Effect of boehmite sol on the performance of alumina microfiltration membranes

Precursor film method was used to prepare highly permeable ceramic microfiltration membranes in this work. The performance of alumina microfiltration membranes was improved by adding boehmite sol in the membrane precursor film. With increasing the boehmite sol content, the effective average pore size of the membrane was continuously decreased and the separation efficiency of the membrane was increased. These improvements were due to that the boehmite sol not only helped the dispersion of the α-Al₂O₃ powder in the membrane forming slurry, but also formed γ-Al₂O₃ in the gaps among the existed α-Al₂O₃ particles which was beneficial for membrane sintering and pore size decreasing. For the membrane prepared with 45 wt% boehmite sol, the effective average filtration pore diameter was 178 nm and the water permeance of the membrane reached 1691 Lm⁻²h⁻¹bar⁻¹, which was much better than the values reported before. Moreover, the reusability of the membrane was confirmed using a recycling test.     

Specific signatures of α-alumina powders prepared by calcination of boehmite or gibbsite

The specific signatures of α-Al₂O₃ powders by a combination of X-ray diffraction (Rietveld analysis), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, thermal analysis (TG-DTA) and cathodoluminescence (CL) were investigated. Thus, α-alumina was prepared by calcination of boehmite or gibbsite at 1573 K for 24 h. The size of α-alumina crystallites obtained using boehmite precursor was smaller than that obtained using gibbsite precursor. The difference in oxygen vacancies (F⁺-centers) amount between α-Al₂O₃ powder obtained by calcination of gibbsite and boehmite was confirmed by CL spectra. Furthermore, the Ti³⁺ emission at 1.71 eV is absent in α-Al₂O₃ powder obtained by calcination of gibbsite. CL has been demonstrated as a possible method for differentiation between the various α-alumina powders.     

Boehmite coating on θ-Al2O3 particles via a sol–gel route

θ-Al₂O₃ powders in slurry form were coated by boehmite nanoparticles prepared from alumina sol that consists of aluminum tri-sec-butoxide (Al(OC₄H₉)₃) as a precursor. The θ-Al₂O₃ powders presented an iso-electrical point (IEP) at pH ∼ 9.2, and formed stable suspensions without notable gravity settling at acidic conditions (pH 3–4). Boehmite nanoparticles with a mean particle size ca. 68 nm were derived from the hydrolysis of Al(OC₄H₉)₃ sol. Well-dispersed boehmite/θ-Al₂O₃ aqueous mixtures consisting of 3 and 9 wt.% of boehmite were then prepared by addition of θ-Al₂O₃ powder into the Al(OC₄H₉)₃ sol followed then by vigorous agitation at 90 °C and in a pH range 3–4. IR and particle-size measurement both revealed possible coating of boehmite on the θ-Al₂O₃ particles. From TEM observation, “thickness” of the boehmite layer did not vary much with the boehmite concentration. Agglomeration of the particles became nonetheless apparent as the boehmite concentration was increased. The boehmite coating on θ-Al₂O₃ surface inhibited grain growth during the θ- to α-phase transformation upon heating. Mean grain size of the boehmite-coated alumina was substantially smaller than that of the un-coated one.     

Synthesis of mesoporous TiO2/γ-Al2O3 composite granules with different sol composition and calcination temperature

Mesoporous TiO₂/γ-Al₂O₃ composite granules were prepared by combining sol-gel method and oil-drop method. After calcination at 450 °C, the composite granules showed anatase and γ-Al₂O₃ phases with the specific surface area of 240-310 m²/g. The phase composition and pore structure of the granules can be controlled by calcination temperature and the mixing ratio of boehmite sol and titania sol. The product granules can be used as a photocatalyst or adsorbent in moving or fluidized bed reactors.     

A model for ceramic membrane formation by dip-coating

The wet ceramic membrane formation on a porous substrate in the dip-coating process plays a key role for high quality separation layers of the asymmetric membranes. A model to quantitatively describe the membrane growth under a driving force of the capillary filtration is derived on the basis of the slip-casting model. As an example, α-Al₂O₃ microfiltration membranes were prepared by dip-coating. The influence of the dipping time on the membrane thickness was examined. The data for α-Al₂O₃ microfiltration membranes fit the predictions made by the model well. In addition, the results of γ-Al₂O₃ membranes demonstrate that this model is also valid for ultrafiltration membrane formation by dipping sols.     

Synthesis and characterization of Al2O3 nanopowders by a simple chitosan-polymer complex solution route

Al₂O₃ nanopowders were synthesized by a simple chitosan-polymer complex solution route. The precursors were calcined at 800–1200 °C for 2 h in air. The prepared samples were characterized by XRD, FTIR and TEM. The results showed that for the precursors prepared with pH 3–9 γ-Al₂O₃ and δ-Al₂O₃ are the two main phases formed after calcination at 800–1000 °C. Interestingly, when the precursor prepared with pH 2 was used, α-Al₂O₃ was formed after calcination at 1000 °C, and pure α-Al₂O₃ was obtained after calcination at 1200 °C. The crystallite sizes of the prepared powders were found to be in the range of 4–49 nm, as evaluated by the XRD line broadening method. TEM investigation revealed that the Al₂O₃ nanopowders consisted of rod-like shaped particles and nanospheres with particle sizes in the range of 10–300 nm. The corresponding selected-area electron diffraction (SAED) analysis confirmed the formation of γ- and α-Al₂O₃ phases in the samples.     

Microstructural investigations of tubular α-Al2O3-supported γ-Al2O3 membranes and their hydrothermal improvement

γ-Al₂O₃ meso-porous membranes supported by tubular α-Al₂O₃ substrates were prepared by using the sol-gel method and their nanostructural characterizations were performed for the first time with high-resolution transmission electron microscopy (HRTEM) before and after hydrothermal treatment at 500 °C. The HRTEM images and pore size distribution (PSD) analyses revealed that the morphologies as well as the characteristics of the powder and membrane samples prepared from the same boehmite are not identical. γ-Al₂O₃ and La₂O₃-Ga₂O₃ doped-γ-Al₂O₃ (LGA) membranes supported by α-Al₂O₃ were also fabricated and characterized under thermal and hydrothermal conditions for the purpose of comparisons. Finally, two type α-Al₂O₃/γ-Al₂O₃/SiO₂ (AA-SiO₂) and α-Al₂O₃/La₂O₃-Ga₂O₃-γ-Al₂O₃/SiO₂ (ALGA-SiO₂) membranes have been synthesized and the gas permeance of the membrane were measured in the temperature range 100–500 °C.     

Influences of pH value on the microstructure and phase transformation of aluminum hydroxide

The effects of pH value on the composition, structure, morphology, and phase transformation of aluminum hydroxides prepared by chemical precipitation were studied. Aluminum hydroxide precipitated at the pH values of 5 and 6 is amorphous and transforms to α-Al₂O₃ at 950 °C via the amorphous aluminum hydroxide → amorphous Al₂O₃ → α-Al₂O₃ transformation path. Aluminum hydroxide precipitated at pH = 7 is boehmite and transforms to α-Al₂O₃ at 950 °C via the γ-AlOOH → γ-Al₂O₃ → α-Al₂O₃ path. Aluminum hydroxide precipitated at pH values in the 8 to 11 range is bayerite and transforms to α-Al₂O₃ at 1000 °C via the α-Al(OH)₃ → γ-Al₂O₃ → ε-Al₂O₃ + θ-Al₂O₃ → α-Al₂O₃ path. Moreover, the pH value affects not only the morphology of aluminum hydroxide particles which changes from ultrafine floccules through 50 nm blowballs then to 150 nm irregular agglomerates with increasing pH value but also the microstructures of final decomposition products of aluminum hydroxides.     

Carbon–Al2O3–Ag composite molecular sieve membranes for gas separation

Phenolic resins loaded with two different inorganic fillers (boehmite (γ-AlO(OH)) and silver (Ag)) were used to prepare composite carbon membranes. Polymer solutions containing γ-AlO(OH) and AgNO₃ were prepared and the effect of Ag on the transport properties of the composite membrane was evaluated. The polymer solutions were coated on α-Al₂O₃ tubes and carbonized in a single dipping-drying-pyrolysis step. After pyrolysis at 550 °C, γ-AlO(OH) yielded γ-Al₂O₃, and Ag agglomerated forming spherical nanoparticles of 30 nm in diameter. Ag loading enhanced the carbon membrane performance for several gas pairs of interest, especially for C₃H₆/C₃H₈ separation, where the C₃H₆/C₃H₈ permselectivity increased from a maximum of 15 to 38.     

Ti addition effect on α-Al2O3 flakes synthesis using a mixture of boehmite and potassium sulfate

Single-crystal α-Al₂O₃ hexagonal flakes with a diameter of about 200 nm and 20 nm in thickness were obtained by mixing different molar ratios of potassium sulfate to boehmite and heating at 1000 °C. Co-doping 1 mol% TiO₂ can increase the shape anisotropy of α-Al₂O₃ hexagonal flakes, increasing the diameter to 400 nm. The effects of potassium sulfate, Fe₂O₃ and TiO₂ on the phase transformation and morphology development of alumina were investigated using X-ray diffraction analysis (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The results indicate that co-doping potassium sulfate, Fe³⁺ and Ti⁴⁺ can promote γ → α-Al₂O₃ phase transformation and change the morphology from a vermicular structure into hexagonal platelets. The shape anisotropy of α-Al₂O₃ hexagonal flakes can be increased by adding TiO₂ due to the segregation of Ti⁴⁺ ions onto the surfaces of basal planes of α-Al₂O₃ single crystal particle.     

Fabrication and microstructural characterization of the novel optical ceramic consisting of α-Al2O3@amorphous alumina nanocomposite core/shell structure

In this study, α-Al₂O₃@amorphous alumina nanocomposite core-shell structure was synthesized from AlCl₃ and the commercial α-Al₂O₃ nanoparticles as the starting materials via a wet chemical route. The results indicated that the shell material mainly comprised of ammonium chloride and boehmite phases. Boehmite was transformed to the amorphous and γ-Al₂O₃ phases after the calcination process and the shell material was completely converted to γ-Al₂O₃ at 1000?°C. However, for the α-Al₂O₃@amorphous alumina core-shell nanoparticles were completely converted to α-Al₂O₃ at 1000?°C. It can be concluded that α-Al₂O₃ core particles, as the seed crystalline, help to transforming of γ-Al₂O₃ phase as the shell material directly without forming transitional phases to α-Al₂O₃. The optical polycrystalline alumina was fabricated using spark plasma sintering of α-Al₂O₃@amorphous alumina core-shell nanocomposite. The body sintered has a final density of ～99.8% and the in-line transmittance value is ～80% within the IR range.     

Oxidation-bonded SiC membrane for microfiltration

Porous SiC is a proven viable material for microfiltration membranes, but its application has been limited by high fabrication cost. In this study, the oxidation bonding technique was used for the first time to fabricate SiC microfiltration membrane. The study was divided into two parts: optimization of the slurry used to dip coat the SiC particles over a porous SiC ceramic support and controlling the oxidation behaviour of SiC with respect to temperature. The oxidation behaviour during different thermal treatments was related to pore morphology and ultimately the membrane permeance. By coating the clay-bonded SiC support with oxidation-bonded SiC and sintering the coating at 1100 °C for 1 h, we prepared a defect-free microfiltration membrane with pure-water membrane permeance of >210 L m^?2 h^?1 bar^?1, an average pore size of 93 nm, and a narrow pore-size distribution.     

Growth mechanism of single-crystal α-Al2O3 nanofibers fabricated by electrospinning techniques

Crystal-growth-related microstructures and the length-to-diameter ratio of a single-crystal-type α-Al₂O₃ nanofiber were examined using HR-TEM techniques. The fibers exhibited diameters ranging from 50 to 100 nm and lengths of several tens of micrometers. During thermal treatments, the alumina fiber went through phase transformations similar to boehmite. Therefore, the phase evolution, especially the final θ- to α-Al₂O₃ stage of the phase transformation, may be the determining factor in the microstructural evolution of the nanofibers. HR-TEM techniques were utilized to demonstrate that the single crystals were formed by the coalescence of well-elongated α-Al₂O₃ colonies. The fibers grew in the [1 1 0] or [1 1 2] direction instead of [0 0 1]. A thermodynamic analysis revealed that if the α-Al₂O₃ nanofiber that transformed from θ-Al₂O₃ behaved in a stable manner, there could be a size ratio limit for the length and diameter of each α-Al₂O₃ colony. The smallest potential diameter was calculated to be around 17 nm.     

Densification ability of combustion-derived Al2O3 powders

Nanocrystalline Al₂O₃ powders containing different amounts of MgO (0.1–5.0 mol%) or added boehmite (AlOOH) have been synthesized by combustion synthesis from aluminium nitrate and magnesium nitrate, using urea or sucrose as fuels. The as synthesized alumina powders were deagglomerated, compacted by dry pressing and sintered at 1625 °C for 2 h. For comparison purposes, a commercial high purity α-Al₂O₃ powder (ACC) was also processed following the same route. The sintered materials were characterized for bulk density (BD), apparent porosity (AP), and water absorption (WA) capacity, microstructure using SEM, and XRD phase composition. In comparison to boehmite, the MgO had a considerable effect on the densification behaviour of combustion-synthesized powder.     

Influence of sintering temperature and holding time on the densification, phase transformation, microstructure and properties of hot pressing WC-40 vol.%Al2O3 composites

WC-40 vol.%Al₂O₃ composites were prepared by high energy ball milling followed by hot pressing. The tungsten carbide (WC) and commercial alumina (Al₂O₃) powders composed of amorphous Al₂O₃, boehmite (AlOOH) and χ-Al₂O₃ were used as the starting materials. The phase transformation during sintering, the influence of sintering temperature and holding time on the densification, microstructure, Vickers hardness and fracture toughness and the toughening effects of WC-40 vol.%Al₂O₃ composites were investigated. The results showed that the amorphous Al₂O₃, AlOOH and χ-Al₂O₃ were transformed to α-Al₂O₃ completely during the sintering process. With the increasing sintering temperature and holding time, the relative density increased and both the Vickers hardness and fracture toughness increased initially to the maximum values and then decreased. When the as milled powders were hot pressed at 1540 °C for 90 min, a relative density of 97.98% and a maximum hardness of 18.65 GPa with an excellent fracture toughness of 10.43 MPa m^1/2 of WC-40 vol.%Al₂O₃ composites were obtained.     

Catalytic dehydration of methanol to dimethyl ether over modified γ-Al2O3 catalyst

A γ-Al₂O₃ catalyst was modified with metal oxide (Nb₂O₅) in order to improve its activity and stability for dimethyl ether (DME) synthesis from methanol. A series of modified γ-Al₂O₃ catalysts were prepared with Nb₂O₅ and characterized by X-ray diffraction and temperature programmed desorption of ammonia. Results showed that the γ-Al₂O₃ catalyst containing 10 wt.% of Nb₂O₅ exhibited the highest surface area among the modified ones. The number of acid sites of the modified catalysts was increased by the Nb₂O₅ modification. In the chemical reaction of DME synthesis, it was found that the Nb₂O₅ modified γ-Al₂O₃ catalyst exhibited a higher activity in the low temperature region (240 °C-260 °C) and a higher activity than did the untreated γ-Al₂O₃ catalyst.     

Microstructure and optical properties of alumina sintered from various phases

Self-synthesized and commercial alumina (boehmite, γ-Al₂O₃, α-Al₂O₃) powders were consolidated using an identical spark plasma sintering cycle, and optically translucent samples were obtained. The benefit of higher pressure is remarkable grain growth suppression. Additionally, the shorter dwell time at higher pressure advantageously leads to a reduction in grain size while conserving the optical transparency and without affecting the density.     

Beneficial effect of adding γ-AlOOH to the γ-Al2O3 washcoat of a PdO catalyst for methane oxidation

The beneficial effect of adding γ-AlOOH to the γ-Al₂O₃ washcoat of a ceramic cordierite (2MgO · 2Al₂O₃ · 5SiO₂) monolith, used to support a PdO catalyst, is reported for methane oxidation in the presence of water at low temperature (<500°C). The mini-monolith (400 cells per square inch (CPI), 1 cm diameter × 2.54 cm length; ~52 cells) was washcoated using a suspension of γ-Al₂O₃ plus boehmite (γ-AlOOH), followed by calcination and then deposition of Pd by wet impregnation. An optimum solid content of 25 wt% in the washcoat suspension was used to obtain a ~25 wt% washcoat on the monolith. The presence of γ-AlOOH enhanced the thermal and mechanical stability of the washcoat, provided that the γ-AlOOH content was <8 wt%. Temperature-programmed methane oxidation (TPO) showed that the addition of γ-AlOOH to the γ-Al₂O₃ washcoat decreased the catalyst activity. However, when H₂O (2 vol% and 5 vol%) was present in the feed gas, the γ-AlOOH improved the catalyst activity and stability. A γ-AlOOH content of ~5 wt% in the washcoat was determined to provide the highest catalyst activity and stability for CH₄ oxidation in the presence of water.     

Enhanced performance of a macroporous ceramic support for nanofiltration by using α-Al2O3 with narrow size distribution

Enhanced performance of a macroporous disk alumina support was fabricated through colloidal filtration route, by using α-Al₂O₃ powder with an average particle size of 1.1 μm. The support, sintered at 1250 °C, showed relative high permeances towards water (101 L h⁻¹ m⁻² bar⁻¹) and nitrogen (∼2×10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹), with an average surface roughness of ∼175 nm and a high mechanical strength of 61.1 MPa. Titania supported γ-Al₂O₃ mesoporous layers were deposited onto this promising disk α-Al₂O₃ support through dip-coating. The disk membrane A1100/TiO₂/γ-Al₂O₃, with pore size of ca. 4.4 nm, showed a pure water flux as high as 4.5 L m⁻² h⁻¹ bar⁻¹, which is four times higher than that of γ-Al₂O₃ membrane reported in literature. This mesoporous membrane showed relative high retention rate (∼80%) towards di-valent cations like Ca²⁺, Mg²⁺, but not for the mono-valent cation (Na⁺).     

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.