Mullite Zirconia Research Articles

Low sintering shrinkage and high strength of porous mullite–zirconia ceramics: Effect of AlF3 and ZrSi2 contents

AbstractLow‐shrinkage porous mullite–zirconia ceramics were prepared using Al and ZrSi2 as raw materials and AlF3 as the whisker catalyst through foam‐gelcasting and freeze‐drying methods. The oxidation of Al and ZrSi2 combined with the Kirkendall effect and volume expansion was employed to reduce the sintering shrinkage, while the mullitization was promoted by the reaction between AlF3 and extra ZrSi2, and ZrO2 were generated to enhance the compressive strength as the reinforcing phase. The effect of AlF3 and ZrSi2 content on the microstructure and properties of porous mullite‐zirconia were explored. These ceramics exhibited high porosity (73.75%–85.02%) and compressive strength (2.98–17.71 MPa), low shrinkage (0.47%–10.89%) and thermal conductivity (0.228–0.462 W/(m·K)). In particular, the porous mullite–zirconia ceramics with great compressive strength (4.01 MPa) companied with near‐zero shrinkage (0.47%) at the AlF3 content of 15 wt% and extra ZrSi2 content of 20 wt% by introducing the reinforcing phase (ZrO2). Additionally, the temperature difference around 950°C of the porous ceramics during the heating test proved the exceptional application on thermal insulation.

Microstructure and mechanical properties of a porous ceramic composite with needle‐like mullite and zirconia

AbstractPorous mullite ceramics have good properties for high‐temperature applications, but porosity gives place to ceramics with low mechanical strength, which restricts the service life in their potential applications. Therefore, performing modifications at the microscale to increase the mechanical strength has become a current challenge to expand its application fields. This work describes the properties of a porous mullite–zirconia composite produced by ceramic processing, using industrial kaolin and stabilized zirconia as raw materials. The growth of mullite needle‐like grains to reinforce the ceramic was promoted by the addition of a molybdenum oxide precursor. The effect of zirconia on the composite was analyzed through an experimental multi‐technique approach and considering a pure mullite sample, identically processed, as a reference. The novel composite has a porosity of about 50%, and presents a homogeneous microstructure, with interlocked mullite needle‐like grains and dispersed rounded zirconia grains. This morphology restricts the mullite tendency to shrink during sintering, giving the material a higher stiffness. In particular, the presence of zirconia in the composite improves both the flexural strength and the apparent Young modulus of the material (about 20% and up to 600%, respectively). These results encourage further investigations to establish this composite for different technological applications.

Microwave heat treatment on kyanite for mullite formation and the preparation of zirconia toughened mullite

This article deals with the investigations on mullite ceramics formation from Kyanite sample- an alumino-silicate mineral with microwave heating. In the present investigation kyanite was heated in microwave sintering furnace with calcined alumina, silicon carbide, zirconium dioxide and binder for mullite formation as well as for the preparation of zirconia toughened mullite product. The results of these studies reveal that how a small amount of zirconium dioxide helps in toughening the mullite formation for refractory and ceramics applications. The results shows that microwave heating method is one of the most novel and eco-friendly process for such toughened mullite formation, well within very short period of time. The observations with mullite formation and zirconia toughened mullite formation were observed with the structural data using X-ray diffraction data, field effect scanning electron microscope and energy dispersive spectroscopy. Future studies are still in process.

Introducing MnO2 and ZnO Additives for the Development of Alumina–Mullite–Zirconia Composites

Introducing MnO2 and ZnO Additives for the Development of Alumina–Mullite–Zirconia Composites

Characterization of in-situ zirconia/mullite composites prepared by sol-gel technique

ABSTRACT The main objective of this study was to investigate the role of zirconia addition to mullite through an in-situ reaction aimed at improving both the mechanical properties and the sinterability behavior. In this work, mullite–zirconia composites were produced using a sol-gel technique. Different amounts of zirconia (0, 10, 15, and 20 wt.%) were added to the mullite, and the calcined gels were sintered at 1550–1700°C for 1 h. The apparent porosity and bulk density of the blank and zirconia/mullite composites were estimated in accordance with ASTM C-20. The phase composition and sample morphology were evaluated via X-ray diffraction (XRD) and scanning electron microscopy analysis (SEM), respectively. Furthermore, the mechanical properties and thermal expansion coefficient (TEC) were also evaluated. The results revealed that the apparent porosity decreased and the density of the zirconia/mullite composites increased when the sintering temperature was increased from 1550 to 1700°C. However, the mechanical properties improved with increasing zirconia content and MZ20 sintered at 1700°C exhibited the maximum bending strength. The TEC results reflected the influence of the composition on the sample TEC. Samples with higher ZrO2 content yielded higher TEC figures than those with lower content.

Mullite–zirconia composite for the bonding phase of refractory bricks in hazardous waste incineration rotary kiln

Although mullite–zirconia composites made from zircon, alumina, and andalusite meet the requirements for many refractory applications, little effort has been made to transfer these composites to the bonding phase (the ‘matrix’) of refractory bricks. In this paper, we investigate how this could be achieved through better control of secondary oxides. The high temperature phases were simulated with thermodynamic software and linked to the microstructures, mineralogy, and properties of the composites. The results revealed that the system is very sensitive to Na2O, which harmed the microstructure considerably. By contrast, TiO2 and P2O5 additions proved beneficial, allowing complete zircon decomposition at 1550 °C while providing the required green strength. Decohesion between the matrix and aggregates due to high matrix shrinkage can be prevented by partially substituting andalusite with the volume-increasing mineral kyanite. Based on these findings, a novel refractory brick was developed and tested with success at an industrial scale.

Effect of the La2O3 Addition on Thermal, Microstructure and Mechanical Properties of Mullite–Zirconia Composites

Effect of the La2O3 Addition on Thermal, Microstructure and Mechanical Properties of Mullite–Zirconia Composites

Thermomechanical Characterisation of Mullite Zirconia Composites Sintered from Andalusite for High Temperature Applications

Mullite-Zirconia refractories are well known for their good resistance to corrosion and thermal shock. In this study, several mullite-zirconia composites were developed from andalusite, alumina and zircon sintered at 1600 °C for 10 hours. The samples were subjected to thermal shock carried out after heating at 1200 °C, in order to study the mechanical and thermomechanical behaviour as a function of the amount of zirconia dispersed in the mullite matrix. It appears that that the amorphous phase (SiO2), determined by X-ray diffraction, produced by the decomposition of andalusite, increases considerably with the amount of final zirconia in the composite and has a very important influence on the porosity. This amorphous phase seems also to have an important influence on the mechanical properties of the material. The characterisation of the thermomechanical behaviour (elastic properties and damage monitoring) was carried out thanks to ultrasonic techniques (US echography and Acoustic Emission). The “surprising” evolution (increase) of the Young’s modulus E of the material after being submitted to repeated thermal shocks is highlighted and explained. The acoustic emission technique carried out at high temperature and also coupled to 4-points bending tests (at room temperature) demonstrates its effectiveness for providing a better understanding of the chronology of the involved mechanisms involved at microstructural scale.

Characterization and production of slip casts mullite–zirconia composites

In this study, mullite–zirconia composites were prepared by reaction sintering of alumina, kaolinite zircon and colemanite powder. Slip casting method was also employed for production of these composites. Two different compositions were prepared with and without colemanite additive. The main purpose of this work was to determine the optimum conditions for mullite–zirconia synthesis in addition to the effects of starting composition, solid concentration of aqueous suspensions and sintering temperature. The resulting sintered materials were characterized in terms of bulk and relative density, firing shrinkage, XRD and SEM. XRD peaks suggested fully developed mullite and zirconia phases by reaction sintering of zircon, kaolinite, alumina and colemanite phases. Zircon usually dissociates at a temperature higher than 1650 °C. However, it has been shown in this study that zircon dissociates at about 1450 °C due to the presence of colemanite. The results of the study indicate that 90% of relative density was achieved as the solid concentration is increased up to 55% in case for slip-casted specimens with colemanite additive (MZC55 with 55% solid concentration). The microstructure of all composites is composed of irregularly shaped mullite grains and round-shaped zirconia grains which are distributed intragranularly and intergranularly.

Microstructure and mechanical properties of hot-pressed Al2O3–mullite–ZrO2–SiC composites

In this study, the effects of SiC content, sintering temperature and Al2O3/SiO2 ratio on the microstructure and mechanical properties of hot-pressed alumina–mullite–zirconia–silicon carbide (Al2O3–mullite–ZrO2–SiC) composite ceramics were investigated systematically. The research results indicate that the composite ceramics have excellent combined properties when SiC content is 20 vol% and sintering temperature is 1530 °C, with Vickers hardness, fracture toughness and flexural strength of 17.5 GPa, 12.3 MPa m1/2 and 970 MPa, respectively. The fracture toughness and flexural strength are higher than those of monolithic Al2O3 ceramics and Al2O3-based composites, which are fairly attributed to the refined microstructures of composites and the synergistic effects of crack deflection, pull-out, branching, and phase transformation toughening mechanisms induced by short columnar Al2O3 grains and SiC platelets formed in the ceramics, as well as the ZrO2 particles existed in the ceramics.

Study on mechanical properties of hot pressing sintered mullite-ZrO2 composites with finite element method

In this study, mullite–zirconia (ZrO2) composites were fabricated by hot pressing sintering method. The effects of sintering temperature and holding time on the microstructures, phase compositions and mechanical properties of the composites were investigated. The results indicated that the size of t-ZrO2 grain varies with sintering temperature and holding time, and the maximum flexural strength of 674.05 MPa and fracture toughness of 12.08 MPa m1/2 are obtained when the sintering temperature is 1500 °C with holding times of 20 and 60 min, respectively. Finite element method was employed to analyze the relationship between grain size and mechanical properties of mullite–ZrO2 composites for the first time. The results showed that the maximum stress on mullite–ZrO2 interface increases with the growth of t-ZrO2 grain size, which enhances the generation and propagation of cracks on grain boundaries significantly and degrades the flexural strength and fracture toughness of the mullite–ZrO2 composite ceramics.

Enhanced mechanical and thermal properties of anisotropic fibrous porous mullite–zirconia composites produced using sol-gel impregnation

A new lightweight and thermal insulating material was successfully prepared by impregnating Al2O3–SiO2 aerogel into a porous mullite–zirconia fiber network with a quasi-layered microstructure. The microstructures, thermal and mechanical properties of the porous mullite–zirconia ceramics were investigated before and after the impregnating of Al2O3–SiO2 aerogel. It was observed that the mullite and zirconia fibers combined well with aerogels. Compared with fibrous porous mullite-zirconia ceramics, the Al2O3–SiO2 aerogel/porous mullite–zirconia composite exhibited higher compressive strength (i.e. 1.36 MPa in the z direction) and lower thermal conductivity (i.e. 0.052 W/(m·K)). The present work provides an efficient way to fabricate highly porous ceramics with high strength and ultra-low thermal conductivity, which find potential use in high-temperature thermal insulation materials.

The effect of colemanite addition on the properties of mullite-zirconia composites prepared by slip casting

Purpose: This study deals with the effects of colemanite (Ca2B6O115H2O) on properties ofslip-cast mullite–zirconia composites prepared via reaction–sintering of kaolinite, aluminaand zircon powders.Design/methodology/approach: Colloidal processing (slip casting) is the routetowards preparing these materials using 45 vol.% aqueous suspensions of a mixture offine powders stabilized with polyacrylate solution as a dispersant.Findings: The influence of powder composition on physical, mineralogical properties, andmicrostructure of these composites after firing at 1450, 1500 and 1550°C are observed.The results show that the density of composites tends to increase with the addition of7 wt.% colemanite. XRD analyses reveal that using colemanite during the synthesis ofmullite-zirconia composites lowers the reaction temperature. All of the composites consistof irregularly shaped mullite and round-shaped zirconia grains, which are distributedhomogenously.Practical implications: Mullite–zirconia composites, owing to their chemical inertnessand good resistance against chemical attack-corrosion by siliceous and metallic melts, theyare employed in the glass industry.Originality/value: Reaction sintering process is considered as a promising technology forpreparing mullite-zirconia composites, because it has some advantages such as low costof traditional raw materials, straightforward production technology and low manufacturingcost.

Thermo-mechanical properties of mullite—zirconia composites derived from reaction sintering of zircon and sillimanite beach sand: Effect of CaO

Mullite—zirconia composites containing 20% zirconia (mass fraction) were prepared by reaction sintering route utilizing Indian coastal zircon flour and sillimanite beach sand. 4%–12% of CaO (mole fraction) with respect to zirconia was used as additive. The effect of additive on densification, microstructure as well as various mechanical and thermo-mechanical properties was studied. Incorporation of CaO reduced the densification temperature of the composites to 1550 °C compared to 1600 °C (for CaO free samples). CaO formed small amount of liquid phase (calcium aluminosilicate), which facilitated sintering. Average grain size of the composites decreased up to 4% CaO addition, afterwards grain size increased with further addition of CaO. Samples with 4% CaO exhibited ∼225 MPa of flexural strength, ∼6 MP·m1/2 of fracture toughness and significant improvement in thermal shock resistance. CaO stabilized the tetragonal zirconia phase and thus improved the mechanical properties.

Phase Evolution, Microstructure, and Mechanical Properties of Alumina-Mullite-Zirconia Composites Prepared by Iranian Andalusite

This paper investigates the effects of Iranian andalusite and short milling times on alumina–mullite–zirconia composites. Andalusite powder was added at 0, 2.5, 5, and 10 wt% to an alumina–zircon mixture and the raw materials were milled for 1 or 3 h. The sintering of samples was carried out at the temperatures of 1550°C, 1600°C, and 1650°C for 3 h. Microstructural changes, phase composition, physical properties, and mechanical strength of the sintered composites were characterized by scanning electron microscopy, X-ray diffraction, density, and strength measurement tests. Results show that andalusite promoted the decomposition of zircon and accelerated the reaction sintering of alumina–zircon, which leads to the formation of much more mullite phase and improvements to the composites’ thermal shock resistance up to about 50%.

Effect of colemanite on properties of traditional mullite zirconia composite

AbstractIn this study, mullite zirconia composites are synthesized via reaction sintering method by using zircon, kaolinite and alumina mixtures including colemanite (Ca2B6O115H2O) (7 wt.%). In mullite zirconia composites, the effect of colemanite additive on mineralogical, thermal, microstructure and mechanical properties and also on formation are studied. Mixtures are sintered at 1,450, 1,500 and 1,550°C for 5 h. in air. The resulting sintered materials are characterized in terms of bulk density, DTA, X-ray diffraction (XRD), SEM and mechanical properties such as flexural strength and elastic modulus. Various effects of colemanite addition on the composites are observed with regard to thermal, mineralogical and mechanical properties. Both DTA and XRD analyses show that colemanite addition in mullite zirconia composite reduces the reaction temperature. Zircon alone in the mixture resulted in a 1,550°C reaction temperature, whereas the compositions including colemanite is attenuated to 1,450°C. While zirc...

Microwave heat treatment on red sediment sillimanite–zircon–alumina composites for fused zirconia–mullite products

ABSTRACTThe present paper shows a convenient and eco-friendly microwave heating method for the synthesis of fused zirconia mullite formation on combining placer sillimanite and zircon with the mixtures of alumina powder and reducing agent silicon carbide powder with lesser time. The X-ray diffraction data field-emission scanning electron microscope spectrum and scanning electron microscopy/energy-dispersive X-ray spectroscopy analyses in this paper predict that alumina is rich in mullite grains, whereas the grain boundary region becomes rich with zirconia after microwave process. The presence of small percentage of zirconia in the intragrain region reveals that mullite compound is surrounded by well-compacted zirconia. These data also reveal the presence of fused zirconia mullite phase along with crystalline SiC and monoclinic zirconia in the composites.

Fabrication and properties of fibrous porous mullite–zirconia fiber networks with a quasi-layered structure

A novel anisotropy porous mullite fiber network with a quasi-layered microstructure was prepared by vacuum squeeze moulding and sintering. The effects of the initial pressure used in vacuum moulding on the microstructure, thermal properties and mechanical properties of the fibrous porous mullite ceramic were studied. Zirconia fiber was introduced into the ceramic, and the effects of altering the fiber content on the pore structure, porosity and properties of the final ceramic were investigated. Results show that the ceramics prepared in this study exhibited high compressive strength (∼5.16MPa in the x/y direction) and low thermal conductivity (∼0.094W/(mK)). This study also reports an optimal processing method for the production of fibrous porous mullite–zirconia composites, which have the potential for use as ultra-high-temperature thermal insulation material.

Influence of zircon particle size on conventional and microwave assisted reaction sintering of in-situ mullite–zirconia composites

Mullite–zirconia composites were fabricated by reaction sintering of ZrSiO4 and α-Al2O3 using conventional heating and microwave processing. The powder mixtures were prepared from sub-micron zircon powders with three different particle sizes and CIPed as coin shaped samples. The samples sintered both in a muffle furnace and microwave furnace. The open porosities, bulk and true densities were measured. Phase transformations were characterized by X-ray diffraction and microstructures were evaluated by scanning electron microscopy. The effects of zircon particle size on the in-situ transformation system and mullitization was evaluated for both methods. As a result, decreasing zircon particle size decreases the in-situ transformation temperature for 25°C (1575°C) in conventional heating. Microwave assisted sintering (MAS) lowers the transformation temperature at least 50°C by lowering the activation energy more efficiently and gives better densification than conventional sintering. Furthermore, milling also produces structures having finer mullite grains.

Fabrication of highly porous honeycomb-shaped mullite–zirconia insulators by gelation freezing

Highly porous mullite-based thermal insulators were fabricated using a gelation freezing technique. In this process, a honeycomb-like arrangement of pores is created by forming ice crystals within a gel containing dispersed mullite particles, followed by sublimation of the ice under vacuum and subsequent sintering. The uniform arrangement of pore channels and cell walls results in a mullite insulator with a very high compressive strength, even for porosities of up to 91.5vol%, together with low thermal conductivity. The mechanical strength of this insulator was found to be strongly influenced by the amount of antifreeze protein, the sintering additive in the raw mixture, and the sintering temperature. The use of 0.25wt% antifreeze protein with the additive 8Y–ZrO2 gave a compressive strength of 11.3MPa, a porosity of 89.1vol%, and a thermal conductivity of 0.28W/mK. This approach can be used to produce macrocellular insulators with specific porosity, thermal conductivity and mechanical properties, suitable for a variety of industrial applications.