Foam-gelcasting preparation of high-strength self-reinforced porous mullite ceramics

High-strength self-reinforced porous mullite ceramics were prepared via foam-gelcasting using mullite powder as a main raw material, AlF₃·3H₂O (0–8 wt%) as an additive, Isobam-104 as a dispersing and gelling agent, sodium carboxymethyl cellulose as a foam stabilizing agent, and triethanolamine lauryl sulfate as a foaming agent. The effects of AlF₃·3H₂O content on rheological and gelling behaviors of the slurries, and porosity and mechanical properties of self-reinforced porous mullite samples were examined. Addition of AlF₃·3H₂O promoted the in-situ formation of elongated mullite in the fired porous samples, which improved considerably their mechanical properties. Compressive strength and flexural strength of 67.0% porous mullite ceramics prepared with addition of 6 wt% AlF₃·3H₂O was as high as 41.3 and 13.9 MPa, respectively. Its hot modulus rupture (HMOR) increased initially with the testing temperature, and peaked (with a maximum value of 16.6 MPa) at 800 °C above which it started to decrease with the testing temperature. Nevertheless, it was still retained as high as 6.7 and 2.8 MPa at 1200 and 1400 °C, respectively.     

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.     

Characteristics of porous mullite developed from clay and AlF3·3H2O

Porous mullite with interlocked needle shape microstructure was developed from China clay and aluminium fluoride trihydrate (AlF₃·3H₂O). The effects of various parameters like sintering temperature, sintering time and the amount of AlF₃·3H₂O on the phase evolution, microstructure and porosity have been studied. Quantitative analysis of mullite was carried out using X-ray diffraction combined with Rietveld-RIR method (Internal standard method). Porous mullite ceramics with 62% open porosity have been prepared at a relatively lower temperature of 1400?°C. The results show that the nucleation of mullite can be achieved from 700?°C onwards using hydrated aluminium fluoride without the formation of intermediate topaz crystals. The high amount of water vapour produced within the system during the in-situ reaction has a crucial role in deciding the reaction mechanism.     

Effects of AlF3 and MoO3 on properties of Mullite whisker reinforced porous ceramics fabricated from construction waste

Mullite-whisker-reinforced anorthite-mullite-corundum porous ceramics were prepared from construction waste and Al₂O₃ powder by adding AlF₃ and MoO₃ as the additive and crystallization catalyst, respectively. The effects of AlF₃ and MoO₃ content on the properties of mullite whiskers, such as open porosity, mechanical properties, pore size distribution, microstructure and phase structure, were investigated in detail. The results showed that the morphology of the mullite whiskers and properties of the porous ceramics were greatly influenced by the AlF₃ and MoO₃ content. The specimen obtained by co-adding 12 wt% AlF₃ and 3 wt% MoO₃, and sintering at 1350 °C for 1 h, exhibited excellent properties, including an open porosity of 67.4±0.5% and biaxial flexural strength of 24.0±0.8 MPa. The mullite whiskers were uniformly distributed; the whiskers had a diameter of 0.05–0.5 µm, length of 8–10 µm, and aspect ratios (length to diameter ratio) of 20–30 on average.     

Fabrication and characterization of anorthite–mullite–corundum porous ceramics from construction waste

In this work, anorthite–mullite–corundum porous ceramics were prepared from construction waste and Al₂O₃ powders by adding AlF₃ and MoO₃ as mineralizer and crystallization catalyst, respectively. The effects of the sintering temperature and time on open porosity, mechanical properties, pore size distribution, microstructure, and phase composition were characterized in detail. The results showed that the formation of the mullite whiskers and the properties of the anorthite–mullite–corundum porous ceramics depended more on the sintering temperature than the holding time. By co-adding 12 wt% AlF₃ and 4 wt% MoO₃, mullite whiskers were successfully obtained at sintering temperatures upon 1350 °C for 1 h. Furthermore, the resultant specimens exhibited excellent properties, including open porosity of 66.1±0.7%, biaxial flexural strength of 23.8±0.9 MPa, and average pore size of 1.32 µm (the corresponding cumulative volume percent was 37.29%).     

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.     

Industrial waste silica-alumina gel recycling: Low-temperature synthesis of mullite whiskers for mass production

The increasing demand for mullite whisker-reinforced, toughened ceramic materials and mullite raw materials that meet industrial requirements has prompted the search for new and alternative sources, as well as effective technologies to obtain the target products. In this work, mullite whiskers of high purity were synthesized by a vapor-liquid–solid (V-L-S) process using industrial waste silica-alumina gel and Al₂(SO₄)₃·18H₂O as raw materials, with AlF₃·3H₂O and Na₂SO₄ as additives. The effects of sintering temperatures on the mullitization reactions and mullite morphology were investigated by XRD, TG-DTA, SEM and so forth. The results suggest that the introduction of AlF₃·3H₂O and Na₂SO₄ alters the mullitization reaction path, which leads to an initial mullitization reaction temperature of 720 °C. The SEM results demonstrate that mullite whiskers transformed from secondary growth to anisotropic growth when the sintering temperature was increased from 720 °C to 825 °C. By analyzing the experimental results, the mechanism of AlF₃·3H₂O-assisted growth of mullite whiskers with Na₂SO₄ as the liquid phase template is proposed based on the “dissolution-precipitation” process. Herein, a novel and feasible solution for the recycling of silica-rich industrial waste is proposed, which offers new and simple insights into the high value-added recycling of industrial waste, which provides new ideas for the actual mass production of mullite whiskers.     

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

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

Influence of V2O5 content on ammoxidation of 3-picoline over V2O5/AlF3 catalysts

V₂O₅/AlF₃ catalysts with V₂O₅ loadings ranging from 2 to 15 wt% were prepared by the conventional wet impregnation method, using nonporous AlF₃·3H₂O sample as the support for impregnating NH₄VO₃. It was found that the catalysts evolve porous structures upon calcination at 723 K. The influence of V₂O₅ content was studied on ammoxidation of 3-picoline on the reduced catalysts. The catalyst with 15 wt% V₂O₅ exhibited the highest selective ammoxidation acitivity towards nicotinonitrile. The XRD and oxygen chemisorption studies revealed that vanadia is in a highly dispersed state in the catalysts.     

Porous acicular mullite ceramics fabricated with in situ formed soot oxidation catalyst obtained from waste MoSi2

Porous acicular mullite (3Al₂O₃·2SiO₂) ceramics containing Cu₃Mo₂O₉ as a soot oxidation catalyst was fabricated by a novel approach using commercial powders of Al₂O₃ and CuO, and powder obtained by controlled oxidation of ground waste MoSi₂. The obtained material consisted of elongated mullite grains which are known to be effective in carbon soot removal from diesel engine exhaust. The presence of in situ created Cu₃Mo₂O₉ was found to catalyze the carbon burnout which is an extremely important feature when it comes to filter regeneration, i.e., the captured soot removal. The carbon burnout temperature in the sample containing 12 wt% CuO was by 90 °C lower than that in the sample without CuO. Effect of sintering temperature as well as the effect of amount of CuO additive on mullite properties were studied. It was found that the increase in amount of CuO in samples sintered at 1300 °C decreased porosity and increased compressive strength of the porous mullite ceramics. The addition of 12 wt% CuO increased the strength of the porous mullite ceramics up to 70 MPa, whereas the porosity was reduced from 62% in the mullite without CuO to 44% in the mullite ceramics containing 12 wt% CuO. Although affected by the amount of CuO, the microstructure still consisted of elongated mullite grains.     

Preparation of Al2O3-mullite thermal insulation materials with AlF3 and SiC as aids by microwave sintering

Al₂O₃-mullite composites were prepared under the synergy effect of AlF₃ and SiC aids by microwave heating. The phase composition, microstructure, porosity, flexural strength, thermal shock resistance, and thermal conductivity were investigated. The XRD results revealed that the content of mullite phase steadily increased with the increasing of AlF₃ content. The microstructure showed that the lower content (≤1 wt%) of AlF₃ led to the formation of granular mullite and the higher content (≥3 wt%) of AlF₃ led to the formation of mullite whiskers, which could form an interlocking structure. In addition, the SiC hot spots can also promote the generation of mullite whiskers by microwave sintering. The thermal shock resistance was significantly improved by the interlocking structure of mullite whiskers. The residual rate of flexural strength of the composite with 3 wt% AlF₃ was 86%. The composite with 3 wt% AlF₃ additives got its optimized thermal conductivity from 30°C to 950°C, the value was between 0.819 and 1.021 W/(mK), which possess excellent thermal insulation performance.     

Preparation and characterization of mullite whisker reinforced ceramics made from coal fly ash

Ceramics with mullite whiskers were prepared from coal fly ash and Al₂O₃ raw materials, with AlF₃ used as an additive. The phase structures and microstructures of the ceramics were identified via X-ray diffraction and scanning electron microscopy, respectively. The results show that pickling of coal fly ash is an effective method for enhancing the flexural strength of ceramics. Sintering temperature and AlF₃ addition were also key factors influencing the creation of ideal ceramics. The ceramic made from pickled coal fly ash, 6?wt% AlF₃, and sintered at 1200?°C, exhibited the highest flexural strength of 59.1?MPa, and had a bulk density of 1.32?g/cm³ and porosity of 26.8%. The results show that ceramic materials made under these conditions are ideal candidates for manufacturing ceramic proppants for the exploitation of unconventional oil and gas resources.     

Kinetic study on the anisotropic grain growth of elongated iron-containing mullite

Elongated iron-containing mullite exhibiting anisotropic grain growth was prepared using fused silica, α-alumina, and Fe₂O₃ powders as raw materials at elevated temperatures. The effects of different Fe₂O₃ contents on the anisotropic growth of mullite columnar crystals were investigated, and phase compositional analysis and structural characterisation were carried out by X-ray diffraction and field emission scanning electron microscopy. The kinetic index and activation energy for the anisotropic grain growth of elongated mullite crystals were calculated, and the surface energy of each crystal plane of the elongated crystal and the solid solution behavior of the iron ions in the mullite were calculated using the density functional theory (DFT) approach. It was found that the addition of Fe₂O₃ promotes the anisotropic grain growth of mullite, and the aspect ratio of the elongated crystals increased upon increasing the Fe₂O₃ content. The solid solubility upper limit of the iron-containing mullite in this system is 9 wt% of iron addition. After sintering at 1973 K, for Fe₂O₃ contents of 3, 6, and 9 wt%, the growth kinetics indices obtained by calculations in the longitudinal direction were 2.22, 2.50, and 3.85, while the reaction activation energies were 457.3, 442.1, and 411.9 kJ mol⁻¹. The calculation results on the surface energy of the elongated mullite crystal showed that the smallest surface energy is on the (001) crystal plane, which is the preferred growth direction. Upon comparison of the surface energies for different Fe₂O₃ contents on the (001) surface, a decrease was observed upon increasing the quantity of Fe₂O₃, and both the growth tendency of the mullite crystals and the aspect ratio of the elongated crystals also increased. The refractoriness under load of the prepared elongated iron-containing mullite was 1664 °C, demonstrating its excellent high-temperature mechanical properties.     

Sintering and microstructural study of mullite prepared from kaolinite and reactive alumina: Effect of MgO and TiO2

Mullite ceramic was prepared using kaolinite and synthesized alumina (combustion route) by solid-state interaction process. The influence of TiO₂ and MgO additives in phase formation, microstructural evolution, densification, and mechanical strengthening was evaluated in this work. TiO₂ and MgO were used as sintering additives. According to the stoichiometric composition of mullite (3Al₂O₃·2SiO₂), the raw materials, ie kaolinite, synthesized alumina, and different wt% of additives were wet mixed, dried, and uniaxially pressed followed by sintering at different temperature. 1600°C sintered samples from each batch exhibit enhanced properties. The 1 wt% TiO₂ addition shows bulk density up to 2.96 g/cm³ with a maximum strength of 156.3 MPa. The addition of MgO up to 1 wt% favored the growth of mullite by obtaining a density and strength matching with the batch containing 1 wt% TiO₂. These additives have shown a positive effect on mullite phase formation by reducing the temperature for complete mullitization by 100°C. Both additives promote sintering by liquid phase formation. However, the grain growth, compact microstructure, and larger elongated mullite crystals in MgO containing batch enhance its hardness properties.     

Recycling photovoltaic silicon waste for fabricating porous mullite ceramics by low-temperature reaction sintering

In this study, porous mullite ceramics with coral-like structures were fabricated at a low temperature of 900 °C by using photovoltaic silicon waste (PSW) as the silicon source directly. The effects of additive content and sintering temperature on the mullitization reaction of green bodies were studied. The results showed that ammonium molybdate tetrahydrate molybdenum (H₂₄Mo₇N₆O₂₄·4H₂O) as an additive could reduce the reaction temperature for mullitization from 1100 °C to 900 °C. The research on the influence of catalyst on material properties showed that porous mullite ceramics with a flexural strength of 52.83 MPa, a 41.78 % porosity, a sintering expansion rate of 0.49 % and an average pore size of 0.23 μm could be fabricated by introducing 7.5 % H₂₄Mo₇N₆O₂₄·4H₂O at the sintering temperature of 1000 °C. This study develops an environment-friendly recycling method of PSW and provides a new idea for the low-cost preparation of porous mullite ceramics with high purity.     

Mechanical properties of reinforced porcelain slabs with mullite whiskers introduced by aluminum silicate fiber

Mullite whiskers (3Al₂O₃·2SiO₂) reinforced porcelain slabs with high bending strength and fracture toughness were successfully prepared by calcining a kaolin-potash feldspar-albite mixture with aluminum silicate fibers (ASF, Al₂O₃·SiO₂). Effects of ASF content on the properties of porcelain slabs were investigated. Bending strength and fracture toughness of the samples reach the peak values, 39% and 19% higher than that of the blank samples when the amount of ASF is 5 wt%. The elongated and needle-like mullite whiskers are obtained in the substrates with ASF fiber addition, differing from the rod-shaped mullite whiskers in the blank ones. During the sintering process, ASF provides growth environment and ingredient for elongated mullite whiskers. The in-situ generated elongated mullite whiskers link the porcelain slabs substrate as a bridge and consume more energy before the fracture occurred. Therefore, mullite whiskers induced by ASF can offer reliable opportunity for preparation of porcelain slabs with good mechanical properties.     

Properties and characterization of alumina platelet reinforced geopolymer composites

Geopolymers are porous, amorphous, alkali-aluminosilicate hydrate materials formed at room temperature via a solution process. Geopolymer based on metakaolin had a relatively homogeneous microstructure that offered consistent behavior but suffered from dehydration cracking and large densification shrinkages when heated. It was found that by reinforcing a metakaolin geopolymer of composition (K₂O·Al₂O₃·4SiO₂·11H₂O) with 50 µm diameter alumina platelets, dehydration cracking could be prevented, and shrinkage could be reduced by an order of magnitude. Samples were reinforced with 30, 50, and 70 wt% of alumina platelets. Although the properties of the 30 and 50 wt% conditions were better than those of unreinforced geopolymer, those samples still showed warping, cracking, and strength losses on heating. The 70 wt% samples did not warp or crack when heated to temperatures of up to 1500°C. The room-temperature 4-point flexural strength of these samples remained at around 20 MPa regardless of heat treatments. The in situ measured flexural strength increased to almost 40 MPa at 600°C, and remained higher than 20 MPa until 1200°C. Samples subjected to propane-torch thermal shock heating and subsequent quenching did not crack or fragment. Dilatometry, X-ray diffraction, and scanning electron microscopy were used for additional characterization. Given these properties, this material showed promise as a castable refractory.     

Preparation of self-reinforcement of porous mullite ceramics through in situ synthesis of mullite whisker in flyash body

Self-reinforced porous mullite ceramics were fabricated by a starch consolidation method with flyash, different aluminium sources (Al(OH)₃ and Al₂O₃) and the additive AlF₃ as raw materials. The reinforcement mechanism of needle-like mullite whiskers through in situ synthesis in ceramic body was investigated. The bulk density, apparent porosity and bending strength of the samples were tested. Phase compositions and microstructures of the sintered samples were measured by XRD and SEM, respectively. It showed that AlF₃ as additive was helpful to the formation of mullite whiskers at a low temperature. As the aluminium sources, Al(OH)₃ was more suitable for the preparation of mullite whiskers than Al₂O₃. The in situ synthesized mullite whiskers formed an interlocking structure, which enhanced the mechanical strength of the porous mullite ceramics. Porous mullite ceramics with bending strength of about 100 MPa and apparent porosity of about 55% were made at 1550 °C.     

Corrosion behaviors of porous reaction-bonded silicon carbide ceramics incorporated with CaO

Reaction-bonded SiC (RBSC) porous ceramics were fabricated at 1450?°C in air by incorporating CaO using ZrO₂ as sintering aids, activated carbon as pore-forming agent, and mullite fibers as reinforcing agent. The effects of CaO content on the properties of the porous RBSC ceramics were studied. Corrosion behaviors of the prepared RBSC porous ceramics in different environments were also investigated. The optimal open porosity, bending strength, average pore size and gas permeability of the ceramics with 0.5% CaO were 40%, 22.5?MPa, 42.9?µm, and 2100?m³/m² h?kPa, respectively. A well-developed neck reaction-bonded by calcium zirconium silicate (Ca₃ZrSi₂O₉) was identified. The porous RBSC ceramics exhibited excellent corrosion resistance in acid and basic solutions. The anti-oxidation temperature of the porous RBSC ceramics could reach 1200?°C in air. The RBSC ceramics maintained the bending strength of 17.5?MPa after 60 cold-hot cycles in air (0–800?°C). The porous RBSC ceramics also exhibited relatively good corrosion resistance in molten salts (NaCl, Na₂SO₄ and CaCl₂). Melten NaOH can aggravate the reaction by breaking the SiO₂ layers on the SiC surface. Overall, these findings offer significant insights into expanding the applications porous RBSC ceramics incorporated with CaO.     

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.