Mullite Ceramics Research Articles

Improvement in sinterability and high-temperature mechanical properties by grain boundary design for high purity mullite ceramics: Crystallization of grain-boundary glassy phase

Improvement in sinterability and high-temperature mechanical properties by grain boundary design for high purity mullite ceramics: Crystallization of grain-boundary glassy phase

Effects of SiC content on thermal shock behavior and elastic modulus of cordierite–mullite composites

Cordierite (Mg2Al4Si5O18) is a commercially available ceramic with low fracture toughness that hampers its broad industrial applications. Although several studies have reported the mechanical improvement of cordierite using various reinforcements, modulating its mechanical and thermal shock characteristics is not explored precisely. In the present research, we investigated the manufacturing of cordierite–mullite ceramics and the role of SiC on their thermomechanical properties. The in-situ formed mullite particles were obtained by mixing andalusite-talc-alumina and addition of SiC. It was found that thermal shock behavior and elastic moduli are dependent on SiC content and retained porosity. Furthermore, the addition of SiC to cordierite-based ceramics could enhance the thermal shock resistance via proper activation of the crack bridging mechanism in the matrix of the prepared composite.

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

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

Mullite phase evolution in clay with hydrated or anhydrous AlF3

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

Corrosion Resistance Properties of Porous Alumina–Mullite Ceramic Membrane Supports

Cost‐effective macroporous alumina–mullite membrane supports are fabricated by sintering coarse α‐alumina and colloidal silica, using cornstarch as the foaming agent at 1550 °C for 2 h in air atmosphere. To evaluate the corrosion resistance of such membrane supports, the corrosion behaviors of the supports before and after corroding in the H2SO4 and NaOH solutions (80 °C) are systematically investigated. Interestingly, with increasing Al2O3/SiO2 ratio, the support presents numerous alumina grains and little glass phases, resulting in excellent corrosion resistance especially in acidic media. Benefiting from the alumina grains and little glass phases among the grain boundaries, lower mass loss, less variations in pore size and porosity, high residual strength, and an almost unchanged gas flux are obtained for a higher Al2O3/SiO2 ratio support. The porous support, herein, is suitable for microfiltration membranes used in harsh environments.

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

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

Fabrication and microstructures improvement of porous mullite ceramics based on sol-treated sawdust

Fabrication and microstructures improvement of porous mullite ceramics based on sol-treated sawdust

Porous mullite ceramics with a fully closed-cell structure fabricated by direct coagulation casting using fly ash hollow spheres/kaolin suspension

Porous mullite ceramics with a fully closed-cell structure were fabricated by direct coagulation casting (DCC) using fly ash hollow spheres (FAHSs) as the non-sacrificial pore-forming agent. Kaolin powder was used as the filler and the suspension stabilizer, and bentonite was used as the rheological additive, respectively. The effects of bentonite on the stabilization mechanism of suspension and properties of the porous ceramics were investigated. It is found that 1 wt% additional bentonite is efficient in improving the suspension stability from 2 h to at least 24 h, due to increased suspension viscosity, thixotropy and yield stress. Densification of the kaolin powder during sintering eliminates the open pores, and the closed pores from FAHSs remain to yield a fully closed-cell structure. The porous mullite ceramics have a total porosity ranging from 44.73% to 46.12%, in which 99% of the pores are closed. Bentonite also increases the compressive strength of porous mullite ceramics by 14.4%, and compressive strength of 58.07 ± 5.44 MPa can be achieved.

Mechanical Properties and Thermal Conductivity of Ytterbium-Silicate-Mullite Composites.

Various Ytterbium-Silicate-Mullite composites were successfully fabricated by adding Yb2SiO5 into mullite ceramics and then using pressureless sintering at 1550 °C. The influence of Yb2SiO5 addition on the microstructure, mechanical properties, and thermal conductivity of ytterbium-silicate-mullite composites was investigated. Results show that the composites mainly consisted of a mullite matrix and some in situ formed Yb2Si2O7 and Al2O3 phases. By the addition of Yb2SiO5, the Vickers hardness of composites reached ~9.0 at an additive concentration of 5 mol %. Fracture toughness increased to ~2.7 MPa·m1/2 at the additive concentration of 15 mol %, owing to the trans-granular fracture and crack deflection of the pinning effect of the Al2O3 phase in the composites. With the increase of the Al2O3 phase in the composite, the thermal conductivity for the 15YbAM reached around 4.0 W/(m·K) at 1200 °C. Although the thermal conductivity increased, it is still acceptable for such composites to be used as environmental barrier coatings.

Preparation and Thermal Shock Resistance of Mullite and Corundum Co-bonded SiC Ceramics for Solar Thermal Storage

Mullite and corundum co-bonded SiC-based composite ceramics (SiC-mullite-Al2O3) were prepared by using SiC, calcined bauxite and kaolin via pressureless carbon-buried sintering. The low-cost SiCbased composite ceramics designed in this study are expected to be used as thermal storage materials in solar thermal power generation based on the high density and excellent thermal shock resistance. The influences of calcined bauxite addition and sintering temperature on the microstructures, phase compositions, and physical properties of the samples were investigated. Results demonstrated that the introduction of calcined bauxite containing two bonding phases greatly reduced the lowest sintering temperature to 1 400 °C. The SiC-mullite- Al2O3 composite with 40 wt% calcined bauxite sintered at 1 500 °C exhibited optimum performance. The density and bending strength were 2.27 g·cm−3 and 77.05 MPa. The bending strength increased by 24.58% and no cracks were observed after 30 thermal shock cycles, while general clay would reduce the thermal shock resistance of SiC. The SiC-mullite-Al2O3 composites with satisfied performance are expected to be used as thermal storage materials in solar thermal power generation systems.

Low-temperature densification and mechanical properties of monolithic mullite ceramic

Dense mullite ceramics with excellent mechanical properties were produced using three kinds of mullite powders, namely micro-, submicro-, and nano-powders. The influence of sintering temperature on the microstructural and mechanical properties of the produced mullite ceramics was investigated. As the sintering temperature increased from 1200 to 1500 °C, the relative density of the mullite ceramic produced using micro-powder varied from 65.3 to 99.2%, leading to an increase in the flexural strength and fracture toughness from 87.6 to 245.7 MPa, and 1.4 to 2.5 MPa m1/2, respectively. The densification temperature of mullite ceramics significantly reduced when submicro- and nano-powders were used as raw materials. The relative density and mechanical properties of mullite ceramics fabricated using submicro- or nano-powders increased at first, then decreased following an increase in the sintering temperature. The mullite ceramic produced using submicro-powder became compact after being sintered at 1200 °C, and exhibited a high flexural strength of 254.7 MPa and fracture toughness of 2.9 MPa m1/2, indicating that low-temperature densification of mullite ceramics can be realised by enhancing the sintering activity of the mullite powder.

Compressive Strength and Thermal Conductivity of Porous Mullite Ceramics

Porous ceramics are a form of ceramic material with widespread applications, such as filters, isolators, and acoustics. This research aimed to investigate the compressive strength and thermal conductivity of porous mullite ceramics (3Al2O3.2SiO2 or Si2Al6O13) produced using dry yeast powder as a PFA (pore-forming agent). Kaolin powder (Al2O3.2SiO4.2H2O or Al2Si2O5(OH)4) was used as a raw material for producing mullite ceramics. Kaolin powder and dry yeast powder were mixed at a variety of dry yeast powder weight fractions: 0%, 5%, 10%, 15%, 20%, and 25%. The composition was mixed using Turbula Mixer for 1 hour. The cylindrical green body (diameters of 12 mm, 15 mm, and 30 mm) of every constituent was formed by the uniaxial pressing method at 10 MPa. Monolithic kaolin was sintered at variable temperatures (1,100 oC, 1,200 oC, 1,300 oC, 1,400 oC, 1,450 oC) for 2 hours then subjected to several tests for its density. From the bulk density tests, it was found that the optimum temperature for 2-hour sintering was 1,450 oC. This temperature was then used for the sintering process of the kaolin specimens which contained dry yeast powder. Testing was performed on the microstructure, bulk density, burning waste in mass and volume, compressive strength, and thermal conductivity. According to the literature, kaolin will transform into mullite (3Al2O3.2SiO2 or Si2Al6O13) and cristobalite (SiO2) at 1,450 oC. It was found that with the increase in the content of dry yeast powder as PFA in the mixture with kaolin, the bulk density decreased (from 2.44 gr/cm3 to 1.521 gr/cm3), the porosity increased (from 23.77% to 52.48%), the compressive strength decreased (from 38.04 MPa to 4.51 MPa), and the thermal conductivity decreased (from 3.76 W/moC to1.34 W/moC), each from yeast powder content 0% to 25%.

High-strength thermal insulating mullite nanofibrous porous ceramics

Mullite fibrous porous ceramics is one of the most commonly used high temperature insulation materials. However, how to improve the strength of the mullite fibrous porous ceramics dramatically under the premise of no sacrificing the low sample density has always been a difficult scientific problem. In this study, the strategy of using mullite nanofibers to replace the mullite micron-fibers was proposed to fabricate the mullite nanofibrous porous ceramics by the gel-casting method. Results show that mullite nanofibrous porous ceramics present a much higher compressive strength (0.837 MPa) than that of mullite micron-fibrous porous ceramics (0.515 MPa) even when the density of the mullite nanofibrous porous ceramics (0.202 g/cm3) is only around three quarters of that of the mullite micron-fibrous porous ceramics (0.266 g/cm3). The obtained materials that present the best combination of mechanical and thermal properties can be regarded as potential high-temperature thermal insulators in various thermal protection systems.

The formation of phases with low or negative linear thermal expansion coefficient in porous mullite ceramics

The formation of phases with low or negative linear thermal expansion coefficient in porous mullite ceramics

Microstructure and mechanical performance of porous mullite ceramics added with TiO2

Porous mullite samples were manufactured using granular mullite powders as major materials, and TiO2 as the additive agent. Influences of TiO2 contents on rheological characteristics of slurries, and microstructure and bending strength at 25–1400 °C of final porous mullite products were investigated. Adding appropriate content of TiO2, a large amount of columnar mullite with mean length of 23.7 μm and the length-to-diameter ratio of 6.7 was in-situ formed, which could improve the mechanical performance of the resultant porous samples. The bending strength at 25 °C of porous products containing 67% porosity fabricated by using 7 wt% TiO2 reached 12.5 MPa, and its bending strength at 1200 °C remained as high as 6.1 MPa.

Preparation and structure control of a scalelike MoSi2-borosilicate glass coating with improved contact damage and thermal shock resistance

The scalelike coatings with MoSi2 as emittance agent, flake fused quartz as coarse fillers, silica sol as dispersive medium of coating slurry, were prepared on rigid mullite fibrous ceramics to avoid fatal radial cracks and improve thermal shock resistance via combining slurry method with sol-gel method. And the particle size distribution of fused silica was changed to get different crack structure in scalelike coatings. Microstructure and phase composition of the coatings with different crack structure were investigated comprehensively. Contact damage resistance, thermal shock resistance and infrared radiating property were also studied. The results showed that only the coatings with mixing large and small particles of fused silica as coarse fillers could form crack network. The scalelike coatings with crack network went through 25 thermal cycles between 1500 °C and room temperature without peeling and spalling, and the bonding strength did not decrease after test. The scalelike structure with crack network also avoided radial cracks and exhibited some softness and less stiffness in Hertzian indentation test. The emissivity of coatings with different crack structure were all higher than 0.85; and the coatings with lower proportion of crack areas had higher emissivity.

Fabrication of Al2O3-Bonded Fibrous Ceramics by An Aqueous Gel-Casting Process

The traditional gel-casting method mostly uses organic compounds, which are complex process, expensive and pollute the environment. In order to overcome the disadvantage of the traditional gel-casting method, an environment friendly aqueous gel-casting process is introduced in this paper. Al2O3-bonded fibrous ceramics with high porosity and low thermal conductivity have been prepared by a novel hydrated alumina gel-casting process using the ρ-Al2O3and mullite fibers or YSZ fibers as raw materials and the deionized water as hydration agent. A bird-nest-patterned three-dimensional reticular skeleton structure was established by the fibers to ensure a certain mechanical strength. The effects of the amount of mullite fibers andYSZ fibers on the properties of porous ceramic, such as phase composition, microstructure, sintering behavior and relative properties were investigated. Results showed that the as-prepared Al2O3-bonded fibrous mullite ceramics have high porosity of 66.9%∼78.6%, low thermal conductivity of 0.236∼0.479 W/m·K, and relatively high compressive strength of 0.80∼2.78 MPa. The as-fabricated fibrous YSZ ceramics had a high porosity (71.1%∼72.7%), a low thermal conductivity (0.209∼0.503 W/m·K) and a relatively high compressive strength (3.45∼4.24 MPa). Comparing to the as-fabricated fibrous mullite ceramics, the compressive strength of as-fabricated fibrous YSZ ceramics is promoted obviously.

Effect of Micro SiC Addition on the Microstructure and Thermal Shock Resistance of 3D Printed Mullite Contained Ceramics

Currently, mullite contained ceramics printed by DIW 3D printing cannot be used in a high temperature environment for multicycle. In the fields of environmental protection and filtration of high-temperature flue gas, 3D printed mullite contained ceramics is in widely demanded owing to the variety of structures. Thus, application of these products have been limited due to their low thermal shock resistance. In order to improve the thermal shock resistance of 3D printed mullite contained ceramics for high-temperature flue gas filter, in this work, the effect of SiC micron particles addition on the thermal shock resistance was investigated. The thermal shock behavior of 3D printed mullite ceramics was explored using a conventional water quenching technique. Furthermore, XRD and SEM were used for determining phase composition and microstructure evaluation, respectively.

Fabrication, microstructure and physico-mechanical properties of highly porous acicular mullite ceramic foams by freeze casting method

In this article, highly porous mullite ceramics with whisker-interlocked microstructure were successfully prepared by freeze casting method. The ceramics possessed very high porosity (up to 94%) and high compressive strength (up to 3.65 MPa). It was found that polyvinyl alcohol (PVA) using as a binder could also affect the microstructure of ceramics significantly. Whisker-interlocked structure could be obtained when PVA concentration was high (4 wt%), while low PVA concentration (1%) led to the aligned lamellar pore structure. Moreover, the ceramic samples had small shrinkages of less than 3%, and solid loading in slurry was found to influence the porosity and compressive strength of the ceramics greatly.

Synthesis and mechanical properties of mullite ceramics with coal gangue and wastes refractory as raw materials

AbstractWith coal gangue and high alumina refractory solid wastes as raw materials, needle‐like mullite powder, with an average diameter of about 1 μm, was synthesized at 1300°C by using the conventional solid‐state reaction method. Mullite ceramics were derived from the inexpensive needle‐like powder. Phase composition was examined by using X‐ray diffraction (XRD), while morphologies of the ceramics were observed by using scanning electron microscopy. The content and distribution of elements in the sintered samples were characterized with energy dispersive spectrometer and X‐ray fluorescence spectroscopy. Mechanical properties of the mullite ceramics were studied by using the three‐point bending method. The aspect ratio of the needle‐like mullite particles was up to 6. The mullite sample sintered at 1500°C for 3 hours had a density of 2.515 g·cm−3, which was slightly lower than the theoretical density. Maximum fracture toughness and bending strength of the mullite ceramics were 1.82 MPa·m1/2 and 71.76 MPa, respectively. This study realizes the resource utilization of gangue and high alumina refractory solid wastes, and the prepared mullite ceramics have good application prospect.