Mullite Formation Temperature Research Articles

Investigations on kaolin mixtures: Impact on mullite formation kinetics and microstructure evolution

AbstractThis study evaluates Algerian kaolin (Djebel Debbagh (DD1) and Tamazart (KT2)) as potential substitutes for commercial kaolin (Lab) in the production of mullite‐based ceramics. Three compositions were prepared by incorporating the appropriate percentage of alumina to each calcined kaolin to achieve stoichiometric mullite precursors. The phase evolution of individual kaolin powders, as well as their mixtures with alumina, depends strongly on the calcination temperature and kaolin impurities. The differential scanning calorimetry combined with thermogravimetric analysis (TGA) showed lower secondary mullite formation temperature for the KT2‐based mixture. However, X‐ray diffraction revealed a complete mullitization in DD1 mixture. The K2O hindered cristobalite formation and reduced secondary mullite formation rate. Microstructure analysis showed lath‐shaped primary mullite and equi‐axed secondary mullite particles. After sintering at 1600°C, The KT2‐based sample (M3) exhibited higher density (3.013 g/cm3) and hardness (9.9 GPa), whereas the DD2‐based sample (M2) showed moderate densification (2.91 g/cm3) and higher flexural strength (159.42 MPa). Impurities (mainly Fe2O3, and K2O) promoted liquid phase sintering, resulting in greater densification in M3, whereas M2 showed more homogeneous microstructure, refined grains, and lower glassy phase content, contributing to enhanced strength.

Effect of SiC whiskers on the anti‐oxidation properties of Yb2Si2O7/mullite/SiC tri‐layer environmental barrier coating for CMCs

AbstractThe mullite and ytterbium disilicate (β‐Yb2Si2O7) powders as starting materials for the Yb2Si2O7/mullite/SiC tri‐layer coating are synthesized by a sol–gel method. The effect of SiC whiskers on the anti‐oxidation properties of Yb2Si2O7/mullite/SiC tri‐layer coating for C/SiC composites in the air environment is deeply studied. Results show that the formation temperature and complete transition temperature of mullite were 800–1000 and 1300°C, respectively. Yb2SiO5, α‐Yb2Si2O7, and β‐Yb2Si2O7 were gradually formed between 800 and 1000°C, and Yb2SiO5 and α‐Yb2Si2O7 were completely transformed into β‐Yb2Si2O7 at a temperature above 1200°C. The weight loss of Yb2Si2O7/(SiCw–mullite)/SiC tri‐layer coating coated specimens was 0.15 × 10−3 g cm−2 after 200 h oxidation at 1400°C, which is lower than that of Yb2Si2O7/mullite/SiC tri‐layer coating (2.84 × 10−3 g cm−2). The SiC whiskers in mullite middle coating can not only alleviate the coefficient of thermal expansion difference between mullite middle coating and β‐Yb2Si2O7 outer coating, but also improve the self‐healing performance of the mullite middle coating owing to the self‐healing aluminosilicate glass phase formed by the reaction between SiO2 (oxidation of SiC whiskers) and mullite particles.

AlF3-assisted flux growth of mullite whiskers and their application in fabrication of porous mullite-alumina monoliths

Mullite is a promising material with its competitive thermochemical and mechanical properties. Although mullite could be obtained by several synthesis methods, the flux method emerges with its advantages over other methods. However, obtaining mullite whiskers with a high aspect ratio and length for ceramic reinforcements is still challenging. In this work, mullite whiskers were grown from AlF3-assisted flux. The addition of AlF3 to flux salt not only decreases the formation temperature of mullite to as low as 700 ​°C and suppresses the formation of corundum side phase, but also increases the length and aspect ratio of the whiskers. The obtained mullite whiskers were used as reinforcement for porous alumina monoliths prepared by the freeze casting route and subsequent sintering at 1500 ​°C. The fabricated mullite-alumina monoliths show competitive compressive strength of 25.7 ​MPa while having as high as 70.6% porosity, which makes them a potential candidate for membrane applications.

Development of mullite fibers and novel zirconia-toughened mullite fibers for high temperature applications

Polycrystalline mullite fibers and novel zirconia-toughened mullite (ZTM) fibers with average diameters between 9.7 and 10.3 μm containing 3, 7 and 15 wt.-% tetragonal ZrO2 (ZTM3, ZTM7, ZTM15) in the final ceramic were prepared via dry spinning followed by continuous calcination and sintering in air. A shift in the formation of transient alumina phases and tetragonal ZrO2 to higher temperatures with increasing amounts of ZrO2 was observed. Concomitantly, the mullite formation temperature was lowered to 1229 °C for ZTM15 fibers. X-ray diffraction revealed formation of the desired tetragonal crystal structure of ZrO2 directly from the amorphous precursor. Room temperature Weibull strengths of 1320, 1390 and 1740 MPa and Weibull moduli of 9.5, 7.1 and 9.0 were determined for mullite, ZTM3 and ZTM15 fibers, respectively. Average Young’s moduli ranged from 190 to 220 GPa. SEM images revealed crack-free fiber surfaces and compact microstructures independent of the amount of ZrO2.

Waste rice husk ash derived sol: A potential binder in high alumina refractory castables as a replacement of hydraulic binder

This work provides a new source of binder system, i.e., waste rice husk ash (RHA) derived sol for unshaped refractories. A facile (alkali extraction) route is used to synthesize the sol from the waste RHA. The sol consists of single phase amorphous silica with average particle size around 22 nm. The estimated cost of 30 wt% solid containing RHA derived sol per litre is around 9.35 $. The temperature of mullite (3Al2O3·2SiO2) formation from waste derived sol and reactive alumina containing system is 1250 °C or higher. The sol (30 wt% SiO2) and the dry sol (99.32 wt% SiO2) is used to formulate the high alumina castable samples. The sintering is performed at 1400 °C, 1500 °C and 1550 °C. The different physico-mechanical characterizations and blast furnace slag (BFS) corrosion test at 1500 °C are comprehensively investigated with the prepared castables. The properties of 5 wt% silica (3 wt% from sol and 2 wt% dry sol) containing castable specimens are also equated with the commercially available different silica containing high alumina castable systems. The obtain results are good agreements with the source data. These promising characteristic confirms the possibility of using waste RHA derived sol for manufacturing of cement free high alumina castable and leads to the economic and ecological benefits in the refractory industries.

Thermally stable boron-containing mullite fibers derived from a monophasic mullite sol

In this work, we reported the synthesis of monophasic boron-containing mullite (MBM) fibers using basic aluminum acetate (BAA) and tetraethyl orthosilicate (TEOS) as starting materials and performed a comparative study between our newly synthesized MBM and commercial Nextel™ 312 fibers. The morphology and the tensile strength are investigated for the new MBM fibers. Continuous and uniform green fibers with a smooth surface were produced via hand drawing method. After the pyrolysis process, amorphous inorganic MBM fibers with a tensile strength of 1.18 ± 0.17 GPa were obtained. After various heat treatment processes (1000–1400 °C for 1.5 h), we found the mullite formation temperature is within the temperature range from 1050 to 1075 °C for both MBM and Nextel™ 312 fibers. However, the results show that the MBM fibers fabricated in this study have a better thermal stability than Nextel™ 312 fibers despite the fact both have the same chemical composition. The improved stability of our MBM fibers is owing to their higher Al4B2O9 formation temperature and lower Al18B4O33 decomposition rate in comparison to those of Nextel™ 312 fibers. Moreover, the newly synthesized MBM fibers have more stable microstructure and less loss in tensile strength at elevated temperatures.

Effect of Bi2O3 on phase formation and microstructure evolution of mullite ceramics from mechanochemically activated oxide mixtures

Mullite ceramics with hollow whisker structure have been synthesized firstly through ordinary sintering process. The effects of Bi2O3 and processing, on mullitization behavior and morphology development of mullite ceramics, derived from the mechanochemically activated mixture of Al2O3 and SiO2, were investigated in this paper. When the content of Bi2O3 was less than 10 mol%, the mullite grains show a short rod-like morphology, without the formation of whisker. As the content of Bi2O3 was increased to more than 10 mol%, the formation temperature of mullite was decreased from 1400 °C to 1100 °C. After sintering at 1400 °C, well-developed mullite whiskers with hollow structure were formed. The formation process and growth mechanism of hollow structural whiskers in mullite ceramic doped with high content of Bi2O3 were discussed in detail.

Phase transformation and properties of high-quality mullite ceramics synthesized using desert drift sands as raw materials

The desert drift sands were used as raw materials to synthesize mullite ceramics in order to save mineral resources and protect the environment. The experimental results show the formation temperature of mullite was reduced significantly and the density and mechanical properties were remarkably improved compared to the reference sample prepared using analytical reagents. The impurity cations formed low thermal expansion of glass phase, contributing to the decrease of thermal expansion coefficient. Finally, the obtained mullite ceramics had excellent densification, mechanical properties and low thermal expansion coefficient.

A novel fabrication approach for improving the mechanical and sound absorbing properties of porous sound-absorbing ceramics

Low-cost porous sound-absorbing ceramics were prepared from steel slag and kaolin, with graphite and NaHCO3 as the pore former. V2O5 and AlF3 were used as additives to promote the secondary mullitization reactions at lower temperatures. The influence of the additives and sintering temperature on phase constituents, open porosity, microstructure and mechanical performance were characterized in detail. Results show that the introduction of the steel slag into porous sound-absorbing ceramics decreases the temperature of mullite formation. Adding 3 wt% V2O5 and 4 wt% AlF3, mullite whiskers with interlocking microstructure were successfully obtained at sintering temperatures upon 1230 °C for 2 h. Furthermore, the resultant samples exhibited excellent performance, including open porosity of 64.7% and compressive strength of 6.78 MPa.

The effect of citric acid on the kaolin activation and mullite formation

The mullite phase in porcelain is important to the porcelain strength; however, its formation from clay minerals usually requires high temperature. Here we found that the activation of kaolin by citric acid would lower the formation temperature of mullite phase to 935.1°C, which is 67.5°C lower than that for the raw kaolin. Needle-like mullite crystal, a desired crystal shape in kaolin, could be observed in activated kaolin at 1050°C. The reduction of mullite formation temperature is due mainly to the surface change in kaolin. Upon citric acid activation, the pseudohexagonal platelets of kaolinite crystal are disintegrated to non-uniform flakes, and the specific surface area increases from 8.18m2/g to 31.21m2/g, both leading to improved activity of kaolin and increased sintering driving force. The results are of interest in producing high-strength porcelain at reduced sintering temperatures and consequent energy saving for sintering process.

Waste-to-Resource Strategy To Fabricate Highly Porous Whisker-Structured Mullite Ceramic Membrane for Simulated Oil-in-Water Emulsion Wastewater Treatment

Industrial waste coal fly ash, containing hazardous metal oxides, poses potential threats to the environment and humans. Efficient recycling of such kind of solid state waste is highly desired yet still challenging. This work addressed waste-to-resource fabrication of a highly porous whisker-structured mullite ceramic membrane for separation of simulated oil-in-water emulsion wastewater by recycling of waste fly ash and natural bauxite with addition of WO3. The formation and characterizations of membranes were systematically studied including reaction mechanism, dynamic sintering behavior, open porosity, mechanical property, pore size distribution, microstructure, and pure water flux. The results show mullite formation temperature was decreased about 100 °C with addition of 20 wt % WO3, whereas open porosity significantly increased with WO3 content due to the formation of a highly porous interlocked whisker structure. Even without any pore formers, interestingly, the membrane with addition of 20 wt % WO3 ...

High-K tungsten-mullite composite for electronic industrial application: synthesis and study of its microstructure, phase behavior and electrical properties

Highly crystallized mullite synthesis with different concentration of tungsten ions has been achieved by sol–gel technique and the effect of tungsten ion doping on mullite was examined at 1,100 and 1,400 °C. Characterizations were done by DTA/TGA, XRD, FTIR, LCR and FESEM instruments at the room temperature. Mullite formation was found to depend on the concentration of the doping ion up to a certain extent. With the addition of tungsten ion, the mullite formation temperature was decreased. The result showed a decrease in mullite phase at higher concentrations of doping ion with respect to undoped mullite. Oxide phases continued to increase with increasing doping concentration, which is mainly responsible for the increment in dielectric value. Dielectric value of the doped mullite was found to decrease with the increase in frequency for all the samples and saturates at higher frequency, which is normal behavior for dielectric ceramics. A.C. conductivity increased with frequency by following Jonscher’s power law. The composite showed maximum dielectric constant of 124.02 sintered at 1,400 °C for 0.05 M at 20 Hz frequency. Highly crystalline mullite whiskers of average dimension 3 μm were obtained at 0.02 tungsten concentration sintered at 1,400 °C. Due to the good dielectric response, comparatively low loss and appropriate nature of the doped mullite it can be used as ceramic capacitors, packaging material for integrated high speed devices. Also, it’s electrical and thermal suitability may prove to be significant as the insulator material of the spark plug.

Densification behavior and microstructure of mullite obtained from diphasic Al2O3–SiO2 gels

Stoichiometric and silica-rich diphasic Al2O3–SiO2 gels were prepared through a sol–gel process. Structural evolution of the gels with heat-treatment up to 1300°C was carried out by combination of differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. Results showed that the homogeneity level of the gels was the determining factor in the phase transformation sequence and the mullite formation temperature. For the gels with high homogeneity level, the crystallization to mullite occurred at low temperature. The presence of ammonium nitrate in the gels promoted the initial formation of alumina–silica spinal at about 950°C, which further reacted with free silica to form elongated orthorhombic mullite grains at about 1250°C.Linear shrinkage, density measurement and scanning electron microscopy (SEM) analyses were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 200°C. It was found that the homogeneity level and amount of silica both have profound effects on the densification behavior. For the gels with high homogeneity level and thus low mullite formation temperature, the densification behavior was worse, owing to the overlapping with the temperature for densification by viscous flow and reaction sintering. In addition, the inhibitory effect for the densification of elongated mullite grains was stronger than that of equaxial ones. Herein, the derived ceramics containing elongated mullite grains and an excess amount of silica sintered to higher densities, since the presence of excess silica promoted the densification by viscous phase sintering.

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.

Effect of La 2 O 3 additives on the strength and microstructure of mullite ceramics obtained from coal gangue and γ-Al 2 O 3

Self-strengthened mullite ceramics with interlocking columnar grains formed through enhanced anisotropic growth by adding La2O3 were prepared from coal gangue and g-Al2O3 within the temperature range 1400–1550 1C and holding time of 4 h. The effects of La2O3 on the bending strength, microstructural evolution, and phase composition were studied. The results showed that the bending strength of the as-prepared ceramics was significantly improved by the addition of La2O3. For samples sintered at 1550 1C, the bending strength increased from 64 MPa to 218 MPa as the La2O3 content increased from 0 mol% to 10 mol%. X-ray diffraction analysis suggested that the formation temperature of secondary mullite was lowered by about 50 1C by adding La2O3. Scanning electron micrographs revealed that the La2O3-added mullite mostly existed in long, columnar form, with aspect ratios of 3–6, forming an interlocking network structure. The interlocking mullite columns, together with the enhanced densification behaviour, contributed to the improved bending strength.

Influence of Titanium Sol on the Sintering Behaviour, Microstructure and Properties of Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> Ceramic Cores

A series of Al2O3-SiO2 ceramic cores with titanium sol sintered at 1350, 1450, 1500 and 1550for 2h, respectively, were prepared, and the phase and microstructure were characterized by X-ray Diffraction and Scanning Electron Microscopy. The influence of titanium sol on the phase transformation, shrinkage rate and flexural strength at room temperature has been investigated. The mullite phase formation temperature decreased by at least 150with the addition of 1wt% to 5wt% TiO2. Additionally, the titania additive promoted the combination of Al2O3 and SiO2, producing mullite phase which reduced the shrinkage and improved the flexural strength.

Low-Temperature Preparation of Porous Cordierite-Mullite Ceramics Using Rice Husk as Silica Source and Pore-Forming Agent

Si and C of rice rusk were used as silica source and pore-forming agent to prepare cordierite-mullite ceramics at 1380°C for 5 h soaking. The Nd2O3 additive was added to improve the sintering properties. The bending strength, porosity, and thermal expansion coefficient were measured. The composition was characterized by X-ray diffraction (XRD). The microstructures of the sintered samples were observed by scanning electron microscopy (SEM). The cordierite-mullite ceramic doped with 2% Nd2O3 have high prorosity and low thermal expansion coefficient, its bending strength reaches 20.55 MPa. The addition of Nd2O3 decreases the mullite formation temperature to 1200 °C

Effect of TiO2 addition on zirconia-mullite composites fabricated by in-situ controlled crystallization of Si-Al-Zr-O amorphous bulk

Zirconia-mullite composite ceramics were fabricated by in-situ controlled crystallization of Si-Al-Zr-O amorphous bulk. The effects of TiO2 addition on the fabrication of zirconia-mullite composites were investigated. The ultra-fine zirconia-mullite composite ceramics were prepared from the amorphous bulk treated. at 980 ℃ for nucleation and 1 140 ℃ for crystallization. The phase transformation of the ceramics was examined using differential scanning calorimetry (DSC) and X-ray diffractometry (XRD). The microstructural features of the samples were evaluated with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and transmission electron microscopy (TEM). The mechanical properties were also determined using Vickers indentation. The results show that the TiO2 additives with mass fraction of 1％-7％ reduce the formation temperature of t-ZrO2 and mullite. When the mass fraction of TiO2 additives is less than 5％, the phases do not change, and most of TiO2 dissolves in ZrO2. When the mass fraction of TiO2 additives is over 5％, the excessive TiO2 forms a new phase, ZrTiO4. Meanwhile, the results also show that TiO2 additives have a great impact on the microstructure and mechanical properties of zirconia-mullite composites. As the TiO2 content increases from 1％ to 7％ (mass fraction),the grain size and the Vickers hardness of zirconia-mullite composites increase. The composite with 3％ (mass fraction) TiO2 additives attains relatively higher fracture toughness.

Synthesis and Characterization of Ti, Fe-Doped Mullite-I

A series of gels with 3Al2O3•2SiO2 were prepared by Sol-gel method and heated at several temperatures for 2 h to synthesize Ti, Fe-doped mullite. The powers were characterized by differential thermal analysis (DSC-TG) and X-ray powder diffraction (XRD). Phase separation was promoted by doping both TiO2 and Fe2O3; with increasing the amount of dopant ions the formation temperature of Si-Al spinel decreased and the formation temperature of mullite increased by TiO2 doping but decreased by Fe2O3 doping. The formation temperature of pure mullite was about 1250-1350 °C.

Boron-doped mullite derived from single-phase gels

Mullite with low dielectric constant and high transparency in infrared and microwave range has potential applications in communication industry. To improve the above properties of mullite, boron-doped mullite single-phase gels with a constant molar ratio of Al/Si = 3/1 and various B/Al ratios (B/Al = 0–0.4/3) were prepared in this study by slow hydrolysis of aluminum nitrate, boric acid and tetraethoxysilane. It was found that boron reduces the mullite formation temperature and suppresses spinel formation. The cell unit lattice parameters and cell volume in boron-doped mullite generally decrease with the increase of boron amount. The SEM observation shows that a small amount of boron reduces the grain sizes of mullite sintered bodies while a large amount of boron facilitates the formation of elongated grains and the amorphous glass phase. Boron decreases the transmittance of mullite ceramic and produces additional intensive absorption bond at 3.9 μm and also reduces the dielectric constants in the frequent range of 1 M–1 GHz.