Mullite Matrix Research Articles

Performance Optimization of Sol-Derived C/Mullite Composites by Reducing the Sintering Shrinkage of Mullite Matrix

Performance Optimization of Sol-Derived C/Mullite Composites by Reducing the Sintering Shrinkage of Mullite Matrix

Oxidation of silicon particles suspended in a mullite matrix with dual-mode vacancy diffusion of oxygen

Mullite bond coats filled with silicon particles have recently been proposed for the next generation of environmental barrier coatings (EBC). The sacrificial oxidation of the silicon particles eliminates some of the environmental durability issues associated with traditional EBC designs. To investigate this novel design approach, a mathematical formulation and resulting numerical method have been developed for simulating the oxidation of silicon particles embedded in a mullite matrix. The focus is on the oxidation of the silicon in a dry oxygen environment where the oxidant is presumably oxygen. Oxygen transport from the exterior environment through the mullite is assumed to occur via two lattice defect diffusion mechanisms. As a result, the rate of oxygen transport is dictated by the self-diffusivity of the lattice defects. The numerical method is implemented using the COMSOL Multiphysics® Program. The method is applied to simulate the oxidation of silicon particles in a mullite pellet in recent oxidation experiments. Numerical results compare favorably with the oxidation behavior observed in the oxidation experiments. The numerical method is also used to investigate the effects of various features of the mullite/silicon particle system on the oxidation behavior.

Densification of SiCf/mullite composite via vacuum pressure impregnation process towards excellent mechanical and microwave absorbing performance

A simple and low-cost method for preparing SiCf/mullite composites with high density, mechanical, and absorbing performance using a vacuum pressure impregnation (VPI) system has been developed. The densification mechanism of the mullite matrix was investigated, and the effects of impregnation pressures on the flexural strength, interfacial bonding strength, and the electromagnetic wave (EMW) absorbing performance of SiCf/mullite composites were also studied. The dense composite was prepared under 3 MPa pressure with a porosity of 6.23 % and median pore diameters (MPDs) of 13.02 nm. The preparation efficiency was improved with the impregnation circles decreased from 20 times to 13 times. The flexural strength of SiCf/mullite composites prepared by VPI is 297 MPa, which is improved 230 % of the one prepared by the traditional vacuum impregnation method. Benefiting from the synergistic effects of appropriate impedance matching and multiple polarization loss, the effective absorption bandwidth (EAB, RL < −10 dB) of the SiCf/mullite composites increases from 2.17 GHz to 2.80 GHz in the X band.This work has a positive application value for the manufacture of high-strength EMW absorbing composites in terms of significantly reducing the preparation cost and cycle duration.

Phase field investigation on the alumina nanocrystalline growth controlled by mullite-alumina phase transition in alumina-mullite fibers

The overgrowth of alumina nanocrystalline is the most important cause for the degradation of alumina-mullite fibers at high temperatures. A profound understanding on the kinetics and mechanisms of alumina nanocrystalline growth controlled by the mullite-alumina phase transition is of great significance to the development and service of alumina-mullite fibers. Here, by using the phase field simulations coupling to materials thermodynamic data, we systematically investigated the growth of alumina nanocrystalline and the mullite-alumina phase transition in alumina-mullite fiber at different temperatures and concentrations of the mullite matrix. The growth rates and exponents of alumina grains at different conditions are quantified and agree well with the experimental results of Nextel 720 fibers. We found that the growth of polycrystalline alumina grains undergoes two stages: rapid growth in the very early stage and then grains coarsening. The mullite-alumina phase transition dominates the rapid growth of alumina grains and the grain coarsening proceeds mainly through grain annexation and dissolution-precipitation. Our results clarify the influence of mullite-alumina phase transition on alumina grain growth from mullite matrix, and quantify the mechanisms and kinetics of alumina grain growth from mullite matrix in the alumina-mullite system.

Study on sintering behavior and properties of lithium slag-based foamed ceramics

To research the value of lithium slag in the development of construction materials, foamed ceramics with different lithium slag/kaolin have been prepared using the conventional high-temperature foaming method. In this study, the effects of sintering temperature and formulation on the morphology and mechanical properties of foamed ceramics are investigated by relying on the formula scheme designed by the Na2O-Al2O3-SiO2 ternary phase diagram. The characterization of the ceramic raw materials and samples by XRF, XRD, SEM, and TGA-DSC reveals that the doping contents of lithium slag have a great influence on the densification of the sample. With the amount of lithium slag increases, the amount of liquid phase in the melt increases, which intensifies the liquid phase flow and accelerates the mass transfer process, promoting the sintering and densification of ceramics. However, too much lithium slag is easy to agglomerate, which is not conducive to the close bonding between the internal components of the ceramic, resulting in a decrease in bulk density. In addition, the doping of spodumene effectively reduces the sintering temperature of the mullite matrix, promotes the formation of mullite crystals, and enhances the mechanical properties of the ceramics. The lithium-slag ceramics sintered at high temperature in this experiment had a maximum compressive strength = 5.70 MPa. average porosity = 73.68%, average apparent density = 0.40 g/cm3 and average water absorption = 2.24%. Overall, the lithium slag foamed ceramics prepared by the high-temperature foaming method have good mechanical properties, which are essential for resource utilization and high value-added utilization of solid waste.

X-band electromagnetic absorption and mechanical properties of mullite/Ti3AlC2 composites

Aiming to enhance the electromagnetic wave absorption (EWA) property of mullite ceramics, MAX phase of Ti3AlC2 was introduced into mullite ceramics for the first time. Significantly improved EWA and mechanical performance of mullite ceramics were achieved simultaneously by introducing Ti3AlC2 via a pressureless sintering process. Results indicated the increasing sintering temperature promoted the densification of as-prepared composites and induced the decomposition of Ti3AlC2 to different degrees. The composites sintered at 1450 °C possessed the optimal mechanical performance with flexural strength, fracture toughness and Vickers hardness of 211.78 MPa, 5.28 MPa m1/2 and 9.36 GPa, respectively. The improved mechanical properties were mainly attributed to the enhanced densification and even-distributed TiC particles in the mullite matrix. Moreover, polarization loss resulting from the unpaired defect and newly formed interfaces, and conductance loss originating from the highly conductive TiC particles, played a major role in the considerable improvement in EWA properties of mullite-based composites. The minimum RL value of composites sintered at 1450 °C reached −35.5 dB with a thickness of 2.65 mm. By regulating the decomposition degree of Ti3AlC2, the mechanical and EWA performance of mullite ceramics can be effectively enhanced. This study will provide an effective reference for the preparation of EWA materials with excellent comprehensive properties.

Near‐net‐size preparation of porous mullite matrix ceramics with in situ formed 3D mullite whisker network

AbstractPorous mullite matrix ceramics have excellent thermal and mechanical properties suitable for applications such as in thermal insulation. However, their applications are limited by processing defects from nonuniform sintering shrinkage and the trade‐off between high porosity (preferred for low thermal conductivity) and high mechanical strength. Herein, we seek to minimize the sintering shrinkage by near‐net‐size preparation and improve the strength by in situ formed whisker network structure. Gelcasting forming technology and pressureless sintering were used to prepare porous mullite matrix ceramics using kyanite and α‐Al2O3 powders as the starting materials and using MoO3 to promote the growth of mullite whiskers. The results showed that the sintering shrinkage could be compensated by the volume expansion from solid‐state reaction during reaction sintering. The in situ formed three‐dimensional (3D) whisker network further reduced sintering shrinkage and effectively improved the strength of the ceramics. An ultralow sintering shrinkage of .78% was achieved. The near‐net‐shape porous mullite matrix ceramics strengthened by 3D whisker network had a high porosity of 63.9%, a high compressive strength of 83.8 MPa and a high flexural strength of 53.5 MPa.

Fabrication and characterization of ceramic hollow shaped from the local kaolin intended for the filtration

ABSTRACT Ceramic membranes are one of the emerging separation technologies with extremely high separation factors. In this work, natural Kaolin clay collected from the western of Algeria was characterized in detail to see its feasibility in a ceramic membrane preparation. The ceramic membrane was prepared by solid-state reactive firing under the slip casting method. The aim is to favor the mullite phase, once the alumina is added a part will be dispersed in the mullite matrix to form a mullite-alumina composite. The mineralogical, morphological structure, particle size distribution, thermal stability, and chemical composition of prepared alumina oxide membrane were carried out using several techniques. They reveal the apparition of two main phases the mullite and the alumina. The kaolin provides the membrane with a well homogeneous morphology, good hydrophobicity, and thermal stability. The obtained membrane presented a mesoporous membrane character that can use successfully for ultrafiltration applications..

Static and dynamic corrosion behavior of SnO2‐doped AZS slip cast refractory in SLS glass

AbstractThe corrosion behavior of a tin IV oxide‐doped AZS‐refractory, subject to static and dynamic corrosion testing at 1370˚C in soda‐lime‐silica glass, was studied considering the effect of the microstructural features on corrosion. The refractory was synthesized by slip cast methods through reaction sintering of alumina and zircon raw materials using SnO2 as a sintering agent. SnO2 had a considerable influence in the enhanced alumina/zircon reaction sintering and the subsequently evolved microstructures of an interlocked Zr(1‐x)Sn(x)O2 solid solution reinforced alumina‐mullite composite. The process kinetics of the refractory corrosion followed reasonably well the predicted dependence on the square root of angular velocity under forced convection corrosion. Glass chemical corrosion and erosion of the refractory, under static and dynamic glass conditions, respectively, revealed the Zr(1‐x)Sn(x)O2 solid solution‐rich mullite matrix as providing the most corrosion resistance and glass compatibility.

Effect of Sintering Temperature on the Interfacial and Mechanical Properties of SiC Fiber Reinforced Mullite Matrix Composites

In the present work, SiC fiber reinforced mullite matrix composites (SiCf/Mu composite) were prepared through a sol-gel process. The effect of sintering temperature on the interfacial and mechanical properties of the SiCf/Mu composite was studied comprehensively and deeply. The experimental results indicated that the micro-mechanical properties of mullite matrix and fiber/matrix interface were closely related to the sintering temperature. When sintering temperature reached 800 and 1000 °C, the matrix was composed of the γ-Al2O3 and amorphous SiO2. As the sintering temperature rise to 1200 °C, mullite phase was formed via the chemical reaction between γ-Al2O3 and amorphous SiO2, leading to significantly increased Young’s modulus and hardness of mullite matrix. It was also found that the interfacial shear strength was enhanced remarkably with sintering temperature due to the interfacial interaction between fiber and matrix. The interface controlled macro-mechanical properties of SiCf/Mu composites were sharply depressed at sintering temperature of 1000 °C, exhibiting a ductile-to-brittle fracture behavior transition.

Effect of C/SiC interphase on interfacial and mechanical properties of SiC fiber reinforced mullite matrix composites

Continuous SiC fiber reinforced mullite (SiCf/Mu) matrix composite was fabricated via sol–gel process. The capability of fiber coatings to weaken the interfacial interaction, thus to enhance the fracture toughness of the SiCf/Mu composite was explored. The results show that the SiC interphase fabricated by chemical vapor deposition (CVD) process was columnar-grain structured. A thin carbon layer was also produced during the CVD process, forming C/SiC interphase. The interfacial shear strength of the SiCf/Mu composite was significantly reduced from ≈537 to ≈115 MPa after the introduction of C/SiC interphase, as quantified by in situ fiber push-in tests. Accordingly, the macro fracture toughness of the composite was remarkably enhanced from ≈0.9 to ≈10.5 MPa m1/2. The work highlights the efficiency of using C/SiC interphase to weaken the interfacial bonding, thus greatly toughen the SiCf/Mu composite.

Thermal decomposition and reaction sintering for synthesis of mullite-zirconia composite using kaolin, γ-alumina and zirconia

The refractory materials used in the wall of the furnaces for glass melting can be prepared from mullite-zirconia composite material. The composite of mullite/zirconia was synthesized from Iraqi kaolin, γ-alumina, and zirconia using thermal decomposition with reaction sintering at 1600 °C. Several batches were prepared with various ratios of kaolin, γ-alumina, and zirconia, and the composite compositions were selected from the Al2O3-SiO2-ZrO2 phase diagram. The mullite-zirconia composite was prepared with different steps beginning with milling the starting materials, semi-dry uniaxial pressing, and then reactive sintering at various temperatures (1200, 1400, and 1600 °C). The predicted phases ZrO2 and Al6Si2O13 were identified by X-ray diffraction patterns according to the phase diagram for all the batches. The lower amount of the zirconia added to mullite reduced porosity and improved the bulk density of the mullite/zirconia composite. The thermal expansion coefficient slightly increased with the addition of zirconia. It also enhanced the thermal shock resistance of the composite. Finally, the mechanical properties were improved by increasing the amount of zirconia particles in a matrix of mullite due to the phase transformation of zirconia from tetragonal to monoclinic phase.

Preparation of mullite-alumina composite by reaction sintering between Algerian kaolin and amorphous aluminum hydroxide

The objective of this work is to obtain a multifunctional ceramic composite material at low cost with improved properties and that can be used in many structural applications like: electrical, refractory support and tribological elements. A mullite-alumina composite were prepared by using Algerian kaolin and amorphous aluminum hydroxide as starting materials. Two mixtures were used by varying the amorphous aluminum hydroxide content 61.5% and 50 wt %. an abundant kaolin has been chosen with a high alumina content and an amorphous alumina trihydrate to lower the temperature of the reaction with the excess of silica in the kaolin. The advantage of adding an amount of amorphous aluminum hydroxide is that it reacts at high temperature with the amorphous phase of silica issue from kaolin. The objective is to increase the amount of the mullite phase formed. When alumina is added in excess, a part will be dispersed in the mullite matrix to form a mullite-alumina composite.The results revealed that mullite is the major phase and alumina the minor phase in the samples sintered at 1600 °C. At this firing temperature, no silica phases (SiO2) are detected. The microstructure shows a higher aspect ratio of mullite crystals. The addition of alumina reduces the size of mullite grains and their aspect ratio. The density and hardness of the resulting mullite-alumina composites increase with sintering temperature and alumina content.

Modulation of magnetic and dielectric response of mullite coated Cu-substituted Co-Zn-ferrite multiphase nanocomposites

Cu-substituted Co-Zn ferrite (CoZCF)) nanoparticles have been prepared by sol-gel method and incorporated in non-magnetic matrix of mullite. The crystallographic phases of CoZCF nanoparticles and CoZCF-mullite nanocomposites were confirmed by XRD and a variation of average crystallite diameter in the range of 15–58 nm has been observed between CoZCF nanoparticles and CoZCF-mullite nanocomposites. Magnetic study of CoZCF and CoZCF-mullite confirmed the presence of superparamagnetic relaxation as well as high magnetic orderings and high coercivity of 1514 Oe in CoZCF-mullite at room temperature. Dielectric observation reveals the presence of sufficiently high dielectric permittivity with a value of 16,372 at 102 Hz frequency and 427 at 104 Hz frequency in CoZCF-mullite nanocomposites. Dielectric response study reveals the presence of ferroelectric to paraelectric phase transition at a temperature of 348 °C. The presence of both magnetic and dielectric orderings of novel CoZCF-mullite nanocomposites would be very interesting for various magneto-dielectric device applications.

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.

Toughening Mechanism of Mullite Matrix Composites: A Review

Mullite has high creep resistance, low thermal expansion coefficient and thermal conductivity, excellent corrosion resistance and thermal shock resistance, and plays an important role in traditional ceramics and advanced ceramic materials. However, the poor mechanical properties of mullite at room temperature limit its application. In order to improve the strength and toughness of mullite, the current research focuses on the modification of mullite by using the second phase. The research status of discontinuous phase (particle, whisker, and chopped fiber) and continuous fiber reinforced mullite matrix composites is introduced, including preparation process, microstructure, and its main properties. The reinforcement mechanism of second phase on mullite matrix composites is summarized, and the existing problems and the future development direction of mullite matrix composites are pointed out and discussed.

SmMn2O5 catalysts modified with silver for soot oxidation: Dispersion of silver and distortion of mullite

The mullite SmMn2O5 (SMO) was modified with silver for soot oxidation by two methods, i.e. sol-gel (Ag/SMO-sg) and impregnation (Ag/SMO-im). The catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectrometry (ICP-AES), H2-O2 titration, H2 temperature-programmed reduction (H2-TPR), CO temperature-programmed reduction (CO-TPR), soot temperature-programmed reduction (soot-TPR), O2 temperature-programmed desorption (O2-TPD), cycled H2 temperature-programmed reduction (cycled H2-TPR), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). Metallic silver nanoparticles were observed in the mullite matrix for both the catalysts. For Ag/SMO-sg, the possible incorporation of silver into the mullite crystal cell and strong electronic interaction between the highly dispersed metal and the support increase the Mn4+/Mn3+ ratio and create more structural defects in the mullite, resulting in boosted NO oxidation and thereby NO2-assisted soot combustion. The introduction of Ag can promote the generation/regeneration of active oxygen, which accelerates the ignition of soot in O2 at low temperatures. Therefore, the sol-gel-synthesized Ag/SmMn2O5 is highly promising for application in diesel particulate filters.

Microstructure and physical properties of a mullite brick in blast furnace hearth: influence of temperature

ABSTRACT Mullite brick is one of the most extensively utilized blast furnace hearth lining materials that feature several advantageous properties. In this study, the microstructure and physical properties of a mullite brick under different thermal treatment temperatures were investigated. The results showed that the temperature could not influence the micro-morphology of the mullite aggregate, but it directly affects the microstructure of the mullite matrix. With the increase in temperature, the degree of bonding of fine grains gets higher and the bonding body becomes smooth in the matrix. The bulk density changes little after different thermal treatments. With the increase in temperature, the compressive strength and modulus of rupture increase, which are beneficial for the mullite brick to the washing resistance of slag/iron. The permanent linear values of mullite brick are negative and relatively small under different thermal treatment temperatures, reflecting an excellent performance of the volume stability of mullite brick.

Mechanical Properties and Thermal Conductivity of Ytterbium-Silicate-Mullite Composites.

Various Ytterbium-Silicate-Mullite composites were successfully fabricated by adding Yb2SiO5 into mullite ceramics and then using pressureless sintering at 1550 °C. The influence of Yb2SiO5 addition on the microstructure, mechanical properties, and thermal conductivity of ytterbium-silicate-mullite composites was investigated. Results show that the composites mainly consisted of a mullite matrix and some in situ formed Yb2Si2O7 and Al2O3 phases. By the addition of Yb2SiO5, the Vickers hardness of composites reached ~9.0 at an additive concentration of 5 mol %. Fracture toughness increased to ~2.7 MPa·m1/2 at the additive concentration of 15 mol %, owing to the trans-granular fracture and crack deflection of the pinning effect of the Al2O3 phase in the composites. With the increase of the Al2O3 phase in the composite, the thermal conductivity for the 15YbAM reached around 4.0 W/(m·K) at 1200 °C. Although the thermal conductivity increased, it is still acceptable for such composites to be used as environmental barrier coatings.

Thermomechanical Characterisation of Mullite Zirconia Composites Sintered from Andalusite for High Temperature Applications

Mullite-Zirconia refractories are well known for their good resistance to corrosion and thermal shock. In this study, several mullite-zirconia composites were developed from andalusite, alumina and zircon sintered at 1600 °C for 10 hours. The samples were subjected to thermal shock carried out after heating at 1200 °C, in order to study the mechanical and thermomechanical behaviour as a function of the amount of zirconia dispersed in the mullite matrix. It appears that that the amorphous phase (SiO2), determined by X-ray diffraction, produced by the decomposition of andalusite, increases considerably with the amount of final zirconia in the composite and has a very important influence on the porosity. This amorphous phase seems also to have an important influence on the mechanical properties of the material. The characterisation of the thermomechanical behaviour (elastic properties and damage monitoring) was carried out thanks to ultrasonic techniques (US echography and Acoustic Emission). The “surprising” evolution (increase) of the Young’s modulus E of the material after being submitted to repeated thermal shocks is highlighted and explained. The acoustic emission technique carried out at high temperature and also coupled to 4-points bending tests (at room temperature) demonstrates its effectiveness for providing a better understanding of the chronology of the involved mechanisms involved at microstructural scale.