Ferrofluids |
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Synthesis of nanoparticles and ferrofluids preparation |
| We synthesized iron oxide nanoparticles and other AFe2O3 (A = Co, Mn and Ni) by chemical co-precipitation. The nanoparticles that we synthesized are then coated with oleic acid. The structure of the ferrofluid consists of a magnetic core (brown circle) and coated with the surfactant (small gray circles), which in our case is oleic acid. What the oleic acid does is that it attaches its polar head to the magnetic core and the nonpolar tail disperses in the nonpolar solvent. So the typical behavior is that when you bring near a bar magnet, the liquid is pulled towards the magnet without the particles separating…and that is always very amusing to see. We are also looking at coating the nanoparticles with polyacrylic acid (PAA), pluronic and citric acid to make the particles suspend in water. For the dispersing liquids, we use hexane and dodecane. These 2 are identical in composition only that dodecane has a longer hydrocarbon chain than hexane. They have large difference in viscosity and freezing temperatures. |
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| The TEM images of Fe3O4 and CoFe2O4 nanoparicles are shown in Figures A and B, respectively. They are spherical and shape and have mean diameters of 14 nm (Fe3O4) and 15 nm (CoFe2O4). XRD peaks (not shown) confirm the cubic structure of the nanoparticles. |
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| Magnetic Characterization |
| All the magnetic measurements were done using the Physical Property Measurement System (PPMS) we have in our lab. We do zero-field cooled and field cooled measurements with 100 Oe external DC field. We can identify the blocking temperature, TB , from this graph to be the temperature at which the ZFC and FC curves start to separate. In hexane, the blocking temperature is right about the freezing point of the liquid while in dodecane, the blocking is below the freezing. These are two distinct cases that are very interesting to look at. Also note that all ZFC curves show a broad feature, which denotes that the particles have a wide-range of grain sizes. |
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| AC Characterization. We measured the complex susceptibility of the ferrofluids in hexane as a function of temperature taken at different frequencies, 10, 100, 1000 and 10000 Hz. We find that the prominent peaks in the 4 curves actually coincide with the freezing temperature of hexane. The positions of the maximum values do not change even if the measuring frequency increases. So clearly, the blocking temperature of the particles is obscured by the freezing of the hexane. |
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| Relevant Publications |
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• R. E. Rosensweig, Ferrohydrodynamics (Cambridge University Press), Cambridge, England, 1985. • Thermal Motion of Magnetic Iron Nanoparticles in a Frozen SolventM. Klokkenburg and B. H. Erne and A. P. Philipse, Langmuir, 21, 1187-1191 (2005). • Two Mechanisms and a Scaling Relation for Dynamics in Ferrofluids, Jinlong Zhang, C. Boyd, and Weili Luo, Phys. Rev. Lett. 77, 390-393, (1996). |
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