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.     

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.     

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.     

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.     

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.     

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.     

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.     

Synthesis and characterization of LaPO4-coated α-Al2O3 powders

LaPO₄-coated α-Al₂O₃ powders were successfully synthesized through the heterogeneous precipitation method. The coated powders were characterized by XRD, TEM, EDS, and zeta-potential measurements. According to the XRD results, the coated powders consisted of only α-Al₂O₃ and monoclinic LaPO₄. The TEM examinations and EDS analysis showed that under alkaline condition a homogeneous coating with thickness of ～10 nm was obtained on the α-Al₂O₃ powder surface. The zeta-potential measurements showed the coated powders to have a pH value of 7.85 at isoelectric point, similar to that of LaPO₄ (pH value of 8.21). This was in good agreement with the TEM and EDS results, indicating a successful coating process in our experiment.     

Atomistic simulation of α-Al2O3 nanoparticle plastic anisotropy under compression

Alumina (α-Al₂O₃) is one of the major ceramic oxides commonly used for its advanced mechanical properties. Since recently, nanoscale α-Al₂O₃ requires more in-depth characterization related to ceramic powder compaction and sintering as well as for applications in the field of biomedical engineering. In this study, we use classical molecular dynamics simulations with a 2/3-body interatomic potential to investigate the mechanical response and the elementary deformation processes of α-Al₂O₃ nanoparticles under compression. Results show that α-Al₂O₃ nanoparticles resist up to particularly elevated critical force before yielding due to a surface nucleation process including various kinds of dislocations and nanotwins strongly sensitive to orientation and temperature. Results are rationalized in terms of stacking-fault energy and shear stress analysis and are discussed in the light of recent micromechanical tests as well as more fundamental observations performed in the bulk material.     

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.     

Ultrafine α-Al2O3. Explosive Method of Synthesis and Properties

An explosive method for producing ultrafine -Al₂O₃ is developed and optimal synthesis parameters are determined. Particles of ultrafine -Al₂O₃ have a spherical shape and are separated from one another. The size distribution is log-normal (number-averaged size 70 nm and variance 1.9). Special features of phase transitions in ultrafine aluminum oxide under shock-wave action are studied. Results of x-ray phase analysis suggest stabilization of the new high-pressure phase -Al₂O₃ with a face-centered cubic lattice with a parameter a = 8.53 Key words: metastability, corundum, shock-wave synthesis, surface, modification.     

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.     

Effects of doping cations on the densification and microstructure of flash-sintered α-Al2O3

The effects of dopants with different valences on the flash sintering behavior of α-Al₂O₃ are investigated. The results indicate that regardless of their valence and ionic radius, all the tested dopants reduce the onset temperature more effectively compared to undoped Al₂O₃. Interestingly, the relative density and grain size of the flash-sintered samples exhibit an inverse linear relationship. The data for the samples doped with non-trivalent cations fall on a different line than those for the samples doped with trivalent cations and the undoped samples. Dopants have an effect on the flash sintering behavior of Al₂O₃ because flash sintering increases the solubility of the dopants in alumina, creating more point defects, and thereby increasing the electrical conductivity of the material. The mechanisms for point defect generation in trivalent and non-trivalent dopants are different.     

Ultrafast high-temperature sintering (UHS) of fine grained α-Al2O3

Ultrafast High-temperature Sintering (UHS) allows consolidation of ceramics in just a few tens of seconds. The green body is placed within a carbon felt heated by the Joule effect at temperatures up to 3000 °C. Here, we propose a combined experimental and numerical analysis to enable the fabrication of fully dense and fine-grain α-Al₂O₃ samples using a multistep computer-controlled current profile. Reference samples processed using a single step current formed cracks at the onset of shrinkage due to their uneven temperature distribution. Fully coupled simulations accounting for electric current, voltage, power, temperature and shrinkage allowed us to improve the experimental setup and to define the current profiles required to UHS highly dense (i.e., relative density > 99 %) 3 mm thick samples with an average grain size of 0.77 μm.