Investigation of colloidal stability and insulation characteristics of magnetic oils.

Superparamagnetic iron-oxide (SPIO) particles were synthesized by the co-precipitation method and the oleic acid-coated SPIO (OA-SPIO) was then obtained by a surface grafting procedure. A stock sample of magnetic oil (MO) with 1.6% particle volume fraction (VF) was obtained by dispersing the OA-SPIO in insulating naphthenic oil. The MO stock sample was diluted in the same naphthenic oil to yield MO with 0.1, 0.04, 0.02, and 0.01% VF. Moreover, the 0.04% VF MO sample was manipulated to yield MO samples with water content of 26, 37, and 63 mg L(-1). The spinel structure of OA-SPIO was assessed by XRD and the average diameter of 8.3 nm was provided by TEM analysis. The saturation magnetization at room temperature (RT) was 70 emu/g and no remanence or coercivity was observed. The average hydrodynamic diameter (D(H)) of the colloidal particles suspended within the 0.04% VF MO sample was 58 nm. After aging for 30 days at RT no change was observed for the lowest water content MO sample (26 mg L(-1)). However, D(H) equals to 270 nm was observed for the highest water content MO sample (63 mg L(-1)). The MO samples with 26 mg L(-1) water content were found stable under heating at 90 degrees C for all VF investigated. We found the insulation resistance dropping significantly as VF and temperature increases. The lowest value found was 11 GOhms for the 0.1% VF at 60 degrees C, which is an acceptable value for MO.

Tailoring magnetic nanoparticle for transformers application.

In this study photoacoustic spectroscopy was used to investigate the effect of dilution of an oil-based magnetic fluid sample on the magnetic nanoparticle surface-coating. Changes of the photoacoustic signal intensity on the band-L region (640 to 830 nm) upon dilution of the stock magnetic fluid sample were discussed in terms of molecular surface desorption. The model proposed here assumes that the driving force taking the molecules out from the nanoparticle surface into the bulk solvent is the gradient of osmotic pressure. This gradient of osmotic pressure is established between the nanoparticle surface and the bulk suspension. It is further assumed that the photoacoustic signal intensity (area under the photoacoustic spectra) scales linearly with the number of coating molecules (surface grafting) at the nanoparticle surface. This model picture provides a non-linear analytical description for the reduction of the surface grafting coefficient upon dilution, which was successfully-used to curve-fit the photoacoustic experimental data.