A heat spreader transfers energy as heat from a hotter source to a colder heat sink or heat exchanger. There are two thermodynamic types, passive and active. The commonest sort of passive heat spreader is a plate or block of material having high thermal conductivity, such as copper, aluminum, or diamond. An active heat spreader speeds up heat transfer with expenditure of energy as work supplied by an external source. A heat pipe uses fluids inside a sealed case. The fluids circulate either passively, by spontaneous convection, triggered when a threshold temperature difference occurs; or actively, because of an impeller driven by an external source of work. Without sealed circulation, energy can be carried by transfer of fluid matter, for example externally supplied colder air, driven by an external source of work, from a hotter body to another external body, though this is not exactly heat transfer as defined in physics. Exemplifying increase of entropy according to the second law of thermodynamics, a passive heat spreader disperses or "spreads out" heat, so that the heat exchanger may be more fully utilized. This has the potential to increase the heat capacity of the total assembly, but the additional thermal junctions limit total thermal capacity. The high conduction properties of the spreader will make it more effective to function as an air heat exchanger, as opposed to the original source. The low heat conduction of air in convection is matched by the higher surface area of the spreader, and heat is transferred more effectively. A heat spreader is generally used when the heat source tends to have a high heat-flux density,, and for whatever reason, heat can not be conducted away effectively by the heat exchanger. For instance, this may be because it is air-cooled, giving it a lower heat transfer coefficient than if it were liquid-cooled. A high enough heat exchanger transfer coefficient is sufficient to avoid the need for a heat spreader. The use of a heat spreader is an important part of an economically optimal design for transferring heat from high to low heat flux media. Examples include:
