Mullite Crystals Research Articles

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.

Low water absorption and pyroplastic deformation of alumina‐strengthened porcelain with petalite and Gairome clay

AbstractThe pyroplastic deformation (PD) of porcelain is a key issue for reducing production loss. In this study, material design to realize both a low water absorption (WA) and low PD in alumina‐strengthened porcelain, which have a trade‐off relationship, was proposed. In this porcelain material design, a rigid microstructure composed of entangled needle‐like mullite crystals is formed by the addition of Gairome clay, which has the positive effect of suppressing PD but negatively affects densification. This was balanced by the controlling the amount and viscosity of the liquid phase in the porcelain fired at elevated temperatures with optimized quantities of added petalite and Gairome clay to obtain a low WA and low PD. As a result, a WA below 0.5% and PD index below 1.5 × 10−6 mm−1 in a wide firing temperature range from 1194 to 1384°C were obtained for porcelain containing Amakusa clay at 37.8 mass%, Gairome clay at 17.0 mass%, petalite at 13.7 mass%, and alumina at 31.5 mass%.

In situ synthesis of melt-grown mullite ceramics using directed laser deposition

While mullite is a significant oxide ceramic material, its current preparation process has many limitations such as complicated process, low efficiency, and high cost. In this paper, directed laser deposition (DLD) was used to prepare mullite ceramics directly using α-Al2O3 and SiO2 powder as raw materials. The phase composition, microstructure, and primary defects of prepared samples were investigated by XRD, Raman spectroscopy, and SEM/EDS. Vickers indentation and three-point bending experiments were used to evaluate mechanical properties. The results showed that microstructure was mainly composed of a mullite crystal phase and Si-rich glass phase. It is shown that mullite ceramics were successfully synthesized in situ during the DLD process with the use of Al2O3 and SiO2 powder. Morphology of mullite crystal inside cylindrical sample was particular “tabular cellular,” which transformed into “rod cellular” at the edge. Two mullite crystals were arranged along deposition direction basically, surrounded by the Si-rich glass phase. The defects included pores with various sizes and crack, in which large pores were mainly distributed at the edge of the sample, and small pores were throughout the section. The crack was distributed in the center of the sample and spread to the edge. Mechanical properties test showed flexural strength, microhardness, and fracture toughness were 62.8 ± 30.3 MPa, 12.7 ± 0.34 GPa, and 1.84 ± 0.14 MPa·m1/2, respectively. It is considered that pores and crack were responsible for poor mechanical properties. This research is expected to provide a new option for the rapid preparation of mullite ceramics.

Mullite phase evolution in clay with hydrated or anhydrous AlF3

Porous mullite ceramics having needle-shaped interlocking microstructure have been developed from China clay with hydrated aluminium fluoride (AlF3.3H2O) or anhydrous AlF3. The mullite phase evolution, microstructure and the reaction mechanism in both the systems have been compared. In the presence of anhydrous AlF3, needle-shaped mullite crystals grow mainly by the decomposition of topaz crystals, which are formed during the intermediate stages of sintering. Whereas in the case of hydrated AlF3 samples, the presence of water vapour and HF generated in the reaction zone help in direct nucleation of mullite crystals without the formation of topaz. However, in the powder mixture containing clay and 40 wt.% AlF3.3H2O, the mullite nucleation happens from the topaz decomposition also, resulting in high mullitisation overall. Mullite crystals formed from the 40 wt.% AlF3.3H2O and clay have good needle-shaped interlocking microstructure, and can be used as a better alternative for anhydrous AlF3 to reduce the cost.

Chemical, phase and structural change of mullite synthesized during sintering of kaolin

AbstractMullite is of great technological relevance but rarely occurs in nature and as a result different approaches have been adopted in its synthesis from alumina bearing minerals. In this study, chemical, phase and structural change of mullite synthesized from sintering of natural kaolinite clay is investigated. Thoroughly beneficiated kaolinite clay powder was obtained from Nigeria and uniaxially pressed into cylindrical compact of 40 × 30 mm followed by sintering at temperatures of 1200°C and 1300°C, respectively. The chemical composition, microstructure change, phase transformation, and reaction bonding were carried out using EDXRF, SEM, XRD, and FT‐IR, respectively, to assess the synthesized mullite. The results showed that a well‐dispersed primary mullite phase was obtained which was fully developed at increased temperature of 1300°C. Better mullite phase was also obtained with increasing alumina content at more elevated temperature of 1300°C while Si‐O‐Al bonding of mullite crystals was also obtained from the FT‐IR spectra. However, the needle‐shaped mullite structure was not achieved which might be attributed to the sintering temperatures 1200°C‐1300°C utilized.

Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures

Colloidal silica as nanostructured binder for refractory castables has attracted many attentions in recent years. In the present study, phase composition, microstructure and reactivity evolution of silica gel at different heating conditions were investigated to find suitable system for colloidal silica application. The results showed that atmosphere and carbon slightly affected phase composition of the silica gel at elevated temperatures, and the crystalline phases were composed of major α-cristobalite and minor α-tridymite. The morphology and particle size of the silica gel were greatly affected by atmosphere and carbon during heating. The spherical nano-silica particles with sizes of 40–50 nm rapidly grew into macroscale rod-like particles with temperature increasing from 800-1000 °C to above 1200 °C in air, and sintering of silica particles was observed. However, the size and morphology of the spherical nano-silica particles retained at high temperature in a reducing atmosphere, and many well developed columnar mullite crystals and some SiC whiskers formed on heating silica gel, alumina fines and carbon at 1500 °C, which was due to carbon inclusions retarding the growth of nano-silica particles and the nano silica remained high reactivity at high temperature. Thus, colloidal silica was suitable for application in carbon-containing refractory castables.

Fabrication of mullite nano ceramic through addition of long-chain carbohydrates

Compared to common coarse-grained ceramics, nano-scaled mullite ceramics exhibit unique microstructures and mechanical properties. However, fabrication of homogeneous and ultra-fine ceramic crystals still remains a major challenge. In this study, fine-grain mullite nanocrystals with uniform particle distribution have been successfully synthesized by addition of agarose, which is a long-chain carbohydrate to the precursor. The effect of length of the carbohydrate chain on the size of mullite crystals has been investigated. The results show that long-chain carbohydrate agarose results in smaller grain size of mullite crystals as compared with glucose. Moreover, nano-scaled mullite ceramics with high hardness (18.36 GPa) and fracture toughness values (3.7 MPa m1/2) were obtained owing to the high relative density and small grain size. The present work highlights its potential application as long-chain carbohydrate in producing high-performance nanoceramics.

Ceramic Composition Material with Mullite Crystals Obtained from the Clayy Raw Material of Yakutia

The article deals with the manufacture of modern structural ceramic materials from clay and loam deposits of the Republic of Sakha (Yakutia). The importance and relevance of the development of the production of building materials from local raw materials is emphasized, since this will certainly affect the effectiveness of the construction complex as a whole. The successful development of the construction complex is capable of not only stimulating growth in all sectors of the economy, but also contributes to solving the most pressing social problems. Today, Yakutia has huge reserves of mineral raw materials for the production of a wide range of building materials and products. Of practical interest are wall materials made from clay soils. Given the features of the region’s raw material base, this work focuses on additional processing of traditional material. Controlling the complex physicochemical and structural-mechanical transformations that occur during heat treatment, a methodology has been developed for creating a composite material that will allow competitive innovative materials with enhanced strength properties to be produced with a reinforcing element with a glassy phase matrix of mullite crystals. The fabricated samples have a wide range of physical and mechanical properties and allow using it as a high-quality structural building ceramics, as well as industrial floor technical tile.

Crystallization kinetics and TCV prediction of coal ash slag under slag tapping conditions in an entrained flow gasifier

Slag blockages in the bottom of gasifiers cause unplanned shutdowns and huge losses for syngas utilization downstream of gasification. The sudden viscosity increase by crystal precipitation leads to slag blockage during slag tapping in entrained flow gasifiers. The temperature at which the viscosity abruptly increases is the temperature of critical viscosity (TCV), which is the strict low limit temperature for slag tapping. A comprehensive investigation of slag crystallization kinetics was performed to illustrate TCV in this work. Five slag samples with three different dominant mineral phases that commonly appear in coal ash slag were selected by thermodynamic calculations. The single hot thermocouple technique (SHTT) was applied to analyze the crystallization kinetics based on Avrami and JMA theory. The kinetic analysis indicated that anorthite showed heterogeneous nucleation and grew with a two-dimensional crystalline pattern. The temperature range for anorthite crystallization was approximately 150 °C. Corundum exhibited heterogeneous nucleation and one-dimensional growth pattern. The maximum crystal ratio was 52% for corundum at the temperature of 98 °C. Mullite showed a low Ec value, resulting in a high Ton. Rapid crystallization of mullite was observed at 60 °C due to homogeneous nucleation and two-dimensional crystallization. In this work, TCV was revealed by crystallization kinetics for the first time. The relationship between TCV and the activation energy, maximum crystalline rate, and initial crystallization temperature was built to illustrate the influence of crystallization on TCV. Three equations were used for TCV prediction to guide coal selection and slag tapping in entrained flow gasifiers.

Characterization of High-Temperature Hierarchical Porous Mullite Washcoat Synthesized Using Aluminum Dross and Coal Fly Ash

Mixture of aluminum dross (AD) and coal fly ash (CFA) was used to produce high-temperature porous mullite for washcoat application. CFA is the combustion by-product of pulverized coal in a coal-fired power plant, while AD is a waste product produced in secondary aluminum refining. In this study, 80 wt% of AD and 20 wt% of CFA was used to prepare a mullite precursor (MP) via acid leaching and dry-milling. The precursor was coated on a substrate and subsequently fired at 1500 °C. The results showed that the precursor transformed to a hierarchical porous microstructure assembled by large interlocked acicular mullite crystals. The pore structures consisted of large interconnected open pores and small pores. The specific surface area of the mullite washcoat was 4.85 m2g−1 after heating at 1500 °C for 4 h. The specific surface area was compatible with the specific surface area of other high-temperature washcoats.

Ceramic Composition Material with Mullite Crystals Obtained from the Clayy Raw Material of Yakutia

The article deals with the manufacture of modern structural ceramic materials from clay and loam deposits of the Republic of Sakha (Yakutia). The importance and relevance of the development of the production of building materials from local raw materials is emphasized, since this will certainly affect the effectiveness of the construction complex as a whole. The successful development of the construction complex is capable of not only stimulating growth in all sectors of the economy, but also contributes to solving the most pressing social problems. Today, Yakutia has huge reserves of mineral raw materials for the production of a wide range of building materials and products. Of practical interest are wall materials made from clay soils. Given the features of the region’s raw material base, this work focuses on additional processing of traditional material. Controlling the complex physicochemical and structural-mechanical transformations that occur during heat treatment, a methodology has been developed for creating a composite material that will allow competitive innovative materials with enhanced strength properties to be produced with a reinforcing element with a glassy phase matrix of mullite crystals. The fabricated samples have a wide range of physical and mechanical properties and allow using it as a high-quality structural building ceramics, as well as industrial floor technical tile.

Effects of different aluminum sources on morphologies and properties of ceramic floor tiles from red mud

Ceramic floor tiles with various surface morphologies of mullite were prepared from red mud and kaolin, α-Al2O3, ρ-Al2O3, γ-Al2O3 and Al(OH)3 as alumina sources. AlF3 and V2O5 were used as catalyst to promote the formation of mullite crystals at low temperature. Influence of Al2O3 crystal types and sintering temperature on microstructure and development of mullite, and performance of ceramic floor tiles were investigated. Results indicate that both the abilities of α-Al2O3, ρ-Al2O3, γ-Al2O3 and Al(OH)3 reacting with SiO2 and the reactivities of them increase successively, therefore the amount of generated needle-like mullite rise in that order. The better dispersion of acicular mullite throughout the ceramic matrix could significantly improve the properties of the ceramics. The ceramic floor tile with Al(OH)3 sintered at 1230 ℃ was found to possess the optimal performance. Under this condition, bulk density, flexural strength, porosity and water absorption of the ceramic floor tile were 1.83 g/cm3, 185.46 MPa, 18.56% and 7.37% respectively, meeting the requirements of building material. The results of this study will improve our understanding of recycling red mud for the fabrication of ceramic materials.

Fabrication and assessment of reinforced ceramic electrical insulator from bamboo leaf ash waste

This is an attempt to study the effect of bamboo leaf ash (BLA) substitution on quartz in the ceramic insulator sintered at 1250 °C for 2 h. The Physico-mechanical properties, structural, morphological and dielectric characteristics of the ceramic samples were examined by using X-ray diffraction (XRD), Differential thermal analysis (DTA), Transform Infrared spectroscopy (FTIR) and Scanning Electron Microscope (SEM) and dielectric measurement techniques respectively. The study revealed that 10% BLA replacement was better in physico-mechanical performance than the commercial ceramic insulator attributed to the increase of secondary mullite crystals in the BLA reinforced treated sample. Also the incorporation of BLA showed no harmful effect in the ceramic matrix.

Influence of type and amount of a quartz­containing additives on the properties of mullite­corundum concrete and samples from it

An influence of type and amount of a quartz­containing additive (silica fume and pulverized silica) on the properties of concrete and samples from it before and after thermal treatment was investigated. It has been established that, the partial replacement of expensive high­alumina cement in the concrete with the quartz­containing additive improves a flowability of concrete and provides an increase in the strength of concrete samples. Optimal was the addition of domestic pulverized sili-ca in an amount of 2 wt. %.
 The compositions of mullite­corundum and mullite­siliceous mixtures with the pulverized silica addition in the optimal amount and a reduced content of high alumina cement (in comparison with the mullite­corundum concrete manufactured by JSC “URIR named after A. S. Berezhnoy”) have been developed.
 Concrete samples from mullite­corundum concrete of the developed composition with the pulverized silica addition were characterized by a higher cold crushing strength after 3 days of air hardening and thermal treatment at 110 and 1400 C (87 and 108 N/mm2, respectively) compared with samples from mullite­corundum concrete without the quartz­containing additive (52 and 70 N/mm2).
 Petrographic studies have established that, the introduction of quartz­containing additives in the optimum amount in the concrete composition contributes to better sintering of concrete. In the presence of quartz­containing additives in the binder mass of concrete samples, crystallization of mullite and calcium hexaaluminate was intensified, as well as anorthite formation.
 The production of mullite­corundum concrete with the quartz­containing additive (pulverized quartz) has been mastered at JSC “URIR named after A. S. Berezhnoy”.

High temperature phase transition behavior of schorl particles modified with rare earth

In this study, rare earth/schorl composites were prepared using co-precipitation technique with Xinjiang schorl and cerium nitrate as raw materials. The phase transitions and the lattice parameters were investigated simultaneously during the heat treatment process utilizing a high temperature X-ray diffraction. The microstructures of the composites treated at 800, 850, 900 and 950 °C were characterized by scanning electron microscope, transmission electron microscope and high resolution transmission electron microscope. The results clearly indicate that the cerium ions exchanged the interfacial structure of schorl particles, which led to the reduction in phase transition temperature. The interfacial migration and mass transfer were promoted during the phase transformation, rendering a better transformation of mullite crystals from schorl.

Microarc oxidation of pure aluminium in alumina containing electrolytes

ABSTRACT In this study, oxide coatings produced on commercially pure Al via 40-min microarc oxidation process in electrolytes containing Na2SiO3, KOH and α-Al2O3 powder were characterized by SEM-EDS, profilometry, XRD and microhardness tests. The addition of α-Al2O3 particles into the electrolyte resulted in increased coating thickness and roughness. The coating thickness values of the samples varied between 35 and 135 µm. Increasing the amount of α-Al2O3 particles in the electrolyte resulted in mechanical deposition/encapsulation of α-Al2O3 particles and/or chemical reaction with constituents of the coatings. This chemical reaction increased the microhardness of the coatings and resulted in preferential growth of mullite (3Al2O3.2SiO2) crystal in the outer glassy region of the coatings. Outer regions of the coatings comprised 3Al2O3.2SiO2, γ-Al2O3 and κ-Al2O3 phases with chemically or mechanically bonded α-Al2O3 particles, and in the inner regions were mostly γ-Al2O3 and κ-Al2O3 phases along with α-Al2O3 phase formed through phase transformations.

Effect of alumina bubble modification on properties of mullite castables

Mullite castables are widely used in the hot metal pretreatment processing equipment. In this work, the effects of modification of alumina bubbles with silica sol on the properties of mullite castables were studied. After the alumina bubbles were modified by silica sol, the bulk density of the samples was slightly increased, and the apparent porosity was decreased. The cold modulus of rupture values of experimental castables increased remarkably after drying at 110 °C and heating at 1400 °C for 3 h. The thermal expansion of the samples was obviously increased, while the thermal conductivity of the samples was decreased significantly. More columnar mullite crystals were formed at the alumina bubble/matrix interface after the alumina bubble modification and form a continuous inter-locking network structure, which was helpful to improve castables’ thermal shock resistance.

Preparation and Characterization of An All Oxide Ceramic Matrix Composite by A Novel Method

Due to the characteristics of high temperature oxidation resistance and high reliability, all oxide composites are ideal materials for components in hot end of aeronautics and astronautics. In this paper, all oxide ceramic matrix composite(CMC) was prepared by a novel method which combined warm pressing of prepreg with PIP using polysilialumoxane as the precursor of mullite. The morphology, structure and mechanical properties of the all oxide CMC were characterized. The results show that polysilialumoxane with a ceramic yield of 53wt%can be completely changed to ceramic at 1200°C and form mullite crystals. The composite with alumina and mullite matrix derived from alumina powder slurry indicated the best mechanical properties, the flexural strength and fracture toughness of the composite reach 208.3Mpa and 128.6Mpa respectively, and the pull-out effect of the fibers is obvious in the process of tensile fracture, and the composites show ductile fracture. whereas the mechanical property decreases significantly after heating at 1200°C for100 hours due to the densification of matrix and abnormal growth of alumina grains in fibers.

Preparation of Cordierite-mullite Ceramics for Solar Thermal Storage

We developed cordierite-mullite composite ceramic materials to package and encapsulate PCM, and presented a preparation process from raw materials of kaolin, talc and alumina. The properties and microstructre of cordierite-mullite composite ceramic were studied. Due to the strengthening effects of mullite, the sample C2 (80 wt% of cordierite and 20 wt % of mullite) sintered at 1 420 °C possessed excellent physical properties. Determined by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis, cuboid-shaped cordierite crystals and needle-like or long quadrilateral-prism mullite crystals with staggered patterns were found, which endowed the composites preferable mechanical strength. After 30 cycles of thermal shock (room temperature to about 1 100 °C, air-cooled), the sample presented superior thermal shock resistance, which is suitable to be applied as solar thermal storage materials.

Influence of dispersing agents on microstructure-related properties of slip cast cordierite ceramics

Abstract This study aims to reveal the effects of the concentrations of solids and dispersing agents on the rheological properties of the suspensions prepared from multicomponent cordierite precursor powder system and on the final properties of the cordierite bodies consolidated by slip casting. For this purpose, the suspensions having solid loadings of 40, 45, and 50 wt% were prepared. In these suspensions, Darvan 821A and Dolapix CE-64 were used as dispersants. The dispersant-induced changes in the properties of both casting suspensions and their slip-cast bodies were characterised. The results indicate that Dolapix CE-64 had less impact on reducing the shear stress values compared to the Darvan series, and thus Darvan 821A rendered suspensions with lower shear stress values than observed with Dolapix CE-64. Both dispersants also caused distinct effects on the crystallinity, secondary phase formation, and porosity, as well as on the physico-mechanical properties. Dolapix CE-64 provided good physico-mechanical properties, which may be attributed to the formation of clustered and interlocked mullite crystals within the cordierite glass matrix. Furthermore, the results revealed that Darvan 821A confers higher thermal conductivities to the cordierite ceramics, while Dolapix CE-64 favours the sintered density and enhances the dielectric constant and dielectric loss tangent values. Consequently, it was found that the slip-cast bodies prepared from the suspensions dispersed with the different type of dispersants exhibit different microstructure-related properties.