High ZT Nanoparticle/Polymer Composites for Cooling Applications
Agency / Branch:
DOD / ARMY
Infrared imaging devices (like night-vision goggles), rely on solid-state coolers to cool and stabilize the temperature of the optical detectors in these systems. Primarily, this cooling is performed by thermoelectric (TE) devices. These highly reliable devices have no moving parts, operate around room temperature and are easily integrated into thermal systems. In operation, electrical power is used to drive a current through the device creating a temperature difference across the faces. Despite the benefits of these solid-state devices, engineering applications have been limited by the relatively low intrinsic efficiency of the semiconductor materials comprising the devices. In fact, in night vision goggles, most of the energy drain is due to the need to cool these IR detector elements. More efficient TE materials would mean less batteries for the soldier to carry; in general, smaller power requirements means smaller weight for any defense system. Cooling systems are characterized by a coefficient of performance (COP) which is defined as the cooling power (output) divided by the input power. Typically, a thermoelectric device consists of a p-type semiconductor leg (hole majority carriers) and an n-type semiconductor leg (electron majority carriers). A figure of merit is defined for each material and is given by ZT=¿Ñ2¿a/¿U, where ¿Ñ is the Seebeck coefficient, ¿a is the electrical conductivity and ¿U is the thermal conductivity. In a thermoelectric device, the COP is related to a dimensionless figure of merit (ZT) of the constituent materials as well as the ratio of hot to cold junction temperature. A figure of merit greater than 1 for the combined n- and p-type thermoelectric elements is required to meet the COP objectives stated in the SBIR solicitation A06-118.
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