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Abstract High-quality SSZ-13 membranes with enhanced permeances were synthesized on the symmetric mullite supports by modifying the synthesis recipes, and shown to be useful for light gas separations. The best SSZ-13 membrane displayed doubled CO 2 permeance and comparative CO 2 /CH 4 selectivities compared to our previous mullite supported SSZ-13 membranes. Single-gas permeance of CO 2 , H 2 , N 2 , CH 4 , C 2 H 6 and C 3 H 8 decreased at 298 K with increasing kinetic diameter, except for CO 2. The CO 2 /CH 4 , CO 2 /N 2 , H 2 /CH 4 and N 2 /CH 4 ideal selectivities were 360, 20, 62 and 16, respectively, at room temperature. The mixture selectivities for the same equimolar gas pairs at 298 K were 406, 32, 43 and 16, and the faster-gas permeances of 4.4, 3.2, 1.8 and 0.22 mol/(m2 s Pa), respectively. Calcination times, long-time operation and long-term storage had few effects on membrane performance. Graphical abstract Image 1 Highlights • High-quality SSZ-13 membranes were synthesized on symmetric supports. • The SSZ-13 membrane exceeded the upper bounds of polymer membranes for CO 2 and H 2 separations. • The SSZ-13 membrane showed the highest N 2 /CH 4 separation selectivity. • SSZ-13 membranes displayed excellent stabilities in multi-calcinations, long-time operation and long-term storage. [ABSTRACT FROM AUTHOR]

Graphical abstract Pure-phase La x Sm 1-x Mn 2 O 5 (x = 0.1,0.3, 0.5) was successfully prepared; Outstanding oxygen reduction activity for La x Sm 1-x Mn 2 O 5 (x = 0.1,0.3, 0.5), as is shown in Koutecky–Levich plots. Highlights • Pure-phase La x Sm 1−x Mn 2 O 5 were firstly achieved through one step calcined solution combustion synthesis with high yield. • The main impact of La/Sm is the amount of surface adsorbed oxygen and surface Mn4+ ions as revealed by XPS results. • The La 0.1 Sm 0.9 Mn 2 O 5 exhibited the highest activity, which has an appropriate ratio between oxygen vacancies and Mn3+. • The cell produced with La 0.1 Sm 0.9 Mn 2 O 5 exhibits flat discharge curves in 40 h above 1.5 V. Abstract Compared the most reported synthesis methods, a series of La x Sm 1−x Mn 2 O 5 (x = 0, 0.1, 0.3, 0.5) catalysts were first synthesized through sol-gel solution combustion synthesis with a short cycle and high yield. The main impact of different La/Sm molar ratios is the amount of surface adsorbed oxygen (O ads) and surface Mn4+ ions as revealed by XPS results. Electrochemical properties are characterized by rotating disk electrode (RDE) and the full dischargeable Al-air battery with 4 M NaOH electrolyte. The measurements of RDE reveal that the electron transfer number of La x Sm 1−x Mn 2 O 5 (x = 0, 0.1, 0.3, 0.5) are all closed to 4, and the sample prepared with x = 0.1 possesses the most excellent electrocatalytic activity for oxygen reduction reaction (ORR) and good discharge performance in alkaline medium. All of the above results indicate that the La x Sm 1−x Mn 2 O 5 (x = 0, 0.1, 0.3, 0.5) are promising catalysts with a high catalytic activity toward ORR. [ABSTRACT FROM AUTHOR]

This paper addresses the effects of processing technique on the synthesis of mullite from pyrophyllite and alpha-alumina powder. The influence of firing temperatures were examined in order to optimize the fabrication process to achieved a suitable mechanical properties. Feedstock powders, with stoichiometric composition, were wet milled in an Attritor mill. The dried powders were consolidated using spark plasma sintering (SPS) in the temperatures range 1400 °C to 1700 °C under pressure of 50 MPa, with a heating rate of 100 °C/min at different holding times. Densities of 3.25 g/cm 3 and 3.17 g/cm 3 were obtained for the samples sintered at 1400 °C and 1600 °C respectively, with a 10-min holding time. The XRD and SEM/EDS were employed to characterize the firing transformation and microstructure of the samples. The SEM micrograph of samples, sintered at 1600 °C with 10 min holding times, revealed that the sample retained its equiaxed grain structure. The XRD results show a reduction in alpha-alumina content after the sample was fired. Hardness and fracture toughness values up to 11.73 GPa and 1.99 MPa•m 1/2 respectively were obtained. [ABSTRACT FROM AUTHOR]

Mullite-type Bi 2 (Ga 1- x Fe x ) 4 O 9 solid solutions, with 0.1 ≤  x  ≤ 0.9, have been synthesized by a combination of mechanical and thermal treatments of a Bi 2 O 3 /Ga 2 O 3 /α-Fe 2 O 3 stoichiometric mixture. The microstructure of the as-prepared materials on the long-range and local atomic scales was investigated by X-ray diffraction and 57 Fe Mössbauer spectroscopy, respectively. The XRD data analysis revealed in all cases linear dependence of the lattice parameters related on x . Due to the ability of the applied Mössbauer spectroscopy to probe the local environment of Fe cations, the local structural disorder in investigated solid solutions is provided. It was shown that the presence of Fe 3+ cations in octahedral sites of the orthorhombic structure causes a local distortion of polyhedra in the material. The preferential occupation of Fe in octahedral site was revealed. Detailed quantitative information on both the cation distribution and the bond lengths provided is discussed in relation to the derived hyperfine parameters. [ABSTRACT FROM AUTHOR]

In this study, the mullitization temperature was drastically reduced to 900 °C through adding boehmite sol to the kaolin/α-Al 2 O 3 reaction couple. The low mullitization temperature was probably ascribed to: (1) high solubility of γ-Al 2 O 3 in the SiO 2 -rich liquid during sintering, (2) adhesive effect of boehmite sol and (3) high reactivity of calcined kaolin. Also, starch particles and polystyrene (PS) micro-spheres were utilized as porogens to prepare high-porosity mullite. Compared to starch particles, the PS micro-spheres were more suitable for fabricating high-porosity mullite. The effects of PS content and sintering temperature on the mullitization reaction and properties of porous mullite were studied. The result demonstrated that single-phase mullite could only be achieved at 1300 °C for samples with porogen. Excessive addition of PS micro-spheres (>32 wt%) would inhibit the porosity increase due to the sample shrinkage during sintering. As the amount of PS increased from 22% to 32%, porosity of the samples increased from 73.29% to 80.10%, compressive strength decreased from 15.55 ± 0.52 to 9.89 ± 0.12 MPa and thermal conductivity decreased from 0.26 to 0.16 W/m/K. [ABSTRACT FROM AUTHOR]

A novel approach to fabricate mullite ceramic foams with high porosity and mechanical strength using fly ash hollow spheres (FAHSs) as the only raw material was reported for the first time, in which FAHSs served as both matrix and pore-forming agent. It was demonstrated that uniform FAHS ceramic foams without cracks could be fabricated by both direct stack sintering method and gel-casting route. The effect of sintering temperature on linear shrinkage, porosity, phase composition and mechanical properties was investigated. With the increase of sintering temperature from 1200 °C to 1350 °C, the fracture mechanism changed from fracturing along FAHSs to fracturing across FAHSs, and the compressive strength of the FAHS ceramic foams increased from 4.4 MPa to 33.4 MPa. The FAHS ceramic foams exhibited high porosity in the range of 73–81% and high mechanical strength, which benefited from the dense assembling of small FAHSs with natural diameter distribution and excellent wall-thickness uniformity. [ABSTRACT FROM AUTHOR]

This paper shows the results of the solid-state synthesis of mullite from spent catalysts discarded from fluid catalytic cracking (FCC); the catalysts are mainly composed of silica and alumina but are polluted with SO X , forming a non-crystalline network. The synthesized mullite was used as a feedstock to thermally spray a coating onto a silica-alumina refractory brick, and its chemical resistance at high temperature was subsequently evaluated by contact with K 2 CO 3 at 950 °C. Initially, the spent catalyst was thermally treated for 2 h at 600, 900, and 1200 °C to eliminate the SO X pollutant. The heat treatment at 1200 °C completely removed the SO X in the sample. Additionally, four thermal processes were performed by heating the spent FCC catalyst in an electrical furnace to 1500 and 1600 °C and by using an oxyacetylene flame to synthesize mullite. Thermal treatments at 1500 °C were performed with and without alumina added to the spent FCC catalyst, whereas those conducted at 1600 °C and using a flame were performed using only added alumina. In the powders thermally treated at 1500 °C, silica-rich mullite (3Al 2 O 3 .2SiO 2 ) accompanied by an excess of alumina or silica was obtained with or without alumina added, respectively. In contrast, the materials treated at 1600 °C formed alumina-rich mullite (2Al 2 O 3 .SiO 2 ), which was accompanied by an excess of alumina. Mullite was not synthesized in the flame-heated powder. The silica-rich mullite accompanied by an excess of alumina was used as feedstock powder to modify the surface of a refractory brick, improving its resistance to chemical attack by K 2 CO 3 at high temperature. [ABSTRACT FROM AUTHOR]

High-alumina coal fly ash (HAFA) is a special solid waste due to the existence of more than 45% alumina and 35% silica, which can be applied to prepare Al-Si series ceramics if the impurities can be removed and the Al/Si mass ratio can exceed 2.55 (Mullite: 3Al 2 O 3 .2SiO 2 ). In this work, a new mechanical–chemical synergistic activation–desilication process is proposed, and the contents of different impurities can be lowered up to less than 1%, and the Al/Si mass ratio can be elevated from 1.26 to 2.71. Especially, the mechanism of this process is investigated in detail. The analysis of the mechanism shows that the decrease of Q 4 (3Al) and the increase of Q 4 (0Al), Q 4 (1Al), and Q 4 (2Al) improve amorphous silicate reactivity through synergistic activation, and the exposed amorphous Si-O-/Si-O-Si can be removed by OH- during the desilication process (desilicated ratio>55%), which help the fine mullite to exhibit excellent properties (bulk density > 2.85 g/cm 3 , apparent porosity < 0.5%) during the sintering process. Finally, this process not only decreases the pollution but also alleviates the shortage of Al/Si resources and promotes the clean development of coal-fired power generation. [ABSTRACT FROM AUTHOR]

This work investigates the difference in viability of primary human foetal osteoblast cells on a glass-ceramic surface with nanoscale topography relative to viability on a smooth glass-ceramic surface containing a bioactive phase. Apatite-mullite glass-ceramics containing bioactive fluorapatite (Ca 10 (PO 4 ) 6 F 2 ) and bioinert mullite (Si 2 Al 6 O 13 ) were synthesised and subsequent heat-treatment was optimised to form nano-sized fluorapatite crystals. Etching was used to selectively remove the bioactive phase, producing a surface with disordered nanoscale topography. Cells were seeded onto a smooth polished glass-ceramic substrate with the bioactive phase intact, an etched nanostructured glass-ceramic with the bioactive phase removed, and a borosilicate glass control. Cell viability after 24 h and 48 h was significantly greater on the nanostructured surface compared to the smooth bioactive surface, while cell viability at both time points was significantly greater on both nanostructured and smooth bioactive surfaces compared to the control. [ABSTRACT FROM AUTHOR]

Injecting lance is a key equipment for hot metal pretreatment. In this work, the lance castables with high mechanical properties and thermal shock resistance were obtained by adding alumina bubbles, due to its spherical shape and hollow structure. It is found that the addition of alumina bubble improves the flow values of castables. The bulk density values of samples with the addition of 4 wt% alumina bubbles (AB4) were the best of all the experimental castables. Alumina bubble addition is effective in heightening the thermal expansion, thermal conductivity of mullite castables and can resist the sharp decrease in thermal expansion above 1300 °C. The addition of alumina bubble can improve the mechanical strength of mullite castables. After heating at 1400 °C, a strong chemical bond forms between the bubbles and the binding phases. Due to the appropriate bonding at the alumina bubble/matrix interface, AB4 castable exhibits a relatively better thermal shock resistance. [ABSTRACT FROM AUTHOR]

In this paper, the influences of variation of spark plasma sintering (SPS) temperature on the microstructure and mechanical properties of mullite-(10 wt%)WC composites were investigated. It was found that the increment of the SPS temperature from 1300 °C to 1400 °C improves the densification behaviour and mechanical properties of this composite. However, with the further increasing of the SPS temperature to 1450 °C, the grain size of mullite matrix considerably increased and tungsten carbide reinforcements partially decomposed to W 2 C phase. Due to inferior mechanical properties of W 2 C in comparison with WC and grain growth of mullite matrix, mechanical properties of this sample were deteriorated. As a consequence, the SPS temperature of 1400 °C was chosen as the optimum consolidation condition for this composite. The comparison of microhardness, fracture toughness and strength of mullite-(10 wt%)WC composite (15.89 GPa, 4.07 MPa·m 1/2 and 298 MPa, respectively) with the monolithic mullite samples SPS-processed at the temperature of 1400 °C confirms the positive effect of WC addition to mullite matrix. [ABSTRACT FROM AUTHOR]

In this work, controllable preparation of micro/nanostructured bismuth ferrites (BFOs) were used to investigate multifarious heterogeneous catalyses, including Fenton/Fenton-like reaction, photocatalysis, photo-Fenton oxidation, and peroxymonosulfate (PMS) activation. Results showed that BFO can be used as a novel catalyst to activate switchable catalytic degradation of organic matters. Additionally, a novel catalytic system for degradation of organic pollutants, which integrating all-above heterogeneous catalyses is denoted as BFO/H 2 O 2 /PMS hybrid reaction, is introduced for the first time. BFO/H 2 O 2 /PMS system effectively degraded > 99% for both methyl orange (MO) and sulfamethoxazole (SMX) within 60 min, which shows better efficiency than above BFO-driven catalyses. The major SMX degradation pathway in BFO/H 2 O 2 /PMS system is proposed via detecting intermediates using LC/MS/MS. It was found that catalytic activities of BFOs are in the order of BFO-L (co-precipitation, micro/nanosize, single crystals exposing facet (0 0 1)) > BFO-H (hydrothermal, nanocluster with a higher surface area than other BFOs) > BFO-C (fabricated using calcination process, microsize), which demonstrated that crystallographic orientation is more significant in heterogeneous catalyses than specific surface area at micro/nanoscale. Besides, the required H 2 O 2 consumption for achieving 99% TOC removal was identified in BFO-driven photo-Fenton oxidation. The other effects on degradation efficiency, such as H 2 O 2 dosage and pH, were investigated as well. In Fenton/Fenton-like reaction, reaction conditions suggested are ∼61.5 mM H 2 O 2 dosage and pH ≥ 4.5 to avoid quenching of HO into HO 2 by excessive H 2 O 2 and Fe leaching. [ABSTRACT FROM AUTHOR]

Lithomargic clay has low plasticity and therefore makes it difficult to form ceramic bodies without addition of binder using conventional sintering technique. Plasticity can be improved by addition of clay binders. However, clay binders increase SiO 2 content and thus lower the refractoriness of the sintered compacts. In this study, mullite was fabricated from lithomargic clay using spark plasma sintering without addition of clay binder. The mullite compacts were produced at a varying temperature range of 1300–1400 °C for a holding time of 300 s under pressure of 30 MPa. Densification and physico-mechanical properties were studied. Densification and hardness values of mullite increased with temperature while apparent porosity and water absorption decreased with temperature. The hardness of mullite sintered at 1400 °C reached 1112 HV. Mullite produced has the potential to be used for lightweight applications. [ABSTRACT FROM AUTHOR]

In this study, the mechanism and kinetic parameters of Al-Si spinel crystallization from Algerian Tamazarte kaolinite was studied by Differential thermal analysis (DTA) technique, which were carried out on samples between room temperature and 1400°C at different heating rates (10-40°C/min). X-ray diffraction was used to identify the phases present in the samples. The activation energies measured by both isothermal (Johnson-Mehl-Avrami (JMA) theory using Ligero method) and non-isothermal (Kissinger methods) treatments are 870 and 810kJ/mol, respectively. The growth morphology parameters n (Avrami parameter which indicates the crystallization mode) were found to be almost equal to 1.08, using non-isothermal treatments, and equal to 1.17 using isothermal (Ligero method) and m (the numerical factor which depends on the dimensionality of crystal growth) was 1.07 obtained by Matusita and al. equation. Analysis of the results showed that bulk nucleation was dominant in Al-Si spinel crystallization followed by two-dimensional growth of mullite crystals with plates morphology controlled by diffusion from a constant number of nuclei. [ABSTRACT FROM AUTHOR]

Porous mullite microspheres with a highly open porosity and average diameter of more than 800 µm were fabricated via an oil-drop molding method accompanied by a freeze casting process. After sintering, a highly porous structure was formed due to interlocking whisker-shaped mullite grains and formation of interconnected skeletons during the freeze-casting process. Additionally, it was found that a high porosity and large pore size in the microspheres green bodies are favorable for the synthesis of mullite whiskers with high aspect ratio. [ABSTRACT FROM AUTHOR]

Sacrificial pore former removal and mullite binder phase transformation ~ the two major stages of processing of oxide bonded porous SiC ceramics ~ were studied by conducting thermal analysis of the precursor powders (SiC +20 wt% Al 2 O 3 +15–41 vol% petroleum coke pore former). Kinetic computation of reactions occurring in these stages was done by fitting the non-isothermal DTA data to empirical models. The ceramics prepared from the precursor powders were characterized by the combined approach of chemical and XRD analyses and electron microscopy and the results in combination with the kinetic data were used to explain the formation of the pore phase and the mullite binder phase of the ceramics. [ABSTRACT FROM AUTHOR]

Results are given for a study of the effect of LiF, ZnO, and CeO modifying additions on synthesis and properties of cordierite-mullite ceramic from Kazakhstan resources. It is established that addition of LiF, ZnO, and CeO facilitates activation of synthesis for cordierite-mullite ceramic based on natural raw material. Addition of LiF reduces the temperature for the start of eutectic melt development, facilitating intensification of cordierite formation, and ZnO and CeO act by a solid-phase mechanism. Introduction of LiF, ZnO, and CeO into the composition of cordierite-mullite ceramic in an amount of 0.5 - 2.0% makes it possible to reduce its firing temperature and increase density, strength, and chemical and thermal stability. [ABSTRACT FROM AUTHOR]

A comparison among various sol-gel bonding systems has been done for high alumina castable refractory. Four different sol systems, namely, alumina, boehmite, mullite, and spinel, have been synthesized and used individually as sole binder in high alumina castable system with particle size distribution coefficient of 0.23 as per Dinger and Funk model. The castable compositions were processed conventionally and properties were evaluated after heat treatment at different temperatures. These developed sol-containing compositions were also compared against commercially available silica sol-containing composition prepared under exactly similar conditions. Mullite sol-bonded composition showed higher cold strength, whereas spinel sol resulted in better hot properties. Improved thermal shock resistance against commercial sol binder was observed in spinel and mullite sol-containing compositions, which may be associated with the microcracking effect produced from the thermal expansion mismatch. The slag corrosion resistance was also observed better for the spinel sol-containing composition. The commercial silica sol-bonded compositions showed higher strength due to its higher solid content but resulted in inferior hot and corrosion properties. [ABSTRACT FROM AUTHOR]

Low-cost porous sound-absorbing ceramics were prepared from steel slag and kaolin, with graphite and NaHCO 3 as the pore former. V 2 O 5 and AlF 3 were used as additives to promote the secondary mullitization reactions at lower temperatures. The influence of the additives and sintering temperature on phase constituents, open porosity, microstructure and mechanical performance were characterized in detail. Results show that the introduction of the steel slag into porous sound-absorbing ceramics decreases the temperature of mullite formation. Adding 3 wt% V 2 O 5 and 4 wt% AlF 3 , mullite whiskers with interlocking microstructure were successfully obtained at sintering temperatures upon 1230 °C for 2 h. Furthermore, the resultant samples exhibited excellent performance, including open porosity of 64.7% and compressive strength of 6.78 MPa. [ABSTRACT FROM AUTHOR]

The mullite phase in porcelain is important to the porcelain strength; however, its formation from clay minerals usually requires high temperature. Here we found that the activation of kaolin by citric acid would lower the formation temperature of mullite phase to 935.1 °C, which is 67.5 °C lower than that for the raw kaolin. Needle-like mullite crystal, a desired crystal shape in kaolin, could be observed in activated kaolin at 1050 °C. The reduction of mullite formation temperature is due mainly to the surface change in kaolin. Upon citric acid activation, the pseudohexagonal platelets of kaolinite crystal are disintegrated to non-uniform flakes, and the specific surface area increases from 8.18 m 2 /g to 31.21 m 2 /g, both leading to improved activity of kaolin and increased sintering driving force. The results are of interest in producing high-strength porcelain at reduced sintering temperatures and consequent energy saving for sintering process. [ABSTRACT FROM AUTHOR]

Development of high pure alumina castable is studied by using synthesized mullite sol as the sole binder. Mullite sol is prepared by wet chemical route and is characterized by its solid content, particle size, thermal analysis, phase development with temperature, microstructure, etc. This sol is used at two different percentages in high alumina castable compositions with two different particle size distribution patterns. Conventional castable processing is done on the compositions and the characterizations are done after heat treatment at three different temperatures. Finally, the best composition is also compared with the commercially available silica sol containing high alumina castable with similar particle size distribution. Considerably improved hot strength, high corrosion resistance and flexural strength (hot modulus of rupture) are obtained for the mullite sol containing composition but with relatively lower thermal shock resistance. [ABSTRACT FROM AUTHOR]

The method of in situ synthesis of mullite whiskers by gas-phase deposition and reaction was applied to improve the compressive strength of the mullite fiber brick. During the preparation process, silica sol, Al(NO 3 ) 3 solution and NH 4 F solution were introduced into the fibrous brick in the form of ions or sol through vacuum impregnation and freeze drying, and the silica sol, Al(NO 3 ) 3 and NH 4 F served as the silica sources, aluminum source and catalyst, respectively. Effects of process parameters (concentration of impregnation solutions, holding time, sintering temperature) on compressive strength and elastic modulus of the fibrous brick during the in situ toughening process were analyzed. SEM and XRD analysis results demonstrated that the mullite whiskers were synthesized on the surface of mullite fibers based on the reaction of AlOF and SiF 4 . What is more, the whiskers on adjacent fibers intersected with each other and formed many unfixed lap-jointing points, resulting in the increase of compressive strength and elastic modulus. Although the density and thermal conductivity of the sample after the generation of mullite whiskers fabricated with the optimum process were 0.406 g/cm 3 and 0.1262 W/(m K), respectively, which were slightly higher than that of the raw fibrous brick (0.375 g/cm 3 density and 0.1069 W/(m K) thermal conductivity, respectively), the corresponding compressive strength and elastic modulus of the sample reinforced with the whiskers increased to 1.45 MPa and 42.03 MPa, respectively, which were much higher than that of the raw fibrous brick (0.39 MPa compressive strength and 6.5 MPa elastic modulus). [ABSTRACT FROM AUTHOR]

The effect of a fiber filler on properties and phase composition of a ceramic based on a corundum-mullite matrix is studied. It is established that use of a steel fiber for reinforcement during preparation of cordierite ceramic is impossible. It is shown that with use of a mullite-silica fiber within the mix composition for cordierite ceramic preparation it is possible to achieve an increase in material mechanical strength by a factor of 1.2 - 2.0; ceramic LTEC at 800°C is 4.1 × 10- 5.0 × 10 K. It is shown that with introduction of mullite-silica fiber into the ceramic composition the relative amounts of cordierite and mullite increase insignificantly. [ABSTRACT FROM AUTHOR]

The hierarchical network was prepared by firing the mixture of mullite fiber and a kind of active powder which was made up of alumina gel, silica gel and AlF 3 powder. During which, the smooth surface of mullite fiber was coated with the secondary structure which was controllable by adjusting the firing temperature or by changing the mass ratio of active powder/mullite fiber. The secondary structure created new pore size distribution for the network. Some new pores with diameters of about 1 μm were found in the modified network, while the network constructed by the smooth mullite fiber only had pores with diameters of about 50 μm. Because of the introduction of the secondary structure, the particle filtration efficiency of the network substantially increased. The filtration efficiency for the 0.5 μm particles increased from 12.9% to 94.2%. The filtration efficiency for the 0.3 μm particles also increased from 0.6% to 56.6%. [ABSTRACT FROM AUTHOR]

We report for the first time using diatom frustules, the porous silica shells of diatoms, as Si precursor for synthesis of porous mullite ceramics with fibrous pore morphology. The porous mullite ceramics prepared by mould pressing method without using any porogen and binder showed high open porosity (up to 60%) due to the highly porous nature of diatom frustules and near zero sintering shrinkage of mullite whiskers. The original circular pores of diatom frustules were turned into fibrous pores of mullite ceramics. The porous ceramics showed flexural strength of 38.6 MPa at the porosity of 55.5% when sintered at 1600 °C for 3 h. [ABSTRACT FROM AUTHOR]

Near-3:2 mullite crystals have been grown successfully with the optical floating zone technique. The growth direction of mullite crystals is along the c axis and growth rate can be reached 10 mm/h. The effect of growth rates on the crystal quality has been studied. The crystalline phase, crystal quality, microstructure, composition distribution of the grown crystals were characterized by powder X-ray diffraction, single crystal X-ray diffraction, scanning electron microscope, electron probe micro-analyzer and Raman spectra, respectively. The grown mullite crystal is formed by peritectic structure with near 3:2 mullite crystal as main face and lamellar or needle-like SiO 2 -rich aluminosilicate as secondary phase that parallel to the crystal growth direction. The measured dielectric properties show large dielectric anisotropy and good temperature stability in near-3:2 mullite crystals. [ABSTRACT FROM AUTHOR]

The consideration of planetary ball mill processing is attempted to obtain modified fly ashes powders that are used as substitutes in a cement paste. Ball milling conditions are related to structural modifications of processed powders. These modifications are retrieved from X-ray diffraction, infrared spectroscopy, powder granulometry, and scanning electron microscopy. As an industrial application, the milled fly ashes are substituted to two different cements to show possible effect on compressive strength of cement pastes. Structural analysis of milled fly ashes show loss of crystallinity of the magnetite phase, nanosizing trend of quartz and mullite phases with some straining that takes place after only 1 h of milling. The drop of particle size of the milled fly ashes is also evidenced but with tendency to agglomeration and particle shape modification from spherical to irregular forms. Mechanical performance results related to modified cement pastes indicate successful substitution of cements by milled fly ashes up to 50%. Substitution results in levels of compressive strength as large as 70 MPa for particular combination of milling time, fly ash content and curing time. [ABSTRACT FROM AUTHOR]

An efficient method to produce highly ordered mullite nanowhiskers using B 2 O 3 -doped molten salt synthesis was reported in this work. The morphology and optical properties of the obtained nanowhiskers were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) spectroscopy. The results show that highly ordered mullite nanowhiskers with uniform morphology in large scale are obtained at 1000 °C for 3 h in air. The structure of mullite is Al-rich and single crystalline. The diameter is in the range of 90–110 nm and length up to 20–30 μm. The reaction mechanism of the highly ordered mullite nanowhiskers is attributed to local concentration gradient. Due to the Al-rich structure, mullite nanowhiskers demonstrate strong photoluminescence (PL) emission at 399 nm, 452 nm and 468 nm, implying mullite nanowhiskers can be applied as a promising candidate in optical and electronic fields. [ABSTRACT FROM AUTHOR]

Starch is used as porosity generator in membranes and other porous ceramics. A study was conducted to evaluate the effects of the characteristics of starches (impurities, particle size), obtained from potato, pea, maize and wheat, on the properties of microfiltration membranes formulated with traditional raw materials (the ceramic matrix was composed of quartz, albite and mullite). The results indicated that the pore size distribution and the permeability coefficient of the membrane can be controlled by modifying only the particle size of starch. In this way, correlations were obtained between surface mean diameter of starch and membrane properties (characteristic pore diameters d 16 and d 50 , and permeability coefficient). Moreover, it is necessary to use a starch having a mean particle size greater than about 50 μm in order to obtain a significant change in the pore size distribution and an increase in the permeability of the membrane. [ABSTRACT FROM AUTHOR]

In our report, a fibrous ceramics with the mullite fibers as the matrix and SiO 2 –AlPO 4 sol as the high-temperature binder were fabricated by the infiltration method. The effects of sintering temperature on the properties, such as porosity, phase transition, microstructure, compressive strength and fracture mechanism, were investigated. Compared with the fibrous ceramics with the SiO 2 –B 2 O 3 binder, the fibrous ceramics with the SiO 2 –AlPO 4 binder exhibited lower densities (0.560–0.595 g/cm 3 ), lower thermal conductivities (0.157–0.165 W/mK), high strain (7–8%), high rebound-resilience (77–90%) and the same level of compressive strengths (1.1–2.1 MPa). During the cooling stage of the heat treatment process, α→β-cristobalite displacive phase transition in the SiO 2 –AlPO 4 binder was successfully suppressed. The fracture behavior of the ceramics tends to be a layer-by-layer failure, which is quite different from that of the traditional ceramics. The non-brittle fracture mode and the excellent elasticity made this fibrous ceramics promising high-temperature elastic insulation materials. [ABSTRACT FROM AUTHOR]

Mullite is one of the popular ceramic materials for different engineering applications due to some of its important properties which include good thermal and chemical stability, high creep resistance, etc. Mullite synthesized by solgel process generally requires lower temperature for processing and generate product with better homogeneity and more purity compared with other conventional processes. Coating of solgel-derived mullite film on surface of a ceramic substrate can improve its thermomechanical properties, making it suitable for applications in different aggravated conditions. This study embarks a comprehensive review on synthesis, characterization and applications of solgel-derived mullite coating. [ABSTRACT FROM AUTHOR]

A mechanical mixture of alumina and pulverized zircon sand in 3:2 M ratio has been plasma sprayed to obtain mullite coating. Thereafter, the top layer of the coating has been remelted using laser. The presence of a mullite phase in the as-sprayed and laser remelted coatings has been confirmed qualitatively using X-ray diffraction. Both as-sprayed and laser remelted coatings have been characterized for their microstructure, hardness and porosity. The ultrafine grain structure of the coating produced by rapid quenching has been analyzed using transmission electron microscope. Presence of a mullite phase in the coatings has also been confirmed using small angle electron diffraction. Laser remelting has resulted in an appreciable reduction in porosity and increase in hardness in the coatings. [ABSTRACT FROM AUTHOR]

The thermal behavior resulting from size reduction of α-Al 2 O 3 and cristobalite powders during mullite formation were examined to understand the effects of the individual size reduction on the initiation and completion temperatures and the reaction rate of mullite formation. Both powders with particle sizes ( D 50 ) of approximately 200, 300, and 400 nm were mixed at a stoichiometric composition of 3Al 2 O 3 ·2SiO 2 and thermally treated up to 1400–1600 °C. The results indicated that the cristobalite amorphized in advance and then reacted with the α-Al 2 O 3 particles. The size of α-Al 2 O 3 particles determined that of the mullite particles formed. Decreasing α-Al 2 O 3 particles reduced the temperature of the amorphization and resulted in lowering and determining the temperature at which mullite formation initiated. While reducing the cristobalite sizes narrowed the temperature ranges of amorphization. It accelerated the rate of mullite formation, allowing the formation reaction to be completed at lower temperatures. [ABSTRACT FROM AUTHOR]

Nano-composite mullite–MoSi 2 with 40 wt.% MoSi 2 was produced from the raw materials of MoO 3 , Al and Si by mechanochemical processing. Different Al amounts (stoichiometric amounts of 1, 1.5 and 2 mol) and milling times (15, 23, 30, 60, 180, 300 min) were applied to obtain this composite. A planetary ball mill with a rotation speed of 250 rpm and a ball-to-powder ratio (BPR) of 40:1 was employed. The composite was formed using 1.5 mol of Al and occurrence of mechanically induced self-propagating reaction (MSR) in 23–30 min. The evolutions of phases and their microstructure were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDX). By means of transmission electron microscopy (TEM) the particle size and morphology of the obtained powders were examined. The mean crystallite size of the composite changed from 60 nm in 30 min to 20 nm in 5 h; the average particle size was about 155 nm in 5 h. [ABSTRACT FROM AUTHOR]

Abstract: Biomorphic silicon carbide–mullite ceramics were prepared from beech wood using liquid Si infiltration and molten salts synthesis. The resulting mullite whiskers coating, as well as the growth mechanism in molten Al2(SO4)3–Na2SO4 environment, have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) techniques. The biomorphic SiC ceramics derived from the beech wood template have coarse pore walls consisting of β-SiC grains with diameters ranging from 5 μm to 20 μm. After the molten salts reactions between biomorphic SiC substrate and mixture molten salts (Al2(SO4)3–Na2SO4), porous Silicon carbide–mullite ceramics with cilia-like microstructure were obtained. This unique structure has potential application in hot gases filters. An oxidation–dissolution cycle was proposed to explain the mullite whiskers growth in molten salts environment. [Copyright &y& Elsevier]

This work studies the development of varying weight percentages (0.5, 1.0, and 1.5 wt%) of surface functionalized mullite reinforced unsaturated polyester (UP) composites and their thermal, dielectric, water absorption, and mechanical properties. The synthesized mullite was functionalized with vinyltriethoxysilane (VTES). The introduction of vinyl groups on the surface of mullite was confirmed by FT-IR, TGA, and X-ray diffraction (XRD) analyses. Varying weight percentages (0.5, 1.0 and 1.5 wt%) of vinyl functionalized mullite (VFM) were incorporated into UP resin with a benzoyl peroxide initiator to obtain composites. The resultant data obtained from thermal, mechanical, dielectric, and water absorption studies, indicate that incorporation of VFM, leads to a significant improvement in the thermo mechanical, dielectric, and moisture resistant properties of the UP composites, compared with those of neat UP matrices. The molecular dispersion of VFM fiber in reinforced UP matrix composites was confirmed by SEM analysis. [ABSTRACT FROM AUTHOR]

Nanoscaled cristobalite and α- Al2 O3 powders were used as the starting materials for synthesizing mullite by solid-state reaction. The thermal reaction of the cristobalite with α- Al2 O3 during the thermal treatment was examined. Cristobalite powder with a D50 value of 430 nm was adopted to mix with α- Al2 O3 powders with a D50 values of 230, 310, and 400 nm in a stoichiometric composition of 3 Al2 O3∙2 SiO2 (71.8 wt% α- Al2 O3 and 28.2 wt% SiO2). Samples for thermal reaction were prepared using uniaxial pressed from the three mixtures that showed various particle number ratios of SiO2/ Al2 O3 due to the different particle sizes of α- Al2 O3. Examinations were performed by differential thermal analysis, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy techniques. The results showed that cristobalite particles amorphized during the thermal treatment, and then reacted with the α- Al2 O3 particle to form mullite via nucleation and growth. The amorphization temperature can be reduced by using finer-sized α- Al2 O3 powders, thus leading to a lower temperature for mullite formation. Mullite crystals with a multidomain structure were observed in the α- Al2 O3 particle matrixes. The crystal orientation of the mullite was controlled by the α- Al2 O3 matrix, that is, [001] α- Al2 O3 → [001] mullite. These results indicate that the amorphization of cristobalite may trigger the reaction of SiO2 with α- Al2 O3, initiating the nucleation of mullite. The α- Al2 O3 particles act as the hosts for mullite formation and determine the size of the mullite particles. [ABSTRACT FROM AUTHOR]

This work is aimed at producing a dense mullite/MoSi2 composite from a reactive blend of MoO3, SiO2 and Al by under load-combustion synthesis. Thermodynamic analyses proved the development of the entire combustion product in molten state with an adiabatic temperature of 2340K. Diluting the reaction mixture with 25vol.% of the combustion product was necessary to produce homogeneity between the obtained phases. The sample porosity was reduced to ~3.0vol.% by controlling the pressing load, Al particle size and the delay time. The coefficient of thermal expansion of the obtained product was 5.96–8.93×10−6 K−1. The obtained dense composite exhibited good oxidation resistance at 1300°C and a low electrical resistivity (1.80Ω·cm). [ABSTRACT FROM AUTHOR]

It is established that the use of an ammonium fluoride treatment on quartz-pyrophyllite rock (45% pyrophyllite and 53% quartz) at 650°C changes the traditional mechanism by which mullite is synthesized; it is formed not from metapyrophyllite but from fluoridated pyrophyllite, with the formation of aluminum trifluoride and topaz being intermediate stages of the synthesis process. The optimum ratio of quartzpyrophyllite rock to ammonium hydrodifluoride is 1:1.6, which at 1300°C increases the yield of acicular mullite (with particles 20 - 30 μm long and 3 - 5 μm in cross section) from 12 to 68 wt.%. The content of residual quartz in the mullite is no greater than 15 - 18 wt.%. [ABSTRACT FROM AUTHOR]

Fused mullite, a commonly used refractory material has been synthesized and adopted in the present study as reinforcement in the nickel composite coating. The composite has been developed through electrodeposition method. The thermal stability of Ni–mullite composite in terms of microhardness was studied at temperatures up to 800°C and compared with Ni–SiC, a commercialized wear resistant coating. The microhardness of as deposited electroforms was similar for both Ni–mullite and Ni–SiC composites (400Hk). A marginal decrease in the microhardness of Ni–mullite occurred at 600°C while, significant reduction was observed beyond 400°C for Ni–SiC coating. Thus, the Ni–mullite composite has higher thermal stability compared to Ni–SiC. The tribological studies showed that the wear volume loss for Ni–mullite is 2.38×10−5 mm3/m while, of Ni–SiC it is 9.58×10−5 mm3/m under identical conditions. The potentiodynamic polarization and electrochemical impedance showed that the corrosion resistance of Ni–mullite composite is better compared to Ni–SiC. Thus, Ni–mullite is a multifunctional coating. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mullite whiskers on the surface of SiC ceramics prepared by molten salts reactions. [•] SEM images show mullite whiskers with nanometer diameters and micrometer lengths. [•] Chemical reaction process was demonstrated by TG–DSC–MS–FTIR coupling analysis. [ABSTRACT FROM AUTHOR]

Abstract: The effects of CeO2 doping of aluminosilicate on mullite formation and the morphology of mullite whiskers are evaluated herein. CeO2-doped mullite precursors were generated by a sol–gel process. The introduction of CeO2 into the aluminosilicate precursor (3Al2O3·2SiO2) not only lowered the mullite formation temperature, but also promoted the crystallization of mullite grains. The mullitization activation energies calculated based on non-isothermal differential scanning calorimetry (DSC) were 473 and 722kJmol−1 for the 2mol% CeO2-doped and undoped samples, respectively. [Copyright &y& Elsevier]

Abstract: Ceramic plates composed of topologically interlocked osteomorphic blocks with mono- and bimodal pore size distributions are fabricated by combining freeze gelation and sacrificial templating and trialled for sound absorption. These blocks present a concavo-convex geometry that constrains their movement solely by the contact with the neighbors. Our studies show that by using porous osteomorphic blocks, enhanced sound absorption coefficient (α) of 0.97 at 420Hz can be reached, indicating that the gaps between blocks have a significant effect on the sound absorption at frequencies lower than 600Hz, whereas the bimodal porosity of the block material enhances sound absorption for higher frequencies. Indentation tests confirm the advantages of the topologically interlocked structures, such as a spectacularly high flexural compliance, as compared to monolithic parts made from the same material. We concluded that materials design based on topological interlocking leads to an excellent combination of mechanical and sound absorption properties. [Copyright &y& Elsevier]

Porous mullite ceramics with unidirectionally oriented pores were prepared by an extrusion method using rayon fibers as the pore formers. Rayon fibers of 8.1, 9.6, 16.8, and 37.6 μm in diameter were used as the pore formers and were kneaded with alumina powder, kaolin clay, China earthen clay, and water to form pastes. These pastes were extruded into cylindrical tubes, dried, and fired at 1500 °C for 4 h. The apparent porosities ranged from 45.7 to 48.2 %. The pore size distributions showed a sharp peak at 9.4, 10.0, 15.6, and 30 μm with increasing fiber diameters. The height of the capillary rise was 1780, 1670, 1320, and 950 mm with increasing fiber diameter. The maximum capillary rise is much higher than previously reported. The contact angle and effective pore radius that determine the capillary rise ability were calculated by fitting the capillary rise curves using the Fries and Dreyer's equation. [ABSTRACT FROM AUTHOR]

Submicrometric mullite fibers were produced, for the first time, by the solution blow spinning technique, SBS. This new methodology was successful in producing crack free mullite fibers, with mean diameter of approximately 800 nm. Fibrous mats with a high surface area (24 m 2 /g ) after firing at 1000 °C were obtained. Moreover, SBS proved to be a very efficient technique to produce mullite fibers at high output rate as compared to the current techniques used in the production of oxide ceramic submicrometric fibers. This novel method might find application to produce a wide range of ceramic submicrofibrous materials. [ABSTRACT FROM AUTHOR]

The effect of fiber content and sintering temperature on sintering behavior and mechanical properties of mullite fibers/hydroxyapatite composites was studied. The composites were fabricated by hydrothermal synthesis and pressureless sintering. The amount of fibers was varied from 5 wt % to 15 wt % through hydrothermal synthesis, mullite fibers and hydroxyapatite composite powders were subsequently sintered at temperatures of 1150, 1250, and 1350 °C. The composites presented a more perturbed structure by increasing fiber content. Moreover, the composites experienced pore coalescence and exhibited a dense microstructure at elevated temperature. X-ray diffraction indicated that the composites underwent various chemical reactions and generated silicate glasses. The generation of silicate glasses increased the driving force of particle rearrangement and decreased the number of pores, which promoted densification of the composites. Densification typically leads to increased hardness and bending strength. The study proposes a densification mechanism and opens new insights into the sintering properties of these materials. [ABSTRACT FROM AUTHOR]