Heat pipe
Sealed copper tube with a working fluid (water typical) + wicking structure. Phase-change cooling: hot-end vaporizes fluid, cold-end condenses, capillary action returns. More efficient than solid copper for heat transport.
Standard in modern CPU coolers (Noctua, be quiet!, Deepcool). 4-8 heat-pipes per tower cooler. Sintered-vs-grooved-wick: sintered handles any orientation (laptop-friendly), grooved is cheaper but gravity-dependent. Performance: 1500-2500 W/cm² heat-transfer capacity. Laptops: thin profile = flat heat-pipes used. Failure mode: dry-out (after years/temp-cycles), vacuum-leak — pipe failure dramatic temp spike. Distinct from vapor chamber: heat-pipe is linear, vapor chamber is 2D spread. Modern GPU coolers use a combination of heat-pipes + vapor chamber.