Mullite Fibers Research Articles

CPAM-assisted synthesis of NaA zeolite membrane on macroporous mullite hollow fiber for ethanol dehydration by pervaporation

Due to the low transport resistance, mullite hollow fiber (HF) could be regarded as a promising support for preparation of NaA zeolite membranes. However, insufficient coverage and undesired seed penetration often occur in the seeding step of the mullite support due to the rough surfaces with ultra-large macropores (2–4 μm). This leads to poor intergrowth of the membrane layer. To solve this issue, cationic polyacrylamide (CPAM) was employed to modify the chemical and electrostatic properties of the mullite HF surface, where the electrostatic attraction between the surface and seed crystals could significantly promote the coating effect. In the seeding, the original seeds (3.2 μm) were mixed with the ball-milled seeds (0.3 μm) to improve the seed coverage, i.e., the original seed was used to block the macropores on the support surface to prevent the penetration of small seeds, while the decked ball-milled seed was used to provide sufficient active sites to induce the intergrowth of the zeolite crystals. The effects of the concentration of CPAM, mixed mass ratio of seeds, and seed concentration on the preparation of NaA zeolite membranes were systematically examined, where the separation performances were characterized by pervaporation (PV) dehydration of 90% (mass) ethanol/water mixtures at 75 °C. The results indicated that under a CPAM concentration of 0.7% (mass), an equivalent mass ratio of mixed seeds in a seed concentration of 1.0% (mass), the prepared mullite HF supported NaA zeolite membrane yielded the best PV performance, where the permeation flux and separation factor corresponded to 5.45 kg·m−2·h−1 and 10000, respectively, being sufficient for industrial applications.

TiO2-embedded dual-layer mullite hollow fiber membranes for efficient removal of Persistent organic pollutants from wastewater

Persistent organic pollutants (POPs) are toxic pollutants that harm the environment and ecosystems. This study presents a novel approach to develop a dual-layer hollow fiber ceramic membrane for phenolic compound removal. A co-extrusion-based phase inversion process and co-sintering techniques were employed to fabricate the membrane, and the effects of TiO2 loadings on the outer layer were investigated. Characterization techniques, including FTIR, SEM, EDX, and zeta potential analysis, were used to analyze the mullite and TiO2 powders and membranes. The membrane’s performance was evaluated through rejection tests of POPs at various concentrations (10, 500, and 1000 ppm) and fouling assessments were conducted. The results showed that the M−MT0.6 membrane, with a mean pore size of 0.0245 μm, effectively rejected benzoic acid (81.45 %), gallic acid (91.45 %), and hydroquinone (74.49 %) from wastewater, achieving the highest permeate flux (1320.50 L/m2·h) for gallic acid at 10 ppm. Additionally, M−MT0.6 exhibited minimal fouling over a 1-h filtration cycle, with the lowest fouling rate (35.1 %) recorded at 1000 ppm for BA, attributed to effective backwashing. However, higher fouling rates were observed at 10 ppm for BA (90.10 %) and HQ (83.50 %). Overall, this study demonstrates the potential of the novel membrane for effective removal of POPs from industrialwastewater.

High-throughput production of low-cost hydrophobic and oleophilic mullite fiber sponges for high-temperature PM filtration

Particulate matter (PM) from high-temperature emissions like chemical plants, coal stoves and vehicle exhausts poses a gravel challenge to human health. To address this issue, researchers have explored various fiber filters, yet the bulk struggle to withstand high temperatures. In this study, mullite fiber sponges were developed utilizing low-cost materials and Kármán vortex solution blow spinning, using surfactants to improve the spinnability of the sol. Optimized sponges demonstrate ultralight (19 mg cm−3), temperature-resistant reversible compressibility (50% strain) and a water contact angle of 135°. These sponges exhibited exceptional thermal insulation (thermal conductivity: 0.0256 W m−1 K−1) and performed well in high-temperature air filtration. At 800 °C, the mullite sponge with a base weight of 35 mg cm−2, achieved an average filtration efficiency of 98.18 % and 99.57 % for PM2.5 and PM2.5−10, respectively, with a quality value of 0.98 Pa-1 at a wind speed of 4 cm s−1. This low-cost mullite fiber sponge offers a promising avenue for designing high-performance filtration materials.

Thermal-Insulation Fillers’ Influences on the Heating Resistance of PDMS-Based Aerogel Layer

PDMS-based aerogel layers were synthesized as insulation layers by adopting mullite fiber (MF), hollow glass microspheres (HGM) and silica aerogel (SA) as the main fillers, and their loading amounts and content ratios were checked to investigate their effects on the thermal insulation properties in PDMS composites by thermal conductivity, thermal stability, and thermal insulation. The loading amount of nanofillers can significantly influence the insulation-layer performance, and the best performance with the lowest thermal conductivity of 0.0568 W/(m·K) was obtained by 10 wt% loading in PDMS with MF:SA:HGM = 2:2:1, which can achieve a temperature difference (∆T) of 67 °C on a 200 °C hotplate. Moreover, the variation of the filler content ratios can also affect the thermal insulation behavior when the loading amount is fixed at 10 wt%, and the best thermal barrier performance can be found for the sample with more SA as the filler (MF:SA:HGM = 1:3:1). The formed sample had the best thermal stability and thermal insulation property, which can stand a 9 min flame test without burning by butane spray gun, and the backside of the sample showed ∆T > 500 °C for the whole test.

Mullite-Fibers-Reinforced Bagasse Cellulose Aerogels with Excellent Mechanical, Flame Retardant, and Thermal Insulation Properties.

Cellulose aerogels are considered as ideal thermal insulation materials owing to their excellent properties such as a low density, high porosity, and low thermal conductivity. However, they still suffer from poor mechanical properties and low flame retardancy. In this study, mullite-fibers-reinforced bagasse cellulose (Mubce) aerogels are designed using bagasse cellulose as the raw material, mullite fibers as the reinforcing agent, glutaraldehyde as the cross-linking agent, and chitosan as the additive. The resulted Mubce aerogels exhibit a low density of 0.085 g/cm3, a high porosity of 93.2%, a low thermal conductivity of 0.0276 W/(m∙K), superior mechanical performances, and an enhanced flame retardancy. The present work offers a novel and straightforward strategy for creating high-performance aerogels, aiming to broaden the application of cellulose aerogels in thermal insulation.

Preparation and Properties of Elastic Mullite Fibrous Porous Materials with Excellent High-Temperature Resistance and Thermal Stability.

Mullite fiber felt is a promising material that may fulfill the demands of advanced flexible external thermal insulation blankets. However, research on the fabrication and performance of mullite fiber felt with high-temperature resistance and thermal stability is still lacking. In this work, mullite fibers were selected as raw materials for the fabrication of mullite fibrous porous materials with a three-dimensional net structure. Said materials' high-temperature resistance and thermal stability were investigated by assessing the effects of various heat treatment temperatures (1100 °C, 1300 °C, and 1500 °C) on the phase composition, microstructure, and performance of their products. When the heat treatment temperature was below 1300 °C, both the phase compositions and microstructures of products exhibited stability. The compressive rebound rate of the product before and after 1100 °C reached 92.9% and 84.5%, respectively. The backside temperature of the as-prepared products was 361.6 °C when tested at 1500 °C for 4000 s. The as-prepared mullite fibrous porous materials demonstrated excellent high-temperature resistance, thermal stability, thermal insulation performance, and compressive rebound capacity, thereby indicating the great potential of the as-prepared mullite fibrous porous materials in the form of mullite fiber felt within advanced flexible external thermal insulation blankets.

Effect of mullite fiber on the properties of glass composite sealing materials

Glass-based sealing materials exhibit significant potential for utilization in solid oxide fuel cells (SOFCs). We conducted a study on the impact of mullite fibers with varying particle sizes on glass composite sealing materials. We evaluated and discussed properties such as coefficient of thermal expansion (CTE), softening temperature (Ts), gastightness, interfacial compatibility, and flexural strength of the sealing materials with different ranges of fiber particle sizes. Our findings indicate that the three different particle sizes of mullite fiber-glass composite sealing materials met the required coefficients of thermal expansion, displayed good interfacial compatibility, and exhibited gastightness lower than 0.0016 sccm cm−1. Notably, the mullite fiber-reinforced sealing material that underwent ball-milling for 1 h outperformed the other two variants. It demonstrated fewer defects, a 57.1 % increase in flexural strength, and a 117.4 % higher maximum strain value.

Multifunctional mullite fiber reinforced SiBCN ceramic aerogel with excellent microwave absorption and thermal insulation performance

Mullite fibers have been widely used in thermal protection materials, but they still face challenges related to inefficient wave absorption performance. SiBCN aerogel has the advantage of tunable dielectric properties, outstanding microwave absorption performance, low density and low thermal conductivity, making it an ideal candidate for enhancing mullite fibers. Herein, a novel SiBCN aerogel/mullite fiber composite (SiBCN/MF composite) was prepared by sol-gel, freeze-drying and high temperature thermal treatment for the first time. By tuning the concentration of polyborosilazane (PBSZ), the density, microwave absorption performance, and thermal insulation properties were regulated. The obtained composites displayed low density (0.151–0.190 g/cm3), excellent microwave absorption performance (the minimum reflection loss as low as −67.83 dB and effective absorption bandwidth of 5.40 GHz) and low thermal conductivity (0.056–0.081 W/(m·K)). The outstanding comprehensive performance of the fabricated composites makes them promising candidates for use in the thermal protection system of aerospace vehicles.

Preparation and heat-insulating properties of hollow mullite fibers achieved via template impregnation method to replicate the structure of metaplexis fibers

Mullite fibers possess low thermal conductivity, excellent high-temperature stability, and thermal shock resistance, making them extensively utilized in high-temperature heat-insulating components. However, the majority of current mullite fibers have solid structures, limiting the potential for further enhancement of their heat-insulating properties. Metaplexis fiber, a plant fiber with a hollow structure, effectively obstructs heat transfer, and therefore exhibits excellent heat-insulating properties. In this study, we employed metaplexis fibers as templates to fabricate mullite fibers with hollow structures. First, a precursor solution was prepared using Al(NO3)3·9H2O and ethyl orthosilicate as the solute and anhydrous ethanol as the solvent. The metaplexis fibers were immersed in the solution, removed, and dried to obtain precursor fibers. Finally, the precursor fibers were heated to the target temperature to obtain hollow mullite fibers. The investigation focused on the impact of preparation parameters such as sintering temperature, precursor solution concentration, and aluminum–silicon molar ratio on the morphologies, phases, pore-size distributions, and thermal conductivities of the resulting hollow mullite fibers. The results demonstrated that the prepared mullite fibers inherited the hollow structure of metaplexis fibers, with the cavity diameter in the range of 3–6 μm and porosity of 91 %. Compared to traditional solid mullite fibers, the thermal insulation performance was improved by more than 59 %. These findings highlight the significant potential of hollow ceramic fibers for the development of heat-insulating materials. Using the unique characteristics of their hollow structure, these fibers offer improved heat-insulating properties compared to their solid counterparts.

Enhanced high-temperature dimensional accuracy by fibers in silica ceramic cores prepared through vat photopolymerization 3D printing

3D printing technology provides new ways to complex-shaped ceramic cores forming inner structures in aero-engine blades, but its high-temperature dimensional accuracy is highly impacted by their volume shrinkage and creep deformation. Silica ceramic cores reinforced by mullite fibers were fabricated by 3D printing in terms of vat photopolymerization in this work. Due to the scraper paving the ceramic slurry in 3D printing, majority of the mullite fibers displayed orientation distribution inside the printing layers and decreased the bending strength as the fiber content was below 5 wt%. With 12.5 wt% of mullite fibers, some fibers showed disorder distribution and arranged to connect the interlayer cracks, therefore improving the strength by fiber pull-out. Even though the reinforcement effect of mullite fibers was inadequate, the porosity and leaching rate were significantly improved. Furthermore, the mullite fibers were beneficial to lower shrinkage during sintering and casting, and enhanced the creep resistance due to the high softening temperature, which resulted in enhanced dimensional accuracy.

Robust, thermally insulating, and high-temperature resistant phosphate-enhanced mullite fiber porous ceramic composites

Porous mullite ceramics with high porosity and low thermal conductivity are urgently required as high-temperature thermal insulators in harsh environments. How to improve the strength of Porous mullite ceramics dramatically under the premise of no sacrificing its thermal insulating properties has remained an extremely challenging. Inspired by the freezing rain phenomenon in nature, high-strength and high-temperature resistant phosphate coatings were applied to the surface of mullite fibers, which significantly improved the mechanical properties of porous mullite ceramics without significantly increasing their thermal conductivity. The Al(H2PO4)3 solution concentration significantly affected the mechanical and thermal properties of the composite material, where the mechanical performance improved with an increasing Al(H2PO4)3 solution concentration. The phosphate coating phase stabilized with an increasing calcination temperature. The mechanical properties of the ceramics were improved by the excellent interfacial bonding strength between the coating and mullite fiber, and the forms of phosphate in the porous ceramics. The optimum composite preparation process increased its compressive strength by approximately nine-fold to 4.1 MPa and resulted in a thermal conductivity of only approximately 0.09 W/(m·K), demonstrating that the composite exhibited a significantly higher comprehensive performance than other previously reported porous ceramics and aerogels. Such new composites with excellent mechanical and thermal insulation properties exhibit excellent potential for high-strength thermal insulation in the chemical, metallurgical, pharmaceutical, aerospace, and other fields.

Combined use of mullite and basalt fiber to inhibit the strength decline of oil well cement stone under alternating conditions of temperature rise and fall

During the development process of heavy oil with steam huff and puff, the wellbore undergoes alternating conditions of temperature rise and fall, leading to a significant decline in the strength of the cement sheath. This work studied the inhibition effect and mechanism of the composite material composed of mullite and basalt fiber on the strength decline of cement stone (the hardened body of cement slurry) under simulated wellbore temperature alternating conditions from 80 °C to 260 °C. The pore structure distribution of cement stone was investigated by mercury intrusion porosimetry (MIP), the microstructure and phase composition of cement stone was characterized by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), together with quantitative X-ray diffraction (XRD) analysis. After 4 rounds of curing under alternating temperature conditions, the compressive strength of cement stone containing mullite and basalt fiber composite material was 28.6 MPa at 260 °C, which was only 9.2 % lower than that at 80 °C. However, the compressive strength of the blank cement stone without composite materials decreased to 22.5 MPa at 260 °C, which was 20 % lower than that at 80 °C. After 4 rounds of alternating temperature cured, the harmful pores of the cement stone without composite materials increase, and the xonotlite flakes and stacks together. The cement stone added with mullite has a finer pore structure after 4 rounds of alternating temperature cured because mullite reacts with portlandite to form C-A-S-H gel. The xonotlite remains fibrous, and a small amount of new phase hibschite is formed. Furthermore, basalt fiber can react with C-A-S-H gel at high temperatures, and the two phases are closely combined to further improve the high-temperature resistance of cement stone.

Silica Sol as a Filler in Process of Materializing Mullite Fiber (SiO2.Al2O3)

Research has been carried out on the effect of adding silica sol as a filler in making mullite fiber (SiO2.Al2O3). The process of making mullite fiber is carried out using the cast molding method. Sample preparation of (I)silica sol and alumina fiber. (II) silica sol with alumina fiber and kaolin. (III) alumina fiber with kaolin and distilled water. Then the samples were mixed until homogeneous in their respective containers. Then printed in a PVC pipe measuring 2.81 cm in diameter and 1.5 cm high. Mullite fiber samples were dried at a temperature of 110°C for 1 hour, then sintered at a temperature of 1200°C, 1300°C, 1400°C, 1500°C. The density of mullite fiber increases linearly, the porosity decreases linearly, and the compressive strength of mullite fiber increases linearly as the sintering temperature increases. XRD characterization shows that in the three mullite fiber samples the same phase appears, namely mullite, sillimanite has an orthorhombic structure, and cristobalite has a tetragonal structure.

Reusable, high specific surface areas, and excellent thermal stability Al2O3–SiO2 aerogel composites as high-temperature thermal insulators for radome applications

In this study, monolithic Al2O3–SiO2 aerogels with excellent thermal stability and high specific surface area were synthesized using boehmite nanorods (L=20–30 nm, D=3–5 nm). The high specific surface area of aerogels was maintained after calcination at 800 °C (663.54 m2/g), 1000 °C (486.91 m2/g), and 1100 °C (230.76 m2/g). The anti-sintering mechanism of the Al2O3-SiO2 aerogels was discussed. Thanks to the composite of Al2O3-SiO2 aerogel and mullite fibers, the disadvantages of brittleness and low strength of aerogel are overcome. The thermal conductivity of the composite is extremely low at 0.034 W/(m·K)-0.089 W/(m·K) (25 °C-1000 °C). Five muffle cycle back-temperature tests at 1000 °C and a butane blowtorch back-temperature test at 1350 °C also proved that the composites have excellent thermal insulation properties. Composites with excellent thermal insulation properties combined with lightweight, thinness, machinability, and good dielectric properties meet the need for high-temperature thermal insulating-wave transmitting integrated materials for hypersonic missile radomes.

Preparation and thermal insulation performance of Al2O3[sbnd]Y2O3[sbnd]SiO2 ternary composite aerogels with high specific surface area and low density

Against the background of global energy shortages and the need for long-term flight safety of aerospace vehicles, traditional silica aerogels have the problems of insufficient temperature resistance and poor thermal insulation at high-temperatures, and are no longer able to meet the demand. In this study, a series of monolithic Al2O3Y2O3SiO2 ternary aerogels were successfully synthesized via synchronous sol–gel and ethanol supercritical drying. Both the prepared aerogels and the calcined samples at 1000 °C exhibited a complete three-dimensional network structure and a high specific surface area (up to 675.7 m2/g and 214.4 m2/g), which was attributed to the supporting effect of the Al2O3 component on the overall skeleton and the inhibiting effect of the Y2O3 component on the crystal transformation. Mullite fiber reinforced Al2O3Y2O3SiO2 aerogel composites were synthesized by vacuum impregnation using mullite fiber felt as a matrix. The prepared sample exhibit very low thermal conductivity (as low as 0.079 W⋅m−1·K−1 at 1000 °C). In summary, the good structural stability and high temperature thermal insulation performance make Al2O3Y2O3SiO2 ternary aerogel have the potential of efficient service in more application scenarios. This study provides a product optimization reference idea for the thermal insulation material industry.

Enhanced performance triboelectric nanogenerator based on mullite/PVA composites for self-driven sensing and smart home control

Polyvinyl alcohol (PVA) has good biocompatibility, a simple fabrication process, and environmental protection, which is very suitable for the production of triboelectric nanogenerator (TENG) applied to smart home control. However, the output performance of the TENG composed of PVA and PDMS films is not high. Previous research has explored the enhancement of PVA-based TENG performance by doping with conductive materials to modify the dielectric properties of PVA composite films. Nevertheless, this approach is associated with issues of high production costs and energy consumption. This work prepared a mullite/PVA composite material TENG (MP-TENG), the introduction of mullite induced interfacial polarization in the composite film. This effect resulted in the appearance of polarization centers, thereby enhancing the charge-sensing capability of the composite film. Consequently, the triboelectric output performance of the MP-TENG was improved. MP-TENGs with different amounts of mullite fiber doping were prepared, and the maximum output performance was obtained when the doping level reached 3 wt%. At this concentration, the composite film exhibited an open-circuit voltage of 70.89 V and a short-circuit current of 2.45 μA. An enhancement of 1.78 and 1.71 times was achieved with respect to the pure PVA-TENG, respectively. In addition, MP-TENG exhibited excellent sensing characteristics, a smart home control system was designed in conjunction with a hardware circuit, which captured hand motions and encoded them to generate binary codes to control the on/off state of the indoor home.

A CFD Analysis of the Desalination Performance of Ceramic-Based Hollow Fiber Membranes in Direct Contact Membrane Distillation

In this numerical study, the performance of ceramic-based mullite hollow fiber (HF) membranes in a direct contact membrane distillation (DCMD) process was evaluated. Three types of membranes were tested: (i) hydrophobic membrane C8-HFM, (ii) rod-like omniphobic membrane (C8-RL/TiO2), and (iii) flower-like omniphobic membrane (C8-FL/TiO2). The CFD model was developed and validated with experimental results, which were performed over a 500 min period. The initial mass flux of C8-HFM was 30% and 9% higher than that of C8-FL/TiO2 and C8-RL/TiO2, respectively. However, the flower-like omniphobic membrane C8-FL/TiO2 had the lowest drop in flux, around 11%, while the rod-like omniphobic membrane C8-RL/TiO2 had a 15% reduction, both better than the 23% reduction in the hydrophobic membrane C8-HFM over the 500 min. The study also analyzed the impact of fouling by examining the variation in mass transfer coefficient (MTC) over time. The results indicated that the ceramic-based mullite HF membranes with TiO2 flowers and rods demonstrated a high resistance to fouling compared to C8-HFM. The modified membranes could find applications in the desalination and handling of seawater samples containing organic contaminants. The CFD model’s versatility can be utilized beyond the current investigation’s scope, offering a valuable tool for efficient membrane development solutions, particularly for challenges such as the presence of organic contaminants in seawater.

Low-cost preparation and characterization of high-purity mullite nanofibrous membranes

Among the family of high-quality fiber materials, mullite fiber membrane is a promising one that possesses abilities of both the thermal resistance and oxidation resistance under high temperature. High-purity mullite nanofibrous membranes were prepared by electrospinning method with low-cost aluminum chloride hexahydrate (AC) as the aluminum source. The effects of sintering temperature and AC content on the phase structure, microstructure, thermal and mechanical properties of the mullite nanofibrous membranes were investigated. The results show that the sintering temperature as low as 1000 °C is the very appropriate temperature for obtaining the high-purity mullite nanofibers with smooth surface. The minimum thermal conductivity of 0.029 W m−1 K−1 and maximum tensile strength of 6.11 MPa are gained from the mullite nanofibrous membranes with AC content of 2 mmol. The mullite nanofibrous membranes with excellent properties can be selected as an ideal candidate for thermal insulation and reinforced applications.

Facile preparation of high-strength SiC/C aerogels from pre-reacted resorcinol–formaldehyde and siloxane

Carbon aerogels are ideal thermal insulation materials in the field of aerospace. However, their poor oxidative resistance at high temperatures is a significant drawback. Therefore, there is considerable interest in developing SiC/C aerogels. In this study, using pre-reacted resorcinol–formaldehyde gel (RF) as the carbon source, homogeneous silica/phenolic resin (SiO2/RF) hybrid aerogels were prepared via atmospheric pressure drying using a simple process. The SiC/C aerogels were subsequently obtained via carbon thermal reduction. As the C/Si ratio decreased (from 6 to 4.5 and 3), the density, thermal conductivity, and compressive strength of the aerogels decreased from 0.19 to 0.26 g·cm−3, 0.0330 to 0.0983 W/(m·K), and 0.57 to 4.83 MPa, respectively. The linear shrinkage rate was 16.4–18.3 %, which is lower than those of most reported SiC/C aerogels. Here, the carbon improved the strength of the SiC-based aerogels while maintaining their thermal insulation characteristics. A composite material prepared using the relatively low-strength aerogels and mullite fibre felt could achieve long-term insulation at 1400 °C in an air environment. This study lays the foundation and outlines various options for achieving an optimal balance between the strength and thermal conductivity of SiC/C aerogels for industrial applications.

Mullite fiber porous ceramic with high quality factor for high-temperature PM filtration

Porous ceramic filters with high porosity and high filtration are in urgent need of high-temperature gas filtration. Here a lightweight and low thermal conductivity mullite fiber porous ceramic filter was prepared through vacuum filtration method using solution blow spun mullite fiber as the matrix material and silica sol as the high-temperature binder. Mullite fiber porous ceramic with a mass ratio of 10:1 has uniform pore structure, low density (0.58 g cm−3), high porosity (78.11%), high compressive strength (1.79 MPa), as well as low thermal conductivity (0.095 W m−1 K−1) at a sintering temperature of 1300 ℃. When the air flow is 6 L min−1, the pressure drop is 180 Pa, the filtration efficiency of PM2.5 and PM10–2.5 is 98.85% and 99.61%, respectively, and the filtration quality factor is 0.025 Pa−1. The mullite fiber porous ceramic filter have outstanding quality factor, which has broad application prospects in high-temperature gas filtration.