Multi-functional mullite fiber-based porous ceramics with a multilevel pore structure assembled by alumina platelets and mullite whiskers

Mullite fiber-based porous ceramics have been widely used in the field of heat insulation. To further broaden their applications in other fields, such as filtration and sound absorption, mullite whiskers and alumina platelets were introduced as the secondary structural materials in mullite fiber-based porous ceramics by a sol-gel combining heat-treating method, and new fiber-based porous ceramics with a unique multilevel pore structure were developed. By adjusting the molar ratios of aluminium tri-sec-butoxide to aluminium fluoride and calcination temperature, these fiber-based porous ceramics not only presented the characteristics of lightweight (maximum density of 0.38 g/cm³) and good heat insulation (minimum thermal conductivity of 0.11 W/mK) comparable to traditional fiber-based porous ceramics, but also showed a superior specific surface area (up to 11.5 g/m²) and excellent sound absorption performance (average sound absorption coefficient as high as 0.728). Owing to these outstanding characteristics, the corresponding porous ceramics are expected to be promising multifunctional materials in diverse fields, especially thermal insulation and sound absorption.     

High-strength thermal insulating mullite nanofibrous porous ceramics

Mullite fibrous porous ceramics is one of the most commonly used high temperature insulation materials. However, how to improve the strength of the mullite fibrous porous ceramics dramatically under the premise of no sacrificing the low sample density has always been a difficult scientific problem. In this study, the strategy of using mullite nanofibers to replace the mullite micron-fibers was proposed to fabricate the mullite nanofibrous porous ceramics by the gel-casting method. Results show that mullite nanofibrous porous ceramics present a much higher compressive strength (0.837 MPa) than that of mullite micron-fibrous porous ceramics (0.515 MPa) even when the density of the mullite nanofibrous porous ceramics (0.202 g/cm³) is only around three quarters of that of the mullite micron-fibrous porous ceramics (0.266 g/cm³). The obtained materials that present the best combination of mechanical and thermal properties can be regarded as potential high-temperature thermal insulators in various thermal protection systems.     

The preparation and properties of high-strength porous mullite ceramics by a novel non-toxic gelcasting process

A novel approach to fabricate porous mullite ceramics with homogeneous pore size and high-strength using green non-toxic and cost-effective poly-γ-glutamic acid (γ-PGA) gelling system was reported for the first time. Effect of γ-PGA addition, additive amount and solid loading on rheological behavior of the slurries, and microstructure and properties of samples were investigated systematically. By optimizing the solid loading of mullite samples, we are able to get the sample with small pores (< 200 µm) dominating (93.3% of the total pores), and compressive strength of the sample reaches up to 26.62 MPa. In addition, the mullite ceramics exhibited high porosity of 75.7% with low thermal conductivity of 0.279 W/(m·K) at room temperature. This study not only provides a green and non-toxic gelling system but also offers porous mullite ceramics with low thermal conductivity and excellent mechanical strength as an energy-saving thermal insulation material.     

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.     

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.     

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.     

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.     

A novel way to fabricate fibrous mullite ceramic using sol-gel vacuum impregnation

Inspired by bird's nest structure, fibrous mullite ceramic was fabricated by vacuum impregnation with mullite fibers as raw material and zirconia sol-gel as inorganic binder. The effect of impregnation times on the properties of the fibrous mullite ceramic, such as porosity, microstructure, compressive strength and room-temperature thermal conductivity were investigated. The results showed that low density (0.45–0.66?g/cm³), relative high compressive strength (0.62–3.34?MPa) and low thermal conductivity (0.037–0.125?W/mk) were exhibited for the sample. The toughness of fibrous mullite ceramic was enhanced due to the micro-creaks caused by transformation of zirconia. From the experimental results, it is suggested that it was an optimal method which have the potential use in high-temperature thermal insulation materials to produce fibrous mullite 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.     

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.     

Fabrication of ultralight heat-insulating hollow-fiber mullite fiberboards

Mullite fiberboards have been extensively used in heat-insulating refractory materials. To further improve the thermal insulation properties and reduce the density, we fabricated mullite fiberboards by vacuum filtration using hollow mullite fibers based on a ceiba fiber template. The effects of sintering temperature and type and content of high-temperature adhesives on the density, compressive strength, thermal conductivity, microstructure, and volume stability of mullite fiberboards were investigated. The results showed that the obtained mullite fiberboards have ultralow density of 0.1–0.2 g/cm³, low thermal conductivity of 0.0988–0.1230 W/(m·K) (at 500 °C), and compressive strength of 0.08–0.12 MPa, and they exhibit good volume stability at 1300 °C.     

Porous ceramics based on high-thermal-stability Al2O3–ZrO2 nanofibers for thermal insulation and sound absorption applications

Al₂O₃ fibers are promising candidates for porous ceramics, but the sudden growth of grains in the fibers above 1200 °C will limit their applications for high temperature. Herein, we reported the successful fabrication of the Al₂O₃–ZrO₂ nanofibers by electrospinning and the nanofiber-based porous ceramics by a combination of gel-casting, freeze-drying and high-temperature sintering. Results show that the addition of Zr could greatly improve the thermal stability (up to 1400 °C) of the Al₂O₃-based nanofibers, owing to the inhibition of the sudden growth of the grains in the fibers at high temperature. The Al₂O₃–ZrO₂ nanofiber-based porous ceramics after sintering at 1100–1400 °C possessed a multi-level pore structure and exhibited high thermal stability, ultra-high porosity (97.79–98.04%), ultra-low density (0.075–0.091 g/cm³) and thermal conductivity (0.0474–0.0554 W/mK), and excellent sound absorption performance with the average sound absorption coefficient of 0.598–0.770. These porous ceramics are expected to be employed in the fields of high-temperature thermal insulation and sound absorption.     

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.     

Lightweight and thermally insulating aluminum borate nanofibrous porous ceramics

Aluminum borate porous ceramics are excellent candidates for high-temperature insulation applications. Current research on aluminum borate-based porous ceramics mainly focuses on porous ceramics made up of aluminum borate whiskers, whose low aspect ratio leads to a relatively dense porous structure; this results in porous ceramics with low porosity and relatively high thermal conductivity. In this study, we report the manufacturing of aluminum borate nanofibrous porous ceramics by an agar-based gel casting method using electrospun nanofibers with a high aspect ratio as the three-dimensional skeleton structure. We explored the effect of the alumina/boron oxide molar ratio on the microscopic morphology and crystal phase composition of the aluminum borate nanofibers and that of the sintering temperature on the micro and macro properties of porous ceramics based on the nanofibers. The results showed that aluminum borate nanofibers with an alumina/boron oxide molar ratio of 7:2 had the densest microscopic morphology, and the corresponding porous ceramics exhibited a higher porosity (91%) and lower thermal conductivity (0.11 W m^?1 K^?1) after sintering at 1200 °C than aluminum borate porous ceramics with aluminum borate whiskers as the skeleton. The successful synthesis of aluminum borate nanofibrous porous ceramics provides new insights into the development of high-temperature insulators.     

Multilayered mullite ceramics with anisotropic properties

Anisotropic layered porous ceramics are crucial to satisfy the demand for devices with directional functions. However, challenges, such as complicated preparation processes and difficulties in regulating the oriented structure, severely limit their application. Here, multilayered mullite ceramics (MLMCs) with specific porosities and strongly anisotropy properties, were prepared by designing porous thin-layered units and an interlayer layout, in combination with a simple gel-casting. Benefiting from the suitable slurry properties, the samples did not show obvious defects, and a perfect bonding was observed between the layers. The optimized MLMCs exhibited a porosity of 65.12%, the differences in compressive and flexural strengths of 14 ( ± 0.7) and 5 ( ± 0.2) MPa in different pressure directions respectively, and the anisotropy factor of thermal conductivity up to 0.98, as well as exhibited good high temperature thermal insulation properties. Ultimately, the MLMCs formed transverse heat conductor and longitudinal heat insulation heat-transfer patterns, with promising applications in thermal insulation.     

Lightweight porous silica-alumina ceramics with ultra-low thermal conductivity

Thermal protection has always been an important issue in the energy, environment and aerospace fields. Porous ceramics produced by the particle-stabilized foaming method have become a competitive material for thermal protection because of their low density and low thermal conductivity. However, the study of porous ceramics for composite systems using particle-stabilized foaming method was relatively rare. Here, silica-alumina composite porous ceramics were prepared by particle-stabilized foaming method, which was achieved by tailoring the surface charges of silica and alumina through adjustment of the pH. Porous ceramics exhibited porosity as high as 97.49% and thermal conductivity (25 °C) as low as 0.063 W m^?1 K^?1. The compressive strength of porous ceramics sintered at 1500 °C with a solid content of 30 wt% could reach 0.765 MPa. Based on the light weight and excellent thermal insulation properties, the composite porous ceramic could be used as a potential thermal insulation material in the spacecraft industry.     

Mechanical evaluations of mullite fibrous ceramics processed by filtration and in situ pyrolysis of organic precursor

Highly porous mullite fiber-based ceramics with a three-dimensional cross structure were successfully designed and fabricated by filtration method. To address the thermal migration of silica sol, polysiloxane was introduced in the bonding system as high temperature binder to form more stable nods in the skeleton as opposed to silica sol which would migrate to the surface of fibers during the sintering process. Due to the lapped structure of mullite fibers, all products showed high porosity(74.1%～80.9%), low density(0.554g/cm³～0.608g/cm³) and relatively low thermal conductivity(<0.14 W/(m*K)). The reaction process of in situ porolysis and fracture mechanisms were illustrated, respectively. Besides, results showed that both sintering temperature and binder content had significant influence on the microstructure and mechanical properties of the products.     

Synthesis of hierarchically porous mullite ceramics with improved thermal insulation via foam-gelcasting combined with pore former addition

ABSTRACT

To further improve the thermal insulation performance of porous mullite ceramics used in important industrial sectors, a combined foam-gelcasting and pore-former addition approach was investigated in this work, by which hierarchical porous mullite ceramics with excellent properties, in particular, thermal insulation property, were prepared. Both mesopores (2–50?nm) and macropores (117.8–202.7?μm) were formed in porous mullite ceramics resultant from 2?h firing at 1300°C with various amounts of submicron-sized CaCO₃ pore former. The former mainly arose from the decomposition of CaCO₃, and the latter from the foam-gelcasting process. The porous samples prepared with CaCO₃ addition had low linear shrinkage of 2.35–4.83%, high porosity of 72.98–79.07% and high compressive strength of 5.52–14.82?MPa. Most importantly, they also exhibited a very low thermal-conductivity, e.g. 0.114?W?m^?1?K^?1 at 200°C, which was much lower than in the cases of their counterparts prepared via the conventional foam-gelcasting route.     

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.     

Fabrication of lightweight ZrO2 fiberboards using hollow ZrO2 fibers

ZrO₂ fiberboards with ultra-low densities (0.34–0.40 g/cm³) were fabricated using biomorphic ZrO₂ hollow fibers, which have a lower density and better thermal insulation than traditional ZrO₂ solid fibers. The effects of sol binder content, sintering temperature, and proportion of solid fibers on the density, microstructure, compressive strength, linear shrinkage, and thermal conductivity of lightweight ZrO₂ fiberboards were investigated. The results showed that the hollow features of biomorphic ZrO₂ fibers were successfully maintained after they were made into ZrO₂ fiberboards, which made them less dense and thermally conductive. The best conditions were found to be a sol binder content of 30 vol%, sintering temperature of 1400 °C, and 20 wt% sintered solid fibers to balance thermal insulation and compressive strength. The results show that the density and thermal conductivity of lightweight ZrO₂ fiberboard gives it obvious advantages as a heat-insulating ceramic. Specifically, when the sintering temperature was 1400 °C, the sample had an ultra-low density of 0.34–0.40 g/cm³, a thermal conductivity of 0.101–0.116 W/(m·K) (at 500 °C), a compressive strength of 0.05–0.24 MPa, and a linear shrinkage of 9.4–13%.