碳化硅材料之间的界面清晰，基体内部无任何金属渗入，但氧化的材料表面有氧化物粘附。     

氮化物结合碳化硅耐火材料的研究现状

分别概述了以氮化硅、赛隆和氧氮化硅作为结合相 的SiC材料的结构特点、理化性能、生产工艺和应用情况,详细 介绍了国内这3种材料的研究现状,并对今后氮化物结合SiC 材料的研究内容提出了自己的观点。     

浅谈碳化硅制品形状设计的改进

应根据碳化硅制品的独特性能和制品的实际工作需要来设计形状和尺寸。文中的建议简而易行；本设计对节约资源、提高经济效益、减轻炉体结构重量、延长炉衬使用寿命，对促进经济发展有着重要意义。     

Synthesis of tungsten carbide nanopowders by direct carbonization of tungsten oxide and carbon: Effects of tungsten oxide source on phase structure and morphology evolution

In the paper, WC nanopowders are successfully prepared by carbothermal reduction method, and the effect of tungsten oxide source on the phase structure evolution and products properties of the as-synthesized WC nanopowders has been investigated. Four tungsten oxide powders are chosen as tungsten oxide sources, e.g., rods-like WO₃ , WO₃ nanopartiles, WO₃ micro-particles and WO_2.9 micro-particles. Compared with other three tungsten oxide sources, the WO₃ micro-particles possesses small particle size, less agglomerates and good dispersity and the uniform tungsten oxide-carbon mixture after ball milling can be easily obtained. The appropriate tungsten oxide source can result in lower processing temperature (≤1200 °C) and shorter holding time (≤3 h). Single-phase WC powders with average particle size of 100 nm and uniform particle distribution can be achieved by micro-particle-like WO₃ at 1100 °C for 3 h. The as-prepared WC products by other three tungsten oxide sources exhibit problems of more aggregates, non-uniform particle size and large particle size (250 nm), respectively. In addition, the method can provide a facile, low-cost, efficient, and industrially feasible pathway for large scale preparation of WC nanopowders.     

Synthesis and characterisation of a submicrometre silicon carbide powder

A beta-silicon carbide powder with a surface area of 30m²g^?l and a mean particle size of < 1μm was produced from the thermal conversion of silicon resin in an atmosphere of hydrogen. The amount of product increased with increasing iron content (0–2.1 wt%) and firing temperature (1200–1500°C). Chemical analysis, X-ray diffraction and i.r. absorption spectrometry were used to follow the conversion reaction.     

Rapid fabrication of Al4SiC4 using a self-propagating high-temperature synthesis method

As a promising high-temperature ceramic, aluminum silicon carbide (Al₄SiC₄) has attracted much attention. Al₄SiC₄ is usually synthesized at high temperatures with a long reaction time in an electric furnace. Self-propagating high-temperature synthesis (SHS) is a promising technique for rapid synthesis. In this study, Al₄SiC₄ was prepared by the SHS method from a mixture of silicon, aluminum and carbon black with the addition of poly(tetrafluoroethylene) (PTFE) as an exothermic promoter. The experimental results showed that the use of a high-pressure Ar atmosphere could retain the gaseous materials in the pellet mixture, and the PTFE additive promoted the formation of silicon carbide. In addition, the oxide layer present on the surface of silicon particles inhibited the reaction between silicon and carbon. As a result, high-purity Al₄SiC₄ could be synthesized from aluminum, silicon, and carbon black with 15 wt% PTFE under 1.0 MPa Ar atmosphere in several seconds by the SHS method.