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%).     

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.     

Design and preparation of high permeability porous mullite support for membranes by in-situ reaction

The natural mineral kaolin combined with alumina additives Al(OH)₃,α-Al₂O₃ and AlF₃ was used to prepare porous mullite ceramic membrane supports using an in-situ reaction. The effects of composition and sintering temperature on the sintering behavior, pore structure, permeability and microstructure of the resulting porous mullite supports were extensively investigated. The experimental results showed that excess SiO₂ in kaolin can be consumed by adding alumina precursors, which resulted in a stiff skeleton of interlinked needle-like mullite crystals in-situ during the sintering. The needle-like mullite crystals touched each other and formed a short network, which acted as a porous skeletal network structure. This network resulted in a highly permeable porous structure. The resulting support is suitable for the preparation of asymmetric ceramic membranes. The densification and pore structure of the support can be effectively adjusted by control of the quantity of alumina precursors in the composition and the sintering temperature. Sintering the subject mullite compositions at 1500 °C for two hours resulted in support structures with an average porosity of 45.9%, an average pore size of 1.3 µm and a penetrating porosity of 35.9%.     

Low-cost porous mullite ceramic membrane supports fabricated from kyanite by casting and reaction sintering

Porous mullite supports are firstly fabricated by casting and reaction sintering based on kyanite with Al(OH)₃ as porogenic agent. The effects of composition and sintering temperature on phase evolution, microstructure, apparent porosity, pore size distribution, linear shrinkage, gas permeation flux and mechanical property of supports are systematically investigated. Results show that the mullitization of kyanite generates needle-like mullite crystals, which form skeleton structures and improve the apparent porosity and strength of supports. Al(OH)₃ addition not only promotes the formation of needle-like mullite but also enhances the apparent porosity of supports. Temperature promotes the development of mullite, from 1450 to 1500 °C, the amount and size of needle-like mullite crystals increase, ≥1500 °C, they reveal columnar morphology. The support prepared with kyanite+40 wt%Al(OH)₃ sintered at 1500–1550 °C exhibits high apparent porosity, good gas permeation flux, excellent mechanical performance and interlocked network structure composed of well development needle-like mullite.     

Microstructure and property of porous mullites with a whiskers framework obtained by a sol–gel process

Porous mullites with a whiskers framework and high porosities were fabricated by the reaction sintering (1100 to 1600 °C, 1 h, in an airtight container) of an aerogel block shaped by the sol–gel transition of a mullite precursor composed of SiO₂ sol, Al₂O₃ and AlF₃ powders (as reaction catalyst). The effect of heating temperatures on porosity, whisker formation, microstructure feature and compressive strength of the porous mullites was determined by XRD, SEM and compressive test. The results indicate that after heating at temperatures from 1100 to 1600 °C, the porosities of the mullites varied within the range of 84.1–80.2%. The whiskers in the framework well lap-jointed each other to form the large space and became elongated and smooth at high temperatures due to the accelerated vapor–solid reaction rate. A maximum compressive strength of 16.1 MPa was obtained for the whiskers framework heated at 1600 °C; this strength was attributed to the strong bonding among the smooth whiskers.     

Effects of in situ synthesized mullite whiskers on flexural strength and fracture toughness of corundum-mullite refractory materials

The method of in situ synthesis of mullite whiskers was introduced to improve the fracture toughness of the corundum-mullite refractory materials. Effects of process parameters (sintering temperature, holding time and addition amount of V₂O₅) on flexural strength and fracture toughness of corundum-mullite during the in situ toughening course were analyzed. The optimum process parameters (the sintering temperature of 1350 °C, the holding time of 2 h, and the V₂O₅ addition amount of 5%) for in situ synthesized mullite whiskers to toughen corundum-mullite were obtained by the response surface method combined with single factor analysis. SEM and EDS analysis results demonstrated that the mullite whiskers had been synthesized in corundum-mullite and they could bridge the cracks during the fracture process. After in situ toughening, the flexural strength versus deflection curves of corundum-mullite showed obvious zigzag or waveform characteristics, indicating in situ toughening effects. At the same time, the flexural strength and corresponding deflection increased remarkably.     

Preparation and properties of in-situ mullite whiskers reinforced aluminum chromium phosphate wave-transparent ceramics

In-situ mullite whisker reinforced aluminum chromium phosphate wave-transparent ceramics were designed and prepared. The phase transformation, microstructure, mechanical and electrical properties of the ceramics were investigated, and the mechanisms of in-situ growth and toughening were discussed. Results indicated that the in-situ growth of mullite whisker significantly improved the mechanical properties of the matrix, especially the high temperature flexural strength. The room temperature flexural strength, 1000 °C flexural strength and fracture toughness of the ceramics were 135.60 MPa, 121.71 MPa and 4.52 MPa m^1/2. After sintering at 1500 °C, the optimum properties of ε^'_r, tanδ and microwave transmittance at region 8–12 GHz were <3.6, <0.03 and>80%, respectively. The sinterability of ACP matrix was improved by the in-situ process of high mullization above 1450 °C. Using ACP binder as the raw material can avoid the phase transformation from B-AlPO₄ to T-AlPO₄. The synthesized mullite whiskers played a role in toughening by whiskers fracture, crack deflection and whisker pulling out.     

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 and thermal shock resistance of corundum-mullite composite ceramics from andalusite

Corundum-mullite composite ceramics have high hardness, small plastic deformation and other excellent performances at high temperature. Corundum-mullite composite ceramics were fabricated from andalusite and α-Al₂O₃ by in-situ synthesis technology. Effects of mullite/corundum ratio and sintering temperatures on the water absorption, apparent porosity, bulk density, bending strength, thermal shock resistance, phase composition and microstructure of the sample were investigated. Results indicated that the in-situ synthesized mullite from andalusite combined with corundum satisfactorily, which significantly improved the thermal shock resistance as no crack formed after 30 cycles of thermal shock (1100 °C-room temperature, air cooling). Formula A4 (andalusite: 37.31 wt%, α-Al₂O₃: 62.69 wt%, TiO₂ in addition: 1 wt%, mullite: corundum=6:4 in wt%) achieved the optimum properties when sintering at 1650 °C, which were listed as follows: water absorption of 0.15%, apparent porosity of 0.42%, and bulk density of 3.21 g⋅cm⁻³, bending strength of 117.32 MPa. The phase composition of the sintered samples before and after thermal shock tests were mullite and corundum constantly. The fracture modes of the crystals were transgranular and intergranular fractures, which could endow the samples with high thermal shock resistance.     

Microwave dielectric properties of BiNbO4 ceramics with CuO–V2O5 addition

BiNbO₄ ceramics were developed by using CuO–V₂O₅ as a liquid phase sintering agent. The resultant dielectric properties were analyzed in terms of the densification and the amount of CuO–V₂O₅ sintering agent. The addition of 0.8 wt.% CuO–V₂O₅ as its sintering agent was observed to perform most satisfactory. At 850 °C, uniform and enhanced microstructure was observed for the BiNbO₄ specimen with 0.8 wt.% CuO–V₂O₅ addition. Furthermore, the effect of CuO–V₂O₅ addition on the microwave dielectric properties of BiNbO₄ was also investigated. As the sintering temperature increased to 900 °C, the dielectric constant increased but nearly constant and the quality factor (QF) showed a maximum at 850 °C and then decreased for all compositions of the 900 °C sintered specimens. With an increase in CuO–V₂O₅ content, the temperature coefficient of frequency (TCF) increased in accordance with the dielectric mixing rule and microstructural behavior.     

Effect of rare earth oxides on the microstructure and properties of mullite/hBN composites

Mullite/hBN composites were fabricated with different rare earth oxides additives (ReO: Er₂O₃, CeO₂, La₂O₃, Lu₂O₃) by pressureless sintering at 1600 °C for 4 h. The impacts of ReO on the phase composition, microstructure, mechanical, dielectric and tribological properties of the composites were investigated. XRD results showed that all the ReO additives were beneficial to the formation of mullite phase. With the decrease in the ionic radius of the ReO, the mullite grains of the composite were refined while their mechanical properties were increased. The sample sintered with Lu₂O₃ showed the smallest grain size and the most excellent mechanical properties, e.g., its relative density, flexure strength, fracture toughness and hardness reached 93.7%, 222 MPa, 3.2 MPa m^1/2 and 6.02 GPa, respectively. Due to the different porosity and phase composition of the composites, the sample sintered with La₂O₃ had the lowest dielectric constant while the sample sintered with Er₂O₃ exhibited the lowest dielectric loss. Attributing to the highest hardness and fracture toughness, the sample sintered with Lu₂O₃ showed the best tribological properties.     

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.     

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.     

Fabrication and properties of porous mullite ceramics from calcined carbonaceous kaolin and α-Al2O3

High purity calcined carbonaceous kaolin and α-Al₂O₃ powders were employed to prepare porous mullite ceramics (Sample A) using graphite as pore former with the reaction sintering method. For the purpose of comparison, porous mullite ceramics (Sample B) was also fabricated from the uncalcined carbonaceous clay incorporated with α-Al₂O₃ powders. Mullitization in the two samples was both nearly complete at 1500 °C, despite the fact that calcination of the clay remarkably depressed mullitization and promoted the formation of glass phase. The Sample A sintered at 1500 °C fractured mainly in an intergranular way, while the Sample B mainly underwent transgranular fracture. The experimental results revealed that densification behavior/open porosity of the Sample A was far more sensitive to sintering temperature. The pore size of the Sample A as well as the Sample B sintered at 1500 °C was in a narrower range of 0.3–5 μm.     

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.     

A new and economic approach to fabricate resistant porous membrane supports using kaolin and CaCO3

This new and economic approach to fabricate resistant porous membrane supports consists of Algerian kaolin and calcite (CaCO₃) instead of Al₂O₃. The porous mullite (3Al₂O₃·2SiO₂) and anorthite (CaO·Al₂O₃·2SiO₂) based ceramics were obtained by solid state reaction. Different calcite amounts (10–28 wt%) have been added into kaolin halloysite type (Al₂O₃·2SiO₂·4H₂O) in order to control pores forming with appropriate distribution and sizes. Based on a pore distribution and formed phases, a kaolin + 15 wt% calcite (K15C) mixture was selected for flat and tubular configurations. A porosity of 45–52% was also obtained when K15C compacts were sintered at 1100–1250 °C. For example, porosity, average pore size (APS) and 3 point flexural strength were 49%, 3 μm and 87 MPa (same as Al₂O₃ value), respectively when K15C compacts were sintered at 1250 °C for 1 h. Finally, a correlation between microstructure and mechanical properties of elaborated supports has been discussed.     

The effect of BaO and Al2O3 addition on the crystallization behaviour of cordierite glass ceramics in the presence of V2O5 nucleant

The effect of V₂O₅ nucleant on crystallization of stoichiometric cordierite glass ceramics in the presence of various amounts of BaO and Al₂O₃ additives were investigated by DTA, XRD and SEM. It was shown that 3 wt.% V₂O₅ and 1.5 wt.% BaO were the optimum amounts of the additives effective in inducing both surface and bulk crystallization in the above glass ceramics.This resulted in ～90 wt.% cordierite after a 3 h heat treatment at 1020 °C. The specimens possessing 4–5 wt.% Al₂O₃ in excess of the stoichiometric cordierite composition, developed mullite along with cordierite in the temperature range of 1045–1055 °C, whereas in the specimen containing 6 wt.% excess Al₂O₃, mullite was detected as the sole crystallization product.     

Effect of boron oxide on the microstructure of mullite-based glass-ceramic glazes for floor-tiles in the CaO–MgO–Al2O3–SiO2 system

The effect of increasing replacement of Al₂O₃ by B₂O₃ in a parent glass on the sintering and further crystallization of mullite was investigated. The composition of the parent glass was chosen in the mullite primary phase field of the CaO–MgO–Al₂O₃–SiO₂ quaternary system. Glass powder pellets were heated under standard (10 °C/min and 2 h of hold time) and fast heatings (25 °C/min and 5 min of hold time) at different temperatures from 700 to 1190 °C. Sintering of B₂O₃-containing glasses took place in the range between 850 and 1050 °C. X-ray diffraction results showed that mullite formed as unique crystalline phase for glasses containing amounts of B₂O₃ larger than 6 wt%. For lower amounts of boron oxide cordierite was formed as secondary crystalline phase. Quantitative determination of mullite by Rietveld analysis indicated that the higher amount of mullite present in the glass-ceramic fast heated at 1160 °C was 19.5 wt% for the glass containing 9 wt% of B₂O₃. The final microstructure of the glass-ceramic glazes showed the presence of well shaped, long acicular mullite crystals dispersed within the residual glassy phase. Results of glass-ceramic glazes when applied as slurry and under industrial heating conditions pointed out promising mechanical properties.     

Effects of sintering temperature on mechanical properties of 3D mullite fiber (ALF FB3) reinforced mullite composites

A new method to weaken the interfacial bonding and increase the strength of 3D mullite fiber reinforced mullite matrix (Mu_f/Mu) composites is proposed and tested in this paper. Firstly, Mu_f/Mu composites were fabricated through sol–gel process with varied sintering temperature. Then, the effects of sintering temperature on mechanical properties of the composites were tested. As sintering temperature was raised from 1000 °C to 1300 °C, the three-point flexural strength of the composites firstly decreased from 66.17 MPa to 41.83 MPa, and then increased to 63.17 MPa. In order to explain the relationship between composite strength and sintering temperature, morphology and structure of the mullite fibers and mullite matrix after the same heat-treatment as in the fabrication conditions of the composites were also investigated. Finally, it is concluded that this strength variation results from the combined effects of matrix densification, interfacial bonding and fiber degradation under different sintering temperatures.     

Effect of TiO2 doping on the characteristics of macroporous Al2O3/TiO2 membrane supports

A cost-effective tubular macroporous ceramic support consisting of alumina and titania was prepared by extrusion and subsequent heat treatment. An Al₂O₃/TiO₂ composite support with high porosity (41.4%), an average pore size of 6.8 μm and sufficient mechanical strength (32.7 MPa) was obtained after sintering at 1400 °C. The formation mechanism of this support as investigated with X-ray micromapping, SEM and XRD indicated that the appearance of Al₂TiO₅ plays a key role in the fabrication of high performance composite membrane supports at relatively low temperature. The amount of Al₂TiO₅ present in the composite has a strong impact on the properties of supports, especially with regard to the mechanical strength. A composite of 85 wt.% Al₂O₃/15 wt.% TiO₂ sintered at 1400 °C for 2 h exhibited both high permeability (pure water flux of 45 m³ m^?2 h^?1 bar^?1), together with an excellent corrosive resistance towards hot NaOH and HNO₃ solutions.