Multidimensionality in fluidized nanopowder agglomerates

Abstract

Abstract. In recent years, the interest in fluidization as a mean to process nanoparticles is strongly increasing. Due to the small size of the nanoparticles, which makes van der Waals forces predominate, they do not fluidize as single particles but as agglomerates. Various researchers using settling experiments and bed expansion measurements conclude that fluidized agglomerates are fractal structures with a single fractal dimension of 2.5. Based on microscopy results, Wang et al. "Powder Technology 124, 152-159 (2002)" propose a hierarchical structure, which seems in contradiction with the use of only one fractal dimension as a descriptor of the whole structure. Moreover, it is not clear whether the structure presented by the agglomerates ex-situ is the same during fluidization. Hence, in this work we have characterized in-situ the internal structure of fluidized agglomerates by means of spin-echo small-angle neutron scattering (SESANS). We show that the structure of the agglomerates present at least two fractal dimensions. One of them is ∼ 2.1 and characterizes the primary strong agglomerates. The second one is ∼ 2.8 and characterizes the larger agglomerates formed by the primary agglomerates.

Characterization of the Stratified Morphology of Nanoparticle Agglomerates

“…The estimation of the theoretical polarization requires knowledge on the density–density correlation function for the sample, the projection of the density–density correlation function in the form of an Abel transform, and the solid fraction of the sample. A detailed description of the SESANS data analysis can be found in the publications by de Martin et al , and Andersson et al , …”

On the fluidization of cohesive powders: Differences and similarities between micro‐ and nano‐sized particle gas–solid fluidization

“…For nanoparticles, it was found that agglomeration takes place in distinctive steps: primary particles to aggregates (sometimes called sub‐agglomerates) to primary or simple agglomerates to complex agglomerates, which is a phenomenon that was discovered by Yao et al [ 33 ] Each stage of the multi‐stage agglomeration (MSA) has its own density and fractal dimension. [ 139 ] The latter is a more recent insight since a fractal dimension of 2.5 was earlier used for all stages of nanoparticle agglomeration. [ 115,140,141 ] No MSA has been described for microparticles, making the final agglomerates present in the fluidized bed of microparticles significantly different.…”