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.     

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.     

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.     

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.     

Method for fabricating microwave absorption ceramics with high thermal conductivity

In this work, a high-performance microwave absorption ceramic together with high thermal conductivity was proposed through the introduction of phenolic resin as the carbon source into AlN ceramic substrate. The phenolic resin was initially mixed with AlN powder and sintering additives in ethanol to form homogeneous slurries, followed by drying, pyrolysis, dry pressing and pressureless sintering to develop nano carbon in situ in AlN substrate. The well controlled microstructure with homogeneous distribution of nano sized carbon lead to the high thermal conductivity and excellent microwave absorption properties. by adjusting the phenolic resin content, the ceramic showed a minimal reflection coefficient of about ?30 dB with an effective bandwidth of about 2 GHz together with a high thermal conductivity of about 135.1 W/m K.     

Recycling of waste red mud for fabrication of SiC/mullite composite porous ceramics

SiC/mullite composite porous ceramics were fabricated from recycled solid red mud (RM) waste. The porous ceramics were formed using a graphite pore forming agent, RM, Al(OH)₃ and SiC in the presence of catalysts. The influence of firing temperature and the pore-forming agent content on the mechanical performance, porosity and the microstructure of the porous SiC ceramics were investigated. Optimal preparation condition were determined by some testing. The results indicated that the flexural strength of specimens increased as a function of firing temperature and a reduction in graphite content, which concomitantly decreased porosity. The ceramic prepared under optimal conditions having 15?wt% graphite and sintered at 1350?°C, demonstrated excellent performance. Under optimal preparation conditions the flexural strength and porosity of the ceramic were 49.4?MPa and 31.4%, respectively. Scanning electron microscopy observation result showed that rod-shape mullite grains endowed the samples with high flexural strength and porosity. X-ray diffraction analysis indicated that the main crystallization phases of the porous ceramics were 6H-SiC, mullite, cristobalite and alumina. This work demonstrates that RM can be sucessfully reused as a new raw material for SiC/mullite composite porous ceramics.     

Foam gel-casting preparation of SiC bonded ZrB2 porous ceramics for high-performance thermal insulation

Lightweight SiC-ZrB₂ porous ceramics is of great potential as thermal insulation material used in aerospace, chemical and energy industries. In this work, a series of SiC bonded ZrB₂ (SiC_b-ZrB₂) porous ceramics with porosity high up to 86.9% were prepared by a simple foam gel-casting method. The SiC_b-ZrB₂ porous ceramic prepared at 1573 K exhibited a low thermal conductivity of 0.280 W/(m?K) and a reasonable compressive strength of 0.52 MPa. It could maintain the original geometric shape and microstructure after a secondary heat treatment at 1473 K in inert atmosphere. When heating the samples with thickness of 30 mm for 12 min with an alcohol spray lamp (～1273 K), the temperatures of the cold sides of SiC_b-ZrB₂ ceramics were all lower than 432 K, demonstrating their exceptional insulation capabilities. The present work provides a simple route to produce robust and thermally-insulating non-oxide porous ceramics for use under high temperature.     

In situ synthesis and thermal shock resistance of mullite used for thermal storage ceramics in solar thermal power generation

Abstract

Mullite ceramic, as one of high performance thermal storage ceramics for solar thermal power generation systems, was in situ fabricated via semidry pressing and pressureless sintering in the air. Andalusite (57–68 wt-%) and calcined bauxite (24–29 wt-%) were used as the raw materials, with kaolin and a tiny of boric acid being added to promote the densification and improve the mechanical properties. The best physical properties and thermal shock resistance were obtained on an optimum A3 sample sintered at 1600°C for 3 h, i.e. a bending strength of 120·44 MPa and 30 cycles thermal shock cycling without cracking (wind cooling from 1000°C to room temperature) with a loss of bending strength of 8·7%.     

Influence of ZrO2 and WO3 doping additives on the thermal properties of porous mullite ceramics

Porous mullite-corundum refractory ceramics were produced by a patented slurry slip casting method from compositions based on commercially available α-Al₂O₃ and γ-Al₂O₃, fused SiO₂ and kaolin. Pores were formed as a result of a chemical reaction of aluminium with water. The influence of usage of raw materials and doping additives such as micro-size ZrO₂ and WO₃ on the sintering temperature, formation of crystalline phases, linear thermal expansion, thermal conductivity and thermal shock resistance of mullite-corundum ceramic was studied. The best thermal shock resistance and, simultaneously, lower thermal conductivity was achieved for the samples doped with WO₃. This was due to the influence of micro-sized WO₃ on the change in γ-Al₂O₃ modification to α-Al₂O₃ and on the structure of mullite ceramics.     

Investigation of the thermal shrink mechanism,thermal conductivity and compressive resistance of TaTiP3O12 ceramics

Dense TaTiP₃O₁₂ ceramics were synthesized by the solid-state method and spark plasma sintering (SPS) with 6 wt% V₂O₅ as a sintering aid, and their phase, microstructure, thermal conductivity, hardness, compressive strength, and expansion property and mechanism were investigated. Results show that the pure phase can be achieved by the two methods. In particular, the sample prepared by SPS possesses a relative density of 97.62% and a porosity of 3.07%, and has better properties than that prepared by the solid-state method. The SPS sample has a thermal conductivity at room temperature of 2.03 w/(m· °C), a Vickers hardness of 4.34 GPa and a compressive strength of 175.98 MPa, which are 0.95, 1.49 and 1.59 times greater than those of the sample prepared by the solid-state method, respectively. In addition, the TaTiP₃O₁₂ ceramic prepared by SPS exhibits a linear ultralow negative thermal expansion property with a coefficient of thermal expansion of ?0.74 × 10^?6 °C ^?1 (-100–400 °C). The negative thermal expansion in TaTiP₃O₁₂ is induced by the coupling effect of [Ta(Ti)O₆] octahedron and [PO₄] tetrahedron caused by the transverse vibration of bridging oxygen atoms.     

Porous alumina ceramics with enhanced mechanical and thermal insulation properties based on sol-treated rice husk

To improve the properties of porous alumina ceramics, which were typically prepared by adding pore-forming agents, rice husk (RH) as pore-forming agent was pretreated with zirconia sol. The effects of sol-treatment on the thermal conductivity and compressive strength of the resultant ceramics were characterized. Furthermore, the pore size distribution, pore shape, microstructure, and phase evolution also were studied. The results showed that the RH pretreatment optimizes the microstructure of the ceramic pores. Moreover, complete morph-genetic RH is clearly observed in the pores, which is established as a key factor in improving the properties of the resultant ceramic. The thermal insulation properties are determined to significantly improve, although the thermal conductivity increases slightly with the increment of zirconia sol concentration from 5 to 10?wt%. Meanwhile, after sintering at 1550?°C, the compressive strength is significantly greater for the specimen prepared with 10?wt% zirconia sol-treated RH (65.56?MPa) than that with untreated RH (43.37?MPa). Hence, it was demonstrated that the use of zirconia sol-pretreated RH as a pore-forming agent could enhance the mechanical and thermal insulation properties of porous alumina ceramics.     

Fabrication of in-situ self-reinforced Si3N4 ceramic foams for high-temperature thermal insulation by protein foaming method

To meet demand for lightweight and high-strength ceramic foams, in-situ self-reinforced Si₃N₄ ceramic foams, with compressive strength of 13.2–45.9 MPa, were fabricated by protein foaming method combined with sintered reaction-bonded method. For comparison, ordinary protein foamed ceramics with irregular block microstructure were fabricated via reaction-bonded method, which had compressive strength of 3.6–20.5 MPa. Physical properties of these two types of samples were systematically compared. When open porosity was about 80%, both types of Si₃N₄ ceramic foams had excellent thermal insulation properties (<0.15 W m^?1 K^?1), while compressive strength of in-situ self-reinforced samples increased by more than 158% compared with ordinary samples. Under high-temperature oxidation conditions, microstructures of both types of samples were deformed with increase in oxidation temperature. Moreover, after oxidation temperature was increased to 1400 °C, oxidation weight gain decreased from 18.07% for ordinary samples to only 2.18% for self-reinforced samples. Thus, high-temperature oxidation resistance of Si₃N₄ ceramic foams was greatly improved.     

Effects of pore structure on thermal conductivity and strength of alumina porous ceramics using carbon black as pore-forming agent

In the present work, carbon black (CB) works as a pore-forming agent in the preparation of alumina porous ceramics. The pore structures (i.e. mean pore size, pore size distribution and various pores size proportions) were characterized by means of Micro-image Analysis and Process System (MIAPS) software and mercury intrusion porosimetry. Then their correlation and thermal conductivity as well as strength were determined using grey relation theory. The results showed that the porosity and mean pore size increased against the amount of CB, whereas the thermal conductivity, cold crushing strength and cold modulus of rupture reduced. The <2 μm pores were helpful for enhancing the strength and decreasing the thermal conductivity whereas the >14 μm pores had the opposite effects.     

Effect of recoating slurry compositions on the microstructure and properties of SiC reticulated porous ceramics

Silicon carbide reticulated porous ceramics (SiC RPCs) were fabricated by polymer sponge replica technique, followed by recoating with SiC slurries of two different sintering additives of MgO–Al₂O₃–SiO₂ (Slurry 1) and polycarbosilane (Slurry 2). The sintering temperature of SiC RPCs recoated with Slurry 2 was 1100 °C, which was 200 °C lower than that for one recoated with Slurry 1. The prepared SiC RPCs exhibited homogeneous microstructure and contained pores with different sizes which was entrapped in the strut of SiC RPCs, small pores with diameter lower than 4 μm and large pores with diameter higher than 10 μm. Bending strength of SiC RPCs recoated with Slurry 1 was two times higher than that for the non-recoated samples, which was 1.88 MPa and was a little higher than that for one recoated with slurry 2. At the same time, high thermal shock resistance and high refractoriness were achieved for SiC RPCs recoated with Slurry 2.     

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.     

A novel and green preparation of porous forsterite ceramics with excellent thermal isolation properties

This work provides a novel and green approach to preparing porous forsterite ceramics by a transient liquid phase diffusion process based on fused magnesia and quartz powders without detrimental additives. The size of quartz particles markedly affected the sintering behaviors, phase composition, microstructure and properties of the porous forsterite ceramics. Fine quartz particles (D₅₀, 3.87?µm) accelerated the rate of the forsterite formation at elevated temperatures and promoted solid-state sintering behavior of the porous ceramics. Conversely, coarse quartz particles (D₅₀, 25.38?µm) reduced the rate of the solid state reaction and a large amount of unreacted SiO₂ and enstatite (MgSiO₃) phases transformed into a transient liquid-phase during the firing process. This effect resulted in a high porosity (approximately 58.89%) and formation of many large pores (mean pore size of 42.36?µm). These features contributed to the excellent thermal isolation properties of the prepared porous forsterite ceramics. The strength of the obtained porous ceramics (about 23.6?MPa) is relatively high compared with those of conventional ceramics.     

Preparation and performance study of cordierite/mullite composite ceramics for solar thermal energy storage

The employment of solar energy in recent years has reached a remarkable edge. It has become even more popular as the cost of fossil fuel continues to rise. Energy storage system improves an adjustability and marketability of solar thermal and allowing it to produce electricity in demand. This study attempted to prepare cordierite/mullite composite ceramics used as solar thermal storage material from calcined bauxite, talcum, soda feldspar, potassium feldspar, quartz, and mullite. The thermal physical performances were evaluated and characterized by XRD, SEM, EPMA, and EDS. It was found that the optimum sintering temperature was 1280°C for preparing, and the corresponding water adsorption was 11.25%, apparent porosity was 23.59%, bulk density was 2.10 mg·cm^?3, bending strength was 88.52 MPa. The residual bending strength of specimen sintered at 1280°C after thermal shock of 30 times decreased to be 57 MPa that was 36% lower than that before. The thermal conductivity of samples sintered at 1280°C was tested to be 2.20 W·(m·K)^?1 (26°C), and after wrapped a PCM (phase change materials) of K₂SO₄, the thermal storage density was 933 kJ·kg^?1 with the temperature difference (ΔT) ranged in 0‐800°C. The prepared cordierite/mullite composite ceramic was proved to be a promising material for solar thermal energy storage.