Liquidlike sintering behavior of nanometric Fe and Cu powders: Experimental approach
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I.
INTRODUCTION
THE principal interest in materials presenting nanocrystalline structures stems from the strong modifications of their solid-state properties compared to conventional polycrystalline solids.[1] At present, a common approach to bulk production is pressing the powders in the same synthesis apparatus.[2] From manufacturing, user, and economical points of view, it is desirable to perform the same processing steps in nanometric powders as in conventional powder metallurgy. Correct sintering is of paramount importance to the powder metallurgical process to ensure the development of the strength and physicochemical properties needed for the part to fulfill its intended role as an engineering component. While the optimization of sintering rates for any material must be the ultimate objective of sintering studies on that material, little can be achieved in this direction unless some basic knowledge of the predominant mass flow mechanisms exists. Nevertheless, sintering studies on nanometric powders have been almost qualitative[3,4,5] and only lately a more quantitative approach has been possible[6] due to new powder production techniques presenting a higher process efficiency.[7] The present work is an analysis of the sintering process and the mass flow mechanisms in nanometric Fe and Cu powders (n-Fe and n-Cu), comparing their behavior with that of micrometric powders. The principal purpose of this work was to study the densification, mass transport mechanism, and microstructure evolution in n-Fe and n-Cu during the sintering process. II.
EXPERIMENTAL DETAILS
Nanometric powders were produced by levitation melting in liquid nitrogen[7] and then shaped by die compaction, as in previously reported studies.[8,9] For the synthesis of n-
O. DOMINGUEZ, Research Associate, is with the Instituto de Metalurgia - UASLP, 78210 San Luis Potosı´, S.L.P./Mexico. Y. CHAMPION, Research Associate, and J. BIGOT, Associate Professor, are with the Centre d’Etudes de Chimie Me´tallurgique, CNRS, F94407 Vitry Sur Seine, France. Manuscript submitted October 24, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fe and n-Cu, the starting materials were industrial metals (99.95 pct). Chemical analysis of powders using inductively coupled plasma emission spectroscopy indicates that the levitation technique used to produce the nanometric powders decreases significantly the impurity content of low vapor pressure elements, oxygen at the surface of the particles being the more important impurity. X-ray diffraction (XRD), transmission electron microscopy (TEM), and Xray photoelectron spectroscopy were used to characterize the oxide film at the surface of the particles.[9,10] In general, the oxide film was 2-nm thick and identified as Fe3O4 on the Fe nanoparticles and Cu2O on the Cu nanoparticles. Figure 1 shows that the as-prepared n-Fe and n-Cu are nearly spherical, lightly aggregated, and exhibit a narrow size distribution. All these properties are quite important due to the fact that basically the theory of sintering is based on th
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