Porous Silicon Carbide Ceramics Research Articles

Possibility of using boric acid and glutinous rice flour as additive for producing silicon carbide ceramic via pressureless solid-state sintering

This research aimed to investigate the possibility of using the mixture of boric acid and glutinous rice flour as a cost-efficient alternative additive for producing silicon carbide (SiC) ceramic sintered at 1800℃ via pressureless solid-state sintering. The results indicated that the addition of boric acid- glutinous rice flour mixture (10 wt%) in an appropriate ratio (0.5 by molar ratio) into the SiC mixture (90 wt%) enhanced the densification and flexural strength of sintered SiC samples. However, increasing boric acid/glutinous rice flour ratio reduces density and flexural strength. The 5.45 wt% carbon content was sufficient to remove the oxide layer of the SiC surface and improve the properties of SiC ceramic. In summary, the boric acid-glutinous rice flour mixture has a high potential to be used as an additive for producing dense and porous SiC ceramics via pressureless solid-state sintering.

Effect of pore size on the flexural strength of porous silicon carbide ceramics

Porous silicon carbide (SiC) ceramics with three different pore sizes (∼7, ∼45, and ∼98 μm) and three different porosities (∼40 %, ∼54 %, and ∼63 %) were prepared from mixtures of SiC, polysiloxane, boron carbide, phenolic resin, and polymer microbeads to investigate the effect of the pore size of porous SiC ceramics on their flexural strength. As the pore size increased from ∼7 to ∼98 μm, the flexural strength decreased from 116.3 to 80.8 MPa at a constant porosity of 40 %, owing to the increased defect size. The typical flexural strengths of porous SiC ceramics fabricated using ∼8 and ∼100 μm templates were 84.9 and 60.6 MPa respectively, at 54 % porosity; the flexural strengths were 58.7 and 43.9 MPa, respectively, at 63 % porosity. These results suggest that depending on the application, the mechanical properties of porous SiC ceramics can be successfully tuned by adjusting their pore size and porosity.

Reconstructed Hierarchical Pore Structure of Porous SiC Ceramics Using Micro‐X‐Ray Computed Tomography

Porous ceramics prepared by freeze casting are strongly appealing for filtration, separation, heat insulation, energy storage, and bioengineering. Herein, a unidirectionally freeze‐casting process is effectively adopted to fabricate porous silicon carbide (SiC) ceramics. The freezing rates of 0.29, 1.59, and 22.80 °C min−1 are presented at the bottom, which are simulated by finite‐element analysis. The pore structure characterize is systematically investigated through the 3D‐reconstructed micro‐X‐Ray computed tomography. Porous SiC ceramics with reasonable densification present the layer‐stacked texture and the hierarchical interconnected porous structure. The pore sizes are in the range of 30–40 μm. With the decreasing of freezing temperatures, the pore sizes reduce, while the number of channels increases. After sublimation of the fine ice crystals, there are a large number of channels (5–10 μm). The open porosities decrease from 82.3% to 78.2%, and a maximum compressive strength of 1.97 MPa is obtained. The pore size distribution is bimodal at the top and bottom of porous ceramics. The surface of porous SiC ceramics with obviously lamellar structure is connected.

Porous silicon carbide ceramics with directional pore structures by CVI combined with sacrificial template method

The porous silicon carbide ceramics with multilevel directional pores were prepared by chemical vapor infiltration (CVI) combined with sacrificial template method. Carbon fiber preform with specific structure was used as template, and silicon carbide was deposited by CVI process. Then, the carbon fiber template was oxidized and removed, so that the porous silicon carbide ceramics were obtained. The influence of deposition time on pore structure, mechanical properties and thermal properties was studied. Compared with the traditional pore-making process, the pores formed by sacrificial template method are more regular, which brings excellent mechanical properties and thermal insulation properties. The porosity and flexural strength of the sample deposited for 72 h can reach 66.00% and 67.52 MPa, respectively, the thermal conductivity can be kept below 1 W/(m·K) from room temperature to 1200 °C.

Effects of solid solution, grain size and porosity on the thermal conductivity of aluminum- and boron-added porous silicon carbide ceramics with in-situ grain growth

The effects of solid solution, grain size and porosity on the thermal conductivity of aluminum (Al)- and boron (B)-added porous SiC ceramics with in-situ grain growth were investigated. Al-added porous SiC ceramics showed higher porosity, higher solubility of Al in SiC lattice, and lower thermal conductivity with the range of 8.5 – 18.2 W/m⋅K. The higher solubility of Al in SiC lattice was considered as the reason of their low thermal conductivity. B-added porous SiC ceramics have lower porosity, lower solubility of B in SiC lattice, and higher thermal conductivity with the range of 30.6 – 79.0 W/m⋅K. The high thermal conductivity of B-added porous SiC ceramics would result from the low solubility of B in SiC lattice and the low porosity. The effect of grain size was found to increase the phonon mean free path for B-added SiC ceramics.

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

AbstractThe 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.

Controlling the electrical resistivity of porous silicon carbide ceramics and their applications: A review

AbstractPorous silicon carbide (SiC)‐based ceramics are widely used in numerous applications of technical importance owing to their exceptional structural (e.g., excellent chemical, mechanical, and thermal stability) and functional (e.g., controlled electrical resistivity) properties. Porous SiC with controlled electrical resistivity is required for various advanced applications, for example, power electronic devices, semiconductor processing parts, fusion reactors, thermoelectric energy conversion, electromagnetic shielding, and environmental applications such as heatable filters. The electrical properties of sintered porous SiC are significantly affected by its chemical composition, processing conditions, and microstructure. This article reviews the influence of certain critical factors, such as the polytype, doping conditions, porosity (%), additive composition (oxide additives, element additives, metal nitride/carbide additives, etc.), and processing conditions on the electrical resistivity of porous SiC. Novel applications of porous SiC with controlled electrical resistivity are also discussed in this review.

Porous SiC Ceramics with Controllable Porosity Achieved by Micro-sized Powder through Recrystallization Sintering

Porous silicon carbide (SiC) ceramics with various porosity and pore size were successfully fabricated by adjusting the particle size and content of fine powder via recrystallization sintering process. The effects of the addition of fine powder with different particle size on the micromorphology of porous SiC ceramic and the flexural strength were investigated. With the increase of the fine powder content from 5wt% to 20 wt%, the porosity and pore size decrease, and the crystal grains gradually become larger and tend to round. The neck area among grains increases, leading to the improvement of flexural strength. The mechanism for promoting the sintering of porous SiC ceramics lies in the evaporation-condensation process of fine powder at high temperature. The finer particles are easier to be evaporated to promote the growth of coarse grain and the development of grain neck, so that the porosity and pore size were smaller, and the flexural strength was enhanced. At the addition of F4 powder of 20 wt%, the porous SiC ceramics with the porosity and MFP size of 39.1% and 1.38 μm respectively, and the flexural strength of 54.49MPa were obtained.

Enhanced mechanical strength of SiC reticulated porous ceramics via addition of in-situ chopped carbon fibers

Due to the poor strength of silicon carbide reticulated porous ceramics prepared by polymer sponge replica method, vacuum infiltration using a low viscosity infiltration slurry is performed to fill the flaws created during pyrolysis of the polymer sponge to improve the compressive strength. However, the flexural strength and thermal shock resistance of the porous silicon carbide ceramic are not significantly improved through this method. This paper investigates the improvement in thermal shock resistance and flexural strength of porous silicon carbide ceramics through the addition of different lengths and amounts of carbon fibers to the infiltration slurry. With the addition of 3 mm long carbon fibers at a concentration of 0.5 wt% to the infiltration slurry, the flexural strength of the porous silicon carbide ceramic was improved by 142%. Moreover, the thermal shock resistance of the SiC specimens was effectively enhanced through the addition of carbon fibers, as evidenced by a 74.4% increase in the residual strength ratio of the specimens. Due to the low thermal expansion coefficient of the carbon fibers, compressive residual stresses were generated in coating layers of SiC RPCs after sintering, which was inhibited the propagation of cracks in the coating layers. Thus, adding carbon fibers to the infiltration slurry can open more possibilities for application of SiC reticulated porous ceramics.

Low-temperature sintering of a porous SiC ceramic filter using water glass and zirconia as sintering aids

The fabrication of porous silicon carbide (SiC) ceramics prepared by existing reaction bonding technology requires high-temperature sintering beyond 1400 °C. We used low-cost industrial grade water glass and zirconia as sintering aids to prepare porous SiC ceramic filters to reduce the sintering temperature while improving the commerciality. Additionally, we investigated the effects of different sintering temperatures and the addition of water glass on the samples. Loading water glass and zirconia reduced the sintering temperature to 600 °C. The additive-loaded ceramic filter sintered at 600 °C exhibited an average pore size of 0.58 μm, a porosity of 47.4%, a bending strength reaching 56 MPa, a pure water permeance exceeding 4800 L m−2h−1bar−1, and good acid corrosion resistance. In addition, the SiC filter prepared at 1000 °C showed good chemical resistance and an average pore size of 0.75 μm as well as mechanical strength and pure water permeance of over 80 MPa and 6294 L m−2h−1bar−1, respectively. The low-cost, high-performance porous SiC ceramic filter exhibits broad commercial application prospects in water treatment.

Structural Properties of Porous Silicon Carbide Bonded with Bentonite, Prepared by a Gas-Emitting Chemical Reaction Method

Direct foaming and bonding techniques were used to fabricate porous silicon carbide ceramics. Silicon carbide is characterized by a high melting temperature; it is always referred to as being bonded with low melting point materials such as bentonite. The technique of generating gases from the reaction of hydrochloric acid HCl with aluminum was used in silicon carbide-bentonite suspension, and by means of which high porosity bodies were prepared by adding different bentonite (10,15,20 and 25 wt.%). After molding them, it is dried and fired at 1100°C. The results of the X-ray diffraction analysis was founded that the main phases appearing after the firing process are α and β–SiC with quartz and mullite, where a part of (β–SiC) is transformed to (α-SiC) and transformation a part of Quartz to Tridymite, which means an increase in the stability of the crystalline phases. The porosity of specimens was decreased with bentonite increases after the firing process, and the values ranged between 62.3% and 72.3%. The compressive strength was increased with bentonite additions increases in the range from 4.35 MPa to 5.08 MPa. From the above results, it is clear that this type of porous (SiC-bentonite) composites can be used in fluid filtration with high efficiency, especially at high temperatures.

Effects of carbon and silicon on electrical, thermal, and mechanical properties of porous silicon carbide ceramics

The properties of porous SiC ceramics were investigated as a function of Si/C content. The addition of C to SiC decreased the electrical resistivity, thermal conductivity, and compressive strength of the porous SiC ceramics for an increase in C content from 0 to 50 wt%. In contrast, the addition of Si to SiC increased both electrical resistivity and compressive strength and decreased thermal conductivity for an increase in Si content from 0 to 30 wt%. The addition of 5 wt% C increased the electrical conductivity, compressive strength, and thermal conductivity of porous SiC ceramics because of the partial densification of SiC by the removal of silica via carbothermal reduction. The current results suggest that both the electrical and thermal conductivities of porous SiC ceramics can be adjusted in the range of 9.0 × 10−2 –3.9 × 101 Ω cm and 1.3–31.4 W/(m·K), respectively, by controlling the Si or C content.

Influence of clay content on microstructure and flexural strength of in situ reaction bonded porous SiC ceramics

The aim of the present study is to reveal the effects of clay contents on in situ reaction bonding of SiC, phase development, microstructure, porosity and mechanical properties of porous SiC ceramics. Porous silicon carbide (SiC) ceramics are prepared in air at 1400 °C for 1 h using alumina as bond phase additive and variable amount of clay as sintering aids and compared their properties. The ceramics prepared with only alumina additive obtained with flexural strength of 28 MPa at porosity 38 vol% without any characteristic peak for mullite. The mullitization was almost completed at 1400 °C with addition of 3 wt% clay. It was found that the addition of clay strongly promoted the phase transformation towards mullite at lower temperature. A high flexural strength of 72 MPa was achieved at porosity level of 33 vol% in the sample prepared with 5 wt% clay due to the enhancement of necks at the contacting points.

Relationship between bending strength of bulk porous silicon carbide ceramics and grain boundary strength measured using microcantilever beam specimens

The relationship between the bending strength of bulk porous SiC ceramics and the grain boundary strength measured using microcantilever beam specimens of SiC bicrystals was investigated. The average value of the grain boundary strength was 39.2 GPa, and its higher value was roughly equal to that derived using an ab-initio calculation. The strengths of the specimens having only one neck were estimated by analyzing the effect of the specimen size on the strength of bulk porous SiC ceramics and by also analyzing the grain boundary strength and the stress concentration at the neck surface. The estimated strengths were generally consistent of the order of several hundred MPa, meaning that the strength of porous SiC ceramics should be dependent on the stress concentration at the neck and the grain boundary strength. Furthermore, they were in a better agreement using smaller neck curvature, smaller neck diameter, and lower grain boundary strength.

Effects of SDBS and ZrO2 additives on the microstructure and properties of silicon-bonded SiC porous ceramics

Porous silicon-bonded silicon carbide (SBSC) ceramics were prepared under argon atmosphere, with silicon as pore former and bonding material, simultaneously, sodium dodecyl benzene sulfonate (SDBS) and ZrO2 as sintering additives, the effects of SDBS and ZrO2 on the porosity, pore size, mechanical, physical and thermal properties and microstructures were investigated. The results suggested that suitable content of SDBS and ZrO2 could not only effectively lower the sintering temperature to 1450 °C due to the sticky flow of molten silicon, but also increase the pore structure and improve the bending strength. The reason for this is that SDBS decomposed into Na2O which reacted with ZrO2 and impurity SiO2, which was the native oxide film on the surface of SiC particles, to form a bonding phase between SiC particles to improve the bending strength; meanwhile, the disappearances of impurity SiO2 would benefit the bond of molten silicon and silicon carbide particles, and silicon melt leaving pores in its original position to increase the pore structure. The optimal apparent porosity, bending strength, average pore size, gas permeance and residual bending strength after thermal shock cycles of SBSC porous ceramic sintered at 1450 °C with 5 wt% SDBS and 6 wt% ZrO2 were 38.33%, 55.4 MPa, 11.3 μm, 106.4 m3/m2·h·kPa and 28.2 MPa, respectively.

Preparation of Biomorphic Porous SiC Ceramics from Bamboo by Combining Sol-Gel Impregnation and Carbothermal Reduction.

This study investigated the feasibility of using bamboo to prepare biomorphic porous silicon carbide (bio-SiC) ceramics through a combination of sol–gel impregnation and carbothermal reduction. The effects of sintering temperature, sintering duration, and sol–gel impregnation cycles on the crystalline phases and microstructure of bio-SiC were investigated. X-ray diffraction patterns revealed that when bamboo charcoal–SiO2 composites (BcSiCs) were sintered at 1700 °C for more than 2 h, the resulting bio-SiC ceramics exhibited significant β-SiC diffraction peaks. In addition, when the composites were sintered at 1700 °C for 2 h, scanning electron microscopy micrographs of the resulting bio-SiC ceramic prepared using a single impregnation cycle showed the presence of SiC crystalline particles and nanowires in the cell wall and cell lumen of the carbon template, respectively. However, bio-SiC prepared using three and five repeated cycles of sol–gel impregnation exhibited a foam-like microstructure compared with that prepared using a single impregnation cycle. Moreover, high-resolution transmission electron microscopy and selected area electron diffraction revealed that the atomic plane of the nanowire of bio-SiC prepared from BcSiCs had a planar distance of 0.25 nm and was perpendicular to the (111) growth direction. Similar results were observed for the bio-SiC ceramics prepared from bamboo–SiO2 composites (BSiCs). Accordingly, bio-SiC ceramics can be directly and successfully prepared from BSiCs, simplifying the manufacturing process of SiC ceramics.

Thermal Shock Resistance of Porous Silicon Carbide Ceramics Prepared Using Clay and Alumina as Additives

ABSTRACTPorous silicon carbide ceramics were prepared by an in situ reaction bonding process using clay and alumina as additives. The effects of alumina additive, pore former on phase composition, microstructure, flexural strength and thermal shock resistance of the ceramics were studied. Thermal shock resistance of porous SiC ceramics due to cooling was evaluated as a function of quenching temperatures and quenching cycles using water- and air-quenching technique. It was observed that residual strength of the quenched samples in water decreased with increase in the quenching temperature but was almost independent of quenching cycles. In water quenching, the surface of the sample cooled almost instantly but the inside remained hot which created an uneven thermal profile and generated microcracks in the sample; as a result sudden reduction of flexural strength was observed. The results showed that flexural strength and thermal shock resistance properties of the ceramics prepared with alumina are better than those of the ceramics prepared without alumina and the material was found suitable for hot gas filtration application.

Fabrication of porous silicon carbide ceramics at low temperature using aluminum dihydrogen phosphate as binder

In the present work, phosphate-bonded porous silicon carbide (SiC) ceramics were prepared by using aluminum dihydrogen phosphate as binder. The morphologies, phase composition, microstructures, mechanical strength, porosity and bulk density of the porous ceramics were investigated. It was found that the aluminum dihygrogen phosphate binder bonding method was a novel and low cost process to prepare near net-shape and high strength porous SiC ceramics at low temperature. The bonding mechanism was A type and B type aluminum metaphosphate which formed from aluminum dihygrogen phosphate during heat treatment bonded the SiC particles together to form porous ceramics. With temperature increasing, the phase of B type gradually transformed into A type aluminum metaphosphate, and the transformation was completed at 900 °C. The A type aluminum metaphosphate began to decompose into AlPO4 and P2O5 when the sintered temperature higher than 908 °C. The flexural strength of the porous SiC ceramics reached the maximum at 900 °C and decreased sharply with temperature increasing. The decomposition of A type aluminum metaphosphate made the strength reduce. The bulk density increased with aluminum dihydrogen phosphate content and decreased with temperature, in contrast, the porosity decreased with increasing binder content and temperature.

Effects of aluminum, boron and carbon additives on microstructure of porous silicon carbide ceramics and their properties

Abstract Porous SiC ceramics with in-situ plate-like grain growth were fabricated by pressureless-sintering using Al, B and C additives, and the effects of Al, B and C additives on the microstructure of porous SiC ceramics and their properties were investigated. Whereas the porous SiC ceramics sintered with B-C additives mostly consisted of fine SiC grains without grain growth, Al additives with B-C system enhanced plate-like grain growth of SiC in the porous SiC ceramics. Notably, the porous SiC ceramics sintered with 0.1 wt% Al and B-C additives consisted of plate-like SiC grains of around 10-20 μm, and the plate-like SiC grains stuck out of the walls of pores and the sintered surface in the porous SiC ceramics. XRD results suggested that the plate-like SiC grains formed in the porous SiC ceramics sintered with Al-B-C additives mainly consisted from 6H (α-SiC) phase. Compressive strength and thermal conductivity of the porous SiC ceramics sintered with 0.1 wt% Al and B-C additives at room temperature were 50 MPa and 10 W/m·K, respectively. From these results, it is concluded that high performance porous SiC ceramics with unique morphology of entangled large plate-like SiC grains were successfully achieved based on in-situ grain growth.

Characterization and mechanism of porous silicon carbide ceramics processed by high temperature chemical corrosion

Characterization and mechanism of porous silicon carbide ceramics processed by high temperature chemical corrosion