Heck, ice would be pretty good, and would maintain a large ∆T until it melted. Copper's a great conductor, but the specific heat is pretty poor: http://hyperphy

Heck, ice would be pretty good, and would maintain a large ∆T until it melted. Copper's a great conductor, but the specific heat is pretty poor: http://hyperphysics.phy-astr.gsu.edu/hbase/tables/sphtt.html http://hyperphysics.phy-astr.gsu.edu/hbase/tables/sphtt.html I guess it also depends on the relative importance of mass vs. volume optimization.