Mixing and characterization of nanosized powders: An assessment of different techniques

The objective of this paper was to gain an understanding of the mixing and characterization of nanosized powders. Three different nanosized material systems were selected based on their physical and chemical properties. Mixing experiments of the selected nanopowders were performed using a variety of environmentally friendly dry powder processing devices and the rapid expansion of supercritical CO₂ suspensions (RESS process) and compared with solvent-based methods coupled with ultrasonic agitation. A number of imaging techniques, including FESEM, AFM, TEM, EELS and EDS were used to characterize the degree of mixing or homogeneity of the mixtures obtained. The results indicate that only some of the imaging techniques are capable of determining the quality of nanoparticle mixing, depending on the physical and chemical properties of the nanopowders. For example, field emission scanning electron microscope (FESEM) is suitable for characterizing powder mixtures having a distinct difference in particle shape, or with a large difference in atomic number of the metallic element of the two constituents. Only electron energy loss spectroscopy (EELS) was able to fully characterize nanopowder mixtures of SiO₂ and TiO₂ at the nanoscale. Energy dispersive X-ray spectroscopy (EDS) provided information on mixing quality, but only on a scale of about 1 μm. The results also show that solvent-based mixing methods coupled with ultrasonic agitation, and RESS generally perform better than dry powder processing systems, with the exception of the hybridizer, in generating a homogeneous mixture.