Effect of raw materials on carbothermal reduction synthesis of γ-aluminum oxynitride spinel powder

The effect of starting Al₂O₃ materials on synthesis of -aluminum oxynitride spinel (-AlON) powder by carbothermal reduction has been investigated. Whether using Al(OH)₃, boehmite, dried gel, or calcined alumina as an alumina source, the reaction rate is faster than that using -Al₂O₃. That is because the oxygen distribution in -Al₂O₃ is cubic close packed as the non-metal arrangement in AlON spinel is whereas that in -Al₂O₃ is hexagonal close packed. When hydrated aluminas used lose structural water and transform into -Al₂O₃ in heating cycle, -Al₂O₃ is easily reduced and nitrided into AlON spinel after a slight adjustment in the cation positions and proportions is made in the reduction process of alumina. The particle morphology of the product after carbothermal reduction was strongly influenced by the type of alumina source employed, which suggests the mechanism of Al₂O₃/C powder mixtures in a flowing N₂ stream is the absorption of aluminum-containing gas, such as Al(g), Al₂O(g), etc., on the surface of aluminum oxide in the initial stages, followed by reaction with N₂ gas to form AlON and AlN phases with nucleation, and growth on Al₂O₃ particle surfaces.     

The effect of an added seed on the phase transformation and the powder properties in the fabrication of Al2O3 powder by the sol-gel process

-Al₂O₃ powder was produced by the sol-gel process. The prepared sol-gels were seeded with 1.5 wt% powder (0.12m). The phase transformation of Boehmite into -Al₂O₃ and also the particle size distribution of the transformed -Al₂O₃ were strongly influenced by seeding and the heating rate during calcination. -Al₂O₃ seed particles have been shown to act as a nuclei for the transition of - to -Al₂O₃ and also to increase the driving force of the phase transformation, which consequently lowers the transformation temperature by about 200 °C. The particles derived from the seeded sol-gels retarded the formation of vermicular microstructures and were finer than those in the unseeded case. The seeding and the control of the heating rate during calcination could inhibit the grain growth due to transformation into -Al₂O₃. Fine particles which are homogeneous and have a high sinterability at lower temperatures could be obtained.     

The formation of α-Al2O3 from θ-Al2O3: The relevance of a “critical size” and: Diffusional nucleation or “synchro-shear”?

The coarsening of -Al₂O₃ crystals to a 'critical size' is often interpreted as the first step in the shear nucleation of -Al₂O₃. The existence of this so-called critical size has also been used to explain the observation that -Al₂O₃ nuclei are generally twice as large as the crystals in the -Al₂O₃ matrix. This paper discusses the important issues in the nucleation of -Al₂O₃ from -Al₂O₃. A few key experiments are also presented to clarify the nucleation process. It is concluded that a critical -Al₂O₃ crystal size is not a prerequisite for -Al₂O₃ nucleation, but is primarily a result of the incubation time required to produce -Al₂O₃ nuclei by diffusional nucleation. It is proposed that the large observed -Al₂O₃ crystal size also does not result from a shear nucleation event in a 'critical size' -Al₂O₃ crystal, but is due to the intrinsically low -Al₂O₃ nucleation density, together with rapid growth of -Al₂O₃ after nucleation.w     

Inviscid melt-spun high-temperature alumina-magnesia fibres

Al₂O₃-MgO (AM) fibres containing 98.16 wt% Al₂O₃ and 1.84 wt% MgO, were produced via inviscid melt spinning. By using scanning electron microscopy it was found that the as-spun AM fibres were hollow and their surfaces were very rough. The X-ray diffraction pattern of the as-spun AM fibre showed -Al₂O₃ as a major phase and -Al₂O₃ as a minor phase. The DTA curve of the as-spun AM fibre showed a single endothermic peak representing the phase transformation of -Al₂O₃ to -Al₂O₃. This phase transformation was readily confirmed by analysing the X-ray diffraction pattern of heat-treated AM fibres.     

Ceramic flake formation in the aluminosilicate system by plasma spraying

Alumina and aluminosilicate flakes with compositions Al₂O₃, 3Al₂O₃2SiO₂ and Al₂O₃2SiO₂ have been produced from commercial raw materials using a direct current plasma spray process in air. The microstructure and phase constitution of the as-sprayed flakes were examined with optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Changes in phase constitution of the flakes with heat-treatment were examined using differential thermal analysis (DTA) and XRD of heat-treated samples. The as-sprayed Al₂O₃ flakes consisted of -Al₂O₃ phase plus a minor -Al₂O₃ phase. The -phase could be removed by heat treating the flakes at 1300C for 2 h. The aluminosilicate flakes consisted of 3Al₂O₃:2SiO₂ (mullite) and an amorphous phase which crystallised to 3Al₂O₃:2SiO₂ (mullite) after heat treatment at 1100C for 2 h. These flakes may find applications as high temperature thermal insulation materials.     

Phase and microstructural development in alumina sol–gel coatings on CoCr alloy

Phase transformation of -Al₂O₃ to -Al₂O₃ in alumina sol gel coatings on biomedical CoCr alloy was studied as function of heat treatment temperature and time. Transformation in unseeded coatings was significant only above 1200 °C. Addition of -Al₂O₃ seed particles having an average size of approximately 40 nm lowered the phase transformation temperature to around 800 °C. These particles were considered to act as heterogeneous nucleation sites for epitaxial growth of the -Al₂O₃ phase. The kinetics and activation energy (420 kJ/mol) for the phase transformation in the seeded coatings were similar to those reported for seeded monolithic alumina gels indicating that the transformation mechanism is the same in the two material configurations. Avrami growth parameters indicated that the mechanism was diffusion controlled and invariant over the temperature range studied but that growth was possibly constrained by the finite size of the seed particles and/or coating thickness. The phase transformation occurred by the growth of -Al₂O₃ grains at the expense of the precursor fine-grained -Al₂O₃ matrix and near-complete transformation coincided with physical impingement of the growing grains. The grain size at impingement was 100 nm which agreed well with that predicted from the theoretical linear spacing of seed particles in the initial sol.     

Thermodynamic properties of the superconductorsBaPb 0.7 Bi 0.3 O 3 andBa 0.7 K 0.3 BiO 3 using Eliashberg theory

We present results of a thermodynamic analysis for the superconductors compounds BaPb_0.7Bi_0.3O₃ and Ba_0.7K_0.3BiO₃. The physical quantities are calculated making use of the Eliashberg theory and the electron-phonon spectra ²()F() as calculated by Shirai et al. For the superconductor BaPb_0.7Bi_0.3O₃, several models of the ²()F() were studied looking for a better agreement with experimental data. The best fit is achieved with a simple constant scaling (C = 1.25) of the Shirai's spectra. The functional derivative of the deviation function D(t) with respect to changes in ²()F() is also calculated.     

Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part I Compositions and properties

X-ray diffraction was utilized to follow the transformation from -SiC (3C) to the various -SiC polytypes in the presence of AlN and Al₂O₃ additives after hot pressing from 1700 to 2100°C. The 2H- and 6H-polytypes of -SiC were the predominate polytypes with additions of only AlN or Al₂O₃, respectively. The amount of 2H- and 6H-polytypes, and subsequently the microstructural morphology of the SiC materials, were found to be controlled by varying the amount of AlN and Al₂O₃. Improvements in fracture toughness to 9 MPa-m were achieved with flexural strengths ranging from 600 to 900 MPa. These results suggest that accurate control of the polytypic make-up of SiC-based materials, along with their mechanical properties, can be achieved through AlN and Al₂O₃ additions.     

Mullite formation from xerogels of (0.84–2.2) Al2O3·1SiO2

Xerogels of (0.84–2.2) Al₂O₃·1SiO₂ prepared by chemical coprecipitation of Al(NO₃)₃·9H₂O and Si(OC₂H₅)₄ experience three thermal reaction paths for mullite formation. Those with pseudoboehmite are found to form mullite via the paths of either Al-Si spinel mullite transformation or -Al₂O₃ -Al₂O₃ + amorphous SiO₂ mullite, depending upon the ratios of Al₂O₃/SiO₂. Higher SiO₂ content may prefer the former reaction. Xerogels composed of bayerite form mullite via the route -Al₂O₃ -Al₂O₃ + amorphous SiO₂ mullite. Mullite thus formed exhibits a different crystal size, being 20–25 nm for that obtained from pseudoboehmite and around 37 nm for bayerite. The highest yield of mullite formation may be achieved with xerogels of pseudoboehmite with the stoichiometric mullite compositions, 3Al₂O₃·SiO₂.     

Equal-channel angular pressing of an Al-6061 metal matrix composite

An Al-6061 metal matrix composite, reinforced with 10 vol % Al₂O₃ particulates, was subjected to equal-channel angular (ECA) pressing at room temperature to a total strain of 5. It is shown that the intense plastic straining introduced by ECA pressing reduces the grain size from 35 m to 1 m and this leads to an increase in the microhardness measured at room temperature. Inspection revealed some limit cracking of the larger Al₂O₃ particulates as a consequence of the ECA pressing. Tensile testing after ECA pressing gave a maximum ductility of 235% at a temperature of 853 K when testing at strain rates from 10^–4 to 10^–3 s^–1. It is suggested that high strain rate superplasticity is not achieved in this material after ECA pressing due to the presence of relatively large Al₂O₃ particulates.     

Alumina-mullite-zirconia composites

A boehmite-silica-zirconia precursor powder with a uniform spatial distribution of constitutive phases was prepared from the corresponding ternary sol by compressing the electrical double layer (EDL) without altering the magnitude of the surface potential at a pH of 3.5. Both the rheological data and the calculated potential energy of interaction indicated that the mixed ternary sol formed weakly coagulated units in a shallow potential energy well after the compression of EDL at a pH of 3.5. The effects of -Al₂O₃ seeding on the phase-formation characteristics were then systematically examined. It was observed that the -Al₂O₃ seeding facilitated the formation of -Al₂O₃ but that it suppressed the growth of mullite in the alumina-mullite-zirconia composite. These observations were attributed to the epitaxial effect of -Al₂O₃ seeds which preferentially induced the formation of corundum phase over mullite formation.     

Microstructural development of a ZrO2 Naβ″-Al2O3 composite

Microstructural development in Na-Al₂O₃ containing 15 vol% ZrO₂ particles is described. Intergranular ZrO₂ particles inhibit abnormal grain growth of the Na-Al₂O₃. The growth of the Na-Al₂O₃ grains and the intergranular ZrO₂ followed a cubic time law. Direct particle coalescence consisting of encounter and spheroidizing processes was found to be the basic growth mechanism for the intergranular ZrO₂     

Microwave and conventional sintering of rapidly solidified Al2O3-ZrO2 powders

The Al₂O₃-ZrO₂ eutectic composition was rapidly solidified, forming amorphous and crystalline structures. The as-quenched material was crushed and pressed into pellets which were sintered conventionally or with microwaves. Conventional and microwave sintering at temperatures up to 1600 °C resulted in a microstructure where 100–200 nm ZrO₂ grains were present intergranularly in the -Al₂O₃ grains. Larger ZrO₂ grains (1 m) were found intergranularly. The as-quenched lamellar structure spheroidized during sintering at high temperatures. Boron contamination of the powders resulted in more homogeneous and dense as-fired samples but promoted the ZrO₂ tetragonal-to-monoclinic transformation, which was attributed to increased grain boundary diffusivity. Conventional sintering at low temperatures resulted in the formation of rods of an Al₂O₃-rich phase which grew from a low-melting B₂O₃-rich liquid.     

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.     

Phase transformation and densification during pressureless sintering of Si3N4 with MgO and Y3Al5O12 additives

The - transformation of Si₃N₄ during liquid-phase sintering appears to be controlled by the growth of the -Si₃N₄ grains in the direction perpendicular to thec-axis in the case of MgO additive. The diffusion through the liquid is the rate-controlling step in the case of the Y₄Al₅O₁₂ additive. The density of the sintered body at the solid skeleton stage was influenced by the change in the - transformation rate and/or by a change of the transformation mechanism. The indirect proportionality between the -phase content in the starting powder and the density at the solid skeleton stage was found. The microstructure of the sintered body is influenced by both the -phase content in the starting powder and the chemical composition of the additive. Fine, uniform microstructure with a high aspect ratio of -grains is obtained, when the -phase content in the starting powder is as small as possible and when the - transformation is controlled by grain growth.     

Sintering and crystallization of mullite powder prepared by sol-gel processing

Mullite powder with the stoichiometric composition (3Al₂O₃.2SiO₂) was synthesized by a sol-gel process, followed by hypercritical drying with CO₂. Within the limits of detection by X-ray diffraction, the powder was amorphous. Crystallization of the powder commenced at 1200 °C and was completed after 1 h at 1350 °C. In situ X-ray analysis showed no intermediate crystalline phases prior to the onset of mullite crystallization and the pattern of the fully crystallized powder was almost identical to that of stoichiometric mullite. The synthesized powder was compacted and sintered to nearly theoretical density below 1250 °C. The microstructure of the sintered sample consisted of nearly equiaxial grains with an average size of 0.2 m. The effect of heating rate (1–15 °C min^–1) on the sintering of the compacted powder was investigated. The sintering rate increased with increasing heating rate, and the maximum in the sintering curve shifted to higher temperatures. The sintering kinetics below 1150 °C can be described by available models for viscous sintering.     

Thermal evolution of thin boehmite films

Using transmission electron microscopy, electron diffraction and resistance change measurements, we studied the process of thermal evolution of thin boehmite (-Al₂O₃·H₂O) films. It has been shown that the crystal lattice of boehmite decomposes at a temperature of 670 K, with simultaneous partial dehydration. Above 875 K crystallization of - Al₂O₃ occurs and the morphology of the film changes from fibrous to granular. Annealing of - Al₂O₃ at 1300 K results in its transformation into - Al₂O₃. At temperatures of 1475 to 1575 K the transformation of -Al₂O₃ into - Al₂O₃ was observed. Depending on the temperature of annealing, thin -Al₂O₃ films were composed of fine crystals or of large (micrometre) corundum crystals. The method of producing of the various aluminas in the form of thin films presented in this report makes investigation of the structure transitions in aluminium oxides by means of transmission electron microscopy possible, and such films can be also used as supports for transmission electron microscopy studies of the model metal-oxide supported catalysts.     

Phase transformation of inviscid melt spun (IMS) alumina-zirconia eutectic fibres

Al₂O₃-ZrO₂ (AZ) eutectic fibres containing 41.05 wt% ZrO₂ and 2.03 wt% Y₂O₃ were produced via inviscid melt spinning (IMS). Scanning electron microscopy (SEM) was used to examine the morphology of the as-spun AZ fibre. Differential thermal analysis (DTA) and X-ray diffraction (XRD) were used to investigate the phase transformations in this fibre. The XRD pattern of the as-spun AZ fibre showed tetragonal ZrO₂, -Al₂O₃, and non-equilibrium -Al₂O₃ phase formation. The DTA curve of the as-spun AZ fibre showed only one endothermic peak representing the phase transformation of -Al₂O₃ to -Al₂O₃. This phase transformation was confirmed by analysing the XRD pattern of heat-treated AZ fibre.     

Phase composition and mechanism of formation of Ba-β-alumina-type systems for catalytic combustion prepared by precipitation

A novel preparation method is proposed for Ba--Al₂0₃-type systems to be used in high-temperature catalytic combustion, consisting of precipitation in aqueous medium. Differential thermal analysis-thermogravimetry, X-ray diffraction, Fourier transform-infrared, TEM and surface area data are presented for Ba-Al-O samples with Al/Ba atomic ratios in the range 14-9 calcined at different temperatures up to 1670 K. The final materials consist of a barium-poor Ba-₁Al₂O₃ phase together with 1% -Al₂O₃ in the case of Al/Ba = 14 and of a barium-rich Ba-₁₁-Al₂O₃ phase together with 2% BaAl₂O₄ in the case of Al/Ba = 9. A mono-phasic sample with a Ba--Al₂O₃ structure is obtained in the case of Al/Ba = 12; this phase is constituted by the simultaneous presence of both ₁ and ₁₁ structure types. Two routes operate above 1370 K in the formation of the Ba--Al₂O₃ phases involving solid-state reactions between -Al₂O₃ and BaAl₂O₄, and -Al₂O₃ and dispersed barium compounds. Based on the analogies between the structures of -Al₂O₃ and of the Ba--Al₂O₃ phases it is suggested that the formation of the latter occurs via diffusion of barium ions within oxygen close-packed planes of the -Al₂O₃-type spinel structure.     

The effect of a Cr2O3-addition on the phase transformation and catalytic properties of γ-Al2O3 in treatment of lean-burn exhausts

The solid-state thermal behaviour of -Al₂O₃ doped with 10 mol% Cr (oxide) was studied with respect to phase-transition behaviour and the co-ordination of the dopant Cr cations. A series of transformations: -Al₂O₃ -Al₂O₃ -Al₂O₃ -Al₂O₃ was observed for Cr₂O₃-doped alumina samples between 500 °C and 1100 °C. Rapid grain growth occurred at temperatures close to 1100 °C. The electron spin resonance (ESR) spectra for the sample heat-treated at 500 °C corresponded to the resonance of -Cr³⁺ in an amorphous Cr oxide impregnated onto the - and -alumina support. The change of ESR spectrum indicated the existence of Cr³⁺, suggesting the formation of a solid solution with the same structures as -Al₂O₃ and/or -Al₂O₃ at 800–1000 °C. The evaluation of catalytic activities for model exhaust was performed under lean-burn (an excess of oxygen) condition of air/fuel ratio A/F = 18 and space velocity SV = 100 000 h^–1 . The modified Al₂O₃ catalyst heat-treated at 1000 °C in air showed removal conversion of 100% for hydrocarbon (C₃H₆), 92% for CO and 5% for NO at 550 °C. Present results suggest that Cr-modification to Al₂O₃ leads to catalytic improvement with good thermal durability.