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.     

Enhanced sound absorption properties of porous ceramics modified by graphene oxide films

Under the condition of constant thickness, improving the low-frequency sound absorption performance of conventional sound-absorbing materials is a challenging research topic. To address this issue, a new reduced graphene oxide/polyvinyl alcohol (RGO/PVA) porous composite ceramic was fabricated using freeze-drying and optimized by redesigning the internal connecting pores of porous ceramic matrixes with a reticular microstructure using RGO and PVA. The as-prepared porous structure showed significant enhancement in the low-frequency sound absorption band compared with pristine porous ceramics. In addition, the hybrid porous ceramics exhibited low thermal conductivity. These favorable properties indicate that the hybrid sound-absorbing ceramics have potential application prospects for noise reduction in the fields of construction and electrical and mechanical devices.     

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.     

Microstructure and properties of porous anorthite/mullite whiskers ceramics with high porosity

Porous anorthite/mullite whiskers ceramics with high porosity (>91%) and high strength (>0.45 MPa) have been successfully prepared by foam gel-casting method. Effects of extra mullite whiskers on properties including thermal conductivity and compressive strength at different temperatures were investigated and discussed in terms of microstructure observed through SEM and TEM. The results showed that the addition of extra mullite whiskers in certain content could effectively reduce thermal conductivity, improve the compressive strength both at room and high temperature at same time. When the mullite whiskers content was 20 mol%, the porosity was as high as 91.6 ± 0.19%, the thermal conductivity was low to 0.034 ± 0.003 W/(m·K), and the compressive strength at 1000°C was high to 0.64 ± 0.11 MPa three times to the pure one. Small pores, small grains, and more phase interface or grain boundary caused by the addition of extra mullite whiskers were the main factors for low thermal conductivity. Meanwhile, small pores, closely bonded small grains, and the stable three-dimension network formed by mullite whiskers helped to improve strength.     

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.     

Fabrication and characterization of a mullite-foamed ceramic reinforced by in-situ SiC whiskers

In this study, a new mullite-foamed ceramic, reinforced with in-situ SiC whiskers (MCS) and applied as the insulating lining of thermal equipment used in cement production, was investigated. Compared with a conventional mullite-foamed ceramic (MC), the MCS phase composition, microstructure, compressive strength, thermal conductivity and alkali corrosion resistance were investigated by using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and Factsage® software. The results showed that after being fired in granular coke, the SiC whiskers formed into MCS struts and were distributed in the pores between the interconnected needle-like mullite. Although the formation of SiC with higher conduction slightly increased MCS thermal conductivity, it significantly enhanced the compressive strength and alkali corrosion resistance of the foamed ceramic. Compared with the MC, although the MCS had higher bulk density (3.9%), and higher thermal conductivity (9.5% at 800 °C), it was more important that greatly improved compressive strength (by 60%) and better alkali corrosion resistance was achieved.     

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.     

Preparation and characterization of mullite microspheres based porous ceramics with enhancement of in-situ mullite whiskers

High-strength insulating ceramic materials were prepared using lightweight mullite microspheres with dense surfaces and high internal porosity as the main raw material and silica sol as a binder. The effects of AlF₃·3H₂O content on the in situ formation and growth of mullite whiskers were analyzed by X-ray diffraction and scanning electron microscopy. The obtained results showed that mullite whiskers were formed in large quantities at 1200 °C using AlF₃·3H₂O and V₂O₅ as additives; their optimal growth was observed at 4 wt% AlF₃·3H₂O and 1 wt% V₂O₅. The apparent porosity of the produced specimens was 39%; the MOR and CCS of the specimens were 31 and 152 MPa, respectively; the HMOR at 1300 °C was 11.32 MPa; and the thermal conductivity at 900 °C was 0.783 W m⁻¹ K⁻¹. The staggered whisker network structure formed between mullite microspheres not only improved the mechanical properties of the material, but also refined its pore size, reduced the thermal conductivity, and enhanced the thermal insulation properties.     

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.     

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.     

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).     

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.     

Processing and properties of silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity

Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity were prepared by sintering nano-SiC powder-carbon black template compacts at 600–1200 °C for 2 h in air. The microstructure of the silica-bonded porous nano-SiC ceramics consisted of SiC core/silica shell particles, a silica bonding phase, and hierarchical (meso/macro) pores. The porosity and thermal conductivity of the silica-bonded porous nano-SiC ceramics can be controlled in the ranges of 8.5–70.2 % and 0.057–2.575 Wm⁻¹ K⁻¹, respectively, by adjusting both, the sintering temperature and template content. Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity (0.057 Wm⁻¹ K⁻¹) were developed at a very low processing temperature (600 °C). The typical porosity, average pore size, compressive strength, and specific compressive strength of the porous nano-SiC ceramics were ∼70 %, 50 nm, 2.5 MPa, and 2.7 MPa·cm³/g, respectively. The silica-bonded porous nano-SiC ceramics were thermally stable up to 1000 °C in both air and argon atmospheres.     

Constructing secondary-pore structure by in-situ synthesized mullite whiskers to prepare whiskers aerogels with ultralow thermal conductivity

Ultralow thermal conductivity and ultralight mullite fibers/mullite whiskers composite aerogels (MF/MW) with secondary-pore structure have been prepared via vacuum impregnation and high-temperature treatment. The in-situ generation of mullite whiskers during vapour-solid reaction process and the mechanism of improving thermal stability have been discussed in detail. Under catalysis condition at 1200 °C, the zero-dimensional nanoparticles of SiO₂-Al₂O₃ aerogels are guided to in-situ transform into one-dimensional mullite whiskers. The secondary-pore structure formed by the overlapped fibers and whiskers in MF/MW reduces the thermal conductivity [as low as 0.0488 W/m^?1 K^?1 compared with that of MF preform (0.0698 W/m^?1 K^?1)] and exhibits excellent thermal stability after 1400 °C heat treatment (0.0503 W/m^?1 K^?1) due to the macropores are decreased and gaseous heat transfer being further weakened effectively. Moreover, the MF/MW exhibits good mechanical performance with high critical compressive stress of 0.2809 MPa, which is more than 317% higher than that of MF preform (0.0673 MPa) at room temperature.     

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.     

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.     

Mechanical and thermal behavior of cellular mullite materials

In this paper, the mechanical behavior and thermal properties of cellular mullite bodies obtained by thermal direct-consolidation of foamed aqueous suspensions of mullite-bovine serum albumin (BSA) and mullite-BSA-methylcellulose (MC) were studied. The mechanical behavior of cellular mullite materials sintered at 1600 °C was evaluated by diametral compression at room temperature, 1000 °C and 1300 °C. The variation in the thermal diffusivity and thermal conductivity at temperatures up to 900 °C was determined using the laser-flash method. The results of the mechanical and thermal evaluation were analyzed based on the porosity features of the sintered materials, which was determined in turn by the starting system used for shaping the bodies.     

Fibrous porous ceramics with devisable phenolic resin reinforcing layer

Fibrous porous ceramics with devisable phenolic resin reinforcing layer were fabricated using low cost atmospheric impregnation technology at room temperature. In combination with additional sealing method, phenolic resin reinforcing layer with controllable thickness could be obtained on the surface of fibrous porous ceramics. Typical gradient profile was observed along the thickness direction of impregnation. The effects of the phenolic resin reinforcing layer on mechanical properties and thermal insulation properties were studied. The results revealed that compressive strength increased from 1.70?MPa to 2.61?MPa, tensile strength increased from 0.78?MPa to 0.91?MPa, and flexural strength increased from 9.55?MPa to 10.89?MPa with the phenolic resin layer increasing from 0?mm to 9?mm. Simultaneously, room-temperature thermal conductivity increased from 0.051?W/(m·K) to 0.055?W/(m·K). In addition, the impact resistance of the surface of the material was obviously improved. The contact angel of the surface of the material exceeded 125°, which effectively improved the environmental adaptability.     

Macroporous mullite materials prepared by novel shaping strategies based on starch thermogelation for thermal insulation

A rigorous microstructural analysis of porous mullite materials developed using novel shaping strategies based on the starch consolidation casting, and their thermal properties in relation to the processing and starch type were accomplished in view of their use as thermal insulators. In order to characterize the size and morphology of pore, basic size and 2D shape factors, and global 3D stereological parameters were determined using microscopy techniques. Results indicated that the porosity volume, pore connectivity degree, and mean free path were the determining factors of the lowest heat transfer by conduction registered in materials prepared with cassava starch. This material is the best candidate to be used in thermal insulation.     

The preparation of mullite foamed ceramics reinforced by in-situ SiC whiskers and their reinforcement mechanism

A SiC whisker-bonded mullite foamed ceramic was prepared by using white clay, industrial alumina and silicon powder as raw materials without solid carbon sources. The XRD, SEM, EDS, and Factsage® software were used to investigate the effect of sintering temperature on the phase composition, microstructure, compressive strength, and Young's modulus of foamed ceramics. Additionally, the synthesis reaction of in-situ SiC whiskers and the effect of their formation on the properties of ceramics were studied. The results showed that the in-situ SiC whiskers with dendrite shapes were formed after firing above 1300 °C at the expense of Si/SiO vapors as well as CO vapor, though there were no solid carbon sources in raw materials, which provided a new idea for the synthesis of SiC whiskers. The formation of SiC whiskers was helpful for improving the compressive strength and Young's modulus of mullite foamed ceramics remarkably. Furthermore, the reinforcement mechanism has been investigated systematically.