Mullite Crystals Research Articles

Polymer templates effects on microstructure and mechanical properties of electrospun mullite nanofibers

Polymer templates are important additives for electrospun ceramic nanofibers and have strong relationship to its properties. In this study, polyvinylpyrrolidone (PVP) and polyvinyl butyral (PVB) were used as templates to prepare mullite nanofibers. The morphology, microstructure and mechanical properties of the nanofibers were investigated and compared. Results show that after 1000 °C calcination, fibers prepared using PVB had a porous structure with surface area of 40.11 m2/g, and were composed of mullite crystals with elastic modulus of ∼27.5 GPa. However, the fibers prepared using PVP had a dense structure with surface area of 8.14 m2/g, and were composed of γ-Al2O3 and mullite crystals with a higher elastic modulus of ∼79.4 GPa. The influence mechanism was confirmed that the different binding states of polymer and aluminasilica precursor were the main reason for the fiber differences. These results give a reference for polymer template categories selection for preparing mullite nanofibers applied in different areas like filtration, catalysis and insulation.

Microstructure evolution and mechanical behavior of a mullite fiber heat‐treated from 1100 to 1300°C

AbstractIn this paper, the effect of phase transformation on microstructure evolution and mechanical behaviors of mullite fibers was well investigated from 1100 to 1300°C. In such a narrow temperature range, the microstructure and mechanical properties showed great changes, which were significant to be studied. The temperature of the alumina phase transformation started at below 1100°C. The main phases in fibers were γ‐Al2O3 and δ‐Al2O3 with amorphous SiO2 at 1150°C. The stable α‐Al2O3 formed at 1200°C. Then the mullite phase reaction occurred. As the alumina phase reaction took place, the tensile strength increased with the increasing temperature. In particular, the filaments achieved the highest strength at 1150°C with 1.98 ± 0.17 GPa, and the Young's modulus was 163.08 ± 4.69 GPa, showing excellent mechanical performance. After 1200°C, the mullite phase reaction went on with the crystallization of orthorhombic mullite. The density of surface defects increased rapidly due to thermal grooving, which led to mechanical properties degrade sharply. The strength at 1200°C was 1.01 ± 0.15 GPa with a strength retention of 63.13%, and the Young's modulus was 184.14 ± 10.36 GPa. While at 1300°C, the tensile strength was 0.64 ± 0.14 GPa with a strength retention of only 40.00%.

Preparation of porous mullite ceramics consisting of needle-like mullite whiskers

ABSTRACT Two steps were involved in the preparation of the porous mullite ceramics: preparation of porous green body by foaming method combined with gelcasting, using alumina, silica and AlF3 as raw materials and synthesis of needle-like mullite whiskers by vapor-phase reaction. Owing to the special pore structure of foaming method, the needle-like mullite crystals had enough space to develop with good structure. Most of the needle-like mullite crystals have diameters around 1.5 μm and lengths of >80 μm. By adjusting the amount of foaming agent, the open porosity varied from 66.3 to 85.5%, and the compressive strength and flexural strength decreased evidently, from 23.4 ± 1.8 to 4.9 ± 0.4 MPa and 9.7 ± 0.9 to 1.0 ± 0.1 MPa.

Factors Influencing Mullite Formation in Refractory Grog

The mineral mullite has many outstanding refractory properties. The content of aluminum oxide in the raw material is the basic factor influencing the content of mullite in the final product. Depending on Al2O3 content, silica-alumina materials can be divided into fireclay and high-alumina with the Al2O3 content of 45 % and more. The study describes the influence of raw material, the influence of homogenization of raw material and influence of firing temperature on the mullite formation in fireclay refractory material. Three kinds of refractory clay with different chemical composition were used as raw material for fireclay grog production. Three kinds of homogenization and three different temperatures were studied as factors influencing mullite formation. Powder X-ray diffraction was used to determine mullite content in material. Quantitative phase analysis was conducted by the Rietveld method. Mullite crystals morphology was observed by scanning electron microscopy.

Compound Fabry-Pérot interferometer for simultaneous high-pressure and high-temperature measurement.

We have proposed and experimentally demonstrated a sapphire-derived fiber (SDF) and silica capillary-based compound Fabry-Pérot interferometer (FPI) for high-pressure and high-temperature sensing. The SDF owns high alumina dopant concentration core, which can generate a mullite crystallization region during an arc discharge process. The crystallization region acts as a reflective interface to form one FPI in the SDF. The other FPI contains an air cavity constructed by the silica capillary and is used for high-pressure sensing. Both gas pressure within a range from 0 MPa to 4 MPa and temperature within a range from 20°C to 700°C are measured. Experimental results show that the wavelength shift of the FPI versus the applied pressure is linear at each tested temperature. The pressure sensitivity is measured to be 5.19 nm/MPa at a high temperature of 700°C, and the linear responses show excellent repeatability with linearity of 0.999. Meanwhile, the proposed FPI can stably function at a high temperature of 700°C with a temperature sensitivity of 0.013 nm/°C. The proposed FPI sensor provides a promising candidate for simultaneous measurement of high pressure and high temperature in extreme conditions.

Advanced Al2O3–SiC–SiO2–C refractories with B2O3 addition

This study investigates the effect of B2O3 addition (0.4 wt%, 0.8 wt%, 1.2 wt%, and 1.6 wt%) on the properties and microstructure of Al2O3–SiC–SiO2–C (ASSC) refractories. The results indicate that B2O3 plays an important role in accelerating mullite formation at low temperature. The substitution with Si by B in mullite crystal resulted in the formation of short and strong B–O bonds after firing at 1450 °C, causing the formation of boron-doped mullite with a smaller cell volume and leading to a decrease in the overall expansion of ASSC refractories. The addition of B2O3 to the ASSC refractories accelerated the growth of mullite along the direction parallel to the c axis, resulting in the improvement of mullite morphology (needle-like). The presence of in situ boron-doped mullite allowed the performance improvement of this class of refractory materials. The mechanical properties of the specimens improved significantly at various thermal-treatment temperatures. Moreover, the addition of B2O3 was beneficial for increasing the bulk density of the specimens, owing to the formation of a boron-rich liquid phase, which could accelerate sintering and densification. The optimal amount of B2O3 added in the specimens was <1.2 wt%, as above this value, an excessive liquid phase was formed, which degraded the high temperature performance.

Effects of sintering temperature on microstructure and properties of low‐grade bauxite‐based ceramic proppant

AbstractLow density and high strength ceramic proppant was prepared by sintering high aluminium type low‐grade bauxite and high iron type low‐grade bauxite at the temperature range from 1300℃ to 1360℃. The phase composition and micromorphology of ceramic proppant were, respectively, characterized by X‐ray diffraction (XRD) and scanning electron microscope (SEM). The bulk density, apparent density, and breakage ratio of ceramic proppant were tested using standard methods. The results show that mullite crystal and corundum crystal are fully developed, the spatial network structure of rod‐shaped mullite is formed, and corundum crystal is evenly distributed in the sample when the sintering temperature is 1340℃. At this temperature, the bulk density of the sample is 1.42 g/cm3, the apparent density is 2.67 g/cm3, and the breakage ratio is 5.1% under the closure pressure of 52 MPa. Then, the growth mechanism of mullite crystal in ceramic proppant was explored. The results reveal that the mullite crystal in ceramic proppant conforms to the layer growth theory. The secondary crystal nucleus is formed in the growth process and shows a step growth mechanism. With the increase in sintering temperature, the preferred orientation growth of mullite crystal finally forms needle‐like morphology.

Effects of zirconium source and content on zirconia crystal form, microstructure and mechanical properties of ZTM ceramics

ZTM ceramics were successfully derived from coal gangue. The effect of zirconium source (ZrO2 and ZrOCl2∙8H2O) and content on properties of the ZTM ceramics has been studied. The phase composition, density, and microscopic morphology were characterized with X-ray diffraction (XRD), Archimedes method and scanning electron microscopy (SEM). The influence of zirconium source, sintering temperature, zirconia content on flexural properties and fracture toughness was studied. The sample, with ZrOCl2∙8H2O added 12% zirconia (Z2TM12) sintered at 1475 °C for 3 h has the highest density of 2.83 g/cm3. Partially stable t-ZrO2 was present in samples with zirconium oxychloride (ZrOCl2∙8H2O) as the zirconium source, due to the constraints of mullite crystals. Therefore, Z2TM12 had both microcrack toughening and stress phase transformation toughening mechanisms. The flexural strength was increased from 162.40 MPa to 285.04 MPa, while the fracture toughness was improved to 3.55 MPa m1/2 from 2.38 MPa m1/2. Our achievement can be used as a reference to fabricate ZTM ceramics from coal gangue with high-value additions.

Shaping quality, microstructure, and mechanical properties of melt-grown mullite ceramics by directed laser deposition

Directed laser deposition (DLD) has demonstrated the potential for rapid preparing high-performance melt-grown ceramics applied for the aero-engine hot section components. Mullite has excellent potential for applications in harsh environments and extreme conditions. However, there are defects (pores and cracks) in melt-grown mullite ceramics (MGMC), limiting its application, related to unclear processes, microstructure, and properties. In this work, we investigated the impact of z-increment on shaping quality, microstructure, and mechanical properties. The results reveal that oxygen vacancy concentration is responsible for the variety of sample colors. Low z-increment is beneficial to decreasing porosity, while high z-increment is beneficial to crack suppression. The formation and propagation mechanism of internal cracks at low z-increment are discussed in detail. Mullite crystals in the sample center are "tabular cellular", which are 2:1 high-alumina mullite. While mullite crystals at the sample edge are 3:2 mullite, undergo four stages as z-increment increase; "tabular cellular", "facet dendrite", "rod cellular", and mixed crystal. This evolution of crystal morphology is attributed to the decrease in temperature gradient and the increase of supersaturation. Besides, the flexural strength and microhardness increase first and then decrease with the rise of z-increment, which is the function of defects, compactness, and microstructure.

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.

Mineral and textural transformations in mixtures of Al-rich and Al–K-rich clays with firing: Refractory potential of the fired products

The present work analyses a set of cylinders manufactured by pressing and fired from 1000 °C to 1270 °C, which are composed of pure Al-rich rocks (bauxite) and mixtures of bauxite with Al–K-rich clays. The aim of the study is to determine the mineral and textural transformations that take place in the mixtures with firing and their influence on the physical properties of the final products in order to evaluate their refractory potential by comparing them with various fired commercial bauxites. To this end, raw and fired samples were analysed by X-ray diffraction, X-ray fluorescence, and optical and field emission scanning electron microscopy, and significant physical properties (e.g. density, linear shrinkage, porosity, colour, water absorption, thermal conductivity and point load resistance) were determined in the fired cylinders. The raw bauxite was mainly formed by boehmite, kaolinite and hematite whereas the illite- and kaolinite-rich clay was formed by quartz, illite and kaolinite. Kaolinite, illite and boehmite are not detected at 1000 °C and, from this temperature upwards, mullite, corundum, γ-Al2O3, hercynite, ilmenite, cristobalite and vitreous phase are formed. Changes in physical properties with rising temperature are associated with the mineralogical changes. The increases in density and linear shrinkage are related to the formation of vitreous phase and the crystallization of mullite, and correlate with the decrease in porosity, water absorption and thermal conductivity. The changes in the colour of the samples are related to the hematite content at each temperature, whereas the point load resistance is greater with higher corundum content. Both the firing temperature and the clay content play an important role in the refractory potential of the mixtures, since samples mixed with illite- and kaolinite-rich clay present similar properties at lower temperatures to those of the fired commercial bauxites.

Substitution of quartz and clay with fly ash in the production of architectural ceramics: A mechanistic study

Quartz and clay are substituted gradually by fly ash using a triaxial ceramic formulation under simulated industrial conditions and the effects of fly ash substitution on the macroscopic properties and microstructures of the sintered ceramics are evaluated systematically. With the substitution of 35 wt% (1250 °C), the ceramic sample exhibited optimal properties, including linear shrinkage of 15.61%, bulk density of 2.39 g cm-3, water absorption of 0.62% and flexural strength of 41.70 MPa, due to the accelerated densification and fly ash-spurred needle-shaped mullite. The microstructure analysis shows that the sintered matrix consists of three types of particles, quartz-, clay- and feldspar-like particles showing sintering behavior with respect to filling the glassy matrix with preserved morphology, precipitating mullite crystals, and fusing with the surrounding glassy matrix, respectively. The strength of the fly ash - containing ceramics is analyzed by the dispersion-strengthening mechanism and porosity and the results indicate that the fly ash particles affect the mechanical strength due to Griffith flaws when the total porosity is less than 25% and pores at higher total porosity. This study provides a viable strategy to recycle industrial fly ash in the production of architectural ceramics.

Study of the Influence of Nanotechnogenic Raw Materials and Pyrophyllite on the Orientation of Clay Particles and the Water Permeability of Acid-Resistant Materials

The use of nanotechnogenic alumina-containing raw material and pyrophyllite in the compositions of ceramic bodies for the production of acid-resistant materials promotes reduction of the anisotropy of shrinkage during drying and firing and water permeability but in terms of water permeability the samples with nanotechnogenic aluminum-containing raw material have the best metrics. The introduction of pyrophyllite into the composition of ceramic bodies for the production of acid-resistant materials promotes formation of a significant accumulation of well-developed needle-shaped crystals of mullite in the samples and the introduction of nanotechnogenic raw material promotes the formation of filamentary crystals of mullite.

Vysokoteplotní fázové transformace kaolinitu v závislosti na krystalinitě

Although kaolinite is one the most important industrial minerals, the processes of its transformation to mullite have not been completely explained so far. The study is focused on kaolinite crystallinity calculation and its effect on high-temperature phases transitions in the series kaolinite-mullite. Samples of purified natural kaolins from several sites were analysed using X-ray diffraction (XRD). Besides the determination of the complex mineral composition, kaolinite crystallite size was calculated from XRD data by the Rietveld method, Scherrer equation and using the Hinckley crystallinity index. Thermal analysis (DSC/TG) was used as the principal approach to examine endothermic and exothermic effects of kaolinite transformations. The course and maximum temperatures of the observed effects were correlated with the original crystallite size of kaolinite. Two samples with different kaolinite crystallinity were also analysed by high-temperature X-ray diffraction (ht-XRD) to study the formation of mullite. Scanning electron microscope (SEM) was used to visualize morphology of kaolinite.It was found out that the original crystallinity of kaolinite affects all three examined processes-kaolinite dehydroxylation, formation of crystalline phases from metakaolinite and development of mullite crystal structure. Dehydroxylation of samples with higher kaolinite crystallinity takes place at higher temperatures. Similar effect applies for the reaction(-s) at the temperature about 980 °C observed at heat flow curve where crystallization of spinel type phase and mullite with very low crystallinity occurs. Broadening of FWHM of the exothermic effect points to decreasing kaolinite crystallinity. Crystallization of mullite exhibits different dependence on kaolinite crystallinity than the previous processes. The results show that mullite with larger crystallite size develops faster from kaolinite of low crystallinity and vice versa.

Oxidation Kinetics of Soot on Acicular Mullite Membrane Filter Using Electron Microscopy and Thermogravimetric Analysis

This paper investigates the oxidation kinetics of carbon black on acicular mullite using thermogravimetric (TGA) and mullite microstructures by scanning electron microscopy (SEM) for the development of diesel particulate filters (DPFs). It is observed that the amount of each chemical composition strongly affects the structure of mullite. The addition of AlF3 and V2O5 to mullite promotes the growth of needle-shaped mullite crystals. Thermogravimetric analysis was used to investigate and characterize chemical kinetics of soot oxidation for better understanding of designs and configurations of diesel particulate filters. The mass conversion of soot on the acicular mullite (ACM) is oxidized faster than that on the mullite (ML) membrane at all temperatures examined. The calculation of apparent activation energy (Ea) of soot oxidation with isothermal methods on mullite was presented. The results showed that activation energy of soot oxidation is enhanced with ACM than with ML at all temperatures examined. The average calculated apparent activation energy of soot oxidation on ACM and ML are 146.4 kJ/mole and 155.3 kJ/mole, respectively.

Development and evolution of a novel (Zr1-xSnx)O2 toughened alumina-mullite slip cast refractory: Effect of SnO2

Zr1-xSnxO2 reinforced alumina-mullite refractory was manufactured by slip cast methods using SnO2 as a sintering agent. SnO2 had considerable influence in the enhanced reaction kinetics of zircon dissociation and subsequent reaction sintering of alumina and zircon. Presence of pro-eutectic transient Al2O3-SnO2 liquid and SnO2-SiO2-ZrO2 amorphous phases, enhanced densification and mullitisation through liquid phase sintering and, mitigated SnO2 volatilisation by (Zr1-xSnx)O2 solid-solution formation. Based on their morphology and aspect ratios, three types of mullite crystals, MI, MII and MIII, were evolved across the matrix microstructure. Characterization of the evolved microstructure revealed coalescence and grain growth of matrix alumina grains including the presence of an acicular tertiary MIII mullite which acts as both a reinforcement and bridging network of matrix to aggregate grains. SnO2 had the effect of lowering the monoclinic – tetragonal phase transformation temperature. Hot flexural strength values remained nearly unchanged (±2%) compared with the AZS composition without SnO2 doping.

A novel monolithic mullite microfiltration membrane for oil-in-water emulsion separation

Monolithic ceramic microfiltration membranes made up of an interlocking network of mullite crystals have been developed from low-cost raw materials, consisting of China clay, AlF3·3H2O and Al2O3. The characteristics of the developed membranes have been studied. The membrane made from clay+40 wt% AlF3·3H2O powder mixture with 10 wt% Al2O3 displayed excellent properties, with apparent porosity of 64%, median pore diameter of 0.3 μm, flexural strength of 43 MPa, and pure water permeance of 1031 L/m2 h bar. The membrane also displayed excellent pore connectivity and chemical stability owing to the crosslinked needle-like mullite microstructure. The performance of the mullite microfiltration (MF) membrane in oil-in-water emulsion separation was studied with three test solutions having oil concentrations of 200 mg/L, 500 mg/L and 1000 mg/L. The membrane showed excellent permeate flux of more than 200 L/m2. h at a transmembrane pressure of 2 bar and high oil rejection of more than 96% with all the feed concentrations. The fabrication of the monolithic mullite MF membrane in comparison to the conventional asymmetric ceramic MF membranes is highly economical and can be used as a cost-effective alternative for oil-in-water emulsion separation.

Synthesis and characterization of sol-gel derived monophasic mullite powder

Abstract High-purity microfine mullite precursor powder of stoichiometric chemical composition, 3Al2O3.2SiO2, was synthesized through the sol-gel route using aluminum isopropoxide and tetraethyl orthosilicate. The derived mullite precursor powder was characterized by BET surface area, particle size distribution, Fourier-transform infrared spectroscopy, thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy. Phase analysis of the precursor powder and calcined samples at different temperatures was done by XRD. Non-isothermal crystallization kinetics of mullite precursor was studied by TG-DTA in static air using heating rates of 5.0, 7.5, 10.0, 12.5, and 15 K.min-1. Grain morphology was studied by SEM. The results showed that synthesized mullite precursor powder possessed homogeneity of Al and Si components, and the amorphous precursor powder was converted to monophasic mullite crystal when heat-treated at 970 °C. Based on Flynn-Wall-Ozawa and Kissinger equations, the activation energy associated with the crystallization of mullite was determined to be 1189.8 and 1189.0 kJ.mol-1, respectively.

Pressure-less joining of SiCf/SiC composites by Y2O3–Al2O3–SiO2 glass: Microstructure and properties

In this study, Y2O3–Al2O3–SiO2 (YAS) glass was prepared from Y2O3, Al2O3, and SiO2 micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 × 10−6, matching-well with that of SiCf/SiC composites. SiCf/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiCf/SiC composite. The optimal brazing parameter was recorded as 1400 °C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 °C was 35.6 MPa, equivalent to 69% of that without thermal shock.

Influence of heating temperatures on structure and microstructure of chamotte–carbon composites

The reaction between chamotte and active carbon at high temperature and low oxygen partial pressure conditions has been investigated. Both components react at temperatures above 1400°C where cristobalite is reduced by the carbon while increases the mullite content. At 1600°C mullite tends to disappear due to the reduction reaction leading to the formation of alumina. The reaction between chamotte and active carbon in these conditions produces the reduction of silica and mullite. This process increases with the reaction time although not all the active carbon is consumed. The graphite nanodomain size of the active carbon tends to decrease with respect to the original size with both the temperature and the reaction time. The microstructures of the obtained materials present a glassy phase due to the inherent presence of impurities in chamotte, and where small particles, pores and mullite needles are also observed. The presence of this glassy phase becomes more pronounced with the treating temperature while solid particles and pores tend to disappear, moreover, the formation of the glassy phase occurs in a less extent due to the higher concentration of active carbon. The presence of the liquid glassy phase also favours the formation of the long-needle like mullite crystals.