Department: Mechanical & Aerospace Engineering
David Miller | Robert Continetti
Name: Silvia De Dea
Email: sdedea @ ucsd.edu
Grad Year: 2007
New synthetic routes for the preparation of magnetic nanoparticles and thin films are under constant investigation. In particular iron oxide magnetic particles have attracted interest due to their application as recording media, ferrofluids, catalysts and targeted drug delivery. Ferric acetylacetonate Fe(acac)3 is known to undergo thermal decomposition to form either Fe3O4 or a-Fe2O3 magnetically ordered materials when heated above 180 oC. We have recently observed that magnetic nanoparticle films can be formed near room temperature when a supercritical solution of Fe(acac)3 in CO2 is decompressed onto a cold Si substrate. Two different processes were studied: one based on the Rapid Expansion of a Supercritical Solution (RESS process) and another based on the depressurization of the saturated supercritical mixture in a fixed volume (BATCH process). The rate of decompression varies dramatically between the two processes, RESS (msec scale) and BATCH (sec scale). We have grown films in background pressures from vacuum to atmosphere and in both air and inert gases. The resulting cluster films have particles in the range from 13 nm to 700 nm, depending on experimental conditions, and show magnetic order. Structural and magnetic data for these thin particle films have been obtained by SQUID and SEM measurements and compared as a function of experimental variables (P, T, geometry, substrate conditions, growth time). In general we did not observe a significant shift in coercivity changing growth conditions in our range and the values of coercivity obtained are smaller than the bulk values for either a-Fe2O3, g-Fe2O3 or Fe3O4. This result is consistent with the formation of magnetic nanoparticles. The mechanism for the decomposition of Fe(acac)3 in our RESS and BATCH processes on a cold substrate with no heat treatment is difficult to explain. We are in the process of coupling the high pressure supersonic free-jet RESS expansion directly to a time-of-flight mass spectrometer to identify size and composition of the clusters and nanoparticles formed in the jet.
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