Porous Mullite Research Articles

Recycling photovoltaic silicon waste for fabricating porous mullite ceramics by low-temperature reaction sintering

In this study, porous mullite ceramics with coral-like structures were fabricated at a low temperature of 900 °C by using photovoltaic silicon waste (PSW) as the silicon source directly. The effects of additive content and sintering temperature on the mullitization reaction of green bodies were studied. The results showed that ammonium molybdate tetrahydrate molybdenum (H24Mo7N6O24·4H2O) as an additive could reduce the reaction temperature for mullitization from 1100 °C to 900 °C. The research on the influence of catalyst on material properties showed that porous mullite ceramics with a flexural strength of 52.83 MPa, a 41.78 % porosity, a sintering expansion rate of 0.49 % and an average pore size of 0.23 μm could be fabricated by introducing 7.5 % H24Mo7N6O24·4H2O at the sintering temperature of 1000 °C. This study develops an environment-friendly recycling method of PSW and provides a new idea for the low-cost preparation of porous mullite ceramics with high purity.

Preparation of environmentally friendly low‐cost mullite porous Ceramics and the effect of Waste Glass Powder on structure and mechanical Properties

Environmentally friendly low-cost porous ceramics are prepared with 98.2 wt% mining waste as the raw materials at 1180 °C. The glass powder forms liquid phase at high temperatures, promoting the densification of the materials. X-ray diffraction (XRD) results show that the formation of mullite is affected by sintering temperature and content of liquid phase. The enhancement in densification is verified by the scanning electron microscopy (SEM) analysis results. According to the evolution of pore structure, the effect of the glass powder on the structure is confirmed. The number ratio of pores of < 8 μm to those of 8 ~ 20 μm is varied from 7:2 to 2:7. The minimum apparent porosity is 31.22 %, while the maximum density and linear shrinkage are 1.75 g/cm3 and 8.48 %, respectively. The flexural strength (three-point bending method) was increased from 7.47 MPa to 28.36 MPa, indicating that the addition of glass powder could increase mechanical properties of the porous ceramics. Therefore, solid wastes can be used as the resource to develop porous mullite ceramics.

Fabrication and characterization of porous mullite ceramics with ultra-low shrinkage and high porosity via sol-gel and solid state reaction methods

Porous mullite ceramics with ultra-low shrinkage and high porosity were prepared by solid state reaction between MoO3 and mullite precursor powders which were synthesized from tetraethylorthosilicate and aluminium nitrate nonahydrate via sol-gel methods. The synthetic process of mullite precursor powder and effects of MoO3 amount on the phase composition, microstructure, physical properties such as firing shrinkage, open porosity, bending strength, water absorption and bulk density of porous mullite ceramics were investigated. The results indicated that the addition of MoO3 not only lowered the mullite forming temperature from 985.4 to 853.3 °C, but also restrained densification behavior of samples due to the formation of mullite and Al2O3–MoO3 solid solution, besides, MoO3 also improves the formability, open porosity and bending strength of samples. The optimal amounts of MoO3 is 8 wt%, and the resultant samples exhibit outstanding properties, including a low shrinkage rate of 1.86 ± 0.07%, an open porosity of 61.91 ± 0.16% and a bending strength of 9.35 ± 1.11 MPa.

Preparation and strengthening mechanisms of in-situ HF catalyzed highly porous mullite with whisker frame structure

Using HF as catalyst and based on sol-gel chemical precursor powder molding and sintering process, porous mullite with whisker frame structure was in-situ synthesized in the blank body. Due to the high efficiency catalysis of HF, this structure has both of high porosity and high strength. The effects of HF addition and sintering temperature on the formation of mullite whisker frame, phase transition process, porosity and strength of porous materials were discussed, and the optimal process parameters were determined. The mechanism of strengthening the porous materials was preliminary analyzed, and under the catalysis of HF, the close connection between whisker frame lap joints was formed. When the porosity of mullite is up to 83.22%, the compressive strength of can reach 16.48 MPa.

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.

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.

Characterization of High-Temperature Hierarchical Porous Mullite Washcoat Synthesized Using Aluminum Dross and Coal Fly Ash

Mixture of aluminum dross (AD) and coal fly ash (CFA) was used to produce high-temperature porous mullite for washcoat application. CFA is the combustion by-product of pulverized coal in a coal-fired power plant, while AD is a waste product produced in secondary aluminum refining. In this study, 80 wt% of AD and 20 wt% of CFA was used to prepare a mullite precursor (MP) via acid leaching and dry-milling. The precursor was coated on a substrate and subsequently fired at 1500 °C. The results showed that the precursor transformed to a hierarchical porous microstructure assembled by large interlocked acicular mullite crystals. The pore structures consisted of large interconnected open pores and small pores. The specific surface area of the mullite washcoat was 4.85 m2g−1 after heating at 1500 °C for 4 h. The specific surface area was compatible with the specific surface area of other high-temperature washcoats.

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%.

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.

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.

Studies on the Densification and Strengthening of Porous Mullite Ceramic in the Presence of Titania Additive

Abstract: Porous mullite was prepared using clay and alumina, with naphthalene used as a pore former. Titania additives of 2, 4 and 6 mass-% were used to improve strength, mullitization and sintering. The additive accelerates the mullite phase formation process by entering into the mullite grain by solid solution formation. The additive was also found to be beneficial as it entered into the glassy phase, reduced the high-temperature viscosity of the glass and thereby helped with sintering. An aluminium titanate phase was noted in the presence of a higher quantity of additive.

TEM analysis and mechanical strengthening mechanism of MnO2 sintering aid in selective laser sintered porous mullites

A novel preparation method for significantly improving mechanical properties of SLS-formed porous mullites was proposed. MnO2 sintering aid was coated on mullite powder surface via a liquid phase co-precipitation reaction between MnC4H6O4·4H2O (Mn(Ac)2·4H2O) and KMnO4. With increasing Mn(Ac)2·4H2O content from 6 to 48 mL, the generated MnO2 content increased from 3.64 to 5.17 wt% which promoted the high-temperature sintering performance and accelerated the re-arrangement of mullite particles to enhance the densification of porous mullite. Then the microstructure evolution and grain growth of mullite grains and the effect of MnO2 sintering aid on mechanical strengthening mechanism of SLS-formed porous mullites were evaluated in depth from the perspective of TEM analysis. It was found that the aspect ratio of mullite grains decreases gradually with increasing Mn(Ac)2·4H2O content, and the accelerated radial growth of mullite grains contributed an increase of fracture toughness from 0.11 ± 0.04 to 0.53 ± 0.01 MPa m1/2. With increasing Mn(Ac)2·4H2O content, the MnO2 crystal changed from ramsdellite structure γ to a stable tetragonal structure β type. Moreover, the fine MnO2 particles preferentially precipitated between interwoven mullite grains at the initial stage and then grew into the large ones pinned between mullite grains, resulting in an improvement of compressive strength from 3.8 ± 0.1 to 22.1 ± 1.0 MPa. These values of mechanical properties were significantly improved by 1–2 orders of magnitude compared with that of mullites adding no MnO2.

Modified cenospheres as non-sacrificial pore-forming agent for porous mullite ceramics

Porous mullite ceramics were produced using mullite precursor and modified cenospheres as a non-sacrificial pore-forming agent. The cenospheres used are aluminosilicate hollow spheres with high silica and alumina content, which are obtained from coal-fired power plant. In this study, the cenospheres were modified using aluminum trichloride hexahydrate (AlCl3·6H2O), alkali/acid leaching and heat treatment. Various types and amounts of the modified cenospheres were mixed with mullite precursor to produce porous mullite ceramics for subsequent firing at 1500 °C. Graphite powder, as sacrificial pore-forming agent, was also used to prepare porous mullite ceramics by the same processing conditions for comparison. The study found that the use of graphite powder was unable to increase the porosity of the mullite ceramics as a result of excessive shrinkage. It acted more as a sintering aid rather than as sacrificial pore-forming agent. On the other hand, addition of modified cenospheres as non-sacrificial pore-forming agent leads to the increment of both total porosity and closed porosity, with the reduction of open porosity. The results showed that with the addition of 40 wt% of modified cenospheres to the mullite precursor, the resultant porous mullite ceramic has a total porosity of 50.2%, thermal conductivity of 1.28 Wm−1K−1, linear shrinkage of 4%, and biaxial flexural strength of 45.9 MPa. Porous mullite ceramic with majority closed pores has potential application for high temperature thermal barrier.

Porous mullite ceramics with enhanced mechanical properties prepared by SLS using MnO2 and phenolic resin coated double-shell powders

Novel double-shell mullite powders with good flowability and formability for selective laser sintering (SLS) were prepared for the fabrication of porous mullite ceramics with enhanced mechanical properties. The inner MnO2 shell was prepared by the co-precipitation reaction between MnC4H6O4·4H2O (Mn(Ac)2·4H2O) and KMnO4, and the outer phenolic resin shell was obtained via the solvent evaporation method. The influence of Mn(Ac)2·4H2O solution content on phase composition, microstructure, pore size distribution and mechanical properties of porous mullitie ceramics was investigated. With increasing Mn(Ac)2·4H2O content from 0 to 48 mL, the compressive strength and fracture toughness of porous mullite ceramics increased from 0.8 ± 0.1 MPa to 22.1 ± 1.0 MPa and from 0.03 ± 0.01 MPa m1/2 to 0.53 ± 0.01 MPa m1/2, respectively. It was found from TEM analysis that the remarkable mechanical property enhancement was attributed to the pinning effect of the presence of MnO2 sintering additive precipitated between the interwoven mullite grains. Correspondingly, the shrinkage increased sharply from 5.27 ± 1.3% to 23.5 ± 1.4%, while the open porosity decreased dramatically from 64.1 ± 0.6% to 48.7 ± 1.2% with the gradual reduction of open pore size distribution from 17.25 to 11.33 μm. Finally, the honeycomb mullite ceramics with very complex shapes of intersecting pore tunnels were successfully prepared by SLS. This paper provide a novel and feasible route to dramatically improve the mechanical properties of SLS-formed porous ceramic products.

Preparation and properties of porous mullite ceramics with high-closed porosity and high strength from fly ash via reaction synthesis process

Fly ash is a typical industrial solid waste that seriously affects human health and ecological balance. In order to recycle the fly ash, in this work, porous mullite ceramics were successfully fabricated with fly ash and bauxite via reaction synthesis process. Effects of firing temperature (1450–1550 °C), silicon carbide addition amount (0–15 wt%), and potash feldspar addition amount (0–16 wt%) on the mullite porous ceramics were systematically investigated. It was found that increasing the silicon carbide addition amount or raising firing temperature was favourable for improving the cold compressive strength and thermal shock resistance of the porous ceramics. Consequently, the porous ceramics with 10 wt% silicon carbide, 4–12 wt% potash feldspar had optimal overall performances. The closed porosity and cold compressive strength ranges were 14.79%–18.57% and 217.18–236.67 MPa, respectively. The thermal cycles were 7–9 times, and the thermal conductivity was reached to 2.19–2.52 W m−1 K−1 at 800 °C. This work provides a convenient and promising method for the utilization of fly ash.

Ultra-low cost porous mullite ceramics with excellent dielectric properties and low thermal conductivity fabricated from kaolin for radome applications

Abstract Near-net-shape mullite ceramics with high porosity were prepared from ultra-low cost natural aluminosilicate mineral kaolin as raw material and polystyrene micro-sphere (PS) as pore-forming agent. Microstructure, flexural strength, thermal conductivity and dielectric properties of the ceramics were systematically researched. Results show that the porous mullite ceramics possess fibrous skeleton structure formed by a large quantity of interlocked mullite whiskers, which results in good mechanical properties and low-to-zero sintering shrinkage. Flexural strength of the porous mullite ceramics can be up to 41.01 ± 1.12 MPa, even if the porosity is as high as 62.44%. The dielectric constant and loss tangent of the porous mullite ceramics at room temperature are lower than 2.61 and 5.9 × 10−3, respectively. Besides, dielectric constant is very stable with the rising of temperature, and the dielectric loss can be consistently lower than 10−2 when the temperature is not higher than 800 °C. In addition, thermal conductivity at room temperature is as low as 0.163 W/m/K when the porosity of mullite ceramics is 80.05%. The infiltration of SiO2 aerogels (SiO2 AGs) can further decrease the thermal conductivity to 0.075 W/m/K, while has just little effects on the dielectric properties. Excellent mechanical, thermal and dielectric properties show that the porous mullite ceramics have potential applications in radome fields. The porous mullite ceramics prepared from kaolin not only have low cost, but also can achieve near-net-shape.

Effect of recoating slurry with different particle size on the properties of reticulated porous mullite ceramics

Mullite reticulated porous ceramics (RPCs) were fabricated with fly ash, alumina, and andalusite by polymer sponge replica technique, followed by the process of vacuum infiltration with recoating slurry included. And the effect of different recoating slurry with different particle size fly ash on the properties of mullite RPCs was studied. The matrix slurry and recoating slurry were investigated and optimized. And then, the microstructure, composition of phases, and mechanical properties of mullite RPCs were examined by SEM (scanning electron micrograph), XRD (X-ray diffraction), and hydraulic universal testing machine. Moreover, the influence of particle size of fly ash on the thermal shock stability was analyzed by calculating residual stress model and grey relational model. The result indicated that the specimens that were recoated by the slurry with the smaller particle size of fly ash might have more excellent properties.

Fabrication and properties of porous anorthite/mullite ceramics with both high porosity and high strength

AbstractPorous anorthite/mullite ceramics with both high porosity and high strength have been successfully fabricated by foam‐gelcasting and pressureless sintering technology, using α‐Al2O3, SiO2, and CaCO3 as starting materials and MnO2 as sintering aids. The porous mullite ceramics prepared in this study had 83.3% porosity and 0.3 W/m·K thermal conductivity, exhibited compressive strength value as high as 6.1 MPa. The samples fabricated with mullite content of 30 mol% possessed 79.4% porosity and 5.9 MPa compressive strength showed thermal conductivity as low as 0.19 W/m·K. With the addition of MnO2, the properties of the prepared materials varied slightly when mullite content changed in a large scale. The results showed that the addition of MnO2 promoted the reaction, affected sintering and grain growth, and contributed to high strength and low‐thermal conductivity.