Amorphous-like Grain Boundary Research Articles

Strengthening-softening transition and maximum strength in Schwarz nanocrystals

Recently, a Schwarz crystal structure with curved grain boundaries (GBs) constrained by twin-boundary (TB) networks was discovered in nanocrystalline Cu through experiments and atomistic simulations. Nanocrystalline Cu with nanosized Schwarz crystals exhibited high strength and excellent thermal stability. However, the grain-size effect and associated deformation mechanisms of Schwarz nanocrystals remain unknown. Here, we performed large-scale atomistic simulations to investigate the deformation behaviors and grain-size effect of nanocrystalline Cu with Schwarz crystals. Our simulations showed that similar to regular nanocrystals, Schwarz nanocrystals exhibit a strengthening-softening transition with decreasing grain size. The critical grain size in Schwarz nanocrystals is smaller than that in regular nanocrystals, leading to a maximum strength higher than that of regular nanocrystals. Our simulations revealed that the softening in Schwarz nanocrystals mainly originates from TB migration (or detwinning) and annihilation of GBs, rather than GB-mediated processes (including GB migration, sliding and diffusion) dominating the softening in regular nanocrystals. Quantitative analyses of simulation data further showed that compared with those in regular nanocrystals, the GB-mediated processes in Schwarz nanocrystals are suppressed, which is related to the low volume fraction of amorphous-like GBs and constraints of TB networks. The smaller critical grain size arises from the suppression of GB-mediated processes.

Intensified spin‐dependent‐transport and localized‐spin freezing in magnetite sinter made from low size‐dispersion hematite nanoparticles with low temperature calcination

The spin-dependent-transport (SDT) has been studied for magnetite (Fe3O4) nano-particle sinter (MNPS) made from low size-dispersion hematite (α-Fe2O3) nanoparticles (LSDHN's) with low temperature calcination. Two kinds of LSDHN's are grown by the hydrothermal synthesis. The average sizes of them are 30 nm and 60 nm. The MNPS is produced by calcining the LSDHN's at 500 °C for 5 hours in the atmosphere of Ar(90%)/H2(10%) mixed gases. As compared with a bulk single crystal, the considerable intensification of negative-differential-magnetoresistance (ND-MR) has been observed for the MNPS. We have not observed abrupt change of the electrical resistivity in the vicinity of the temperature of the Verwey transition (which is the metal-insulator transition) appeared for a bulk single crystal. The ND-MR for 30 nm shows larger values than that of 60 nm on the temperature dependence. From the X-ray diffraction experiment, the MNPS is found to include crystalline magnetite regions. We consider that the MNPS is composed of large amorphous-like grain-boundaries and small crystalline grains. The electrical current is inferred to flow in grain-boundary regions. In grain-boundary regions, since the localized spins are relatively random distributed, the spin-polarized conduction electrons show the SDT. Below the Verwey temperature, we have observed the magnetization difference between zero-field cooling (ZFC) and field-cooling (FC). This phenomenon indicates that the localized spins in the amorphous-like grain-boundaries are frozen in some degree. Above the Verwey temperature, the magnetoresistance is well fitted by the square of the Langevin function. We consider that the localized spins in the amorphous-like grain-boundaries do not form perfectly random configuration and are somewhat ordered in a short range region. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)