Fluidization is a technique used to process large quantities of nanopowder with no solvent waste and a large gas–solid contact area. Nonetheless, nanoparticles in the gas phase form clusters, called agglomerates, due to the relatively large adhesion forces. The dynamics within the fluidized bed influence the mechanism of formation, and thus, the morphology of the agglomerates. There are many theoretical models to predict the average size of fluidized agglomerates; however, these estimates of the average lack information on the whole size range. Here, we predict the agglomerate size distribution within the fluidized bed by estimating the mode and width using a force balance model. The model was tested for titania (TiO₂), alumina (Al₂O₃), and silica (SiO₂) nanopowders, which were studied experimentally. An in-situ method was used to record the fluidized agglomerates for size analysis and model validation.

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The release of nanosized particles from fluidized beds of ceramic oxide nanopowders, namely, TiO₂ (P25), Al₂O₃ (AluC) and SiO₂ (A130) has been assessed for the first time. Previous models and experiments for processing engineered nanoparticles (ENP) using fluidized beds reported only the formation of micron-sized cluster agglomerates in the gas phase. In this work, aerosol spectrometry techniques such as scanning mobility particle sizing (SMPS) and optical particle counting (OPC) have been combined with powder technologies, such as the borescope high-speed camera system, to determine the particle size distribution from 5 nm to 1 mm above a fluidized bed. Furthermore, the morphology of nanoparticulate aerosol at different locations in the bed was determined by offline electron microscopy. The results demonstrate that free nano- and micron-sized particles are released from fluidized beds. Since the structures found above the bed are also expected to be present within fluidized beds, a revision of existing nanoparticle fluidization models, and improved safety and control measures in reactors for gas-phase ENP processing are needed to avoid nanoparticle release.

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Synthesizing pure phase Ba₃Si₆O₉N₄ by the conventional solid-state reaction method is challenging because of easily formed secondary phase Ba₃Si₆O₁₂N₂ showing similar crystal structure. In this work, an alternative low temperature synthesis method is presented, and a series of green to blue emitting (Ba, Sr)₃Si₆O₉N₄: Eu²⁺ phosphors were prepared by a mechanochemical activation route. Variations in photoluminescence properties and crystal structure, as induced by the change in phosphor composition, were investigated. Under ultraviolet-light excitation, Ba₃Si₆O₉N₄: Eu²⁺ phosphor exhibited a strong narrow green emission at 518 nm and simultaneously a weak emission at 405 nm, which are ascribed to different Eu/Ba sites in Ba₃Si₆O₉N₄ lattice proved by Density Functional Theory (DFT) calculations. A continuous green to blue emission in (Ba, Sr)₃Si₆O₉N₄: Eu²⁺ phosphors could be achieved by tuning the crystal structure and local coordination environment acting on Eu²⁺ with Sr/Ba substitution. More Sr/Ba substitution improved thermal quenching and resulted in a different characteristic of emission peak shift upon increasing the temperature.

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Efficient nanopowder processing requires knowledge of the powder’s mechanical properties. Due to the large surface area to volume ratio, nanoparticles experience relatively strong attractive interactions, leading to the formation of micron-size porous structures called agglomerates. Significant effort has been directed towards the development of models and experimental procedures to estimate the elasticity of porous objects such as nanoparticle agglomerates; however, none of the existing models has been validated for solid fractions below 0.1. Here, we measure the elasticity of titania (TiO2, 22 nm), alumina (Al2O3, 8 nm), and silica (SiO2, 16 nm) nanopowder agglomerates by Atomic Force Microscopy, using a 3.75 μm glass colloid for the stress–strain experiments. Three sample preparations with varying degree of powder manipulation are assessed. The measured Young’s moduli are in the same order of magnitude as those predicted by the model of Kendall et al., thus validating it for the estimation of the Young’s modulus of structures with porosity above 90 %.@en

Nanoparticles surrounded by gas agglomerate in a hierarchical fashion. From production until powder processing in the gas phase, nanoparticles go from individual particles to aggregates, simple agglomerates, and complex agglomerates. Even though the structures at each level have unique properties, they are commonly assessed as a whole. Additionally, the effect of external factors on the morphology of these structures during gas processing is not well understood and challenging to study due to the limited techniques for in situ analysis of the dynamic phenomenon. Here, we study three materials in their hydrophobic and hydrophilic version. We describe the structural characteristics of each hierarchical level of complex agglomerate formation obtained from two in situ techniques. The first scale, namely aggregates, are open structures with a fractal dimension of about 1.5, which then form simple agglomerates with a fractal dimension close to 3, that later cluster into complex agglomerates that present a fractal dimension of about 2. Furthermore, gas dynamics were found to densify the simple agglomerates, increasing their fractal dimension by more than 0.1.@en

Nanoparticles have properties of interest in biology, physics, ecology, geology, chemistry, medicine, aerospace, food science, and engineering among many other fields, due to their intrinsic properties arising from their large surface area to volume ratio and small scale. Most nanoparticle applications require particle’s surface adaptations, for which numerous methods have been developed. For this purpose, the characteristics of fluidization that make it an attractive processing technique are the large gas-solid contact area, no solvent, potential scalability, and suitability for continuous processing. Nanoparticles are not fluidized individually, but rather as clusters, which formdue to the relatively large interparticle forces. As a result, fluidization dynamics is strongly linked to nanoparticle agglomeration.@en