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Thermoelectrics

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In a thermoelectric device, a heat source creates a temperature differential across a semiconductor. The temperature differential drives the flow of current within the semiconductor, thereby converting heat to electrical power. To facilitate the flow of current, thermoelectric materials should exhibit high electrical conductivity. But to maintain a large temperature differential, the thermal conductivity should be low. Unfortunately, these two aims are often inconsistent. Motivated by the seminal contribution of Millie Dresselhaus, MIT EECS, Ted Harmon of MIT Lincoln Labs, and Millie's student Lyndon Hicks, recent work has focused on reducing the thermal conductivity by manipulating the nanostructure of the materials.

fig. 3 Schematic of thermoelectric device courtesy of Gang Chen, MIT MechE
fig. 4. Performance of thermoelectric power generators
Fig. 3. A schematic representation of a thermoelectric device. Image, courtesy of Gang Chen, MIT MechE.
  Fig. 4. The performance of thermoelectric power generators is quantified by the figure-of-merit, ZT=S2σT/k, where S is the Seebeck coefficient, σ the electrical conductivity, k the thermal conductivity, and T the absolute temperature. The use of nanomaterials, pioneered by Dresselhaus, et al., aims to increase thermoelectric efficiency by increasing the ratio of electrical to thermal conductivity, σ/k. Image courtesy of Gang Chen, MIT Mechanical Engineering.

High Power Density Thermoelectric Generation
Rajeev Ram

We have obtained thermoelectric power densities of up to 6W/cm2 from a 4.9µm-thick InGaAs-based superlattice incorporating semimetallic self-assembled ErAs nanodots. The ErAs dots and the superlattice both scatter phonons participating in cross-plane heat transport, reducing the thermal conductivity to below the alloy limit by nearly a factor of two. In addition to measuring the high power density, we have performed nanoscale thermal imaging of the devices with a temperature resolution on the order of 10 mK. The imaging technique allows us to examine heat transport in nanostructured thermoelectric materials.

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