Characterization of Porous Alumino-Silicate Bonded SiC-Ceramics Prepared by Hand-Pressing and Extrusion Methods

Silicon carbide was mixed with alumina and kaolin to obtain porous alumino-silicate bonded SiC ceramics. Starch was added as sacrificial template. The mixtures were processed by hand-pressing and extrusion method. The effect of firing temperature (1450 ^°C) and the addition of starch on the composition and characteristics of fired specimens were studied. The changes in phase composition and microstructure of fired SiC specimens were followed through x-ray diffraction analysis and scanning electron microscopy, respectively. The sintering parameters and thermal oxidation in air of such specimens were determined. The results indicated that silicon carbide is oxidized during firing into silica which reacted with silica-alumina mixture forming alumino-silicate bonding. Meanwhile, starch burnt out leaving pores inside the specimens. Porous SiC specimens of 1.72 to 1.79 g.cm^?2 bulk density, 40 to 45% open porosity and 250 to 350 N.cm^?2 compressive strength could be obtained by using a mixture of 80 mass% SiC and 20 mass% alumina and kaolin as starting materials. The properties of porous SiC specimens depend on the type and amount of used starch. The extrusion method is favorable for preparing porous SiC articles of homogeneous microstructure and good properties.     

Electrical,thermal, and mechanical properties of porous silicon carbide ceramics with a boron carbide additive

The effects of the boron carbide (B₄C) content and sintering atmosphere on the electrical, thermal, and mechanical properties of porous silicon carbide (SiC) ceramics were investigated in the porosity range of 58.3%–70.3%. The electrical resistivities of the nitrogen-sintered porous SiC ceramics (∼10^–1 Ω·cm) were two orders of magnitude lower than those of argon-sintered porous SiC ceramics (∼10¹ Ω·cm). Both the thermal conductivities (3.3–19.8 W·m^–1·K^–1) and flexural strengths (8.1–32.9 MPa) of the argon- and nitrogen-sintered porous SiC ceramics increased as the B₄C content increased, owing to the decreased porosity and increased necking area between SiC grains. The electrical resistivity of the porous SiC ceramics was primarily controlled by the sintering atmosphere owing to the N-doping from the nitrogen atmosphere, and secondarily by the B₄C content, owing to the B-doping from the B₄C. In contrast, the thermal conductivity and flexural strength were dependent on both the porosity and necking area, as influenced by both the sintering atmosphere and B₄C content. These results suggest that it is possible to decouple the electrical resistivity from the thermal conductivity by judicious selection of the B₄C content and sintering atmosphere.     

碳化物衍生碳及其在吸附领域中的应用研究进展

碳化物衍生碳（CDC）是近年来开发的一种具有纳米结构的新型多孔碳材料。该材料是将碳化物前体通过物理（如热分解等）或化学（如卤化等）过程,由表及里逐层刻蚀掉非碳原子得到的,可以实现原子水平上的调控。本文介绍了CDC的制备工艺、性质（孔和形貌）及其在吸附领域中的应用。结果表明,可以通过控制刻蚀温度、碳化物前体、后处理等合成条件实现CDC比表面积大范围可调、孔径及孔径分布等微观结构精确可控,得到从无定形碳到高度有序的石墨、碳纳米管或石墨烯等多种结构。最后指出了解制备过程-结构-性质之间的关系有利于精确调控CDC以满足特定应用的要求。     

Thermal and Electrical Properties in Plasma-Activation-Sintered Silicon Carbide with Rare-Earth-Oxide Additives

This study investigates the thermal and electrical properties of SiC ceramics with a combination of Y₂O₃ and rare-earth-oxide additions as sintering additives, by comparing four types of SiC starting powders varying in particle size and chemical composition. The powder mixtures were plasma-activation sintered to full densities and then annealed at high temperatures for grain growth. The thermal conductivity and electrical resistivity of the SiC ceramics were measured at room temperature by a laser-flash technique and a current–voltage method, respectively. The results indicate that the thermal conductivity and electrical resistivity of the SiC ceramics are dependent on the chemical composition and particle size of the starting powders. The thermal conductivities observed for all of the annealed materials with a rare-earth La₂O₃ sintering additive were >160 W·(m·K)⁻¹, although low electrical resistivity was observed for all materials, in the range 3.4–450 Ω·cm. High thermal conductivity, up to 242 W·(m·K)⁻¹, was achieved in an annealed material using a commercial 270 nm SiC starting powder.     

多孔碳化硅陶瓷的原位氧化反应制备及其性能

以SiC为陶瓷骨料,Al2O3作为添加剂,通过原位氧化反应制备了Sic多孔陶瓷,并对其氧化反应特性及性能进行了研究.结果表明:在1 300～1 500℃,随烧结温度的升高,SiC的氧化程度增加,SiC多孔陶瓷的强度逐渐增加,但开口孔隙率有所降低.莫来石相在1 500℃开始生成·当烧结温度升高到1 550℃时,莫来石大量生成,得到了孔结构相互贯通且颈部发育良好的莫来石结合SiC多孔陶瓷;由于在SiC颗粒表面上覆盖了致密的莫来石层,SiC的氧化受到抑制,开口孔隙率因而升高,SiC多孔陶瓷的强度因莫来石的大量生成而增加.由平均粒径为5.0um的SiC,并添加20%(质量分数)Al2O3,经1 550℃烧结2h制备的SiC多孔陶瓷具有良好的性能,其抗弯强度为158.7MPa、开口孔隙率为27.7%.     

Production of porous SiC by liquid phase sintering using graphite as sacrificial phase: Influence of SiO2 and graphite on the sintering mechanisms

Porous silicon carbide (SiC) is a promising ceramic for high-temperature applications due to its unique combination of properties. In the present work, a fabrication route for porous SiC is presented using graphite spherical powder as sacrificial phase to introduce porosity. By varying the initial amount of sacrificial phase, high-performance SiC materials with porosities in the range 30–50% were manufactured and characterized in terms of microstructure, density, thermal conductivity and flexural strength. The materials were fabricated by liquid phase sintering in presence of 2.5 wt.% Al₂O₃ and Y₂O₃ as sintering additives. The results indicate that the SiO₂ present in the starting SiC powders interacts with the sintering additives forming liquid phases that promote densification and weight loss. Besides, an Al-Si liquid phase is formed at higher sintering temperatures, whose contribution to densification is inhibited in presence of graphite due to the formation of Al-rich carbides.     

Porous silicon carbide structures with anisotropic open porosity for high- temperature cycling applications

We have prepared porous silicon carbide by a novel two-step template method. Graphite/SiC composites of required size and shape are first fabricated by hot pressing at 2125 °C, followed by the removal of the graphite template by controlled heat treatment. The anisotropy in the composite structure is restored after the removal of the template and porous SiC with anisotropic properties is obtained. The composite can be easily machined by electrical discharge machining because of the presence of graphite, and porous SiC can be obtained by heat treatment, solving the inherent difficulty in the machining of SiC. The mechanical properties, thermal conductivity, and thermal shock resistance of porous SiC have been studied in both directions. The material shows good thermal shock resistance in the perpendicular to pressing direction even at 1400 °C. Hence porous SiC suitably machined preserving the proper direction can be a potential candidate for thermal cycling applications.     

Effects of porosity on electrical and thermal conductivities of porous SiC ceramics

The effects of porosity on the electrical and thermal conductivities of porous SiC ceramics, containing Y₂O₃–AlN additives, were investigated. The porosity of the porous SiC ceramic could be controlled in the range of 28–64 % by adjusting the sacrificial template (polymer microbead) content (0–30 wt%) and sintering temperature (1800–2000 °C). Both electrical and thermal conductivities of the porous SiC ceramics decreased, from 7.7 to 1.7 Ω⁻¹ cm⁻¹ and from 37.9 to 5.8 W/(m·K), respectively, with the increase in porosity from 30 to 63 %. The porous SiC ceramic with a coarser microstructure exhibited higher electrical and thermal conductivities than those of the ceramic with a finer microstructure at the equivalent porosity because of the smaller number of grain boundaries per unit volume. The decoupling of the electrical conductivity from the thermal conductivity was possible to some extent by adjusting the sintering temperature, i.e., microstructure, of the porous SiC ceramic.     

Tribological behavior and applications of carbon fiber reinforced ceramic composites as high-performance frictional materials

Due to the favorable tribological, mechanical, chemical, and thermal properties, carbon fiber reinforced ceramic composites, especially carbon fiber reinforced carbon and silicon carbide dual matrix composites (C/C–SiC), has been considered as high-performance frictional materials. In this paper, current applications and recent progress on tribological behavior of C/C–SiC composites are reviewed. The factors affecting the friction and wear properties, including the content of silicon carbide and carbon matrix, carbon fiber preform architecture, as well as the matrix modification by alloy additives and C/C–SiC composites under various test conditions are reviewed. Furthermore, based on the current status of researches, prospect of several technically available solutions for low-cost manufacturing C/C–SiC composites is also proposed.     

Synthesis and properties of macroporous SiC ceramics synthesized by 3D printing and chemical vapor infiltration/deposition

Open porosity cellular SiC-based ceramics have a great potential for energy conversion, e.g. as solar receivers. In spite of their tolerance to damage, structural applications at high temperature remain limited due to high production costs or inappropriate properties. The objective of this work was to investigate an original route for the manufacturing of porous SiC ceramics based on 3D printing and chemical vapor infiltration/deposition (CVI/CVD). After binder jetting 3D-printing, the green α-SiC porous structures were reinforced by CVI/CVD of SiC using CH₃SiCl₃/H₂. The multiscale structure of the SiC porous specimens was carefully examined as well as the elemental and phase content at the microscale. The oxidation and thermal shock resistance of the porous SiC structures and model specimens were also studied, as well as the thermal and mechanical properties. The pure and dense CVI/CVD-SiC coating considerably improves the mechanical strength, oxidation resistance and thermal diffusivity of the material.     

Effect of Impurity and Carrier Concentrations on Electrical Resistivity and Thermal Conductivity of Sic Ceramics Containing BeO

Silicon carbide ceramics with BeO as an additive exhibit unusually high electrical resistivity and thermal conductivity compared to conventional SiC ceramics. Studies concerning the effects of carrier concentration in the SiC grains on electrical properties and thermal conductivity are described. The low carrier concentration in this ceramic is responsible for the high electrical resistivity. The thermal conductivity, however, decreases gradually with increasing impurity concentration.     

Effect of carbon content on electrical,thermal, and mechanical properties of porous SiC ceramics with B4C and C additives

The electrical, thermal, and mechanical properties of porous SiC ceramics with B₄C-C additives were investigated as functions of C content and sintering temperature. The electrical resistivity of porous SiC ceramics decreased with increases in C content and sintering temperature. A minimal electrical resistivity of 4.6 × 10^?2 Ω·cm was obtained in porous SiC ceramics with 1 wt% B₄C and 10 wt% C. The thermal conductivity and flexural strength increased with increasing sintering temperature and showed maxima at 4 wt% C addition when sintered at 2000 °C and 2100 °C. The thermal conductivity and flexural strength of porous SiC ceramics can be tuned independently from the porosity by controlling C content and sintering temperature. Typical electrical resistivity, thermal conductivity, and flexural strength of porous SiC ceramics with 1 wt% B₄C-4 wt% C sintered at 2100 °C were 1.3 × 10^?1 Ω·cm, 76.0 W/(m·K), and 110.3 MPa, respectively.     

Effect of picosecond laser texturing on the friction behavior of silicon carbide in hybrid ceramic bearings under dry and water lubrication

Hybrid ceramic bearings consisting of silicon carbide (SiC) and GCr15 steel have a wide range of applications. However, under unlubricated and water lubricated conditions, friction between SiC and GCr15 steel inevitably occurs due to interfacial contact (e.g. dry friction and boundary friction). Laser surface texturing, as a promising surface modification technique, has a significant impact on the material friction and wear characteristics aspects. In this paper, the UV picosecond laser was used to process a groove-textured structure on the surface of silicon carbide ceramics with the aim of comparatively investigating the effect of frictional properties of textured SiC surfaces and untextured SiC surfaces under dry friction and water lubrication conditions. The results showed that the picosecond laser successfully machined surface textures on the hard-to-process SiC surface, and the picosecond laser texturing had a significant effect on the tribological behavior of SiC ceramics. Under dry friction conditions, the coefficient of friction (COF) of picosecond laser-textured SiC with GCr15 steel increased (from 0.249 to 0.296) due to the micro-cutting effect and high surface roughness caused by the texture. In contrast, the COF of picosecond laser-textured SiC with GCr15 was significantly reduced under water-lubricated conditions (from 0.22 to 0.167), due to the facilitated frictional chemical reaction on the laser-textured surface, which favors the improved tribological properties of the silicon carbide friction interface.     

Porous SiC ceramics with dendritic pore structures by freeze casting from chemical cross-linked polycarbosilane

In this study, a commercial polycarbosilane (PCS) and divinylbenzene (DVB) were used as the preceramic polymer precursor and crosslinking agent, respectively to form porous silicon carbide (SiC) ceramics by freeze casting DVB/camphene/PCS solutions. Porous silicon carbide (SiC) with a dendritic pore structure and connecting bridges was obtained after pyrolysis at 1200 °C. The effects of DVB and PCS content on the rheological properties of the solution and the morphological characteristics and the compressive strengths of SiC ceramics were investigated. The use of DVB and the resulting chemical cross-linking yielded modified pore characteristics and much lower oxygen content in pyrolyzed SiC compared to the conventional thermal curing method. A compressive strength of 18.7 MPa was obtained for pyrolyzed SiC prepared with 20 wt% PCS and a 0.2 DVB/PCS mass ratio.     

Sintering additives for SiC based on the reactivity: A review

Silicon carbide (SiC) is one of the most attractive materials for high temperature applications, being used in many areas, such as gas turbines, heat exchangers, and space shuttles, because of its excellent strength, oxidation resistance and chemical stability at high temperatures. Moreover, SiC and its composites are being considered as structural materials for advanced fission reactors and future fusion reactors owing to its additional low induced radioactivity under neutron irradiation conditions. On the other hand, pure SiC can only be densified by sintering at high temperatures and pressures because of its high covalent bonding nature and low self-diffusivity. Therefore, the addition of sintering additives is essential for enhancing the densification of SiC. This paper reviews the criteria for the selection of effective SiC sintering additives based on the Gibbs free energy to predict the reactivity between the sintering additive and SiC, particularly for liquid phase sintering at 1700–1900 °C. The thermodynamic simulation was verified by offering the experimental results for various types of sintering additives, such as main group metals, metal oxides, and rare earth elements. This review suggests a guideline for the selection of sintering additives for SiC.     

Control of Electrical Resistivity in Silicon Carbide Ceramics Sintered with Aluminum Nitride and Yttria

The electrical properties of β‐SiC ceramics were found to be adjustable through appropriate AlN–Y₂O₃ codoping. Polycrystalline β‐SiC specimens were obtained by hot pressing silicon carbide (SiC) powder mixtures containing AlN and Y₂O₃ as sintering additives in a nitrogen atmosphere. The electrical resistivity of the SiC specimens, which exhibited n‐type character, increased with AlN doping and decreased with Y₂O₃ doping. The increase in resistivity is attributed to Al‐derived acceptors trapping carriers excited from the N‐derived donors. The results suggest that the electrical resistivity of the β‐SiC ceramics may be varied in the 10⁴–10^?3 Ω·cm range by manipulating the compensation of the two impurity states. The photoluminescence (PL) spectrum of the specimens was found to evolve with the addition of dopants. The presence of N‐donor and Al‐acceptor states within the band gap of 3C–SiC could be identified by analyzing the PL data.     

Effect of graphite and Mn3O4 on clay-bonded SiC ceramics for the production of electrically conductive heatable filter

Electrically conductive porous SiC ceramics are attracting substantial attention due to their application in heatable filters, vacuum chuck, and semiconductor processing parts, etc. The main problem is their high processing cost. Ideal candidates from an engineering ceramic perspective will be mechanically durable and have the required electrical properties with sufficiently low fabrication costs. To decrease the sintering temperature, kaolin has been added, but it tended to render the material an insulator. Graphite was used to effectively decrease the electrical resistivity. Additionally, manganese oxide was used to decrease the quantity of kaolin (the component that leads to an insulator material after sintering) and decrease the electrical resistivity while maintaining the mechanical properties. In our study, we found that SiC with 35% kaolin, 20% graphite and 10% manganese oxide can produce samples with 6.5 × 10^?1 Ω cm electrical resistivity and 43.5 MPa flexural strength at a low sintering temperature of 1200 °C.     

Thermal and electrical properties of additive-free rapidly hot-pressed SiC ceramics

The thermal and electrical properties of newly developed additive free SiC ceramics processed at a temperature as low as 1850 °C (RHP0) and SiC ceramics with 0.79 vol.% Y₂O₃-Sc₂O₃ additives (RHP79) were investigated and compared with those of the chemically vapor-deposited SiC (CVD-SiC) reference material. The additive free RHP0 showed a very high thermal conductivity, as high as 164 Wm⁻¹ K⁻¹, and a low electrical resistivity of 1.2 × 10⁻¹ Ω cm at room temperature (RT), which are the highest thermal conductivity and the lowest electrical resistivity yet seen in sintered SiC ceramics processed at ≤1900 °C. The thermal conductivity and electrical resistivity values of RHP79 were 117 Wm⁻¹ K⁻¹ and 9.5 × 10⁻² Ω cm, respectively. The thermal and electrical conductivities of CVD-SiC parallel to the direction of growth were ∼324 Wm⁻¹ K⁻¹ and ∼5 × 10⁻⁴Ω⁻¹ cm⁻¹ at RT, respectively.     

Effects of Elemental Additives on Electrical Resistivity of Silicon Carbide Ceramics

Effects of various elemental additives on the electrical resistivity of hot-pressed SiC ceramics were studied. The electrical resistivity at room temperature of dense SiC ceramics varied greatly depending on the additives used. SiC ceramics with added Be had an extremely high electrical resistivity of 3 × 10¹²°.cm. On the other hand, SiC ceramics with added B and Al had electrical resistivities of 2 × 10 and 0.8 °cm, respectively. The differences in the electrical resistivity of the dense SiC ceramics were considered to be due to different solubilities of the additives in SiC grains. SiC ceramics with added Be had a low level of impurities in the SiC grains as a result of the low solubility of Be in these grains.     

Synthesis of One-Dimensional Nanostructured Silicon Carbide by Chemical Vapor Deposition

Nanorods of the wide-bandgap semiconductor silicon carbide belong to a promising group of one-dimensional materials with potential applications extending from reinforcement of composites to applications as building blocks that can be logically assembled into appropriate two- (and three-) dimensional architectures, permitting researchers to exploit their unusual electronic, optical, and other properties. Specific to the most common silicon carbide polytypes are a low intrinsic carrier concentration, an exceptionally high breakdown electric field, high thermal conductivity, high-temperature stability, and resistance to an aggressive environment. This should permit one to develop even submicron-level SiC-based devices operating under high-temperature, high-power, and/or high-radiation conditions, under which conventional semiconductors cannot function. Detailed control of the conditions favorable for the nucleation and growth processes of nanorods of a given SiC polytype is necessary because the electrical and optical properties of each SiC polytype are very different. Therefore, a systematic investigation of factors that primarily influence the morphology and polytype of a vapor-phase-grown SiC has been made in the present work. These factors were the temperature, the flow rates of the gaseous precursors, and the Si/C molar ratio in the gas phase. In order to investigate the role of these factors, the “cold gas-hot substrate” chemical vapor deposition (CVD) method has been applied, because it permits them to be closely controlled in a wide range. While in the overwhelming majority of previous investigations nanorods of the 3C SiC polytype have been grown, the present work delineates conditions that are favorable for the growth of single-phase 2H, 3C, 15R, and 6H SiC nanorods, respectively.Original English Text Copyright © 2005 by Fizika i Khimiya Stekla, Pampuch, Gorny, Stobierski.This article was submitted by the authors in English.