Synthesis and Characterization of Oxide/Metal Exchange-Coupled Nano-Composite Materials for Permanent Magnetic Applications

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, June 2016
Dr. Javier Garay Chairperson

Permanent magnets (PMs) are essential to an amazing variety of current and future devices, causing widespread interest in improving PM performance. A promising approach to improving PM performance is exchange coupling between hard and soft magnetic phases. Exchange coupling has been shown  to improve  the  energy  product  of  nano-composite  magnets,  compared  to  their  single  phase counterparts. This dissertation presents a simple and scalable material engineering route that produces exchange coupling in nano-composite PMs. Notably, no rare-earth or precious metals are used. The
composites are ferrite based. In one system, SrFe₁₂O₁₉ is used as the hard phase and Fe₃O₄ as the soft phase. In the second system SrFe₁₂O₁₉ is the hard phase while Co is the soft phase, leading to oxide/metal nano-composites. In order to maximize the beneficial effect of exchange coupling, a fine degree of mixing between the hard and soft phases is required. In order to achieve well intermixed phases at the nano-scale, soft phase precursor is precipitated on SrFe₁₂O₁₉ flakes through heterogeneous precipitation by  decomposition  of  urea.  The  soft  phase  precursor  is  reduced  and  core-shell  hard/soft  magnetic composite  is  synthesized.  A  clear  processing  window  is  established  to  control  composition.  This requires temperatures high enough to reduce the soft phase precursor, yet low enough to keep the hard/soft  interphase  reaction  free,  producing  a  hard/soft  ratio  that  maximizes  the  energy  product.  The resulting nano-composite powder outperforms the energy product of pure hard phase, SrFe₁₂O₁₉ by 37%.

The  energy  product  of  hard/soft  magnetic  nano-composite  powder  is  further  improved  by applying a similar synthesis route to a SrFe₁₂O₁₉/Co composite (Co replacing Fe₃O₄ as the soft phase). In order to optimize microstructure and composition ratio, the amount of Co precipitated on SrFe₁₂O₁₉ is varied by controlling precipitation time and precipitation SrFe₁₂O₁₉:Co ratio. Synthesizing optimized SrFe₁₂O₁₉/Co composite powder leads to an energy product improvement of 162% compared to pure SrFe₁₂O₁₉ powder.

Bulk dense nano-composite materials have been difficult to synthesize due to grain growth attributed from slow heating rates of traditional sintering techniques. High processing temperatures leads to high density, minimizing property diluting porosity. However, a thermodynamically favored reaction
at elevated temperatures deprives the composite of improved magnetic properties. Core-shell SrFe₁₂O₁₉/Co  nano-composite  powders  are  processed  into  bulk  samples  through  Current  Activated  Pressure Assisted  Densification  (CAPAD).  Relatively  high  processing  pressure  and  heating  rates  are  taken advantage of during CAPAD and a processing window that leads to high density, as well as reaction free samples is established. The result is oxide/metal nano-composites, which would not have been possible though traditional sintering. The processing route developed produces bulk SrFe₁₂O₁₉/Co composite material with a 70% improvement in energy product compared to the bulk SrFe₁₂O₁₉. First order reversal curve (FORC), δM and recoil loop analysis is used to provide evidence of exchange coupling.