Synthesis of Yttrium Iron Garnet based ferrofluids and study of their colloidal, magnetic and optical properties

Abstract

Ferrofluids are colloidal systems of superparamagnetic nanoparticles suspended in a carrier liquid and coated with a stabilizing agent. The presence of an external magnetic field gradient affects the direction of the magnetic nanoparticles. Ferrofluids are applied for the separation of waste plastics via magnetic density separation but the black appearance of the ferrofluids makes it difficult to observe the processes due to the strong optical absorption from UV to NIR region. Ferrofluids based on Yttrium Iron Garnet nanoparticles (YIG-NPs) were synthesized due to the bulk properties of YIG considered to be lower in optical absorbance than magnetite and maghemite, where it is expected YIG ferrofluids to be more transparent than conventional ferrofluids based on magnetite and maghemite. Maghemite nanoparticles were formed from magnetite nanoparticles by refluxing in acidic environment to promote the ion exchange of Fe2+ and Fe3+. The stabilization part to obtain stable maghemite nanoparticles in a carrier liquid was not applicable to obtain stable YIG-NPs in a carrier liquid and stable magnetite nanoparticles in a carrier liquid. Magnetite nanoparticles were synthesized by co-precipitation and coated by adsorbing citrate molecules on the magnetite nanoparticles as a result of transferring the magnetite nanoparticles in acidic environment. YIG dispersions were formed via solvothermal synthesis by heterogeneous seeded growth of YIG-NPs on Yttrium Aluminium Garnet (YAG) seeds. The coating was executed with oleic acid as the stabilizing agent dispersed in cyclohexane. The coating in the oil phase showed improvement in the ferrofluid behaviour, flow of magnetic dispersion towards higher magnetic field gradient, compared to the stabilization method of maghemite and magnetite nanoparticles in the water phase. The ferrofluids were analyzed to study the colloidal, magnetic and optical properties. The colloidal properties were observed from DLS and TEM measurements. Magnetic sedimentation experiments were done to study the colloidal properties in magnetic field. Magnetometry was applied to study the magnetic properties. For the optical properties, UV-Vis measurements were done to observe the wavelength-dependent optical density profiles. Size control of the YIG-NPs was not achieved due to the inconsistency of the formed YAG seeds. YIG-on-YAG nanoparticles were formed but showed aggregation. Optical transmission of the YIG ferrofluids was affected by the optical scattering of the aggregates. The aggregates were detected by DLS and magnetic sedimentation. The characterization techniques indicated that the presence of aggregates affected the ferrofluid stability and explained the rapid sedimentation of the magnetic colloids