Today I returned to a book about Density Functional Theory, A Practical Introduction, 2009, by David Sholl and Janice Steckel. In their first chapter, pp 4-5, they describe the successful use of DFT to determine the reason why copper, ordinarily maleable, becomes brittle when bismuth is added, at a very low level — less than 100 ppm. Ditto with lead and mercury inclusions. They describe the work of Schweinfest, Paxton and Finnis (not seen) which determined that the cause was the large bismuth atoms, migrating to grain boundaries within the copper, causing a lack of cohesion between grains. The original paper by SPF is in Nature, vol 432 (2004), pg 1008 ff.
That reminds me of Goldschmid’s description of the diffusion of copper into bismuth telluride (Bi2Te3), sectrion 6.3, pp 86-87 of his Introduction to Thermoelectricity, 2010. Copper diffuses very readily into Bi2Te3. Hence copper contacts soldered to a Bi2Te3 thermoelectric element give poor performance. Nickel is a better choice of contact material.
There is more to be studied along these lines. For instance, gold atoms, are almost as large /heavy as lead, etc. Yet I would expect gold, because of its electronic structure, to be very compatible with copper, and not drastically affect the maleability of copper. Something to investigate, theoretically and experimentally.
Best wishes,
Ken Roberts,
24-Aug-2015
Logic, Astronomy, Science, and Ideas Too 