Achieving high permeability of in-situ formed lightweight Cf/SiC(rGO)px/SiC porous ceramics with improved interfacial compatibility for transpiration cooling

Making lightweight porous ceramics with excellent permeability applied for transpiration cooling is still challenging. Herein, an ingenious fabrication method is proposed to successfully prepare C_f/SiC(rGO)_px/SiC porous ceramics possessing low density, high permeability and satisfactory mechanical properties. The introduction of carbon fibers for constructing channels and SiC(rGO)_p with three-dimensional (3D) honeycomb cellular net-like structure, could effectively decrease density and improve porosity. Meanwhile, self-supporting porous skeleton, high open porosity and uniform pores distribution contribute to brilliant permeability of the products. Good interfacial compatibility among SiC(rGO)_p, carbon fibers and β-SiC/SiOxCy/C_free matrix, as well as toughening effects of carbon fibers are beneficial for enhancing fracture toughness and compressive strength. Particularly, C_f/SiC(rGO)_p0.6/SiC porous ceramics exhibit low density (1.12 g·cm^?3), low linear shrinkage (3.22%), especially high permeability (1.36 ×10^?7 mm²), satisfactory fracture toughness (1.77 MPa·m^1/2), excellent hardness (3.88 GPa) and compressive strength (6.41 MPa), focusing on potential applications as coolant medium in transpiration cooling.     

Permeability and corrosion resistance of porous SiOC-bonded SiC ceramics prepared by the preceramic polymer

Porous SiC ceramics have been used in high temperature flue gas filtration fields because of their excellent properties such as high strength, high temperature resistance, corrosion resistance, and long service time. This work reports the porous SiOC-bonded SiC ceramics prepared at low temperature. The properties of porous SiC ceramics were first investigated with silicone resin content from 10 to 25 wt%, and then the effects of different pore-forming agent contents on the behaviors of porous SiC ceramics were discussed by adjusting poly (methyl methacrylate) PMMA microbeads from 5 to 20 wt%. The prepared porous SiC ceramics showed apparent porosity from 17.3% to 57.7%, compressive strength from 6 to 216 MPa, and Darcy permeability k₁ ranging from 7.02 × 10⁻¹⁴ to 1.45 × 10⁻¹² m². The corrosion behavior of porous SiC ceramics was investigated in acidic and alkaline media. The porous SiC ceramics showed better corrosion resistance in acidic solutions.     

Fabrication and properties of SiC porous ceramics using a polyurethane preparation process

SiC porous ceramics can be prepared by introducing the polyurethane preparation method into the production process of ceramic biscuits, followed by sintering at 1300?°C for 2?h under N₂ flux after the cross-linking of polycarbosilane at 220?°C for 4?h in air. The microstructures, mechanical properties and infiltrations of the SiC porous ceramics are investigated in detail. The best dispersal effect comes from the SiC slurry with xylene as the solvent and a mixture of Silok®7096 (1?wt%) and Anjeka®6041 (4?wt%) as the dispersant. The compressive strength of SiC porous ceramics with high porosity (69.53%) reaches 16.9?MPa. The heat treatment can increase infiltration, the rate of which (4.296?×?10^?7 mm²) after the heat treatment at 750?°C in air is approximately two times faster than that before the heat treatment. The SiC porous ceramics fabricated in this study will have potential application in active thermal protection systems.     

Performance study of foam ceramics prepared by direct foaming method using red mud and K-feldspar washed waste

In this study, foam ceramics were prepared via a direct foaming method at high temperatures (1080–1120 °C), using red mud (RM) and K-feldspar washed waste (KFW) as the raw materials and SiC as the foaming agent, respectively. The chemical compositions and crystalline phases of the raw materials as well as the structural and mechanical properties of the foam ceramics were investigated. By adjusting the formulation and sintering process parameters, the porous structure of the foam ceramics could be effectively modulated. In addition to some residual crystalline phases in the raw materials, new phases, including rutile (TiO₂) and anorthite (CaAl₂Si₂O₈), were generated in foam ceramics. The compressive strength of the foam ceramics decreased with an increase in the KFW/RM ratio and sintering temperature, which was mainly related to the low density of the foam ceramics and the poor support of the pore walls to the structure. Among all the foam ceramics investigated, the foam ceramic with the KFW/RM ratio of 1:1, SiC content of 1 wt%, sintering temperature of 1100 °C and sintering time of 60 min showed the best overall performance with a bulk density, an apparent porosity, an average pore size and a compressive strength of 0.77 g/cm³, 61.89%, 0.52 mm, and 3.64 MPa, respectively. Its excellent porous structure and mechanical properties rendered it suitable for application as insulation materials or decorative materials for building partition walls.     

Preparation and properties of high-porosity ZrB2-SiC ceramics by water-based freeze casting

The development of novel cermet composites based on porous ceramics with high porosity, interconnected pore structure and good mechanical property has attracted considerable attention in engineering application. In this work, water-based freeze casting process was employed to fabricate ZrB₂-SiC porous ceramic with aligned lamellar-channels structure using PAA-NH₄ as the dispersant. The results revealed that the well-dispersed suspension with best rheological behavior was obtained using 1.0 wt% PAA-NH₄ at pH 9. The crack-free porous ceramic exhibited small volume shrinkage ranging from 2.59 % to 1.87 %. By varying the solid loading, the fabricated samples displayed a tailored porosity ranging from 76.12% to 59.37% and an excellent compressive strength of 7 MPa to 78 MPa. After oxidation, the samples displayed a decreased porosity and an increased compressive strength. The ZrB₂­SiC porous ceramic fabricated in this work will be a promising candidate for the framework of cermet composite.     

Hierarchical Structure of Porous Silicon Nitride Ceramics with Aligned Pore Channels Prepared by Ice‐Templating and Nitridation of Silicon Powder

A unique hierarchical porous structure of silicon nitride ceramic with 76.5% porosity is fabricated by combining an ice‐templating method and nitridation for a silicon powder. The porous silicon nitride ceramics were composed of a lamellar structure with aligned pore channels and ceramic walls filled with fibrous whiskers. This study is focused on the influences of freezing rate on the microstructures and properties of the silicon nitride ceramics. The properties were characterized by compressive strength and gas permeability, which were shown to vary with controlled microstructure. The compressive strength and the permeability reached up to 32.2 MPa and 0.035^?12 m², respectively.     

Controllable preparation of porous ZrB2–SiC ceramics via using KCl space holders

In this work, a novel and facile technique based on using KCl as space holders, along with partial sintering (at 1900 °C for 30 min), was explored to prepare porous ZrB₂–SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB₂–SiC samples possess high porosity of 45–67%, low average pore size of 3–7 μm, high compressive strength of 32–106 MPa, and low room temperature thermal conductivity of 13–34 W m⁻¹ K⁻¹. The porosity, pore structure, compressive strength and thermal conductivity of porous ZrB₂–SiC ceramics can be tuned simply by changing KCl content and its particle size. The effect of porosity and pore structure on the thermal conductivity of as-prepared porous ZrB₂–SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB₂–SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated.     

Recycling of waste red mud for fabrication of SiC/mullite composite porous ceramics

SiC/mullite composite porous ceramics were fabricated from recycled solid red mud (RM) waste. The porous ceramics were formed using a graphite pore forming agent, RM, Al(OH)₃ and SiC in the presence of catalysts. The influence of firing temperature and the pore-forming agent content on the mechanical performance, porosity and the microstructure of the porous SiC ceramics were investigated. Optimal preparation condition were determined by some testing. The results indicated that the flexural strength of specimens increased as a function of firing temperature and a reduction in graphite content, which concomitantly decreased porosity. The ceramic prepared under optimal conditions having 15?wt% graphite and sintered at 1350?°C, demonstrated excellent performance. Under optimal preparation conditions the flexural strength and porosity of the ceramic were 49.4?MPa and 31.4%, respectively. Scanning electron microscopy observation result showed that rod-shape mullite grains endowed the samples with high flexural strength and porosity. X-ray diffraction analysis indicated that the main crystallization phases of the porous ceramics were 6H-SiC, mullite, cristobalite and alumina. This work demonstrates that RM can be sucessfully reused as a new raw material for SiC/mullite composite porous ceramics.     

Effect of technological parameters on densification of reaction bonded Si/SiC ceramics

Si/SiC composite ceramics was produced by reaction sintering method in process of molten silicon infiltration into porous C/SiC preform fabricated by powder injection molding followed by impregnation with phenolic resin and carbonization. To optimize the ceramics densification process, effect of slurry composition, debinding conditions and the key parameters of all technological stages on the Si/SiC composite characteristics was studied. At the stage of molding the value of solid loading 87.5% was achieved using bimodal SiC powder and paraffin-based binder. It was found that the optimal conditions of fast thermal debinding correspond to the heating rate of 10?°C/min in air. The porous C/SiC ceramic preform carbonized at 1200?°C contained 4% of pyrolytic carbon and ～25% of open pores. The bulk density of Si/SiC ceramics reached 3.04?g/cm³, silicon carbide content was 83–85?wt.% and residual porosity did not exceed 2%.     

Permeability behavior of silicon carbide-based membrane and performance study for oily wastewater treatment

Porous SiC ceramic is considered as a suitable material for hot gas filtration, microfiltration, and many others industrial applications. However, full utilizations of porous SiC ceramics have been limited by high-processing costs. In this study, mullite-bonded porous SiC ceramics membranes were prepared using commercial SiC powder, alumina, clay, and different sacrificial pore formers. The effect of different pore formers on the microstructure, mechanical strength, porosity and pore size distribution, air, and water permeability of porous SiC ceramics were investigated. The average pore diameter, porosities, and flexural strength of the final ceramics varied in the range 3.7-6.5 µm, 38-50 vol. %, and 28-38 MPa, respectively, depending on the characteristics of pore former. The Darcian (k₁) and non-Darcian (k₂) permeability evaluated from air permeation behavior at room temperature was found to vary from 1.48 × 10⁻¹³ to 4.64 × 10⁻¹³ m² and 1.46 × 10⁻⁸ to 6.51 × 10⁻⁸ m, respectively. All membranes showed high oil rejection rate (89%-93%) from feed wastewater with oil concentration of 1557 mg/L. The membrane with porosity ~48 vol% and mechanical strength 31.5 MPa showed and highest pure water permeability of 13 298 Lm⁻²h⁻¹bar⁻¹.     

Processing and properties of silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity

Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity were prepared by sintering nano-SiC powder-carbon black template compacts at 600–1200 °C for 2 h in air. The microstructure of the silica-bonded porous nano-SiC ceramics consisted of SiC core/silica shell particles, a silica bonding phase, and hierarchical (meso/macro) pores. The porosity and thermal conductivity of the silica-bonded porous nano-SiC ceramics can be controlled in the ranges of 8.5–70.2 % and 0.057–2.575 Wm⁻¹ K⁻¹, respectively, by adjusting both, the sintering temperature and template content. Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity (0.057 Wm⁻¹ K⁻¹) were developed at a very low processing temperature (600 °C). The typical porosity, average pore size, compressive strength, and specific compressive strength of the porous nano-SiC ceramics were ∼70 %, 50 nm, 2.5 MPa, and 2.7 MPa·cm³/g, respectively. The silica-bonded porous nano-SiC ceramics were thermally stable up to 1000 °C in both air and argon atmospheres.     

High gas permeability and directional channels of mullite porous ceramics prepared by pectin-based freeze-drying method

New gel system for preparing mullite porous ceramics by gel-casting freeze-drying was proposed, using pectin as gel source and alumina and silica as raw materials. Directional channels were formed due to sublimation of water during freeze-drying and decomposition of pectin during high temperature sintering to prepare porous mullite ceramic membranes. Effects of solid content on the properties of mullite ceramics in terms of phase composition, microstructure, apparent porosity, bulk density, pore size distribution, compressive strength, thermal conductivity, pressure drop, and gas permeability were investigated. It was found that prepared porous mullite possessed high apparent porosity (56.04%–75.34%), low bulk density (.77–1.37 g/cm³), uniform pore size distribution, relatively high compressive strength (.61–3.03 MPa), low thermal conductivity (.224–.329 W/(m·K)), high gas permeability coefficient (1.11 × 10⁻¹⁰–4.73 × 10⁻¹¹ m²), and gas permeance (2.18 × 10⁻²–9.32 × 10⁻³ mol⋅m⁻²⋅s⁻¹⋅Pa⁻¹). These properties make prepared lightweight mullite ceramic membranes promising for application in high temperature flue gas filtration. Proposed gel system is expected to provide a new route to prepare porous ceramics with high porosity and directional channels.     

Influence of additive contents on the properties of SiC ceramic membranes and their performance in oil-water separation

The high processing cost is the key challenge for the economic industrial use of SiC membrane The aim of this research was to fabricate mullite bonded porous SiC ceramic membrane at low temperature from novel combinations of fly ash and alumina (FA) in weight ratio 44.5:55.5 as sintering additives. The influences of FA and pore former content on the porosity, morphology, crystalline phase composition, mechanical performance, permeability properties were investigated. The membrane prepared at 1300°C using 20 wt% FA showed pure water permeability 3690 Lm⁻²h⁻¹bar⁻¹ and exhibited high oil removal efficiency of ∼ 98% from the synthetic oil-water emulsion having oil concentration of 1000 mg/L. The corrosion behaviour of silicon carbide membrane in the strong acid and alkali solution and its mechanism were investigated. The utilization of fly ash successfully reduced the raw material cost and sintering temperature and the use of alumina reduced the amount of oxidation of SiC as well as increased the amount of mullite bond phase which resulted excellent mechanical strength to the final ceramics.     

Effects of inert filler addition on the structure and properties of porous SiOC ceramics derived from silicone resin

Porous SiOC ceramics were obtained from a new self-blowing precursor silicone resin DC217, by pyrolysis at 1200 °C in argon. Silicon carbide powders were incorporated into the silicone resin as inert fillers. The effects of the mean particle size of SiC fillers on the porosity, compressive strength and microstructure of the porous ceramics were investigated. With the mean particle size of SiC powders increasing from 5 μm to 10 μm, the porosity (total and open) of the porous ceramic increased and the compressive strength decreased. However, the porosity, compressive strength and cell morphology of the porous ceramics showed no evident changes when the mean particle size of fillers increased from 10 μm to 15 μm. Micrographs indicated that, when the mean particle size of fillers exceeded 5 μm, the porous ceramics could have a well-defined and regular pore structure. Furthermore, comparing with the porous ceramics which fabricated under the same condition with the SiOC powders as fillers, the cell morphology was similar. But the compressive strength and the oxidation resistance of the porous ceramics with SiC powders as fillers were much better.     

Low thermal conductivity in porous SiC–SiO2–Al2O3–TiO2 ceramics induced by multiphase thermal resistance

The introduction of multiple heterogeneous interfaces in a ceramic is an efficient way to increase its thermal resistance. Novel porous SiC–SiO₂–Al₂O₃–TiO₂ (SSAT) ceramics were fabricated to achieve multiple heterogeneous interfaces by sintering equal volumes of SiC, SiO₂, Al₂O₃, and TiO₂ compacted powders with polysiloxane as a bonding phase and carbon as a template at 600 °C in air. The porosity could be controlled between 66% and 74% by adjusting the amounts of polysiloxane and the carbon template. The lowest thermal conductivity (0.059 W/(m·K) at 74% porosity) obtained in this study is an order of magnitude lower than those (0.2–1.3 W/(m·K)) of porous monolithic SiC, SiO₂, Al₂O₃, and TiO₂ ceramics at an equivalent porosity. The typical specific compressive strength value of the porous SSAT ceramics at 74% porosity was 3.2 MPa cm³/g.     

Three-dimensional (3D) flexible nanofibrous network knitting on hierarchical porous architecture

Hierarchical structured porous ceramics have attracted tremendous research interests because of their numerous excellent properties including robust mechanical strength and large surface area. In this work, silicon carbide (SiC)-based porous ceramics with three levels of pore hierarchy are fabricated from silicon particle-stabilized foams and a subsequent one-step calcination after they were embedded with coke. Three-dimensional (3D) flexible nanofibrous network is adhered and wrapped on cell walls of porous ceramics, which is readily fine-tuned and tailored by the temperature to provide optimized pore structure. The resultant SiC-based porous ceramics present a density of 1.03 g/cm³ at a porosity of 72% with a large quantity of hierarchical micro- and macropores. This hierarchical structure leads to robust compressive strength (23.52 MPa) and large surface area (64.32 m²/g). The fabrication method is straightforward and sought-after, providing a facile technical route for advanced hierarchical porous ceramics used in filtration and catalysis fields.     

Low-temperature preparation of porous SiC ceramics using phosphoric acid as a pore-forming agent and a binder

Porous SiC ceramics combine the properties of both SiC ceramics and porous materials. Herein, we design a facile method via pressureless sintering at relatively low temperatures for the synthesis of porous SiC ceramics. In the synthesis process, phosphoric acid was used as the sintering additive that reacted with SiO₂ on the surface of SiC to form phosphates. The formed phosphates acted as a binder to connect the SiC particles. At a fixed temperature, the phosphates were partially decomposed and released a large amount of gas. This changed the pore structure of the ceramics and greatly improved their porosity. Finally, we obtained the porous SiC ceramics with high porosity and high strength. We investigate the effects of H₃PO₄ content on the phase composition, microstructure, porosity, mechanical properties and thermal expansion coefficient of the prepared porous SiC ceramics. It was shown that at the sintering temperature of 1200 °C, the highest porosity of the samples can reach 70.42% when the H₃PO₄ content is 25 wt%, and their bending strength reaches 36.11 MPa at room temperature when the H₃PO₄ content is 15 wt%. In addition, the porous SiC ceramics show good high-temperature stability with a bending strength of 42.05 MPa at 1000 °C and the thermal expansion coefficient of 3.966 × 10⁻⁶/°C.     

Corrosion behaviors of porous reaction-bonded silicon carbide ceramics incorporated with CaO

Reaction-bonded SiC (RBSC) porous ceramics were fabricated at 1450?°C in air by incorporating CaO using ZrO₂ as sintering aids, activated carbon as pore-forming agent, and mullite fibers as reinforcing agent. The effects of CaO content on the properties of the porous RBSC ceramics were studied. Corrosion behaviors of the prepared RBSC porous ceramics in different environments were also investigated. The optimal open porosity, bending strength, average pore size and gas permeability of the ceramics with 0.5% CaO were 40%, 22.5?MPa, 42.9?µm, and 2100?m³/m² h?kPa, respectively. A well-developed neck reaction-bonded by calcium zirconium silicate (Ca₃ZrSi₂O₉) was identified. The porous RBSC ceramics exhibited excellent corrosion resistance in acid and basic solutions. The anti-oxidation temperature of the porous RBSC ceramics could reach 1200?°C in air. The RBSC ceramics maintained the bending strength of 17.5?MPa after 60 cold-hot cycles in air (0–800?°C). The porous RBSC ceramics also exhibited relatively good corrosion resistance in molten salts (NaCl, Na₂SO₄ and CaCl₂). Melten NaOH can aggravate the reaction by breaking the SiO₂ layers on the SiC surface. Overall, these findings offer significant insights into expanding the applications porous RBSC ceramics incorporated with CaO.     

Preparation and properties of porous SiC–Al2O3 ceramics using coal ash

In this paper, we first reported that porous SiC–Al₂O₃ ceramics were prepared from solid waste coal ash, activated carbon, and commercial SiC powder by a carbothermal reduction reaction (CRR) method under Ar atmosphere. The effects of addition amounts of SiC (0, 10, 15, and 20 wt%) on the postsintering properties of as-prepared porous SiC–Al₂O₃ ceramics, such as phase composition, microstructure, apparent porosity, bulk density, pore size distribution, compressive strength, thermal shock resistance, and thermal diffusivity have been investigated. It was found that the final products are β-SiC and α-Al₂O₃. Meanwhile, the SEM shows the pores distribute uniformly and the body gradually contacts closely in the porous SiC–Al₂O₃ ceramics. The properties of as-prepared porous SiC–Al₂O₃ ceramics were found to be remarkably improved by adding proper amounts of SiC (10, 15, and 20 wt%). However, further increasing the amount of SiC leads to a decrease in thermal shock resistance and mechanical properties. Porous SiC–Al₂O₃ ceramics doped with 10 wt% SiC and sintered at 1600°C for 5 hours with the median pore diameter of 4.24 μm, room-temperature compressive strength of 21.70 MPa, apparent porosity of 48%, and thermal diffusivity of 0.0194 cm²/s were successfully obtained.     

Lightweight porous silica-alumina ceramics with ultra-low thermal conductivity

Thermal protection has always been an important issue in the energy, environment and aerospace fields. Porous ceramics produced by the particle-stabilized foaming method have become a competitive material for thermal protection because of their low density and low thermal conductivity. However, the study of porous ceramics for composite systems using particle-stabilized foaming method was relatively rare. Here, silica-alumina composite porous ceramics were prepared by particle-stabilized foaming method, which was achieved by tailoring the surface charges of silica and alumina through adjustment of the pH. Porous ceramics exhibited porosity as high as 97.49% and thermal conductivity (25 °C) as low as 0.063 W m^?1 K^?1. The compressive strength of porous ceramics sintered at 1500 °C with a solid content of 30 wt% could reach 0.765 MPa. Based on the light weight and excellent thermal insulation properties, the composite porous ceramic could be used as a potential thermal insulation material in the spacecraft industry.