High thermoelectric energy conversion efficiency of a unicouple of n-type Mg3Bi2 and p-type Bi2Te3

Reliable N-type Mg3.2Sb1.5Bi0.49Te0.01/304 stainless steel junction for thermoelectric applications

Micro-thermoelectric devices

By ball milling alloyed bulk crystalline ingots into nanopowders and hot pressing them, we had demonstrated high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this study, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 in the temperature range of 75 and 100 °C has been achieved. This process is more economical and environmentally friendly than starting from alloyed bulk crystalline ingots. The ZT improvement is caused mostly by the lower thermal conductivity, similar as the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the increased grain boundaries of the nanograins, precipitates, nanodots, and defects. Our material also exhibits a ZT of 0.7 at 250 °C, similar to the value obtained when ingot was used. This study demonstrates that high ZT values can be achieved in nanostructured bulk materials with ball milling elemental chunks, suggesting that the approach can be applied to other materials that are hard to be made into ingot, in addition to its advantage of lower manufacturing cost.

http://absimage.aps.org/image/MAR09/MWS_MAR09-2008-006039.pdf

Enhanced Thermoelectric Figure-of-Merit in p-type Nanostructured Bismuth Antimony Tellurium Alloys Made from Elemental Chunks

Bismuth telluride (Bi2Te3)-based thermoelectric materials are well-known for their high figure-of-merit (zT value) in the low-temperature region. Stable joints in the module are essential for creating a reliable device for long-term applications. This study used electroless Co-P to prevent a severe interfacial reaction between the joints of solder and Bi2Te3. A thick and brittle SnTe intermetallic compound layer was successfully inhibited. The strength of the joints improved, and the fracture mode became more ductile; furthermore, there was no significant degradation of thermoelectric properties after depositing the Co-P layer after long-term aging. The result suggests that electroless Co-P could enhance the interfacial stability of the joints and be an effective diffusion barrier for Bi2Te3 thermoelectric modules.

Interfacial Stability in Bi2Te3 Thermoelectric Joints.

Traditional deposition methods of electroplating, sputtering and evaporation can produce weak and inconsistent interfaces between the electrode contact and Bi2Te3-based nanostructured thermoelectric (TE) materials. The electrode contact layers with low bonding strength are mainly due to the severe damage to the top layer of the TE material during surface preparation. We report a novel and cost-effective method for the surface preparation of thermoelectric materials (Bi2Te3) using pulsed light annealing prior to nickel electroplating. Short duration high-energy light absorption at the surface of the TE material causes reflow and annealing of surface structures, which enhance the electrical and thermal flow across the interface through more consistent and stronger bonding between the electrode metal and TE material. An electrical contact resistance reduction on the order of 5 μΩ cm2 and a pull strength of around 8 MPa were reproducibly measured for both p-type (Bi0.5Sb1.5Te3) and n-type (Bi2Te2.7Se3) compositions compared to electroplated nickel samples that did not receive pulsed light annealing treatment. This novel technique for surface preparation prior to metallization is applicable to a range of TE materials, and will lead to a significant improvement in the performance of TE modules and devices.

Pulsed-light surface annealing for low contact resistance interfaces between metal electrodes and bismuth telluride thermoelectric materials

All stars exhibit intensity fluctuations over several timescales, from nanoseconds to years. These intensity fluctuations echo off bodies and structures in the star system. We posit that it is possible to take advantage of these echoes to detect, and possibly image, Earth-scale exoplanets. Unlike direct imaging techniques, temporal measurements do not require fringe tracking, maintaining an optically-perfect baseline, or utilizing ultra-contrast coronagraphs. Unlike transit or radial velocity techniques, stellar echo detection is not constrained to any specific orbital inclination. Current results suggest that existing and emerging technology can already enable stellar echo techniques at flare stars, such as Proxima Centauri, including detection, spectroscopic interrogation, and possibly even continent-level imaging of exoplanets in a variety of orbits. Detection of Earth-like planets around Sun-like stars appears to be extremely challenging, but cannot be fully quantified without additional data on micro- and millisecond-scale intensity fluctuations of the Sun. We consider survey missions in the mold of Kepler and place preliminary constraints on the feasibility of producing 3D tomographic maps of other structures in star systems, such as accretion disks. In this report we discuss the theory, limitations, models, and future opportunities for stellar echo imaging.

/pdf/stellar-echo-imaging-of-exoplanets-2ey3eswfl5.pdf

Dan Mitchell

Papers

Pulsed-light surface annealing for low contact resistance interfaces between metal electrodes and bismuth telluride thermoelectric materials

Stellar Echo Imaging of Exoplanets