Properties Of Porous Mullite Ceramics Research Articles

Robust, thermally insulating, and high-temperature resistant phosphate-enhanced mullite fiber porous ceramic composites

Porous mullite ceramics with high porosity and low thermal conductivity are urgently required as high-temperature thermal insulators in harsh environments. How to improve the strength of Porous mullite ceramics dramatically under the premise of no sacrificing its thermal insulating properties has remained an extremely challenging. Inspired by the freezing rain phenomenon in nature, high-strength and high-temperature resistant phosphate coatings were applied to the surface of mullite fibers, which significantly improved the mechanical properties of porous mullite ceramics without significantly increasing their thermal conductivity. The Al(H2PO4)3 solution concentration significantly affected the mechanical and thermal properties of the composite material, where the mechanical performance improved with an increasing Al(H2PO4)3 solution concentration. The phosphate coating phase stabilized with an increasing calcination temperature. The mechanical properties of the ceramics were improved by the excellent interfacial bonding strength between the coating and mullite fiber, and the forms of phosphate in the porous ceramics. The optimum composite preparation process increased its compressive strength by approximately nine-fold to 4.1 MPa and resulted in a thermal conductivity of only approximately 0.09 W/(m·K), demonstrating that the composite exhibited a significantly higher comprehensive performance than other previously reported porous ceramics and aerogels. Such new composites with excellent mechanical and thermal insulation properties exhibit excellent potential for high-strength thermal insulation in the chemical, metallurgical, pharmaceutical, aerospace, and other fields.

Thermal Properties of Porous Mullite Ceramics Modified with Microsized ZrO2 and WO3.

Mullite ceramics are well known as materials with a high temperature stability, strength and creep resistance. In this research, the effect of a modification with magnesia-stabilized zirconia and yttria-stabilized zirconia, separately, as well as in a mixture with WO3, in 1:1 and 1:2 ratios on the thermal properties of porous mullite ceramics was investigated. The porous mullite-containing ceramics were prepared by a slip casting of the concentrated slurry of raw materials with the addition of a suspension of Al paste for the pore formation due to the H2 evolution as a result of the reaction of Al with water. The formed samples were sintered at 1600 °C and the holding time was 1 h. The materials were characterized using X-ray diffractometry, scanning electron microscopy, mercury porosimetry, the laser flash contactless method, thermal shock resistance testing and the non-destructive impulse excitation method for determining the elasticity modulus. The modification of the porous mullite ceramic with a mixture of ZrO2 and WO3 oxides had a positive effect by decreasing the thermal conductivity, due to the increased porosity, in comparison to the undoped samples and samples with only ZrO2. The doubling of the WO3 amount in the modifying oxide mixtures improved the ceramic thermal shock resistance. The porous mullite ceramics which were modified with magnesia-stabilized zirconia (2.8 mol% MgO) and WO3 had a lower thermal conductivity and improved thermal shock resistance than the samples with yttria-stabilized zirconia (8 mol% Y2O3) and WO3.

The fabrication of porous mullite ceramics based on the different sizes of aluminum hydroxide

In this research, for studying the influence of size and heat treatment temperature of initial Al(OH)3 on the physical properties of porous mullite ceramics, porous mullite ceramics were prepared by in situ reaction sintering of amorphous silica and treated Al(OH)3. The transition phases χ-Al2O3, к-Al2O3, and stable phase α-Al2O3 can be obtained in turn when the treatment temperature of raw Al(OH)3 is 500, 1000, and 1500°C, respectively. The coarser the raw Al(OH)3, the higher the strength of porous mullite ceramics. When the sintering temperature is 1500°C, the bending strengths of PS500-C, PS1000-C, and PS1500-C (PSx-C represents that the specimen was prepared by the coarse grade Al(OH)3, which was previously treated at x°C) are 40.3 ± 2.1, 54.9 ± 5.2, and 64.8 ± 4.8 MPa, respectively. In addition, although the activated Al2O3 can decrease the formation temperature (∼100°C) of porous mullite ceramics, the strength and density of porous mullite ceramics prepared by activated Al2O3 will decrease at the same sintering temperature. It is believed that the increase of defects and pores during the phase transformation should be responsible for this phenomenon.

In-situ growth of mullite whiskers and their effect on the microstructure and properties of porous mullite ceramics with an open/closed pore structure

Porous mullite ceramics with an open/closed pore structure were prepared by protein foaming method combined with fly ash hollow spheres. Both the open porosity and total porosity of samples were enhanced by increasing the hollow sphere content. Mullite whiskers with a diameter of 0.2–4 μm were grown in-situ in the porous mullite ceramics with an AlF3 catalyst, conforming to a vapor-solid growth mechanism. The pore structure of the porous mullite ceramics was significantly affected by the mullite whiskers which increased the open porosity and total porosity. Moreover, the median pore size was reduced from 65.05 μm to 36.92 μm after the introduction of mullite whiskers. The flexural strength and the thermal conductivity of the samples decreased with increasing total porosity. The porosity dependence of the thermal conductivity was well described by the universal model, providing a reference for the prediction of thermal conductivity of porous ceramics with open/closed pores.

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.

Influence of ZrO2 and WO3 doping additives on the thermal properties of porous mullite ceramics

Porous mullite-corundum refractory ceramics were produced by a patented slurry slip casting method from compositions based on commercially available α-Al2O3 and γ-Al2O3, fused SiO2 and kaolin. Pores were formed as a result of a chemical reaction of aluminium with water. The influence of usage of raw materials and doping additives such as micro-size ZrO2 and WO3 on the sintering temperature, formation of crystalline phases, linear thermal expansion, thermal conductivity and thermal shock resistance of mullite-corundum ceramic was studied. The best thermal shock resistance and, simultaneously, lower thermal conductivity was achieved for the samples doped with WO3. This was due to the influence of micro-sized WO3 on the change in γ-Al2O3 modification to α-Al2O3 and on the structure of mullite ceramics.

Effects of Al2O3 crystal types on morphologies, formation mechanisms of mullite and properties of porous mullite ceramics based on kyanite

Porous mullite ceramics with different crystal shapes of mullite are fabricated by in-situ reaction with middle-grade kyanite as raw material, Al(OH)3, γ-Al2O3, ρ-Al2O3 and α-Al2O3 as alumina sources. Effects of Al2O3 crystal types on morphology evolution and formation mechanisms of mullite, and properties of porous ceramics are investigated. Results show that mullite in the sample with Al(OH)3 mainly shows acicular morphology, because its (001) plane has the minimum interplanar crystal spacing and maximal attachment energy, it grows fast along [001] direction by screw dislocation mechanism. With a successive slowdown in reactivities of Al(OH)3, γ-Al2O3, ρ-Al2O3 and α-Al2O3, the amount and aspect ratio of mullite reduce, its growth mechanism gradually transforms into two-dimensional nucleation. Acicular mullite not only reinforces samples, but makes effective pore sizes smaller, which enable the sample with Al(OH)3 to present low bulk density, high apparent porosity and linear changes, small average pore size and good mechanical strength.

Influence of pore characteristics on the properties of porous mullite ceramics

Porous mullite (3Al2O3·2SiO2) ceramics were fabricated by mixing a commercial alumina (Al2O3) with silica (SiO2) from different sources, a commercial silica and rice husk silica (RHSiO2), in this study. Activated carbon (AC) was added as a pore former. Different compositions of starting raw materials used were fabricated to study pore characteristics and mullite phase evolutions. The pressed samples were sintered in an electric oven at 1450 °C for 4 h with a heating and cooling rate of 120 °C/min. An X-ray diffractometer (XRD) was used to characterize mullite phase present. Microstructures of pores formed at the external and internal surfaces of the sintered mullites were mainly investigated using scanning electron microscopes (SEMs). Evaluation of pore sizes and their distributions on the surfaces was performed using an image analysis technique based on SEM images observed on the top and cross-sectional surfaces of the sintered mullites. The quantitative information on pore characteristics related to starting chemicals, mullite densities, and mechanical properties has been extensively discussed.

Effects of firing temperature on the microstructures and properties of porous mullite ceramics prepared by foam-gelcasting

Porous mullite ceramics were prepared at 1300–1600°C for 2 h via a foam-gelcasting route using industrial-grade mullite powders as the main raw material, Isobam 104 as the dispersing and gelling agent, triethanolamine lauryl sulphate as the foaming agent and sodium carboxymethyl cellulose as the foam stabilising agent. The effects of firing temperature on the sintering behaviour of green samples as well as microstructures and properties of final porous mullite products were investigated. With increasing the temperature from 1300 to 1600°C, linear shrinkage and bulk density values of fired samples increased, whereas their porosity decreased. Mechanical strength and thermal conductivity values of fired samples decreased with increasing their porosities. Even at a porosity level as high as 79.4%, compressive and flexural strengths of fired samples (with average pore size of 314 μm) remained as high as 9.0 and 3.7 MPa, respectively, and their thermal conductivity (at 200°C) remained as low as 0.21 W (m−1 K−1).

Capillary rise properties of porous mullite ceramics prepared by an extrusion method with various diameters of fiber pore formers

Porous mullite ceramics with unidirectionally oriented pores were prepared by an extrusion method using rayon fibers as the pore formers. Rayon fibers of 8.1, 9.6, 16.8, and 37.6 μm in diameter were used as the pore formers and were kneaded with alumina powder, kaolin clay, China earthen clay, and water to form pastes. These pastes were extruded into cylindrical tubes, dried, and fired at 1500 °C for 4 h. The apparent porosities ranged from 45.7 to 48.2 %. The pore size distributions showed a sharp peak at 9.4, 10.0, 15.6, and 30 μm with increasing fiber diameters. The height of the capillary rise was 1780, 1670, 1320, and 950 mm with increasing fiber diameter. The maximum capillary rise is much higher than previously reported. The contact angle and effective pore radius that determine the capillary rise ability were calculated by fitting the capillary rise curves using the Fries and Dreyer’s equation.

Fabrication of Porous Mullite Ceramics with High Porosity Using Foam-Gelcasting

In this paper, porous mullite ceramics with an apparent porosity up to 81 % were fabricated by foam-gelcasting using mullite powder as raw material with solid loading of 40 vol.%. The monomers content and sitering temperature have obvious effect on the properties of porous mullite ceramics. The apparent porosity of the prepared samples was in the range of 75~82%, compressive strength, was in the range of 3.0~16.02 MPa, and thermal conductivity was between 0.14 and 0.47 W/(m•K). A complex porous microstructure was formed, where large spherical pores contained small cellular pores on their internal walls.

Fabrication and properties of porous mullite ceramics from calcined carbonaceous kaolin and α-Al 2O 3

High purity calcined carbonaceous kaolin and α-Al 2O 3 powders were employed to prepare porous mullite ceramics (Sample A) using graphite as pore former with the reaction sintering method. For the purpose of comparison, porous mullite ceramics (Sample B) was also fabricated from the uncalcined carbonaceous clay incorporated with α-Al 2O 3 powders. Mullitization in the two samples was both nearly complete at 1500 °C, despite the fact that calcination of the clay remarkably depressed mullitization and promoted the formation of glass phase. The Sample A sintered at 1500 °C fractured mainly in an intergranular way, while the Sample B mainly underwent transgranular fracture. The experimental results revealed that densification behavior/open porosity of the Sample A was far more sensitive to sintering temperature. The pore size of the Sample A as well as the Sample B sintered at 1500 °C was in a narrower range of 0.3–5 μm.

Capillary rise properties of porous mullite ceramics prepared by an extrusion method using organic fibers as the pore former

Porous mullite ceramics with unidirectionally oriented pores were prepared by an extrusion method to investigate their capillary rise properties. Rayon fibers 16.5 μm in diameter and 800 μm long were used as the pore formers by kneading with alumina powder, kaolin clay, China earthen clay and binder with varying Fe 2O 3 contents of 0, 5 and 7 mass%. The resulting pastes were extruded into cylindrical tubes (outer diameter (OD) 30–50 mm and inner diameter (ID) 20–30 mm), dried at room temperature and fired at 1500 °C for 4 h. The bulk densities of the resulting porous ceramics ranged from 1.31 to 1.67 g/cm 3, with apparent porosities of 43.2–59.3%. The pore size distributions measured by Hg porosimetry showed a sharp peak at 10.0 μm in the sample without Fe 2O 3 and at 15.6 μm in the samples containing Fe 2O 3; these pores, which arose from the burnt-out rayon fibers, corresponded to total pore volumes ranging from 0.24 to 0.34 ml/g. SEM showed a microstructure consisting of unidirectionally oriented pores in a porous mullite matrix. Prismatic mullite crystals were well developed on the surfaces of the pore walls owing to the liquid phase formed by the Fe 2O 3 component added to color the samples. The bending strengths of the tubular samples ranged from 15.6 to 26.3 MPa. The height of capillary rise, measured under controlled relative humidities (RH) of 50, 65 and 85%, was greater in the ceramics containing Fe 2O 3 than in those without Fe 2O 3, especially in the thinner samples. The maximum capillary rise reached about 1300 mm, much higher than previously reported. This excellent capillary rise ability is thought to be due to the controlled pore size, pore distribution and pore orientation in these porous mullite ceramics.

Preparation and Some Properties of Porous Mullite Ceramics Using Needle-like Crystals

Porous mullite ceramics with porosity of 26-57% were prepared from needle-like mullite crystals of 3-5μm long and 0.5-1.0μm in diameter synthesized by firing New Zealand kaolin at 1650°C. Pore characteristic (porosity, pore diameter), three points bending strength and thermal conductivity were examined on the porous mullite ceramics sintered at 1550°-1700°C in air. With increasing sintering temperature, the pore volume decreased, resulting in an increase in the bending strength. However, the pore size distribution was almost independent of the sintering temperature. The average pore diameters for the porous mullite ceramics sintered at 1550°-1700°C were in the range of 0.46-0.58μm. Thermal conductivity at room temperature decreased linearly with increasing porosity.