The two semiconductors are positioned thermally in parallel and joined at one end by a conducting cooling plate (typically of copper or aluminium).Ī voltage is applied to the free ends of two different conducting materials, resulting in a flow of electricity through the two semiconductors in series. TECs are constructed using two dissimilar semi-conductors, one n-type and the other p-type (they must be different because they need to have different electron densities in order for the effect to work). Fortunately the cleavage planes generally run parallel to the C-axis, so the material is quite strong when assembled into a thermoelectric cooling module. As a result, crystalline Bismuth Telluride cleaves readily along these Tellurium–Tellurium layers (like Mica sheets). While alternate layers of Bismuth and Tellurium are held together by strong covalent bonds, adjacent layers of Tellurium are held together only by weak van der Waals bonds. The thermoelectric elements must be incorporated into a cooling module so that the crystal growth axis is parallel to the length of each element (perpendicular to the ceramic plates), so that this anisotropy is harnessed for optimum cooling.Īnother interesting characteristic of Bismuth Telluride is that Bismuth Telluride (Bi 2Te 3) crystals are made up of hexagonal layers of similar atoms. Hence the anisotropic behavior of resistance is greater than that of thermal conductivity, and the highest Figure-of-Merit occurs in the parallel orientation. Thermal conductivity, on the other hand, is about double parallel to the crystal-growth axis that perpendicular direction. Its electrical resistance is about four times greater parallel to the axis of crystal growth than perpendicular to it. Due to its crystal structure, Bismuth Telluride is highly anisotropic. Other thermoelectric materials include Lead Telluride (PbTe), Silicon Germanium (SiGe), and Bismuth-Antimony (Bi-Sb) alloys, which may be used in specific situations however, Bismuth Telluride is the best material in most computer cooling scenarios.īismuth Telluride has two characteristics worthy of note. The thermoelectric semiconductor material most often used in today's TE coolers is an alloy of Bismuth Telluride (Bi 2Te 3) that has been suitably doped to provide individual blocks or elements having distinct "N" and "P" characteristics. These are usually manufactured by either directional crystallization from a melt or pressed powder metallurgy. In practice semi-conductors are the material of choice. This tends to work against any heat gradient produced, and lowers their overall ZT value. If the temperature of the hotter junction (heat sink) is kept low by removing the generated heat, the temperature of the cold plate can be cooled by tens of degrees.Īt first glance metals with their low electrical resistance might seem like a good choice for TEC construction however they also have high thermal conductivity. (This electrostatic potential is called the contact potential.)Ĭurrent passing across the junction results in either a forward or reverse bias, resulting in a temperature gradient. When two conductors with different Fermi levels make contact, electrons flow from the conductor with the higher level, until the change in electrostatic potential brings the two Fermi levels to the same value. The Fermi level represents the demarcation in energy within the conduction band of a metal, between the energy levels occupied by electrons and those that are unoccupied. The reason for this is a difference in the so-called Fermi level between the two conductors. When two conductors are placed in electric contact, electrons flow out of the one in which the electrons are less bound, into the one where the electrons are more bound. Peltier himself did not appreciate the potential of his discovery, and it was not efficiently exploited until the end of the 20th century. This effect can be harnessed to transfer heat, creating a heater or a cooler. In fact, depending on the direction of the current, the overall effect could be either heating or cooling. What Peltier observed was that when electric current passed across the junction of two dissimilar conductors (a “thermocouple”) there was a heating effect that could not be explained by Joule heating alone. The principle of thermoelectric cooling dates back to the discovery of the Peltier Effect by Jean Peltier in 1834.Īll electric current is accompanied by heat current (Joule heating). Passing a current though a TEC transfers heat from one side to the other, typically producing a heat differential of around 40☌-or as much as 70☌ in high-end devices-that can be used to transfer heat from one place to another. Thermoelectric coolers (TECs), also known as Peltier coolers, are solid-state heat pumps that utilize the Peltier effect to move heat.
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