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.     

Effects of V2O5 addition on the synthesis of columnar self-reinforced mullite porous ceramics

Spearhead columnar mullite was synthesized by in-situ reaction with V₂O₅ as additive. When the content of V₂O₅ was 7 wt%, the length of the spearhead columnar mullite was the longest with an aspect ratio of about 3.5. Furthermore, columnar self-reinforced mullite porous ceramics were prepared by a foam-gelcasting method, and the effects of V₂O₅ content on the rheological and gelling properties of mullite slurries as well as the microstructure, physical property and thermal insulation property of the prepared mullite porous ceramics were studied. The results showed that the flexural strength and compressive strength of the porous ceramics with 63% porosity prepared by using 2 wt% V₂O₅ additive were respectively as high as 13.9 and 41.3 MPa, and the thermal conductivity was about 1.04 W m^?1 K^?1.     

High-strength thermal insulating porous mullite fiber-based ceramics

How to improve the strength of fibrous porous ceramics dramatically under the premise of no sacrificing its low density and thermal conductivity has remained a challenge in the high-temperature thermal insulation field. In this paper, a new kind of high-strength mullite fiber-based ceramics composed of interlocked porous mullite fibers was prepared by nanoemulsion electrospinning and dry pressing method. Results show that as to the porous ceramics with the same density (～ 0.8 g/cm³), the three-dimensional skeleton structure composed of porous mullite fibers was much denser than that composed of solid mullite fibers. Therefore, porous mullite fiber-based ceramics exhibited a higher compressive strength (5.53 MPa) than that of solid mullite fiber-based ceramics (3.21 MPa). In addition, porous mullite fiber-based ceramics exhibited a superior high-temperature heat insulation property because the porous structure in fibers could reduce the radiant heat conduction. This work provides new insight into the development of high-temperature thermal insulators.     

In-situ growth of mullite whiskers and their effect on the microstructure and properties of porous mullite ceramics with an open/closed pore structure

Porous mullite ceramics with an open/closed pore structure were prepared by protein foaming method combined with fly ash hollow spheres. Both the open porosity and total porosity of samples were enhanced by increasing the hollow sphere content. Mullite whiskers with a diameter of 0.2–4 μm were grown in-situ in the porous mullite ceramics with an AlF₃ catalyst, conforming to a vapor-solid growth mechanism. The pore structure of the porous mullite ceramics was significantly affected by the mullite whiskers which increased the open porosity and total porosity. Moreover, the median pore size was reduced from 65.05 μm to 36.92 μm after the introduction of mullite whiskers. The flexural strength and the thermal conductivity of the samples decreased with increasing total porosity. The porosity dependence of the thermal conductivity was well described by the universal model, providing a reference for the prediction of thermal conductivity of porous ceramics with open/closed pores.     

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.     

Preparation and characterization of porous mullite ceramics via foam-gelcasting

Porous mullite ceramics were prepared via foam-gelcasting using industrial grade mullite powder as the main raw materials, Isobam-104 as the dispersing and gelling agent, sodium carboxymethyl cellulose as the foam stabilizing agent, and triethanolamine lauryl sulfate as the foaming agent. The effects of processing parameters such as type and amount of additive, solid loading level and gelling temperature on rheological properties and gelling behaviors of the slurries were investigated. The green samples after drying at 100 °C for 24 h were fired at 1600 °C for 2 h, and the microstructures and properties of the resultant porous ceramic samples were characterized. Based on the results, the effects of foaming agent on the porosity level, pore structure and size and mechanical properties of the as-prepared porous mullite ceramics were examined. Porosity levels and pore sizes of the as-prepared samples increased with increasing the foaming agent content up to 1.0%, above which both porosity levels and pore sizes did not change. The compressive strength and flexural strength of the as-prepared sample with porosity of 76% and average pore size of 313 μm remained as high as 15.3±0.3 MPa and 3.7±0.2 MPa, respectively, and permeability increased exponentially with increasing the porosity.     

Microstructure and properties of lightweight fibrous porous mullite ceramics prepared by vacuum squeeze moulding technique

A novel fibrous porous mullite network with a quasi-layered microstructure was produced by a simple vacuum squeeze moulding technique. The effects of organic binder content, inorganic binder and adsorbent on the microstructure and the room-temperature thermal and mechanical properties of fibrous porous mullite ceramics were systematically investigated. An anisotropy microstructure without agglomeration and layering was achieved. The fibrous porous mullite ceramics reported in this study exhibited low density (0.40 g/cm³), low thermal conductivity (~0.095 W/(m K)), and high compressive strength (~2.1 MPa in the x/y direction). This study reports an optimal processing method for the production of fibrous porous ceramics, which have the potential for use as high-temperature thermal insulation material.     

Preparation of porous Y2SiO5 ceramics with relatively high compressive strength and ultra-low thermal conductivity by a TBA-based gel-casting method

Porous Y₂SiO₅ ceramics with relative high compressive strength (as high as 24.45 MPa) and ultra-low thermal conductivity (～0.08 W/m K) were successfully fabricated by a tert-butyl alcohol based gel-casting method. The formation mechanism of the 3D interconnected pores and the properties of the green body are discussed. The porosity, pore size, compressive strength and thermal conductivity could be controlled by varying the initial solid loading and the sintering temperature. When regulating the initial solid loading (from 20 to 50 wt%) and sintering temperature (from 1200 to 1500 °C), the porosity can be controlled between 47.74% and 73.93%, and the compressive strength and the thermal conductivity of porous Y₂SiO₅ ceramics varied from 3.34 to 24.45 MPa and from 0.08 to 0.55 W/m K, respectively. It should be noted that the porous Y₂SiO₅ ceramics with 30 wt% solid loading and sintering at 1400 °C had an open porosity of 61.80%, a pore size of 2.24 μm, a low room-temperature thermal conductivity of 0.17 W/m K and a relatively high compressive strength of 13.91 MPa, which make this porous Y₂SiO₅ ceramics suitable for applications in high-temperature thermal insulators.     

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.     

Porous mullite ceramics with low thermal conductivity prepared by foaming and starch consolidation

Porous mullite ceramics were prepared from an industrial grade mullite powder by foaming and starch consolidation. The viscosities of the original suspensions and the foamed ones with solid loading of 62.5 and 67.5 wt% were measured. After the steps of forming and drying, the green bodies were sintered under different temperatures from 1,200 to 1,600 °C for 2 h. The influence of solid loading of suspension and sintering temperature on the porosity and compressive strength was evaluated. The sintered mullite ceramics, with porosity from 86 to 73 vol% and corresponding compressive strength from 1 to 22 MPa, contained a multi-modal microstructure with large spherical pores and small pores on internal walls. Thermal conductivity measurement carried out by the transient plane source technique at room temperature resulted in values as low as 0.09 W/mK. In addition, the relationship between thermal conductivity and porosity was discussed in detail.     

Fabrication and properties of porous anorthite/mullite ceramics with both high porosity and high strength

Porous anorthite/mullite ceramics with both high porosity and high strength have been successfully fabricated by foam-gelcasting and pressureless sintering technology, using α-Al₂O₃, SiO₂, and CaCO₃ as starting materials and MnO₂ as sintering aids. The porous mullite ceramics prepared in this study had 83.3% porosity and 0.3 W/m·K thermal conductivity, exhibited compressive strength value as high as 6.1 MPa. The samples fabricated with mullite content of 30 mol% possessed 79.4% porosity and 5.9 MPa compressive strength showed thermal conductivity as low as 0.19 W/m·K. With the addition of MnO₂, the properties of the prepared materials varied slightly when mullite content changed in a large scale. The results showed that the addition of MnO₂ promoted the reaction, affected sintering and grain growth, and contributed to high strength and low-thermal conductivity.     

Fibrous porous mullite ceramics modified by mullite whiskers for thermal insulation and sound absorption

In order to meet the demand for thermal insulation and sound absorption, fibrous porous mullite ceramics (FPMC) with high porosity and an interconnected pore structure were prepared, followed by a pore structure modification with in situ grown mullite whiskers on the three-dimensional framework of the FPMC. The resultant hierarchical material exhibited superior sound absorption performance in the low-to-medium frequency to most reported sound-absorbing materials, as well as a sufficient compressive strength of 1.26 MPa with low thermal conductivity of 0.117 W·m^?1·K^?1. Moreover, the effects of solid content and mullite whiskers on the microstructure and physical properties of the material were analyzed. The increase of solid content led to increased compressive strength and thermal conductivity and decreased frequency corresponding to the first sound absorption peak. The thermal conductivity and compressive strength of the material increased as the mullite whiskers grew, while the median pore size decreased.     

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.     

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.     

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.     

Preparation and properties of porous mullite-based ceramics fabricated by solid state reaction

In this study, the effect of the alumina particle size on the formation of mullite using a silica gel powder and micro- and nano-scale Al₂O₃ powders as raw materials was investigated. The optimized Al₂O₃ source was then reacted with the silica gel to prepare porous mullite-based ceramics. The results revealed that the highly reactive nano-Al₂O₃ powder could form mullite at a relatively low firing temperature. Therefore, the nano-Al₂O₃ powder was used to prepare porous mullite-based ceramics by firing at 1600 °C, 1650 °C and 1700 °C. The pore size of the prepared porous mullite-based ceramics ranges from tens to hundreds of micrometres, with the apparent porosity being 42.8–58.0%. Further, the mullite content in the samples increased with increasing firing temperature, and a higher firing temperature promoted sintering, resulting in improved strength of the sample. After calcination at 1700 °C, the mullite content in the sample reached 81.8%, and the sample showed excellent thermal shock resistance. The strengths of the samples before and after thermal shock were found to be 23.6 and 15.58 MPa, with the residual strength ratio being 66%.     

Effects of firing temperature on the microstructures and properties of porous mullite ceramics prepared by foam-gelcasting

Porous mullite ceramics were prepared at 1300–1600°C for 2?h via a foam-gelcasting route using industrial-grade mullite powders as the main raw material, Isobam 104 as the dispersing and gelling agent, triethanolamine lauryl sulphate as the foaming agent and sodium carboxymethyl cellulose as the foam stabilising agent. The effects of firing temperature on the sintering behaviour of green samples as well as microstructures and properties of final porous mullite products were investigated. With increasing the temperature from 1300 to 1600°C, linear shrinkage and bulk density values of fired samples increased, whereas their porosity decreased. Mechanical strength and thermal conductivity values of fired samples decreased with increasing their porosities. Even at a porosity level as high as 79.4%, compressive and flexural strengths of fired samples (with average pore size of 314?μm) remained as high as 9.0 and 3.7?MPa, respectively, and their thermal conductivity (at 200°C) remained as low as 0.21?W?(m^?1?K^?1).     

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.     

Low cost foam-gelcasting preparation and characterization of porous magnesium aluminate spinel (MgAl2O4) ceramics

Porous MgAl₂O₄ ceramics were prepared via a low cost foam-gelcasting route using MgAl₂O₄ powders as the main raw material, ammonium polyacrylate as a dispersant, a small amount of modified carboxymethyl cellulose as a gelling agent, and TH-IV polymer as a foaming agent. The effects of additive's content, solid loading and gelling temperature on slurry's rheological behavior were investigated, and microstructures and properties of as-prepared porous MgAl₂O₄ ceramics examined. Based on the results, the roles played by the foaming agent in the cases of porosity, pore structure, pore size, mechanical properties and thermal conductivity were clarified. Porosity and pore sizes of as-prepared porous MgAl₂O₄ ceramics increased with increasing the foaming agent from 0.05 to 0.6 vol%. Porous MgAl₂O₄ ceramics with porosity of 75.1% and average pore size of 266 µm exhibited a compressive strength as high as 12.5±0.8 MPa and thermal conductivity as low as 0.24 W/(m K) (at 473 K).     

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.