Large-scale precipitation synthesis of α-alumina with poly aluminum chloride: Optimization of synthesis parameters

α-Alumina production has been industrialized since many years ago. Nevertheless, there are still challenges in the efficient synthesis of α-alumina with subtle specifications for high-tech applications, specifically on large scales. So, here, we investigated the large-scale synthesis of α-alumina by the precipitation method and with industrial grade poly aluminum chloride (PAC) as the aluminum precursor, for the first time. The synthesis procedure was optimized in terms of synthesis yield and facilitation of α-alumina formation by adjusting the concentration of the precursor (PAC) and precipitant (NaOH) solutions. The samples were characterized by conducting several analyses, including X-ray diffraction, X-ray fluorescence, field emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, dynamic light scattering, and gas adsorption. The results indicated the significant effects of PAC and NaOH concentrations on the synthesis yield and facilitation of the α-alumina formation. According to the results and process considerations, working at higher levels of PAC (.75 g/ml) and NaOH (1.0 M) concentrations was found to be the optimum synthesis conditions. Moreover, the potential of the optimized sample as the abrasive material and the catalyst support was evaluated. The results confirmed the high potential of the optimized sample in these applications. This study therefore introduces a promising and cost-effective method for the large-scale synthesis of versatile α-alumina.     

Formation of porous α-alumina from ammonium aluminum carbonate hydroxide whiskers

Ammonium aluminum carbonate hydroxide (AACH) whiskers prepared by hydrothermal technique were employed as precursor material for development of porous alumina. After compaction of AACH whiskers at 8 bars, calcination was performed at 650?°C followed by sintering at different temperatures. The sintered samples were characterized by XRD, FTIR, SEM and mercury intrusion porosimetry. Mechanical strength was determined by compression testing. At sintering temperatures of 1200?°C to 1400?°C, the % age porosity was around 80%. At 1500?°C, the percentage porosity decreased to 71%. The as-prepared AACH consisted of bundles of whiskers with diameters as thick as 0.7?µm, while an individual whisker had a diameter of about 100?nm with an aspect ratio of about 33. A two-phase mixture consisting of θ- and α-alumina was obtained at 1100?°C, while at 1200?°C and above, single phase α-alumina was formed. θ-alumina retained the bundle-like morphology. However, transformation to α-alumina was accompanied by formation of bead-like morphology. These beads were joined together through necks/stems within the whiskers as well as across the parallel-lying whiskers. These necks grew at 1300?°C to form aggregates with smooth surfaces. At 1400?°C, these aggregates started joining with each other by neck formation and at 1500?°C, a three-dimensional network was formed. For sintering temperatures of up to 1400?°C, pores with sizes around 260?nm were very stable. At 1500?°C, significant pore growth took place along with an overall densification. Therefore, number of pores with sizes of around 260?nm decreased and those with sizes around 10?µm, 1?µm and 5?nm increased. The compression strength of samples sintered at 1100?°C to 1300?°C was in the range of 3.4–4.3?MPa. At 1400?°C, the strength increased to 5.2?MPa, while at 1500?°C, it jumped to 10.8?MPa due to the formation of three-dimensional network.     

Microreactor-assisted synthesis of α-alumina nanoparticles

In this paper, nanosized α-Al₂O₃ powders were synthesized with the aid of a high throughput impinging stream microreactor. It was demonstrated that the as-prepared powders exhibited better dispersity and more homogeneous distribution of particle size than that prepared by conventional parallel flow precipitation method due to the drastic collisions and homogeneous explosive nucleation in microchannel during the precipitation process. The effects of calcinating temperature and time on the morphology, phase composition and particle size of α-Al₂O₃ were systematically studied. Comparatively well dispersed and crystallized α-Al₂O₃ powders with higher sintering activity were obtained by calcinating the as-prepared precursors at 1200 °C for 2 h in air. The specific surface area of α-Al₂O₃ powders was above 20 m² g⁻¹ and average particle size was about 110 nm with higher sintering behavior. From room temperature to 1520 °C, the green compact exhibited shrinkage of 19.34%. This work opens a door for developing ultrafine α-Al₂O₃ powders with uniform size distribution, high crystallinity, and excellent thermal expansion property.     

Shock activation of α-alumina from calcinated Al-rich sludge

Enormous quantities of Al-sludge from aluminum surface treatments are yearly generated all over the world. Despite of being known as important supplier of alumina, the Al-sludge also have some other components that need to be removed. Calcination has revealed to be an inevitable treatment to achieve such objective. It was also found that conventional densification processes are not efficient for this kind of powders and, to overcome that difficulty, a dynamic compaction approach, using detonation generated shock waves have been followed. The calcinated powders compacted by this way have shown up values of density and hardness, after sintering, comparable to the ones of commercial alumina processed by the conventional methods.     

Synthesis of thermally stable micro spherical χ-alumina by thermal decomposition of aluminum isopropoxide in mineral oil

Thermal decomposition of aluminum isopropoxide (AIP) in mineral oil at 250–300 °C over a 2 h duration results in χ-alumina powders having high thermal stability. The mechanism of the process involves the formation of amorphous complex before further decomposition takes place. Phase transformation of the obtained products was also investigated. It was found that χ-alumina synthesized by this method transformed directly to α-alumina at temperature higher than 1000 °C.     

Low-temperature formation of α-alumina from various polyhydroxoaluminum-hydroxy acid composite gels

Low-temperature α-alumina formation was attempted using various polyhydroxoaluminum (PHA)-hydroxy acid composite gels, which were prepared from PHA solutions containing different amounts of hydroxy acids, such as lactic acid, glycolic acid, malic acid, citric acid or mandelic acid. The composite gels began to transform into α-alumina when heated at lower temperatures of around 500 °C and the α-alumina fraction of the heat-treated products increased with increasing temperature. The α-alumina fraction was also dependent on both the type and amount of hydroxy acid additive. Among the composite gels studied, significant low-temperature α-alumina formation was observed for the PHA-mandelic acid, PHA-citric acid and PHA-lactic acid series. Low-temperature α-alumina formation was further promoted by employing a two-step heat-treatment method. The interaction between the functional groups of PHA and hydroxy acid and the seeding effect appear to play important roles in the course of the gelation and calcination processes for low-temperature α-alumina formation.     

Low-temperature synthesis of α-alumina using NaCl–Na3AlF6 flux

A low-temperature synthesis of α-alumina was studied using a flux consisting of salts and additives. Al(OH)₃ was used as a precursor for the preparation of alumina; NaCl–KCl, NaCl, Na₂SO₄ and K₂SO₄ were used as the salts in the flux; and cryolite (Na₃AlF₆) was used as an additive. Using a flux consisting of Na₃AlF₆ with each salt confirmed that a phase change to α-alumina was possible even at a low temperature of 600 °C. Among the salts used, NaCl was the most effective. The flux with NaCl and Na₃AlF₆ could produce highly pure and highly crystalline platelet α-alumina crystals at 700 °C. Herein, NaCl exhibited interfacial activity during the phase change process and contributed to the growth of the α-alumina plate-like crystals with a clear crystal surface. Na₃AlF₆ was found to contribute significantly to the formation of crystal nuclei for the initiation of the phase change to α-alumina even with the addition of small amounts, namely, a 0.02–0.03 M ratio (Na₃AlF₆/Al(OH)₃).     

Size-controlled synthesis of nano α-alumina particles through the sol–gel method

Nano α-alumina particles were synthesized by a sol–gel method using aqueous solutions of aluminum isopropoxide and 0.5 M aluminum nitrate. 1/3-benzened disoulfonic acid disodium salt (SDBS) and sodium bis-2-ethylhexyl sulfosuccinate (Na(AOT)) were used as surfactant stabilizing agents. Solution was stirred for different periods (24, 36, 48 and 60 h) at 60 °C. The samples were then analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Introduction of surfactant stabilizing agents and different stirring times will affect the size and shape of particle formed and also the degree of aggregation. SDBS, however, produced better dispersion, finer particles and spherical shape nanoparticles, compared to Na(AOT). The finest particle size (20–30 nm) was obtained at 48 h stirring time with SDBS surfactant.     

Low temperature synthesis and characterization of rutile TiO2-coated mica–titania pigments

Rutile TiO₂-coated mica–titania pigments were prepared by hydrolysis of TiCl₄ ethanolic solution in water at 70 °C. MnO₂ as a rutile promoting additive was deposited onto mica prior to TiO₂. X-ray diffraction and Raman spectra analysis verified that use of only 2.07 wt% MnO₂ with respect to mica weight began to provide a complete rutile TiO₂ coating without calcination. The rutile promoting effects of MnO₂ could be ascribed to the structural similarity of rutile and pyrolusite. Scanning electron microscopy analysis showed that MnO₂ also had a pronounced effect on the morphology of TiO₂ coatings. The prior deposition of MnO₂ onto mica lead to the formation of rutile TiO₂ films composed of highly oriented fine needles on the mica surface and nanoflower structures on the needle structures. The as-obtained rutile-TiO₂ coated mica–titanium pigments are shown to exhibit a high photostability under UV illumination.     

Effective synthesis of well graphitized high yield bamboo-like multi-walled carbon nanotubes on copper loaded α-alumina nanoparticles

A high-yield bamboo like multiwalled carbon nanotubes (CNTs) were successfully synthesized on copper substituted alumina nanoparticles by thermal chemical vapor deposition (CVD) technique under atmospheric pressure. The obtained products were characterized by various techniques like FESEM with EDX, HRTEM and Raman spectroscopy, which reveals the formation of CNTs and are of bamboo shaped (stacking arrangement) multiwalled type with graphene layers having a diameter between 4 and 9 nm. The appearance of two peaks at 1597 cm^− 1 and 1302 cm^− 1 in Raman spectra are noticed as G-band and D-band for graphitic nature and defects due to bending & curvature of bamboo like carbon nanotubes (b-CNTs), respectively. The influence of reaction parameters such as time, temperature and flow rate was also studied to increase the carbon yield.     

Low temperature synthesis of single-phase α-Fe2O3 nano-powders by using simple but novel chemical methods

Two simple chemical methodologies have been described for the preparation of single-phase α-Fe₂O₃ nano powder with particle size ∼ 20-30 nm. Precursor powders were synthesized by reacting aqueous solutions of ferric nitrate and PVA and sucrose (method 1) and EDTA (method 2) and then evaporating the resulting solutions to dryness. The precursors were subsequently calcined in air for 2.30 hr at different temperatures ranging from 250 to 450 °C. The synthesized powders were characterised using TG-DSC analysis, X-Ray Diffraction, BET surface area measurement, SEM and TEM. DC electrical resistivity of the synthesized materials was measured by the two-probe method. The synthetic routes described here provide simple but cost-effective methods to produce single phase, α-Fe₂O₃ nanopowder at a comparatively low temperature.     

Plastic forming and microstructural development of α-alumina ceramics from highly compacted green bodies using extrusion

High density (>99% TD) and microstructurally controlled α-alumina ceramics were produced from seeded nano-size boehmite (γ-AlOOH) sols with a very fine crystallite size (2–3 nm). A totally wet processing technique comprising vacuum filtering and pressure filtration (PF) was applied in order to increase the solids-loading of the sol and hence form an extrudable paste suitable for plastic forming using extrusion. High packing densities (>68% TD in the green state) are achieved by PF starting from the slurry state resulting in the formation of a consolidated paste which is further consolidated by extrusion. This combined processing technique was successfully applied, in an attempt to reduce the γ-Al₂O₃ formation temperature, and hence lower the θ- to α-Al₂O₃ transition temperature. The microstructure of dense α-Al₂O₃ bodies derived from seeded boehmite sol contains very fine (250 nm) alumina grains after sintering at 1200 °C for 2 h. Although the DTA evidence points to a θ- to α-Al₂O₃ transition temperature of 1208 °C for a seeded (with 30 nm TiO₂) sample, X-ray analysis indicates that a seeded, pressure filtrated and extruded sample is transformed to α-Al₂O₃ phase after sintering at 1100 °C for 2 h.     

Low temperature pressureless sintering of α-SiC with Al2O3 and CeO2 as additives

The combination of Al₂O₃ and CeO₂ was testified as suitable sintering additive for liquid phase sintering of SiC ceramics, which has lower sintering temperature than that sintered with Al₂O₃ and Y₂O₃ as sintering aids. However, the mechanical properties including flexural strength, Vickers’ hardness and fracture toughness of this system were similar to those of the samples sintered with Al₂O₃ and Y₂O₃ as sintering aids. The good wettability of the eutectic liquid phase on SiC plate, the high solubility of SiC particles into the liquid phase and the penetration of the liquid phase along the SiC–SiC grain boundaries all confirmed the suitability of the combination of Al₂O₃ and CeO₂ as liquid phase sintering additive for SiC.     

Low temperature synthesis of novel SrCe0.9Yb0.1O3−α-chlorides composite electrolytes for intermediate temperature protonic ceramics fuel cells

In this study, novel SrCe_0.9Yb_0.1O_3−α-(Na/K)Cl and SrCe_0.9Yb_0.1O_3−α-NaCl-BaCl₂ composite electrolytes were synthesized at low temperature (750 °C). The structural properties of the composite electrolytes based on mixtures of SrCe_0.9Yb_0.1O_3−α (SCY) and chlorides ((Na/K)Cl, NaCl-BaCl₂) are characterized. The microstructure of the composite electrolytes are observed by scanning electron microscopy (SEM). The electrical conductivity is determined by impedance spectroscopy. The ionic conductivities of the composite electrolytes are higher than that of SCY. Finally, high-performance intermediate temperature fuel cells of the composite electrolytes are obtained.     

Location of Mn in MnAPO‐11: influence of synthesis from aqueous and non‐aqueous media

MnAPO11 samples were synthesized from aqueous (MnAPO11(A)) and ethylene glycol (MnAPO11(NA)) media. The crystallinity of the samples was more when the synthesis was carried out in ethylene glycol. Chemical and thermogravimetric analyses reveal greater incorporation of Mn in the framework of MnAPO11(NA) than in MnAPO11(A). At least five different types of Mn(II) species are detected in the samples by ESR. The studies suggest that Mn is more homogeneously distributed in MnAPO11(NA) than in MnAPO11(A).     

Combustion synthesis of β-SiAlON from a mixture of aluminum ferrosilicon and kaolin with nitrogen-containing additives using acid enrichment

In the work, single-phase β-SiAlON was prepared from a powder mixture based on aluminum ferrosilicon and aluminosilicate (kaolin) with nitrogen-containing additives (product of nitriding of the starting mixture and NH₄F) by combustion synthesis followed by acid enrichment of the synthesized product. Self-propagating combustion synthesis was conducted under natural nitrogen filtration. The synthesis parameters (nitrogen pressure, sample diameter, density of the starting mixture) and the conditions of acid enrichment of the synthesized products (acid concentration, temperature of acid enrichment and dispersion of the starting powder) were determined. After acid enrichment of the synthesized product, which contained β-SiAlON and α-Fe phases, the phase content of β-SiAlON was 99.5 wt%.     

Effects of α- and γ-tocopherols on formation of hydroperoxides and two decomposition products from methyl linoleate

The antioxidant effects of α-and γ-tocopherols (at 0, 10, 100, 500, and 1000 ppm) were evaluated in a model system based on the autoxidation of methyl linoleate in bulk for 4 d at 40°C. Samples were collected every 24 h and analyzed for the 9 cis,trans, 9 trans,trans, 13 cis,trans, and 13 trans,trans isomers of hydroperoxide, hydroxy, and ketodiene oxidation products by high-performance liquid chromatography. Results showed that both α- and γ-tocopherols are effective hydrogen donors as evidenced by their abilities to inhibit the formation of hydroperoxides, hydroxy compounds, and ketodienes and the cis,trans to trans,trans isomerization of hydroperoxides. Compared with γ-tocopherol, α-tocopherol was a more efficient antioxidant at very low concentrations (10 ppm) but a less efficient antioxidant at the high concentrations (100–1000 ppm). This paradoxical behavior is explained on the basis of differences in ease of hydrogen donation between the two tocopherol homologs. Although α-tocopherol shows some loss of efficiency with increasing concentration, it is not a prooxidant when compared to the control void of antioxidants.     

Mechanochemical synthesis of zinc ferrite from zinc oxide and α-Fe2O3

Mechanochemical synthesis of zinc ferrite (ZnFe₂O₄) from a powder mixture of zinc oxide (ZnO) and hematite (α-Fe₂O₃) by room temperature grinding using a planetary ball mill was investigated. The grinding enables us to obtain the amorphous mixture of the starting materials. Most of ZnO reacts with α-Fe₂O₃ to convert into insoluble amorphous zinc and iron compounds within 2h-grinding. Prolonged grinding enhances the crystallization of ZnFe₂O₄ from the amorphous compounds. ZnFe₂O₄ crystallized by the grinding for 3 h or more consists of nanocrystalline particles with high specific surface area.     

A new procedure for the synthesis of α-acyloxy aldehydes from ketones via α,β-epoxy sulfides

α-Acyloxy aldehydes, as O-protected α-hydroxy aldehydes, have been prepared in good yields via a new procedure involving the sequence: (1) reaction of ketones with sulfur ylides to give α,β-epoxy sulfides, (2) ring opening with carboxylic acids, (3) oxidation of the α-acyloxy-β-hydroxy sulfide products by mCPBA to the corresponding sulfones; and (4) acyl migration followed by elimination of the sulfinate group. The last transformation was readily accomplished, in very good yields, by equivalent amounts of triethylamine in dichloromethane at room temperature (method A); or by acyl migration assisted by complexation of the liberated alkanthiol accomplished in the α-acyloxy-β-hydroxy sulfides by Ag₂O in acetonitrile to give the same α-acyloxy aldehydes (method B). The presented procedure, compared to alternative procedures, has advantages including easily available key intermediates (α,β-epoxy sulfides), mildness of the reactions conditions, more general procedure (applied to different types of α-hydroxy aldehydes).