Characterization of the Stratified Morphology of Nanoparticle Agglomerates

Abstract

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.

Experimental investigation on the microstructure of fluidized nanoparticle agglomerates by TEM image analysis

“…It is reported that the density of simple agglomerates of ~40 μm is about 137 kg/m 3 . [ 16 ] This is reasonable considering the existence of more pores among the aggregates inside simple agglomerates with increasing size. The total porosity of the FNPAs ε a can be calculated by Equation ) from the two hierarchical porosities obtained.…”

“…This is a typical three‐level hierarchical structure first revealed by Yao et al [ 5 ] and broadly accepted by many authors. [ 9,15,16 ]…”

“…The simple agglomerates present in the as‐received nanopowders can be characterized by some sizing method analyses. [ 15,16,22 ] In our work, their size distributions were obtained by using a laser granulometer (Master‐sizer 2000, Malvern Instruments) after the dispersion of the powders in pure alcohol solution by ultrasonication for 30 minutes. The suspension was transported to the measuring zone of the laser granulometer and the dispersed agglomerates passed through the laser beam.…”

“…Yao et al [ 5 ] first revealed the three‐level hierarchical structures of FNPAs, namely aggregates, simple agglomerates, and fluidized nanoparticle agglomerates, which was broadly accepted by many authors. [ 9,15,16 ] In light of this, more parameters are needed to get a detailed description of the different substructures of FNPAs (ie, aggregates and simple agglomerates). De Martín et al [ 15 ] and Fabre et al [ 16 ] suggested a quantitative description of the substructures of FNPAs using the small‐angle neutron scattering, but little further research has been reported.…”

“…[ 9,15,16 ] In light of this, more parameters are needed to get a detailed description of the different substructures of FNPAs (ie, aggregates and simple agglomerates). De Martín et al [ 15 ] and Fabre et al [ 16 ] suggested a quantitative description of the substructures of FNPAs using the small‐angle neutron scattering, but little further research has been reported. The two‐dimensional image analysis has been widely proven as an effective method to estimate the fractal dimension of agglomerates.…”