Mullite Materials Research Articles

Mechanical Properties of Mullite Investigated by Nanoindentation

The mechanical behavior of sintered mullite material was studied using nanoindentation tests. Mullite compact was obtained by cold pressing sol-gel synthesized mullite precursor powder and sintering at 1550 °C. Analysis of the microstructural parameters and phase composition was done by XRD (X-ray diffraction) and SEM-EDS (scanning electron microscopy with energy dispersive X-ray spectrometry). A Berkovich indenter was employed for nanoindentation measurements at various loads (1000-9000 µN). After each test, in situ SPM (scanning probe microscopy) imaging was performed. The XRD pattern of sintered mullite displayed peaks of mullite (93.3%) and corundum (6.7%). Results revealed average values of hardness and elastic modulus of sintered mullite as 15.55 GPa and 174.37 GPa, respectively. Moreover, nanoindentation results indicated that mullite follows the Hall-Petch hardening relation due to the presence of grains with a size range of 0.2-2 µm. Indentation in areas with smaller grains exhibits higher hardness values. Post-test SPM images disclosed the presence of pile-ups around the indents, which were formed under loads higher than 3000 µN.

Microstructure and alkali attack resistance of novel mullite materials prepared from low-grade raw bauxite and kyanite tailings

Microstructure and alkali attack resistance of novel mullite materials prepared from low-grade raw bauxite and kyanite tailings

Microstructure and properties of highly porous mullite materials fabricated by direct foaming-gelcasting

Microstructure and properties of highly porous mullite materials fabricated by direct foaming-gelcasting

Effect of lanthanum content on physicochemical properties and thermal evolution of spent and beneficiated spent FCC catalysts

Two FCC spent catalysts with different amounts of La2O3 and similar Al2O3/SiO2 ratios and the corresponding residues of an optimized process of La-extraction are studied. The potential of the powders as feedstock for mullite refractories is established. The four powders have been fully characterized – chemical composition, crystalline phases, particle size, specific surface area, porosity, and morphology. Their thermal evolution up to 1500°C has been studied using dynamic techniques – hot stage microscopy, DTA-TG – and isothermal thermal treatments. The effect of powder characteristics on high temperature phase development, in particular, on mullite formation is discussed.Besides La, the La-extraction process also implies partial extraction of the matrix in which zeolites are dispersed, resulting in reduced impurities contents and Al2O3/SiO2 ratios.The powders present differences in the textural properties– porosity and specific surface area – that do not affect the high temperature behavior, which is determined by the impurities and the Al2O3/SiO2 ratio. The four powders can be proposed as precursors for mullite materials. The compositions of the spent catalysts can be envisaged for application up to 1200°C. The compositions of the residues of the La-extraction process can be proposed for higher temperatures once their composition is corrected by adding alumina to avoid extensive cristobalite formation together with mullite.

Effect of Food Waste Condensate Concentration on the Performance of Microbial Fuel Cells with Different Cathode Assemblies

The aim of this study is to examine the effect of food waste condensate concentration (400–4000 mg COD/L) on the performance of two microbial fuel cells (MFCs). Food waste condensate is produced after condensing the vapors that result from drying and shredding of household food waste (HFW). Two identical single-chamber MFCs were constructed with different cathodic assemblies based on GoreTex cloth (Cell 1) and mullite (Cell 2) materials. Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements were carried out to measure the maximum power output and the internal resistances of the cells. High COD removal efficiencies (>86%) were observed in all cases. Both cells performed better at low initial condensate concentrations (400–600 mg COD/L). Cell 1 achieved maximum electricity yield (1.51 mJ/g COD/L) at 500 mg COD/L and maximum coulombic efficiency (6.9%) at 400 mg COD/L. Cell 2 achieved maximum coulombic efficiency (51%) as well as maximum electricity yield (25.9 mJ/g COD/L) at 400 mg COD/L. Maximum power was observed at 600 mg COD/L for Cell 1 (14.2 mW/m2) and Cell 2 (14.4 mW/m2). Impedance measurements revealed that the charge transfer resistance and the solution resistance increased significantly with increasing condensate concentration in both cells.

Direct electron transfer process-based peroxymonosulfate activation via surface labile oxygen over mullite oxide YMn2O5 for effective removal of bisphenol A

The development of efficient and eco-friendly Mn-based catalysts for peroxymonosulfate (PMS) activation and revealing the mechanism are highly desired for pollutants removal in wastewater. In this study, a ternary mullite oxide YMn2O5 (YMO) was synthesized for bisphenol A (BPA) degradation in the presence of PMS for the first time. Combined with XPS, EPR and TG, abundant surface labile oxygen species (Olab) with a ratio of 39.7% were identified in the mullite materials, which was higher than that for α-, β-, γ- and δ-MnO2. YMO also exhibited higher interfacial conductivity and stronger interactions among the catalyst, PMS and BPA than other MnO2. Superior performance in BPA degradation was observed with YMO, and Olab was suggested as the main active sites, for the normalized first-order rate constant was linearly dependent on the ratio of the oxygen species. Chemical quenching experiments, EPR and open circuit potential suggested that the degradation of BPA was mainly caused by the direct electron transfer process through the reactive Olab-PMS complexes. The influence of YMO concentration, PMS concentration, solution pH, different anions and reaction temperature was thoroughly studied. Moreover, the intermediates were identified and the possible degradation pathways were proposed. This work demonstrated that mullite oxide YMO with enriched Olab species can be used as eco-friendly catalysts for efficient PMS activation to remove recalcitrant organic compounds in wastewater.

Corrigendum to “Optical characterisation of alumina–mullite materials for solar particle receiver applications” [Solar Energy Mater. Solar Cell. 230 (2021) 111170

Corrigendum to “Optical characterisation of alumina–mullite materials for solar particle receiver applications” [Solar Energy Mater. Solar Cell. 230 (2021) 111170

A Study on the Wear Behaviour of Monolithic Mullite Materials for Dental Applications

The aim of this study is to evaluate the potential use of the monolithic mullite samples for the dental applications. For this purpose, at first the monolithic mullite samples obtained by the powder metallurgy method have been characterized by the scanning electron microscopy (SEM) together with the EDS analysis. The pin-on-disk tribometer has been used to conduct the wear test in order to determine the volumetric loss. After the wear tests, the worn surfaces of the sintered mullite samples have been studied. The presence of the 3Al2O3.2SiO2 structure has been confirmed by the FT-IR spectroscopy through the observation of the characteristic Al-O-Si linkages at the spectra. The wear rate results derived from the volumetric losses have been measured to be around 2.42×10-6 mm3/Nm. This value has been found to be lower than those of alumina and alumina-mullite composites in the previous studies, which explains the higher wear resistance of these mullite materials. The room temperature friction coefficients of AS-1 and AS-2 samples have been determined to be 0.88 and 0.80, respectively. The wear behaviour of the samples has been correlated to the sintering temperature and the resultant relative density of the samples. The mullite samples obtained in this study have indicated better wear performance than the other materials (alumina and alumina-mullite composites) in spite of the usage of monolithic material. Therefore, this study suggests that the monolithic mullite materials have a potential to be exploited for the replacement of the previous ceramic-based dental materials. Further studies might contribute to the improvement of these materials to be utilized in the oral environment.

Optical characterisation of alumina–mullite materials for solar particle receiver applications

Alumina–mullite particles are used in high-temperature solar thermal applications such as solar particle receivers. In this study, optical properties of alumina–mullite materials with variable content of alumina and mullite are determined in the spectral range of 0.193–1.69 μm. Variable angle spectroscopic ellipsometry is performed for alumina–mullite thin films, which are fabricated by magnetron sputtering. The thin films are characterised by scanning electron microscopy, atomic force microscopy, and energy dispersive spectroscopy methods. The B-spline model is employed to generate ellipsometric parameters to fit the measured data and to obtain the optical properties. The investigated materials of variable content of alumina and mullite have a similar refractive index in the considered spectral range. The absorptive index of the alumina–mullite materials in the spectral range of 0.193–0.4 μm is higher than in the range 0.4–1.69 μm. The absorptive index decreases with increasing content of alumina in the spectral range of 0.193–0.4 μm. The material composed of similar proportions of alumina and mullite yields the highest absorptive index in the spectral range of 0.4–1.1 μm. The optical properties determined for the alumina–mullite materials are applied to obtain the radiative properties of spherical homogeneous particles. Mie theory is used to calculate absorption and scattering efficiency factors, as well as the scattering phase function. In addition, the scattering phase functions are obtained using the Henyey–Greenstein approximation and the transport approximation. The Monte Carlo ray-tracing method is employed to study the radiative transfer in a model one-dimensional particle curtain containing polydisperse particles exposed to high-flux solar irradiation. It is found that the overall reflectance, absorptance and transmittance of the particles only weakly depend on the optical properties of the materials investigated.

Effect of MgO addition on the sinterability and mechanical properties of mullite ceramics

Mullite is a refractory material with singular properties, although high temperatures and long sintering times are required to obtain this material with good densification. In this study, aluminum hydroxide and colloidal silica were used to produce mullite through reactive sintering and MgO was employed as a sintering additive. The compositions were prepared with different amounts of MgO and sintered at 1350, 1450, and 1550 °C, and then analyzed using X-ray diffraction, scanning electron microscopy, and measurements of apparent porosity (AP) and flexural strength. The results showed that the raw materials used allowed the mullite formation at relatively low temperatures (1350 °C), regardless of the amount of MgO added but with the increase in MgO content, a spinel phase appeared, resulting in a fraction of residual α-alumina. The MgO addition lowered the densification temperature at around 50 °C. Furthermore, the higher the sintering temperature and the MgO content, the larger and more anisotropic the mullite grains were. At sintering temperatures above 1450 °C, AP was reduced to approximately 10%. The MgO addition and increase in sintering temperature improved the flexural strength of mullite materials.

Structural, micro-structural, optical and dielectric behavior of mullite ceramics

Mullite ceramics of two different compositions Al0·60Si0·40O and Al0·75Si0·25O have been synthesized by pyrophoric method followed by reaction sintering at 1050°C. The structural, micro-structural, optical, and dielectric characterization of mullite has been carried out through XRD, Raman, SEM, UV–Visible and dielectric studies. In this work, the synthesized mullite materials possess quite high dielectric constant value with negligible dielectric loss factor. The room temperature dielectric constant values for Al0·60Si0·40O and Al0·75Si0·25O stoichiometric mullites are found to be 110 and 2665 with a loss factor of 2.04 and 2.29 at 100 Hz frequency, which is comparatively high than the corresponding values of dielectric constants reported in previous literatures of mullite. The dielectric constant values for both the ceramics are very high (>104) for temperatures greater than 250°C. The room temperature dielectric constant (6.03 and 5.08, at 1 MHz), dielectric loss (0.07 and 0.05, at 1 MHz), electrical conductivity (1.5 × 10−5 S/m and 8.1 × 10−6 S/m, at 300 °C) results for Al0·60Si0·40O and Al0·75Si0·25O prove these ceramics suitable for electronics-related applications. The Arrhenius type conductivity, non-Debye type relaxation mechanism is confirmed from AC conductivity, complex impedance, and modulus study of these ceramics. These ceramics possess high optical band gap values of 3.46 eV and 3.36 eV for Al0·60Si0·40O and Al0·75Si0·25O, respectively which is suitable for using as electronic circuit packaging material or as a dielectric substrate material for any kind of storage devices.

Mechanical and thermal behavior of cellular mullite materials

In this paper, the mechanical behavior and thermal properties of cellular mullite bodies obtained by thermal direct-consolidation of foamed aqueous suspensions of mullite-bovine serum albumin (BSA) and mullite-BSA-methylcellulose (MC) were studied. The mechanical behavior of cellular mullite materials sintered at 1600 °C was evaluated by diametral compression at room temperature, 1000 °C and 1300 °C. The variation in the thermal diffusivity and thermal conductivity at temperatures up to 900 °C was determined using the laser-flash method. The results of the mechanical and thermal evaluation were analyzed based on the porosity features of the sintered materials, which was determined in turn by the starting system used for shaping the bodies.

Recycling of mullite from high-alumina coal fly ash by a mechanochemical activation method: Effect of particle size and mechanism research

High-alumina coal fly ash (HAFA) is a special solid waste since its alumina content can reach 40–50 wt%, which is seen as a potential resource for mullite material production. However, obtaining an ideal mullite material from HAFA is difficult because of its low Al2O3/SiO2 mass ratio. In this work, the microstructure characteristics of HAFA were systematically analyzed by combining multiple characterization techniques. It was found that HAFA had a core-shell structure with a mullite/corundum crystal core and a silica-rich amorphous phase shell. The novel mechanochemical activation–desilication process was used to remove amorphous phase from HAFA and elevate the Al2O3/SiO2 mass ratio. In particular, the effect of particle size after mechanical treatment and mechanism of the desilication process were extensively investigated. On decreasing the particle size, a high leaching rate of alumina was achieved during mechanochemical activation, thus generating a hydroxysodalite coating layer as desilication was suppressed, and the amorphous phase was effectively removed. The mineralogical phase of the desilicated HAFA is mainly mullite and corundum, and the Al2O3/SiO2 mass ratio was elevated from 1.29 to 3.02. Mullite refractory obtained from the desilicated HAFA exhibited excellent physical properties. This study provides insights into further high-valued utilization of HAFA.

Mining Wastes of an Albite Deposit as Raw Materials for Vitrified Mullite Ceramics

In this work, an examination of mining wastes of an albite deposit in south Spain was carried out using X-ray Fluorescence (XRF), X-ray diffraction (XRD), particle size analysis, thermo-dilatometry and Differential Thermal Analysis (DTA) and Thermogravimetric (TG) analysis, followed by the determination of the main ceramic properties. The albite content in two selected samples was high (65–40 wt. %), accompanied by quartz (25–40 wt. %) and other minor minerals identified by XRD, mainly kaolinite, in agreement with the high content of silica and alumina determined by XRF. The content of Na2O was in the range 5.44–3.09 wt. %, being associated with albite. The iron content was very low (<0.75 wt. %). The kaolinite content in the waste was estimated from ~8 to 32 wt. %. The particle size analysis indicated values of 11–31 wt. % of particles <63 µm. The ceramic properties of fired samples (1000–1350 °C) showed progressive shrinkage by the thermal effect, with water absorption and open porosity almost at zero at 1200–1250 °C. At 1200 °C, the bulk density reached a maximum value of 2.38 g/cm3. An abrupt change in the phase evolution by XRD was found from 1150 to 1200 °C, with the disappearance of albite by melting in accordance with the predictions of the phase diagram SiO2-Al2O3-Na2O and the system albite-quartz. These fired materials contained as main crystalline phases quartz and mullite. Quartz was present in the raw samples and mullite was formed by decomposition of kaolinite. The observation of mullite forming needle-shape crystals was revealed by Scanning Electron Microscopy (SEM). The formation of fully densified and vitrified mullite materials by firing treatments was demonstrated.

Microscopic regulation of plant morphological pores on mechanical properties of porous mullite materials

AbstractGenerally, a multilayer structure is present inside a walnut shell, and the residual structure of the walnut shell is retained after impregnation and firing. When the walnut shell is used as a pore‐forming agent, this structure helps in improving the mechanical and thermal insulation properties of the lightweight porous materials. In this study, porous mullite materials (PMMs) with plant morphological structure pores were prepared using a‐Al2O3 and silica powder as the raw materials with addition of sol‐impregnated walnut shell powder (WSP). The influence of sol type and firing temperature on the pore structure of the PMMs was analyzed, which affected the compressive strength and thermal conductivity. The plant morphological porous structure was observed in the samples after sol impregnation. After firing at different temperatures, the porous structure gradually contracted and supported the pores, improving the mechanical properties, while the complex porous structure increased the heat conduction path, thereby improving the insulation performance. Using WSP impregnated with silica‐sol and zirconia‐sol as pore‐forming agents, PMMs with higher compressive strength and relatively low thermal conductivity (TC) were prepared.

The potential of La-containing spent catalysts from fluid catalytic cracking as feedstock of mullite based refractories

The potential of La-containing spent FCC catalysts as feedstock of mullite refractories is discussed. A simple methodology to determine the maximum use temperatures is proposed. It combines the analysis of the microstructural evolution with temperature with strain-controlled (2 × 10−4s-1 to 3% maximum strain) and constant stress compressive creep tests at high stresses (15−30 MPa) at 1100–1200 °C. Complete reaction was assured by milling the waste before processing mullite materials. Phase and microstructural evolutions with temperature were established combining dynamic techniques (DTA-TG and dilatometry) with quenching experiments. Isothermal treatments at 1300 and 1400 °C allowed selecting the optimum thermal treatment: 1400 °C-3 h.The formation of a La2O3 rich Al2O3-SiO2 liquid at low temperatures ≈1200 °C leads to the development of a microstructure constituted by a skeleton of mullite tabular grains trapping the remaining glass. Such microstructure endows the material with sufficient creep resistance at temperatures 1100−1200 °C to be used as constituent of refractories for temperatures ≤1200 °C.

Improving the usability of biodiesel blend in low heat rejection diesel engine through combustion, performance and emission analysis

Utilization of biodiesel which is of renewable in nature was enhanced in single cylinder, 4-S diesel engine based on combustion, performance and emission analysis. Esterified jatropha biodiesel (EJO) at 4%, 10% and 20% was added in normal diesel by v/v ratios to form B04, B10 and B20 blends. The crown surfaces of piston, cylinder head and valves were coated with 250 μm thick mullite material (Metco 6150) to form low heat rejection engine (LHRE). The optimum EJO blend for the conventional engine (CE) as well as LHRE was investigated by comparing their characteristics from all biodiesel blends with the normal diesel of CE operation. B10 was obtained as an optimum blend for both engines. For CE at B10 operation, the CO, HC emissions (except NOx) and brake thermal efficiency (BTE) were reduced as compared with CE at diesel fuel. For LHRE operation, the emissions were further reduced but BTE was slightly reduced. Finally to improve the usability of biodiesel, 5% anhydrous ethanol was added into optimum blend to form E05B10 mixture. Ethanol required higher latent heat for evaporation; this caused reduction of NOx and improved BTE. Emission load on environment (GWP, RIP and TAP) was reduced by optimum blend operation.

Evaluation of mineral matter transformations in low-temperature ashes of South African coal feedstock samples and their density separated cuts using high-temperature X-ray diffraction

ABSTRACTCoal mineral matter is the leading cause of inevitable ash problems (fouling and slagging) during the combustion of coal. Low-temperature ash (LTA) samples of feed coals and the density separated fractions were subjected to high-temperature X-ray diffraction (HT-XRD) to identify the mineral reactions occurred at elevated temperatures under oxidizing conditions. Excluded minerals were mainly contained in the sink fractions, while included minerals were mainly present in the float fractions. QEMSCAN results indicate that the mineral associations in carbominerites and included minerals in Feeds A, B and C and sinks B and C are responsible for the melt formation during HT-XRD experiments. HT-XRD results indicate the presence of mullite, anorthite, and amorphous aluminosilicate materials formed in the thermally treated LTA samples. The formation of these slagging crystalline and glassy phases could be attributed to either crystallization during the cooling of the molten solution, or via solid-state reactions at elevated temperatures. A comprehensive knowledge of the included and excluded minerals can be used to prepare a blended feedstock for combustion processes. The operational ash-related problems in the combustion and gasification processes could be minimized by implementing this comprehensive knowledge of the transformation of coal minerals at elevated temperatures.

Cellular mullite materials processed by direct foaming and protein casting

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

RETRACTED: Mechanical properties analysis and microstructure of ceramic materials of mullite ceramic material

Two unique multiphase volume elements were developed in this paper against the mechanical performance analysis problem of mullite ceramic material. They realized consideration of multiple materials...