Mullite Composites Research Articles

Thermal and Microstructure Stability of Cordierite–Mullite Ceramics Prepared from Natural Raw Materials

In this study, attempts were undertaken to explore the possibilities to prepare cordierite–mullite composites with potentiality to be used as refractory materials from natural raw materials. Solid state reaction sintering different sample powders at different temperatures from 1,250 to 1,400°C has been applied to prepare cordierite–mullite ceramics. XRD has proved the presence of cordierite and mullite in different ratios as major phases. Thermal shock test method based on heating–quenching procedure, supplemented by scanning electron microscopy and mercury intrusion porosimeter, has been used to demonstrate the thermal stability of different samples.

Fabrication and Properties of 3D-C<sub>f</sub>/Mullite Composites with SiC Interfacial Coatings

Three-dimensional carbon fiber reinforced mullite composites (3D-Cf/mullite) were fabricated by sol-gel method with diphasic mullite sol as raw materials. SiC interfacial coatings were deposited by polycarbosilane infiltration and pyrolysis before sol-gel process. The as-fabricated composites showed 285.7MPa in flexural strength, relatively close to 277.6MPa of those without coatings. After treated at 1300°C and 1500°C in Ar atmosphere for 1h, the retention rates of flexural strength were 80.0% and 69.9%, respectively, which showed a great improvement compared to the composites without coatings. This was attributed to the prevention of carbothermal reduction between carbon fiber and matrix by SiC coatings.

Highly porous corundum–mullite ceramics – Structure and properties

The aim of this study was to improve the mechanical properties of porous corundum ceramics by adding various types of SiO2 source (SiO2, SiC and Si3N4), but at the same time retaining high porosity (at least 55%). Ceramics were fabricated by slip casting. Pores were formed using aluminium's reaction with water. It was found that the bending strength of the material can be improved and relatively high porosity retained by producing corundum–mullite composites. Addition of 3.7equivalent wt% of SiO2 source increased the bending strength by up to 250% in comparison with unmodified corundum ceramics. The apparent porosity decreased by up to ca. 8%. If the amount of SiO2 source was increased from 3.7equivalent wt% to 7.3equivalent wt%, the bending strength decreased. The best mechanical properties were achieved with samples that were modified with SiC and Si3N4 nanopowders. This is due to better dispersion in Al2O3 matrix.

Role of Cr2O3 on formation of reaction sintered dense ZrO2–Al2O3–SiO2 based refractory materials

Extremely dense zirconia–mullite composites were developed by reaction sintering of microcalcined fine alumina powder and zircon flour taken in different ratios with Cr2O3 as an additive. The powder mixes were isostatically pressed and sintered at 1550°C. The sintered compacts were characterised by different physicomechanical properties, microstructure developed and phase compositions. Cr2O3 exhibited positive influence up to certain level of addition on the thermomechanical properties of reaction sintered zirconia–mullite composites including corrosion resistance and microstructure.On a développé des composites extrêmement denses de zircone et mullite par frittage réactif de poudre fine d’alumine micro calcinée et de farine de zircone en différentes proportions, avec du Cr2O3 comme adjuvant. On a pressé les mélanges de poudres au moyen d’une presse isostatique et on les a frittés à 1550°C. On a caractérisé les différentes propriétés physico mécaniques, la microstructure développée et la composition de phases des compacts frittés. Le Cr2O3 exhibait une influence positive jusqu’à un certain niveau d’addition sur les propriétés thermomécaniques des composites de zircone et mullite à frittage réactif, incluant la résistance à la corrosion et la microstructure.

Powder Injection Molding of Ceria-Stabilized, Zirconia-Toughened Mullite Parts for UAV Engine Components

Powder injection molding (PIM) of ceria-stabilized, zirconia-toughened mullite composites were investigated in the present article with the goal of obtaining performance enhancement in complex geometries for energy and transportation applications. A powder-polymer mixture (feedstock) was developed and characterized to determine its suitability for fabricating complex components using the PIM process. Test specimens were injection molded and subsequently debound and sintered. The sintered properties indicated suitable properties for engine component applications used in unmanned aerial vehicles (UAVs). The measured feedstock properties were used in computer simulations to assess the mold-filling behavior for a miniature turbine stator. The results from the measurements of rheological and thermal properties of the feedstock combined with the sintered properties of the ceria-stabilized, zirconia-toughened mullite strongly indicate the potential for enhancing the performance of complex geometries used in demanding operating conditions in UAV engines.

Application of the statistical Taguchi method to optimize X-SiAlON and mullite formation in composite powders prepared by the SRN process

Abstract The formation of an X-sialon/mullite nanocomposite by a combination of mechanical activation and silicothermal reduction and nitridation is reported. The starting materials (andalusite, Si and various aluminium sources) were activated in a planetary mill and fired in N2 at 1450 °C. Microstructure and phase analysis was carried out on fired and unfired samples using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Taguchi statistical experiments were carried out to optimize the formation and crystallinity of the X-sialon component of the sialon–mullite composite. The milling time is the factor of greatest influence on the formation and crystallite size of X-sialon; milling for 15 h increases the amount of X-sialon formed but longer milling times decrease the crystallinity of the products and the sialon/mullite ratio. High-energy milling of an andalusite/Si/alumina mixture produces a mullite-sialon nanocomposite powder. TiO2 additive stabilises the mullite component whereas Y2O3 enhances the formation of X-sialon and Al(OH)3 increases the crystallinity. The optimized conditions for the production of X-sialon/mullite nanocomposite powders are the use of Al2O3 as the Al source, milling time 15 h, avoiding the use of TiO2 as an additive.

Effects of sintering temperature on mechanical properties of 3D mullite fiber (ALF FB3) reinforced mullite composites

A new method to weaken the interfacial bonding and increase the strength of 3D mullite fiber reinforced mullite matrix (Muf/Mu) composites is proposed and tested in this paper. Firstly, Muf/Mu composites were fabricated through sol–gel process with varied sintering temperature. Then, the effects of sintering temperature on mechanical properties of the composites were tested. As sintering temperature was raised from 1000 °C to 1300 °C, the three-point flexural strength of the composites firstly decreased from 66.17 MPa to 41.83 MPa, and then increased to 63.17 MPa. In order to explain the relationship between composite strength and sintering temperature, morphology and structure of the mullite fibers and mullite matrix after the same heat-treatment as in the fabrication conditions of the composites were also investigated. Finally, it is concluded that this strength variation results from the combined effects of matrix densification, interfacial bonding and fiber degradation under different sintering temperatures.

Toughened carbon nanotube–iron–mullite composites prepared by spark plasma sintering

Carbon nanotube–iron–mullite nanocomposite powders were prepared by a direct method involving a reduction in H 2 –CH 4 and without any mechanical mixing step. The carbon nanotubes are mostly double- and few-walled (3–6 walls). Some carbon nanofibers are also observed. The materials were consolidated by spark plasma sintering. Their electrical conductivity is 2.4 S/cm whereas pure mullite is insulating. There is no increase in fracture strength, but the SENB toughness is twice than the one for unreinforced mullite (3.3 vs. 1.6 MPa m 1/2 ). The mechanisms of carbon nanotube bundle pullout and large-scale crack-bridging have been evidenced.

Electrical and dielectric properties of sol–gel derived mullite doped with transition metals

Highly crystallized mullite composite has been synthesized at 1000 °C and 1300 °C via sol–gel technique in the presence of transition metal ions such as cobalt, nickel and copper. The dielectric properties (dielectric constant, loss tangent and a.c. conductivity) of the composites have been measured at room temperature and their variation with increasing frequency and concentration of the doped metals has been investigated. The composite doped with nickel exhibits minimum dielectric constant of 3.15 at 0.002(M) concentration at a frequency of 1.5 MHz. Experimental data shows a linear increase in a.c. conductivity with increasing concentration of metal doping ions .The dielectric constant and loss tangent of the composites are within the range of requirements for commercial use in electronics.

Preparation of porous mullite composite by microwave sintering

Microwave sintering method was carried out to prepare porous mullite composite. An insulation structure based on hybrid heating mode was well designed with the wall of mullite and the aided heaters of SiC. The obtained samples were characterized by XRD analysis, apparent porosity detection, and bending strength measurement. SEM was used to observe the microstructure of the sample. It is found that the porous mullite composite could be prepared through the microwave sintering within 2 h at relatively low temperatures around 1000 °C. The lasted samples show comparatively superior properties to the products prepared by conventional processing.

Influence of Ti addition on boehmite-derived aluminum silicate aerogels: structure and properties

Aluminosilicate aerogels offer potential for extremely low thermal conductivities at temperatures greater than 900 °C, beyond where silica aerogels reach their upper temperature use limits. Aerogels have been synthesized at two Al:Si ratios, a 3Al:1Si mullite composition, and an 8Al:1Si alumina rich composition. Boehmite (AlOOH) is used as the Al source, and tetraethoxysilane as the Si precursor. The influence of Ti as a ternary constituent, introduced through the addition of titanium isopropoxide in the sol–gel synthesis, on aerogel morphology and thermal properties is evaluated. Four different boehmite precursor powders are evaluated. Morphology, surface area and pore size, and thermal transformation vary with the crystallite size of the starting boehmite powder, as does incorporation of titanium and evolution of Ti-containing crystalline phases. The addition of Ti influences sol viscosity, gelation time, surface area and pore size distribution, as well as phase formation on heat treatment.

Boron mullite: Formation and basic characterization

A series of boron doped mullites (B-mullite) was prepared from single-phase gels with initial compositions based on a 1:1 isomorphous substitution of Si by B, starting from a 3:2 mullite composition (Al4.5Si1.5O9.75). A high amount of boron (>10mol.%) can be incorporated into the crystal structure of mullite where it most likely replaces Si. In situ phase formation of B-mullites was studied with high temperature X-ray diffraction and thermal analysis. A decrease of the formation temperature for B-mullite with increasing boron content was observed. With increasing boron content lattice parameters b and c significantly decrease, while no systematic evolution of a is observed. Long annealing at 1400°C results in decomposition of B-mullite to boron free mullite and α-alumina. At 800°C B-mullite appears to be stable over a period of at least 12 days. The mean thermal expansion coefficient was reduced by 15% upon incorporation of boron which makes the material technologically interesting.

Microwave Sintered Porous Mullite Composite

In order to aviod much high energy and CO2 generation. Microwave sintering (MS) is always carried out to prepare refractory. In this work，MS was proposed to sinter mullite composite. An insulation structure based on hybrid heating mode was well designed with the wall of high-aluminum refractory and the aided heaters of SiC. The sintered samples under different sintering processing were characterized by appearent porosity detection and volume density. Phase composition was investigated by X-ray Diffraction (XRD) analysis. Surface and fracture Flied Emission Scanning Electron Microscopy (FESEM) was applied to observe pore distribution and grain growth. Sintering profile was provided in this work. It was found that mullite composite could be prepared within 1h at relative lower temperature around 1300°C. Compared with samples prepared by convitional sintering (CS), MS samples presented better densification and more uniform grain growth. Microwave plasma phenomena was explored inside MS samples, however, not found in CS ones.

Effect of Milling Time on the Physical and Mechanical Properties of Celsian-Mullite Composites Synthesized from Coal Fly Ash

ABSTRACTFour Celsian (Ba0.75Sr0.25Al2Si2O8)/Mullite (Al6Si2O13) composites, with potential structural applications at high temperatures, are synthesized from coal fly ash (byproduct of a Mexican coal-burning power plant, constituted mainly by SiO2 and Al2O3). Nominal Celsian/Mullite weight ratios studied are 80/20, 60/40, 40/60 and 20/80. Mullite is synthesized separately at 1600ºC/2h and then mixed with a Celsian precursor mixture previously calcined at 900°C/5h. During this process the Celsian phase is formed by a solid state reaction at 1100-1400ºC/5h. Prior to this, the reacting mixture is milled in a planetary mill during 1 or 2h and then compacted by uniaxial and cold isostatic pressing. The microstructure and phase composition of the synthesized composites are characterized by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM/EDS). Their dynamic Young’s modulus is measured by an ultrasonic technique, and their mechanical strength is evaluated from flexural tests carried out at room temperature. The expected phases are obtained in all cases, although with some differences with respect to their expected relative proportions, according to the studied nominal compositions. In general, the longest milling time employed produced samples with the largest degree of crystallinity and density, as well as with the best microstructural characteristics and mechanical properties.

Growth habit of mullite composites

Under open-system hydrothermal conditions, the observed growth habit for mullite composites is columnar. Based on theoretic calculations, performed with the Cambridge serial total energy package, of mullite crystal surface energies for facets (001), (210), (110), and (120), we analyze and discuss the relationship between growth rate and growth habit. Following the above facet order, these surface energies are 0.337, 0.022, 0.217, and 0.039 keV nm-2. According to the Curie-Woolf principle, growth rates would follow the sequence (001)>(110)>(120)>(210). Thus mullite crystals would favor theoretically a c -axis columnar growth habit. This result supports in this instance growth rate determinations based on theoretic calculations of surface energies.

Effect of MgO in the microstructure formation of zirconia mullite composites from sillimanite and zircon

The effect of MgO additive on the structural, microstructural and hardness properties of zirconia mullite (MUZ) has been discussed. The MgO additive in MUZ not only stabilizes the cubic zirconia phase but also acts as a sintering aid for the formation of cross-linked mullite grains. The electron micrographs of plasma fused MgO–MUZ shows a uniform arrangement of platelet structure of mullite and dendrite structure of zirconia on mullite surface. The micrograph of plasma sintered composites shows a ladder like structure and a complete cross-linked mullite grains whereas the surface morphology of conventionally sintered composites clearly indicates the presence of small and big grains close packed to each other. Appreciable hardness and higher optical band gap have been observed for plasma fused MgO–MUZ composites. A complete dissociation of sillimanite and zircon has been occurred in plasma fused composites for the complete conversion of MUZ whereas the complete dissociation of sillimanite and zircon has not observed in plasma sintered and conventionally sintered composites. These observations have been realized from the X-ray diffraction and Fourier transform infrared studies.

CASTEP Calculation of Surface Property of Mullite Composites and Analysis of Crystal Growth Habit

According to the conditions of preparation of mullite composites by open-system hydrothermal crystallization method, surface energies of mullite crystal were calculated by the first principle plane pseudo-potential. Growth habit plane of mullite composites was also discussed and analyzed. The calculation results demonstrate that surface energies of (001), (010), (100), (210) and (120) of mullite crystal are 0.337KeV•nm-2, 0.029KeV•nm-2, 0.027KeV•nm-2, 0.032KeV•nm-2, 0.037KeV•nm-2, respectively. That is to say, the surface energies of mullite crystal follow the order of (001) >> (210) > (120) > (100) > (010). It is also found that E(001) is much larger than others. According to Curie-Wulff principle, growth rate of crystal face is directly proportional to its surface energy, which shows that growth rate of mullite crystal in (001) direction is the fastest and mullite crystal is favorable for one-dimensional growth to columnar.

Preparation Cordierite-Mullite Composites with Iron Tailing

Cordierite–mullite composites were prepared by using iron tailing, burn talc, suzhou soil, knar clay and alumina powder as raw materials. The main influencing factors such as the molar ratio of cordierite to mullite, amount of iron tailing and calcination temperature were discussed. The microstructure of the sintered sample was analyzed with X-ray diffraction (XRD) and Scanning electron microscopy (SEM). The results show that the optimal prescription was V sample (36.4635wt% of iron tailings, 13.6813 wt % of burn talc, 10.2154 wt % of suzhou soil, 3.8405 wt % of knar clay and 35.7984 wt% of alumina), and its sintered material contains about 50% of mullite and 50% of cordierite;The optimal sintered temperature was 1250°C and the holding time was 2 hours. The bending strength of the sample is 29.79MPa and the thermal shock resistance expriments are above 60 times.

An X-ray micro-fluorescence study to investigate the distribution of Al, Si, P and Ca ions in the surrounding soft tissue after implantation of a calcium phosphate-mullite ceramic composite in a rabbit animal model

Synthetic calcium phosphates, despite their bioactivity, are brittle. Calcium phosphate- mullite composites have been suggested as potential dental and bone replacement materials which exhibit increased toughness. Aluminium, present in mullite, has however been linked to bone demineralisation and neurotoxicity: it is therefore important to characterise the materials fully in order to understand their in vivo behaviour. The present work reports the compositional mapping of the interfacial region of a calcium phosphate--20 wt% mullite biocomposite/soft tissue interface, obtained from the samples implanted into the long bones of healthy rabbits according to standard protocols (ISO-10993) for up to 12 weeks. X-ray micro-fluorescence was used to map simultaneously the distribution of Al, P, Si and Ca across the ceramic-soft tissue interface. A well defined and sharp interface region was present between the ceramic and the surrounding soft tissue for each time period examined. The concentration of Al in the surrounding tissue was found to fall by two orders of magnitude, to the background level, within ~35 μm of the implanted ceramic.

Spark plasma sintering to restrict sintering reactions and enhance properties of hydroxyapatite–mullite biocomposites

Herein, we demonstrate how spark plasma sintering (SPS) can be useful in restricting the sintering reactions and faster densification in Hydroxyapatite–Mullite system, which otherwise shows extensive sintering reactions during conventional pressureless sintering, as reported in a recent study [Nath et al. J. Am. Ceram. Soc. 93 (2010) 1639–1649]. The microstructure of SPSed Hydroxyapatite (HAp)-20 wt% mullite composites was characterized by submicron sized HAp and equiaxed mullite grains. Another important result has been the achievement of higher hardness of 7 GPa, which is much higher than pressureless sintered composites. The cell culture study including cellular viability using MTT analysis establishes good cytocompatibility of SPSed composites.