Application of lanthanum to pseudo-boehmite and γ-Al2O3

Elements such as lanthanum, zirconium, thorium and cerium are known to raise the thermostability of -Al₂O₃. Among these elements lanthanum is the most effective. Three procedures have been used to apply lanthanum homogeneously to the surface of pseudo-boehmite and -Al₂O₃, namely, incipient wetness impregnation, deposition-precipitation from a homogeneous solution, and specific adsorption of a La(EDTA)-complex. The latter procedure results in supports covered homogeneously with lanthanum. The minimum amount of lanthanum required to render the catalyst supports thermostable, depends on the procedure to apply the lanthanum. The specific adsorption procedure offers the possibility to obtain thermostable supports with loadings as low as 0.5 wt % La.High-resolution electron microscopy was used to assess the mechanism of the stabilization by application of foreign metal ions in amounts less than a monolayer. It appeared that addition of foreign metal ions brings about crystallization of the amorphous surface layer of -Al₂O₃.     

Sintering of pseudo-boehmite and γ-Al2O3

Sintering of pseudo-boehmite, acicular-Al₂O₃ produced by dehydration of pseudo-boehmite, and-Al₂O₃ ex alum was investigated. The sintering process was studied by X-ray diffraction, transmission electron microscopy with selected area electron diffraction and BET surface area measurements. The solid state reaction to-Al₂O₃ causes a steep drop of the surface area to less than 10 m²g^–1. The acicular pseudo-boehmite and-Al₂O₃ supports exhibit an intermediate state where the acicular particles assume a rod-like shape and the surface area falls from about 300 to 100 m²g^–1. It was established that reaction to -Al₂O₃ and, hence, sintering proceeds via a nucleation and growth mechanism. The rate-limiting step is nucleation of -Al₂O₃. Consequently, the contacts between the elementary alumina particles dominate the sinter process. The contact between the acicular elementary particles of pseudoboehmite and-Al₂O₃ studied leads to the reaction to -Al₂O₃ to be almost complete after keeping samples for 145 h at 1050 °C. Decomposition of alum produces very small particles showing negligible mutual contacts. Consequently an elevated thermal stability is exhibited. Treatment of the alumina ex alum with water and drying results in a xerogel in which contact between elementary particles is much more intimate. Accordingly, treatment at 1050 °C causes a sharp drop in surface area.     

Fabrication and mechanical properties of α-Al2O3/β-Al2O3/Al/Si composites by liquid displacement reaction

Al₂O₃/metal composites were fabricated by heating three kinds of commercial mullite refractories in contact with Al, and their mechanical properties were investigated. Aluminum reacted with the mullite and SiO₂-glass constituting the mullite refractories and changed them into -Al₂O₃ and Si. Simultaneously, -Al₂O₃ was formed by the reactions among -Al₂O₃ and sodium and potassium oxides in the glassy phase. Also, Al penetrated into the -Al₂O₃/-Al₂O₃/Si composite by partly dissolving Si. Finally, the mullite refractories were changed into -Al₂O₃/-Al₂O₃/Al/Si composites. The phase contents, microstructures and mechanical properties of the resulting composites varied with the composition of the refractories. The content of -Al₂O₃ in the composite was lowest at the lowest Na₂O and K₂O contents in the refractories. Silicon in the composite had its highest content at the highest SiO₂ content. The composite fabricated from SiO₂/Al₂O₃ (in mol) (SAR)=1.85 consisted of 2–5 m Al₂O₃ grains embedded in metal, but that from SAR=1.05 showed a complicated microstructure with small and large grains. The bending strength of the composites fabricated from the refractories of SAR=1.85, 1.24, and 1.05 were 327, 405 and 421 MPa, respectively. Also, the corresponding fracture toughness values were 5.2, 6.1, and 5.5 MPam , respectively.     

Microwave sintering of nanocrystalline γ-Al2O3

The densification and phase transformation behavior of gas condensation synthesized nanocrystalline γ-A1₂O₃ sintered with microwave radiation has been studied. The polymorphic nucleation and growth phase transformations which occurred as the material was heated through the temperature range of 800–1300°C present significant obstacles in the achievement of specimens which possess high bulk densities. These phase transformations are accompanied by a change in particle morphology, crystallite size, and surface area. Alumina derived from a chemically synthesized boehmite precursor has been shown to exhibit the same nucleation and growth phase transformation behavior when conventionally heated. It is concluded that nanocrystalline γ or δ alumina will not be a viable starting material for the production of dense bodies with grain sizes of less than 100 nm.     

Changing the surface mechanical properties of silicon and α-Al2O3 by ion implantation

Microhardness indentation testing has been used as a means of introducing controlled localized deformation and fracture in both ion-implanted and unimplanted {1 1 1} silicon and {1 0 ˉ1 2} sapphire single crystal surfaces. The microstructural alterations due to implantation with N ₂ ⁺ and Al⁺ into silicon and Y⁺ into sapphire have been characterized using channelled Rutherford backscattering, transmission electron microscopy and electron channelling in the scanning electron microscope. It was found that sapphire only became amorphous at doses ⪞3×10¹⁶ Y⁺cm⁻² which corresponds to a total energy deposition of ∼3×10²³ keV cm⁻³ (∼44 kJ mm⁻³). The low-load microhardness (<50 gf) was found to be sensitive to the thickness of the amorphous layer produced by implantation into both silicon and sapphire. Compared with the parent crystal, this layer was found both to be softer and to behave in a relatively plastic manner with considerable plastic pile-up occurring around indentations in the higher dose specimens. The indentation fracture behaviour was found to be dominated by the presence of implantation-induced compressive stresses. The resulting effects were: (a) a decrease in the size of the radial crack traces (henceK _IC is apparently increased when evaluated using indentation fracture mechanics), (b) a decrease in the frequency of occurrence of lateral break-out in silicon and subsurface lateral cracking in sapphire, (c) initiation of lateral cracks further below the surface in both silicon and sapphire. Thus in general, it is concluded that hardness and surface plasticity are associated with softer amorphous layers whilst indentation fracture modifications are principally stress related.     

High-purity disperse α-Al2O3 nanoparticles synthesized by high-energy ball milling

The preparation of disperse fine equiaxed α-Al₂O₃ nanoparticles with narrow size distribution, high purity, and high yield is essential for producing Al₂O₃ nanocrystalline ceramic of fine grains which may exhibit a good toughness. In this work, micron-sized α-Al₂O₃ particles were directly ball-milled and subsequently washed with hydrochloric acid at room temperature. Fracture of large α-Al₂O₃ particles and cold welding of fine α-Al₂O₃ nanoparticles occur simultaneously during ball milling. It leads to the reduction of particle size with increasing milling duration below 80?h and reaches to a dynamic equilibrium with a minimal average particle size of 6.4?nm for milling durations over 80?h. Using the optimized high-energy ball milling parameters, we prepared high-purity disperse equiaxed α-Al₂O₃ nanoparticles with an average particle size of 8?nm and a purity of 99.96% (mass percent) in a high yield. After fractionated coagulation separations, disperse fine equiaxed α-Al₂O₃ nanoparticles with narrow size distribution were obtained. Finally, Al₂O₃ nanocrystalline ceramic with a relative density of 99.8% and an average grain size of 34?nm was sintered from the disperse fine equiaxed α-Al₂O₃ nanoparticles with an average particle size of 4.8?nm and a size distribution of 2–10?nm by pressureless two-step sintering.     

Improvement of structural and mechanical properties of alumina coatings by incorporation of TiO2 and α-Al2O3 nanoadditives

Abstract

Alumina coatings embedded with different nanoadditives were fabricated on aluminium alloy by microarc oxidation (MAO). Incorporation of nanograins into the prepared coatings was accomplished by dispersing nanoadditives into different electrolytes during the MAO process. Our results show that nanograins are successfully embedded in the ceramic coatings, and the embedded coatings are compact and have lower porosity. The mechanical properties of the nanograin embedded coatings such as hardness, adhesion and wear resistance are consequently improved, and the samples prepared in aluminate electrolyte with α-Al₂O₃ nanoadditive have better mechanical properties than those prepared in other electrolytes. Our results also show that the mechanical properties of MAO coatings are closely related to the surface structure. The introduction mechanism of nanograins into the ceramic coatings resulted from the reactions occurring in the microarc discharge channels such as diffusion and electrophoresis, which is believed to improve the structure of the prepared coatings.     

Electron emission measurements and the defect structure of α-Al2O3

In this article, electron emission is used to study the defect structure of alumina. The need of a direct measurement of the position of the Fermi level (or the electron concentration in the conduction band) is shown by discussing the actual electrical data on alumina. The emission has been measured over a large temperature range (1400 to 2400 K) and the emission of a technical polycrystalline alumina is reported up to the melting temperature under a controlled oxygen partial pressure. Additional results are reported for titanium- and iron-doped polycrystalline aluminas. The results are discussed from two points of view. First the quantitative data concerning the work function are taken into account and the contribution of the surface layer is discussed. Secondly, the dependency of the electron emission on the oxygen partial pressure is explained by the defect chemistry of the oxide. The absence of variation of the electron concentration in a certain range of is due to a self compensation between donor and acceptor impurities.     

Interfacial reactions between titanium film and single crystal α-Al2O3

Titanium is commonly used to join metals and ceramics by active metal brazing methods. In this work, titanium was sputter deposited on to single-crystal -Al₂O₃ substrates and the interfacial reactions between the titanium film and the Al₂O₃ substrate were studied. Al₂O₃ was reduced by titanium when samples were annealed at 973 and 1173 K for 300 s in an argon gas flow. Metallic aluminium was produced at the interface, and this diffused from the interface into the titanium film. At 1173 K, the intermetallic compound Ti₃Al and the intermediate titanium oxides, such as Ti₂O and TiO, were formed. The Al⁰ diffusion is important in stimulating interfacial reactions.     

Structural behavior of laser-irradiated γ-Fe2O3 nanocrystals dispersed in porous silica matrix : γ-Fe2O3 to α-Fe2O3 phase transition and formation of ε-Fe2O3

The effects of laser irradiation on γ-Fe₂O₃ 4 ± 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of γ-Fe₂O₃ phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that γ-Fe₂O₃ nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined in situ under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that ε-Fe₂O₃ is an intermediate phase between γ-Fe₂O₃ and α-Fe₂O₃ and a series of distinct Raman vibrational bands were identified with the ε-Fe₂O₃ phase. The structural transformation of γ-Fe₂O₃ into α-Fe₂O₃ occurs either directly or via ε-Fe₂O₃, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.     

Formation of mixed TiC/Al2O3 layers and αa- and κ-Al2O3 on cemented carbides by chemical vapour deposition

Al₂O₃ and Ti(C, O) were codeposited as a mixed chemical vapour deposition (CVD) layer from AlCl₃-TiCl₄-CH₄-CO₂-H₂ gas mixtures on cemented carbides and pure alumina substrates. A thermodynamical approach of this CVD system is presented. The coatings were described by SEM and X-ray diffraction analysis. They consist of large facetted-Al₂O₃ crystals containing some titanium and surrounded by a fine grained Ti(C, O) matrix. Carbon diffusing from the cemented carbide substrate can considerably influence the morphology and the composition of the mixed coating.Methane in a AlCl₃-CO₂-H₂ environment stabilizes the-Al₂O₃ phase which can be deposited as a compact layer without whisker formation on a WC-Co substrate even without a TiC underlayer.     

Self-diffusion in α-Al2O3 and growth rate of alumina scales formed by oxidation: effect of Y2O3 doping

In many cases, alumina scales are assumed to grow predominantly by oxygen diffusion, but some authors have found that the growth can be controlled by aluminium diffusion. These mechanisms can be modified by active elements. The problem with alumina is that there is a lack of data about self-diffusion coefficients, and, due to the stoichiometry of alumina, diffusion data correspond to an extrinsic diffusion mechanism so that it is not possible to compare oxygen and aluminium diffusion coefficients. In order to obtain information about the alumina scale growth mechanism, oxygen (¹⁸O) and aluminium (²⁶Al) self-diffusion coefficients in Al₂O₃ were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. For both isotopes, bulk and sub-boundary diffusion coefficients were determined in single crystals of undoped alumina. Grain-boundary diffusion coefficients have been computed only for oxygen diffusion in polycrystals. Oxygen diffusion has been also studied for yttria-doped -alumina in the lattice, sub-boundaries and grain boundaries. Oxygen and aluminium bulk diffusion coefficients are of the same order of magnitude. In the sub-boundaries, aluminium diffusion is slightly faster than oxygen diffusion. Yttria doping induces a slight increase of the oxygen bulk diffusion, but decreases the grain-boundary diffusion coefficients on account of segregation phenomena. These results are compared with the oxidation constants of alumina former alloys (alloys which develop an alumina scale by oxidation). It appears that neither lattice self-diffusion nor grain boundary self-diffusion can explain the growth rate of alumina scales. Such a situation is compared to the case of Cr₂O₃.     

Auto ignition processing of nanocrystalline α-Al2O3

This paper reports the results of an investigation aiming at synthesizing nanocrystalline α-alumina using the “Auto Ignition” processing technique. The process utilizes in-situ exothermic reactions occurring between a suitable oxidizer (e.g., aluminum nitrate) and a fuel (urea). Due to generation of high temperatures for a short time, the resulting crystalline phase(s) is(are) able to nucleate but cannot grow. This work systematically examines and characterizes the resulting powders obtained from various ratios of the oxidizer and fuel. Both X-ray diffraction and transmission electron microscopy results confirm that under certain conditions it is possible to obtain pure α-alumina in nanocrystalline size. It is expected that sinterability of such powders will be better because the question of γ to α phase transformation does not arise.     

Influence of yttrium on microstructure and point defects in α-Al2O3 in relation to oxidation

Although the influence of yttrium on transport properties of alumina has been the object of many studies, the mechanisms by which this element acts have not yet been elucidated. The method of modification by yttrium of the microstructure of polycrystalline alumina and the nature of the point defects created by this doping element were studied. The results obtained are discussed in relation to alumina transport properties and especially in relation with the effect of yttrium on the oxidation mechanism of alumina former alloys, taking into account the doping amount.     

CO2 pretreatment and hydrothermal treatment of commercial γ-Al2O3 powders for purification and refinement

Ultrapure, nanosized alumina (Al₂O₃) powders are highly required for high performance Al₂O₃ ceramics. However, the synthesis of the powders via an efficient and low-cost way is still a challenge. In the present research, we treated commercial γ-Al₂O₃ powders via hydrothermal treatment combined with CO₂ pretreatment technique. The effect of hydrothermal pressure on the crystal phase, particle size and purity of the treated powders were investigated. In addition, the effect of CO₂ pretreatment on the purification of the powders was discussed. Commercial γ-Al₂O₃ powders are fully converted to boehmite (AlOOH, a derived form of Al₂O₃) at a hydrothermal pressure of 3.5 MPa. The boehmite powders reduce to the minimum particle size of 50–100 nm after being hydrothermal treated at 3.5 MPa. CO₂ pretreatment has been found to be very efficient in the purification of the powders. The Al₂O₃ content of the powders after being CO₂ pretreated at 1 MPa could reach up to 99.9410% which is much larger than that of commercial γ-Al₂O₃ powders (99.5096%). The as-received ultrapure, nanosized boehmite powders are promised raw materials for high performance Al₂O₃ ceramics.     

Wetting and adhesion of Ni-Al alloys on α-Al2O3 single crystals

The effect of Al additions on the wetting and adhesion of Ni on an -Al₂O₃ single crystal was studied. Contact angles were measured by the sessile drop technique under vacuum or in He atmosphere. The morphological and chemical features of metal-vapour and metal-oxide interfaces were determined by scanning electron microscope (SEM), microprobe analysis and profilometry. The work of adhesion of Ni-Al alloys on Al₂O₃ substrates was significantly higher than for pure Ni and Al components. This result was explained by co-operative adsorption of aluminium and oxygen atoms at the Ni-Al₂O₃ interface. The influence of oxidation of the alloy on wetting and bonding is also discussed.     

Preparation of High-Performance SmBa2Cu3O7−x Superconducting Films by Chemical Solution Deposition Approach

Pure and Co³⁺-doped SmBa₂Cu₃O_7?z (SmBCO) superconducting films were prepared on (00l) LaAlO₃ single crystal substrate by self-developed fluorine-free chemical solution deposition approach. According to the X-ray diffraction and SEM observation, SmBCO films with biaxial texture possess dense, smooth, and microcrack-free surface microstructures. However, critical transition temperature (T _c) of a Co³⁺-doped SmBCO film is lower than that of pure SmBCO film, which may be attributed to the Co³⁺ doping decreasing the concentration of hole carriers for doped film. In addition, the Co³⁺-doped film has higher normalized critical current densities (J _c) in the whole magnetic fields, indicating better magnetic flux pinning properties. These results show that Co³⁺ doping by this chemical method is one of the promising ways to prepare high-performance SmBCO films.     

Ultralong and Mesoporous ZnO and γ-Al_2O_3 Oriented Nanowires Obtained by Template-assisted Hydrothermal Approach

Highly mesoporous Zn O and g-Al2O3nanowires（NWs） are both synthesized by a hydrothermal method using commercially available porous anodic aluminium oxide（AAO） as template. AAO membrane acts as template for Zn O NWs and both as template and precursor for g-Al2O3 NWs. The formation of intermediate phases of porous Zn6Al2（OH）16CO3and boehmite（g-Al OOH） were observed, both occurring during the hydrothermal synthesis of porous Zn O and g-Al2O3 NWs, respectively, and disappearing after annealing at 600 C. This novel template-assisted hydrothermal process leads to the formation of porous Zn O and g-Al2O3NWs（specific surface area of 192 m2 g 1and 263 m2 g 1, respectively）, showing pore sizes around 4 nm in diameter. The influence of the reaction parameters on the nanostructure morphology was also investigated. A Zn O seed layer, deposited on the AAO channels prior to the hydrothermal synthesis, leads to more compact Zn O nanowires（99 m2 g-1） protecting the AAO host from the chemical attack of the precursor solution.