Pure Mullite Research Articles

Structural, morphological, dielectric properties, impedance spectroscopy and electrical modulus of sintered Al6Si2O13–Mg2Al4Si5O18 composite for electronic applications

In this study, the sol-gel method of low-temperature synthesis was utilized to create mullite/cordierite precursor powder. The materials (TEOS) Si(C2H5O)4, Al(NO3)3.9H2O and Mg(NO3)2 were utilized as source of SiO2, Al2O3, and MgO oxides respectively in order to create mullite/cordierite precursor gel with various concentrations and designations (MC00, MC10, MC20, MC30, MC40, and MC50). Crystalline phases were seen and described using scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). After the powder was formed at 1600 °C for an hour, its sintering was looked at. For all mixes, as the cordierite concentration rises, the bulk density rises and the open porosity decreases. The existence of a vitreous phase may be the cause of the rise in bulk density and reduction in open porosity seen with rising temperatures.The dielectric characteristics of the samples have been examined at room temperature; the relative dielectric constant (εr), loss tangent (tanδ), and dielectric characteristics were assessed at frequencies of 0.1 Hz, 1, 10, 100, and 1000 kHz. At 1 kHz, the relative dielectric constant values are closest to those of mullite (εr = ∼5 to ∼ 6), and c at 1 kHz. On the other hand, the lowest dielectric losses were observed (tanδ from ∼0.06 to ∼ 0.04). The Electrical properties, electrical resistivity, AC conductivity, impedance spectroscopy, electrical modulus, and the relative quality factor (QF) of pure mullite and mullite/cordierite composite sintered at 1600 °C for 1 h (MCyy) samples were investigated as a function of cordierite content at varied frequencies (0.01 Hz, 1 kHz, 100 kHz, and 1000 kHz). The hardness (HV) and coefficient of thermal expansion (CTE) of the composite material that underwent sintering at a temperature of 1600 °C were determined by the use of a hardness tester and a dilatometer, respectively. The rise in temperature, coupled with an equal quantity of cordierite, resulted in an elevation of the apparent density and a reduction in the open porosity. In the case of the specimen subjected to sintering at a temperature of 1600 °C, the introduction of cordierite content in the range of 0–40% resulted in an observed rise in HV value from 9.15 to 12.99 GPa. However, a further increase in cordierite content to 50% led to a decrease in HV value to 10.99 GPa. Nevertheless, the CTE value throughout the temperature range of 50–1200 °C exhibited a consistent decline, ranging from 5.37 × 10−6 to 2.32 × 10−6 K−1. Notably, the composite material consisting of 50 wt% cordierite had elevated HV and the most minimal CTE value.

Development of a new composite ceramic membrane from mullite, silicon carbide and activated carbon for treating greywater

In this experimental research, it is aimed at evaluating a novel composite ceramic membrane for treating and recycling on-site greywater. Therefore, three types of tubular microfiltration ceramic membrane including Mullite, Mullite/SiC and Mulite/SiC/AC composite were fabricated and sintered at a relatively low temperature (1250 ° C) under N 2 atmosphere. Field Emission Scanning Electron Microscopy (FE-SEM) and Atomic Force Microscopy (AFM) analyses showed that the presence of silicon carbide and activated carbon can improve the surface roughness. Besides, the changes in open porosity, radial and longitudinal shrinkage, mean pore size and mechanical strength were measured and compared for the pure Mullite and composite membranes to determine the most appropriate membrane for the treatment of greywater. The highest pure-water permeability of 3954 kg/m 2 h.br was associated with the Mullite/SiC/AC membrane with the highest porosity of 64.7% and an acceptable appropriate mechanical strength (17.2 MPa approx.). Furthermore, the highest removal efficiency was related to Mullite/SiC/AC in comparison with Mullite and Mullite/SiC membranes. It was observed that using the Mullite/SiC/AC membrane, the removal efficiencies for chemical oxygen demand (COD), biochemical oxygen demand (BOD), total sulfate, total nitrate, phosphate, and detergent (ABS) were 88.0%, 87.0%, 71.5%, 86.5%, 94.3%, and 40.4%, respectively.

Transient liquid phase sintering of high-purity mullite for high-temperature structural ceramics

High-purity mullite ceramics, promising engineering ceramics for high-temperature applications, were fabricated using transient liquid phase sintering to improve their high-temperature mechanical properties. Small amounts of ultrafine alumina or silica powders were uniformly mixed with the mullite precursor depending on the silica-alumina ratio of the resulting ceramics to allow for the formation of a transient liquid phase during sintering, thus, enhancing densification at the early stage of sintering and mullite formation by the reaction between additional alumina and the residual glassy phase (mullitization) at the final stage of sintering. The addition of alumina powder to the silica-rich mullite precursor resulted in a reaction between the glassy silica and alumina phases during sintering, thereby forming a mullite phase without inhibiting densification. The addition of fine silica powder to the mullite single-phase precursor led to densification with an abnormal grain growth of mullite, whereas some of the added silica remained as a glassy phase after sintering. The resulting mullite ceramics prepared using different powder compositions showed different sintering behaviors, depending on the amount of alumina added. Upon selecting an optimum process and the amount of alumina to be added, the pure mullite ceramics obtained via transient liquid phase sintering exhibited high density (approximately 99%) and excellent high-temperature flexural strength (approximately 320 MPa) at 1500 °C in air. These results clearly demonstrate that pure mullite ceramics fabricated via transient liquid phase sintering with compositions close to those of stoichiometric mullite could be a promising process for the fabrication of high-temperature structural ceramics used in an ambient atmosphere. The transient liquid phase sintering process proposed in this study could be a powerful processing tool that allows for the preparation of superior high-temperature structural ceramics used in the ambient processing atmosphere.

Kinetics of Mullitization from Polysilsesquioxane and Boehmite Precursors

Mullite ceramics was synthesized using boehmite (-AlOOH) as alumina source and polymethysilsesquioxanes (PMS), a novel class of preceramic polymer, as silica source. Quantitative mullitization was calculated from the X-ray diffractograms of powder samples of appropriate stoichiometry of the precursors calcined for various holding times between 1200o and 1300oC, using Rietveld analysis, and the kinetics of phase formation was studied. The rate constant and activation energy of mullitization was calculated by the Johnson-Mehl-Avrami equation. This approach holds great potential for the commercial use of polymeric silica precursor for the fabrication of highly pure mullite and other silicate systems for various application such as thermal and environmental barrier coatings.

Corrosion degradation of mullite subject to carbon monoxide atmosphere at 1000oC–1600°C

AbstractThus far, studies on the damage to refractory materials under carbon monoxide atmospheres have mostly concentrated on the effects of carbon deposition, and the testing temperature was always set at approximately 500°C to promote the deposition of carbon. However, this testing temperature is far below the operating temperature of most refractories. In this study, mullite, a widely used high‐temperature structural material, was subjected to a carbon monoxide atmosphere at 1000°C‐1600°C to investigate its phase and microstructural evolutions. Changes to the grain boundaries were initially observed in mullite specimens treated at 1000°C and 1200°C. After treatment at 1400°C, the specimen surface comprised α‐Al2O3, a glass phase, and a small amount of mullite. However, treatment at 1600°C resulted in only α‐Al2O3 and a small amount of glass phase on the surface. Additionally, pores and voids were found in the glass phase on the surface and in the bulk of the specimens treated at 1400°C and 1600°C. This study demonstrated the stability of pure mullite in a carbon monoxide atmosphere and revealed that impurities accelerating generation of the liquid phase in Al2O3–SiO2 system significantly affect the stability of mullite in a carbon monoxide atmosphere.

Thermal and Microstructural Study on Mullite-Based Ceramics from Rice Husk-Alumina Mixtures

This study focuses on the physical and microstructural properties of mullite-based ceramic synthesized by solid-state reaction of rice husk ash (RHA) and alumina (Al2O3). Laser Flash analysis (LFA), Field Emission Scanning Electron Microscopy (FESEM) and Energy-dispersive X-ray spectroscopy (EDX) characterized the effects of the mixtures of RHA and Al2O3 matrix. The results show that the Al2O3rich samples sintered at 1500 °C exhibit the highest thermal diffusivity radiated at 500 °Cwith values varied from 0.258-0.369 mm2/s. The addition of Al2O3 (30-60 wt.%) into RHA enhance the crystallization of mullite on the surface of vitreous particles. The presence of diphasic mullite densified the green bodies,and its crystallites size keep increases (400.78-650.52 nm) by theincrement of the sintering temperature (1200-1500 °C). These results suggested that addition of Al2O3into RHA enhance the thermal diffusivity as the values closer to the thermal diffusivity of pure mullite and have high potential application as thermal insulation material. The properties of mullite-based ceramics listed above such samples M1, where the composition closes to mullite (3:2) have comparable properties to commercialize mullite ceramics.

Fused deposition modeling of mullite structures from a preceramic polymer and γ-alumina

Thermoplastic filaments for Fused Deposition Modeling fabrication of mullite ceramic components were produced from a polymethylsiloxane ceramic precursor, γ-Al2O3 powder and ethylene vinyl acetate (EVA) as an organic, elastomeric binder. Two micron-sized γ-Al2O3 powders with different particle size distributions were used. Both type of powders could be successfully 3D printed, however γ-Al2O3 with finer particle size resulted in better quality of printed parts with much smoother surface.Sintering experiments showed that mullite starts forming already at 1250 °C, but a final sintering of 1550 °C is necessary to achieve pure mullite using the coarse powder, while some residual α-Al2O3 was present when using finer powder.Results demostrate that Fused Deposition Modeling can be used to shape preceramic polymers to generate large scale components with a complex shape in which, during firing, the siloxane produces highly reactive silica that allows for the in-situ formation of silicate ceramic phases, such as mullite.

Investigation of sintering behavior of sanitaryware ceramics by controlling glassy phase

The focus of this study was to investigate the effects of glassy and crystalline phases in sintering behavior of the sanitaryware body. To achieve this aim, two different phase analysis methods, the ratio of slopes and Rietveld methods, were compared. This research was carried out in three steps by using X-ray analysis values. In the first step, different amounts of metallic Si were added to pure mullite and quartz materials and their reference analysis lines were drawn. Secondly, sanitaryware samples were analyzed and their analysis lines were also drawn and compared with references. Therefore, a comparison between the slopes of the reference and analysis lines was allowed for the determination of the amount of quartz, mullite, and amorphous phases as 22.1, 18.8, and 59.1 wt.%, respectively. These results were used to estimate the chemical composition of the glassy phase that formed in standard sanitaryware microstructure. In the last step of research, new glassy phase compositions were prepared based on a standard sanitaryware glassy phase composition. Different mixture ratios of alkaline and earth alkaline oxides (Na2O/K2O, Na2O + K2O, SiO2/Al2O3, and MgO/CaO) were used to modify the glassy phase compositions. Results show that softening point and viscosity of the glassy phase can be controlled by controlling these certain parameters, and this helps to consume energy in the sintering process.

High dielectric response of cobalt aluminate mullite (CAM) nanocomposite over cobalt aluminate mullite polymer (CAMP) nanocomposite in PVDF matrix

In this communication we have compared the dielectric behavior of cobalt aluminate mullite (CAM) nanocomposite and cobalt aluminate mullite polymer (CAMP) nanocomposite with different molar concentration of Co +2 ions. The study of dielectric property of the CAM samples as well as that of the CAMP samples at room temperature shows that at all concentrations the dielectric constant is higher than pure mullite and there is a critical concentration of Co +2, where maximum enhancement of dielectric property occurs. This paper demonstrates that the loading of a conductive component into a highly insulating matrix is an effective way to fabricate composites with high permittivity as well as a charge storage material. We have designed a device using CAM as the electrode material and CAMP as the separator to compare it with a commercial Li-polymer ion mobile battery. We observed that the charge storage ability of the composite system is better than the commercial Li-polymer ion mobile battery. Our device persists for more than 24 h while the maximum voltage recorded by the device is 0.885 V, whereas the maximum voltage recorded by the conventional commercial Li-polymer ion mobile battery is 0.566 V.

Ultra low-density mullite foams by reaction sintering of thermo-foamed alumina-silica powder dispersions in molten sucrose

Ultra low-density mullite foams are prepared by thermo-foaming followed by reaction sintering of alumina-silica powder dispersions in molten sucrose. The foaming & setting time, foam rise, sintering shrinkage, porosity, cell size and compressive strength are studied as a function of ceramic powder loading, foaming temperature and magnesium nitrate (blowing agent and setting agent) concentration. Phase pure mullite is produced by reaction sintering at 1600°C. The mullite foams produced without magnesium nitrate have porous struts and cell walls due to improper densification. The magnesium nitrate drastically decreases the foaming & setting time and increases the foam rise and cell interconnectivity. The MgO produced from the magnesium nitrate assists the densification of the mullite as evidenced from the non-porous struts and cell walls at higher magnesium nitrate concentrations. The maximum porosity of 94.92 and 96.28vol.% achieved without and with magnesium nitrate, respectively, is the highest reported for mullite foams.

Research in the Field of Preparing Molded and Unmolded Refractories Based on High-Alumina HCBS. Part 6. Mullitization and Thermal Expansion of Materials Based on Compound Composition HCBS1

Sintering and mullitization are studied for specimens prepared on the basis of compound composition HCBS in the system bauxite – very fine quartz glass (VFQG), and also with addition of refractory clay. The nature of sintering and mullitization is governed to a considerable extent by SiO2 content in the original composition (VFQG + clay). Dilatometric studies of specimens previously fired at 1600°C established that depending on composition thermal expansion values in the range 20 – 1000°C are within the limits of 0.5 – 0.6%, which is close or comparable with similar data for pure mullite.

Effect of Yb2SiO5 addition on the physical and mechanical properties of sintered mullite ceramic as an environmental barrier coating material

In this study, ytterbium monosilicate (Yb2SiO5)-added sintered mullite ceramics are prepared as candidate materials for environmental barrier coatings (EBCs). The effect of adding Yb2SiO5 on the physical and mechanical properties of the sintered mullite ceramics is investigated. The Yb2O3–SiO2–Al2O3 ternary phase diagram indicates that adding Yb2SiO5 to the mullite goes beyond simply mixing; instead, liquid sintering occurs. Therefore, when we add Yb2SiO5 to the mullite, the sintered body possesses a denser microstructure and faster densification rate than does pure mullite. The density rapidly increases with the addition of 6wt% Yb2SiO5 in the mullite, and almost full densifications are achieved with the addition of 12wt% and 18wt% Yb2SiO5. In this study, mullite ceramic that contains 12wt% Yb2SiO5 exhibits the smallest plastic deformation and the highest elastic modulus among ceramics containing 6, 12, and 18wt% Yb2SiO5, according to Hertzian indentation results. The results suggest that 12wt% Yb2SiO5-doped mullite may be expected to act as a potential EBC material based on its excellent elastic properties, dense microstructure, and appropriate coefficient of thermal expansion.

Tensile and creep performance of a novel mullite fiber at high temperatures

The novel fiber CeraFib75 with a composition near to pure mullite was analyzed with respect to its potential for high-temperature applications. This mullite fiber free of glass phase was aimed to overcome the strength of commercial oxide fibers at high-temperatures. Tensile tests at room and high temperatures ranging from 900 to 1400°C and creep tests were performed. Nextel™720, another crystalline mullite-alumina fiber, was tested as a reference. Microstructure and crystal phase analysis of the new fiber revealed mullite grains with traces of γ- and α-alumina in-between; it contains occasionally defects causing a reduced strength at room-temperature. Remarkably, at temperatures beyond 1200°C, CeraFib75 presented a higher tensile strength than Nextel™720. During tensile tests at 1400°C, an extended region of inelastic deformation was observed for CeraFib fibers only, which was related to a grain boundary sliding mechanism. Creep rates were of the same order of magnitude for both fibers.

Effect of the addition of alkaline earth sulfates to mullite ceramics on the corrosion and wetting by Al–Mg alloy

Novel ceramics showing a high corrosion resistance were obtained by adding alkaline earth sulfates to mullite (Al6Si2O13). Powder samples of mullite, mullite–SrSO4 and mullite–BaSO4 were prepared. Disk pellets of 4cm in diameter were conformed by uniaxial pressing at 78MPa and sintered at 1450°C for 6h. Wetting experiments were carried out by the sessile drop test at 900°C using an Al–7Mg alloy. After sintering, substrates of mullite, mullite–SrAl2Si2O8 or mullite–BaAl2Si2O8 were obtained. The contact angles between samples surface of alkaline earth sulfates containing mullite and molten alloy were higher than that corresponding to pure mullite, showing a non-wetting behavior. Cross-section analysis by SEM and EDS of samples after the wetting test showed that the corrosion products were spinel (MgAl2O4) and periclase (MgO). The BaAl2Si2O8-containing samples showed a higher corrosion resistance.

Porous acicular mullite obtained by controlled oxidation of waste molybdenum disilicide

Porous acicular mullite was fabricated by using waste MoSi2 heating element and Al2O3. Careful calcination of the pulverized heating element led to the formation of a mixture of MoO3 and amorphous SiO2. This mixture was employed as both SiO2 precursor and pore former. The oxidation of MoSi2 and mullite formation were studied. The effect of fabrication temperature on phase composition, porosity, grain morphology, and compressive strength of sintered mullite was examined. Pure mullite with porosity of more than 60% and compressive strength of ∼20MPa was obtained at temperature as low as 1300°C. The microstructure consisted of elongated, rectangular, prism-like grains which are known to be effective in filtration of diesel engine exhaust. The increase in sintering temperature caused the change of grain morphology and reduction in compressive strength.

Characterization of cordierite-mullite ceramics prepared from natural raw materials

Abstract Series of six cordierite-mullite ceramics were synthesized via solid state reaction at various temperatures from 1250 °C for pure cordierite to 1500 °C for pure mullite. Then the samples were submitted to the test of thermal shock resistance based on cycling heating-quenching procedure. X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Mercury intrusion porosimeter (MIP) have been used to characterize the samples before and after cycling heating-quenching method. Sample 6 was broken after 35 heating-quenching cycles, while the five other reminded stable. The refractoriness of samples is found to be higher than that of commercial ones. XRD shows that heating-quenching procedure has led to crystallization of cordierite and mullite phases. Apart from sample 6, the pore structure is stable with slight consolidation. The microstructure images confirm the results of XRD and MIP showing crack in sample 6 only, but compact and larger particles resulting from crystal growth in other samples due to the repeated action of heating.

Toughened carbon nanotube–iron–mullite composites prepared by spark plasma sintering

Carbon nanotube–iron–mullite nanocomposite powders were prepared by a direct method involving a reduction in H 2 –CH 4 and without any mechanical mixing step. The carbon nanotubes are mostly double- and few-walled (3–6 walls). Some carbon nanofibers are also observed. The materials were consolidated by spark plasma sintering. Their electrical conductivity is 2.4 S/cm whereas pure mullite is insulating. There is no increase in fracture strength, but the SENB toughness is twice than the one for unreinforced mullite (3.3 vs. 1.6 MPa m 1/2 ). The mechanisms of carbon nanotube bundle pullout and large-scale crack-bridging have been evidenced.

Rietveld Quantitative Phase Analysis of OPC Clinkers, Cements and Hydration Products

It has been more than twenty years since the excellent volume of Reviews in Mineralogy dedicated to Modern Powder Diffraction was published (Bish and Post 1989). That volume contained a series of key articles ranging from the basic of powder diffraction to sample preparation and synchrotron and neutron powder diffraction. Within that volume, quantitative phase analysis was extensively discussed in a specific chapter (Snyder and Bish 1989). Snyder and Bish (1989) discussed the Reference Intensity Ratio approach (also known as Chung method), the method of standard additions (also known as spiking method) and the full pattern-fitting approach using both the Rietveld method and the observed patterns method. The reader is referred to that volume for the basics of powder diffraction, and to the specific chapter by Snyder and Bish (1989) for the history of quantitative phase analysis from powder diffraction and for a discussion or the early findings. Quantitative phase analysis by X-ray powder diffraction dates back to 1925 (Navias 1925). In this work, the amount of mullite obtained by firing selected clays (and a feldspar) was determined by the direct comparison of the intensities of two diffraction lines of the fired samples with those of pure mullite. The patterns were recorded on photographic negatives after an X-ray exposure of 165 hours (almost a week). Quantitative phase analysis (QPA) from diffraction data can be obtained from a number of methods explained in classical books (Klug and Alexander 1974; Cullity 1978; Snyder and Bish 1989; Zevin and Kimmel 1995; Jenkins and Snyder 1996). However, it is now safe to say that QPA from powder diffraction data is nowadays mainly based on the Rietveld methodology (Rietveld 1969; Hill and Howard 1987; Bish and Howard 1988; Bish and Post 1993; Madsen and Scarlett 2008). Hence, …

Temperature feature on synthesizing mullite whiskers from coal fly ash and andalusite-sericite phyllite

Mullite whiskers were successfully synthesized using coal fly ash, andalusite-sericite phyllite, NH4Al(SO4)2·12H2O and SiO2, with a slight addition of NaH2PO4·2H2O by designing composition after sintered at different temperatures. The mullite whisker morphology, structure and formation mechanism were investigated by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The results show that the forming temperature of pure mullite from andalusite-sericite phyllite is lower than that from coal fly ash owing to more silica glass phase. The mullite microcrystals begin to grow at approximately 1200 °C for andalusite-sericite phyllite and 1300 °C for coal fly ash, and both grow into long whiskers at around 1400 °C. With the prescription of Al2O3/SiO2 molar ratio of 0.32, sintered at 1400 °C for 6 h and chemically leached with HF solution to remove glass matrix in the products, uniform mullite whiskers with 0.2–0.5 μm (aspect ratio 30, prepared from high-aluminium coal fly ash) and 0.3–3 μm (aspect ratio 10–30, prepared from andalusite-sericite phyllite) in diameter can be well prepared.

Synthesis and Characterization of Ti, Fe-Doped Mullite-I

A series of gels with 3Al2O3•2SiO2 were prepared by Sol-gel method and heated at several temperatures for 2 h to synthesize Ti, Fe-doped mullite. The powers were characterized by differential thermal analysis (DSC-TG) and X-ray powder diffraction (XRD). Phase separation was promoted by doping both TiO2 and Fe2O3; with increasing the amount of dopant ions the formation temperature of Si-Al spinel decreased and the formation temperature of mullite increased by TiO2 doping but decreased by Fe2O3 doping. The formation temperature of pure mullite was about 1250-1350 °C.