Amorphous and Nanocrystalline High Temperature Magnetic Material for Pwr

We have used transmission electron microscopy (TEM) to observe the morphologies of ferrite nanoparticles synthesized using a radio frequency plasma torch. These nanoparticles were faceted, having cuboctahedral polygonal growth forms with exclusively the {111} and {100} type faces. A model for the critical nucleus shapes and the growth forms of small cuboctahedral particles is proposed considering the Helmholtz free energies for nucleation and growth. The critical nucleus shape is predicted to be either a perfect cube or a perfect octahedron, while the equilibrium growth forms are the truncated shapes. High-resolution TEM observations of individual nanoparticles reveal that the smallest nanoparticles exhibit perfect octahedral shapes, while larger particles have truncated octahedral shapes. Theseobservations are consistent with the model predictions. The morphology observations are also consistent with 0:95 < c1 0 0c1 1 1< 1:14, wherec1 0 0 andc1 1 1 are the surface energies of the (100) and (111) surfaces, respectively.2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.