Porous SiC Ceramics Research Articles

Effect of V2O5 addition on the properties of reaction-bonded porous SiC ceramics

Mullite-bonded porous SiC ceramics were prepared with and without V2O5 addition in air by an in situ reaction bonding technique. This process is based on the oxidation of SiC and the mullitization between oxidation-derived SiO2 and Al2O3. The effects of V2O5 addition on the phase composition, microstructure, open porosity, flexural strength and pore size distribution of porous SiC ceramics were investigated. With the addition of V2O5, the mechanical properties of porous SiC ceramics were improved. It was found that the V2O5 addition strongly promoted the phase transformation towards mullite and reduced the temperature of mullitization by nearly 400°C. When 4wt% V2O5 was added, a high strength of 30.11MPa was achieved at an open porosity of 42.1%.

Processing, microstructures and mechanical properties of aqueous gelcasted and solid-state-sintered porous SiC ceramics

Abstract Porous solid-state-sintered SiC ceramics with controllable structures and excellent mechanical performances were prepared by aqueous gelcasting, combined with direct protein foaming technique. Dispersion of SiC and carbon black powders in water, the rheological behavior of steady SiC-B 4 C-C slurries, and solidification process of foamed suspensions were systematically investigated. Porosities as well as the microstructures of sintered bulks were tailored by varying the solid content of slurries and amount of foaming agent. The relationship between the compressive strength (41–151 MPa) and porosity (49–68%), and the fracture behaviors were discussed. For improving the connectivity, grain grading was adopted and its effects were revealed.

STUDIES ON PERMEABILITY PROPERTIES AND PARTICLE CAPTURE EFFICIENCIES OF POROUS SiC CERAMICS PROCESSED BY OXIDE BONDING TECHNIQUE

Porous SiC ceramics bonded with mullite MBS of fractional porosity (epsilon) of 0.29-0.56, average pore size (d(pore)) of 5-11 mu m, flexural strength (sigma) of 9-34 MPa, and elastic modulus (E) of 7-28 GPa] and cordierite (CBS with epsilon of 0.33-0.72, d(pore) of 6-50 mu m, sigma of 5-54 MPa, and E of 6-42 GPa) were prepared by heating in air at 1350-1500 degrees C compacts of desired amounts of SiC, Al2O3, and MgO powders and petroleum coke dust as the pore former. Air permeation behavior of well-characterized samples was studied with fluid superficial velocity (v(s)) from 0.08 to 1.0 m s(-1) and at RT to 750 degrees C. The Darcian (k(1)) and non-Darcian (k(2)) permeability coefficients were evaluated by fitting the Forchheimer's equation to experimental pressure drop-superficial velocity data. Porosity dependence of permeability coefficients was explained in terms of structural characteristics. Changes in pressure drop experienced by the porous ceramics at high temperatures were explained by temperature dependence of permeability coefficients and variation of fluid properties. Collection efficiency (E-overall) of filter ceramics operating on removal of solid NaCl nanoaerosol particles (of 7-300 nm size) was determined from particle counts before and after filtration at v(s) = 0.05-0.10 m s(-1). Experimental results showed variation of E-overall from 96.7 to 99.9% for change of epsilon from 0.56 to 0.68. The size-selective fractional collection efficiency at different porosity levels was derived using the well-known single-collector efficiency model considering some boundary conditions and the model data were validated with experimental results. The test results were used to examine the applicability of the filter ceramics in nanoparticle aerosol filtration processes.

Testing of porous SiC with dense coating under relevant conditions for Flow Channel Insert application

Thermally and electrically insulating porous SiC ceramics are attractive candidates for Flow Channel Inserts (FCI) in dual-coolant blanket concepts thanks to its relatively inexpensive manufacturing route. To prevent tritium permeation and corrosion by Pb-15.7 a dense coating has to be applied on the porous SiC. Despite not having structural function, FCI must exhibit sufficient mechanical strength to withstand strong thermal gradients and thermo-electrical stresses during operation. This work summarizes the results on the development of coated porous SiC for FCI. Porous SiC was obtained following the sacrificial template technique, using Al2O3 and Y2O3 as sintering additives and a carbonaceous phase as pore former. Sintering was performed in inert gas at 1850–1950°C during 15min to 3h, followed by oxidation at 650°C to eliminate the carbonaceous phase. The most promising bulk materials were coated with a ∼30μm thick dense SiC by CVD. Results on porosity, bending tests, thermal and electrical conductivity are presented. The microstructure of the coating, its adhesion to the porous SiC and its corrosion behavior under Pb-17.5Li are also shown.

Effect of SiC whiskers on properties of porous silicon carbide ceramic

The porous SiC ceramics reinforced by silicon carbide whisker were produced by mixing SiC whisker with core–shell structure powder precursor. The surface morphology, phase composition, porosity, mechanical and thermal properties of the products were analysed. The morphology images show that small part of the SiC whiskers are inserted in the holes of SiC matrix, and most of the SiC whiskers are well dispersed in the matrix. The X-ray diffraction (XRD) pattern indicates that the crystalline phase of porous SiC ceramics reinforced by SiC whisker is pure β-SiC. The bending strength of porous SiC ceramics is strengthened by SiC whisker through the reinforcement mechanisms of whisker bridging, crack deflection and pulling-out whisker effect. The influence of addition proportion of SiC whisker on the bending strength was studied. Along with increasing SiC whisker additive, the bending strength of the porous ceramics performs an increasing trend. When the percentage of SiC whisker in the SiC ceramics increases to 15%, the bending strength of the ceramic runs up to 2·4 times of the SiC ceramic without addition of SiC whisker. Thermal shock resistance tests reveal that the composite has both good bending strength and good high temperature performance, which are very meaningful for the application of composite ceramics in the high temperature fluid filtration field.

Fabrication and microstructure of porous SiC ceramics using suspension emulsions as pore-forming agents

Using emulsions of cyclohexane-in-water as pore-forming agent, porous SiC green bodies were prepared by gelcasting technology. Effects of the amount of oil and emulsifier on the microstructure, the porosity, the compressive strength and the drying linear shrinkage of the porous SiC green bodies were examined. By adjusting the amount of oil and emulsifier, the average pore size varied from 10.77μm to 26.40μm, the porosity varied from 63.57% to 80.19%, and the compressive strength varied from 14.7MPa to 1.7MPa. The compressive strength of the typical sintered specimens with porosity of 62.17% reached 60.69MPa.

Investigation of thermal oxidation of Al2O3-coated SiC powder

a b s t r a c t SiC powder (-form) of sub-micrometre size (d50 of 0.65 m) was coated with a thin layer (∼12 nm) of boehmite by sol–gel technique. The final powder having ∼24 wt% boehmite was characterized by particle size and surface area determination, chemical analysis, surface potential measurement and Fourier transform infrared (FTIR) spectroscopic analysis. The thermal oxidation behaviour of the coated powder was examined using the simultaneous TG/DTA technique in dry air. The results indicated crystallization of mullite phase and the activation energy of crystallization was determined to be 368.9 ± 15.1 kJ mol−1. Synthesis of porous SiC ceramics of fine pore structure (with average porosity and pore size ranging from 47% to 54% and 170–260 nm, respectively) using thermal oxidation of coated powder was also demonstrated.

Preparation and Properties of <i>In Situ </i>Formed YAG Bonded Porous SiC Ceramics

Yttrium aluminium garnet (YAG) possesses many good properties, and usually uesd as sintering additives for silicon carbide ceramics. The YAG precursor solution was preformed by the coprecipitation process, then 8wt% prepared precursor solution was added and uesed as binder for samples' green bodies dry mould pressing. The dried pressed samples were directly sintered at 1850¡æ under vaccum atmosphere, and the precursor Al2O3-Y2O3 could be formed YAG during sintering process. The oxidation resistance property was performed at 1500¡æ in air. Bulk density and porosity were about 2.42g/cm3 and 24.98% respectively, and three-point bonding strength was up to 90MPa. Phase composition of sintered specimen was determined by X-ray diffraction and fracture morphology was detected by scanning electron microscope (SEM). The results showed that the as-fabricated porous SiC ceramics exhlibted excellent oxidation resistance abilities, its oxidation resistance improvement was benefited from silicon carbide particle coated with a compact YAG layer. The relationship between microstructure and properties was analyzed, and the finally formed phase composition was discussed.

Novel fabrication route for porous silicon carbide ceramics through the combination of in situ polymerization and reaction bonding techniques

ABSTRACTFor the first time, an in situ polymerization technique was applied to produce mullite‐bonded porous SiC ceramics via a reaction bonding technique. In this study, SiC microsized particles and alumina nanopowders were successfully coated by polyethylene (PE), which was synthesized from the particle surface in a slurry phase reactor with a Ziegler–Natta catalyst system. The thermal studies of the resulting samples were performed with differential scanning calorimetry and thermogravimetric analysis. The morphology analysis obtained by transmission electron microscopy and scanning electron microscopy (SEM) confirmed that PE was successfully grafted onto the particle surface. Furthermore, the obtained porous ceramics were characterized in terms of their morphologies, phase composition, open porosity, pore size distribution, and mechanical strength. SEM observations and mercury porosimtery analysis revealed that the quality of the dispersion of nanosized alumina powder into the microsized SiC particles was strongly enhanced when the particles were coated by polymers with in situ polymerization. This resulted in a higher strength and porosity of the formed ceramic porous materials with respect to the traditional process. In addition, the X‐ray diffraction results reveal that the amount of mullite as the binder increased significantly for the samples fabricated by this novel method. The effects of the sintering temperature, forming pressure, and polymer content on the physical and mechanical properties of the final porous ceramic were also evaluated in this study. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40425.

Preparation and properties of mullite-bonded porous SiC ceramics using porous alumina as oxide

Mullite-bonded porous silicon carbide ceramics were prepared by an in situ reaction bonding technique and sintering in air with SiC, porous Al2O3, and graphite as starting materials. The pores in the ceramics were formed by burning graphite and by stacking particles of SiC and Al2O3. The surface of SiC was oxidized to SiO2 at high temperature. With a further increase in temperature, SiO2 reacted with Al2O3 to form mullite. The reaction-bonding characteristics, phase composition, open porosity, mechanical strength as well as the microstructure of porous SiC ceramics were investigated.

An advanced method to manufacture hierarchically structured carbide-derived carbon monoliths

The preparation of carbide-derived carbon (CDC) monoliths with a hierarchically structure in the nm and μm range is presented. Basis is the manufacturing of porous cellular SiC ceramics based on a biomorphous approach with μm porosity and subsequent conformal conversion to CDC by reactive extraction with chlorine. The SiC ceramics can be sintered at low temperatures and short times (1500°C, 2h) compared to classical preparation methods. The SiC ceramics show a macro pore volume (1–10μm channel size) of 0.56mlg−1, which corresponds to 1.5mlg−1 in the resulting CDC. The final carbon material exhibits an additional nano pore volume of 0.525mlg−1 with a mean slit pore size of 0.86nm. Mechanical stabilities of the highly porous CDC are excellent (bending strength 2.1±0.2MPa, corrected Weibull modulus 8.7, characteristic strength 2.2MPa and Youngs modulus 10.0±0.5GPa). The reactive extraction of the carbide monoliths shows very high reaction rates, approx. two dimensions faster (95×) compared to non-porous samples. Thus the manufacturing of the structured carbide and CDC can be performed at lower costs.

Permeability and Nanoparticle Filtration Assessment of Cordierite-Bonded Porous SiC Ceramics

Cordierite bonded porous SiC ceramics having pore fractions (e) between 0.33 and 0.72 and pore sizes of 6–50 μm, flexural strength of 5–54 MPa, and elastic modulus of 6–42 GPa were prepared by oxide bonding at 1350 °C in air compacts of SiC, Al2O3 and MgO powders with petroleum coke (PC) as the sacrificial pore former. To test the applicability of the porous ceramics in the fluid flow field, air permeation behavior was studied with fluid superficial velocity from 0.083 to 0.90 m s–1 and at 26–750 °C. The Darcian, k1, and the non-Darcian, k2, permeability coefficients were evaluated by fitting Forchheimer’s equation to the experimental results. The temperature dependence of the permeability coefficients was explained from structural changes occurring during test conditions. The collection efficiency of filter ceramics (e = 0.62–0.68) operating on removal of nanosized aerosol particles with sizes varying from 7 to 300 nm was determined by counting particles before and after filtration at a fluid superficial...

Preparation and Hydrogen Permeability of SiC-Y2O3Composite Membranes

ABSTRACTSiC-Y 2 O 3 porous composites were fabricated using Y 2 O 3 powders synthesized by sol-gel process to control physical and thermo-chemical properties. Y 2 O 3 powders were mixed with SiC powders by co-pressing with HPCS (hydridopolycarbosilane) binder atmoderate temperature. The properties of membranes were characterized by XRD, FE-SEM, and BET surface analysis. Hydrogenpermeability was performed at various temperatures.Key words : SiC, Y 2 O 3, HPCS, Hydrogen Permeance tests 1. Introduction ydrogen selective SiC membranes have been used forapplication in membrane reactors. 1,2) SiC as a struc-tural material has advantages such as thermal-mechanicalstability and chemical inertness. SiC also has disadvantagein terms of its porosity and mechanical property of hard-ness. Since SiC has the covalent character of Si-C bonds,SiC ceramics can be sintered at high temperature and arelimited in terms of applications. However, very little hasbeen reported on the fabrication of porous SiC ceramics.

Fabrication and properties of cordierite–mullite bonded porous SiC ceramics

Cordierite–mullite bonded porous SiC ceramics were prepared by an in situ reaction bonding technique using a silicon carbide, aluminum hydroxide, MgO, and graphite as starting materials. The starting materials were mixed in different ratios and heated at different temperatures (1300–1450°C) to obtain good mechanical properties. The pores in the ceramics were formed by burning graphite and by stacking particles of SiC and Al(OH)3. The surface of SiC was oxidized to SiO2 at high temperature, and fine Al2O3 grains produced by the decomposition of aluminum hydroxide. With further increasing the temperature, SiO2 reacted with Al(OH)3 and MgO to form cordierite–mullite. SiC particles were bonded by the oxidation-derived SiO2 and cordierite–mullite. The reaction bonding behavior, phase composition, open porosity, microstructure and mechanical strength as well as pore size distribution of porous SiC ceramics were investigated. The results show that when 4.5wt% MgO was added, in the absence of graphite, the flexural strength and the open porosity of the porous SiC ceramics were 78.71MPa and 21.63%, respectively. In addition, when 30% graphite was added to the above composition, a flexural strength of 41.29MPa was achieved at an open porosity of 42.11%.

Fabrication of porous silicon carbide ceramics with high porosity and high strength

Abstract Unique porous SiC ceramics with a honeycomb structure were fabricated by a sintering-decarburization process. In this new process, first a SiC ceramic bonded carbon (SiC/CBC) is sintered in vacuum by spark plasma sintering, and then carbon particles in SiC/CBC are volatized by heating in air at 1000 °C without shrinkage. The honeycomb structure has at least two different sizes of pores; ∼20 μm in size resulting from carbon removal; and smaller open pores of 2.1 μm remaining in the sintered SiC shell. The total porosity is around 70% and the bulk density is 0.93 mg/m3. The bending and compressive strengths are 26 MPa, and 105 MPa, respectively.

Effect of Y<sub>2</sub>O<sub>3</sub> addition on the properties of mullite bonded porous SiC ceramics prepared by an infiltration technique

espanolSe han sintetizado materiales porosos de SiC-Mullita mediante la infiltracion de polvo prensado de SiC y Y2O3 con un precursor liquido de mullita, el cual con un tratamiento termico posterior a 1300-1500 ° C da lugar a los materiales porosos de SiC-Mullita. Se estudio el efecto del contenido de Y2O3 y la temperatura de sinterizacion en la constitucion mineralogica, en la microestructura, en el grado de oxidacion del SiC, la resistencia a la flexion, la porosidad total y su distribucion de tamano. Debido a la oxidacion y a la mejora en la formacion de los cuellos por la adicion de Y2O3, se alcanzan altos valores de resistencia, 49 MPa, para estos materiales porosos de SiC-Mullita con porosidad 28 % en volumen. EnglishMullite bonded porous SiC ceramics were synthesized by infiltrating a powder compact of SiC and Y2O3 with a liquid precursor of mullite which on subsequent heat treatment at 1300-1500 oC produced mullite bonded porous SiC ceramics. The effect of Y2O3 content and sintering temperature on phase composition, microstructure, oxidation degree of SiC, flexural strength, porosity and pore size distribution were studied. Due to enhance oxidation and well developed neck formation by the addition of Y2O3 a high strength 49 MPa was achieved for the porous mullite bonded SiC ceramics with porosity 28 vol %.

Development of elongated 6H-SiC grains in reaction-bonded porous SiC ceramics

Porous 6H-SiC ceramics were fabricated at a low temperature by an in situ reaction bonding process. After subsequent heat treatment, porous SiC ceramics with elongated 6H-SiC grains and improved strength were obtained. The results indicated that in situ growth of elongated 6H-SiC grains was caused by rare earth elements together with the liquid Ba–Al–Si–O system. The type and content of rare earth oxides had a large influence on the growth of elongated 6H-SiC grains.

Effect of Al(OH)<sub>3</sub> Addition on the Properties of SiC/Al<sub>2</sub>O<sub>3</sub> Composite Porous Ceramics

SiC/Al2O3 composite porous ceramics were prepared by an in situ reaction bonding technique and sintering in air with SiC and A1(OH)3 as starting materials. The pores in the ceramics were formed by stacking particles of SiC and A12O3. The surface of SiC was oxidized to SiO2 at high temperature. With further increasing the temperature, SiO2 reacted with A12O3 to form mullite. The reaction bonding characteristics, phase composition, and mechanical strength as well as microstructure of porous SiC ceramics were investigated.

Fabrication of mullite-bonded porous SiC ceramics via a sol–gel assisted in situ reaction bonding

Abstract In the present work, mullite-bonded porous SiC ceramics were fabricated using reaction bonding techniques. The morphologies, phase composition, open porosity, pore size distribution and mechanical strength of porous ceramics were examined as a function of alumina sources (calcined nano-sized alumina powder and alumina sol prepared from hydrolysis of aluminum isopropoxide) and contents. It was found that the addition of alumina in powder form effectively enhanced the strength and decreased the porosity. In contrast, when alumina was added in sol form, a reverse effect was observed. Moreover, it was revealed that when a portion of calcined alumina was replaced by alumina sol, the mechanical properties improved significantly (more than 30%) as well as porosity compared to the traditional method. Pore size distribution analysis showed that the dispersion of nanosize alumina powder and SiC micro-particles in alumina sol is strongly improved compared to mixing in ethanol.

A new approach for the net-shape fabrication of porous Si3N4 bonded SiC ceramics with high strength

Abstract In this study, Si 3 N 4 bonded porous SiC ceramics with high strength had been net-shapely fabricated by a new approach. In this approach, we proposed a two-step processing route composed of freeze casting and carbothermal reduction reactions in which carbon aerogels, derived from sol infiltration and pyrolysis, involved. The phase components, microstructures and properties of the prepared ceramics were investigated. The results showed that carbon aerogels with high apparent surface area had been completely reacted and new SiC and Si 3 N 4 grains had been produced. The porous ceramics with flexural strength of 164.3 MPa at 33% porosity and 80.5 MPa at 46% porosity were obtained, whose linear shrinkages were only 1.06% and 1.94% during the whole processing respectively.