Phase Control and Processing Methods for High-Coercivity Magnets based on Metastable Fe2O3
High coercivity magnets are crucial in modern technology. Current state-of-the-art, high coercivity magnets rely on rare earth elements and/or precious metals which raises environmental and economic concerns. For this reason, the development of high-coercivity permanent magnetic materials comprised of earth-abundant and environmentally friendly materials is of world-wide interest. The focus of this work was the development of bulk, nanocomposite permanent magnetic materials based on the high-coercivity, metastable -Fe2O3 phase. A reverse-micelle/sol-gel process was used to synthesize Fe2O3/SiO2 core-shell structures, with Ba used as an aid in the formation of -Fe2O3 nanorods. Initial samples were prepared with an Fe/Ba concentration of 15 mol%. The SiO2 acted as a long range diffusion-mediating matrix, which allows for the controlled growth and stabilization of the metastable Fe2O3 phases. Annealing at 1050˚C resulted in -Fe2O3 nanorods with a maximum coercivity of 17.5 kOe. The diffusivity of Fe/O was increased through modification of the Si-O matrix with Na addition which resulted in -Fe2O3 nanorod formation with comparable dimensions and magnetic properties at 875˚C. This significant decrease in growth temperature confirms the hypothesis that -Fe2O3 stability is size dependent and is controlled by long range diffusion. Annealing experiments were also done using an externally applied magnetic field to investigate the effect of a magnetic field on the growth of nanometric Fe2O3. It was found that magnetic ordering was induced at 600°C, which is 300°C lower than samples annealed without field. The Si-O and Na-Si-O shells thicknesses were decreased to increase the Fe/Ba concentration, and therefore the magnetic content, to 90 mol%, resulting in samples with coercivites ≥13.0 kOe. Finally, bulk, high-coercivity magnets comprised primarily of the metastable -Fe2O3 phase were produced by rapidly densifying the composite Fe-O/Si-O composite powders via Current Activated Pressure Assisted Densification (CAPAD), followed by annealing. The Na-Si-O shell powders with 15 mol% Fe/Ba were also densified via CAPAD processing, resulting in relative densities ~99%, and high coercivites similar to the ideal Fe-O/Si-O powder samples. The nanocomposites produced in this work have some of the highest coercivities ever reported in dense millimeter-sized magnets that do not contain rare earths or precious metals.