Synthesis of γ-Al2O3 nanowires through a boehmite precursor route

By regulating the pH values of the reaction solution, the boehmite (γ-AlOOH) nanowires and nanoflakes were successfully synthesized with a simple hydrothermal route using anhydrous AlCl₃, NaOH and NH₃·H₂O as starting materials. Crystalline γ-Al₂O₃ nanowires with diameter of 10–30 nm and length of several hundreds of nanometer have been prepared by thermal decomposition of γ-AlOOH precursor. X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), and high resolution transmission electron microscope (HRTEM) were used to characterize morphology and structure feathers of the synthesized samples. The pH values of the solution play important roles in the formation of γ-AlOOH nanowires. After calcinated at 500 °C for 2 h, the obtained γ-Al₂O₃ with a linear structure is similar to the γ-AlOOH precursor.     

Novel synthesis and characterization of nanosized γ-Al2O3 from kaolin

This paper reports the synthesis of nanosized γ-Al₂O₃ from acid-leachates of calcined kaolin. Al (hydr)oxide was precipitated with ammonia from the leachate in the presence of polyethylene glycol. A white powder of nanosized γ-Al₂O₃ particles was obtained after calcination. X-ray diffraction (XRD), different scanning calorimetries and thermogravimetry (DSC-TG), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and magic angle spinning nuclear magnetic resonance (MAS NMR) were used to characterize the samples. Typical nanosized γ-Al₂O₃ particles showed rod-like morphology with width of about 7 nm and length of approximately 20 nm. A possible mechanism from kaolin to nanosized γ-Al₂O₃ is proposed.     

Sintering of α-Al2O3-seeded nanocrystalline γ-Al2O3 powders

This paper demonstrates that seeding nanocrystalline transition alumina powders is a viable option for producing high quality, alumina-based ceramics. By using α-Al₂O₃ concentrations of ⩾1.25 wt.% α-Al₂O₃ seed particles (equivalent to 5 ×10¹⁴ seeds/cm³ of γ-Al₂O₃) the sintering temperature is reduced from 1600°C for unseeded γ-Al₂O₃ to 1300–1400°C in dry pressed powders. The scale of the sintered microstructure is related to N_v^−1/3 and thus a 100-nm grain size is obtained. It is apparent that seeding is necessary for producing dense, alumina-based ceramics from nanocrystalline transition alumina powders.     

Microwave-assisted synthesis of nanosized α-Al2O3

Nanosized alumina powder was prepared from a mixture of aluminium nitrate and carbon black through microwave heating (2.45 GHz and 900 W) for different times. The products were characterized by powder X-ray diffraction. The results showed that γ-Al₂O₃ was the main phase for powder samples heated for 4 and 6 min. When heating was extended to 8 min, weak peaks of α-Al₂O₃ also appeared. For heating times longer than 10 min, α-Al₂O₃ was the only crystalline phase present. The resultant particles were observed by SEM and TEM methods. The average particle size was found to be 96 nm. The surface area of powder was 48 m²/g after 15 min heating.     

One-pot synthesis of α-Fe2O3 nanospheres by solvothermal method

We have successfully prepared α-Fe₂O₃ nanospheres by solvothermal method using 2-butanone and water mixture solvent for the first time, which were about 100 nm in diameter and composed of very small nanoparticles. The as-prepared samples were characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results showed that the product was α-Fe₂O₃ nanosphere, and the temperature was an important factor on the formation of α-Fe₂O₃ nanospheres.     

Triton X-100 directed synthesis of mesoporous γ-Al2O3 from coal-series kaolin

Mesoporous γ-Al₂O₃ has been successfully synthesized by using calcined coal-series kaolin as raw material and Triton X-100 (TX-100) as template. The effect of TX-100/Al^3 + ratio on the structural and textural properties of mesoporous γ-Al₂O₃ was investigated. Physical properties of obtained samples were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, transmission electron microscopy (TEM), thermogravimetric analysis (TG), scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDAX) and Fourier transform infrared spectroscopy (FTIR). The results indicated that the amount of TX-100 influenced the structure and porous properties of mesoporous γ-Al₂O₃ significantly. When TX-100/Al^3 + ratio was in the range of 0.03–0.15, all samples had mesoporous structures with BET surface area of 193.0–261.0 m²/g and pore size of 5.04–6.71 nm. In addition, the reaction mechanism involved in the process was proposed and discussed.     

Catalytic conversion of CCl2F2 on a γ-Al2O3 catalyst

Reaction of hydrogen with CCl₂F₂ has been studied on a -Al₂O₃ catalyst at 300°C. Pure -Al₂O₃ appears to be unselective in the catalytic conversion of CCl₂F₂, however, halogenated alumina showed high activity in a reaction leading to the formation of disproportionation products.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

Mechanochemical-molten salt synthesis of α-Al2O3 platelets

The polyaluminium chloride (PACl) precursor was used for a simple and scaled-up mechanochemical-molten salt synthesis of α-Al₂O₃ platelets. PACl, as a low temperature α-Al₂O₃ precursor, was firstly mechanically activated by high-energy ball milling for 5 min, followed by a next 5 min ball milling in the presence of a NaCl–KCl salt mixture. The starting formation temperature of the α-Al₂O₃ phase was 600 °C. In the subsequent annealing in the temperature range of 660–1000 °C, the α-Al₂O₃ phase with a well developed plate-like morphology was obtained. The products were characterized by X-ray powder diffractometry, scanning electron microscopy (SEM), and thermal analysis (DTA, TG) and solution ²⁷Al NMR spectroscopy.     

Mechanochemical synthesis of α-Al2O3-Cr3+ (Ruby) and χ-Al2O3

In this work we present a unique method to synthesize χ-Al₂O₃ and α-Al₂O₃ doped with Cr³⁺ (ruby). The ruby is synthesized by mechanical milling of pseudoboehmite that is doped in-situ with chromium. The doping is carried by adding chromium sulfate hydrate to an aqueous solution rich in aluminum sulfate hydrate. The pH in the solution is controlled to be between 9 and 10 by using ammonia, which induces the pseudoboehmite precipitation. The Cr³⁺ is added for its remarkable effects on the detectability of ruby emitting luminescent R₁ and R₂ bands that are traceable in Raman spectroscopy. The formation of ruby is detected at milling times as short as 5 hours and increased with the milling time. Ruby phase is further confirmed by means of true atomic resolution Transmission Electron Microscopy (TEM).     

Hydrothermal synthesis of hierarchical micron flower-like γ-AlOOH and γ-Al2O3 superstructures from oil shale ash

The hierarchical micron flower-like γ-AlOOH and γ-Al₂O₃ superstructures were synthesized using oil shale ash (OSA) as a new raw material. In order to prepare the better quality alumina with uniform morphology, the effects of ethanol and surfactant CTAB on the formation of alumina particles were investigated and the formation mechanism of flower-like composite particles was also discussed. The structural and morphological properties of the uncalcined and calcined powders were characterized by X-ray diffractometer (XRD), Scanning electron microscope (SEM), Brunauer Emmett Teller (BET) and Fourier transform infrared (FT-IR). The results indicated that uniform flower-like γ-AlOOH and γ-Al₂O₃ assembled by nanosheets which thickness is less than 100 nm were prepared. After calcined at 600 °C, morphology of the flower-like superstructure was maintained perfectly. The results obtained in this work prove that the oil shale ash can be used for production of hierarchical superstructures alumina and open a new way for comprehensive application of OSA.     

Preparation and characterization of mesoporous γ-Al2O3 membranes

以粒径为0.3～0.4 μm的α-Al₂O₃为原料,通过悬浮液真空抽吸法,制备出平均孔径约为70 nm的完整无缺陷的片状α-Al₂O₃支撑体;以仲丁醇铝为前驱体,采用颗粒溶胶路线制备出稳定的Boehmite溶胶,以此溶胶采用浸浆法,在制备的α-Al₂O₃支撑体上制备出完整无缺陷的γ-Al₂O₃中孔膜,并考察了烧成温度对γ-Al₂O₃中孔膜性能的影响。结果表明,本文制备出的γ-Al₂ O₃膜的孔径约为3 nm,对PEG的截留分子量为2800～5300,纯水渗透通量为11.5～25.9 L.m^-2.h^-1［7.6×10⁵ Pa,（14±1）℃］。说明在孔径为70 nm左右的载体上直接制备孔径为3 nm的完整的中孔膜是可行的。     

Vacancy ordering in the structure of γ-Al2O3

Defect structure of γ-Al₂O₃ prepared by the thermal decomposition of well-crystallized, high purity boehmite (γ-AlOOH) has been studied by HREM. It was shown that the intrinsic feature of γ-alumina structure is a presence of almost hexagonal closed loops formed due to the ordering of cation vacancies over octahedral positions on (110) and (111) planes. These defects are relatively stable; they are preserved, though being changed in shape, in the γ-alumina sample upon its further calcination until the appearance of traces of δ-alumina phase.     

Solution combustion synthesis of α-Al2O3 using urea

The processes involved in the solution combustion synthesis of α-Al₂O₃ using urea as an organic fuel were investigated. The data describing the influence of the relative urea content on the characteristic features of the combustion process, the crystalline structure and the morphology of the aluminium oxide are presented herein. Our data demonstrate that the combustion of stable aluminium nitrate and urea complexes leads to the formation of α-alumina at temperatures of approximately 600–800 °C. Our results, obtained using differential thermal analysis and IR spectroscopy methods, reveal that the low-temperature formation of α-alumina is associated with the thermal decomposition of an α-AlO(OH) intermediate, which was crystallised in the crystal structure of the diaspore.     

Novel synthesis of advanced composites of α-Al2O3 reinforced with Ce TZP through co-precipitation process

Abstract

The presence of zirconia grains in alumina matrix as a discrete second phase plays an important role in the development of high performance ceramics subjected to intensive thermal as well as mechanical stress. The recent development on transformation toughening by addition of CeO₂, Y₂O₃, MgO or CaO to zirconia in alumina matrixes offers a new generation ceramics with improved mechanical properties. The stabilisation of tetragonal or metastable tetragonal phases much depends on the type of additives, mole concentration and the processing route adopted. The present paper describes the synthesis of ultra fine powders of CeO₂ based tetragonal or metastable tetragonal zirconia polycrystals for developing suitable toughening characteristics of advanced ceramic composites of α-Al₂O₃ matrixes with particular concentration of Ce TZP by wet chemical simultaneous precipitation process. The degree of stabilisation of different percentages by mole varying from 6 to 12 mol.-%CeO₂ based zirconia polycrystal ceramics and advanced composites of α-Al₂O₃ matrixes with 10 wt-% Ce stabilised tetragonal zirconia polycrystals (Ce TZP) was studied through thermal and X-ray diffraction (XRD) analysis. The powder morphology, sintering behaviour, microstructure characteristic and mechanical properties such as hardness and fracture toughness of composites were also evaluated.     

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.     

SO2 Reactions over γ-Al2O3,Pt/γ-Al2O3,Sn/γ-Al2O3 and Pt–Sn/γ-Al2O3

Platinum and Platinum–tin bimetallic catalysts supported on alumina were prepared by co-impregnation of both metallic precursors on the support and used as catalysts for the oxidation of SO₂. Platinum dispersion was determined by means of H₂–O₂ titration. Tin addition (1 and 2 wt%) only slightly decreased the exposed platinum atoms suggesting that tin is mainly over the support. At temperatures lower than 300 °C, SO₂ did not react with oxygen. Nevertheless, when the temperature was increased, the SO₂ oxidation began. The ignition temperatures for SO₂ oxidation (taken at 50% conversion) were 345 °C for 1% Pt/Al₂O₃ and 520 °C for 1% Pt–2% Sn/Al₂O₃. The strong displacement on activity suggests that tin plays an important role as inhibitor of the SO₂ oxidation reaction.     

Thermal-assisted cold sintering study of Al2O3 ceramics: Enabled with a soluble γ-Al2O3 intermediate phase

Thermal-assisted cold sintering process (TA-CSP) has been applied to fabricate high dense α-Al₂O₃ ceramics with submicron grain sizes. The α-Al₂O₃ (80 wt%) and γ-Al₂O₃ (20 wt%) powders are firstly mixed and then cold sintered at 300 °C to produce a green bulk with a relatively high density of ～ 86.9 %, and then later a second heat treatment (800–1350 °C) is applied to finally fabricate (～ 98 % dense) α-Al₂O₃ ceramics with grain sizes of 720 nm. A microstructural analysis with XRD and TEM suggests that the TA-CSP samples not only complete the final densification but also drive a phase transition of γ-Al₂O₃ to α-Al₂O₃. To put into perspective the Hardness and Young's modulus of TA-CSP samples reach ～ 14 GPa and ～ 335 GPa, respectively, which is comparable to conventional sintered samples processed at higher temperatures of 1500–1700 °C. Therefore, it is feasible to utilize TA-CSP to prepare α-Al₂O₃ ceramics with small grain sizes at low sintering temperatures.     

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.     

Physicochemical Properties of Supported γ-Al2O3 and TiO2 Ceramic Membranes

Abstract

Supported γ-Al₂O₃ and TiO₂ ceramic membranes were prepared by sol-gel techniques from alkoxide precursors. Tests were conducted to measure the permeabilities of these membranes to solvent under a variety of operating conditions. Variables studied were feed temperature, length of time on stream, and feed pH. The stabilities of the membranes in harsh chemical environments were also determined. An alternative method for preparing supported ceramic membranes is also suggested.