Porous mullite honeycomb by hydrothermal treatment of fired kaolin bodies in NaOH

A conventional hydrothermal treatment with various concentrations of NaOH was used at 150° and 190°C to dissolve excess silica glass and thus make porous mullite ceramics from a fired New Zealand kaolin body. The effect of hydrothermal treatment time on the dissolution of the glass was examined. At 150°C, the dissolution of glass was almost complete after treatment for 8 hrs in 5N-NaOH solution and about 40–43 wt% of the glass was removed from the fired kaolin body leading to porous mullite. However, when the fired kaolin body was treated for more than 5 hrs in 5N-NaOH at 190°C, a composite of mullite and a nonporous crystalline phase of unknown symmetry resulted. These crystals formed from the dissolution and recrystallization of the glass. After the dissolution of glass in 2N-NaOH solution at 190°C for 5 hrs, a porous mullite body of 52.8% porosity with an average pore diameter of 0.57 m could be obtained, and this was only composed of mullite whiskers. Growth of unidentified nonporous crystals in the body which was treated in 5N-NaOH solution at 190°C led to a decrease in specific surface area and therefore, these crystals should be avoided.     

Microwave Versus Conventional-Hydrothermal Synthesis of NaY Zeolite

This paper focuses on the synthesis of NaY zeolite crystals by using microwave-hydrothermal (M-H) processing. NaY zeolites were synthesized from colloidal silica, sodium aluminate, sodium hydroxide and deionized water at 100°C, and some properties were studied. NaY zeolite crystals with high surface area, in the range of 439 to 716 m²/g, have been prepared at 100°C for 1 to 3 hrs. Compared to the formation of NaY zeolite via conventional-hydrothermal (C-H) treatment, the M-H treatment led to increased rate of formation by 3 to 4 times.     

Pore Structure and Thermal Stability of Mesoporous Mullite Fibers Prepared by Crystallization and Selective Leaching of Al2O3-SiO2 Glass Fibers

Porous mullite fibers were prepared by crystallization and selective leaching of Al2O3-SiO2 glass fibers using buffered HF-NH4F(BHF) aqueous solutions. The optimum concentration of BHF solution for selective leaching of the fibers was 0.9 mass% HF and 17.0 mass% NH4F. By firing at 1000–1300°C, the glass fibers changed into composite texture of mullite and glassy phase. Since the pores in the fibers were formed by selective leaching of glassy phase among mullite grains, they were tunable by changing the firing conditions of fibers. Pore size of the samples changed from around 4 nm in the 1100°C fired sample to 16 and 40 nm in the 1200 and 1300°C fired samples, respectively. The highest specific surface area obtained was around 30 m2/g, when the fibers were heat treated at 1200°C for 24 h and leached for 20 h in 0.9 mass% HF-17.0 mass% NH4F solution. From the thermal stability tests of the porous mullite fibers, its specific surface area was found to be maintained up to 1200–1300°C.     

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

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.     

Structural study of mullite based ceramics derived from a mica-rich kaolin waste

Mullite-based ceramics have been synthesized by reactive sintering of a mixture containing kaolin and a mica-rich kaolin waste. Samples fired in the temperature range from 1300 to 1500 °C were characterized by X-ray diffraction (XRD). The quantitative phase analysis and unit cell parameters of the mullite were determined by Rietveld refinement analysis of the XRD data. Mullite-based ceramics with 1.2 wt% quartz, 56.3 wt% glass (amorphous phase), 2.64 g/cm³ of apparent density, and 35±1.2 MPa of flexural strength were obtained after firing at 1500 °C. A liquid phase sintering mechanism activated by a total mica content of 13.3 wt% allowed to increase the mullite content to 47.6 wt% (2.3 wt% quartz and 50.1 wt% glass phase) and improve the flexural strength (70±3.9 MPa) after firing at 1400 °C.     

Microwave-hydrothermal synthesis of barium titanate under stirring condition

The role of in situ stirring under microwave-hydrothermal (M-H) conditions on the synthesis of barium titanate was investigated for the first time by powder X-ray diffraction and scanning and transmission electron microscopy. Stirring under M-H conditions in the temperature range of 150–200 °C led to enhanced crystallization of Ba titanate as revealed by yields compared to the static condition. In addition, stirring led to smaller and more uniform crystals under M-H conditions compared to those crystallized without stirring. Powder X-ray diffraction revealed the formation of only cubic polymorph of Ba titanate at or below 200 °C in 4 h with or without in situ stirring under M-H conditions. These results show that stirring is an important parameter during M-H synthesis of nanophase Ba titanate.     

Dependence of the properties of chamotte on the firing regime of the briquette

Conclusions The physicomechanical properties and phase composition of chamotte depends on the composition of the gas medium, the firing temperature, and the cooling regime. In the briquettes fired at 1500°C in an oxidizing medium with the cooling temperature lowered from 1500 to 1200°C, the apparent density increases and the porosity decreases. In the briquettes fired in a reducing medium with the cooling temperature lowered from 1500 to 1300°C, there is a reduction in the apparent density and an increase in the porosity but with a further reduction of the cooling temperature to 1200°C, these properties change in the opposite direction.The apparent density of chamotte and its concentration of mullite is increased with a reduction in the cooling rate. The apparent density of the chamotte from the Polozhsk kaolin under analogous cooling conditions increases more rapidly than that from the Novoselitsk kaolin chamotte.The temperature interval in which the cooling rate effectively increases the apparent density of the chamotte fired in an oxidizing medium is 1500–1300°C. In a reducing medium this interval is shifted 100°C toward lower temperatures.The experiments have shown that an increase in the cooling rate of the fired briquettes at temperatures below 1200°C has only a very slight effect on the apparent density and porosity of the Novoselitsk kaolin chamotte. When the chamotte has been obtained from Polozhsk kaolin, a reduction in the cooling rate at temperatures below 1200°C, and particularly in a reducing medium, has a quite perceptible effect on the increase in apparent density and reduction of porosity of the material.Translated from Ogneupory, No. 3, pp. 22–26, March, 1982.     

Sintering and characterisation of ceramics containing paper sludge, glass cullet and different types of clayey materials

The present paper reports on the sintering behaviour of several ceramics prepared using a previously selected mixture of incinerated paper mill sludge and glass cullet in the ratio 60/40 which was blended with 10, 20, 30 and 40 wt.% of three different natural materials. The three natural products were: a red quartzitic clay, a yellow quartzitic clay and a kaolin. All mixtures were blended by attrition milling and dried; powders were sieved, pressed into specimens and fired for 1 h at temperatures ranging from 1040 to 1140 °C. The resulting materials were characterized by water absorption, shrinkage, crystallographic composition, microstructure and physico-mechanical properties. It was observed that materials containing kaolin display the best overall behaviour independently of the quantity of kaolin introduced. Conversely the optimal sintering temperature, and consequently the best properties of the materials prepared using red or yellow clay were measured on products fired at temperatures above 1080 °C; materials and temperatures are affected by the amount of clay added.     

Formation of Novel ZSM-5/Porous Mullite Composite from Sintered Kaolin Honeycomb by Hydrothermal Reaction

The formation and properties of continuous ZSM-5 film on a porous mullite honeycomb have been investigated. The porous mullite honeycomb coated with ZSM-5 film provides a novel microporous (0.5 nm in diameter)–macroporous (0.5–0.6 μm in diameter) composite. Amorphous silica-glass in the kaolin honeycomb sintered at 1650°C is used as a source for ZSM-5 formation. The honeycomb is hydrothermally treated in NaOH, tetrapropylammonium bromide, and water to prepare a novel honeycomb–zeolite composite by in situ crystallization of ZSM-5 film. This paper describes the effects of hydrothermal conditions—such as reaction temperature, time, and concentration of NaOH solution—on the formation of ZSM-5 film on the honeycomb, and on the mechanical strength and porous properties of the honeycomb composite.     

Influence of heat treatment of fused mullite materials on their properties

Conclusion It is shown that the temperature of initial crystallization of glass spheres of mullite composition with diameters of less than 0.1 mm, in the metastable state, equals 935°C. The phase composition of the spheres of diameters from 0.1 to 0.5 mm varies. With an increase in the diameter of the spheres from 0.5 to 1–1.7 mm their properties vary slightly.Thermal processing of the spheres of mullite composition with different starting contents of mullite and glass in the mullite component influences the crystallization of the mullite and causes increases in the density at temperatures from 900–1300°C. The most rapid change in properties takes place in the spheres containing the maximum amount of glass phase (100%). In the 1300–1500°C range in all spheres, with different contents of glass phase in the starting condition, there is a variation in the density and in the mass proportion of mullite.An increase in soaking from 0.5 to 1.5 h at 1300 and 1500°C leads to an increase in the mullite content and a rise in the density of the glass spheres.During the firing of finely milled powders made of granules and spheres with different contents of glass phase and mullite, and also specimens prepared from these powders, with temperature rise the material's density and the amount of mullite are both increased, and the changes in these factors are identical. It is established that with increase in the mullite concentration in specimens of finely milled powders there are marked reductions in shrinkage, and apparent density; and an increase in the porosity of the specimens fired at 1750°c. There is much less change in the properties at 1700°c.Translated from Ogneupory, No. 4, pp. 7–10, April, 1990.     

Synthesis of high-performance mullite ceramics based on associated rare-earth kaolin

The objective of this work was to prepare high-purity, high-strength mullite ceramics from low-cost, associated rare-earth kaolin (AREK). A reaction sintering process using calcined AREK and γ-Al₂O₃ powders was used to synthesize high-performance mullite ceramics. Mineralogical, morphological, and chemical characteristics of AREK were given. The effects of associated REEs in kaolin and sintering temperature on the microstructural evolution, phase transformation, and physical properties of mullite were studied. The results showed that the mullite contents were 98.8%, the maximum aspect ratio was 8.22 μm, the relative density was 93.04%, and the micro-Vickers hardness and flexural strength were 10.63 GPa and 184.24 MPa, after sintering at 1500°C for 4 h. For comparison, calcined without rare-earth kaolin was also employed as a raw material to synthesize mullite ceramics, and the mullite content prepared by sintering the two kaolin clays at 1320–1480°C for 4 h was quite similar. However, mullite prepared using AREK forms secondary mullite in the temperature range of 1480–1500°C with a significantly higher mullite content, and therefore, the advantages of preparing mullite based on AREK as the raw material are high purity, low mullitization temperature, and high strength.     

Effect of mullite phase formed in situ on pore structure and properties of high-purity mullite fibrous ceramics

Porous mullite ceramics are potential advanced thermal insulating materials. Pore structure and purity are the main factors that affect properties of these ceramics. In this study, high performance porous mullite ceramics were prepared via aqueous gel-casting using mullite fibers and kaolin as the raw materials and ρ-Al₂O₃ as the gelling agent. Effects of addition of mullite fibers on the pore structure and properties were examined. The results indicated that mullite phase in situ formed by kaolin, and ρ-Al₂O₃ ensured the purity of mullite samples and mullite fibers bonded together to form a nest-like structure, greatly improving the properties of ceramic samples. In particular, the apparent porosity of mullite samples reached 73.6%. In the presence of 75% of mullite fibers, the thermal conductivity was only 0.289 W/m K at room temperature. Moreover, the mullite samples possessed relatively high cold compressive strength in the range of 4.9–9.6 MPa. Therefore, porous mullite ceramics prepared via aqueous gel-casting could be used for wide applications in thermal insulation materials, attributing to the excellent properties such as high cold compressive strength and low thermal conductivity.     

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).     

Fabrication and properties of porous mullite ceramics from calcined carbonaceous kaolin and α-Al2O3

High purity calcined carbonaceous kaolin and α-Al₂O₃ 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-Al₂O₃ 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.     

Study of the service of high-alumina and kaolin refractories in the checkers and structure of blast-furnace stoves

Conclusions The structure of kaolin brick in the high-temperature zone of the stove after 6.7–7.2 years service at subcupola temperatures of 1300°C underwent substantive external and structural changes.In the structure of the stove made from high-alumina brick VGO-62 after six years service we detected substantially less change.In the kaolin brick of the upper rows of the checker three clearly defined zones are formed: working (slag), impregnated with alkalis, the transition zone (porcelain-type), and the least-changed zone. During the service of high-alumina refractories mullite crystallizes, which confirms the results of work carried out previously.Owing to the creep of kaolin and high-alumina refractories 4–5% shrinkage of the brick occurs in the upper rows of the checker, and at a depth of 2.5–6.0 m — 1.5–2.0%. The height of the checker diminishes under these conditions by 0.8–1.0 m.The densification of the structure of the upper rows of the checker corresponds to a reduction in porosity and an increase in the density of the brick.The temperature of initial deformation under load of 2 kg/cm² of the slagged kaolin refractories taken from the upper rows of the checker diminishes by 150–200°C, and in the lower layers of the checker it does not alter. This factor for high-alumina refractories in service increases by 160–190°C. In the high-temperature zone of the stoves it is desirable to test dense, high-alumina refractories containing 72–75% Al₂O₃.Translated from Ogneupory, No. 5, pp.14–19, May, 1972.     

High-strength thermal insulating porous mullite fiber-based ceramics

How to improve the strength of fibrous porous ceramics dramatically under the premise of no sacrificing its low density and thermal conductivity has remained a challenge in the high-temperature thermal insulation field. In this paper, a new kind of high-strength mullite fiber-based ceramics composed of interlocked porous mullite fibers was prepared by nanoemulsion electrospinning and dry pressing method. Results show that as to the porous ceramics with the same density (～ 0.8 g/cm³), the three-dimensional skeleton structure composed of porous mullite fibers was much denser than that composed of solid mullite fibers. Therefore, porous mullite fiber-based ceramics exhibited a higher compressive strength (5.53 MPa) than that of solid mullite fiber-based ceramics (3.21 MPa). In addition, porous mullite fiber-based ceramics exhibited a superior high-temperature heat insulation property because the porous structure in fibers could reduce the radiant heat conduction. This work provides new insight into the development of high-temperature thermal insulators.     

Changes in the properties of aluminosilicate refractories under prolonged reducing conditions

Conclusions Studies of corundum and aluminosilicate refractories of dense and granular structures in an atmosphere of hydrogen and dissociated ammonia at 1200, 1500, and 1700°C in periods of 175 and 50 h showed that the resistance of the products increases with an increase in the alumina concentration and density. The maximum resistance is exhibited by corundum products. In the aluminosilicate refractories there is some additional sintering of the material with the separation of mullite and glass. Simultaneously on the surface of the specimens we detected deeper mineralogical changes, accompanied by the decomposition of the mullite, with the formation of corundum, silicon monoxide, and glass.The changes in the phase composition are accompanied by a change in the structure, and an increase in the creep. Considering that a reduction in the temperature of 100°C causes a reduction in the creep by approximately a half [26], it can be recommended that corundum refractories should be used (under a load of 2 kg/cm²) in a reducing atmosphere at temperatures of up to 1550–1600°C, sillimanite up to 1450–1500°C, kaolin and chamotte (high-grog) up to 1300°C, with a reduction in the load and an increase in the density, the temperature of application for the products examined, especially corundum, can be increased.Translated from Ogneupory, No.5, pp.26–32, May, 1972.     

Porous mullite ceramics with enhanced compressive strength from fly ash-based ceramic microspheres: Facile synthesis,structure, and performance

Porous mullite ceramics are widely used in heat insulation owing to their high temperature and corrosion resistant properties. Reducing the thermal conductivity by increasing porosity, while ensuring a high compressive strength, is vital for the synthesis of high-strength and lightweight porous mullite ceramics. In this study, ceramic microspheres are initially prepared from pre-treated high-alumina fly ash by spray drying, and then used to successfully prepare porous mullite ceramics with enhanced compressive strength via a simple direct stacking and sintering approach. The influence of sintering temperature and time on the microstructure and properties of porous mullite ceramics was evaluated, and the corresponding formation mechanism was elucidated. Results show that the porous mullite ceramics, calcined at 1550 °C for 3 h, possess a porosity of 47%, compressive strength of 31.4 MPa, and thermal conductivity of 0.775 W/(m?K) (at 25 °C), similar to mullite ceramics prepared from pure raw materials. The uniform pore size distribution and sintered neck between the microspheres contribute to the high compressive strength of mullite ceramics, while maintaining high porosity.     

Effect of pyrophyllite on the mullitization in triaxial porcelain system

China clay (kaolin) was progressively replaced by pyrophyllite in a conventional porcelain mix. Addition of 5% pyrophyllite as a replacement of china clay improved the fired strength by about 24% compared to that of the conventional body fired at 1300 °C. Percentage of mullite was found to increase in the fired specimens when kaolinite was progressively replaced by pyrophyllite. However, beyond 7.5% pyrophyllite addition, amorphous SiO₂ released from pyrophyllite dehydroxylate inhibited further recrystallization of mullite. There was very insignificant change in the phase compositions with mixes having pyrophyllite content higher than 7.5%. Entire phenomenon has been explained on the basis of structural reorganization of pyrophyllite during dehydroxylation. Presence of large amount of undissolved quartz of smaller size as well as isolated pores in the microstructures of specimens containing pyrophyllite more than 7.5% are assumed to hinder the propagation of crack and thereby improving the mechanical properties. The size and shape of mullite crystals is to a large extent controlled by the fluidity of the liquid matrix from which they grow and this is again a function of temperature and composition.     

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 Fe₂O₃ 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/cm³, 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 Fe₂O₃ and at 15.6 μm in the samples containing Fe₂O₃; 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 Fe₂O₃ 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 Fe₂O₃ than in those without Fe₂O₃, 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.