Mullite Precursor Research Articles

Phase evolution and dynamics of cerium-doped mullite whiskers synthesized by sol–gel process

The effects of CeO2 doping of aluminosilicate on mullite formation and the morphology of mullite whiskers are evaluated herein. CeO2-doped mullite precursors were generated by a sol–gel process. The introduction of CeO2 into the aluminosilicate precursor (3Al2O3·2SiO2) not only lowered the mullite formation temperature, but also promoted the crystallization of mullite grains. The mullitization activation energies calculated based on non-isothermal differential scanning calorimetry (DSC) were 473 and 722 kJ mol−1 for the 2 mol% CeO2-doped and undoped samples, respectively.

Activation energy for mullitization of gel fibres obtained from aluminum isopropoxide

Gel fibres of mullite precursor were prepared from an aqueous solution of aluminum nitrate (AN), aluminum isopropoxide (AIP) and tetraethylorthosilicate (TEOS). A 4:1 molar ratio of aluminum isopropoxide and aluminum nitrate was optimized to obtain spinnable precursor sol for synthesis of fibres. Thermogravimetry–differential scanning calorimetry (TG–DSC), Fourier transform infrared (FTIR) spectra and X-ray diffraction (XRD) analyses were used to characterize properties of the gel and ceramic fibres. The precursor gel completely transformed to mullite at 1200 °C. The activation energy of mullite crystallization was 993·5 kJ/mol by the Kissinger equation.

Synthesis of mullite bonded porous SiC ceramics by a liquid precursor infiltration method: Effect of sintering temperature on material and mechanical properties

Mullite bonded porous SiC ceramics were synthesized by infiltrating a SiC porous compact with a liquid precursor of mullite followed by sintering at 1300–1500°C in air. Different volume fractions (0–0.26%) of petroleum coke powder were used as pore former. SiC particles bonded by mullite and oxidation derived SiO2 formed an interconnected porous network. Porosity varied in a range of 31–48vol%. The flexural strength and the Young's modulus of the porous ceramics varied in the ranges of 19.3–43.8MPa and 16.6–42.9GPa respectively and the mechanical property–porosity relationship could be explained by the minimum solid area (MSA) model.

Incorporating Simulated Zinc Ash by Kaolinite- and Sludge-based Ceramics: Phase Transformation and Product Leachability

Zinc is one of the hazardous metals commonly found in municipal solid waste incineration (MSWI) ash, and this study reveals the stabilization mechanisms when sintering zinc-laden ash and ceramic precursors as a waste-to-resource strategy. Using ZnO to simulate the zinc-laden ash and sintering with kaolinite and mullite ceramic precursors, both zinc aluminate spinel (ZnAl2O4) and willemite (Zn2SiO4) phases were found in the products under the tested thermal conditions. The results also indicate that kaolinite and mullite precursors exhibit different incorporation behavior, and ZnAl2O4 and Zn2SiO4 were found to be competitive as the Zn-hosting phases in the system. A prolonged leaching test was used to evaluate the leachability of potential product phases in the system. The concentrations of zinc in ZnO and Zn2SiO4 leachates were about two orders of magnitude higher than that in ZnAl2O4 leachate, indicating the preference of forming ZnAl2O4 for zinc stabilization. Furthermore, the aluminum-rich sludge generated from waterworks could be beneficially used as a material resource to stabilize zinc in this study. The X-ray diffraction (XRD) pattern collected from the 1150 °C and 3-h sintered sample shows the success of incorporating zinc into the ZnAl2O4 spinel structure with waterworks sludge precursor. The formation of ZnAl2O4 indicates a strong potential for employing aluminum- and silicon-based materials to thermally immobilize zinc and achieve the beneficial use of metal-laden MSWI ash.

Metakaolin–nanosilver as biocide mullite precursor

Abstract Incorporation of silver nanoparticles enhances the biocide functionality of Ag supported kaolinite/metakaolite materials. In this regard, structural evolution of supports in the presence of silver nanoparticles is investigated. For this purpose, 27Al and 29Si magic-angle-spinning nuclear magnetic resonance spectroscopy was applied to investigate the bonding between silver nanoparticles and kaolinite/metakaolin materials. The incorporation of silver nanoparticles, following a chemical or thermal reduction, does not change the structure of kaolin, whereas in the case of metakaolin, a preferential interaction with tetrahedral sheets was observed and some stabilization of hexacoordinated aluminium is favoured.

Metal stabilization mechanism of incorporating lead-bearing sludge in kaolinite-based ceramics

The feasibility and mechanism of incorporating simulated lead-laden sludge into low-cost ceramic products was investigated by observing the reaction of lead with two kaolinite-based precursors under sintering conditions. To investigate the phase transformation process of lead, lead oxide (PbO) mixed with a kaolinite or mullite precursor were fired at 500–950°C for 3h. Detailed X-ray diffraction analysis of sintered products revealed that both precursors had crystallochemically incorporated lead into the lead feldspar (PbAl2Si2O8) crystalline structure. By mixing lead oxide with kaolinite, lead feldspar begins to crystallize at 700°C; maximum incorporation of lead into this structure occurred at 950°C. However, two intermediate phases, Pb4Al4Si3O16 and a polymorph of lead feldspar, were detected at temperatures between 700 and 900°C. By sintering lead oxide with the mullite precursor, lead feldspar was detected at temperatures above 750°C, and an intermediate phase of Pb4Al4Si3O16 was observed in the temperature range of 750–900°C. This study compared the lead leachabilities of PbO and lead feldspar using a prolonged leaching test (at pH 2.9 for 23d) modified from the toxicity characteristic leaching procedure. The results indicate the superiority of lead feldspar in stabilizing lead and suggest a promising and reliable strategy to stabilize lead in ceramic products.

Incorporation of mullite as a bond phase into porous SiC by an infiltration technique

Abstract Porous SiC bodies were fabricated from α-SiC powder and petroleum coke powder as the pore former were infiltrated with a liquid precursor of mullite which on subsequent heat treatment at 1300 °C produced a mullite bonded porous SiC ceramics. Process parameters which control the rate of infiltration were studied. Results showed that infiltration rate could be estimated by using weight gain measurement. Sintering infiltrated bodies at a low temperature of 1300 °C yielded specimens in which silicon carbide, mullite and cristobalite were detected. Depending on the porosity of the starting SiC body, the porosity of the final ceramic could be varied in the range of 36–49 vol.% with a variation of flexural strength from 19 to 33 MPa. The average pore diameter (∼8 μm) was found to be almost independent of porosity.

Low temperature needle like mullite grain formation in sol–gel precursors coated on SiC porous substrates

Dense mullite coating having thickness in the range of 3 to 5 μm was produced from sol–gel mullite precursor coated on SiC porous substrates at heat treatment temperatures as low as 1300 °C. Mullite formed in the coating layer was characterised by X-ray diffraction. The precursors have an average particle size of 170 nm and the mullite formed in the coating in situ has a grain size of 3–5 μm. Mullite grains formed on the SiC have needle like morphology. The mullite formation has been explained on the basis of reaction between the silica–alumina nano precursor and the needle like morphology has been similar to that formed from a liquid phase. The gas permeation analysis shows that there is considerable difference between gas pressure while using SiC substrate before and after coating and hence clearly indicated reduction in pore size. This particular approach is good since usual mullite formation is at high temperature and is difficult to attain small grain size. Further, in situ formed mullite, in this investigation covers the SiC surface protecting the SiC from oxidation at high temperature.

Needle-Like Mullite Ceramic Prepared by Sol-Gel Process

Using tetraethoxysilane, aluminum nitrate and aluminum fluoride as raw materials, the precursor of mullite was prepared by sol-gel process. When the precursor sintered at 1200°C, mullite ceramic was obtained. Differential thermal analysis, X-ray powder diffraction and scanning electron microscope were used to characterize the dried mullite gel and ceramic blocks. The results suggest mullite is synthesized by solid-phase reaction mechanism. X-ray powder diffraction indicates mullite is the main crystals phase in the ceramic specimen. SEM micrograph shows the mullite grains in the shape of short rod with length of 20 um when sintered at 1200 °C for 2 h and the grains grown up to acicular with length of more than 50 um when the treating time under 1200 °C achieved to 4 h.

Thermodynamic and Kinetic Analyses of Calcined Kaolin Precursors Used for Synthesis of Mullite Nanocomposite

Mullite precursor was prepared from kaoline when calcined at 1250 °C，when the concentration of sodium hydroxide of 3 mol/l; crystallization temperature of 80 °C; holding time of 3 h and precursor concentration of 0.15 g/ml, the morphology of the prepared mullite nanocomposite appears spherical-like, and it has good crystallization property and low agglomeration, and the average size of the particles is about 60.9 nm. Both the thermodymics and kinetics of mullite precursor have been studied. The results are as follows: the reaction started when the calcine temperature T﹥495°C; mullite precursor formed when the reaction system temperature 1000 °C; the activation energy of precursor Ea=928.3 KJ/mol; the reaction order n=1.33. We have reported our successful experiments to use kaolin to synthesize mullite nanocomposite with satisfying, spherical morphology, relatively homogenous grain average size of 60.9 nm, and little aggregation[1-2]. This paper presents an investigation of the thermodynamics and kinetics of the precursor preparation process.

In situ interlocking structure in gel-derived mullite matrix induced by mechanoactivated commercial mullite powders

A unique technique was developed to render dense mullite ceramics with an in situ tough interlocking structure. The microstructures were achieved by using a commercial mullite powder mechanoactivated with tungsten carbide milling media as seeds and a gel-derived nanosized mullite precursor as a matrix. Only less than 1 wt.% seed was sufficient to trigger the in situ formation of mullite whiskers uniformly distributed in a dense submicron-grained mullite matrix after sintering at 1450 °C for 5 h.

Review of mullite synthesis routes by sol–gel method

The sol–gel method for the mullite synthesis is reviewed, with particular emphasis on the characterization of monophasic and diphasic gels at low, intermediate and high temperatures and the factors that influence the hydrolysis and condensation rate of the sol–gel process, which in turn determine the properties of the final material. A wide range of studies about mullite precursors synthesized via sol–gel is discussed here.

Grain-Size Characteristics of Mullite Nanocomposites Prepared by Open-System Hydrothermal Crystallization Method

In this research, raw kaolin is processed by oxidation-reduction fining method and then baked to produce mullite precursor, which can prepared multi-phase mullite nanocrystals at low temperature hydrothermal conditions under normal pressure. According to the analysis of the controlling factors of NaOH mole ratio in the hydrothermal system, reaction temperature, holding time and the precursor concentration, etc., we studied the effect of above-mentioned factors on the grain size of product. The results show that when NaOH concentration 3mol/L, hydrothermal reaction temperature 80°C, holding time 3 h, precursor concentration 0.15g/ml, the morphology of the prepared nanocomposite with grains size of 60 ~ 90nm appears spherical-like, also the uniform dispersion can be acquired.

Preparation of Nanometer Mullite via Supercritical CO<sub>2</sub>

The xerogel of mullite precursor with high specific surface area (422m2•g-1) was successfully prepared from the sol of aluminium isopropanol (AIP) and tetraethyl orthosilicate (TEOS) tuned by supercritical CO2 and subsequent supercritical fluid extraction. Nanometer mullite was prepared by calcining the xerogel of the precursor, and the obtained mullite together with the xerogel precursor was characterized by SEM, XRD, N2-adsorption and IR. The results showed that the xerogel precursor exhibit a shape of amorphous structure, with size of no more than 100 nm, and no huge blocks was found, indicating that the supercritical CO2 plays an important role in controlling the process of hydrolyzation, sol-gel transition, in which supercritical CO2 may promote the mixture of Si and Al on atomic scale. SEM indicated that the obtained mullite were mainly 2-3 μm in length and lath in shape with aspect ratios of 6-10, and the laths were interlaced with each other to construct a relative dense mullite aggregation. The mineralogical phases tracing experiment showed that the mullitization process underwent from 1200 °C to 1400 °C as followed by the phase transition from -Al2O3 to -Al2O3, and to -Al2O3, and the temperature of complete phase transformation into mullite was 1400 °C.

Synthesis of precursor for fibrous mullite powder by alkoxide hydrolysis method

Abstract Precursor for fibrous mullite powder was synthesized by simple alkoxide hydrolysis method without special treatment, and the formation mechanism of fibrous mullite precursor was investigated. As-synthesized precursor was amorphous and consisted of nano-scaled particles, and Al2O3 and SiO2 components containing small amount of Si–O–Al components existed in the precursor. During heat-treatment, Al–Si spinel phase and amorphous SiO2 formed around 1000 °C, and the products after heat-treatment above 1200 °C consisted of single mullite phase as fibrous shape of mullite precursor was maintained. In the present synthesis process, fibrous precursor derived from aluminum isopropoxide acted as a template for fibrous mullite precursor. Specific surface area of as-synthesized mullite precursor was 380 m2/g, and it gradually decreased with increasing heat-treatment temperature. Specific surface area of the products significantly decreased around 1200 °C, that corresponded to crystallization temperature, and that of the product after heat-treatment at 1300 °C was 13 m2/g.

Mullite fibers prepared from an inorganic sol–gel precursor

Mullite fibers were prepared by sol–gel process using alumina sol and silica sol. Alumina sol was synthesised from dissolution of aluminum powder in aluminum chloride hexahydrate solution. The optimal spinning alumina sol could be obtain in the composition range of Al/AlCl3·6H2O molar ratio 3.4–3.8. The Al and Si components were mixed at the molecular level and linear molecules were formed in the mullite precursor sol. The dried mullite precursor gel fibers completely transformed to mullite fibers at 1,000 °C with a smooth surface and uniform diameter.

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al 2O 3·2SiO 2 (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H 2O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.

Effect of Precursors on Transmittance and Microstructure of Mullite Ceramics

Two different kinds of mullite precursors with composition 3Al2O3•2SiO2 (3:2) were prepared by conventional drying ethanol solution and spray-drying aqueous solution of aluminum nitrate nanohydrate and tetraethoxysilane, respectively. The results of scanning electron microscope (SEM) indicate that one powder consists of irregular particles with size of 1-10μm, the other powder is made of inhomogeneously sized hollow spherical particles with mean size of 0.5-5μm. The TG-DTA curves indicate the hollow spherical particles are unfavorable to eliminate the decomposed products. After the precursors were sintered by Spark Plasma Sintering at 1450°C for 10min, the microstructures of the former are made of fine equiaxed grains with sizes of around 0.5μm, the latter consist of elongated grains distributed in the matrix of fine grains with imhomogenous size of 0.5~10μm due to the liquid phase forming. The different microstructures lead to the former sintered body is transparent, while, the sample from spray-drying is opaque.

Geopolymer Technique에 의한 Mullite 분말의 합성

Mullite precursors were synthesized with aluminosilicate gels from mixture of aluminum nitrate and sodium orthosilicate by the geopolymer technique at ambient temperature. Then, the gel was heat-treated in air up to 1200℃ at intervals of 100℃. Raw and heattreated gels were characterized by TG-DTA, XRD, FTIR, 29 Si MAS-NMR, TEM. The result to examine the crystallization of behavior though DTA, the synthesized precursors were crystallized in the temperature range from 950℃ to 1050℃. The XRD results showed that the gel compositions were begun to crystallize at various temperature. Also, it was found that the precursors of M-4 begun to crystallize at about 950℃. The M-4 XRD peaks were characterized better than M-1~M-3 at 1000℃. The formation temperature of mullite in this study is much lower than that of previous sol-gel methods, which crystallized at up of 1200℃. TEM investigations revealed that the sample with 10 ㎚ particle size was obtained via heat-treated at 1000℃ for M-4.

Mullite Interlayer of C/Si-C-N Composites Fabricated by PIP

Mullite interlayer of the carbon fiber reinforced Si-C-N matrix composite (C/Si-C-N) was fabricated by liquid precursor impregnation and pyrolysis (PIP). The mixture of aluminium secbutoxide (A1OBu) and tetraethoxysilicate (TEOS) were used as starting materials. The monophase polymer gel used as mullite precursor could be formed from the mixture in the semi-closed air. A mullite interlayer with a thickness about 500nm is obtained at 850°C- 1200°C. The fiber/interlayer bonds are tight. The carbon fibers are not damaged in the fabrication procedure.