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Description
A commercial micrometric Cu powder is mixed with Ag nanowires (diameter 0.2 µm, length 30 µm) synthesized in-house, in order to prepare a powder with a Ag content of 1 vol. %. Two powder batches are prepared for consolidation into cylinders (diameter 8 mm, length 33 mm) by spark plasma sintering.
One cylinder is sintered at 400 °C, where the solubility of Ag in Cu is below 0.1 vol. %, which allows one to obtain a composite microstructure with pure Ag nanowires dispersed in a pure Cu matrix. The other cylinder is sintered at 600 °C, where the solubility of Ag in Cu is equal to about 2.4 vol. %, which allows the Ag nanowires to dissolve into the surrounding Cu volume to form Cu/Ag alloy nanowires.
The diameter of the cylinders is reduced by wire-drawing, in several passes, thus producing progressively finer wires (diameter in the range 1-0.2 mm). Wires with ultrafine Cu grains (200-700 nm for a 0.5 mm diameter wire) elongated along the drawing axis are prepared. The nanowires (pure Ag or Cu/Ag alloy) are dispersed along the Cu grain boundaries.
Both kind of wires show a similar ultimate tensile strength (1100 MPa at 77 K), reflecting an equivalent strengthening effect provided by the pure Ag and Cu/Ag alloy nanowires.
However, the electrical resistivity of the Cu/Ag-Cu wires (0.56 µΩ.cm at 77 K) is significantly higher compared to that of the Ag-Cu wires (0.49 µΩ.cm at 77 K). This shows that despite the very local nature of the Cu/Ag alloy, its formation is to be avoided by controlling the sintering conditions, which thus have to be changed depending on the chosen Ag content.
Pure Ag nanowires - Cu composites wires are to be preferred in order to obtain the most suitable properties for high magnetic fields.
