Superlattice Thin-film Thermoelectric Materials and Devices
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Superlattice Thin-film Thermoelectric Materials and Devices Rama Venkatasubramanian1, Brooks O’Quinn, Edward Siivola, Kip Coonley, Pratima Addepally, Mary Napier, and Thomas Colpitts Research Triangle Institute, Research Triangle Park, NC 27709, USA
Abstract Thin-film nano-structured materials offer the potential to enhance the performance of thermoelectrics, with near-term capabilities like small-footprint coolers for lasers and microprocessors. Our recent focus has been to transition the enhanced figure-of-merit (ZT) in ptype Bi2Te3/Sb2Te3 and n-type Bi2Te3/Bi2Te3-xSex superlattices to performance at the module level with several device demonstrations. We have been able to obtain a best ZT of ~2 in a p-n couple, the fundamental cooling or power conversion unit in an operational module. In addition, we have been able to demonstrate p-n couple ZT of as much as 1.6 from heat-to-power efficiency data. The thermal interface resistances between the active device and the external heat source have been optimized. A power level of 38 mW per couple for a ∆T of about 107K, with 4-micron-thick element, was obtained. This translates to an active power density of ~54 W/cm2 and a mini-module power density of ~10.5 W/cm2. In short, power devices with thin-film superlattices are a real possibility. In the cooling arena, we have been able to obtain over 50K active cooling with thin-film modules, useable in several laser and microprocessor cooling needs. This is in spite of severe thermal management issues that had to be overcome noting that the “true” hot-side temperature, and hence the “true” ∆T, across the device are much higher. Even so, we have p-n superlattice couples that show twice the cooling ∆Tmax, compared to the best bulk p-n couples at cryogenic temperatures. Some of the challenges that remain to be addressed in the full development of this materials technology and thoughts on further progress in nano-structured materials are presented. Introduction Thin-film nano-structured materials offer the potential to dramatically enhance the performance of thermoelectrics, thereby offering new capabilities ranging from efficient cooling of small footprint communication lasers to eliminating hot spots in microprocessor chips in the near-term, to CFC-free refrigeration, portable electric power sources for replacing batteries, thermo chemistry-on-a-chip, etc. in the long-term. We demonstrated [1] a significant enhancement in the thermoelectric figure-of-merit (ZT) at 300K; a ZT of about 2.4 in 1-nm/5-nm p-type Bi2Te3 /Sb2Te3 superlattice structures and more recently, up to a ZT of 1.7 to 1.9 at 300K in 1-nm/4-nm in n-type Bi2Te3 /Bi2Te3-xSex superlattices have been measured. These improvements have been realized using the concept of phonon blocking, electron-transmitting superlattice structures. The phonon blocking arises from a complex localization-like behavior for phonons in nanostructured superlattices [2] and the electron transmission is facilitated by optimal choice of band-offsets in these semiconductor heterostructu
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