Processing of microcellular silicon carbide ceramics with a duplex pore structure

A novel processing route for producing microcellular SiC ceramics with a duplex pore structure has been developed using a polysiloxane, carbon black, SiC, Al₂O₃, Y₂O₃, and two kinds of pore former (expandable microspheres and PMMA spheres). The duplex pore structure consists of large pores derived from the expandable microspheres and small windows in the strut area that were replicated from the PMMA spheres. The presence of these small windows in the strut area improved the permeability of the porous ceramics. The gas permeability coefficients of porous SiC ceramics were 0.13 × 10¹² m² for the porous SiC without PMMA spheres, 0.47 × 10¹² m² for the porous SiC with 10 wt% PMMA spheres, and 0.82 × 10¹² m² for the porous SiC with 20 wt% PMMA.     

Preparation of calcium hexaluminate porous ceramics by gel-casting method with polymethyl methacrylate as pore-forming agent

Calcium hexaluminate (CA₆) porous ceramics were prepared by gel-casting method, with α-Al₂O₃ and CaCO₃ as raw materials and polymethyl methacrylate (PMMA) microspheres as pore-forming agent. The effects of the amount of pore-forming agent PMMA microspheres on the phase composition, bulk density, apparent porosity, flexural strength, microstructure, thermal shock stability and thermal conductivity of CA₆ porous ceramics were systematically studied. The pores of CA₆ porous ceramics are mainly formed by the burning loss of PMMA microspheres and the decomposition of organic matter. Adding an appropriate amount of PMMA microspheres as pore-forming agent has a positive effect on the thermal shock stability of CA₆ porous ceramics. When the amount of pore-forming agent is 15 wt%, the volume density of CA₆ porous ceramics is 1.33 g/cm³, the porosity is 63%, the flexural strength is 13.9 MPa, the thermal shock times can reach 9 times, and the thermal conductivity is 0.293 W/(m·K), which can meet the application in refractory, ceramics or high temperature cement industries.     

Targeted design and analysis of microwave absorbing properties in iron-doped SiCN/Si3N4 composite ceramics

The SiCN(Fe)/Si₃N₄ composite ceramics containing C, β-SiC and α-Fe were designed by impregnating polysilazane followed by pyrolysis at 1–5 times with porous silicon nitride matrix. After the heat treatment, these new phases gradually formed and filled in the pores of the matrix to form a large whole. They bond together to facilitate the formation of conductive networks, which can improve the storage and transport capacity of the charge, thus improving the dielectric properties of the material. In this way, the highest dielectric and magnetic loss tangent values were achieved as 0.49 and 0.29 after third and fourth cycles, respectively. The electromagnetic wave can be absorbed ~80–90% in the range of 8–18 GHz with the thickness of 2 mm. The minimum reflectivity was ?12.5 dB at 16 GHz with the thickness of 5 mm, showing >90% electromagnetic wave absorption at this frequency.     

Characteristics of microbubbles generated by porous mullite ceramics prepared by an extrusion method using organic fibers as the pore former

Porous mullite ceramics with unidirectionally oriented pores were prepared by an extrusion method using rayon fibers as the pore formers and the characteristics of microbubbles generated by these porous ceramics were investigated. The 1200 mm long ceramics were tubular and of thick or thin types of 20–30 mm inner diameter and 30–50 mm outer diameter, respectively. The thin and thick samples had porosities of 47 and 49% and average pore radii of 7.8 μm. The gas permeabilities of the thick and thin samples were 4.1 × 10^?14 and 5.4 × 10^?14 m², respectively. Microbubbles were generated by introducing N₂ gas through the ceramic tube by immersing it into water. The minimum pressure (bubble point pressure) for generation of microbubbles was 20 kPa, much lower than for other bubble-forming methods. The average microbubble radii ranged from about 70 to 105 μm at flow rates of 0.15–0.25 L/min in the thin sample and 0.3–0.7 L/min in the thick sample. These bubble sizes are much smaller than calculated for a Fritz-type bubble such as generally formed by bubbling from pores and/or orifices. However, the present bubble sizes agree well with the calculated value based on nanobubbles, indicating that bubble formation occurs by mixing the gas with water in small pores. Since microbubbles enhance the dissolution rate of a gas phase in water, they are potentially useful for improving water environments, especially oxygen-deficient water. The effectiveness of gas dissolution in water was confirmed by determining the dissolution behavior of CO₂ gas using these porous ceramics.     

The sound absorption performance of the highly porous silica ceramics prepared using freeze casting method

Highly porous silica ceramics with unidirectional pores were prepared using the freeze casting method. By adjusting the solid content and freezing temperature, the porosity of the ceramics was tailored in the range of 78.20%-84.59% and pore size in the range of 8.4-71.4 μm, respectively. Sound absorption properties of porous silica ceramics was studied and the effect of structural factors was systematically investigated. The results showed that higher porosity and smaller pores of the porous ceramics favored the sound absorption in the entire sound wave frequency. By backing the sample with small pore size porous ceramics, the sound absorption property was enhanced, particularly in the low and medium frequency range, thus the sound absorption peak shifted towards lower frequency. The presence of air gap in the back would also favor sound absorption in low and medium frequency range. The as-fabricated porous silica ceramics owed excellent sound absorption properties due to their unidirectional pores and low flow resistances.     

The capillary and thermal performance of the porous silicon nitride ceramics with nearly spherical pore structure

The capillary and thermal performance of porous Si₃N₄ ceramics with nearly spherical pore structure has been investigated by altering the addition and diameter of pore-forming agent polymethyl methacrylate (PMMA) in this work. An exponential model is used to evaluate the liquid uptake capacity of porous Si₃N₄ ceramics. Porous Si₃N₄ ceramics fabricated by 5 μm PMMA with 40 wt.% addition possess the lowest capillary time constant and show the best capillary performance owing to the perfect balance between friction resistance and capillary force. The thermal conductivity of porous Si₃N₄ ceramics is significantly impacted by their porosity. Alexander model with an exponent of .96 is suitable for predicting the thermal conductivity of porous Si₃N₄ ceramics due to its R-squared up to .99. Moreover, with the addition and diameter of PMMA decrease, the flexural strength of porous Si₃N₄ ceramics increases. These results support the application of porous Si₃N₄ ceramics in the field of mass and heat transfer.     

Fabrication of porous forsterite-spinel-periclase ceramics by transient liquid phase diffusion process for high-temperature thermal isolation

Porous forsterite-spinel-periclase ceramics with low thermal conductivity were synthesized via a transient liquid phase diffusion process by using pre-synthesized pellets and fused magnesia powder. The effects of sintering temperature on the pore formation, phase composition, sintering behavior, and properties of the resulting porous ceramics were investigated. The pre-synthesized pellets had a porous structure and contained a large amount of cordierite and enstatite. During the sintering progress, the pellets were converted into a transient liquid phase, which diffused into the solid MgO matrix. The liquid phase diffusion reaction promoted forsterite and spinel formation, which resulted in the in-situ formation of large pores. At elevated temperatures, the liquid phase disappeared and a large number of well-developed grains were simultaneously precipitated from the liquid phase. Porous ceramics with thermal conductivities of 0.42–0.48 W/(m·K) and refractoriness under load values of 1588 °C and 1624 °C were obtained after sintering at 1600 °C for 3 h.     

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.     

Mechanical property and gas filtration behavior of porous reaction bonded Si3N4 with monodispersed PMMA as pore former

Porous Si₃N₄ ceramics were prepared by gel casting combined with a reaction bonding route using monodispersed PMMA as the pore former, and Isobam was used as a gel agent. With the PMMA addition varying from 0 to 20 wt.%, the bending strength was degraded from 94.0 to 39.1 MPa owing to the increased porosity and decreased bulk density. The β-Si₃N₄ prismatic grains and round pores introduced by the monodispersed PMMA micro balls would endow the samples with high strength, and the permeability of the resulting samples was increased obviously with the increase of PMMA addition. Flue gas filtration test exhibited that the filtration efficiency of the porous ceramics filter was not degraded with introducing of the PMMA pore former, even though the permeability was increased obviously. The block type of the sample with 20-wt.% PMMA additions during filtration was cake filtration, which indicates that the sample has the characteristic of being reusable after back-blowing in flue gas filtration applications. Porous Si₃N₄ with high strength and permeability fabricated via the reaction bonding route exhibits great potential for low-cost high-performance ceramics filters.     

Near‐Net‐Shaped Porous Ceramics for Potential Sound Absorption Applications at High Temperatures

We present an interesting processing route for obtaining alumina/mullite‐based ceramics with controlled porosity and airflow resistance leading to promising microstructures for application as sound absorbers. The use of ceramic materials aims for potential applications where high temperatures or corrosive atmospheres are predominant, e.g., in combustion chambers of gas turbines. For the production of the porous ceramics we combined freeze gelation and sacrificial templating processes to produce near‐net‐shaped parts with low shrinkage (<3%) based on environmental‐friendly and low cost conditions. The obtained microstructure presents a bimodal pore size distribution, with small pores derived from the freeze gelation process (~30 μm) connecting large pores (2–5 mm diameter) originated from the expanded polystyrene template particles. These connections, called “windows” in this study, show a significant impact on the sound absorption properties, allowing the pressure diffusion effect to take place, resulting in a significant improvement of the sound absorption coefficient. By varying the template particle content and the slurry solid content, it is possible to control the sound absorption behavior at different frequencies of the open‐celled ceramics. These ceramics feature a high open porosity, from 77% to 82%, combined with sufficient compressive strength ranging from 0.27 to 0.68 MPa and sound absorption coefficients of 0.30–0.99, representing a highly promising combination of properties for noise control and reduction at corrosive environments and high temperatures.     

Preparation and characterization of lotus ceramics with different pore sizes and their implication for the generation of microbubbles for CO2 sequestration applications

Four kinds of porous mullite ceramics, named lotus ceramics because of the similarity of their microstructure with lotus roots, were prepared by an extrusion method using rayon fibers of four different diameters (8.1, 9.6, 16.8 and 37.6 μm) as the pore formers. The physicochemical properties of these samples were characterized to test their applicability for the generation of microbubbles. The lotus ceramic samples contained pores of 9.4, 10, 15.6 and 30 μm size and porosities of 45–48%. SEM micrographs confirmed that the cylindrical pores were oriented unidirectionally along the extrusion direction and the degree of alignment was greater with larger fiber diameter. The permeability for gaseous CO₂ increased with increasing pore size from 3×10^?13 to 8×10^?13 m². The four lotus ceramic samples, a commercial air stone (72 μm) and two simple tubes (1000 and 3500 μm) were used to generate microbubbles in water under ambient conditions from a gas mixture of CO₂ and air. It was found that the bubble size could be decreased with bubblers of smaller pore size. In the bubble size measurements for pure CO₂ and air, the air bubbles were larger than the CO₂ bubbles due to partial dissolution of CO₂ into the water during bubbling. In order to generate smaller size bubbles using porous ceramic bubblers, the liquid must penetrate through the pores of the lotus ceramics before the gas is introduced into the system.     

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.     

Capillary rise properties of porous mullite ceramics prepared by an extrusion method using organic fibers as the pore former

Porous mullite ceramics with unidirectionally oriented pores were prepared by an extrusion method to investigate their capillary rise properties. Rayon fibers 16.5 μm in diameter and 800 μm long were used as the pore formers by kneading with alumina powder, kaolin clay, China earthen clay and binder with varying Fe₂O₃ contents of 0, 5 and 7 mass%. The resulting pastes were extruded into cylindrical tubes (outer diameter (OD) 30–50 mm and inner diameter (ID) 20–30 mm), dried at room temperature and fired at 1500 °C for 4 h. The bulk densities of the resulting porous ceramics ranged from 1.31 to 1.67 g/cm³, with apparent porosities of 43.2–59.3%. The pore size distributions measured by Hg porosimetry showed a sharp peak at 10.0 μm in the sample without Fe₂O₃ and at 15.6 μm in the samples containing Fe₂O₃; these pores, which arose from the burnt-out rayon fibers, corresponded to total pore volumes ranging from 0.24 to 0.34 ml/g. SEM showed a microstructure consisting of unidirectionally oriented pores in a porous mullite matrix. Prismatic mullite crystals were well developed on the surfaces of the pore walls owing to the liquid phase formed by the Fe₂O₃ component added to color the samples. The bending strengths of the tubular samples ranged from 15.6 to 26.3 MPa. The height of capillary rise, measured under controlled relative humidities (RH) of 50, 65 and 85%, was greater in the ceramics containing Fe₂O₃ than in those without Fe₂O₃, especially in the thinner samples. The maximum capillary rise reached about 1300 mm, much higher than previously reported. This excellent capillary rise ability is thought to be due to the controlled pore size, pore distribution and pore orientation in these porous mullite ceramics.     

Enhanced high‐temperature thermoelectric performance of CdO ceramics with randomly distributed micropores

CdO ceramics with randomly distributed micropores were synthesized by the conventional solid‐state reaction method via adding polymethyl methacrylate (PMMA) as the void forming agent. The introduction of microsized pores can significantly affect both electrical and thermal transport properties of the CdO matrix, and possible underlying mechanisms are discussed. Due to the dramatically reduced thermal conductivity, a highest ZT value of ~0.51 has been achieved in the porous CdO at about 1000 K, increased by about 52% as compared to that of the dense sample without PMMA addition. This work demonstrates that introducing microsized pores is a very simple, low‐cost, and efficient strategy to improve the high‐temperature thermoelectric performance of CdO ceramics.     

Porous yttria‐Stabilized Zirconia Ceramics Fabricated by Nonaqueous‐Based Gelcasting Process with PMMA Microsphere as Pore‐Forming Agent

Porous yttria‐stabilized zirconia (YSZ) ceramics were fabricated using tert‐butyl alcohol (TBA)‐based gelcasting with monodisperse polymethylmethacrylate (PMMA) microspheres as both pore‐forming agent and lubricant agent. The TBA‐based slurry of 50 vol% solid loading with excellent rheological properties appropriate for casting was successfully prepared by using a commercial polymer dispersant DISPERBYK‐163 as both dispersant and stabilizer. The distribution of the spherical pores made from PMMA microspheres was very homogeneous. Their average diameter decreased from 16.9 to 15.7 μm when the sintering temperature was increased from 1350°C to 1550°C. The compressive strength increased from 14.57 to 142.29 MPa and the thermal conductivity changed from 0.17 to 0.65 W/m·K when the porosity decreased from 71.6% to 45.1%. The results show that this preparation technology can make all the main factors controllable, such as the porosity, the size and shape of pores, the distribution of pores, and the thickness and density of pore walls. This is significant for fabricating porous ceramics with both high compressive strength and low thermal conductivity.     

Influence of the content of polymethyl methacrylate on the properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics are highly regarded as ideal materials for radomes due to their unique characteristics. However, the slurry used for the preparation of porous Si₃N₄ ceramics suffers from a low cure depth, making it challenging to fabricate ceramic components using DLP technology. In this study, porous Si₃N₄ ceramics were prepared by combining DLP technology with pore-forming agent method. The addition of polymethyl methacrylate (PMMA) powders with lower refractive index than that of Si₃N₄ powders can improve the penetration depth of ultraviolet light in the Si₃N₄ slurry. A systematic study was conducted to investigate the influence of the addition of PMMA powders on the properties of Si₃N₄ slurries and porous Si₃N₄ ceramics. When PMMA powders were added at 10 wt%, the slurry with a lowest viscosity of 0.13 Pa s (the shear rate is 30 s⁻¹) and cure depth of 40.0 μm (the exposure energy is 600 mJ/cm²) was obtained. With the increase of PMMA content, porous Si₃N₄ ceramics experienced a gradual decrease in both the flexural strength and bulk density, while the porosity increased from 14.41% to 27.62%. Specifically, when 20 wt% PMMA was added, the resulting porous Si₃N₄ ceramics had a lowest bulk density (2.41 g/cm³), a maximum porosity (27.62%), and a flexural strength (435.87 MPa). The study is of great significance in establishing an experimental foundation for fabricating porous Si₃N₄ ceramics by using DLP technology.     

Effects of mono-dispersed PMMA micro-balls as pore-forming agent on the properties of porous YSZ ceramics

Porous yttria-stabilized zirconia (YSZ) ceramics were successfully fabricated by the dry pressing method with different size (1.8–20 μm) and amount (2–60 vol.%) of mono-dispersed poly methyl methacrylate (PMMA) micro-balls. Different PMMA additions with different size and amount were investigated to achieve optimal thermal and mechanical properties. With increases of the amount of PMMA, the porosity of porous YSZ ceramics ranges from 7.29% to 51.6%, the flexural strength increases firstly and then decreases, and the thermal conductivity decreases continuously. With decreases of the diameter of PMMA micro-balls, the mean pore size and thermal conductivity of porous YSZ ceramics decrease, and the flexural strength of porous YSZ ceramics with same porosity increases firstly and then decreases. The porous YSZ ceramics with a higher porosity (18.44 ± 1.24%), the highest flexural strength (106.88 ± 3.2179 MPa) and low thermal conductivity (1.105 ± 0.15 W/m K) can be obtained when the particle diameter and the amount of PMMA are 5 μm and 20 vol.%, respectively.     

Preparation and properties of porous alumina ceramics with oriented cylindrical pores produced by an extrusion method

Porous alumina ceramics with unidirectionally-oriented pores were prepared by extrusion. Carbon fibers of 14 μm diameter and 600 μm length to be used as the pore-forming agent were kneaded with alumina, binder and dispersing agent. The resulting paste was extruded, dried at 110 °C, degreased at 1000 °C and fired at 1600 °C for 2 h. SEM showed a microstructure of dispersed highly oriented pores in a dense alumina matrix. The pore area in the cross section was 25.3% with about 1700 pores/mm². The pore size distribution of the fired body measured by Hg porosimetry showed a sharp peak corresponding to the diameter of the burnt-out carbon fibers. The resulting porous alumina ceramics with 38% total porosity showed a fracture strength of 171 MPa and a Young's modulus of 132 GPa. This strength is significantly higher than the reported value for other porous alumina ceramics even though the present pore size is much larger.     

Preparation and characterization of porous anorthite ceramics from red mud and fly ash

The porous anorthite ceramics with high porosity, good mechanical strength and low heat conductivity were prepared using red mud and fly ash as raw materials via the pore forming method. The effects of sintering temperature and fly ash on phase evolution, densification, compressive strength, thermal conductivity and microstructure of the ceramic materials were investigated. The results showed that the compressive strength of the porous ceramics had an obvious improvement with the increase in fly ash, and the densification and heat conductivity decreased firstly and then increased. In particular, specimen S2 containing 30 wt% red mud and 40 wt% fly ash sintered at 1150°C had the better performances. It had the water absorption of 18.18%, open porosity of 38.52%, bulk density of 1.29 g/cm³, compressive strength of 42.46 MPa, and heat conductivity of 1.24 W/m·K. X-ray diffraction analysis indicated that mullite, anorthite, α-quartz, and diopside ferrian were the dominant phases in the specimens. Scanning electron microscopy micrographs illustrated that plenty of open pores with strip shape and closed pores with axiolitic shape existed in the specimens. Furthermore, the existence of mullite could prevent crack propagation to enhance the energy of inter-granular fracture. It endowed the porous anorthite ceramics with high porosity, good compressive strength, and low heat conductivity.     

Nano-SiC-decorated Y2Si2O7 ceramic for enhancing electromagnetic waves absorption performance

To improve the electromagnetic (EM) wave absorption performance of rare earth silicate in harsh environments, this work synthesized dense SiC–Y₂Si₂O₇ composite ceramics with excellent EM wave absorption properties by using the polymer permeation pyrolysis (PIP) process, which introduced carbon and SiC into a porous Y₂Si₂O₇ matrix to form novel composite ceramics. SiC–Y₂Si₂O₇ composite ceramics with different numbers of PIP cycles were tested and analysed. The results show that the as-prepared composites exhibit different microstructures, porosities, dielectric properties and EM wave absorption properties. On the whole, the SiC–Y₂Si₂O₇ composite ceramics (with a SiC/C content of 29.88 wt%) show superior microwave absorption properties. The minimum reflection loss (RL_min) reaches ?16.1 dB when the thickness is 3.9 mm at 9.8 GHz. Moreover, the effective absorption bandwidth (EAB) included a broad frequency from 8.2 GHz to 12.4 GHz as the absorbent thickness varied from 3.15 mm to 4.6 mm. In addition, the EM wave absorption mechanism was analysed profoundly, which ascribed to the multiple mediums of nanocrystalline, amorphous phases and turbostratic carbon distributed in the Y₂Si₂O₇ matrix. Therefore, SiC–Y₂Si₂O₇ composite ceramics with high-efficiency EM wave absorption performance promise to be a novel wave absorbing material for applications in harsh environments.