Mullite Composites Research Articles

Oxidation behavior of sol-derived C/Mullite composites in air

Thanks to the reinforcing and toughening of carbon fiber, carbon fiber reinforced mullite (C/Mullite) composites exhibit favorable application potential as thermal structural materials. To further reveal the failure mechanism of the C/Mullite composites in service environment, the oxidation resistance and failure behavior of the composites were systematically investigated in this study. It was found that the oxidation mechanism of the composites inclined to diffusion control at 1300 °C∼1500 °C, the cracks and pores of the composites provided diffusion channels for oxygen to oxidize the composites, the oxidation degree of carbon fibers and interfacial coating after oxidation was responsible for the weight loss and performance degradation of the composites. Additionally, the oxidation resistance of C/Mullite composites can be obviously enhanced by using the SiO2-rich (the Al/Si atomic ratio was 1/2) Al2O3–SiO2 sol to densify at the later stage of fabrication, strength retention and modulus retention of the composites were increased by 28 % and 40 % after oxidation at 1500 °C in air, respectively.

Mixed Oxide Aerogels with High‐Performance Insulating Properties for High‐Temperature Space Application

Herein, a direct comparison of the thermal conductivity and stability of different silica‐fiber‐reinforced silica aerogels is presented. The thermal performance under high‐temperature exposure within an arc‐heated facility is evaluated by means of thermal propagation and stability. All aerogel materials, tested within this study, outperform the high‐temperature fiber‐based insulation mat in terms of thermal protection. Modification of the aerogels in the composition and opacification leads to an improvement of thermal insulation properties as well as of thermal stability against the high temperature. All materials are prepared in the form of cylindrical disks of 100 mm diameter having a thickness of 20 mm. Based on classic silica aerogel, an opacified silica aerogel and an aerogel in mullite composition, inorganic fiber‐reinforce composites have been prepared and tested, and their performance is discussed comparatively. The specimens with mullite composition are intended to form mullite in situ during application to avoid energy‐costly pretreatment.

Mechanical, thermal, and dielectric properties of glass mullite composites for low‐temperature cofired ceramic and radome applications

AbstractSiO2‐Al2O3‐CaO‐based glass (10–60 wt%)/mullite composites were investigated for the LTCC and radome applications. The optimum densification temperatures decreased from 1550°C (10 wt% glass) to 1400°C (30 wt% glass) by means of liquid‐phase sintering, and to 850°C–825°C (50–60 wt% glass) by means of viscous phase sintering. XRD analysis showed that mullite was the main phase as well as in situ crystallized anorthite after 825°C. The composite with 20 wt% glass was a suitable candidate for the radome applications (bulk density = 2.86 g/cm3 after sintering at 1450°C, dielectric constant (loss) = 7.12 (0.0025) at 5 MHz, thermal expansion coefficient = 4.27 ppm/°C between 25°C and 800°C, thermal shock resistance parameter = 162°C), and the composite with 50 wt% glass was a suitable candidate for the low‐temperature cofired ceramic applications (bulk density = 2.64 g/cm3 after sintering at 850°C, dielectric constant (loss) = 6.79 (0.0043) at 5 MHz, thermal conductivity = 2.11 W/m⋅K at 25°C, and thermal expansion coefficient = 3.93 ppm/°C between 25°C and 300°C).

Radiation shielding and mechanical properties of mullite-zirconia composites fabricated from investment-casting shell waste

Low-cost zirconia-toughened mullite composites were prepared from investment casting shell waste and alumina. The influence of sintering temperature on the composites' properties, including crystalline phase composition, microstructure, degrees of densification, mechanical properties, and radiation shielding characteristics, was investigated. The results show that the higher the sintering temperature, the higher the degree of densification is, improving the mechanical and radiation shielding properties of prepared composites. The mullite-zirconia composite sintered at 1600 °C presents a good mechanical strength, with flexural and compressive strength values of 190 and 308 MPa, respectively. These values are comparable to or even better than mullite ceramics prepared from other waste materials. Furthermore, the composites' gamma-ray and neutron attenuation characteristics suggest that they can be promising as radiation shields.

The Influence of Mullite Shape and Amount on the Tribological Properties of Non-Asbestos Brake Friction Composites

For investigating the effect of mullite as a reinforced fiber of the non-asbestos brake friction material on the performance of brake pads, mullite reinforced composites with different contents (5% and 10%) and shapes (powder-based and fiber-based) were developed, and the physical and mechanical properties of the composites were analyzed. The tribological properties of the composites were tested by a Chase tester followed by the IS-2742 standard, and the worn surface was investigated by three-dimensional surface topography and SEM. The results show that the brake friction material with 5% powdered mullite performs best, having the highest stable friction performance (0.86), the lowest wear rate (3%), the lowest friction variation performance (0.263), and the best fade-recovery performance. With the increase of mullite content, the friction variation, wear resistance, and friction stability of the composites become worse. Meanwhile, the performance of powder-based mullite composites is better than that of fiber-based. The worn surface analysis shows that the fiber-based mullite composite has a higher surface roughness, fewer contact platforms, more wear debris, and peeling pits. In contrast, the powder-based mullite composites have a better surface performance. It provides a practical basis for mullite-reinforced non-asbestos brake friction materials.

Novel cordierite-acicular mullite composite for diesel particulate filters

Cordierite-acicular mullite composites containing 0, 25, 50, 75 and 100 wt% of mullite were fabricated from waste MoSi 2 and commercial powders of Al 2 O 3 and spinel (MgAl 2 O 4 ). Careful oxidation of pulverised waste MoSi 2 rendered a precursor mixture of MoO 3 and amorphous SiO 2 , which served as pore forming agent and SiO 2 source, respectively. Evaporation of MoO 3 at ∼750 °C allowed production of highly porous cordierite-mullite ceramic composite after sintering in air at 1350 °C for 4 h. The combination of equiaxed cordierite grains and elongated (prism-like) mullite grains, resulted in unique microstructure with open porosity between 53.3 and 55.6 vol% which makes the obtained composite convenient for application as diesel particulate filter material. The presence of mullite affected four key thermo-mechanical properties which determine the thermal shock resistance of cordierite-mullite composite. The best thermal shock resistance was measured in composite containing 75 wt% of mullite. It was a result of improved thermal conductivity (1.081 W/mK) and bending strength (3.62 MPa) and relatively low values of coefficient of thermal expansion (3.8 × 10 −6 K −1 ) and elastic modulus (2.27 GPa).

Mechanical properties of unidirectional laminated hybrid SiC–Nextel™ 720 fiber-reinforced oxide matrix composites fabricated by a novel precursor infiltration and pyrolysis method

Unidirectional laminated hybrid SiC–Nextel™ 720 fiber-reinforced yttria-stabilized zirconia–mullite composites were fabricated by a novel precursor infiltration and pyrolysis method. The effect of annealing atmosphere (Ar and air) at 1200 °C on the mechanical properties and failure mechanism of the composites was investigated. The Ar-annealed composite had a superior flexural strength of 337.1 ± 5.7 MPa and a good fracture toughness of 13.3 ± 0.1 MPa m1/2, and exhibited non-brittle failure behavior. However, the Air-annealed composite had a poor flexural strength of 102.8 ± 0.8 MPa and a low fracture toughness of 5.3 ± 0.3 MPa m1/2, and it exhibited brittle failure behavior. Further analysis indicated that the presence of a considerable amount of residual carbon in the Ar-annealed composite effectively weakened the fiber/matrix interface by reducing the residual radial compressive stress and the interfacial coefficient of friction. The weak interface could trigger toughening mechanisms in the composite, such as fiber pullout and debonding, and crack deflection. The mechanical degradation of the Air-annealed composite was also associated with the slight damage of Nextel™ 720 and the oxidation of SiC fibers after exposure in a high-temperature air environment. The findings of this study provide guidance for designing new composites exhibiting high performance.

Metal-insulator multilayer precursors for enhanced Sm2+ absorption and Tm2+ luminescence in SiAlO thin films prepared by magnetron sputtering

Silicon-aluminum-oxigen (SiAlO) coatings doped with Sm2+ and prepared by reactive magnetron co-sputtering of Si, Al, and Sm targets, are attractive for luminescence solar concentrator applications but suffer from the low absorption between 300 and 600 nm. This article reports that the main cause of low absorption is a high concentration of undesired Sm3+. This finding is supported by optical transmittance, photoluminescence emission and excitation characterization, and X-ray photoelectron spectroscopy data of the Sm's 3d5/2 edge. We present an alternative deposition process for obtaining Sm doped SiAlO layers with enhanced Sm2+ absorption by incorporating Sm through the use of multilayer thin-film precursors composed of metallic Sm and SiAlO layers. After thermal post-deposition treatments, diffusion and reaction of the metallic Sm layers with the SiAlO host results in coatings showing the characteristic 5d → 4f transitions of Sm2+ in the region between 250 and 600 nm which were not detectable in Sm-doped single layers. This same deposition strategy produces Tm doped SiAlO coatings with Tm2+‘s characteristic luminescence at 1132 nm when the SiAlO host is in the mullite composition region. The photoluminescence excitation spectrum of Tm2+ is compared to phosphor with similar composition and covers the range between 300 and 700 nm.

Microstructure and properties evolution of C/Mullite composites during fabrication process

Carbon fiber reinforced mullite (C/Mullite) composites with favorable mechanical properties have been fabricated through sol-impregnation-drying-heating (SIDH) route in our previous work. For the further optimization of fabrication technology, it is very necessary to elucidate the evolution of microstructure and properties of C/Mullite composites during SIDH process. For this purpose, C/Mullite composites were fabricated at different cycles through SIDH route in present paper, and the effect of fabrication cycles on microstructure, mechanical properties and oxidation resistance of the composites were investigated. The results show that mullite matrix grew on the surface of single carbon fiber firstly, then grew from inside to outside of carbon fiber bundle during fabrication. Finally, it grew completely on the surface of the composites and formed a “mullite coating”. Porosity of the composites decreased gradually and pores of the composites transformed from connected ones into isolated ones as the fabrication cycles increased. When the fabrication cycles reached 24, the porosity of the composites was unchanged basically. At the same time, the composites possessed the optimal mechanical properties and oxidation resistance.

Rheological, physico-mechanical and microstructural properties of porous mullite ceramic based on environmental wastes

Highly porous mullite-corundum bodies were prepared by slip casting with the utilization of porogen sources as environmental wastes. The stoichiometric composition of mullite was established by the application of clay and alumina. Pore formers were saw dust or activated charcoal. Bodies were processed, dried and fired between 1180 and 1450°C. Microstructure and the mechanical properties of the product were evaluated. The results showed that, the porosity decreased with raising the firing temperature but the mullite formation increased. The porosity increased with the addition of pore forming agent while modulus of rapture decreasing. The results concluded that the porosity ranged (40–56%) with the addition of pore forming agent (1–10%) with decreasing modulus of rapture (30–8.5MPa) respectively.

Fabrication and properties of three-directional orthogonal aluminosilicate fiber fabric-reinforced mullite composite

In this study, a three-directional orthogonal aluminosilicate fiber fabric-reinforced mullite composite with excellent properties was prepared by the precursor impregnation and pyrolysis (PIP) method. The influence of the number of PIP cycles on the microstructure and mechanical properties of the composite was studied. The density and mechanical properties of the composite enhanced with increasing number of PIP cycles. After the 6th PIP cycle, the density of the composite increased by only 0.8%, and the flexural strength and fracture toughness of the composite reached 130.1 MPa and 4.91 MPa m1/2, respectively. Moreover, the thermal shock resistance of the composite was investigated by the high-low temperature (from 1300 °C to room temperature) thermal treatment and water quenching method. The strength retention rate of the composite after 10 high-low temperature thermal cycles was 90.1%; the retention rate dropped to 79.5% after 15 thermal cycles. The composite with the critical thermal shock temperature difference 843.9 °C displayed excellent thermal shock resistance.

Oxide ceramic fibers via dry spinning process—From lab to fab

AbstractOxide fibers preparation and manufacturing capabilities at Fraunhofer‐Center HTL are introduced, showing the development and preparation of oxide ceramic fibers from lab scale to pilot scale up to near production scale. As a specific example, the development of an aluminosilicate fiber with mullite composition is discussed in more detail. Fiber development started from nonaqueous sol‐gel precursors in the early lab scale. With increasing fiber spinning volume, precursors were switched to water‐soluble systems. Transformation from green fiber to ceramic fiber was monitored by thermogravimetric and differential thermal analysis, X‐ray diffraction, and scanning electron microscopy. The evolution of ceramic phases, microstructure formation, and the effects on tensile strength and Young's modulus were investigated. Weibull statistics and fracture analysis helped to understand the results. Next step will be the transition from large lab scale to pilot scale, demonstrating manufacturing capability.

Performance optimization of sol-derived C/Mullite composites by introducing a PyC-SiC double-layer interfacial coating

Three-dimensional carbon fiber preform reinforced mullite (C/Mullite) composites have been prepared through the sol-impregnation-drying-heating route in our previous studies. To further optimize the performance of C/Mullite composites, a PyC-SiC double-layer interfacial coating was introduced by polymer pyrolysis in this paper. The effects of PyC-SiC interfacial coating on the mechanical properties and thermal shock resistance of C/Mullite composites were investigated. It was found that the PyC-SiC interfacial coating played a positive role in protecting carbon fiber, weakening interfacial bonding and alleviating thermal mismatch between fiber and matrix. Consequently, both flexural strength and fracture work of C/Mullite composites were increased by more than 40%, and the thermal shock resistance in air was also improved.

Preparation of Mullite Fiber Reinforced Mullite Composites

In this paper, mullite fiber reinforced mullite composites were prepared by stitched structural perform, the silica sol and alumina sol. In order to improve the compatibility of mullite fiber and silica sol matrix, interface coating was made by infiltration. The composites were prepared by the method of liquid phase infiltration, and the suitable ceramic heat treatment temperature was 750∼900°C though testing the damage of fiber strength after heat -treatment at different temperatures. Results showed that SiO2-Al2O3-ZrO2 interface coating could uniformly distribute on the surface of mullite fibers, and the appropriate interfacial bonding strength was obtained, which made a contribution to the high density mullite fiber reinforced mullite composite, whose density reached 1.75∼1.8g/cm3, tensile strength, bending strength and compressive strength reached 48.7MPa,65.4MPa and 82.7MPa, respectively. Dielectric constant was 3.2±0.1, Loss angle tangent was less than8×10-3.So mullite fiber reinforced mullite composites became a new generation of high temperature wave- transparent material.

Densification behaviour and microstructure of spark plasma sintered alumina–mullite nanocomposite

Spark plasma sintering (SPS) is a consolidation technique that combines high heating and cooling rates with a uniaxial applied pressure which results in short processing time. SPS is successful in producing ceramics with novel microstructures, which are often reported to be made at temperatures lower than ones used in conventional densification techniques. In this work, the densification and microstructure development sequences of the alumina–15 vol% mullite composite during the SPS and the conventional process were compared. Through SPS technique, the fully dense 0.5 µm grain sized alumina composite (99.5% T.D.) was obtained at 1600°C and its microstructure exhibited nanocrystalline in submicron-sized grains. The nanocrystalline grains were arranged in ground-shaped clusters. Vickers hardness and fracture toughness values for spark plasma sintered composite were calculated as 2098 ± 66 kgf/mm2 and 3.61 ± 0.06 MPa m1/2, respectively.

Fabrication and Properties of Three-dimensional Braided Carbon Fiber Reinforced SiOa-rich Mullite Composites

In order to enhance the fracture toughness of mullite, three-dimensional braided carbon fiber reinforced mullite (C/mullite) composites were prepared using the Al2O3-SiO2 sol with a high solid content as raw material. Mullitization behavior of the sol was characterized. Then, the microstructure, mechanical properties and oxidation resistance of C/mullite composites were investigated. It is found that the SiO2-rich mullite with desirable sintering shrinkage can be synthesized at 1 300 °C from the sol with an Al203/SiO2 mass ratio of 1:1. The C/mullite composites with a total porosity of 21.5% were fabricated by repeating 18 cycles of vacuum impregnation-drying-heat treatment, showing a flexural strength of 234.5 MPa and a fracture toughness of 13.1 MPa·m1/2. Since carbon fibers were protected by compact matrix, the C/mullite composites show favorable oxidation resistance during 1 200 °C-1 600 °C even if an open porosity of 10.3% was detected.

Non-Isothermal Dehydration Kinetics of Diphasic Mullite Precursor Gel

Aluminosilicate gel precursor having mullite composition was synthesized from inorganic salts of aluminum and silicon by employing the sol-gel method. Chemical analysis, surface area, and bulk density measurements were performed to characterize the dried gel. The course of the palletization was examined by FT-IR analysis which confirmed the diphasic nature of the gel. SEM and XRD analysis were performed to study microstructure and phase development. ThermoGravimetric (TG) analysis of the dried gel was performed at multiple heating rates and from the results obtained; kinetics of thermal dehydration was studied by applying Friedman differential and Kissinger-Akahira-Sunose integral isoconversional procedures. It was observed that the total dehydration process of the gel was accomplished by two different stages and both the stages followed second-order rate kinetics. The first stage was assigned to the dehydration of silicon hydroxide gel whereas the second stage was associated with aluminum hydroxide gel dehydration.

Mechanical properties and thermal stability of carbon fiber cloth reinforced sol-derived mullite composites

For the wide application as thermal protection materials, it is very necessary for mullite ceramics to improve fracture toughness. In this paper, the laminated and stitched carbon fiber cloth preform reinforced mullite (C/mullite) composites were prepared through the route of sol impregnation and heat treatment using the Al2O3—SiO2 sol with a high solid content as raw materials. The C/mullite composites showed a flexural strength of 228.9 MPa that was comparable to that of dense monolithic mullite although the total porosity reached 13.4%. Especially, a fracture toughness of 11.2 MPa·m1/2 that was 4–5 times that of dense monolithic mullite was obtained. Strength deterioration due to the carbothermal reduction between carbon fiber and the residual SiO2 in matrix was found above 1200 °C. A pyrolytic C (PyC) coating was deposited on carbon fibers as interfacial coating. The chemical damage to carbon fibers was obviously alleviated by the sacrifice of PyC coating. Accordingly, the C/PyC/mullite composites kept strength unchanged up to 1500 °C, and showed much higher strength retention ratio than C/mullite composites after annealing at 1600 °C.

Mechanical properties of zirconia toughened mullite composites fabricated by slip casting

Mechanical properties of zirconia toughened mullite composites fabricated by slip casting

Development of high temperature material for cellular ceramic regenerative storage heater

Ceramic Regenerative Storage Heater (RSH) is the preferable solution to provide clean hot air in Hypersonic Wind Tunnel (HWT) and Scramjet Test facilities to prevent the liquefaction of air during expansion in the high Mach number nozzles. Selection of the heater storage material for such operational requirements is a challenging task. As per the theoretical background; alumina, zirconia or mullite compositions have the capability to withstand at such high operating conditions. In the current study, a series of experiments were carried out to estimate the thermal characteristics of the material. This paper highlights the simplified approach by combining theoretical and experimental methods for economic (reduced time and cost) selection process of high temperature material. The desired material must possess good thermal shock resistance and low dust generation characteristics. Al2O3 and SiO2 based matrix composition fused in a transcrystalline phase exhibits significant improvement on the overall material integrity. Several Al2O3 and SiO2 matrix compositions were examined and the final matrix composite ratio of 90% of Al2O3 and 10% of SiO2 is considered as the desired candidate material. The ceramic matrix was sintered at a temperature of 2123 K, under the specific pressure of 1400 kg/cm2 for a period of 2 h, at a heating rate of 3.5 °C/min. The results of the thermal shock tests on various samples are presented. To improve the anti-dust characteristics, the final samples were coated with fine particles (5 µm) of Al2O3 and SiO2 matrix. Based on the experimental evaluation, Al2O3 and SiO2 fused matrix composition found to be meeting all the requirements of high temperature storage material.