K. Coonley

Enhanced thermoelectric performance in PbTe-based superlattice structures from reduction of lattice thermal conductivity

We have fabricated two-dimensional n-type PbTe∕PbTe0.75Se0.25 structures using an evaporation process. In optimized films exhibiting a high-quality superlattice structure, a significant reduction in lattice thermal conductivity has been experimentally measured. The reduction would indicate enhanced thermoelectric device performance compared to standard PbTeSe alloys given that the electrical components, specifically, the Seebeck coefficient and electrical resistivity, were not observed to deteriorate from bulk values. The analysis of these films shows continuous layers with a true two-dimensional superlattice structure, as opposed to the PbTe∕PbSe system that exhibits zero-dimensional structures from self-assembly. The room-temperature measurement of cross-plane figure-of-merit in a n-type PbTe∕PbTe0.75Se0.25 device structure by the transient method has been combined with temperature-dependent measurements of in-plane resistivity and Seebeck coefficient to yield evidence of enhanced thermoelectric perform...

High efficiency segmented bulk devices cascaded with high-performance superlattice cold-stage

Segmented bulk single-couple devices have been fabricated using SiGe, PbTe, and TAGS materials. Initial optimization studies have yielded power generation efficiencies in excess of 12%, with cold-side temperatures of /spl sim/175/spl deg/C and hot-side temperatures of /spl sim/700/spl deg/C. The goal is to cascade these devices with high-performance Bi/sub 2/Te/sub 3/-superlattice cold-stage operating between 25/spl deg/C to 175/spl deg/C. We will be discussing the trade space between segmented and cascaded assemblies as it relates to the thermal and electrical matching between the different layers and device complexity. It will be shown how layer matching affects overall device performance and how this knowledge can be used to determine the optimal design. We will also discuss the methodologies used to meet the various challenges of high temperature materials assembly including ohmic contacts, diffusion barriers, and CTE induced stresses. Measurement results of device performance will be provided to illustrate the consequences of the methodologies used. We will also include results from early integration of these 2-stage segmented devices to thin-film superlattice cold-stage device to yield three stage power devices.

Thermal Stability of p-type Bi 2 Te 3 /Sb 2 Te 3 and n-type Bi 2 Te 3 /Bi 2 Te 2-x Se x Thermoelectric Superlattice Thin Film Devices

Thermolectric devices have been constructed using Bi 2 Te 3 /Sb 2 Te 3 and Bi 2 Te 3 /Bi 2 Te 2-x Se x superlattice thin films. Since these devices are intended for use in systems that will operate at elevated temperatures over their lifetime as in many power conversion applications, thermal stability of the thermoelectric figure of merit is an important consideration. The ZT e of p-type and n-type superlattice thin film elements was evaluated at specific intervals during exposure to elevated temperatures of 150°C for up to 60 hrs. Results indicate that the figure of merit for p- and n-type superlattice films is not compromised over time when exposed to these operating temperatures. In fact, both p- and n-type superlattice thin film ZT e remains very constant or improves slightly when subjected to continuous exposure to elevated temperatures. Evaluation of these thin film thermoelements is reported and implications of these results are considered for thin film thermoelectric modules.

Developing PbTe-based superlattice structures with enhanced thermoelectric performance

The fabrication of n-type PbTe/PbTe/sub 0.75/Se/sub 0.25/ structures using a simple evaporation technique has yielded high-quality superlattice films, a significant reduction in lattice thermal conductivity and potentially enhanced thermoelectric device performance, compared to standard PbTeSe alloys. The room temperature lattice thermal conductivity of PbTeSe alloys have been reduced by a factor of two or more using PbTe/PbTeSe superlattices in the cross-plane direction. Using this advantage, we have begun characterizing the cross-plane ZT of PbTe/PbTeSe superlattice devices, including the development of appropriate Ohmic contacts for the PbTe-material system. We will discuss various device process technologies for improved Ohmic contacts. The room-temperature measurement of cross-plane figure-of-merit in n-type PbTe/PbTe/sub 0.75/Se/sub 0.25/ device structure by the transient method will be reported. Also, these results will be combined with temperature dependent measurements of in-plane resistivity and Seebeck coefficient to yield evidence of enhanced thermoelectric performance. The results from similar p-type films, as well as preliminary data on heteroepitaxial films grown on Bi/sub 2/Te/sub 3/ will be discussed.

Development of wafer-scale cooling/heating thermoelectric arrays using thin-film superlattice devices

Thin-film superlattice thermoelectric material was used to fabricate 2-inch wafer scale thermoelectric module arrays. These arrays employ a promising thermoelectric device technology that exhibits a significant enhancement in the thermoelectric device figure of merit (ZT) at 300 K, cooling/heating power densities in excess of 100 Watts/cm/sup 2/, and response times significantly faster than bulk devices. To power and characterize these devices, we have developed a high-speed computer controlled multi-channel power supply with integrated real-time infrared imaging. This system allows creation of individual temperature control profiles, and exploits the rapid response time of the thin-film device. Using this system, we demonstrate high-speed operation and compare the response times of bulk and thin-film devices in the same format. Finally, applications of thin-film thermoelectric arrays are considered.