Mullite Whiskers Research Articles

Fabrication of high-performance SiC ceramic membranes modified using in situ grown mullite whiskers and high-temperature filtration

The gas permeance and particle removal efficiency of silicon carbide (SiC) membranes are the most important performance indicators for high-temperature gas filtration. The rational design of SiC membrane microstructure has a positive impact on the improvement of performance indicators. This work prepared SiC membranes modified by in situ grown mullite whiskers, optimized their preparation conditions and investigated their performance improvement. The membrane suspension composition and sintering temperature were optimized using Taguchi’s method and gas permeance as the target. The thickness of the membrane was regulated by adjusting the spraying cycles and solid content in membrane suspension. Results showed that best membrane performance was obtained when the content of sintering aids, MoO3, and AlF3·3H2O were 8, 12, and 12 wt%, respectively; the solid concentration in membrane suspension was 30 wt%; the number of spraying cycles was 2, and the temperature was 1350 °C. The resulting membrane obtained a pore size of 5.8 µm and the Darcy's permeability coefficient k was 8.58 × 10−12 m2. It also had good interfacial adhesion and thermal shock and corrosion resistances. The membrane achieved a PM2.5 removal rate of more than 99.9 % at room temperature and high temperatures (100 °C–400 °C) with a low pressure drop below 1.4 kPa. Overall, the SiC ceramic membranes are prospective candidates for dust-laden gas filtration at high temperatures.

Lightweight porous Al2O3‐based ceramics with highly controllable performance via binder jetting

AbstractThis study introduces a feasible approach for preparing Al2O3‐based ceramics with highly controllable performance via binder jetting. The Al2O3‐based ceramics were prepared using Al2O3 and H3BO3 as raw materials. The results showed that the samples prepared with 0.24 mol H3BO3 exhibited high apparent porosity (64%–66.67%), well‐bending strength (3.46–8.53 MPa), and low thermal conductivity (0.113–0.329 W·m−1·k−1 at 200–800°C). It was observed by an electron microscope that the increase of boric acid content led to the in‐situ formation of more aluminum borate grains in the sample. Meanwhile, the remaining Al2O3 reacts with silicon oxide in the impregnation solution to form mullite whiskers. The whiskers ensured that the sample had superior mechanical properties and lower thermal conductivities. Based on the above properties, the possibility of Al2O3‐based ceramics application in the aerospace field can be speculated.

Biomimetic supported polyoxometalate structures designed and its applications for the ultra-deep oxidative desulfurization of fuels

Inspired by the fact that human nose cilia can effectively contact with particles to prevent their entry into our body, this kind novel biomimetic structures are designed and synthesized to enable the loading efficiency between the polyoxometalate catalytic species (POMs) and the carrier, thereby improving their loading rate and promoting their catalytic performance. The mullite whiskers-uniformed cordierite (CMW) is used as a carrier to support POMs, in which mullite whiskers regarding as the cilia in our nose, this kind catalyst could not only overcome the shortcomings of simple POMs with low specific surface and easy to self-agglomerate, but also could greatly increase their contact probability with the carrier. At the same time, the advantage of the cordierite block structure also greatly conducive to their actual application value on the reusing operation. A series of Keggin-type POM-loaded bulk materials, POM@CMW, [VimAm]Br@POM@CMW, [DVim]Br@POM@CMW, have been developed with various ionic liquid-based POMs (ionic liquid = ILs, chosen these two kinds of [VimAm]Br and [DVim]Br) to a comparative study. The oxidative desulfurization reaction (ODS) for fuels was performed using the obtained materials as catalysts and H2O2 as oxidant, without the additional extractants for the existing of ILs in catalyst. Through experimental evaluations, [DVim]Br@POM@CMW was found the excellent efficiency on the Sulfur removal, with an DBT removal of 96.34 % at 70 °C for 120 min. According to the FT-IR and SEM results after the reaction, the catalyst could retain their original structure and the dibenzothiophene (DBT) removal could still be maintained above 95 % after 7 reuses. Furthermore, the oxidation reactivity of different substrates was in the following order: DBT > 4,6-dimethyldibenzothiophene (4,6-DMDBT) > thiophene (BT) and the high activity in actual diesel oil could be also exhibited. The enhanced reaction activity, excellent structural stability, and recyclability can be attributed to the abundance of active sites and outstanding mechanical properties provided by cordierite/mullite whiskers as a carrier, indicating their potential in the novel biomimetic-structured POMs-loaded catalysts.

The corrosion behavior of Al2O3-SiO2 refractory in reducing atmosphere

Hydrogen metallurgy technology is essential for reducing CO2 emissions. Hydrogen (H2) serves as an ideal energy carrier to replace carbon-based fuels, producing only water as a byproduct. Al2O3-SiO2 refractories are commonly used in hydrogen metallurgy technology, including in technologies like HyCROF(Hydrogen-enriched Carbonic oxide Recycling Oxygenate Furnace) and gas heating furnaces. It is important to systematically study their corrosion in high-temperature reducing gas environments. The results indicated that the Al2O3-SiO2 refractory remained stable in pure H2 atmosphere at temperatures below 1200 °C. However, SiO2 and mullite were reduced by H2 when the temperature was increased above 1200 °C. Carbon precipitation was observed in both pure CO atmosphere and H2-CO mixed atmosphere, resulting in an increase weight of the sample. Additionally, the sample developed mullite whiskers after heating treatment at 1200 °C in H2-CO mixed atmosphere. This study aims to clarify the corrosion mechanism of Al2O3-SiO2 refractory in reducing atmosphere and to provide a theoretical basis for the selecting and optimizing refractories for hydrogen metallurgy technology.

Waste electric porcelain-based refractory bricks with significantly enhanced mechanical properties: preparation, characterization and mechanism

Using waste electric porcelain to produce refractory bricks is a viable technology. However, these bricks currently have lower mechanical strength compared to those made from mineral raw materials. This study optimized the proportions and sintering process to develop high-strength refractory bricks from waste electric porcelain. The impact of physical phase, crystallinity, sintering temperature, and fly ash content on the bricks properties was examined. The research found that low crystallinity in waste electric porcelain leads to disorganized growth of mullite nano whiskers at high temperatures, reducing mechanical strength due to insufficient support structures and stress concentration zones. The addition of fly ash promotes the formation of uniformly dispersed mullite columns through non-uniform nucleation, significantly improving mechanical properties by creating a collective tetrahedral structure. Bricks with 20 wt% fly ash sintered at 1440 °C achieved a flexural strength of 77.3 MPa, a compressive strength of 156 MPa, and an apparent porosity of 10.89%. High-temperature simulations indicate that these bricks exhibit good compressive strength and deformation resistance, with no adhesion observed. These high strength refractory bricks have promising potential for use in various high-temperature applications, including kiln linings, high-temperature flue, high-temperature support components and other industrial processes where durability and thermal stability are critical.

Fabrication and characterization of mullite whiskers reinforced Al2O3-SiC-C castables

As a refractory material used in the iron runner of blast furnaces, the thermal shock resistance and oxidation resistance of Al2O3-SiC-C (ASC) castables are important factors affecting their service performance and service life. In this study, mullite whiskers were introduced in to ASC castables by in-situ growth using AlF3 as catalysts to improving its performance. The catalyst addition amount and heat treatment time affect the phase composition, microstructure, and performance of castables were investigated. Combined with characterization and first-principles calculation, it is suggested that the mullite whiskers grow along the c-axis direction, and their growth mode conforms to a vapor-solid (V-S) mechanism. As expected, the in-situ growth mullite whiskers significantly improved performance of ASC castables, especially when 1.2 wt% AlF3 was added as catalyst. Compared to the samples without AlF3 additives, the thermal shock resistance (RSR = 82.26 %) and oxidation resistance (I = 27.75 %) increased by 50.04 % and 49.21 %, respectively. Mullite whiskers were dispersed in the material, improved the thermal shock resistance of the castables through whisker bridging. Meanwhile, the intertwining of the whiskers forms an interlocking structure that hinders the oxidation process of the castables. Mullite whiskers not only serves as a bridging role but also prevents the diffusion of oxygen into the interior, which provide a novel approach for strengthening castables.

Molybdenum oxide catalyzed low temperature preparation of mullite whisker from silica-alumina gel slag

Catalyzed by MoO3, mullite whiskers were synthesized through a molten salt method. This study comprehensively explored the impact of reaction temperature, MoO3 mass fraction, and calcination temperature on the morphology of mullite whiskers. Employing analytical techniques such as XRD and SEM, the phase composition and morphology of the samples were meticulously scrutinized. The results unveiled that at a MoO3 mass fraction of 7–8% (molar fraction) and a calcination temperature of 800 °C, mullite whiskers demonstrated a heightened level of purity. Building on these observations, the study delved into the mechanistic underpinnings of MoO3 as a catalyst in mullite whisker synthesis. It is proved that the metal oxides contained in the silica-alumina glue slag can also form a local liquid phase at low temperatures, reducing the viscosity of the reaction system and liquefaction temperature. The addition of molybdenum oxide makes the reaction process of mullite whisker growth shorter. At the same time, the addition of molybdenum oxide can effectively reduce the viscosity of the reaction system, thus lowering the reaction temperature and increasing the reaction efficiency. In addition molybdenum oxide can be used as a templating agent to induce the growth of mullite whiskers along specific crystal planes. Compared with aluminum fluoride catalysis, molybdenum oxide does not produce the hazardous gas HF, the sublimated molybdenum oxide gas can be recycled, and the molybdenum oxide attached to the whiskers can also be dissolved in water. These theories provide experience in the large-scale preparation of mullite whiskers.

Enhanced comprehensive properties of alumina ceramic shells by a pennisetum fiber/AlF3•3H2O powder hybrid system

The strength and permeability to gas of ceramic shells are key factors affecting the quality of castings. This research aims to improve the properties of alumina-based ceramic shells (i.e., Al2O3 and SiO2) using a combination of pennisetum fiber and AlF3•3H2O powder. The pennisetum fiber can improve the green strength of ceramic shells, while AlF3•3H2O powder can improve the high-temperature strength of ceramic shells by inducing the generation of mullite (3Al2O3•2SiO2) whiskers at high temperatures. Meanwhile, the mixture of pennisetum fibers and mullite whiskers can also improve the permeability to gas of the ceramic shell. As a result, the modified ceramic shell shows a clear increase in the above-mentioned properties, including 43 % of the green strength, 100 % of the permeability to gas, and 30 % of the thermal diffusion coefficient. In addition, the high-temperature bending strength and deformation under the weight of the ceramic shells are also obtained.

Cordierite ceramic membrane preparation using gas solid reaction growth for nanoparticles filtration

Mullite whiskers have been successfully grown on the surface of cordierite honeycomb ceramics by the gas-phase growth method. The cilia-like structure can significantly improve the filtration precision. The acicular mullite whiskers grew on the cordierite present nanometer-sized (200–300 nm) diameters and micrometer-sized (1–2 μm) lengths. Owing to mullite whiskers growth on surface of pores, the morphology of pores is changed and the microstructure change will lead to filtration precision increasing. The composites with the largest ratio of length/diameter mullite whiskers possess the highest filtration precision, which can reach 122.42 nm, 255.00 nm and 458.67 nm in intercepting different particle size of nano-SiO2, respectively. Moreover, the bending strength of the cordierites were enhanced due to the generation of mullite whiskers, these generated whiskers intersected with each other forming many unfixed lap-jointing points. The porous ceramics membrane with the unique microstructure has great potential application in gas-solid separation field and liquid-solid separation field.

Finite element simulations of thermal stress distribution in thermal barrier coatings with different mullite whisker arrangements

In recent years, thermal barrier coatings (TBCs) have been extensively employed to safeguard turbine engine blades against high temperatures and harsh operating conditions. Mullite whiskers are frequently utilized to improve the mechanical properties of ceramic matrix composites because of their exceptional characteristics. This study investigates thermal stress in coating systems by analyzing the impact of varying content, aspect ratio, and deflection angle of mullite whiskers using finite element simulations under static conditions. Results demonstrated that incorporating mullite whiskers can mitigate the sudden stress variation between the ceramic coating and the substrate. The addition of mullite whiskers reduced the maximum stress value in the coating system, thereby enhancing its thermal stability and service life. The arrangement of mullite whiskers was controlled during the preparation procedure to ensure their proper orientation. The coating system exhibited higher mechanical properties at a certain position. This study provided a theoretical basis for the design of mullite whisker–reinforced TBCs.

Preparation and Properties of High Sound-Absorbing Porous Ceramics Reinforced by In Situ Mullite Whisker from Construction Waste.

Porous sound absorption ceramic is one of the most promising materials for effectively eliminating noise pollution. However, its high production cost and low mechanical strength limit its practical applications. In this work, low-cost and in situ mullite whisker-reinforced porous sound-absorbing ceramics were prepared using recyclable construction waste and Al2O3 powder as the main raw materials, and AlF3 and CeO2 as the additives, respectively. The effects of CeO2 content, AlF3 content, and sintering temperature on the microstructure and properties of the porous ceramics were systematically investigated. The results showed that a small amount of CeO2 significantly promoted the growth of elongated mullite crystals in the resultant porous ceramics, decreased the growth temperature of the mullite whiskers, and significantly increased the biaxial flexural strength. When 2 wt.% CeO2 and 12 wt.% AlF3 were added to the system, mullite whiskers were successfully obtained at a sintering temperature of 1300 °C for 1 h, which exhibited excellent properties, including an open porosity of 56.4 ± 0.6%, an average pore size of 1.32-2.54 μm, a biaxial flexural strength of 23.7 ± 0.9 MPa, and a sound absorption coefficient of >0.8 at 800-4000 Hz.

Reinforcement effect of fly ash with different morphologies on aluminum foam prepared via powder metallurgy

Powder metallurgy has outstanding advantages in the preparation of aluminum foam fillers and special-shaped aluminum foams, which can realize near-net-shape forming. To improve the mechanical properties of aluminum foams prepared by powder metallurgy, low-cost solid waste fly ash (FA) was selected as the reinforcement phase, and the strengthening effect of the FA particles with different morphologies on the mechanical properties of the aluminum foam was analyzed. The results show that the strengthening effect of 1.0 wt% mullite whiskers prepared from the original FA on aluminum foam is superior to that of spherical raw FA or irregular granular FA. In particular, a reinforced aluminum foam with a yield strength of 20.76 MPa was obtained after precursor foaming at 610 °C for 10 min because the wettability between the whiskers and matrix is improved when the whiskers were coated with Sn in advance and the foamable precursor was obtained via hot-pressing.

The effect of multiple factors on the synthesis of mullite whiskers based on the S-L-S mechanism and the comparison of strengthening performance

The effect of multiple factors on the synthesis of mullite whiskers based on the S-L-S mechanism and the comparison of strengthening performance

Fabrication of high-permeance SiC filters reinforced with in situ-grown mullite whiskers for gas/solid filtration

The pursuit of cost-efficiency, superior strength, and excellent gas permeance has driven the development of silicon carbide (SiC) membranes or filters for gas/solid filtration. In this study, SiC filters with high gas permeance reinforced with in situ-grown mullite whiskers were successfully prepared through reaction bonding. Low-cost kaolin and bauxite were used as sintering aids and raw materials for mullite whisker formation, and MoO3 was used as an additive to promote mullite whisker growth. The effects of different sintering temperatures and the addition of kaolin, bauxite, and MoO3 on filter performance were systematically investigated. When the sintering aid and MoO3 content additions were 6 and 1.5 wt%, respectively, and the sintering temperature was 1450 °C, the resulting ceramic filter exhibited a porosity of 45.5 %, a bending strength of 17.9 MPa, and an average pore size of 47.7 µm. Moreover, the filter exhibited a high Darcy’s permeability coefficient of 2.42 × 10−11 m2, satisfactory chemical stability, and thermal shock resistance. The addition of MoO3 facilitated the consumption of the molten phases and increased the anisotropic growth rate of the mullite whiskers, which contributed to the fabrication of the high-permeance filters. Finally, the ceramic filters demonstrated good performance for the filtration of dusty gases loaded with fly ash particles with a mean size of 15.3 µm at room temperature and 300 °C, and the removal rate reached > 99.9 %. The pressure drop was a little lower than that of our commercially available SiC filters. The SiC filters showed good prospects in the field of gas/solid filtration.

Effects of SiCnw/Al2O3 composite powders on properties of Al2O3–SiC refractory castables

The SiCnw/Al2O3 composite powders synthesized through combustion synthesis method were added to the Al2O3–SiC refractory castables used in waste incinerators to improve their properties. The properties of Al2O3–SiC refractory castables with SiCnw/Al2O3 composite powders (0–8 wt%) were evaluated which contained apparent porosity, bulk density, mechanical strength, high temperature wear resistance, hot modulus of rupture, thermal shock resistance and slag corrosion resistance. The results showed that various properties of the castables were enhanced with the addition of SiCnw/Al2O3 composite powders. When 6 wt% composite powders were added, the hot modulus of rupture increased by 84.1% from 1.63 to 3.00 MPa, the high temperature wear volume decreased by 71.0% from 6.2 to 1.8 cm³, and the residual strength rate increased by 22.6% from 42.5% to 52.1%. Therefore, the properties of Al2O3–SiC castables were improved through the synergistic reinforcement effect of SiC nanowires and acicular mullite whiskers on the matrix.

Shape‐controllable synthesis of mullite whisker and its effect on tribological behavior of resin‐based friction materials

AbstractThe development and design of fillers aid in improving the tribological performances of friction materials. Herein, the effects of the aspect ratio of mullite whisker on the tribological properties were studied. Mullite whiskers were prepared by the sol–gel process and the aspect ratio of the whisker was controlled by introducing aluminum fluoride (AlF3). The aspect ratio increased significantly at first and then decreased with increasing the contents of AlF3. The experimental results show that higher aspect ratio whiskers stabilized the friction coefficient while increased the wear rate. In addition, the composite with mullite whisker grown in excessive AlF3 (20%) resulted in a higher friction coefficient compared to adding inadequate AlF3 (10%). The Scanning Electron Microscope (SEM) and Energy Dispersive X‐Ray Spectroscopy (EDX) results indicated that higher aspect ratio of whisker was more likely to fall off from matrix which sequentially being compacted and formed contact plateaus during the friction process. In conclusion, it was found that abrasive wear and adhesive wear were the dominant wear mechanism.Highlights The aspect ratio of mullite whisker was controlled within the range of 6 ~ 21. The tribological properties of whiskers reinforced composites were studied. The morphology of the mullite whisker affects the friction stability. The flaky mullite whisker can improve wear resistance of the composites.

In situ mullite whisker network formation for high strength and lightweight ceramic proppants

In this study, a facile in situ sintering method has been proposed for fabricating high strength lightweight mullite whisker network reinforced proppants. The effect of TiO2 on the microstructure, properties and fracture behaviours of the mullite whisker network was researched in detail. The addition of TiO2 promoted the formation of a modified mullite whisker network structure, which effectively improved the flexural strength. The sample containing 4 wt.% TiO2 exhibited an excellent flexural strength of 215.25MPa and low bulk density of 1.53 g/cm3 at 1400 ?C. The TiO2-added mullite whiskers mostly exhibited rod-like crystals with about 0.5- 0.8 ?m in diameter and 10-15 ?m in length. The as-prepared ceramic proppant showed excellent properties, such as low breakage ratio (2.19%under 52MPa), low density (1.36 g/cm3) and low acid solubility (2.28 wt.%).

A study on the efficient separation of oily water using mullite whiskers membrane through combined filtration and electrofiltration.

This study explores the efficacy of a ceramic membrane combining filtration, electrofiltration, and backwashing for oily water treatment. A secondary mullite membrane was synthesized, showcasing high permeate flux (534 LMH), biaxial flexural strength (75.21 MPa), and cost-effectiveness. Operational parameters, set at 2 bar pressure and 0.727 m s-1 cross-flow velocity, were optimized for desirable permeate flux and oil removal rates. Critical electric field intensity (E crit) ranged from 50 to 55 V, guiding optimal voltage selection for electrofiltration. Electrokinetic phenomena, such as electrophoresis and electroosmosis, addressed fouling issues. Higher salt concentrations exacerbated fouling and reduced electric field efficiency. Energy analysis revealed potential savings, dropping from 3.88 kW h m3 without voltage to 2.71 kW h m3 at 65 V for salt-free solutions. However, higher salt concentrations increased fouling, elevating energy consumption. These findings affirm the value of affordable ceramic membranes for oily water treatment, stressing the need for parameter optimization to enhance performance and energy efficiency.

Research on preparation and related properties of macro-micro porous mullite ceramic skeletons via twice pore-forming technology.

A lot of solid waste coal gangue is produced every year in the process of coal mining and coal washing, which poses a great threat to human health. How to deal with coal gangue properly is still a serious problem. In this study the macro-micro composite porous mullite ceramic skeletons were successfully prepared using solid waste coal gangue and α-Al2O3 as main raw materials via twice pore-forming technology. The main phase composition of the porous ceramic skeletons was mullite tested by X-ray Diffractometer (XRD). The morphology and microstructure of the porous ceramic skeletons were analyzed by Scanning Electron Microscope (SEM). The results show that the microstructure of porous ceramic skeletons was mainly composed of mullite whiskers. With the increase of sintering temperature from 1200 °C to 1350 °C, the maximum length of mullite whiskers grew up from 2.68 μm to 8.10 μm and their average length grew up from 0.78 μm to 2.98 μm. The maximum compressive strength of the porous ceramic skeletons with 30 PPI and 45 PPI were 1.25 MPa and 1.54 MPa tested by Universal Testing Machine (UTM) at the sintering temperature of 1250 °C, respectively. The linear shrinkage, bulk density and pore stem density of the porous ceramic skeletons became larger with the rising of sintering temperatures from 1150 °C to 1350 °C. However, the corresponding performance values of 45 PPI porous ceramic skeletons were greater than that of 30 PPI at the same sintering temperature. The prepared porous ceramic skeletons will be used in ceramic-metal wear-resistant composites for the later research and the study provides a new idea for coal gangue on the comprehensive utilization with high added value and brings both good environmental and economic benefits.

High Emissivity MoSi2-SiC-Al2O3 Coating on Rigid Insulation Tiles with Enhanced Thermal Protection Performance.

High emissivity coatings with sol as the binder have the advantages of room temperature curing, good thermal shock resistance, and high emissivity; however, only silica sol has been used in the current systems. In this study, aluminum sol was used as the binder for the first time, and MoSi2 and SiC were used as emittance agents to prepare a high emissivity MoSi2-SiC-Al2O3 coating on mullite insulation tiles. The evolution of structure and composition at 1000-1400 °C, the spectral emissivity from 200 nm to 25 μm, and the insulation performance were studied. Compared with the coating with silica sol as a binder, the MoSi2-SiC-Al2O3 coating has better structural uniformity and greater surface roughness and can generate mullite whiskers at lower temperatures. The total emissivity is 0.922 and 0.897, respectively, at the wavelength range of 200-2500 nm and 2.5-25 μm, and the superior emissivity at a low wavelength (<10 μm) is related to a higher surface roughness and reduced feature absorption. The emissivity reduction related to the oxidation of emittance agents at a high temperature (-10.2%) is smaller than that of the silica-sol-bonded coating (-18.6%). The cold surface temperature of the coated substrate is 215 °C lower than the bare substrate, suggesting excellent thermal insulation performance of the coating.