Nano-Scale Magnetic Film Formation by Decompression of Supercritical Carbon Dioxide/Ferric Acetylacetonate Solutions.

The formation of nano-scale magnetic films is investigated by decompressing supercritical CO2/Ferric acetylacetonate (Fe(acac) 3) solutions. The primary technique used is the Rapid Expansion of Supercritical Solutions (RESS), which consists of expanding a supercritical solution through a micron-sized nozzle aperture and directing the resulting supersonic free-jet onto both hot and cold silicon substrates. To model the thermodynamics, kinetics, and fluid mechanics, we also completed solubility measurements using a specially designed UV-Visible high pressure cell. The nano-scale films grown using RESS have particles in the range from 13 nm to 700 nm and, surprisingly, show magnetic order (coercivity, Hc = 50-100 Oe) even when grown on cold silicon substrates. Magnetic iron oxides formed from Fe(acac)3 thermal decomposition were expected only for temperatures above 180C. To understand whether the high kinetic energies achieved in the RESS expansion initiated the decomposition of Fe(acac)3 into a magnetic material, the same saturated supercritical mixture was decompressed in a fixed volume BATCH process. The rate of decompression varies dramatically between the two processes, RESS (musec scale) and BATCH (sec scale). The nano-scale films deposited near room temperature were again magnetically ordered (H c = 35-55 Oe). These results suggest that decomposition of Fe(acac) 3 occurs in the supercritical phase. The high pressure RESS expansion was then coupled directly to both a time of flight mass spectrometer and to a quadrupole mass spectrometer to identify size, composition, and translational energies of the clusters and nanoparticles formed in the jet. While CO 2 clusters up to N = 40 were observed, no evidence of solute clusters nor mixed CO2-solute clusters was found. These experimental results suggest that most of nanoparticle growth in the RESS process occurs at the silicon surface. Furthermore, Fe(acac)3 translational energies measured were too small to be responsible for the energies necessary to achieve the chemical transformation to the magnetically ordered state found in the cold nano-scale films. The velocity distribution data for CO2 clusters and solutes provide interesting new evidence for velocity slip, and non equilibrated thermal expansions for both supersaturated vapor jets with condensation and superheated liquid jets undergoing fragmentation.