Mullite Fibers Research Articles

Stabilizing the pore structures of foam glasses for improving the mechanical and physical properties at a low sintering temperature

We investigate the foaming stabilizing effects of K3PO4 and Li2CO3 to the physical and mechanical properties of foam glasses and find an optimal amount of 0.75 wt% K3PO4 and 1.5 wt% Li2CO3 that enables a balanced combination of superior low bulk density (0.33 g/cm3) and high compressive strength (3.28 MPa). The chemical stability of foam glasses with Li2CO3 exhibits a low weight loss of 0.50 wt% and ion dissolution in aqueous solutions with pH of 3 and 7, showing that Li2CO3 can effectively enhance the chemical stability of the foam glasses subject to acid solution and water. Next, the combinational effects of MnO2, polycrystal mullite fiber, K3PO4, and Li2CO3 are investigated, where the physical and mechanical properties are overall improved especially in compressive strength, but other properties show minor improvement. The foam glasses stabilized by K3PO4 and Li2CO3 are weaker than those stabilized by commonly-used stabilizer Na2HPO4, but are lighter with a homogeneous foaming structure of more uniformly-distributed pores. This work finds more alternative ways in effectively stabilizing the foaming, and should inspire more systematic approaches to controlling the foaming for diversified engineering needs by establishing fundamental structure-property-process relationships, thereby customizing the design of foam glasses with desirable properties.

Controlled synthesis of biomimetic materials with protruding structures by in situ growth of silica nanorods via hydroxyl-localized droplet template method

Inspired by the Lotus-Effect in nature, biomimetic materials with a protruding structure have been prepared through a hydroxyl-localized droplet templating method, in which the droplets are formed by polyvinylpyrrolidone as a surfactant and sodium citrate as a stabilizer. Silica nanorods grow uniformly and densely unidirectionally on the surface of high silica fibers. With the adjustment of reaction conditions and reagent concentration, the aspect ratio of the nanorods is adjustable between 1.73 and 17.58, where the length and diameter are controllable in the range of 180–368 nm and 194–1489 nm, respectively. Meanwhile, the structure also undergoes changes from straight rod-like to curved linear shapes with curvature values up to 1.50. It is found that the hydroxyl group on the fiber surface, which adsorbs the attachment of emulsified droplets and participates in the condensation reaction of tetraethyl orthosilicate, serves as the key to the in-situ growth of silica nanorods. In addition, silica nanorods are also successfully grown on mullite fibers and basalt fibers yielding biomimetic materials with protruding structures. The mean lengths of silica nanorods on mullite and basalt fibers are 833 nm and 1361 nm, correspondingly, along with respective mean diameters of 379 nm and 271 nm.

Lightweight and thermal insulation mullite fibers/whiskers hierarchical framework materials: Low-temperature in-situ growth method and mechanism

Mullite fibers/whiskers hierarchical structure materials (MFWs) were prepared via the three-stage method, i.e. seeds breeding, precursors introducing and whiskers growth. The mechanism of low-temperature in-situ synthesis of mullite whiskers during gas-phase reaction process has been discussed in detail. The seeds bred on mullite fibers (MFs) are the growth points and can effectively reduce the subsequent growth temperature of mullite whiskers (MWs). The precursors composed of aluminum source, silicon source and catalyst provided raw materials for whiskers growth. Under the heat treatment temperature of 800 °C, mullite seed grains were guided to in-situ transform into MWs. Moreover, MFWs fabricated via low-temperature in-situ growth mechanism on the MFs present low density (0.103–0.147 g/cm3) and ultralow thermal conductivity (0.0426–0.0514 W·m−1·K−1). Due to the lower whiskers growth temperature in this work than the ones in the most recent literatures, the three-stage method can be regarded as a viable strategy for low-temperature in-situ growth whiskers.

A Facile Method for Fabricating a Monolithic Mullite Fiber-Reinforced Alumina Aerogel with Excellent Mechanical and Thermal Properties.

Alumina aerogels are considered to have good application prospects in the high-temperature field. In this study, monolithic mullite fiber-reinforced alumina aerogels with excellent mechanical and thermal properties were synthesized via a facile method without the use of any chelating agents. This method successfully avoids the introduction of impurities during the use of catalysts and chelating agents while greatly reducing gelation time, and thus helps mullite fibers to uniformly disperse in the sol. The compressive stress at 80% strain of the obtained mullite fiber-reinforced alumina aerogels was as high as 16.04 MPa—426% higher than that of the alumina aerogel without the addition of mullite fibers. Regarding thermal properties, the shrinkage of the mullite fiber-reinforced alumina aerogels (AM) samples was less than 1% after heat treatment at 1300 °C for 2 h. Furthermore, the rear-surface temperature of the AM samples burned by a butane blow torch was only 68 °C. These outstanding properties make AM samples promising for application in thermal insulation materials in high-temperature fields such as aerospace and industrial thermal protection in the future.

Mechanical and Thermal Properties of Phosphoric Acid Activated Geopolymer Materials Reinforced with Mullite Fibers.

This article investigates several important properties, such as thermal resistance, mechanical properties, and phase evolution, of geopolymer ceramics reinforced with mullite fibers. This particular fiber reinforcing geopolymer composites was prepared from kaolinite and mullite fibers with phosphoric acid as activator. X-ray diffraction (XRD), thermogravimetry and differential scanning calorimetry, Fourier-transform infrared spectroscopy, and scanning electron microscopy were used to determine the phase evolution and strengthening mechanisms. With the addition of mullite fibers, the mechanical properties increased by at least 20%. The optimum flexural strength exceeded 13 MPa. It was found that mullite fibers had desirable interface bonding with this type of geopolymer, promoting both crack deflection and fiber pullout strengthening mechanisms. This was correlated with a significant strengthening effect of the fibers. The linear shrinkage after heat treatment at 1150 °C~1550 °C was investigated and correlated with XRD analyses. The addition of mullite fibers reduced the linear shrinkage significantly up to 1350 °C. The large linear shrinkage above 1450 °C was correlated with the decomposition and melting of the AlPO4 phase.

Thermal Radiation Shielding and Mechanical Strengthening of Mullite Fiber/SiC Nanowire Aerogels Using In Situ Synthesized SiC Nanowires.

Traditional solid nanoparticle aerogels have been unable to meet the requirements of practical application due to their inherent brittleness and poor infrared shielding performance. Herein, combining vacuum impregnation and high-temperature pyrolysis, a novel micro/nano-composite fibrous aerogel was prepared via in situ synthesis of silicon carbide nanowires (SiC NWS) in mullite fiber (MF) preform. During this process, uniformly distributed SiC NWS in the MF preform serve as an enhancement phase and also act as an infrared shielding agent to reduce radiation heat transfer, which can significantly improve the mechanical properties of the mullite fiber/silicon carbide nanowire composite aerogels (MF/SiC NWS). The fabricated MF/SiC NWS exhibited excellent thermal stability (1400 °C), high compressive strength (~0.47 MPa), and outstanding infrared shielding performance (infrared transmittance reduced by ~70%). These superior properties make them appealing for their potential in practical application as high-temperature thermal insulators.

Experimental study on toughening of porous cordierite ceramic prepared by particle-stabilized emulsions

The majority of work has focused on the toughening of porous cordierite ceramics (PCC), which has a uniform and homogeneous porous structure prepared by the particle-stabilized emulsions after sintering at ∼1300 ℃ for 2 h. The particle sizes of raw materials were adjusted to 20 nm to ensure that all samples maintained the same porous structure, which prevented crack propagation from separating. PCC's bending strength increased by up to ∼214% at ∼4 wt% ZrO2 content, which absorb energy to improve fracture toughness. The addition of mullite fibers created new intermolecular forces between colloidal particles and filler parameters, which played a negative impact on the stability of the micelle shape, resulting in unsuccessful foaming and toughening. Rod-shaped cordierite crystals with a large aspect ratio were formed on the inner wall of PCC with 3 wt% AlF3 and created a short network in situ, which improved the bending strength of the tensile structure by strengthening crack deflection and crack bridging.

An investigation on tribological properties and mechanical properties of UHMWPE/polycrystalline mullite fiber

An investigation on tribological properties and mechanical properties of UHMWPE/polycrystalline mullite fiber

Effect of mullite phase formed in situ on pore structure and properties of high-purity mullite fibrous ceramics

Porous mullite ceramics are potential advanced thermal insulating materials. Pore structure and purity are the main factors that affect properties of these ceramics. In this study, high performance porous mullite ceramics were prepared via aqueous gel-casting using mullite fibers and kaolin as the raw materials and ρ-Al 2 O 3 as the gelling agent. Effects of addition of mullite fibers on the pore structure and properties were examined. The results indicated that mullite phase in situ formed by kaolin, and ρ-Al 2 O 3 ensured the purity of mullite samples and mullite fibers bonded together to form a nest-like structure, greatly improving the properties of ceramic samples. In particular, the apparent porosity of mullite samples reached 73.6%. In the presence of 75% of mullite fibers, the thermal conductivity was only 0.289 W/m K at room temperature. Moreover, the mullite samples possessed relatively high cold compressive strength in the range of 4.9–9.6 MPa. Therefore, porous mullite ceramics prepared via aqueous gel-casting could be used for wide applications in thermal insulation materials, attributing to the excellent properties such as high cold compressive strength and low thermal conductivity.

Super-insulated, flexible, and high resilient mullite fiber reinforced silica aerogel composites by interfacial modification with nanoscale mullite whisker

Fiber-reinforced aerogel composites are extensively used in thermal insulation and energy-saving fields, while the inherently weak interface combination between fiber and aerogel undermines the effect of fiber reinforcement . To deal with this problem, mullite whiskers were in-situ synthesized on the surface of alumina silicate fiber through a facile and environmentally benign method. The nanoscale mullite whiskers growing on the fiber strengthen the interfacial bonding , thereby improving the mechanical properties of the composite. The alumina silicate fiber/mullite whisker/silica aerogel composites delivered ultrahigh compression stress and modulus of 6.45 MPa and 69.74 MPa at 50% strain, which is 6.14 times and 9.07 times higher than those of bare fiber reinforced silica aerogel . Meanwhile, the composite maintained a high retention rate of maximum stress of 97.8% after being compressed at 30% strain for 200 cycles. Moreover, the composite exhibits an ultralow thermal conductivity of 27.5 mW m −1 K −1 and a low density of 0.26 g cm −3 . The strong, resilient, lightweight composite with ultralow thermal conductivity is applicable for heat preservation and energy saving. • Mullite nanowhiskers were in-situ synthesized on alumina silicate fiber without F catalysts contaminating environment. • The stress (6.45 MPa) and modulus (69.74 MPa) at 50% strain increased dramatically due to interfacial enhancement. • The tough and resilient composite can be compressed at 30% strain for 200 cycles. • The composite also exhibited a low density (0.26 g cm −3 ) and a low thermal conductivity (27.5 mW m −1 K −1 ).

Thermal insulation and anti-vibration properties of MoSi2-based coating on mullite fiber insulation tiles

Thermal protection materials with excellent thermal insulation properties and high reliability are crucial for aerospace vehicle. Mullite fiber insulation tiles coated with MoSi2-borosilicate glass ([email protected]2) were prepared by a simple slurry method. Results shown that the surface temperature rapidly reached 1043.1 °C under the heat flux of 450 kW/m2, while the cold-surface remained at room temperature. The adjusting effects of MoSi2-based coating on the thermal response and transfer properties of MFIT were investigated systematically. Compared with MFIT, the surface and internal temperatures of [email protected]2 were obviously suppressed, due to the existence of MoSi2-based coating with high emissivity, which effectively enhanced the thermal radiation of the surfaces. Finally, the structural reliability under coupled environment of heating (q0 = 450 kW/m2) and random vibration with high magnitude (20–2000 Hz, Grms = 20 g) was also investigated. Destructive cracking and peeling of the coating were not observed, due to the excellent bonding strength between the coating and the substrate. The results provided an important basis for the potential application of insulation tiles with mullite fibers in aerospace vehicles and the design of lightweight thermal protection systems.

Mechanical and Thermal Properties of Phosphoric Acid Activated Geopolymer Materials Reinforced with Mullite Fibers

Mechanical and Thermal Properties of Phosphoric Acid Activated Geopolymer Materials Reinforced with Mullite Fibers

Low-Cost and Temperature-Resistant Mullite Fiber Sponges with Superior Thermal Insulation and High-Temperature Pm Filtration

Low-Cost and Temperature-Resistant Mullite Fiber Sponges with Superior Thermal Insulation and High-Temperature Pm Filtration

Application of 3D mullite fiber matrix as a lithiophilic interlayer in lithium metal anodes.

The lithium (Li) metal anode is an intriguing choice for Li metal batteries because of its considerably high theoretical capacity. However, the commercialization of unimproved lithium metal batteries is impeded due to the severe security issues related to uncontrollable Li dendrite growth. Herein, we fabricated three-dimensional (3D) mullite sheets via a high temperature annealing process in the presence of silica sol and used such sheets as interlayers between Li anodes and separators for assembling cells. As both Al2O3 and SiO2, the two main components of mullite fibers, are lithiophilic, the deposition of Li during charging processes could be regulated and the growth of lithium dendrites and accumulation of dead Li could be efficiently suppressed. In a Li||Cu half-cell, the mullite fiber/Li composite electrode delivers a coulombic efficiency of above 98% under a current density of 1.0 mA cm-2. For a Li||LiFePO4 full cell, the same composite electrode exhibited improved capacity retention (>89%, after 800 cycles) and rate capability at 5.0C.

Foreign element doping and thermal stability of alumina aerogels

AbstractThe addition of foreign elements is considered as an effective method to improve the thermal stability of alumina aerogels at a higher temperature. However, the location and stabilizing mechanism of the foreign elements in the alumina aerogel have not been carefully studied. In this work, Si or La was introduced into the network of alumina aerogels through a sol‐gel strategy. The Si‐doped alumina aerogel maintained high surface area (92 m2/g) and pore volume (0.572 cm3/g) even at 1300°C. The dopants prevented α‐Al2O3 transformation at elevated temperatures (1200°C–1300°C). The distribution of foreign ions and their stabilizing mechanism were discussed in detail. The doped alumina aerogels reinforced by mullite fiber felt, with quite low density and thermal conductivity, can be used as high‐temperature thermal insulations.

Customizing the properties of borosilicate foam glasses via additions under low sintering temperatures with insights from molecular dynamics simulations

We computationally and experimentally investigate the impact of a series of additions to the physical and mechanical behaviors of borosilicate foam glasses sintered at low temperatures for customized engineering needs. A trend of enlarging and reducing in density and strength is shown when adding Ca(NO3)2 and Ba(NO3)2, and constantly increasing trends are found for CaSO4 and CaCO3. The molecular mechanism of the CaCO3-added foam glasses is elucidated with molecular simulations, where the enlarged strength and density are attributable to more uniformly distributed pore microstructures. The reinforcing effects of mullite fibers are explored, where a general trend of enlarging in density and strength is shown. Total porosity and volume-absorption rate are inversely correlated with strength and density, respectively, guiding a potential pathway for targeted design. By understanding the effects of pore microstructures, this study establishes a novel material process-structure-property perspective, inspiring future efforts in mass-producing low cost, high-performance environmentally friendly materials.

Microstructure evolution and mechanical behavior of a mullite fiber heat‐treated from 1100 to 1300°C

AbstractIn this paper, the effect of phase transformation on microstructure evolution and mechanical behaviors of mullite fibers was well investigated from 1100 to 1300°C. In such a narrow temperature range, the microstructure and mechanical properties showed great changes, which were significant to be studied. The temperature of the alumina phase transformation started at below 1100°C. The main phases in fibers were γ‐Al2O3 and δ‐Al2O3 with amorphous SiO2 at 1150°C. The stable α‐Al2O3 formed at 1200°C. Then the mullite phase reaction occurred. As the alumina phase reaction took place, the tensile strength increased with the increasing temperature. In particular, the filaments achieved the highest strength at 1150°C with 1.98 ± 0.17 GPa, and the Young's modulus was 163.08 ± 4.69 GPa, showing excellent mechanical performance. After 1200°C, the mullite phase reaction went on with the crystallization of orthorhombic mullite. The density of surface defects increased rapidly due to thermal grooving, which led to mechanical properties degrade sharply. The strength at 1200°C was 1.01 ± 0.15 GPa with a strength retention of 63.13%, and the Young's modulus was 184.14 ± 10.36 GPa. While at 1300°C, the tensile strength was 0.64 ± 0.14 GPa with a strength retention of only 40.00%.

Microstructural and mechanical characterization of a mullite fiber

The effect of thermal exposure on a mullite fiber was analyzed. This type of mullite fiber, consisting of γ-Al2O3 and amorphous SiO2, was developed for high-temperature applications. Heat treatments at temperatures ranging from 900 °C to 1500 °C for 1h were performed in air. Investigations showed that the tensile strength of the initial fiber was about 1.60 GPa. And the elastic modulus was about 133.51 GPa. The bundles’ strength decreased at 900 °C slightly after thermal treatment, then increased and got a maximum at 1100 °C with 1.65 GPa. At above 1100 °C, the strength degraded sharply due to the mullite phase transformation and the exaggerated grain growth. At 1300 °C, the phase reaction almost finished with a tensile strength of 0.86 GPa. And the strength retention was only 47.50%. When the heat-treated temperature got to 1500 °C, the density of surface defects in the fiber surged, making it too fragile and weak to go through the tensile tests.

Influences of mullite fibers on mechanical and thermal properties of silica‐based ceramic cores

AbstractMullite fibers composite silica‐based ceramic cores were successfully prepared by injection molding. The effects of mullite fibers on the mechanical and thermal properties of ceramic cores were investigated. The results indicated that the linear shrinkage was significantly decreased and the porosity was gradually increased with the increase of mullite fibers. In addition, the flexural strength for the room temperature and the simulated casting temperature of 1500°C was increased to a maximum value when the content of mullite fibers was about 1 wt.%, and then decreased with the increase of mullite fibers. The mullite fibers of 1 wt.% presented excellent mechanical properties with a linear shrinkage of .65%, a porosity of 6.96%, and a flexural strength of 17 MPa at room temperature and 34.83 MPa at the simulated casting temperature of 1500°C. Besides, the change in microstructure and properties in various contents of mullite fibers were analyzed.

Fabrication and electromechanical characterization of mullite ceramic fiber/thermoplastic polymer piezoelectric composites

AbstractVarious types of sensors such as piezoelectric ceramics and strain gauges are widely used to monitor the damage of automobile components, buildings, and so on. Among these sensors, piezoelectric sensors have a high‐energy conversion efficiency, that is a high sensitivity. Therefore, we fabricated a mullite fiber‐reinforced resin sheet with piezoelectric particles (Piezo‐MFRS) and evaluated their sensor performance. Piezo‐MFRS was fabricated by the hand layup method—the polyamide resin was mixed with piezoelectric particles and infused into the mullite fiber fabric. After that, the piezo‐MFRS was polarized by a corona discharge. We investigated the relationship between the applied impact load or vibration load to the piezo‐MFRS and the generated voltage. The piezo‐MFRS induced an output voltage of 20 mV under a compressive stress of 4 MPa, and an output voltage amplitude of 2 mV due to bending vibration of amplitude 0.5 mm. Therefore, it seems that the piezo‐MFRS is suitable for use as a load sensor. Furthermore, we confirmed that the tensile properties of piezo‐MFRS were not reduced by the addition of piezoelectric particles. Consequently, it is likely that piezo‐MFRS is a promising material for a rugged piezoelectric sensor and opens the door to monitoring large structures.