Neither do I. But aren't you baffled that a few grams of sugar in a kilo of dough will make bread tough and full of holes, or that a miniscule addition of carbon to iron yields steel? Compare the thermal conductivity of a solid block of concrete to that of gravel, sand, or cement. The lattice structure is the thing.
Maybe there's a thermally ideal lattice arrangement for copper we can get by by adding some impurity. Maybe.
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