Thin-film Superlattice Thermoelectric Devices for Power Conversion and Cooling
Thin-film nanostructured materials offer the potential to dramatically enhance the performance of thermoelectrics and offer new capabilities ranging from microelectronics cooling to thermochemistry-on-a-chip to miniature power sources. We have recently demonstrated a significant enhancement in thermoelectric figure-of-merit (ZT) at 300K, of about 2.4 in p-type Bi2Te3/Sb2Te3 superlattices and ~1.2 in n-type Bi2Te3/Bi2Te3-xSex superlattices. For power conversion, it is also of interest to evaluate the ZT at higher temperatures. The ZT of the p-type superlattice material appears to increase from 2.45 at 300K to 2.92 at 400K. We will also discuss the progress in n-type Bi2Te3/Bi2Te3-xSex superlattices and our initial understanding on the reasons behind the less-than-dramatic performance of these materials compared to the p-type superlattices. We will discuss approaches to achieve an average ZT of ~2.5 with both p- and n-type superlattice couples through the temperature range of 450K to 300K, to achieve a power device conversion efficiency of 11.4% with a temperature differential of only 150oC. We have begun fabricating thin-film power device modules utilizing the p and n-type superlattice materials. We have achieved current densities in the range of 3.3A/cm2 in mini-modules with a DT across device of only 25K; bulk thermoelectric modules, for similar DT, offer 16 mA/cm2. Similarly, we have obtained an order or more larger power conversion levels with thin-film superlattice devices compared to commercial thin-film devices for similar DT. These material developments are projected to lead to thin-film power conversion device modules with an intrinsic (excluding fuel mass and other system components like thermal management) specific power in the range of 571 W/gm. This can be compared to a potentially achievable 4 W/gm using bulk modules. The thin-film devices, resulting from microelectronic processing, also allow cooling devices with ability to remove high heat-flux levels. We have obtained 32K and 40K sub-ambient cooling at 298K and 353K, respectively, in single p-type superlattice micro-thermoelements with potential localized active-cooling power densities approaching 700 W/cm2. In addition to high-performance and large cooling power densities, these thin-film microdevices are also extremely fast acting, within ~10 msec or about a factor of 23,000 faster than bulk thermoelectric technology. We will describe progress in p-n couple and module fabrication. We will discuss cooling module development including early demonstrations of a 2.5 cm x 3.5 cm large-area thin-film thermoelectric module with about 630 elements. We will discuss outstanding issues such as heat removal from the heat sink towards the full exploitation of this technology for both power conversion and cooling. Our approach is to develop thermal management compatible with the concept of High-Active-Flux, Low-Input-Output-Flux device. Thermal modeling data will be presented that indicates that it is possible to couple two orders of magnitude larger active heat-flux through the thermolements, by having low packing fraction of active thermoelectric devices on a high-efficiency heat spreader. Thus, while we operate with heat flux levels in the range of 500 to 1500 W/cm2 through the elements, depending on cooling or power conversion, heat flux levels in the range of 5 to 15 W/cm2 are expected at the heat sink.