Columnar Mullite Research Articles

In-situ grown columnar mullite reinforced SiC porous ceramic membrane support with excellent mechanical performance and corrosion resistance: Using TiO2 as the key to form columnar mullite

In this study, in-situ growth of columnar mullite reinforced silicon carbide porous membrane supports were prepared, and the addition of TiO2 helped to reduce the sintering temperature for the anisotropic growth of necked columnar mullite crystals. The synthesis of in situ columnar mullite makes reasonable use of the oxidized silica of silicon carbide, and in turn reduces the loss of the silicon carbide porous membrane support under strong corrosive conditions. The material showed a flexural strength of up to 72.95 MPa, and a water flux of 31612.83 L·m−2·h−1·bar−1 when sintered for 2 h in an air environment at 1250 °C. The flexural strength after corrosion in 20 vol% H2SO4 and 1 wt% NaOH solution (90 °C) exceeded 93 %. This shows that this porous silicon carbide ceramic membrane scaffold has potential applications in the field of water treatment under harsh environments.

Preparation of β-SiAlON/SiC composite ceramic foam filters and their oxidation resistance

In this study, ceramic foam filters with high porosity, strength, and oxidation resistance are produced at low temperatures by preparing porous β-SiAlON/SiC composite ceramic foams using the foam impregnation method with Si, Si, α-Al2O3, AlN, and SiC as the raw materials. The presence of SiC inhibits the growth of β-SiAlON grains, which reduces the amount of β-SiAlON produced. As the proportion of β-SiAlON increases, the porosity of the β-SiAlON/SiC composite ceramic foam decreases, whereas the bulk density and compressive strength increase. Non-isothermal oxidation experiments show that β-SiAlON/SiC composite ceramic foams exhibit high oxidation resistance and their macroscopic morphology does not change significantly after oxidation. As the proportion of β-SiAlON increases, the smooth SiO2 oxide layer on the surface of the ceramic foam gradually changes to acicular and columnar mullite crystals, and the oxidation resistance increases. Therefore, β-SiAlON acts as a high-temperature binder when it is added to SiC. Hence, the prepared β-SiAlON/SiC composite ceramic foams can be used as ceramic filters in the field of molten metal filtration.

Absorption rate and thermal shock resistance of Co2O3-Fe2O3-mullite-based ceramics for solar heat absorbers

Mullite ceramic materials have a number of excellent properties that make them an excellent choice for solar heat absorber materials in Concentrating Solar Power (CSP) systems. In this study, the effects of adding Co2O3 and Fe2O3 on the properties of mullite-based ceramics were studied, aiming to get excellent absorption rate and thermal shock resistance. The results show that the sample C2 (45.05 wt% 75 calcined bauxite, 45.05 wt% Suzhou clay, 9 wt% Fe2O3, 3 wt% Co2O3) sintered at 1400 °C has the best comprehensive performance, with a bending strength of 143.28 MPa and a solar absorption rate of 92.2 % in the wavelength range of 0.3–2.5 μm. The bending strength increased by 31.36 % after thermal shock cycles for 30 times (1000 °C ∼ room temperature, air cooling). Generation of iron cobalt spinel and lattice distortion caused by Fe3+ and Co3+ into the mullite lattice combined to promote the absorption rate. In the meantime, the main crystalline phase columnar mullite showed a tight continuous interlaced network structure. The fracture mode was mainly transgranular fracture, which endowed C2 excellent thermal shock resistance.

Properties and mechanism of mullite whisker toughened ceramics

High-strength ceramics were prepared from high alumina fly ash (HAFA) and activated alumina as raw materials with magnesia as a sintering additive. The growth kinetics and influence mechanism of secondary mullite whiskers were investigated. Meanwhile, the effects of the Al2O3/SiO2 mass ratio (A/S) and the amount of magnesia on the content and morphology of mullite in the green body were investigated, so as to emphasize the effect of the liquid phase in the sintering process on the growth of secondary mullite whiskers. The results showed that the aspect ratio of secondary mullite whiskers increased significantly after adding activated alumina to increase the A/S ratio of raw materials. When 30 wt% activated alumina was added, the mullite content increased by 5.39%, and the whisker length increased from 1.36 μm to 4.18 μm. The addition of magnesia improved the liquid phase formed during the sintering process and the K value method was used to determine the sintering liquid phase content under various conditions. It was observed that increasing the magnesia level by 1 wt% could raise the liquid phase content by 5–7%. When the total liquid content of the system was 30–40%, the growth activation energy in the diameter direction of the whisker reduced significantly, promoting the growth of secondary mullite whiskers along the C axis. The morphology of mullite gradually developed from fibrous to long columnar crystal, making it combine more densely with the green body matrix. Furthermore, the staggered long columnar mullite crystal structure changes the fracture mode of ceramics from intergranular to transgranular fracture, which fully uses the high mechanical strength of mullite. As a result, the fracture energy and strength of ceramics are significantly improved.

Corrosion resistance mechanism of postmortem corundum-mullite refractories used in hot stoves

ABSTRACT Alkali attack of aluminosilicate refractories has been a long-term problem in the industry. In this paper, XRD, SEM, and CT scanning methods were used to analyze the phase and structure characteristics of postmortem corundum-mullite refractory used in hot stoves for more than 10 years, and the anti-corrosion mechanism of the refractory was explored. Results show that the corundum-mullite refractory after long-term corrosion by hot stove gas maintains excellent physical properties, and its cold compressive strength can still reach 92.7 MPa. The mullite network formed by the interpenetration of short columnar mullite endows the corundum-mullite refractory with excellent resistance to gas corrosion: Under the working conditions of the hot stoves, mullite network plays a role in dispersing impurities in refractory, so that the CaO entrained in hot stove gas can be transformed into high melting point CAS2 instead of low melting point phase with high Ca content in the working layer of refractory; CAS2 has good resistance to alkaline gases such as K2O(g) and Na2O(g), and the blocking of pores in the working layer of refractory by CAS2 finally makes the refractory free from the corrosion of hot stove gas.

Effects of V2O5 addition on the synthesis of columnar self-reinforced mullite porous ceramics

Spearhead columnar mullite was synthesized by in-situ reaction with V2O5 as additive. When the content of V2O5 was 7 wt%, the length of the spearhead columnar mullite was the longest with an aspect ratio of about 3.5. Furthermore, columnar self-reinforced mullite porous ceramics were prepared by a foam-gelcasting method, and the effects of V2O5 content on the rheological and gelling properties of mullite slurries as well as the microstructure, physical property and thermal insulation property of the prepared mullite porous ceramics were studied. The results showed that the flexural strength and compressive strength of the porous ceramics with 63% porosity prepared by using 2 wt% V2O5 additive were respectively as high as 13.9 and 41.3 MPa, and the thermal conductivity was about 1.04 W m−1 K−1.

Effect of in-situ formation of columnar mullite and pore structure refinement on high temperature properties of corundum casatbles

The effects of in-situ synthesis columnar mullite and pore structure on the hot modulus of rupture (HMOR), thermal shock resistance and corrosion resistance of corundum castables have been investigated in this paper. When 2% nano silica was added, the pore diameters of castables could be decreased to 15 nm (at 110 °C), 1 μm (1100 °C) and 6 μm (1500 °C), respectively. The corresponding reducing magnitude of pore size is 98.5%, 83.3% and 33.3%. The HMOR of castables fired at 1500 °C increased by 110% to 3.64 MPa. Furthermore, after three thermal shock cycles, the residual strength ratio of castables increased from 5.2% to 15.3%. A large amount of cross-distributed columnar mullite was formed between nano silica and α-Al2O3 by the two-dimensional nucleation mechanism, which remarkably enhanced the high temperature properties. The penetration index reduced from 30.86% to 19.88%, suggesting that smaller pore size and higher viscosity had a great influence to the penetration process.

Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures

Colloidal silica as nanostructured binder for refractory castables has attracted many attentions in recent years. In the present study, phase composition, microstructure and reactivity evolution of silica gel at different heating conditions were investigated to find suitable system for colloidal silica application. The results showed that atmosphere and carbon slightly affected phase composition of the silica gel at elevated temperatures, and the crystalline phases were composed of major α-cristobalite and minor α-tridymite. The morphology and particle size of the silica gel were greatly affected by atmosphere and carbon during heating. The spherical nano-silica particles with sizes of 40–50 nm rapidly grew into macroscale rod-like particles with temperature increasing from 800-1000 °C to above 1200 °C in air, and sintering of silica particles was observed. However, the size and morphology of the spherical nano-silica particles retained at high temperature in a reducing atmosphere, and many well developed columnar mullite crystals and some SiC whiskers formed on heating silica gel, alumina fines and carbon at 1500 °C, which was due to carbon inclusions retarding the growth of nano-silica particles and the nano silica remained high reactivity at high temperature. Thus, colloidal silica was suitable for application in carbon-containing refractory castables.

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.

Effect of alumina bubble modification on properties of mullite castables

Mullite castables are widely used in the hot metal pretreatment processing equipment. In this work, the effects of modification of alumina bubbles with silica sol on the properties of mullite castables were studied. After the alumina bubbles were modified by silica sol, the bulk density of the samples was slightly increased, and the apparent porosity was decreased. The cold modulus of rupture values of experimental castables increased remarkably after drying at 110 °C and heating at 1400 °C for 3 h. The thermal expansion of the samples was obviously increased, while the thermal conductivity of the samples was decreased significantly. More columnar mullite crystals were formed at the alumina bubble/matrix interface after the alumina bubble modification and form a continuous inter-locking network structure, which was helpful to improve castables’ thermal shock resistance.

Effect of sintering temperature on microstructure and mechanical behavior of alumina-based ceramic shell by SLS

Selective laser sintering (SLS) was adopted to rapidly prepare silica-based ceramic shell for superalloy turbine blade, and combined with high-temperature sintering process to further improve the mechanical properties of ceramic shell. The effects of different sintering temperatures (1450~1600 ℃) on the flexural strength of alumina-based ceramic shells were investigated. The phase constitution and fracture morphology of alumina-based ceramic shell molds were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that ceramic shell with excellent mechanical properties can be obtained by SLS+high-temperature sintering quickly and effectively. As sintering temperature increases from 1450 ℃ to 1600 ℃, flexural strength of ceramic shell at room temperature increases, and reaches 38.03 MPa at 1600 ℃. Columnar mullite phase is the main strengthening phase, with the increase of sintering temperature, the content of mullite phase in the shell increases, the content of quartz phase decreases, and the content of cristobalite phase increases first and then decreases. The crack propagation form changes from slow expansion to rapid expansion and causes transient fracture. The fracture changes from a tearing shape to a flush small section, and appear the intergranular and transgranular fracture mode, the crack is tended to expand to the mullite particles.

Properties of inorganic high-temperature adhesive for high-temperature furnace connection

A novel type of inorganic high-temperature adhesive that can be cured at room temperature and applied to a high-temperature furnace was prepared using alumina and micro-silica as materials, liquid aluminum dihydrogen phosphate as a binder, and then sintered at a high temperature. To evaluate the mass ratio of Al2O3:SiO2 and the optimal sintering system, the physical properties of the adhesive after sintering were tested, and the phase and microstructure were characterized. The results revealed that the physical properties of the adhesive at 1300–1600 °C were better at a mass ratio of Al2O3:SiO2 = 5:1. When the sintering temperature was 1300 °C, columnar mullite was formed. At 1500 °C, the adhesive had a high shear strength.

Effects of the content of in-situ grown mullite on the microstructure and mechanical properties of 3Y-TZP ceramics fabricated by gel-casting

The effects of different content of mullite on the microstructure and mechanical properties of in-situ mullite toughened 3Y-TZP ceramics (Mullite/3Y-TZP composite ceramics) prepared by gel-casting combined with pressureless sintering were observed in this study. The results indicated when the content of mullite was low (2–4 mol%), especially in 4 mol%, mullite showed regular columnar structure and the size of columnar mullite was larger. When the content got higher (> 6 mol%), mullite showed a granular or even fluid shape and was connected to each other. In addition, the bending strength and fracture toughness of the samples showed a trend of rising first and then decreasing with the increase of the content of mullite, but the hardness had been declining. The sample containing 4 mol% mullite had the best mechanical properties (hardness reached 13.02 GPa, bending strength reached 907.31 MPa and fracture toughness reached 10.38 MPa m1/2). Furthermore, mullite, alumina and zirconia particles played an important role in toughening reinforcement due to pinning effect.

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500°C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4MPa, the fracture toughness of 10.2MPa.m1/2, the Vickers hardness of 13.2GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.

Microstructural evolution, phase transformation, and variations in physical properties of coal series kaolin powder compact during firing

Coal series kaolin (CSK) from coalmine in Shanxi, China was studied in the form of powder compact by using die-pressing technique. Mineralogical, morphological, and chemical characteristics of CSK were given. The microstructural evolution, phase transformation, and relevant variations in physical properties of the fired CSK with elevating temperature were investigated. Results indicated that mullite began to form at the temperature as low as 1002.8°C. Columnar mullite appeared when firing at 1500°C. Transformation of cristobalite to liquid phase favored the densification of the fired powder compacts and made them acquire the densest microstructure in 1580–1600°C temperature interval. Die-pressing technique and flake-like nature of CSK particle induced the interlocking texture on the plane perpendicular to die-pressing direction. Low content of impurities as TiO2 and Fe2O3 increased the formation temperature of the columnar mullite and endowed samples with high refractoriness (as high as 1600°C). The advantages of using CSK for industrial ceramic preparation are its high purity, low mullitization temperature, and high refractoriness.

Synthesis of Mullite Using Low-Grade Bauxite and Coal Gangue

The total bauxite reserves of China are about 3.7 billion tons, however the principal and the first-class bauxite reserves account for only about 20%. A large number of coal gangue was accumulated on the surface of earth, which not only encroaches land but also pollutes the environment. In this study, low-grade bauxite with alumina content of 53.18 wt.% originating from Shanxi Yangquan and coal gangue with alumina content of 38.07 wt.% originating from Shanxi Datong were used to synthesize mullite materials. Bauxite and gangue were mixed by the ratio of 1 to 1. The ingredients were prepared according to the alumina content of 45.63 wt.% and 70 wt.%, by adding a certain amount of industrial alumina, respectively. The samples were sintered at 1350°C to 1600°C after being milled, dried and formed. The influence of alumina content and sintering temperature on the phase, microscopic morphology and properties of samples were discussed. The results show that, the main phases of the sample with the alumina content of 70 wt.% were mullite (more than 60 %) and corundum after being sintered for 4h at 1600 °C. The phenomenon of columnar mullite and granular corundum staggering had been observed. The bulk density and the bending strength of sample is 3.21 g/cm3and of 220.73 MPa respectively.

Phase transformations in mullite-based nanocomposites

Abstract Chemical vapor deposition (CVD) has been used to deposit mullite (3Al 2 O 3 · 2SiO 2 ) based nanocomposite coatings and powders. The nanocomposite coatings typically comprised of Si-rich nanolayer close to the coating-substrate interface which serves as a template for the growth of columnar mullite grains. Finally a third zone composed of Al-rich nanocrystallites is obtained towards the top surface of the coating. The phase stability/transformation of these three zones in the coatings, which is critical for high temperature applications, was investigated in the range 1000–1600 °C. Si-rich nanolayer converts to mullite after a two-step anneal, columnar mullite grains undergo a tetragonal–orthorhombic transformation and the Al-rich nanocrystallite powders completely transform into mullite and α Al 2 O 3 above 1500 °C. Various aspects of these phase transformations which eventually lead to the formation of mullite-α Al 2 O 3 nanocomposite coating are discussed.

Functionally Graded CVD Mullite Coatings

AbstractFunctionally graded mullite (3Al2O3.2SiO2) coatings were deposited on SiC substrates by Chemical Vapor Deposition (CVD) using the AlCl3-SiCl4-CO2-H2 system. It was found that due to preferential adsorption, coatings on SiC started off being Si-rich, while coatings on alumina substrates started out being Al-rich. In either case, if the coating composition was not close to stoichiometric mullite, the microstructure consisted ofg-Al2O3 nanocrystallites imbedded in a vitreous SiO2-rich matrix (nanocrystalline microstructure). On grading the composition, mullite grains nucleated when the composition of the growing nanocrystalline coatings reached a narrow surface composition range of Al/Si molar ratio of 2.9-3.4. Once nucleated, columnar mullite grains could be graded to highly nonstoichiometric Al-rich compositions. However, if the nucleated mullite grains were graded to be Si-rich, the mullite structure could not be sustained, and the coating reverted back to the nanocrystalline microstructure. This phenomenon is explained on the basis of the linkage of coordination polyhedra in the atomic structure of mullite.

Formation of mullite coatings on silicon-based ceramics by chemical vapor deposition

Dense, uniform, mullite coatings have been deposited by chemical vapor deposition on SiC substrates, using a AlCl 3-SiCl 4-CO 2-H 2 system. The typical coating microstructure consisted of a thin layer of nanocrystallites of γ-Al 2O 3 in vitreous silica at the coating-substrate interface, with columnar mullite grains over this interfacial layer. The composition of the coating was graded such that the outer surface of the coating was highly alumina rich. The changes in the coating microstructure with processing parameters are discussed. The ability of mullite to incorporate such large composition variations is discussed in the light of vacancy formation as theAl/Si ratio is increased and the ordering of these vacancies leads to changes in lattice parameters. The formation of domains was studied by measuring the spacing of superlattice spots in electron diffraction patterns and the relationship between domain size andAl/Si ratio is discussed.