Tuning of Orientation and Magnetic Properties of Ferromagnetic Aerogels produced through a Magnetic-Field Assisted Synthesis

Ferromagnetic aerogels possess many desirable properties such as low densities, high porosities and surface areas, and interesting magnetic properties which allow opportunities in a wide range of applications including sensing, energy storage, switches, theranostics, catalysis, and many others. In a majority of these applications, the aerogels consist of some type of host material such as silica or a polymer that is doped with a ferromagnetic nanomaterial. However, ferromagnetic aerogels that consist only of the magnetic material without a host are highly desirable because they allow higher magnetizations per unit mass, better exposed surfaces, and simpler structures. We have prepared magnetic nanowire aerogels using a magnetic-field assisted synthesis which allows formation of magnetic nanowire aerogels in a fast, simple, and scalable manner. In a typical synthesis, a metal salt such as iron or cobalt chloride was placed inside a magnetic field, and sodium borohydride was added dropwise as a reducing agent. This resulted in rapid formation of gels that could then be supercritically dried into aerogels or conformed into thin films depending on the desired application. The applied magnetic field strength was varied from 0 mT to 150 mT and had impact on the nanowire aspect ratios and orientation, the magnetization and coercivities of the aerogels, the mechanical properties, and the observed surface areas and porosities. Typically, low applied field strengths showed low ordering and small aspect ratios while higher applied fields increased the orientation and aspect ratios of the nanowires. Bimetallic iron-nickel aerogels were also prepared and the influence of the metal counterion was studied. The counterion not only impacted the rate of the reaction, but also the shape of the nanostructures within the formed aerogels. The iron-to-nickel ratio also influenced the nanowire length and magnetic properties. The magnetic field assisted synthesis strategy allowed rapid formation of template free ferromagnetic aerogels and can be expanded to achieve a wide range of bimetallic materials.