Effects of Al2O3 crystal types on morphologies,formation mechanisms of mullite and properties of porous mullite ceramics based on kyanite

Porous mullite ceramics with different crystal shapes of mullite are fabricated by in-situ reaction with middle-grade kyanite as raw material, Al(OH)₃, γ-Al₂O₃, ρ-Al₂O₃ and α-Al₂O₃ as alumina sources. Effects of Al₂O₃ crystal types on morphology evolution and formation mechanisms of mullite, and properties of porous ceramics are investigated. Results show that mullite in the sample with Al(OH)₃ mainly shows acicular morphology, because its (001) plane has the minimum interplanar crystal spacing and maximal attachment energy, it grows fast along [001] direction by screw dislocation mechanism. With a successive slowdown in reactivities of Al(OH)₃, γ-Al₂O₃, ρ-Al₂O₃ and α-Al₂O₃, the amount and aspect ratio of mullite reduce, its growth mechanism gradually transforms into two-dimensional nucleation. Acicular mullite not only reinforces samples, but makes effective pore sizes smaller, which enable the sample with Al(OH)₃ to present low bulk density, high apparent porosity and linear changes, small average pore size and good mechanical strength.     

NMR study on reaction processes from aluminum chloride hydroxides to alpha alumina powders

Starting from gelatinous aluminum chloride hydroxide, the transformation process toward α-Al₂O₃ was examined using ²⁷Al NMR, both in the liquid and solid states, as a main analytical tool. By increasing the hydrolysis ratio (h, defined as [OH⁻]/[Al³⁺]) of the starting aqueous precursor up to h = 2.5, the transition temperature to the final product, α-Al₂O₃, decreased to as low as 500°C. In this case, the structural change from amorphous alumina to α-Al₂O₃ took place without intermediate transition Al₂O₃ phases. Examining the process of networking during the transition from aqueous sol–through the state of xerogel–to final anhydrous oxide by nuclear magnetic resonance (NMR) revealed the presence of highly polymeric species mainly ascribed to δ-[Al₂O₈Al₂₈(OH)₅₆(H₂O)₂₄]¹⁸⁺ (δ-Al₃₀). δ-Al₃₀ species were found in the solution phase and became predominant after drying. We conclude that the lower temperature synthesis of α-Al₂O₃ became possible due to preformation of polymerized AlO₆ construction units in the precursor, reducing the energy barrier for the nucleation of the final α-Al₂O₃ phase.     

Improvement of interface between Al and short carbon fibers by α-Al2O3 coatings deposited by sol–gel technology

This paper describes an investigation on an improvement of the interface between Al and short carbon fibers (SCFs) with α-Al₂O₃ coating by sol–gel technology. The composites of Al/uncoated SCFs and Al/α-Al₂O₃-**coated SCFs were fabricated successfully by vacuum press infiltration. The formation of α-Al₂O₃ coating during calcination was analysed by Fourier transform infrared (FTIR) and X-ray diffraction (XRD). Scanning electron microscopy (SEM), energy-dispersive analysis of X-ray (EDAX) and transmission electron microscope (TEM) were used to observe the coated SCFs and the interface of composites. The results showed that the average thickness of the α-Al₂O₃ coating was about 200–250 nm and the formation of Al₄C₃ at the interface between Al matrix and SCFs was controlled by the α-Al₂O₃ coatings.     

Synthesis of plate-like α-Al2O3 single-crystal particles in NaCl–KCl flux using Al(OH)3 powders as starting materials

Plate-like α-Al₂O₃ single-crystal particles were successfully synthesized in NaCl–KCl flux using Al(OH)₃ powders as starting materials, and the influence of pre-calcining of Al(OH)₃ powders on the phase formation and morphology of α-Al₂O₃ powders was focused. When Al(OH)₃ powders are used as starting materials, the synthesized product at 900 °C is mainly composed of α-Al₂O₃ and κ-Al₂O₃, and most synthesized particles show alveolate morphology. At 1100 °C, single-phase α-Al₂O₃ powders are developed, in which there are many aggregations of intensively bound plate-like particles. In contrast, using porous amorphous Al₂O₃ powders obtained by pre-calcining Al(OH)₃ powders at 550 °C for 3 h as the starting material, plate-like α-Al₂O₃ single-crystal particles can be well developed above 900 °C. The reason of the influence of pre-calcining of Al(OH)₃ powders on the phase formation and morphology of α-Al₂O₃ powders is also discussed in the paper.     

The activation mechanism of oxalic acid on γ-alumina and the formation of α-alumina

Converting the γ phase into the α phase completely is necessary in the presintering stage of industrial alumina (Al₂O₃), which requires high temperature and energy consumption. To reduce the presintering temperature, γ-Al₂O₃ was activated by oxalic acid. XRD, ²⁷Al-MAS-NMR and TG-DSC were used to characterize the γ - alumina before and after activation, and the phase transformation was studied. The formation temperature of α-Al₂O₃ decreased to 1029 °C for oxalic acid activated γ-Al₂O₃, and the α-fraction was 100% for activated γ-Al₂O₃ at 1300 °C. After oxalic acid activation, the diffraction peak intensity of γ-Al₂O₃ decreased significantly; the results of ²⁷Al-MAS-NMR suggested that octahedral [AlO₆] in γ-Al₂O₃ was easier than tetrahedral [AlO₄] to be attacked by oxalic acid, and the formation of pentavalent [AlO₅] with higher reaction activity, which was in favour of the lowering formation temperature of α-Al₂O₃. The dissolution concentration of Al increased after oxalic acid activation, and the dissolution process was controlled by surface reactions. Oxalic acid mainly attacked the octahedral aluminium in γ-Al₂O₃ and extracted Al as three complexes of [Al(C₂O₄)]⁺, [Al(C₂O₄)₂]^- and [Al(C₂O₄)₃]^3-. Oxalic acid activated γ - Al₂O₃ with a lower phase transformation temperature has broad application prospects in the alumina industry.     

Methods and Approaches of the Sol–Gel Technology for the Surface Modification of Aluminum Oxide Powders

The results of studies, sol–gel synthesis, and the sedimentation stability of complex multicomponent sol–gel systems of the “silica sol modified with Co(NO₃)₂ · 6H₂O, Al(NO₃)₃ · 9H₂O with α-Al₂O₃ or γ-Al₂O₃ as highly dispersed filler” type are generalized. The physical–chemical processes accompanying the formation of modifying layers on the powder oxide particles are examined. The promising prospects of applying α-Al₂O₃ powders modified with a silicate layer of the composition (wt %) 1.2K₂O · 3Al₂O₃ · 3.2CaO · 12.5Na₂O · 28.1B₂O₃ · 52SiO₂ in the fabrication of ceramic materials with improved strength characteristics are demonstrated.     

Study on α-alumina precursors prepared using different ammonium salt precipitants

Nano-sized α-Al₂O₃ platelets have been produced by the precipitation method employing the starting material of Al(NO₃)₃·9H₂O and ammonium salt precipitants, such as the NH₄OH, (NH₄)₂CO₃ and NH₄HCO₃. The effects of chemical composition of ammonium salt precipitants and aging time of precipitated product on the formation of precursor and final product of α-Al₂O₃ particles were studied. The precursors with different crystal structures were formed depending on the chemical composition of precipitant and the agglomeration of final α-Al₂O₃ particles was found to be greatly affected by the precipitant. The aging time of precipitated precursor also influenced the agglomeration of final α-alumina particles.     

Synthesis of α-Al2O3 at mild temperatures by controlling aluminum precursor,pH, and ethylenediamine chelating additive

This study demonstrates the synthesis of α-Al₂O₃ by sol–gel method according to various reaction parameters. Various Al₂O₃ phases were synthesized by a simple sol–gel method using three different aluminum precursors (aluminum isoporoxide (AIP), Al(OH)₃, and AlO(OH)) and pHs (3, 7, and 9). Thermally treating of the synthesized powders at 1200 °C produced rhombohedral structure α-Al₂O₃. When AIP was used as an aluminum precursor, α-Al₂O₃ was synthesized at all pH levels by calcination at 1200 °C. The structure was easily changed to α-Al₂O₃ by the addition of ethylenediamine as a chelating additive at the lower temperature of 1000 °C. In contrast, no α-Al₂O₃ structure was obtained by using Al(OH)₃ or AlO(OH) precursors at higher pH in spite of thermal treatment at 1200 °C. The specific surface areas were larger in α-Al₂O₃ synthesized using AIP precursor compared with that using Al(OH)₃ and AlO(OH) precursors. Electrophoretic light scattering (ELS) measurement in aqueous solution at pH=7 revealed positive surface charges in the α-Al₂O₃ synthesized using AIP precursor, but negative charges in that synthesized using Al(OH)₃ and AlO(OH) precursors. Most significantly, the α-Al₂O₃ synthesized with the ethylenediamine chelating additive had a negative charge, despite the use of AIP precursor, with a higher mobility and larger aggregated particle diameter.     

Effect of TGO evolution and element diffusion on the life span of YSZ/Pt–Al and YSZ/NiCrAlY coatings at high temperature

In this work, the growth of thermally grown oxides (TGO) on Pt–Al and NiCrAlY bond coats and the element diffusion behavior were investigated. During oxidation, TGO initiated at YSZ/Pt–Al interface developed from a α-Al₂O₃ mono-layer to a α-Al₂O₃+NiO/α-Al₂O₃ double-layer with the increase of thermal cycling temperature. While for YSZ/NiCrAlY coating, after exposed at 1100 °C for 240 h, a double-layered TGO was formed at the interface of NiCrAlY/substrate. It is composed of an upper layer of α-Al₂O₃, Cr₂O₃ and NiCr₂O₄ mixture and a bottom layer of α-Al₂O₃. After the coating was thermal cycled at 1200 °C for 96 h, a triple-layered TGO was generated containing a bottom layer of α-Al₂O₃, a middle layer of Al₂O₃ and Cr₂O₃, and an upper layer of mixed α-Al₂O₃, Cr₂O₃ and NiCr₂O₄. The multi-layered structure of TGO is caused by the difference of element diffusion rate and formation energy of oxides. It facilitates the alternative accumulation and release of stress. Thus, the consequent service life of YSZ/Pt–Al coating is better than that of YSZ/NiCrAlY coating.     

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.     

Electrochemical synthesis and the physicochemical properties of a nanodispersed system based on iron and aluminum oxides

An Al₂O₃–Fe₂O₃ dispersed oxide system has been obtained electrochemically in aqueous solutions containing chloride ions with the subsequent thermal treatment. The phase and elemental composition and structural characteristics of the samples have been studied using X-ray phase analysis, Mössbauer spectroscopy, and scanning electron microscopy. The effect of the electrolysis mode and thermal treatment on the characteristics of the samples which were synthesized is shown. It is found that a high-temperature treatment (1100°C) facilitates the formation of a complex oxide material whose phase composition is a combination of the α-Al₂O₃ (corundum) and α-Fe₂O₃ (hematite) crystalline phases and whose particles have nanometer sizes.     

Formation of micron-sized and nanometer-sized single crystal alumina whiskers by displacement reactions

We propose a general methodology for fabricating single crystal α-Al₂O₃ whiskers by displacement reactions. The methodology is based on studies in which Al-rich powder mixtures that contain different kinds of metal oxides (MO_x) were sintered. In some sintered products, the in situ formed Al₂O₃ appeared as particulate while in other it appeared as whiskers. Some conclusions from this study are: growth of the whiskers involves the presence of MO_x−1 vapor and Al₂O vapor during sintering, and the dimension of the whiskers depends on the size of the initial MO_x particles. Micron-sized whiskers were produced in Al–MoO₃ and in Al–WO₃, while nanorods were produced in Al–SiO₂.     

Texture development in Fe-doped alumina ceramics via templated grain growth and their application to carbon nanotube growth

Fe-doped alumina (Fe-Al₂O₃) materials with a controlled microstructure could be designed for some special uses such as a substrate for carbon nanotube growth. In this study, Fe-doped Al₂O₃ ceramics with varying degrees of texture were prepared via Templated Grain Growth method and utilized for carbon nanotube synthesis by Catalytic Chemical Vapor Deposition in order to investigate how α-Al₂O₃ crystal orientation affects carbon nanotube growth in polycrystalline ceramics. The degree of texture increased with the Fe content in the presence of liquid phase. Three kinds of carbon filaments (few-wall carbon nanotubes bundles, individual multi-wall nanotubes and carbon nanofibres) were observed over Fe-doped Al₂O₃ ceramics with varying degrees of texture depending on the surface roughness, crystallographic orientation and the size of the catalyst nanoparticles. While well-textured substrates with a rough surface led to a small amount of randomly oriented carbon nanotube bundles, perpendicularly oriented individual multi-wall nanotubes were obtained over relatively smooth single crystal α-Al₂O₃ platelet surfaces (basal planes) which remained in the matrix without growing.     

The NaF-AlF3-Al2O3-Na2O system—I standard free energy of formation of α-aluminium oxide from emf measurements

The behaviour of oxygen and aluminium electrodes was studied by means of emf measurements in Na₃AlF₆ melts saturated with α-Al₂O₃ in the temperature range of 1233–1310 K. The results indicate that the electrodes are reversible when they are separated by an α-Al₂O₃ diaphragm. However, the aluminium electrode reacts with the melt to a certain extent. This means that the standard free energy data for α-Al₂O₃ are not easily derived from emf measurements in Na₃AlF₆ melts saturated with α-Al₂O₃. However, the emf for the following cell (Pt)O₂/Li₃AlF₆₍₁₎, AlF_3(s), Al₂O_3(s)//Li₃AlF₆₍₁₎, AlF_3(s), Al₂O_3(s)/Al was in excellent agreement with values derived from standard free energy data for α-Al₂O₃ formation in the JANAF Tables in the range of 1020–1275 K.     

In situ formation mechanism of aluminum oxycarbonitride in the unoxidized zone of Al–Al2O3 composites

The Al–Al₂O₃ composites were prepared by fused alumina, α-Al₂O₃ micropowders, and metal aluminum powder. The samples with 3% carbon black (N330), 3% resin powder and α-Al₂O₃ micropowder, 10% α-Al₂O₃ micropowder were named S1, S2, and S3, respectively. They were oxidized at 1500 °C for 3 h in a box-type electric furnace, and then the unoxidized areas were analyzed by X-ray diffraction and scanning electron microscopy. It was found that the aluminum oxycarbonitride formed in situ during the oxidation resistance test inhibited further oxidation of S2. The in situ formation mechanism of aluminum oxycarbonitride in the unoxidized zone is believed to be 7Al + 3Al₃C₄ + 12AlN + 4Al₂O₃ = 12Al₃CON, which is also verified and proven by XRD, SEM and EDS in this work. The oxidation depth of S2 is 46.7% lower than that of S3. Sample S2 hardly has any linear change after fired at 1500 °C × 3 h in coke, which benefits to improve the volume stability and prolongs the service life. Among the three batches, S2 exhibits the minimum creep rate, from 0% to 0.026% at 1500 °C, and the HMOR at 1500 °C in a N₂ atmosphere of S2 is 58 MPa. The Al–Al₂O₃ composites combined with resin strengthened by in situ formed aluminum oxycarbonitride give the composites excellent high-temperature strength owing to the fiber-intersected reinforced microstructure of Al₃CON crystals at high temperature. In the Al–C–N–O system, it was found that the general formula of aluminum oxycarbonitride is expressed by Al_4n+m(C,O,N)_3n+m, and the (n, m) values are (0, 3) for Al₃CON.     

Deposition of Al2O3 ceramic film on copper-based heterostructured coatings by aluminizing process: Study of the electrochemical responses and corrosion mechanism of the coating

The effect of deposition of the Al₂O₃ ceramic film by the aluminizing method on electrochemical responses and corrosion mechanism of copper-based heterostructured coatings was studied. The single layer coatings of Cu and Al₂O₃ and Cu/Al₂O₃ double layers were produced using reverse pulsed current electroplating process followed by powder cementation of aluminum on a substrate made of Inconel 600 superalloy. The produced coatings were then characterized using Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD) methods. In order to evaluate the behavior and corrosion mechanism of the produced coatings, potentiodynamic polarization and electrochemical impedance spectroscopy methods were also used in 1 mol/L HCl solution at immersion times of 1, 12, 24, and 48 hours. The results of the study showed that the mechanism of the formation of Cu/Al₂O₃ copper-based coatings is that in the aluminizing step, first, the diffusion of Al from the surface layers to the interior occurs and then the diffusion of Cu from the plating layer to the exterior takes places. It was also found that the deposition of the Al₂O₃ ceramic film resulted in the formation of α-Al₂O₃ and CuAl₂O₄ phases and increased corrosion resistance in Cu/Al₂O₃ copper-based coatings at all immersion times and the corrosion mechanism has changed from uniform to localized state.     

Synthesis mechanisms,optical and structural properties of η-Al2O3 based nanoparticles prepared by DC arc discharge in environmentally friendly liquids

The staple topic of this work is synthesis of η-Al₂O₃ nanoparticles via direct current (DC) arc discharge in different carrier media. The effects of surrounding liquid environment on composition, crystal structure and optical properties of η-Al₂O₃ nanoparticles were studied. DC arc discharge was made between two pure aluminum electrodes in water, ethanol and methanol as environmentally friendly liquid carrier media. For all media a 40 A electrical current was used for arc discharge. Scanning electron microscopy observations, X-ray diffraction and optical transmission spectroscopy, were employed for characterization of particles size, shape, crystal structure and optical properties respectively. X-ray diffraction results reveal the synthesis of pure η-Al₂O₃, Al/η-Al₂O₃ and a mixture of η-Al₂O₃ and γ-Al₂O₃ in water, ethanol and methanol respectively. The mean particle size for the nanoparticles in ethanol is 50 nm. In deionised water and also in methanol, Al₂O₃ nanoparticles with mean size of 37 nm were observed. Nanoparticles in ethanol and methanol, were completely precipitated within 48 h. In all the liquid media, optical transmissions are the optical characteristics of Al₂O₃ and aluminum nanoparticles in accordance with the data obtained from X-ray diffraction. The lowering of the band gaps with respect to the bulk value of alumina due to some oxygen deficiency reveals gradual oxidation of nanoparticles in water. The composition and formation mechanisms of the nanoparticles are discussed based on the chemical nature of the liquids and the behaviour of carrier medium under DC arc discharge condition. This results demonstrate an environmentally friendly pathway for rapid mass production of η-Al₂O₃ and mixed-Al₂O₃ and γ-Al₂O₃ nanoparticles that is essential for catalytic application of Al₂O₃ nanostructures.     

Surface interactions in the system RhAl2O3

Under oxidizing conditions Rh is easily dispersed on γ-Al₂O₃ up to a saturation concentration of > 10% of the support area. The average oxidation state of the dispersed phase is Rh³⁺. Excess Rh is present in the three-dimensional Rh₂O₃ particles. The interaction with the support in air is weak at <600 °C and both the dispersed and three-dimensional phases are easily reduced. Multiple chemisorption of CO, NO, and H is noted on the dispersed phase. In the limit two adsorbate molecules reside on the surface Rh in very dilute samples. The multiplicity is a strong function of Rh concentration and invalidates the measurement of accessible metal area in dilute samples. H₂-O₂ titration, with proper precautions, gives more reliable results, due to a constant surface oxide stoichiometry. In heat treatment (>600 °C), Rh-oxide interacts with both γ-Al₂O₃ and γ-Al₂O₃, diffusing into the subsurface region and the bulk. This process can only be partially reversed by reduction in H₂ (>550 °C). Thus, the measured value of surface Rh will depend also on the prereduction procedure. Exposure of $\text{Rh}\text{Al}{}_2\text{O}{}_3$ catalysts to high temperature under oxidizing conditions will cause loss of active area by both particle growth and by diffusion into the bulk of the support. In concentrated samples one can distinguish among the three Rh states: dispersed on surface, particulate, and dissolved in the support.     

Properties of aluminum-fluoride catalysts prepared by the fluoridation of aluminum oxide with trifluoromethane

High-purity AlF₃ has been prepared by allowing γ-Al₂O₃ to react with gaseous trifluoromethane at 670–770 K under 101 kPa total pressure. The use of gaseous trifluoromethane is a new, general method for preparing metal fluorides from metal oxides. AlF₃ prepared using this procedure retained the physical form of the starting γ-Al₂O₃. A $\text{1}\text{16}$-in. γ-Al₂O₃ extrudate, for example, yielded an AlF₃ extrudate with comparable physical dimensions and crush strengths. X-Ray diffraction, BET surface area, pore volume, and surface acidity measurements were employed to characterize various AlF₃ samples. Significant decreases in surface area and pore volume as well as surface acidity occurred upon increasing the concentration of AlF₃ from 90 to 100%. This behavior presumably results from the fluoridation of residual γ-Al₂O₃. AlF₃ extrudates were utilized as supports for Pt and Pd catalysts. Specific benzene hydrogenation activities of these catalysts are comparable to those of Pt and Pd on γ-Al₂O₃. In a unique application, $\text{Pd}\text{AlF}{}_3$ was used to hydrogenate m-diethylbenzene in superacid ($\text{HF}\text{TaF}{}_5$) solution.