Mullite Matrix Research Articles

The role of kyanite in the crystallization and densification of the high strength mullite matrix composites

The crystal structure, the unit cell parameters and the extent of mullitization were determined, using the Rietveld method, for range of mullite matrix formulations in which kyanite is used as particles reinforcement. The results combined with the mechanical properties and microstructure indicated the effectiveness of the kyanite particles to enhance the strength (>200 MPa), the Vickers hardness (>11 GPa) and the elastic modulus (150 GPa). The strengthening mechanism was particularly linked action of particles reinforcement. At low temperature, kyanite acts as fillers reducing the porosity and playing the role of nucleation sites for the crystallization of metakaolin to mullite. At high temperature (>1350 °C), kyanite decomposes to mullite avoiding the grain growth of the existing crystals and delaying the densification. The extent of the reduction in porosity and the extreme limitation of the liquid phase ensure the homogeneity and the refractoriness that justify the strength enhancement. The unit cell parameters and the crystal structure confirmed predominance of the mullite 3:2 with their small grain size being one of the most stable mullite phases. The small size of their particles and the continuity into the mullite matrix composites allow good packing process for the optimum characteristics achieved: strength, microstructure and thermal expansion coefficient.

Micro-mechanical properties and interfacial engineering of SiC fiber reinforced sol-gel fabricated mullite matrix composites

The toughening mechanisms of a SiC fiber reinforced sol-gel fabricated mullite matrix composites were studied by combining the microstructure, the micro-mechanical properties (especially the interface) and the macro fracture resistance by a bottom-to-up mechanical characterization method (transmission electron microscopy, nanoindentation, fiber push-in, digital image correction, etc.). The results show a chemical-reaction controlled fiber/matrix interface in the as-fabricated composite, leading to pretty strong interfacial shear strength (~537MPa), measured by the fiber push-in tests. Interfacial engineering by chemical vapor deposited pyrocarbon interphase can effectively hinder the interfacial reactions and weaken the interfacial interactions. The low shear strength of the tailored fiber/matrix interface (~155MPa) could trigger the toughening mechanisms like interface debonding, fiber pull-out, etc., when the composite was subjected to external bending stresses. Finally, the fracture toughness of the novel composite was found significantly enhanced from ~0.8MPa√m to ~8.3MPa√m, after the interfacial engineering with pyrocarbon interphase.

Weak interface dominated high temperature fracture strength of carbon fiber reinforced mullite matrix composites

Weak fiber/matrix interface dominates the toughening properties of ceramic matrix composites. This paper reports a novel sol-gel fabricated carbon fiber reinforced mullite matrix composite, in which the fiber/matrix interface was inherently weak in shear properties (∼25MPa), measured in-situ by fiber push-in tests. The interface microstructure was chemically sharp, characterized by transmission electron microscopy. The outcome of the weak interface was the full trigger of the toughening mechanisms like crack deflection, etc., leading to significant enhancement of the fracture toughness of the composite (∼12MPa√m), measured by single edged notch beam method. Finally, due to the weak fiber/matrix interface and large thermal expansion mismatch of the fiber and matrix, the high temperature fracture strength was enhanced in the temperature range from 25 to 1200°C, which is attributed to the enhancement of the interfacial property at elevated temperatures that favors better load transfers between composite constituents.

Thermal Properties of Carbon Fiber Reinforced Mullite Composites Derived from Sol-Gel

Mullite matrix composites with laminated and stitched carbon fiber cloth preform as reinforcement were fabricated via the route of “infiltration-drying-heat treatment” using Al2O3-SiO2 sol as raw materials. Thermal properties from room temperature to 1673K of the composites were investigated. The coefficient of thermal expansion (CTE) increases first and then decreases, and reaches a maximum of 4.83×10-6K-1 at 1273K. As a result of the further sintering of matrix, the CTE is negative at above 1300°C. The specific heat capacity increases to the maximum of 1.547J·g-1·K-1 at 1473K and remains stable at above 1473K, with a minimum of 0.756J·g-1·K-1 at room temperature. The thermal diffusivity decreases from 1.1mm2·s-1 at room temperature to 0.707 mm2·s-1 at 973K as the temperature was elevated, and remains stable at above 973K. On the contrary, the thermal conductivity is improved with increasing temperature on the whole and varies from 1.859W·m-1·K-1 at room temperature to 2.325W·m-1·K-1 at 1473K.

Low temperature sol-gel based erbium doped mullite nanoparticles: Structural and optical properties

Abstract Thermally stable aluminum-silicate i.e. mullite is doped with erbium for optical device applications. The matrix is synthesized and doped with 0.1 M of Er3+ by the sol-gel precursor method at room temperature. Crack-free and uniform distribution of the species was detected and profile of the various elements showed stoichiometric ratios, thus suggesting a homogeneous dispersion of the rare-earth ions inside the heterogeneous mullite host matrix. Different sizes of mullite grains 167–202 nm are observed by FESEM; the addition of erbium gets smaller size of mullite crystals 40–110 nm. The crystallization process is evaluated by X-ray diffraction (XRD) whereas Fourier Transmission Infrared Spectroscopy (FTIR) is used to monitor the bonding of crystalline phases. Average crystallite size in mullite matrix is calculated to be 35.50 nm, whereas smaller crystallite sizes 21.38 nm are observed in the case of doped sample. Brunauett–Emmett–Teller (BET) analysis shows the lower surface area value of doped matrix to be 20 m2/g, pore size of 0.03 nm and pore volume of 0.33 cm3/g. Thermally stable, transparent, high refractive index, photoluminescent Er3+ doped mullite matrix is beneficial for optical applications at room temperature.

The role of kyanite in the improvement in the crystallization and densification of the high strength mullite matrix

Fine particles of kyanite were used as reinforcement in mullite matrix composite. Mix of metakaolin and meta-alumina (calcined bauxite) received different fraction of fine kyanite particles: 37.5, 42.5 and 50 mass%. The objectives were to improve the crystallization of kaolin-based mullite matrix reducing the amorphous content and the grain growth. Packing process and densification were enhanced, leading to the action of kyanite at low temperature ( 1350 °C), the kyanite particles are decomposed to mullite still without glassy phase and the metastable alumina presented in the mix reacts with amorphous silica to produce additional mullite. The sintering behaviour, the thermal expansion behaviour of ~0.5 % between 200 and 1000 °C and the coefficient of the thermal expansion of 5.92, 5.15 and 6.92 × 10−6/°C for KY3, KY4 and KY5, respectively, between 200 and 1000 °C, and their decrease at higher temperature demonstrated the optimum refractoriness of the mullite matrix composites developed.

A simple and efficient way to prepare porous mullite matrix ceramics via directly sintering SiO2-Al2O3 microspheres

A novel route was proposed to fabricate porous mullite matrix ceramics by directly sintering SiO2-Al2O3 microspheres (SAMs) in air, which was simple and efficient compared with other methods. The effects of sintering temperature on the phase composition, microstructure, porosity and compressive strength of porous ceramics were studied. It is found that the mullite phase generates when sintered at above 1450°C. SEM results shows that the sintering temperature has a significant influence on the formation of pores morphology. As the sintering temperature increases from 1200°C to 1650°C, the original small holes inside and between the SAMs develop into macropores, whose pore size increases to eight times more than that of SAMs at a maximum, and the distribution of pores is relatively uniform. Porous mullite matrix ceramics with the porosity of 81.37% and compressive strength of 6.25±0.91MPa have been obtained through this method at the sintering temperature of 1550°C.

Corrosion of Carbon Free and Bonded Refractories for Application in Steel Ingot Casting

This phenomenological study investigates the corrosion of refractories by a highly corrosive steel (1.6587, 18CrNiMo7‐6) with a high aluminum content and casting temperature of 1580 °C. The applied refractory castables with matrices based on alumina, mullite, and zirconia/titania doped alumina (AZT) are carbon free or low carbon (4 wt%) containing with and without nanoscaled additives. The corrosion is analyzed mainly by microscopy after the corrosion tests. The carbon containing samples are negligibly corroded due to inhibited wetting. The nanoadditives in the carbon containing samples show no influence on the corrosion. The carbon free AZT is attacked most strongly with a corrosion layer of about 14 mm. In the alumina (corrosion layer about 6 mm) and AZT sample, compositions corresponding to manganese aluminates form with manganese from the steel. When also silicon diffuses into the refractory, compositions referring to manganese aluminosilicates form. In the mullite matrix crucible (corrosion layer about 1 mm) compositions corresponding to manganese aluminosilicates form directly with manganese from the steel resulting in a highly viscous melt at the interface which retard the further attack. For a future final evaluation, however, also the steel quality has to be taken into account as will be studied by an Aspex‐SEM.

The Preparation of Heat Insulating Moulded Refractories

The thesis deals with the study of influence of the foaming agent Schäumungsmittel W 53 addition on the specific weight and mechanical characteristics of mullite based heat insulating refractory castables. The effect of dosing and mixing time was observed in this work. The influence of partial substitution of mullite matrix by kaolin was also studied. Kaolin was added in dry matter or in the foamed state. Optimal dosage of the agent was 0,25 % and the optimal mixing time was 10 min. The addition of kaolin was the most effective in the foamed state.

Effect of Y<sub>2</sub>O<sub>3</sub> on the Sintering, Mechanical and Dielectric Properties of Mullite/h-BN Composites

Mullite/10 wt. %h-BN composites with 5 wt. % Y2O3 additive were fabricated by pressureless sintering at different temperatures. The densification, phase composition, microstructure, mechanical and dielectric properties of the mullite/h-BN composites were investigated. With the addition of Y2O3, the sintering temperature of the mullite/h-BN composites declined, while the density, mechanical and dielectric properties all increased. The addition of Y2O3 promoted the formation of liquid phase at high temperature, which accelerated the densification. Besides, Y2O3 particles which were located at the grain boundaries inhibited the grain growth of mullite matrix. For the mullite/h-BN composites with Y2O3 additive, the appropriate sintering temperature was about 1600°C. The relative density, flexural strength, fracture toughness and dielectric constant of the Y2O3 doped mullite/h-BN composite sintered at 1600 °C reached 82%, 135 MPa, 2.3 MPa·m1/2 and 4.9, respectively.

Influence of corn flour as pore forming agent on porous ceramic material based mullite: Morphology and mechanical properties

Porous material was processed by the mixing, molding and pressing the ceramic material, afterward burnout and sintering; through the forming porous, using corn flour at different concentration (10, 15 and 20 wt.%) as a pore forming agent; in order to determinate the influence of porous on the mechanical, morphological and structural properties. The effect of the volume fraction of corn flour in the mullite matrix, at various sintering temperature from 1100, 1200, 1300 and 1500?C were tested by Diffraction X ray, showing changes in crystalline phases of mullite (3Al2O3-2SiO2), as result of sintered temperatures. Presence of talcum powder in formula, also cause the formation of the cordierite and cristobalite crystalline phases, giving stability and adhesion to the structure of ceramic material. When sintering at temperatures between 1300 to 1500?C, and it was used the concentration of corn flour 15-20 wt.% as forming agent porous, it was found the better mechanical properties. The scanning electron microscopy analysis shows the presence of open porosity and anisotropy.

Fabrication and Characterization of Hydroxyapatite/Mullite and Tricalcium Phosphate/Al<sub>2</sub>O<sub>3</sub> Composites Containing 30 wt% of Bioactive Components

Mullite-matrix and Al₂O₃-matrix composites were fabricated with 30 wt% hydroxyapatite (HA) and tricalcium phosphate (TCP), respectively, as additives to give bioactivity. A diphasic gel process was employed to lower the densification temperature of the mullite matrix to 1320℃. A polymer complexation process was used to synthesize a TCP powder that was fully densified at 1250℃, for application to the matrix. For the HA/mullite composite, HA decomposed during sintering by reactions with the matrix components of Al₂O₃ and SiO₂, resulting in a mixture of Al₂O₃, TCP, and other minor phases with a low densification of less than 88% of the theoretical density (TD). In contrast, the TCP/Al₂O₃ composite was highly densified by sintering at 1350oC to 96%TD with no reaction between the components. Different from the TCP monolith, the TCP/Al₂O₃ composite also showed a fine microstructure and intergranular fracture, both of which characteristics are advantageous for strength and fracture toughness.

Features of High-Strength Composite Material Structure Creation

The basis of technology proposed is use of a sol-gel method for preventing polycrystalline corundum fiber from crystallization during heating to high temperature and for low-temperature synthesis of prescribed phases in a corundum matrix with the aim of improving the operating properties of composite materials based on corundum. As a result of firing a charge based on corundum powder modified with tetraethoxysilane and polycrystalline corundum fiber modified with ethylsilicate-32 at 1360°C materials are created with very good strength properties. The materials exhibit electrical insulation properties and are stable in ionized gas streams at the level of known analogs as a result of creating self-reinforced mullite and β-SiC corundum matrix, reinforced with polycrystalline fiber and rapidly sintered due to presence of silicon oxynitride.

Structure-property relationships of mullite-SiC-Al2O3–ZrO2 composites developed during carbothermal reduction of aluminosilicate minerals

Evolution of SiC and ZrO2 in the matrix of Al2O3 or mullite have been reported to enhance a higher toughness, good thermal shock resistance (lowering thermal expansion and improving thermal conductivity) and improved creep resistance of composite materials. In this study, the structure-property relationships of mullite-Al2O3 matrix composites have been investigated in conjunction with the evolution of reinforcing phases such as SiC–ZrO2 by an economical heat treatment process called carbothermal reduction of inorganic minerals (Kaolinite, Andalusite, Zircon). The influence of starting materials in relation with the variation in molar ratio of C/SiO2 on the phase composition, microstructures, physical and mechanical properties have been studied. Light microscopy has been supplemented with scanning electron microscopy, XRD analysis, differential thermal and thermal gravity analysis to follow the structure-property relationships. The experimental results show that with increasing of C/SiO2 ratio in starting materials, very fine SiC whiskers have been formed in the microstructures. Moreover, the densification and strength are considerable higher for ZrO2 + SiC containing composites in comparison to that of only SiC added ones. Furthermore, it has been found that the appropriate ratio of C/SiO2 with the associated firing temperature to develop a higher densification and SiC crystallization have been related to the 3.5, 1550 °C for kaolinite, 3.5, 1450 °C for zircon and 5.5, 1600 °C for andalusite containing composite samples, respectively.

Thermal vibration characteristics of fiber-reinforced mullite sandwich structure with ceramic foams core

This paper researched a sandwich structure composed of fiber-reinforced mullite matrix composite face sheets and ceramic foams elastic layer subjected to high temperature and large gradient thermal environment. The vibration characteristics of the hybrid structure with free–free boundary condition were investigated in room and high temperature thermal environment by both experimental means and finite element method (FEM). Finite element models were established to estimate the vibration characteristics of the hybrid sandwich structure. Modal tests by hammer impulse method and exciter were carried out in room temperature environment to get the natural frequencies and corresponding modal shapes and damping loss factors. Then the exciter experiment was performed with the same setups as room temperature test to get the temp-varying vibration characteristics under high temperature thermal loads. The numerical simulation results agree well with the experimental results. The natural frequencies and damping loss factors of the hybrid sandwich structure show steady state with the temperature increased up to 800°C and so large gradient. Correspondently, the structural stiffnesses and damping characteristics of such composite structures keep excellent performance in thermal environment.

Microstructures and Mechanical Properties of Three-Dimensional Braided Carbon Fiber Reinforced Mullite Composites with Different Sols as Raw Materials

In order to efficiently fabricate dense three-dimensional carbon fiber reinforced mullite matrix composites (3D C/mullite), two kinds of Al2O3-SiO2 sols with high solid content were used as raw materials. The ceramic yield and mullitization behavior of the sols and the densification process were investigated. It is indicated that the two sols have proper solid content and ceramic yield and can be completely transformed into mullite at 1573K, which make them be able to prepare composites. 3D C/mullite composites with a porosity of ∼25% were prepared by repeating less than 20 cycles of infiltration-drying-heating of sols, and the microstructures and mechanical properties were examined. The results suggest that the sol with smaller particle size derived composites exhibit well-sintered dense matrix and physically stronger interfacial bonding, which are beneficial to improve load-bearing and load-transferring capacity of matrix. As a result, this composites show much higher mechanical properties. The flexural strength and modulus are 2.4 times and 1.3 times as those of the sol with larger particle size derived composites, respectively.

Sol–gel synthesis of transition-metal ion conjugated alumina-rich mullite nanocomposites with potential mechanical, dielectric and photoluminescence properties

Mechanical, dielectric and photoluminescence properties of transition-metal ions doped mullite nanocomposite synthesized via alkoxide hydrolysis.

Impregnation of porous mullite with Na2SO4 phase change material for thermal energy storage

The porous mullite/Na2SO4 composites with both novel structure and improved heat storage properties were successfully prepared by infiltration of the molten Na2SO4 into the porous mullite matrix, which was fabricated by TBA-based freezing casting method. The effects of fabrication parameters on pore structure of the porous mullite matrix, the infiltration ratio of molten Na2SO4 and heat storage properties of the composites were investigated extensively. The results show that the infiltration ratio of the molten Na2SO4 decreases with decreasing the porosity and increasing the infiltration temperature and the average pore size of the porous mullite matrix. While it increases initially with increases of the infiltration time, and tends to remain constant thereafter. The optimal infiltration temperature and infiltration time are 950°C and 1h, respectively. The unidirectionally aligned open pores in the porous mullite matrix benefit the infiltration process, and the higher infiltration ratio of the molten Na2SO4 and the relatively larger specific heat capacity of the mullite powders both contribute to the higher heat storage density of the composites.

Thermal diffusivity of kaolinite–mullite ceramic matrix composite with silicon nitride nanoparticle filler

Thermal diffusivity of mullite fused ceramics was determined from room temperature up to 500°C at intervals of 100°C. The ceramic material was formed by combining kaolinite as the matrix and silicon nitride as the filler powder. The ceramics were sintered at 1000°C, 1100°C, and 1200°C to study the structural differences. Thermal diffusivity data showed that the addition of silicon nitride affects the composites heat flow and this behavior is noticeable at 1100°C and 1200°C samples. The thermal diffusivity data was plotted on 1000/T graph to demonstrate a linear relation for filler effects of silicon nitride on the mullite matrix. Additions of 25% and 30% silicon nitride sintered at 1200°C exhibited thermal diffusivity values lower than pure mullite crystal at 0.55–0.45mm2/s. These values suggest that heat conduction across the system was interrupted by silicon nitride nanoparticle additions via micropore formations. These samples have potential applications for high temperature thermal insulation.

Densification, microstructure and mechanical properties of Mullite–TiC composites prepared by spark plasma sintering

Mullite–TiC composites were prepared using Mullite and TiC powders by spark plasma sintering at a temperature range of 1500–1600°C for a holding time of 600s under the pressure of 20MPa. Microstructure, strength and hardness of the Mullite–TiC composites were studied. The Mullite–TiC composite containing 10–30wt% TiC reached over 95% theoretical density at 1550°C and nearly full density at 1600°C. The strength and Vickers hardness of Mullite–(10wt%)TiC composite sintered at 1600°C reached a maximum value of 274MPa and 1355HV, respectively. Novel dense Mullite–TiC composites were fabricated by a Spark plasma sintering process. It was found out that introduction of TiC can enhance the mechanical properties of Mullite matrix effectively.