UC San Diego

The formation of nano-scale magnetic films is investigated by decompressing supercritical CO\₂/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 180\⁰ C. 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 ([mu]sec scale) and BATCH (sec scale). The nano-scale films deposited near room temperature were again magnetically ordered (Hc = 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 CO\₂-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 CO\₂ 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