Porous Mullite Ceramics Research Articles

MoO3 and AlF3 effect on the properties of porous mullite ceramics with needle‐like microstructure

AbstractPorous mullite ceramics were synthesized via ceramic processing using a local industrial kaolin, alumina, and with AlF3 and MoO3 precursor additives to catalyze the formation of pores and acicular mullite grains. The textural properties, crystalline phases, microstructures, and mechanical properties of the obtained samples were comprehensively analyzed and compared with a reference mullite formulated without additives. The obtained ceramics have a well‐defined porous microstructure, with a porosity of about 50%, and a pore size distribution in the 0.1–2 µm range. Ceramics that involve the addition of MoO3 present well‐defined acicular mullite grains, with slightly higher porosity, larger pores, and better mechanical resistance compared to those processed using AlF3.To correlate the ceramic porosity with its mechanical properties, a simple model of spherical pores was proposed to assess the flexural strength of the dense ceramic. It was found that the inclusion of additives promoting needle‐like microstructures increases the mechanical resistance up to four times the value determined for the ceramic without additives (flexural strength up to 390 MPa for zero‐porosity extrapolation). These results, together with the refractoriness of mullite, allow inferring the potential applications of the developed materials as structural ceramics with relatively low density and for filtering applications.

Effect of ZrSiO4 as additive on the mechanical properties of DLP printing porous mullite ceramics

In order to satisfy the demand for mullite porous ceramics with complex geometries in the engineering field, the digital light processing (DLP) technology is employed to prepare mullite porous ceramics in this study. However, the poor mechanical properties of the mullite porous ceramics prepared using the DLP technology limits its further application in engineering field. In this work, the microstructure of mullite crystals was regulated by adding the sintering aid ZrSiO4 to enhance the mechanical properties of the mullite ceramics, and the effects of sintering aid ZrSiO4 content and sintering temperature on the internal microstructure evolutions of mullite porous ceramics were investigated. Results showed that the decomposition of ZrSiO4 promoted the generation of the silica-rich liquid phase at high temperatures, which effectively promoted the nucleation and growth of needle-like mullite crystals. When the sintering temperature was 1600 °C and the ZrSiO4 content was 10 wt%, the well-developed elongated crystals formed an interlocking nonwoven structure. The corresponding mullite porous ceramics exhibited a compressive strength of 2.52 MPa and a porosity of 74.51%. The strategy of this study can be a feasible route for preparing lightweight and high-strength mullite porous ceramics via digital light processing.

Preparation of cost-effective and self-reinforced porous mullite ceramics for advanced heat insulation and sound absorption applications

Self-reinforcing porous mullite ceramics have attracted considerable attention in the field of insulation and sound absorption materials, due to their precise and controllable porous structure, high temperature and corrosion resistance, and low thermal conductivity. However, the high cost and relatively low performance obstruct their practical applications. In this work, self-reinforcing porous mullite ceramics were fabricated by utilizing construction waste via foam-gel casting and freeze-drying methods, and the impact of blowing agent dosage and sintering temperatures on the microstructures, sound absorption, and thermal insulation properties were systematically investigated. In addition, the related mechanism was also discussed in detail. The results show that, at a sintering temperature of 1000 °C and blowing agent dosage of 0.10 wt%, the self-reinforcing porous mullite ceramics can be successfully obtained, which exhibited a bulk density as low as 0.24 g/cm³ and porosity more than 90 %. Consequently, at a frequency of 2000 Hz, its absorption coefficient reached about 0.81. Furthermore, its thermal conductivity also decreased to about 0.06266 W/(m·K). This development not only unveils innovative avenues for the cost-effective fabrication of porous ceramics but also significantly broadens their potential applications within high-performance thermal insulation and sound absorption domains.

Tailoring microstructure and properties of porous mullite-based ceramics via sol-gel impregnation

Nanotechnology has been introduced into the refractory industry decades ago. However, there are still some peculiarities that hinder its routine application. Poor mechanical strength is one of them. Here we present the fabrication technique for mullite porous ceramics using a calcium-free colloidal binder system followed by secondary sol-gel impregnation intended for fabrication of large up-scaled monoliths. The primary foam was prepared via direct foaming of slurry containing colloidal silica, gelling agents, surfactant, silica fume, and different types of alumina. The foam mixture was cast, de-molded and dried using a cascade drying procedure. The investigation of mullitization that occurred during the high-temperature treatment was studied by XRD and TG-DTA analysis. The optimized sintering curve was then applied to obtain the highest mechanical properties. The resulting foam (with open porosity, volume density about 900 kg/m3 and mechanical strength about 10 MPa) was further processed by the impregnation step. The applied silica or combined silica/aluminum sol and unreacted alumina underwent secondary mullitization during following high-temperature treatment, which significantly enhanced the mechanical strength (up to 18 MPa). The secondary mullitization process was also investigated by HT-XRD and TG-DTA analysis. Foam microstructure was observed by an electron scanning microscope. Thermal conductivity of as-prepared ceramics foams varied from 0.3 to 0.4 W/m·K. The foam wall porosity evolution was studied by mercury intrusion porosimeter and estimated from image analysis.

Study on comprehensive properties of mullite porous ceramics prepared from gangue based on sintering temperature

Based on coal gangue and clay with median particle diameters of 0.22 and 0.21 nm as the main raw materials, starch is used as the pore-forming agent, the mullite porous ceramics are prepared by the hydraulic forming method and the solid sintering method, and the sintering temperature and the amount of clay added are studied. The mechanism influence on the performance of mullite porous ceramics. Through thermogravimetric analysis (TG-DSC), flexural properties, X-ray diffractometer (XRD), scanning electron microscopy (SEM), mercury intrusion method and other technical methods, the thermal stability, mechanical strength, crystal phase composition, Performance indicators such as microscopic morphology and pore size are characterized. The results show that the mullite porous ceramics prepared under the conditions of sintering temperature of 1300 °C and clay content of 40 wt.% have the best performance, with a porosity of 34%, a bulk density of 1.79 g/cm3, and a volume shrinkage rate. 11.88%, flexural strength of 33.28 MPa, uniform pore distribution and more mature mullite crystals. The research results open a new way to explore the high-value utilization of coal-based wastes.

Robust, thermally insulating, and high-temperature resistant phosphate-enhanced mullite fiber porous ceramic composites

Porous mullite ceramics with high porosity and low thermal conductivity are urgently required as high-temperature thermal insulators in harsh environments. How to improve the strength of Porous mullite ceramics dramatically under the premise of no sacrificing its thermal insulating properties has remained an extremely challenging. Inspired by the freezing rain phenomenon in nature, high-strength and high-temperature resistant phosphate coatings were applied to the surface of mullite fibers, which significantly improved the mechanical properties of porous mullite ceramics without significantly increasing their thermal conductivity. The Al(H2PO4)3 solution concentration significantly affected the mechanical and thermal properties of the composite material, where the mechanical performance improved with an increasing Al(H2PO4)3 solution concentration. The phosphate coating phase stabilized with an increasing calcination temperature. The mechanical properties of the ceramics were improved by the excellent interfacial bonding strength between the coating and mullite fiber, and the forms of phosphate in the porous ceramics. The optimum composite preparation process increased its compressive strength by approximately nine-fold to 4.1 MPa and resulted in a thermal conductivity of only approximately 0.09 W/(m·K), demonstrating that the composite exhibited a significantly higher comprehensive performance than other previously reported porous ceramics and aerogels. Such new composites with excellent mechanical and thermal insulation properties exhibit excellent potential for high-strength thermal insulation in the chemical, metallurgical, pharmaceutical, aerospace, and other fields.

Near-net-size porous mullite ceramics with tuneable sintering shrinkage for thermal insulation applications

The microstructure and properties of porous ceramics are severely affected by volume shrinkage during the sintering process. Therefore, reducing sintering shrinkage while maintaining the properties of porous ceramics is a serious challenge. Herein, near-net-size porous mullite ceramics with tuneable sintering shrinkage were prepared by the foam-gelcasting method using kyanite as raw material and aluminum fluoride and aluminum powders as additives through the volume expansion generated by the sintering reaction. The effects of Al powder content on the sintering shrinkage, porosity, compressive strength and thermal conductivity of porous mullite ceramics were investigated. The results show that the sintering shrinkage gradually decreases with increasing Al powder content, and expansion is even produced. When the Al powder content was 5 wt%, the sintering shrinkage of porous mullite ceramics was −0.21 %, with porosity of 84.2 %, compressive strength of 2.45 ± 0.15 MPa, and thermal conductivity of 0.19 W/(m·K) at room temperature. The results of the high-temperature fire resistance test showed that the temperature difference between the front and back sides of the sample under the flame gun reached 800 °C, indicating good thermal insulation performance. The near-net-size sintering process of porous ceramics can effectively reduce the subsequent processing cost, open new paths for their production, and broaden the application fields of porous mullite ceramics.

Porous mullite ceramics with hierarchical pores constructed via vat photopolymerization of multi-phase Pickering emulsion

Herein, a novel method for hierarchically porous mullite ceramics with complex and fine structure was proposed, that is vat photopolymerization of multi-phase Pickering emulsion. Pickering emulsions stabilized by mixed particles features water in oil type, high solid content, suitable rheological properties and excellent ultraviolet-curing ability, making them appropriate pastes for vat photopolymerization. The printed components possess hierarchically porous structure involving interconnected mm-scale pores designed by three-dimensional model, uniform μm-scale pores derived from emulsion droplets and less voids generated by the removal of photosensitive resin in the location of continuous phase. Therefore, porous mullite ceramics with octet lattice structure possess excellent compressive strength up to 4.3 MPa at high porosity of 74.08% compared to the counterparts from pure resin-based suspension. The products from this facile, universal and effective method could serve as ideal candidates for catalytic filters, biological scaffolds and electronic components owing to their designable structure and outstanding multifunctionality.

A simple and efficient hydratable alumina gel‐casting method for the fabrication of high‐porosity mullite ceramics

AbstractIn this study, we have developed a method for the preparation of porous alumina‐containing ceramics by inorganic gel‐casting of hydratable alumina system. The gelation behavior of hydratable alumina in water and the high‐temperature phase evolution of alumina gels were clarified. In addition, extremely high‐porosity mullite ceramics were prepared using the hydratable alumina gel‐casting method combined with carbon fibers as the pore‐forming agent method. The as‐prepared mullite ceramics have a pore size of less than 10 μm, controllable high porosity of 90.6%–78.1%, a lightweight of 0.30–0.70 g cm−3, high compression strength of 1.41 ± 0.21–16.30 ± 0.20 MPa, low dielectric constants at 30 GHz of 1.34–1.79, the dielectric loss tangent of less than 3 × 10−3, and low thermal conductivities of 0.083–0.305 W/(m K). Lightweight, high strength, excellent dielectric properties, and outstanding thermal insulation indicate the potential of porous mullite ceramics for use in radar radomes or windows.

Mullite porous ceramics with high strength for high-temperature thermal insulation

To address the low thermal efficiency and serious energy consumption of industrial kilns, the gel casting method is used herein for the successful preparation of porous mullite thermal insulation materials with good thermal and mechanical properties. The effects of poly-γ-glutamic acid (γ-PGA) content (0.6–2.2 wt%) on the performances of the slurries, green bodies, and sintered bodies are comprehensively investigated based on the gelation mechanism. The mullite slurries show good flowability and stability, and the powders are evenly dispersed. Furthermore, the green bodies have sufficiently high mechanical strengths to meet the conditions of processing into complex shapes. The sample open porosity is shown to increase, and the pore size gradually becomes more uniform, as the γ-PGA content is increased. At a γ-PGA content of 1.8 wt%, the open porosity reaches 75.85 %, the compressive strength is 14.02 MP. The thermal conductivity increases from 0.263 W/(m⋅K) to 0.544 W/(m⋅K) as the temperature increases from 25 °C to 1000 °C. At the same time, the Gong model accurately describes the relationship between the thermal conductivity and porosity. In addition, infrared thermographic analysis and alcohol lamp ablation experiments macroscopically demonstrate the excellent thermal insulation performance of the porous mullite at high temperatures.

Porous mullite fiber-based ceramics inspired by biomimetic natural pine wood

How to further improve the strength of fiber porous ceramics has always been the focus of researchers. Herein, inspired by the unique microstructure of natural woods which exhibit excellent mechanical and thermal insulation properties, a high strength mullite fiber-based porous ceramic with wood-like microstructure was successfully designed and fabricated. The wood-like porous ceramics were mainly composed of two kinds of layered structures that are perpendicular to each other. The horizontally oriented lamellar structure was constructed by assembling the fiber membranes layer by layer, and the vertically oriented lamellar structure perpendicular to the fiber layer was constructed by the directional freeze casting method. The results indicated that as to the mullite fiber-based porous ceramics with the similar density (∼0.36 g/cm3), the compressive strength along the X-axis direction of porous ceramics with wood-like microstructure prepared by the directional freeze casting method (3.4 MPa) was much higher than that of porous ceramics prepared by the non-directional freeze casting method (1.04 MPa), owing to the formation of silica lamellar structure perpendicular to the fiber layers. In addition, the wood-like mullite fiber-based porous ceramics also exhibited excellent thermal insulation performance along the X direction due to existence of a large number of pores in the fiber layers. This study provides a new strategy for the preparation of a high-strength thermal insulating fiber-based porous ceramic.

Controllable preparation and properties of gel-casting porous mullite ceramics via lignin fiber assisted pore formation method

The preparation of high-strength porous mullite via gel-casting has emerged as a hot topic, but balancing high strength and high porosity remains challenging. Herein, we successfully prepared porous mullite with uniform pore size and high strength using natural lignin fibers as the pore-forming agent via gel-casting. Moreover, we investigated the properties of mullite slurries, as well as green bodies and sintered ceramics in details. The strong water transport capacity of the lignin fibers enabled the homogeneous dispersion of gel system and improved the flowability of slurry. The flexural strength of green body was significantly increased to 16.07 MPa by optimizing the lignin fiber content. Porous mullite ceramics sintered at 1400 °C exhibited a homogeneous microstructure with average pore size of 5.62 μm. Furthermore, mullite ceramics had a porosity of 64.73% at a total solid loading of 55 wt% and a lignin fiber content of 10 wt%, whereas the compressive strength remains high at 11.32 MPa. Permeability tests confirmed the presence of interconnected pores, its coefficients k1 and k2 increases with the total porosity of ceramics. This work provides new insights into designing the preparation of porous mullite ceramics with high performance.

Porous Mullite Ceramic Modification with Nano-WO3.

Mullite and mullite-alumina ceramics materials with dominance of the mullite phase are used in different areas of technology and materials science. Porous mullite ceramics materials can be used simultaneously as refractory heat insulators and also as materials for constructional elements. The purpose of this work was to investigate the WO3 nanoparticle influence on the evolution of the aluminum tungstate and zircon crystalline phases in mullite ceramics due to stabilization effects caused by different microsize ZrO2 and WO3. The use of nano-WO3 prevented the dissociation of zircon in the ceramic samples with magnesia-stabilized zirconia (MSZ), increased porosity by approximately 60 ± 1%, increased the intensity of the aluminum tungstate phase, decreased bulk density by approximately 1.32 ± 0.01 g/cm3, and increased thermal shock resistance by ensuring a loss of less than 5% of the elastic modulus after 10 cycles of thermal shock.

Physico-Mechanical and Microstructure Characteristics of Porous Mullite Ceramics

Porous mullite ceramics were prepared from mixtures of Egyptian kaolinite-clay and calcined α-alumina powder. The effects of two methods of compaction as well as two types of binder were investigated. The influence of sintering temperature on the physical properties, crystalline phase, compressive strength and microstructure was studied. Mullite bodies processed by uniaxial pressing utilizing 15wt. % water at 1500°C for 2h exhibited the highest compressive strength of 150 MPa and bulk density of 2.1 g/cm3, as well as, an open porosity of 35%. In the present study, the only stoichiometric mullite was achieved by the crystallization of metakaolin. The mullite crystallizing out from the liquid phase was rich in silica, as determined by EDS, while that resulting from alumina grains was rich in alumina. The microstructure showed that primary mullite appeared at 1200°C, beside the unreacted alumina particles. On the other hand, at 1400°C alumina particles started to react with the glassy phase to form parallel laminates of mullite.

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

AbstractNew 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.

Low-temperature synthesis of mullite by molten molybdenum trioxide assisted mesoporous aluminum source dissolution

Porous ceramics with more than 99% mullite (3Al2O3·2SiO2) content were fabricated at the low sintering temperature of 800 °C using silicon debris and mesoporous aluminum oxide (Al2O3) obtained from calcining aluminium hydroxide (Al(OH)3) as silicon source and aluminium source, respectively. The study found that adding ammonium molybdate tetrahydrate (AMT) as sintering additive allowed the complete mullitization reaction temperature of the raw materials to be reduced to approach the melting point of molybdenum trioxide (MoO3). Specifically, the silicon debris is continuously oxidized to silicon dioxide (SiO2) in the sintering process, prompting the formation of a silicon-rich liquid phase in the molten MoO3. Meanwhile, a large number of mesopores in the Al2O3 contributed to the dissolution of aluminum source in the silicon-rich liquid phase. Resultantly, synthesizing mullite can be performed at a temperature as low as 800 °C. Additionally, the research showed that the porous mullite ceramics sintered at this temperature had a flexural strength of 22.07 MPa, an open porosity of 52.58%, a linear shrinkage rate of 0.9%, and a bulk density of 1.49 g/cm3. This study not only presents a novel low-cost method for preparing porous mullite ceramics at low temperatures but also provides a simple method for the complete recycling of waste silicon debris.

Effect of MoO3 addition on fly ash based porous and high-strength mullite ceramics: In situ whisker growth and self-enhancement mechanism

In this paper, using only high alumina fly ash as raw material and MoO3 as additive agent, the high-strength porous mullite ceramics were achieved by in situ mullite whisker growth via a calcination method. The influence of MoO3 addition on pore structure and mechanical properties of as-prepared mullite ceramics was investigated. The mechanism of MoO3 promoting the formation of mullite whiskers and improving the strength of matrix was elucidated. The porous mullite ceramic reached its best performance with a high porosity of 49.4% and an improved compressive strength of 13.03 MPa with 10 wt% MoO3 addition calcinated at 1300 °C. Specifically, MoO3 facilitated the mullite whisks growth mainly by reducing melting point of amorphous SiO2/quartz and changing crystal structure of Al2O3. The larger aspect ratio of mullite grain at 1300 °C was easier to form an interlocking structure, which enhanced the strength of mullite ceramic while maintaining high porosity.

Exploiting kaolinite-alumina heteroaggregation in Pickering emulsion stabilisation and porous mullite fabrication

Clay particles have gained considerable attention as a potential option for stabilising Pickering emulsions in recent years, which are promising templates for the development of porous ceramics intended for high temperature applications. However, the development of porous ceramics using this approach requires control over a number of factors such as stability, microstructure, rheology, and processability of the Pickering emulsions. In this work, we discuss a versatile approach for formulating porous mullite ceramics from emulsions stabilised by oppositely charged kaolinite and alumina particles. In this process, we exploit the electrostatic heteroaggregation phenomena in the oppositely charged mixed colloidal particle system. The aggregates formed by electrostatic interaction readily adsorb to the interface created during emulsification and favor the formation of highly stable Pickering emulsions. The influence of parameters such as concentration of each particle in the mixture, pH, and total particle concentration on the microstructure of Pickering emulsions are studied. We further show that processable Pickering emulsions can be engineered by in-situ surface modification of the hetero-aggregates using a short-chain amphiphile. Porous mullite ceramic is obtained by drying and sintering the optimised emulsion formulation. The phase evolution, microstructure, and porosity of the resulting ceramic are characterised. The macroporous mullite developed through this approach can be used as thermal insulator due to low thermal conductivity and tailored microstructure.

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

AbstractExcessive 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.

Open-cell mullite ceramic foams derived from porous geopolymer precursors with tailored porosity

Porous geopolymer precursors were firstly prepared by the direct foaming method using bauxite, fly ash (FA), and metakaolin (MK) as raw materials, and porous mullite ceramics were prepared after ammonium ion exchange and then high-temperature sintering. The effects of chemical foaming agent concentration, ion-exchange time, and sintering temperature on porous geopolymer-derived mullite ceramics were studied, and the optimal preparation parameters were found. Studies have shown that the concentration of blowing agent had great influence on open porosity (<i>q</i>) and porosity and cell size distributions of geopolymer samples, which in turn affected their compressive strength (<i>σ</i>). Duration of the ion exchange had no obvious effect on the sintered samples, and the amount of mullite phase increased with the increase in the sintering temperature. Mullite foams, possessing an open-celled porous structure, closely resembling that of the starting porous geopolymers produced by directly foaming, were obtained by firing at high temperatures. Stable mullite (3Al₂O₃·2SiO₂) ceramic foams with total porosity (<i>ε</i>) of 83.52 vol%, high open porosity of 83.23 vol%, and compressive strength of 1.72 MPa were produced after sintering at 1400 ℃ for 2 h in air without adding any sintering additives using commercial MK, bauxite, and FA as raw materials.