Mullite Formation by Endothermic Reaction of α-Alumina/Silica Microcomposite Particles

The formation of mullite was investigated using microcomposite powders which consist of α-alumina cores and amorphous silica coatings. Differential thermal analysis and X-ray diffraction showed that the mullitization reaction was endothermic. In contrast, mullite forms exothermically in samples prepared by sol-gel processing. The results are shown to be consistent with available thermodynamic data for mullite formation from different alumina and silica phases.     

凝胶法片状莫来石粉的制备及应用

采用拟薄水铝石和非晶态超细SiO_2微粉为原料。以溶胶-凝胶工艺制取莫来石凝胶,通过干燥、煅烧研制得片状结晶莫来石粉。初步研究了凝胶法莫来石粉在钛酸铝陶瓷和烧结刚玉砖中的应用。实验结果表明:凝胶粉起到了促进烧结、降低烧成温度的作用,并使钛酸铝——莫来石复相陶瓷和刚玉砖的强度得到了显著提高。     

用凝胶法制备片状莫来石粉及其应用

用拟薄水铝石和非晶态ＳｉＯ_２微粉为原料，用溶胶－凝胶工艺制取了莫来石凝胶。通过干燥、煅烧，制得片状结晶莫来石粉．初步研究了凝胶法莫来石粉在钛酸铝陶瓷和烧结刚玉砖中的应用。实验结果表明，片状莫来石起了促进烧结、降低烧成温度的作用，并使钛酸铝－莫来石复相陶瓷和刚玉砖的强度得到显著提高．     

Effect of the processing on the production of cordierite–mullite composite

In this study, effect of process on the production of cordierite–mullite composite was studied. For this reason two different processing methods were used in the production of cordierite–mullite composites. In first process, in situ cordierite–mullite composites were produced from cordierite and mullite layers which were formed by using aqueous tape casting method. In second one, composite was produced by addition of pre-produced mullite powders (in different weight percents, 0–30) into cordierite starting powders. The results show that the addition of pre-sintered mullite powders to the cordierite slip has more effect on densification behavior and mechanical properties of composites than layered production method.     

Contribution to porous mullite synthesis from clays by adding Al and Mg powders

This work has been undertaken to consider the production of porous mullite from kaolinitic clays. X-ray diffraction, scanning electron microscopy, thermogravimetry, differential thermal and dilatometry analyses were performed to study their thermal behavior. Pure and porous mullite was obtained by adding aluminum metal powder to kaolinite and hot processing in oxidizing atmosphere. Comparative studies were carried out to evaluate the influence of various weight ratios of magnesium on both the formation and reactive sintering of mullite. Porosity was shown to be increased with magnesium content while needle-like mullite crystals could be observed.     

Optically Translucent Mullite Ceramics

The optical transmittance of pressureless sintered and of hot isostatically pressed (HIPed) mullite ceramics was studied in the near infrared (NIR), visible light (VIS), and the ultraviolet (UV) spectrum ranges. Both processing methods provided fine-grained microstructures (mean grain size of 5–10 μm) with equiaxed grains and a narrow grain-size distribution. In pressureless sintered mullite, the porosity was 5.5% and transmittance was 20% in the VIS and NIR ranges. In the HIPed mullite, with porosity <1%, a transmittance of 40% was measured in the VIS range and up to 80% in NIR. UV transmittance of both pressureless sintered and HIPed mullite samples was low. The optically translucent mullite was found suitable for optical windows in the VIS and NIR ranges, particularly at elevated temperatures.     

Mullite/Alumina Particulate Composites by Infiltration Processing: III, Mechanical Properties

The effect of incorporating mullite into alumina by an infiltration process on the mechanical properties was investigated. Data for Young's modulus, strength, and fracture toughness for various composite compositions were compared with those for the unreinforced matrix (alumina). Measurements of Young's modulus by a resonance technique showed that the addition of mullite decreased Young's modulus. Up to 14 vol%, these changes were close to those expected, but above this mullite content, the decrease was more dramatic and indicated specimen damage during processing. The addition of mullite led, in some cases, to increases of more than 60% in both the strength (biaxial flexure) and indentation fracture toughness. These increases have been attributed to the method of introducing mullite and the resulting residual compressive surface stresses. The strength of the indented composite bodies deviated from the ideal behavior, indicating the probability of R -curve behavior in these materials.     

Infiltration and Pyrolysis of Polytitanocarbosilane in an Si-Ti-C-O Fabric/Mullite Porous Composite

Incorporating Si-Ti-C-O fabric into a mullite matrix is expected to increase the fracture energy of mullite ceramics. The present paper describes the processing of an Si-Ti-C-O fabric/mullite/polytitanocarbosilane composite. A polytitanocarbosilane (a precursor of Si-Ti-C-O fiber)/xylene solution was infiltrated into a laminated porous mullite composite with 35–37 vol% fabric and thermally decomposed to an amorphous solid at 1000°C, in an argon atmosphere, to decrease the porosity and residual stress induced by the difference in thermal and mechanical properties between the Si-Ti-C-O fabric and the mullite. The decrease in porosity of the composite with pyrolysis of the precursor polymer was analyzed theoretically, and those results were used to control the effective experimental parameters. The infiltration/pyrolysis process was repeated eight times to produce a composite of 90.4% theoretical density. The composite exhibited significant pseudoductility, with a fracture energy of 11.4 kJ/m² and a flexural strength of 290 MPa.     

Strengthening of Porous Mullite and Zirconia CMC Matrices by Evaporation/Condensation

A processing method using evaporation/condensation sintering in an HCl atmosphere was developed for strengthening porous materials without shrinkage. Strengthening without shrinkage is useful in preventing voids and cracks that might be formed during constrained densification, e.g., a porous matrix in a continuous fiber reinforced ceramic composite. Mixtures of mullite and zirconia (monoclinic, tetragonal (3 mol% Y₂O₃), and cubic (8 mol% Y₂O₃)) were studied and exposed to HCl vapor at temperatures up to 1300°C. It was observed that the evaporation–condensation mass transport process produced a porous material with minimal shrinkage. As the crystal structure of the starting tetragonal and cubic zirconia powders did not change after extensive coarsening, it appeared that zirconium and yttrium were transported in the same proportion via evaporation/condensation. The process produced significant coarsening of the zirconia grains, which made the material resistant to densification when heated to 1200°C in air. Because the sintering produced coarsening without shrinkage, the pores also coarsened and a porous microstructure was retained. Mixtures of mullite and zirconia were used because mullite does not densify under the processing conditions used here, namely, heat treatments up to 1300°C. The mullite particles acted as a non-densifying second phase to further inhibit shrinkage when the mullite/zirconia composite was heated up to 1200°C in air. The coarsened cubic zirconia plus mullite mixture had the least densification after heat treatments in air of 100 h at 1200°C.     

Comparative study of conventional and microwave sintered mullite fibres: structural study

Mullite fibres were synthesised from a mixture of aluminium, aluminium chloride and acidic silica solutions as monophasic salts using a sol–gel technique. The viscosity and rheological behaviour of the mullite precursor sol were examined. Mullite fibres were synthesised using both conventional and microwave sintering techniques. The samples were characterised by X-ray diffraction and scanning electron microscopy. Pure mullite fibres were synthesised by microwave sintering at a relatively low temperature of 1200°C. The grain size of the fibre samples sintered using the microwave technique was finer than that produced by conventional sintering. These results show that microwave sintering is a promising technique for processing mullite fibres.     

Colloidal processing of a mullite matrix material suitable for infiltrating woven fibre preforms using electrophoretic deposition

Commercially available alumina and silica precursors for the preparation of mullite ceramic via colloidal processing and viscous transient sintering have been identified, including fumed nanosize powders and colloidal suspensions. These materials were chosen due to the fact that they can be used in the form of a sol, as mullite matrix precursors, to infiltrate woven fibre preforms using electrophoretic deposition. The sintered density of the mullite matrices sintered for 2 h, at the upper temperature for fabricating SiC-fibre reinforced composites (1300 °C) is only ≈ 90% of theoretical. However, by exploiting a viscous flow densification mechanism, it is envisaged that hot-pressing can be used to produce fully dense mullite matrix composites at the required temperatures. Additionally. using a simple pressureless sintering route, almost fully dense (98% of theoretical density) monolithic mullite has been obtained from the pre-mullite powders. A very homogeneous and fine microstructure was achieved by sintering for 5 h at a temperature of ≈ 1450 °C.     

莫来石基纳米复合陶瓷

纳米陶瓷是八十年代中期发展起来的一个新兴材料领域,纳米陶瓷的力学性能较同化学组成陶瓷相比有数量级的提高,并具有一些新的特性,为材料开拓了新的应用领域。本文介绍了莫来石基纳米陶瓷结构特点,主要制备方法,工艺要点及相关性能,并总结了纳米粒子在莫来石基体中的作用。     

In situ mullite foam fabrication using microwave energy

In situ mullite ceramic foams were fabricated using polymeric sponge replication method from ceramic slurry containing alpha alumina and kaolin mixtures. Ceramic preforms were processed using microwave energy and conventional heating. The sintered foam samples were characterized by SEM and XRD observations, density measurements and compression tests in order to observe the effect of two different sintering techniques on the structure and properties. It was found that the microwave processing was completed in a shorter burning out and sintering cycle and produced structures having higher mullite transformation ratio and fine grains.     

Processing of Silicon Carbide-Mullite-Alumina Nanocomposites

Nanocomposite materials in the form of nanometer-sized second-phase particles dispersed in a ceramic matrix have been shown to display enhanced mechanical properties. In spite of this potential, processing methodologies to produce these nanocomposites are not well established. In this paper, we describe a new method for processing SiC-mullite-Al₂O₃ nanocomposites by the reaction sintering of green compacts prepared by colloidal consolidation of a mixture of SiC and Al₂O₃ powders. In this method, the surface of the SiC particles was first oxidized to produce silicon oxide and to reduce the core of the SiC particles to nanometer size. Next, the surface silicon oxide was reacted with alumina to produce mullite. This process results in particles with two kinds of morphologies: nanometer-sized SiC particles that are distributed in the mullite phase and mullite whiskers in the SiC phase. Both particle types are immersed in an Al₂O₃ matrix.     

Transient liquid phase sintering of high-purity mullite for high-temperature structural ceramics

High-purity mullite ceramics, promising engineering ceramics for high-temperature applications, were fabricated using transient liquid phase sintering to improve their high-temperature mechanical properties. Small amounts of ultrafine alumina or silica powders were uniformly mixed with the mullite precursor depending on the silica-alumina ratio of the resulting ceramics to allow for the formation of a transient liquid phase during sintering, thus, enhancing densification at the early stage of sintering and mullite formation by the reaction between additional alumina and the residual glassy phase (mullitization) at the final stage of sintering. The addition of alumina powder to the silica-rich mullite precursor resulted in a reaction between the glassy silica and alumina phases during sintering, thereby forming a mullite phase without inhibiting densification. The addition of fine silica powder to the mullite single-phase precursor led to densification with an abnormal grain growth of mullite, whereas some of the added silica remained as a glassy phase after sintering. The resulting mullite ceramics prepared using different powder compositions showed different sintering behaviors, depending on the amount of alumina added. Upon selecting an optimum process and the amount of alumina to be added, the pure mullite ceramics obtained via transient liquid phase sintering exhibited high density (approximately 99%) and excellent high-temperature flexural strength (approximately 320 MPa) at 1500 °C in air. These results clearly demonstrate that pure mullite ceramics fabricated via transient liquid phase sintering with compositions close to those of stoichiometric mullite could be a promising process for the fabrication of high-temperature structural ceramics used in an ambient atmosphere. The transient liquid phase sintering process proposed in this study could be a powerful processing tool that allows for the preparation of superior high-temperature structural ceramics used in the ambient processing atmosphere.     

Residual Stress in Alumina–Mullite Composites

Lattice strains have been measured by neutron diffraction from the alumina matrix in alumina–mullite composite cylinders. Infiltration processing results in a macroscopic variation of mullite concentration and residual stress throughout the samples. At the midheight of the cylinder, the stress component parallel to the cylinder axis is in a compression of −130 ± 30 MPa with respect to the value near the top surface. At the surfaces, the stress component parallel to the surface is in a compression of −130 ± 30 MPa with respect to values near the midheight of the cylinder. An overall tensile shift of the macroscopic stress profile in a fully infiltrated specimen is interpreted as a grain interaction effect between the alumina and mullite constituents. The positional dependence of the mullite phase concentration is also determined by neutron diffraction.     

粉煤灰合成莫来石晶须过程中的相变特征

采用循环流化床(CFB)高铝粉煤灰和煤粉炉(OF)高铝粉煤灰为原料,以AlF_3·3H_2O为导向剂,V_2O_5为催化剂,通过固相反应制备莫来石晶须。利用XRD和SEM等手段,分析了产物的物相,对产物的形貌进行了表征。探讨了原料结晶程度、合成温度及保温时间、导向剂含量等工艺条件对产物的影响,优选出合理的制备工艺参数,分析了莫来石晶须的合成反应机制。     

Effect of Bi2O3 on phase formation and microstructure evolution of mullite ceramics from mechanochemically activated oxide mixtures

Mullite ceramics with hollow whisker structure have been synthesized firstly through ordinary sintering process. The effects of Bi₂O₃ and processing, on mullitization behavior and morphology development of mullite ceramics, derived from the mechanochemically activated mixture of Al₂O₃ and SiO₂, were investigated in this paper. When the content of Bi₂O₃ was less than 10?mol%, the mullite grains show a short rod-like morphology, without the formation of whisker. As the content of Bi₂O₃ was increased to more than 10?mol%, the formation temperature of mullite was decreased from 1400?°C to 1100?°C. After sintering at 1400?°C, well-developed mullite whiskers with hollow structure were formed. The formation process and growth mechanism of hollow structural whiskers in mullite ceramic doped with high content of Bi₂O₃ were discussed in detail.     

Cellular mullite materials processed by direct foaming and protein casting

In this paper, cellular mullite bodies were developed by thermal direct-consolidation of foamed aqueous mullite-bovine serum albumin (BSA) and mullite-BSA-methylcellulose suspensions, burning out (650 °C, 2 h) and sintering (1600 °C, 2 h). Some modifications to the shaping route conventionally used in protein casting were incorporated in the proposed processing to obtain bodies with controlled and homogeneous microstructures. The materials were characterized by porosity measurements, analysis of phases by XRD, and microstructural analysis by SEM and Hg-porosimetry. Characteristic parameters of cell size distributions, percentage of open and closed cells, window size and interstitial pore size distributions, and microstructural features of the mullite matrix were determined. Moreover, basic 2D cell size parameters and global 3D stereological parameters were analysed. The obtained results showed that mullite bodies with hierarchical porosity and different microstructural features were developed from the design and control of processing routes, which use BSA as a foaming and binder/consolidator agent.     

Characterization of textured ceramics containing mullite from phyllosilicates

Organized ceramics are obtained from kaolinite and muscovite suspensions and shaped by aqueous tape casting or centrifugation. These processes favor the preferential orientation of particles in the powder compact. After sintering at 1400 °C, this study analyzed sample microstructures using QTA to determine the degree of the mullite orientation. The analyses revealed two main texture components, a planar texture along the c-axis of the mullite and a preferred orientation along the a-axis, which were aligned parallel and perpendicular to the casting plane, respectively. The important role of processing parameters in the organization degree of the mullite was apparent during the study. The elastic properties at different measurement scales were obtained using US echography and nanoindentation and were closely related to the organization degree of the mullite crystals obtained from the QTA analyses that were consistent with the development of an interconnected mullite network. The Young's moduli due to the nanoindentations were also determined parallel and perpendicular to the layers, and indicated the samples' anisotropic behavior. Both the Young's modulus and the anisotropy of the Young's modulus were correlated with the texture index. In particular, the anisotropy of the Young's moduli was linearly related to the overall texture index, highlighting the microstructures' anisotropic nature.