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.     

Effect of carbon and α-Al2O3 on controlled synthesis of AlON powder

The α-Al₂O₃/C mixtures were prepared by ball milling, and then AlON powders were synthesized by carbothermal reduction and nitridation (CRN). The effects of α-Al₂O₃ particle size, carbon powders morphology and particle size on the morphology and particle size of the synthesized AlON powders were studied. The results showed that as the particle size of α-Al₂O₃ increases, the particle size of the synthesized AlON powders will also increase, but the surface morphology will not be affected. The increase of the carbon particle size will increase the particle size of the synthesized AlON powders. The pore size and number of pores of the synthetic AlON powders were very similar to the morphological characteristics of the carbon powders. In addition, the mechanism of controlling the synthesis of AlON powder with α-Al₂O₃ and carbon as raw materials was also discussed.     

The spheroidization process of micron-scaled α-Al2O3 powder in hydrothermal method

Understanding the spheroidization process of micron-scaled α-Al₂O₃ powder in hydrothermal method is of great importance but still not completely revealed. The results demonstrated that SO₄^2? played a significant role in the formation of spherical powder, while the bubble generated from the reaction of urea didn't work in the spheroidization process. The spheroidization process was summed up as two steps. The first was that SO₄^2? limited the hydrolysis of Al³⁺ and reacted with Al³⁺ and OH^- to form Al₄(OH)₁₀SO₄, which nucleated and agglomerated into granular precipitates. The second was Ostwald ripening, which gave the spherical precursors a double-layered structure. When the spherical precursors obtained 120 °C were sintered at 1200 °C, α-Al₂O₃ were got and the spherical morphology still maintained with a large number of nano-sized pores. We anticipate the spherical α-Al₂O₃ with nano-sized pores can be applied in adsorption and filtration industries.     

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.     

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.     

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.     

Low-temperature synthesis of α-Al2O3 powder aided by ball milling and a trace amount of sodium chloride

It is crucial to synthesize α-Al₂O₃, which is a multifunctional material, at high temperatures due to the nature of the material. However, this high-temperature preparation process is not energy efficient, which goes against the global aim of carbon neutrality. In this study, we explored the effect of adding 1 wt.% (NaCl) as an additive and a suitable amount of Al(OH)₃ in a ball mill to form a precursor. The impact of NaCl and ball milling duration on the phase transition from Al(OH)₃ to α-Al₂O₃ at low temperatures was investigated. After the conversion from Al(OH)₃ to γ-Al₂O₃, the NaCl particles on the surface of γ-Al₂O₃ act as diffusion channels, helping to accelerate the substance diffusion during the transition from γ-Al₂O₃ to α-Al₂O₃, resulting in a lower formation temperature of α-Al₂O₃, which is 700°C. Additionally, the presence of NaCl results in α-Al₂O₃ particles growing into hexagonal plates. The α-Al₂O₃ plates produced from calcining the mixture of Al(OH)₃ and NaCl at 700°C have an average diameter of 3 µm, an average diameter/thickness ratio of 10 and a specific surface area of 12.0085 m²/g. These results indicate that low-temperature synthesis of α-Al₂O₃ powder is possible by using NaCl as an additive instead of molten salt in the ball milling process.     

Preparation and evaluation of α-Al2O3 supported lithium ion sieve membranes for Li+ extraction

Spinel lithium manganese oxide ion-sieves have been considered the most promising adsorbents to extract Li⁺ from brines and sea water. Here, we report a lithium ion-sieve which was successfully loaded onto tubular α-Al₂O₃ ceramic substrates by dipping crystallization and post-calcination method. The lithium manganese oxide Li₄Mn₅O₁₂ was first synthesized onto tubular α-Al₂O₃ ceramic substrates as the ion-sieve precursor (i.e. L-AA), and the corresponding lithium ion-sieve (i.e. H-AA) was obtained after acid pickling. The chemical and morphological properties of the ion-sieve were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Both L-AA and H-AA showed characteristic peaks of α-Al₂O₃ and cubic phase Li₄Mn₅O₁₂, and the peaks representing cubic phase could still exist after pickling. The lithium manganese oxide Li₄Mn₅O₁₂ could be uniformly loaded not only on the surface of α-Al₂O₃ substrates but also inside the pores. Moreover, we found that the equilibrium adsorption capacity of H-AA was 22.9 mg·g⁻¹. After 12 h adsorption, the adsorption balance was reached. After 5 cycles of adsorption, the adsorption capacity of H-AA was 60.88% of the initial adsorption capacity. The process of H-AA adsorption for Li⁺ correlated with pseudo-second order kinetic model and Langmuir model. Adsorption thermodynamic parameters regarding enthalpy (∆ H), Gibbs free energy (∆ G) and entropy (∆ S) were calculated. For the dynamic adsorption–desorption process of H-AA, the H-AA exhibited excellent adsorption performance to Li⁺ with the Li⁺ dynamic adsorption capacity of 9.74 mg·g⁻¹ and the Mn²⁺ dissolution loss rate of 0.99%. After 3 dynamic adsorption–desorption cycles, 80% of the initial dynamic adsorption capacity was still kept.     

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.     

Effect of Industrial Alumina Powder Particle Size on Preparation of Active α-Al2O3 Micropowder

To study the effect of the particle size of industrial alumina powder on the properties of active alumina micropowder(α-Al2O3 micropowder),the fine powders with...     

Effect of citrate to nitrate ratio on the sol-gel synthesis of nanosized α-Al2O3 powder

This paper studied the effect of the variation of citrate to nitrate ratio on the sol-gel synthesis of nanocrystalline α-alumina powder. The process involves the formation of a sol using aluminium nitrate and citric acid which then transferred to a transparent gel and finally to a powdery mass. The prepared alumina powder was thoroughly characterised by different techniques such as phase analysis, thermal analysis, microstructure study, etc. Variation in citrate to nitrate molar ratio (C/N) influenced the thermal decomposition behaviour, particle size distribution, phase transformation temperature and specific surface area of the developed powder. The complete formation of α- Al₂O₃ was obtained at around 950 °C for C/N=1.5. The unimodal distribution of the particles within a narrow size range and higher particle to particle linking through solid bridging are the characteristics of the powder prepared at C/N=1.5. It was found higher citrate to nitrate ratio in the fuel rich condition is responsible for better properties of synthesised alumina powder.     

Coke formation on the surface of α-Al2O3 in the catalytic pyrolysis of naphtha

The catalytic pyrolysis of naphtha has been carried out in a quartz reactor loaded with 5 mm α-A1₂0₃ spheres. The yields of ethylene and propylene exhibit about 10% and 5% higher values compared to the thermal pyrolysis in the absence of α-A1₂0₂ spheres at the same reaction conditions. The coke formation on α-A1₂0₃ spheres increased continuously along with the axial length of the reactor as well as with reaction time. Results suggest that the concentration of the coke precursors in the gas phase may increase along with the axial length of the reactor. Coke filled up completely the internal pore of the sphere near the exit of the reactor after reaction for 4 hr. The coke film on the external surface of the sphere grew continuously thicker. The coke formation was influenced strongly by the physical properties of the α-Al₂O₃ spheres. Coke deposition was the least on the α-A1₂0₃ sphere with the lowest surface area and pore volume among the tested α-A1₂0₃ spheres.     

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.     

Fabrication of mesoporous Cr2O3 with highly distributed α-Fe2O3 and Pt nanoparticles for ultrafast H2 evolution rate

In the present contribution, p-n type heterojunction α-Fe₂O₃/Cr₂O₃ S-scheme system photocatalyst has been fabricated utilizing a sol-gel approach with assisted nonionic surfactant for a highly effective H₂ evolution rate under visible illumination. Pt NPs have been reduced by photodeposition during the photocatalytic reaction to collect Pt@α-Fe2O3/Cr2O3 finally. XRD analysis of Fe₂O₃/Cr₂O₃ nanocomposites verified the construction of Fe₂O₃ and Cr₂O₃ with rhombohedral phases. TEM images of Cr₂O₃ NPs were almost spherical and uniform in shape and size (20 ± 5 nm), and very small Fe₂O₃ NPs (3-5 nm) were distributed on the mesoporous Cr₂O₃ networks. The obtained α-Fe₂O₃/Cr₂O₃ photocatalyst exhibited noteworthy photocatalytic H₂ evolution with high efficiency and stability for 45 h. Interestingly, the photocatalytic H₂ evolution rate gradually boosted with the extent of Fe₂O₃ percentage up to 15% and its rate of 2215.4 μmol g^-1h^-1, which was fostered 7.25 folds larger than that of Cr₂O₃ NPs (305.7 μmol g^-1h^-1). The enhancement H₂ evolution rate of Fe₂O₃/Cr₂O₃ photocatalyst in comparison with bare Cr₂O₃ NPs was ascribed to facilitate the separation of photocarriers and existing considerable reactive sites. In addition, constructing n-type Fe₂O₃ and p-type Cr₂O₃ with close contact is essential in improving the H₂ evolution rate. The possible photocatalytic mechanism over Fe₂O₃/Cr₂O₃ nanocomposite was addressed based on electrochemical measurements. The construction of the S-scheme system of Fe₂O₃/Cr₂O₃ nanocomposite could be suggested to improve the separation of photocarriers through optimal transfer channels owing to the formation of synergistic characteristics. Our results provide avenues for constructing stable photocatalysts with high efficiency for H₂ evolution through visible exposure.     

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。     

Investigation of the formation process of highly porous α-Al2O3 via citric acid-assisted sol-gel synthesis

Macroporous α-alumina can be obtained by polymer-induced phase separation during the sol-gel process based on dissolved aluminum salts. Polyethylene oxide (PEO) acts as phase separating agent, propylene oxide as proton scavenger. Calcination at 1200 °C yields the thermodynamically stable α-Al₂O₃. It usually exhibits little to no porosity, with internal surface areas below 5 m²/g. We report on the successful establishment of citric acid as a novel phase separating agent in this process. Macropores ranging from 115 nm up to several μm have been successfully introduced in α-Al₂O₃. Compared to the PEO-route, BET surface areas doubled, going up to 12 m²/g. The materials prepared via the new method also exhibit a significantly different microstructural habitus.¹³C-NMR studies and TG/DTA measurements revealed a complete incorporation of citric acid into the gel network. This implies a change in phase separation mechanism, giving rise to a more particulate structure and hence the formation of larger macropores.     

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).     

Preparation of spherical α-Al2O3 nanoparticles by microwave hydrothermal synthesis and addition of nano-Al seeds

Due to their special appearance, spherical α-Al₂O₃ nanoparticles play an important role for obtaining high-performance structural and functional ceramics. However, there are still problems such as easily agglomerates to form worm-like structures at high temperatures and difficult availability of spherical nanoparticles. In this study, spherical α-Al₂O₃ nanoparticles with high dispersion were prepared by a combination of a microwave hydrothermal method and an addition of nano-Al particles as seeds. First, spherical amorphous alumina precursors were synthesized by the microwave hydrothermal method at 100°C for 30 min using Al₂(SO₄)₃·18H₂O, Al(NO₃)₃·9H₂O, and urea, as raw materials, and then spherical α-Al₂O₃ nanoparticles with a diameter of about 66 nm were acquired after calcined the precursor at 1050°C for 90 min by adding nano-Al seeds, which reduced the calcination temperature by 50°C and holding time by 30 min compared to that without seeds. Kinetic analysis shows that 5 wt.% nano-Al seeds can reduce the activation energy of crystalline transition of γ-Al₂O₃ to α-Al₂O₃ from 516.51 to 474.37 kJ/mol. Moreover, the microscopic mechanism of nano-Al particles as seeds was investigated. The characterizations of sintering properties show that spherical α-Al₂O₃ nanoparticles facilitate the acquisition of uniform microstructure for resulting ceramic and the fracture modes include both intergranular and transgranular fractures.     

Preparation and self-healing performance of spherical α-Al2O3@MoSi2 particles using the precipitation method

α-Al₂O₃ was coated on the powder surface using the precipitation method to improve the pre-oxidation resistance of molybdenum disilicide in thermal barrier coatings (TBCs). The coating effects of four different aluminum sources (Al(NO₃)₃·9H₂O) to MoSi₂ mass ratios were evaluated by viscosity and micro-morphology. The shell-forming effects on the powder size and post-treatment were thoroughly analyzed to meet the requirements for practical application in TBCs. The results showed that the precipitation method was superior to the sol-gel process in terms of the shell thickness obtained. Optimal shell-forming was generated at 1200 °C in an inert atmosphere. A bonding layer (Al₆Si₂O₁₃) would be formed at the core-shell interface, further preventing oxidation penetration. The self-healing particles MoSi₂@α-Al₂O₃ can effectively seal micro-cracks (width below 300 nm) because of the fluidity of SiO₂ at working temperatures.     

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.