Liquidlike sintering behavior of nanometric Fe and Cu powders: Experimental approach

Abstract

Nanometric Fe and Cu powders were sintered in vacuum, He, and H 2 atmospheres after uniaxial cold pressing. The shrinkage behavior of samples was studied using three different dilatometric techniques: constant heating rate, isothermal annealing, and the Dorn method. Density greater than 90 pct was obtained at sintering temperatures of 900 ЊC. In nanometric powders, densification and grain coarsening occurred in a narrow temperature interval. Despite the low oxide content in the starting powders (1.5 to 4 wt pct), the reducing atmosphere plays a relevant role in the sintering process. The self-diffusion activation energies obtained for nanometric Fe were 116 and 60 kJ/mole in vacuum and H 2 , and those obtained for nanometric Cu were 70 and 43 kJ/mole in He and H 2 . According to the present results, the activation energies obtained from both nanometric powders in H 2 could be associated with those for self-diffusion in liquid Fe (65 kJ/mole) and Cu (41 kJ/mole).

Effect of size-dependent grain structures on the dynamics of nanoparticle coalescence

“…As such, a special core/shell structure of nanoparticles can make the coalescence process unlike that for conventional sintering. It has been reported that the activation energy for the sintering of Fe and Cu nanoparticles below the melting point is close to that for the self-diffusion of liquid Fe and Cu, 35 being attributed to the existence of the amorphous surface layer. Changes in temperature and grain size during the coalescence of nanoparticles can induce melting and recrystallization, which in turn can affect temperature and grain evolution again.…”

“…While amorphous-shell dynamics dominate initial neck formation, lattice mismatch of the cores controls the behavior afterwards. The liquid-like sintering behavior of Fe and Cu nanoparticles, 35 and the associated activation energies, arising from amorphous surface layers, is relatable to the current phenomenon. Moreover, compared to a rapid shape evolution of the 2 nm 1273 K case (with an initial temperature of T/T melt ¼ 0.85), the 3 nm 1500 K case exhibits a much slower shape evolution upon neck formation, which can be attributed to the different grain structures (i.e., entirely amorphous versus core-shell) of the 2 nm and 3 nm particle cases.…”

Analysis of Iron Oxide Reduction Kinetics in the Nanometric Scale Using Hydrogen

“…In this regard, a qualitative model is proposed to further explain the reduction of the surface oxide of iron nanopowder based on the knowledge developed from the reaction kinetics of iron oxide nanopowder (Figure 17) where the oxide layer is assumed to consist of Fe 2 O 3 (Figure 3). The densities of the oxide layers being 5.2 and 5.7 g/cm 3 Catalytic reaction: The freshly formed metallic iron surfaces exhibit an auto-catalytic nature, supporting the chemisorption and disassociation of hydrogen molecules to yield active hydrogen atoms which are transferred or transported from metal surface to the metal/oxide interfaces through 'portholes' of water vapour [25,27].…”

“…Nanoscale materials are of interest owing to their modified solid-state properties compared to conventional solids [3]. Nanosized metal powder are being produced and extensively used for a wide range of applications.…”

Competing regimes of rate dependent plastic flow in ultrafine grained metals

“…The UFG copper samples with grain size of 100 nm were produced by adapted powder metallurgy techniques [17][18]. To confirm the existence of the domain III, we performed variable strain rates experiments in compression within the upper level of the quasistatic conditions.…”