Heat sink
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A heat sink is an environment or object capable of absorbing heat from another object with which it is in thermal contact (either direct contact or radiational "contact"). In common use, it is a device made of metal brought into contact with the hot surface of a component, such as a microprocessor chip or other power handling semiconductor in order to stablise its temperature through increased thermal mass and heat dissipation. Primarily by conduction and convection and to a lesser extent by radiation. Heat sinks are widely used in electronics, and have become almost essential to modern central processing units.
A heat sink usually consists of a metal structure with one or more flat surfaces to ensure good thermal contact with the components to be cooled, and an array of comb or fin like protrusions to increase the surface contact with the air, and thus the rate of heat dissipation. A heat sink is often used in conjunction with a fan in order to increase the rate of airflow over the heat sink, thus maintaining a larger temperature gradient by replacing warmed air faster than would be by convection, this is known as a forced air system.
Heat sinks are commonly made of a good thermal conductor such as copper or aluminum. Copper is significantly more expensive than aluminum but is also a better thermal conductor. The contact surface of a heat sink must be highly polished in order to ensure the best thermal contact with the object to be cooled. Sometimes a thermally conductive grease is employed to ensure the best thermal contact, such greases often contain colloidal silver (an even better thermal conductor than copper.) It is claimed that some brands of thermal grease that are advertised as containing silver or silver oxide actually contain neither, most notably that of CompUSA.
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Temporary heat sinks are sometimes used in soldering circuit boards in order to prevent the heat from damaging sensitive nearby electronics - in the simplest case, this means gripping part of a component to be soldered with a crocodile clip or similar.
Other Meanings
In urban planning, an urban area may be referred to as a heat sink (or urban heat island) because of its tendency to absorb sunlight and re-radiate it as heat, together with heat from energy usage.