MgO as a non-pyrolyzable pore former in porous membrane supports

The gas permeability of Y_0.03Zr_0.97O₂ (3Y-TZP) porous supports from thermoplastic feedstocks has been improved by adding MgO as a non-pyrolyzable pore former. Common pyrolyzable pore formers such as graphite often produce tortuous and narrow pore channels with low gas permeability, limiting the performance of oxygen transport membranes or other membranes relying on gas transport to the active membrane surface. Thermoplastic feedstocks for extrusion of tubular 3Y-TZP supports were prepared with four different amounts of pyrolyzable pore formers and/or MgO as non-pyrolyzable pore former. The MgO was removed after sintering by leaching in acetic acid. With this technique we obtained porosities above 70 vol% and gas permeabilities above 3?10^?14?m². Compared to samples with only pyrolyzable pore formers, the non-pyrolyzable pore former increases the gas permeability and reduces the tortuosity.     

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.     

Microstructure and permeability of porous Si3N4 supports prepared via SHS

Porous Si₃N₄ ceramics with tailored pore structures were fabricated via self-propagating high temperature synthesis (SHS) using Polymethylmethacrylate (PMMA) as pore forming agent. The pore structures, mechanical properties and permeation performance of porous Si₃N₄ ceramics were investigated by altering the particle sizes and amount of PMMA. With the increasing content of PMMA, the flexural strength of samples decreased from 102.5 MPa to 9.4 MPa. The tortuosity which showed irregular variation affected gas permeability directly. The samples with 20 wt% content of PMMA exhibited the maximum Darcian and non-Darcian constants with the smallest tortuosity. Moreover, the comparison of permeability coefficients with other ceramics via different pore forming methods in literature was presented. The specimens exhibited great permeability due to the large pore sizes created by the elongated and coarsened β-Si₃N₄ grains during the SHS process, providing a low-cost and environmentally friendly method for preparing high permeability porous Si₃N₄ supports.     

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.     

Permeability of the porous Al2O3 ceramic with bimodal pore size distribution

Porous Al₂O₃ ceramics with bimodal pore size distribution were fabricated by partial sintering with monodispersed PMMA micro balls as pore agent. The porosity of the fabricated porous Al₂O₃ is increased with content of the pore agent increase, the bulk density and bending strength are decreased, accordingly. Relations between pressure drop and flow velocity of the air through the porous Al₂O₃ fit the Forchheimer's equation well for compressible fluid. Due to pores introduced by the pore agent, the Darcy permeability and inertial permeability of the porous Al₂O₃ are increased obviously. For given flow velocity, with increase of the PMMA content, the Forchheimer's number of the fluid through the porous Al₂O₃ is decreased, which results in decrease of the inertial resistance ratio to the total pressure drop. The porous Al₂O₃ ceramics with pores introduced by the monodispersed PMMA micro balls show higher permeability while the filtration selectivity is not deteriorated.     

Extrusion of highly porous silicon nitride ceramics with bimodal pore structure and improved gas permeability

Highly porous Si₃N₄ ceramics with bimodal pore structure were prepared by the extrusion processing with petroleum coke of 30 μm as pore‐maker. The microstructure, mechanical strength, and gas permeability were investigated. The microstructure with petroleum coke contained not only numerous fine pores by interlocking the high aspect ratio β‐Si₃N₄ grains, but also some large pores of 15‐25 μm left by the burnout of petroleum coke. The resultant samples obtained an improved gas permeability of 1.2 × 10^?12 m², which is approximately two times that of samples without petroleum coke addition. Furthermore, the mechanical strength is still superior even at a porosity of 67% in comparison with the other porous ceramics used in the current diesel particulate filter.     

Gas permeability and mechanical properties of porous alumina ceramics with unidirectionally aligned pores

Porous alumina ceramics having unidirectionally aligned cylindrical pores were prepared by extrusion method and compared with porous ceramics having randomly distributed pores prepared by conventional method, and their gas permeability and mechanical properties were investigated. SEM micrographs of the porous alumina ceramics prepared by the extrusion method using nylon fibers as the pore former showed excellent orientation of cylindrical pores. The bending strength and Weibull modulus of the extruded porous alumina ceramics with 39% porosity were 156 MPa and 17, respectively. These mechanical properties of extruded samples were higher than those of the conventional porous alumina ceramics. The strength decreased from 156 to 106 MPa with increasing pore size from 8.5 to 38 μm. The gas permeability of the extrusion samples is higher than that of the conventional samples and increased with increasing of porosity and pore size.     

Microstructure and permeability of porous YSZ ceramics fabricated by freeze casting of oil-in-water suspension

Porous yttria-stabilized zirconia (YSZ) ceramics are fabricated through freeze casting of oil-in-water suspension followed by sintering at 1250−1550 °C. The pore structure, compressive strength and permeability of porous YSZ ceramics are tailored via altering the emulsion content and sintering temperature. The samples obtained using higher emulsion content or at lower sintering temperature show larger Darcian and non-Darcian constants due to their higher open porosity and larger pore size. Furthermore, the investigation on individual contributions of viscous and inertial resistances on the total pressure drop during permeation process indicates that the viscous resistance increases but the inertial resistance decreases with increasing the emulsion content or decreasing the sintering temperature for samples. Porous YSZ ceramics obtained in this work with a k₁ range of 3.14 × 10⁻¹³–1.12 × 10⁻¹² m² are appropriate for applications in filters and membrane supports.     

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 of near net size porous alumina-calcium aluminate ceramics by gelcasting-pore-forming agent processs

In the processing of porous ceramics, shrinkage from green body to sintered compact during drying and sintering is one of the key concerns which affect microstructure and properties of porous ceramics. Through releasing gases from the burning of the pore forming agents, and volume expansion from the formation of low density resultants during sintering, the sintering shrinkage can be effectively compensated and near net size preparation can be achieved. Herein, near net size porous alumina-calcium aluminate ceramics with controllable shrinkage have been prepared using a combination of gelcasting and pore-forming agent process by adjusting the amount of CaCO₃ and polymethyl methacrylate (PMMA) microspheres added. Al₂O₃ and CaCO₃ were used as raw materials, PMMA microspheres were used as pore-forming agent, isobutylene/maleic anhydride copolymer (Isobam104) was used as gelling agent and dispersing agent. The effects of the addition amounts of CaCO₃ and PMMA in the slurry on the phase composition, shrinkage, porosity, and strength of porous alumina-calcium aluminate ceramics were investigated. The results show that as the CaCO₃ addition amount increases from 0 to 20 wt%, the shrinkage of the samples gradually decreases from 7.3% to −1.4%, and the consequent porosity increases from 58% to 66%, while the compressive strength increases from 5.9 to 15.5 MPa. When PMMA content increases from 10 to 50 wt%, the shrinkage of the samples decreases first and then increases, the porosity increases from 51% to 74%, and the compressive strength decreases from 12.5 to 5.3 MPa. The mechanisms for controlling shrinkages during preparation of porous alumina-calcium aluminate ceramics can be attributed to the following aspects: on one hand, gas release from burning of PMMA and decomposition of CaCO₃ during sintering; on the other hand, volume expansion due to the formation of lower density calcium aluminates which come from the reactions between CaO and Al₂O₃. The near net size preparation technique is of great significance for the manufacture of porous ceramics since the subsequent machining cost can be effectively reduced.     

Fabrication of mullite-bonded porous silicon carbide ceramics by in situ reaction bonding

An in situ reaction bonding technique was developed to fabricate mullite-bonded porous silicon carbide (SiC) ceramics in air from SiC and α-Al₂O₃, using graphite as the pore-former. Graphite is burned out to produce pores and the surface of SiC is oxidized to SiO₂ at high temperature. With further increasing the temperature, the amorphous SiO₂ converts into cristobalite and reacts with α-Al₂O₃ to form mullite (3Al₂O₃·2SiO₂). SiC particles are bonded by the mullite and oxidation-derived SiO₂ to obtain porous SiC ceramics. The reaction bonding behavior, open porosity, pore size distribution and mechanical strength of porous SiC ceramics were investigated as a function of the sintering temperature, forming pressure and graphite content. In addition, the phase composition and microstructure were also studied.     

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.     

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.     

Membrane Porous Materials from Sialon

Carbothermal reduction and simultaneous nitriding of kaolin in pressings from a mixture with graphite are used to fabricate a ceramics based on β-sialon with a porosity of 50 – 60%, a pore size of about 1 μm, and a gas permeability of about 0.04 μm². Mercury porosimetry of one of the specimens shows a narrow size distribution of the pores with a maximum at 0.7 μm. The method used for fabricating the porous sialon is classified as reaction sintering, and the elevated permeability of the obtained sialon materials is associated with the negative volume effect of the reaction of carbonitriding of kaolin.     

Fabrication and properties of porous mullite ceramics from calcined carbonaceous kaolin and α-Al2O3

High purity calcined carbonaceous kaolin and α-Al₂O₃ powders were employed to prepare porous mullite ceramics (Sample A) using graphite as pore former with the reaction sintering method. For the purpose of comparison, porous mullite ceramics (Sample B) was also fabricated from the uncalcined carbonaceous clay incorporated with α-Al₂O₃ powders. Mullitization in the two samples was both nearly complete at 1500 °C, despite the fact that calcination of the clay remarkably depressed mullitization and promoted the formation of glass phase. The Sample A sintered at 1500 °C fractured mainly in an intergranular way, while the Sample B mainly underwent transgranular fracture. The experimental results revealed that densification behavior/open porosity of the Sample A was far more sensitive to sintering temperature. The pore size of the Sample A as well as the Sample B sintered at 1500 °C was in a narrower range of 0.3–5 μm.     

Fabrication and characterization of porous CaSiO3 ceramics

Porous CaSiO₃ ceramics were prepared via a solid-state reaction method using CaCO₃ and SiO₂ as raw materials and active carbon as a pore-forming agent. The results indicated that porous CaSiO₃ ceramics could be obtained under a low sintering temperature of 1320?°C. The addition of active carbon significantly affected the volume density, microstructure, pore size distribution and mechanical strength of porous CaSiO₃ ceramics. With the increase of active carbon content, the volume density decreased, meanwhile the pore size and porosity increased gradually. Besides, the three-point bending tests demonstrated that the mechanical strength was decreased with increasing active carbon content. However, all the porous ceramics still exhibited high mechanical strength. These results implied that the increase of active carbon content not only enlarged the pore size and enhanced the porosity, but also kept a remarkable mechanical strength of porous CaSiO₃ ceramics. Therefore, these rationally designed CaSiO₃ porous ceramics will be a highly potential material in various applications due to its high mechanical strength, low sintering temperature and narrow pore size distribution.     

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.     

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.     

The role of Al in the reactive synthesis of porous Mo2Ti2AlC3 ceramics

The mixed powders of TiH₂, molybdenum, aluminum and graphite with molar ratios of 2/2/n/2.85 (n ranges from 1.0 to 1.4 mol with an interval of 0.1 mol) were used as raw material powders for this work, and a novel porous Mo₂Ti₂AlC₃ was synthesized via reactive synthesis. Through systematic research on the pore structure parameters of porous Mo₂Ti₂AlC₃ prepared with different aluminum content, the results show that there is a clear correlation between the aluminum content and the pore structure parameters. With the aluminum content rising from 1.0 to 1.2 mol, the viscous permeability coefficient and pore size decreased, while the porosity increased; When the aluminum content increased from 1.2 to 1.4 mol, the pore structure parameters of porous Mo₂Ti₂AlC₃ displayed an opposite trend. The reasons for the evolution laws of these pore structure parameters were also discussed in depth. In addition, the pore structure forming mechanism of porous Mo₂Ti₂AlC₃ ceramics during the activation reaction sintering process has been explored. This work can provide an important reference for the subsequent preparation of quaternary porous MAX phase ceramics.