Shock-compression response of magnetic Fe3O4 nanoparticles

The shock-compression response of magnetic nano-Fe3O4 powders of ∼12 nm and ∼6 nm, and their bimodal mixture (∼12 nm + ∼6 nm in a 4:1 ratio), was investigated to determine the densification behavior of nanoparticles and properties of the consolidated magnets. Compacts recovered following shock compaction using a three-capsule, plate-impact, gas-gun setup at impact velocities of ∼800 m s−1 and ∼1100 m s−1 showed maximal densification of ∼80% theoretical maximum density (TMD) for ∼12 nm particles, ∼70% TMD for ∼6 nm particles, and ∼75% TMD for the bimodal mixture (∼12 nm + ∼6 nm in a 4:1 ratio) nanoparticles. Pressure–volume compressibility (Hugoniot) plots of nano-Fe3O4 powder compacts were constructed to assess their dynamic densification behavior. Scanning electron microscopy imaging of the recovered compacts showed the presence of a laminar structure, with inter-layer voids and microscopic gaps between nanoparticle agglomerates, which were responsible for the incomplete consolidation. X-ray diffraction and transmission electron microscopy analyses showed the retention of nanostructure in the recovered compacts, although the shock-energy deposition caused some grain growth. Hysteresis loop measurements revealed that the magnetic properties were retained, and shock compaction did not cause any magnetic transition. The potential causes for the lack of complete densification observed in the recovered shock-consolidated compacts of nano-Fe3O4 particles are discussed in light of the shock-consolidation mechanisms and interparticle bonding in nano-sized powders.