Elastic properties and damping behavior of alumina–zirconia composites at room temperature

Alumina–zirconia composite ceramics (AZ composites) have been prepared in the whole range of compositions from pure alumina to zirconia (in steps of 10 vol.%) by slip casting, followed by sintering at 1350 °C and microstructural characterization via the Archimedes method (relative densities 0.93–0.99). Young's modulus has been measured at room temperature via the impulse excitation technique (IET) and, after appropriate porosity correction (linear, power-law, exponential), found to be in good agreement with the Hashin–Shtrikman bounds. The damping factor (internal friction), which has been measured for dense AZ composites (also via IET at room temperature), is found to increase with increasing zirconia content. Damping factors measured for porous AZ composites with porosities 25–71%, prepared with corn starch as a pore former, have been found to depend only slightly on porosity, unless the porosities are extremely high (>70%). At these porosities, however, where the Young's moduli approach zero, the damping factors exhibit a steep increase.     

Detection of tetragonal zirconia in alumina–zirconia powders by thermoluminescence

The thermoluminescence (TL) after excitation by UV or X-rays radiation of alumina-zirconia powders is investigated. The composites present five of the characteristic peaks of zirconia at −170, −145, −90, 0 and 95 °C. After a thermal treatment of mixed oxides, a new peak is observed at −35 °C in TL. This peak reveals the presence of stabilized tetragonal zirconia in the material. Moreover by comparing this analysis with those realised by X-ray diffraction (XRD), it can by shown that the TL has one better limit of detection than the XRD.     

Mechanochemical synthesis of alumina–zirconia nanocomposite powder

Abstract

The alumina–zirconia nanocomposite powder has been synthesised by mechanical activation of a dry powder mixture of AlCl₃, ZrCl₄ and CaO. Mechanical milling of the above raw materials with the conditions adopted in this study resulted in the formation of a mixture consisting of crystalline CaO and amorphous aluminium and zirconium chlorides phases. There was no sign of chemical reaction occurring during milling stage as evidenced by X-ray diffraction (XRD) studies. Subsequent heat treatment of the milled powder at 350°C resulted in the occurrence of displacement reaction and the formation of ZrO₂ and Al₂O₃ particles within a water soluble CaCl₂ matrix. The effect of higher temperature calcination on the phase development in this powder mixture was followed by XRD analysis. Scanning electron microscopy and differential thermal analysis were also used in the characterisation of the powders. Perhaps the most important observation in this study was the formation of α-Al₂O₃ phase at a very low temperature of 400°C.     

Two-step sintering of fine alumina–zirconia ceramics

This work investigates the feasibility to the fabrication of high density of fine alumina–5 wt.% zirconia ceramics by two-step sintering process. First step is carried out by constant-heating-rate (CHR) sintering in order to obtain an initial high density and a second step is held at a lower temperature by isothermal sintering aiming to increase the density without obvious grain growth. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range between 1400 and 1450 °C is effective for the first step sintering (T₁) due to its highest densification rate. The isothermal sintering is then carried out at 1350–1400 °C (T₂) for various hours in order to avoid the surface diffusion and improve the density at the same time. The content of zirconia provides a pinning effect to the grain growth of alumina. A high ceramic density over 99% with small alumina size controlled in submicron level (0.62–0.88 μm) is achieved.     

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.     

Fractal analysis of cracks in alumina–zirconia composites

The crack paths, induced by Vickers indentation in alumina–zirconia composites, were analyzed using fractal geometry. The fractal dimension n_S was calculated for each crack. This parameter refers to a corresponding three-dimensional fracture surface and indicates how its geometry varies by changing the magnification. An interesting correlation between K_IC and n_S was found: it suggests that the samples with high percentages of alumina and also the pure zirconia are characterized by an intergranular fracture mode, while the composites with high zirconia content present a transgranular fracture mode. This result is confirmed by analyzing the energies of fracture calculated using both the classical and fractal approaches. The results obtained in this research not only made it possible to understand the fracture behavior of the analyzed composites, but also confirmed the good potential of fractal analysis to explain complex mechanisms such as those involved in the fracture of brittle materials.     

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.     

Processing of different alumina–zirconia composites by slip casting

Two Al₂O₃–ZrO₂ mixture preparation routes: classical powder mixing and addition of a Zr (IV) precursor solution to a well dispersed Al₂O₃ suspension, were used to produce alumina (Al₂O₃)–zirconia (ZrO₂) slip cast composites. For the conventional powder mixing route, two commercial 3 mol% yttria-partially stabilized zirconia powders, 0.3 wt% Al₂O₃-doped (Al-doped Y-PSZ) and without Al₂O₃ (Y-PSZ), were employed. The influence of the zirconia content and the solid loading on the rheological properties of concentrated aqueous Al₂O₃–ZrO₂ slips were investigated. The density of green samples was studied and related to the degree of slip dispersion. In addition, the influence of the processing conditions on the density and microstructure development of sintered samples were investigated. By using the Zr (IV) precursor route, nano-sized ZrO₂ (ZN) particles homogeneously distributed on the Al₂O₃ particle surfaces were obtained; however, it let to aggregates of some Al₂O₃ particles with very fine ZrO₂ uniformly distributed. The viscosity and yield stress values of Al₂O₃–ZN suspensions were markedly higher than those of Al₂O₃–Al-doped Y-PSZ and Al₂O₃–Y-PSZ ones, for all the compositions and solid loading studied and resulted in a less dense packing of cast samples. However, for the composite with 10.5 vol% ZN a high sintered density and a smaller ZrO₂ grain size distribution compared with the conventional powder mixing route could be obtained.     

Residual stresses in alumina–zirconia laminates

Significant residual stresses can arise in hybrid ceramic laminates during the densification and cooling processing cycles. The densification stresses in alumina–zirconia laminates were calculated assuming the layers to be linear viscous with data obtained by cyclic loading dilatometry. These stresses placed the zirconia layers in biaxial tension and even at 1 MPa or less, they were sufficient to cause a type of linear cavitation damage. The methodology was also applied to asymmetric laminates, successfully predicting their observed curling behaviour. Thermal expansion mismatch stresses arise during cooling, again placing the zirconia layers in residual biaxial tension and leading to the formation of transverse (channelling) cracks. The stresses were calculated using both elastic and viscoelastic formulations and were confirmed with indentation measurements. Additions of alumina to the zirconia layers were effective in reducing both sources of residual stress and allowed crack formation during processing to be avoided. Residual stresses were also shown to improve mechanical performance.     

Use of a design-of-experiments approach for preparing ceria–zirconia–alumina samples by sol–gel process

In this work we successfully obtained ceria–zirconia–alumina samples by the sol–gel technique. These materials were prepared under acidic or basic conditions, using either nitric acid or ammonium hydroxide as the catalyst. A design-of-experiments approach was used in order to optimize the specific surface area, pore structure, and thermal stability of the prepared samples. It was observed that the addition of ceria and zirconia did not affect the formation of γ-Al₂O₃. The highest surface areas and smallest pore sizes were observed for specimens obtained under acidic conditions and with low to intermediate concentrations of cerium and water. The increase of the heat treatment temperature from 600 °C to 1000 °C led to both a decrease of the surface area and an increase of the mean pore size. This behavior is due to the coalescence of pores upon calcination. Samples with a high concentration of ceria showed an expressive thermal instability at high temperatures. On the other hand, the addition of zirconia increased the thermal stability of these materials. In general, samples with improved thermal stabilities were obtained under basic conditions.     

Sintering,microstrusture and hardness of different alumina–zirconia composites

Two commercial 3 mol% yttria-partially stabilized zirconia powders, 0.3 wt% Al₂O₃-doped (Al-doped Y-PSZ) and without Al₂O₃ (Y-PSZ), were used to produce alumina (Al₂O₃)-zirconia (ZrO₂) slip cast composites. The influence of the substitution of Al₂O₃ either by different Al-doped Y-PSZ contents or 50 vol% Y-PSZ on the sintering kinetic at the intermediate stage was investigated. In addition, the microstructure of Al₂O₃ and the different composites at temperatures in the range of 1100–1600 °C was studied and related to the sample hardness. An increase in the sintering rate was observed when Al-doped Y-PSZ increased from 22 to 50 vol% or when 50 vol% Y-PSZ was substituted by 50 vol% Al-doped Y-PSZ. 50 vol% ZrO₂ was the most effective concentration to reduce the rate of Al₂O₃ grain growth in the final sintering stage; the Al₂O₃ grain growth began at lower temperatures and became greater with decreasing the Al-doped Y-PSZ content. On the contrary, the ZrO₂ grain growth slightly increased with increasing the Al-doped Y-PSZ concentration. However, for 50 vol% Al-doped Y-PSZ a smaller ZrO₂ grain size distribution compared with 50 vol% Y-PSZ could be achieved. As the average Al₂O₃ grain size of the sintered samples became greater than about 1 µm a markedly decrease in the hardness was found; this occurred at temperatures higher than 1400 °C and 1500 °C for Al₂O₃ and the composite with 10.5 vol% Al-doped Y-PSZ, respectively.     

Influence of dispersant,storage time and temperature on the rheological properties of zirconia–paraffin feedstocks for LPIM

The dependence of the rheological properties of zirconia–paraffin feedstocks for low-pressure injection moulding (LPIM) on binder composition, storage time and temperature was investigated. Feedstocks with varying amounts of dispersant were fabricated to work out the required quantity for a monolayer of dispersant molecules on zirconia particles. Experimental results revealed that a shear rate dependent characteristic of the viscosity against the amount of dispersant exists. The observations were compared with calculated values according to an adsorption model, which overestimated the required quantity of dispersant to form a monolayer. Feedstocks stored at elevated temperature for several days exhibited a time-dependent decrease of the yield stress and viscosity, which is supposed to be caused by physical or chemical interactions among zirconia and the dispersant. Increasing working temperature and decreasing solids loading were found to significantly decrease the yield stress as well as the viscosity. Flow activation energies and flow indices were calculated and compared with literature. This study shows that the dispersant used in this investigation has a remarkable influence especially on the time-dependent flow behaviour of zirconia–paraffin feedstocks that affects further processing and reproducibility.     

Preparation of a mesoporous ceria–zirconia supported Ni–Fe catalyst for the high temperature water–gas shift reaction

A Ni–Fe/ceria–zirconia catalyst with ordered mesostructure was prepared by the hard-template method employing mesoporous silica (KIT-6) as a template to impart its highly ordered structure to the ceria–zirconia mixed oxide support. Catalytic activities of the Ni–Fe/CeO₂–ZrO₂ catalyst for the water–gas shift reaction were superior to those of a commercial Fe–Cr-based catalyst. The ordered structure of Ni–Fe/CeO₂–ZrO₂ catalyst became more stable compared to one prepared without zirconia due to structural stabilization of the mixed oxide by added zirconia in the framework. Alloying of Ni and Fe and enhanced mobility of lattice oxygen in the oxide support may promote its catalytic activity and selectivity for the water–gas shift reaction.     

Dispersion,rheology and aqueous tape casting of alumina–zirconia composites

Abstract

Alumina and zirconia were dispersed individually in aqueous media using Darvan C as the dispersant and at optimised pH condition. Based on sedimentation, rheology, yield stress, electrodeposition and zeta potential measurements, 2 wt-% of the dispersant and a pH of 10·5 were found to be the optimum condition for the codispersion of alumina and zirconia. Aqueous tape casting slurries with a solid loading of 32 wt-% were prepared under the optimised conditions of dispersion. Alumina–zirconia (50 : 50) composite tapes of 40 μm thickness and 56% green density were obtained.     

Mesoporous ceria–zirconia–alumina nanocomposite-supported copper as a superior catalyst for simultaneous catalytic elimination of NO–CO

A series of transition metal (Mn, Fe, Co, Ni, Cu and Ag) oxides supported on ceria–zirconia–alumina nanocomposite catalysts were prepared through wetness impregnation method. The catalytic performance of these catalysts were evaluated in the catalytic elimination of NO–CO. Activity results revealed supported copper catalyst gave the optimal catalytic activity, which was related to high dispersion of copper species (XRD and Raman), low-temperature reducibility (TPR), and more oxygen vacancies (DRS).     

Properties of alumina 10 vol% zirconia composites—The role of zirconia starting powders

Zirconia toughened alumina (ZTA) materials are applied for cutting tools, wear parts and in biomedical applications. Due to the constraint of the rigid alumina matrix, ZTA materials with up to 10 vol% zirconia addition (AZ10) do not require addition of stabilizer oxides. AZ10 materials based on submicron sized alumina and four different submicron to nanoscale zirconia powders were manufactured by hot pressing at temperatures between 1475?1600 °C. Results show that the powder choice has a strong influence on mechanical properties, evolution of microstructure and phase composition. Best results with strength up to 850 MPa, fracture toughness values of 8.5 MPa√m and invulnerability to overfiring were obtained with zirconia powders showing the coarsest yet most homogeneous primary particle size and a low degree of agglomeration. Ultrafine but hard agglomerated powders lead to materials with extremely inhomogeneous microstructure and inferior properties.     

Experimental determination of the Hamaker constants for solid–water–oil systems

The van der Waals interaction (vdW) is a fundamental interaction in colloid and interface science. Regardless of the methods used in deriving the vdW interaction between two bodies as a function of their separation distance, the Hamaker constant is always an essential parameter involved. In this paper, a simple experimental method is presented to determine the Hamaker constant. As an example, the Hamaker constant of a solid-water-oil system is related to its surface and interfacial energies, which can be measured accurately. Based on the proposed method, the effects of two typical solid surfaces and three kinds of aqueous solutions on the Hamaker constant and wettability of the solid-water-oil system are studied. It is found that hydrophilic and hydrophobic solid surfaces will lead to rather different Hamaker constants and wettability behaviour. The detailed experimental results also show that the ionic surfactant solutions have a strong influence, whereas the pH value of the aqueous phase has a limited effect on the Hamaker constant. In addition, the electrolyte solutions do not strongly affect the Hamaker constant for the oil phase interacting with the solid surface across an electrolyte solution. Such determined Hamaker constants are in reasonable agreement with the reported Hamaker constants for oils (dodecane and hexadecane), mica, and metals (Ag, Au, and Cu) interacting across a pure water phase.     

Chemical vapor deposition of ordered structures in YAG–alumina eutectic system

The eutectic structure of metals and ceramics is the result of a self-organization phenomenon in which multiple solid phases solidify with an ordered structure from a liquid phase. Hence, a melt-solidification process was the only way to generate ordered structures in a ceramic eutectic system. Here, we prepared ordered structures of Y₃Al₅O₁₂ (YAG)–Al₂O₃ composites via chemical vapor deposition in the YAG–Al₂O₃ eutectic system. Spatially periodic YAG rod and lamellar structures were generated in an alumina matrix homoepitaxially grown on the sapphire substrates on the Al₂O₃-rich side of the eutectic composition, whereas α-Al₂O₃ rod and lamellar structures were generated in a YAG matrix on the Y₂O₃-rich side. The results reveal that the pattern formation of ordered structures in a ceramic eutectic system can occur not only during in the melt-solidification process but also during the vapor deposition process. The Ce³⁺-doped YAG–Al₂O₃ composite film converts some of the blue light into yellow light, allowing some of it to pass through, and emitting white light. The Eu³⁺-doped YAG–Al₂O₃ composite film can be utilized as a scintillation screen for a high-resolution X-ray imaging test to see though a semiconductor storage device.     

Filamentous alumina–chitosan porous structures produced by gelcasting

Porous filamentous macroelements with tunable properties were developed using alumina–chitosan fibers produced by gelcasting extrusion. The initial suspension was prepared as a dilute aqueous acetic acid solution containing 13 vol% of alumina particles and 1.3 vol% of dissolved chitosan. After extrusion, coagulation in an NaOH bath and drying, 300 μm diameter continuous fibers (9 vol% of chitosan) were compacted and sintered at different temperatures (1100–1500 °C) to produce 40×40 mm² cylindrical macroelements. The effects of the thermal treatment temperature on the porosity, specific surface area, mechanical strength and microstructure of the macroelements were evaluated. It was verified that these properties can be controllably modified in a wide range, depending on the sintering conditions.     

Ytterbia–neodymia–costabilized TZP—Breaking the limits of strength–toughness correlations for zirconia?

TZP ceramics were manufactured by hot pressing of pyrogenic zirconia nanopowder which was costabilized by 1 mol% ytterbia and 2 mol% neodymia (1Yb–2Nd–TZP) via the nitrate route. The evolution of microstructure, phase composition and mechanical properties with variation of sintering temperature from 1200 °C to 1400 °C was investigated. 1Yb–2Nd–TZP consists of a bimodal microstructure of small very transformable tetragonal grains and large cubic grains. At intermediate sintering temperature the materials combine a bending strength of 1250 MPa with a fracture resistance >13 MPa √m. The high threshold stress intensity of 7 MPa √m indicates high resistance to subcritical crack growth. An increase in fracture resistance before the crack tip induced by compressive residual stress shifts the strength–toughness correlations to higher values than previously considered possible.