Advanced ceramics based on aluminum,silicon, chromium,and lanthanum oxides: Preparation and applications (an overview)

Overviewed is the state-of-art in the synthesis and applications of advanced Al₂O₃-Cr₂O₃, SiO₂-Cr₂O₃, and Cr₂O₃-La₂O₃ composite ceramics.     

Effect of particle size distribution of raw powders on pore size distribution and bending strength of Al2O3 microfiltration membrane supports

To lower the sintering temperature of Al₂O₃ microfiltration membrane support, Al₂O₃ powders with particle size distribution of tri-modal are chosen. The results show that the function of fine Al₂O₃ grains depends on their agglomeration state: if fine Al₂O₃ grains distribute discretely, the bending strength of the support increases along with a slight increase in porosity; however, the aggregated fine grains are harmful to both bending strength and pore size distribution of the support. The bridging of medium Al₂O₃ grains between coarse grains contributes to increase the bending strength, but has less effect on porosity. The addition of medium (and/or fine) Al₂O₃ powder has less effect on the pore size distribution of the support if only coarse Al₂O₃ grain forms the support's framework, which suggests a new way to prepare the support with both high bending strength and high porosity at low temperature.     

Hydrogen production for fuel cell by oxidative reforming of diesel surrogate: Influence of ceria and/or lanthana over the activity of Pt/Al2O3 catalysts

A series of Pt catalysts supported on Al₂O₃ (Pt/A), Al₂O₃-CeO₂ (Pt/A-C), Al₂O₃-La₂O₃ (Pt/A-L) and Al₂O₃-La₂O₃-CeO₂ (Pt/A-L-C) have been prepared and tested in the oxidative reforming of diesel surrogate with the aim of studying the influence of ceria and lanthana additives over the activity and stability toward hydrogen production for fuel cell application. Several characterization techniques, such as adsorption-desorption of N₂, X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduction, H₂ chemisorption, and thermogravimetric analysis, have been used to define textural, structural, and surface properties of catalysts and to establish relationships with their behaviour in reaction. This physicochemical characterization has shown that lanthana inhibits the formation of α phase in alumina support and decreases ceria dispersion. Activity results show a better performance of ceria-loaded catalysts, being the Pt/A-C sample the system that offers higher H₂ yields after 8 h of reaction. The greater H₂ production for ceria-loaded catalysts, particularly in the case of the system Pt/A-C, is attributed to the Pt-Ce interaction that may change the electronic properties and/or the dispersion of active metal phase. Also, the Ce^III form of Ce^IV/Ce^III redox pair enhances the adsorption of oxygen and water molecules, thus increasing the catalytic activity and also decreasing coke deposition over surface active Pt phases. Stability tests showed that catalysts in which Pt crystallites are deposited on the alumina substrate covered by a lanthana monolayer, give rise to an increase in stability toward H₂ production.     

Effect of washcoat modification with metal oxides on the activity of a monolithic Pd-based catalyst for methane combustion

The deposition of Ni, Co, Ce or Fe oxides onto the washcoat surface in the 0.5%Pd/Al₂O₃ catalyst enhances conversion of CH₄. Catalytic activity of the Pd-catalysts containing cobalt oxide depends on the incorporated amount of cobalt oxide and the method of incorporation. The highest activities were those of the 0.5%Pd/0.3%Co/Al₂O₃ and 1%Pd/0.3%Co/Al₂O₃ catalysts (cobalt oxide deposited onto the surface of Al₂O₃) and the 0.5%Pd/5%Co₃O₄–Al₂O₃ catalyst (mixed washcoat). Total SSA, Pd dispersion and Pd crystallite size in the x%Pd/y%Co/Al₂O₃ catalysts depend on the incorporated amount of PdO and cobalt oxide. Pd dispersion in the 1%Pd/Al₂O₃ catalyst increases from 4% to 20% upon deposition of 14 wt.% Co₃O₄ (by mass Al₂O₃) onto the Al₂O₃ surface (1%Pd/0.3%Co/Al₂O₃). This increase in Pd dispersion influence the increase in the activity of the 1%Pd/Al₂O₃ catalyst. On the surface of the 0.5%Pd/5%Co₃O₄–Al₂O₃ catalyst Pd occurs mainly in the form of PdO and displays considerable mobility under conditions of temperature variations—cyclically undergoing reduction and oxidation. At 500 °C, in vacuo, the reduction was irreversible and parallelled by the agglomeration of metallic Pd crystallites. At room temperature, cobalt occurred on the catalyst surface in the form of Co⁺² ions (CoAl₂O₄) and was reduced to Co⁰ at 500 °C (in vacuo). Up to 500 °C, the reduction of Co was reversible.     

Microstructure and mechanical characterization of Cu–Ni/Al2O3 nanocomposites fabricated using a novel in situ reactive synthesis

Chemical-based synthesis of co-formed oxide (CuO–NiO–Al₂O₃) nanoparticles, followed by selective hydrogen reduction of the Cu and Ni oxides and ultimately consolidation into pellets, produced various compositions of Cu–Ni/Al₂O₃ nanocomposites. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy analyses were used to characterize the powders. The generated powders ranged in size from 20 to 70 nm, with a considerable presence of agglomerates. According to SEM examination, the powders were homogeneous in shape and particle size. Cold pressed nanocomposite powders were sintered for 2 h at 950 °C. SEM with energy dispersive spectroscopy (EDS) was also used to study the microstructure of the sintered specimens. In addition, the physical and mechanical properties of sintered specimens were studied. When the Al₂O₃ content increased, a more uniform distribution of nanosized Al₂O₃ particles in the Cu–Ni matrix was attained, resulting in a reduction in particle size. The results also demonstrated that as the Al₂O₃ concentration was raised, the microhardness and compressive strength of the nanocomposites rose by 74% and 67% compared to pure alloy, with the exception of fracture strain, which decreased dramatically.     

Selective Corrosion Preparation and Sintering of Disperse α‐Al2O3 Nanoparticles

Disperse fine equiaxed α‐Al₂O₃ nanoparticles with a mean particle size of 9 nm and a narrow size distribution of 2–27 nm were synthesized using α‐Fe₂O₃ as seeds and isolation via homogeneous precipitation‐calcination‐selective corrosion processing. The presence of α‐Fe₂O₃ acting as seeds and isolation phase can reduce the formation temperature to 700°C and prevent agglomeration and growth of α‐Al₂O₃ nanoparticles, resulting in disperse fine equiaxed α‐Al₂O₃ nanoparticles. These α‐Al₂O₃ nanoparticles were pressed into green compacts at 500 MPa and sintered first by normal sintering to study their sintering behavior and finally by two‐step sintering (heated to 1175°C without hold and decreased to 1025°C with a 20 h hold in air) to obtain nanocrystalline α‐Al₂O₃ ceramics. The two‐step sintered bodies are nanocrystalline α‐Al₂O₃ with an average grain size of 55 nm and a relative density of 99.6%. The almost fully dense nanocrystalline α‐Al₂O₃ ceramic with finest grains achieved so far by pressureless sintering reveals that these α‐Al₂O₃ nanoparticles have an excellent sintering activity.     

Synthesis of cubic aluminum nitride by carbothermal nitridation reaction

Cubic aluminum nitride (AlN) was synthesized by the carbothermal nitridation reaction of aluminum oxide (Al₂O₃). The effects of Al₂O₃ particle size, reaction temperature and reaction time on the synthesis of cubic AlN were investigated, and the reaction mechanism was also analyzed. The results showed that cubic AlN could be formed at a lower temperature with fine Al₂O₃ powder than with coarse Al₂O₃ powder. The cubic AlN may be the product of Al₂₃O₂₇N₅ synthesized from Al₂O₃ and hexagonal AlN, and transforms into hexagonal AlN at temperatures above 1800°C.     

Preparation and properties of porous Al5BO9 for high-temperature wave-transparent and thermal insulating applications

Porous Al₅BO₉ is a promising high-temperature wave-transparent material. However, method for the preparation of this material is not readily available. Herein, porous Al₅BO₉ ceramics with controlled porosity and small volume shrinkage are successfully prepared by using Al₂O₃ and B₂O₃ as starting materials without pore formers. The SEM and pore size distribution studies show that the as-prepared porous Al₅BO₉ ceramics exhibit a uniform pore structure and a narrow pore size distribution. Intriguingly, simply adjusting the densities of the green bodies, the density and porosity of the porous Al₅BO₉ ceramics can be controlled. The pore-forming mechanism is presumed to be a combination of boron oxide volatilization during the high-temperature synthesis and lap of elongated grains. Porous Al₅BO₉ ceramics have good high-temperature stability, which can maintain dimensional and composition stability up to 1673 K. The compressive strength can reach 211 MPa at 32.4% porosity and the dielectric constant can be as low as 3.02 at 43.2% porosity. In addition, the dielectric constant and loss tangent keep almost unchanged with temperature.     

Pore growth during the initial stages of sintering ceramics

Mercury porosimetry was used to measure changes in pore size distribution during initial stage sintering of compacts of submicron size particles of several oxides. Pore growth was observed in MgO and Fe₂O₃, and in Al₂O₃ under certain conditions. Pores can grow by these mechanisms: surface diffusion, particle size distribution effects, particle coalescence, phase transformation, and evaporation/condensation. Surface diffusion may be the mechanism in the case of an alpha alumina. Phase transformation was shown to be the cause when sintering gamma alumina. In the case of magnesia and ferric oxide, particle coalescence appears to be operating. Since pore growth competes with densification for the use of surface energy, it is an important sintering process.     

Preparation of high-porosity Al2O3 ceramic foams via selective laser sintering of Al2O3 poly-hollow microspheres

In this paper, high-porosity Al₂O₃ ceramic foams called Al₂O₃ PHM ceramics were fabricated through selective laser sintering (SLS) via Al₂O₃ poly-hollow microspheres (Al₂O₃ PHMs). SLS parameters were optimized by an orthogonal experiment as to be laser power = 6 W, scanning speed = 1800 mm/s, and scanning space = 0.15 mm. The effect of sintering temperature on microstructure, shrinkage, porosity, phase composition, mechanical properties and pore size distribution of Al₂O₃ PHM ceramics were investigated. When sintering temperature increased, Al₂O₃ PHM ceramics contained only Al₂O₃ phase and were gradually densified. With the raise of sintering temperature, the porosity of Al₂O₃ PHM ceramics decreased gradually from 77.09% to 72.41%, but shrinkage in H direction and compressive strength of Al₂O₃ PHM ceramics increased from 6.63% and 0.18 MPa to 13.10% and 0.72 MPa, respectively. Sintering temperature had little effect on pore size distribution of Al₂O₃ PHM ceramics, which only declined from 24.2 to 21.4 μm with the increase of sintering temperature from 1600 to 1650 °C. This method can not only directly prepare ceramic foams with complex shapes, but also control properties of ceramic foams. It provides a simple preparation method for many kinds of ceramic foams with complex structure and high porosity by using PHMs with different composition.     

Effects of LaB6 on the microstructures and ablation properties of 3D C/C-SiC-ZrB2-LaB6 composites

C_f/LaB₆ preform was used to prepare LaB₆ doping C/C-ZrC-SiC composites by precursor infiltration and pyrolysis method, and effects of LaB₆ on the microstructure and ablation resistance of C/C-SiC-ZrB₂-LaB₆ composites were investigated. Results show that LaB₆ was reacted with Zr-precursor or its products to generate ZrB₂ and other compounds. Thanks to the reactive sintering effect of LaB₆, a compact ceramic skeleton was established in the substrate, which played a vital role in densifying and resisting ablation. The oxidation of the evenly-distributed La- and Zr-compounds produced the homogeneous oxide scale. The ternary phases of La₂Si₂O₇, La_0.71Zr_0.29O_1.65 and La₂Zr₂O₇ were participated in the formation of the oxide scale, evolving the layer from SiO₂ to SiO₂-La₂Si₂O₇ and then to SiO₂-La₂Si₂O₇-La_0.71Zr_0.29O_1.65-La₂Zr₂O₇-ZrO₂ layer. After plasma ablation for 360 s, the mass and linear ablation rates were 0.3848 mg/s and 0.3694 μm/s, respectively. The seamless multi-phase layer can effectively prevent the composites from long-time ablation.     

Electrical and thermal studies of the distribution of carbon black in a polyester matrix in the presence of aluminum oxide

The distribution of a filler in a polymeric matrix is one of the most important factors affecting the physical properties of the final product. For this reason, the main objective of this study was to introduce aluminum oxide (Al₂O₃), acting as a dispersing agent, to reduce the filler–filler interaction and enhance the filler–polymer interaction. To achieve this aim, the electrical behavior of a styrenated polyester resin filled with different amounts of high‐abrasion furnace black in the presence of 5% Al₂O₃ was studied in the vicinity of the percolation threshold to evaluate the effect of the addition of Al₂O₃ in an attempt to reduce the filler–filler interaction through the polyester matrix. At a certain concentration of carbon black, an abrupt increase was noticed through electrical conductivity, permittivity, and dielectric loss investigations. With this increase, the tendency of conductive chain formation increased through the aggregation of a carbon black particle network. The addition of 5% Al₂O₃ improved the filler distribution by lowering the aggregate size and consequently enhanced the formation of the network. From the Arrhenius temperature dependence of the electrical conductivity, the activation energy and pre‐exponential factor were obtained, and they confirmed the validity of the compensation law for the semiconducting composite systems. The composites were also analyzed by thermogravimetric analysis. Al₂O₃ improved the thermal stability of the composites in comparison with that of a sample free of Al₂O₃. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Stabilizing effect of the carbon shell on phase transformation of the nanocrystalline alumina particles

Intensive phase transformations of alumina are known to occur at temperatures above 1000 °C. In the present work, high temperature behaviour of pure Al₂O₃ and the carbon coated Al₂O₃@C sample with core-shell structure was comparatively studied using low-temperature nitrogen adsorption, transmission electron microscopy, powder X-ray diffraction (XRD) analysis and solid-state nuclear magnetic resonance (NMR). The solid-state NMR ²⁷Al method has allowed us to identify and estimate the concentration of all phases appeared during the transformation of pseudoboehmite γ-Al₂O₃ into corundum α-Al₂O₃. The data obtained correlate well with the results of XRD analysis and low-temperature nitrogen adsorption. It is shown that the deposition of carbon coating with formation of core-shell Al₂O₃@C system stabilizes the size of oxide core and prevents the formation of corundum phase until the temperatures of 1350–1400 °C, which are close to the temperature of carbothermal reduction of alumina. The stabilization of the size of the oxide core nanoparticles was considered as a main factor preventing the formation of corundum phase at high temperatures. At the same time, the carbon coating does not affect the phase transformation of γ-Al₂O₃ to δ-Al₂O₃.     

Emulsion–ion extraction technique for synthesis of hollow bicomponent oxide microspheres

Abstract

H ollow ox ide microspheres in the bicomponent systems Al₂O₃-28 wt-%SiO₂ (AS) ( mullite) , Al₂O₃- 13 wt-%T iO₂ (AT) , and Z rO₂-10 wt-%Y₂O₃ ( Z Y) were prepared by the emulsion-ion extraction technique. Monodisperse microsphere formation was found to depend on the experimental parameters adopted during ion ex traction and the surfactant concentration present in the emulsion system. Powder characteristics were investigated using X-ray diffraction, optical and scanning electron microscopy, and particle size analysis. The gel microspheres in the AS, AT , and ZY systems started crystallising at about 900, 800, and 400 °C respectively. The oxide microspheres were mostly spherical in morphology and the sphericity was retained even after calcination at 1300 °C for 1 h. Formation of hollow microspheres with a single spherical cavity was identified by SEM . All oxide microspheres calcined at 1300 °C for 1 h ex hibited a particle size distribution within the range 5-60 μm, the average size ( d₅₀) varying from 19 to 22 μm. BCT / 537     

Solid-state sintering of core-shell ceramic powders fabricated by particle atomic layer deposition

The properties of technical ceramics are highly dependent on their microstructure, which evolves during sintering. Sintering is the process by which ceramic parts are subjected to high temperatures to activate chemical diffusion and the consumption of porosity. During the initial stage of sintering, interparticle necks between neighboring particles form and subsequently increase in size, consuming porosity as the particle centers move closer together. To experimentally determine how this process depends on particle surface composition, particle atomic layer deposition (ALD) was used to deposit a thin film of amorphous aluminum oxide (Al₂O₃) onto yttria-stabilized tetragonal zirconia (3YSZ) particles, producing core-shell structured powders. The uniformity of the Al₂O₃ film was confirmed with transmission electron microscopy and energy dispersive spectroscopy. Scanning electron microscopy was used to observe microstructural evolution during sintering, and the dihedral angles of Al₂O₃ and 3YSZ grains were measured to determine the ratio of interfacial energies between the 3YSZ|3YSZ, 3YSZ|Al₂O₃, and Al₂O₃|Al₂O₃ interfaces. Analysis of the densification kinetics revealed that the initial stage of densification is dependent on the material at the surface of the particles (ie, the Al₂O₃ film) and is controlled by the diffusion of Al³⁺ cations through Al₂O₃. Once the Al₂O₃ film has coalesced, the sintering behavior is controlled by the densification of the core material (3YSZ). Thus, core-shell powders fabricated by particle ALD sinter by a two-step process where the kinetics are dependent on the material present at interparticle contacts.     

Memory characteristics of multi-stacked thin films using La2O3 and LaAlO3 as charge trap layer

Al₂O₃/La₂O₃/Al₂O₃ (ALA) and Al₂O₃/LaAlO₃/Al₂O₃ (A/LAO/A) multi-stacked films were deposited on Si substrates by MOCVD. No interfacial layers (Al_xSi_yO_z) were observed in TEM images, and the thickness ratio of the tunnel oxide (bottom oxide), trap layer (middle oxide), and blocking oxide (top oxide) was about (1:1.3:3) in both films. Memory windows of the (ALA) and (A/LAO/A) films were 1.31 V and 3.13 V, respectively. Each value in the program/erase cycle test was maintained for up to 10⁴ cycles.     

Flash sintering of yttria-stabilized zirconia powders coated with nanoscale films of alumina by atomic layer deposition

The addition of small quantities of aluminum oxide (Al₂O₃) to 8 mol% yttria-stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al₂O₃ are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD-coated powders were flash sintered using voltage-to-current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al₂O₃ films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al₂O₃ can enhance grain growth and sintering of 8YSZ. This suggests that Al₂O₃ dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.     

Grain Growth Behavior of Aluminum Oxide Reinforced with Carbon Nanotube During Plasma Spraying and PostSpray Consolidation

Grain-boundary mobility of the plasma sprayed aluminum oxide (Al₂O₃)–carbon nanotube (CNT) composites is evaluated in the current work. Grain mobility is evaluated from the grain growth within the spray-dried particles and thermal history experienced during high-temperature plasma processing. CNTs form an interfacial grain boundary layer during thermal exposure, limiting the grain growth of plasma-sprayed coatings. Consequent hot isostatic pressing (HIPing) of CNT-reinforced Al₂O₃ at 1773 K shows differences in grain growth kinetics, degree of densification, and pore shrinkage. Densification of HIPed coatings is observed to be dictated by CNTs, phase transformation, initial grain size, and time of thermal processing. CNTs have shown to impede the Al₂O₃ grain growth by serving as grain pinning obstacles. Impediment of grain-boundary mobility with variation of CNT content, and time and temperature of the heat treatment of aluminum oxide (Al₂O₃)–CNT nanocomposite is addressed in detail.     

Effect of Alumina Reactivity on the Densification and Properties of Al2O3–Cr2O3 Refractories

Alumina‐chrome (Al₂O₃–Cr₂O₃) refractories with Al₂O₃:Cr₂O₃ molar ratio 1:1 were synthesized in the temperature range of 1400–1700°C by conventional solid–oxide reaction route. The effect of different aluminas (viz., hydrated and calcined) on the densification, microstructure, and properties of Al₂O₃–Cr₂O₃ refractories was investigated without changing the Cr₂O₃ source. The starting materials were analyzed to determine the chemical composition, mineralogy, density, surface area, and particle size. Sintered materials were characterized in terms of densification, phase assemblage, and mechanical strength at room temperature and at higher temperatures. Microstructural evolution at different sintering temperature was correlated with sintering characteristics. It can be concluded that the Al₂O₃–Cr₂O₃ refractories prepared with hydrated alumina as Al₂O₃ source show better densification and hot mechanical strength than corresponding calcined variety.     

Studies on the Crystallization Behavior of Al2O3-HfO2 Ceramic Fibers Prepared by Melt-spinning of Polymer Precursors

In this paper, Al₂O₃-HfO₂ ceramic fibers with different element distributions were prepared by melt-spinning of polymer precursors. The crystallization behavior of the two fibers was investigated and compared. From in-situ XRD, the critical crystal size and initial crystal size of the crystal structure transition from γ-Al₂O₃ to α-Al₂O₃ were calculated to be 14 nm and 40 nm, respectively. The activation energy calculated by the non-isothermal crystallization kinetic accounted for the difference in the crystal structure transition process. As a result of the lower crystallization activation energy, the γ-Al₂O₃ reached the critical size earlier in the Al₂O₃-HfO₂ ceramic fibers with a relatively uneven distribution of elements, and the fibers exhibited a lower crystal structure transition temperature of α-Al₂O₃. While the Al₂O₃-HfO₂ fibers with uniform distribution of elements showed better crystal size stability at high temperatures.