Surface modification of α-Al2O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions

Stereolithography of UV-curable ceramic suspensions can benefit from the preparation of stable, low viscosity and high solid loading ceramic suspensions without yield stress. Appropriately adding dispersants could optimize the rheological behavior to meet the requirements of stereolithography. In this work, short-chain dicarboxylic acids were utilized to modify the alumina particles and achieve well dispersed ceramic suspensions. The maximum adsorption capacities of dicarboxylic acids were determined by the method of High Performance Liquid Chromatography and the mechanism of surface modification and dispersion was also discussed. Dicarboxylic acids’ influence on the rheology behavior was systematically studied. When doses of dicarboxylic acids reach their maximum adsorption capacities, the alumina suspensions would achieve their lowest viscosities and yield stresses. 45 vol% alumina suspension with a viscosity ˂2 Pa s at shear rate 30 s⁻¹ was successfully formulated. A sintering density of 96.5% can be achieved for the sebacic acid-modified alumina UV-curable suspension.     

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.     

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.     

A novel approach to process phase pure α-Al2O3 coatings by solution precursor plasma spraying

The present work represents a significant new approach in the field of thermal spraying to deposit nanostructured phase-pure α-Al₂O₃ coatings in a single step. In order to understand the mechanism of coating formation, a detailed investigation of in flight formed particles and splat morphologies has been carried out. A plausible deposition mechanism has been proposed based on the understanding derived from the above studies that can form the basis for developing novel ceramic coatings employing the solution precursor plasma spray technique.     

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.     

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).     

Using modified Pechini method to synthesize α-Al2O3 nanoparticles of high surface area

A modified Pechini method for the preparation of a high surface area α-alumina is proposed. The synthesis of these nanoparticles was carried out using a polymer as a chelating agent. The polymer was prepared from citric acid and acrylic acid by the melt blending method. The resulting α-alumina (98.16%) after calcination at 900 °C consisted of cylindrical nanoparticles of 100–200 nm in length and <25 nm in diameter with a relatively high surface area (18 m² g^?1).     

Preparation of high pure α-Al2O3 nanoparticles at low temperatures using Pechini method

A Pechini process was successfully used to synthesize alpha-alumina (98.95% mass fraction) at relatively low calcination temperature (925 °C). The synthesis of these nanoparticles was carried out using a polymer prepared from citric acid and ethylene glycol by the melt blending method. This polymer worked as a chelating agent for aluminum cations. The final products were produced after a dual-stages thermal treatment. The resulting α-alumina consisted of nanoparticles of 8–16 nm in diameters with a surface area (～8 m² g^?1). The mass fraction of α-alumina was dependent on the concentration of aluminum salt and polymer precursor's solutions, while the surface area of the final product was dependent on the mass fraction of θ-alumina.     

A critical role of pH in the colloidal synthesis and phase transformation of nano size α-Al2O3 with high surface area

This paper demonstrates the critical role of pH in the colloidal (i.e. sol-gel) synthesis of nano size α-Al₂O₃. Investigation, based on X-ray diffraction pattern, indicates that the transformation of α-Al₂O₃ from gels obtained at high pH⩾7, undergoes via formation of boehmite (gel→AlOOH→γ-Al₂O₃→α-Al₂O₃). On the other hand, the transformation of α-Al₂O₃ at low pH (⩽6) follows the gel→γ-Al₂O₃ →α-Al₂O₃ sequence. The transmission electron microscopy reveals that the particle size of the α-Al₂O₃ decreases from ≈750 to ≈70 nm with decreasing pH from 12 to 2.5, respectively. The α-Al₂O₃ with large surface area (≈130 m²/g) is obtained when it is processed at low pH (2.5). The true powder density of the α-Al₂O₃ sample derived at pH=2.5 is observed to be 3.92 g/cm³ after sintering at 1450 °C, due to fine particles. The isoelectric point (iep) and ξ of the α-Al₂O₃ are found to be 8.7 and 9.2 when synthesized at pH 2.5 and 12, respectively.     

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.     

A Novel α-Al2O3 Diesel Particulate Filter with Alkali Metal-Based Catalyst for Diesel Soot Oxidation

A diesel particulate filter (DPF) substrate was fabricated from alkali-resistant α-Al₂O₃ to make a practical use of alkali metal catalysts for diesel soot oxidation. The fundamental properties of the α-Al₂O₃-DPF, including its particle filtration efficiency, pressure drop, and thermal durability were comparable to those of the conventional DPFs. The diesel soot oxidation activity of the catalyzed α-Al₂O₃-DPF with a washcoat of alkali metal-based catalyst was, even after thermal aging, much higher than that of the conventional catalyzed-DPF with platinum group metal catalysts.     

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.     

Novel Ti2O3–Al2O3 raw materials with controllable Ti2O3 content from the aluminothermic reduction of ilmenite

Titanium-containing refractories have excellent performance, but the high cost limits their application. A series of Ti₂O₃–Al₂O₃ raw materials with different Ti₂O₃ contents were prepared by aluminothermic reduction of ilmenite, while the generated ferrotitanium alloy can be used as raw materials for special steels. Regulating the amount of aluminum added in the system regulates the degree of titanium reduction, and the formed ferrotitanium alloy can achieve separation from the oxides. With decreasing aluminum content of, the Ti₂O₃ content increased, and the continuous distribution of the corundum area decreased, resulting in a continuously distributed Ti₂O₃ area. Our results indicated that the molar ratio of aluminum to ilmenite should be higher than 1.4 to achieve slag iron separation. The reaction model for the aluminothermic reduction was established, and the formation mechanism of Al₂O₃ and Ti₂O₃ in the system was 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.     

Catalytic Oxygen Removal from Synthetic Coke Oven Gas: A Comparison of Sulfided CoMo/γ-Al2O3 and NiMo/γ-Al2O3 Catalysts with Pt/γ-Al2O3 as Benchmark Catalyst

For the oxygen removal from coke oven gas (COG) the catalytic activity of commercial catalysts CoMo/γ-Al₂O₃ and NiMo/γ-Al₂O₃ was evaluated after a sulfidation pretreatment and compared to the Pt/γ-Al₂O₃ reference catalyst. Elemental analysis and temperature-programmed desorption showed that the oxidation reaction and the associated oxidation of active sulfidic centers is the main cause of deactivation despite the presence of other reductants, such as hydrogen. This approach could allow an appropriate sulfide catalyst to be designed for oxygen removal corresponding to the typical COG composition in the presence of H₂S.     

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.     

液相法制备微晶 α-Al2O3的研究进展

综述了水热法、溶胶凝胶法、沉淀法及微乳液法制备微晶α-Al2 O3的研究成果,重点介绍了各个方法中影响微晶α-Al2 O3尺寸和形貌的影响因素,并对其影响机理做了简要分析.评述了各个制备方法的优缺点,其中溶胶凝胶法和沉淀法是工业上最常使用的方法,最后对微晶α-Al2 O3制备方法的发展前景进行了展望.     

ZrO2-Y2O3-Al2O3系陶瓷中α-Al2O3纤维的形成

查明,采用在高频率放电的等离子中使盐溶液脱硝基盐的方法制取的由80%(按质量计)ZrO_2〔3% Y_2O_3(克分子)〕 20%(按质量计)Al_2O_3组成的相当不均衡的粉料,在加热处理和随后烧结过程中于ZrO_2质陶瓷材料的组织结构中形成了不同方向的长度为数微米的氧化铝纤维。     

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.