Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material's thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but since then-despite investigation of various approaches-there has been only modest progress in finding materials with enhanced ZT values at room temperature. Here we report thin-film thermoelectric materials that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys. This amounts to a maximum observed factor of approximately 2.4 for our p-type Bi2Te3/Sb2Te3 superlattice devices. The enhancement is achieved by controlling the transport of phonons and electrons in the superlattices. Preliminary devices exhibit significant cooling (32 K at around room temperature) and the potential to pump a heat flux of up to 700 W cm-2; the localized cooling and heating occurs some 23,000 times faster than in bulk devices. We anticipate that the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications: for example, in thermochemistry-on-a-chip, DNA microarrays, fibre-optic switches and microelectrothermal systems.

Experimental I-V-Tc-ΔT data of thin-film superlattice thermoelectric modules is used to determine the internal ΔT, cross-plane Seebeck coefficient, effective thermal interface resistance, device ZT, and Qmax. We demonstrate 55K of external cooling at 300K (Tcmin=244.8K), with an estimated heat pumping capacity of 128W∕cm2. The average ZT300 for the best superlattice devices is 0.75, compared to 0.66 for a bulk BixSb2−xTe3∕Bi2SexTe3−x device. Our model indicates a significantly higher internal ΔT occurs across the active thermoelectric element, which was verified using buried thermocouples.

Te 3 -based superlattice (SL) thermoelectric (TE) devices are an enabling technology for high-power and low-temperature applications, which include low-noise amplifier cooling, electronics hot-spot cooling, radio frequency (RF) amplifier thermal management, and direct sensor cooling. Bulk TE devices, which can pump heat loads on the order of 10 W/cm 2 , are not suitable in these applications due to their large size and low heat pumping capacity. Recently, we have demonstrated an external maximum temperature difference, DT max , as high as 58 K in an SL thin-film p-n couple. This state-of-the-art couple exhibited a cold-side minimum temperature, T cmin , of À30.9°C. We regularly attain DT max values in excess of 53 K, in spite of the many significant electrical and thermal parasitics that are unique to thin-film devices. These measurements do not use any complex thermal management at the heat sink to remove the heat flux from the TE deviceÕs hot side. We describe here multistage SL cooling technologies currently being developed at RTI that can provide useful microcooling cold-side temperatures of 200 K. This effort includes a three-stage module employing independently powered stages which produced a DT max of 101.6 K with a T cmin of À75°C, as well as a novel two-wire three-stage SL cascade which demonstrated a T cmin of À46°C and a DT max of nearly 74 K. These RTI modules are only 2.5 mm thick, significantly thinner than a similar commercial three-stage module (5.3 mm thick) that produces a DT max of 96 K. In addition, TE coolers fabricated from these thin-film SL materials perform significantly better than the extrapolated performance of similar thickness bulk alloy materials.

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