Effect of zirconia content on mechanical and thermal properties of mullite–zirconia composite

Abstract

Mullite–zirconia composites were prepared by adding various zirconia contents in the mullite ranging from 0 to 30 wt-% and sintering at 1400–1600°C for 2 h. The phase composition examined by X-ray diffraction showed that mullite was the major phase combined with developed t-ZrO₂ and m-ZrO₂ phase as a function of zirconia content, especially at 1600°C, wherein m-ZrO₂ predominated. Density increased when the zirconia content and sintering temperature were increased ranging from 2·2 to 3·53 g cm^?3. The morphology of mullite grain showed elongated grains, whereas dispersed zirconia showed equiaxed and intergranular grains. Flexural strength was continuously improved by adding zirconia during the sintering temperature ranging from 1400 to 1500°C, whereas flexural strength was initially improved up to 5 wt-% of zirconia addition and deteriorated with more than 5 wt-% of zirconia content during sintering between 1550 and 1600°C. The maximum strength, 190 MPa, was obtained when sintering mullite with 30 wt-% of zirconia content at 1500°C. The degradation of strength at high sintering temperature may be a result from more occurrence of m-ZrO₂ phase. Thermal expansion of sintered specimens indicated linear change and hysteresis loop change. The hysteresis loop obtained with increased zirconia content resulted in the t–m phase transformation. Martensitic start temperature M_s was determined to be 530°C for 15 wt-% zirconia sintered at 1500°C, implying that the t–m phase transformation occurred.     

Phase composition of alumina–mullite–zirconia refractory materials

Refractories in the Al₂O₃–SiO₂–ZrO₂ system are widely used in many applications, for ceramic rollers in particular, and are characterized by high mechanical strength, excellent thermal shock resistance, resistance to corrosion by alkaline compounds and low creep at high temperature. Their performances greatly depend on the amount and chemical composition of crystalline and glassy phases, which were investigated by quantitative XRPD (RIR–Rietveld) and XRF in order to assess the effect of various Al₂O₃/SiO₂ ratios of starting batches and different alumina particle size distributions. Refractories consist of mullite, corundum, zirconia polymorphs and a vitreous phase in largely variable amounts. The mullite percentage, unit cell parameters and composition vary with sintering temperature, being mostly influenced by the Al₂O₃/SiO₂ ratio of the batch. Its orthorhombic unit cell increased its volume from 1400 to 1500 °C, while its stoichiometry became more aluminous. The corundum stability during firing is strongly affected by the Al₂O₃/SiO₂ ratio, but not by the particle size distribution of alumina raw materials. Zirconia raw materials are involved in the high temperature reactions and about one-third of the available ZrO₂ is dissolved in the glassy phase, ensuring excellent resistance to alkali corrosion, mainly depending on the fraction of coarse alumina. The phase composition of the vitreous phase increased with sintering temperature, being over 20% when the fractions of coarse alumina in the starting batch are between 0.2 and 0.5.     

Tailoring of functionally graded zirconia–mullite/alumina ceramics

A new tailored zirconia–mullite/(0–100 vol%) alumina as functionally graded ceramics (FGCs) was designed and synthesized by reaction sintering of zircon and alumina. Zircon and alumina powder mixtures were mixed, stacked, compacted in a cylindrical die and sintered. The sintered samples made of 11 layers and varied gradually in composition by 10 vol% from one layer to the other layer (i.e. from zirconia–mullite layer to alumina layer) resulted in continuous functionally graded ceramics without sharp interfaces. Phase composition and densification behaviors of the samples were investigated. Microstructure, mechanical and thermal properties of FGC and its non-layered composites were studied. Results showed that the tailored FGZM/A gave continuous homogenous structure with highly improved physical, mechanical and thermal properties. The different properties of tailored FGZM/A recorded average values or rather better of its non-layered composites which gave a new way for material design.     

Zirconia toughening of mullite–zirconia–zircon composites obtained by direct sintering

Although multi-phase ceramic materials were always used, nowadays composite materials have an important industrial and technological role, because they enlarge the design capability of the manufacturer in properties and behaviors.Some mullite–zirconia–zircon composites were recently processed and characterized which presented satisfactory properties for structural applications under severe chemical and thermomechanical conditions. The objective of the present work is to study the influence of the starting composition in the mechanical and fracture properties of mullite–zirconia–zircon composites, with different microstructures, obtained by direct sintering of binary mixtures of electrofused mullite–zirconia (MZ) and micronized zircon. The materials were consolidated by slip casting of concentrated aqueous suspensions in plaster molds from a wide range of powder compositions (between 15–85 wt% and 85–15 wt% of the two raw materials used).Flexural strength (σ_f), dynamic elastic modulus (E), toughness (K_IC) and fracture surface energy (γ_NBT) were evaluated. The results were explained by microstructure and the XRD-Rietveld analysis.At low proportion, the zircon was thermally dissociated. The ZrO₂ was a product of this reaction and also influenced the mechanical and fracture properties of these materials through several combined mechanisms, principally as a result of the development of microcracks due to the volume change of the zirconia grains caused by the martensitic transformation during the cooling of these composites from sintering temperature.Composites prepared with higher MZ in the starting powders showed a higher fracture toughness and initiation energy. Microstructure consisting of mullite as a continuous predominant phase in which zircon and zirconia grains were distributed showed better mechanical and fracture properties.     

Crystallization studies in mullite and mullite–YSZ beads

Mullite and mullite/Y-ZrO₂ (25 and 50 vol.%) beads were produced by thermal spraying in water the corresponding atomised compositions. The amount of amorphous phases in the as-sprayed beads was comparatively determined by X-ray diffraction (XRD) and differential thermal analysis (DTA) methods. Characteristic XRD peaks associated to crystallization of the mullite and Y-ZrO₂ (t) phases were compared with the intensity of the exothermic events in the DTA for each composition. A direct linear relationship between the amount of amorphous phases estimated by XRD methods and the area under DTA peaks per unit mass was found. The crystallization microstructure of the different composition beads after heat treatments was followed by scanning electron microscopy (SEM).     

Processing and crystallisation of rapidly solidified Al2O3–Y2O3 fibres

Abstract

Amorphous fibres of the Al₂O₃–Y₂O₃ system were prepared by a melt extraction technique, and subjected to crystallisation. The quality of the melt extracted fibres is controlled by the wheel edge and rotational speed, with both having a significant effect on fibre diameter and avoidance of irregularities and instabilities along the fibre length. Tensile strength in the glassy state varied from 0·6 to 1·0 GPa. Crystallisation activation energies calculated from scan-rate dependence of DTA peaks are 741 and 1374 kJ mol^-1 for E1 (Al₂O₃–yttrium aluminium garnet (YAG) eutectic), 390 kJ mol^-1 for YAG, and 438 kJ mol^-1 for E2 (YAG–yttrium aluminium perovskite (YAP) eutectic) by the Kissinger method; and 698 and 1346 kJ mol^-1 for E1, 352 kJ mol^-1 for YAG, and 399 kJ mol^-1 for E2 by the Augis–Bennett method.     

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.     

Sintering and microstructure of mullite–Mo composites

Mullite–Mo composites of different compositions (0–100 vol.% Mo) were sintered to near theoretical density by pulse electric current sintering (PECS). The densification behaviour and the microstructure of mullite–Mo composites as a function of Mo content were studied. The addition of 10 vol.% Mo significantly enhanced the strength and toughness of monolithic mullite to 556 MPa and 2.9 MPa m^1/2, respectively. SEM observations revealed the modification of discrete isolated Mo particles to continuosly interconnected network with the increase in the Mo content. Mo grains were located at the grain boundaries as well as inside the mullite grains. The addition of Mo to monolithic mullite led to a change in the fracture mode.     

Mechanical properties and microstructures of reaction sintered mullite–zirconia composites in the presence of an additive — dysprosia

Mullite–zirconia composites were prepared from Indian zircon flour and calcined alumina following the reaction sintering route. Zircon flour and calcined alumina with 0–4.5 mol% dysprosium oxide were attrition milled followed by isostatic pressing and sintering at 1400–1650°C for 2 h. Significant densification was achieved after dysprosia addition as an additive. The thermal expansion coefficient values were found to be reduced in the presence of dysprosia. Dysprosia helps in densification by liquid phase formation as well as by stabilisastion in tetragonal zirconia state. The thermo-mechanical and microstructural characteristics of the composites were discussed.     

Microstructure tailoring of the nickel–yttria stabilised zirconia (Ni–YSZ) cermet hollow fibres

Nickel–yttria stabilised zirconia (Ni–YSZ) hollow fibres have been prepared by the phase inversion/sintering technique followed by a reduction process with hydrogen. This work is particularly focussed on tailoring the microstructure and the properties of hollow fibres by ethanol addition into the spinning hollow fibre suspension. Microstructure evolution change is demonstrated by increasing the amount of ethanol from 0 to 35 wt% e.g. the hollow fibre cross-section is modified from a sponge-like structure sandwiched by two thin finger-like layers to the sponge-like structure only. Higher ethanol content translates to denser hollow fibres. This trend also correlates with the shrinkage, mechanical strength and electrical conductivity of the hollow fibres. As the ethanol content is increased, shrinkage reduces, mechanical strength improves and electrical conductivity increases. The Ni–YSZ hollow fibres made from suspensions containing 15–25 wt% ethanol are considered the best option as anode supports for micro-tubular solid oxide fuel cells in terms of their median porosity values, since insufficient porosity would hinder the fuel and product transport, whereas excessive porosity would deteriorate the mechanical strength of the fibres.     

Hierarchical mullite structures and their heat-insulation and compression–resilience properties

Near-net-shaped hierarchical structure-adjustable short mullite fibers/mullite whiskers frameworks (MF/MW frameworks) were prepared by slurry-filtration and heat-treating method. The main structure of MF/MW framework was constituted by lap-jointed mullite fibers. Every single fiber in the framework was densely covered by mullite whiskers which formed through fluorine-catalyzed gas-phase reaction, and the fibers actually served as curved substrates for the mullite whiskers' growth. The lap-jointing points of the fibers were served by movable intersected mullite whiskers. Moreover, the microstructure of the frameworks could be adjusted by tailoring the raw materials mass ratio. The volume densities, the apparent porosities and the thermal conductivities of the MF/MW frameworks in different raw materials mass ratios were 0.459–0.487 g/cm³, 79.7–82.8% and 0.1356–0.1965 W/k m, respectively. The compression–resilience property of the samples was tested under 0.4 MPa at room temperature. The compression ratio and resilience ratio of the MF/MW frameworks in different raw materials mass ratios were 1.63–2.25% and 92.67–98.16%, respectively. The MF/MW frameworks with advanced thermal and mechanical properties were considered to be promising high-temperature heat-insulation material.     

A study of the acidic and catalytic properties of pure and sulfated zirconia–titania and zirconia–silica mixed oxides

Protonated pyridine (PyH⁺) was not found on ZrO₂ (Z) or ZrO₂–TiO₂ (ZT), but was detected on sulfated oxides (ZS, ZTS) by IR spectroscopy. In contrast, ZrO₂–SiO₂ samples containing about 30–80 mol% ZrO₂ showed Brønsted acidity both in nonsulfated (ZS) and sulfated (ZSS) forms. The total acidity was determined by NH₃TPD. Introduction of sulfate ions increased the sitespecific catalytic activity (TOF) in the conversion of cyclopropane or nhexane. The effect of sulfate ions was more significant on samples rich in zirconia. Results suggest that Zr is homogeneously distributed in ZS samples rich in silica. Zirconiabound dimeric sulfate, generating strong acidity, could not be formed in these preparations due to the absence of fairly large ZrO₂ domains.     

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 3.7 equivalent 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 3.7 equivalent wt% to 7.3 equivalent 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.     

Low-temperature sintering and microstructure control of mullite–Mo composites

Dense mullite–Mo (45 vol%) composites with homogeneous microstructure have been obtained by plasma activated sintering of a mixture of Mo and mullite precursors at a relatively low temperature (1350 °C) and a pressure of 30 MPa. The mullite precursor was synthesized by a sol–gel process followed by a heat-treatment at 1000 °C. The influence of different mullite precursors on the densification behavior and the microstructure of mullite–Mo composites has been studied. The precursor powder heat-treated at 1000 °C with only Si–Al spinel but no mullite phase shows an excellent sintering activity. Following the sintering shrinkage curves, a two-stage sintering process is designed to enhance the composite densification for further reducing the sintering temperature. The study reveals that viscous flow sintering of amorphous SiO₂ at low temperatures effectively enhances the densification. Moreover, microstructure of these composites can be controlled by selecting different precursors and sintering temperatures.     

Effective diffusion constant and adsorption constant of synthesized alumina,zirconia, and alumina–zirconia composite material

In this study, Al₂O₃, ZrO₂, and Al₂O₃–ZrO₂ composite materials were prepared with the sol–gel technique. X-ray diffraction analysis, differential scanning calorimetry–thermogravimetry, scanning electron microscopy–energy-dispersive X-ray spectrometry, nitrogen adsorption isotherm measurements, and helium pycnometry were used to characterize the resultant materials. Effective diffusion coefficients of helium and hydrogen and the adsorption equilibrium constant of hydrogen in the resultant materials were determined using single-pellet moment technique. The effective diffusivities of helium and hydrogen in both ZrO₂ and Al₂O₃–ZrO₂ composite pellets were found to be smaller than the value found for Al₂O_3, due to the lower tortuosity factor values of the Al₂O₃ pellet. It was found that hydrogen was weakly adsorbed on all resultant materials.     

Mullite–zirconia–zircon composites: Properties and thermal shock resistance

In the refractory field mullite and zirconia are the basis of materials used in the glass industry or when high chemical stability and corrosion resistance are necessary. In this work various mullite–zirconia/zircon compositions were investigated to improve the thermal shock (TS) resistance of dense composites produced by slip casting and sintering at 1600 °C. Zircon (SiZrO₄) acts as bonding phase and its thermal decomposition adds zirconia and silica to the material. Resultant composites were characterized by density and dilatometric measurements, XRD and SEM techniques. TS behavior was tested by quenching in water with quenching temperature differentials ΔT from 400 to 1200 °C. The degree of damage after the TS was experimentally evaluated through the variation of the elastic modulus E which is measured by the excitation technique. The severity of the TS test and the effect of the number of thermal cycles on E for each ΔT employed were determined.The tested materials retained their original mechanical properties for temperatures below a critical temperature ΔT_c near 600 °C. Materials quenched from ΔT of 1000 °C showed as much as 30% reduction in E indicating the important microstructure damage. The TS resistance improved with increasing zircon addition to 35 wt% in agreement with the behavior predicted from R parameter for crack initiation.     

Ceria–zirconia mixed oxides: Synthetic methods and applications

The primary objective of this review was to illustrate the significance of ceria–zirconia (CZ) mixed oxides as catalysts and catalyst supports as employed for a wide variety of catalytic applications both in the liquid and gaseous phases. In particular, we were interested in bringing together the recent literature pertaining to these mixed oxides with catalysis perspective. The most prominent application of CZ mixed oxides is in three-way catalysis (TWC) as oxygen storage and release material for several years by replacing cerium dioxide as it shows better efficiency and a high thermal stability. Doping with zirconium oxide, as it is alone a non-reducible oxide, makes the CZ mixed oxide a highly reactive, thermally stable, and more reducible with elevated oxygen storage capacity (OSC) that are important for TWC applications. Apart from the TWC use, the CZ mixed oxides have a huge number of applications, as a direct component or a support, ranging from water–gas shift reaction, reforming of hydrocarbons, dehydration of alcohols, CO₂ utilization, catalytic combustion of pollutants, fine chemicals production, photocatalysis, and so on. All these applications are mainly dependent on three parameters of the mixed oxides, namely, OSC or redox nature, acid–base properties, and crystalline phases. Besides, most of the applications are influenced by the physical properties such as specific surface area, pore volume, pore diameter, crystallite size, and so on. In this review, many details pertaining to the synthesis of these mixed oxides by various conventional and non-conventional methods, their characterization by several techniques, and their application for various reactions of energy and environmental significance, as reported in the literature, are assessed.     

Fabrication and characterization of anorthite–mullite–corundum porous ceramics from construction waste

In this work, anorthite–mullite–corundum porous ceramics were prepared from construction waste and Al₂O₃ powders by adding AlF₃ and MoO₃ as mineralizer and crystallization catalyst, respectively. The effects of the sintering temperature and time on open porosity, mechanical properties, pore size distribution, microstructure, and phase composition were characterized in detail. The results showed that the formation of the mullite whiskers and the properties of the anorthite–mullite–corundum porous ceramics depended more on the sintering temperature than the holding time. By co-adding 12 wt% AlF₃ and 4 wt% MoO₃, mullite whiskers were successfully obtained at sintering temperatures upon 1350 °C for 1 h. Furthermore, the resultant specimens exhibited excellent properties, including open porosity of 66.1±0.7%, biaxial flexural strength of 23.8±0.9 MPa, and average pore size of 1.32 µm (the corresponding cumulative volume percent was 37.29%).     

The influence of Ti addition on fracture toughness and failure of directionally solidified LaB6–ZrB2 eutectic composite with monocrystalline matrix

Fracture toughness and failure mechanism of directionally solidified eutectic composites LaB₆–ZrB₂ and LaB₆–(Zr_0,9Ti_0,1)B₂ were investigated. The addition of Ti increases the cohesion strength of the matrix–fiber interface, causes redistribution of stresses in the whole composite, and thus influences the mechanism of crack propagation.Fracture toughness of composites was determined by Brazilian test. The central crack was introduced in the plane, parallel to the axis of the eutectic rod, in direction, perpendicular to the axis. Such experimental setup enables to investigate the interaction of crack with fiber–matrix interface and eliminates the effect of other factors on K_1C.The present investigation shows that Ti additions to LaB₆–ZrB₂ result in up to 25% increase of fracture toughness of the composites.