Preparation and properties of mullite-bonded porous fibrous mullite ceramics by an epoxy resin gel-casting process

Porous fibrous mullite ceramics with a narrow range of pore size distribution have been successfully prepared utilizing a near net-shape epoxy resin gel-casting process by using mullite fibers, Al₂O₃ and SiC as raw materials. The effects of sintering temperatures, different amounts of fibers and Y₂O₃ additive on the phase compositions, linear shrinkage, apparent porosity, bulk density, microstructure, compressive strength and thermal conductivity were investigated. The results indicated that mullite-bonded among fibers were formed in the porous fibrous mullite ceramics with a bird nest pore structure. After determining the sintering temperatures and the amount of fibers, the tailored porous fibrous mullite ceramics had a low linear shrinkage (1.36–3.08%), a high apparent porosity (61.1–71.7%), a relatively high compressive strength (4.4–7.6 MPa), a low thermal conductivity (0.378–0.467 W/m K) and a narrow range of pore size distribution (around 5 µm). The excellent properties will enable the porous ceramics as a promising candidate for the applications of hot gas filters, thermal insulation materials at high temperatures.     

Synthesis and characterization of mullite-ZrO2 porous fibrous ceramic for highly efficient oil-water separation

This work aimed to proposing a new strategy for preparing the mullite-ZrO₂ porous fibrous ceramic used as alternative matrix material for oil-water separation by the aqueous gel-casting method. The properties of the fabricated porous fibrous ceramics in terms of microstructure, phase composition, apparent porosity, bulk density and compressive strength were investigated and the separation behavior was predicted by analyzing the structural changes. It is demonstrated that the phase composition of green bodies consisted of bayerite, boehmite, ZrSiO₄ and YSZ, and the sintered sample contained mullite, ZrO₂ and YSZ. As the YSZ fibers increased, the porosity of the fabricated porous ceramic increased with the maximum value of 70.65% due to the formation of more pores caused by YSZ fibers. Moreover, a significant increase in compressive strength (up to 9.52–21.86 MPa) was observed with the increase of YSZ fibers. Therefore, the fabricated porous ceramics could be appropriative for advanced applications of separation membranes for oil-water separation.     

Structure and properties of Al2O3-bonded porous fibrous YSZ ceramics fabricated by aqueous gel-casting

To meet requirements for high porosity and high strength, novel aqueous gel-casting process has been successfully developed to fabricate Al₂O₃-bonded porous fibrous YSZ ceramics with ρ-Al₂O₃ and YSZ fibers as raw materials. Microstructure, phase composition, apparent porosity, bulk density, thermal conductivity, and compressive strength of fabricated porous ceramics were investigated, and effects of fiber content on properties were discussed. According to results, bird nest 3D mesh with interlaced YSZ fibers and Al₂O₃ binder was formed, ensuring the ability to obtain high performance, lightweight ceramics. An increase in the number of YSZ fibers led to more complex interlaced arrangement of fibers and denser network structure of porous ceramics at retaining their stability. Furthermore, their apparent porosity and bulk density increased, whereas thermal conductivity and compressive strength decreased with increasing the fiber content. In particular, comparatively high porosity (71.1–72.7%), low thermal conductivity (0.209–0.503 W/mK), and relatively high compressive strength (3.45–4.24 MPa) were obtained for as-prepared porous ceramics, making them promising for applications in filters, thermal insulation materials, and separation membranes.     

Porous YSZ Ceramics Reinforced by Different Kinds of Fibers

Porous YSZ ceramics reinforced by different fibers were prepared by gel‐casting with 15% solid content and pressureless sintering. The four kinds of fibers (mullite, aluminosilicate, Al₂O₃, and YSZ fibers) were added into the YSZ ceramics with the same 10% vol content. After sintered at 1500°C for 2 h, aluminosilicate and mullite fibers could not be found in the samples of porous YSZ ceramics, which showed they reacted with YSZ ceramics at high temperature, while YSZ and Al₂O₃ fibers still kept perfect after sintering. Furthermore, the influences of fiber content, sintering temperature, porosity of matrix materials on compressive strength and porosity of the porous YSZ ceramics were studied. The results showed that Al₂O₃ fiber showed more obvious reinforcing effect than YSZ fiber on porous YSZ ceramics. The fiber‐reinforcing effects depend on fiber content, sintering temperature, and porosity of matrix materials. The fiber addition can improve the shrinkage behavior of porous ceramics during sintering and strengthen the skeleton of porous ceramics.     

Effect of mullite phase formed in situ on pore structure and properties of high-purity mullite fibrous ceramics

Porous mullite ceramics are potential advanced thermal insulating materials. Pore structure and purity are the main factors that affect properties of these ceramics. In this study, high performance porous mullite ceramics were prepared via aqueous gel-casting using mullite fibers and kaolin as the raw materials and ρ-Al₂O₃ as the gelling agent. Effects of addition of mullite fibers on the pore structure and properties were examined. The results indicated that mullite phase in situ formed by kaolin, and ρ-Al₂O₃ ensured the purity of mullite samples and mullite fibers bonded together to form a nest-like structure, greatly improving the properties of ceramic samples. In particular, the apparent porosity of mullite samples reached 73.6%. In the presence of 75% of mullite fibers, the thermal conductivity was only 0.289 W/m K at room temperature. Moreover, the mullite samples possessed relatively high cold compressive strength in the range of 4.9–9.6 MPa. Therefore, porous mullite ceramics prepared via aqueous gel-casting could be used for wide applications in thermal insulation materials, attributing to the excellent properties such as high cold compressive strength and low thermal conductivity.     

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.     

A new Al2O3 porous ceramic prepared by addition of hollow spheres

A method for making porous ceramic prepared by adding hollow spheres was developed, and the resulting porous ceramic was named as hollow spheres ceramic. Water soluble epoxy resin was used as a gel former in the gelcasting process of the Al₂O₃ hollow sphere and Al₂O₃ powder, the porous ceramic porosity varies from 22.3 to 60.1 %. The influence of amount of Al₂O₃ hollow sphere and sintering temperature on the microstructure, compressive strength and thermal conductivity were investigated. With an increasing amount of hollow sphere in the matrix, the porosity increases, which leads to decreased bulk density, compressive strength and thermal conductivity. The compressive strength of the porous ceramics has a power law relation with the porosity, and the calculated power law index is 4.5. The equations of the relationship between porosity and thermal conductivity of porous ceramics are proposed. The thermal conductivity of samples with 60.1 % porosity is as low as 2.1 W/m k at room temperature.     

Investigating the effect of SPRM on mechanical strength and thermal conductivity of highly porous alumina ceramics

For recycling waste refractory materials in metallurgical industry, porous alumina ceramics were prepared via pore forming agent method from α-Al₂O₃ powder and slide plate renewable material. Effects of slide plate renewable material (SPRM) on densification, mechanical strength, thermal conductivity, phase composition and microstructure of the porous alumina ceramics were investigated. The results showed that SPRM effectively affected physical and thermal properties of the porous ceramics. With the increase of SPRM, apparent porosity of the ceramic materials firstly increased and then decreased, which brought an opposite change for the bulk density and thermal conductivity values, whereas the bending strength didn’t decrease obviously. The optimum sample A2 with 50 wt% SPRM introducing sintered at 1500 °C obtained the best properties. The water absorption, apparent porosity, bulk density, bending strength and thermal conductivity of the sample were 31.7%, 62.8%, 1.71 g/cm³, 47.1 ± 3.7 MPa and 1.73 W/m K, respectively. XRD analysis indicated that a small quantity of silicon carbide and graphite in SPRM have been oxidized to SiO₂ during the firing process, resulting in rising the porous microstructures. SEM micrographs illustrated that rod-like mullite grains combined with plate-like corundum grains to endow the samples with high bending strength. This study was intended to confirm the preparation of porous alumina ceramics with high porosity, good mechanical properties and low thermal conductivity by using SPRM as pore forming additive.     

Low sintering shrinkage porous mullite ceramics with high strength and low thermal conductivity via foam-gelcasting

Excessive sintering shrinkage leads to severe deformation and cracking, affecting the microstructure and properties of porous ceramics. Therefore, reducing sintering shrinkage and achieving near-net-size forming is one of the effective ways to prepare high-performance porous ceramics. Herein, low-shrinkage porous mullite ceramics were prepared by foam-gelcasting using kyanite as raw material and aluminum fluoride (AlF₃) as additive, through volume expansion from phase transition and gas generated from the reaction. The effects of AlF₃ content on the shrinkage, porosity, compressive strength, and thermal conductivity of mullite-based porous ceramics were investigated. The results showed that with the increase of content, the sintering shrinkage decreased, the porosity increased, and mullite whiskers were produced. Porous mullite ceramics with 30 wt% AlF₃ content exhibited a whisker structure with the lowest shrinkage of 3.5%, porosity of 85.2%, compressive strength of 3.06 ± 0.51 MPa, and thermal conductivity of 0.23 W/(m·K) at room temperature. The temperature difference between the front and back sides of the sample reached 710°C under high temperature fire resistance test. The low sintering shrinkage preparation process effectively reduces the subsequent processing cost, which is significant for the preparation of high-performance porous ceramics.     

Near-net-size preparation of porous mullite matrix ceramics with in situ formed 3D mullite whisker network

Porous mullite matrix ceramics have excellent thermal and mechanical properties suitable for applications such as in thermal insulation. However, their applications are limited by processing defects from nonuniform sintering shrinkage and the trade-off between high porosity (preferred for low thermal conductivity) and high mechanical strength. Herein, we seek to minimize the sintering shrinkage by near-net-size preparation and improve the strength by in situ formed whisker network structure. Gelcasting forming technology and pressureless sintering were used to prepare porous mullite matrix ceramics using kyanite and α-Al₂O₃ powders as the starting materials and using MoO₃ to promote the growth of mullite whiskers. The results showed that the sintering shrinkage could be compensated by the volume expansion from solid-state reaction during reaction sintering. The in situ formed three-dimensional (3D) whisker network further reduced sintering shrinkage and effectively improved the strength of the ceramics. An ultralow sintering shrinkage of .78% was achieved. The near-net-shape porous mullite matrix ceramics strengthened by 3D whisker network had a high porosity of 63.9%, a high compressive strength of 83.8 MPa and a high flexural strength of 53.5 MPa.     

Porous mullite ceramics with enhanced compressive strength from fly ash-based ceramic microspheres: Facile synthesis,structure, and performance

Porous mullite ceramics are widely used in heat insulation owing to their high temperature and corrosion resistant properties. Reducing the thermal conductivity by increasing porosity, while ensuring a high compressive strength, is vital for the synthesis of high-strength and lightweight porous mullite ceramics. In this study, ceramic microspheres are initially prepared from pre-treated high-alumina fly ash by spray drying, and then used to successfully prepare porous mullite ceramics with enhanced compressive strength via a simple direct stacking and sintering approach. The influence of sintering temperature and time on the microstructure and properties of porous mullite ceramics was evaluated, and the corresponding formation mechanism was elucidated. Results show that the porous mullite ceramics, calcined at 1550 °C for 3 h, possess a porosity of 47%, compressive strength of 31.4 MPa, and thermal conductivity of 0.775 W/(m?K) (at 25 °C), similar to mullite ceramics prepared from pure raw materials. The uniform pore size distribution and sintered neck between the microspheres contribute to the high compressive strength of mullite ceramics, while maintaining high porosity.     

Effect of alumina fiber content on pore structure and properties of porous ceramics

Porous Al₂O₃ ceramics with different contents of alumina fibers were prepared by gel-casting process. The effects of Al₂O₃ fiber content on pore size distribution, porosity, compressive strength, and load-displacement behavior of the ceramic materials were investigated. Initial results showed that with the increase of Al₂O₃ fiber content, the pore size and porosity of the material is increased, and the compressive strength is decreased. However, upon increasing the fiber content from 50 wt% to 67 wt%, the performance of the samples changed greatly. The compressive strength of the material increased, while the porosity remained unchanged, the pore size increased greatly, and the shape of the load displacement curve changed. It showed that when the fiber content increased from 50 wt% to 67 wt%, the loading body in the fiber-reinforced porous ceramics changed from particles to fibers.     

Microstructural comparison of porous oxide ceramics from the system Al2O3–ZrO2 prepared with starch as a pore-forming agent

In this paper we show examples of microstructures of porous oxide ceramics prepared by traditional slip casting (TSC) and starch consolidation casting (SCC) and present results obtained using different microstructural characterization techniques; Archimedes method (open and total porosity), shrinkage measurement, mercury intrusion porosimetry (pore size distribution) and microscopic methods – optical microscopy with microscopic image analysis (pore size distribution) and scanning electron microscopy (detailed investigation of the local microstructure). In particular, microstructures are compared for porous ceramics from the system Al₂O₃–ZrO₂ prepared with rice and corn starch. It is shown that maximum values of the total porosity of porous ceramics prepared with starch as a pore-forming agent were approx. 50%. A major finding by using SEM with respect to starch-produced porous ceramics is the existence of pore fillings in the form of small sintered ceramic shell inside the pores, as a result of starch granule shrinkage during the drying and burn-out steps.     

Seed assisted in-situ synthesis of porous anorthite/mullite whisker ceramics by foam-freeze casting

Porous anorthite/mullite whisker ceramics with both high strength and low thermal conductivity have been successfully prepared by combining seed-assisted in situ synthesis and foam-freeze casting techniques. The addition of mullite seed was conducive to a reduction in the sintering shrinkage, pore size, and anorthite grain size. This increased the high aspect ratio of mullite whiskers, which enhanced the strength and diminished the thermal conductivity. Mullite whiskers overlapped to form a stable three-dimensional network structure similar to the bird's nest, which was also beneficial to heighten the mechanical properties of the prepared porous ceramics. Through this method, the prepared materials had a high apparent porosity of 87.7–90.2%, a low bulk density of 0.29–0.36 g/cm³, a high compressive strength of 0.65–3.31 MPa, and low thermal conductivity of 0.067–0.112 W/m·K. The results indicated that the method described here can fabricate porous ceramics with excellent properties for further thermal insulating applications.     

Ultra-low cost porous mullite ceramics with excellent dielectric properties and low thermal conductivity fabricated from kaolin for radome applications

Near-net-shape mullite ceramics with high porosity were prepared from ultra-low cost natural aluminosilicate mineral kaolin as raw material and polystyrene micro-sphere (PS) as pore-forming agent. Microstructure, flexural strength, thermal conductivity and dielectric properties of the ceramics were systematically researched. Results show that the porous mullite ceramics possess fibrous skeleton structure formed by a large quantity of interlocked mullite whiskers, which results in good mechanical properties and low-to-zero sintering shrinkage. Flexural strength of the porous mullite ceramics can be up to 41.01 ± 1.12 MPa, even if the porosity is as high as 62.44%. The dielectric constant and loss tangent of the porous mullite ceramics at room temperature are lower than 2.61 and 5.9 × 10⁻³, respectively. Besides, dielectric constant is very stable with the rising of temperature, and the dielectric loss can be consistently lower than 10⁻² when the temperature is not higher than 800 °C. In addition, thermal conductivity at room temperature is as low as 0.163 W/m/K when the porosity of mullite ceramics is 80.05%. The infiltration of SiO₂ aerogels (SiO₂ AGs) can further decrease the thermal conductivity to 0.075 W/m/K, while has just little effects on the dielectric properties. Excellent mechanical, thermal and dielectric properties show that the porous mullite ceramics have potential applications in radome fields. The porous mullite ceramics prepared from kaolin not only have low cost, but also can achieve near-net-shape.     

Preparation of self-reinforcement of porous mullite ceramics through in situ synthesis of mullite whisker in flyash body

Self-reinforced porous mullite ceramics were fabricated by a starch consolidation method with flyash, different aluminium sources (Al(OH)₃ and Al₂O₃) and the additive AlF₃ as raw materials. The reinforcement mechanism of needle-like mullite whiskers through in situ synthesis in ceramic body was investigated. The bulk density, apparent porosity and bending strength of the samples were tested. Phase compositions and microstructures of the sintered samples were measured by XRD and SEM, respectively. It showed that AlF₃ as additive was helpful to the formation of mullite whiskers at a low temperature. As the aluminium sources, Al(OH)₃ was more suitable for the preparation of mullite whiskers than Al₂O₃. The in situ synthesized mullite whiskers formed an interlocking structure, which enhanced the mechanical strength of the porous mullite ceramics. Porous mullite ceramics with bending strength of about 100 MPa and apparent porosity of about 55% were made at 1550 °C.     

Fabrication and properties of mullite fiber matrix porous ceramics by a TBA-based gel-casting process

A bird nest-like structure was designed by using the mullite fiber as the matrix and SiO₂ as the high temperature binder. This special material was successfully prepared by a TBA-based gel-casting process. The randomly arranged fiber laps bonded by SiO₂ binder was the most important structure characteristic of this porous material. The effect of sintering temperature on the properties, i.e. porosity, bulk density, linear shrinkage, compressive strength, thermal conductivity and the microstructure was studied. The composite exhibited significant pseudoductility. The fracture mechanism of this composite under compression was discussed. The results indicated that the sintering temperature ranging from 1500 to 1600 °C was suitable for yielding mullite fiber matrix porous ceramics which had a low thermal conductivity (0.19–0.22 W/m K), a relatively high compressive strength (3–13 MPa) and a high resilience (66–70%) for applications in the thermal insulators and high-temperature elastic seal field.     

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.     

Transient liquid phase diffusion process for porous mullite ceramics with excellent mechanical properties

Porous mullite ceramics were fabricated by the transient liquid phase diffusion process, using quartz and fly-ash floating bead (FABA) particles and corundum fines as starting materials. The effects of sintering temperatures on the evolution of phase composition and microstructure, linear shrinkage, porosity and compressive strength of ceramics were investigated. It is found that a large amount of quartz and FABA particles can be transformed into SiO₂-rich liquid phase during the sintering process, and the liquid phase is transient in the Al₂O₃-SiO₂ system, which can accelerate the mullitization rate and promote the growth of mullite grains. A large number of closed pores in the mullite ceramics are formed due to the transient liquid phase diffusion at elevated temperatures. The porous mullite ceramics with high closed porosity (about 30%) and excellent compressive strength (maximum 105?MPa) have been obtained after fried at 1700?°C.     

Synthesis and characterization of porous Y2SiO5 with low linear shrinkage,high porosity and high strength

The emerging porous Y₂SiO₅ ceramic is regarded as a promising candidate of thermal insulator owing to its very low thermal conductivity. However, recent works on porous Y₂SiO₅ are confronted with severe problems such as large linear shrinkage (18.51–20.8%), low porosity (47.74–62%) and low strength (24.45–16.51 MPa) at high sintering temperatures (1450–1500 °C). In this work, highly porous Y₂SiO₅ ceramic with low shrinkage and excellent high-temperature strength was fabricated by in-situ foam-gelcasting method at 1550 °C. The as-prepared sample has unique multiple pore structures, low linear shrinkages of 6.3–4.5%, controllable high porosities of 60.7–88.4%, high compressive strengths of 38.2–0.90 MPa, and low thermal conductivities of 0.126–0.513 W/(m K) (porosity: 87.1–60.2%). The effects of relative density on relative strength, as well as porosity on thermal conductivity were quantitatively discussed. The present results indicate that porous Y₂SiO₅ is the potential high-temperature thermal insulation material of light weight, low thermal conductivity, and high strength.