D.L. Huber, J. Watt, G.C. Bleier, T.C. Monson, J.C. Neely
Sandia National Laboratories,
Keywords: magnetic nanoparticles, nanocomposites, inductors
Summary:Magnetic nanoparticles can exhibit a property known as superparamagnetism where they have extremely high susceptibility and a complete absence of magnetic hysteresis. This would represent an ideal inductor material if it could be maintained in a bulk material. To do this, magnetic nanoparticles must be densely packed without touching, as contact would lead to the formation of ferromagnetic domains and loss of superparamagnetism. The formation of bulk superparamagnetic materials can be discussed in terms of two fundamental steps. The first is the large scale synthesis of precisely size controlled magnetic nanoparticles, and the second is the densification of these particles into a highly magnetic material. While both are important, the first is perhaps the more technologically challenging and will be the focus of this discussion.The properties of magnetic nanoparticles vary dramatically with size, with an abrupt transition from superaparmagnetic to ferromagnetic behavior. The highest magnetic susceptibility, however, is typically found just before this transition to ferromagnetism. Therefore, careful control of particle size and reproducibly of particle size are critical for real world application. The Extended LaMer Mechanism is a general method for size control in the synthesis of nanoparticles by establishing steady state growth through the continuous, controlled addition of precursor. The steady state growth regime is characterized by a constant concentration of unreacted precursor as well as a uniform rate of growth in particle volume. This approach allows reproducibility in particle size from batch to batch, as well as prediction of size produced later in a reaction by monitoring early stages of growth. This method has been demonstrated using important synthetic systems including magnetite, iron, and gold nanoparticles. Independent reactions have been shown to extremely reproducible, with sizes reproduced with a coefficient of variation of less than 2%. Still, the Extended LaMer reaction requires a slow addition of product and long reaction times. We have also demonstrated a reversible magnetic agglomeration method for controlling particle size. This takes advantage of a phase transition, where magnetic nanoparticles become insoluble when the magnetic interactions between them reach a critical value. This approach allows rapid synthesis of precisely size controlled magnetic nanoparticles, yielding 10s of grams of nanoparticles per liter of solvent per hour. Nanoparticles formed through both methods have been densified into inductor materials and tested. They represent a tunable inductor material with excellent properties that extend to the MHz frequency regime with low losses and little to no decrease in susceptibility. We are currently investigating their application in high frequency switching circuits for power electronics. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.