Spark Plasma Sintering of Copper-Matrix Composites as Cylinders with Micrometric Grain Size towards the Preparation of High-Strength Conducting Wires for Resistive Magnets

Spark plasma sintering (SPS) resembles hot-pressing because a uniaxial pressure is applied to the sample, but a key difference is that heating is performed by the application of a dc pulsed current to the pressing die and powder compact. Therefore, SPS allows higher heating and cooling rates than hot-pressing, allowing for lower sintering temperatures and shorter holding times. It is thus possible to avoid grain growth and prepare samples with a very fine microstructure. Moreover, a key limitation compared to pressureless sintering was lifted when the possibility to sinter complex-shape samples was demonstrated. The basics of SPS will be presented.<br/>We will in particular present the case of cylinders that serve as precursors for the preparation of wires of pure copper, carbon nanotube - copper composites and silver nanowire - copper composites. Such wires may find some applications when ultra-high strength must be combined to a high electrical conductivity, as required for high-field magnets, or when lightweight materials are needed such as for aerospace power and engineering.<br/>SPS allows one to prepared copper cylinders with micrometer-sized grains, typically ten times lower than conventional copper cylinders prepared by routes involving melting-solidification, and therefore a relatively high temperature, either for alloying or for the preparation of the pure copper sample to be used in a composite. SPS cylinders with only about 90% of their full density were found suitable for the rest of the study, because a too high density hampers the deformability of the cylinder during wire-drawing, resulting in sample breaking.<br/>The microstructure of the powders, cylinders (8 or 24 mm in diameter) and wires will be presented as well as the mechanical properties and electrical resistivity of the wires measured at 293 K and 77 K.