Sintering kinetics of disperse ultrafine equiaxed α-Al2O3 nanoparticles

The sintering kinetics of ceramic nanoparticles is essential for preparing dense nanocrystalline ceramics with fine grains, but the sintering kinetics of disperse ultrafine α-Al₂O₃ nanoparticles has not been systematically explored so far. In this paper, the sintering kinetics of disperse ultrafine equiaxed α-Al₂O₃ nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without any agglomeration was studied systematically. Finally, α-Al₂O₃ nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered in air by two-step sintering (heated to 1100 °C without hold and then cooled down to 950 °C with a 40 h hold). The sintering temperature is the lowest for pressureless sintering of α-Al₂O₃ and almost fully dense α-Al₂O₃ nanocrystalline ceramic obtained also has the finest grain so far.     

Sintering kinetics and microstructure evolution in α-Al2O3 nanocrystalline ceramics: Insensitive to Fe impurity

Diffusion kinetics, processing, and microstructures of alumina ceramics have been of great scientific and technological interests. Here we reported the sintering kinetics and microstructure evolution in high-purity dispersed ultrafine α-Al₂O₃ nanoparticles with superior sinterability. An extremely fine grain size of 34 nm with 99.6 % theoretical density and a uniform microstructure can be produced by two-step pressureless sintering. Systematic kinetic analysis further points out to a surprising insensitivity of α-Al₂O₃ nanocrystalline ceramics to Fe and other impurities, which contrasts with prior reports in the literature. We propose this insensitivity is originated from extensive grain boundaries in nanocrystalline ceramics, which allows segregation of impurities and lowers their concentrations at grain boundaries.     

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.     

Pixelated sintering of α-Al2O3

The sintering of α-alumina by a brand new and innovative technique, called pixelated sintering (PS), is here studied. Densification and grain growth by PS of perfectly controlled granular compacts are analysed and compared to results obtained using Spark Plasma Sintering (SPS) and Pressure-Less SPS (PL-SPS). Materials are exposed to the same temperature profiles whatever the sintering technique used in order to assess the potential of PS in terms of microstructure control. It is shown that PS can be used as an alternative technique to SPS for fast sintering with the advantages of a much simpler and cost-effective set-up, as well as a better control of the localised heat input. PS also appears to be a very modular technology in the way it controls the temperature gradients allowing its implementation for multi-step sintering approaches, as well as for the fabrication of large and complex parts.     

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.     

Preparation of highly active nickel catalysts supported on mesoporous nanocrystalline γ-Al2O3 for CO2 methanation

In this study, a group of Ni catalysts supported on mesoporous nanocrystalline γ-Al₂O₃ with high surface area and with different Ni contents was prepared and employed in carbon dioxide methanation reaction. The obtained results showed increasing in nickel content from 10 to 25 wt.% decreased the specific surface area of catalysts from 177 to 130 m²/g and increased the nickel crystallite size from 12 to 24 nm. In addition, increasing in nickel content increased the reducibility of catalyst. The catalytic results revealed that the catalyst with 20 wt.% of Ni possessed high activity and stability in CO₂ methanation reaction.     

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.     

Phase metastability of nanosized α-Al2O3 crystallites

The reversal of the α- to θ-Al₂O₃ phase transformation and the induced microstructure evolution of boehmite-derived discrete nanosized α-crystallites are examined. Three categories of α-crystallites smaller than 100 nm were examined and found to have similar behavior: (1) pre-existing α-crystallites, (2) α-crystallites formed in situ during the calcination of θ-crystallites of sizes near the critical size, 25 nm, and (3) α-crystallites formed in situ by the thermal treatment of as-received θ-crystallites. The α-crystallite may transform back to the θ-phase above 800 °C. The backwards θ-crystallite may also re-transform to the α-phase again. Because of the density difference between α- and θ-Al₂O₃, the strain involved in the volume expansion and shrinkage during the phase transition eventually results in the formation of a twinned and/or mosaic structure for the θ- and α-crystallites. A strain release model representing the microstructure evolution of the α- to θ-phase and the θ- to α-Al₂O₃ phase transformation is proposed.     

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.     

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。     

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).     

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.     

Determination of the shape,size and porosity of fine α-Al2O3 powders prepared by emulsion evaporation

Stable water in oil emulsions were prepared from Al(NO₃)₃ solutions of different molarities. Dark brown precursors were obtained from these emulsions which were evaporated by adding them dropwise into hot mineral oil. The precursors containing diaspore as raw alumina were calcined at 1000 °C and fine α-Al₂O₃ powders were obtained. The photographs of the powders were taken by the use of an electron microscope and their particle size distributions were determined. The shapes and sizes of the powders were discussed. Their mesopore size distributions were obtained from the data of the desorption of nitrogen at 77 K. The relation between the obtained results and the concentrations of the Al(NO₃)₃ solutions were discussed.     

Microstructure and thermoelectric properties of α-and γ-Al2O3 doped ZnO under high pressure

Using high pressure and high temperature (HPHT) technology to improve the thermoelectric properties of oxides is a feasible solution. To further understand the micro-physical mechanism improving the thermoelectric performance of ZnO in a higher pressure environment, the effects of α and γ lattice structure Al₂O₃ (α-Al₂O₃, γ-Al₂O₃) on doped ZnO were systematically studied. ZnO samples with different Al doping ratios (α-x, γ-x; x = 0.02, 0.04, 0.06, 0.08) were prepared by HPHT. Test results show that the high pressure and high temperature synthesis method effectively improves the solid solubility of α-Al₂O₃ and γ-Al₂O₃ in ZnO, among which γ-Al₂O₃ is more easily dissolved into the zinc oxide crystal lattice. Under the same doping ratio, the electronic conductivity of the sample synthesized with γ-Al₂O₃ is higher than that of the sample synthesized with α-Al₂O₃. In addition, the ZnAl₂O₄ phase precipitated out of the sample with increased Al doping. Although the ZnAl₂O₄ phase decreases the electrical properties of the sample, a small amount of ZnAl₂O₄ is uniformly dispersed in the ZnO sample, which can inhibit the growth of crystal grains, and assist grain refinement. The optimized high pressure sintering temperature was 1123 K, and the zT value of γ-0.04 was 0.17 at 973 K.     

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.     

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.     

Comparative investigations of the catalytic properties of an anodic Al2O3-coated catalyst and of α- and γ-Al2O3 bulk catalysts

The catalytic properties of an Al₂O₃-coated catalyst formed by anodic oxidation of aluminium were compared with those of α- and of γ-Al₂O₃- bulk catalysts in the dehydration of 2-propanol and in the isomerization of n-butenes. In the dehydration reaction both the Al₂O₃-coated catalyst and the γ-Al₂O₃ bulk catalyst show approx. the same activities and activation energies. However, the reaction rate based on the unit of the surface area for the Al₂O₃-coated catalyst is approx. 30 times larger than that for the γ-A1₂O₃ bulk catalyst. In the isomerization of the individual isomeric n-butenes the Al₂O₃-coated catalyst was more active than the γ-Al₂O₃, bulk catalyst and under otherwise equal conditions equilibrium was reached at approx. 80 K lower temperature.     

Role of B2O3 in formation of mullite from kaolinite and α-Al2O3 mixtures

Abstract

The microstructure, phase constitution, and physical properties of mullite bodies prepared from α-Al₂O₃- kaolin mixtures with added B₂O₃ were investigated. Densification was found to be enhanced with small additions of B₂O₃. The results indicate that 0.5 wt-% B₂O₃ increases the content and growth rate of the mullite. It was found to be the optimum addition with respect to densification and resulting properties.     

Preparation of equiaxed α-Al2O3 by adding oxalic acid

The preparation of α-Al₂O₃ powders with equiaxed architecture for the fabrication of advanced ceramics is of great importance but still challenging. A new and facile approach for the fabrication of equiaxed α-Al₂O₃ adopting alumina hydroxide and oxalic acid as the raw materials was reported in this paper for the first time. The current work demonstrated that the adding 0.16 mol/L oxalic acid solution made α-Al₂O₃ heterogeneously nucleate at a temperature as low as 800 °C, and the amount of nucleation was high enough to remove the vermicular microstructures during the morphology evolution of α-Al₂O₃, resulting in the formation of equiaxed α-Al₂O₃ particles with an average size of 205.72 nm at 1300 °C.