Mullite Phase Research Articles

Growth mechanism of in situ diamond-shaped mullite platelets and their effect on the properties of Al2O3–SiC–C refractories

Abstract The microstructure evolution and properties of Al 2 O 3 –SiC–C refractories with different Fe 2 O 3 content (0, 0.5, 1.0 wt%) heat-treated at 1400 °C were investigated. Also, the in-situ growth mechanism of diamond-shaped mullite platelets was discussed. The experimental results indicate that there were only mullite and SiC whiskers formed in the absence of the additive. However, both whisker-shaped and diamond-shaped mullite phases could be generated aside from SiC whiskers, which were satisfactorily correlated with Fe 2 O 3 addition. Interestingly, the introduction of Fe 2 O 3 changed the morphology of mullite whiskers from the curved structure (without additive) to the straight and elongated shape (with additive). Furthermore, these two-dimensional mullite platelets were better developed resulting in the larger size and higher yield with 1.0 wt% Fe 2 O 3 . XRD, SEM, TEM and HRTEM analyses reveal that with the addition of Fe 2 O 3 , mullite phases in refractory samples grew preferentially toward the crystal facets along (130) and (4 2 − 0) directions of mullite nanocrystals, thus, a diamond-shaped structure was generated. As the well-developed mullite platelets were formed, the cold modulus of rupture (CMOR) and the cold crushing strength (CCS) increased from 6.5 MPa to 10.3 MPa and from 38.7 MPa to 55.7 MPa, respectively. Meanwhile, after three thermal shock cycles, the CCS of samples containing 1.0 wt% Fe 2 O 3 only decreased by 2.6 MPa, and the residual strength ratio of CCS (CCSst) reached the maximum value of 95.3%. These results suggest that the in situ synthesized platelet-shaped mullite phase observably improved the thermal shock resistance of Al 2 O 3 –SiC–C refractories.

Synthesis of nanocrystalline photoluminescent mullite using sacrificial cotton wool and filter paper templates

AbstractNanocrystalline mullite has been synthesized by sintering of absorbed monophasic aluminosilicate precursors into cotton wool and filter paper cellulosic structures. Effects of molybdenum and tungsten doping on template mediated mullite at 1200°C and 1400°C have been investigated using differential thermal analysis (DTA), X‐ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron micrographs (FESEM). Microstructures of the sintered samples revealed successful replication of cellulosic structure with the formation of dense mullite phases having grain diameter in the range of 50‐80 nm. Strong photoluminescence emissions in the UV and visible regions result from the oxygen vacancies and Al–O bonds present in mullite structures.

B4C mineralizing role for mullite generation in Al2O3-SiO2 refractory castables

Abstract Based on refractory end-users’ requirements, continuous efforts have been made to design engineered products able to withstand high temperatures (800–1500 °C) and severe thermal gradients. One alternative to enhance the mechanical properties of alumina-based compositions consists of inducing in situ generation of phases with platelet or acicular shape within their matrix fraction, which may improve crack deflection and grain bridging mechanisms. Mullite and Al 18 B 4 O 33 are some compounds that present such interesting features. Thus, this work addresses the evaluation of alumina refractory castables bonded with SioxX-Zero and/or microsilica, containing 0 or 1 wt% of B 4 C (sintering additive), aiming to: (i) induce transient liquid sintering, (ii) point out which silica source could favor a more effective mullite formation at high temperatures, and (iii) analyze the influence of B 4 C in the phase transformation and thermo-mechanical properties of the designed compositions. Comparing SioxX-Zero and microsilica-bonded refractories, the latter showed more likelihood to give rise to the mullite phase during the samples’ thermal treatments. Moreover, adding B 4 C to the castables containing 3 wt% of SiO 2 induced the generation of a boron-rich liquid phase with transient features during the samples’ firing step, favoring earlier sintering and faster mullite and Al 18 B 4 O 33 formation. These transformations resulted in refractories with enhanced creep, thermal shock resistance and HMOR behavior in a broader temperature range (600–1550 °C), which may enable them to be used in various industrial applications (petrochemical, steel-making, etc.).

Al2O3\u2013TiO2/ZrO2\u2013SiO2 based porous ceramics from particle-stabilized wet foam

The porous ceramics based on Al2O3–TiO2/ZrO2–SiO2 from particle-stabilized wet foam by direct foaming were discussed. The initial Al2O3–TiO2 suspension was prepared by adding TiO2 suspension to partially hydrophobized colloidal Al2O3 suspension with equimolar amount, to form Al2TiO5 on sintering. The secondary ZrO2–SiO2 suspension was prepared using the equimolar composition, and to obtain ZrSiO4, ZrTiO4, and mullite phases in the sintered samples, the secondary suspension was blended into the initial suspension at 0, 10, 20, 30, and 50 vol%. The wet foam exhibited an air content up to 87%, Laplace pressure from 1.38 to 2.23 mPa, and higher adsorption free energy at the interface of approximately 5.8×108 to 7.5×108 J resulting an outstanding foam stability of 87%. The final suspension was foamed, and the wet foam was sintered from 1400 to 1600 °C for 1 h. The porous ceramics with pore size from 150 to 400 μm on average were obtained. The phase identification was accomplished using X-ray diffraction (XRD), differential thermal analysis (DTA), and thermogravimetric analysis (TGA), and microstructural analysis was performed using field emission scanning electron microscopy (FESEM).

Heat-Resistant Porous Composite Based on Ordinary Clay for Thermal Insulation of Hot Surfaces

Compositions of porous compounds (density 300 to 500 kg/m3) for thermal insulation of hot surfaces up to 1300oC were developed. Ordinary clay (at a clinkering temperature of 1150...1200oC), was chosen as a main substance for structure formation. The increasing thermo-resistance fraction of a ceramic matrix composite was obtained via embedding a refractory clay and dinas fillers into compositionб as well as by means of optimizing the relationship «ordinary clay – high aluminous cement», for which the formation of additional amount of mullite phase is observed when first heating of material.

Crystallization of mullite from kaolin according to optical spectroscopy of impurity ions

The luminescence spectra of Cr3+ and Eu3+ ions embedded in mullite ceramic samples are studied. The samples were annealed at temperatures from 600 to 1200°С. Beginning at a temperature of 890°С, the spectra demonstrated the presence of a newly formed crystal phase. At temperatures from 920 to 1200°С, the crystalline form manifested itself as a stable mullite phase. A temperature of 920°С corresponded to a sharp change in the symmetry of the crystal field around Eu3+ (phase transition). The parameters of the spectral lines of ions were used to estimate the mullite crystal-lattice disorder and the residual deformations in ceramic samples. The spread of the mullite crystal-lattice parameter is determined as |Δа/а| ≈ 0.028 and is independent of the annealing temperature.

The relation of pyroplastic deformation and liquid viscosity in vitreous ceramics

Experiments were conducted to study the relation of pyroplastic deformation and liquid viscosity in vitreous ceramics. The pyroplastic deformation was researched through experimentation in a muffle furnace. The quenching method was introduced to gain the chemical composition of liquid phase in the sample. The experiment was carried out in a tube furnace. The Riboud model was selected to calculate the liquid viscosities based on the comprehensive consideration of different viscosity models. Generally, the small viscosity value of liquid phase in the sample at high temperature accelerates the deformation of the sample. On the other hand, the deformation is retarded if the viscosity value of the generated liquid phase is thicker than 2 × 104 Pa s. The chemical composition of the sample and the secondary mullite phase in the sample are other parameters which strongly impact the pyroplastic deformation of vitreous china.

Reuse of spent FCC catalyst, waste serpentine and kiln rollers waste for synthesis of cordierite and cordierite-mullite ceramics

Spent fluid catalytic cracking (FCC) was gathered from several petrochemical plants and calcined in a rotary furnace between 1000 and 1100°C in order to remove sulphur and hydrocarbon based impurities. Calcining process on FCC led to formation of AlVO4 ceramic phase, so converted the hazardous waste to non-hazardous applicable raw material. In this study, two ceramic bodies as cordierite and cordierite-mullite were synthesized with calcined spent FCC, waste serpentine, kiln rollers waste and high grade kaolin as raw materials. The XRD results showed that the cordierite and cordierite-mullite were synthesized successfully so that 96.4% of F1 (cordierite) sample fired at 1400°C was cordierite phase and F2 (cordierite-mullite) sample fired at 1450°C was completely cordierite and mullite phases. The synthesized cordierite and cordierite-mullite samples had lower porosity values and coefficient of thermal expansion (CTE) than similar industrial products. The negative CTE value that obtained from the cordierite sample up to 800°C is favorable for some applications. The considerable results of the synthesized cordierite and cordierite-mullite from this work present cost reduction of the two ceramic bodies production and may help to solve the environmental problems with the use of three waste sources in large scales.

Thermal exposure effects on the long‐term behavior of a mullite fiber at high temperature

AbstractThe behavior of an oxide fiber at elevated temperatures was analyzed before and after thermal exposures. The material studied was a mullite fiber developed for high‐temperature applications, CeraFib 75. Heat treatments were performed at temperatures ranging from 1200°C to 1400°C for 25 hours. Quantitative high‐temperature X‐ray analysis and creep tests at 1200°C were carried out to analyze the effect of previous heat treatment on the thermal stability of the fibers. The as‐received fibers presented a metastable microstructure of mullite grains with traces of alumina. Starting at 1200°C, grain growth and phase transformations occurred, including the initial formation of mullite, followed by the dissociation of the previous alumina‐rich mullite phase. The observed transformations are continuous and occur until the mullite phase reaches a state near the stoichiometric 3/2 mullite. Only the fibers previously heat treated at 1400°C did not show further changes when exposed again to 1200°C. Overall, the heat treatments increased the fiber stability and creep resistance but reduced the tensile strength. Changes observed in the creep strain vs. time curves of the fibers were related to the observed microstructural transformations. Based on these results, the chemical composition of the stable mullite fiber is suggested.

Performance enhancement of the ceramic products by adding the sand, chamotte and waste brick to a porous clay from Bir Mcherga (Tunisia)

Investigations were undertaken to assess the chemical, physical and mechanical changes of porous clay and non-plastic clays mixtures fired between 900 and 1150°C. Five mixtures (S1: 70 mass % clay +30 mass % chamotte, S2: 70 mass % clay +15 mass % chamotte +15 mass % sand, S3: 70 mass % clay +30 mass % sand, S4: 70 mass % clay +30 mass % waste brick and S5: 70 mass % clay +15 mass % waste brick +15 mass % sand) from Tunisia were evaluated for their possible ceramic applications. X-ray diffraction (XRD), thermal analysis (DTA/TG), dilatometry, scanning electron microscopy (SEM) and physical-mechanical analyses such as tensile strength, water absorption, linear shrinkage and bulk density were used to assess the phase evolution and microstructure of the sintered mixtures. The formations of both the anorthite and mullite phases and the vitreous phase have improved the required ceramic product performance. S4 and S5 had the highest values of tensile strength at 1150°C. These mixtures, according to the European Norm EN 14411, are suggested for porcelainized and fully vitrified stoneware. S3 sample is suitable for the production of vitrified stoneware. S1 and S2 mixtures are suitable the production of earthenware and unglazed stoneware.

Study the Effect of Glass and Carbon Fibers on the Firebrick Properties

This work was carried out to investigate effect of glass fiber (GF) and carbon fiber (CF) on kaolin-clay firebrick properties. GF and CF are considered inorganic fibers with application of high temperatures. Kaolin clay is mixed with short GF and CF separately by different percentages (0, 0.5, 1, 1.5 and 2) wt%. Kaolin-GF and kaolin-CF mixtures were compacted by using the semi-dry pressing method. The compacted specimens were fired at different temperatures (1100, 1200 and 1300)°C. Properties which include bulk density, apparent porosity, water absorption, thermal conductivity and fracture strength were obtained from the firebrick specimens. The results show that the addition of GF has beneficial in lowering of firing temperature, and consequently accelerating the densification via enhanced grain boundary diffusivity. Increasing GF content in the firebrick mixture enhances the fracture strength due to increase amount of glassy and mullite phases. Incorporation of CF has inversely affected than GF on the firebrick properties. As the percentage of CF increased the density of firebrick decreased, and the porosity and water absorption increased.

Effect of ulexite concentrator waste on the physical and mechanical properties and sintering behaviour of floor tile bodies

AbstractThe effect of ulexite concentrator waste on the physical and mechanical properties and sintering behaviour of a standard floor tiles (FT) body fired at 1080°C was studied. The linear firing shrinkage, water absorption and fired bending strength of the floor tile/tiles were determined. X-ray diffraction analyses identified quartz, albite, mullite and hematite phases in the floor tiles containing 3 wt.% waste material. The FT body with the smallest water absorption (∼0.2%), the greatest bending strength (∼335 kgf/cm2) and the second greatest linear firing shrinkage values had optimal composition. In light of the excellent physical-mechanical properties and the results of the scanning electron microscopy-energy dispersive X-ray analyses, the authors determined that the sintering temperature of the FT body containing 3 wt.% ulexite concentrator waste was reduced by 100°C.

Zircon bonded with Iraqi Kaolin

Clay is one of many types of raw materials that have plasticity and advantage of securing so used as abinder in refractory products. Iraqi white kaolin used as a binding material in ratio (15, 20, 25, 30)%. Thesamples were formed by dry pressing and left at room temperature for (24)hr then fired at (1200)˚C withsocking time for (2)hr. The Study included Linear shrinkage, Apparent density, Porosity, water absorption,loss in mass, wettability and microstructure by (SEM) as physical properties, Thermal conductivity, andthermal shock resistance as thermal properties. The microstructure of the refractories consist of zirconiumsilicate, mullite phase, and glass binding phase. The refractory showed medium porosity. Increasing ofIraqi white kaolin percentage leads to Increasing the Linear shrinkage ratio, Apparent density, loss inmass and thermal conductivity and decreasing the Apparent Porosity and water absorption ratio , becauseof the Increasing of the liquid phase ratio in refractory. The effect of thermal shock on diametrical strengthstudied, where it is noticed the thermal shock temperatures between (400-600)˚C.

Recovery of Alumina from Coal Fly Ash by CaCl2 Calcination Followed by H2SO4 Leaching

In this paper, a calcination-leaching method by using CaCl2 and H2SO4 to recover alumina from coal fly ash (CFA) is developed. The process conditions including the parameters of CaCl2 content, calcination temperature and time, acid concentration and leaching time are studied. Heat treatment under 900°C for 1 h of the ash with 80 wt.% CaCl2 significantly enhances the alumina leachability (>96.6%) in comparison with the directing acid-leaching (2.31-4.78%) method. The recovery rate increases dramatically with the increasing of CaCl2 content from 20% to 80%, and a H2SO4 leaching time from 10 minutes to 1 hour as well, but it remains near constantly as the increasing of CaCl2 dosage >80% or acid-leaching time >1 h. The reaction mechanism is studied by X-ray diffraction (XRD) analysis to determine the crystalline phases change. The results indicate that quartz, mullite and amorphous phases have been destroyed after calcination with CaCl2 and transformed to gehlenite, calcium aluminum silicate, andradite, wollastonite and mayenite, which are susceptible to acid leaching. The mineralogical characteristics of roasted clinker indicate that molten CaCl2 can promote the incorporation of Ca into silica and silicates and result in the progressive susceptibility to acid attack. The potential chemical reactions during calcinating and leaching processes are also studied via chemical equations. The reuse and resource of coal fly ash can reduce the environmental impact of ash heap and the heavy metal pollution to the surrounding environment.

Manufacturing of Refractory Bricks from Iraqi Flint

It was done in this study the preparation of bricks refractory from Iraqi Flint and adding Iraqi white kaolin as a binding material in ratio (25, 30, 35, 40, 45)%. The samples were formed by dry pressing, dried at room temperature for 24hr then at (110)̊C for 6hr then fired at (1200)˚C with soking time for 2hr. Linear shrinkage, Apparent density, Porosity, water absorption and microstructure by (SEM) as physical properties, diametrical strength and surface roughness as mechanical testing, and thermal conductivity, linear thermal expansion and thermal shock resistance as thermal properties were studied in this study. According to (SEM) and (XRD) results, the refractories consist of mullite phase and glass phase. The results show the increase of linear shrinkage and decrease of apparent porosity and water absorption with the increasing of kaolin adding because of the increasing of ceramic raw material, which mean increasing of glass phase. For the same reason linear thermal expansion increase with the kaolin adding increasing.

The impact of kaolin dehydroxylation on the porosity and mechanical integrity of kaolin based ceramics using different pore formers

Thermally induced lattice defects in kaolin play a key role in the mechanical integrity of kaolin based ceramics during heat treatment due to their phase transformations. The effects of three types of pore formers (sawdust, styrofoam and powdery high density polyethylene) on the porosity and mechanical integrity of kaolin based ceramics have been studied based on their batch formulations. The porosity of the ceramic bodies was experimentally determined, while the structure and chemistry of the materials were elucidated via X-ray diffraction (XRD), scanning electron microscopy (SEM), FTIR, DSC/TG/DTA, and zeta potential. The kaolin-based porous samples sintered at 1150°C exhibited mullite phase transformation attributed to recrystallization effects. Morphologically, open pores were distributed on the sample surfaces due to larger pathways and available channels of eviction of the pore formers. Compressive strength decreased linearly as apparent porosity increased signifying its correlation. The compressive strength showed to be much more sensitive to defects created by dehydroxylation as micro cracks were observed on the sample with HDPE as pore former which could be as a result of large volume change accompanying phase transformations and sensitivity to the dehydroxylation phase. It is noted that the new pore former (HDPE) used in this study produced ceramic bodies with porosity as high as 67% and hence the least compressive strength.

Microstructural, physical and mechanical behavior of pastes containing clays and alumina waste

The purpose of this study is to analyze the influence of an aluminum oxide-rich precursor added to clays (smectite/kaolinite) during heat treatment on their mineralogical and microstructural changes, emphasizing the presence of mullite phase. Three clays were used. The samples were subjected to physical, chemical, thermal and mineralogical characterizations. Clay compositions containing alumina waste were then formulated and heat-treated at temperatures ranging from 900 to 1400°C, applying a heating rate of 5°C/min and 60min dwell time, after which mineralogical and morphological analyses were performed. The results indicated that mullite can be obtained from clay formulations containing alumina waste, and that the amount of the mullite phase formed is determined by the composition, particle size and applied temperature.

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300°C. In slurry-Si, silicon was oxidized into SiO2 in the temperature range from 1300°C to 1400°C and it reacted with Al2O3 into mullite phase at 1450°C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300°C and the secondary mullitization occurred at 1450°C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.

Modification of Porous Cordierite Ceramic

Highly porous cordierite ceramic by using of talcum, kaolinite and γ-alumina was obtained by method of slip casting of concentrated suspension. Additives of amorphous silica and non-stabilized zirconia in the amount of 5 wt% were used. Sintering temperature of dried samples was in range of 1250 – 1450°C. All the samples contain crystalline phases of cordierite, mullite and corundum but, depending on the additives, as a result of sintering in addition forms spinel, cristobalite or zircon (ZrSiO4). Porosity of obtained materials changes in large scale from 42 to 59 per cent; it is influenced by both sintering temperature and composition. Compressive strength increases with the addition of zirconia. In comparison, compressive strength of samples without additives or with silica additive does not exceed 3.5 MPa. The increase of coefficient of thermal expansion depends both on the composition and sintering temperature. ZrO2 additive increases the coefficient of thermal expansion considerably.

Preparation and anti-oxidation mechanism of mullite/yttrium silicate coatings on C/SiC composites

In order to enhance the oxidation resistance of C/SiC composites, mullite/yttrium silicate coatings were fabricated on C/SiC composites through dip-coating route. Al2O3-SiO2 sol with high solid content was selected as the raw material for mullite and “silicone resin + Y2O3 powder” slurry was used to synthesize yttrium silicate. The microstructure and phase composition of coatings were characterized, and the investigation on oxidation resistance and anti-oxidation mechanism was emphasized. The as-fabricated coatings consisting of SiO2-rich mullite phase and Y2Si2O7 phase show high density and favorable bonding to C/SiC composites. After oxidized at 1 400 °C and 1 500 °C for 30min in static air, the coating-containing C/SiC composites possess 91.9% and 102.4% of the original flexural strength, respectively. The desirable thermal stability of coatings and the further densification of coatings due to viscous flow of rich SiO2 and Y-Si-Al-O glass are responsible for the excellent oxidation resistance. In addition, the coating-containing composites retain 99.0% of the original flexural strength and the coatings exhibit no cracking and desquamation after 12 times of thermal shock from 1 400 °C to room temperature, which are ascribed to the combination of anti-oxidation mechanism and preferable physical and chemical compatibility among C/SiC composites, mullite and Y2Si2O7. The carbothermal reaction at 1 600 °C between free carbon in C/SiC substrate and rich SiO2 in mullite results in severe frothing and desquamation of coatings and obvious degradation in oxidation resistance.