A) the best heatsinks are pure copper, because aluminum is worse at taking heat in then copper, so it hurts thermal performance. Copper dissipates heat fine on its own, just look at thermalright heatsinks (they are all either copper or aluminum, and the best in the market)
B) The point of any cooling device is to move the heat away from what it is cooling (with the exception of fans, I mean heatsinks). So you want something that takes in heat.