(608c) Deagglomeration of Nanoparticle Agglomerates by Rapid Expansion of Supercritical Carbon Dioxide through a Nozzle

A major challenge in making and utilizing materials based on nanoparticles and nanocomposites is the tendency for individual nanoparticle constituents to aggregate due to Van der Waal forces and form large fractal structures on the order of tens of microns. Intimately mixing two or more constituents at the nanoscale requires one to first break the original agglomerates and then mix the individual nanoparticle constituents. To that extent, the focus of this study is to investigate the deagglomeration mechanism in a novel mixing method that utilizes the rapid expansion of supercritical suspensions (RESS). The expansion results in the simultaneous deagglomeration and mixing of the nanoparticle constituents. The focus of the present study is on the deagglomeration process where suspension containing agglomerates of nanoparticles in supercritical CO2 expands to atmospheric conditions.

Agglomerates of a single nanoparticle constituent, alumina (dp = 13 nm) was stirred in a high pressure vessel with CO2 at pressures in the range of 550 to 1700 psi and at a temperature of 45oC. The suspension was then rapidly expanded through a variety of sub-millimeter nozzles into a small expansion chamber. The deagglomerated nanoparticles, which were in an aerosol form, were sized by an in-situ method based on laser diffraction, and were examined by two ex-situ methods: differential mobility analysis and electron microscopy imaging of agglomerates collected by inertial impaction. Various pre-expansion pressures were considered and their influence on the size reduction of alumina agglomerates was studied. This work also investigated deagglomeration of other types of nanoparticles.

To better understand the deagglomeration mechanisms in RESS, a theoretical analysis based on one-dimensional mass, momentum, and energy conservation principles was conducted to determine the changes in flow and thermodynamic properties in the RESS process. The calculations indicated that the flow in a thin capillary nozzle was governed by friction and was always subsonic. When CO2 exited the nozzle, it expanded very rapidly and the flow became supersonic. After the pressure dropped to a level that matched the ambient condition, this supersonic flow experienced a sudden deceleration as it traveled through a normal shock, which is a step change in pressure, temperature, and density and became subsonic again. Thus, we believe that deagglomeration in our experiments was due to two independent mechanisms: first the strong velocity gradient in the thin nozzle, and then the impact with the normal shock.