Mullite Structures Research Articles

Coordinated regulation for α-Al2O3 and mullite structures in the alumina-mullite fiber based on the different adding form of Fe element

The coordinated regulation for α-Al2O3 and mullite structures in the alumina-mullite duplex fiber poses a challenging yet crucial task in achieving a fiber with exceptional high-temperature properties. In this study, Fe element was added into the alumina-mullite fibers in the form of iron sol and ferric nitrate solution, respectively, with the aim of obtaining a fiber consisting of fine mullite grains embedded with nano α-Al2O3 grains. The results revealed that the addition of Fe element significantly enhances the crystallization process of mullite in alumina-mullite fibers. Specifically, adding iron sol resulted in the reduce of α-Al2O3 formation temperature to 1300 °C, while introducing Ferric nitrate solution promoted the formation of mullite phase. Simultaneous introduction of 1.3 wt% iron sol and 1.3 wt% Ferric nitrate solution contributed to achieve alumina-mullite fiber with tensile strength of 2.06 GPa. In addition, the influence mechanism of Fe source on the phase transformation and microstructures of alumina-mullite fiber were discussed.

Thermal and structural performance of additively manufactured ceramic porous media burners

This study examines the effect of porosity, topology and material composition on the performance of additively manufactured ceramic porous media burners. Thermal-structural simulations of triply periodic minimal surfaces (TPMS) were performed to investigate thermal-stress magnitude and distribution. Alumina and mullite TPMS structures were 3D-printed and tested in a methane-air combustion experiment, where the mullite structures showed superior durability compared to alumina. X-ray imaging revealed notable correlation between predicted high tensile stress regions and experimental crack formation. At equivalent porosity and cell size, TPMS structures with higher specific surface area and tortuosity were found to have lower thermal strain and propensity for structural failure. Structures with these features, namely diamond and I-WP, demonstrated favorable thermal and structural behavior under identical thermal conditions. These findings can guide the design and manufacturing of high-temperature applications with embedded porous structures, such as heat exchangers, catalytic converters, and thermal management systems.

Mullite-silica scaffolds obtained by stereolithography and reaction sintering

Mullite is a widely used refractory ceramic due to its good properties and high temperature stability. The necessity to obtain complex mullite architectures has encouraged the development of new AM techniques with improved resolution and surface finishing quality. In the present work, mullite-based scaffolds were obtained by stereolithography using a commercial silica resin and alumina nanoparticles following two routes: infiltration of printed porous silica parts with a colloidal alumina sol and printing of silica/alumina parts by developing a photocurable alumina resin that is mixed with the silica one. Post-processing of the obtained parts was carried out, including debinding and subsequent reaction sintering of the starting ceramic materials to give rise to mullite phase. All materials were characterised in terms of composition and microstructure, demonstrating that the second route results more effective to produce mullite structures in the final parts due to the higher alumina concentration and the larger reaction surface.

Flaş sinterleme yönteminin uçucu kül mikroyapısına etkisi

Bu çalışmada termik santral atığı olan cam seramik malzemelerden uçucu kül’ ün yoğunlaştırılması (sinterlenmesi) elektrik alan destekli flaş sinterleme yöntemiyle gerçekleştirilmiştir. Uçucu Kül numuneleri Tunçbilek Termik Santralinden (Kütahya, Türkiye) temin edilmiştir. Flaş sinterleme deneyinde 0,85 A akım üst limiti belirlenerek 450 V/mm elektrik alan uygulanmıştır. XRD analizine göre yapıda camsı faz, kuvars, hematit, manyetit, magnezyoferrit ve müllit yapıları bulunmaktadır. Kimyasal analizi (XRF) ile içeriğinde belirlenen yüksek orandaki SiO2, Al2O3 ve Fe2O3’ün tek başlarına ve/veya birlikte oluşturdukları cam-seramik malzemelerin sinterlenmesi 1500℃ ve üzeri sıcaklıklarda gerçekleştiği literatürde rapor edilmiştir. Elektrik alanlı flaş sinterleme işlemi uçucu külün ortalama sinter sıcaklığını 1500℃’lerden 1137℃’ye düşürürken sinter süresini de 3 saatten 70 saniyeye çekmiştir. Bu yöntem ile gerçekleştirilen sinterleme işlemi geleneksel sinterleme yöntemlerinden daha düşük sıcaklık ve daha kısa sürede gerçekleştirilmiştir. Literatürdeki uçucu kül sinterleme işlemlerinin SEM mikrografları ile karşılaştırıldığında daha yoğun bir yapı sinter işleminin başarılı bir şekilde gerçekleştiğini kanıtlanmıştır.

Low‐cost inkjet printing of thin‐film mullite structures

AbstractA commodity inkjet printer was modified to print layered ceramic structures on stiff glass substrates, using inks in the form of sols with mullite stoichiometry. The sol composition was optimized in terms of ink viscosity, surface tension, and evaporation properties. The ink was then fed into a multi‐nozzle commodity printer head and deposited by ink‐jetting. Thin lines without bulges with a thickness of ∼1.8 μm with concave profile and a line width of ∼100 μm were successfully printed on fused silica substrates. The presence of mullite was confirmed by Raman spectroscopy and XRD after sintering of printed structures.

Low-cost inkjet printing of thin-film mullite structures

Low-cost inkjet printing of thin-film mullite structures

Effect of MgO sintering additive on mullite structures manufactured by fused deposition modeling (FDM) technology

An optimized recipe for 3D printing of Mullite-based structures was used to investigate the effect of MgO sintering additive on the processing stages and final ceramic properties. To achieve dense 3:2 mullite, ceramic filaments were prepared based on an alumina powder, a methyl silicone resin, EVA elastomeric binder and MgO powder. Using 1 wt% MgO and a dwell time of 5 h at 1600 °C, a dense mullite structure could be obtained from filaments with a diameter of 1.75 mm. Ceramic structures with and without sintering additive were printed in vertical and horizontal direction, to investigate the effect of printing direction on mechanical strength after sintering. Using four-point bending test, it was demonstrated that by using MgO, the printing orientation did not affect the mechanical strength significantly anymore. The low Weibull modulus could be explained by the closed porosity that emerge during the degassing of the preceramic polymer due to cross-linking.

An advanced ash fusion study on the melting behaviour of coal, oil shale and blends under gasification conditions using picture analysis and graphing method

• Ash fusion behaviour was investigated via an image-based characterisation method. • Oil shale ash is an effective refractory additive to delay ash fusion of coal. • Low melting aluminosilicates reduces sintering temperature of ash samples. • Sinter strength of ash pellet increases with dilatometric shrinkage. • Oil shale improves ash chemistry without adversely affecting reaction quality. This study investigates the potential of solid fuel blending as an effective approach to manipulate ash melting behaviour to alleviate ash-related problems during gasification, thus improving design, operability and safety. The ash fusion characteristics of Qinghai bituminous coal together with Fushun, Xinghua and Laoheishan oil shales (and their respective blends) were quantified using a novel picture analysis and graphing method, which incorporates conventional ash fusion study, dilatometry and sintering strength test, in a CO/CO 2 atmosphere. This image-based characterisation method was used to monitor and quantify the complete melting behaviour of ash samples from room temperature to 1520 °C. The impacts of blending on compositional changes during heating were determined experimentally via X-ray diffraction and validated computationally using FactSage. Results showed that the melting point of Qinghai coal ash to be the lowest at 1116 °C, but would increase up to 1208 °C, 1161 °C and 1160 °C with the addition of 30%–50% of Laoheishan, Fushun, and Xinghua oil shales, respectively. The formation of high-melting anorthite and mullite structures inhibits the formation of low-melting hercynite. However, the sintering point of Qinghai coal ash was seen to decrease from 1005 °C to 855 °C, 834 °C, and 819 °C in the same blends due to the formation of low-melting aluminosilicate. Results also showed that blending directly influences the sintering strength during the various stages of melting. The key finding from this study is that it is possible to mitigate against the severe ash slagging and fouling issue arising from high calcium and iron coals by co-gasification with a high silica-alumina oil shale. Moreover, blending coals with oil shales can also modify the ash melting behaviour of fuels to create the optimal ash chemistry that meets the design specification of the gasifier, without adversely affecting thermal performance.

Designing Colloidal Silica-Bonded Porous Structures of In-situ Mullite for Thermal Insulation

Abstract: Colloidal silica (CS) is a promising raw material for refractory castable ceramics. It consists of stable suspensions of synthetic amorphous silica nanoparticles that behave simultaneously as liquid medium and binder for ceramic particles and as a porogenic agent and highly reactive source of silica to promote in-situ reactions. The setting mechanism of CS balances two opposite effects. Adding more CS to a suspension increases the bonding potential for gelling reactions and strengthening; on the other hand, it also introduces more water into the system, enhancing pore content. Such effects can be advantageously employed in the preparation of porous structures from aqueous suspensions and applied as high-temperature thermal insulators. The present study addresses the production of porous structures of in-situ mullite attained from aqueous suspensions of highly porous transition alumina particles bonded with colloidal silica. Different grades of CS and transition aluminas were combined to present suitable workability (flowability and gelling time) and to generate stoichiometric mullite or mullite-alumina porous structures after sintering.

Refractory and heat-insulating materials based on aluminosilicate SHS compositions

The phase composition, structure, and physical and mechanical characteristics of refractory aluminosilicate SHS materials (AS materials) were investigated taking into account the properties of initial components and the production method. Computational methods were used to reveal macrokinetics of processes and determine the possible formation of mullite structures and corundum in aluminosilicate refractory SHS materials during the synthesis reactions. Heat losses caused by the small thickness of a coating or the walls of the material between pores were found to have a significant effect on the synthesis reaction in thin coatings and porous AS materials, as well as the on the formation of various chemical compounds.

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.

Complex mullite structures fabricated via digital light processing of a preceramic polysiloxane with active alumina fillers

By taking advantage of the multi-functional properties of preceramic polymers, their transformation into ceramic material at low sintering temperatures and the processing capabilities of polymer manufacturing processes, mullite components were fabricated by additive manufacturing. A photocurable silicone preceramic polymer resin containing alumina particles was shaped into complex structures via Digital Light Processing. Dense and crack-free, highly complex porous mullite ceramics were produced by firing a mixture of a commercially available photosensitive polysiloxane as the silica source, containing alumina powder as active filler, in air at a low sintering temperature (1300 °C). In particular, the developed formulations, coupled with the additive manufacturing approach, allow for precise control of the architecture of the porous ceramic components, providing better properties compared to parts with stochastic porosity.

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.

K–Al-based mixed oxides as high-capacity carbon dioxide adsorbents

K–Al-based mixed oxides (KAl6O9.5) with mullite structures were synthesized as CO2 adsorption materials using a polymerized complex method. Al3+ sites in the octahedral AlO6 units of K–Al-based mixed oxides were substituted with various metal ions with +2 or +3 valence states to enhance basicity. Among these samples, the Fe-introduced sample (KAl5.5Fe0.5O9.5) showed 130 times higher CO2 adsorption capacity than that of Li4SiO4. Raman spectra of these samples indicated that large distortions of the AlO6 unit were observed only for the Fe-introduced sample. Local polarization caused by such distortions could induce increased basicity of this sample.

Porous co-continuous mullite structures obtained from sintered aluminum hydroxide and synthetic amorphous silica

Porous materials produced from sintered Al(OH)3 show a potentially useful α-Al2O3-based coral-like co-continuous microstructure of high porosity (above 70%) and chemical resistance. However, due to the lack of efficient connections among the particles of the solid phase, their poor mechanical properties limit their use in biomechanical and thermo-mechanical applications, as scaffolds for bone tissue and hot air filters, respectively. In this study, authors improved these connections reinforcing the structure with a sintering aid (synthetic amorphous silica, SAS). Al(OH)3 particles (previously sintered at 1500°C, 5h) were imbibed with SAS particles, compacted and sintered at 1300°C, which generated a coral-like mullite-based porous structure. The porosity levels of the material (47%) were similar to those of the initial green state (50%) and achieved high levels of mechanical properties (flexural strength of 50.29MPa, elastic modulus of 26.00GPa), with small linear thermal shrinkage (lower than 6% at 1500°C).

Hierarchical mullite structures and their heat-insulation and compression–resilience properties

Near-net-shaped hierarchical structure-adjustable short mullite fibers/mullite whiskers frameworks (MF/MW frameworks) were prepared by slurry-filtration and heat-treating method. The main structure of MF/MW framework was constituted by lap-jointed mullite fibers. Every single fiber in the framework was densely covered by mullite whiskers which formed through fluorine-catalyzed gas-phase reaction, and the fibers actually served as curved substrates for the mullite whiskers' growth. The lap-jointing points of the fibers were served by movable intersected mullite whiskers. Moreover, the microstructure of the frameworks could be adjusted by tailoring the raw materials mass ratio. The volume densities, the apparent porosities and the thermal conductivities of the MF/MW frameworks in different raw materials mass ratios were 0.459–0.487g/cm3, 79.7–82.8% and 0.1356–0.1965W/km, respectively. The compression–resilience property of the samples was tested under 0.4MPa at room temperature. The compression ratio and resilience ratio of the MF/MW frameworks in different raw materials mass ratios were 1.63–2.25% and 92.67–98.16%, respectively. The MF/MW frameworks with advanced thermal and mechanical properties were considered to be promising high-temperature heat-insulation material.

Synthesis of the mullite refractory ceramic coating under local heating

The possibility of performing the thermochemical synthesis in the M1 oxide ceramic coating under surface heating in order to produce mullite structures is considered with the aim of extending the field of application of protective oxide ceramic coatings and decreasing the power consumption. The coating is applied to a ShA chamotte brick. Then, the synthesis of mullite and mullite-like structures is provided with the use of surface heating by thermochemical compositions and the thermal insulation prepared from the refractory cellular concrete for the control of the heat removal.

Silica-Free Mullite Structures in the Al2O3-B2O3-P2O5 Ternary System

The stability of silica-free mullite structures in the Al2O3-B2O3-P2O5 ternary system was investigated at T ) 900-1000 °C. A monophasic mullite was obtained for the composition Al8BPO16, showing that substitution of P and B for Si is possible for 2:1 mullite, Al4SiO8, while the same substitution for the 3:2 mullite yielded a biphasic mixture. Another monophasic sample was obtained in the same temperature range, with composition Al8.6P0.4B1.6O16.3. Further preparations close to the above compositions enabled us to assess the homogeneity range for these silica-free mullites. At 1000 °C it can be expressed by the following notation: Al8+xP1-xB1+xO16+x/2 with 0 e x e 0.6 ( 0.1. The stability range expressed by the above relation results lie on the join 2:1 silica-free mullite-Al9B2O16.5. Neutron and X-ray powder diffraction were performed on the two limit compositions (with x ) 0 and x ) 0.6), showing that phosporus is likely to enter the tetrahedral T site. Boron occupies a site with triangular coordination, between T and T* sites, at a distance of 0.6 A from the center of the T polyhedron.

Electron paramagnetic resonance and Mössbauer spectroscopy of transition metal ion doped mullite

Appreciable difference in the properties of undoped and oxide-doped mullite are observed. The oxidation state of cation, its concentration and the position of the mullite lattice occupied by it appear to be the responsible factors. Mullite has, therefore, been doped with four transition metal ions, Mn, Fe, Cr and Ti. With the help of EPR and Mossbauer spectroscopy (supplimented by X-ray diffractometry) the oxidation states of these ions and the mullite lattice sites where they enter has been investigated. It was observed that Mn ion was present in Mn2+ and Mn3+ states, the former remained as clusters and the latter occupied the octahedral sites in the mullite lattice. Only Fe3+ ion was detected and conclusive evidence was obtained for the entry of Fe3+ in the octahedral lattice position of mullite from the analysis of Mossbauer spectra with the help of a specially written computer programme. The Cr ion entered the mullite structure only in the Cr3+ state. The change in lattice parameters of Cr doped mullite were measured by the XRD technique. The results showed that the expansion of b-axis was more than that of the a-axis which supported the presence of Cr3+ ion in the octahedral site of mullite lattice. The absence of signal in the EPR spectra of Ti doped mullite suggested the presence of only Ti4+ (3d0) ion. Very low electrical resistivity of Ti doped mullite and close similarity between mullite and Al2TiO5 structures stood as evidence for incorporation of Ti4+ ion in the octahedral site of mullite lattice by replacing Al3+ ion.

Elongated mullite crystals obtained from high temperature transformation of sillimanite

The high temperature transformation of sillimanite to mullite has been studied by means of dilatometry, X-ray diffraction, FT-IR and scanning electron microscopy (SEM-EDS) techniques. The results obtained showed that total transformation of sillimanite to mullite takes place between 1550 and 1600°C. A rapid nucleation occurs above 1550°C due to the resemblance of sillimanite and mullite structures. As a consequence of the high temperature of mullitization of sillimanite, the grain growth of elongated mullite crystals takes place almost simultaneously with the nucleation stage. Finally, a step of acid leaching was employed to remove the glassy phase and high-grade purity mullite powders were obtained.