Nanotech 2010

Interaction of Silver Nanopowder with Copper Substrate under Air Atmosphere

J. Sopousek, J. Bursik, J. Zalesak, V. Bursikova, P. Broz
Masaryk University, CZ

Keywords: Silver nanopowder, copper substrate, electron microscopy, sintering, soldering, nanoindentation


The silver nanopowder was investigated in details by different techniques (TEM, XRD, DSC, TGA, etc.). The results of DSC/TGA measurements under slightly oxidizing Ar atmosphere and XRD analyse show new results that silver nanopowder covers sequentially by different oxides: Ag2O (cubic - Pn-3n), Ag2O (hexagonal - P), and Ag2O3 (cubic - Pn-3m), see Fig.1. The silver nanopowder decomposes to metal silver and oxygen at 230ºC. At higher temperature region 230-370°ºC the exothermal effect indicates superposition of thermal effect of silver oxide decomposition and agglomeration of uncovered metal silver nanoparticles. Finally, melting temperature of bulk silver 961ºC is occurred. The new results on interaction of silver nanopowder with copper substrate were obtained experimentally under atmospheric air at temperatures above silver oxide decomposition. The Cu/Ag-nano/Cu samples were prepared under defined small mechanical load at different temperatures and annealing times. The Ag inter layer yields a firm junction between copper plates. The microstructure of the prepared experimental joints, composition profiles and local mechanical properties were studied on cross-sectional samples by means of analytical electron microscopy and depth-sensing nanoindentation technique. The compactness of the silver interlayer is dependent on annealing time. The formation of the porous silver interlayer is evident at early stages. The investigation of the interface between copper substrate and nanosintered silver showed that this region reveals thin interlayer, which corresponds to Cu2O formation at the interface. The measurements of thickness of the Cu2O interlayer in the samples annealed at different temperature/time conditions reveal that kinetics of oxide is close to parabolic growth. Original nanoindentation tests were carried out on cross sections of the Cu/nano-Ag/Cu sandwich samples annealed at 400°C using depth-sensing indentation method (Fig. 2). Three groups of indentation response curves corresponding to Cu, Cu2O and sintered nano-silver layer are clearly distinguishable. Metal nanopowders are promising materials for the preparation of lead-free nanosolders applicable at temperatures higher than processing temperature. Nano silver represents suitable base material, which can be used for study of nanosintering. The sintering temperature is 0.42 Tm of bulk silver. The joining using this process seems to be a promising alternative to standard soldering using bulk alloys. Joints of Cu/Ag/Cu can be prepared on air but Cu2O oxide interlayer is formed between silver layer and copper substrate. The mechanical load plays an initiation role for sintering and have to be more clarified. The system Cu/Ag nano/Cu seems to be good experimental model for both theoretical calculations and the development of the nanoparticle soldering. The work has been supported by GACR 106/09/0700 and MSM0021622410.
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