In situ transmission electron microscopic investigations of reduction-oxidation reactions during densification of nickel
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The consolidation of crystalline powders to obtain dense microstructures is typically achieved through a combination of volume and grain boundary diffusion. In situ transmission electron microscopy was utilized to study neck formation between adjacent nickel particles during the early stages of sintering. It was found that the presence of carbon during consolidation of Ni lowers the reduction temperature of nickel oxides on the particle surface and therefore has the potential to accelerate consolidation. In the absence of carbon, the surface oxides remain present during the early stage of sintering and neck formation between particles is limited by selfdiffusion of nickel through the oxide layer. This study provides direct experimental evidence that corroborates related earlier hypotheses of self-cleaning on the surface of the nanoparticles that precedes neck formation and growth.
I. INTRODUCTION
Sintering describes the diffusion-driven process for the densification of metals and ceramics to generate dense polycrystalline microstructures.1,2 The consolidation process is commonly described through three primary stages following the initial particle contact formation. The first stage involves neck formation and growth, whereas the second and third stages relate to densification before and after the isolation of the pores, respectively. Activation of diffusion and, hence, sintering can be achieved by the addition of minor phases,3,4 elevated temperatures,3 applied isostatic or uniaxial pressure, or the application of external electromagnetic fields.5–7 Often, a combination of the above parameters is applied. Common consolidation routines are hot isostatic pressure (HIP) sintering, microwave sintering, and electric field-assisted sintering (EFAS), which includes spark plasma sintering (SPS), field-assisted sintering technology (FAST) and pulsed electric current sintering. Currently several hundred publications and patents per year report that EFAS techniques have the potential to increase the rate of densification, reduce sintering temperatures, shorten holding times,5,6,8 and suppress grain growth while achieving full density.8–11 The resulting microstructures often provide unique macroscopic properties such as improved mechanical properties,12,13 oxidation or corrosion resistance,13,14 optical transmission,15 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.256 J. Mater. Res., Vol. 27, No. 18, Sep 28, 2012
and electrical properties.16,17 Despite the abundance of sintering studies reported in the literature and the wide application of EFAS, the atomic scale consolidation mechanisms that occur in the presence of electrical fields and relatively high heating rates during EFAS remain mostly unclear.8 The presence of surface films covering individual powder particles can have a significant influence on the kinetics of neck formation during the initial stage of sintering. For instance, several authors have reported studies on insulating surface oxide layers covering metallic nanopartic
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